WORKS OF
PROF. IRA O. BAKER
PUBLISHED BY
JOHN WILEY & SONS, Inc.
A Treatise on Masonry Construction.
Materials and Methods of Testing Strength,
etc.; Combinations of Materials— Composi-
tion, etc.; Foundations — Bearing Power of
Soils, etc.; Masonry Structure — Dams, Re-
taining Walls, Abutments, Piers, Culverts,
Vousoir Arches, Elastic Arches. Tenth Edi-
tion, Re-written and Enlarged. 8vo, 745
pages, 244 figures, cloth, $5.00.
Engineers' Surveying Instruments.
Their Construction, Adjustment and Use.
Second Edition, Revised and Greatly En-
larged, 12mo, ix+391 pages, 86 figures,
cloth, $3.00.
A Treatise on Roads and Pavements.
8vo, xi + 667 pages, 235 figures, 80 tables,
cloth, $5.00.
A TREATISE
ON
ROADS AND PAVEMENTS
BY
IRA OSBORN BAKER, C.E., D. ENG'G
Professor of Civil Engineering, University of Illinois; Author of Masonry
Construction, Engineer's Surveying Instruments; Member of
American Society of Civil Engineers, Western
Society of Engineers, Society for Promo-
tion of Engineering Education
THIRD EDITION
RE-WRITTEN AND ENLARGED
FIRST IMPRESSION
NEW YORK
JOHN WILEY & SONS, INC.
LONDON; CHAPMAN & HALL, LIMITED
1918
Copyrighted 1903, 1913, 1918,
BY
IRA OSBORN BAKER
PRESS OP
• RAUNWORTH It CO.
BOOK MANUFACTURERS
BROOKLYN. N. V.
PREFACE
FIRST. EDITION
THE object of this book is to give a discussion from the point of
view of an engineer of the principles involved in the construction of
country roads and of city pavements. The attempt has been made
to show that the science of road making and maintenance is based
upon well-established elementary principles, and that the art depends
upon correct reasoning from the principles rather than in attempting
to follow rules or methods of construction. In some cases prac-
tical experience has not yet determined the best method of pro-
cedure, and in these cases the conflicting views with the reasons for
each are fully stated.
Considerable space has been given to the economics and location
of country roads and to the construction and maintenance of earth
roads, since such roads constitute more than ninety-five per cent of
the mileage of the public highways and are greatly in need of careful
consideration. It is frequently claimed by engineers that the public
would be benefited by placing the care of the roads in the hands of
engineers; but there is no evidence that any considerable number of
engineers comprehend either the principles of road making necessary
for the improvement and maintenance of our country roads, or the
economic limitations and political d fficulties of the problem. The
first five chapters of this book are offered as a contribution to this
phase of the good-road problem.
The remainder of the book, the portion that considers roads
having permanently hard surfaces, which may not unfittingly be
said to relate to urban and suburban roads, is based chiefly upon
American experience, because the principles of road making worked
out in this country are probably best suited to American conditions,
and also because in most particulars American roads and pave-
ments are superior to any other in the world. Some countries
have more hard roads than this, because of a difference in condi-
tions; but in no country does the quality of such roads average
iii
382053
PREFACE — FIRST EDITION
better than in this. In some foreign cities the pavements seem to
be better cared for than in this country, owing chiefly to different
controlling conditions; but the principles of construction employed
here are equal to the best. Notwithstanding the general excellence
of the best American practice in constructing hard roads and pave-
ments, there is still room for improvement in adapting the particular
form of construction to the local conditions and also in preserving
the surface from ruthless destruction. These two phases of the
subject have been emphasized in the proper places in this volume.
Throughout the attempt has been to state fully and clearly the
fundamental principles of the construction and maintenance of roads
and pavements.
In the preparation of the book the endeavor has been to observe
a logical order and a due proportion between the different parts;
and great care has been taken in classifying and arranging the matter.
It will be helpful to the reader to notice that the volume is divided
successively into parts, chapters, articles, sections having small-
capital black-face side-heads, sections having lower-case black-face
side-heads, sections having lower-case italic side-heads, and sections
having simply the serial number. In some cases the major subdi-
visions of the sections are indicated by small numerals. The con-
stant aim has been to present the subject clearly and concisely.
Every precaution has been taken to present the work in a form
for convenient practical use and ready reference. Numerous cross
references are given by section number; and whenever a table or a
figure is mentioned, the citation is accompanied by the number of
the page on which it may be found. The table of contents shows
the general scope of the book; the running title assists in finding
the different parts; and a very full analytical index makes every-
thing in the book easy of access.
The author will esteem it a favor if any errors that may be found
are at once brought to his notice.
I, O. B.
CHAMPAIGN, ILL.,
November 27, 1902.
PREFACE
THIRD EDITION
THE numerous changes in methods of road and pavement con-
struction have made necessary a radical revision of this volume.
Five years ago, before many of the changes had become well estab-
lished, a second edition was issued containing a supplementary
chapter which briefly treated some of the new forms of construction.
The present edition has been thoroughly revised and entirely re-
written. Five chapters of minor importance have been dropped to
make room for an equal number of new ones. The number of illus-
trations has been greatly increased. No pains have been spared to
bring the book up to date and to fully present the best modern
practice.
Attention has been given to materials and forms of construction
that affect the quality and cost of the road and pavement, rather
than to the machines employed and the methods of doing the work.
In other words, the book is intended more for the one who designs
and inspects the road or pavement than for the contractor who
constructs it. In recent years there have been developed a number
of machines for doing road work and for handling road-building and
paving materials that have greatly reduced the cost of road and
pavement construction; but to have included an adequate discussion
of such appliances and of modern methods of operation and organi-
zation would have greatly increased the size of the book.
Photographs from which illustrations have been made were
obtained from the following:
Austin Manufacturing Co., Chicago, 111.
Baker Manufacturing Co., Springfield, 111.
Barber Asphalt Paving Co., Philadelphia, Pa.
Barrett Manufacturing Co., New York City, N. Y.
Cressy Contracting Co., Boston, Mass.
Granite Paving Block Manufacturing Association of the U. S., Boston, Mass.
Illinois Paving Brick Publicity Bureau, Chicago, 111.
Illinois State Highway Department, Springfield, 111.
VI PREFACE — THIRD EDITION
Metropolitan Paving Brick Co., Canton, Ohio.
National Paving Brick Manufacturers Association, Cleveland, O.
Portland Cement Association, Chicago, 111.
Standard Oil Co., New York City, N. Y.
U. S. Office of Public Roads and Rural Engineering, Washington, D. C.
Mr. Walter Buehler, Chicago, 111.
Mr. John S. Crandell, New York City, N. Y.
Mr. Harlan H. Edwards, Danville, 111.
Mr. Richard H. Gillespie, Chief Engineer of Streets, Bronx, New York City.
Dr. Herman von Schrenk, St. Louis, Mo.
The following persons have generously given valuable suggestions
on the subject stated.
Mr. Arthur N. Johnson, Chicago, Concrete Roads.
Mr. Walter Buehler, Chicago, Wood-block Pavements.
Mr. Philip P. Sharpies, New York City, Bituminous Materials.
Mr. John W. Stipes, Champaign, 111., many matters.
Mr. D. T. Pierce, Philadelphia, Sheet Asphalt Pavements.
Mr. W. C. Perkins, Conneaut, Ohio, Brick Pavements.
Mr. George N. Norton, Buffalo, N. Y., Cost of Maintenance of Sheet Asphalt
Pavements.
Mr. Walter L. Weeden, Boston, Granite Block Pavements.
Valuable data were received from many, which are specifically
acknowledged in the text.
To all of these the author gratefully acknowledges his obligations.
I. O. B.
URBANA, ILLINOIS,
January 8, 1918.
TABLE OF CONTENTS
PART I. COUNTRY ROADS
PAGE
INTRODUCTION 1
CHAPTER I. ROAD ECONOMICS AND ROAD ADMINISTRATION
ART. 1. ROAD ECONOMICS. Advantages of Good Roads. Cost of
Wagon Transportation. Financial Value of Road Improvements. Tractive
Resistance. Power of Horse. Travel Census. Weight and Width of
Vehicles 3
ART. 2. ROAD ADMINISTRATION. Administrative Unit. State Aid.
National Aid. Classification of Roads. Road Taxes 31
CHAPTER II. ROAD LOCATION
Elements Involved. Distance. Grade. Rise and Fall. Curves.
Width. Cross Section. Placing the Line. Establishing Grade. Example
of Re-location 41
CHAPTER III. EARTH ROADS
ART. 1. CONSTRUCTION. Width. Cross Section. Grades. Drainage.
Excavation and Embankment. Improving Old Roads. Road Building
Machinery. Cost of Earthwork. Bridges. Waterways. Culverts. Retain-
ing Walls. Guard Rails. Guide Posts. Artistic Treatment 70
ART. 2. MAINTENANCE. Destructive Agents. Care of Surf ace : Road
Drags and Rules for Using; Scraping Grader and Rules for Using; V Road-
Leveler. Care of Side Ditches. Care of Roadside. Obstruction by Snow.
Systems of Maintenance. Expenditures for Maintenance 115
ART. 3. SURFACE OILING. Preventing Dust. Effect of Oiling on
Maintenance. Preparing Surface. The Oil. Applying the Oil. Cost 133
CHAPTER IV. SAND AND SAND-CLAY ROADS
ART. 1. SAND ROADS. Drainage. Grading. Shade. Hardening the
Surface 139
ART. 2. SAND-CLAY ROADS. The Design. Natural Mixture of Sand
and Clay. Sand on Clay Subgrade. Clay on Sand Suhgrade. Cost.
Maintenance 140
vii
Vlll CONTENTS
CHAPTER V. GRAVEL ROADS
PAGE
ART. 1. THE GRAVEL. Requisites for Road Gravel. Distribution of
Gravel. Characteristics of Different Gravels 150
ART. 2. CONSTRUCTION. Drainage. Width. Maximum Grade. Crown.
Forms of Construction: Surface Construction; Trench Construction.
Bottom Course. Screening the Gravel. Hauling the Gravel. Measuring
the Gravel. Cost. Economic Value of Gravel Surface. Specifications. . . . 165
ART. 3. MAINTENANCE. Destructive Agents. Method of Maintenance.
Cost 178
ART. 4. DUST PALLIATIVES. Dust Preventives: Fresh Water, Sea
Water, Deliquescent Salts, Proprietary Compounds, Sprinkling with Oil. . . . 181
CHAPTER VI. WATER-BOUND MACADAM ROADS
HISTORY 185
ART. 1. THE STONE. Requisites for Road Stone. Methods of Testing
Stone 186
ART. 2. CONSTRUCTION. Forms of Construction. Width. Crown.
Thickness. Cross Section. Permissible Grades. Preparing Subgrade.
Crushing the Stone. Spreading the Stone. Road Rollers. Rolling the
Stone. Binding the Stone. Cost of Construction. Specifications 189
ART. 3. MAINTENANCE. Leveling. Ruts. Patching. Rolling. Sprink-
. ling. Cost 223
CHAPTER VII. PORTLAND-CEMENT CONCRETE ROADS
DEFINITIONS. HISTORY .• 227
ART. 1. THE MATERIALS. Cement. Fine Aggregate. Coarse Aggre-
gate. Theory of Proportions. Methods of Proportioning. Data for
Estimates 227
ART. '2. THE CONSTRUCTION. Drainage. Preparing the Subgrade.
One- vs. Two-course Pavements. Cross Section. Maximum Grade.
Width. Thickness. Crown. Side-forms. The Concrete: proportions,
mixing, consistency, placing, striking, finishing, curing, protecting. Con-
traction Joints. Reinforcement. Shoulders. Curbs. Cost of Concrete
Roads. Characteristics of Concrete Roads. Concrete Street Pavements. . 238
ART. 3. MAINTENANCE. Character of Work Required. Cost of Main-
tenance 264
CHAPTER VIII. BITUMINOUS ROAD MATERIALS
DEFINITIONS 267
ART. 1. ASPHALT. Definitions. Characteristics of Asphalt. Sources
of Asphalt. Specifications for Asphalt: for Bituminous Surfaces, Binder for
Bituminous Macadam, Binder for Bituminous Concrete, Sheet Asphalt,
Filler for Block Asphalt. Cost .267
ART. 2. PETROLEUM. Classification. Methods of Refining. Ship-
ping. Asphaltic Content of Road Oils. Specifications for Oil: for Park
Drives, Earth Roads, Gravel Roads, Macadam Roads. Cost 283
CONTENTS IX
PAGE
ART. 3. TAR. Definitions. Characteristics of Tar. Specifications:
for Bituminous Surface, Bituminous Macadam, Bituminous Concrete, Joint
Filler for Block Pavements. Cost. . 289
CHAPTER IX. BITUMINOUS SURFACES FOR ROADS
KINDS OF BITUMINOUS SURFACES 296
ART. 1. PROTECTIVE COATING. Bituminous Material 297
ART. 2. BITUMINOUS CARPETS. Bituminous Material. Cleaning Road
Surface. Applying Bituminous Material. Value of Bituminous Carpets.
Maintenance. Cost 298
CHAPTER X. BITUMINOUS MACADAM AND BITUMINOUS CON-
CRETE ROADS
ART. 1. BITUMINOUS MACADAM ROADS. Foundation. Maximum Grade.
Crown. Wearing Course. Bituminous Binder. Tar-sand Macadam. Char-
acteristics of Bituminous Macadam. Costs. Maintenance 306
ART. 2. BITUMINOUS CONCRETE ROADS. The Aggregate. The Binder.
Mixing. Laying. Seal Coat. Cost. Comparison of Bituminous Mac-
adam and Bituminous Concrete , . . . 312
PART II. STREET PAVEMENTS
CHAPTER XI. PAVEMENT ECONOMICS AND PAVEMENT
ADMINISTRATION
ART. 1. PAVEMENT ECONOMICS. Benefits of Pavements. Investment
in Pavements 318
ART. 2. PAYEMENT ADMINISTRATION. Importance of Problem. Ap-
portionment of Cost. Special Assessments. Guaranteeing Pavements.
Tearing Up Pavements 321
CHAPTER XII. STREET DESIGN
Street Plan: checkerboard, diagonal, concentric. Size of Lots and
Blocks. Width of Streets. Area of Streets. Width of Pavement. Street
Grades. Crown of Pavement. Cross Sections of Side-hill Streets. Plans
and Specifications. Street Trees 336
CHAPTER XIII. STREET DRAINAGE
Underdrainage. Catch Basins. Gutters. Drainage at Street Inter-
section. Surface Drainage. Crown, Rules for 361
CHAPTER XIV. CURBS AND GUTTERS
Curb: Stone, Concrete, Combined Concrete Curb and Gutter. Radius
of Curb at Street Corner. Combined Curb and Walk . . 378
X CONTENTS
CHAPTER XV. PAVEMENT FOUNDATIONS
PAGE
ART. 1. PREPARATION OF SUBGRADE. Drainage. Rolling the Sub-
grade. Filling Trenches 392
ART. 2. THE FOUNDATION. Portland-Cement Concrete: Thickness,
Proportions, Mixing and Placing, Finishing, Curing. Cost. Old Macadam.
Bituminous Concrete Foundation 399
ART. 3. FOUNDATIONS FOR STREET-RAILWAY TRACKS. Foundation.
Ties. Rails. Paving 407
CHAPTER XVI. ASPHALT PAVEMENTS
ART. 1. SHEET ASPHALT PAVEMENTS. Classification. History. Foun-
dation: portland-cement concrete, bituminous concrete, other forms. Binder
Course: open, closed; cement; bitumen in binder; mixing; laying; thick-
ness. Wearing Coat: sand; filler; cement; bitumen in wearing coat; mixing;
laying; rolling; thickness. Asphalt Adjacent to Tracks. Causes of Fail-
ure. Methods of Repairing. Cost of Construction. Cost of Mainte-
nance. Maximum Grade. Crown. Merits and Defects. Specifica-
tions 411
ART. 2. ASPHALT CONCRETE PAVEMENTS. Definitions: Bitulithic,
Warrenite, Amiesite, Topeka mixture, stone-filled sheet-asphalt, asphalt-
concrete pavement. Mixing and Laying. Cost. Merits and Defects.
Specifications 461
ART. 3. ROCK ASPHALT PAVEMENTS. Construction 469
ART. 4. ASPHALT BLOCK PAVEMENTS. The Blocks. Cost. Merits and
Defects... . 470
CHAPTER XVII. BRICK PAVEMENTS
ART. 1. THE BRICK. The Clay. Manufacture of the Brick. Kinds
of Brick. Size. Testing the Brick. Service Tests 474
ART. 2. CONSTRUCTION. Foundation. Bedding Course* sand
cement-sand, mortar. Laying the Brick. Joint Filler: sand, hydraulic
grout, bituminous cement, tar-sand. Expansion Joints. Comparison of
Types. Brick Adjacent to Track. Maximum Grade. Brick Streets.
Brick Roads. Cost. Merits and Defects. Specifications 503
ART. 3. MAINTENANCE. Repairs: soft brick, shrinkage of sand cush-
ion, settlement of foundation, settlement of trench, defective grouting,
bulges, re-laying pavement, cracks. Re-surfacing: asphalt top, tar top,
turning the brick, monolithic brick top. Cost of maintenance 552
CHAPTER XVIII. STONE-BLOCK PAVEMENTS
Nomenclature: Roman roads, cobble-stone, Belgian-block, oblong-block,
durax pavements 566
ART. 1. THE STONE: granite, Medina sandstone, Potsdam sandstone,
Sioux Falls quartzite, Kettle River sandstone, limestone 569
CONTENTS
PACE
ART. 2. CONSTRUCTION. Foundation. Bedding Course: sand, mortar.
The Blocks: dressing, re-cutting, size, measuring. Setting. Ramming.
Filling Joints. Expansion Joints. Paving Adjacent to Track. Maximum
Grade. Merits and Defects. Durax Pavement. Cost: the blocks, granite-
block, Medina-sandstone block, and durax pavements , . 572
ART. 3. MAINTENANCE. Re-laying. Re-filling Joints. Spalling Joints.
Raising Low Blocks. Settlement of Foundation. Settlement of Trench. . . . 597
CHAPTER XIX. WOOD-BLOCK PAVEMENTS
KINDS OF WOOD-BLOCK PAVEMENTS. HISTORY 601
ART. 1. MATERIALS AND TREATMENT. The Timber. Causes of Decay.
Preservative. Treatment 603
ART. 2. CONSTRUCTION. Bedding Course: sand, cement-sand, mor-
tar, bituminous cement. Laying the Blocks. Joint Filler: sand, tar pitch.
Open-joint Construction. Expansion Joints. Cost. Merits and Defects.
Specifications 612
ART. 3. MAINTENANCE. Removing Poor Blocks. Raising Low Spots.
Re-laying over Trenches. Lowering Bulges. Bleeding. Cost of Main-
tenance 628
CHAPTER XX. SELECTING THE BEST PAVEMENT
KINDS OF PAVEMENTS 633
ART. 1. THE DATA FOR THE PROBLEM. Durability. Requirements of
the Ideal Pavement: cost of construction, cost of maintenance, tractive
resistance, slipperiness, ease of cleaning, noiselessness, healthfulness, freedom
from dust and mud, temperature 635
ART. 2. THE SOLUTION OF THE PROBLEM. Economic Solution. Non-
economic Solution , 651
ROADS AND PAVEMENTS
INTRODUCTION
THE problems involved in the construction and maintenance
of rural highways differ materially from those which are encountered
in the improvement and care of city streets, and therefore this
discussion of the subject of Roads and Pavements will be divided
into Part I, Country Roads, and Part II, City Pavements. In
each division of the subject certain general principles will first be
considered, and the further discussion will be divided according
to the several materials in use for road surfaces. It will not always
be possible to keep the several portions entirely distinct, but a
knowledge of the intention in this respect will make it easier to
understand the method of presentation or to turn readily to the
discussion of any particular phase of the subject.
The classification of road surfaces into country Roads and
City Pavements is partly according to the most general use of
each, and partly according to the elaborateness of the construction.
According to the somewhat loose classification here adopted road
surfaces for country roads consist of two parts, subgrade and
wearing coat; while city pavements consist of four parts, sub-
grade, foundation, a binder or cushion course, and a wearing coat.
1
PART I
COUNTRY ROADS
PART I will include matters relating to earth, sand and sand-
clay, gravel, water-bound macadam, bituminous macadam, and
concrete roads in rural districts, although some of the discussion
is also applicable to these road surfaces when employed in city
streets.
CHAPTER I
ROAD ECONOMICS AND ROAD ADMINISTRATION
ART. 1. ROAD ECONOMICS
1. ADVANTAGES OF GOOD ROADS. Good roads are so im-
portant to the financial, social and educational well-being of a rural
community that no enumeration of their advantages is likely to
include all the benefits; but a brief consideration of some of the
chief advantages of good roads will be of value in determining the
amount of money that may justifiably be expended to secure road
improvement and in deciding who should in equity bear this ex-
pense. The principal advantages of good roads, i. e., of permanently
hard ones, are as follows:
1. Good roads decrease the cost of transportation, — at some
seasons of the year considerably, but at others very little. This
item will be considered more fully later (see § 4-9).
2. Good roads make the marketing of crops easier. This ad-
vantage results in extending the area devoted to the cultivation
of fruits and vegetables, and is most effective in the vicinity of
a large city.
3. Good roads give a wider choice of time for the marketing of
crops. In some instances good roads permit the marketing of crops
when the labor of production is not pressing; but this advantage
3
4 ROAD ECONOMICS AND ROAD ADMINISTRATION JCHAP. I
accrues only to the producers of imperishable crops, and is not
of great importance since the labor required to market the product
is small in comparison with that of production.
4. Good roads permit the marketing to be done when prices
are most favorable. This advantage is more important with per-
ishable than with imperishable products. As far as perishable prod-
ucts are concerned, this advantage is virtually included in par-
agraph 2 above. As far as imperishable products are concerned,
this advantage is important only near a large city, i. e., where the
producer hauls to the market. Prices of staple farm products (not
garden products) are not much affected by roads, since the con-
dition of the roads is local while prices are governed by world-wide
conditions. Writers on good-road economics usually greatly over-
estimate this advantage as far as the ordinary producer of imper-
ishable products is concerned. If this advantage were anything
like as great as is frequently claimed, producers would store such
products at the local shipping point, or in the great city, or at the
port of export, awaiting a favorable price. Such storage would
also permit the delivery at a time when other work was least
pressing. The expense of storage at the local shipping point is a
small per cent of the value of the product. It is frequently, but
erroneously, claimed that hard roads would save the Illinois farmer
3 to 5 cents per bushel — an amount 10 to 15 times the cost of
storage. Since producers do not so store their products, it is safe
to assume that this advantage of good roads as a rule, is not very
great. The present method of doing business makes this advantage
comparatively unimportant.
5. Good roads give a wider choice of the market place. This
advantage affects perishable products chiefly, and for geographical
reasons is, as a rule, not very great.
6. Good roads tend to equalize the produce market between
different weather conditions. In the absence of railroad transpor-
tation and cold storage, this advantage might be of considerable
local importance; but under ordinary conditions it is comparatively
unimportant.
7. Good roads tend to equalize railroad traffic between the
different seasons of the year. Impassable wagon roads over any
considerable area materially decrease the amount of agricultural
products to be transported by railroads, and a return of good roads
will for a time congest the railroad transportation facilities. The
effect of good roads in equalizing railroad transportation is partially
ART. 1] ROAD ECONOMICS
neutralized by the fact that agricultural products are only one of
many classes of commodities transported by the railroads; and
also by the fact that most railroads transport agricultural products
originating over a considerable area, and bad wagon roads are not
likely to occur over all the contributory area at the same time;
and further by the fact that the storage capacity of warehouses
helps to equalize the traffic.
8. Good roads tend to equalize mercantile business between
different seasons of the year. Merchants having a considerable
rural custom could do business more economically if the trade
were distributed uniformly throughout the yea*. However, the
succession of good and bad wagon roads is only one cause of the
unequal distribution of rural patronage.
9. Good roads permit more easy intercourse between the mem-
bers of rural communities, and also between rural and urban pop-
ulations. This is an important benefit, particularly under a demo-
cratic form of government.
10. Good roads facilitate the consolidation of rural schools, and
thereby increase their economy and efficiency. This is an impor-
tant matter to coming generations.
11. Good roads facilitate rural mail delivery, and thereby tend to
improve the social and intellectual condition of the rural population.
12. Good roads sometimes change rural into suburban property.
13. Good roads are a material factor in stimulating tourist
travel and making rural communities attractive to vacation residents.
2. It is customary to include the increase in the price of farming
land as one of the benefits of good roads; but the increase in price
of land is simply the measure of the value of all the above advan-
tages, and hence should not be included.
3. Notice that the first eight advantages mentioned above relate
to the financial benefits of good roads, and the last four to the
social benefits. In the past writers upon good-road economics have
given much attention to the supposed financial benefit of hard roads
and little or none to the social advantage. Any considerable ex-
penditure for the improvement of rural highways can not be justi-
fied on financial grounds alone (§ 12). Good roads are desirable
for the same reason that a man buys an automobile or builds a
fine house, i. e., because they are a comfort and a pleasure. Good
roads should be urged principally for the same reason that good
schools are maintained, namely, because they increase the intel-
ligence and value of the citizen to society.
6 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
4. COST OF WAGON TRANSPORTATION. The chief financial
advantage of hard roads is the decreased cost of transportation.
It is proposed to inquire briefly concerning the cost of wagon trans-
portation with a view of determining the proportion of this cost
that may be saved by road improvement.
In this connection, a distinction must be made between the
cost to those whose chief business is to sell transportation, and
the cost to those to whom transportation is a mere incident of a
business organized for some other purpose. The first class is rep-
resented by a freighter, and the second by a farmer. The former
maintains his teams and wagons only to transport freight, while
the latter ordinarily keeps his teams and wagons primarily for
general farm work of which transportation on the roads is only
a small part. In some cases the traffic to be considered is prin-
cipally that by freighters, but usually the chief traffic over country
roads is that connected with agricultural operations.
Again, consideration should be given only to hauling in which
the load is equal to the full capacity of the team for the particular
condition of the roads. A farmer may employ a two-horse team
to take a bushel of potatoes to town, or a grocery wagon may make
a trip to deliver a pound of cheese; but the partial load is entirely
independent of the condition of the roads.
Further, it is necessary to notice that only the rate for full loads
should be considered. If a number of packages are carried in the
same load for different parties, part of the charge is to cover the
cost of collection, distribution, possible partial loads, etc.; and
therefore only part of the charge is for transportation proper.
5. Cost to Freighters. The cost will vary greatly with the
conditions of the service, i. e., with the character of the road surface,
the average grade of the road, the maximum grade, return load, etc.
Except in rare cases, the cost per ton-mile for loads one way
upon earth roads will not be more than 25 cents, and ordinarily it
will not be more than 15 to 20 cents;* while with easy grades and
favorable road surface it may be as low as 10 to 15 cents, and with
long hauls, return loads, and favorable road surface, it may be 8
to 10 cents. When the last price is obtained there is little need or
opportunity for road improvement.
6. Cost to Farmers. In this division of the subject, a distinc-
tion should be made between producers of perishable products and
* See "Cost of Wagon Transportation," by the author, in Proceedings of Illinois Society
of Engineers, Vol. 16 (1901), p. 3&-44; full abstract of the same in Engineering News, Vol.
45 (1901), p. 86.
ART. 1] ROAD ECONOMICS
producers of non-perishable products. The first class is represented
by gardeners, dairymen, fruit-growers, etc.; and the second, by
producers of hay, grain, cotton, etc.
The cost of transportation is much greater for perishable than
for non-perishable products. In the first place, the marketing of
perishable products is an important factor in comparison with
the cost of production, and frequently necessitates an independent
transportation department; while the labor of marketing non-
perishable products is comparatively small — particularly as in most
localities where there is much of this class of produce, the distance
from the farm to the railroad station is short. Further, perishable
products must go to market whatever the condition of the roads,
while non-perishable ones can wait for comparatively favorable
conditions; and finally, the former frequently go to market in
partial loads, and the second usually in full loads. Except in com-
paratively limited districts, non-perishable products make up the
bulk of the traffic on the country roads. According to the U. S.
Census of 1890, the gardeners, fruit-growers, dairymen, vine-growers,
florists, and nurserymen constitute only 1.8 per cent of the so-called
farming class.
7. The cost of transporting perishable products is probably
greater than that for any other class of traffic over the country
roads; but as it is next to impossible to secure any reliable data
no attempt will be made to present any general conclusions. For
several reasons, this traffic will usually justify larger expense for
road improvement than any other.
The cost of transportation to farmers proper, i. e., producers of
non-perishable products, depends chiefly upon the condition of the
road surface and upon the demands of general farm work. Loam
or clay roads are reasonably good when dry, and are therefore at
least passable most of the year; while sand roads are at their worst
when dry, and are therefore in their worst condition during the
greater part of the year. Fortunately sand roads are less common,
the country over, than clay or loam roads. In the crop season,
with a little choice as to the time of doing the work the cost on
fairly level loam or clay roads is probably not more than 10 to
12 cents per ton-mile; and when farm work is not pressing, the cost
is not more than 8 to 10 cents per ton-mile.*
* See "Cost of Wagon Transportation," by the author, in Proceedings of Illinois Society
of Engineers, Vol. 16 (1901), p. 36-44; full abstract of the same in Engineering News, Vol.
45 (1901), p. 86.
8
ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
8. A Conflicting View. In current literature on road economics,
the claim is frequently made that the cost of wagon transportation
to the farmer is considerably more than stated in § 7. Apparently
most of these claims are based, either directly or indirectly, upon
data published in Circular 19 of the Road Inquiry Office of the
United States Department of Agriculture under date of April 4,
1896. Table 1 is a brief summary from that circular.
The conclusions of this circular have been so widely quoted and
so generally accepted as to justify a brief consideration. In former
editions of this book these conclusions were somewhat carefully
considered, and the following is a brief summary of that investigation.
TABLE 1
AII/EGED COST OP WAGON TRANSPORTATION
Ref.
No.
Locality.
Average
Distance
Hauled,
Miles.
Average
Load
Hauled,
Tons.
Average
Cost per
Ton-mile,
Cents.
Total Cost
from Farm
to Market,
per Ton.
I
Eastern States.
5 9
1.108
32
$1 89
9
Northern States
6 9
27
1 86
3
Middle-Southern States
8.8
31
2 72
4
Cotton States
12.6
0.688
25
3 05
5
Prairie States
8.8
1.204
22
1 94
6
Pacific Coast and Mountain. . . .
23.3
1.098
22
5.12
7
Whole United States
12 1
1 001
25
3 02
Circular 19 concludes that 313,349,227 tons were hauled over
the highways of the United States in 1895 at a cost of $3.02 per
ton-mile, or a total cost of $946,314,665.54; and that the annual
cost of transporting the crops of the United States to market was
26.6 per cent of the price of the crops at the market. These con-
clusions are greatly in error chiefly for the following reasons:
1. The investigation was not very elaborate, since replies were
received from only one county in thirty.
2. The average distance hauled seems to be about twice too
great.
3. The value for the average load hauled is approximately
twice too great. The mean between the maximum and the mini-
mum load may be one ton; but the great bulk of teaming is done
when the roads are at least in fair condition, when the load is con-
ART. 1] ROAD ECONOMICS 9
siderably more than one ton. The author has examined the records
of several grain buyers in central Illinois, where at times the roads
are as bad as anywhere, and finds that the average load is nearly
a ton and a half. Statistics for marketing over 300,000 bushels
of corn in Illinois gives the average load as almost exactly 2
tons.
4. The cost per ton-mile indicates that this value was obtained
by assuming that the wages of wagon, team, and driver are 35 cents
per hour; that the team travels 3 miles per hour; and that the
team hauls a load only one way. The price per hour for a team
is too great, since the cost per day as reported by 316 farmers in
76 counties in Illinois varied during crop time from $1.40 to $2.74,
the average being only $2.13, or say 21 cents per hour.
5. No account was taken of the relative amounts of traffic in
the several states.
9. Under date of Feb. 28, 1907, the Bureau of Statistics of the
U. S. Department of Agriculture published Bulletin 49 — Cost of
Hauling Crops from Farm to Shipping Point. In the latter investi*.
gation the cost of hauling the twelve leading farm products from
the farm to the shipping point during the crop year of 1905-06
is said to be $84,684,000; whereas in the first investigation (§8),
the cost in 1895 of hauling all crops from the farm to the market
was said to be $652,000,000. Notice that the cost according to
the later and more elaborate investigation is only about one eighth
of that by the former investigation, notwithstanding the fact that
the weight of the seven leading crops was almost exactly 50 per
cent greater in 1905 than in 1895; in other words, on the face of
the returns, the result in the first bulletin is about sixteen times
too great.
Further, the result of the second investigation is subject to
errors 2, 3, and 4 of § 8. Besides, the second bulletin is greatly
in error for the following reasons: It finds the cost of hauling crops
from the farm to the market to be $72,984,000; and then adds
$11,700,000 as the cost of hauling wheat to local mills to be ground.
This allowance is altogether too great, since it assumes that more
than one third of the wheat not used for seed is ground at the local
mill; while only an inappreciable quantity is so used.
Correcting the above errors would reduce the total of the second
bulletin to one half or one third, and make the result in the first
bulletin thirty to fifty times too great. Unfortunately the results
of the first investigation are frequently used in discussions on road
10 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
economics, and the object of this note is to show their utter unre-
liability.*
10. Possible Annual Saving. The Office of Road Inquiry, in
Circular No. 19, to which reference has been made, estimates the
possible annual saving by road improvement as $628,000,000.
This estimate is based upon a comparison of the data in Circular
No. 19 with that on the "Cost of Hauling Farm Products to Market
or Shipping Point in European Countries, Collected by U. S. Con-
sular Agents," published in Circular No. 27 (Feb. 5, 1897) of the
Office of Road Inquiry of the U. S. Department of Agriculture.
The average cost as given in the latter circular is 10 cents per ton-
mile, and the difference between this and the average stated in
Table 1 is 15 cents per ton-mile, which is two thirds of the average
value in Table 1.
Concerning the data for America, notice that they are taken
from Circular No. 19, and are greatly in error as has already been
shown. Concerning the data for Europe, notice in the first place
that they are open to most of the criticisms made against the data
in Table 1. In the second place, the twelve results given in Cir-
cular 27 vary from 4 to 30 cents per ton-mile, which is too wide a
range and too few results for an accurate determination of the
average cost of wagon transportation in Europe. In the third
place, some of the results are professedly the cost to transportation
companies, and some the cost to farmers to whom the hauling of
the crops to market is merely an incident of farm work. And,
finally, the data for the cost of hauling not done by transportation
companies are for hauling garden products, etc., to large cities,
and are therefore not representative of the cost of transporting
general farm products to market.
11. It is very unfortunate that the conclusions from the two
Circulars referred to above, have been so generally accepted by
speakers and writers upon good-road economics. Country roads
are used chiefly by farmers, and if improvements are made they
must be paid for largely, if not entirely, by farmers; and therefore
the cooperation of farmers must be secured before any improve-
ment of the country roads is possible. Farmers know that con-
clusions such as are deduced above from Table 1 are ridiculous;
and not unnaturally distrust the motives prompting the argument,
* For a further discussion of the Circular see the following: In defense of the Circular,
Engineering News, Vol. 34, p. 410-11; do., Vol. 45, p. 50-51. Controverting the Circular:
Engineering News, Vol. 34, p. 377-78; do., Vol. 34, p. 410-11; do., Vol. 44, p. 337-44; do.,
Vol 5, p. 48-49; do., Vol. 57, p. 428.
ART. 1] ROAD ECONOMICS 11
and are hostile to all propositions for road improvement supported
by such arguments.
Further, it is not possible to determine either the cost of wagon
transportation or the financial value of road improvement in the
wholesale manner proposed in the above Circulars. The cost of
haul and the value of unproved roads vary greatly with local con-
ditions; and consequently a special investigation should be made
for each particular case.
However, it should be borne in mind in discussing road eco-
nomics that financial profit is only one of the advantages of good
roads (see § 1-3).
12. FINANCIAL VALUE OF ROAD IMPROVEMENT. It is not
possible to present any valuable general conclusions as to the saving
in cost of transportation attainable by any proposed road improve-
ment.
For any particular road where the traffic is principally by
"freighters" as defined in §5, it is possible to arrive at a rough
approximation by (1) taking a census of the traffic, (2) making
an estimate of the present cost per ton-mile, and (3) making an
estimate of the cost after the improvement. The amount of traffic
varies with the condition of the road surface, and the chief difficulty
is to determine the advantage of being able to move freight at any
time. This advantage will depend upon the proportion of the time
that the roads are "good," which depends entirely upon the local-
ity and the nature of the road surface, and varies greatly from
year to year. Ordinarily the road is used by a variety of team-
sters, and the cost varies with the particular circumstances of each.
There will rarely be conditions to which this method of investiga-
tion can be applied with any degree of certainty. At best the
results of such an investigation must be regarded as mere approx-
imations, since no factor of the problem can be determined accu-
rately, and since any slight error in the estimated saving per
ton-mile is greatly magnified when multiplied by the number of ton-
miles. Nevertheless such an investigation is desirable to aid the
judgment, but its approximate nature must not be forgotten.
For roads where the travel is by farmers the difficulties are still
greater. The number of users is greater, the cost of transportation
to the different users varies very greatly, and the value of being
able to use the road at any time is very different with different
users, and for the same class of users varies with the locality and
the nature of the road.
12 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
The amount of money that may justifiably be expended for any
proposed road improvement will depend upon the present condition
of the road, the amount and the nature of the travel, and the cost
of constructing and maintaining the improved road. The ques-
tion is a local one, and can be answered approximately correctly
only after careful study of the conditions. Ordinarily the saving
in transportation, except near large cities, will not justify any
radical road improvement; but with a miscellaneous travel, the
social advantages of road improvement should be taken into con-
sideration, even though they can not be computed in dollars and
cents.
13. TRACTIVE RESISTANCE. The solution of some problems con-
nected with road improvement requires a knowledge of the tractive
resistance. Until recently all vehicles were horse-drawn; but now
many are propelled by motors. The passenger automobile with
its wide range of speed and power is able to surmount almost any
grade likely to be encountered upon a road used also by horse-
drawn vehicles; and automobile trucks are not common, at least
yet, upon rural roads, and besides the factors governing the tractive
power of such vehicles are not yet well established. Hence the
road problems involving tractive resistance relate almost exclu-
sively to horse-drawn vehicles; and consequently only this class
will be considered under this head.
The resistance to traction of a vehicle on a road consists of three
independent elements: axle friction, rolling resistance, and grade
resistance.
14. Axle Friction. The resistance of the hub to turning on
the axle is the same as that of a journal revolving in its bearing,
and has nothing to do with the condition of the road surface. The
coefficient of journal friction varies with the material of the journal
and its bearing, and with the lubricant. It is nearly independent
of the velocity, and according to observations made by the author
seems to vary about inversely as the square root of the pressure.
For light carriages when loaded, the coefficient of friction is about
0.020 of the weight of the axle; for heavier carriages when loaded,
it is about 0.015; and for the ordinary thimble-skein American
wagon when loaded, it is about 0.012. The above results are for
good lubrication; if the lubrication is deficient, the axle friction
is two to six times as much as above. The above figures agree
reasonably well with results obtained for journal friction of ma-
chines. Apparently the value of this coefficient in Morin's experi-
ART. 1]
ROAD ECONOMICS
13
ments (§ 20) was 0.065.* The greater axle friction is probably
due to the inferior mechanical construction of French carriages
and wagons of that date.
The tractive power required to overcome the above axle friction
for American carriages of the usual proportions is about 3 to 3J Ib.
per ton of the weight on the axle; and for truck wagons, which
have medium-sized wheels and axles, is about 3J to 4J Ib. per ton.
15. Rolling Resistance. The resistance of a wheel to rolling
along on a road is due to the yielding or indentation of the road,
which causes the wheel to be continually climbing an inclination.
The resistance is measured by the horizontal force necessary at the
axle to lift the wheel over the obstacle or to roll it up the inclined
surface; and varies with the diameter of the wheel, the width of
the tire, the speed, the presence or absence of springs on the vehicle,
and the nature of the road surface.
16. Diameter of Wheel. The rolling resistance varies inversely
#s some function of the diameter of the wheel, since the larger the
wheel the less the force required to lift it over the obstruction or
to roll it up the inclination due to the indentation of the surface.
Table 2 shows the results obtained by Mr. T. I. Mairs at the Mis-
souri Agricultural Experiment Station,* with three different-sized
TABLE 2
EFFECT OF SIZE OF WHEELS ON TRACTIVE RESISTANCE
Resistances in Pounds per Ton
Ref.
No.
Description of Road Surface.
MEAN DIAMETER OF FRONT
AND REAR WHEELS.
50"
38"
26"
1
2
3
4
5
6
7
8
9
10
Macadam: slightly worn, clean, fair condition. . . .
Gravel road: dry, sand 1" deep, some loose stones .
Gravel road: up grade 2.2%, \" wet sand, frozen
below
57
84
123
69
101
132
173
178
252
61
90
132
75
119
145
203
201
303
70
110
173
79
139
179
281
265
374
Earth road : dry and hard
" " £" sticky mud, frozen below, rough. .
Timothy and blue-grass sod' dry grass cut
"wet and spongy
Corn-field: flat culture, across rows, dry on top. . .
Plowed ground : not harrowed, dry and cloddy . . .
Average value of the tractive power
130
148
186
* Lowe's Strassebaukunde, page 75. Wiesbaden, 1895.
t Missouri Agricultural Experiment Station, Bulletin No. 52, Columbia, 1902.
14 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
wheels. The 50-inch represents 44-inch front and 56-inch hind
wheels; the 38-inch represents 36-inch front and 40-inch hind wheels;
and the 26-inch represents 24-inch front and 28-inch hind wheels.
The tires were 6 inches wide. The load was practically If tons
in each case.
Morin concluded that the resistance varies inversely as the first
power of the diameter of the wheel; Dupuit that it varies as the
square root; and Clarke claims that it varies as the cube root.*
According to some experiments made in England in 1874,f the
tractive resistance varied more rapidly than the first power of the
diameter of the wheels. The mean results in Table 3 vary nearly
inversely as the square root of the mean diameter — certainly more
nearly than as either the first power or the cube root. For obvious
reasons, the experiments can not be very exact; and apparently
the tractive resistance varies differently for different surfaces. The
exact determination of the law of variation is of no great importance.
17. Width of Tire. If the wheel cuts into the road surface, the
tractive resistance is thereby increased; but with surfaces for which
there is little or no indentation, the traction is practically inde-
pendent of the width of tire.
Table 3, page 15, shows the results of an elaborate series of
experiments by the Missouri Agricultural Experiment Station, t
The load in each case was 1 ton. These results show that on poor
macadam, poor gravel, and compressible earth roads, and also on
agricultural land, the broad tire gives less resistance except as
follows: (1) when the earth road is sloppy, muddy, or sticky on top
and firm underneath; (2) when the surface is covered with a very
loose deep dust and is hard underneath; (3) when the mud is very
deep and so sticky that it adheres to the wheel; or (4) when the
road has been rutted with the narrow tire. The last conclusion
was established by a large number of experiments not included in
Table 3.
Table 4, page 16, gives data on the effect of width of tire upon
the tractive power, obtained by the Studebaker Bros. Manufactur-
ing Co., South Bend, Ind., in 1892, with an ordinary 3J-inch thimble-
skein wagon. Notice that on a hard and incompressible road sur-
face, e. g., wood block pavement and gravel, the narrower tire draws
* Clarke's Construction of Roads and Streets, p. 294. London, 1890.
t Clarke's Manual of Rules, Tables and Data for Mechanical Engineers, p. 962. London,
1877.
t Missouri Agricultural Experiment Station, Bulletin No. 39, Columbia, Mo., July, 1897.
ART. 1]
ROAD ECONOMICS
15
TABLE 3
TRACTIVE RESISTANCE OF~ BROADBAND NARROW TIRES *
Resistances in Pounds per Ton
Ref
WIDTH (
)F TIRE.
Nr» of
No.
Description of the Road Surface.
li".
6'V
Trials.
1
Broken Stone Road:
Hard, smooth, no dust, no loose stones, nearly
level
121
98
2
2
Gravel Road:
Hard and smooth, few loose stones size of black
walnuts. . *
182-
134
2
3
4
Hard, no ruts, large quantity of sand which
prevented packing
New, gravel not compact, dry
239
330
157
260
1 '
1
5
Wet loose sand 1" to 1\" deep
246
254 *
2
6
7
8
9
10
11
Earth Roads:
Loam, — dry, loose dust 2" to 3" deep
. ' ' " hard, no dust, no ruts, nearly level.
stiff mud, drying on top, spongy below .
1 ' mud 1\" deep, very sticky, firm below.
Clay, — sloppy mu,d 3" to 4" deep, hard below .
dry on top but spongy below, narrow
tires cut in 6" to 8"
90
149
497
251
286
472
106"
109
307
325?
406*^
422
2
3
1
1
1
2
12
13
dry on top but spongy below
stiff deep mud
618
825
464
551
5
1
14
Mowing Land:
Timothy sod, — dry, firm, smooth, narrow tire
cuts in 1"
317
229
\
15
16
moist, narrow tire cuts in 3^". .
soft and spongy, grass and
stubble 3" high, narrow tire
cuts in 6"
421-
569
305
327
1
1
17
18
19
20
21
Pasture Land:
Blue-grass sod, — dry, firm, smooth
soft, narrow tire cuts in 3". . .
narrow tire cuts in 4"
Stubble Land:
Corn stubble, — no weeds, nearly dry enough
to plow
some weeds and stalks, dry
enough to plow
218
420
578
631
423
156
273
436
418
362
2
2
1
2
1
?,?,
in autumn, dry and firm
404
256
2
23
24
Plowed Land:
Freshly plowed, not harrowed, surface rough . .
harrowed, smooth, compact. . .
510
466
i* /•>
283
323
M
1
1
* Missouri Agricultural Experiment Station, Bulletin No. 39, Columbia, Mo., July, 1897.
16
ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. 1
TABLE 4
EFFECT OF WIDTH OF TIRE UPON TRACTIVE POWER *
Resistances in Pounds per Ton
Ref.
No.
Description of the
Road Surface.
DIAMETERS OF THE FRONT AND REAR WHEELS RESPECTIVELY.
3' 6" and
3' 10".
3' 6" and
3' 10".
3' 8" and
4' 6".
3' 6" and
3' 10".
3' 8" and
4' 6".
WIDTH OF THE TIRE.
H"
4"
H"
4"
U"
4"
H"
3"
If"
3"
228
114
228
1
2
3
4
5
• 6
7
Sod
283
152
239
152
189
114
265
Earth Road, hard. . . .
" " muddy..
Sand Road, hard
" " deep.. ..
Gravel Road good
199
371
108
243
162
351
268
171
304
164
236
141
254
168
98
61
117
70
83
35
80
46
. ; . .
' 54
66
28
76
38
Wood Block, round . .
51
49
the easier; while upon the soft or spongy surface the wider tire
draws the easier.
Morin experimented (see § 20) with tires 2|, 4J, and 6J inches
wide; and concluded that on a solid road or pavement the resist-
ance was independent of the width of the tire, but on a compressible
surface the resistance decreased as the width of the tire increased,
the rate depending upon the nature of the surface.
For a further discussion of the relative merits of broad and narrow
tires, see § 200-202.
18. Effect of Speed. The rolling resistance increases with the
velocity, owing to the effect of the shocks or concussions produced
by the irregularities of the road surface. This increase is less for
vehicles having springs than for those not having them, and is also
less for smooth road surfaces than for rough ones.
Table 5, page 17, is a summary of Morin's results (see § 20)
showing the effect of a variation of speed for vehicles provided
with springs. In a rough way the three speeds are 2J, 5, and 7J
feet per second, or about 2, 4, and 6 miles per hour respectively.
According to these results the resistance on broken-stone roads
increases roughly as the fourth root of the speed, and on stone-block
pavement about as the square root. For springless vehicles the
increase would be greater. The above is for metal tires; for pneu-
* Pamphlet by Studebaker Bros, Manufacturing Co., South Bend, Ind., 1892,
ART. 1]
ROAD ECONOMICS
17
TABLE 5
EFFECT OF SPEED ON TRACTIVE POWER *
The figures give the resistance in pounds per ton
Ref.
No.
Description of the Road Surface.
STAGE COACH.
CARRIAGE.
Walk.
Trot.
Fast
Trot.
Walk.
Trot.
Fast
Trot.
1
2
3
4
5
6
7
8
9
10
11
Broken Stone Road:
Good condition, dry and compact .
Very firm large stones visible. . .
42
59
49
77
95
112
146
-164—
49
75
75
92
108
127
161
100
50
81
88
100
117
134
169
41
58
48
76
93
110
145
Ifi2
48
73
74
91
108
126
160
302
49
81
88
99
116
132
168
Little moist, or little dirty. .......
Firm, little soft mud
' ' ruts and much mud
Portions worn out, thick mud
Much worn, ruts 3" deep, thick mud
Vrrv hnd rutr 1" dccn vcr« fnmrh
Stone Block Pavement:
Very smooth narrow joints
32
35
35
48
52
49
55
61
56
31
34
44
47
51
60
54
67
67
Fair condition, dry
Moist covered with dirt
matic tires there is very little increase of resistance with increase
of speed, f
The 'preceding data refer to the effect of speed upon the tractive
power after the load is in motion. It requires from two to six or
eight times as much force to start a load as to keep it in motion at
2 or 3 miles per hour. The extra force required to start a load is
due in part to the fact that during the stop the wheel may settle
into the road surface, in part to the fact that the axle friction at
starting is greater than after motion has begun, and further in part
to the fact that energy is consumed in accelerating the load.
19. Effect of Springs. Springs decrease the tractive resistance
by decreasing the concussions due to irregularities of the road sur-
face, and are therefore more effective at high speeds than at low
ones, and on rough roads than on smooth ones. Apparently no
experiments have been made upon the effect of springs; but a few
data on this subject may be obtained by comparing the last and the
sixth columns of Table 6, page 18.
20. Results of Early French Experiments. Immediately before
and shortly after the introduction of railroads, European engineers
made many experiments on the force necessary to draw different
vehicles over various surfaces. The experiments by Morin, * made in
* Experiences sur le tirage des voitures et sur les effets destructeurs qu'elles exercent sur
les routes, executees en 1837 et 1838 par ordre du Ministre de la Guerre, et en 1839 et 1841
par ordre du Ministre des Travaux Publics, A. Morin. Paris, 1842.
t Proc. of lust, of Mecb. Engrs. (London), for 1890, Part No. 2, p. 195.
J.
18
ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
TABLE 6
TRACTIVE RESISTANCE OF DIFFERENT VEHICLES ON VARIOUS LEVEL ROADS, AT THREE MILES AN HOUR, IN POUNDS PER TON |
| VEHICLES WITH SPRINGS.
sjlb5 £,$0
i-l TH T-H <M (M i-H —I
^ ^ ^
ricoojcoio
1 II II II II
CO • GOCOOO TH GO 00 CO CO O »C 00 i-HTfHTtH 00
£tj^ ii ii ii
'.'.'.'.'.
£*-•*)* Oi CO «C CO TH HH 1C !>• Cl i— i rfi CO CO CO ^f rP
| VEHICLES WITHOUT SPRINGS.
Freight Wagon.
t = 4.5"
a = 2.5"
^00
1 .... cOC^ COOOrH CO^ ^ COt^O^C^CO CNC^CO °<
ebb
COCO
II II
^_^^ . . . ^.^^ ^^.^
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rHrHC^ i— ti— IrHC^C^ r— 1 rH
Freight Cart.
t = 4.5"
a = 2.5"
b
00
II
rH i 1 . 1 rH
s
II
Q
1C *C CO 00 O O O 1C CO »C GO O *C 00 CO >C CO »C
ICO TJHCDOOO5 CO CO ^ 1C CO 00 O I-H (N (M CO CO
O E II
II II
b-OOO5T^ . ICO Tt^b-0005 CO CO -tf »C b- 00 O CN (N <N CO CO
i— 1 • _ _ _ _ _ rHrH
1
05
1
»
1
*o
fl
Q,
i
5
.
••••«. . . . . .^ ...
f3
• • • • rC ^ 73 .
• • "§ ° '. gii • • •
>j • ®^ s. C§ "0 • • • • -^ §
•g £»; g" " -g s^ • "d^^i •
1 -il 1 - II -: :- -:1 ^ ^ : :
^s i 1 L = !
Hft?»i^tjiwH?«S
gK^S^SK" ^J>O e rH*CQ THrH"oOK*r-^PH Pn^K* o*->f^'^ CH
^5 B^ ^ ' ^fe GQ S^
SI
rH<NCOTt^iC COI> OOOSOi-H (N CO T^ 1C CO b- 00 O5 OI-HIM CO
ART. 1]
ROAD ECONOMICS
19
1837-41 for the French Government, were much the most elaborate
and appear to have been made with great care. Table 6, page 18,
is a summary of Morin's results showing the tractive resistance for
different vehicles on various road surfaces. The table represents
about 700 experiments.
21. Results of Modern American Experiments. Table 7, page
20, shows data obtained by the author. The tractive power was
determined with a Baldwin dynograph, Fig. 1 . The instrument con-
sists of two long flat springs fastened together at their ends and
TOP VIEW
BOTTOM VIEW
FIG. 1. — BALDWIN DYNOGRAPH.
20
ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP.
having their centers slightly farther apart than their ends. One
end of the apparatus is attached to the wagon, and the team is
hitched to the other. The pull of the team causes the centers of the
flat springs to approach each other. One spring supports a gradu-
ated disk, and the other is connected to an index arm which is pivoted
at the center of the disk. From one end of this index arm, the pull
can be read directly from the graduated disk. There are two extra
index arms — one to indicate the maximum power developed and one
to indicate a rough average. The former (the upper one in Fig. 1) is
simply pushed around by the main index arm and is left at the highest
point. The latter (the middle arm in Fig. 1) has a transverse slot
in which plays a stud on the main index arm. When making an
TABLE 7
TRACTIVE RESISTANCE ON LEVEL PAVEMENTS
Expt.
No.
Location and Description of the Pavement.
Pounds
Ton.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
IS
19
20
21
22
23
24
2f>
26
Asphalt: Chicago — Calumet Ave., bet. 43d and 44th Sts.; smooth, clean,
no cracks, 52° F
Chicago — Calumet Ave., bet. 43d and 44th Sts.; smooth, clean,
no cracks, 84° F
Chicago — Washington Boul., bet. Halsted and Green Sts.;
smooth, clean, no cracks, 42° F
Brick: Champaign — University Ave., west of New St.; 3 X 9-in. brick
on concrete, corners rounded, sand filler, not worn, clean
Champaign — Second South St.; same as No. 4, except newer and
covered with 2-in. of dust
Champaign — First South St.; same as No. 4, except cement filler,
just completed
Chicago — Peoria St., between Washington and Randolph; 2£ X 8-
in. brick on concrete, pitch filler, new ,
Chicago — Laurel St. Stock Yards; 3 X 8-in. brick on gravel and
cinders, sand filler, corners not rounded
Chicago — Exchange Ave., Stock Yards; 2j X 8-in. brick on sand
and old macadam, tar filler, new
Granite block: Chicago-^Exchange Ave., Stock Yards; smoothly dressed
3 X 9-in. blocks on concrete, joints J in., tar filler, not
worn „
Chicago — Randolph St., between Desplaines and Halsted;
smoothly dressed blocks on concrete, pitch filler, new. .
Chicago — Halsted St., between Randolph and Washing-
ton; ordinary granite, 9 years old
Macadam: Chicago— Michigan Ave., between 42d and 43d Sts.: granite
top, no dust, no mud . .
Plank road: Chicago— Packer's Ave., Stock Yards; oak plank, 3 X 12-in.,
nearly new
Exactly same as above after worn down \ in. in many places,
clean
Substantially same as above; covered with i-in fine loose dirt
bteel wheelway: Chicago— Transit Ave., Stock Yards; 8-in. 1H lb.
channel on 2 X 8-in. pine, that on macadam, covered
with 7f-in. powdered stone
Same when scraped with a shovel
Same when covered with J-in. fine dust
Wood block: Rectangular blocks 3 X 12-in. considerably worn
Cylindiical cedar block covered with 5-in. silica pea gravel
Exactly same as above covered with J-in. crushed gravel . .
Cylindrical cedar block; clean, blocks slightly convex on top
Cylindrical cedar block on 2-in. plank and 2 in. of sand, clean,
not worn
Same as above; clean, slightly worn
Same as above; clean, considerably worn i
37
70
34
17
31
22
24
37
25
29
30
36
18
32
38
40
40
10
28
30
90
50
53
37
51
54
ART. 1]
ROAD ECONOMICS
21
experiment the main index arm is continually in motion, and the
position of the auxiliary arm roughly indicates the average power
exerted. The end of the index arm opposite the graduated arc
records the amount of tractive resistance upon a strip of paper which
is wound from one cylinder to another by clock-work located behind
the lower right-hand corner of the top view of Fig. 1. The auto-
graphic record is more accurate than the indicated reading.
The wagon employed was the usual thimble-skein four-wheel
farm wagon with a 2-inch tire. Experiments 3, 4, and 5 were made
with wheels averaging 42 J inches in diameter, and the remainder
with wheels averaging 47 inches.
22. From a study of the preceding experiments and also others
not here described, it is concluded that the average tractive resist-
ance on different road surfaces is about as in Table 8 which is given
for use in comparing different roads and pavements.
TABLE 8
STANDARD TRACTIVE RESISTANCE OF DIFFERENT ROADS AND PAVEMENTS
Ref.
TRACTIVE ]
IESISTANCE.
No.
Kind of Road Surface.
Pounds per Ton.
In Terms of Load.
1
2
Asphalt — artificial sheet
Brick.
30- 70
15- 40
A-A
T^T— JG ,
3
Cobble stones .
50-100
13l3 5l°
4
5
6
.7
8
Portland cement concrete, unsurfaced
Earth roads — ordinary conditions. . .
Gravel roads
Water-bound macadam
Plank road
27- 30
50-200
50-100
20-100
30- 50
li
-Ar-A
TTTo~2~o
TT1--46
9
Sand — ordinary condition
100-200
2TT-T&
10
Stone block
30- 80
11
11
Steel wheelway
15r- 40
iJPS
1?
Wood block — rectangular .
30- 50
X-t£
13
cylindrical
40- 80
S-S
23. GRADE RESISTANCE. This is the force required on a grade
to keep the load from rolling down the
slope. It is independent of the nature
of the road surface, and depends only
upon its angle of inclination.
In Fig. 2, P is the grade resistance,
and W is the weight of the wheel and
its load. From the diagram it is easily seen that P —. W X B C -r-
A C. For all ordinary cases, A C may be considered as equal to
A B, and then P = WXBC+AB.
FIG. 2.
22 ROAD ECONOMICS AND ROAD ADMINISTRATION
The preceding analysis is approximate for three reasons: (1)
assuming A C = A B, i. e., assuming the sine of inclination to be
equal to the tangent; (2) assuming the normal pressure on the
inclined road surface to be equal to the weight, i. e., assuming the
cosine of the inclination to be unity; and (3) neglecting the fact that
the hind wheels carry a greater proportion of the load on an inclina-
tion than on the level. The resulting error, however, is wholly
inappreciable.
Grades are ordinarily expressed in terms of the rise or fall in feet
per hundred feet, or as a per cent of the horizontal distance. If
A B be 100 feet, then the number of feet in B C is the per cent of the
grade; and therefore the grade resistance is equal to the load mul-
tiplied by the per cent of the grade. Or the grade resistance is
equal to 20 Ib. per ton multiplied by the per cent of the grade.
24. POWER OF A HORSE. The horizontal pull which a horse can
exert depends upon its weight, its build, the method of hitching, the
foothold afforded by the surface, the speed, the length of duration of
the effort, the rest-time between efforts, and the tax upon the future
efficiency of the horse. The chief of these are the weight, the speed,
and the length of the effort.
Horses vary in weight from 800 to 1,800 Ib. The larger horses
do not usually travel more than 2| or 3 miles per hour. With
reasonably good footing a horse can exert a pull equal to one tenth
of its weight at a speed of 2j miles per hour (3| feet per second)
for 10 hours per day for 6 days per week and keep in condition.
This is a common rate of exertion by farm horses in pulling plows,
mowers, and other agricultural implements. On this basis a 1500-
Ib. horse would develop 550 foot-pounds per second (the conven-
tional horse-power), and 16,500,000 foot-pounds per day. This
may be considered about the limit of endurance. A lighter horse will
exert a proportionally less force. If the time of the effort is decreased,
the draft may be proportionally increased; or if the speed is increased,
the draft must be decreased in a like proportion. In other words, the
foot-pounds of energy that can be developed per day by any particular
horse,is practically constant.
The maximum draft for a horse is about half of its weight,
although horses have been known to exert a pull of two thirds of
their weight. Most horses can exert a tractive power equal to half
their weight, at a slow walk for about 100 feet. On the road in
emergencies, as in starting the load or in overcoming obstacles, a
horse may be expected to exert a pull equal to half its weight, but
ART. 1] ROAD ECONOMICS 23
at this rate it would develop a day's energy in about 2 hours; and
consequently if it is expected to work all day, it should not be
called upon to exert its maximum power except for a short time.
Similarly, a horse can exert a draft equal to one quarter of its weight
for a longer time. The. working tractive power of a horse may be
taken as one tenth of its weight, with an ordinary maximum of one
quarter, and in great emergencies a maximum of one half its weight.*
25. Increasing the number of horses does not increase the power
proportionally — for somewhat obvious reasons. It is stated that
for a two-horse team the efficiency of each horse is about 95 per
cent; for a three-horse team, about 85 per cent. Of course such
data are not much more than guesses.
26. Effect of Grade. The effective tractive power of a horse
upon an inclined road surface is decreased by the fact that the
horse must lift his own weight up the grade. If T= the tractive
power, W = the weight of the horse, t = the tractive power on the
level in terms of the weight of the horse, and g = the rise of the
grade per unit of horizontal distance, then, with sufficient accuracy,
T = tW -gW = (t- g)W (1)
If it be assumed that the average working tractive power of the
horse is one tenth of its weight, then t = 10 per cent; and equation
(1) shows that on a 1 per cent grade the horse can exert an effective
tractive power of 9 per cent of its weight, and also that it will be
able to carry its own weight up a 10 per cent grade. If it be as-
sumed that the horse exerts a tractive power equal to 20 per cent of
its weight, then equation (1) shows that on a 10 per cent grade it
can take its own weight up and in addition exert a tractive power of
10 per cent of its weight upon the load. By assigning values to t
and g, equation (1) readily shows the effective draft of a horse upon
any grade.
Equation (1) is not mathematically , correct, since it assumes
that the weight of the horse is always normal to the road surface.
However, the formula is sufficiently accurate for use in comparing
the relative tractive power of a horse on different grades (§ 27).
At best such a formula can be only approximate, since the tractive
power varies greatly with the foothold.
* For the results of experiments made at the Kansas State Agricultural College, showing that,
a horse in pulling from 500 to 1500 feet probably exerted from 26 to 42 per cent of its weight,
see Engineering and Contracting, Vol. 38 (1912), p. 515.
ROAD ECONOMICS AND ROAD ADMINISTRATION [cHAP. I
PA
W
27. Maximum Load on a Grade. On a grade the effective trac-
tive power as given by equation (1) is used up in moving the load
over the road surface and in lifting the load vertically. If L = the
load, and M the coefficient of road resistance, then
(t - gW =
and
* +
(2)
(3)
Equation (3) gives the load that a horse can draw up any grade.
Table 9 is computed from equation (3) for a value of t equal
to one tenth of the weight ,of the horse^ The top line of the
table shows the loads that a horse can draw on the level on the
various road surfaces; and any column of the table shows the load
at a horse can pull on any grade for that particular road surface.
As showing the different effects of grades upon different roads,
notice that on a muddy earth road a 1 per cent grade reduces the
load less than one tenth, while on asphalt a 1 per cent grade reduces
,e load more than one half; or, again, notice that with a 5 per
cent grade, on iron rails the load is less than one twentieth of the
load on the level, while on the best earth road the load is one fifth
of that on the level.
TABLE 9
EFFECT OF GRADE UPON THE LOAD A HORSE CAN DRAW ON DIFFERENT ROADS
The Load is in terms of the Weight of the Horse
±
j,
EARTH ROAD.
Rate of
Iron
Sheet
Broken
Stone
NT '
Grade.
Rails.
Asphalt.
Stone.
Block.
T
No.
Per Cent.
M = 5*0-
M = iJu-
Best.
Spongy.
Muddy.
M= A-
/* = ]&•
M = its1'
1
0
20.00
10.00
6.00
5.00
3.00
2.00
1.00
2
1
6.00
4.50
3.33
3.00
2.09
1.50
0.91
3
2
3.20
2.67
2.16
2.00
1.51
1.14
0.67
4
3
2.00
1.75
1.49
1.40
1.11
0.87
0.54
5
4
1.33
1.20
1.05
1.00
0.82
0.66
0.43
6
5
0.91
0.83
0.75
0.71
0.60
0.50
0.33
7
6
0.62
0.57
0.52
0.50
0.43
0.36
0.25
8
9
7
8
0.40
0.2?
0.38
0.22
0.34
0.21
0.33
0.20
0.29
0.18
0.25
0.15
0.18
0.11
10
9
0.15
0.10
0.09
0.09
0.08
0.07
0.05
11
10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Table 9 shows the load a horse can draw upon different grades
and different road surfaces when exerting a uniform pull equal to
AKT. 1]
ROAD ECONOMICS
25
one tenth of its weight. If we desire to know the maximum load
which a horse can draw up any grade, we must insert in equation
(3) the maximum value of t and compute the corresponding value
of L. The value of t to be used in this computation will depend
upon the length of the grade and upon the frequency with which it
occurs. If the grade is only a few hundred feet long, it will probably
be safe to assume a pull equal to one fourth of the weight of the
horse; but this value should be assumed only for the maximum grade,
since such pulling is too exhausting for continuous work.
Table 10 presents the same data as Table 9, but in a slightly
different form.
TABLE 10
LOAD WHICH A HORSE CAN DRAW ON A GRADE IN TERMS OF THE LOAD ON THE
LEVEL WHEN EXERTING A UNIFORM FORCE EQUAL TO ONE TENTH OF ITS
WEIGHT
EARTH ROAD.
Rate of
Iron
Sheet
Broken
Stone
Ref.
Grade,
Rails.
Asphalt.
Stone.
Blocks.
No.
Per Cent.
M = ui0.
M = yJo-
M=&.
« = A.
Best.
Spongy.
Muddy.
M= A-
M= A-
P-A.
1
0
1.00
1.00
1.00
1.00
1.00
1.00
1.00
2
1
0.30
0.45
0.56
0.60
0.62
0.75
0.91
3
2
0.16
0.27
0.36
0.40
0.50
0.57
0.67
4
3
0.10
0.18
0.25
0.28
0.37
0.44
0.54
5
4
0.07
0.12
0.17
0.20
0.27
0.33
0.43
6
5
0.04
0.08
0.12
0.14
0.20
0.25
0.33
7
6
0.03
0.06
0.08
0.10
0.14
0.18
0.25
8
7
0.02
.0.04
0.06
0.06
0.10
0.12
0.18
9
8
0.01
0.02
0.04
0.04
0.06
0.08
0.11
10
9
0.01
0.01
0.02
0.02
0.03
0.04
0.05
11
10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
28. The maximum load which a horse can draw upon any road,
particularly upon the steepest grade, is not, however, necessarily
proportional to the rate of grade and to the resistance, since the
pull that a horse can exert depends upon the foothold. Owing
to the danger of slipping on steep grades, particularly when the
road is wet or icy, it is not customary to lay sheet asphalt on grades
of more than 4 per cent, or ordinary stone blocks on grades of more
than 10 per cent. On steeper grades, special forms of stone blocks
are sometimes employed to increase the tractive power by affording
better foothold for the horses.
29. TRAVEL CENSUS. A knowledge of the use made of a road or
pavement has an important bearing on questions of construction,
26 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
maintenance, and cleaning. The travel determines the amount of
money that may economically be spent in reconstruction, and also
fixes the width and character of the improved portion. The use
made of the road or street is necessary to determine the amount of
service obtained from any particular road surface. Further, the
cost of cleaning a pavement depends upon the character of the sur-
face and the travel; and unless the latter is known, it is impossible
to make any instructive comparison between the cost of cleaning
different surfaces.
30. History. It is surprising that but few travel censuses have
ever been taken. Except in France, not much attention has been
given to this subject in Europe. The French engineers have made a
very careful study of the amount and effect of travel on the rural
roads. Previous to 1844 travel data had been collected in certain
localities; but from 1844 to 1903 ten censuses of national scope
were taken, and it is planned to take one every 10 years.
The results of the French observations are not of much value in
America because of the difference in conditions; and results before
about 1906 are not of much value because of the recent introduction
of the automobile.
31. American Roads. The Massachusetts Highway Commission
in 1909 had a travel census taken upon state-aid highways for 14
hours per day for seven days at 238 stations, and in 1912 a similar
count was made at 156 stations, and in 1915 at 192 stations. At
the same time a travel census was taken at a number of points on
the roadways of the Metropolitan and the Boston Park Systems.
The methods and results are presented in the respective annual
reports.
The observer separated motor-driven vehicles into three classes,
and horse-drawn into four. These classes and the assumed weight
of each (taken according to the prevailing practice in Great Britain)
are as follows :
MOTOR-DRIVEN VEHICLES: WEIGHT
Runabouts 1 . 35 tons
Touring cars 2 . 23 ' *'
Trucks 6.25 "
HORSE-DRAWN VEHICLES:
1-horse, light 0.36 "
1- " heavy 1.12 "
2 or more horses, light 0 . 54 ' '
2 " " heavy 2.46 "
ART. 1]
ROAD ECONOMICS
27
In the first three years motor travel on the rural roads increased
130 per cent, and in the second three years 150 per cent; while the
horse-drawn vehicles decreased almost exactly 20 per cent in each
period. In 1912 the motor-driven vehicles were 63 per cent of the
horse-drawn, but in 1915 they were 497 per cent. The total travel
increased about in proportion to the number of motor cars registered.
As the number of motor cars in this country is rapidly increasing
(for example, the number made in 1916 was 80 per cent greater than
in 1915), it is likely that travel on public highways will continue to
increase.
A summary of this census for a few roads is shown in Table 11;
and incidentally this table also shows the use of travel census
data in determining the unit cost of maintenance. The wide
variation in the cost of maintenance of these roads is probably
due to differences in the age or character of the surface and to dif-
ferences in the character of the traffic.
TABLE 11
TRAVEL AND COST OF REPAIRS ON MASSACHUSETTS STATE-AID ROADS
Road.
MOTOR-DRAWN
VEHICLES.
HORSE-DRAWN
VEHICLES.
TOTAL
TRAFFIC.
COST OF
MAINTE-
NANCE.
Runabout.
O
bf
a
I
a
Single
Horse.
Two or
More.
Tons per Day.
g^
X£
1°
Jr
1
flj
iij
£
Cents per Ton-
Mile per
Year
>>
>>
1
1
t*
a
<u
w
Ashley
14
60
86
194
44
15
15
76
7
115
65
278
334
1365
121
50
58
407
63
533
4
8
31
13
49
0
78
17
1
30
70
66
75
28
47
30
25
64
15
167
16
46
39
19
198
77
190
60
14
98
5
4
2
1
2
2
3
4
1
5
14
12
27
14
193
88
65
36
3
59
271
1 618
1 199
3468
1332
1 140
1 022
1305
186
1918
81 150
485 220
359 730
1 040 430
399 570
342 210
306 660
391 550
55770
575 280
$ 266
1 104
200
1 081
1031
592
1334
510
143
1 040
0.38
0.23
0.06
0.10
0.26
0.17
0.44
0.13
0.25
0.18
Beverly (No. 1)
Hamilton
Lynn . .
Medford-Somerville.
Milton
Sangus
Shrewsbury
Truro
Weston .
The use of a travel census in determining the character of a road
surface fitted to particular traffic conditions is incidentally illustrated
in Table 26, page 177.
32. In 1906 the Illinois Highway Commission took a census of
travel at 71 stations at various times during one year.* Observa-
* Annual Reports for 1906 and 1907.
28 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
tions were made of only the number of vehicles without distinction
as to their character or weight. Twelve of the roads had about 75
vehicles per day, twenty-seven about 145, and ten about 250. The
results seem to show that there is no relation between the travel on a
road and the population of the near-by town; or, in other words,
that there are roads in the vicinity of even very small towns that
have as much travel as roads near large cities.
33. In the summer of 1917 the Iowa Highway Commission took a
census of travel on a few of the main roads. Observations were made
at each station for ten days; and all vehicles were actually weighed.
A record is to be made of farm traffic, of town and city traffic, and of
tourist travel, with the hope of securing data for an equitable appor-
tionment of road expenditures. A preliminary report on observa-
tions made during the tourist season at eight stations on an earth-
surface tourist highway leading into important market centers,
showed 3 per cent tourist travel, 87 per cent interurban traffic, and
10 per cent farm traffic.
34. American Streets. The first travel census in the United
States was carried out by the Barber Asphalt Co. in New York City
and less elaborately in ten other cities in 1885.* The record
shows the total number of vehicles and the number of tons per
foot of width of pavement. The same company took a similar
census in New York City in 1904.f In both cases the count was
limited mainly to asphalt and granite-block pavements. Since 1913
the city of St. Louis, Mo., has taken an annual travel census — at
first on business streets, but later also on residence streets. | In
New York and St. Louis there was a very great annual increase in
the amount of travel. In St. Louis from 1914 to 1915 the increase
was 20 per cent, the increase in motor-driven traffic being 53 per cent
and the decrease in horse-drawn 15 per cent. A few other records
of various kinds have been made in several cities.
35. Classification of Traffic. The data collected in a travel census
should be such that, in addition to being used for local comparisons,
they should be such as to permit comparisons with data taken in
other localities. There is no standard method of classifying the
vehicles, or of the assumed weight of the different vehicles, or of
fixing the width of the traveled way. Further, the density of travel
is sometimes stated by giving simply the number of vehicles per day
* Trans. Amer. Soc. of C.E., Vol. 15 (1886), p. 123.
t Ibid., Vol. 57 (1906), p. 181-90.
t Engineering News, Vol. 76 (1916), p. 832-34.
ART. 1] ROAD ECONOMICS 29
or per year; but usually by giving the number of tons per year per
foot of width. The unit for comparing the cost of maintenance
is either the tons per year per foot of width or the ton-miles per
year.
The classification and weights of the vehicles in the Massachusetts
census are shown in § 31. Apparently the width of the traveled
way was taken as the full width of the improved portion, — as it prob-
ably is in a narrow rural road. For a somewhat similar classifica-
tion and schedule of weights employed by several road and pave-
ment constructing companies, see page 149 of the 1912 Proceedings
of the American Society of Municipal Improvements. s
36. Weight of Vehicles. The following classification and sched-
ule of weights has been recommended.* The weight of the horse is
to be considered as a part of that of the vehicle ; and the ton is 2,000
pounds.
HORSE-DRAWN VEHICLES MOTOR-DRIVEN VEHICLES
Number of Horses. ^ons*' Style of Automobile. ™tona*'
Single. horse without vehicle. ... 0.50 Motorcycle or bicycle 0.15
1-horse vehicle, light 1 .20 2-passenger automobile 1 .30
1-horse vehicle, heavy 2 . 00 Over 2-passenger automobile. . . 2 . 20
2-horse vehicle, light 2.00 Freight motor-truck, light 6.30
2-horse vehicle, heavy 4.00 " " medium. . 6.00
3-horse vehicle 5.00 " " heavy 8.50
4-horse vehicle 6 . 00
37. Width of Traveled Way. The effective traveled width of a
street is sometimes taken as 3 feet less than the width between curbs.
In many cases, particularly adjacent to car tracks and where auto-
mobiles are parked along the curb, most of the travel is concentrated
upon a comparatively narrow portion of the pavement.t The
results should be stated in tons per foot of total width, and also in
tons per foot of maximum traveled width.
If the several classes of traffic are segregated into different lines
of travel, the details should be stated.
38. Diversion of Travel. In any investigation of traffic conditions
preliminary to any improvement in either the location or the surface
of a road, careful attention should be given to the probable effect
of the improvement in diverting travel to the road or street. Some-
* W. H. Council, Chief of Bureau of Highways, Philadelphia, Engineering and Contracting,
Vol. 47 (1917), p. 227.
t For diagrams showing this concentration, see Engineering News, Vol. 78 (1917), p. 201-2.
30 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
times a small change in the condition of the road makes a radical
change in the amount and character of the travel.
39. WEIGHT AND WIDTH OF VEHICLES. Formerly the only
excessive loads hauled over rural roads or city streets were heavy
pieces of building or bridge material, machinery, etc., hauled on
horse-drawn vehicles, and steam traction-engines; but as these
vehicles were not very numerous and as the speed was low, no serious
harm was done, particularly where traction engines were required to
remove or cover the mud lugs. In the last ten years the advent of
heavy high-speed motor trucks has greatly increased the loads and
speeds of vehicles using the highways. Certain types of motor
vehicles now in use are too heavy for the present roads or pavements,
and much damage is being done; and furthermore the number and
weight of such vehicles is rapidly increasing. It is inequitable and
impracticable to reconstruct all or even many of the roads and pave-
ments so as to enable them to carry safely such motor-driven vehicles.
Therefore special taxes are being levied upon heavy motor-driven
vehicles, partly to make them partially pay for the damage done to
the highways, but chiefly to prevent further increase in their number
and weight. Some of the states have laws regulating the load per
width of tire and also the speed, and some regulate also the diameter
of the wheel.* The following is from an ordinance recently passed
in New York City.f
LICENSE SCHEDULE FOR MOTOR TRUCKS IN NEW YORK CITY
" (a) Vehicles carrying or intending to carry a total gross load of 6,000 Ib.
or less upon any wheel shall be charged the following annual license fee:
Load in Pounds License Fee
per Inch Width for
of Tire. Each Vehicle.
700 or less $1
751 to 800 3
801 to 850 6
851 to 900 12
901 to 950 25
951 to 1 000 50
" (6) In addition to the fees provided in subdivision (a), further fees shall be
charged for loads greater than 6,000 Ib. upon any wheel, but not exceeding 10,000
Ib., as follows:
* For the laws regulating motor trucks in a number of states and cities, see Engineering News,
Vol. 76 (1916), p. 938-39.
t Engineering Record, Vol. 73 (1917), p. 790,
ART. 2] ROAD ADMINISTRATION 31
Weight in License'Fee
Pounds per for
Wheel. Each Vehicle.
6 000 to 6 500 $75
6 501 to 7000 110
7 001 to 7 500 150
7 501 to 8 000 200
8 001 to 8 500 : . . . 300
8 501 to 9 000 500
9 001 to 9 500 • 750
9 501 to 10 000 1 000
" For loads greater than 10,000 Ib. per wheel, license fees shall be charged
for each vehicle at the additional rate of $500 for each 1,000 Ib. per wheel increase
in weight, provided no load greater than 1,000 Ib. per inch width, of wheel shall
in any case be permitted, except as specified in subdivision (d).
" In lieu of the fees hereinabove provided for in subdivisions (a) and (6) for
loads of 6,000 Ib. or more on any wheel, special permits may be issued for single
trips and fees charged therefor at the rate of 10 per cent of the fees therein pro-
vided, except that no single fee shall be less than $25.
" (c) Vehicles 6 feet 6 inches or more in width over all shall be charged, in
addition to the fees specified in subdivision (a) and (6), the following annual
fee;
WIDTH OF VEHICLE LICENSE FEE
for each inch in
width in excess
of 6 feet 6 inches.
6 feet 6 inches to 7 feet 0 inches $5
7 " 0 " 7 " 6 " 10 :,
7 " 6 " 8 " 8 " 15
8 " 0 " 8 " 6 " 20
8 " 6 " 9 " 6 " 25
" (d) In lieu of the fees provided in subdivision (c) for excess width of vehicle,
special permits for single trips may be granted upon payment of single fee of not
less than $10."
ART. 2. ROAD ADMINISTRATION
41. ADMINISTRATIVE UNIT. In this country until recently, the
management of roads rested upon local authorities, either those of
townships or counties. In those cases in which the administration of
road affairs was nominally in the hands of the county authorities,
nothing was usually done except to divide the county into road dis-
tricts and virtually transfer all authority to local officials appointed
for that purpose. Apparently it is impossible to secure either good
roads or an efficient road administration by the action of officials
who have only local authority, and who are necessarily swayed by
purely local, if not individual, interests. This is not peculiar to
32 ROAD ECONOMICS AND ROA±> ADMINISTRATION
America, since great difficulties have always been encountered in
maintaining a system of public highways by any locally governed
community.
The fundamental difficulty is that the small administrative unit
makes it impossible to secure efficient supervision, since the time
necessarily required in road administration is but an incident among
private or official duties. Another difficulty is that the official is
usually elected for political reasons, rather than for his ability in
matters relating to the roads. A further difficulty is that the tenure
of office is short, and successive officials have conflicting views as to
road administration and road improvement.
Another objection to the small administrative unit is the improb-
ability of the district's having suitable machinery in sufficient
quantity to effectively and economically care for the roads.
42. State Aid. — In 1891 the state of New Jersey inaugurated a new
departure in road administration in the United States — that of state
aid in road construction. In 1917 all of the states except Mississippi
and South Carolina had adopted some form of state aid. The fun-
damental principle of state aid is that some roads are built at the
joint expense of the state and local authorities. The states differ
greatly as to (1) the proportion paid by the state, (2) the amounts
paid by the county, township, and abutting property, (3) the amount
and the method of the supervision over the construction, and (4)
the authority that maintains state-aid roads.
The adoption of state aid led to the establishment of state high-
way departments in many of the states; but to participate in federal
aid (see § 45) it was necessary for a state to have a state highway
department, and hence all of the states now have such departments.
One of the most important factors in bringing about the rapid
adoption of state aid and state supervision, has been the introduction
of the automobile and the consequent development of greater interest
in good roads.
43. Table 12 gives data concerning road improvement in the
several states.
44. The principle of state aid is defended on the ground that
(1) it secures centralized, and therefore more efficient, control; (2)
makes the wealth of the city bear part of the expense of maintaining
the country roads; and (3) compels the railroads and other state-
wide corporations to bear part of the expense of local improvements.
The chief advantage of state aid is that it secures uniform and more
intelligent supervision than is possible — on state-aid roads at least —
ART. 2]
ROAD ADMINISTRATION
33
TABLE 12
ROAD MILEAGE AND ROAD EXPENSES IN THE SEVERAL STATES
MILES OF
RURAL PUB
Lie ROADS.
State.
Total
Surfaced
Roads in
State.
Total
Public
Rural
Roads in
State.
Percent-
age of
Surfaced
Roads in
1915.
Year
Original
State-
Aid Law
Passed.
State
Aid in
in 1915.
Total
Cash
Expendi-
tures in
1915.
Alabama
Arizona . .
5915
350
55446
12075
10.7
2.9
1911
1909
$126 134
476 178
$4 283 207
1 076 178
Arkansas
1 200
50743
2.3
1913
25000
2 803 000
California
13 000
61 038
21.3
1895
8 301 149
20 753 281
Colorado
Connecticut
1 750
3200
39691
14 061
4.4
22.7
1909
1895
203 000
2 084 944
2 193 000
3 484 944
Delaware
Florida
300
3 500
3674
17 995
8.0
19 4
1903
1915
31000
1 135
397500
5 501 135
Georgia
13000
84770
15.3
1908
3 700 000
Idaho
950
23 109
4 1
1905
200000
1 974 636
Illinois
11 000
94 141
11.7
1905
818 638
9 263 995
Indiana
27 000
63 370
42 6
1917
13 000 000
Iowa. .
1 000
106 847
1 0
1904
80935
13 606 299
Kansas
Kentucky
1250
13 000
111 536
57 916
1.1
22 1
1911
1912
10000
573 715
5 510 000
3 122 430
Louisiana. .
2 250
24 562
9 1
1910
144 821
3 569 709
Maine
3 000
25 528
11 7
1901
1 009 345
3 293 902
Maryland .
2950
16 458
17.9
1898
3 330 000
5 630 000
Massachusetts. . . .
Michigan
8800
8 600
18681
74 089
46.6
11 6
1892
1905
2 634 567
975 000
6 557 279
10 174 738
Minnesota
Mississippi
Missouri . .
5500
2500
8 000
93500
45778
96 124
5.9
5.5
8 3
1905
i907
1580000
369 i89
8 292 000
2 900 000
8 369 189
Montana
Nebraska. . .
Nevada
775
500
75
39204
80338
15 000
2.0
.6
5
1913
1911
1911
18346
120000
3 676 318
3 520 000
250 000
New Hampshire. . .
New Jersey
New Mexico
New York
1800
4600
450
17 500
14020
14817
11 873
80 112
12.8
31.0
3.8
21 8
1903
1891
1909
1898
666 339
1 163 308
152 122
13 983 769
2 363 414
7 163 584
584 919
24 255 648
North Carolina. . . .
North Dakota ....
Ohio
6500
1 100
30 920
50758
68000
86 453
12.8
1.6
35 8
1901
1909
1904
10000
3 442 604
5 510 000
2 500 700
12 975 688
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina. . . .
300
7780
9883
1246
3 500
107 916
36819
91556
2 121
42220
.3
21.1
10.8
58.8
8 3
1911
1913
1903
1902
10000
230 000
6 541 257
204 119
3410000
6 182 000
12 541 257
594 119
1 000000
South Dakota
850
96306
.9
1911
1 450 000
Tennessee
Texas
8625
12 000
46050
128 960
18.7
9 3
1915
1917
3-500
3 503 500
9 500 000
Utah
1 053
15 000
7 0 •
1909
121 000
1 213 100
Vermont
3 478
15082
23 i
1898
485 145
1 475 145
Virginia
4 760
53 388
8 9
1906
526 645
4 018 399
Washington
West Virginia
Wisconsin
5460
1 200
14 050
42428
32024
75 702
12.8
3.7
18 5
1905
1909
1911
1 435 020
9212
1 389 51 5
6 670 702
2 759 212
9 960 980
Wyoming. .
500
14381
3.5
1911
5000
441 291
Total
276 920
2 451 660
11.3
53 491 651
266 976 399
34 ROAD ECO JMICS AND ROAD ADMINISTRATION [CHAP. I
with a smaller administrative unit; and besides the standards set on
state-aid roads tend to become the ideals for the other roads.
45. National Aid. In 1916 Congress passed a law granting to
the several states federal aid in the construction of roads, which
was another new departure in road administration in this country.
The law appropriated $5,000,000 for federal aid in 1917, and pro-
vided to increase the amount $5,000,000 each year until in 1921, when
the appropriation will be $25,000,000. The U. S. Department of
Agriculture may deduct 3 per cent for administrative expenses, and
the remainder is divided among the several states as follows: One
third in proportion to the areas of the states, one third in proportion
to the mileage of star and rural mail routes in the states, and one
third in proportion to the population according to the preceding
federal census. The federal money can be used to pay not to exceed
one half of the total cost of the construction of any road or system
of roads, the plans for which have been previously approved by
the proper federal authority. Table 13, pages 36-37, shows the
official figures employed in making the distribution, and the amounts
for each state in 1917.
In Europe nearly all countries give national aid in some form for
building roads.
46. CLASSIFICATION OF ROADS. It has long been known by
close students that the problems of road administration would be
greatly improved if the roads were classified according to their im-
portance, into state, county and township roads, or into county,
township and neighborhood roads, the roads of each class to be under
a corresponding administrative authority. One of the incidental,
but not unimportant, results of the adoption of the state aid has been
the classification of the wagon roads. It has been found that 10
to 15 per cent of the roads carry from 80 to 90 per cent of the travel.
These principal roads are called state or county roads, and are the
ones upon which the state aid is expended, either directly or indirectly
under the supervision of state authorities.
The modification of the state road laws incident to the introduc-
tion of the principle of state aid has usually resulted in giving to some
county authority supervision over township road officials.
47. ROAD TAXES. How shall the expense of constructing and
maintaining roads be distributed? This question has been answered
in various ways in different parts of this country and in different
countries of Europe. There are three forms of road taxes which
have long been in use; viz.: (1) a tax upon the traveler, (2) a cap-
ART. 2] ROAD ADMINISTRATION 35
itation tax, and (3) a property tax. The first leads to toll roads;
and the second is usually called a poll tax.
In 1901 the State of New York introduced a method of raising
revenue for road purposes, viz. : a license for operating automobiles.
For present purposes this will be referred to as the automobile road
tax.
48. Toll Roads. These are conducted on the theory that the
travelers over a road are the recipients of its benefits and should
pay for its support. Toll roads are justifiable only in a new country
where the amount of taxable property is small, and where for long
stretches of territory there are few inhabitants, since such roads
induce the investment of capital that possibly the pioneer or the new
community could not afford; and even under these conditions they
are practicable only where there is considerable traffic. In early
times the government collected the toll and used it for the main-
taining and extension of the road; but later toll roads were usually
in the hands of private capitalists.
Toll roads are objectionable owing to the proportionally great
expense of collecting the revenue, and owing to the fact that they
are managed solely with reference to securing returns upon the
capital invested and without regard to the interests of the public.
The only remedy for the evils of the system is for the public to sup-
port the roads. Roads are an indispensable public convenience
—a benefit to every citizen, whether a direct user of the road or not,—
and consequently should be maintained by a universal tax. At
present the toll system is regarded as unwise for both economic and
political reasons; and toll roads have almost entirely been abolished
both in this country and in Europe.
49. Poll Tax. Notwithstanding the fact that most writers on
political economy consider a capitation tax an undesirable form
of taxation, nearly all of the states levy a poll tax for road purposes.
Apparently it is the only capitation tax in this country. It is not
wise to occupy space here to inquire into either the wisdom or reason
for this form of road tax.
Almost universally the law permits the payment of the poll
road-tax in money or labor, and it is usually paid in labor. In the
poorer and less populous states, this tax is nearly the sole support
of the road system. In many states there are numerous exemptions,
and in all states the tax is difficult to collect, and consequently
the poll tax is an unimportant element in road construction and
maintenance.
36
ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. 1
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ART. 2]
ROAD ADMINISTRATION
37
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38 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
50. Property Tax. There are three forms of the property road
tax: the special assessment, the direct tax, and the general tax.
In many states when any considerable road improvement is
contemplated, part or all of the cost of the same is laid as a special
tax or assessment on all real estate within some certain distance of
the improvement. In Indiana at one time, this distance was two
miles; and in Wisconsin, three. Ordinarily this tax is not uniform
over the included area, but is graded according to the supposed
benefits. This tax is usually payable in money.
In most of the states, the territory is divided into small units,
called road districts, and a uniform road tax is laid upon all prop-
erty within the district. Usually this tax may be paid in either
money or labor; and when so permitted, is usually paid in labor.
In most states there is also a general property tax for road and
bridge purposes, which must be paid in money.
In poorer communities the roads are cared for principally by
the district road tax, which is usually paid in labor; but in wealthier
communities the general property road and bridge tax (cash tax)
is greater than the district road tax (labor tax).
51. Labor vs. Money Tax. In most of the states the labor tax
is still regularly employed, although it is gradually disappearing.
The labor-tax system was inherited from England, and is a survival
of the feudal method of requiring all able-bodied men to render public
service. England and France have a labor road-tax, but upon a
much less extensive scale than has this country.
The roads and streets of the cities, towns, and villages are usually
under the control of the municipalities, in which as a rule the labor
tax does not exist; and therefore the labor-tax system applies chiefly-,
if not wholly, to rural communities. Further, since a very large pro-
portion of the roads are of Dearth, the labor-tax system is usually
applied to the construction and care of only earth roads.
It is common to assume that the labor-tax system is all wrong,
and that its evils would be escaped by paying road taxes in money.
The labor tax has inherent disadvantages, but many of the defects
charged to it belong rather to defective administration and to the
system that leaves the control of the public highways to a small
locally-governed community.
The objections to the labor-tax system are: 1. The labor is
indifferent and inefficient. 2. It is impossible to get the work done
at the most suitable time. 3. The system allows no selection of the
laborer. All of these are important considerations.
ART. 2] ROAD ADMINISTRATION 39
The reply to the above objections is usually about as follows:
1. The farmer is willing to pay more in labor than in money, which
compensates in part, at least, for the objections to the labor-tax
system. This preference is not peculiar to the American farmer.
In France, if the road tax is paid in money, a reduction of 40 to 50
per cent is made; but still 60 per cent of the people prefer to pay in
labor. Farmers not infrequently give more both in labor and
money than is exacted as road taxes, because they are interested
in better roads. 2. In many rural communities it is impossible to
secure any one to do road work at reasonable wages at the most
suitable season. 3. If the tax were paid in money, there is no
certainty that the labor would be any more efficient. Streets are
maintained under the cash-tax system, but the labor is not ideally
efficient. The authority that virtually wastes the labor tax will
probably also waste the cash tax.
52. The labor tax is not necessarily the cause of inferior roads,
nor the cash-tax system in itself the cause of improved roads. Town-
ships under the labor-tax system often have better roads than
adjoining townships under the cash-tax system. The one thing
absolutely necessary for successful road management is effective
supervision of the work. Without it neither system .will accom-
plish much, and with it either system will do reasonably well.
Many townships have changed from the labor-tax system to the
cash-tax system with a marked improvement in the condition of the
roads — due chiefly, if not wholly, to better administration. In these
cases the public sentiment that demanded road improvement secured
the change from the labor tax to the cash tax; and, consciously or
unconsciously, also secured a more efficient road administration. In
many of these cases the so-called cash-tax system is practically
only a change in the method of administering the labor-tax system,
since farmers desiring to do so are given an opportunity to work out
their road taxes under the cash system. Under the labor-tax system
those working upon the roads receive credit on their road taxes,
while in the so-called cash system the laborer usually receives an
order which is accepted as cash in paying taxes.
The labor-tax system is more objectionable with roads having a
hard surface than with earth ones, since the construction of the
former is more difficult and their maintenance requires intimate
knowledge and constant attendance, and also since the former are
built only where there is much travel and where the labor of main-
tenance is greater, This subject will be considered incidentally
40 ROAD ECONOMICS AND ROAD ADMINISTRATION [CHAP. I
under Maintenance in the chapters on earth, gravel, and broken-
stone roads.
53. Automobile Tax. Since 1901 the several states have adopted
the system of licensing automobiles as a means of securing revenue
for road purposes. In 1916 the gross revenue from this source
amounted to $25,865,370, of which 92 per cent was applicable for road
work; and the net amount so applied was nearly 9 per cent of the
total expenditures for rural roads and bridges in the United States.
About 70 per cent of the total automobile revenue is expended under
the supervision of the State Highway Departments.
In 1916 there was an average of 1.4 motor cars for each mile of
rural public roads in the United States, and the number of motor cars
is increasing, the increase in 1915 being 40 per cent and in 1916 43
per cent. The average annual registration or license fee per motor
varies between the several states from 50 cents to $19.67. The
tendency in all of the states is to increase the fee.
54. Comparison of Road Expenditures. From 1904 to 1915 the
annual expenditures on the rural roads and bridges in the United
States increased from about $80,000,000 to about $282,000,000, an
increase of more than 2j times. During the same period the annual
expenditures for state-aid road and bridge construction and main-
tenance increased from $2,550,000 to $53,492,000, an increase of
20 times. In 1904 the local bond issues for roads and bridges
amounted to $3,530,000, but in 1915 amounted to about $40,000,000.
In 1904 the expenditure for roads under state supervision was 6 per
cent of the total road expenses; but in 1915 it was 30 per cent, or
more than the total expenditure for roads in 1904.
In 1904 about one fourth of the total expenditures for roads and
bridges was paid in labor. From 1904 to 1915, while the total
expenditures for roads and bridges have increased 3J times, the por-
tion from local bond issues about 1 1 times, and that from state aid
20 times, the portion from the labor road-tax decreased about half.
In 1904 the actual cash road and bridge expenditure in the
United States averaged slightly less than $28 per mile of rural roads;
but in 1915 it had increased to an average of $109 per mile of road.
55. The magnitude of the above sums shows the importance of
the present road expenditures, and also the probability that such
expenditures will increase greatly in the future. Road construction
and maintenance is already a matter of great importance to the public
and to the engineering profession, and is likely to increase rapidly.
CHAPTER II
ROAD LOCATION
58. ELEMENTS INVOLVED. In general the determination of the
best location for a road requires a study of the topographical fea-
tures of the region through which the road is to pass, and also an
investigation of the nature and amount of the traffic to be provided
for. Viewed as a question of economics, the best location is that
for which the sum of the interest on the cost of construction and of
the annual cost of maintaining the road and of conducting trans-
portation over it, is a minimum. The location of a wagon road is
not, however, entirely a question of economics, since the location
should be made with reference to the convenience and comfort,
and perhaps also to the pleasure, of those who use it; and is frequently
more of a social question than one of economics. Only the economic
features of location will be considered here, and they but briefly.
However, in locating a new road, it is well to remember that the
location will probably serve for many generations, and perhaps for all
time, as the growing importance of the surrounding country and the
location of buildings and of division lines of the land with reference
to the road make it increasingly more difficult and expensive to change
the location. Thus the location of a road is a field where costly
errors and permanent blunders may creep in and forever fasten
themselves upon the road and its users; and, worst of all, these errors
become more costly as the use of the road increases.
Over most of the United States, the roads are in the main already
located, and the necessity for the location of new ones does not often
arise; and hence as a rule, the only application of the principles of
economic location will be in the re-location of comparatively short
stretches of road. The original location may have been fit and proper
when the region was new and undeveloped, but the increase in the
amount and the change in the character of the traffic may justify a
very considerable change. There are many rural roads that could
be materially improved by a careful re-location.
41
42 ROAD LOCATION [CHAP. II
59. Rural roads are used by both horse-drawn and motor-driven
vehicles; and strictly each class of vehicles should be considered in
solving problems of road location. However, the passenger auto-
mobile need not be considered, since with the variable speed and high
power of its engine, it can overcome any grade that can be econom-
ically used by horse-drawn vehicles, and since the cost of transpor-
tation by a motor-driven vehicle is small it may be neglected in
computing the effect of slight differences in distance; and therefore
the passenger automobile may be disregarded in problems of road
location. Automobile trucks need not be considered for the same
reasons as above, and also as they are much less common on typical
rural roads than horse-drawn vehicles.
60. The principles to be observed and the methods to be em-
ployed in making the location of a wagon road are substantially the
same as those used in the location of a railroad. The method of
examining the country and of making surveys will not be considered
here, as such subjects are elaborately presented in treatises on rail-
road location.
The fundamental principles applicable in locating a new rural road
or in improving an old one will be briefly considered; but no hard
and fast rules can be laid down, for each road must be designed for
the place it is to occupy and the service it is to render. In the loca-
tion of any road there will always be an opportunity to exercise
keen insight and good judgment.
The subject will be considered under the five heads: distance,
grades, curves, width, and placing the line.
61. DISTANCE. Other things being equal, the shorter the road
the better, since any unnecessary length causes a constant threefold
waste: (1) the interest on the cost of constructing the extra length;
(2) the ever-recurring cost of repairing it, and (3) the time and labor
employed in traveling over it. However, the advantage of straight-
ness, i. e., of shortness, is usually greatly over-estimated. The dif-
ference in length between an absolutely straight line and one deflect-
ing a little to one side is not very great. For example, in Fig. 3, if
A B = B C = 1,000 feet, and B D '= 10
feet, the line A B C is only one tenth of a
foot longer than the line ADC. If
A B = B C = 1 mile, and B D = 300
feet, the line A B C is only 17 feet longer
than ADC. " If a road between two places ten miles apart were
made to curve so that the eye could nowhere see more than a quarter
DISTANCE 43
of a mile of it at once, its length would exceed that of a perfectly
straight road between the same points by only about one hundred
and fifty yards/*
One of the most common defects of ordinary country roads is
that distance has been saved by a disregard of the desirability of
easy gradients. The curving road around a hill may often be no
longer than the straight one over it. The latter is straight only
with reference to the horizontal plane, but curved as to the vertical
plane; while the former is curved as to the horizontal plane, but
straight as to the vertical plane. Both lines curve, and the one
passing over the hill is called straight only because its vertical curva-
ture is less apparent to the eye.
62. Value of Saving Distance. Theoretically the value of a dif-
ference in length may be computed by determining (1) the amount of
traffic, (2) the cost per ton-mile, and (3) the total coat of conducting
the traffic; and then assuming that the value of any difference of
length is to the total cost of transportation as the difference of the
length is to the total length. If the annual cost of conducting trans-
portation over a given road is known, then this cost divided by the
length of the road gives the annual interest upon the sum that may
be reasonably expended in shortening the road 1 mile, i. e., the value
of a saving of a mile of distance; and of course dividing this sum by
the number of feet in a mile will give the value of saving 1 foot of
distance.
Unfortunately it is not possible to determine the amount of
traffic with any considerable degree of accuracy. At some railroad
stations the sole freight shipped out is agricultural produce, in which
case the traffic over any particular wagon road can be approximated
by distributing the shipments according to 'the contributing area.
The average load can be determined with sufficient accuracy by con-
sulting the records of the grain dealers. In addition to the above,
which may be called the heavy freight traffic, there is a considerable
amount of light freight and passenger traffic which would be bene-
fited by a saving of distance.
For the sake of working out an example, it will be assumed that
the cost of transportation is 10 cents per ton-mile. This cost is
made up of the cost of loading and unloading, of driving, of feed,
and of wear and tear of horses, wagon, and harness. The cost of
loading and unloading is independent of distance. The cost of
driving nominally varies as the time, i. e., as the distance (see third
paragraph of § 63). The cost of wear and tear varies as the distance;
44 KOAD LOCATION [CHAP. II
but the cost of feed does not so vary. It is impossible to assign
reliable values to these several factors of the cost, but it is certain
that only part of the cost of transportation varies as the distance;
and for the sake of completing the illustration, it will be assumed that
8 cents per ton-mile varies as the distance. This sum multiplied by
the number of tons passing over the road in a year will give the sum
that may be spent annually to secure a saving of 1 mile of distance.
For example, a road leading to a certain village was originally
laid out on the east and north sides of a quarter-section, but on
account of low ground on the northeast corner another road was
opened on the south and west sides. The quarter-section was one
large field. How much expense would the traffic justify in order to
secure a road diagonally through the quarter-section. The heavy
freight traffic was approximately 3,000 loads of 1 ton each per annum.
The annual value of saving 1 mile would then be 8 cents X 3,000 =
$240. The saving in distance by going through the quarter-section
is 0.29 mile; and the annual value of saving this distance is $240 X 0.29
= $69.60. The diagonal road would occupy 2\ acres less land
than the longer one; and as the land rented for $3 per acre, this
adds $3 X 2 J = $7 per annum to the value of the diagonal road. The
annual saving from these two items is then $69.60 + $7.00 = $76.60.
This is the interest at 5 per cent on $1,532, which sum, according to
the above computations, could be borrowed, and used to secure this
improvement, and the community be no worse off financially.
In addition, there would be some advantage to the light freight
and passenger traffic by shortening the road, but it was difficult, if not
impossible, to estimate this saving; and as the benefit per trip
would probably be less than for the heavy freight traffic, it was
neglected. There would be a slight saving in the cost of mainte-
nance of the shorter road, as in this case the soil and drainage was
as good on one line as on the other. Further, there would be some
saving on the return trip by the shorter road. On the other hand,
it is probable that the smaller number of acres required for the
diagonal road would cost at least as much as for the road around
the quarter-section, owing to the farmers' justifiable dislike for non-
rectangular fields, and because the diagonal road would divide the
quarter-section.
63. There are several matters that materially affect the relia-
bility of the method of the above investigation. In the first place,
the cost of transportation can not be known with any degree of
reliability. The farmers concerned would stoutly contend that the
DISTANCE 45
price assumed above is much too great; while freighters would
claim that it was too low (§ 4-9).
In the second place, not all of the computed annual saving is
available for making the improvement, since some of it should be
set aside to form a sinking fund to be used ultimately in extinguish-
ing the debt. It is not the part of wisdom to extend the debt very
far into the future, since the conditions may materially change.
For example, a new railroad may divert the traffic from this par-
ticular road, or improvements in the condition of the surface of the
road may decrease the cost of transportation, — either of which
would decrease the value of the proposed improvement. Of course,
certain contingencies may increase the traffic and thereby add to
the value of the improvement; but it is not wise to incur a definite
debt for an equal and somewhat problematic saving. Road reformers
sometimes overlook the fact that interest is a yearly charge and that
the debt must finally be paid.
In the third place, the cost of transportation does not necessarily
vary proportionally to the distance, as was assumed above. If the
difference in distance is sufficient to make a difference of one trip
per day, then the value of the saving in distance is tangible; but
where the saving in length is insufficient for an additional trip, the
value of the difference in distance depends upon the value, for
other work, of the small portions of time of men and teams which
may be saved by the shorter route, — a value which exists, but which
is difficult to estimate.
Therefore any estimate as to the value of a saving of distance is
necessarily only a rough approximation; and at best it should be
used only as a guide to the judgment.
64. The problem to find the value of saving distance is very dif-
ferent for wagon roads than for railroads. In the case of railroads
the cost of the various elements has been carefully investigated for
many years, and the transportation is all conducted under a single
management and by the same party that maintains the road surface;
while in the case of wagon roads, a multitude of private parties
conduct the transportation under various conditions, and the main-
tenance of the road is in the hands of public officials.
65. GRADE. A level road is most desirable; but as it can
seldom be obtained, we must investigate the effect of grades upon
the cost of constructing and operating the road, and also determine
what is the steepest allowable grade.
The grade may be reduced (1) by going round the hill or by
46 fcOAb LOCATION [CHAP, n
zigzagging up the slope, or (2) by cutting down the hill. If the
slope to be ascended is a long one, the first method must be em-
ployed ; but if the grade is short, the second is usually the cheaper.
Increasing the length adds to the cost of construction and of trans-
portation, while cutting down the hill adds only to the cost of con-
struction. The maintenance of the longer and flatter line may
cost either more or less than the shorter and steeper one according
to the circumstances of the case. In a broken or rough country,
a proper adjustment of the grades is the most important part of the
art and science of road building, and the better the road surface the
more necessary is such an adjustment.
66. All grades are objectionable for two distinct reasons, viz.:
because a grade increases the amount of power required to move a
load up it, and because a grade may be so steep as to limit the amount
of the load that can be moved over the road. The first applies to
all grades whatever their rate or height; while the latter applies
only to the steepest grade on the road, and in a measure is inde-
pendent of its height and depends only on its rate. At present only
the first objection to grades will be considered; and subsequently
the second objection will be discussed (§ 74).
67. Ordinary Effect of Grade. Table 9, page 24, shows the
load (in terms of the weight of the horse) which a horse with a pull
equal to one tenth of its weight can draw up various grades on several
road surfaces. To emphasize the effect of the grade upon the load,
the same data are presented in a slightly different form in Table 10,
page 25, which shows at a glance the load on any grade in terms of
the load on the level. Tables 9 and 10 show that the better the con-
dition of the road surface, i. e., the less the rolling resistance, the
more objectionable a grade. For example, according to Table 10,
on iron rails on a 3 per cent grade a horse can draw only 10 per cent
as much as on a level; while on a water-bound macadam road on a
3 per cent grade it can draw 25 per cent as much as on a level.
A horse can occasionally and for a short time exert a pull equal
to more than one tenth of its weight. If the grade is not too long, a
horse can safely exert a force equal to one quarter of its weight,
and in emergencies one half.
To move a load over an ordinary earth road requires a tractive
force of 100 Ib. per ton (see Table 8, page 21); and therefore a team
of 1200-lb. horses exerting a force equal to one tenth of their weight
can draw 2.4 tons on the level. The reserve power to take the
load up the hill is (0.25 - 0.10) X 1200 X 2 = 360 pounds. The
GRADE 47
total load to be carried up the grade is the wagon and its load plus
the weight of the team, or 2.4 + (1200 X 2 -f- 2000) = 3.6 tons. The
grade resistance is 20 Ib. per ton for each per cent of inclination
(§23); and the grade resistance for this load on a 1 per cent grade
is 3.6 X 20 = 72 Ib. Therefore, the grade up which a pull of 360 Ib.
will take the 3.6 tons is 360 -f- 72 = 5 per cent, which is the maximum
permissible grade for an earth road in ordinary condition. The
team could probably pull this load up 400 to 500 feet of such a grade.
By the same method of analysis, the load for the same team on a
level, muddy earth road having a tractive resistance of 200 Ib. per
ton is 1.2 tons, and the maximum permissible grade is 7.5 per cent.
For a water-bound macadam road having a tractive resistance of
33 Ib. per ton, the load on the level is 7.3 tons, and the permissible
maximum grade is 2.2 per cent.
68. What load can the above team take up a 4 per cent maxi-
mum grade on a water-bound macadam road having a tractive
resistance of 33 Ib. per ton? The grade resistance is 20 X 4 = 80 Ib.
per ton; and the tractive resistance is 33 Ib. per ton; therefore the
total resistance is 80 + 33 = 113 Ib. per ton. The maximum tractive
power of the team is equal to one quarter of its weight, or 600 Ib.;
and the grade resistance for the weight of the team = 2400 -r- 2000 X
80 = 96 Ib.; therefore the net tractive power of the team is 600 —
96 = 504 Ib. Then the weight of the wagon and the load which the
team can draw up this grade is 504 -f- 113 = 4.4 tons.
69. Rise and Fall. By rise and fall is meant the vertical height
through which the load must be lifted in passing over the road in each
direction. One foot of rise and fall is a foot of ascent with its cor-
responding foot of descent. In passing over a ridge 10 feet high
standing in the middle of a level plain, there is only 10 feet of rise
and fall; and not 10 feet of rise plus 10 feet of fall. If the road is
level, Fig. 4, then an elevation or depression of, say, 1 foot produces
FIG. 4. Fio. 5.
literally 1 foot of rise and a corresponding foot of fall; but if the
road is on a steep grade, Fig. 5, an elevation of 1 foot above the grade
line or of a like amount below the grade line, literally speaking,
produces no rise and fall, because in either case it is a continuous
48 ROAD LOCATION [CHAP. II
up grade. However, as far as operation is concerned, the two
cases are exactly alike, and each has a foot of rise and fall.
Rise and fall is measured by the number of vertical feet of rise,
as shown by the differences of elevation on the profile.
70. The introduction of rise and fall is a question either (1)
between the increased cost of operation and the increased cost of
construction required to fill up the hollow or to cut down the hill,
or (2) between the cost of operation of the rise and fall and of the
increased distance necessary to go around the obstruction.
The following example is often cited as showing the improve-
ment that can be made in locating roads. " An old road in Anglesea
rose and fell between its extremities, 24 miles apart, a total vertical
amount of 3,540 feet; while a new road laid out by Telford between
the same points, rose and fell only 2,257 feet; so that 1,283 feet of
vertical height is now done away with, which every horse passing
over the road had previously been obliged to ascend and descend
with its load. The new road is, besides, more than two miles shorter.
Such is one of the results of the labors of a skilful road maker."
The road may have been economically re-located, but the citation
fails to show whether the increased cost of construction to eliminate
rise and fall was justified by the decreased cost of operation.
The following example from the same author, also frequently
quoted, shows that rise and fall was eliminated by increasing the
distance, although no attempt is made to show that the increased
distance was more economical than the rise and fall thereby elimi-
nated. " A plank road, laid out between Cazenovia and Chitten-
ango, N. Y., is an excellent exemplification of the true principles of
road making. Both these villages are situated on the Chittenango
Creek, the former being 800 feet higher than the latter. The most
level wagon road between these villages rises more than 1,200 feet in
going from Chittenango to Cazenovia, and rises more than 400 feet in
going from Cazenovia to Chittenango, in spite of this latter place
being 800 feet lower. It thus adds one half to the ascent and labor
going in one direction; and in the other direction it goes up hill one
half the height, which should have been a continuous descent. The
line of the plank road by following the creek (crossing it five times)
ascends only the necessary 800 feet in one direction, and has no
ascents in the other, with two or three trifling exceptions of a few
feet in all, admitted in order to save expense. There is a nearly
vertical fall in the creek of 140 feet. To overcome this, it was
necessary to commence far below the falls, to climb up the steep
RISE AND FALL 49
hillside, following up the sides of the lateral ravines until they were
narrow enough to bridge, and then turning and following back the
opposite sides till the main valley was again reached. The extreme
rise is at the rate of 1 foot to the rod (1 in 16J), and this only for
short distances, and in only three instances, with a much less grade
or a level intervening."
71. Classes of Rise and Fall. In discussing the effect of rise and
fall upon the operation of a road, a distinction must be made be-
tween three classes of rise and fall, as follows:
Class A. Rise and fall on grades at a less slope than the angle
of repose (the grade on which a vehicle by its own weight will main-
tain a uniform speed), and so situated as not to require any addition
to the total power required to move a load over the road.
Class B. Rise and fall on grades so steep as to require either the
holding back of the load by the team or the application of brakes.
Class C. Rise and fall on the maximum grade.
72. An example of the first class of rise and fall is shown in
Fig. 6. The team is relieved on the down grade an amount exactly
equal to the extra tax upon the up ^_ -^
grade, and the only effect upon the ^^^
team is that the effort is concentrated
on the up grade instead of being uniformly distributed over the road ;
but as the slope is assumed to be equal to or less than the angle of
repose, the maximum effort is equal to or less than twice the normal.
If the grade line rises above the level instead of dipping below it,
the case is not changed except that the rise is a little more unfavor-
able, since the team has no relief before the increase in effort is
required. Therefore this class of rise and fall costs little or nothing.
In the preceding examples, a change of velocity would alter the
power required at any particular instant; but in wagon-road traffic
the speed is always small and consequently the effect of variations
of speed are quite small, and may be entirely neglected. On rail-
roads a variation of the velocity materially affects the cost of rise
and fall.
If the grade is greater than the angle of repose, the team in descend-
ing must hold back the load, which is lost energy, or brakes must
be applied, which tend to destroy the road; and in ascending, the
demand upon the team is greater than twice the normal. There-
fore in either case this class of rise and fall adds to the cost of oper-
ating the road.
If the grade is the maximum, it may be sufficient to limit the
50 KOAD LOCATION [CHAP. II
amount of the load a team may draw over the more level portions
of the road, and therefore greatly add to the cost of transportation.
As a chain is no stronger than its weakest link, so a road is no better
than its steepest grade.
73. Cost of Rise and Fall. What does it cost to develop the
power required to haul a load up a grade less than the grade of
repose? In other words, what is the cost of Class A rise and fall?
The cost of transportation consists chiefly of the cost of driving,
of feed, and of the wear and tear on the team. Usually the cost of
driving will be approximately half of the total cost of transportation;
and as a team can draw a load up the grade of repose at practically
the same speed, at least for short stretches, as upon the level, there
will usually be no material increase in the cost of driving. Even
though the team may travel slower because of the grade, the cost
of the increased time can scarcely be computed because of the impos-
sibility of determining the value of fractions of time for other pur-
poses. The cost of wear and tear on the team and part of the cost of
feed must vary approximately as the total power developed. There-
fore the conclusion may be drawn that rise and fall belonging to
Class A will not add appreciably to the cost of transportation.
This conclusion is corroborated by the popular belief that a gently
undulating road is less fatiguing to horses than one which is perfectly
level. The argument in support of this belief is that alternations of
ascents,, descents, and levels call into play different muscles, allowing
some to rest while others are exerted, and thus relieving each in turn.
The argument is false, and probably originated in the prejudices of
man in his quest for variety, rather than in the anatomy of the horse;
but the above theory would not have gained its wide popularity
if a gently undulating road were appreciably more fatiguing to a
horse than a perfectly level one. A perfectly level road is the best
for ease of transportation.
74. Limiting Effect of a Grade. If the grade is steeper or longer
than that up which the team can draw the normal load by exerting
twice the tractive power required on the level, i. e., if the rise and fall
belongs to Class C, then the grade has the effect of limiting the load
that can be drawn over the level portion of the road, and conse-
quently increases the cost of transportation. The load which a team
can draw up any grade can be approximately computed as in § 68.
If the load that can be drawn up any particular grade is, for example,
three fourths of the normal load on the level; then it will cost as much
to haul three fourths of a load with this grade as a full load without
RISE AND FALL 51
the grade. If the cost with a grade less than the maximum is 10
cents per ton-mile (§ 4-7 and § 12), then the cost with the maximum
grade will bel0^f=13j cents per ton-mile; and therefore for
each ton going over the road, the maximum grade adds 3| cents per
ton mile. In determining the amount of traffic, only full loads
should be included; but notice that the full load varies with the
speed. A ton may be a full load at 3 miles per hour, while half a
ton may be a full load at 6 miles per hour.
Knowing the load on the maximum grade and also the cost per
ton-mile for a level road or for a grade less than the maximum, the
justifiable expenditure to reduce the maximum grade may be com-
puted as follows: The difference in cost per ton-mile with and with-
out the maximum grade may be determined as in the preceding
paragraph; and this multiplied by the number of loads annually
going over the road gives the sum that may be spent annually to
reduce the maximum grade to the lesser value. This sum may be
used to pay interest on the cost of cutting down the hill or of filling
up the hollow.
The data are so uncertain that the result must be regarded only
as a rough approximation; and yet it is worth while to make an
investigation as above as a guide to the judgment.
75. Class B rise and fall is intermediate between Class A and
Class C, and its cost is even more difficult to compute than that of
Class C. The chief difficulty is in determining the relative cost of
developing power on a level and up a grade. Only an estimate can
be made, and the estimate will vary greatly with the point of view.
For example, farmers usually have a surplus of power (horses) as
far as transportation is concerned, and therefore they would con-
sider a slight increase in the demand for power as a matter of small
moment. Again, teamsters differ greatly as to what is a proper
or economical load for a horse, and also as to the effect of a tem-
porary over-load.
There are two methods of computing the cost of this class of
rise and fall, neither of which is more than roughly approximate.
1. Assume that the cost of Class B rise and fall bears the same
relation to that of Classes A and C, that the grade of B bears to that
of A and C. Then if the grade for Class B is only a little greater
than the angle of repose, the cost is only a trifle greater than that
of Class A ; and if the grade is nearly a maximum, then the cost of
the rise and fall closely approximates that of Class C.
2. Assume that the energy developed on a grade over and above
52 ROAD LOCATION [CHAP. II
uhat required on the grade of repose, costs the same per unit as that
of an equal amount of energy developed on the level. For example,
assume that the rise is 1 foot more than the angle of repose; and
assume that the cost of drawing a load on a good water-bound
macadam road is 5 cents per ton-mile (§ 5-6), and that the tractive
power is 40 Ib. per ton. Then, moving a ton 1 mile will develop
5,280 X 40 = 211,200 foot-pounds of energy, which will cost 5 cents.
The cost of 1 foot-pound of energy, then, is 5 4- 211,200 = 0.000,023,7
cents. Drawing a ton over a rise 1 foot high develops 2,000 foot-
pounds, the cost of which is 0.000,023,7 -f- 2 X 2,000 = 0.023,7
cents. In going up the above grade, the team must develop enough
power to move the load up the grade of repose and in addition must
develop enough to lift the load 1 foot vertically. Therefore the
cost of the 1 foot of rise assumed above is 0.023,7 cents for each
ton going over the road.
It was assumed above that the load is retarded in the descent by
the application of brakes; but if the grade in question is situated in
a flat country where brakes 'are not usually placed upon vehicles,
the team must hold back on the descent an amount equal to the
extra energy required on the ascent, and therefore the cost of the
foot of rise and fall will be, almost or quite, doubled.
With data similar to the above, and with a knowledge of the
amount of traffic, it is a simple arithmetical process to compute the
sum that may be spent annually to eliminate one or more feet of
rise and fall. Notice that in this case only the full loads should be
considered (see the first paragraph of § 74). For example, assume
that a water-bound macadam road has a traffic of 20 tons per day
one way for 300 days of the year, or an annual traffic of 20 X 300
= 6,000 tons. The cost of a foot of rise and fall per ton of traffic
is 0.023,7 cents, and the annual cost on this particular road is 0.000,237
X 6,000 = $1.42. This is the amount which, according to the above
investigation, can be spent annually to cut down the hill or to fill
up the hollow sufficiently to eliminate 1 foot of rise and fall.
Similarly, for an earth road having a cost of 15 cents per ton-
mile and a tractive power of 100 Ib. per ton, 1 foot of rise costs
0.028,4 cents, and the foot of ascent assumed above will cost 0.028,4
cents for each ton going over the road. If this road has a traffic
of 5 tons one way for 300 days of the year, the annual cost of the
foot of rise and fall is 0.028,4 X 5 X 300 = 44.6 cents, which is
the sum that can be spent annually to eliminate the foot of rise and
fall.
RISE AND FALL 53
From the point of view of the last solution, it appears that the
cost of Class A rise and fall increases with the steepness of the
grade, that is, increases as the rate of the grade approaches the
angle of repose. In all probability this is correct, but all the data
involved ^re too uncertain to warrant any further discussion of
the subject here. However, the engineer should bear such relations
in mind in solving a particular problem.
76. Distance vs. Rise and Fall. In locating a road the question
may arise between the relative desirability of introducing rise and
fall and of increasing the length of the road. The problem then is
to determine the relative value of distance and of rise and fall.
If the conclusion in § 73' is correct, that the cost of Class A rise
and fall is not appreciable, then the distance should not be increased
at all to eliminate Class A rise and fall.
77. For Class B rise and fall an approximate solution can be
obtained by assuming that it costs the same to develop a certain
amount of energy in overcoming Class B rise and fall as to develop
a like amount of energy in moving a load on a level road. This
assumption is probably reasonably correct.
For example, the tractive resistance of the best water-bound
macadam road is 33 Ib. per ton, and the work necessary to raise 1 ton
through 1 foot of rise is 2,000 foot-pounds; therefore to develop
2,000 foot-pounds of work on a level water-bound macadam road,
a ton must be moved 2,000 -*- 33 = 60 feet. Hence the cost of
operating 60 feet of distance on this road may be considered as equiv-
alent to 1 foot of rise and fall. Therefore to eliminate a foot of rise
and fall of Class B, the length of the road may be increased 60 feet.
Table 14 gives the corresponding distance for other road surfaces.*
78. Apparently writers on roads have not made a distinction
between the several classes of rise and fall. Herschel says: f " To
determine whether it is more advisable to go over than around a
hill, all other considerations being equal, we have this rule: Call the
difference between the distance around on a level and that over the
hill d (the distance around being taken as the greater), and call h
* The above relations are for a load transported on wheels. It may be interesting to know
the corresponding relations for pedestrians. The work (energy) required of a man in walking
is practically independent of the nature of the road surface. A man makes progress in walking
by allowing his body to fall through a small space and then raising it again preparatory to
another fall. For an average man, the energy expended in walking 16 to 20 feet horizontally
is sufficient to raise his body through 1 foot vertically. Therefore, for pedestrians 1 foot of
rise and fall is equivalent to, say, 18 feet of horizontal distance.
t Clemens Herschel, Science of Road Making, Prize Essay of the State Board of Agriculture
of Massachusetts, Boston, 1869, p. 207-63; revised edition, Engineering News, New York,
1890, p. 9.
54 ROAD LOCATION [CHAP. II
TABLE 14
HORIZONTAL DISTANCE EQUIVALENT TO 1 FOOT OF CLASS B RISE AND FALL
Earth roads, muddy (tractive resistance 200 Ib. per ton) .... 10 feet
ordinary " 100 " 20 "
dry and hard " 80 .... 25 "
Stone-block pavement, best " 40 " 50"
ordinary; " 80 " .... 25 "
Gravel, best " 50 " .... 40"
" ordinary " 80 . ... 25 "
Water-bound macadam road, best " 33 " 60"
ordinary., " 50 " .... 40'"
Brick on concrete " 25 " 80"
Sheet asphalt " 20 " .... 100 "
Iron rails, clean " 10 " .... 200 "
the height of the hill. Then in case of a first-class road, we go round
when d is less than 16ft; and in case of a second-class road, we go
round when d is less than 10ft." Although not specially so stated,
the above rule was plainly intended for water-bound macadam
roads.
The above rule (which has been frequently quoted) recognizes
no distinction between the several classes of rise and fall. It makes
the avoidance of a foot of rise in going over a small culvert or of a
foot of fall in crossing an open ditch, equally as important as the
elimination of a foot of rise and fall on the maximum grade. It is
not possible to draw sharp lines between the several classes of rise
and fall, but it is certain that there is a great difference in cost
between a foot of rise and fall on a flat grade and the same quantity
on the maximum or limiting grade. Notice that the above rule
makes the horizontal distance equivalent to a foot o£ rise much less
than that stated in Table 14.
79. Maximum Grade. The fixing of the proper maximum or
ruling grade is the most important matter connected with the loca-
tion of a road. To do this intelligently, the maximum grade must be
considered both as an ascent and as a descent. Viewed as an
ascent, the maximum or ruling grade chiefly concerns the draught of
heavy loads; and viewed as a descent, it chiefly concerns the safety
of rapid traveling. In both respects, the effect of the grade in
limiting the load depends upon its rate, and is practically inde-
pendent of its length.
80. As an Ascent. The load which a team can draw over any
road is determined by the length and steepness of the maximum
grade; or, in other words, the length and rate of the permissible
MAXIMUM GRADE 55
maximum grade depends upon the endurance of the team. TL
method of computing the load that a team can draw up any grade
was explained in § 68, page 47. That investigation shows that the
permissible maximum grade varies greatly with the conditions of
the surface; and that the better the surface the less should be the
ruling grade. In other words, unless the maximum grade is light,
the amount that can be hauled on a water-bound macadam road
does not differ greatly from that on an earth road.
A team could probably pull the maximum load up a stretch
of the maximum grade 400 to 500 feet long; and if the maximum
grade does not occur too often, it could probably pull the load up a
stretch two or three times as long. On long maximum'grades, it is
wise to provide a little stretch of nearly level grade upon which to
let the team rest. In the above computation, the team is assumed
to have a reserve power equal to that exerted on the maximum
grade; but the power required to start the load may be four or
five times the normal tractive resistance, and hence a nearly level
resting place is required, so that the team may readily start the
load.
81. Many books on roads state that if the maximum grade
is long, the slope should be flattened toward the summit to com-
pensate for the decreased strength of the fatigued horses. This
reasoning is incorrect and the remedy is impracticable. The argu-
ment is incorrect since it assumes that if the horse is to develop
energy to lift the load up the incline, it should not work at a uniform
rate. Universally the race horse goes fastest on the home stretch;
and if it is urged to its utmost speed at first, it is sure to lose the race.
The recommendation is impracticable, since the topography would
rarely permit the flattening of the grade at the top without increased
expense, and it would not be wise to incur extra cost for this pui-
pose,
82. If the loads are much heavier in one direction than in the
other, it is permissible to oppose the lighter traffic with the steeper
ruling grade.
83. As a Descent. Viewed as a descent, the maximum grade
concerns chiefly the safety of rapid travel. Many writers on roads
claim that the descending grade should not exceed the angle of
repose, i. e., should not exceed the inclination down which the
vehicle will descend by its own weight. This limit is impracti-
cable, since the angle of repose varies with the kind of vehicle,
degree of lubrication, amount of load, size of wheels, etc. Besides,
56 ROAD LOCATION [CHAP. II
this limitation is unnecessary, since the resistance of traction in-
creases as the speed, and in going down it is only necessary to
drive faster to prevent the vehicle from unduly crowding upon
the team; but of course this remedy has its limitations. Further,
the speed in descending may be checked by the application of the
brake; but it should be remembered that the use of the brake is
detrimental to the road surface, particularly on the maximum
grade.
Grades twice as steep as the angle of repose are operated without
inconvenience or danger. In Europe it is usually assumed that on a
good water-bound macadam road, of which the angle of repose is
about 2 or 2j per cent, a 5 per cent grade is the maximum that can
be descended safely at a trot without brakes; and, if the stretch
is long, 3 per cent is considered the maximum for safety. On moun-
tain roads having a water-bound macadam surface, freight wagons
descend 12 per cent grades by the use of brakes, but only with
expert drivers.
84. Safety at Summit. The grade each side of the summit
should be such that two automobiles approaching the summit should
be able to see each other when at least 200 feet apart, which is prac-
tically equivalent to limiting the grade for 100 feet on each side of
the summit to 5 per cent, or in other words to limiting the sum of
the grade for 100 feet either side of the summit to 10 per cent. This
is particularly important on a road having only a one-track improved
surface.
Where a highway crosses a steam or electric railway on the same
level, and where the highway has a steep grade as it approaches the
crossing, there should be sufficient level road at the top of the grade,
say 50 feet, to permit a wagon or an automobile, particularly the
latter, to stand while the train passes. If this condition does not
obtain, an automobilist is liable to kill his engine at the top of the
grade just as he is starting to cross the track and just as a train is
coming. Automobilists quite frequently encounter such dangerous
crossings.
The crossing should be wide enough, say 18 feet, to permit two
vehicles to meet upon the crossing.
85. Table 15, page 57, are the limits recommended by a special
committee of the American Society of Civil Engineers.* They are
presented here for convenience of reference and comparison.
*Proc. Amer. Soc. of C. E., Vol. 42 (1916), p. 1612.
MAXIMUM GRADE 57
TABLE 15
MAXIMUM PERMISSIBLE GRADE
Kind of Road Surface.
Grade.
Stone block with bituminous filler
15
Gravel
12
Brick with bituminous filler.
12
Water-bound macadam
12
Stone block with portland-cement filler
9
g
Bituminous concrete
g
Portland-cement concrete . .
g
Bituminous carpet
6
Brick with portland-cement filler
6
Sheet asphalt
5
Wook block
4
86. Minimum Grade. Considering only the cost of transporta-
tion, a perfectly level road is the best; but it costs less to maintain
a road upon a slight grade than one perfectly level. All roads
should be higher in the center than at the sides, so as to shed the
rain to the side ditches, but on any road longitudinal ruts are lia-
ble to form and interfere with the surface drainage; and therefore if
the road is perfectly level in its longitudinal direction, its surface
can not be kept free from water without giving it so great an incli-
nation transversely as to expose vehicles to the danger of overturn-
ing or skidding. On a perfectly level road, every rut will hold water,
which will soak into the road and soften it whether it be earth or
broken stone; whereas with even a slight longitudinal grade, every
wheel track becomes a channel to carry off the water. It is a com-
mon observation that earth roads running up hill and down dale
have surfaces better to travel upon than more level ones. This is
largely due to the better longitudinal surface drainage.
The harder the road material the less the necessity for longitudi-
nal drainage of the surface. An earth road surface is certain to
wear into ruts, and hence is greatly benefited by having a longi-
tudinal slope. Gravel and broken-stone roads are liable to wear
into longitudinal ruts, and hence need longitudinal drainage. Water-
bound macadam roads built with the hardest limestones or trap are
not easily worn into ruts, and therefore the necessity for a longi-
tudinal grade is less with this class of construction.
A longitudinal grade decreases the cost of maintenance, and the
advisability of introducing a grade for such a purpose depends upon
the relative cost of constructing it and upon the capitalized value
58 ROAD LOCATION [CHAP. II
of the cost of maintaining it. With earth roads the expenditures
for maintenance are ordinarily too slight to justify much expense in
securing a longitudinal grade; but with high class broken-stone
roads, which naturally have a heavy traffic, a considerable expense
to secure a slight longitudinal grade is usually justifiable. Engi-
neers whose experience has been largely upon railroads and canals
are prone to spend money to secure an absolutely level road, where
a slight grade could be secured at less expense. In filling up a
hollow or cutting down a hill, the employment of a light longitudinal
grade may decrease the cost of construction and also the cost of
maintenance without increasing the cost of transportation (§ 71-73).
The important principle to remember is that a slight longitudinal
grade is an advantage; although over a long stretch of level country
it may not be practicable to secure it.
The following is the minimum grade adopted by leading engi-
neers for water-bound macadam roads: in England 1 in 80 or Ij
per cent; in France, by the Corps des Fonts et Chaussees, 1 in 125
or 0.8 per cent; in the United States 1 in 200 or 0.5 per cent.
87. CURVES. Theoretically the shortest radius of curvature
allowable on roads depends upon the width of the road, and upon the
maximum length of horse teams frequenting the road or upon the
speed of the shorter teams. Since the length of a four-horse team and
vehicle is about 50 feet, to permit such a team to keep upon a 12-foot
roadway would require a radius of the inside of the curve of about
100 feet; on a 16-foot roadway a radius of about J5 feet would be
required; and on an 18-foot roadway, a radius of about 66 feet.
In France the minimum radius is as follows: on main and depart-
mental roads of which the trackway is 20 to 22 feet wide, 165, and
in extreme cases 100 feet; on principal country roads which are
20 feet wide, 50. In Saxony the minimum radius on principal roads
is 82 feet, and on ordinary country roads it is 40 feet.
" On mountain roads with grades of 1 or 2 per cent, heavy teams
require curves of 40 feet radius, and light ones 30 feet; and with
grades of 3 or 4 per cent, heavy teams require 65 and light ones 50
feet." " In extreme cases on mountain roads four- and six-horse
teams haul maximum loads over 16-foot roads having a radius at
their outer edge of 30 feet." However, in this case the roads on
the curves must be level, as the rear team is expected to do all of
the pulling on the curve.
88. For safety of automobile- travel, curves should be so flat
that two automobiles in approaching will be able to see each other
CtJRVES
when at least 200 feet apart; and where this is not feasible, a con-
spicuous sign should be placed. When the curve is located on a grade,
the tadius of the curve should be not less than 300 or 400 feet, even
if the view is unobstructed.
At the corners or intersections of roads, the hedges, trees, etc.,
should be removed so that automobilists approaching the corner
can have an unobstructed view of the side road for 200 or 300
. feet.
89. 90° Curves. There are many 90° curves in highways,
especially in that part of the country where the land was surveyed
according to the U. S. public land system. If a pavement 15 feet
wide is constructed in
the middle of a 50-foot
right-of-way, and if the
improvement is to be
kept within the right-of-
way at the corner, the ~t
radius of the center line
of the curve can be only
52.9 feet. But by pur-
chasing a comparatively
small area on the corner,
the length of the radius
can be greatly increased.
Fig. 7 shows a solution
of thi's problem.* " The
piece of land L K M N
Contains Only 0.055 acres, FIG. 7.— CURVE OF PAVED WAY AT 90° CORNER.
and often the saving in
the decreased amount of paving will more than pay for the extra
land. In addition the right-of-way is not contracted at the corner
as it is by the shorter radius curve, so that the 17.5-foot margin
between the inner edge of the pavement and the proper tyline is pre-
served for use as an earth road around the corner as wfcll as on tan-
gents."
Fig. 8, page 60, shows the solution of the above problem at the
intersection of two paved roads, f " Sections like GHJKLPQR
* H. E. Bilger, Road Engineer, Illinois Highway Department, Illinois Highways, January,
1917, p. 5.
t H. E. Bilger, Road Engineer, Illinois Highway Department, in Engineering News-
Record, Vol. 79 (1917), p. 134.
60
ROAD LOCATION
[CHAP, ii
should be built monolithic with the usual convexity of surface at
JK, although J is depressed. Areas like HJK will come out warped
surfaces, but are easily built by an experienced contractor. The
FIG. 8. — CURVES AT INTERSECTION OF Two PAVED ROADS.
ten construction joints shown should be nothing more than planes
of cleavage. Sections like KEFJ, which are built last, have, their
corner elevations fixed by the main pavement. Therefore the usual
convexity of surface is preserved; and the inner edge FJ is depressed
to meet the required elevation. In areas like KLE the surface of
the ground should be kept about 1 inch below that of the surface
of the pavement adjacent. The catch basins and drains will keep
the ground dry."
90. Super-elevation. It is natural for vehicles to keep to the
inside of the curve, partly to save distance and partly to get the
benefit of the crown of the road to prevent tipping outward. If
the curve has no super-elevation on the outside, the slew of the
vehicle, particularly a fast-moving motor-driven one, will materially
grind out the surface of the road.
The theoretically perfect super-elevation is given by the formula
E =
W2S2
32.2 R'
(1)
CURVES 61
in which E is the elevation in feet, W the width of the road in feet,
S the speed in miles per hour, R the radius of the curve in feet.
However, the maximum super-elevation is limited by the transverse
slope suitable for horse-drawn traffic.
The method adopted by the Illinois Highway Department is
very simple and effective. It is as follows: " Whatever the char-
acter of the road surface, the inner half of the curve is carried around
on the level; and the outer half of the curved roadway is elevated
so that the surface is a right line from inside to outside on any
radial line. For example, if the road surface is concrete 16 feet wide
with a 2-inch crown, then the outer edge of the outer half will be
elevated 2 inches, and the super-elevation of the curve proper will be
4 inches; and including the slope of the extra width (§97), the total
super-elevation will be nearly 5 inches."
The California Highway Department employs the following
method: The super-elevation on all curves is f inch per foot, which
on a 300-foot radius is perfect compensation for a speed of 17 miles
per hour and on a 200-foot radius for 13 miles per hour.
When curves have the proper super-elevation, the tendency
to keep to the inside of the curve will be less, and the damage due
to slewing will be nearly or wholly eliminated. For the best results
the super-elevation should begin a short distance before the tangent
point and not reach its full amount until an equal distance past the
tangent point.
91. Aesthetic Value of Curves. On a curved road there is a
constantly changing panorama or vista before the traveler, rather
than the constant and uninteresting vanishing point on a straight
road. However, in most cases the location of buildings and the tillage
of fields have fixed the location of the road within narrow limits; and
hence there is but little opportunity to consider the aesthetic or
artistic features of the location of the ordinary highway. In the loca-
tion of park drives the artistic feature is the controlling element.
92. WIDTH. Under this head will be considered the width of
the right-of-way and also the width of the improved portion.
93. Width of Right-of-Way. The legal width of right-of-way
varies greatly in different states. In an early day, before any attempt
was made to improve the wheel way, the legal width was often 100
feet, and sometimes 10 rods (165 feet). In some of the states where
land is cheap, the former width to some extent still prevails. In
most of the states of the Mississippi Valley, particularly those in
which the land was divided according to the system of U. S. public
62 ROAD LOCATION [CHAP. II
land survey, the legal width of right-of-way is usually 66 feet. A
few of these states classify the roads, making the less frequented
ones narrower; for example, in Texas the widths of first, second, and
third class roads are 60, 30, and 20 feet, respectively. In the earlier
settled states along the Atlantic coast, 3 rods (49J feet) is a common
width, although some of the less frequented roads are only 2 rods
(33 feet) wide.
If the surface is loam or clay, a considerable width of traveled
way is required that the traffic may not cut the surface up so badly
when it is soft. This is probably the explanation of the 60 or 66
feet so common in the Mississippi Valley. In some of the states,
for example, Illinois, the law specifies that, " if possible," a strip
equal in width to one tenth of the right-of-way shall be reserved for
pedestrians on each side between the property line and the ditch.
This leaves 53 feet for the wheelway and ditches, which is probably
none too much for a loam or clay road. If the ditches are deep and
consequently wide, the sidewalk is usually curtailed rather than the
wheelway.
In Massachusetts the roads improved by state aid usually have a
right-of-way of 50 feet wide, and in localities where there was a
possibility of space being required by an electric road, they are 60
feet, the latter being considered sufficient to accommodate a double-
track electric road, wagon ways, and sidewalks.
94. In England the principal roads, especially those near popu-
lous cities, are laid out 66 feet wide, 20 or 22 feet being covered
with broken stone.
In Holland the usual width is 38 feet, of which 14 feet is
improved.
In France the standard widths are to the nearest foot as follows:
Class of Road. Right-of-Way. Width Improved.
National roads 66 feet 22 feet
Departmental roads 40 " 20 "
Provincial " 33 " 20 "
Neighborhood " 26 " 16 "
95. Width of Improved Portion. In view of the cost of improv-
ing or paving the roadway, it is important to determine the proper
or best width of the improved portion. The best or economic width
of! the improved portion depends upon (1) the cost of the paved
portion, (2) the cost of constructing the shoulders, i. e., of partially
improving or hardening the natural soil at the edges of the improved
WIDTH 63
portion, (3) the amount of travel, and (4) the proportion of motor-
driven vehicles.
Except for cost, the wider the improved way the better; but length
is more valuable than width, and it is often difficult to get an improved
road because of the expense. Hence it is wise to make the paved way
only wide enough to accommodate the travel reasonably well.
The width necessary for ordinary rural traffic is often over-
estimated. Two wagons having a width of wheel base of 5 feet and
a width of load of 9 feet can pass on a 16-foot roadway and leave
6 inches between the outer wheel and the edge of the paved way
and a clearance of 1 foot between the inner edges of the loads. This
extreme case will rarely occur, and hence a width of 16 feet will
certainly be enough unless there is considerable rapid traffic.
The Massachusetts Highway Commission carefully measured
the width of traveled way on numerous crushed-stone roads, and
found that with an improved width of 15 to 24 feet, — the average
being 16.1 feet, — the maximum width of traveled way averaged
14.92 feet and the width commonly traveled averaged 11.05 feet.*
On this evidence the Commission concludes that " a width of 15
feet is ample except in the vicinity of the larger towns, and that
12 feet is sufficient for the lighter traveled ways, bat that 10 feet
is too narrow unless good gravel can be obtained for the shoulders."
The average width commonly traveled on forty-six of the 15-foot
roads was 9.58 feet.
In New Jersey the improved width for state-aid roads is 9 to
16 feet, mostly 10 to 12 feet. The improved width of French roads
varies from 16 to 22 feet (§ 94); in Austria, from 14 to 26 feet; and
in Belgium there are many roads surfaced only 8| feet wide.
96. The preceding data for the width of the improved portion were
fixed before automobiles became numerous. Naturally provision
should be made to permit automobiles to pass safely at considerable
speed ; and hence the widths stated above are too small. Two auto-
mobiles can not safely pass at low speed upon less than 12 feet, and
usually it is considered that a road having any considerable motor
travel should have a width of 14 or 16 feet. The Massachusetts
Highway Commission once built double-track roads 18 feet wide;
but in consideration of the large number of wheels that went off
the side of the improved way, increased the width to 19| feet, after
which few, if any, wheels went off the side.
* Report of the Massachusetts Highway Commission for 1897, p. 31. For a summary of
similar data for each township for five years, see Report for 1901, p. 47-55.
64 ROAD LOCATION ICHAP. n
97. Width on Curves. If the deflection angle is more than about
30°, the traveled way should be widened on the curve. If there is
likely to be much motor-driven traffic, the width at the center of the
curve should be increased 30 to 40 per cent, the increase tapering
to nothing at the tangent points.
The slope of the inner half of the curve should be continued over
this extra width.
If the improved way is so narrow that a considerable number
of vehicles turn off upon the shoulders, then the proper construction
of the shoulders becomes a considerable item; and it may be wiser
to improve a wider portion and spend less money upon the shoulders.
Obviously the best width depends upon the amount of travel, the
relative cost of the pavement and of improving the shoulders. It
has been said that if a vehicle is compelled to turn off on the
shoulder more than five times in going a mile, the improved portion
should be widened.
Since earth roads have the same material in the shoulders as in
the traveled way, and since the cost of an improved earth road is so
small, the whole width between the side ditches should be improved.
Since gravel roads are comparatively cheap to construct, and since
there is only a little difference between the cost of the improved way
and that of the shoulders, gravel roads can appropriately be wider
than roads of higher unit cost. For current practice concerning the
width of gravel roads, see Figs. 44 and 45, page 170. For examples
of the way in which these principles have been applied in water-
bound macadam, see Figs. 48-56, pages 197-99; and for concrete
roads, see Fig. 75, page 243.
98. Location of the Wheelway. The improved portion is some-
times placed in the middle of the traveled way, and sometimes at
one side. Apparently the natural position is in the middle with an
earth track on each side; but in this case, if the pavement is crowned,
as is usual, one half of the storm water falling on it is discharged
upon the shoulder at each side of the pavement, i. e., upon that por-
tion of the road the harder to keep in proper condition. On the
other hand, if the improved portion is placed at one side of the trav-
eled way, and if at the same time it is given a uniform slope toward
the nearer side ditch, all of the storm water falling upon the im-
proved portion will be discharged upon the unused shoulder next to
the side ditch and therefore do no harm. Further, in many cases
grass will grow upon the unused shoulder and protect it, so no harm
will be done if an occasional wheel does turn off onto this shoulder.
PLACING THE LINE
65
The heavier loads usually go toward town; and therefore if
the single-track improved portion is placed upon the right-hand side
going toward town, the heavier loads will have the right-of-way (in
the United States at least), and will not turn off from the paved
portion.
99. CROSS SECTION. The cross section of a road or pavement
depends upon the material of the road surface, and hence will be
considered in the respective chapters following.
However, the data in Table 16 on the crown or transverse slope
of the road surface are given here for convenience of reference and
comparison. These values were recommended by a special com-
mittee of the American Society of Civil Engineers.*
TABLE 16
CROWN OF ROADWAY
MATERIAL OF ROADWAY.
TRANSVERSE SLOPE,
Inches per Foot.
Maximum.
Minimum.
Earth
1
1
1
I
1
i
|
4~
1
4
1
4
!
Gravel
\Vater-bound IVIacadam
Bituminous IVIacadarn
Bituminous Concrete
Bituminous Carpet
Stone-block
Portland-Cement Concrete
Brick
Wood Block
Sheet Asphalt
100. PLACING THE LINE. The controlling points of a line are
certain points at which the position of the road is restricted within
narrow limits and is not subject to change. These may be points
where the location is governed by the necessity of providing an out-
let for the traffic, or points where the position of the line is restricted
by topographical considerations — such as a summit over which the
road must pass, or a suitable location for a bridge.
After the reconnoissance of the locality is completed and the
Proc. Amer. Soc. of Civil Engr's, Vol. 42 (1916), p. 1615.
86 ROAD LOCATION [CHAP. II
position and elevation of the controlling points are known, the line
must be marked upon the ground. For example, assume that it is
desired to run a road from A to D, Fig. 9, page 67, D being a pass
over the ridge. If the road follows the line A B C D, it will have
the profile shown near the bottom of Fig. 9. The average grade
from A to B is 1 per cent, and from B to C 5 per cent. If it is de-
sired to locate a road that shall have a grade no steeper than 5 per
cent, we may begin at D and locate a line having an uniform 5 per
cent grade. It is best to commence the location from D, since
usually the slopes nearer the foot of the hills are flatter than those
at the summit, and consequently there is more choice of position of
the line there than at the summit. Frequently in rough country,
the only controlling point fixed before beginning the location survey
is the lowest pass over a ridge or mountain range.
Beginning at D, a line may be located either (1) by setting off
the angle of the gradient on the vertical circle of a transit or on a
gradienter, and sighting upon a rod which is moved until the line
of sight strikes it at the same height from the ground that the instru-
ment is above grade; or (2) the points for the line may be found
by running a line of levels ahead of the transit, and measuring the
distances by which to reckon the rate of the grade. The line DEC,
Fig. 9, has a uniform gradient of 5 per cent.
If a contour map is at hand, the line can be located approxi-
mately by opening a pair of dividers until the distance between the
points corresponds to 100 feet, setting one point on the place of
beginning and the other on the next lower contour, which gives a
line 100 feet long with a grade equal to the distance between con-
tours— in Fig. 9, 5 feet.
The line D F G has a uniform grade of 5 per cent. From H to A
the road will have considerably less grade than 5 per cent, and can
have a comparatively wide range of position.
The average grade from A to D is a little less than 5 per cent, but
the slopes are so steep between D and C that it is impossible, within
the limits of the map, to locate such a line. If such a gradient is
located from D toward A, it will necessarily -make a number of short
turns on itself, which, although undesirable, are sometimes un-
avoidable. These short turns seriously impede traffic, since vehi-
cles can not easily pass each other on such short curves — particu-
larly if each is drawn by a long team. Short turns are also danger-
ous in descending, in case control of the vehicle is lost or the team
runs away.
PLACING THE LINE
67
101. The line A B C D may be considered as an old road which it
is proposed to improve by reducing the grades. Substituting the
line C E D for CD changes the maximum grade from 10 to 5 per
cent.
68 ROAD LOCATION [CHAP. H
102. In placing the line attention should be given to the nature
of the soil on alternative lines, since on one side of the valley the
surface may be clay, upon the opposite gravel; in the bottom of
the valley the soil is usually alluvial, while higher up it is generally
better for road purposes. It should be remembered that in almost
all steep slopes covered with loose material, the debris is either slowly
moving down the slope or has attained a state of repose so deli-
cately adjusted that an excavation for a road-bed on the inclined
surface will again set the mass in motion. Such movements are
particularly common in loose materials in countries where the frost
penetrates deeply and the ground becomes very soft when thawing,
and frequently entail long-continued and serious expense in main-
tenance.
If the road is to have a surface of gravel or broken stone, the
relative proximity of the materials for the original construction as
well as for repairs should be considered in deciding between possible
locations. However, it should be remembered that after the road
is completed, the amount of hauling required to supply materials
for maintenance must of necessity be small in comparison with the
ordinary traffic over the road; and hence this consideration should
not have undue weight.
Attention should also be given to the disposal of the drainage
water, and to the question of danger from high water in streams.
For example, in Fig. 9 it is possible to locate a line on the upper side
of the map with an uniform grade of 4 per cent, but such a line will
lie so near the branch entering the main stream at B as to be in
danger from floods. The matter of crossing streams should receive
most careful study. Bridges are comparatively expensive to build
and to maintain.
It may be cheaper to carry the road across the gully on an em-
bankment or a trestle than to make a detour around the head of the
valley. This question can be determined by comparing the greater
cost of construction of the shorter line with the capitalized value
of the greater cost of operating the longer line.
In some localities the protection of the road against snow is an
important matter. Deep cuts almost always catch snow; and for
this reason it is sometimes better to go around a point by a sup-
ported grade than to cut through it. In a snow country, roads
should be located on slopes facing south and east in preference to
slopes facing north and west, as the sun has greater power on the
former to melt the snow.
EXAMPLE OF RE-LOCATION
69
" Nothing pays like first cost in road building," i. e., money
expended in intelligent study of the location is the most economical
expenditure in the construction of a road.
103. EXAMPLE OF RE-LOCATION. Fig. 10 shows the old and
the new location of a road. The old location, in the back-ground,
FIG. 10. — RE-LOCATION OF ROAD.
had many sharp curves, an undulating profile, and two stream
crossings; while the new location has easy curves, no needless rise
and fall, and no stream crossings.
104. ESTABLISHING THE GRADE LINE. After placing the center
line, the topography should be taken on each side of the line for
some distance — the distance depending upon the lay of the land; —
and then a map should be drawn showing the center line and the con-
tours. This will serve to show whether the line is placed to the best
advantage, and whether any changes are desirable. This is especially
necessary over rough ground or where the line is on a maximum
grade.
The center line for a final location should be carefully run and
permanently marked, so that it may be re-located if necessary. A
line of levels should be run and a profile drawn, upon which the
grades may be established and from which the earthwork may be
estimated (§ 138).
CHAPTER III
EARTH ROADS
106. In 1915 the surface of 87 per cent of the roads of the United
States was the native earth (Table 12, page 33); and in all prob-
ability 70 to 80 per cent of these roads will always remain earth
roads.
The earth road is the cheapest road in first cost. It is a light-
traffic road, and only when the travel becomes considerable is it
possible to procure the money with which to improve the surface by
the use of some foreign material, as gravel or broken stone. For-
tunately, the best form for the earth road is also the best preparation
for any improved surface. This surface, whatever its nature, is
only a roof to protect the earth from the effects of weather and travel,
and any preparation that will enable the native soil when unprotected
to resist these elements will enable it the better to serve as a founda-
tion for the improved surface. Because of the importance of earth
roads as a means of transportation and also because of the importance
of a properly formed and well-drained road-bed for all improved road
surfaces, earth roads will be considered somewhat fully.
107. The term earth road will be used as applying to roads whose
surface consists of the native soil; and, unless otherwise stated, it
will be understood as meaning a road whose surface is loam or clay.
Roads on loam and clay will be discussed in this chapter; and
roads on sand or sand and clay mixed will be considered in the next
chapter.
AKT. 1. CONSTRUCTION
108. WIDTH. The width of the right-of-way varies greatly
but is usually between 40 and 66 feet (§93). With a 66-foot right-
of-way it is customary to reserve about 6 feet outside of the ditch
on each side for a foot-way, and to grade the remaining 54 feet.
With a 40-foot right-of-way it is customary to reserve 6 feet on each
side for a foot-way, thus leaving 28 feet for ditches and wheelways.
70
ART. 1] CONSTRUCTION 71
For equally good surface drainage, the greater width requires deeper
ditches and more cost in construction; but permits a wider distribu-
tion of the travel which is an advantage when the roads are muddy
or rough. The deep ditches are harder to maintain, and as a rule
the native soil from the bottom of deep ditches is not so good for road
building purposes as that nearer the surface. The cost of main-
taining the road varies with the amount of travel, and is practically
independent of the width. Therefore the width to be improved
depends chiefly upon the width of the right-of-way, the character of
the soil, the climate, and the first cost. In a wet climate, with soil
easily working into mud, a wide wheelway is desirable; while in a
dry climate, or with a soil not readily forming mud, a narrow wheel-
way is satisfactory.
109. Width on Curves. For a rule for widening the wheel-
way on curves, see § 97. This rule hardly applies to earth roads,
but it is well to bear it in mind in locating or improving earth roads
that may ultimately have a hard-surfaced wheelway.
110. CROSS SECTION. The cross section or transverse contour
of an earth road is an important matter with reference to the cost of
construction and maintenance, and depends mainly upon the tools
or machinery used in construction and maintenance and upon the
form required for drainage. The subject is discussed fully in
§ 129-31.
111. Super-elevation on Curves. For a discussion of the super-
elevation of the outer edge of the wheelway on curves, see § 90.
112. GRADES. For a general discussion of the effect of both
maximum and minimum grades upon the use and maintenance of
a road, see § 79-86.
The principal problem in reference to grades is the determina-
tion of the maximum grade permissible. This problem does not admit
of exact mathematical determination; and therefore recourse must
be had to experience. For obvious reasons there are not many
definite data under this head on record. In hilly country short
grades of 1 in 3 (33%) are occasionally found — particularly in a
newly settled country, — and grades of 1 in 4 (25%) are somewhat
common. In a comparatively flat country, grades of 1 in 8 (12J%)
are not infrequent.
In improving the celebrated Holy head road, Telford found in
old roads many grades of 1 in 6 and 1 in 7. A number of roads
improved by state aid in New Jersey originally had grades of 14
per cent. Of course only the roads having the most traffic were
72 EAKTH ROADS [CHAP. Ill
improved; and probably less frequented roads in each locality have
much greater grades.
For mountain roads, where the bulk of the traffic is usually
down hill, the maximum grade is often 8 per cent and sometimes
as much as 12 per cent. " Experience in heavy freighting has shown
that wagons can be satisfactorily controlled in all weather on
12 per cent grades, but they can not be safely controlled on steeper
grade."
113. DRAINAGE. Drainage is the most important matter to
be considered in the construction of roads, since no road, whether
earth or stone, can long remain good without it.
A perfectly drained road will have three systems of drainage,
each of which must receive special attention if the best results are to
be obtained. This is true whether the trackway be iron, broken
stone, gravel, or earth, and it is emphatically true of earth. These
three systems are underdrainage, side ditches, and surface drainage.
114. Underdrainage. Any soil in which the standing water in
the ground comes at any season of the year within 3 "feet of the
surface will be benefited by drainage; that is, if the soil does not
have a natural underdrainage, it will be improved for road purposes
by artificial subsurface drainage. It is the universal observation
that roads in low places which are thoroughly underdrained dry out
sooner than undrained roads on high land. Underdrained roads never
get as bad as do those not so drained. Underdrainage without
grading is better than grading without drainage; and, in general,
it may be said that where the soil does not have natural under-
drainage, there is no way in which road taxes can be spent to better
advantage than in subsurface drainage. Underdrainage is the very
best preparation for a gravel or stone road. Gravel or broken
stone placed upon an undrained foundation is almost sure to sink
(perhaps slowly, but none the less surely), whatever its thickness;
whereas a thinner layer upon a drained road-bed will give much
better service.
115. The Object. The opinion is quite general that the sole
object of underdrainage is to remove the surface water, but this is
only a small part of the advantages of the underdrainage of roads.
There are three distinct objects to be gained by the artificial under-
drainage of a wagon road.
1. The most important object is to lower the water level in the
soil. The action of the sun 'and the wind will finally dry the surface
of the road; but if the foundation is wet? it will be soft a.nd spongy.
ART. 1] CONSTRUCTION 73
the wheels will wear ruts, and the horses' feet will make depressions
between the ruts. The first shower will fill these depressions with
water, and the road will soon be a mass of mud. A good road can
not be maintained without a good foundation, and an undrained soil
is a poor foundation, while a dry subsoil can support almost any
load.
2. A second object of underdrainage is to dry the ground quickly
after a freeze. When the frost comes out of the ground in spring,
the thawing is quite as much from the bottom as from the top. If
the land is underdrained, the water when released by thawing from
below will be immediately carried away. This is particularly im-
portant in road drainage, since the foundation will then remain solid
and the road itself will not be cut up. Underdrainage will usually
prevent the " bottom dropping out " when the frost goes out of the
ground.
3. A third, and sometimes a very important, object of subdrainage
is to remove what may be called the underflow. In some places
where the ground is comparatively dry when it freezes in the fall, it
will be very wet in the spring when the frost comes out — surpris-
ingly so considering the dryness before freezing. The explanation
is that after the ground freezes, water rises slowly in the soil by
the hydrostatic pressure of water in higher places; and if it is not
drawn off by underdrainage it saturates the subsoil and rises a.
the frost goes out, so that the ground which was comparatively dry
when it froze is practically saturated when it thaws.
116. The underdrainage of a road not only removes the water,
but prevents, or greatly reduces, the destructive effect of frost.
The injurious effect of frost is caused entirely by the presence of
water; and the more water there is in the road-bed the greater the
injury to the road. The water expands on freezing, the surface of
the road is upheaved, and the soil is made porous; when thawing
takes place, the ground is left honeycombed and spongy, ready to
settle and sink, and under traffic the road " breaks up." If the
road is kept dry, it will not break up. Underdrainage can not pre-
vent the surface of the road from becoming saturated with water
during a rain, but it is the best means of removing surplus water,
thus allowing the surface to dry and preventing the subsequent
heaving by frost.
That frost is harmless where there is no moisture, is shown on a
large scale in the semi-arid regions west of the Mississippi river.
The ground there is normally so dry that during the winter, when
74 EARTH ROADS [CHAP. Ill
it is frozen, cracks half an inch or more wide form, owing to the dry-
ing and consequent contraction of the soil, which shows that there
is no expansion by the freezing of water in the soil; and therefore
in this region there is no heaving or disturbance by frost.
117. The Tile. The best and cheapest method of securing under-
drainage is to lay a line of farm tile 3 or 4 feet deep on one or both
sides of the roadway. The ordinary farm tile is entirely satis-
factory for road drainage. It should be uniformly burned, straight,
round in cross section, smooth inside, and have the ends cut off
square. Tile may be had from 3 to 30 inches in diameter. The
smaller sizes are usually a little over a foot long, — the excess length
being designed to compensate for breakage; and the larger sizes
are nominally 2 or 2J feet long, but usually a little longer. The
cost of tile free on board at the factory is usually about as in Table
17, page 75. Y's for connections can be had at most factories, but
they cost four or five times as much as an ordinary tile. With
patience and a little experience ordinary tile can be cut to make
fairly good connections.
Before the introduction of tile for agricultural drainage, it was
customary to secure underdrainage by digging a trench and deposit-
ing in the bottom of it logs or bundles of brush, or a layer of stone ;
or a channel for the water was formed by setting a line of stones
on each side of the trench and joining the two with a third line
resting on these two. Apparently it is still the practice in some
localities to use such substitutes for ordinary drain tile. Tiles are
better, since they are more easily laid and are less liable to get
clogged. Tiles are cheaper in first cost, even when shipped consid-
erable distances by rail, than any substitute; and the drains are
much more durable.
Tiles are laid simply with their ends in contact, care being taken
to turn them until the ends fit reasonably close. In some localities
there is apparently fear that the tile will become stopped by fine
particles of soil entering at the joints, and consequently it is specified
that the joints shall be covered with tarred paper or something of the
sort; but in the Mississippi Valley, where immense quantities of tile
have been laid, no such difficulty has been encountered. With a
very slight fall or even no fall at all, tiles will keep clean, if a free
outlet is provided, and they are not obstructed by roots of trees —
particularly willow.
In some localities it is apparently customary to use collars
around the ends of the tile to keep them in line. If the bottom of
ART. 1]
CONSTRUCTION
75
the trench is made but little wider than the diameter of the tile, or
if a groove is scooped out in the bottom of the trench to fit the tile,
no difficulty need be apprehended from this source.
TABLE 17
COST AND WEIGHT OF DRAIN TILE
Inside
Diameter.
Price per 1000 Ft.
f.o.b. Factory.
Weight
per Foot.
Number of Feet
in a Car Load.
3 inches
$10.00
51b.
7000
4
15.00
7
6500
5
20.00
9
5000
6
27.00
12
4000
7
35.00
14
3000
8
45.00
18
2500
9
55.00
21
1800
10
65.00
25
1600
12
90.00
33
1000
14
120.00
43
800
16
150.00
50
600
18
240.00
70
400
20
300.00
83
330
24
360.00
112
300
118. The Fall. There is no danger of the grade of the tile being
too great, and the only problem is to secure sufficient fall. A num-
ber of authorities on farm drainage and also several engineering
manuals assert that a fall of 2| or 3 inches per 100 feet is the lowest
limit that should be attempted under the most favorable conditions;
but practical experience has abundantly proved that a much smaller
fall will give good drainage. In central Illinois and northern
Indiana there are many lines of tile having falls of only £ to J of an
inch per 100 feet which are giving satisfactory drainage; and not
infrequently ordinary tile laid absolutely level directly upon the
earth in the bottom of the trench, without collars or other covering
over the joints, has given fairly good drainage without trouble from
the deposit of sediment. Of course, extremely flat grades are less
desirable than steeper ones, since larger tile must be used, and
greater care must be exercised in laying them, and since there is
more risk of the drain's becoming obstructed; but these extremely
flat grades are sometimes all that can be obtained, and even such
drains abundantly justify the expense of their construction.
If possible at reasonable expense, the grade should be at least
2 inches per 100 feet; and unless absolutely necessary should never
76 EARTH ROADS [CHAP. Ill
be less than J inch per 100 feet. On level or nearly level ground
the fall may be increased by laying the tile at the upper end shallower
than at the lower.
119. Size of Tile. The following formula has frequently been
employed to determine the size of tile. :
*»L0;y£*, (i)
in which A is the number of acres for which a tile having a diameter
of d inches and a fall of / feet in a length of I feet will remove 1
inch in depth of water in 24 hours.
Equation (1) is based on the formula ordinarily employed for
the flow of water through smooth cast iron pipe, and is only roughly
applicable to tile. It probably gives too great a capacity Tor tile.
However, all the factors of the problem are too uncertain to justify
an attempt at mathematical accuracy. For example, we can not
know with any certainty the maximum rate of rainfall, the duration
of the maximum rate, the permeability of the soil, the amount of
water retained by the soil, the effect of surface water flowing onto the
road from higher ground, the area to be drained, etc. The above
formula is useful only in a locality where there is no local experience
with tile; and its chief value consists in showing the relation between
capacity and grade, and the effect of a variation in the diameter of
the tile.
The object of under draining a road is to prevent the plane
of saturation from rising so near the surface as to soften the
foundation of the road even during a wet time, and therefore the
provision for underdrainage should be liberal; but what will be
adequate in any particular case depends upon the amount of traffic,
the local topographic conditions, the character of the soil, etc. The
best practice in agricultural drainage provides for the removal of
0.5 to 1 inch of water per day; but since the side ditches will assist
in removing rain water from the road, it is probable that a provision
for the removal of half an inch per day is sufficient for the under-
drainage of a road. If there is an underflow of water from higher
ground, or if the ground is " springy," then the ordinary provisions
for underdrainage should be increased.
120. It is not wise to lay a smaller tile than a 4-inch one, and
probably not smaller than a 5-inch. Tile can not be laid in exact
line, and any tilting up of one end reduces the cross section. Again,
ART. 1] CONSTRUCTION 77
if there is a sag in the line equal to the inside diameter, the tile will
shortly become entirely stopped by the deposit of silt in the depres-
sion.
It is sometimes wiser to lay a larger tile than to increase the fall.
Ordinarily, the deeper the tile the better the drainage, although 3J
or 4 feet deep is usually sufficient.
121. Laying the Tile. It is unwise to enter upon any entailed
discussion of the art of laying tile. The individual tiles should
be laid in line both vertically and horizontally, with as small joints
at the end as practicable. Care should also be taken that the tile
is laid to a true grade, particularly if the fall is small, for if there is
a sag it will become filled with sediment, or if there is a crest silt
will be deposited just above it. The drain should have a free and
adequate outlet. The end of the line of tile should be protected
by masonry, by plank nailed to posts, or by replacing three or four
tiles at the lower end by an iron pipe or a wooden box.
122. Cost of Laying Tile. On the basis of 15 cents an hour for
common labor, the prevailing cost of laying tile in loam with clay
subsoil is about as follows: for 8-inch tile or less, 10 cents per rod
for each foot of depth; for 9-inch, 11 cents; for 12-inch, 14 cents;
for 15-inch, 17 cents; and for 16-inch, 18 cents. To aid in remem-
bering the above data, notice that the price is 10 cents per rod
per foot of depth for 8-inch tile or less, with an increase of 1 cent
for each additional inch of diameter.
The cost of a mile of 5-inch tile drain is usually from $200 to
$250, exclusive of freight on the tile. If there is any considerable
amount of tiling, the above prices for the smaller tile can be reduced
10 to 20 per cent; and often there is enough discount on the prices
given in Table 7, page 75, to cover the railroad freight-charges.
A tile drain is a permanent improvement with no expense for main-
tenance, the benefit being immediate and certain; and therefore it
is doubtful if money can be spent on earth roads to better advan-
tage than in laying tile.
123. One vs. Two Lines. Usually a line of tile 2J to 3 feet deep
under the ditch at one side of the road will give sufficient drainage.
In case of doubt as to whether one or two lines of tile are needed,
put in one and watch the results. If both sides of the road are
equally good, another tile drain is not needed. In making these
observations care should be taken not to overlook any of the con-
tingent factors, as, for example, the difference in the effect of the sun
upon the south and the north sides of the road, the effect of shade
78 EARTH ROADS [CHAP. Ill
or of seepage water, the transverse slopes of the surface of the
road, etc.
124. Location of Tile. Some writers on roads recommend a
line of tile under the middle of the traveled portion. With the
same depth of digging, a tile under the side ditch is more effective
than one under the center of the road. Further, if the tile is under
the center, there is liability of the settling of the soil in the trench,
which will make a depression and probably a mud hole; and if the
tile becomes stopped, it is expensive to dig it up, and the doing so
interferes with traffic. Finally, if the road is ever graveled or
macadamized, the disadvantage of having the tile drain under the
center of the road is materially increased.
Some writers advocate the use of a line of tile near the surface,
on each side of the trackway. The object of placing the tile in this
position is to secure a rapid drainage of the surface; but very little,
if any, water from the surface will ever reach a tile so placed, since
the road surface when wet is puddled by the traffic, which pre-
vents the water's percolating through the soil. It is certain that
in clay or loam the drainage thus obtained is of no practical value.
Many farmers have tried to drain their barn-yards by laying tile
near the surface, but always without appreciable effect. The
deeper the tile the better the drainage.
The .rapid surface drainage sought by putting a tile or its equiva-
lent near the surface, can best be secured by giving the surface of
the road a proper crown and keeping it free from ruts and holes
(§ 205.)
While a line of tile on one side of the road is usually sufficient,
there is often a great difference as to the side on which it should be
laid. If one side of the road is higher than the other, the tile should
be on the high side to intercept the ground water flowing down the
slope under the surface. Sometimes a piece of road is wet because
of a spring in the vicinity, or perhaps the road is muddy because
of a stratum which brings the water to the road from higher ground ;
in either case, the source of supply should be tapped with a line
of tile instead of trying to improve the road by piling up earth.
125. Side Ditches. The side ditches are to receive the water
from the surface of the traveled way, and should carry it rapidly and
entirely away from the roadside. They are useful, also, to inter-
cept and carry off water that would otherwise flow from the side
hills upon the road. Ordinarily they need not be deep; but, if
possible, should have a broad, flaring side toward the traveled way,
ART. 1] CONSTRUCTION 79
to prevent accident if a vehicle should be crowded off the side of
the roadway. The outside bank should be flat enough to prevent
caving.
If the road is tiled as above recommended, the side ditch need
not be very large; but it should be of such a form as to permit its
construction with the road machine or scraping grader (§ 155) or
with a drag scraper (§ 150), instead of by hand. On comparatively
level ground, the proper form of side ditch is readily and cheaply
made with the usual road machine. Fig. 11, page 82, shows a shal-
low ditch of the proper form; and Fig. 12 shows a deeper one of the
same general form. If a larger ditch is needed, it should be of such
a form as to be made chiefly with the drag-scoop scraper.
A deep narrow ditch is expensive to maintain, since it is easily
obstructed by caving banks, by weeds, and by floating trash. For-
tunately the shallow ditch is easy and cheap to construct and also
to maintain. If it is necessary to carry water along the side of the
road through a rise in the ground, it is much better to lay a line
of tile and nearly fill the ditch than to attempt to maintain a narrow
deep ditch. A tile is much more effective per unit of cross section
than most open ditches.
126. The side ditch should have a uniform grade and a free out-
let into some stream, so as to carry the water entirely away from
the road. No good road can be obtained with side ditches that
hold the water until it evaporates. For this reason much ostensible
road work is a positive damage. Piling up the earth in the middle
of the road is perhaps in itself well enough, but leaving undrained
holes at the side probably more than counterbalances the benefits
of the embankment. A road between long artificial ponds is always
inferior and is often impassable. It is cheaper and better to make
a lower embankment, and to drain thoroughly the holes at the side
of the road. Public funds often can be more widely used in making
ditches in adjoining private lands than in making ponds at the
roadside in an attempt to improve the road by raising the surface.
It is cheaper and better to allow the water to run away from the road
than to try to lift the road out of the water.
When the road is in an excavation, great care should be taken
that a ditch is provided on each side to carry away the water so that
it shall not run down the middle of the road. Every road should
have side ditches, even one that runs straight down the side of a
hill. Indeed, although it often has none, the steepest road needs the
side ditch most. Frequently the water runs down the middle of
80 EARTH ROADS [CHAP. Ill
the road on a side hill and wears it into gullies, which are a discom-
fort, and often dangerous, in both wet weather and dry.
In a slightly rolling country, the side ditch frequently has no
outlet, and the water is allowed to accumulate at the foot of the
slope and there remain until it is absorbed by the ground or seeps
into the tile drain. The water seeps away very slowly because the
fine silt carried down by the water fills up the pores of the native
soil and renders it nearly impervious. The difficulty could be
remedied by providing an inlet from the open ditch to the tile. This
may be a well, walled with plank or masonry without mortar (except
near the topj and having a grating in the side or top through which
the water may pass. The well should be large enough to allow a
man to enter it to clean it, and should extend a foot or more below
the bottom of the tile. Earth roads in villages and towns are usually
better provided with such inlets than country roads, but both could
be materially improved at comparatively small expense by pro-
viding inlets from the side ditch into the tile.
127. If it can be prevented, no attempt should be made to carry
water long distances in side ditches; for large bodies of water are
hard to handle, and are liable to become very destructive. Side
ditches should discharge frequently into the natural watercourses,
though to compass this, it may in some cases be necessary to carry
the water from the high side to the low side of the road. This is
sometimes done by digging a gutter or by building a dam diagonally
across the road, but both are very objectionable. A better way
is to lay a tile or put in a culvert (Fig. 53, page 198), the amount
of water determining which shall be done.
It is sometimes necessary to carry water a considerable distance
in the side ditches, as, for eaxmple, when the road is in excavation.
This requires deep ditches, which are undesirable and dangerous;
and if the grade is considerable, the ditches wash rapidly. In such
cases, it is wise to lay a line of tile under the side ditch, and at
intervals turn the water from the surface ditch into the tile drain.
This can be accomplished readily by inserting in the line of porous
tile a Y section of vitrified pipe, with the short arm opening up hill.
Of course, the short arm, i. e., the vertical arm, need not be as large
as the body. If necessary, two or three lengths of porous tile may
be added at the upper end of the Y to make connections with the
bottom of the open ditch. Earth, sods, or stones, can be piled
around the upper end of the tile to make a dam and to hold the tile
in place.
ART. 1] CONSTRUCTION 81
Some road engineers lay a line of tile under the side ditch, and
fill the trench with broken stone, thus making the tile carry both
the surface water and the underdrainage. This practice probably
affords better surface drainage, but it costs more than to allow the
surface water to flow away in the side ditches. This construction
is sometimes defended on the ground that the broken stone prevents
the wheels from striking the tile when vehicles in passing are forced
into the ditches. This danger does not seem very great, and would
not occur at all if the tile were laid at the proper depth; but this is
sometimes impossible owing to a hard substratum.
128. As a rule side ditches will not have too much fall; but
sometimes a ditch straight down a hill will have so much as to wash
rapidly, in which case it is an advantage to put in an obstruction of
stone or brush. In extreme cases the bottom of the ditch is paved
with stones.
129. Surface Drainage. The drainage of the surface of a road
is very important, and is provided for by crowning the surface
and keeping it smooth. It should be remembered that water upon
the surface of the road can not be carried away by the underdrains,
since the water can reach them only after it has penetrated and
softened the road surface. The slope from the center to the side
should be enough to carry the water freely and quickly to the side
ditch; and if the surface is kept free from ruts and holes, less crown
will suffice than if no attention is given to keeping the surface smooth.
If there is not enough crown, the water can not easily reach the
side ditches; and hence the road soon becomes watersoaked.
On the other hand, the crown may be too great. If the side
slopes are so steep that traffic keeps continually in the middle, the
road will be worn hollow and retain the water instead of shedding it
promptly to the side ditches. If the crown is too great, it is difficult
for vehicles to turn out in passing each other. Again, if the earth
is piled too high in the middle, the side slopes will be washed into
the side ditches, which not only damages the road but also fills up
the ditches. Further, if the side slopes are steep, the top of the
wheel will be further from the center of the road than the bottom;
and the mud picked up by the bottom of the wheel will be carried to
the top of the wheel and then dropped farther from the center of the
road than it was before, each passing vehicle moving the earth
from the center toward the side of the road. Finally with the
ordinary method of caring for earth roads, more water stands on a
very convex road than on a flatter one.
82
EARTH ROADS
[CHAP, in
The slope from the center to the side should be at least half an
inch to a foot, or 1 foot in 24 feet; and it should not be more than 1
inch to a foot, or 1 foot in 12 feet. If the surface is well cared for,
the former is better than the latter; but in no case is it wise to
exceed the latter slope.
Some claim that theoretically the cross section of the surface
should be the arc of a circle, and others that it should consist of
two planes meeting at the center and having their junction rounded
off with a short curve. Great refinement in this matter is neither
possible nor important. Two examples of a properly crowned
road are shown in Figs. 11 and 12. The crown can be easily and
cheaply constructed with the scraping grader (§ 155).
FIG. 11. — ROAD SURFACE AN ARC. SHALLOW SIDE DITCH.
The drainage of the surface of a road is chiefly a matter of main-
tenance (see Art. 2 of this chapter); and one of the most common
defects of maintenance is the failure to fill ruts and keep the surface
smooth so that the water will .be promptly discharged into the side
ditches. A comparatively shallow rut will nullify the effect of any
f — en
0.5V/?. Tile
FIG. 12. — ROAD SURFACE AN ARC. DEEP SIDE DITCH.
reasonable amount of crown. Seldom is a mile of road seen which
does not have a number of ruts and saucer-like depressions which
catch and hold the water. On undulating roads, ruts and holes are
naturally drained; and this is the reason why undulating roads are
better than perfectly flat ones (see Minimum Grade, §86).
Fig. 13 shows a form of cross section sometimes adopted for
earth roads in villages and towns. The gutter usually is made next
to the sidewalk, which is objectionable, since horses must stand
in the mud and water when hitched in front of the abutting prop-
erty. The form shown in Fig. 12 is free from this objection. A nar-
row berm is left between the sidewalk and the edge of the slope to
prevent crowding the gutter too close to the shade trees, which are
ART. 1] CONSTRUCTION 83
usually planted just outside of the sidewalk. The gutter shown in
Fig. 12 decreases the available wheel way, and consequently in some
localities would be undesirable. This cross section also can be made
and maintained with the ordinary scraping grader.
j< aft >i< — Qff -** /? rft -»i
Walk \ ] ^^-— TI L-^. fl
I* 4ft <n -^^-^^r- /2fff, j
FIG. 13. — CROSS SECTION OF VILLAGE STREET.
130. The crown should be greater on steep grades than on the
more level portions, since on the grade the line of steepest descent is
not perpendicular to the length of the road, and consequently the
water in getting from the center of the road to the side ditches travels
obliquely down the road. If the water once commences to run
down the center of the roadway on a steep grade, the wheel tracks
are quickly deepened, and the road becomes rough and even danger-
ous. Under these circumstances, it is necessary to construct catch-
waters (" water-breaks," " hummocks," or " thank-you-marms ")
at intervals to catch the water which runs longitudinally down the
road, and to convey it to the side ditches. These catch-waters may
be either broad shallow ditches or low flat-ridges constructed across
the road; and they may slope toward one or both side ditches. In
the former case, they should cross the road diagonally in a straight
line; and in the latter case, in plan they should be a broad angle
with the apex at the center of the road and pointing up hill. There
is little or no difference between the merits of the ditch and the ridge,
unless the bottom of the former is paved with gravel, broken stone,
or cobbles. The ridges are more common, but usually are so narrow
and so high as to form a serious obstruction to travel, a fact which
is especially objectionable since the introduction of the automobile.
However, neither the ditches nor the ridges should be used except
on steep grades where really necessary, since either form is at best
an obstruction to travel. The angle that the catch-waters shall
have with the axis of the road should be governed by the steepness
of the grade — the steeper the grade the more nearly should the
catch- waters run down the road. They should have a considerable
breadth so that wheels may easily ascend them and horses will not
stumble over them.
Catch-waters should be constructed also in a depression where
an ascending and a descending grade meet, in order that they may
collect the water that runs down the traveled way and convey it
84 EARTH ROADS [CHAP. Ill
into the side ditches. These catch-waters should run square across
the road, and should be quite shallow ditches, the bottom of which
should be hardened with gravel, broken stone, or cobbles.
131. Some writers recommend that the surface of a road on the
face of a hillside should consist of a single slope inclining inwards
(see Fig. 14). This form of surface is advisable on sharp curves, but
Fio. 14. — IMPROPER CROSS SECTION OF ROAD ON SIDE HILL.
is of doubtful propriety elsewhere. The only advantage of this
form is that the water from the road is prevented from flowing down
the outer face of the embankment; but the amount of rain water
falling upon one half of the road can not have a very serious effect
upon the side of the embankment. With a roadway raised in the
center and the water draining off to either side, the drainage will be
more effectual and speedy than if the drainage of the outer half
must pass over the inner half. If the surface is formed of one plane,
as in Fig. 14, the lower half of it will receive the greater share of the
travel as the tendency is to keep away from the edge; and as this
part of the surface will bo more poorly drained, it is nearly certain
to wear hollow. This will interfere with the surface drainage; and
consequently a road with this section will require excessive attention
to keep it in good condition. Figs. 53 and 54, page 198, show two
forms of Swiss hillside roads having the center higher than either
side.
Whatever the form of the road surface, if the hillside is steep
there should be a catch-water above the road to prevent the water
from the hillside above flowing down on the road. Fig. 14 shows
such a catch-water. It should be, say, 6 feet back from the
excavation, and should have a width and depth according to the
amount of water to be intercepted.
132. EXCAVATION AND EMBANKMENT. Side Slopes. The
angle of the slopes of the cuts and fills is designated by the ratio of
the horizontal to the vertical distance. Thus, if the face of the fill
ART. 1]
CONSTRUCTION
85
has an inclination of 1 \ feet horizontal to 1 foot vertical, the slope is
designated as If to 1.
The slope of the excavations varies with the nature of the soil,
being for economy as steep as the tenacity of the soil will permit.
Solid rock may be cut with a slope of \ to 1. Common earth will
stand 1 to 1, or 1 1 to 1 — the latter being safer arid more usual. Gravel
requires \\ to 1. Some clays will stand 1 to 1, while some require
a much flatter slope — in extrene cases 6 to 1. Fine sand requires
a slope of 2 to 1, or 3 to 1.
The slope of embankment has less range than that of excava-
tions, since there is less variety in the nature and the condition of
the materials, and is usually \\ to 1.
133. In both railroad and wagon-road work, it is customary to
establish all earthwork slopes as planes intersecting each other in
right lines. The original form is never maintained, since it is not a
form of equilibrium and stability. Storm water soon washes away
the angle formed by the intersection of the two plane surfaces at the
top of the embankment, and the water flowing down the slope soon
rounds out the angle at the foot. Such construction violates one of
the fundamental principles of stability, and it is a. needless expense to
build laboriously a form of construction which nature will inevitably
destroy.
The transverse contours of the embankment and excavation
shown in Figs. 15 and 16 are designed to meet the above objections
FIG. 15. — CROSS SECTION FOB EMBANKMENT.
to the ordinary forms of construction. These sections are de-
signed in accordance with the forms of railroad excavations and
'to /
IP /
I / OT/te OT/'/e
Fio. 16. — CROSS SECTION FOR EXCAVATION.
\»
embankments recommended by D. J. Whittemore, the distinguished
chief engineer of the Chicago, Milwaukee and St. Paul Railroad,
EARTH ROADS [CHAP. Ill
whose forms have met with the unanimous approval of leading
engineers.
It is customary in railroad construction to make the top of the
earth embankment wider than the base of the gravel or broken-
stone ballast, which gives a berm between the base of the ballast
and the outer edge of the earth embankment. This berm has been
omitted in Figs. 15 and 16, since with an earth surface there is
nothing corresponding to the ballast.
134. If the natural slope above the cut is long or steep, a catch-
water drain should be constructed along the upper edge of the exca-
vation slope to prevent the surface water from above washing
down over the face of the cut; but the catch-water should be well
back from the edge of the excavation, to prevent the water in the
drain from softening the upper angle of the slope (Fig. 14, page 84).
The slopes of both excavations and embankments should be
sowed with grass seed. Sometimes the material of the embank-
ment is such that grass seed will not grow, in which case it may be
necessary to lay sod; but of course this is very expensive. The
loots of the grass will hold the earth from slipping, and prevent
the face of the slope from being gullied out and washed down.
135. There is a tendency for workmen in order to decrease the
amount of labor required to leave the side slopes of embankments
hollow and those of excavations rounding. When inspecting the
work, this tendency should be borne in mind.
136. Setting Siope Stakes. For instructions as to methods of
staking out the ground preparatory to beginning the work of exca-
vating and embanking, see any of the standard volumes on railroad
engineering.
137. Computing Earthwork. For the methods employed in
computing the contents of excavations and embankments, see any
of the various treatises on that subject; or for a briefer presentation
of the subject, see books on surveying or railroad engineering.
138. Balancing Cuts and Fills. Other things being equal, the
most economical position of the grade line is that which makes the
amount of cuts and fills equal to each other. If the cuts are the
greater, the earth therefrom must be wasted, i. e., deposited in spoil
banks; and if the fills are the greater, the difference must be ob-
tained from borrow pits, — both of which operations involve addi-
tional expense for labor and land. Sometimes it is more economical
to make an embankment from near-by borrow pits than to bring
the necessary material from a far-distant cut; or, vice versa, it is
ART. 1] CONSTRUCTION 87
sometimes more economical to waste the material from a cut than
to send it to a remote fill. The most economical use of the material
depends upon the machinery used in moving the earth, the char-
acter of the earth in both cuts and fills, the road over which the
earth must be transported, the cost of haul, the price of land, the
liability of cuts being filled with snow, etc.; and the matter must
be decided by the engineer to the best of his judgment in each
particular case.
When the road lies along the side of a hill, one side of the road is
usually in cut and the other in fill; and it is customary so to place
the center line that these two parts are at least nearly equal. How-
ever, where the side slopes are steep, it is better to make the road
mostly in cuts on account of the difficulty of forming stable fills on
steep slopes.
139. In railroad work it is the custom to balance cuts and fills
on the longitudinal profile of the road, but in wagon-road work the
fills as shown by the profile of the center line should be slightly in
excess, to provide a place for the earth taken from the side ditches.
On account of the expense, wagon roads follow the surface more
nearly than railroads; and consequently the earth from the ditches
is proportionally more in wagon-road construction than in railroad
construction.
140. Shrinkage of Earthwork. With the ordinary soil, the act of
excavation so breaks it up that it occupies more space after excava-
tion than before; but when the material has been placed in an
embankment it will usually occupy less space than in its original posi-
tion. The expansion due to excavation is usually 8 to 12 per cent
of the volume, and in extreme cases may be 40 per cent; but in
placing the material in the embankment, it is compacted by the
weight of the embankment itself, by the pounding of the hoofs and
by the action of the wheels, until usually the final volume is less
than the original.
At first thought it seems strange that earth should occupy less
space when placed in an embankment than when in its original
position, seeing that it is not so hard and firm, and that it will
usually settle still farther after the embankment is completed.
The following facts account for this phenomenon: 1. The continued
action of frost has made the soil in its natural position more or less
porous. 2. Earths which have been lying in situ for centuries
become more or less porous through the slow solution of their soluble
constituents by percolating water. 3. The surface soil is rendered
£8 EARTH ROADS [CHAP. Ill
more or less porous by the penetration of vegetable roots which
subsequently decay. 4. There is ordinarily more or less soil lost
or wasted in transporting it from the excavation to the embank-
ment.
The amount of shrinkage depends chiefly upon the character of
the material and the means by which it is put into the embankment,
and somewhat upon the moisture of the soil, the rainfall conditions
while the work is in progress and soon afterwards, and the depth to
which frost usually penetrates. If the soil is moist when placed in
the bank, it will become more compact than if it is dry. Rain
greatly affects the shrinkage, and embankments put up during a
rainy season will be more compact than those built during a dry
time. Soil from above the usual frost line is more porous than that
not subject to the heaving effect of alternating freezing and thawing,
and consequently shrinks more when put into an embankment.
The natural shrinkage of the ordinary soils is in the following
order: (1) sand and sandy gravel least, (2) clay and clayey soil
intermediate, and (3) loams most. The shrinkage according to
the method of handling is in the following order, beginning with
the least: (1) drag scrapers, (2) wheel scrapers, (3) wagons, (4) cars,
(5) wheelbarrows. The usual allowance for shrinkage for drag-
scraper work is as follows: gravel 8 per cent, gravel and sand 9
per cent, clay and clayey earth 10 per cent, loam and light sandy
earth 12 per cent, loose vegetable surface-soil 15 per cent. The
above results are for ordinary earth, and do not apply to such
unusual materials as " buckshot," gumbo, very fibrous soil, etc.,
which have a much greater shrinkage. Solid rock will expand 40
to 50 per cent.
The shrinkage of earth should be considered in locating the grade
lines to balance the cuts and fills.
141. Settlement of Embankments. The shrinkage of earth-
work referred to above takes place chiefly during construction, but
the continued action of the weight of the embankment and the
effect of rain and traffic will usually cause a comparatively small
settlement after completion. Sand or gravel embankments built
with wheel scrapers will usually settle 1 to 2 per cent after comple-
tion, and clay or loam embankments about 2 to 3 per cent. With
drag scrapers the settlement will usually be a little less than the
above; and with dump carts or wagons, a little more. With wheel-
barrows the settlement is usually about 10 per cent, but may be as
much as 25 per cent, depending upon the moisture in the soil, the
. 1] CONSTRUCTION
rain during construction, and the length of time under construc-
tion.
The settlement of the embankment after completion should be
taken into account when determining whether the bank has been
raised to the proper height. The .embankment should be built to
such a height that after it has ceased to settle it will be at grade.
The length of time required for this settlement depends upon the
weather conditions. The proper adjustment of the height of the
embankments to compensate for future settlement is an important
matter with broken-stone roads and with pavements.
142. The above remarks about settlement do not apply to em-
bankments built with the elevating grader (§ 161). The settle-
ment of earth roads put up by these machines is of no importance,
and depends upon the amount of rolling they receive.
143. Rolling the Embankment. Many writers on roads rec-
ommend the rolling of all new earth embankments. In view of
the usual settlement of banks built with drag or wheel scrapers, it
does not appear that rolling with a farm roller would be very effect-
ive, and a heavier roller is seldom available. Simply rolling the
top of the finished bank is not worth much, since the effect of the
roller does not reach very deep ; and, besides, no roller will compact
loose earth so that wheels and hoofs will not make depressions in
it.* Further, it is not practicable to roll the bank during the
progress of construction, except when the scraping and elevating
graders are used. Finally, those who travel the road most are gen-
erally the ones who pay for the construction, and almost univer-
sally they prefer to compact the earth by traffic.
It is customary to roll the foundation of pavements, but the
chief object of so doing is to discover soft places rather than to con-
solidate the surface; and, besides, the foundation of a pavement is
protected from rain and the action of wheels, and therefore the
effect of the rolling is permanent, while with an earth road it is not.
144. Over-haul. When earthwork is done by contract, the bid
includes the cost of removing excavated material and depositing
it in embankments, provided the necessary length of haul does not
exceed a specified limit. When the material must be carried beyond
this limit the extra distance is paid for at stipulated price per cubic
yard per 100 feet of haul. This extra distance is known by the
name of " over-haul " or simply " haul." For an explanation
* The heaviest steam rollers give a pressure of about 600 pounds per linear inch, while
wagons frequently give twice, and occasionally three times, as much,
90 EARTH ROADS [CHAP. Ill
of the method of computing " haul," see treatises on earthwork or
books on railroad surveying.
The specified limit, i. e., the distance of free haul, depends upon
the conditions. It is sometimes made as low as 100 feet, and is
sometimes 2,000 feet — the latter usually only in street work. In
railroad work 500 feet is a common limit.
145. Frequently all allowances for over-haul are disregarded.
The profiles, estimates of quantities, and the required disposal of
material are shown to bidding contractors; and they must then
make their own allowances, and bid accordingly. This method has
the advantage of avoiding possible disputes as to the amount of the
over-haul allowance, and on this account is adopted by some railroads.
146. Stability of Embankments. The principles to be observed
in the formation of an embankment depend somewhat upon the
machinery employed in doing the work, but a few general considera-
tions are not out of place here.
Specifications usually require that " all matter of vegetable
nature must be carefully excluded from the embankment." It is
impracticable to do this when the road passes through grass land —
particularly if the grade is built with a scraping grader (§ 155).
It is desirable to remove brush, tall grass, and high weeds from the
space to be occupied by the embankment and the borrow pit; but
small twigs, leaves, and sod are no material detriment, and their
removal is a needless expense — except at the point where the road
passes from cut to fill. It is essential that all vegetable matter
and loose porous soil should be removed at this point, otherwise
there will be a soft place which will soak up water and make a mud
hole and also weaken the bank just below it. When an embank-
ment is to be made across a swamp, bog, or marsh, the site should
first be drained as thoroughly as possible.
Perfect solidity should be the aim, and all necessary precautions
should be taken to prevent or lessen the tendency of the bank to
slip. To secure stability, embankments should be built in successive
layers not more than 3 or 4 feet thick, and the vehicles conveying
the materials should be required to pass over the bank, so as to con-
solidate the earth. Specifications sometimes state that the layers
shall be made concave, but this refinement is scarcely ever necessary,
although it is well to see that the layers are never very much convex.
Embankments are sometimes first built up in the center, and after-
wards widened by tipping or dumping earth over the side; but this
never should be allowed.
ART. 1] CONSTRUCTION 91
When embankments are to be formed on sloping ground, it may
be necessary to plow the ground or to cut steps in a rocky surface
to keep the filling from sliding down the natural surface. In many
cases where roads are to be constructed along steep slopes, it is
found cheaper to use retaining walls (§ 192) to sustain the road
upon the lower side and the earth-cut on the upper side than to
cut long slopes or form high embankments.
147. IMPROVING OLD ROADS. Country roads may be improved
in any of several ways:
1. By changing the location, to secure better alignment or lower
gradients. The method of doing this has been discussed in Art. 2,
Chapter I.
2. By cutting down the hills and filling up the hollows, to secure
easier gradients. A hill may be cut down without seriously inter-
fering with traffic by cutting one side of the roadway down a foot
or two with drag or wheel scrapers (§ 154), and then turning traffic
on this portion and lowering the other side, continuing to cut down
each side alternately until the desired depth is reached. If the
earth is deposited upon the embankment in the hollow, the traffic
will consolidate the road as it is built up, which is very desirable.
3. By laying tile and cutting open ditches, to improve the drain-
age, as has been discussed in § 114-28.
4. By re-forming the surface by the use of the scraping grader
to improve surface drainage, as discussed in § 155-58.
5. By adding sand or gravel to a clay road, or clay to a sand
road, to improve the surface, as considered in Chapter IV.
148. ROAD-BUILDING MACHINERY. In recent years there has
been a great advance in the machinery employed in building earth-
roads. The wheelbarrow was formerly much used for short hauls,
but has been superseded by some form of drag scraper (§ 150) drawn
by horses, and is never used now except for very small jobs, or in
wet and swampy places. Formerly an embankment was constructed
with plows and drag-scoop scrapers (Fig. 17, page 92), while now it is
built much more cheaply and better with either the scraping grader
(Fig. 23, page 96), or with the elevating grader (Fig. 28, page 101).
Years ago earth was thrown into wagons or carts by hand and hauled
to its destination, while now it is moved with the two- or four-
wheel scrapers (Fig. 21 or 22, page 95). Earth was formerly
moved considerable distances with the drag scraper, while now the
wheel scraper is employed. Formerly the surface of the excavation
was finished with the drag-scoop scraper, while now it is done much
92 EAKTH ROADS [CHAP. Ill
better and more cheaply with the tongue scraper (Fig. 18, page
93) or the scraping grader (Fig. 23, page 96).
There are a variety of plows, dump carts, wagons, etc., used in
moving earth, which need not be considered here. The dump cart
is much in favor in the New England States, but is never used in the
Mississippi Valley. The steam shovel and dump cars afford the
most economical method of handling earth when the amount to be
moved justifies the outlay for the plant; but as that would seldom
be the case in highway work, this method will not be considered.
149. Scrapers. Scrapers are generally used to move material
after it has been loosened by plowing. There are two principal
kinds — the drag and the wheel scraper.
150. Drag Scrapers. There are three forms of the drag scraper
— the scoop (Fig. 17), the pole-scraper (Fig. 18, page 93), and the
Fresno scraper (Fig. 19, page 93).
151. The drag-scoop scraper, Fig. 17, is sometimes referred to
as the drag scraper or simply the drag, and also as the slip scraper or
the slip. It is made in
three sizes. The smallest,
for one horse, has a capacity
of 3 cubic feet; and the two
larger sizes, for two horses,
have a capacity of 5 and
FIG. 17.— DRAG-SCOOP SCRAPER. . . . a
7 feet respectively. Some
have metal runners on the bottom and others have practically a
double bottom, both of which devices decrease draft and increase
durability.
The drag-scoop or slip scraper is much used for moving earth
short distances; but with it there is difficulty in building a bank
of uniform solidity, since each scraperful is deposited in a compact
mass by itself, with low loose places between them. Nor is the
slip scraper suitable for finishing an embankment, since the surface
made with it is a succession of humps and hollows which is very
trying to drive over when dry, and when it rains the low places fill
with water which speedily softens the remainder of the road, and
finally produces mud holes. The pole or tongue scraper (§152) is
much preferable for finishing the surface.
The drag-scoop or slip scraper is sometimes employed in loading
wagons. This is done by building an elevated platform under which
the wagons are driven, and to the top of which the earth is drawn in
a scoop scraper upon an inclined runway. In the middle of the plat-
ART. 1]
CONSTRUCTION
93
form is a hole through which the scraper is dumped. This arrange-
ment of platform and runways is called a trap.
152. The pole or tongue scraper, Fig. 18, is ordinarily used for
leveling up the road surface in excavations, and is frequently
FIG. 18. — POLE OB TONGUE SCRAPER.
employed in preparing the subgrade for pavements. It may be
used to transport earth short distances, but is not so good for this
purpose as the scoop scraper. It is made in two sizes, 36 and 48
inches wide.
153. The Fresno scraper, Figs. 19 and 20 is the outgrowth of
FIG. 19. — FRESNO SCRAPER, READY FOR LOADING.
experience in irrigation, and has some advantages over the com-
mon scoop scraper. (1) The proportions are such that it is more
readily loaded to its full capacity. (2) It distributes the earth on
94
EARTH ROADS
[CHAP, in
the bank better, as it can be adjusted to deliver in layers from 1 to
12 inches thick. (3) The runners make it more durable. (4) It
is more easily loaded. (5) It will follow up a steep bank without
dumping, and hence runways are not required.
Fresno scrapers are made in three sizes, the cutting edge being
FIG. 20. — FRESNO SCRAPER, DUMPED
3J feet, 4 feet, and 5 feet; and their respective capacity is 8, 10, and
12 cubic feet.
Under favorable conditions this form of scraper will push con-
siderable earth along in front of it, and consequently the capacity
is frequently stated as much greater than that given above.
154. Wheel Scrapers. There are two forms of wheel scrapers, —
those with two wheels and those with four. The two-wheel scraper
consists of a steel box mounted on wheels and furnished with levers
for raising, lowering, and dumping, Fig. 21, page 95. It is made in
three sizes, No. 1, 2, and 3, having a capacity of 9, 12, and 16 cubic
feet, respectively. Some manufacturers make an automatic front
end-gate which adds materially to the load the scraper will carry, par-
ticularly on a rough down-hill road.
The four-wheel scraper is a steel box or scoop suspended from
a frame supported upon four wheels, see Fig. 22, page 95. It is made
in two sizes, a half-yard and a yard capacity. It may be loaded by
a snatch team or by steam power, — either a traction or a hoisting
engine. The larger size is loaded with a 20 H.P. steam traction
ART. 1]
CONSTRUCTION
95
engine or a 30-60 H.P. gasoline tractor, or by a hoisting engine;
but when loaded, it is drawn by a two-horse team. The pan
FIG. 21.— TWO-WHEEL SCRAPER, FILLING.
is raised and lowered by the tractive power. Work can be done
cheaper with the four-wheel scraper than with the two-wheel
scraper, because the former carries larger loads and also because it
is self-loading and self-dumping.
The four-wheel scraper has been
called a self-loading and self-
dumping wagon.
The four-wheel or Maney
scraper was first made in 1909,
and has been used in the far
west in railroad work. It is used
extensively in preparing the sub- FIG. 22.— FOUR-WHEEL SCRAPER.
grade for pavements.
155. Scraping Grader. There are several forms of scraping
graders of the type shown in Fig. 23, which differ in minor details
but all of which accomplish substantially the same work. Each
consists of a frame carried on four wheels, supporting an adjustable
scraper-blade, the front end of which plows a furrow while the rear
end pushes the earth toward the center of the road or distributes it
uniformly to form a smooth surface. The blade can be set at any
angle with the direction of draft, or at any height; and it may also
be tilted forward or backward. This machine will work in almost
any soil — even where a plow will not. It is hauled by horses or
traction engine, usually the former, and makes successive rounds
EARTH ROADS
[CHAP. Ill
or cuts until the desired depth of ditch and crown of road is ob-
tained.
This machine is often called a road grader and sometimes a blade
grader; but it is here designated as a scraper grader to distinguish
FIG. 23. — SCRAPING GRADER.
it from the elevating grader (§ 161). The scraping grader is an
important machine in both the construction and the care of earth
roads. As an instrument of maintenance it has been called a road
hone, but could more properly be called a road plane.
Fig. 24 to 27, pages 98 to 99, show the various kinds of con-
struction work that may be done with this type of machine. For
a discussion of the work of this machine in maintenance, see § 208-1 1 .
Various devices are employed to neutralize the lateral resistance
of the earth to being pushed sidewise by the blade. In some types
the whole rear end of the machine may be thrown to one side or the
other by operating a hand-wheel; in other forms the rear axle is
shifted lengthwise so that one wheel may bear against the unplowed
bank of the ditch; in other cases either rear wheel can be moved in
or out independently; in still other types either the front or rear
wheels or both may be set at any inclination by operating a hand-
wheel; and in other forms the wheels have a flange which cuts into
the earth and resists the lateral thrust.
The scraping grader is of inestimable value in constructing
•earth roads, as it does the work better and much cheaper than it
ran be done either by hand or with plows and scrapers. The work
done with the scraping grader is also superior to that done with
plows and drag scrapers, since the plow cuts deeper in some places
ART. 1] CONSTRUCTION 97
than others and these places are left full of loose earth and soon
form holes which catch and hold water.
156. There are scraping or blade graders on the market having
only two wheels, and also those having no wheels; but such machines
are neither common nor efficient. They have been superseded by
the road drag (§ 206).
157. Operating the Scraping Grader. To build a road with the
scraping grader, first plow a light furrow with the point of the blade,
where the outside of the ditch is to be (see Fig. 24, page 98). To
make the blade penetrate hard or stony ground, elevate the rear end
considerably and use only the point. On the second round, with
the front and rear wheels in line (see Fig. 25), drive the team so
that the point of the blade will follow the furrow made the first
round, plowing a full furrow with the advance end of the blade, and
dropping the rear end somewhat lower than before. The third time
round, move the earth previously plowed over toward the middle of
the road. In moving the earth toward the center of the road,
elevate the rear end of the blade to allow the earth to distribute
under it, so as to build the road at the side of the proper crown
before filling the -center; and if the machine slides sidewise instead
of pushing the ridge of earth toward the center, either slue the
whole rear end of the machine toward the center, or move one hind
wheel or the whole rear axle laterally until the rear wheel bears
against the bottom of the unplowed bank at the ditch, or incline
the rear wheels, according to the construction of the machine.
Finally, return to the ditch and plow it out deeper, moving the earth
over toward the middle whenever as much is plowed as the machine
can move at once. Repeat this until the ditches are of the proper
depth, and the road as full and round as required.
A ridge should not be left in the middle of the road. Usually a
skillful handling of the machine will prevent the formation of such
a ridge by elevating the rear end of the scraping blade, thus allowing
the earth to lose out under it as the center of the road is approached.
If the road is very rough, it may not be possible to fill all the ruts
without at some places forming a ridge in the center of the road.
If the ridge is formed, it can be flatted down by setting the blade
square across the road and allowing the earth to flow under it; or
with most machines the center ridge can be leveled down by revers-
ing the blade and using the back of it.
If the ground where a road is to be constructed is covered with
weeds and grass, it should be cleared by burning or by mowing and
98
EARTH ROADS
[CHAP. Ill
FIG. 24.— SCRAPING GRADER MAKING FIRST ROUND.
FIG. 25. — SCRAPING GRADER MAKING SECOND ROUND.
ART. 1]
CONSTRUCTION
99
FIQ. 26. — SCBAPING GRADER PLOWING BETWEEN FRONT WHEELS.
FIG. 27. — SCKAPING GRADER CUTTING AWAY OLD BANK.
100 EARTH ROADS [CHAP. Ill
raking. With sod ground the best road can be obtained by first
cutting the sod as thin as possible and moving it to the center of the
road, and then going back to the ditch and continuing the grading
as described above. To cut a thin slice of sod, the scraper blade
should be as sharp as possible. When the ground to be moved is
covered with sod or wee'ds, some operators make the first cut on the
inside of the ditch and at each successive round cut a little farther
out, thus distributing the sod through the earth forming the road-
way. This requires too much cutting with the unsharpened end of
the blade, and is therefore not as good as the method described above.
158. It is best not to put more than 4 to 6 inches of loose earth
into the road at one working, as that is all that can be thoroughly
packed by traffic. If a greater amount is thrown up at one time,
the bottom of the grade will remain soft and cause the road to cut
into deep ruts as soon as the top has become thoroughly soaked by
rain. As far as possible the grading should be done early in the
summer, giving ample time for the loose earth to settle and pack
before the fall rains. If worked in the fall, there should never
be more than 4 inches of loose earth put upon the road at one
working.
If the maximum amount of earth is to be placed upon the road
at once, it is wise to roll each successive layer with as heavy a roller
as is available or as a team can draw, as otherwise traffic will con-
solidate only the surface, and the bottom of the grade will long
remain soft and spongy.
159. The scraping grader is usually drawn by four or six horses,
depending upon their size, and the character and condition of th&
soil. One man can operate the machine, and one or two men are
required to drive.
A traction engine is sometimes used; and it is a better power,
since it gives a steady draft and does not need to stop to rest. At
certain seasons of the year, the traction engine is the cheaper power,
and at other times horses are the cheaper, depending upon the re-
quirements of horses for farm work and the demands for the trac-
tion engine in threshing and shelling.
160. The cost of building an ordinary prairie road with horse-
power with this machine is about $30 to $40 per mile, with a width
of 30 or 35 feet and a crown of 6 inches above the natural surface.
The first is the cost when there is no sod, and the second when there
is a stiff sod. A second 6 inches may be added for about $30 per
inile,
ART. 1] CONSTRUCTION 101
If the ground is very dry and hard, another team and driver will
be required, and the above prices may be nearly doubled.
161. Elevating Grader. The best known form of the elevating
grader is shown in Fig. 28. It consists of a frame resting upon four
wheels, from which is suspended a plow and a frame carrying a
wide traveling belt. The carrier is built in sections and its height
is adjustable. The larger carrier will deliver earth 14, 17, 19, or 22
feet horizontally and 8 feet vertically from the plow; while the
smaller size will deliver 14 and 17 feet horizontally and 7 feet verti-
cally. The smaller machine is designed for highway work. The
FIG. 28. — ELEVATING GRADER.
plow loosens the soil and throws it upon the traveling inclined belt,
which delivers it upon the embankment direct or into wagons.
This is an exceedingly effective machine for building open ditches,
earth embankments, or filling wagons. By changing the length of
the carrier and by properly distributing the earth, the machine will
build either a broad low embankment from a narrow deep cutting,
or a narrow high embankment from a broad shallow cutting; or
the machine will excavate a deep narrow ditch with flat spoil banks,
or a shallow ditch with narrow spoil banks. This machine is espe-
cially adapted to building earth roads in a prairie country, for
which purpose it has been very largely used.
The large machine is usually propelled by twelve horses — eight
in front and four behind, — and the smaller by eight in front. Often
a traction engine is cheaper than horses. One man can operate the
machine; and at least two men, and usually three, are required to
drive the larger machine, but usually two drive the smaller one.
102
EARTH ROADS
[CHAP, in
162. Operating the Elevating Grader. To build a new road of
the sections shown in Fig. 11 and 12, page 82, first mark by stakes
a line 10 feet on each side of the center of the proposed road. With
the machine arranged to throw the earth 17 feet horizontally, drive
along the left-hand row of stakes and back on the other side of the
road in the same way. The streams of earth as delivered will overlap
5 or 6 feet. Start the machine on the second round with the right-
hand forward wheel in the furrow of the previous round, and complete
the round. A harrow should follow the machine to break up the
sod and level the bank. Continue to make rounds until the ditches
are as wide as desired.
Commence the second plowing by bringing the left-hand wheel
of the machine to the left-hand edge of the first furrow cut, which
brings the plow one furrow to the left of the point of commencing
the first plowing, and keep this relative position while making this
round. Make the second round with the left-hand forward wheel
in the furrow of the previous round; and continue to make rounds
until the outside of the ditch is reached again. For the best results
a harrow and roller — the heavier the better — should follow the
grader during the second and subsequent rounds — see Fig. 29.
When the second plowing has been completed, the grade will be
FIG. 29. — ELEVATING GRADER BUILDING EARTH ROAD.
high and narrow; and therefore the carrier should be shortened to
14 feet. Then start the machine so that the plow will take a furrow
ART. 1] CONSTRUCTION 103
from the center of the ditch, and continue the third plowing, as
described above for the first and second, to the outside of the ditch.
For the fourth plowing take a couple of furrows from the outside of
the excavation to deepen the ditch.
The final result should be about as in Fig. 11 or 12, page 82. Most
operators, however, leave a berm at the inside edge of the ditch
(Fig. 37, page 122), which is undesirable since it interferes with the
operation of the scraping grader in maintaining the road.
163. For loading wagons, the carrier is arranged to deliver at 17
or 19 feet horizontally from the machine, the wagons are driven so
that the earth falls from the carrier into the wagon, and both move
at the same speed until the wagon is loaded; and then the grader
slows down while the loaded wagon drives out and an empty one
drives in. Common wagons with dump boards are not so easily
loaded as the usual dump wagon, since they are narrower and longer.
It is customary to estimate three dump wagons for the first 100 feet
of haul, and an additional wagon for each 100 feet thereafter.
164. COST OF EARTHWORK. Of necessity, general estimates of
the cost of earthwork can not be very exact, since the cost will vary
with the condition of the soil, the wages, the hours constituting a
day's work, the relative amount paid for supervision, the effective-
ness of the supervision, the facilities for preventing one part of the
crew from interfering with the work of another, the proper adjust-
ment of the number of shovelers per wagon or cart, or of scraper
holders to scrapers, etc. The following data have been checked by
engineers and contractors of wide experience and are believed to be
reasonably reliable.*
165. In the analysis of the cost of earthwork to follow, the price
for a man will be assumed to be $1.50 per day of 10 hours, and that
for a team and driver $3.50 per day. These were the usual wages
formerly paid by contractors, which are the prices to be considered
here; for if the work is done under the labor-tax system ordinary
estimates will not apply (§ 51-52), and if the farmer hires out to
do the work of a teamster he usually demands the ordinary pay for
that class of work. These were about the prices current for a number
of years in a number of states, before the rise of prices incident to
the Great European War. Of course, if wages are greater than as
stated above, the following prices can be changed proportionally.
166. Cost with Scraping Grader. In prairie soil, two men and
* For an instructive discussion of methods of Handling Earth in Road Construction, see an
article by Chas, R, Thomas in Engineering and Contracting, Vol. 47 (1917), pp. 406-08.
104 EARTH ROADS [CHAP. Ill
four horses with a scraping grader can build a mile of road 36 feet
wide from inside to inside of ditch with a crown of 6 inches at the
center after being compacted, for $30 to $40, which is equivalent to
if or 2J cents per cubic yard. The first is the cost when there is
no sod, and the last when there is sod, The cost for a crown of 12
inches will be about $70 per mile, or If cents per cubic yard. The
above prices do not include interest, or wear and tear of grader,
which would be about £ cent per cubic yard.
In hard soil requiring an extra team and hence another driver,
add one half to the above prices.
167. Cost with Elevating Grader. The elevating grader, Fig. 28
and 29, pages 101 and 102, will, in light prairie soil, deposit on a road
1,000 cubic yards per day of 10 hours; and will load into wagons
500 yards per day. The outfit required is: seven two-horse teams
at $2.50 each plus 2 drivers at $2.00 each plus 1 operator at $2.00 and
1 at $2.50 = $29.50. For the earth deposited on the road this is 2.9
cents per cubic yard, and for that loaded into wagons 5.8 cents,
exclusive of interest, depreciation, and administration.
168. Cost with Drag-Scoop Scraper. Drag scrapers are admirably
adapted for borrowing at the sides of embankments and for wasting
from cuts or ditches, and also for opening the mouth of large cuts;
but are not economical except for short distances. There is no
danger of the scraper getting out of order until it is worn out and
unfit for use, and the manner of using it is quickly learned by any
one. Drag scrapers are made in three sizes having a capacity of
3, 5, and 7 cubic feet, respectively; but it must not be assumed
that each scraper will carry to the embankment an amount equal to
its rated capacity, since in the first place it is difficult to completely
fill the scraper, and in the second place the scraper carries loose
earth which will shrink about 25 per cent when compacted in the
embankment. Unless the soil is very loose and easily loaded, it is
not safe to assume that each trip of the scraper will make of com-
pleted embankment more than one half of its rated capacity. The
larger size is most economical, but the relative advantage is not
proportional to the size, since the larger size is not as easy to handle
nor as easy to fill. Scrapers should be used in gangs of not less
than six to decrease the cost of loading, superintendence, spreading,
etc.
169. Cost of Loosening. Sand or sandy loam can be scraped
without plowing. In loam a two-horse team and plow will loosen
400 cubic yards per day, at a cost of $3.50 for team, plow, and driver,
ART. 1] CONSTRUCTION /105
and $1.50 for the plow holder, making a total of $5.00, or 1J cents
per cubic yard. Sometimes the driver can also hold the plow, in
which case loosening will cost about 1 cent per cubic yard, since the
team will not do quite as much work as when there is a plow holder
and also a driver. If the ground is hard it will be necessary to add
another team and also a man to " ride " the beam of the plow. If
the ground is not very hard, this force will loosen 400 cubic yards
per day at a cost of 2.1 cents per yard.
170. Cost for 25-foot Haul. The cost of building an embank-
ment from a borrow pit at the side of the road will first be considered.
For a 60-foot right-of-way and a light embankment, the length of
haul or " lead " from center of gravity of the fill to the center of
gravity of the cut will be about 25 feet. This distance will be a
little more or a little less according to the height and width of the
bank, and the width reserved for sidewalk; but slight difference in
length of short hauls make comparatively little difference in the
cost of moving the earth, because in the first place a considerable
part of the cost of hauling is due to time consumed in turning and
loading, and in the second place the cost of transportation is only
about half the total cost of moving the earth.
On the road, an ordinary team will travel 220 feet per minute
(2J miles per hour), but in scraping considerable time is consumed
in turning, waiting to load, etc., and besides, the distance traveled is
more than that from the center of cut to the center of fill; therefore
the ordinary speed of the team is no guide in this connection.
Experience shows that a team will use from a minute to a minute
and a half in making a round trip at the above distance, or, say, li
minutes per trip. A foot vertically is equivalent to 10 to 25 feet
horizontally (see § 77), and in estimating the length of haul this
fact must be taken into account.
Using the large scraper, a scraperful will make 3J cubic feet
of compacted embankment, or will require eight trips per cubic
yard. Therefore a team will place a yard of earth in the fill every
10 minutes, or 6 yards per hour and 60 yards per day. In light loose
earth, where it is easy to fill the scrapers full, a team may make 70
yards; but if the ground is hard, or obstructed with roots and grass,
50 yards may be the maximum. Assuming a day's work to be 60
yards, the cost of hauling is $3.50 -5- 60 = 5.83 cents per cubic
yard.
One man will hold and fill the scraper for two teams at a cost of
$1.50 -T- (2 X 60) = 1.25 cents per yard. One man on the dump
106 EARTH ROADS [CHAP. Ill
will distribute and level the earth deposited by six teams, at a cost
of $1.50 -f- (6 X 60) = 0.4 cents per cubic yard. One foreman
will be required at, say, $2.50 per day; or $2.50 ^ (6 X 60) = 0.69
cents per cubic yard. For wear and tear of scraper we may allow
10 cents per day for each, or 60 cents for the lot; and for wear of
plow and cost of sharpening, say, 30 cents, making a total of 90 cents
or 0.25 cents per cubic yard. In very hard ground the above prices
may be doubled.
The total cost of moving earth 25 feet will then be as in Table 18,
page 108.
171. Cost for 50-foot Haul. We will next consider the cost for a
50-foot haul. At this distance a scraper holder can fill for three
teams. Each team can put in about 50 cubic yards per day. The
other items will be substantially as for a 25-foot haul, and the total
cost will be as in Table 18.
172. Cost for 100-foot Haul. Each team will make a trip in
about 2| minutes, and will put in 40 cubic yards per day. The
total cost will be as in Table 18.
173. Cost for 200-foot Haul. At this distance a scraper holder
can fill for four teams. Each team will make the trip in about 3J
minutes, and put in about 35 cubic yards per day. The total cost
will then be as in Table 18. .
174. Cost for Hard Ground. If the ground is so difficult to plow
as to require a second team and a man to ride the beam, add 1 or 1|
cents to the values in Table 18 for the extra cost of loosening; and
add, say, one fifth to the cost of hauling to allow for the fact that
in hard ground the scrapers are not as well filled as in loose light
soil. Also add one half to the above estimated cost of wear and
tear. The results for hard ground are then as in Table 18.
175. Cost with Two-Wheel Scrapers. Two-wheel scrapers are
excellent for hauling earth distances up to 600 or 700 feet. They
are made in three sizes, No. 1, 2, and 3, having a capacity of 9, 12,
and 15 cubic feet, respectively. With No. 1 the team fills its own
scraper, while with No. 3 an extra team (a snatch team) is required
to fill the scrapers reasonably full; and unless the ground is very
loose and light an extra team is required to fill No. 2. Most con-
tractors use either No. 1 with a single team or No. 3 with a snatch
team. It usually takes about five loads with No. 1 to make a cubic
yard in place; four, with No. 2; and three, with No. 3.
176. Cost for 100-foot Haul. It is assumed that the scrapers
will be worked in a gang of six, which will require one foreman, one
ART. 1] CONSTRUCTION 107
plow, three scraper holders, and one man on the dump. The
expense for these items will be the same as for the drag scrapers,
and are so entered in Table 19, page 109. At this distance a trip
will occupy 2J minutes, and a yard will be deposited every 10 min-
utes, or 60 yards per day, at a total cost $3.50, or 5.83 cents per
cubic yard for hauling.
The wear and tear is computed on the assumption that a scraper
will, last for 200 days' continuous work, making a cost for deprecia-
tion and repairs of, say, 20 cents per day per scraper. The wear
and tear on the plow will be estimated at 30 cents per day. The
total cost will then be as in Table 19.
177. Notice that the cost for 100 feet with the two- wheel scraper
is 9.99 cents per cubic yard, while with the drag scraper for the same
distance it is 12.67 cents. The difference is in the cost of hauling,
v, hich is due to the difference in the capacity of the scrapers.
178. Cost for 200-foot Haul. A trip will be made in about 4
minutes, and each scraper will put in 50 cubic yards per day. The
three scraper holders can fill an additional scraper, making nine
in all. The cost will thgn be as in Table 19.
179. Cost for 300-foot Haul. In this case another scraper can
be added, making four scrapers to each scraper holder. A trip can
be made in about 5j minutes, and each team will move 45 yards
per day. The cost will be as stated in Table 19.
180. Cost for 400-foot Haul. It is difficult to determine the
most economic distance for each size of scraper, since the several
sizes are seldom available for making the test. However, at 300
feet, the cost with a No. 2 scraper is about the same as with a
No. 1 at 200 feet; and at 400 feet the cost with a No. 2 is about the
same as with a No. 1 at 300 feet. But at 400 feet a No. 3 is more
economical than a No. 2.
A snatch team is required in filling No. 3 scrapers. The extra
. force acquired by using the extra team completely fills the scraper,
and also packs the load so that it is less liable to spill than when
loaded by a single team. For this distance it is most economical
to work the scrapers in a gang of eight; and two men will be re-
quired to hold the scrapers while being filled. Each team will put
into place 45 cubic yards, or 360 for the gang. The total cost will
be as shown in Table 19.
181. Cost for Other Distances. For each additional 100 feet of
lead, add 1 cent per cubic yard to the cost of haul; and the total
cost will be approximately as shown in Table 19.
108
EARTH ROADS
CHAP. Ill
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ART. 1]
CONSTRUCTION
109
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110 EARTH ROADS [CHAP. Ill
When the amount of earth to be moved is considerable and the
length of haul is great, something must be allowed for keeping in
repair the road over which the earth is transported. A wheel
scraper is prone to wear a series of fyumps and hollows along the
road it traverses, and these must be kept in subjection, if the work
is to be done at reasonable cost. The proper allowance will vary
greatly with the soil, the weather, etc. Trautwine recommends
0.1 cent per cubid yard per 100 feet for this expense.
182. It is difficult to determine at what distance wagons should
supersede two-wheel scrapers; but usually the economic limit for
two-wheel scrapers is 600 to 800 feet, and it is seldom wise to use
such scrapers beyond 800 feet — unless they are at hand and wagons
are not.*
183. Cost with Four-Wheel Scrapers. The cost of moving earth
with the four-wheel scraper is not well established. With enough
scrapers to keep the loading engine reasonably busy, the cost of
power for loading — operator, fuel, rent, etc., — will be about 2 or
3 cents per cubic yard, depending upon the hardness of the soil.
The saving in loading is 5 to 6 cents over that of wagons — see
§184.
184. Cost with Wagons. It will be assumed that the wagons are
filled by hand, that they are used in gangs of nine, and that the haul
is 700 to 800 feet. If the roads are level and fairly smooth, a load
will make about 1| cubic yards in place; and with ordinary roads, 1
yard will make a load ; but if the roads are soft and steep, f of a yard
may make a load. The amount a team can deliver will vary greatly
with the time consumed in waiting to load and in loading. With
short wagon-hauls and well-organized work, half of the time is thus
consumed, and often much more is thus consumed. The time of
the wagon while loading should be considered as a part of the cost
of loading, and this will be discussed more fully in the next para-
graph. For the above distance, the round trip will consume 15
minutes; and assuming a yard as a load, each wagon will deliver
50 cubic yards per day, at a cost of 7.00 cents per cubic yard for
hauling.
There is very great variation in the amount of earth a shoveler
will load in a day. In well managed work, the shovelers are not
* For the results of an elaborate time study of cost of moving various kinds of soils dif-
ferent distances with two-wheel scrapers and Fresno scrapers, see Engineering and Contracting,
Vol. 41 (1914), pp. 629-36.
ART. 1] CONSTRUCTION 111
actually engaged in loading much more than half of the time; while
under poor management, they do not really work half of the time.
With short intervals of rest equal to the working time, a man should
load, in a day of 10 hours, 20 cubic yards of light sandy soil, 17
yards of loam, and 15 of heavy soil — provided all are loosened by
plowing or picking. Usually five or seven men are set to load a
wagon — two or three on each side and one at the rear. Seven men
will load a wagon with loam in 5 minutes, 8 minutes will be con-
sumed in traveling to and -from the dump, 1 minute in dumping
and 1 minute in getting into and out of the cut — making in all 15
minutes for a round trip; and therefore the cost for wagon and
team is 8.75 cents per cubic yard, as above. In this case the team
works only about half the time. If only five men are engaged in
loading a wagon, 7 minutes will be consumed in loading, and the
time for a round trip will be 17 minutes, and each wagon will deliver
only 35 cubic yards, making the cost 10 cents per cubic yard. In
this case, the team really works less than half the time. If the
men shovel only 12 to 15 cubic yards each, as is very common, the
loss by the wagon's waiting for a load is considerably more than
above. The proper number of men to be set to loading a wagon
depends upon the relative wages of shoveler and wagon, upon the
length of the haul, and upon the quantity loaded per day per man.
Usually seven shovelers are employed to each wagon, but this
number is not enough to secure the greatest economy. In the fol-
lowing estimates it will be assumed that nine shovelers are em-
ployed to each wagon. At the above distance, nine shovelers, each
loading 17 yards per day, will be required to keep three wagons
going, each of which deposits 45 cubic yards per day. The cost of
loading will then be 8.8 cents per cubic yard.
The cost of leveling the dump is small, if dump wagons are used
and the earth is dumped over the end of the embankment or wasted ;
and it may be taken the same as for scrapers, i. e., at 0.40 cents
per cubic yard. But dump wagons are so heavy and expensive
that they are seldom used; and if ordinary wagons with dump
boards are employed, the expense for labor on the dump will be
about three times as great as above, or, say, 1.20 cents per cubic
yard.
The driver furnishes his own wagon, and hence no account is
taken of the wear and tear of it. There should be a small allowance
made for the wear and care of shovels, say, 0.1 cent per cubic
yard.
112 EARTH ROADS [CHAP. Ill
The total cost of moving earth 700 to 800 feet with wagons,
then, is as follows:
1. Loosening 1 . 25 cts. per cu. yd.
2. Shoveling 8.80
3. Depreciation of shovels 0. 10 " "
4. Hauling 700 to 800 feet 7.00
5. Helper on dump 1 .20 " "
6. Superintendence 0.69 " "
7. Water boy 0.16 " "
Total cost 18.20
185. For longer distances add 1 cent per cubic yard for each
100 feet of distance — the usual charge for over-haul.
186. Other Methods. When the haul is more than 600 or 800
fe^et, and when the amount of work to be done is sufficient to justify
the initial expense, it is more economical to use portable track and
small dump cars than to use wagons. However, such conditions
seldom occur in wagon-road construction.
187. Finishing the Slopes. In addition to the elements of
cost discussed above, there is always some expense in leveling off
the bottom of the cut, in digging ditches, in trimming up the slopes
of embankments and excavations, and in cutting a catch-water at
the top of the slope in excavation. The cost of these items will
very greatly with the degree of finish required and also with the
depth of the cut or the fill; and it may amount to 0.25 or 0.50 of
a cent per cubic yard. If the bottom of the cut can be leveled off
with the scraping grader, and if the ditch also can be made with
this machine, the cost of this item will be considerably reduced.
188. Profits and Administration. The proper allowance under
this head will vary according to the magnitude of the work, the
risks involved, etc.; but will usually be 5 to 15 per cent. Out of
this the contractor must pay the expense of assembling the plant,
the cost of tool house, the wear and tear on small tools, interest
on investment, profits, etc.
189. BRIDGES. This subject will not be considered here, since
the space available is not sufficient, and since there are a number
of elaborate treatises on bridges. None of these, however, gives
an adequate treatment of the very small highway bridge, or fairly
represents current practice for moderate spans.
ART. 1] CONSTRUCTION 113
190. WATERWAYS. The determination of the amount of
waterway required for any particular bridge or culvert is a matter
of importance. Although the problem does not admit of an exact
mathematical solution, it requires intelligent treatment. For a
discussion of this subject, see pages 564-69 of the tenth edition of
author's Masonry Construction.*
191. CULVERTS. For a discussion of the cost and method of
construction of culverts — wood box, vitrified pipe, cast iron pipe,
stone box, and masonry-arch culverts, — see the author's Masonry
Construction, pages 569-605.*
One common defect of earth roads is that culverts are made too
short, which concentrates the traffic upon the portion of the road
usually least able to bear it. A short culvert may be permissible
when the cost per unit of length is great, but the defect is common
where this cost is quite small.
192. RETAINING WALLS. Retaining walls are masonry struc-
tures employed to support the sides of roads on hillsides or in places
where land for the ordinary earth slopes is not readily obtainable.
For a discussion of retaining walls, see the author's Masonry Con-
struction, pages 489-534.*
193. GUARD RAILS. Roads on steep hillsides or on high em-
bankments, and particularly on sharp curves in mountain roads,
should be protected to insure vehicles against the possibility of
falling down the slope. In Europe such protection is usually
afforded by a stone wall, or by stone posts set at frequent intervals.
In this country the usual protection is by means of wood posts and
wood guard rails. The description of the guard rails used on the
state-aid roads in Massachusetts is as follows: " Posts of cedar, or
other wood which endures well in the soil, are set at intervals of 8
feet, and 1 foot in from the edge of the embankment. These posts
are planted to the depth of 3 feet, and project for 3 feet 6 inches
above the ground. The top of this post is transversely notched, so
as to receive one half of a rail 4 inches square. Half-way down,
the post is notched to receive another rail 2 by 6 inches in size.
These rails, preferably of planed spruce wood, are spiked to the
posts. To insure the better preservation of the wood and it
visibility in the night-time, it is painted with two coats of oil paint
of some light color." For a diagram of this guard rail, see Fig. 52,
page 198.
* A Treatise on Masonry Construction, by Ira O. Baker, 745 +xv pp., 6X9 inches, cloth,
10th edition. John Wiley & Sons, New York. Price $5.00.
114 EARTH ROADS [CHAP. Ill
The Massachusetts Highway Commission wherever practicable
widens the base of the embankment until a slope of 1 to 4 is ob-
tained, and then dispenses with the guard rail. This plan is believed
to be more economical, and to give a better appearance.
194. GUIDE POSTS. Some states, by statute, require guide
posts at all intersections; and their value to the occasional traveler
is sufficient to justify the expense. The guide post may be a plain
post, supporting near its top a board upon which is the name of
the place reached by the road, with figures showing the distance,
and a OP^ to show the direction.
195. ARTISTIC TREATMENT. Engineers are accustomed to
study chiefly or only the economic side of construction, and are
therefore likely to neglect the artistic treatment of the highway.
In the attempt to beautify the roadside, it may be necessary to sac-
rifice a little of utility to secure a pleasing effect. Masses of foliage
and shade add beauty to the roadside, but tend to keep the traveled
way damp — usually the bane of good earth roads. Trees are a
necessary adjunct to a beautiful highway, but are anything but a
benefit to the traveled way. If beauty is desired at the expense of
utility, highways can scarcely be too much shaded by over-arching
boughs. However, a happy medium will suffice in most places.
The varieties of trees, suitable for the ornamentation of highways
are almost infinite. The elm, with its graceful arching branches
and delicate lace-like foliage, is not surpassed; and the hard maple
and the oaks are very handsome for this purpose. The walnut, the
butternut, the hickory, the beech, the poplar, and the pine, ranging
from the most delicate to the most somber and rugged, are all more
or less adapted to particular' requirements and circumstances.
Trees such as willow, the roots of which spread extensively or seek
water vigorously, should not be permitted to grow near tile drains,
as the small roots frequently entirely obstruct the tile. Trees
should not be planted close to the traveled way, but near the edge of
the right-of-way, or if possible on the private property bordering
the right-of-way.
196. The roadside fences are usually the property of the adjoin-
ing land owner, and may mar or beautify the landscape. The
hedge rows of England and the stone fences of New England are all
that can be desired for appearance; but in localities where there is
much snow they catch the drifting snow and so obstruct the high-
way. The only thing favorable to the appearance of the common
wire fence is that it is inconspicuous.
ART. 2] MAINTENANCE 115
ART. 2. MAINTENANCE
197. Proper maintenance is as important as good construction.
A distinction should be made between maintenance and repairing.
The former keeps the road always in good condition; the latter
makes it so only occasionally. If the road is not properly main-
tained, it deteriorates in a geometrical ratio. A small depression
fills with water and soon becomes a mud hole which travel makes
deeper and deeper; or an obstructed side ditch forces the water to
run down the center of the road, and gullies out the surface. A
defect which could be remedied in the beginning with a shovelful of
earth and a minute's time, if neglected may require a wagon load of
earth or an hour's time, besides being in the meantime an annoyance
or a damage to travel. The better the state in which a road is kept,
the less are the injuries to it by ordinary travel and the weather.
198. DESTRUCTIVE AGENTS. The agents tending to destroy
the road are: water, narrow tires, the tracking of the front and rear
wheels, the horse not being hitched before the wheel.
199. Water. Water is the natural enemy of good earth roads.
The chief object of maintenance should be to keep the surface smooth
and properly crowned so that rain will be shed into the side ditches.
These should be kept open so that the water may be carried entirely
away from the road. This subject is fully considered in § 205-15.
200. Narrow Tires. It is desirable that a wagon in passing
over the road should help to make or preserve it, and not to destroy
it; and therefore as far as the road is concerned, within reason-
able limits, the broader the tire the better.
Tables 3 and 4, pages 15 and 16, show the relative tractive resist-
ance of broad and narrow tires. Although there is not much differ-
ence between the tractive power of broad and narrow tires, the latter
are much more destructive on any road, particularly on an earth
one. But in deciding upon the proper width of tire, there are other
factors besides the tractive resistance and the preservation of the
road, that must be considered.
If wagons were employed only upon the public highway, it
might be wise to use wide tires and sacrifice some tractive power
for the benefit of the roads. Other things being equal, a wagon
with broad tires is not so easily managed as one with narrow tires.
To be equally easy to turn, the broad-tired wagon should have a
narrower bed, or a longer front axle, or a smaller front wheel. In
116 EARTH ROADS [CHAP. Ill
Europe it is customary to adopt the smaller front wheel, which is
very destructive of the broken-stone roads so common in that
country. Increasing the length of axle interferes with getting the
wagon up to cribs, warehouses, etc., and increases the difficulty in
going through gates, passing buildings, etc. It is not clear that laws
should be passed regulating the width of tires, many claims to the
contrary notwithstanding.
" The best argument against the enactment of laws concerning
broad tires is found in the fact that the numerous and long-enforced
English statutes on this matter have of late years been abrogated,
a century of experience having shown that they are difficult to
administer and generally disadvantageous." The Massachusetts
Highway Commission, after an elaborate discussion of the subject,*
says: " It is a matter of doubtful expediency to endeavor, in the
present state of our highways, by general legislation to control the
width of tires or the diameter of wheels."
201. Many European countries have laws regulating the width
of tires. In England for 100 years the law required 1 inch of tire
for each 500 pounds of load, but all general laws in that country
regulating the width of tires have been repealed. In France the tires
of market carts vary from 3 to 10 inches in width, being generally
from 4 to 6 inches, with the rear axle about 14 inches longer than the
forward one.
In this country a number of the states have statutes concerning
the width of tires, many of which take the form of a rebate, either
cash or part of the road tax, to those using tires of a prescribed
width. The following is the legal width in Ohio for vehicles using
gravel or broken-stone roads:
Minimum width of tire for load of 2,500 to 3,500 Ib 3 inches
" " 3,500 " 4,000 " 3£ "
4,000 " 6,000 " 4 "
6,000 " 8,000" 5 "
8,000 or more " 6 "
According to wagon manufacturers about 60 per cent of the
wagons used on country roads have tires 1^ to If inches wide, those
of the remaining 40 per cent being 2 to 4 inches. The broad tire is
of comparatively recent introduction on rural roads in this country.
202. There is greater justification for limiting the load per unit
of width of tire on pavements than on earth roads, since with the
* Report of the Massachusetts Highway Commission for 1893, p. 56-62.
ART. 2] MAINTENANCE 117
latter the damage is not so great and can be more easily repaired.
For a discussion of limiting loads on pavements, see § 39. Notice,
however, that these regulations apply only to loads heavier than any
likely to traverse earth roads.
203. Equal Axles. Since with equal axles the hind wheel follows
in the track of the fore wheel, it increases the depth of the rut, and
consequently increases the destructive effect of the wagon upon the
road. The remedy would be to make the lengths of the two axles
unequal, but this would make the wagon more difficult to manage
and would also increase the tractive resistance. The advantage of
not permitting one wheel to exactly follow another, is shown by the
fact that there are no ruts at a corner or a sharp turn in the road; but
it is not practicable to secure this advantage generally, either by
making the two axles of unequal length or by preventing a wagon
from traveling in the ruts already made.
204. Horse not Hitched before Wheel. On broken-stone roads,
the horses' feet loosen fragments of stone, which tends to destroy
the surface; and if the horses were hitched directly in front of the
wheels, the stones loosened by the horses' feet would be rolled down
by the wheels of the wagon. This is a matter of some moment with
broken-stone roads, but is not important with earth roads. How-
ever, a few teamsters using earth roads hitch their horses in front of
the wheel, to enable their horses and wheels to run in the beaten
track made by the feet of preceding horses not hitched in front of
the wheel.
205. CARE OF THE SURFACE. The most important work in
maintaining an earth road is to keep the surface smooth so that the
rain water will flow quickly into the side ditches. If the surface of
an earth road is properly formed and kept smooth, the water will be
shed into the side ditches and do comparatively little harm; but if it
remains upon the surface, it will be absorbed and convert the road
into mud.
There are three classes of machines in use for filling ruts and
depressions and in keeping the surface smooth. They are the road
drag, the scraping grader, and the V road-leveler.
206. Road Drag. The road drag is the simplest, the cheapest,
and the most effective implement for smoothing the surface of a
road. Four type forms of the road drag are shown in Fig. 30, 31,
32, and 33, pages 118 and 119. Each consists essentially of one or
more cutting edges which stand obliquely across the road as the
implement is drawn along the road. The road drag is designed to do
118
EARTH ROADS
[CHAP. Ill
three things, viz., (1) smooth the surface by paring off the high
places and filling up the low spots; (2) move a little earth toward the
center of the road to compensate for the wash of the water in reducing
the crown; and (3) puddle or harden the surface by dragging it
when wet. To accomplish the first purpose, the cutting edge must
make a considerable angle with the line of draft, so that the earth
that is pared from the high places will drift along in front of the cut-
ting edge and fill up the low spots. To secure the second object,
the front end of the cutting edge should be toward the outside of
the traveled way, so the drifted earth will be moved toward the center
of the trackway. To accomplish the third purpose, the road should
be dragged when it is wet. Working the soil when it is wet, puddles
it; when it dries, it will be hard and more nearly waterproof. Under
favorable conditions each successive dragging adds a thin layer of
tough and impervious material; and consequently the frequent
dragging of an earth road builds up a crust that does not easily become
muddy.
The slicker, or lapped-plank drag, Fig. 30, is easily and cheaply
made; and is to be preferred when the road is quite soft, and also in
soil that is too sticky to flow along the blade of the other forms of
drags. In these cases the slicker smooths the road partly by com-
FIG. 31. — SPLIT-LOG ROAD DRAG.
FIG. 32. — PLANK ROAD DRAG.
pressing the high places and partly by cutting them off, and thus
fills up the low places and gives the water an opportunity to run off.
ART. 2]
MAINTENANCE
119
The split-log drag, Fig. 31, is commendable chiefly because of the
ease with which it can be made when a suitable log is at hand. The
log should be 7 or 8 feet long, and 10 or 12 inches in diameter. The
braces between the two halves of the log may be roughly hewn and
the ends be made to fit into 2-inch auger holes in the slabs. The
braces should be long enough to hold the two slabs about 30 inches
apart. The braces should be fastened in place by driving wedges
FIG. 33. — ADJUSTABLE THREE-BLADE STEEL FiG. 34. — ROAD SLICKER AT WOBK.
ROAD DRAG.
Fio. 35. — WORK OF ROAD SLICKER.
FIG. 36. — SPLIT-LOG DRAG AT WORK.
into their outer ends. Fig. 36 shows a crude split-log with removable
platform doing excellent work.
The dimensions and construction of the plank drag are shown
in Fig. 32. If the main planks are hard wood and 2 J or 3 inches thick,
the reinforcing planks may be omitted.
Fig. 33 shows an adjustable steel road drag. The cutting blades
can be tilted forward or backward by the hand lever. Similar steel
drags are made with two blades. Non-adjustable 2-blade and
3-blade steel road drags also are upon the market.
120 EARTH ROADS [CHAP. Ill
207. Rules for Using Road Drag. The following rules should
be observed in dragging earth roads:
1. Remove all loose stones from the road before dragging it.
2. The depth of cutting may be regulated by the length of the
hitch chain. A short chain causes an uplift, and hence a lighter cut;
and vice versa, a long hitch causes less uplift, and hence a, deeper
cut. The depth of cut can also be varied by the driver moving
forward or backward on the drag, or e\en by inclining his body.
3. The driver should stand upon the drag, and be ready to shift
his position as the circumstances demand. However, if an unusually
soft portion of the road is encountered, it may be best for the driver
to walk.
4. Drive the team in a walk.
5. Drive a horse on each side of the right-hand wheel track, with
the front end of the cutting edge on the outside of the traveled way,
and proceed to the end of the portion of the road to be dragged; and
then on the return do similarly for the other wheel track. Of course,
if the traveled way is wide, it may be necessary to make more than
one round trip.
6. The best time to use the drag is when the road is drying out,
while the soil cuts easily, and is damp or wet enough to puddle ; but the
soil should not be so sticky as to cling to the cutting edges of the drag.
7. Do not move too much earth toward the center of the road.
There are two reasons for this rule. First, it is important to keep
the road surface as hard as possible, and hence no more earth should
be loosened than just enough to make the surface smooth. An excess
of loose earth will make the road dusty in dry weather and muddy
in wet weather. Second, only enough earth should be moved toward
the center to counteract the effect of the rain in reducing the crown.
An excess crown will concentrate travel at the center and cause deep
ruts to form, which will hold water and make mud. If the crown
becomes too high, drag it once or twice in such a way as to work
the soil away from the center.
8. By hitching the team close to the front end of the drag, the
blade will cut like a plow ; and the machine can then be used to remove
weeds or deepen the side ditch. With this hitch, the driver should
stand near the forward end of the blade, and should throw his
weight forward or backward as the work may require. Care must
be taken that the entire drag is not tipped over forward. When
weeds clog the cutting edge, they can be removed by the driver's
shifting his weight to the rear end of the cutting edge.
ART. 2] MAINTENANCE 121
9. The road should be dragged after it has been roughened by
being used when muddy; but the dragging should not be postponed
until the soil has ceased to be mellow and easily moved. It is a
waste of time to postpone the dragging until the road has become
dry and hard.
10. The most effective time to drag a road is in the spring imme-
diately after the frost is out.
11. If the roadway is very rough and contains many deep ruts and
holes, it may be best to drag it when it is quite slushy. In this case
a lapped-plank smoother, or slicker, is better than the split-log,
plank, or steel drag. The hitch to the slicker should be such that the
outer end of the machine is a little ahead, so as to fill up the holes
and ruts.
12. If a slushy road is dragged or smoothed just before freezing
weather, the surface will freeze and make a fine, smooth, hard road.
208. Cost of Dragging. The cost of dragging the road depends
somewhat upon the condition of the road and also upon the demand
for men and teams for agricultural work; but usually the cost per
mile per round trip of an 8-foot drag will be about 50 to 60 cents.
See § 229 for further details.
209. Scraping Grader. This machine is described in § 155-56,
and its use in road construction is explained in § 157-60. It is pro-
posed here to consider the use of this machine in the maintenance
of a road.
If the road drag (§ 206) were frequently and efficiently used, the
road would be kept in a fairly good condition; but for one reason
or another, the roads are not usually dragged when the work can be
done most efficiently, and consequently are allowed to dry out
rough and full of ruts. After they have reached this stage, it is
nearly or quite impossible to secure a smooth surface with any form
of road drag. Under these conditions the surface can be restored by
running a scraping grader over the road so as to plane off the ridges
and fill up the ruts.
210. Operating the Scraping Grader. Commence at the ditch and
work toward the center, scraping with the entire length of the blade.
The blade should stand nearly square across the road, and consider-
able earth should be shoved along in front, — enough to fill the depres-
sions;— but only enough earth should be moved toward the center of
the roadway to replace that washed down by the rains. The sur-
plus earth should be uniformly distributed over the surface, by
carrying the rear end of the blade a little higher than the point. A
122 EARTH ROADS [CHAP. Ill
ridge of earth should not be left in the center of the road, since it
will but slowly consolidate and is likely to be washed into the side
ditches to make trouble there.
This work should be done early — before the ground becomes
hard and difficult to work, before traffic has been compelled par-
tially to do the work of the road leveler, and while the surface is in
condition to unite with the loose earth left by the machine, and
when the roots of grass and weeds do not interfere with the work of the
blade. Unfortunately this work is often postponed until the ground
is so hard that it is impossible to do a thoroughly good job. If
the ground is a little too wet for tillage, it is all the better for road
making, since it will pack and harden better than though it were
drier. After the ground becomes dry and hard, it is not only more
laborious and expensive to secure a smooth surface; but the newly
repaired road may for weeks be in a worse condition than before
it was worked, since the loose earth is too dry to pack under traffic.
211. A common error in scraping roads is not to begin far enough
down in the ditch, thus leaving a shoulder which prevents the water
from flowing from the roadway into the side ditch. Fig. 37 shows a
FIG. 37. — OBJECTIONABLE SHOULDERS LEFT BY SCRAPING GRADER.
road finished in this way. The shoulders not only dam back the
water, but also narrow the roadway; and after weeds and grass have
got a good start, it is improbable that the shoulder will be cut off
next time the road is scraped, and in all probability each successive
scraping will make a bad matter worse. However, with a skilful
use of the scraping grader these shoulders can be cut off.
Not infrequently writers claim that material from the side
ditches should not be placed upon the roadway. Unquestionably
silt from the bottom of the ditches is undesirable material with
which to built or repair a road; but in ditches properly constructed
and cared for, there is not much, if any, of such material, and if any
of it is removed with the scraping grader it is so thoroughly mixed
with good material before it reaches the roadway as to be practi-
cally harmless. The advice against fine material from the side
ditches originated when the drag scraper was the chief tool used in
repairing roads, and the advice has unfortunately outlasted its
usefulness.
ART. 2] MAINTENANCE 123
212. Cost with Scraping Grader. The scraping grader may be
drawn by three 2-horse teams or by a traction engine; but unless
the roads are very hard and tough, horses are more economical than
the tractor, since the grader is too small to be used economically with
the ordinary tractor.
To shape up the road in the spring, six horses and three men are
required to operate the scraper. The wages of a team and driver
will usually be $3.00 or $3.50 per day, since generally the scraping
should be done when farmers are busy with farm work, and since
the work is hard on teams. The cost of operating the grader is then
$9.00 to $10.50 per day. A scraper will on the average smooth up
3 or 4 miles per day, at an expense of $3.00 to $3.50 per mile, or, in
round numbers, including repairs and loss by bad weather, say,
$4.00 per mile. If the road is not very rough, two rounds are
enough; and if it is very bad four may be required, but usually three
rounds are sufficient. If the work is postponed too long, the cost may
be nearly double the above.
The cost of smoothing up city streets would be considerably
more than the above, because of the time consumed in passing side-
walk crossings or in turning to avoid them. Particularly under such
conditions, the amount of work accomplished in a day depends greatly
upon the training of men and horses.
213. The V Road-Leveler. One form of this machine is shov/n
in Fig. 38. It consists of two cutting blades 18 or 25 feet long,
suspended from a platform which
carries the operating machinery.
The spread of the cutting blades
and also the relative height of
the two ends are adjustable.
The machine is drawn behind a
15 to 25 H.P. tractor. With the
longer blades the leveler will
smooth up a maximum width of
30 feet at one time; and with
the shorter blades 22 feet.
Either size machine can shape FIG. ss.— THE v ROAD-LEVELER.
up a roadway 10 or 12 feet
wide. There are at least two other somewhat similar forms of this
machine.
The scraping grader is too large for a convenient number of horses,
and too small for a tractor; and hence the V road-leveler was
124 EARTH ROADS [CHAP. Ill
invented to economically utilize the full power of an ordinary 'trac-
tion engine.
Sometimes three large adjustable steel road drags are hitched
behind a traction engine, and do substantially the same work as a
V road-leveler.
Sometimes a drag or a roller is hitched behind the V road-leveler,
to level down or consolidate the loose earth left between the cutting
blades.
214. Filling Holes. After the road has been smoothed by the
scraping grader or the V road-leveler, it is a good plan, particularly
if the road is very rough, to send a man with a shovel to fill up all
ruts and depressions that were too deep to be filled by the scraper.
If a deep hole has been filled by the scraper, it is well to add a little
more earth to provide for settlement in order to prevent the re-
appearance of the hole. The new material should be trodden or
tamped solidly into place.
Holes and ruts in an earth road should never be filled with stone,
brick, or coarse gravel. The hard material does not wear uniformly
with the rest of the road, but produces bumps and ridges, and
usually results in making two holes, each larger than the original
one. It is a bad practice to cut a gutter from a hole to drain it to
the side of the road. Filling the hole is the proper course, whether
it is dry or contains mud.
215. Removing Stones. All loose stones larger than 2 inches
in diameter should be removed; and stones projecting above the
surface should be dug out. They should be taken entirely away,
or be piled beyond the side ditches; and should never be left just
outside of the trackway, as is sometimes done, where they restrict
traffic and obstruct the flow of water from the center of the side
ditches.
216. CARE OF SIDE DITCHES. The side ditches should be ex-
amined in the fall to see that they are free from dead weeds and
grass; and late in the winter they should be examined again to see
that they are not clogged with corn stalks, brush, etc., washed in
from the fields. The mouth of culverts should also be cleared of
rubbish, and the outlet of tile drains should be opened. Attention
to side ditches will prevent overflow and washing of the road-bed,
and will also prevent the formation of ponds at the roadside and
the consequent saturation of the road-bed. The road care-taker
should frequently go over his portion of the road just as a heavy
fall of snow is going off, for it is then that water does most damage.
ART. 2] MAINTENANCE 125
217. CARE OF ROADSIDE. It is desirable that the roadside
should be so cared for as to secure a coating of grass instead of un-
sightly and noxious weeds. This can usually be accomplished at a
slight expense by an occasional mowing.
218. Care of Trees and Hedges. Earth roads should have
plenty of light and air. Trees along the road may add beauty to
the landscape (§195), but shade is nearly sure to breed mud holes.
In some localities and under some conditions, shade upon the road
surface should be eliminated by cutting down the trees or by trim-
ming them so as not to keep the breeze and sunlight from the road;
but in other localities and under other conditions, a little of the utility
of the road may be sacrificed to secure attractiveness in the general
surroundings.
A tall hedge cuts off the view of the adjacent country, shuts out
the breeze, in a dry time keeps in the dust, and in a wet time retards
the drying of the road. The hedges usually belong to the adjacent
private property, but in most states the height is limited by statute;
and in such cases the road officials should enforce the law. If there
is no law governing hedges and trees near the road on private
property, the road officials should use all possible diplomacy to have
trees and hedges trimmed with reference to the benefits of the road.
In this connection, see § 219.
219. OBSTRUCTION BY SNOW. In localities subject to heavy
falls of snow, it is an important matter to keep the roads from be-
coming obstructed by it during the winter. In some countries where
there is only an occasional fall of snow, as in France, it is customary
to remove it from the surface of the road; but where there is much
snow, it is only necessary to compact it so as to make the road pass-
able. This is done by driving horses or cattle back and forth along
the road, or by rolling the road with a heavy farm-roller. The use
of the roller should commence with the first storm of the season and
be continued as often as necessary through the winter. In the case
of a very heavy storm, the roller should be sent over the roads at
intervals during its continuance. Obviously this work must be
done by the residents along the road.
Snow and ice frequently accumulate in the side ditches to such a
height as to make the surface of the road the principal line of drain-
age. In the spring, when this occurs on earth roads, a large volume
of snow-water flows down the road, and often seriously damages it
by washing gullies in the surface. The best water-bound macadam
roads may be seriously injured in this way; and in some localities
HAT.
to remove the snow
of the character. The cfiffieofty and
free from
jlrrp and narrow, particularly
to maintain a eulitat or cohered
and private drives with the
%'V3"% h&bie tiO Mt^ff^F^* ^M^fBBPCi ^ili>
to
:
This difficulty could be obviated. 01
detreascd, by oaoBtmftmg dbaflow ade ditches
a large tile drain tinder the ditch to cam
K orMMnnm ny snow can ue oecreasea uy proper
Dees, UDdaUudi, efcCL, along the side of the road.
•ed by the obstruction of the currents of air near
tint cany the drifting snow. In forests the winds
tent wfiuul5 to cany the enow, and consequently
of a uniform depth, but in the open country
mind. Fences and shrubbery which retard the
the mam to blow through, cause the snow to pile
1 fide and possibly to block the road and ditches.
he either quite open or very dose. A High tight
wind, and causes the snow to pile up on the wind-
inadaiiie is partially obstructed, the wind moves
o earth cots and also into the beaten snow path,
Fifihig the snow trackway gradually raises the
f the road until turning out into the loose snow
nt, •" in many townships the cost of keeping the
ti*fr winter is one third, and in some one half, of
rinrmifd on the highways, and the average for
boV or $UO per mile per annum.
bfc coat of maintenance on account of snow should
(Bating a road (§ 100-02).
OT MADrTEHAJfCE. The administration of the
•Hi toads k a matter of great difficulty. The
Bae justifiable expense is comparatively small;
wo does the work must have charge of a consider-
•BqpBntrjr can get over the road only at infre-
•Mny states the maintenance is done in
W»r fax, which at best is inefficient. 3. The
. 2] MAINTENANCE
imount of work required may vary suddenly and greatly with local
storms. 4. The administration is usually in the hands of an inexpert
man or board to whom the care of the roads is only an incident in
orivate or official duties. In this connection, see § 41.
As a rule inadequate attention has been given in this country
to the maintenance of roads, and this is particularly true of earth
roads. For the latter there have been only a few attempts to develop
an efficient system, and even they are still in the experimental stage.
Further, the conditions vary so greatly in different parts of the
country that a system that is reasonably successful in one locality
may be wholly inapplicable in another.
The various systems that have been attempted may be classified
somewhat roughly as follows: (1) intermittent repairs; (2) continu-
ous repairs; (3) continuous maintenance; and (4) contract system.
These systems differ greatly and over-lap as applied in different
localities.
223. Intermittent Repairs. This system is with propriety often
called the pathmaster system. In this system the care of the roads
is left to the official pathmaster, who has charge of 8 or 10 miles of
roads, and who superintends the working out of the labor road-tax.
This is the most common but least efficient system. This system
has all the objections enumerated in § 222.
In practice this system is a method of intermittent repairs rather
than maintenance, that is, under this system the roads are allowed
to get into a comparatively bad condition before they are repaired
or restored.
224. Continuous Repairs. This system consists in putting
the care of the roads of a township or its corresponding administrative
road-unit into the hands of a smaU squad of men who give all or
substantially all of their time to the care of the roads. These men
ire provided with a scraping grader (§ 155) or a V road leveler
(§213), and teams or a traction engine, shovels, picks, etc.
The theory of this system is that the roads, or at least the main
)nes, will not get very bad, or be bad very long, before the repair
2;ang will be along. The advantages of this system are: (1) the squad
is employed continuously, and hence becomes more expert; and (2)
:he amount of road cared for is so great as to warrant providing the
squad with a good outfit. The disadvantages are: (1) the work is
lot done at the most advantageous time, i. e., when the soil is most
3asily worked; and (2) the roads are not in good condition all the
ime.
128 EARTH ROADS [CHAP. Ill
225. Continuous Maintenance. The essential feature of this
system is that the care of a definite road is allotted to one man, who
makes this his first business. The system is sometimes called the
patrol system, and takes this name from the method long employed,
chiefly in Europe, in caring for water-bound macadam roads in which
a man devoted all of his time to patrolling and caring for a compara-
tively short piece of road.
It is not usual to attempt formally to maintain any but the most
traveled earth roads. Even on these roads the amount of work
required varies so much with the season and with th3 frequency
of storms, that the section must be comparatively short; and there-
fore at times there is not work enough to require the full time of the
patrol. To meet this condition it is customary to employ a man who
lives near the road, to labor upon the road when directed. The
direction of the patrols is in the hands of a township official or
foreman; and a general supervision of all the foremen is in the hands
of the county engineer.
The only work ordinarily attempted is to drag the road as needed.
The overseer or superintendent communicates with the patrols by
telephone, both to inquire as to the condition of the roads and
to order work done. The patrol is usually required to report by
postal card as to the work done and the time required. In some cases
a report is required for each day's work, but sometimes only a weekly
report is demanded. In some states a portion of the road tax is set
apart by statute to pay for dragging, and can not be used for any other
purpose.
The length of the sections vary with the possibility of securing
competent patrols; but are preferably not more than a mile or two
each, so as not to interfere too much with the other duties of the
patrol.
Prizes are sometimes given for the best work, the money for the
same in some cases being taken from the road taxes and in other cases
is contributed by Chambers of Commerce, etc.
By this system of maintenance the roads are kept all the time
in a fairly good condition. In a number of cases where tried the con-
clusion reached was that the condition of the roads was much better
under this than under other systems, and at the same time the total
cost was less.*
226. In those states in which an attempt is made to continuously
*For one example, see Engineering Record, Vol. 73 (1916), p. 643-44; and for another
see Engineering and Contracting, Vol. 38 (1912), p. 714.
ART. 2] MAINTENANCE 129
maintain the earth roads, there is usually a law prohibiting the use
of the dragged surface until it has dried out so that a wheel will not
make a rut.
227. Maintenance by Contract. In view of the ordinarily
inefficient system of caring for roads, it has frequently been pro-
posed to maintain them by contract. As a rule, work done under
the supervision of a contractor who has pecuniary interest in the
result is more economical than that performed under the direction
of a public official; but it is not wise to do work by contract unless
the amount required can be approximately known beforehand, and
also unless the character of the performance can be easily deter-
mined after completion. Neither of these important conditions
would be present in a contract for the maintenance of a public high-
way. Owing to the indefiniteness as to the amount and character
of the work to be done, it is not at all certain that the maintenance
could be provided for by contract for a sum less than the public
officials could do the work under the present system. The inspec-
tion would finally depend upon the road official, and the letting of a
contract would increase the difficulties and expense of supervision.
It is claimed that the contractor could maintain a trained corps,
and therefore do better work than can be done by the present system;
but it is doubtful if contract work would be any cheaper or better
than the method described in § 225.
228. EXPENDITURES FOR MAINTENANCE. There are but few
data concerning cost of maintaining earth roads, and much of that
is very indefinite since the conditions of soil, weather, etc., are not
stated and also since no definite information can be stated as to the
quality of the maintenance.
229. Dragging. The cost of systematically dragging a road in
Arkansas was $11 per mile per annum, or 50 cents for each dragging.*
In Tennessee 30 miles of roads in sections of 3 miles each were
dragged during the months of December, January, February, and
March, under the continuous maintenance system. The price for a
man and a 2-horse team was 30 cents per hour. The county furnished
the drags. Prizes were offered for the best kept road, and the prize
was awarded to a road for which the cost was $5.00 per mile -per
annum.
In Hale Township, Carroll County, Missouri, an overseer is in
charge of every 8 miles of road, and has ten patrols, each in charge
of a section. After each rain the overseer by telephone calls upon the
* Bui. No. 48 of the U. S. Office of Public Roads, p. 46-7.
130 EARTH ROADS [CHAP. Ill
patrols to drag the roads. The cost of maintaining the roads dur-
ing April, May and June is from $10 to $15 per mile, including $15
for each overseer.
Clayton County, Iowa, from 1913 to 1916 maintained the county
roads, i. e., 226 of the 1350 miles in the county, by the continuous
maintenance system. The patrol section was from 7 to 10 miles.
Some of the patrolmen put in all their time, but some only part
time. The patrolmen hired help as was necessary. Each patrolman
furnished his own team and wagon. Usually three horses were used
on a drag. The average pay was 27f cents per hour for patrolmen,
47 J cents per hour for man and team, and 10 cents per hour for each
extra horse. Patrolmen's assistants were paid 25 cents per hour, and
45 cents per hour for man and team. The average cost of dragging
was 56J cents per mile for one round trip. The total cost for main-
tenance and repairs averaged about $56 per mile per year. The
average cost of dragging in Clayton County was 56J cents per mile
per round trip; while that in other counties of the state was 71.3
cents per mile per round trip.*
230. Total Cost of Maintenance. The following data are for
the maintenance of 70 miles of road during the years 1909 and 1910
in northern Michigan, f The roads were maintained by the patrol
system under the direction of the County Engineer. The roads
were " floated," i. e., dragged with the slicker or lapped-plank
drag, once when the frost was partly out and once after it was com-
pletely out. The roads were dragged after every heavy or protracted
rain during the season. After the roads had settled in the spring,
every hole was filled, and the roads otherwise put into good condition.
The patrols were required to be on the road and work upon either
the road-bed or the drainage system, two specified days in each month.
They were expected to mow weeds and brush on the roadsides, break
through the snow in winter, and keep the road to the standard of a
good earth road. The cost was as shown in the tabular statement
on the opposite page.
" The greater cost of dragging in 1909 was because the roads
had not previously been dragged; and the greater cost of general
repairs in 1910 was because of the higher standard of maintenance." J
231. Table 20, page 132, gives the results of a test in maintenance
* Engineering Record, Vol. 73 (1916), p. 643.
t K. I. Sawyer, in Proc. 1915 Short Course in Highway Engineering, University of Michigan,
p. 62-64.
J Private letter from Mr. Sawyer.
ART. 2] MAINTENANCE 131
conducted by the U. S. Office of Public Roads.* " Before the main-
tenance was undertaken the county repaired the road and put it in
good shape. The repairs consisted in shaping parts of the road with
a scraping grader, clearing and widening the ditches and clearing
the culverts, and applying gravel to a section of the road. The cost
1909 1910
Length maintained, miles 70 . 5 72 . 5
Length of patrol section, miles 6 4-6
Average cost, per mile :
Dragging $26 . 17
Patching surface, culvert and ditch work, cutting
weeds and brush . . 8 . 56
Total cost per mile . . $34.73 $28.42
Cost of dragging one mile one time 0 . 925
of the repairs was $700. On the 8 miles of road there are 4 bridges,
19 culverts, 54 drain pipes under driveways, 59 intersecting roads
with drain pipes, 42 intersecting roads without drain pipes, and 10
small wooden bridges across the gutter. The entire 8 miles of road
is well traveled, and there is considerable heavy teaming over parts
of it A portion of the road is also used by United States cavalry.
There is also considerable automobile traffic on some portions.
A travel census for 3 days in March on one section of the road shows
the following : Loaded 1 -horse wagons, 15; unloaded 1 -horse wagons,
58; loaded 2-horse wagons, 38; unloaded 2-horse wagons, 49; loaded
4-horse wagons, 9; unloaded 4-horse wagons, 4; saddle horses, 96;
and motor runabouts, 1. The patrolman furnished a horse, cart,
and small tools. He was supplied with a plank road-drag, and re-
quired to furnish two horses to drag the road whenever it was in
suitable condition for dragging, usually following each rain. He
was paid $60 per month and $1.00 per day extra whenever he
used two horses to drag the road. His presence was required on
the road from 8 a.m. to 4:30 p.m., with thirty minutes allowed for
lunch."
" The cost of dragging was approximately $1.25 per mile for each
dragging of three round trips. The item of $169.88 for repairing,
clearing and improving ditches and underdrains was large, because
it was found necessary as the year progressed to rebuild entirely
portions of the gutters and ditches.
* Engineering and Contracting. Vol. 38 (1912), p. 714.
132
EARTH ROADS
[CHAP, in
TABLE 20
COST OF MAINTENANCE OF 8 MILES OF ROAD IN ALEXANDRIA COUNTY, VIRGINIA,
From December 17, 1911, to June 30, 1912
Ref.
No.
Kind of Work.
DAYS.
COST.
Total.
Per Cent.
Total.
Per Mile.
1
2
3
4
5
6
7
Dragging. .
38.5
73.
26.5
10.5
10.
5.5
6.
22.7
42.9
15.6
6.2
5.9
3.2
3.5
$128.89
169.88
61.78
24.55
23.36
12.67
13.86
$16.11
21.22
7.72
3.06
2.92
1.58
1.73
Repairing, cleaning and improving
ditches and underdrains
Cutting brush, etc
Picking off stones
Taking census
Inspection during storms
Clearing fallen trees, building
guard rails, etc
Total for 6.5 months
170.00
100.0
$434.99
$54.34
" The following conclusions are clearly demonstrated by the
experiment: (1) The use of the drag has greatly improved the
daily condition of the road and rendered it smooth and comfortable
for travel for a greatly increased number of days in bad weather.
(2) A width of earth road in excess of 24 feet is unnecessarily expensive
to maintain. (3) The presence of the patrolman during storms and
immediately after, saves considerable expense for repairs due to the
wash of surface water. (4) The existence of poorly drained private
driveways and intersecting roads is a constant expense for main-
tenance. (5) The use of small tiles for side drains and the building
of wooden bridges over gutters at driveways is a serious obstacle
to proper drainage. The pipe is usually laid at insufficient depth,
and becomes broken and clogged. It would appear that paved gut-
ters at driveways would not be unduly expensive in the long run, and
would certainly provide better surface drainage. (6) It is not
economical to employ a patrolman during the winter months, unless
his time can be used to advantage in clearing brush and rubbish from
the right-of-way; but a man should be constantly in charge of every
mile of road to inspect it during storms, and to free the ditches.
(7) The presence of old cobble stones and poorly consolidated
coarse gravel is a serious impediment to the use of the drag. The
stones must be removed from the road before dragging can be suc-
cessful. (8) There is ample work for one man continuously during
8 or 9 months of the year; and there is difficulty in combining road-
patrol work with the dragging of earth roads."
ART. 3] SURFACE OILING 133
232. Table 21 gives details of the average annual expenses for
roads in Champaign County, Illinois. Notice that part of the
expenditures are for maintenance proper, while part are for im-
provements in the original construction.
TABLE 21
AVERAGE EXPENDITURES PER MILE OF EARTH ROADS IN CHAMPAIGN Co., ILL.
1. New steel bridges — exclusive of county aid * $16.20
2. Drainage 6.32
3. Tile culverts 1 .32
4. Repairs of bridges and culverts 2 . 93
5. Grading (not simply smoothing and leveling) , 1 .43
6. Smoothing and leveling (not grading) 2 . 83
7. Mowing the roadsides 1.14
8. Administration 2 . 69
Total average annual expenditure $34 . 86
It is not known that any data similar to those in Table 21 were
ever before collected, and hence there is no means of knowing
whether these data are representative. These expenditures were
in 1900 before the use of oil in maintaining earth roads (see Art.
3 of this chapter). It is probable that the expenditure for bridges
is considerably larger than the average. Champaign County is
rolling prairie situated in the corn belt. There are no large streams,
and practically all the land is under cultivation. Farm lands with-
out buildings then sold at $80 to $100 per acre. There are 1.97
miles of road per square mile of area outside of cities and villages.
All the roads have a black loam surface.
ART. 3. SURFACE OILING
233. The surface of an earth road is sometimes treated with oil
to prevent dust and also to aid in keeping the surface smooth. In
small towns and villages the former is the chief purpose; while on
rural roads the latter is the main, or sole, object.
234. PREVENTING DUST. The annoyance from dust usually
reaches its maximum in small towns and villages, owing to the
concentrated travel and the presence of more people to be incon-
venienced. The dust can be greatly reduced by properly dragging
the road. The surface should never be dragged when dry, since the
* In Illinois the county pays half the expenses of bridges costing more than a specified per
cent of the assessed value of the township. The expenditures by. the county for new steel
bridges is nearly as much as by the township.
134 EARTH ROADS [CHAP. Ill
resulting loose earth will speedily be ground into dust; and for the
same reason, an excess of loose earth should not be left in the center
of the road.
Within the last decade many dust palliatives and preventatives
have been used; but oil is the agent most frequently employed on
earth roads. Crude tar has been employed as a dust palliative;
but on account of its injury to rubber tires and also on account of
its tracking into houses, it is not satisfactory.
235. EFFECT OF OIL ON MAINTENANCE. Loam and clay roads
are improved by a little moisture — just enough to keep them damp
and dark without making them soft or spongy. In dry climates the
roads not only become excessively dusty, which is a great discom-
fort, but also wear into pot-holes, which are dangerous, since being
filled level-full of dust their presence is not revealed until a wheel or
a horse's foot plunges into them. In some localities the dust at
times is practically hub deep, and is not only an annoyance but
greatly increases the tractive resistance. In arid climates and even
in dry times in humid climates, sprinkling with water is an effective
means of maintenance. A layer of straw is sometimes put upon the
road to subdue or prevent the dust; but of course the effect is only
temporary.
Recently crude petroleum has been employed on highways,
instead of water, to prevent dust. Oil has been used for this purpose
in Southern California more than elsewhere, primarily on account
of the high grade of oil that is available at low cost, but also on
account of the sandy soil, the semi-arid climate, and the absence
of freezing weather.
Oil when applied to, loam and clay roads, reduces the dust, makes
the road-bed at least partially non-absorbent, and gives a dark-
colored surface which is more pleasing to the eye than the ordinary
light, dusty soil. Since the road-bed is less absorbent, it is not so
easily worked into mud; and besides the oily surface more readily
sheds the rain water into the side ditches. In localities where there
is frequent thawing and freezing and also much rain, the effect of
oil on loam and clay roads does not last through the succeeding
winter, except in case of an unusually dry winter and spring. In
comparatively dry climates and upon a sandy soil, the continued
application of suitable oil to the surface tends to gradually improve
the condition of the road-bed. However, whatever the character
of the soil, roads having a considerable hauling are not materially
improved either temporarily or permanently.
ART. 3] SURFACE OILING 135
236. PREPARING THE SURFACE. The surface should be smooth
and properly crowned, so as to shed water into the side ditches.
The surface can be properly prepared with a road drag (§ 206) or
a scraping grader (§ 155), according to the degree of roughness and
the dryness of the soil. If much earth is moved in shaping the sur-
face, the road should be subjected to travel to consolidate the loose
earth; and if depressions appear these should be filled, and be con-
solidated by travel or other means before the oil is applied. The
expense incurred in securing a hard, smooth, properly crowned sur-
face will more than be made up by the increased effectiveness of the
oil treatment. For the best results the upper 2 inches of the road
should be fairly dry; but the surface should be free from dust.
237. THE OIL. For a discussion of the origin and character
of road oils, the method of shipping them, and specifications for oils
suitable for the different kinds of soil, see Art. 2 of Chapter VIII.
238. APPLYING THE OIL. The oil should be applied .at the rate
of one fourth to one half gallon per square yard of surface. If the
road has never been oiled, or if more than one season has elapsed
since a previous oiling, about a half gallon per square yard will be
required. If the road has been oiled earlier in the season, one
fourth to one third of a gallon per square yard will usually be
satisfactory. It is much better to apply a small amount of
oil twice each season rather than to put on the full quantity
in one application. When too much oil is applied, it is not only
wasted, but is often disagreeable to the users of the road. The
first time the road is oiled, the best results may be secured by using
a thin product that will penetrate the road-bed to a considerable
distance and at the same time contain as much binding material as
possible. The oil should be thin at ordinary atmospheric tem-
peratures, and applied cold. If a thick oil is used for the first
application, it will not penetrate to any considerable distance, but
will form a mat upon the surface; and consequently, if the road is
not well underdrained, the accumulation of moisture under the mat
may cause the road to dry out more slowly, and may also cause the
mat to break up. After the top layer of the road has been satu-
rated, a heavier oil may then be used; and it is best applied when
hot.
239. The uniform distribution of the oil is one of the essential
requirements for success. An ordinary street sprinkler or a home-
made device attached to a thresher tank-wagon may be utilized
for distributing the oil, although considerable care is required to
136 EARTH ROADS [CHAP. Ill
secure the right amount and a uniform distribution. Much better
results can be secured by the use of specially designed pressure dis-
tributor tank wagons or trucks. There are a number of such wagons
and trucks on the market. Some of them are equipped with a heat-
ing device so that hot oil may be applied when required; and all
have pumps for distributing the oil uniformly and under pressure,
and some have a device for spraying the oil upon the road.
On earth roads the use of the pressure distributor is useful mainly
to secure the proper quantity; but on a gravel or broken-stone road
the pressure distributor is important, since applying the oil with
force blows the dust off the pebbles or stones and permits a better
adhesion of the oil. The chief advantage of the spray is that it
secures a more uniform distribution.
The distributor should be so regulated that the width to be oiled
will be covered by one or more uniform strips without any over-
lapping. Any spots between the strips that are not covered by the
machine distributor, should be covered with a hand-pouring can
following immediately after the distributor. For the best results
the oil should be applied in two equal coats with an interval of at
least 5 or 6 hours between them to allow the first to be absorbed
before the second is applied. Travel should be barred from the road
for 3 days after the first coat is applied, to allow the oil to be absorbed.
Fig. 39 shows a pressure tank wagon for distributing road oil.
This road oiling-machine is provided with an oil-burning heater and
a jacket around the tank; and also has two pumps for applying
the oil under pressure and regulating the amount. Fig. 40 shows
another form of road oiler.
240. COST OF OILING. The cost of preparing an earth road
for oiling will vary greatly, depending upon the condition of the
surface. If the surface is not already well crowned, the road
should be treated with either the road drag or the scraping grader.
However, such work should not be considered as part of the cost
of oiling; but should be considered as part of the cost of construc-
tion or of maintenance, since the road should be properly crowned
and be kept so whether or not it is oiled. Even though the sur-
face is properly maintained, it will probably be necessary to drag
the road and otherwise shape it up; and this cost may be $10 to
$25 per mile.
The price of oil for earth roads varies from 4 to 8 cents per
gallon (§ 560-61). The oil is usually applied at the rate of J to \
gallon per square yard, and the oiled width is generally 15 feet; and
ART. 3]
SURFACE OILING
137
FIG. 39. — ROAD OILING- MACHINE.
FIG. 40. — MACHINE APPLYING ASPHALT OIL.
138 EABTH ROADS [CHAP. Ill
therefore with the smaller values above, the oil may cost $88 per
mile, and with the larger values $352.
The cost of applying the oil will depend upon the length of the
haul, the size of the tank, and the method of applying; and may
vary from $50 to $150 per mile for machine application. There-
fore, the total cost of oiling, including only slight preparation of
the road surface, and excluding rent and depreciation of equipment,
and also excluding general administrative expense, will vary from
$148 to $527 per mile.
CHAPTER IV
SAND AND SAND-CLAY ROADS
ART. 1. SAND ROADS
243. As a rule roads on pure sand are the worst in existence,
since they are good only when wet, and therefore are at their worst
most of the year; while in most localities clay or loam roads are at
their best most of the time. If the sand is fine, a dry sand road is
worse than any muddy road.
244. DRAINAGE. Roads on pure or nearly pure sand require
very different treatment from those on clay and loam. Dampness
improves a sand road, while it damages a clay or -loam road; and
therefore the preceding rules for the drainage of loam or clay roads
must be reversed for sand roads. Wet sand makes a better road
than dry sand, and therefore draining a sand road is useless and
possibly a damage. Of course, this is not true of quicksand, since
that is improved by drainage; but there is very little, if any, of this
material in roads.
245. GRADING. Sand roads are usually nearly level longitudi-
nally; and hence need little, if any, grading. They should not be
crowned, since they do not need surface drainage. The traveled
portion should be simply leveled off.
246. SHADE. While shade harms a loam or clay road, it im-
proves a road of sand or broken stone, since it prevents the evapo-
ration of the moisture from the road-bed. Therefore a sand road
can be permanently improved by planting trees so as to shade the
traveled way. They will prevent, in part, the drying effect of the
winds, as well as intercept the rays of the sun.
247. HARDENING THE SURFACE. The great disadvantage of
pure sand as a road material is the freedom with which the grains
move one on the other; and therefore to improve a sand road grass
should be encouraged to occupy all the space possible, since its roots
will decrease the movement of the grains under the tread of the
139
140 SAND AND SAND-CLAY KOADS [CHAP. IV
hoofs and wheels. It is an advantage if low growing bushy vege-
tation occupies the surface clear up to the traveled way — both for
the shade and for the binding effect of the roots and the leaves. The
leaves fall into the ruts and also aid in binding the sand.
Where no other recourse is possible, it is advantageous to have
two roadways adjacent to each other, one of which is planted with
grass while the other is in use. If the traffic is not very great, the
effect of the grass will last for a year or two after the road is again
used by the wheels. A fertilizer is sometimes applied to stimulate
a growth of grass upon the wheelway. In some localities the sand
is so fine that it drifts like snow, and fills the partially hardened
way, in which case the road is improved by planting the roadsides
with grass to prevent the sand from being blown into the road.
A road on pure sand is improved temporarily by covering it
with a thin layer of any vegetable fiber, as decaying leaves, straw,
marsh hay, waste from sorghum mills (bagasse), fibrous or string-
like shavings, etc. This fibrous material soon becomes incorporated
with the sand and decreases its mobility; but the vegetable matter
wears out and decays, and consequently the effect is only temporary.
The length of time such expedients will last depends upon the
climate and the amount of travel.
248. In this connection it is a significant fact that the sand
shoulders of a broken-stone road soon become firm and hard, owing
to the infiltration of the fine dirt and stone dust washed from the sur-
face of the roadway. The fine particles of dust between the grains
of sand act mechanically to decrease the mobility of the sand, and
to increase capillary attraction and diminish percolation, which
in turn keeps the sand damp and still further decreases its mobility.
Apparently, then, the incorporation of fine dust in a sand road will
improve it; but it will be difficult to procure sufficient dust for this
purpose.
ART. 2. SAND-CLAY ROADS
250. A sand road is best when wet, and a clay road is worst when
wet; but a road surface constructed of sand and clay mixed in proper
proportions possesses the good qualities of both the sand and the clay,
and frequently is better then either. Such roads are called sand-clay
roads, and give the best results where the ground is not subject to
deep freezing. Materials suitable for the construction of sand-clay
roads are found in greater abundance in the southern states than in
ART. 2] SAND-CLAY ROADS 141
any other portion of this country; and consequently sand-clay
roads are much more common in that portion of the country, although
sand-clay roads have been built to a considerable extent in several
northern states. There are many localities, particularly in the
South, where sand-clay roads are the only improved roads which
are economically possible. In many cases a sand-clay road gives
good service at comparatively low cost of both construction and
maintenance.
Three distinct methods are employed in constructing sand-clay
roads, viz.: (1) a natural mixture of sand and clay is placed on
top of either a sand or a clay road ; (2) a layer of sand is incorporated
in the road-bed of a clay road; or (3) a layer of clay is added to a sand
road.
251. THE DESIGN. The width and thickness adbpted will of
course depend upon the travel and upon the money available.
For a single track the improved width is usually 10 or 12 feet, and
for a double track 14 or 16 feet. For a discussion of the super-eleva-
tion and width on curves, see § 90 and 97, respectively.
The thickness at the center varies from 6 to 10 inches, usually
6 to 8; and at the sides 4 to 8, usually 4 to 6, but feathers out at the
very edge. The crown should be f to f inch per foot of total width.
252. NATURAL MIXTURES OF SAND AND CLAY. Sometimes a
natural mixture of sand and clay is found in such proportions as to
make an excellent road surface for moderate travel under most or
all weather conditions.
The next chapter treats of gravel roads, and a special case of a
gravel road is one in which a layer of natural cementing gravel is
added to a clay or loam road; but such a form of construction will
not be considered here.
253. To Test the Sand-Clay Mixture. To determine the probable
wearing power of natural mixtures of sand and clay proceed as
follows:
1. Examine the existing road to see if there are any portions
that are reasonably good under all weather conditions, to identify
the character of soil desired for other portions of the road.
2. Observe out-crops of sandy clay or clayey sand. The best
mixtures of sand and clay for road building purposes will stand at
relatively steep slopes, will develop few surface cracks in drying, and
will appear dense and firm in dry weather.
3. Determine the percentage of clay and sand in the mixture.
To do this, thoroughly dry a sample of the soil, weigh it, place it in
142 SAND AND SAND-CLAY ROADS (CHAP. IV
a vessel several times larger than the sample, fill the vessel with water,
agitate the water, and pour off the muddy water. Before pouring
off the water, allow it to stand a minute or two, so the sand may
settle and thus prevent its being carried away with the clay. Repeat
the washing until the water remains clear; and then dry the sand
and weigh it. Fair results may be expected if the sand content is
from 50 to 70 per cent; but the best results are obtained when
the sand varies from 60 to 70 per cent; or in other words, for
the best results about two thirds of the mixture should be sand.
Further, the greater the proportion of coarse to fine sand th:3
better.*
4. Sometimes a natural mixture may be improved by combining
it with another natural mixture or with nearly pure clay or pure
sand. A determination of the sand and clay contents of the natural
mixture will give some indication of the element needed to improve
it; but the only way to determine it definitely is to make several
trial proportions and test them. To determine which of several
natural or artificial mixtures will probably give the best results in
the road, proceed as follows: Mix the sample to a stiff mortar;
spread a small quantity of each mixture upon a board or plate of
glass to the thickness of about 1 inch; and with some improvised
equivalent of a biscuit cutter, cut from each mixture two samples
containing 1 to 2 cubic inches each. It is essential to cut only equal
quantities from each mixture. Roll these samples between the palms
of the hands into approximately true spheres; scratch a number on
each; and then place them in the sun to dry. When thoroughly
dried, one sphere of each sample should be tested dry and the other
wet. To test a sphere dry, rub it lightly with the thumb, and if it
has too much sand ii, will disintegrate rapidly; while if it contains an
excess of clay, it will speedily rub into dust. If it has a suitable
proportion of sand and clay, it will simply become slightly glazed,
and will offer considerable resistance to abrasion. To test the
spheres wet, place them in a circle in a flat pan or dish, and gently
pour enough water into the pan to cover them, being careful not to
pour water directly upon any sphere. The specimens containing
too much sand will break down first; those having an excess of clay
will usually disintegrate next; and those having the best proportions
will endure longest. These experiments will indicate the least
* For a series of complete sieve analyses of fourteen samples of material from sand-clay
roads that had given good service, and a discussion of the relative merits of the same sec
Trans. Amer. Soc. of Civil Eng'rs, Vol. 77 (1914), p. 1465-75.
ART. 2]
SAND-CLAY ROADS
143
desirable mixtures, and will also show what other proportions
should be tested. A second test of the most promising mixtures
will probably indicate whether or not any of the samples are worthy
of a trial in an experimental section of road.
254. By proceeding as described above one may determine
within comparatively narrow limits the possibility of successfully
using any available mixtures of sand and clay for a road surface.
255. Construction. When a suitable mixture of sand and clay
is available, it is only necessary to add a layer of this material to a
sand road or to a clay road. The road-bed on clay should be
underdrained and crowned as described in Art. 1 of Chapter III,
and the surface of the sand road should be prepared as stated in
Art. 1 of this chapter. The amount of the initial crown will depend
upon the thickness of sand and clay to be added.
The sand-clay mixture is spread upon the road to the desired
width and thickness (§251), and is smoothed with a scraping grader
(§ 155) or drag (§ 206). Travel may then be admitted; but the
road should not be considered finished until it has been thoroughly
soaked by rains, harrowed to break up lumps, and again shaped
with the scraping grader or drag.
After the road has been in service for a considerable time, if it
develops that either clay or sand is lacking in the surface, a thin layer
of that element may be spread and be incorporated into the road-bed
with a disk or toothed harrow.
Finally the sand-clay road, if properly built, will become smooth,
nearly dustless, and resilient; and under moderate travel should
continue so without much, if any, expense for maintenance.
Single Track -tO f
Double Truck- 14 ft.
FIG. 41. — MIXTURE OF SAND AND CLAY ON NATIVE SUBGRADE.
256. SAND ON CLAY SUBGRADE. The object in this form of
construction is to incorporate sufficient sand with the clay subgrade
to obtain a mixture of sand and clay that will fulfill the conditions
stated in § 253. For the best results the amount of clay in the fin-
ished road surface should not much exceed the amount required to
144
SAND AND SAND-CLAY ROADS
[CHAP. IV
fill the voids in the sand. Ordinarily about two parts of sand to one
part of clay gives satisfactory results.
Fio. 42. — CLAY ON SAND STJBQRADE.
257. The Sand. The sand to be added to a clay subgrade
should preferably be a coarse-grained pure silica sand. Any sand
containing any considerable percentage of mica is not desirable.
For the best results, not less than 45 per cent nor more then
60 per cent of the sand should be finer than that caught on a stand-
ard No. 10 sieve, and coarser than that caught on a No. 60 sieve;
and that caught on No. 20, 40 and 60 sieves should be about equal
to each other.
258. The Proportions. The proportions of sand and clay can
be determined approximately by finding the amount of water that
can be poured into a vessel full of sand. To do this determine
the weight of water required to fill any suitable vessel; and then
fill the vessel with sand. Next determine the weight of water that
can be poured into the vesselful of sand. For example, if the pail
holds 12.4 Ib. of water, and 3.5 Ib. of water can be poured into the
pailful of sand, then the voids in the sand are: 3.5 -r- 12.4 = 28
per cent. Therefore in the finished road the proportion of clay should
be about 28 per cent, and that of sand about 72 per cent. This
proportion and others differing slightly therefrom should be tested
by the method described in paragraph 4 of § 253.
However, since exact mathematical proportions are not possible
in incorporating the sand with the clay in the road surface, mathe-
matical refinement in these experiments is inappropriate.
259. The Thickness. For average rural-road travel, the depth
of the sand-clay surface should be about 6 to 8 inches at the center
and 4 to 6 inches at the edges of the traveled way feathering out
on the shoulders; and hence the thickness of the layer of sand
to be added to the road should be roughly two thirds of the above
depth, the exact thickness depending upon the best proportion of
sand and clay as determined by the method described in § 253.
ART. 2] SAND-CLAY ROADS 145
260. Construction. The surface of the clay road is shaped up as
described for an earth road (§ 129-31), and is then thoroughly plowed
to a depth of 6 or 8 inches according to the thickness of sand to be
added. Next the thickness of sand determined as above is spread
upon the surface and leveled down with the scraping grader or the
road drag. The road is then plowed again, as deep as possible, to
mix the clay and the sand; and after this plowing the sand and clay
are further mixed by harrowing with a disk harrow, which can be
done best when the road is wet. Attempting to mix the materials
dry is usually unsuccessful. After the sand and clay are thoroughly
mixed, the surface is smoothed and crowned with the blade grader
or the road drag, and travel is admitted to complete the mixing and
to compact the road. While the road is new it should be watched
carefully, and the surface should be kept free from ruts and saucer-
like depressions by going over it when necessary with the scraping
grader or the V road leveler. When the road has begun to solidify,
the road drag is not very effective — at least neither the split-log nor
the plank drag. It will likely be necessary to add clay in spots where
the road surface is loose, and sand where it is sticky. Probably
the road will not arrive at its best condition for a year or two or at
least not until after several long-continued wet spells during which
the travel will thoroughly consolidate the road.
261. CLAY ON SAND SUBGRADE. The clay is added to serve
as a binder to hold the grains of sand together. For the form of
construction under consideration here, clay is the only material
available for permanent effect. For the proportions of clay and sand
to be attained, see § 253.
262. The Clay. Clay varies more in its suitability for road build-
ing purposes than sand; and it is difficult to determine in advance
the result of a service test in the road.
All clays contain more or less sand, and as the clay is to serve
as a binder for the sand, the less sand the clay contains, that is the
more nearly it is a pure clay, the better; and hence loam, which
is a mixture of clay, sand, and vegetable matter, is not the most suit-
able for this purpose.
The less the clay is affected by the presence of water the better.
Clays are known as slaking and non-slaking. The former absorb
water freely and slake or fall to pieces when put into water, some-
what like a piece of quick-lime; and are deficient in binding power,
and hence are undesirable as a binder for a sand-clay road.
To compare the slaking qualities of several clays, make small
146 SAND AND SAND-CLAY ROADS [CHAP. IV
bails of each of approximately the same size by rolling between
the hands, leave the balls in the sun or put them into an oven until
they are well dried out, and then place them under water. The
ball which holds its shape longest has the highest resistance to
slaking, and contains the clay most suitable for use as binder in a
sand-clay road. To make a fairly trustworthy comparison the
samples should contain substantially the same proportion of sand;
and therefore the percentage of cand in each sample of clay should
be determined (see § 253) before beginning the above test, and sand
should be added so as to give all samples the same proportion of
sand.
The above method of testing may be employed also to determine
the slaking qualities of the clay when mixed with different propor-
tions of sand, and therefore may afford valuable information in fix-
ing the proportions of clay and sand to be used in the road surface.
Other things being equal, the clay which shrinks least in drying
out is best suited for use in a road surface. The relative shrinkage
may be determined by observing the balls used in the test above,
while they are being dried out before being immersed.
263, The Construction. The road-bed should be provided with
side ditches and be graded as described for earth roads (§ 129-31),
except that the surface should have but little, if any, crown. Then
spread a layer of clay of such thickness that when thoroughly mixed
with the sand of the read-bed, the mixture will have the required
depth,— 6 or 8 inches as the case may be. The thickness of the loose
clay required will usually be a little greater than one third of the
ultimate thickness of the combined sand and clay. The exact pro-
portion of clay to secure this condition is to be determined as de-
scribed in § 253.
After the clay has been spread and leveled off with a road drag,
the clay and the sand are to be thoroughly mixed by successively
plowing and then harrowing with a disk harrow. Roughly the
plowing should extend into the sand to twice the depth of the layer
of clay added. It is better at first to have too little sand rather than
too much, for it is easier to correct the proportions by adding sand
from the subgrade then by hauling clay from a distance. When
the sand and clay have been thoroughly mixed in the correct propor-
tion, the road should be properly shaped by the use of a scraping
grader or road drag, and then travel may be permitted. After the
first soaking rain, plow and harrow the surface again until the sur-
facing material practically becomes mud, after which shape up
ART. 2]
SAND-CLAY ROADS
147
the surface and keep it in shape by repeated dragging until it has
dried out and is thoroughly compacted. At this stage the road
roller should not be used, since it will harden only the surface and
prevent the travel from consolidating the mixture from top to bottom.
The crust formed by the roller will carry the travel until the first
wet spell, and then it will cut through and the road will break up;
and nothing permanent will have been gained by the use of the
roller.
The road should be watched carefully for several months, and ruts
and other depressions should be filled by the use of the steel road
drag or the scraping grader. Any deficiencies in sand or clay that
are revealed should be corrected by adding the lacking ingredient.
264. COST. In the Southern states the cost of constructing a
TABLE 22
COST OF SAND CLAY ROADS*
Exclusive of grading and materials
Items.
Mixture of
Clay and
Sand.
Sand on Clay
Subgrade.
Clay on Sand Subgrade.
Brook-
ville,
Fla.
Gray
Head,
Miss.
Mos-
cow,
Miss.
Jack-
son,
N. C.
Pear-
shall,
Tex.
San
An-
tonio,
Tex.
Tar-
boro,
N. C.
Sayre,
Okla.
Length surfaced, miles
Width graded, feet:
Cuts
Fills
0.45
28
20
16 .
660
0.75
3
830
0.50
7
$0.175
.50
.031
'.'425
.374
.019
.0035
.0035
.0157
2.19
22
22
16
1 700
0.125
6
2 300
0.10
6
$0.20
.50
.010
.223
.344
.003
.010
.004
.006
0.78
30
30
17
1524
1.5
7
0.45
30
30
14
890
1.00
7
0.79
26
26
15
1.00
40
26
16
2.50
22
22
18
0.75
28
28
14
Width surfaced, feet
Materials :
Sand, cu. yd
Distance hauled, miles. . .
Depth applied, inches. . . .
Clay, cu. yd
1556
0.19
8
$0.12
.30
.0064
.091
'.9ii
.032
.0017
.0005
.003
1815
0.057
7
$0.15
.345
.0088
.072
1 .286
.0069
.0015
2815
1 383
1.42
8
$0.16
.30
.0096
.134
";567*
.072
.007
Distance hauled, miles. . . .
Depth applied, inches. . . .
Wages per hour:
$0.10
.25
.003
"!49
.007
.006
$0.125
.35
.002
0.29
.84
.077
.002
.003
6
$0.08
.24
.0018
.005
" ! 255
.0178
.0024
.0022
.0007
Teams
Cost:
Subgrade, per sq. yd
Stripping surfacing ma-
terial, per sq. yd
Hauling sand, per sq. yd .
Hauling clay, per sq. yd. .
Spreading material, per
sq. yd
Mixing sand jand clay, per
sq. yd
Final shaping, per sq. yd .
General expense, per sq.yd .
Total cost, per sq.yd.
0.198
0.082
0.105
0.233
.121
.089
.036
.187
* Constructed by U. S. Office of Public Roads and Rural Engineering, Bui. No. 463
(1917), p. 45.
148 SAND AND SAND-CLAY ROADS [CHAP. IV
16-foot sand-clay surface, exclusive of grading, usually ranges between
$500 and $1500 per mile and nearly proportionally for other widths.
For details of the cost of some such roads, see Table 22.
In Michigan, which state seems to have more sand-clay roads
than any other state except Georgia, the cost of a well-built
9-foot sand-clay road consisting of 6 inches of clay on a sand sub-
grade ranges from $1,000 to $1,800 per mile.
265. MAINTENANCE. The sand-clay road is really a superior
type of earth road; and therefore all that has been said in Chapter
III concerning the maintenance of earth roads applies also the sand-
clay surfaces. As in the case of ordinary earth roads, economical
maintenance depends largely upon proper original construction. The
use of too fine sand and the insufficient mixing of the sand and clay
are the common defects of construction that should be remedied by
proper maintenance.
The surface should be kept smooth and properly crowned by the
use of the road drag, particularly after severe or long-continued
wet spells. If a hole forms because of an excess of clay, it should
be filled mainly or entirely with coarse sand, after first loosening the
material in the bottom of the hole with a pick so that the new mate-
rial will bond with the old. Sometimes the fine sand washes to the
side of the road and leaves an excess of clay, in which case a thin
layer of new coarse sand should be spread upon the surface when it
is soft or at least wet. The sand that has washed out should never
be used again.
If the whole surface has a tendency to rut and form holes in
wet weather, it usually means that too much clay has been used in
the construction of the road; and it may then be necessary to cover
the entire surface with a layer of sand and harrow it in. If, on the
other hand, the surface is too sandy, clay must be added in the same
way.
If lack of proper dragging has allowed the road to become badly
worn, then it must be plowed up with a rooter plow, after which it
should be harrowed, preferably with a disk harrow, and be re-shaped
with a grader.
266. The ability of a sand-clay road to carry travel, particularly
motor-driven vehicles, is indicated roughly by Table 23, page 149.
267. Apparently, in the Southern States, representative sand-clay
roads are maintained in reasonably good condition at an expense of
$5 to $10 per year per mile, the chief or sole expense being for
dragging.
ART. 2]
SAND-CLAY ROADS
149
TABLE 23
APPROXIMATE TRAVEL ON SAND-CLAY ROADS IN GEORGIA
Average Numbers of Vehicles both Ways,
per Day.
lype 01 vemcie.
e
c8
1-5
ja
£
si
B
<:
&
§
•-)
>>
3
>->
M
3
•<
"ft .
£
"S
o
>'
0
fc
1
Horse-drawn:
Buggies . .
20
60
60
4
30
90
90
6
35
100
120
6
25
75
75
4
20
60
60
4
20
60
60
4
20
60
60
4
25
70
70
5
40
120
120
8
40
120
120
8
25
75
75
4
20
40
40
4
1-horse wagon (net load 750 Ib.)
2-horse wagon (net load 1,500 Ib.)
4-horse wagon (net load 3 000 Ib )
Total
144
.10
6
2
2
216
10
6
2
2
261
15
8
4
3
179
20
10
6
4
144
20
10
6
4
144
30
15
8
2
144
30
15
8
2
170
20
10
8
2
288
30
15
8
4
288
30
15
8
4
179
20
10
6
2
104
10
6
2
2
Motor-driven :
2-passenger automobile .
6-passenger automobile
Trucks (net load 1,500 Ib )
Total
20
12
20
8
30
10
40
18
40
22
55
28
55
28
40
19
57
14
57
14
38
26
20
16
Per cent motor-driven
t Trans. Amer. Soc. of Civil Eng'rs, Vol. 77 (1914), p. 1492.
CHAPTER V
GRAVEL ROADS
270. Gravel may be defined as a mass of small, more or less
rounded fragments of stone which have been broken out and shaped
by the action of water or ice. When properly used gravel makes
an excellent road surface; and is much used on account of its wide
distribution, the ease with which it can be applied, the good results
obtainable under widely varying conditions of soil, climate and travel,
and the low cost of construction and maintenance under a moderate
amount of travel. Many gravel roads have been poorly constructed
or inadequately maintained, or over-burdened with travel; and as
a consequence many people believe the building of gravel roads a
waste of money under any condition. Nevertheless good gravel
roads have a large field of usefulness. Gravel roads constitute
about one third of the total mileage of improved roads in the United
States; and in 1914 constituted 41 per cent of the state-aid roads.
A gravel surface is most suitable for. country highways not having
exceedingly heavy traffic, for unfrequented streets in villages and
small cities, and for park roads.
ART. 1. THE GRAVEL
271. REQUISITES FOR ROAD GRAVEL. To be suitable for
road-building purposes, gravel should fulfill the following conditions:
1. The fragments should be so hard and tough that they will not be
easily ground into dust by the impact of wheels and hoofs. 2. The
pebbles should be of different sizes, each in the proper propor-
tion. 3. There should be intermixed with the coarser particles
some material which will cement and bind the whole into a solid
mass.
272. Durability. From the nature of its origin, it is apparent
that gravel may differ widely in the nature of the stones composing
it. Not only do different gravels differ from each other, but any
150
ART. 1] THE GRAVEL 151
particular gravel may be composed of fragments of a variety of
rocks. Having been transported a considerable distance by water
and ice, gravel is usually fairly durable, since the softer and more
friable fragments have been worn away. In many parts of the
country the rocky fragments transported by water and ice are more
durable than any of the native rocks.*
273. Sizes. If the pebbles are too large, the road will not be
homogeneous, and the large stones will work to the surface under the
action of traffic and frost; but, on the other hand, if the pebbles
are too small, the gravel will partake too much of the character of
sand, and will be difficult to bind properly. The best results are
obtained when the largest pebbles are not more than f to 1 inch, or
at most If inches, in greatest dimension. With stones larger than
1 inch, it is difficult to keep the surface from breaking up when dry.
Small gravel makes a pleasanter road to ride upon and one that is
easier to keep in order. If stones larger than Ij or 2 inches are
present in the gravel, they may be screened out and used in the
foundation (§303).
It is desirable that the several sizes should be so proportioned
that the smaller ones are just sufficient to fill the interstices between
the larger ones, since then less binder is required. The binder is
usually the least durable ingredient, and hence the less there is of it
the better. Gravel can often be improved by screening — either co
remove an undesirable size or to separate it into several sizes
afterward to be combined in new proportions. The proper pro-
portion depends upon the nature of the gravel — whether the binding
material is already present in the form of dust, or whether some of
the pebbles must be crushed to produce the binder.
274. Binder. The most important requisite for good road-
building gravel is that it shall bind or pack well. If it does not pack
well, the wheels will sink into the gravel and increase the tractive
resistance, and the rain water will penetrate the road-bed and soften
it. To bind well, the several fragments should be in contact with
one another at as many points as possible, in order that they may
be firmly supported, and that friction may act to the best advantage
to resist displacement. To secure contact at every point, all the
interstices between the fragments should be filled — those between
the large pebbles, with small pebbles; those between the small
pebbles, with sand grains; and, finally, those between the sand
* For a discussion of the merits of the principal stones for road -building purposes, see Art. 1.
Chapter VI.
152 GRAVEL ROADS [CHAP. V
grains, with some finer material, called a binder. The binding
material must be very finely divided, so that it can be worked into
the smallest interstices; and for this reason, it is the least durable
part of the gravel, being easily washed out or blown away. For
the best results, then, the sizes of the coarser particles should be so
adjusted as to require a minimum amount of binder.
The binding material may consist of clay, loam, silica, stone dust,
iron oxide, etc., or some other ingredient which will crush under traffic
and furnish a fine dust.
Clay is by far the most common binding material; but the only
recommendations for it are (1) that it is easily reduced to an im-
palpable powder by the action of wheels or by water, and (2) that
it is often found already mixed with the gravel, and (3) that if it
must be artificially mixed, it is plentiful and cheap. Clay is an un-
desirable binder, since its binding action depends in a large measure
upon the state of the weather. During a rainy period it absorbs water
and loses its binding power, and the road becomes soft and muddy;
while in dry weather it contracts and cracks, thus releasing the
pebbles and giving a loose surface. Clay is also very susceptible
to the action of frost; and consequently when the frost is going out,
a gravel road with a clay binder ruts up badly and frequently breaks
entirely through. When the weather is neither too damp nor too
dry, a gravel road with clay binder is very satisfactory. The clay
should be no more than enough to fill the voids in the pebbles and
sand, and for a good road-gravel should not exceed 15 to 20 per cent
of the mass. Not infrequently much greater quantities of clay are
present. This surplus may sometimes be removed by screening;
but often it can be removed only by washing — a process which is
usually so expensive as to be prohibitive.
Loam is chiefly clay mixed with sand and a little vegetable
matter, lime, etc.; and as a binding material has all the charac-
teristics of clay.
A very finely divided silica, easily mistaken for clay, is occa-
sionally present in gravel, and makes an excellent binding material.
Iron oxide is frequently found as a coating on the pebbles in
such quantities as to cement them firmly together. These ferru-
ginous gravels when broken up and put upon a road, will again
unite — often more firmly than originally, because of the greater
pressure — and form a smooth hard surface, impervious to water.
They are much used in road building, gravel from Shark River,
N. J.,— much used around New York City— and that from the Ohio
ART. 1] THE GRAVEL 153
river near Paducah, Ky., — largely used in the neighboring states —
being examples.
275. Comparatively coarse gravel frequently contains some
other ingredients, as, for example, fragments of limestone or shale,
which under the action of traffic and the weather reduce to a powder
and form a good binding material. Sometimes gravel contains bits of
iron-stone (clay cemented with iron oxide) in the form of thin flat
chips which break and crush easily under the wheels, and if present
in any quantity make a most excellent binding material.
276. The binding action referred to in the preceding discussion
is mechanical; and we come now to the consideration of an action
not yet well understood, but which for the present at least will be
called chemical action. Experiments seem to prove that if fine
powder of certain stones is wetted with water and subjected to
compression, a true chemical cementation takes place. Conse-
quently some stones when broken into small fragments, wetted and
traversed by heavy wheels or by a road-roller will be cemented
together to a considerable degree. This cementation is due to the
fact! that the friction of one small piece of stone upon another pro-
duces a very fine powder at the point of contact, which when wetted
and compressed, forms a weak cement. Owing to the rounded
surfaces of water-worn pebbles, this cementing action is much less
with gravel than with rough angular fragments of broken stone;
but with gravel composed of undecayed rocky fragments this action
takes place to a considerable degree. As a rule, pebbles of bluish
color will thus cement together, while reddish or brown ones will
not, which accounts in part at least for the well known superiority
of blue gravel for road purposes. Trap rock possesses the property
of cementation in a high degree, and hence trap gravel is a very
excellent road-building material. Limestone possesses a fair de-
gree of cementation, but is too soft to wear well. Quartz wears
well but produces little or no dust for cementation, and besides its
surfaces are so smooth and hard that the binder has but little effect;
and therefore it rarely happens that a gravel of which more than
one half of its bulk is white quartz pebbles proves to be a good road
gravel.
The cementation of rocky fragments is much more important in
a water-bound macadam road than in a gravel one, and therefore
the subject will be more fully considered in Chapter VI.
277. The binding elements heretofore discussed exist naturally
in the gravel; but gravels are often found that do not contain any
154 GRAVEL ROADS [CHAP. V
binding material, and in such cases it is necessary to add some
cementing material.
Clay, shale, hard-pan, marl, loam, etc., are often used for this
purpose, chiefly because they are so plentiful and easily applied;
but none of them are suitable for the purpose, as they all have the
characteristics of a clay binder (see § 274). With any of them, it
is difficult to keep the gravel from breaking up — particularly under
heavy traffic.
In some localities a poor iron ore is found, which, when mixed
with gravel, makes an excellent binder and gives a smooth hard
road surface. Bog iron-ore, which occurs in marshes, is usually
very good for this purpose.
The fine dust from a stone crusher, when mixed with gravel, will
bind it together; but it is seldom feasible to use stone dust on
account of the expense. When this method of binding gravel is
resorted to, the construction partakes of the character of a water-
bound macadam road — a subject foreign to this chapter (see Chapter
VI). The chief difference between a gravel and a crushed-stone
road is in the thoroughness of the binding. The binding of a gravel
road is due chiefly, and usually solely, to the mechanical action of
the binder; while the binding of the broken stone is due to both the
mechanical and the chemical action of the binder, and both are
stronger with rough angular fragments of broken stone than with
water-worn pebbles.
278. DISTRIBUTION OF GRAVEL. The gravel beds of the
glacial drift furnish excellent road-making materials. The glacial
ice sheet, often a mile or more thick, covered New England and
Canada and all of the United States north of an irregular line start-
ing on the Atlantic Coast a little south of New York City and run-
ning thence successively to the southwest corner of the State of New
York, to Cincinnati, to a point a little north of the mouth of the
Ohio river, to the mouth of the Missouri river, to Topeka, Kansas,
thence north and west a little west and south of the Missouri river
to the head waters of that stream, and thence west to the Pacific
ocean. All of the area north of the above described line was cov-
ered with the ice sheet except small portions of southeastern Minne-
sota, northeastern Iowa, northwestern Illinois, and a considerable
portion of southwestern Wisconsin. As this ice sheet crept to the
southward, it rent great quantities of stone from the bed rocks; and
these materials were borne southward, either in the slow-moving
ice or hurried along by the violent currents of water which swept
ART. 1] THE GRAVEL 155
forward to the margin of the ice field. Thus impelled the under-
ice streams were able to bear toward the margin of the glacier great
quantities of stone. The original range of the glacial gravels has
been greatly extended here and there by the streams, which, flowing
southward beyond the drift belt, have often carried quantities of
the hard detritus for many miles beyond the limits of the ice-
field.
Unfortunately the glacial gravel deposits have not been studied
from the point of view of the road-maker. However, it is known
that east of the Hudson river the glacial supply of road gravels is
only here and there of economic importance, for in most of that field
the glacial waste lies on native rocks which are suitable for road-
making; and that from the Hudson to the Mississippi, the glacial
deposits of bowlders and gravel afford better road-building mate-
rials than any of the native rocks. Glacial gravels exist in consid-
erable quantities in western Pennsylvania, in the greater part of
Ohio, in northern Indiana, and in northern Illinois, and to some
extent in several of the states of the Northwest.
279. South of the glacial district, the rocks exposed to the
weather have decayed by a process of leaching, which in many cases
has removed strata hundreds of feet thick. The rocky portion is
removed in proportion to its solubility; and, as a result, there are
often left concretions of cherty matter which were originally con-
tained in beds of limestone. This cherty residuum of flinty mate-
rial generally lies in a comparatively thin sheet of fragments min-
gled with sand and clay; but occasionally it is found in deposits
from which the clay and sand have been removed by recent or
ancient streams, leaving the material well suited for spreading upon
a road. Sometimes this cherty residuum is found in layers of
fragments many feet thick, and is valuable for road-building in a
locality where more suitable material is scarce. The presence of
chert is often revealed by the gullies in the plowed fields and along the
streams. In some localities very good roadways are constructed
simply by shoveling these fragments from the stream beds and
depositing them on the road.*
This cherty deposit is a valuable road material in the southern
portion of the Appalachian mountains, and along the Ozark foot-
hills in southern Illinois (particularly in Alexander and Union
counties), in southern Missouri, and in northern Arkansas. Chert
is found in some of the states of the Northwest where the glacial
* For a discussion of chert as a road-building material, see § 295.
156 GRAVEL ROADS [CHAP. V
erosion was small, so that the rocks that had decayed before the
glacial time were not entirely removed. In southwestern Arkan-
sas the gravels consist of fragments of novaculite or razor stone — a
material of nearly the same geological origin and physical character-
istics as chert. In many places in that state the novaculite gravels
form extensive beds, 20 or more feet thick. At the southern
extremity of the Appalachian mountain system is a wide-spread
deposit of gravel, termed the La Fayette formation, whose geologi-
cal origin is not determined. This deposit often attains a thickness
of 40 to 50 feet, and is a valuable source of road-building material.
280. If gravel be defined as material prepared by nature ready
to be laid upon the road, then a few words are in place here concern-
ing iron ore. In some localities there are low-grade iron ores
which, owing to the admixture of various impurities, are unfit for
use in making iron, but may be valuable for road building. These
low-grade ores are widely distributed; and generally wherever
limestone occurs below a considerable thickness of sandstone, the
upper portion of the limy layer will be found to contain iron, and
will probably be a fair road material. A lean iron ore is frequently
found in marshes; and this variety, known as bog ore, usually
makes excellent roads, since it crushes readily and gives a smooth
hard surface.
281. Exploring for Gravel. In searching for gravel in the
glaciated district, the following suggestions by Professor Shaler *
will be useful:
" In the process of retreat of the ice, the deposits which it left
were accumulated under several quite diverse conditions. One of
these produced the till, or commingled coarse and fine materials,
which had been churned up into the ice during the time of its
motion, and came down, when the melting occurred, as a broad,
irregularly disposed sheet which, with rare exceptions, is to be
found in all parts of the glaciated district, save where it has been
swept away by streams.
" Again, from time to time during the closing stages of the ice
age, the prevailing steadfast retreat of the ice was interrupted by
pauses or re-advances. In these stages there was formed along the
margin of the ice-field what is called a frontal moraine, composed of
debris shoved forward by the glacier or melted out of it along its
front. These moraines are in most cases traceable, where they
have not been washed away or buried beneath later accumulations,
* American Highways, N. S. Shaler, Professor of Geology, Harvard University, p. 71-73.
ART. 1] THE GRAVEL 157
in the form of a ridge-like heap of waste, which, as we readily note,
contains much less clay and sand and therefore a larger proportion
of gravel and bowlders, than the sheet-like deposit of till above
described. In some cases these moraines are very distinct features
in the landscape, appearing, from the number of large bowlders
which they expose, much like ruined walls of cyclopean masonry.
More commonly they are found in the form of slight ridges, which
may be covered with fine material, but commonly exhibit here and
there projecting bowlders. In general it may be said that the
moraines afford much better sites for pits from which road mate-
rials are to be obtained than the till, and this because of the pre-
vailing absence of clay and sand in the deposits.
" Here and there in almost all glaciated districts, especially in
the valleys of the greater streams, there may be found narrow ridges,
often of considerable height, and almost always extending in the
direction of the ice movement. These ridges are generally termed
by geologists eskars, and often have a tolerable continuity for
scores of miles at right angles to the ice front. A section of them
shows generally a gravelly mass, nearly always free from clay and
often containing little sand, though occasionally there is an abun-
dance of large bowlders, which have a prevailing rounded or water-
worn form. These eskars were doubtless formed in the caves be-
neath the ice through which the ancient sub-glacial streams found
their way. These under-ice rivers were much given to changing
their position, and as a stream lost its impetus it was apt to fill its
ancient arched-way with debris, which in- its time of freest flow
would have been sent forward to the ice front. At many places in
New England and in New York these eskars contain large and use-
ful deposits of gravel, and also occasionally quantities of bowlders
well fitted for crushing as regards their size and hardness. In the
Western States, because of the general coating of deep soil, these
eskars are less easily found; but they exist there, and should be
sought for.
" Where the eskars terminate, as they commonly do, on a
morainal line, there is almost invariably found, immediately in
front of their southern terminations, a delta-like deposit which,
though generally composed in large measure of sand, frequently
contains near the moraine extensive accumulations of useful gravel
and small bowlders which are fit for crushing.
" Information may be had from the banks of streams, where by
chance they have cut below the deep coating of fine materials. The
158 GRAVEL ROADS [CHAP. V
existence of any distinct up-rise of the surface affords some reason
to expect that the coarse glacial waste may be at that point not
very deeply hidden."
282. When a gravel road is to be built, if the local gravel resources
have not already been thoroughly explored, every reasonable effort
should be made to ascertain the extent and character of the available
deposits of gravel. To test a gravel deposit, test holes or wells should
be sunk at regular intervals over the deposit. These holes should
be large enough for a man to get down into them and to examine
the gravel in place and to collect samples. Care should be taken that
the samples are truly representative. Note should be made of the
amount and character of the overlying material, of the depth of the
gravel deposit, and of the dip of the strata.
283. CHARACTERISTICS OF DIFFERENT GRAVELS. Any gravel
which stands vertical in the bank, showing no signs of slipping
when thawing out in the spring, requiring the use of the pick to
dislodge it, and falling in large chunks or solid masses, is sufficiently
clean and free from clay for use on the road, and usually contains
just enough cementing material to cause it to pack well.
Pit gravel usually contains too much earthy material, and can
be greatly improved by screening. Gravel is still being deposited
in drifts and bars by streams, and this will be found to partake of
the character of the pit gravel of the locality, except that it gen-
erally contains less clay, and may have an excess of sand. This
is often called river gravel, and is one of the best sources of road
material. Lake gravel varies greatly in character. It is usually
free from earth and contains sufficient sharp sand to pack well;
but is liable to be slaty — an undesirable quality.
284. Composition of Representative Gravels. In an endeavor
to determine the composition necessary for a road-building gravel,
samples were obtained of a number of gravels that had given satis-
factory service in the road. The samples in each case were selected
by a person thoroughly conversant with the use of that particular
material, and are believed to be fairly representative.
Table 24, page 160, shows the sieve analysis of these gravels.
Each sample was first washed in successive waters until the water
remained clear, and then the wash water was allowed to stand until
the matter in suspension was precipitated. The precipitate was
dried in an oven to a constant weight and then weighed; and the
washed gravel was air-dried, and then sifted and weighed. The
per cent of voids in the washed gravel was obtained by gently ram-
ART. 1] THE GRAVEL 159
ming the gravel under a measured quantity of water in a small metal
cylinder, the ramming not being severe enough to crush any of the
pebbles or fragments.
Table 25, page 161, shows the results of a mineralogical analysis
of such of these gravels as had passed a screen having J-inch meshes.
The matter recorded in Table 24 as being in suspension is called
clay in Table 25, although part of it was doubtless organic matter
and part fine sand, but the error is not material.
286. To study these gravels further, each will be considered in
order.
1. Urbana. This is a screened drift gravel obtained near Ur-
bana, Champaign Co., 111., which has been used in a few instances
on private driveways. Table 25 shows only 3.8 per cent of clay
present, which will have only a small binding effect. There is
7.6 per cent of iron oxide (Fe2Os) in the clay, but there is so small
a proportion of clay in the gravel that the iron contained in it will
have an inappreciable binding effect. The principal source of binder
is, then, the 65 per cent of ferruginous limestone. Limestone itself
when pulverized makes an excellent binding material. However,
in this case only a small part of the limestone is in the form of flat
chips that may be easily crushed under the wheels, but the most of
the fragments are rounded and not easily crushed except by compara-
tively heavily loaded wheels. There is only a small per cent of
crystalline rocks present, and these are hard and not readily crushed,
and consequently can not materially affect the binding qualities of
the mass. The gravel also contains 22.2 per cent of quartz; but
this material is very hard and not easily crushed, and besides its
dust is almost wholly devoid of cementing properties. Both the
quartz and the crystalline rocks are quite sharp and angular, which
is a very desirable condition, and aids the binding action of the
clay and the limestone dust. This gravel packs slowly in the road,
particularly under the light traffic of a private driveway; but under
moderate traffic makes a fairly good road, and is not much affected
by freezing and thawing.
2. Decatur. This is a gravel much used on the country roads
near Decatur, Macon County, 111., with satisfactory results. This
gravel has a comparatively large amount of fine sand. An examina-
tion of Table 25 shows that it contains more than twice as much
clay as the Urbana gravel, but only about one third enough to
fill the voids. A considerable portion of the limestone both of
the pure and the ferruginous — a total of 30 per cent, — i» in thin
160
GRAVEL ROADS
[CHAP. V
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THE GRAVEL
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162 GRAVEL EOADS [CHAP. V
friable chips, and is easily crushed by the traffic, thus making an
excellent binder. The ferruginous limestone contains an unim-
portant proportion of iron; but the ferruginous sandstone is heavily
charged with iron oxide, which makes a good cementing material.
This gravel makes a smooth, hard surface, reasonably free from
dust in the summer and mud in the winter.
3. Lexington. This gravel is used with entire satisfaction in
and around Lexington, McLean County, 111., for country highways.
Notice that the clay is equal to only one seventh of the voids.
Nearly all of the 21 per cent of ferruginous limestone consists of
thin chips which are easily crushed by the traffic. Some binder is
probably obtained from the 58 per cent of siliceous limestone. The
per cent of crystalline rocks present is very small, and can not
materially affect the quantity of the gravel. The amount of quartz
is less than in the preceding gravels, and is an unimportant ele-
ment.
4. Rockford. This gravel has given satisfactory service in Rock-
ford, Winnebago County, 111., probably under more exacting con-
ditions than any of the preceding. This is considerably the coarsest
gravel in Table 24. Notice that this gravel contains, roughly
speaking, only about one tenth enough clay to fill the voids. The
chief source of binder is the limestone which exists in the form of
pebbles, but contains no considerable amount of iron or silica. The
basic crystalline rocks by decomposing may furnish a little binder;
but as they are round hard pebbles, not easily crushed, the binder
derived from this source can be of no practical importance. Very
little cementing material may be derived from the iron conglomerate
or from the limestone and quartz conglomerate.
5. Peekskill. This gravel is from Roa Hook, a " point " in the
Hudson river near Peekskill, N. Y., and is much used in and around
New York City, where it is considered one of the best road gravels.
Notice that the clay is less than one thirtieth of the volume of the
voids. Considerable binding material is doubtless derived from the
ferruginous limestone, which contains a comparatively small per cent
of iron. The iron in the ferruginous sandstone is too small in amount
to be appreciable. Some binder is doubtless derived from the meta-
morphosed rocks containing iron, silica, and mica. Notice that there
are nearly 30 per cent of crystalline rocks, which upon being finely
pulverized will furnish an excellent cementing material, particularly
after being decomposed. This gravel requires considerable rolling
with a heavy roller to crush the several ingredients and liberal
ART. 1] THE GRAVEL 163
sprinkling to work the pulverized material into the voids, before the
mass binds. All other gravels in Table 24 bind and make fair roads
under ordinary traffic.
6. Buck Hill. This gravel was obtained from the Buck Hill pit
at Tuckahoe, N. J. It was recommended as a representative
gravel by Hon. Henry I. Budd, State Commissioner of Public
Roads of New Jersey. This gravel consists chiefly of clay and
partially rounded quartz pebbles. The metamorphosed rock is
angular and friable. The clay is probably enough to fill the voids
when the gravel has been compacted by traffic. This is the first
of the samples in which the iron contained in the clay is appreciable,
and the iron doubtless has an important part in binding the road.
This gravel is used for road building without rolling.
7. Rock Hill. This sample was obtained from the Rock Hill
pit at Tuckahoe, N. J., and is substantially the same as No. 6,
except in having a greater per cent of voids and in containing some
sandstone which crushes easily and materially reduces the voids
of the gravel after it has been compacted in the road. It is said
that the best results are obtained by mixing this and the preceding
gravel half and half.
8. Shark River. This gravel was obtained from the Manasquan
Gravel Co. of Asbury Park, N. J., and is much used in southern
New Jersey and around New York City. It consists wholly of
clay and small rounded pebbles of pure white quartz, and conse-
quently the only binding material is the clay and the 2 per cent
of iron contained in it.
9. Oaktown. This is a gravel obtained from the Wabash river
by dredging, a few miles above Vincennes, Ind., which has been
used on the roads entering Oaktown, Knox Co., Ind. There is
very little clay in this gravel, — only 7.1 per cent, if the shale be
considered as clay, as it is practically. The chief source of binding
material is the 18.9 per cent of carbonate of lime, much of which
is in the form of flat chips. The metamorphic rocks are also in
thin chips, and are easily pulverized. The crystalline rocks and
the quartz are comparatively rough and angular. In service the
limestone pebbles grind up under traffic, and the road becomes
hard and firm, and is not much affected by freezing and thawing.
10. Shaker Prairie. This gravel is found in a pit on Shaker
Prairie, west of Oaktowp, Knox Co., Ind., and consolidates under
traffic much more quickly than the preceding. This gravel contains
a comparatively small amount of fine sand, being in this respect
164 GRAVEL ROADS [CHAP. V
about on a par with the Peekskill gravel — see No. 5, Table 24.
It contains a comparatively large amount of clay, being in this respect
similar to the New Jersey gravels — No. 7, 8, and 9 in Tables 24
and 25. This gravel has more iron in the clay than any of the
samples except the Tuckahoe gravels — No. 6 and 7. The limestone
is in comparatively large rounded pebbles, and not easily crushed
under traffic. The road is bound almost wholly by the clay and
the iron in it, and by the pulverized limestone.
11. Paducah. This gravel came from a pit about 2 miles west
of Paducah, Ky., on the Ohio river at the mouth of the Tennessee
river. It makes excellent roads that pack quickly under traffic
and are not much affected by freezing and thawing. The coarse
material consists of water-worn chert pebbles, and is cemented
by ferruginous clay. The chert is brittle and crushes with a sharp
splintery fracture, and consolidates readily under traffic, the sharp
angular fragments giving an immobile mass and offering excellent
surfaces for the cementing action of the binder.
12. Rosetta. This gravel comes from the Rosetta pit at Fort
Gibson; near Vicksburg, Miss., and is much used by the Illinois
Central Railroad as ballast. It is here included under the belief
that it will also make good wagon roads. The quartz pebbles are
quite rough and angular, and in the pit seem to be quite firmly
cemented together by ferruginous clay.
286. Conclusion. From the preceding, the following conclusions
may be drawn. 1. The relation between the proportion of voids
and the per cent of clay is no indication of the road-building qual-
ities of a gravel, for under traffic some of the fragments may crush
and decrease the per cent of voids and at the same time increase
the amount of the binding material. 2. The friability of the pebbles
has a greater effect upon the road-building qualities of a gravel
than the per cent of the voids. 3. The binding material may be
clay, or clay and iron, or pulverized limestone, or all of these com-
bined. The less clay the more slowly will the road bind, but the
less it will be affected by frost.
A study similar to the preceding will not certainly determine
the suitability of a gravel for road purposes, but it will throw valu-
able light upon its probable behavior in the road. The only sure
way to determine the road-building qualities of a gravel is to test
it by actual service, for much depends upon the friability of the
pebbles, the weight of the traffic, the climatic conditions, etc. In
applying the test of actual service, particularly to determine the
ART. 2] CONSTRUCTION 165
relative merits of two gravels, account should be taken of (1) the
nature of the soil, (2) the care employed in preparing the founda-
tion, (3) the quantity of material used, (4) the amount and char-
acter of the traffic, (5) the care given to maintaining the road, and
(6) the length of time the material has been in service. The char-
acter of a gravel road is generally indicated by the sound of the
metal tires of the wheels of the vehicles passing over it. If the wheel
makes a continuous crisp gritty sound, the road is reasonably good;
if the gritty sound is absent, there is probably too much earthy
matter on the surface; and if the sound is intermittent, there are
probably too many large pebbles in the surface.
ART. 2. CONSTRUCTION
288. The subgrade for a gravel road should be prepared in sub-
stantially the same manner as for an earth road (see Art. 1, Chapter
III). Indeed a first-class earth road is the best foundation for a
gravel road.
289. DRAINAGE. In no case should the drainage be neglected
— neither the side ditches nor the underdrainage. With the hard,
impervious surface of a gravel road, the water reaching the side
ditches is greater than with an earth surface; and therefore the
side ditches should be larger for gravel and broken-stone roads than
for earth ones.
A gravel road upon an undrained soil entails a needless expense
for maintenance, and is never so good as if the road-bed had been
thoroughly underdrained. Not infrequently a thin coating of
gravel has been thrown upon an undrained foundation, only to
sink out of sight in a year or two, and the attempt to secure a
gravel road has been abandoned. In such cases a comparatively
small expense for underdrainage would probably have resulted in a
fair road instead of a failure. The total amount of good road-build-
ing material in the world is small in comparison with the possible
future demand, and therefore it is a public misfortune to have any of
it wasted in bungling attempts at road building. One purpose of
gravel is to give a more or less rigid layer which will distribute the
concentrated pressure of the wheels over a sufficiently large area
of the earth foundation to enable it to support the load without
indentation. The thickness of gravel required to support the
load depends upon the degree of the drainage, since the more water
in the earth the less load it can support. Underdrainage costs
166 GKAVEL ROADS [CHAP. V
nothing for maintenance, and decreases the amount of gravel re-
quired, as well as the cost of maintaining the surface.
290. The tile should be placed under the side ditches — as de-
scribed for earth roads (§116). Some writers recommend that a
tile be laid under the middle of the gravel with the earth surface
sloping both ways to the tile. There are several objections to
this construction: (1) sloping the earth surface is not of much
advantage, and (2) it needlessly increases the depth of the gravel;
and (3) if the road is otherwise well made, the surface should be
practically impervious to water. See § 124.
Some writers advocate a tile each side of the graveled portion,
with short lines of tile running each way from the center of the
roadway obliquely to the side tile, these "miter drains" to be
placed 15 feet apart in wet places. Clearly this construction is
based upon a misapprehension of the source of the water reaching
a drain tile. The water that enters a tile comes from below and
not directly down from above. It is abundantly proven that in
an earth road needing underdrainage, little or no water penetrates
the surface; and with good gravel roads there will be still less.
Therefore "miter underdrains " below the graveled portion of the
roadway are absolutely worthless, and tiles at the edges of the hard-
ened way are no better than tiles under the side ditches'.
291. WIDTH. "For a discussion of the principles governing the
width of improved way and also whether it shall be located in the
center or at one side of the wheel way, see § 95-98. For a consider-
ation of the excess width on curves, see § 97.
292. MAXIMUM GRADE. For a general discussion of maxi-
mum grades, see § 79-86. A committee of the American Society
of Civil Engineers recommends that the maximum grade permissible
be 12 per cent — see Table 15, page 57. In the matter of permissible
maximum grades, gravel and macadam roads are in the same class.
For data concerning existing maximum grades on water-bound mac-
adam roads, see § 112.
293. CROWN. The same general principles concerning the
crown apply in gravel roads as in earth roads — see § 130-31. The
slope of the gravel surface from the center to the side should be
at least one quarter of an inch per foot, and it should not be more
than three quarters of an inch per foot. The first is about right
for park drives, which have light traffic and are well cared for. If
the drive is narrow, the crown may be a little greater than this;
but if it is broad, the crown should be less, to prevent the surface
ART. 2] CONSTRUCTION 167
from being gullied out near the gutters by the water running from
the center to the tides. The maximum crown, as above, would be
about right for a country gravel road with heavy traffic, or for a
street. If the gravel contains an excess of clay, the crown should
be greater than the above maximum, as the surface will be liable
to rut up.
Frequently gravel roads have an excessive crown, which forces
travel to use a narrow strip in the center — see § 129. This results
from the fact that the gravel is placed thicker at the center than
at the edges; and thus the surface of the gravel is given a greater
crown than the original earth road, while a gravel road should have
a less crown than an earth one.
A committee of the American Society of Civil Engineers recom-
mends a maximum crown of 1 inch per foot of half width and a
minimum of half that amount — see Table 16, page 65.
294. For a rule for super-elevation on curves, see § 90.
295. FORMS OF CONSTRUCTION. There are two forms of con-
struction of country gravel roads, which differ as to the manner
of preparing the subgrade to receive the gravel. In one form the
gravel is simply deposited on the surface in a strip along the middle
of the former earth road; and in the other a trench is excavated
in which the gravel is placed. For convenience of reference the
former will be called Surface Construction, and the latter Trench
Construction.
296. Surface Construction. The crudest form of this method
of construction consists in dumping gravel, as it comes from the
bank, in piles in line on an earth road. The quantity of gravel is
gaged by dumping a load in one, or two, or three lengths of the
wagon. Little or no attention is given to leveling off the top of
the piles, and it is not rolled except as travel is forced upon the
ridge when the earth upon the sides gets muddy. For the first year
or two after construction, such a gravel road is little if any better
than an earth one. The surface is full of cradle holes and is easily
cut into ruts; and the loose material absorbs the rain, and be-
comes mixed with the soil below. If the gravel is good, the road
becomes fairly good after the gravel has been packed by travel and
after the holes have been filled up by the addition of new material.
This form of construction is common where gravel is plentiful, the
work usually being done by labor road-tax.
297. Another form of surface construction consists in setting
up two lines of plank on edge and filling the space between them
168 GRAVEL ROADS [CHAP. V
with gravel. The gage planks are set en edge, 8, 10, or 12 feet
apart according to the importance of the road, and the gravel is
filled in between the planks, 8 or 10 inches deep at the sides and
12 or 15 at the center. Of course, when the boards are moved
forward to be used again, the edge of the gravel spreads out and
takes the natural slope, and under traffic it spreads out still further.
Ordinarily in this form of construction the gravel is not rolled,
and there is little or no driving over it by teams engaged in the
construction. The only advantage of this method over the pre-
ceding one is that it affords a means of gaging the depth of gravel
and of determining the quantity used; and the chief objection to
it is that when gravel is put on in a thick layer, the lower part is
not consolidated well, at least not for a considerable time, and
therefore the surface is liable to break up. This form of construc-
tion is very common.
298. In the best form of surface construction, the former earth
road is first smoothed up with the scraping grader and if necessary
the crown is reduced. If after smoothing the surface with the
grader, the foundation is not already firm and solid, it should be
rolled. Next a layer of gravel 4, or at most 6, inches deep is spread
upon the prepared subgrade, and leveled — either by hand with a
shovel and rake, or with a harrow or scraping grader. In dump-
ing from a wagon or cart, the larger stones will roll to the outer
edge of the heap; and hence in leveling the gravel care should
be taken that these are scattered and covered deeply with fine
material, for otherwise the road will not have an uniform texture
and will wear unevenly and the large stones are liable to work to
the top.
If the teams hauling the gravel are required to drive over that
already placed, the road will be consolidated much sooner, but
as the tractive resistance on loose gravel is very great, there is
some disadvantage in this requirement. If it is to be insisted
upon, the construction of the road should begin at the end nearest
the gravel pit. The gravel can be consolidated with a roller, but not
as effectively as by traffic, since no roller gives so great a pressure
as the wheels of loaded wagons (see § 378). But heavy loads should
not be permitted to go over the road while the surface is wet and
soft, for fear the wheels will cut through and mix the earth and the
gravel. While the gravel is being consolidated by the passage of
the teams employed in the construction or by ordinary traffic, all
ruts should be filled as soon as formed, by the use of a garden rake,
ART. 2]
CONSTRUCTION
169
and all saucer-like depressions should be filled by shoveling in fresh
gravel. The cost of filling ruts and depressions will be more than
saved in future repairs, and besides a much better road will be the
result.
After one layer has been thoroughly consolidated add a second,
and so on until the desired depth is reached. The first layer may
be the poorer gravel, the best being reserved for the top. All the
layers should be added in time to get well packed before the rains
and frosts of winter soften the road-bed.
When finished the gravel should be deepest at the center and
taper off to the sides. It is immaterial whether the first layer is
the widest or the narrowest — there is a little advantage either way.
The depth necessary will depend upon the nature of the soil, the
quality of the gravel, the amount of travel, the maximum weight
per wheel, and the care given to maintenance; but under ordinary
conditions, a depth of 8 or 10 inches of compacted gravel at the
center is sufficient. The width should vary with the amount of
travel, but for a country road a depth of 6 inches at 4 or 5 feet
from the center is sufficient.
Fig. 43 shows the dimensions required in good practice.
4 "t'fi* h
rf & G* \
(S/rface of Loose ^Material
x^
V"
0^ V
^r^^J
Fia. 43. — CROSS SECTION OF GRAVEL ROAD. SURFACE CONSTRUCTION.
For data on the width of the actually traveled way on gravel
roads, see § 95-96.
299. Trench Construction. In this form of construction, a
trench is excavated, 10 or 12 inches deep and of the required width
for the reception of the gravel. The bottom of the trench is usually
made parallel to the finished road surface by sloping it from the center
toward the sides (see § 351). Fig. 44 shows the form when the
finished surface is an arc. Fig. 44 is the standard form for state-
aid roads in Connecticut, except that the width of the graveled way
may be 12, 14, or 16 feet. The crown is f inch per foot of distance
from side to center, or 6 inches for a 16-foot roadway. There is
not much difference whether the road surface is an arc or two planes
meeting in the center. The latter is probably a little the better for
170
GRAVEL ROADS
[CHAP, v
country roads, although the former is the more common. Notice
that in Fig. 44 the intersection of the road surface with the side slope
of the embankment, is rounded off somewhat as recommended in
Half Section in Cut Half Section in Fill
•
FIG. 44. — CONNECTICUT GRAVEL ROAD. THENCH CONSTRUCTION.
Fig. 15 and 16, page 85. The exact method of rounding off the cor-
ners in Fig. 44 is not specified. The thickness of the layers as shown
is after consolidation.
Fig. 45 shows the standard form of construction adopted by
the Texas Highway Commission. Notice the wings in Fig. 45.
The bottom of the trench should be rolled to consolidate it and
to discover any soft places in the foundation. After rolling, any
depressions should be filled and the foundation then re-rolled. The
steam roller is better for this purpose than the horse roller, since it is
heavier and since the horses' feet do not dig up the subgrade. For a
discussion of rollers, see § 378. For precautions to be taken in roll-
ing the subgrade, see § 369.
A layer of 3 or 4, or at most 6, inches of gravel is placed in the
trench, and the gravel is harrowed with a tooth harrow, and is then
consolidated either by throwing the road open to travel or by rolling.
The latter is preferable, since teams in passing each other are liable
to break down the edges of the trench and mix the earth with the
gravel, and since the wheels are liable to break through the thin
layer of gravel — particularly if a wet time intervenes. If the only
FIG. 45.— TEXAS GRAVEL ROAD. TRENCH CONSTRUCTION.
gravel available contains an excess of large pebbles, they may be
used in the lower layer, in which case the layer can not be compacted
either by the wheels or by rolling. If the gravel is only slightly
ART. 2] CONSTRUCTION 171
deficient in binding material, it will be impossible to use a heavy
roller, since the gravel will push along in front of it.
Additional layers are added as rapidly as the preceding one is
compacted, until the desired depth is reached. Before rolling the
last layer the earth at the sides of the trench, i. e., the " shoulders "
or " wings," should be thoroughly rolled; and then the rolling of the
gravel should proceed from the sides toward the center, to prevent
the gravel from slipping outward. The gravel will compact much
better when damp; but if it is sprinkled, care should be taken that
(1) the gravel is not made so wet that the earthy binding material
becomes semi-fluid and collects on the surface, and (2) that the sub-
grade is not unduly softened.
No practical amount of rolling will cause a gravel road to " come,
down " in the sense that a water-bound macadam road does; that
is, a gravel road can not be rolled until the surface is as hard as it
will probably be after it has been opened to traffic for a time, since
even the heaviest rollers do not give as much pressure as the wheels
of heavily loaded wagons. This difference between gravel and
water-bound macadam roads is due to the fact that gravel has the
binding material uniformly distributed throughout the mass, while
with broken stone the binder is spread upon the top and worked in
by rolling and sprinkling.
300. Surface vs. Trench Construction. Surface construction
is cheaper and seems to be much more common than trench con-
struction. Surface construction is the better, since the depth of
the gravel at different distances .from the center is approximately
proportional to the amount of traffic; while in the trench construc-
tion, if the graveled portion is wide the sides are liable not to be much
used, and if the graveled portion is narrow passing vehicles are
forced upon the earth shoulders. Therefore it appears that surface
construction is best for roads having a large amount of traffic. In
park drives and streets, the whole width of the roadway is excavated
and filled with gravel.
Trench construction is a little more economical of gravel, and
is therefore most suitable where gravel is expensive.
301. Earth Road beside the Graveled Way. It is sometimes
advocated that there should be two tracks, an earth road for sum-
mer travel and a graveled way for winter use. This plan has some
advantages and also some disadvantages. When the earth track
is dry, it is preferred by most teamsters to the hard gravel road;
and the use of the earth roadway decreases the wear on the gravel,
172 GRAVEL ROADS [CHAP. V
—which is clearly an advantage, for a gravel road like most other
things will wear out. On the other hand, if the summer track
is immediately adjacent to the hardened way, the earth of the
former will become mixed with the gravel of the latter, much to
the detriment of the gravel. The chief source of expense in the
maintenance of gravel roads is due to the damage done by the
mixing of earth from the side of the road with the gravel, thus
forming a mixture that will hold water and cause the road to cut
up. It has been suggested that the objection to the two tracks
could be obviated by constructing a ditch, or sodding a narrow
space between the two; but this is impracticable. The two tracks
require a wider right-of-way, and therefore for this reason are fre-
quently impossible.
302. For a discussion as to whether the gravel road shall be in the
center of the right-of-way or at one side, see § 98.
303. BOTTOM COURSE. The gravel usually contains many
stones too large to be used in or near the wearing surface, and
therefore it is economy to screen the material and lay the larger
pebbles in the bottom. Some writers object to using pebbles larger
than 1 or 1| inches in diameter for the bottom course, on the ground
that the heaving effect of frost and the vibration due to the pass-
ing wheels will cause the larger stones to rise to the surface and
the smaller ones to descend — like the materials in a shaken sieve.
Unquestionably, if a vehicle is driven over a layer of loose stones
of all sizes, the larger ones will tilt up when the weight comes upon
them and the smaller ones will roll down into the space made vacant
by such tipping; and by a repetition of this process, the large stones
will gradually reach the surface. The heaving action of the frost
acts in a similar way. But it does not follow that a layer of coarse
stones at the bottom of a gravel road will thus work to the top
when the interstices of the gravel above are filled with binding mate-
rial and all is compacted by traffic or by rolling. Experience has
shown that if 2 to 4 inches of the top dressing has suitable binding
material, it is extremely improbable that pebbles 2 to 2| inches in
diameter in the bottom course will ever work to the surface.
304. Other materials than coarse pebbles may be used for the
lower course. In many localities there are large quantities of
coal slack, which is useless as fuel and is too friable for the wearing
surface of a road, but which can be used for the bottom course of
a gravel road. Coal slack has thus been successfully employed,
and is often cheaper than gravel. Blast-furnace slag has also been
ART. 2] CONSTRUCTION 173
used for this purpose. Sometimes broken stone is used for a bot-
tom course; but on account of the expense of breaking, only a
stone found already broken in the quarry is suitable for this pur-
pose. A " flake " stone or quarry chips are the forms generally
used. The celebrated gravel roads of Central Park, New York
City, have a " rubble foundation " — not a Telford foundation
(§ 349). The rubble layer is 10 to 12 inches thick, and the gravel
4 to 6 inches after being thoroughly compacted. The stones, none of
which exceeded 9 inches in greatest dimensions, were dumped upon
the subgrade from carts and " evenly adjusted by a little labor
of the hand."
305. SCREENING THE GRAVEL. As a rule gravel should be
screened to exclude that which is too fine, and also to insure an
even distribution of the fine and coarse material when placed upon
the road. Where a small amount of gravel is required, the ordi-
nary stationary inclined screen is used, the gravel being thrown
against it with a shovel; but where a considerable amount is re-
quired, it is much cheaper to use a rotary screen driven by power.
If the gravel contains a considerable quantity of stones more
than 2j or 3 inches in diameter, a stone crusher can be profitably
employed, in which case it may be economical to use an elevator,
rotary screen, and elevated storage bins, and to put all of the
gravel through the crusher, rotary screen, and storage bin (see Fig. 62,
page 205).
Under favorable circumstances, the cost per cubic yard of screen-
ing by hand will be about an hour's wages for a man for each time
the material is handled with a shovel; while with the rotary screen,
it can be screened to three sizes and be placed in elevated bins for the
same amount.
306. The Michigan State Highway Department, which builds
many good gravel roads, requires the use of a rotary screen not less
than 9 feet long and 30 inches in diameter, divided into three sec-
tions having perforations 3, 2, and f inches in diameter. The
Department divides road-building gravel into two classes as follows:
The best contains at least 60 per cent of material passing a 2j-inch
screen and caught on a J-inch screen; and the second class contains
at least 40 per cent caught between these screens. The Department
limits the clay binder to 10 per cent of the entire mass.
The specifications of the American Society for Municipal Im-
provements for 1916 divide mixtures of gravel, sand, and clay as
follows: No. 1 contains 60 to 75 per cent caught between the l?-inch
174 GRAVEL ROADS [CHAP. V
and the J-inch screen, and of this portion from 25 to 75 per cent
shall be retained on the f-inch screen, and of the portion passing
the J-inch screen from 65 to 85 per cent shall be retained on the
200-mesh sieve. No. 2 contains 60 to 75 per cent caught between the
2i-mch and the J-inch screen, and of this portion from 25 to 75 per
cent shall be retained on the 1-inch screen, and of the portion passing
the J-inch screen 65 to 85 per cent shall be retained on the 200-mesh
sieve. No. 2 is to be used for the two lower courses of the road, and
No. 1 for the top course.
307. HAULING THE GRAVEL. Gravel is usually obtained from
pits, and is generally overlaid with more or less earth, which should
be entirely removed before beginning to haul the gravel. Not infre-
quently this earthy material is allowed to tumble into the pit and
mix with the gravel, greatly to the detriment of the finished road.
The loading of the gravel can be greatly facilitated by using a
board platform 8 to 10 feet long and 4 to 6 feet wide. This plat-
form is placed against the bottom of the bank in such a manner
that when the gravel above is dislodged it falls upon the platform,
from which it is easily shoveled into the wagon. Often the plat-
form can be supported upon legs at a height above the top of the
wagon, and the gravel can be simply pushed off into the wagon
with the shovel. Sometimes the circumstances justify the use
of a drag scraper (§ 150) — drawn by a horse attached to a cable
passing through a block — to drag the gravel to the edge of the plat-
form, whence it drops into the wagon; and sometimes, if a large
quantity is to be loaded and a large number of teams are engaged
in the hauling, the wagons can be loaded with a trap — an elevated
platform upon which the gravel is hauled with a drag or a wheel
scraper, and through which it drops into the wagon below.
308. MEASURING THE GRAVEL. When gravel roads are built
by public officials, the gravel is usually measured in the pit or in
the wagon. The former is the better practice, since it is more definite.
When the road is built by contract, the gravel is measured (1) in
the wagons, or (2) loose in the road by means of gage boards, or (3)
compacted in the road by means of established grades. The first
or second method is generally used with surface construction, and the
third with trench construction. With the last, it is customary to
require that the finished surface shall conform to an established grade;
and consequently a considerable quantity of gravel is liable to be
forced into the subgrade, — particularly if the earth foundation is
made to conform to the grade established for it. The specifications
ART. 2] CONSTRUCTION 175
for state-aid roads in New Jersey specify that " the contractor is to
place sufficient gravel on the road to allow it to shrink 33 per cent
in rolling and settling." Loose gravel with clay or loam binder will
shrink 12 to 15 per cent in rolling, and gravel in which the binder is
produced by crushing part of the material will shrink still more —
possibly twice as much; the above specifications provide, therefore,
for the possibility of forcing 18 to 21 per cent of the gravel into the
subgrade.
If it is expected that part of the gravel may be forced into the
soil, the subgrade may be left a little higher than the established
grade; and then the addition of the stipulated amount of gravel will
bring the finished surface to the specified grade. Or, a thin layer
of sand on the subgrade will sometimes prevent the gravel from
being forced into the soil. For a further discussion of this subject,
see § 377.
309. COST. The cost of gravel roads varies greatly with the
form of construction, the cost of gravel, the amount of grading and
drainage required, the width and thickness of the gravel, etc. An
average depth of 1 foot over a width of 13J feet requires half a
cubic yard per linear foot of road, or 2,640 cubic yards per mile.
The gravel usually costs from 5 to 10 cents per cubic yard stripped
in the bank. The cost of loading will vary from 5 to 10 cents per
cubic yard, not including the time lost by the team in waiting for
a load. Setting gage plank, leveling, etc., may cost from 2 to 10
cents per cubic yard. The cost of hauling varies materially with
the time of year (see § 4), and including the time lost in loading
and unloading, will usually be at least 15 cents per cubic yard
(about 1J tons) per mile and seldom more than 30 cents — the
former when done by farmers in the slack season and the latter
when done by teamsters. For a haul of 1 mile the total cost in
place is 40 to 50 cents per cubic yard.
310. Reports from forty-four counties in Indiana show that the
total cost of construction of gravel roads in that state varies from
$800 to $3,500 per mile; and except in a few counties, the cost
varies from $1,000 to $2,500, and is generally from $1,000 to $2,000.
The cost varies with the distance about as follows: when the gravel
is hauled 1 mile, the total cost of the road is $1,000 per mile; when
the haul is 2 miles, $1,250 per mile; when the haul is 3 miles, $1,500
per mile; when the haul is 4 miles, $1,750 per mile; and if 5 miles,
$2,000 per mile. Numerous data from Ohio and Illinois seem to
show that the above prices are fairly representative.
176 GRAVEL ROADS [CHAP. V
In Missouri in 1912 one-course gravel roads 10 feet wide cost
$800 to $1,800 per mile, and 15 feet wide from $1,500 to $2,500,
exclusive of grading, drainage, culverts, interest, and profits.
In Michigan in 1913 the average cost of 68 miles of state-aid
gravel roads was as follows:
ITEMS. AVERAGE COST
Per Mile. Per Sq. Yd.
Shaping and draining $455.46 $0.086
Gravel, loading and hauling, 1,649 cu. yd 247 . 34 0 . 046
Culverts, etc 71.93 0.013
Surfacing 1661.85 0.316
Total * $2 436.58 $0.460
311. ECONOMIC VALUE OF A GRAVEL SURFACE. The value
to a community of covering an earth road with gravel is a subject
the discussion of which leads different persons to widely different
conclusions, depending upon the point of view and upon the data
assumed.
The advantage of a gravel surface over one of earth is that the
hard and impermeable surface of the former is equally good at all
seasons of the year. The financial value of a road which is good at all
seasons of the year varies greatly with the locality and the occupa-
tion of those who use it. Near a large city such roads are nearly
indispensable to dairymen, fruit growers, and truck farmers; but
permanently hard roads are not of great financial advantage to
grain growers and stock raisers, except in the immediate vicinity
of a large city. A road which is uniformly good at all seasons of
the year is of some economic advantage to a farming community,
since it permits hauling to be done at times when other work is
impossible, and since it makes possible the marketing of commodi-
ties when the price is most favorable. It is impossible to compute
the money value of these factors; but, in general, it is not very
great (see § 4-7). The chief advantage of a road good at all seasons
of the year is its effect upon the social life of the rural district
(§ 1-3).
The amount of a load than can be hauled on an earth road is
often determined by the grades rather than by the nature of the
surface; and unless the grades are light, the maximum load for a
gravel road is not much greater than that for a dry earth road.
Therefore, before adding a gravel surface to an earth road, the
gradients should be carefully studied with a view of deriving the
ART. 2]
CONSTRUCTION
177
II
0)<N
W
O
0
o
8 8|
3 * 33-
8 S8
8 3
3 3
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3 3
o *o
CO t>»
8 g :
3 3 ;
10 o •
11
Tt< *O CO l> 00 OS
178 GRAVEL ROADS [CHAP. V
utmost benefit of the improved surface by securing easy ruling
grades (see § 74).
312. The cost of the improvement is the sum of (1) the annual
interest on the cost of construction, (2) the excess of the annual cost
of maintaining the gravel road over that of maintaining the earth
road, and (3) the annual payment necessary to accumulate a fund
sufficient to make periodic repairs, i. e., to add a new surface at
intervals. The money spent in road improvements is to be con-
sidered as an investment which will return annual interest in the
reduced cost of transportation and in the greater freedom of traffic
and social intercourse.
313. DURABILITY. On account of the low first cost of the gravel,
and the fact that reasonably good gravel roads can be built without
any investment of money in rollers, crushers, and other costly
machinery, they are well suited to light traffic roads, to residence
streets in small cities, and to park drives. A gravel road well built
of good material is excellent for automobiles, and will safely carry
a considerable number daily — see Table 26, page 177.
314. SPECIFICATIONS. The American Society of Municipal
Improvements publishes standard specifications for the material
and workmanship of gravel roads. The specifications are modified
from time to time as is necessary to keep them up to date. Printed
copies may be had of the secretary for a nominal sum.
The various State Highway Departments also publish standard
specifications
ART. 3. MAINTENANCE
316. There are more miles of gravel roads in this country than
of any other type of improved roads; and therefore the proper main-
tenance of these roads is an important matter. If properly con-
structed and reasonably maintained, a gravel road is quite satisfac-
tory except under very heavy travel.
The proper care of side ditches, culverts, shoulders, and the
surface has been considered in the discussion on earth roads —
Art. 2, Chapter III; — and all that has been said there applies equally
well to gravel roads. The maintenance of a gravel road is more im-
portant than that of an earth road, because more is rightly expected
of the former than of the latter; and since the gravel road represents
a greater investment, neglect may result in greater damage.
317. DESTRUCTIVE AGENTS. The destructive agents are the
same for gravel as for earth roads (see § 198-204), except that for
ART. 3] MAINTENANCE 179
gravel roads a gradient is an element of destruction whose impor-
tance varies with its steepness. Horses in drawing a load up a hill
or in holding back a load in coming down, are liable to displace
pebbles with the calks of their shoes, and after the first stone is
displaced it is easier to loosen others. The locking of the wheel,
until it slides in going down hill, is also hard on a gravel road.
Grades have a further disadvantage. Automobiles are likely
to speed up at the bottom of the grade so as to reach the top at a
fair velocity, and the sudden acceleration of the speed is likely to
dislodge pebbles and cause the road to ravel.
318. THE METHOD OF MAINTENANCE. The three methods of
maintenance employed in caring for earth roads (§ 222-26) are also
employed for gravel roads. Since gravel roads represent a greater
investment, and since more is expected of them, it is desirable that
they shall be cared for under the patrol system, that is, the system
of continuous maintenance.
When a gravel road is first thrown open to traffic, it should be
carefully watched and all incipient ruts and depression should be
filled as soon as formed, either by raking in gravel from the sides of
the depression or by adding fresh gravel — in the earlier stages of this
work the former is the better, and in the later stages the latter is
necessary. The new gravel should be finer and contain more bind-
ing material than that employed in the original construction. If the
depression is very shallow, it is wise to roughen the surface with a
garden rake before adding the new material. It is important that
ruts and shallow holes should be filled as soon as they appear,
for they will hold water, which will soften the gravel bed and cause
the road to wear rapidly. At, say, every f mile a small pile of gravel
should be stored to be used in filling depressions.
If the road surface becomes muddy when wet, there is an excess
of clay binder; and therefore a thin layer of coarse clean sand or fine
clean gravel, preferably the latter, should be added. On the other
hand, if the surface shows a tendency to disintegrate or ravel, there
is not enough binder; and therefore a layer of clay should be added
and harrowed into the gravel. However, if the surface does not at
once set up, it should not be concluded that there is not enough
binder, for a road that binds quickly is likely to cut up badly, par-
ticularly when wet.
During this stage, all loose stones should be removed from the
roadway, both for the comfort of travelers and the good of the road.
After the gravel has become thoroughly consolidated, i. e., after
180 GRAVEL ROADS [CHAP. V
the wheels no longer make even shallow ruts, the only care the
road is likely to need for several years is to keep the side
ditches and culverts free from weeds and floating trash, and to
attend to the drainage of the surface when the snow is melting
(§ 219).
After a time the gravel will work out to the sides of the road too
far, and the center will wear hollow. It will then be necessary to use
a scraping grader (§ 155-56) to push the gravel back to the center.
In doing this care should be taken not to scrape up the earth with the
gravel. A good time to use the grader is just after a rain, when the
road is soft and easily scraped, and when the gravel scraped to the
center is in the best condition to pack again. The road should never
-be allowed to wear so hollow in the center as to interfere with the flow
of water from the surface to the side ditches.
319. Sprinkling. A gravel road with clay binder needs a little
moisture to hold it together, since the clay shrinks and cracks under
excessive drought, loses its binding power, and permits the road to
break to pieces. Under such circumstances a sprinkling with water
is a means of preserving the road from serious damage, although
on account of the expense this is seldom done except on park
drives.
320. Re-Surfacing. It will finally be necessary to repair the
surface by adding a coating of new gravel. For this purpose the size
of the largest pebbles should vary with the thickness of the coat. It is
usual to put the gravel on by making two or three dumps of a wagon
load, i. e., by stretching a cubic yard over 15 to 25 linear feet, accord-
ing to the thickness of layer required, and spreading the gravel just a
little wider than the wagon track. Traffic will spread it still wider,
and also pack it.
In making repairs, it is better to apply a thin coat often than a
thicker coat less frequently, since a thick coating does not pack
well. A layer of 2 inches of gravel is better than more — unless on a
spot that has cut through.
321. COST. The cost of maintenance varies with the climate,
the amount and nature of the traffic, the quality of the gravel, etc.
Data from Indiana and Ohio show that it varies from $40 to $100
per mile per annum — the former where the traffic is light, the gravel
good, and the snow light; and the latter where the traffic is heavy,
the gravel poor, and the snow heavy. In New Hampshire the cost
is $20 to $100, usually, $20 to $50 per mile per year exclusive of re-
surfacing; or including re-surfacing, $150 to $300 per mile per year
ART. 4] DUST PALLIATIVES 181
for all expense.* In Vermont the average cost of maintaining 175
miles in 1912 varied from $10.97 to $32.33, the average being $20.71.
ART. 4. DUST PALLIATIVES
323. The surfaces of gravel roads are treated for two distinct
purposes, — to lay the dust and to bind the surface materials. The
agents that accomplish the first are called dust palliatives; and
those that secure the second are called road binders, protective
coatings, or bituminous carpets. Only the first will be considered
here.
324. DU.ST PREVENTIVES. The suppression of the dust from
an earth road is in the interests of the people using the road or re-
siding adjacent to it; but the prevention of dust on a gravel road is
important not only to the interests of those using the road or living
near it, but also to the very life of the road itself. The simplest
materials used for this purpose are: Fresh water, salt water, deli-
quescent salts, proprietary compounds, oil, and tar. The two latter
are the most common, and are used primarily as road binders, al-
though they incidentally prevent the binder of the gravel from being
blown away as dust.
325. Sprinkling with Fresh Water. This is the simplest method
of preventing dust. Sprinkling a gravel road with water not only
suppresses the dust, but prevents the disintegration of the surface by
raveling (see § 397). The water should be applied in a fine spray,
and in such quantities as not to run in streams on the surface; that
is, several light sprinklings are better than a single flooding. If
sprinkled too heavily or too often, the road is softened and breaks
up easily.
Reliable and definite data concerning the cost of sprinkling are
rather meager. Dust may usually be kept down on a gravel road
carrying a moderate amount of heavy travel, by sprinkling, for about
2 to 3 cents per square yard per annum.
326. Sprinkling with Sea Water. This is a simple remedy, but
obviously is applicable only to roads located near the sea coast. Sea
water is more effective in laying dust than fresh water owing to cer-
* From an instructive account of the patrol system of maintenance employed by the New
Hampshire Highway Department, in Engineering News, Vol. 74 (1915), p. 1110. For an
article explaining the use of gasoline motor trucks and trailers in maintaining gravel roads in
Alabama and giving data on the cost of the work, see Engineering Record, Vol. 74 (1916), p,
73-74.
182 GRAVEL ROADS [CHAP. V
tain deliquescent salts which it contains; but the presence in the
sea water of salts not possessing hygroscopic properties causes disa-
greeable and destructive mud in wet weather, and renders this form
of treatment rather unsatisfactory.
327. Moistening with Deliquescent Salts. Solutions of water
and various deliquescent salts have been used to moisten the surface
of roads to prevent dust. The effect of these solutions is more lasting
than that of fresh water alone, and they are easily applied by the
ordinary sprinkling wagon. However, some difficulty is encoun-
tered in obtaining a solution of constant strength, and its cost is
considerable.
Among the most common of these salts is calcium chloride,
which may be obtained commercially in a granular condition or in a
concentrated solution. The granular salt may be applied with an
ordinary agricultural drill. When applied in granular form about
three fourths of a pound per square yard is used for the first applica-
tion, and slightly less for succeeding ones. When applied in liquid
form, a 15 per cent solution is ordinarily used for the first application,
and for successive applications an 8 or 10 per cent solution is em-
ployed. Of course, such salts are not suitable in arid or semi-arid
regions, since there is but little moisture in the air to be absorbed;
nor in an extremely humid climate, since the salt is likely to be
washed away by the rains. Calcium chloride is odorless and clean,
but has no permanent effects upon the road. It may cause soreness in
horses' feet, if used in quantities much in excess of those stated above.
328. Sprinkling with Proprietary Compounds. There are a
number of proprietary dust-laying compounds upon the market.
Most of them consist of deliquescent salts dissolved in water, and
some consist of by-products from manufacturing. The names of a
few of the first class are: Aconia, calcite, and panscale. Several of
them seem to be no more efficient than calcium chloride, but are con-
siderably more expensive.
329. Sprinkling with Light Oil. The dust of a gravel road may
be laid by sprinkling it with oil much as was described for an earth
road (§ 236-39); but it is usually more economical to apply a pro-
tective coating or bituminous carpet (Art. 1 and 2 of Chapter IX)
which not only prevents to a certain degree the formation of dust,
but protects the surface of the road.
Oil as a dust layer was used chiefly on park drives and suburban
roads, but has been abandoned where there is any considerable
amount of automobile travel. The following account from the pre-
ART. 4] DUST PALLIATIVES 183
vious edition of this volume describes the former practice in Wash-
ington, D. C.
330. Practice in Washington, D. C. The city of Washington,
D. C., formerly sprinkled the gravel park drives with the light as-
phaltic oil described in § 552. The following is a description of the
method then employed in applying the oil.*
" All ruts and holes in the surface of the road are first repaired
by cleaning out the cavity, filling it with coarse stone which is cov-
ered with a coating of hot, heavy, asphaltic oil, then sprinkling a
light coat of screenings over the oil, and finally compacting the
patch by ramming. When all holes have been thus repaired, the
surface of the road is thoroughly cleaned with rattan brooms, care
being taken to remove all loose materials and caked dirt or dust so
that the stone forming the wearing surface of the road shall be
exposed and clean.
" When the road is entirely free from moisture, and during warm
dry weather, if possible, a light asphaltic oil is spread (without being
heated) by means of special sprinkling wagons. One third to one
half gallon of oil to the square yard usually forms the first applica-
tion. To allow it to penetrate into the surface, the road is closed to
traffic at least 48 hours after the first application.
"At the end of this time the surface of the road is covered with
a thin coating of clean, coarse, sharp sand or stone screenings, free
from dust ; and is then rolled and traffic allowed to go over it. A
cubic yard of sand or screenings usually covers from 75 to 125 square
yards of road surface.
" The oiling described above keeps the surface in excellent con-
dition for a year. It is never dusty, and is muddy for only a few
hours after a heavy thaw when the skid-chains of automobiles tear
up the surface. The subsequent passage of automobiles without
chains soon irons out the roadway. At the end of the year the sur-
face of the road is again thoroughly cleaned, from \ to J gallon of oil
to the square yard under normal conditions being spread over it, and
the road closed for 48 hours and covered with sand or screenings
as before. This treatment is continued from year to year."
" The cost, for the first application, from 2.8 to 4.6 cents per
square yard; and for the second application from 1.3 to 2.8 cents per
square yard."
* Paper by Col. Spencer Cosby, U. S. Army, in Charge of Buildings and Grounds, Wash-
ington, D. C., presented before Section D'of the American Association for the Advancement
of Science, on Dec. 29, 1911.
184 GRAVEL ROADS [CHAP. V
331. The amount of oil applied is often considerably greater than
that employed in Washington, D. C., as mentioned above, being
from 0.3 to 0.4 gallon per square yard, in which case the total cost of
material and labor is from 4.0 to 7.0 cents per square yard. When
the quantity;required is as great as this, it is probable that the ex-
pense is not justifiable, since instead of spending the larger sum for a
light oil, it is more economical to spend a little greater amount for
a heavier oil or for a stronger binding material, and construct a more
durable protective coating (see § 583).
CHAPTER VI >
WATER-BOUND MACADAM ROADS
334. Throughout the entire nineteenth century a road built
by placing small fragments of broken stone on the ground and com-
pacting them into a solid mass by rolling or by travel was called a
macadam road, after John Loudon MacAdam (1756-1836), a famous
English builder of broken-stone roads and one of the first to build
such roads. The broken stone is called macadam, and the work of
construction macadamizing.
A broken-stone road is sometimes called a telford road after
Thomas Telford (1757-1834), a famous English engineer; but the
term telford is usually, and appropriately, restricted to a particular
form of the foundation of a broken-stone road (§ 349).
The fragments of stone in the road referred to above were held
together by the cementing power of the dust of the stone; but the
use of the automobile has shown the desirability of a broken-stone
road having a binder stronger than stone dust; and this led in the
early years of the twentieth century to the introduction of a new
type of broken-stone road — one bound with a bituminous cement, such
as tar or asphalt. Such a road is called a bituminous madacam road ;
and consequently the broken-stone road having a stone-dust binder
is now called a water-bound macadam road.
For more than a hundred years the water-bound macadam was a
leading form of improved road all over the world, and even now it
is exceeded in mileage only by earth and gravel roads; but, since the
early years of this century, it is much less frequently built than
formerly. It is used now only in rural roads having comparatively
little motor-driven travel and on residence streets having but little
through travel. In some particulars the water-bound macadam
road will be discussed a little more fully than its individual merits
may warrant, because many of the principles of its construction are
applicable to bituminous roads, which are becoming increasingly
important.
185
186 WATER-BOUND MACADAM ROADS [CHAP. VI
ART. 1. THE STONE
335. REQUISITES FOR ROAD STONE. The principal requisites
of a material for a water-bound macadam road are hardness, tough-
ness, cementing or binding power, and resistance to the weather.
Usually any stone that is hard and tough will resist the weather
reasonably well; but shales and slates, though hard and tough when
first quarried, often disintegrate when exposed to the weather.
The material for a road surface should also be uniform in quality
or the surface will wear unevenly; and the depressions which occur
where the material is comparatively soft will hold water, thus soften-
ing the road-bed and occasioning damage difficult to repair.
336. Hardness and Toughness. These two qualities are closely
related. Hardness is that property of a solid which renders it dif-
ficult to displace its parts among themselves; while toughness
enables the parts to yield somewhat without being separated or
broken. For road purposes, hardness is the power possessed by
a rock to resist the rubbing or the abrasive action of wheels and
horses' feet; while toughness is the adhesion between particles of
a rock which gives it power to resist fracture when subjected to
blows. A stone may be hard and brittle, and be quickly pounded to
pieces in the road, as quartz; or it rr ay ha\ e a high crushing strength
and yet be deficient in toughness, and grind away speedily under the
abrasion of traffic, as some \arieties of sandstones. A road metal
should have enough resistance to crushing to support the load brought
upon it by the wheels, and enough toughness to prevent its being
readily ground into powder. A large part of the fine material is
inevitably swept away by the rains and winds, or is removed by
scrapers to keep the road in good condition during wet weather;
and therefore it is important that the fragments should be tough
enough not to be unduly pulverized by travel. Toughness is incom-
patible with a high degree of hardness, and in a measure makes up
for a deficiency in resistance to crushing. Hardness could be meas-
ured by the resistance offered by a rock to the grinding of an emery
wheel; and toughness would be measured by the resistance to frac-
ture when struck with a hammer.
337. Cementing or Binding Power. Binding power is the prop-
erty possessed by rock dust to act as a cement between the coarser
fragments composing a stone road. This property is of the highest
value, for the strength of the binder determines the resistance of the
AJRT. 1] THE STONE 1S7
road to the wear and tear of travel more than does the strength of
the fragments themselves. It is possessed in a very much higher
degree by some varieties of rocks than by others, and its absence is
so pronounced in some varieties that they can not be made to com-
pact under the roller or under traffic without the addition of some
cementing agent.
338. METHODS OF TESTING STONE. There are two methods
of determining the qualities of a stone for road-building purposes:
(1) by using the stone in the road and keeping an account of the
cost of repairs over a series of years, or (2) by laboratory experi-
ments. The first is uncertain owing to the variations in climatic
conditions, and in the amount and nature of the traffic, etc.; and
would be very expensive and take a long time. In the second
method of testing, it is difficult to duplicate in the laboratory the
conditions of actual service; but nevertheless much valuable infor-
mation may thus be obtained at a moderate expense and in a com-
paratively short time.
Systematic laboratory tests of road metal are of comparatively
recent origin, and may be said to have been started about 1880 by
the French governmental engineers, who have made extensive use
of this method in determining the quality of the rock used in con-
tract work and in selecting new quarries. Only a little such labora-
tory work has been done in England and Germany. From 1894 to
1899 the Massachusetts Highway Commission conducted a series of
tests of road-making materials, and developed new and important
methods of testing, and deduced much valuable information.
339. Since the latter date the U. S. Office of Public Roads and
Rural Engineering has conducted extensive laboratory tests of the
road-building stones. Bulletin No. 370 (1916) describes the methods
employed and gives the results obtained for 3,650 samples from forty
states and three foreign countries. The Laboratory tests road
materials for public officials without cost; and also gives advice as to
the value of the material for road-building purposes.
340. The principal tests applied to stone for water-bound mac-
adam roads will be very briefly described.
341. Hardness Test. This test determines the hardness of the
stone; and virtually consists in measuring the amount ground off
under certain conditions. The loss in the above tests varied from
1.0 to 32.8 per cent, usually from 2 to 10. The test is made with the
Dorry machine.
342. Toughness Test. This test is to determine the resistance to
188
WATER-BOUND MACADAM ROADS
CHAP. VI
impact. It is made by finding the number of hammer-like blows
required to break a cylindrical specimen. The results range from 3
to 43, about half of the materials requiring from 10 to 20 blows.
The test is made with the Page impact machine.
343. Abrasion Test. The results of this test depend upon both
hardness and the resistance to abrasion. Fragments of the stone are
rotated in a cylinder inclined to the axis of rotation, and the amount
worn off is determined. The result is expressed either in per cent of
wear or in the arbitrary French coefficient of wear, which equals
40 divided by the per cent of wear. The latter is in more common
use. The French coefficient of wear for the test referred to in § 339
ranges from 1.4 to 41.7, usually from 10 to 30. The test is made with
a Deval machine.
344. Cementation Test. This test determines the binding power
or cementing value of the stone dust to hold together the coarser
fragments of a water-bound macadam road. This is the most
important quality of a stone for such a road. The test is made by
placing small fragments of stone and water in a ball mill and grinding
them to a stiff paste, which is then moulded into a briquette under
heavy pressure. The briquette is dried and tested in an impact
machine to determine the number of blows required to break it.
The results range from 0 to over 500. Values below 10 are called
low; from 10 to 25, fair; from 25 to 75, good; from 75 to 100, very
good; and above 100, excellent. The results for a few stones are as
follows:
NAME.
MAX.
MlN.
NAME.
MAX.
Mm.
Andesite
500+
9
Gravel
500
3
Basalt ... .
500+
2
Limestone
500
9
Chert
500 +
2
Marble
85
10
Conglomerate
500+
20
Quartzite
45
o
Diabase
500+
2
Sandstone
500 +
1
Diorite.
148
5
Shale
367
28
Granite
255
2
Slate
500+
1
345. Conclusion. The principal rocks used for water-bound
macadam roads are traps (a popular term which includes diabase,
diorite, and several other igneous rocks), granites, and limestones.
Their value for road-building purposes is in the order named. In
the Eastern States the traps are the ones most used, in the Missis-
sippi Valley limestone is the most common.
For information concerning the road-building materials of the
ART. 2] CONSTRUCTION 189
United States, see Preliminary Report on the Geology of the Common
Roads of the United States, by Nathaniel Southgate Shaler, in U. S.
Geological Survey, Fifteenth Annual Report, 1893-94, p. 255-306.
ART. 2. CONSTRUCTION
347. The principles of construction for earth roads apply also to
the construction of the subgrade for broken-stone roads (see Art.
1, Chapter III). The drainage of the foundation by tile drains and
side ditches should not be neglected (see § 114-24 and § 125-28).
348. FORMS OF CONSTRUCTION. With reference to the method
of preparing the subgrade to receive the stone, there are two forms
of construction — surface construction and trench construction. The
surface construction consists simply in placing a layer of broken
stone upon the earth road and leaving it to be compacted by travel.
In the West many miles of road are constructed on this plan with
limestone. As a rule this material readily pulverizes under the
traffic, and the powder cements well; consequently the road soon
binds together. Such roads are not first class, but they give good
returns on their cost. On account of the simplicity of the con-
struction, this form will not be considered further.
The trench construction consists in excavating a trench of the
required width and depth, and depositing the broken stone in it.
349. With reference to the lower course of stone there are two
systems of construction, — the macadam and the telford (§ 334),
The macadam road consists of two or more layers of crushed stone,
its distinguishing characteristic being that the lower course of crushed
stone is placed directly upon the earth road-bed. The telford road
consists of a foundation or pavement of rough stone blocks set upon
the road-bed, covered with one or more layers of crushed stone, the
distinguishing feature being the paved foundation.
350. Telford vs. Macadam Roads. Each of these systems has
its earnest advocates who contend for its exclusive use.
The most important claims of the advocates of the telford con-
struction are (1) that the open foundation is necessary for drainage;
(2) that the sub-pavement is necessary on soft or poorly drained soil
to prevent the small fragments of broken stone from working down
into the soil and the soil from working up into the stone; and (3)
that the telford is the cheaper, since the expense of crushing is saved.
The most important claims of the advocates of the macadam
system are: (1) that the drainage afforded by the telford construe-
190 WATER-BOUND MACADAM ROADS [CHAP. VI
tion is no better than that with the macadam construction; (2)
that on any well drained soil there is no tendency of the stone to
work down or of the soil to work up; (3) that tile drainage and
macadam construction are cheaper than the telford system; and
(4) . that since the introduction of the machine rock-breaker, it is
cheaper to crush the stone and lay the macadam foundation than
to place the telford.
The view taken by different road builders in this matter is prob-
ably largely due to the conditions in the vicinity in which they have
worked and to the skill with which the two systems have been applied
in work which has come under their observation. The foundation
which is proper in a given case is determined by the nature and con-
dition of the soil upon which it is constructed. If the road-bed is
thoroughly drained and is composed of material which will not readily
soften, there will be no need of a telford foundation. If, on the other
hand, the soil is retentive of moisture and can not be thoroughly
drained, it may be necessary to provide a foundation which will
•prevent the soil from working up into the stone and the road metal
from working down into the soil.
To MacAdam is due the credit of discovering the supporting
power of a layer of comparatively small angular fragments of stone.
351. Forms of the Subgrade. The finished surface of the road
should have sufficient crown to shed the rain water into the side
ditches. There are in common use two methods of securing this
crown. In one the earth surface is made level, and the slope is given
by a greater thickness of metaling at the center than at the sides; in
the other, the slope or camber is given to the earth bed, and the metal
has a uniform thickness. The advocates of the first system say that
there is more wear at the center than at the sides, and that conse-
quently the metaling should be thicker at the center. Those in
favor of the uniform thickness say that as the pressure on the earth
is practically the same at the sides as at the center, the thickness
should be uniform, since the principal object of the layer of stone
is to distribute the concentrated pressure of the wheel over a greater
surface of the earth bed. Both forms of construction are in common
use, although the preference seems to be slightly in favor of making
the subgrade parallel to the finished road-surface and the stone of
uniform thickness. A level subgrade is slightly cheaper to form.
Fig. 46 shows a cross section of the celebrated Shrewsbury
and Holyhead road in the west of England, built by Telford
in 1815. The construction of this road, which formed a link in
ART. 2] CONSTRUCTION
the direct line of communication between England and Ireland,
was made a national undertaking, and resulted in what was at that
FIG. 46. — TELFOKD'S SHREWSBURY AND HOLYHEAD ROAD.
time one of the finest pieces of road construction in the world. Notice
that the subgrade is flat.
Fig. 47 shows a modern telford road as built in New Jersey.
7/7
FIG. 47. — MODERN TELFORD ROAD AS BUILT IN NEW JERSEY.
Notice that the base of the foundation is parallel to the -surface of
the finished road.
Compare the above with Fig. 51-56, pages 197-99.
352. WIDTH. For a discussion of the principles governing the
the width of the improved way, and also whether it shall be in the
center or at the side of the wheelway, see § 95-98.
353. Shoulders. The discussion referred to above deals only
with the width of the paved portion; but there should be an addi-
tional width of earth sufficient to keep the broken stone in place,
particularly while being rolled. This strip of earth is usually called
a shoulder, but sometimes and improperly a wing (see § 364). The
proper width of the shoulder will depend upon the soil, the climate,
and the amount of rolling it receives. Usually 2 or 3 feet is suf-
ficient, although 5 to 7 feet is frequently provided — see Fig. 47.
The Swiss road shown in Fig. 53, page 198, has a shoulder of only 18
inches. An excessive width of shoulder adds greatly to the cost of the
road when in excavation or on embankment. The surface of the
shoulder should conform to the general curve of the finished road-
way. The shoulder serves the double purpose of holding the broken
stone in place and of affording room for vehicles to pass each other.
To improve the shoulders for the second purpose, they are some-
times covered with a thin coat of gravel to harden the surface.
Sand shoulders are speedily hardened by the infiltration of fine stone
192 WATER-BOUND MACADAM ROADS [CHAP. VI
dust and dirt washed from the surface of the road. This effect is
quite noticeable with coarse sand; and is appreciable even with fine
sand.
354. CROWN. The center of the road should be higher than the
sides, so that the water from rains may flow rapidly into the side
ditches. If originally too flat, the road is soon worn hollow, and
the middle becomes a pool if on level ground, or a water course
if on an inclination. In the former case the middle of the road is
sloppy; and in the latter, the fine material washes away and leaves
the larger stones bare. There has been much discussion both as to
the proper amount of crown and the exact form of the transverse
profile of the roadway.
355. Form of the Crown. Some claim that the upper surface
should be curved, and others that it should be two inclined planes
meeting at the center of the road and having their angle slightly
rounded off. Both forms are in common use; the first is the more
common, but apparently the latter is the better.
The following objections are urged against the curved profile:
1. The greater slope near the side causes vehicles to seek the center,
and consequently the road wears unequally. 2. Owing to the excess
of travel at the center, the road soon wears hollow and holds water,
which is both unsightly and a damage to the road. 3. The slope
is so slight near the center that a small settlement of the subgrade
causes a depression of the surface, which holds water.
The only objection to a surface composed of two planes is that
the flanks wear hollow and hold water; but this objection has less
force than any of the three against the curved profile.
Regularity and evenness of crown is more important than the
mathematical form of the cross section. A slight depression be-
comes very conspicuous when filled with water; and besides the
water standing upon the surface softens it and tends to increase
the depression. With a little care in filling the low places devel-
oped during the rolling, it is possible to build a broken-stone road
with an almost mathematically exact crown.
356. Amount of Crown. The proper amount of crown depends
chiefly upon the method of making repairs. If new material is
added only, say, each second or third time the surface is smoothed
up, then the crown should be greater to compensate for future
wear; but if new material is added practically continuously, the
crown may be considerably smaller. The rate of transverse slope
should be smaller on wide than on narrow streets, to prevent the
AUT. 2] CONSTRUCTION 193
water from unduly washing the surface near the sides. There
should be more crown on steep grades than on flat ones, to throw
the water quickly to the side ditch and to prevent it from flowing
down the grade on the surface of the road and washing out the
binder.
Sometimes wide boulevards, with curved profile and maintained
by continuous repairs, have a crown of one sixtieth of the width, or a
rise of 0.4 inch per foot from side to center, or an average slope of
1 in 30. The French roads, which have a curved profile and are
maintained by the system of continuous repairs, have a crown of one
fiftieth of their width, or a rise from side to center of 0.5 inch per
foot or a slope of 1 in 25. Many of the better cared for streets and
park drives have a crown of one fortieth, or a rise from side to center
of 0.6 inch per foot or an average slope of 1 in 20. On the state-aid
roads in Massachusetts (narrow roads and continuous repairs), the
surface consists of two planes meeting in the center, the transverse
slope being f inch to a foot or 1 in 16. Broken-stone roads made of
soft stone and maintained by periodic repairs frequently have an
original crown of one twelfth — an average slope of 1 inch to 1 foot
or 1 in 12.
357. With a broken-stone road, the method of making repairs
has more weight in determining the amount of the crown than in the
case of either an earth road or a gravel road. The earth road is easily
and cheaply maintained by what may be called the system of con-
tinuous repairs with the road drag, which restores or rather main-
tains the crown. With a gravel road, when it is necessary to restore
the crown by adding more gravel, it is usually sufficient to put on
only a thin layer and wait a comparatively short time for travel to
consolidate it. With a water-bound macadam road, if the crown or
rather the surface is to be perpetually maintained, it is necessary to
keep a man upon a short stretch of the road practically all of the
time, adding thin patches of stone in first one place and then another,
a method so expensive that it is practiced in this country only on
park drives, boulevards, etc.; and if the crown is to be restored
periodically, it is necessary to add a considerable layer of stone and
consolidate it by long continued and expensive rolling and sprinkling,
and on' account of the expense of this operation and the obstruction
to traffic it is customary to lay such a thickness of stone and to give
the surface such a crown as not soon to require a repetition of the
process. Therefore it happens that broken-stone roads are often
built with a crown nearly, if not quite, equal to that of gcxxl
194 WATER-BOUND MACADAM ROADS [CHAP. VI
earth roads, and with more perhaps than is given to good gravel
roads.
358. There is a slight advantage of a very high crown for a broken-
stone road, particularly for one that is not frequently cleaned. If
the crown is great, the rains will the better wash the surface. Dirt
upon the surface is not only unsightly, but is also detrimental since
it holds the water and softens the surface. Of course the material
washed by rains into the gutter must eventually be removed; but
this can be removed more cheaply from the gutter at comparatively
long intervals, than from the surface with brooms or scrapers at
short intervals. The practice of making a high crown is somewhat
common in villages using soft road metal and having earth gutters
and only surface drainage.
This advantage of a high crown is less for a country road than
for a village street, since the wind usually gets a better sweep at the
former than at the latter.
359. Super-elevation on Curves. For a rule for the super-eleva-
tion of the road surface on curves, see § 90.
360. THICKNESS. The object of placing a layer c: broken
stone upon the trackway is to secure (1) a smooth hard surface,
(2) a water-tight roof, and (3) a more or less rigid stratum which
will distribute the concentrated pressure of the wheel over so great
an area of the subgrade that the soil can support the load without
indentation. The smoothness and tightness of the surface depends
upon the quantity and quality of the binding material (§ 383-87),
and the rigidity of the layer depends somewhat upon the binder,
but chiefly upon the thickness of the stratum. The supporting
power of the subgrade depends upon the nature of the soil and the
drainage. Therefore the minimum thickness of broken stone depends
upon the nature of the soil, the drainage, the traffic, and the binding
material; and the initial thickness depends upon the amount of wear
permitted before new material is added. If repairs are continuous,
the initial thickness may be the minimum; but if repairs are made
periodically, the initial thickness must be equal to the minimum
thickness plus the amount allowed for wear. After a road has been
worn down 3 inches, it is usually so uneven as to require re-surfacing;
and therefore it is uneconomical if the road in this stage is much or
any thicker than the minimum required to prevent its breaking
through.
There has been much discussion and there is a great difference
of opinion as to the proper depth of a broken-stone road. The
ART. 2] CONSTRUCTION 195
depth considered necessary by the most extreme advocates of thick
roads has decreased with the introduction of more improved methods
of construction — particularly the use of binder and a steam roller, —
and as the advantage of thorough underdrainage has been better
appreciated. Early in the last century, a depth of 18 to 24 inches
was frequently considered necessary for heavy traffic, but later it
was reduced to 12 or 15 inches, while now 6 inches, or less, is usually
considered sufficient.
361. Theoretical Thickness. The concentrated load of a wheel is
transmitted through the broken stone to the earth in lines diverging
downward, and the wheel may be assumed as resting upon the apex
of a cone whose base is upon the earth subgrade. It is not wise to
attempt to find a mathematical relation between tht load on the
wheel and the resulting pressure on the earth, since neither the
angle of the cone nor the distribution of the pressure on the base
of the cone are known.
The Massachusetts Highway Commission assumes the concen-
trated load to be uniformly distributed over an area equal to the
square of twice the thickness of the layer of crushed stone, which is
equivalent to assuming that the sides of the cone make an angle of
48 \ degrees with the vertical and that the pressure is uniformly dis-
tributed over the base. According to this theory, if t = the thick-
ness of the stone in inches, w = the maximum weight in pounds
per wheel, and p = the supporting power of the soil in pounds
per square inch, then
The Commission has applied this formula to roads already con-
structed to determine the safe bearing power of the soil, and con-
cludes that non-porous soils, drained of - ground water, at their
worst will support a load of 4 Ib. per square inch, and that sand and
gravel will safely support 20 Ib. per square inch.*
Although the method of arriving at equation (1) is not correct,
the manner of deducing the supporting power of the soil in a measure
offsets the error, and consequently the formula may be used with
some confidence.
362. Actual Thickness. In Massachusetts the thickness for
state-aid roads varies from 4 to 16 inches, the standard for crushed
* Massachusetts Highway Commission, Report for 1901, p. 15,
196 WATER-BOUND MACADAM ROADS [CHAP. VI
stone with macadam foundation on well-drained sand or gravel being
6 inches, " which appears to be ample for the heaviest traffic."
In New Jersey, on state-aid roads, the depth of stone with mac-
adam foundation varies from 4 to 12 inches, but is generally 6 inches;
and the telford roads are from 8 to 12 inches thick, usually 8 inches.
Most of the roads have a macadam foundation, the telford being
used as a rule only where field, stones suitable for a telford foundation
are found alongside of the road.
363. The experience at Bridgeport, Conn., has been frequently
cited to prove that a comparatively thin road is sufficient. Some-
thing like 60 miles of 4-inch macadam roads built in that place gave
excellent service even under heavy traffic. The conditions were
very favorable for a thin road: (1) the soil was sand or sandy loam,
and had fairly good natural drainage; (2) the subgrade was thor-
oughly rolled Vith a 15-ton roller; (3) the broken stone was trap,
which is hard and durable; (4) the binder was hard and durable,
being either stone dust or siliceous sand, and was free from clay or
loam; (5) the binder was worked in until the voids in the crushed
trap were practically filled, the effect of frost being thus reduced to a
minimum and the soil being prevented from working up from below;
(6) the stone was thoroughly consolidated with a steam roller of
adequate weight; and (7) the roads were maintained by the system
of continuous repairs.
The experience at Bridgeport has been repeated at several other
places; but such experiences should be regarded as the exception,
rather than the rule, since 4-inch roads are adequate only under
favorable natural conditions and with the most painstaking con-
struction and careful maintenance. The fact that a very thin road
can carry the traffic does not prove that such a road is the most
economical, for the increased cost of maintenance may more than
counter-balance the decreased cost of construction. The engineer
should always attempt to construct economically and adapt his
construction to fit the natural conditions.
364. Wings. In the preceding discussion of the thickness of
the road metal it has been assumed that the depth was practically
uniform; but some engineers, in recognition of the fact that there is
less travel nearer the sides than at the center, make the thickness of
a strip on each side considerably less than that at the center. The
thin strips on the sides are called wings. Fig. 48, a portion of the
Swedesboro road in Gloucester County, New Jersey, shows a cross
section of this form. This construction is somewhat common in
ART. 2]
CONSTRUCTION
197
New Jersey, both with telford and macadam foundations, and has
been adopted by the U. S. A. engineers for macadam roads in Porto
Rico. The wings are usually 2 or 2| feet wide. A road with wings
M--- 7ft **?25ft^ 9ft
jrMorzu&n w/nqs-^^ \
— 7ft
I
Walk
wmmmmm
FIG. 48. — NEW JERSEY TELFORD ROAD WITH MACADAM WINGS.
FIG. 49. — STANDARD SECTION FOR NEW YORK STATE-AID ROADS.
is simply a compromise between a narrow thick road and a wide
thin one.
365. EXAMPLES OF CROSS SECTIONS. Fig. 46, page 191, shows
a cross section of a telford road built under Telford's direction in
FIG. 50. — GENERAL SECTION OF FLUSHING AND JAMAICA ROAD.
K3/T- •»*--*- 7/7 6in - - -++* --7ft 6117 »t<5 ft->]
Fia. 51. — STANDARD SECTION IN EXCAVATION FOR MASSACHUSETTS STATE-AID ROADS.
1815. Fig. 47, page 191, shows a New Jersey telford road. Fig.
48 shows a telford road with macadam wings. Fig. 49 shows the
standard cross section for state-aid roads in the State of New York.
Fig. 50 is a section of a road in Flushing, Long Island, near New York
198
WATER-BOUND MACADAM ROADS
CHAP. VI
City. Fig. 51 and 52, are the standard cross sections in excavation
and on embankment, respectively, for state-aid roads in Massachu-
25 ft
15 ft »K- 4ft -
Y/////////////////////////////////,
CROSS SECTION OF ROAD
-fr — ^"-H
LONGITUDINAL SECTION
FIG. 52. — STANDARD SECTION ON EMBANKMENT FOR MASSACHUSETTS STATE-AID ROADS.
FIG. 53. — CLASS-!! ROAD, CANTON OF BERN, SWITZERLAND.
8M%da S
FIG. 54.— CLASS-IH ROAD, CANTON OF BERN, SWITZERLAND
Fio. 55.— TYPICAL ROAD IN DEPARTMENT OF BAS-RHIN, FRANCE.
setts. Fig. 53 and 54 show two Swiss roads. Fig. 55 shows a typi-
cal road in the Department of Bas-Rhin, France. The broken
ART. 2] CONSTRUCTION 199
stone is 6 inches deep. Fig. 56 is a typical French road in the
Department of Seine-et-Oise.
FIG. 56. — TPYICAL ROAD IN DEPARTMENT OF SEINE-ET-OISE, FRANCE.
366. PERMISSIBLE GRADES. For a general discussion of the
subject of maximum and minimum grades, see § 79-86. The fol-
lowing examples of maximum grades for water-bound macadam roads
are instructive.
In France the standard is: on national roads, not exceeding
3 per cent; departmental roads, not exceeding 4 per cent; and
subordinate roads, not exceeding 6 per cent. On the great Alpine
road over the Simplon Pass, built under the direction of Napoleon
Bonaparte, the grades average 1 in 22 (4J%) on the Italian side,
and 1 in 17 (5.9%) on the Swiss side, and in only one case become
as steep as 1 in 13 (7.7%).
In Great Britain, the celebrated Holy head road, built by Tel-
ford through the very mountainous district of North Wales, has an
ordinary maximum of 1 in 30 (3f %), with one piece of 1 in 22 (4|%)
and a very short piece of 1 in 17 (5.9%), on both of which pieces
special care was taken to make the surface harder and smoother
than on the remainder of the road.
On the National Pike over the Alleghenies, built before the intro-
duction of the railroad, the maximum was 7 per cent. At an early
day the New York law limited the grades of turnpikes (toll roads) to
1 in 11 (9%).
In New York on state-aid roads the nominal maximum is 5 per
cent, but grades as steep as 6j per cent have been found necessary
in some cases. In New Jersey are a number of state-aid roads having
grades of 7 and 8 per cent, and one has 10J per cent. In Massa-
chusetts no hard-and-fast standard has been adopted for the state-
aid roads, but a few have 5 per cent grades and a considerable num-
ber have 4 per cent grades. It is said that on some important
Massachusetts roads the grade can not at reasonable expense be
reduced below 7 per cent.
367. In improving city streets it is often impossible to make any
radical change in the grade owing to the resulting damage to abut-
200 WATER-BOUND MACADAM ROADS [CHAP. VI
ting property, and it is almost impossible to avoid the steep grade
by a change of location; and consequently some city streets have
very steep grades which are used with surprisingly good results.
Newton, Mass., has a number of water-bound macadam streets
which have long stretches of 9 and 10 per cent grades, and has one
12 per cent grade 1,000 feet long. Waltham, Mass., has one 400-
foot stretch of water-bound macadam on a 12 per cent grade, and
another on a 13 per cent grade. In the Borough of Richmond
(Staten Island), New York City, are several pieces of 10, 11, and 12
per cent grades, and 100 feet of 14 per cent, two stretches of 200 feet
each of 16 per cent, and one piece 200 feet long of 20 per cent grade.
368. PREPARING THE SUBGRADE. The broken stone is designed
to take the wear of hoofs and wheels, but the earth foundation must
support the load ; and therefore any road which is constructed with-
out giving due ' attention to the earth road-bed is wrong from the
start, and will never be a good road until the defect is remedied.
For instructions concerning the construction of embankments
and excavations, see § 132-35. In building an embankment upon
which broken stone is to be laid, every reasonable care should be
taken to prevent uneven settlement. It is sometimes advisable
to delay the laying of madacam for at least a year in order to give
the embankment time to settle, for it is impossible to construct an
embankment of earth more than a few feet in height without having
subsequent settlement. If this settling took place evenly all along
the embankment, no particular harm would be done to the mac-
adam laid upon it; but owing to the difference in the soils composing
embankments and also in the way the earth is dumped, there is
always a tendency for some parts to settle more than others.
Sometimes the road surface is placed so low that it forms a gutter
to dram the adjacent fields, which of course is very objectionable.
Occasionally the earth from the side ditches and from the trench in
which the stone is placed, is deposited at the side of the right-of-way
instead of being used to raise the road surface. In this connection,
see § 139.
369. After the subgrade has been brought to the proper form
(§ 351), it should be rolled thoroughly— both to consolidate it and to
discover soft spots. For a discussion of road rollers, see § 378-79.
Fig. 57 shows the method of smoothing the subgrade with a
scraping grader; and also shows the rolling of the shoulder. Fig.
58 shows the subgrade after the rolling is completed.
In rolling; quicksand spots are sometimes discovered, in which
ART. 2]
CONSTRUCTION
201
case the troublesome material should be excavated and suitable
material substituted. If the road-bed be of sand or of material of
such a nature as to push along in a wave in front of the roller, a
thin layer of broken stone or gravel strewn over the surface will
FIG. 57. — SMOOTHING SUBGRADE AND ROLLING SHOULDER,
FIG. 58. — SUBGRADE ROLLED AND READY FOR STONE.
enable the roller to consolidate the road-bed. If the surface is clay
that sticks to the roller, sprinkle a thin layer of sand or cinders
over the surface. If the clay is soft and forms a wave in front of the
roller, additional rolling is a detriment, as it increases the plasticity
of the clay.
370. SETTING THE TELFORD. The distinguishing feature of a
telford road is its paved foundation. After the road-bed has been
brought to the proper form and been rolled, rough stones are set
upon the surface to form a pavement 5 to 8 inches thick, the thick-
ness depending upon that to be given to the finished road (§ 360),
the general practice being to make the paved foundation about two
thirds of the total thickness of the road. The practice of Telford
202 WATER-BOUND MACADAM ROADS [CHAP. VI
was to grade the road-bed flat, and then construct his pavement
deeper in the middle than at the sides, using for a roadway 16 feet
wide, stones about 8 inches deep at the middle and 5 inches at the
sides. This practice is still followed by some engineers; but it is now
more common and usually considered preferable to make the surface
of the road-bed parallel to the finished surface, and the pavement of
uniform thickness. Fig. 46, page 191, shows a telford road with a
level subgrade; and Fig. 47, page 191, a telford road with the sub-
grade parallel to the finished surface.
The size of the stones for the telford pavement is of no great
importance, at least there is a great difference in the practice of the
best road builders. The width of the stones varies from 3 to 10
inches, 3 to 6 being most common; and the length varies from 6
to 20 inches, 8 to 12 being most common. It is desirable to have
the width on any particular job somewhat nearly uniform, and the
stones in any course should be still more nearly equal. The stones
are set upon their widest edge with their greatest length across the
road, the -joints being broken as much as possible. Each stone
should stand independently of its neighbor, i. e., one stone should
not lean against another. The irregularities of the upper surface
are then broken off with a hammer, and the interstices between
the stones are filled with spalls lightly driven into place with a ham-
mer or a crow-bar. This knocking off of the projecting points and
the driving of spalls into the interstices should not be done so near
the face of the pavement as to dislocate the stones last set. It is
frequently specified that no wedging shall be done within 10 or 15
feet of the front edge of the pavement. After the projecting points
have been knocked off and the interstices have been filled with stone
chips or ordinary crushed stone, the pavement is usually rolled. It
is usually specified that the roller shall not go nearer to the front of
the pavement than 25 to 30 feet.
The cardinal requisite of a telford foundation is the interlocking
of the stone closely and compactly together by barring, wedging,
and rolling until the entire structure is brought in action to resist
disturbance as a single mass.
371. CRUSHING THE STONE. The introduction of a machine
for breaking the material greatly cheapened the cost of broken-
stone roads. The rock crusher was introduced into America in
1860, before which time the stone was broken by hand with ham-
mers on the side of the road. Coincident with the introduction
of power for breaking the stone, came the revolving screen which
ART. 2]
CONSTRUCTION
203
permitted the fragments to be assorted as to size — an important
feature, as will soon be shown.
372. Forms of Crushers. There are two types of crushers now
in common use. The older one, often called the Blake after the
original inventor, consists of a strong iron frame, near one end of
which is a movable jaw. By means of a toggle-joint and an eccen-
tric, this jaw is moved backward and forward a slight distance. As
the jaw recedes the opening increases and the stone descends; as the
jaw again approaches the frame, the stone is crushed. The maxi-
mum size of the product is determined by the distance the jaw plates
FIG. 59. — OSCILLATORY STONE CRUSHER.
are from each other at their lower edge. This machine is also fre-
quently called the oscillatory breaker, or jaw breaker. Fig. 59 shows
one form of this type. The size of the product is regulated by
raising or lowering the wedge 10, or by inserting a different pair of
toggles,— 7.
The second form of crusher, called the Gates after the original
inventor, consists of a solid conical iron shaft which is supported
within a heavy iron receptacle shaped somewhat like an inverted
bell. By means of an eccentric bearing a rocking and rotary motion
is given to the shaft, so that each point of its surface is successively
brought near to and removed from the surface of the bell, which
causes the stone to be successively crushed as it descends. Fig. 60
204
WATER-BOUND MACADAM ROADS
[CHAP, vi
shows one form of this type of crusher. An adjustment permits a
variation in the size of the product. This form is often called the
rotary breaker or gyratory breaker.
It is not wise here to consider the relative merits of the different
forms and sizes of stone crushers, the power required, the output,
FIG. 60. — GYRATOKY ROCK CRUSHER.
etc., since the construction of a reasonably good macadam road
requires a large equipment of machinery and an experienced con-
tractor, and since the equipment varies with the conditions.
Fig. 61 gives a hint as to the arrangement of the crusher, the
elevator, the screens, and the storage bins. Fig. 62, shows a real
crushing plant at Green Lake, Wis.
373. SIZES OF STONE. The size of stone used for road metal
depends upon the hardness and toughness of the stone and upon
the weight of the traffic. The harder and tougher the material,
the smaller it may be broken without danger of its crushing or shat-
ART. 2]
CONSTRUCTION"
205
FIG. 61. — DIAGRAMMATIC ARRANGEMENT OF STONE-CRUSHING PLANT.
?
FIG. 62. — STONE-CRUSHING PLANT.
206 WATER-SOUND MACADAM ROADS [CHAP, vt
tering under the load of wheels and the impact of hoofs; and the
harder and tougher a stone, the smaller it must be broken in order
that it may compact well in the road. The stones in the top course
should be larger for heavy traffic than for light traffic, to prevent
their being ground to powder. Larger stones CPU be used in the
bottom layers of a road than at the top.
One of Mac Adam's rules was to exclude any fragment weigh-
ing more than 6 ounces. A 1 J-inch cube of compact limestone weighs
about 6 ounces. Another of MacAdam's rules was to exclude any
stone that could not readily be put into a man's mouth. These rules
are frequently quoted, even now, although improvements in road
machinery have made them inappropriate with present methods.
The bottom course of a macadam road built of soft stones is
often composed of fragments 3 to 4 inches in greatest dimensions;
but if it is built of hard tough stone, the sizes are 2 to 2| inches.
The size of rock in the lower courses is not so important as that for
the surface course (see § 374). The top course of hard tough stones
is usually 1 to 2 inches for heavy traffic, and J to 1 inch for light
traffic.
The custom is to lay the stone in courses of substantially one size,
although some road builders prefer to have the sizes mixed when
thrown into the road. The only advantage of the latter practice is
that with a skilful proportioning of the sizes less rolling is required;
but it is objectionable owing to the difficulty of getting the several
sizes properly proportioned and keeping them thoroughly mixed.
There is generally too much fine material in the mixed sizes, which
makes the road wear rapidly and unevenly.
Connected with the crusher and run with the same power is
generally a rotary screen having meshes of three sizes — usually
about i, 1J, and 2J inches.
374. For economic reasons the size of stone in the several courses
and their thickness should be adjusted so as to use, if possible, all
of the output of the crusher. The output of the various sizes varies
considerably with the character of the stone. With a hard stone,
half or more of the product of the crusher will not pass through the
|-inch screen; while with field stones one half may pass through
such a screen. The last gives more " fines " or " screenings "
than can be used profitably during construction, but the surplus is
very useful in maintaining the surface. With some rocks it is diffi-
cult to get enough fine material for use in the original construc-
tion.
ART. 2]
CONSTRUCTION
207
375. SPREADING THE STONE. The stone is usually hauled
from the crusher to the road in wagons or trucks, dumped upon the
roadway, and spread with forks or rakes. Dumping in place is objec-
tionable, since the coarse and fine fragments become separated in
FIG. 63. — AUTO TRUCK DUMPING STONE.
the process, producing a layer of unequal density and an irregular
surface after rolling. It is sometimes specified that the stone
FIG. 64. — SPREADING STONE WITH "RAKES. FIG. 65. — SPREADING STONE WITH SHOVELS.
shall be dumped upon a platform, from which it is distributed
with shovels. This method of spreading costs 4 to 6 cents per
cubic yard — about twice that by dumping and raking and is
appropriate only when the very best results are sought. Wagons and
208
WATER-BOUND MACADAM ROADS
[CHAP. VI
trucks are upon the market which can automatically dump and dis-
tribute the stone in layers of uniform thickness; but owing to their
FIG. 66. — SHTJART GRADER.
cost and weight they are not in very general use. Fig. 63 shows an
auto truck dumping the stone in a ridge on the subgrade.
The stone is sometimes spread by hand with rakes and shovels —
see Fig. 64 and 65. Notice the template in Fig. 65 used to gage
the thickness of the layer of stone.
There are several methods of spreading the stone by machinery.
Some contractors use the Shuart grader, Fig. 66, a machine that
FIG. 67. — HABBOWING STONE.'
68. — LEVELING STONE WITH
SCRAPING GRADER.
was devised for use in leveling ground that is to be irrigated,
Other contractors level the stone with a harrow as shown in Fig. 67.
ART. 2 1 CONSTRUCTION 209
Still other contractors use a scraping grader to level the stone —
Fig. 68. Fig. 69 shows the bottom course of stone ready for rolling.
The stone should be applied in uniform layers, the thickness
of each depending upon the total thickness of the road. Two
methods are in use for gaging the thickness of the layers of stone.
1. On the finished subgrade, wood cubes of a depth equal to the
thickness of the layer are set at frequent intervals, and the loose
FIG. 69.— BOTTOM COTTBSE READY FOB ROLLING.
stone is laid even with the tops of these blocks. This method is
sometimes described as building by blocks, and is the one employed
on the state-aid roads of New Jersey. 2. The soil is brought to an
established grade, and the finished road is required to be brought
to another established grade, in which case neither the absolute
thickness nor the uniformity of the several courses is a matter of
much importance. This method is employed on the state-aid roads
in Massachusetts.
376. SHRINKAGE IN ROLLING. Before beginning to spread the
layers of stone, it is necessary to determine the amount the crushed
stone will shrink in rolling. The shrinkage has an important bearing
upon the thickness and cost of the finished road; but great errors
are sometimes made in estimating the amount of shrinkage. The
following examples from practice show the actual shrinkage.
210 WATER-BOUND MACADAM ROADS [CHAP. VI
In one case,* with trap rock If to 2J inches, rolled with a 12|-ton
steam roller upon a subgrade so hard that the wagons hauling the
stone made no ruts, 5.67 inches of loose stone rolled to 4 inches,
and 7.38 inches rolled to 6 inches. The average thickness of the
loose "stone was determined by dividing the quantity of stone used
by the area covered. The first is a shrinkage of 29 per cent and
the second of 19 per cent. The difference between these two results
is probably due to errors of observation, to variations in the thick-
ness of the finished road, and to the fact that the thicker layers did
not compact as solidly as the thinner ones. The stone was rolled
dry until the desired thickness was reached, when the binder was
added, and sprinkling was commenced.
In another case,f with 2-inch trap laid on the compact surface
of an old crushed-stone road and rolled with a 12-ton roller, 3.9
inches of loose stone rolled to 3 inches. The shrinkage was 23 per
cent. The thickness was determined from the area covered and
the quantity of stone used. No stone could have been forced into
the subgrade, but there was some uncertainty as to the average
elevation of the surface of the old street.
It has been determined { by tests over several miles of road
where the output of the crusher was carefully measured in wagons
and also when rolled in place, that 6 inches of loose hard limestone
rolled down to 4f inches, which is a shrinkage of 20 per cent.
377. It is probable that the maximum actual shrinkage in rolling
is less than 20 per cent. The apparent shrinkage depends upon the
nature and condition of the subgrade, i. e., upon the amount of stone
forced into the earth.
If the soil is clay, the sprinkling required to work the binder
into the interstices may soften the subgrade so that considerable
stone will be forced into the earth. This condition is indicated by
the roller's leaving tracks upon the surface; and when this occurs, the
work should be stopped until the subgrade dries out. To prevent
the crushed stone from being forced into the clay subgrade during
construction or after completion — particularly when the frost is
going out, — a layer of sand, stone screenings, ashes, or the like,
is sometimes interposed. The English engineers often use " hard
core " (a mixture of brick rubbish, old plastering, and broken stone)
on a clay soil, to prevent the mud's working into the metaling. Any
* W. C. Foster in Trans. Amer. Soc. of Civil Eng'rs, Vol. 41, p. 135-38.
t F. G. Cudworth in Trans. Amer. Soc. of Civil Eng'rs, Vol. 41, p. 126-28.
t H. P. Gillette in Economics of Road Construction, p. 19-20. New York, 1901.
ART. 2] CONSTRUCTION 211
material not affected by water is useful for this purpose; and the
finer it is the better, since the smaller will be the apertures in it, and
the more certainly will it prevent the soil from coming up through it.
If the soil is sandy, a thin layer of coarse gravel or broken stone
laid upon the surface and then rolled, will prevent any further loss
of the road metal in the subgrade. If the soil is nearly pure sand,
the wetter it is the less crushed stone will be forced into it; and
therefore if water is plentiful, it may be wise to keep the sand satu-
rated while the rolling is in progress to prevent the loss of the stone.
The Massachusetts Highway Commission used cotton cheese-cloth
on a soft fine sand to prevent the stone from sinking into the sub-
grade. " It is not at all needful that the partition should be endur-
ing, for as soon as the stones in the lower layer have been forced
into contact and have become bound together, there is no further
danger of the mingling of the stone with the sand; and hence the
decay of the fabric is a matter of no consequence. The cloth was
spread in strips lengthwise of the way; and the stone for the bottom
layer was shoveled from the sides upon it with no unusual care. A
section through such a road showed that the stones did not tear
through the cloth. At 3 cents per square yard on the road, the cost
of the cloth may be less than one third that due to the loss of the
broken stone which would occur if it were allowed to come directly in
contact with the sand. Various kinds of strong paper were tried,
but found worthless." A thick coating of straw has been used to
hold up the macadam on a sandy soil.
However, if the sand is firm enough to hold up the stone during
the rolling, it is not necessary to prevent the mixing of the sand
and the stone, since the subgrade may be left a little high, with
the expectation of forcing the stone into the sand. This is equiva-
lent to using the sand of the subgrade as a filler or binder for the
lower portion of the broken stone. If the sand is dry and nearly
pure, it can be thus forced nearly to the top of a 4-inch course of
coarse broken stone.
378. ROAD ROLLERS. The roller is indispensable for the eco-
nomic construction of water-bound macadam roads. Roads can be
built without the use of a roller, but always at large expense to the
traffic and with great waste of the road metal; and such roads never
have as smooth a surface and are not as durable as if a roller had been
employed in their construction. With traffic-consolidated roads,
much of the metal is worn round and smooth before the fragments
become firmly fixed in place: and the dirt brought upon the road by
212 WATER-BOUND MACADAM ROADS [CHAP. VI
the traffic mixes with the stone and prevents it from ever packing as
solidly as the clean stone would, and, besides, the dirt when wet
has a lubricating effect upon the stone which under the action of
traffic causes the surface to break up readily. Further, during
the time travel is consolidating the stone, the surface is not even
approximately water-tight; and therefore the subgrade is softened by
rains, and the stone is mixed with the earth below and virtually lost.
FIG. 70. — THREE-WHEEL OR MACADAM ROLLER.
Ordinarily, it is true economy to compact the road by the use of a
roller.
Formerly both horse and steam rollers were employed; but now
only the latter are used. There are two type forms of steam, or
rather power, road-rollers — see Fig. 70 and 71. The first, or three-
wheel type, is the form employed in macadam road construction;
and the second, or tandem type, is the form used in rolling asphalt
pavements and other bituminous road surfaces. Both types are
driven by steam or by gasoline, but the latter is rapidly gaining in
favor. There are a variety of forms of each type, but the essential
features of all are practically the same.
The total weight of the macadam roller varies from 7 to 15 tons;
and the pressure under the drivers varies from 300 to 500 Ib. per linear
inch. The total weight of the tandem roller varies from 2J to 10
ART. 2] CONSTRUCTION 213
tons, and the pressure under the driving drum from 125 to 300 Ib.
per linear inch and for the lightest roller the compression under the
steering drum is usually about 60 Ib. per linear inch. Of the tandem
FIG. 71. — TANDEM OK ASPHALT ROLLEE.
type the 5- and 8-ton roller are most common, and give a pressure
under the driving drum of about 200 and 280 Ib. per inch respectively.
379. The weight of the roller should be proportional to the hard-
ness of the stone, as too heavy a roller crushes the material instead of
compacting it. An excessively heavy roller will sometimes sink into
light or loose soil, and force it ahead in a wave which the roller can not
surmount. This may sometimes be prevented by spreading a thin
layer of sand or gravel on the surface being rolled. A similar dif-
ficulty sometimes occurs with a heavy roller on a layer of loose
stones. If the front wheels or rollers of the machine were larger,
this difficulty would be decreased. In localities where the soil is of a
loose sandy nature, a roller weighing 10 or 12 tons is usually preferred;
and in districts where the soil is stiff or gravelly clay, a weight of 12
or 15 tons is used. In localities where the road material is hard, a
15-ton roller is necessary; but with the softer stones a weight of 10
or 12 tons is sufficient.
380. ROLLING THE STONE. Rolling is a very important part of
the construction of a water-bound macadam road. The subgrade
should be rolled to prevent the stones' being forced into the earth.
The lower course of the stone should be rolled to compact it so that
the pieces will not move one upon the other under the traffic and the
top course should be rolled to pack or bind the pieces into place.
214 WATER-BOUND MACADAM ROADS [CHAP. VI
Rolling accompanied by sprinkling (see Fig. 72) is necessary also
to work the binding material into the interstices so as to make the
surface water-tight. Roads that have been consolidated by traffic
are largely held together by mud, and after long use are fairly
smooth and hard in dry weather, but become soft and muddy dur-
ing a wet time.
The stone is put on in two or three layers, — according to the total
thickness of the finished road, — and each course is thoroughly rolled
FIG. 72. — SPRINKLING AND ROLLING.
before the next is added. No course should be more than 4 to 6
inches thick. When a telford foundation is used, broken stone is
spread over the pavement to bring the top surface to the proper
form and height, after which it is rolled.
381. The rolling should proceed gradually from both sides toward
the center. If the weight of the roller can be varied, commence
with the unballasted roller, and increase the weight as the stone
becomes consolidated. If the surface of the layer shows a wavy
motion after being rolled three or four times, the subgrade is too wet;
and time should be given it to dry out. Some coarse brittle granitic
rocks begin to crawl and the sharp edges to break off after the roller
has passed over them a few times; but a light sprinkling of sand or
stone screenings will prevent this, and facilitate the consolidation
of the layer. All irregularities of the surface developed by the
rolling should be corrected by filling the depressions with stone of the
size used in the layer.
The rolling should be continued until the stone ceases to. creep
in front of the roller, and until the macadam is firm under the foot
as one walks over it. When the rolling is complete, one of the
ART. 2] CONSTRUCTION 215
larger stones of the course can be crushed under the roller without
indenting the surface of the layer.
When the first course has been consolidated, a second, usually
a thinner one of smaller stones, is added ; and then it is rolled the same
as the first. Finally a third course consisting of about half an inch
of sand or fine stone and stone dust is added. The roller is then
passed over this layer, with the result that the bits are ground to
powder. As the rolling of this course proceeds it is sprinkled, the
aim of the sprinkling and rolling being to work the fine material
into the cavities between the pieces of crushed stone, thus binding
the whole into a solid ma*ss. The proper binding of the road is the
most important part of the construction, and will be more fully
considered presently (see § 383).
382. Amount of Rolling. The total amount of rolling required
varies with the weight of the roller, the hardness and the size of the
stone, and the amount of binder and water used. Trap rock being
very hard requires two or three times as much rolling as most other
stone. An excess of binding material and of water gives a compact
surface with comparatively little rolling, but the road is not as dur-
able as though it had been more thoroughly rolled.
The following examples are representative of the best American
practice.
In New York City, 5 inches of crushed gneiss on telford and
5 inches of trap on the gneiss, bound with trap screenings, was rolled
with a 15-ton steam roller at the rate of 40.6 square yards per hour,
or 10 cubic yards per hour. Although it is common to give the
amount of rolling in terms of the time required, the statement is
somewhat indefinite, since the work accomplished varies with the
speed of the roller and also with the length of run, i. e., with the time
lost in starting and stopping. The usual speed of steam rollers is 2 to
2J miles per hour. The above work is equivalent to 0.553 ton-
miles per square yard, or 2.246 ton-miles per cubic yard. The num-
ber of trips was 130.*
In making repairs, a 6-inch course of 2-inch trap was rolled at
the rate of 26.2 square yards per hour, or 4.4 cubic yards per hour.
The work amounted to about 0.859 ton-miles per square yard, or
5.177 ton-miles per cubic yard. The number of trips over the sur-
face was 201. f
An area of 22,000 square yards 01 a 3-inch course of 2-inch trap
* Trans. Amer Soc. of Civil Engineers, Vol. 8, p. 105-6.
jlbid., p. 107.
216 WATER-BOUND MACADAM ROADS [CHAP. VI
upon an old broken-stone road, bound with trap-rock screenings
and rolled with a 10-ton steam roller, was finished at an average
rate of 47.15 square yards per hour of rolling, the extremes being 38.4
and 61.1 square yards per hour. This was an average of about 4.0
cubic yards per hour. *
A 6-inch course of If- to 2J-inch trap rock, bound with lime-
stone screenings, was rolled with a 12^-ton steam roller at an average
rate of 31.4 square yards per hour, or 5.2 cubic yards per hour.f
The Hudson County Boulevard (Jersey City, N. J.) consists
of 8 inches of telford, 2^ inches of 2|-inch stone, 1J inches of 1|-
inch stone, and then \ to 1 inch of coarse screenings — all trap rock.
The macadam top was supposed to roll down to 4 inches, i. e., 4|
to 5 inches of loose stone was supposed to roll to 4 inches. The
rolling was distributed about as follows: On the telford, 10 to 12
passages; on the 2^-inch course, 8 to 10 passages; on the IJ-inch
course, 10 to 12; and on the screenings, 80 to 90, — making a total
of 100 to 120 passages of the roller over the road.
383. BINDING THE ROAD. The interstices between the frag-
ments of stone should be filled with a fine material which will act
mechanically to keep out the rain water and thereby keep the
subgrade dry, and also to support the fragments and prevent them
from being broken, and which will bind or cement the fragments
into a single more or less solid mass. The proper binding of the
stone is the most important part of the construction of a water-
bound macadam road.
384. Nature of the Binder. The binding material or the filler
should be finely divided so as to be easily worked into the interstices,
should have a considerable resistance to crushing so as to properly
support the pieces of crushed stone, and should not change its phys-
ical condition when wet. Various materials have been employed
— clay, loam, shale, sand, and limestone and trap-rock screenings.
Clay and loam are frequently used. Their merit is that they are
cheap, are easily applied and have a high cementing power; but they
are easily affected by water and frost, and when wet act more as a
lubricant than as a binder. Clay or loam binder will give a smooth
road without much rolling, but such a road is liable to be very dusty
in dry weather, and muddy in wet weather. When clay or loam is
employed as a binder, the utmost care should be taken that no more
is used than just enough to fill the voids.
* Trans. Amer. Soc. of Civil Engineers, Vol. 41, p. 127.
., P. 138.
ART. 2] CONSTRUCTION 217
Shale and slate are only hard and compact clay, and their only
merit is that they give a smooth surface with but little rolling. They
are speedily reduced to dust, and then have all the disadvantages
of clay. They have only fair cementing power.
Sand is often used as a filler, and if composed of fine, clean, hard
grains, gives fair results; but sand which is resistant enough for a
good binding material usually consists of silica or quartz, neither
of which has a high cementing power. If the grains are coated more
or less with iron oxide, or if accompanied by bits of ironstone (clay
cemented with iron oxide), sand makes an excellent binding material,
since the iron possesses considerable cementing power. This form of
binder is particularly valuable in making repairs over an opening
when a roller is not available, or when water for washing in the binder
is scarce. Low-grade iron ore has been used for a binder — either
alone or mixed with stone dust.
Fine screenings — the finest product of the stone crusher, say,
from J or | inch to dust — from the stone used in the body of the
course is the most desirable material for a binder, partly because it
helps to utilize the entire product of the crusher, partly because of
its high crushing strength, and partly because the stone is usually
selected for the high cementing power of its dust. Limestone has
very high cementing power, but is soft and pliable. Trap has .a
fair cementing power, and is hard and durable. Limestone screenings
require less rolling, but the trap dust makes a more durable road.
Sometimes the detritus removed from the surface of a stone road
during maintenance or preparatory to making repairs, is employed
as a binder. At best, such material is very poor for this purpose.
It is worn out and has performed its duty; and, besides, it is composed
largely of manure and vegetable and earthy matter — all of which are
very undesirable in a binder. Such detritus is more valuable as a
fertilizer than as a road material.
385. Applying the Binder. There is a difference of opinion
among competent engineers as to the best method of applying the
binding material. Some apply it on the top of each course, and
some on top of only the last course. In the first case, all the voids
from the bottom to the top of the road are filled with fine material;
in the second case, the binder usually fills the voids of the top course
only. Those who advocate the first method claim that the whole
mass should be filled to prevent the stones from moving under the
traffic, and also to prevent the soil from working up from below;
while the advocates of the second method claim (1) that filling the
218 WATER-BOUND MACADAM ROADS [CHAP. VI
top layer is sufficient to hold the stone in place near the surface, (2)
that the stones of the lower courses have no tendency to move, (3)
that the unfilled voids of the lower course promote drainage, and (4)
that as the upper layer wears away, the dust will wash down into the
lower open spaces in such a manner as always to keep the 3 or 4
inches just below the surface properly bound. If the stone is hard,
or if the lower courses are not thoroughly rolled, applying the binding
material only on the top of the last course practically fills the voids
to the earth foundation; but of course it is cheaper to apply the filler
on the top of each course than to attempt to fill all of the voids by
applying it on the top course only. If the stone in the lower courses
is soft, or if the top of the next to the last course is thoroughly rolled,
applying the binder on the top fills the voids in the top course only.
It is sufficient to fill the voids of the top course.
The binder is applied by spreading a layer of " fines " about half
an inch thick over the partially rolled surface. The filler should
be dumped upon a board platform, and not directly upon the road
surface; and should be distributed evenly over the stone with a
shovel. Under no consideration should loam or vegetable matter
be allowed to contaminate the stone screenings. After the binding
material has been evenly distributed, the surface is then sprinkled
and rolled. The sprinkler should have many fine openings, the
object being to give a gentle shower rather than a violent flooding.
The water washes the fine material into the cavities below, and the
roller crushes the small fragments and makes more dust. The
rolling also aids in working the binder into the mass; in fact, the
binder can be worked in to a considerable extent by dry rolling,
and consequently the quantity of water used varies widely with
the method of doing the work, but is usually about 4 to 6 cubic
feet per cubic yard of stone. Sometimes men with heavy brooms
are kept upon the road sweeping the binding material about to assist
in working it in, and also to secure a more uniform distribution of it.
While applying the screenings care should be taken to pick off any
coarse stone — particularly flat ones, — as they do not bind well and
their subsequent loosening causes the road to ravel (§ 397).
As the rolling and sprinkling proceed, fine material should be
added where needed, i. e., as open spaces appear. All the filler
should not be put on in the beginning, since a thin layer can be
worked in to better advantage than a thick one; and, besides, it is
desirable to use only enough to fill the voids.
Occasionally the surface of the road becomes muddy and sticks
ART. 2] CONSTRUCTION 219
to the roller. This can be remedied in either of two ways: viz.,
by sprinkling the roller and keeping it constantly wet, or by keeping
the sprinkling wagon immediately in front of the roller and having
the binder always fully saturated. The rolling is continued until
the water is forced as a wave in front of the roller and until the sur-
face behind the roller is mottled or puddled and is covered with a
thin paste. The binding, or the puddling of the surface, can not be
done satisfactorily when the surface freezes nightly.
When finished, if the road is allowed to dry and is then swept
clean, the surface will be seen to have the appearance of a rude
mosaic, the flat faces of the fragments of stone being crowded against
one another and the interspaces being filled with the binding material
— the latter occupying about half of the area. Such a surface when
dry will stand considerable sweeping with a steel broom or brush
without the fragments of stone being loosened. The water used in
construction not only aids in working the binder into the interstices,
but also develops the cementing power of the rock dust.
386. Usually after the rolling has been completed a thin coating
of binding material is sprinkled over the surface. Authorities
differ as to the amount of fine material to be left on the finished
surface, some specifying as little as f inch and some as much as
1 inch, the usual quantity being f to J inch. If only enough binding
material to fill the interstices between the coarser fragments is left
upon the road, the fine material will be blown and washed away,
and soon there will not be enough to level up between the large
bits and to hold the surface stones in place, when the wear will
come directly upon the stones. On the other hand, if any con-
siderable quantity of fine material is left upon the surface, it is
speedily ground up, and becomes offensive dust if it is not sprinkled,
and equally objectionable mud if it is sprinkled. It is probably
best to put on a quantity just sufficient to give a thin layer, say,
J to f inch, over the surface, and when this amount is blown or
washed away renew it. By this method, the wear on the body of
the road will be prevented, a minimum amount of sprinkling will
be required, and there will be as little dust as possible. The surface
coat is also serviceable in decreasing the tendency of the binding
material to dry out and to lose part at least of its cementing power.
Fine material over and above that required to fill the interstices is
useful only to prevent raveling and to keep the wear from the sur-
face of the stone ; and therefore sand is as good for the top dressing
as stone dust, and is usually much cheaper. It is desirable that this
220 WATER-BOUND MACADAM ROADS [CHAP. VI
coat of fine material shall be sprinkled and rolled before the traffic
is admitted.
The road is now finished; and after it has dried out for a day or
two, it may be thrown open to traffic.
387. Amount of Binder. The amount of binder required depends
upon the hardness of the stone and the amount of rolling preceding
the application of the binder (§ 385). The voids in the broken stone
can be reduced by rolling to 20 or 25 per cent, say 22 per cent, of
the compacted mass; and the completed road will contain 4 to 7 per
cent, say 5 per cent, of voids; and therefore enough binder must be
added to fill about 17 (=22-5) per cent of voids. The binder
itself usually contains 40 to 50 per cent of voids, and therefore the
volume of filler required is 40 to 50 per cent more than the voids to be
filled, i. e., 40 to 50 per cent more than 17 per cent of the original
volume of stone; or, in other words, the amount of filler required is
25 to 35 per cent of the thickness filled. This allows a little for
waste and for the thin coating spread upon the finished surface. If
the binder is applied before the rolling has progressed very far, more
fine material will be required, since some of it will work in between
the fragments of stone and prevent them from coming into as close
contact as they otherwise would. In this case, part of the surplus
binder will be flushed to the surface during the sprinkling and rolling,
as mortar flushes to the surface in tamping concrete; but in no case
does all the surplus thus work out, and consequently the road is not
as durable as though only enough binder had been used to fill the
voids; and, further, the binder which flushes to the surface must be
removed as mud. An excess of binder is often used to reduce the
cost of construction by decreasing the amount of sprinkling and roll-
ing required; but such a practice adds to the cost of maintenance,
and the road is less durable and more dirty.
388. COST OF CONSTRUCTION. The cost of construction of a
crushed-stone road varies greatly with the size of the job, the con-
ditions of the material and labor markets, the specifications under
which the work is done, etc. It is unwise to give here any details
as to the cost of the several parts of the work; and only a few data
will be given concerning the total cost of a road. The total cost
varies with the amount of grading and drainage required, the length
improved in a single season, the length of railroad and wagon haul,
the specifications, the labor market, etc. The following are a few
representative examples of first-class construction.
The values here given are somewhat out of date; but present
ART. 2] CONSTRUCTION 221
values are quite abnormal owing to the Great European War. The
data given below are interesting chiefly as showing relative cost in
different localities and of the different parts of the work. For cur-
rent prices consult the construction news in technical journals.
389. New Jersey. In northern New Jersey, the total cost of
trap macadam roads 4 to 6 inches deep, where the rock was obtained
near the road, ranged from 20 to 45 cents per square yard; and
telford roads consisting of 8 inches of telford and two courses of
broken stone 2| and 1| inches thick respectively, cost from $1.02
to $1.29 per square yard. In the southern part of that state, where
the stone is transported 20 to 70 miles, 8-inch trap macadam roads
cost from 23 to 70 cents per square yard, the average being from 50 to
60 cents per square yard.*
390. Massachusetts. The average cost of 220 miles of state-
aid roads in Massachusetts built from 1894 to 1899, f reduced to the
TABLE 27
COST OF MASSACHUSETTS STATE-AID WATER-BOUND MACADAM ROADS
Per Cent
Items of Expense. of Total
Cost,
Earthwork at 32.1 cents per cubic yard 16.4
Rock excavation at $1.80 per cubic yard 2.0
Shaping earth subgrade at 2.0 cents per cubic yard 2.4
Gravel for foundation and wings at 55.8 cents per cubic yard 3.5
Telford foundation at 33.9 cents per square yard 0.2
( $1.503 per ton for local stone )
Broken stone at «. „«-»*_ _ ,_ }• 45.3
j $1.503 per ton for local stone \
( $1.920 per ton for trap j
Side drains at 34.5 cents per lineal foot 2.7
Rubble masonry — dry, at $3.133 per cubic yard 2.6
" " in cement, at $5.770 per cubic yard 3.3
Guard rails at 16 cents per lineal foot 1.7
Stone boundary-posts at $1.417 each 0.6
Paved cobble gutters 66.0 cents per square yard 1.1
Vitrified-clay pipe-culverts — 12-inch, at 65 cents per lineal foot 1.2
Land damages, catch basins, and minor items of construction 3.0
Engineering and inspection 14.0
Total 100.0
equivalent cost of a " standard mile " (15 feet wide), was $9,931.23
per mile for construction and engineering expenses, exclusive of cost
of administration and the salaries of the chief engineer and two
* Compiled from the Reports of the State Commission of Highways of New Jersey, 1895-
1900.
t Report of Massachusetts Highway Commission, 1900, p. 150-57.
222
WATER-BOUND MACADAM ROADS
[CHAP, vi
assistants. The maximum average for the roads in any township
was $20,257.48 and the minimum $4,871.30 per " standard mile."
The above gives an average cost of $1.126 per square yard, a maximum
of $2.302, and a minimum of $0.564.
In Massachusetts in 1897, 52 miles were built in 187 towns
(townships), the average cost of the several items being as shown
in Table 27.* An examination of the reports for other years indi-
cates that the above exhibit is fairly representative, except that the
expenditure for stone is smaller than the average. In the state-
aid roads built from 1894 to 1899, the cost of the broken stone was
equal to 55 per cent of the total cost of the road, but in later
years the amount of stone used was decreased.
391. New York. In the State of New York in 1898, 22 miles of
state-aid macadam roads were built in six sections, with an average
cost of 84.0 cents per square yard, the maximum being $1.085 and
the minimum 64.8 cents. The roads consisted of 4 inches of native
stone, and 2 inches of trap rock bound with limestone screenings, f
392. Michigan. In Michigan the average cost of 52 miles of
water-bound macadam roads is as follows:
ITEMS.
AVERAGE.
PER MILE.
PER SQ. YD.
Broken stone, cubic yards
1653
$ 435.85
2264.88
71.80
1621.91
0.313
$0.083
0.523
0.013
0.307
Grading, shaping and draining .
Crushed stone.
Culverts. .
Surfacing, including loading and hauling.
Total
$4394.44
$0.925
393. Missouri. In Missouri the average cost of two-course work
is as stated in the table at the top of page 223. §
394. SPECIFICATIONS. The American Society of Municipal Im-
provements publish specifications for water-bound broken-stone
roads, printed copies of which may be had for a nominal sum. These
specifications are changed from time to time as is necessary to keep
them up to date.
* Report of Massachusetts Highway Commission., 1898, Appendix C, p. 74-75
Report of New York State Engineer and Surveyor, 1899, p. 37,
Engineering and Contracting, Vol. 41 (1914), p 705
76id., Vol. 38 (1912), p. 14,
ART. 3]
MAINTENANCE
223
ITEMS.
Per Sq. Yd. of
Road Surface.
Per Cu. Yd. of
Loose Stone.
Quarry rent.
$0 013
$0 05
Quarrying
0 100
0 40
Crushing
0 075
0 30
Hauling, 1 mile
0 075
0 30
Shaping road-bed
0 025
0 10
Spreading material
0 025
0 10
Rolling
0 013
0 05
Sprinkling
0 013
0 05
Superintendence. ...
0 013
0 05
Incidentals
0 013
0 05
Total, exclusive of interest and depreciation on
plant, profits, and administration
$0 . 364
$1 45
ART. 3. MAINTENANCE
395. After the road has been properly rolled and the surface
has been made compact and smooth, it is very desirable that it
should always be maintained in that condition. Many seem to
believe that a macadam road is a permanent construction which
needs no attention after completion; but proper maintenance is as
important as good construction.
Formerly much attention and study was given to the causes of
wear of water-bound macadam roads; and it was needed, for many
such roads were subjected to a heavy travel, and required great care
to keep them in usable condition. To maintain the roads in good
condition it was necessary to make repairs either at frequent intervals
or continuously, and to add new material as the old was washed off
or blown away. But the coming of the automobile made neces-
sary a radical change in the methods of maintenance and con-
struction of broken-stone roads. With horse-drawn vehicles the
abrasion of the horses' feet and of the metal tires wore off dust or
binder to replace that washed off and blown away; but the fast-
moving low-hung body of the automobile threw more of the road dust
into the air than horse-drawn vehicles, and hence more of the binder
was blown away, and besides the automobile made no dust to replace
that blown away. Further, the action of the wheels of the auto-
mobile, particularly in starting and stopping and in rounding curves,
tends to dislodge the stones.
Therefore the introduction of the motor-driven vehicles radically
changed the method of maintenance of the broken-stone roads.
Instead of trying to maintain a water-bound macadam road having
224 WATER-BOUND MACADAM ROADS [CHAP. VI
any considerable amount of motor-driven traffic, by adding new
material to replace that washed off and blown away, it became the
practice to give the surface of such roads a coating of bituminous
material, such as tar or asphalt, which prevented the formation of
dust and also protected the surface from wear. This method of
treatment is discussed in Chapter IX.
396. Since the introduction of motor-driven vehicles, the only
water-bound macadam roads not protected by a bituminous coating
are those that carry only a small amount of travel, particularly a
small number of motor vehicles; and hence the maintenance of such
roads is not of great importance. Therefore the maintenance will
be considered only briefly.
397. RAVELING. One of the chief evils to be contended with in
the maintenance of a crushed-stone road is the tendency to ravel,
i. e., for one stone after another to work loose on the surface. This
occurs only after a long dry spell or in a road originally deficient in
binding power, and is more likely to occur on lightly traveled roads
than on those having heavy traffic. Raveling may take place
where the wind sweeps away the binding material from the surface,
or on a steep grade where the water has washed the fine material
away from between the fragments; and is chiefly due to the picking
of the horses' shoes, which in a measure is counteracted by the rolling-
action of the wheels.
Two expedients are employed to prevent raveling. 1. Sprink-
ling the road with water effectually stops raveling, and causes the
surface to solidify again. This is the most common remedy on city
streets and suburban roads — where water is usually convenient and
plentiful. For a further discussion of Sprinkling, see § 401. 2.
A thin coating of coarse sand is very effective in preventing raveling.
Ordinarily on country roads a layer half an inch thick is sufficient.
Unless the season is very dry or the road is unusually exposed to the
wind, a single application will be enough for one season.
398. RUTS. Next after raveling, the tendency to form ruts is
the most serious evil to be contended against in the maintenance of
crushed-stone roads. Ruts are due either (1) to a greater wheel
load than the road is capable of standing, or (2) to the use of an
inferior binding material, as loam, or (3) to tracking. Ruts are most
likely to occur in the spring or during a wet time, when the road-
bed is soft, and are more common on country roads than on city
streets, since in the latter the frequent changes in direction to avoid
other vehicles produce a more uniform wear over the whole surface of
ART. 3] MAINTENANCE 225
the road. However, a street-car track in a broken-stone road pre-
vents the distribution of traffic uniformly over the entire surface and
greatly increases the tendency to form ruts.
After ruts appear the only remedy is to fill them either with
new material or by picking down the sides of the ruts and raking
the loosened material into the depression. Usually the latter course
is the wiser, particularly on a new road. Frequently the tendency
to form a rut may be effectually arrested by sweeping into it the loose
detritus from the adjacent parts of the road. If the road surface
is compact and hard, it may be necessary to loosen the bottom and
sides of the rut before adding new material, so that the new will
thoroughly unite with the old. The new material should be of the
same character as the old, as otherwise the surface will wear unequally
and become rough.
399. PATCHING. Formerly much attention was given to the
patching of macadam roads to keep the surface free from ruts and
depressions (see pages 253-57 of the former editions of this treatise) ;
but the only water-bound macadam surfaces now in use are those
having a comparatively small amount of travel, and therefore the
repair of such roads is a comparatively simple matter.
When new stone is added, the old surface should be loosened to
insure that the new stone will unite with the old. The patch should
be rounded rather than square cornered. Care should be taken to
leave no place where water may lodge. When new, the patch should
be a little higher than the adjoining surface. The stone employed
in patching should be a little smaller than that used in the original
construction. It is better to lay the stone in two thin courses than
in a single thick one, and allow the first to become consolidated
before the second is added. Ordinarily, in applying patches uTthin
coats over small areas it is unnecessary to use binding material, since
the road usually has enough detritus to fill the interstices of the new
stone. If laid in damp weather, when the surface of the road is soft,
there is usually no difficulty in getting a layer one-stone thick to
consolidate without any binding material. If the patch is small
and thin, it will usually not be necessary to tamp or roll it.
When the surface of the road has become uneven and rough,
and when the broken stone is thick enough not to require much new
material, the top of the road is loosened, re-graded, and re-rolled.
The loosening is usually done by running over the road, one or more
times, with a steam stone-road roller having spikes in the rear wheels,
or by breaking up the surface with a scarafier, — a cross between a plow
226 WATER-BOUND MACADAM ROADS [CHAP. VI
and a harrow. After the crust is broken up, the surface is leveled
off by the use of a harrow and hand shovels and rakes; and then it is
sprinkled and rolled as in the original construction. Usually no new
binding material is required, the detritus from the old road being
sufficient.
400. ROLLING. In the spring after the frost goes out, the road
bed is soft and porous; and a thorough rolling with a steam roller
at this time, before the subgrade is dry, is one of the best and cheap-
est methods of keeping a macadam road in good condition. Just
before this rolling is the time to add a little fresh surface material,
here and there, as may be needed to fill up slight depressions.
401. SPRINKLING. Moisture is necessary to preserve the cement-
ing power of the binding material, and also to prevent an excessive
removal of dust by the wind; and therefore sprinkling to the extent
required to prevent these injuries is an advantage. The water
should be applied in a fine spray, and not be allowed to run in streams
on the road; that is, several light sprinklings are better than a single
flooding. If sprinkled too heavily or too often, the road is softened
and breaks up easily.
Sprinkling is usually employed on park drives and city streets,
where it is generally conceded to. be true economy, without taking
into consideration the prevention of dust; but it never was much
used on rural roads on account of the expense, and the introduction
of bituminous coatings as dust preventatives has entirely done
away with sprinkling as a road preservative.
402. For a discussion of sprinkling with water, oil, etc., see
§ 325-31 under Gravel Roads.
For a discussion of bituminous coatings for macadam roads,
see Art. 1 and 2 of Chapter IX.
403. COST OF MAINTENANCE. The introduction of automo-
biles greatly increased the cost of maintaining water-bound mac-
adam roads (see § 395). Data on the cost of maintaining water-
bound roads before the advent of motor-driven vehicles are now of
little or no value ; and there are almost no data on the cost of main-
tenance of such roads where the amount and character of the travel is
known.
CHAPTER VII
PORTLAND-CEMENT CONCRETE ROADS
406. DEFINITIONS. The wearing coat of a concrete road is a
layer of portland-cement concrete. The word concrete ordinarily
means a mass of pebbles or broken stone bound together into a solid
mass by hydraulic cement ; and the word cement in engineering liter-
ature ordinarily means hydraulic cement, and hi recent years it
usually means portland cement. Until recently the type of road
considered in this chapter was called a concrete road. However,
the preceding meanings do not now apply in discussions concerning
roads and pavements. Recently a form of construction has been
introduced in which the pebbles or fragments of broken stone are
held together by bituminous cement instead of hydraulic cement;
and therefore to prevent the possibility of misunderstanding it is
wise to employ the terms portland-cement concrete road or simply
Portland-concrete road, and bituminous-cement concrete road or
simply bituminous-concrete road, to designate these two types.
407. HISTORY. Except three comparatively small sections con-
structed earlier, concrete was not used for road surfaces until the
early years of this century. In 1909 there were in this country less
than a half million square yards; but in 1916 there were laid in this
country 24 million square yards of such road surfaces, an increase of
forty-eight times in 7 years. At present about 50 per cent of such
surfaces are on rural roads, and about 25 per cent each on streets and
alleys.
ART. 1. THE MATERIALS
408. The concrete is composed of portland cement, sand or stone
screenings, and pebbles or broken stone. The sand or stone screenings
are often called the fine aggregate, and the pebbles or the crushed
stone the coarse aggregate.
409. CEMENT. For a discussion of the characteristics of the
227
228 PORTLAND-CEMENT CONCRETE ROADS [CHAP VII
different types of hydraulic cement and of the methods of testing the
same, see pages 54-83 of the tenth edition of the author's Treatise on
Masonry Construction.* The only form of hydraulic cement used
in concrete roads is portland cement.
It is almost universally specified that the cement shall meet the
requirements of the latest standard specifications of the American
Society for Testing Materials, which also have been adopted by all
of the national engineering associations. Printed copies of these
specifications can be had of the above Society for a nominal sum;
and they have been republished by various organizations.
410. FINE AGGREGATE. For an extended discussion of the
characteristics and requirements of sand and screenings for making
concrete and the method of testing each, and also for a discussion
of the relative merits of sand and screenings, see pages 85-97 of
the tenth edition of the author's Treatise on Masonry Construc-
tion.*
411. Sand vs. Screenings. The finer particles screened out of
the broken stone are sometimes used in concrete instead of sand;
and there is much discussion as to the relative merits of sand and
screenings. Experiments show that if all conditions are the same,
the screenings make slightly stronger mortar; but in practice it has
been found nearly impossible to exclude dust from the stone screen-
ings, particularly if the stone is soft (as is most limestones) or if the
screenings get wet before being screened, and hence in practice mor-
tar or concrete made from screenings is not usually as strong as that
made of sand of the same degree of fineness. For these reasons
many engineers prohibit the use of stone screenings.
412. The specifications for the fine aggregate recommended by
the 1916 National Conference on Concrete Road Building are as
follows:
"The fine aggregate shall be natural sand or screenings from hard, tough,
durable crushed rock or gravel; and shall consist of quartzite grains or other
equally hard material graded from fine to coarse with the coarse particles pre-
dominating. When dry it shall pass a screen having four meshes per linear inch,
and not more than 25 per cent shall pass a sieve having fifty meshes per
linear inch, and not more than 5 per cent shall pass a sieve having one hundred
meshes per linear inch. It shall not contain vegetable or other deleterious mat-
ter, nor more than 3 per cent by weight of clay or loam. Routine field tests shall
be made on the fine aggregate as delivered. If there is more than 5 per cent of
* A Treatise on Masonry Construction, by Ira O. Baker. 745 pages, 6X9 inches. John
Wiley & Sons, New York City. Tenth Edition, 1909.
ART. 1] MATERIALS 229
clay or loam by volume settled after one hour's shaking in an excess of water,
the material represented by the sample shall be held pending laboratory tests.
"The fine aggregate shall be of such quality that mortar composed of one
part portland cement and three parts fine aggregate, by weight, when made into
briquettes, shall show a tensile strength (at seven and twenty-eight days)
equal to or greater than the strength of briquettes composed of one part by weight
of the same cement and three parts standard Ottawa sand. The percentage of
water used in making the briquettes of cement and fine aggregate shall be such
as to produce a mortar of the same consistency as that of the Ottawa sand bri-
quettes of standard consistency."
413. COARSE AGGREGATE. For a discussion of the requisites
of gravel and broken stone for concrete, see pages 97-103 of the
tenth edition of the author's Treatise on Masonry Construction.
414. Gravel vs. Broken Stone. There is difference of opinion
as to the relative merits of gravel and broken stone for concrete.
The elements to be considered are the strength, density, and cost of
the concrete.
Gravel concrete has only about 80 to 90 per cent of the strength
of broken-stone concrete.
Experience shows that gravel concrete is more easily compacted,
and has fewer cavities than broken-stone concrete; and hence gravel
concrete is denser and more waterproof.
As a rule, the first cost of gravel is less than that of broken stone,
and the former is considerably easier to handle.
415. However, since gravel is liable to contain so much clay or
loam as to materially reduce the strength of the concrete, some
engineers for this reason alone prefer broken stone to gravel. Even
though only portions of the gravel are naturally dirty, or even though
only portions of it are likely to contain an undue amount of the strip-
ping, some engineers, owing to the greater care required in inspection
and to the uncertainty of eliminating all dirty gravel, prefer broken
stone to gravel.
416. The Specifications for the coarse aggregate recommended by
the 1916 National Conference on Concrete Road Building are as
follows :
"The coarse aggregate shall consist of clean, hard, tough, durable crushed
rock or pebbles graded hi size, free from vegetable or other deleterious matter;
and shall contain no soft, flat or elongated particles. The size of the coarse
aggregate shall be such as to pass a l£-inch round opening, and shall range from
1£ inches down; and not more than 5 per cent shall pass a screen having four
meshes per linear inch, and no intermediate sizes shall be removed.
"Crusher-run stone, bank-run gravel, or artificially prepared mixtures of
fine and coarse aggregate shall not be used."
230 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
417. THEORY OF PROPORTIONS. The whole theory of the
proper proportions for concrete is comprised in two laws as follows :
1. For the same fine aggregate and the same coarse material, the
strongest concrete is that containing the greatest per cent of cement
in a unit of volume.
2. For the same per cent of cement and the same aggregates, the
strongest concrete is made with that combination of fine and coarse
aggregate which gives a concrete of the greatest density.
The first law is almost self-evident, and concerns the relative
richness of the concrete. Experiments have shown this law to be
almost mathematically exact.
The second law is very important, and concerns the qualities of
the fine and coarse aggregates. The second law is equivalent to say-
ing that the cement should fill the voids of the sand, and that the
resulting mortar should fill the voids of the coarse aggregate. If the
cement does not fill the voids of the sand, or if the mortar does not
fill the voids of the coarse aggregate, the concrete will obviously be
less dense than though the voids were just filled. If the paste is more
than enough to fill the voids in the sand, or if the mortar is more than
enough to fill the voids in the coarse aggregate, the concrete will be
less dense than though the voids were just filled; since both the paste
and the cement mortar have a less density than ordinary concrete;
and hence the strength due to the increased amount of cement may
be neutralized by the decrease in density, but the possibilities of
this depend upon the plasticity of the mortar, the amount of tamp-
ing, the character of the sand and the stone, and the gradation of
the sizes. Experiments and experience have shown this law to be
almost mathematically exact.
418. METHODS OF PROPORTIONING. There are four methods in
more or less general use for proportioning concrete, which may be
briefly designated as follows: (1) by arbitrary assignment; (2)
by voids; (3) by trial; and (4) by sieve-analysis. These methods
will be considered separately in the above order.
419. Proportioning by Arbitrary Assignment. This is the least
scientific method of proportioning, it virtually assumes the relations
as a matter of judgment, without much, if any, consideration of the
character of the aggregate; that is, the proportions are assigned
without any reference to the fineness or coarseness of the sand and the
stone, or to the gradation of the sizes of each.
420. Proportioning by Voids. To determine the best propor-
tions for any particular sand and aggregate, find the per cent of voids
ART. 1] MATERIALS 231
in the sand and in the stone, and use enough cement paste to fill the
voids in the sand and enough mortar to fill the voids in the coarse
aggregate. Owing to the errors in using this method, and particu-
larly in getting the cement paste to fill the voids of the sand and the
mortar to fill the voids of the stone, this method is not very prac-
tical.
421. Proportioning by Trial. The second law in § 417 leads to a
simple method of finding the best relation of the sand and the stone.
According to that law that combination of sizes of sand and stone
which with a constant quantity of cement gives the least volume of
concrete is the best.
To apply this method procure a vessel of uniform cross section,
say a cylinder, 10 or 12 inches in diameter and 12 or 14 inches deep,
its strength being such that its volume will not be changed in tamping
it full of concrete. Weigh out a unit of cement, and any number of
units of sand, say two, and also weigh out any number of units of
broken stone, say five, taking care that the quantities are such that
when the ingredients are thoroughly mixed and placed in the cylinder,
the mixture will fill it only partly full, — say three quarters full.
Make a concrete of any desired consistency by mixing the cement,
sand and stone with water on a sheet of steel ; tamp the concrete into
the cylinder leaving the upper surface smooth and horizontal, and
then measure the depth of the concrete from the top of the cylinder.
Next empty the concrete from the cylinder, clean it and the tools;
and then make another batch with different proportions of sand and
stone, keeping the quantity of cement and the plasticity of the con-
crete the same as before. If this batch when tamped into the cylin-
der gives a less volume of concrete, this proportion is better than the
first. Continue the trials until the proportion has been found which
will give the least depth in the cylinder.
422. Proportioning by Sieve Analysis.* " Sieve analysis con-
sists in separating the particles or grains of a sample of any material-
such as broken stone, gravel, sand or 'cement — into the various sized
particles of which it is composed, so that the material may be repre-
sented by a curve each of whose ordinates is the percentage of the
weight of the total sample which passes a sieve having holes of a
diameter represented by the distance of this ordinate from the origin
* This method of proportioning the sizes of the sand and stone in concrete was devised by
Wm. B. Fuller, and is described by him in detail on pages 183-215 of Taylor and Thomson's
Concrete Plain and Reinforced (1909 edition), from which these extracts are taken by permis-
sion of the authors,
232
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
in the diagram." The line DBKLA, Fig. 73, is a typical sieve-
analysis curve for crusher-run micaceous-quartz stone; and the line
OF represents a fine sand.
" The objects of sieve-analysis as applied to concrete aggregates
are: (1) to show graphically the sizes and relative sizes of the par-
ticles; (2) to indicate what sized particles are needed to make the
aggregate more nearly perfect, and so to enable the engineer to im-
prove it by the addition or substitution of another material; and (3)
to afford means for determining the best proportions of different
aggregates."
" The experience which the writer [Fuller] has had and the various
experiments which he has made indicate that concrete which works
F E
0.25 0.50 075 1.00 1£5 t.50 1.75
Diameter of Parftcfe3 in Inches
FIG, 73, — SIEVE-ANALYSIS CURVE.
the smoothest in placing and gives the highest breaking strength for
a given percentage of cement, is made from an aggregate whose
sieve analysis, taken after mixing the sand and the stone, forms a
curve approaching a parabola having its beginning at the zero of co-
ordinates and passing through the intersection of the curve of the
coarsest stone with the 100 per cent line, that is, passing through the
upper end of the coarsest stone curve." In Fig. 73, the parabola
OCPA represents a theoretically perfect combination of sizes of
sand and stone all of whose pieces pass a If -inch screen. This curve
shows, for example, that for the best combination of the above
materials 93 per cent of the mixture should pass the li-inch sieve, 76
ART. 1] MATERIALS 233
per cent should pass the 1-inch sieve, 54 per cent the ^-inch, and so
on."
" Where, as in Fig. 73, the materials to be mixed are represented
by only two curves no combination of which will make a curve as
close to a parabola as is desirable, there is another limiting condition
which was brought out by the experiments, viz., that for the best
results the combined curve shall intersect the parabola on the 40
per cent line, at C, and that the finer material shall be assumed to
include the cement."
423. The curve DBKLA, Fig. 73, may be transformed so that
it will pass through C, by changing the distances from the top of the
TPC* f\C\
diagram to the line DBKLA in the proportion -== = — = 61 per cent,
tin Uo
which shows that 61 per cent of the dry materials should be broken
stone. In a similar manner the line OF is re-plotted in the position
OJ. The line OJCGA is assumed to represent the best possible
combination of sizes of this sand and stone. For example, with the
best possible combination of sizes of this stone and sand, 89 per cent
would pass the 1 J-inch sieve, 67 per cent would pass the 1-inch sieve,
46 per cent the J-inch sieve, and so on.
" The proportion of cement to be used to give the required
strength of concrete must always be assumed; and in this example
it will be assumed that the cement is to constitute one eighth of the
dry materials (measured before the sand and stone are mixed together),
which will make the cement one ninth or 11 per cent of the total dry
materials. Since the diagram shows that the sand and cement are
to constitute 39 per cent of the dry materials, the sand must then be
39- 11 =28 per cent."
" The proportions of concrete for 1 part cement to 8 parts of
sand and stone, measured separately, then are: 11 per cent cement,
28 per cent sand, and 61 per cent broken stone, or 1 : 2.5 : 5.5 by
weight. If the proportions are required by volume and the relative
weights of the sand and the stone differ from their relative volumes,
the proportions should be corrected accordingly."
424. " An important feature of the sieve-analysis curves is that
they show how the materials may be improved by adding or sub-
tracting some particular size. For example, if the stone repre-
sented by the curve DBKLA in Fig. 73 had contained more pieces
0.5 and 1.0 inch in diameter, its curve would have more nearly ap-
proached the parabola in the region SG. If a stone giving the line
DRHA were used, the ratio for transforming the line to make it
234 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
EC 60
pass through C would be 7^ = — = 66 per cent, which shows that with
the assortment of sizes of broken stone represented by this line the
best concrete is made by using 66 per cent of broken stone. For a
1 : 8 mixture as before, the proportions would be 11 : 23 : 66, or
1:2: 6, — a cheaper, stronger, and denser concrete than that made
with the stone represented by the line DBKLA."
For further details concerning this method and for later slight
modifications in the form of the ideal sieve-analysis curve, see Taylor
and Thompson's book mentioned above or the author's Treatise on
Masonry Construction.*
" This method affords a means of determining the best propor-
tions in which to mix the fine and coarse aggregate, and also shows
how the aggregate may be improved by adding or subtracting some
particular size. Sieve analyses can be made from time to time as
the work progresses to see whether or not the sizes of the aggregate
have changed; and if the sizes have changed, the proportions can be
varied to secure the most economical and the densest concrete.
In a work of any magnitude the greater labor required in determining
the proportions by sieve-analysis curves is likely to be justified by
the better quality, or the less cost, of the concrete; and the extra
labor required to make sieve analyses during the progress of the work
will be worth all it costs because of the better control of the propor-
tions of the concrete.
:< To secure the maximum benefit of this method of proportion-
ing, it is necessary to screen the aggregate to several sizes and then
combine them in the proportions indicated by the sieve-analysis
curve. As to whether or not the increased cost of screening and
proportioning would be justified by the saving of cement, depends
upon the magnitude of the work and other conditions. The following
example illustrates the possibilities:
'The ordinary mixture for water-tight concrete is about
1 : 2J : 4J, which requires 1.37 barrels of cement per cubic yard of
concrete. By carefully grading the materials by the methods of
sieve analysis the writer [Fuller] has obtained water-tight work with
a mixture of about 1:3:7, which requires only 1.01 barrels of
cement per cubic yard of concrete. This saving of 0.36 barrel is
equivalent, with portland cement at $1.60 per barrel, to $0.58 cubic
* For an explanation of the advantages of plotting sieve-analysis curves on logarithmic
11'1 of an ingenious use °f the
ART. 1] MATERIALS 235
yard of concrete. The added cost of labor for proportioning and
mixing the concrete because of the use of five grades of aggregate
instead of two, was about $0.15 per cubic yard, thus effecting a net
saving of $0.43 per cubic yard."
425. DATA FOR ESTIMATES. Portland Cement. Portland ce-
ment, which is now practically the only cement used in engineering
construction, is usually shipped in cloth bags containing 94 Ib. each.
The cement is usually sold by the barrel, which is considered as four
bags. In computations involving the proportions for concrete, a
bag is usually considered as containing one cubic foot of packed
cement.
Until the disturbance of prices by the Great European War the
price of portland cement at any place east of Omaha, Nebraska,
was from $1.50 to $1.75 per barrel in cloth bags, the bags being
charged at 10 cents each and undamaged bags being returnable at
that price.
426. Sand and Gravel. Before the Great European War the
price of washed, screened, and graded sand at the pit was about 25
cents per ton; and of washed, screened, and graded gravel, about 35
cents per ton. The freight was about 47 to 48 cents per ton per 100
miles in each case.
427. Broken Stone. Before the disturbances of prices by the
Great European War, the price of crushed limestone, screened
and graded, f.o.b. the quarry, was about 55 cents per ton; and the
freight was about 47 to 48 cents per ton per 100 miles. Limestone,
graded and loaded into a freight car weighs about 2500 Ib. per cubic
yard.
The cost of crushed trap f.o.b. the quarries in New Jersey for
several years previous to 1900, was 40 to 50 cents per ton (about 50
to 62 cents per cubic yard) ; but in that year the price was increased
nearly 50 per cent. In Massachusetts, the cost of broken trap on
cars at the end of the railroad transportation, varies from $1.10 to
$1.60 per ton (about $1.32 to $1.93 per cubic yard). In Boston, the
cost of crushed granite delivered on the streets is $1.65 to $1.90 per
ton. In Montreal, syenite delivered on the streets costs an average
of $1.15 to $1.20 per ton.
428. Ingredients for a Cubic Yard. Table 28, page 236, shows
the quantities of cement, sand, and stone required for a cubic yard
of concrete of different proportions, using three grades of broken
stone or gravel. The concrete was mixed wet but not soupy; and
was also mixed very thoroughly. If it had been mixed drier or less
236
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
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ART. 1] MATERIALS 237
thoroughly, it would have been less dense, and consequently less
quantities of materials would have been required to make a yard.
Data like that in Table 28 are affected by the fineness of the
cement, the fineness and dampness of the sand, the kind and coarse-
ness of the stone, the proportions of the several sizes of sand grains
and stone fragments, the thoroughness of the mixing, the amount of
tamping, etc.; and different experimenters have obtained widely
different results. Most experimenters obtain a less quantity because
the concrete is mixed drier and entrains more air, and hence is less
dense.
429. Fuller's Rule. The following rule, devised by Wm. B.
Fuller,* gives the quantities of cement, sand, and stone required to
make a cubic yard of concrete; and is fairly representative of all
classes of materials. This rule is valuable because it is so simple
that it can be carried in the memory.
c = number of parts of cement.
s = number of parts of sand.
g = number of parts of gravel or broken stone.
C = number of barrels of packed portland cement required for 1
cubic yard of concrete.
S = number of cubic yards of loose sand required for 1 cubic yard
of concrete.
G = number of cubic yards of loose gravel or broken stone required
for 1 cubic yard of concrete.
11
~ c + s + g'
" If the coarse material is broken stone screened to uniform
size, it will contain less solid matter in a given volume than average
stone, and hence about 5 per cent should be added to quantities of all
three ingredients as computed by the above rule. On the other hand,
if the coarse material is well graded in size, about 5 per cent may be
deducted from all of the quantities."
* Taylor and Thompson's Concrete Plain and Reinforced, Second Edition, p. 16.
238 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
The preceding formulas are sometimes modified by changing the
constants 11 and 3.8. For example, one engineer substitutes 9.5
for the 11, and 4 for the 3.8.
ART. 2. THE CONSTRUCTION
431. DRAINAGE. The drainage of the road-bed of a concrete
road or pavement is of vital importance. If the subgrade is not well
drained, there is danger that after the concrete is laid, the drying of
the soil under the edges of the concrete may permit the pavement to
settle and thus cause longitudinal cracks on the surface. Further,
if the subgrade is not well drained, there is a possibility that the frost
may lift the edges of the concrete roadway and cause a longitudinal
crack, — at least on the lower side. With some forms of roads a crack
in the surface will heal under travel; but a crack in a concrete pave-
ment not only can not heal under travel, but will continually enlarge.
There is no part of the work of the construction of a concrete road or
pavement that is more worthy of intelligent care and painstaking
labor than the preparation of the subgrade; and the slight addi-
tional cost necessary to insure good results is abundantly justifiable.
If the soil is sandy, there is a probability that the natural under-
drainage is sufficient for the purpose.
If the soil is only moderately retentive, a shallow longitudinal
ditch should be constructed just outside of the edge of the concrete
slab. The ditch should extend about 8 or 10 inches below the sur-
face of the road-bed, i. e., below the bottom of the concrete slab; and
should be filled with coarse gravel or broken stone. From this longi-
tudinal ditch short transverse trenches should be dug across the
shoulder to the ditch at the side of the roadway. These transverse
trenches should have a grade sufficient to permit them to carry the
water promptly and completely to the side ditch. In particularly
retentive soil, these transverse trenches should not be placed more
than 50 feet apart. On the level stretches, these transverse trenches
should be practically at right angles to the direction of the road;
but if the road is on a grade, they should make an acute angle with
the roadway, the amount of this angle depending upon the grade of
the road. These lateral ditches should be filled level full with broken
stone or coarse gravel to a point at least a little beyond the outer
edges of the shoulders and preferably nearly to the bank of the ditch
at the side of the roadway.
If the soil is so retentive that the underground water level is likely
ART. 2]
THE CONSTRUCTION
239
to rise within a foot or so of the surface, then a farm tile should be laid
on one or both sides of the paved portion. For a discussion of the
purposes of underdrainage and the method and cost of laying the
tile, see § 114-24 of Chapter III, — Earth Roads. For a discussion of
the subject of Side Ditches, see § 125-28.
432. V-Drains. Some engineers employ what is usually called
a V-drain, Fig. 74. The subgrade is excavated to a depth below
Concrete I/fearing Cog f
FIG. 74. — THE V-DRAIN.
the base of the concrete slab of 12 to 18 inches at the center and 4 to
6 inches at the side, and this trench is filled with loose stone.
The sizes of the stones are usually limited as follows: the largest
the equivalent of a 6-inch cube, and the smallest a 2-inch fragment.
The largest stones are placed in the bottom of the trench, and no
3-inch stone is allowed within 2* inches of the upper surface of the
stone in the trench. After the trench is filled with stones, it is usually
rolled with a self-propelling roller, and any depressions that appear
are filled with stones.
This form of drain ordinarily costs 50 to 60 cents per linear foot,
and is of doubtful economy, unless where bowlders or loose stones
abound on or near the road, or where the road is on very wet and
retentive soil.
433. PREPARING THE SUBGRADE. The fundamental require-
ment is that the subgrade shall at all times be of uniform density,
so that it will not settle unevenly and cause cracks in the concrete
surface.
434. Before the grading is begun, or at least before it is com-
pleted, the curbs (Chapter XIV) are built, or the side forms (§ 448)
are set, to serve as guides in bringing the subgrade to the proper
surface.
435. On Virgin Soil. All brush, trees, stumps, and large roots
for a width of 25 feet on each side of the center line of the proposed
roadway, should be cut off and be removed from the limits of the
right-of-way.* The road should be grubbed for the full width of the
* If the pavement is to be more than 15 feet wide, this quantity is to be proportionally
greater.
240 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
excavation, and no stumps or large roots should be left within these
limits except when a fill of more than 5 feet is called for on the plans,
in which case all stumps should be cut off to within 12 inches of the
original ground level. Stumps on the cleared portion not within the
grubbed limits, should be cut off to within 12 inches of the original
ground level. Stumps on the cleared portion not within the grubbed
limits, should be cut off not more than 2 feet from the ground.
All soft or spongy spots and all vegetable matter should be
removed, and the space be re-filled with suitable material. It is not
wise to put any dependence upon the ability of the concrete slab to
bridge soft spots. After the subgrade has been brought to the proper
surface, it should be rolled with a 3-wheel 10-ton self-propelling roller
to disclose any soft spots. Any depressions discovered in rolling
should be filled and be re-rolled until no depressions appear. The
rolling should be continued until the roller leaves no material track
or depression. However, clean dry sand can not be consolidated by
rolling; and some plastic clay can be rolled too much.
After the grading is completed, if the natural lay of the ground
is such that there are sumps or pockets which hold water, or if such
sumps or pockets are made during -the progress of the work, they
should be thoroughly drained.
436. On Earth Road. If the concrete road or pavement is to be
constructed upon virgin soil, that is, if it is not to be constructed
on an old road-bed, the precautions described above are sufficient to
secure a reasonably good foundation. But if the pavement is to be
constructed upon an old road-bed of any kind, great care must be
taken in preparing the subgrade. The old road-bed is likely to be
more compact in the center than at the sides; and consequently
there is danger that the pavement will settle more at the sides than
at the center, and therefore will crack longitudinally. Further, it is
likely that the traveled way of the old road will not at all places be
central under the new pavement, and consequently the latter will
settle unevenly and crack.
When the subgrade is an old roadway, since the roller is likely to
balance upon the more compact central core and therefore not
consolidate the soil at the side of the old roadway, it is not sufficient
to roll the subgrade longitudinally. In extreme cases it may be
necessary to plow the old road-bed and then harrow it, and finally
consolidate the entire new road-bed with the roller.
437. On Gravel or Macadam Road. Not infrequently a concrete
slab is constructed on an old gravel or macadam road. There is
ART. 2] THE CONSTRUCTION 241
considerable difference of opinion as to the wisdom and the method
of utilizing the old road. The objections and difficulties are that the
old road usually has too much crown; the surface is full of holes; the
old road often has an undesirable profile; and the old road is narrower
than the new one, and consequently the sides are likely to settle and
crack the concrete slab longitudinally. On the other hand, the old
road is already in place, is usually well consolidated, and a careful
investigation should be made to determine whether it can be econom-
ically used.
The first thing is to establish a grade line for the subgrade. This
may require the cross sectioning of the old road at frequent intervals, —
perhaps each 50 or 100 feet, depending upon the condition of the sur-
face and the regularity of its profile. After having established a
grade line, it is usually necessary to scarify the old road in places to
remove the high spots. The low places should be filled with gravel
or stone to avoid an excessive depth of concrete. The subgrade is
then rolled; and if necessary, the low places are again filled and
again rolled. The permissible degree of variation of the subgrade
from the proposed contour depends uporf the relative cost of the
labor required in finishing and of the concrete required to fill the low
places.
It is particularly important that the side forms for the concrete
(see § 448) should be set up, or that the curbs should be built, before
the old road is scarified, so they may carry the templet used as a
guide in re-shaping the old road.
If the old road is narrower than the new, or not central under it,
great care must be taken in consolidating the soil at the edge of the
old road. These edges should be covered with gravel or broken stone
and be rolled until they are firm and solid.
438. Cross Section of Subgrade. The cross section of the sub-
grade is made either flat or crowned to conform to the finished sur-
face of the concrete. The former seems to be the better and more
common. It is claimed that experience shows that a pavement hav-
ing a flat subgrade is less likely to crack longitudinally. No entirely
satisfactory reason for this has been given; but possibly it is due to
the greater tendency of the two portions to separate when the sub-
grade is crowned.
439. ONE VS. TWO-COURSE PAVEMENTS. Usually the con-
crete is laid in one course at a single operation ; but sometimes, when
material suitable for the wearing surface is expensive, and when local
but less suitable material is much cheaper, the concrete is laid in two
242 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
courses, the lower one of the poorer and cheaper material. The
lower course is usually also a little leaner and the top course a little
richer than for a one-course pavement. At times such construction
may be economical.
On the other hand, the two-course work is objectionable for the
following reasons: 1. It is difficult to keep the two classes of material
separate. This is more serious on a narrow rural road than on a wide
city pavement. 2. There is a possibility of not securing a good union
between the two courses. 3. If the lower course is leaner, it will
absorb water from the subgrade and expand, while the top course
is likely to dry out and contract; and consequently the two are likely
to separate. 4. The bottom course absorbs water and expands, and
is likely to produce a longitudinal crack.
Good two-course pavements have been laid; but they require
more labor and greater care in construction. Statistics show that a
little less than 30 per cent of all concrete roads and pavements are two-
course work.
All the discussions that follow relate to one-course work.
440. If reinforcement fe to be used (§ 469), it is necessary to lay
the concrete in two courses and place the reinforcing net between
them; but the same mixture is ordinarily used for both courses,
and hence such construction is not usually classed as two-course
work.
441. CROSS SECTION OF PAVEMENT. The cross section of the
concrete slab depends upon its width and thickness, and upon the
crown of its upper surface.
Fig. 75 shows the cross sections recommended by the 1916
National Conference on Concrete Road Construction, for cuts
and for fills. For the recommendations of a committee of the
American Society of Civil Engineers concerning crown, see Table 16,
page 65.
442. Crown of Surface. In determining the proper crown
for a concrete road surface, a distinction should be made between a
country road and a city street. The former need be crowned only
enough to afford lateral drainage, particularly after the middle is
worn down somewhat; while on a city street with side curbs the
crown should be enough to prevent an undue portion of the pave-
ment from being covered with water during a rain.
The only advantage in crowning a road surface is to secure surface
drainage, and with perfect work a very small crown would suffice
An excessive crown drives travel to the middle of the road and con-
ART. 2]
THE CONSTRUCTION
243
sequently does not distribute the wear uniformly over the pavement;
therefore the less the crown the better, provided good surface drain-
age is secured. Some crown is necessary on account of (1) inev-
itable imperfections in finishing the surface, (2) the accumulation of
leaves, twigs, straws, etc., on the surface, and (3) the wear of the
pavement.
443. The crown used in practice for concrete roads without
curbs, varies from -£$ to ^w of the width, without much tendency
ofC/rc/e
SECTION ON LEVEL o/=? /N CUT
*tV' Denotes Mdfh of Pbremenf
FIG. 75. — CROSS SECTIONS FOR CONCRETE ROADS.
to group withir any smaller limits. For concrete pavements with
curbs the crown employed in practice varies from -gV to y^ of the
width, the intermediate values being used about equally. The
National Conference on Concrete Road Building recommends a
crown of T^-Q.
444. Super-elevation on Curves. For a discussion of this subject,
see § 90.
445. MAXIMUM GRADE. For a discussion of the general sub-
jects of maximum grades, see § 79-85; and for the recommendations
of a committee of the American Society of Civil Engineers concerning
maximum grades, see Table 15, page 57. The above committee
recommends 8 per cent as the permissible maximum; but numerous
examples could be cited of concrete surfaces on steeper grades. For
example, Sioux City, Iowa, lays concrete on grades up to 16 per cent,
and Mankato, Minn., on 14.3 per cent.
244 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
446. WIDTH. For a general discussion of the width of the im-
proved portion of a road, see § 95-6. Both the 1914 and the 1916
National Conference on Concrete Road Building recommended a
width of concrete of 10 feet for a single-track road, and 18 feet for a
double-track road; and that for 'roads 18 feet or more in width, it is
not necessary to harden the shoulders by applying gravel or broken
stone. For a consideration of extra width on curves, see § 97.
447. THICKNESS OF CONCRETE. The proper thickness depends
to some degree upon the climate, the width of the slab, the character
of the soil, the thoroughness of both the surface and the under-
drainage, the degree of consolidation of the subgrade, the propor-
tions and quality of the concrete, the character of the workmanship
of every part, the care employed in curing the concrete, and the
amount and character of the traffic. These factors are not of equal
importance, and the importance of any one item may vary consid-
erably with local conditions.
It is not possible to compute the thickness required in any par-
ticular case; and all that can be done toward determining a working
rule for the thickness of a concrete road slab, is to accumulate data
concerning practice. In studying the result^ of experience, care
should be taken to consider each of the factors mentioned above.
As a rule engineers make the thickness more at the middle than
at the sides; but some engineers prefer a uniform thickness, on the
theory that traffic is likely to be distributed over the entire surface.
Apparently the latter overlook the heaving effect of frost, and also
the effect of wear, which is more at the middle than at the sides.
In California, part of which state is classed as semi-arid and part
as arid, the State Highway Commission has laid many miles of con-
crete roads 16 and 18 feet wide and only 4 inches thick, which have
given entire satisfaction.
In Oregon, a state that is chiefly semi-arid, many miles have been
built 15 feet wide with a thickness of 5J inches at the sides and 6£
inches at the crown, which competent authority pronounces as being
satisfactory. In 1914 three miles of 16-foot roadway were built in
which the thickness at the sides was only 4 inches and that at the
crown only 5 inches, and in 1917 this road was in excellent con-
dition.
In the Mississippi Valley, the general practice seems to be to
make the thickness 6 inches at the side and 7 or 8 inches (usually the
latter) at the crown.
448. SIDE FORMS. The concrete is laid between side forms or
ART. 2] THE CONSTRUCTION 245
curbs. The former is the custom for rural roads, and the latter
for city pavements. The forms for curbs or combined curbs and
gutters will be considered in Chapter XIV — Curbs and Gutters.
For rural roads, planks or steel channels are set at the edges of the
concrete slab to retain the concrete, the width of the form boards
determining the thickness of the slab. The forms are usually set
before the subgrade is brought to its final surface. The tops of the
forms are used as guides in finishing the surface of the subgrade, and
later are used to guide the template in striking off the the top surface
of the concrete.
The forms may consist of 2- or 2^-inch plank or of steel channels.
The plank forms should be securely held in place by means of stakes
on the outside driven either to such a depth that their tops are below
the upper edge of the forms or at such a distance outside of the forms
as not to interfere with the operation of the template. The planks
should have a continuous bearing on the subgrade, as otherwise
they will sag when the concrete is struck off. Adjoining ends of
the several planks should be fastened together so as to keep them in
line. Fig. 76, page 247, shows plank forms in reasonably good posi-
tion. Fig. 77, page 247, shows plank forms poorly constructed and
poorly set up. In Fig. 77 notice the space under the bottom of the
side form ; and apparently the forms are not in line either in the fore-
ground or the back-ground.
The steel channels are generally more economical than plank;
and are easier set, and keep their place better. They are provided
with telescoping joints, which keep the different sections in line;
and are hung upon steel stakes previously driven to the right depth,
which make it easy to place the forms at the desired grade.
The steel side-forms may be flat and leave a square corner on the
upper edge of the concrete slab, or they may have a projection that
will leave a beveled edge. However, if the edge of the concrete slab
is to be beveled or rounded, it is better to secure this by the use of a
finishing tool than by a bevel-edge form.
449. THE CONCRETE. Proportions. The best proportions for
any particular materials can be determined only by one or the other
methods explained in §421 and §422; that is, either by finding by
trial the proportions that give maximum density and hence maximum
strength, or by sieve-analysis curves.
The proportions originally used are 1 : 2 : 3, 1 : 2 : 5, 1 : 3 : 6,
or 1 : 2J : 7. The efficiency of these proportions can not be known
unless the gradation of the aggregates is known.
246 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
450. The National Conference on Concrete Road Building rec-
ommends (for the fine and coarse aggregates specified in § 412 and
§ 416, respectively) 1:2:3. The Ohio Highway Department uses
1:2:4; and the Pennsylvania Highway Department 1 : 1J : 3
or 1:2:3. The U. S. Office of Public Roads uses 1 : l\ : 3 for
gravel or 1 : 1 \ : 3 for broken stone.
451. Mixing. The ingredients should be mixed, preferably in a
batch mixer, until every fragment of the coarse aggregate is covered
with mortar and until the concrete is of uniform consistency and
color. Ordinarily this can not be accomplished unless all of the
materials are in the drum for at least 1 minute and the drum is run
at a rate of not less than 12 or 16 revolutions per minute. The time
element is as important as the number of revolutions, since time is
necessary to allow the water to diffuse through the mass. Some
engineers specify that the concrete shall be turned for If minutes.
Experiments show the following relation between crushing strength
and the time of mixing.*
Crushing strength when mixed f minute or 9 revolutions = 1400 Ib. per sq. in.
Crushing strength when mixed 1 minute or 17 revolutions = 1587 Ib. per sq. in.
Crushing strength when mixed 1£ minute or 26 revolutions = 1924 Ib. per sq. in.
The drum must be completely emptied before another batch is
added.
452. Fig. 76 shows a typical batch concrete-mixer. In this
case the concrete is delivered with a bottom-dump bucket. The
bucket should close tightly so as not to leak. Sometimes the con-
crete is delivered with a tilting bucket. Again, the concrete is
delivered through a revolving spout having spiral blades on its
inside; and sometimes the mixing of the concrete due to its travel
through the tube is offered as an excuse for decreasing the amount of
mixing in the drum, but this should not be allowed as in its trip
through the revolving spout the concrete is mixed but a little. Some-
times the concrete is delivered by sliding it down a trough; but this
is undesirable as the tendency is to mix the concrete too wet, so it
will slide freely. Fig. 77 shows a concrete mixer delivering the
wearing coat of a two-course concrete road through a gravity spout.
Apparently the mixture is much too wet.
The concrete is sometimes transported in wheel-barrows or bug-
gies; but this is objectionable as in the transportation, particularly if
* Engineering News, Vol. 75 (1916), p. 768.
ART. 2]
THE CONSTRUCTION
247
the distance is more than 100 feet or if the concrete is quite wet, the
coarse aggregate settles to the bottom, and the thoroughness of the(
mixing is destroyed.
453. Batch mixers are made in various capacities from 6 to 60
cubic feet of concrete; but only those having a capacity from 6 to 30
cubic feet, a 1- to 5-bag mixer, are used in rural-road or city-pave-
ment work. A one-bag or two-bag mixer is the most common, as it is
practically impossible in road work to get the material to a larger
mixer fast enough to keep it going economically.
The latest models of batch mixers have a loading skip, a device for
regulating the time of mix, and an automatic water tank (Fig. 76).
FIG. 76. — CONCRETING CREW AT WORK.
FIG. 77. — DELIVERING THE TOPPING OF A
TWO-COURSE CONCRETE ROAD.
454. With continuous mixers it is difficult to control accurately
the proportions and the thoroughness of the mixing. Some con-
tinuous mixers have devices for automatically measuring the several
ingredients, and give fairly satisfactory results when intelligently
operated and kept in good order; but ordinarily specifications do not
permit the use of a continuous mixer.
455. Whatever the type of concrete mixer, it is wise to check
the proportions occasionally, at last once each day, by noting the
quantities of the several ingredients used in a certain area of pave-
ment. Since the volume of concrete produced from stated quanti-
ties of the ingredients can not be computed accurately, this method
is a better check upon uniformity than upon the amount of the
ingredients used; but after a little experience with the particular
materials and consistency, a real check can be obtained of the
amount of the ingredients used.
248
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
FIG. 78.— Box WHEED-BARROW.
Fig. 78 shows a form of wheel-barrow used on the Allentown
and Eastern concrete road in Pennsylvania, which secures greater
accuracy in measuring the ingredients than the usual wheel-barrow
having a pressed steel bowl with
four curved edges. It can be
seen at a glance whether the bar-
row shown in Fig. 78 is full; while
with the form having four curved
edges, it is difficult to determine
whether the barrow contains the
right quantity.
456. Hand Mixing. Only a
few years ago concrete for roads
and pavements was usually mixed
by hand; but now it is practi-
cally all mixed by machine. The
output per man with a machine
is usually about 50 per cent more
than by hand; and the total cost is considerably less by machine
than by hand, the exact difference depending upon the thoroughness
of mixing in both cases, upon the size and type of the machine, and
upon the amount of work per year done with it.
457. Re-tempering. The re-tenipering of concrete, i. e., the re-
mixing of concrete that has partially set, should not be permitted
under any circumstances.
458. Consistency. The materials should be mixed with sufficient
water to produce a concrete which will hold its shape when struck off
with the template. The consist-
ency should not be such as to
cause a separation of the mortar
from the coarse aggregate in
handling. The tendency is to
use an excess of water, which
facilitates the handling of the
concrete, but also tends to sepa-
rate the ingredients and greatly
weakens the concrete.
Fig. 79 shows the proper con-
sistency. With less water the concrete will not flow; and with
more water there will be segregation, the mortar flowing to the
bottom of the pile. The consistency of concrete is not a sure indi-
FIG. 79. — PROPER CONSISTENCY OF CONCRETE.
ART. 2] THE CONSTRUCTION 249
cation of its quality, since a given consistency may be produced by
using more cement and less mixing, or by more mixing and less
cement, or by adding hydrated lime; but nevertheless the above
test is a reasonably good one.
459. Placing. If the subgrade has been rutted up in hauling over
it, the surface should be restored; and then it should be thoroughly
sprinkled, but there should be no pools of water when the concrete
is placed. The thorough sprinkling of the subgrade adds materi-
ally to the wearing quality of the concrete, by keeping it from dry-
ing out too soon.
Immediately after being mixed, the concrete should be deposited
to the required depth and width. The section should be completed
to a transverse joint without the use of intermediate forms or bulk-
heads, or a transverse joint may be placed at the point of stopping
the work. In case the mixer breaks down, the concrete should be
mixed by hand to complete the section.
Where reinforcement is used it should be embedded in the lower
course of concrete before the concrete has begun to harden; and the
concrete above the reinforcement should be placed within 20 minutes
after the placing of the concrete below.
460. The placing of concrete for roads when the temperature is
near freezing is not advisable; but if such work is practically un-
avoidable, the water and the aggregates should be heated before
mixing, and the fresh concrete should be protected from freezing
for at last 10 days, if the temperature remains near freezing
(see § 464). Concrete should never be deposited on a frozen sub-
grade.
461. Striking. As soon as placed the concrete should be struck
off to the established crown and grade by means of a template resting
on the side forms and moving with a combined longitudinal and
transverse motion. The concrete should have originally been de-
posited so high that a little concrete will accumulate in front of the
template ; but this accumulation should not become excessive, and it
should be kept nearly uniform along the template by removing the
excess with a shovel and throwing it ahead or where needed along
the template. As the template approaches a transverse joint most
of the excess should be removed.
The template or strike board for a 10- or 12-foot road consists of
a 2-inch plank 6 or 8 inches wide cut on the under side to fit a crown
slightly in excess of that of the finished surface, and having handles
and a shoe to run on the side forms; and for a 14- to 20-foot road,
250
PORTLAND-CEMENT CONCRETE ROADS
[CHAP. VII
two 2- by 10-inch planks spiked together. When the pavement is
over 20 feet wide a trussed template must be employed.*
462. A variety of devices have been invented to facilitate the
striking of the concrete and at the same time^to consolidate the
FIG. 80. — HAND-TAMPING TEMPLATES FOE CONCRETE ROADS.
FIG. 81. — BAKER FINISHING MACHINE.
concrete by tamping the surface. One such method consists in first
using a template that leaves the concrete a little high, particularly
in the center; and then following with a heavy template having a
* For detailed drawings of two templates running on rollers and having levers for raising,
and also being adjustable for different widths of pavements, see Proc. 1916 National Confer-
ence on Concrete Road Construction, p. 203 and 204.
ART. 2J
THE CONSTRUCTION
251
handle at each end by which it is lifted and dropped at close inter-
vals—see Fig. 80. Another device consists of a self-propelling tem-
plate and tamping machine running upon the side forms. The
front end of the machine strikes the concrete a little high and the
rear end tamps it to the right height by an up and down motion
of a steel plate extending across the pavement. Fig. 81 shows this
machine.
463. Finishing. After the concrete is brought to the estab-
lished grade and crown with the template, the surface is smoothed or
finished, usually with a wood float, the operator working upon a
suitable bridge— see Fig. 82. The wood float is better than a
FIG. 82. — BRIDGE UPON WHICH FINISHER WORKS.
metal trowel, since the latter gives a polished surface and also tends
to work a film of neat cement to the surface.
The time of finishing has a marked effect upon the wearing quality
of the pavement. The tendency is to finish the concrete too soon
after placing. The proper time depends upon the weather conditions
and the wetness of the concrete when placed. The concrete should not
be finished until it is nearly ready to take the initial set; and when in
this condition the surface will contain practically no free water, and
will be of such a consistency that the wood float will leave distinct
marks on the surface. All foreign substances, such as sticks, coal,
lumps of clay, etc., on the surface should be removed before finishing.
The surface is sometimes stippled with a broom or stiff brush to
252 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
prevent it from becoming slippery, but this is unnecessary; and the
making of grooves in the surface is very objectionable even on grades,
since the grooves weaken the slab and greatly increase the wear.
Several devices have been invented to facilitate the finishing
of the surface. One method is to draw an 8- to 12-inch rubber belt
or a rubber-faced canvas belt, or a plain canvas belt back and forth
across the pavement after it has been struck off with the template,
and to move it along the pavement as the surface is finished. For
the best results the surface should be given a second floating just as
the cement takes its initial set. When armored contraction joints
(§465) are used, it is necessary to do a small amount of hand floating
near the joint. The belt finish gives a good surface at small cost.*
The most recent method of finishing the surface is to roll it.
The rolling may be done in either of two ways: 1. With a roller
about 8 inches in diameter and about 6 feet long, made of light sheet
steel and weighing about 70 lb., attached to a long pole, the operator
standing at the side of the road and rolling the concrete back and
forth across the road. 2. With a roller a little longer than the con-
crete road is wide, having a round steel axle projecting at each end,
the roller being operated by a man on each side of the roadway by
means of a suitable handle with a hole in its lower end through which
the axle passes. Although first used in 1917, the method of finishing
by rolling seems to secure greater strength and density than hand
floating; and is rapidly being adopted.
Another method is to draw a rubber garden hose in the form of the
letter U along the pavement. Still another method is to draw a
1-inch plank endwise back and forth over the concrete by means of a
rope attached to each end. A long-handled float has been used, but
opinions differ as to its efficiency.
464. Curing and Protecting. The green concrete may be seriously
damaged by the too rapid drying out of the surface in hot or windy
weather, or by exposure to low temperature, or by being opened to
travel too soon.
If the concrete dries out too rapidly, the surface becomes friable
and chalky, and is covered with shrinkage cracks, which are a source
of weakness. Therefore in hot or windy weather it is usually neces-
sary to cover the concrete with canvas for at least half a day after
it is floated. This canvas is made either in 6-foot strips 2 or 3
feet longer than the pavement is wide, or in long strips a little
* Engineering News, Vol. 77 (1917), p. 197-8; or Illinois Highways, Dec., 1916, p. 155-56.
ART. 2]
THE CONSTRUCTION
253
FIG. 83. — CONCRETE ROAD COVERED WITH
CANVAS.
wider than the pavement, mounted on rollers. In either case, if
the concrete has not set, the canvas should be supported on frames
so it will not touch the concrete.
Fig. 83 shows a concrete road
protected by canvas. Notice
that the earth at the sides of
the concrete has been plowed
preparatory to covering the con-
crete with it.
When the concrete has hard-
ened sufficiently, the canvas is
removed, and the pavement is
covered with at least 2 inches of
earth which should ordinarily be
kept wet for 10 to 15 days.
Shavings or straw are sometimes
used to cover a new concrete
pavement ; but they are liable to
be washed into piles in sprinkling or to be blown off, in either case
leaving exposed patches
If there is danger from frost, the ingredients should be heated
before the concrete is mixed (see § 451); and after being placed the
green concrete should be protected by canvas or building paper.
The former is the easier handled, and is usually more economical.
In extreme cases steam may be blown under the canvas or building
paper. To protect the concrete after the first night, a layer of straw
with a little earth on it has been used.
Great care should be exercised in opening the pavement to travel.
The length of time necessary to keep the pavement closed will depend
entirely upon weather conditions. During warm weather the pave-
ment should be kept closed to travel for at least fourteen days, and
preferably for three weeks. When the conditions are such that the
temperature of concrete is less than 50° when placed, hardening takes
place very slowly. When a concrete pavement has been laid in the
late fall, it is sometimes difficult to determine when it will be safe to
open the road. In rare cases it may be necessary, owing to local
conditions, to open the road or street before it is absolutely safe.
Under such conditions if about 2 inches of straw is placed on the
pavement and this is covered with a few inches of earth, the pave-
ment will be protected sufficiently against abrasion to allow the open-
ing of the road sooner than could be safely done without such pro-
254 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
tection. This cover will, however, not minimize the danger of dam-
age to the pavement by heavy loads, which will tend to crack a
pavement that has not developed its full strength.
465. CONTRACTION JOINTS. Concrete expands and contracts
with changes of temperature and moisture. Since the pavement is
ordinarily laid in warm weather, the contraction is likely to be greater
than the expansion; and besides the concrete can resist the com-
pression due to expansion better than the tension due to contraction.
To prevent the formation -of unsightly and irregular cracks due to
contraction, it is customary to provide contraction joints at regular
intervals. If the pavement has curbs, longitudinal contraction
joints also are provided at each curb. Concrete roads and pave-
ments are usually provided with transverse contraction joints from
25 to 75 feet apart, usually about 50 feet.
466. Contraction joints are made in any of three ways: (1) by
inserting a wood strip or steel plate, and removing it after the con-
ji >
FIG. 84. — ASSEMBLING AKMORED JOINT.
crete is in place, and then pouring in an elastic mastic of tar or
asphalt; (2) by inserting during construction a sheet of mastic pre-
pared for the purpose; or (3) by inserting one or more thicknesses
of tar paper or asphalt felt.
The longitudinal contraction joints are made by placing next
to the curb a layer of bituminous mastic from \ to 1 inch thick, de-
pending upon the width of the pavement.
The transverse joints are sometimes protected by placing a soft-
steel i-inch plate on each side of a J-inch sheet of mastic. The plates
are provided with projections which securely tie or bind them to the
concrete. Fig. 84 shows the two metal plates and the intervening
sheet of mastic or tar paper being clamped together preparatory
ART. 2]
THE CONSTRUCTION
255
to being set into the pavement. Fig. 85 shows the joint being
installed in position in the concrete. Notice that the plates are
suspended from a temporary bar which rests on the side forms.
FIG. 85. — INSTALLING ARMORED JOINT.
Fig. 77, page 247, shows an armored joint almost covered with
concrete; but notice that there is no temporary supporting bar as in
Fig. 77. These protected joints are expensive, complicated to
install, and do not wear down with the concrete; and consequently
are falling into disrepute.
The most popular joint filler for transverse joints is one or more
thicknesses of 3-ply tar paper, which when new projects slightly
above the surface of the pavement. Fig. 86 shows the method
FIG. 86.— FINISHING TAR-PAPER JOINT.
FIG. 87. — TRIMMING TAR-PAPER JOINT.
of finishing the concrete next to one of these joints; and Fig. 87
shows the way of trimming off the surplus tar paper. Some
engineers use two thicknesses in hot weather, and three in cool
256 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
weather. A single thickness of 3-ply tar paper has been entirely
satisfactory. To facilitate the insertion of the tar paper, it is tied
with a small string to a steel plate or 1-inch plank thinned a little at
its lower edge and cut to the proper width and crown, and the plank
and paper are set into place and concrete is deposited on both sides
of the plank; and then the strings are cut, the plank and strings
removed, and the space occupied by the plank is tamped full of con-
crete.
It is important that the face of the joint be truly vertical, or
there is danger of one slab's sliding up and over the next one
A transverse joint is likely to be a source of weakness owing to the
tendency to force an excess of cement, and consequently a deficiency
of stone, next to the joint; and also owing to the tendency of the con-
crete to chip out next to the joint. These tendencies arc reduced
somewhat by the use of a split float (Fig. 86), which will finish
both sides of the joint at the same time.
467. Sometimes the contraction joints are placed at an angle
with the length of the road, which is an advantage if there is either
an elevation or depression at the joint; but if the joints are properly
made and maintained, the extra length is only a needless expense,
and besides it is very difficult to strike the surface adjacent to the
diagonal joint.
468. A number of attempts nave been made to determine mathe-
matically the proper distance between contraction joints; but
there is so much uncertainty in each of the several factors of the prob-
lem that any such computation is practically worthless. There
seems to a growing tendency to narrow the thickness of the joint
filler and to lengthen the distance between the joints. Some rural
roads have been built without any contraction joints, on the theory
that when cracks form they can be filled with pitch. In filling a
crack the mastic is piled up over the crack a little to protect the edge
of the concrete. Of course, all contraction cracks as well as all others
should be kept full of bituminous filler as a part of the maintenance
of the road.
469. REINFORCEMENT. Some engineers claim that concrete -
pavements should be reinforced to prevent cracks due (1) to changes
of temperature and moisture, (2) to improper drainage and defective
foundation, (3) to insufficient thickness of concrete, and (4) to
defective construction. The most simple and most economical
method of eliminating each of the three last classes of cracks is to
remove the cause. The use of reinforcement simply distributes the
ART. 2] THE CONSTRUCTION 257
cracks due to changes of temperature and moisture, thus substi-
tuting many minute cracks for a few large ones. The large cracks
can be protected by filling with tar or asphalt, while the small ones
can not be protected, or rather will not be protected, and hence will
be a cause of deterioration of the pavement.
It is impossible to compute with any degree of accuracy the
amount of reinforcement required to prevent temperature cracks,
and much more so to determine the amount required to prevent
cracks due to the other causes mentioned in the preceding para-
graph. To be most efficient in preventing cracking due to some of
the causes, the reinforcement should be near the top of the slab, and
for others near the bottom. When reinforcement is used, it is gen-
erally placed 2 inches from the top; but when so placed it is not
very effective. With the same depth of embedment the reinforce-
ment in a thin broad slab is much less effective than that in a
narrow deep beam. Further, the reinforcement is expensive, and is
troublesome to install. The reinforcement usually adds 15 to 20
cents per square yard to the cost of the pavement. When reinforce-
ment is used, the concrete must be laid in two courses, which further
adds to the expense, and also there is danger that the two courses
may not thoroughly unite. Those who professedly use reinforce-
ment primarily to prevent temperature cracks, usually recommend
that contraction joints be constructed 75 feet apart; but many
concrete roads have been reasonably satisfactory without either
reinforcement or contraction joints.
However, reinforcement does serve to keep the parts of the slab
from separating after cracks have formed.
It is probably unwise to reinforce a concrete pavement, except
perhaps where the slab rests upon spongy soil which it is not prac-
ticable to replace, or where it is impossible to obtain adequate drain-
age. Reinforcement is more common for wide city pavements than
for narrow country roads. Only 5 per cent of all concrete roads and
pavements laid in this country have been reinforced.
470. SHOULDERS. The shoulders should be partially con-
structed when the subgrade is prepared; and after the concrete is
completed and cured, the shoulder should be finished. It is usual
to reinforce the earth shoulder by adding broken stone or coarse
gravel, making it 4 to 6 inches thick next to the paved roadway and
feathering out to nothing at a distance of 3 to 5 feet out. The
shoulders should be thoroughly consolidated by rolling, and should
be finished flush with the pavement but not any above it.
258
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
Some competent engineers do not strengthen the earth shoulders
of double track roads; but this seems of doubtful widsom, because
there is always more or less turning off from the pavement, even on
double-track roads, — if for no other reason, than that the slow-moving
vehicle may give the right-of-way to the fast-moving one, as is
required by law in some states.
471. CURBS. A concrete road ordinarily does not have a curb,
since it is expected that vehicles will turn off upon the shoulders.
When a road is in a cut or upon a hillside, it may be necessary to
provide a gutter for drainage. If the road surface is water-bound
gravel or water-bound macadam, the gutter must be outside of the
WinnetkaJ//.
V®
/<4
j
IL_
> \v* :Va4;;;?.VMr* Fabnc^b^\ f/M fe£±.$-
Indianapofo,Ir)d ff/mber/y,
FIG. 88. — INTEGRAL CURBS FOR CONCRETE PAVEMENTS.
paved way, since the flowing water would destroy the road surface;
but if the road is a concrete one, flowing water will not damage it,
and consequently instead of building a gutter outside of the paved
way, it is considerably cheaper to build a curb against or on the edge
of the concrete slab, and allow the water to flow down the edge of the
concrete slab.
The curb is most cheaply constructed if it is cast at the same time
as the slab, and hence is called an integral curb. The integral curb
is usually cheaper to build than a durable gutter, and in a cut its use
saves considerable excavation.
The integral curb has been used for concrete driveways for a
number of years, but was first used for public roads about 1914.
Various forms of integral curbs have been used. The simplest is
made by shaping the end of the strike board to form a low curb, but
ART. 2]
THE CONSTRUCTION
259
this form is used only on park drives where a prominent curb is not
desired but where a waterway is necessary. The cost of such curbs
is nearly negligible.
Fig. 88 shows four types of integral curbs that have been used.*
These curbs are constructed by making a form board for the edge
of the roadway slab and the back of the curb, and forming the face of
the curb by clamping with carpenter's screw clamps a form board
against spacing diaphragms. The curbs shown in Fig. 88 are much
cheaper to construct than the type of combined concrete curb and
gutter used with other forms of pavements — see Chapter XIV.
Fig. 89 shows the from used in integral curb construction
in Milwaukee, Wisconsin, f As soon as the pavement is struck
off, the form is set in
place and weighted with
bags of sand to prevent
it from rising when the
concrete is deposited in
it.
The concrete founda-
tion for brick pave-
ments for rural roads
were formerly built with
integral curbs similar to
the last two shown in
Fig. 88 except that the
corner at both the bottom and the top of the inside face was made
square rather than rounded. Strictly speaking the projection on the
concrete foundation is not a curb, since it does not project above
the pavement. It is usually referred to as an integral curb; but a
more appropriate name is concrete edging, which is occasionally used.
At present brick pavements for rural roads are usually built mono-
lithic (§ 982), which does away with the need of any curb or edging.
472. COST OF CONCRETE ROAD. The cost of a concrete road-
slab varies with the specifications and the local conditions, and hence
no record of cost will apply strictly in all cases; but cost data are
useful for comparison and as a guide in preparing estimates.
Some of the following cost data are a little out of date; but prices
for 1917 are abnormal owing to the disturbance, due to the Great
European War, and besides any cost data presented in a book of this
Fio. 89. — FORM FOR INTEGRAL CUBE.
* Engineering Record, Vol. 71 (1915), p. 111.
t Engineering and Contracting, Vol. 45 (1916), p. 544.
260 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
character soon becomes out of date. The costs given below are full,
and are believed to be accurate and representative. They are inter-
esting chiefly as showing relative values in different localities, and of
the different parts of the work. For current prices for concrete
roads, consult the construction news in technical journals.
473. Cost of Materials. For data on the cost of portland
cement, see §425; for the same for sand and gravel, see §426;
and for broken stone, see § 427. For more recent prices, see the
market reports in current technical journals. For information con-
cerning the amount of the several ingredients required for a cubic
yard of concrete, see § 428-29.
474. Cost of Labor. The work of the two following examples
of hand and machine mixing was done under substantially the same
conditions and hence the results are fairly comparable. Notice
that the cost with hand mixing is about a half more than with ma-
chine mixing. Further, it is probable that no practicable amount of
hand mixing will give as good concrete as ordinary machine mixing;
or in other words, if the hand mixing had been as thorough as the
machine mixing, the difference in cost would probably have been still
greater. Hand mixing has practically been abandoned in concrete
road and pavement work.
475. Hand Mixing. The construction was two-course work — a
5-inch 1:3:5 base and a 2-inch 1 : 2 wearing coat; but the cost is
not given separately for the base and the top. There were steel-
protected contraction joints every 25 feet. The pavement was 30
feet wide.*
ORGANIZATION. COST.
CTS. PER SQ. YD.
1 foreman @ 40ff per hour 2 . 10
1 finisher @ 40^
1 finisher's helper @ 20 ^f
Total for finishing 2.88
1 form setter @ 25 £
1 form setter's helper @ 20ff
Total for setting forms 1.76
8 mixers @ 20^
1 cement man @ 20 ^f
1 man on sand @ 20 £
4 men on broken stone @ 20 j£
2 spreaders @ 2Q£
Total for mixing and spreading 12 . 24
1 watchman Q 50
Total for mixing and laying 19 ..
* Engineering and Contracting, Vol. 38 (1912), p. 710.
ART. 2] THE CONSTRUCTION 261
476. Machine Mixing. The width of pavement was 30 feet.
The construction was two-course work, — a 5-inch 1:3:5 base and
a 11-inch 1:1:1 wearing coat.*
ORGANIZATION. COST.
CTS. PER SQ. TD.
BASE:
9 men on broken stone @ 22%£ 2 . 18
3 men on sand @ 22^ , 0.73
1 man at skip @ 22^. 0.24
1 man wheeling cement @ 22%£ 0 . 24
1 man leveling concrete @ 25£ 0 . 27
1 helper leveling concrete @ 22^ 0 . 24
1 tamper @ 22^ 0.24
1 engineer @ 25£ 0.27
1 fireman @ 25£. . . 0.27
1 bucket operator @ 15£ 0.16
1 water boy @ 5£ 0.05
1 sack boy @ 5£ 0.05
1. foreman @ 45ff 0.48
Total for base 5.43
WEARING COAT:
4 men on granite chips @ 22^ .• 0.48
4 men on sand @ 22^ 0.48
2 men at skip @ 22^ 0.24
2 men wheeling cement @ 11\i 0 . 24
2 rough spreaders @ 22^ 0.24
1 fine spreader and tamper @ 25ff 0.13
1 fireman @ 25^f 0.13
1 engine runner @ 25^ 0 . 13
1 bucket operator .@ 15^ 0.08
1 sack boy @ 5?f 0.02
1 water boy @ 5^ 0.02
1 foreman @ 45?f 0 . 24
1 finisher @ 25£ 0.61
1 finisher's helper @ 22^ 0.55
Total for wearing coat 3 . 61
SETTING FORMS:
1 man @ 22^ 0.42
MISCELLANEOUS:
1 man trimming grade @ 22|ff 0 . 43
2 men cleaning up sand and stone 0 . 36
moving machine 1 . 30
Total for miscellaneous labor . . 2 . 09
Grand total 13.50
* Engineering and Contracting Vol. 38 (1912), p. 710.
262
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
477. Relative Cost of Labor and Materials. The following data
are the averages for seven Michigan State-aid roads, eight Wayne
Co. (Mich.) roads, and all the concrete roads built by the Illinois
Highway Commission in 1912-13.
MATERIALS : aggregates 27 . 7 per cent
cement 21.6
expansion joints and supplies 6.4
Total materials 55 . 7 ' '
LABOR.. 44.3 "
Total 100.0
478. Total Cost. The three examples in Table 29 are believed
to be fairly representative.
The values given in Table 30, page 263, are the average of the
contractor's costs exclusive of over-head expenses and profits, but
inclusive of the preliminary shaping of the surface and the finishing
of the earth shoulders; and include representative states.
For more recent data, see the bidding prices in the construction
news of current technical journals.
TABLE 29
COST OF ONE-COURSE CONCRETE SLAB FOR ROADS IN ILLINOIS *
DATA AND DIMENSIONS
ITEMS.
' McLEAN.
CARLJNYILLE.
SPRINGFIELD.
Area of pavement, sq. yd
Thickness of pavement, inches
5000
6
7 111
6.5
5594
7
Width of pavement, feet
Length of haul, miles
45
0 12
16 ;
1 5
18
12
Cost of cement per bbl
Barrels of cement used per sq. yd
Cost of gravel per cu. yd. f.o.b. desti-
nation
Labor, price per hour
$1.06
0.29
$1.25
0.22
0.98
0.33
$1.25
0.225
1.025
0.29
$1.25
0 25
Teams, pi ice per hour
0.50
0.50
0.50
COST OF LABOR AND SUPPLIES
ITEMS.
TOTALS.
Per
Sq.yd.
TOTALS.
Per
Sq. yd.
TOTALS.
Per
Sq. yd.
Superintendence
Shaping subgrade
Loading and hauling sand and stone . . .
Mixing and placing concrete
Watchman and miscellaneous labor. . . .
Cost of sand and stone
$140.00
307.11
267.34
414.63
110.26
1017.63
1 547.15
48.67
30.75
35.00
45.18
$0.028
.061
.053
.083
.022
.204
.309
.010
.006
.007
.010
$ 157.50
108.70
795.05
700.58
131.46
741.00
2307.90
112.40
25.00
31.75
100.66
591.73
$0 . 0220
.0153
.1120
.0986
.0184
.1050
.3246
.0156
.0034
.0047
.oiio
.0840
$ 202.00
343.44
603.50
644.25
383.75
1 622.01
1 551.17
206.74
119.19
18.33
$0.0361
.0415
.1078
.1150
.0686
.2897
.2772
.0369
.0213
.0033
Cost of cement
Expansion joints
Coal, oil, and miscellaneous supplies. . .
Forms and other lumber
Filling curb expansion joints
Reinforcing steel
Excavation . .
2li' .38
' ' ! 0378
Trimming shoulders
Total
$3964.02
$0.793
$5803.07
$0.8176
$5794.76
$1.0352
* Report 111. Highway Com., 1912, p. 240, 241, and 247, respectively.
ART. 2] THE CONSTRUCTION 263
TABLE 30
AVERAGE COST PER SQUARE YARD OF CONCRETE ROAD SLABS IN 1915 *
Connecticut $1 . 13 Missouri $1 .09
Illinois 1 . 03 New Jersey 1 . 23
Indiana 0 . 98 New York Q 98
Iowa 1 . 19 Ohio. • i .02
Kansas 1 .28 Pennsylvania 1 .01
Maryland £ 1 . 08 Texas 1.15
Massachusetts . 0.95 West Virginia 1 .03
Michigan 1 . 10 Wisconsin 1 . 02
Minnesota 1.11
479. CHARACTERISTICS OF A CONCRETE ROAD. The character-
istics of a well-built concrete road are: 1. It is reasonable in first
cost in proportion to its probable durability. 2. It has a low tractive
resistance, but gives a good foothold for horses and automobiles. 3.
It is free from dust. 4. It is easily maintained. 5. It is reasonably
durable when properly maintained. 6. Its only fault is that the
color is somewhat trying to the eyes of the user; although the light
color is some advantage at night.
480. CONCRETE STREET PAVEMENTS. The preceding discussion
has reference primarily to strips of concrete 10 to 20 feet wide for
rural roads, since concrete is in more common use for rural roads
than for city streets. The same methods without material change
can be employed for pavements up to 30 feet wide; but for wider
pavements it is necessary to modify the plan in one of two ways as
follows: (1) Make a longitudinal joint in the middle of the street
and lay half of the street at a time; or (2) insert screeds transversely
to the street, and strike the concrete with a straight edge held parallel
to the curb.
On a wide street the finishing is done with a belt or a board laid
flatwise and reaching half-way across the pavement, the end at
the curb being handled by a man and the end at the center being
guided by a small rope in the hands of a man standing on the remote
curb. The former method is considered the better, since a longi-
tudinal joint is undesirable. Such joints are sometimes protected
with steel plates, and should always be covered with tar; but even
then a rut is likely to form along them.
Concrete pavements having separate curbs require longitudinal
contraction joints at each edge (§ 465-68); but the integral curb
(§ 471) eliminates this complication.
* Data collected by the Portland Cement Association.
264
PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
481. SPECIFICATIONS. Complete specifications for concrete
roads are printed by the several State Highway Departments, of
which copies may doubtless be had by citizens of the respective
states upon request. The Portland Cement Association publishes for
gratuitous distribution the specifications adopted by the American
Concrete Institute and recommended by the 1916 National Con-
ference on Concrete Road Building, copies of which may be had
gratuiously of the Secretary, Portland Cement Association, 111 W.
Washington St., Chicago.
ART. 3. MAINTENANCE
482. CHARACTER OF WORK REQUIRED. The work required to
maintain the concrete slab is: (1) keep the joints and cracks filled
with bituminous cement; (2) fill with bituminous cement any small
pits that appear; (3) clean out any holes left where pebbles have
Fio. 90.— FILLING A DIAGONAL CRACK.
been dislodged or where a friable fragment has disintegrated, and fill
them with bituminous concrete; and (4) repair any places where the
concrete is otherwise defective.
The bituminous material may be either a heavy grade of refined
tar or a corresponding grade of asphalt, After the bituminous
ART. 3] MAINTENANCE 265
material has been applied, its surface should be sprinkled with coarse
sand or fine chips of a hard stone.
Fig. 90 shows the method of repairing a transverse contraction
joint.
Before being filled, all cracks and joints should be swept clean
with rattan or steel brooms. The old tar need not be removed ; but
any matted earth or other foreign material not removed by the first
sweeping should be loosened and removed with a steel hook. It is
usually necessary to cover all joints and fill all cracks twice each year.
The pits and the cup-like holes from which pebbles, sticks, etc.,
have been dislodged, unless early filled, will enlarge rapidly under
travel. Where defects of any considerable size are to be repaired,
the edges should be made vertical with a chisel and the depth of the
hole increased to 1 inch, if it is not already that deep. The hole
should be thoroughly cleaned and painted with hot tar; and then be
filled with bituminous concrete and sprinkled with coarse sand or fine
hard stone chips.
When it is necessary to repair any considerable defective portion
of the concrete, the place to be patched should be trimmed and
cleaned as described above, and painted with neat cement mortar;
and then the hole should be tamped solidly full of good portland-
cement concrete. The patch should be kept damp and protected
from travel until the cement has fully set.
483. The above method of maintenance will probably serve
indefinitely, and will preserve the surface except for the natural
wearing away of the concrete by travel.
484. Bituminous Surface. Many attempts have been made
to cover the surface of a concrete road with a bituminous coating.
This phase of the subject is considered in Art. 2 of Chapter VIII.
485. COST OF MAINTENANCE. The cost of maintenance con-
sists of the annual expense for repairs and the annual contribution to
a fund for rebuilding the slab when it is worn out. The introduction
of concrete roads is so recent, particularly in proportion to their' life,
that no reliable data have been accumulated as to the second item
of the cost of maintenance. There are reasonably accurate data for
the annual cost of repairs, but as a rule there is no information as to
amount of or character of the travel on the road; and therefore it is
not possible to make any accurate comparisons between such data.
Further, no standard has been established as to what constitutes
good maintenance; and no system of doing the work has been
fully tested.
266 PORTLAND-CEMENT CONCRETE ROADS [CHAP. VII
Apparently the most complete data are those obtained in 1915
by the Illinois Highway Department.* The average annual cost for
repairs on 75 miles of concrete rural roads in comparatively short
sections, was 0.4 cent per square yard for supervision, labor, equip-
ment and materials. Most of the roads were built in 1914 or 1915.
Two methods of maintenance were tried, — (1) by the use of a one-
horse wagon drawing a portable heating-kettle, and (2) by an auto-
mobile truck carrying a heating tank. By the first method the total
cost was 0.32 cent per square yard per treatment, and by the second
0.22 cent. In this work it was found that joints, even though pro-
tected by steel plates, required about the same attention as ordinary
cracks.
In Connecticut the average cost of repairs to the concrete slab
was 0.4 cent per square yard per annum and to drainage 0.3 cent
per square yard, or a total of 0.7 cent.
* Illinois Highways, August, 1915, p. 118-22; or Engineering and Contracting, Vol. 47 (1917)
p. 14.
CHAPTER VIII
BITUMINOUS ROAD MATERIALS
486. DEFINITIONS. Bitumen. A mixture of native or pyro-
genous hydrocarbons and their non-metallic derivatives. It may be
a gas, liquid, or solid; and if solid, is soluble in carbon disulphide.
487. Bituminous Material. Any material containing bitumen
or constituting a source of bitumen. Bituminous coal, peat, etc.,
are called pyro-bitumens because a bitumen may be produced from
them by distillation. The ordinary bituminous materials used in
roads and pavements are asphalt, petroleum, and tar.*
488. Cut-back Product. A petroleum or tar residuum which
has been fluxed with distillate.
489. Flux. Fluid oil or tar which is incorporated with asphalt,
petroleum, or tar residuum for the purpose of reducing their con-
sistency.
ART. 1. ASPHALT
490. DEFINITIONS. Asphalt. Solid or semi-solid native bitumen
or solid or semi-solid bitumen obtained by refining petroleum, which
consists of a mixture of hydrocarbons and which melts upon the
application of heat. Asphalt is usually found associated with
various mineral and organic substances. Different varieties of
asphalt are called albertite, grahamite, gilsonite, turrellite uintatite,
wurtzelite, etc.
491. Crude Asphalt. A native mixture of bitumen, sand, clay,
water, organic matter, etc.
492. Refined Asphalt. A native mixture after it has been freed
wholly or in part from water and organic and inorganic matter by
being heated.
* For detailed explanations of the methods employed in testing bituminous road materials,
see Bulletin 314 of the Office of Public Roads and Rural Engineering, U. S. Department
of Agriculture, Washington, D. C., 1915.
267
268 BITUMINOUS ROAD MATERIALS [CHAP. VIII
493. Rock Asphalt. A limestone or sandstone naturally impreg-
nated with asphalt. Rock asphalt is the principal form of asphalt
used in Europe for paving purposes, and there is usually designated
as asphalt.
494. Asphaltic Cement. Refined asphalt which has been mixed
with some solvent to increase its plasticity, adhesiveness, and tenacity.
495. CHARACTERISTICS OF ASPHALT. As usually found asphalt is
of a dark brown or glistening black color. It varies in hardness from
a viscous liquid to about 3J on the Dana scale. When rubbed or
freshly broken, it emits a peculiar bituminous odor, and has a slight
sour smell. . Its specific gravity in the natural state varies from 0.96
to 1.68 according to its porosity and the amount and the character
of the impurities present. It is insoluble in water; but is more or
less soluble in carbon disulphide, alcohol, turpentine, ether, naphtha,
and petroleum.
Asphalt has an appearance somewhat like coal tar. The prin-
cipal method of distinguishing asphalt from coal tar, available
to the layman, is the odor. The tar emits a sharp, acrid odor;
while both the crude and the refined asphalt when cold give a weak
clay-like odor, and must be rubbed to obtain the distinctive bitumi-
nous odor. If tar is mixed with asphalt, the presence of 25 per cent
will be revealed by the odor. When being laid in a road or pave-
ment, tar gives off a bluish vapor, while asphalt emits a white vapor.
496. Asphaltic limestone, when freshly broken, varies in color
from chocolate brown to black, the color being darker as the pro-
portion of asphalt is greater. The percentage of asphalt permeating
the limestone varies in different deposits and in different parts of the
same mine, usually ranging from 1 to 20 per cent.
Asphaltic sandstone contains from 1 to 70 per cent of asphalt.
The grain is sometimes dense and sometimes porous, sometimes very
fine and sometimes coarse.
497. SOURCES OF ASPHALT. Liquid asphalt, or maltha as it is
usually called, is found in large quantities in California; but solid or
natural asphalt is not found to any great extent in the United
States. The principal kinds of the natural asphalts used in this
country are: Trinidad, Bermudez, and California.
498. Trinidad Asphalt. The Island of Trinidad, near the north-
east coast of Venezuela, South America, supplied something like
90 per cent of all the asphalt used in the world from about 1875 to
1900; and at present the Island of Trinidad is the main source of
supply of the native asphalt used in the United States. In south-
ART. 1] ASPHALT 269
west corner of the Island is the so-called pitch-lake, which has an
area of about 115 acres. The surface of the lake has an elevation
of 138 feet above the sea-level, and near the center the asphalt has a
depth of 78 feet. As a rule the surface of the asphalt is sufficiently
hard that teams may be driven over it; but the whole mass is in
constant motion around several vortices, as shown by trunks of
trees which rise and after a time again disappear. Excavations
made during the day close up during the night.
The asphalt is excavated with picks and shovels, conveyed to
the shore in carts, and lightered to vessels off-shore. On the sea
voyage it becomes compacted into a solid mass and must be again
broken up with picks. The crude asphalt is mixed with much
earthy and a little vegetable matter and water, and is dark brown.
The crude material is refined by placing it in kettles or open
tanks and heating it for three or four days, during which time the
water is evaporated, the vegetable matter rises to the surface and
is skimmed off, and the earthy material settles to the bottom. Great
care is required in the refining process not to heat the apshalt to a
point where chemical changes take place. The refined asphalt
must be softened by the addition of some fluxing material before it
is ready for use in the pavement.
499. Bermudez Asphalt. A lake in the State of Bermudez,
Venezuela, South America, supplies large quantities of asphalt.
The crude asphalt consists of bitumen mixed with sand, clay, and
vegetable matter; and is refined in the same way as Trinidad asphalt,
but more rapidly, since it contains less water and mineral matter.
500. California Asphalt. California is the principal producer of
asphalt in the United States; and is said to have not only larger
quantities of asphalt than any other equal area in the world, but a
greater variety of forms — solid and liquid asphalt, and asphaltic
limestones and sandstones — and in more localities. Maltha (" liquid
asphalt ") is found chiefly at Carpinteria, Santa Barbara County,
near the sea shore; and solid asphalt is found at La Patera, Santa
Barbara County, also near the sea shore. Asphaltic limestone and
sandstone are found at a number of places in California, in all degrees
of richness and consistency. The principal deposits are a,t Santa
Cruz, San Luis Obispo, and Kings City. The asphalt is extracted
from the stone by heating the mass in a tank and drawing off the
liquid asphalt.
601. The base of the California petroleums is asphaltic, as dis-
tinguished from the paraffin base of the eastern oils; and the process
270 BITUMINOUS ROAD MATERIALS [CHAP. VIII
of refining petroleum leaves the asphalt or maltha as a residue, and
at several places asphalt is produced in this way from crude petro-
leum.
502. Petroleum Residue. Some crude petroleums on distilla-
tion have an asphalt residue (see § 547). Three fourths of all the
asphalt used in the United States is obtained from asphaltic petro-
leums. Such material is usually called oil asphalt.
503. Other American Asphalts. Considerable asphalt is shipped
to the United States from mines at Inciarte and La Paz, State of
Zulia, Venezuela, South America. It is usually called Maracaibo
asphalt from the gulf and lake of that name near the mines.
Asphalt is found in much smaller quantities, but sufficient to
be of considerable commercial importance, in Utah, Colorado,
Indian Territory, Texas, and Kentucky. One of the most important
of these is Gilsonite, a solid and nearly pure native bitumen found in
Utah and Colorado.
Several deposits of natural asphalt exist in Cuba and along the
Gulf Coast of Mexico.
504. SHIPPING ASPHALT. Refined asphalt is shipped in barrels
or metal drums or in tank cars.
Fig. 91 shows the method of
unloading asphalt binder from
the tank car to the machine
which distributes it upon the
asphalt-bound macadam road.
In the left foreground is the
FIG. QI.-UNLOADING ASPHALT BINDER. mOt°r ' tl>Uck distributor, and
behind it is a portable heater
for heating the asphalt. Notice that the tank is covered with
blankets.
505. DESIRABLE PROPERTIES OF ASPHALT. The character-
istics required in an asphalt differ according to the purpose for
which it is to be used; but in general any asphalt for use in roads
or pavements should have the following properties: 1, chemical
stability; 2, freedom from decomposition products; 3, binding
power; 4, resiliency, and 5, waterproof ness.
Apparently it is impossible to devise any tests to measure directly
some of these properties; and the difficulties of devising a series
of tests is increased by the variation in the character of the different
materials.
506. Chemical Stability. The chemical stability of a bituminous
ART. 1] ASPHALT 271
»
«ement is indicated somewhat by the extent to which the material
is volatilized under standard temperature conditions. The harden-
ing of the bituminous cement due to evaporation and oxidation is
determined by making penetration tests before and after volatiliza-
tion. The temperature at which the material gives off vapor enough
to ignite gives further indication of chemical stability.
607. Freedom from Decomposition Products. If the refining
process has been carried on at a too high temperature, the cement
may have been partially decomposed. This condition is indicated
by the amount of free carbon and other decomposition products that
are separated by certain solvents. If the material is a fluxed nat-
ural asphalt, these tests throw some light upon the character of the
flux.
508. Binding Power. It is important that the bituminous
material shall have cementing or binding power, particularly at
summer temperatures. There is no single test for this property.
The ductility test gives some indication concerning cementing value.
509. Resiliency. It is important that the bituminous cement
shall have the power to absorb shock and thus prevent the blow
of the wheel or the hoof from destroying the road or pavement sur-
face. This requires that the cement shall have resiliency and mal-
leability, which depend somewhat upon consistency.
510. Waterproofness. The bituminous material should be water-
proof so as to prevent water from penetrating the body of the road
and doing damage by freezing or softening the subgrade.
511. TESTS OF BITUMINOUS MATERIALS. Below are the tests
usually applied to bituminous materials, and a brief statement
of the significance of each. All of these tests (§ 512-27) are applied
to asphalts, but only the first eight (§ 512-19) are applied to oils and
tars.*
512. Foam Test. This test is applied to asphalts and tars to
determine the presence of water. Water is chiefly objectionable
since it makes the material difficult to handle when heated above the
boiling point of water, because the steam makes the oil or tar foam
or froth.
513. Specific Gravity. This test is valuable mainly as a means of
* For a detailed account of the method of making the testa and also illustrations of the
apparatus used, see Bulletin 314 of the U. S. Department of Agriculture, December 10, 1915,
or Hubbard's Laboratory Manual of Bituminous Materials, 8vo, p. 159, John Wiley & Sons,
New York, 1916; and for a description of the tests see Proc. Amer, Soc. of Civil Engineers,
December, 1914, p. 3036-50.
272 BITUMINOUS ROAD MATERIALS [CHAP. VIII
: . _ _ : — ^
identifying the material; but in connection with other tests it is
sometimes serviceable in determining the suitability of a material
for road purposes. The specific gravity of crude asphalt varies from
1.04 to 1.40, and asphaltic cement from 0.96 to 1.06. The specific
gravity of crude petroleum varies from 0.73 to 0.98, the paraffin oils
being the lowest and the asphaltic the highest. The specific gravity
of crude tar varies from 1.00 to 1.22, the water-gas tars ranging from
1.00 to 1.10, and the coal tars from 1.10 to 1.22. The specific gravity
of tar depends chiefly upon the amount of free carbon it contains,
the higher the specific gravity the greater the percentage of free
carbon. Refined tar has a higher specific gravity than crude tar,
partly because the light hydrocarbons and the water have been
driven off.
514. Flash Point. The flash point is determined by either the
open-cup or the closed-cup method, the latter being the more accu-
rate. This test is mainly valuable as a quick means of differentiating
between heavy crude oils and cut-back products;* but it also indicates
the temperature at which a refined oil has been distilled. Crude
paraffin oils usually flash lower than crude asphaltic oils.
515. Consistency. The consistency of a bituminous material
is an important factor, since it determines the grade of material
suitable for a particular use, and since this test is a means of securing
uniformity in the product.
There are three methods or instruments in common use for deter-
mining consistency, viz.: the Engler viscosimeter, the New York
Testing Laboratory float apparatus, and the penetrometer.
516. Viscosity. The viscosimeter determines the viscosity ^i. e.,
time required for a specified amount of the material to flow through a
standard aperture. This test is generally used for liquid bituminous
materials. *
517. Float Apparatus. This apparatus determines the time for
a specified quantity of semi-solid or solid material to flow through
an aperture; and is generally used for semi-solid and solid tars and
pitches.
518. Penetration. The penetrometer determines the distance
a needle will penetrate the material in a specified time. Of course,
the size of the needle, the weight on the needle, and the temperature
of the material are carefully standardized. The penetration is usually
stated in tenths of millimeters; but sometimes in degrees, since the
index finger sweeps over an arc of a circle graduated to degrees.
This apparatus is generally employed for asphalts; but it is not used
ART. 1] ASPHALT 273
for tars, because the surface tension and the presence of free carbon
considerably affect the results without materially affecting the con-
sistency.
519. Melting Point. The determination of the melting point is
mainly of value as a means of identification. It is virtually a test
of consistency (§515). As a rule as the melting point of a bituminous
material rises, it becomes harder and more brittle. One of the char-
acteristics of asphalt which peculiarly fits it for use in roads and
pavements is that it has a high melting point without being brittle.
Paraffin also has a high melting point, but it is brittle.
520. Loss by Evaporation. This test determines the amount of
volitilization under standard conditions. The residue is usually
tested for penetration, melting point, and ductility. The compari-
son of the results of these tests before and after the evaporation test
determines the amount of hardening, which is an indication of the
stability of the cement.
521. Distillation. The distillation is carried on at considerable
lower temperatures than the evaporation test, and is usually applied
only to tars. The melting point of the residue is determined,
and also its consistency at several temperatures. This is an impor-
tant test of tars to determine their road-building qualities and also
their method of preparation.
522. Bitumen Soluble in Bisulphide. It is usually assumed that
all matter soluble in cold carbon disulphide is bitumen. Fluid oils
are almost wholly soluble in this material. The amount and char-
acter of the insoluble matter are of most interest in this test. The
insoluble matter is usually free carbon, which is of no value in road
work. The failure to pass this test is an indication that the material
has been overheated, i. e., " cracked."
523. Bitumen Soluble in Naphtha. This test is chiefly valuable
in determining the amount of body-forming hydrocarbons in oil and
oil products. " No oil having less than 4 per cent insoluble in naph-
tha will be of service in road work except as a dust preventive."
Bitumens insoluble in naphtha are commonly called asphaltenes,
while those soluble are called malthenes.
524. Bitumen Soluble in Tetrachloride. The test is made for
purposes of identification and also to determine whether the material
has been over-heated in the process of manufacture. The bitumen
insoluble in carbon tetrachloride but soluble in carbon disulphide is
commonly called carbenes.
525. Fixed Carbon. The amount of fixed carbon shows much
274 BITUMINOUS ROAD MATERIALS [CHAP. VIII
the same results as the bitumen insoluble in naphtha. The amount
of fixed carbon present is an indication of the mechanical stability of
a road oil. Paraffin oils have only little fixed carbon, while asphaltic
oils have more, and asphalts still more. This test can not be applied
to tar, owing to the error introduced by the presence in it of consid-
erable free carbon.
526. Ductility. This test consists in forming a briquette of the
material and observing the amount of elongation before rupture.
It is the only test for determining the cementing value of an asphalt,
and hence is very important.
527. Paraffin Scale. This test consists in determining the amount
of paraffin present. It is made as a means of identification, and is
not a very accurate test; and there is considerable difference of
opinion as to its value.
528. THE FLUX. A flux is a heavy oil or the residue from the
distillation of petroleum which is mixed with refined asphalt to make
it of suitable consistency for use in a sheet asphalt pavement or as
a binder for asphaltic macadam or concrete. Fluxes are usually
obtained from paraffin, semi-asphaltic, or asphaltic oils; and vary
greatly in character with the petroleum from which they are derived.
The lower the specific gravity of the flux, the less the amount required
to produce an asphalt cement of the desired consistency. Different
asphalts require quite different amounts of flux. For example, Ber-
mudez asphalt requires only 7 per cent of a light flux, while Trinidad
asphalt requires 20 per cent.
529. Specifications for Flux. The following are the specifications
of the American Society of Municipal Improvements, adopted
October 14, 1915, for the flux to be used in preparing asphalt for
sheet asphalt pavements.
1. The flux must have a penetration greater than 350 with a No. 2 needle
at 77° F. under a 50-gram weight applied for one second.
2. It shall have a specific gravity at 77° F. between 0.92 and 1.02.
3. When 20 grams of the flux are heated for 5 hours at 325° F. in a tin
box 2? inches in diameter and three quarters of an inch deep after the manner
officially prescribed, the loss shall not exceed 5 per cent by weight; and the
residue left after such heating shall flow at 77° F.
4. The flux shall not flash below 350° F. when tested in a closed oil tester.
5. It shall be soluble in carbon tetrachloride to the extent of not less
than 99 per cent.
530. ASPHALT CEMENT. Asphalt cement is produced by
mixing refined asphalt and a flux. The asphalt should be heated to
ART. 1] ASPHALT . 275
325 to 350° F. and the flux to 150 to 200° F. before they are mixed.
The mixing is done by blowing air or steam, preferably the latter,
through perforated pipes in the bottom of the melting tank. The
mixing should be very thorough, and usually requires three or more
hours. Care should be taken -that the cement is not burned, par-
ticularly if the tank is heated over a fire. The cement will harden
if kept heated for a long time or if the agitation is kept up unduly
long; but the cement can be softened again by adding more flux
and mixing further.
531. SPECIFICATIONS FOR ASPHALT CEMENT. Asphalt cement
is used for a bituminous surface on water-bound gravel or macadam
roads (§ 589) and for binder in bituminous macadam and bituminous-
concrete roads (§611 and 622); but principally for sheet asphalt
pavements (Art. 1 of Chap. XVI), asphaltic concrete (Art. 2 of Chap.
XVI), and also for joint filler for brick, stone-block, and wood-
block pavements (Chap. XVI, XVII, and XIX, respectively).
Usually separate specifications are drawn for each of the above
uses.
532. Whatever the purpose for which the asphalt is to be used,
there are two classes of specifications for it, one known as general
or open or blanket specifications, and the other as restricted or
special or alternate specifications. The former are drawn so as
to include all kinds of asphalt whatever their source or origin;
and the latter consist of special requirements for each type or
asphalt.
Tables 31, 32 and 33, page 278, 280, and 281, show a summary
of restricted specifications of asphalt for different uses; and § 543
contains an example of general specifications for asphalt for sheet
asphalt pavements.
There is a sharp difference of opinion as to the relative merits of
the two classes of specifications. Those who favor restricted speci-
fications claim that there is so much difference between the different
kinds of asphalt that it is impossible to make general requirements
which will apply to all and at the same time define any quality so as
to make it a real test for any particular kind of asphalt. For exam-
ple, assume that it is desired to permit the use of any of the four
asphalts of Table 31, page 278, and that it is stated that the specific
gravity shall be from 0.96 to 1.06. The lower limit is too small to
fix this quality in some of the asphalts, and the upper limit is too
great to fix it for others. Again, if the specifications state that the
penetration shall be between 90 and 160, they will permit the use
276 BITUMINOUS ROAD MATERIALS [CHAP. VIII
of any of the four asphalts of Table 31; but the limits are too wide
to secure uniformity in any of the cements, and an entirely unsuitable
material could be supplied under such specifications.
On the other hand, equally competent asphalt specialists strongly
dissent from the above statements;- and claim that the different
asphalts on the market are so nearly alike in their essential qualities
as not to justify separate specifications. They claim that there is
no more reason for separate specifications for the different asphalts
than for separate specifications for different brands of portland
cement. They claim that the bitumen in all asphalts is practically
the same, and that the seeming difference in asphalts is due to the
mineral matter which they contain. For example, Trinidad refined
asphalt (§ 498), one of the best-known and most extensively used
of the natural asphalts, contains about 36 per cent of mineral matter;
and consequently its specific gravity is greater and its penetration
is less than a more pure asphalt. The finely divided mineral matter
in this asphalt does not injure it for some uses, for example, sheet
asphalt pavements, since in practice a considerable amount of
fine mineral matter is added to the asphalt to give it physical stability
(§ 826). Those who advocate general specifications claim that the
illustration concerning specific gravity in the preceding paragraph is
wide of the mark, since a test for specific gravity is valuable only
as a means of identifying the material, and in no way aids in deter-
mining any essential quality. They also claim that the penetration
of the pure bitumen in all asphalts is substantially the same; and
that the difference in the limits is only to provide for the difference
between heavy and light traffic, a difference in the fineness of the
sand, and differences in climatic conditions.
The divergence of opinion as to the merits of the two types of
specifications is shown by the fact that the standard specifications
of the American Society of Municipal Improvements for bituminous
macadam (§ 537-38), bituminous concrete (§ 539-40), and seal coat,
(§ 541), adopt the restricted or special form of specifications for the
asphalt; while the standard specifications of the same Society for
asphalt concrete and sheet asphalt pavements (§ 542-43) are based
upon general or blanket specifications for the asphalt. However, a
recent vote shows that the weight of the society is in favor of the
general specifications.
533. The writing of specifications for asphaltic cement requires
thorough laboratory knowledge of the chemical and physical char-
acteristics of asphalt, and also practical experience in the use of the
ART. 1] ASPHALT 277
material.* Great care must be used in changing the limits in speci-
fications, since a change in the value for one element may require a
corresponding change in some other factor. Below are specifica-
tions for asphaltic cement that have been successfully used for roads
and pavements.
534. Asphalt for Bituminous Surface on Water-bound Macadam.
The following are the specifications of the Barber Asphalt Company
for liquid asphalt for both cold and hot-surface application to
water-bound macadam : f
635. Liquid Asphalt A. (For Cold Application.) 1. Specific Gravity: The
specific gravity at 60° F. shall not be less than 0.91.
2. Flash Point: The flash point by the Tagliabue open cup shall not be less
than 100° F.
3. Viscosity: The specific viscosity by the Engler apparatus at 77° F., for
the first 50 c.c. shall be between 90 and 100.
4. Bitumen Soluble in Bisulphide : The bitumen soluble in carbon disulphide
shall not be less than 99 per cent.
5. Paraffin Scale: The paraffin scale determined by the Holde method shall
not be more than 0.25 per cent.
6. Distillation : When evaporated to 80 per cent by weight, by heating in an
open dish at 150° F., the residue shall have at 77° F. with a No. 2 needle under a
weight of 50 grams in 1 second, a penetration of not less than 20 mm.; and its
adhesiveness by the Osborne test shall not be less than 200 seconds.
536. Liquid Asphalt B. (For Hot Application.) 1. Specific Gravity: The
specific gravity at 60° F. shall be not less than 1.00.
2. Flash Point. The flash point by the Cleveland cup shall be not less than
325° F.
3. Viscosity: The specific viscosity by the Engler apparatus at 212° F., for
the first 50 c.c. shall be from 23 to 33.
4. Bitumen Soluble in Disulphide: The bitumen soluble in carbon disulphide
shall be not less than 99.0 per cent.
5. Paraffin Scale: The paraffin scale by the Holde method shall not be
more than 0.25 per cent.
6. Distillation: The loss at 325° F. after 5 hours of 50 grams in a 2£ by 1|-
inch dish shall not be more than 1.0 per cent.
7. Residue, loss by evaporation: The residue of a 5.0 mm. penetration at
77° F. under a load of 100 grams with a No. 2 needle at 5 seconds, when evap-
orated at 500° F. in an open dish shall lose not less than 75.0 per cent.
8. Adhesiveness: The adhesiveness at 77° F. by the Osborne test shall not be
less than 400 seconds.
* For the methods and results of tests of asphalts see Hubbard's Dust Preventives and
Road Binders, 8vo, p. 416, John Wiley & Sons, New York, 1910; Richardson's Modern Asphalt
Pavement, 8vo, p. 629, John Wiley & Sons, New York, 1908.
t Letter to the author under date of July 13, 1917.
278
BITUMINOUS ROAD MATERIALS
[CHAP, viii
537. Asphalt Binder for Bituminous Macadam. The American
Society of Municipal Improvements on October 12, 1916, adopted
restricted specifications for four kinds of asphalt, any one of which is
acceptable as a binder for bituminous macadam. The full specifi-
cations for an asphalt cement made from Gilsonite and asphaltic
oil follow. The specifications for the three other asphalts are in
exactly the same form; and Table 31 gives the essential elements
of the specifications of all four.
TABLE 31
COMPARISON OF SPECIFICATIONS FOR ASPHALT CEMENTS FOR BITUMINOUS
MACADAM
Standards of American Society of Municipal Improvements, Adpoted October 1, 1915
Ref.
No.
Items.
KIND OF ASPHALT CEMENT.
Gilsonite
and
Asphalt Oil.
Texas and
California
Oil Asphalt.
Mexican Oil
Asphalt.
Bermudez
Asphalt.
1
2
3
4
5
6
7
8
9
10
Shall not foam at
177° C.
205° C.
0.96-1.00
100-120
>50
> 60° C.
177° C.
205° C.
1.00-1.03
90-110
>15
>30°C.
177° C.
205° C.
1.025-1.045
110-130
>30
> 40° C.
177° C.
163° C.
1.035-1.060
130-160
>30
120-180 sec.
<3.0%
94-98.0
98.5%
75-85%
11-14%
Flash point, not less than
Specific gravity at 25° C
Penetration —
100 grams, 5 sec., 25° C
200 grams, 60 sec., 40° C. . . .
Melting point by cube method.
Viscosity by N. Y. float appa-
ratus
Distillation, loss after 5 hrs
Bitumen soluble in disulphide . .
Bitumen soluble in tetrachloride
Bitumen soluble in naphtha ....
Fixed carbon.
<2.0%
99.5%
99.5%
75-85%
8-12%
<2.0%
99.5%
99.5%
75-85%
9-13%
<2.0%
99.5%
99.5%
70-80%
12-17%
638. Gilsonite and Asphaltic Oil. 1. Foam: The asphalt cement shall be
homogeneous, free from water, and shall not foam when heated to 177° C. (350° F.)
2. Flash Point: It shall show a flash point of not less than 205° C. (400° F.)
when tested in the New York State Board of Health Closed Oil Tester.
3. Specific Gravity: Its specific gravity at a temperature of 25° C. (77° F.)
shall be not less than 0.960 nor more than 1.000.
4. Evaporation: When tested with a standard No. 2 needle by means of a
standard penetrometer, it shall show penetrations within the following limits
for the conditions stated, the penetrations being expressed in hundredths of a
centimeter: 100-gram load, 5 seconds at 25° C. (77° F.), from 100 to 120; 200-
gram load, 1 minute at 4° C. (39° F.), not less than 50.
5. Melting Point: Its melting point as determined by the cube method shall
be not less than 60° C. (140° F.).
6. Distillation: When 50 grams of the material is maintained at a uniform
temperature of 163° C. (325° F.) for 5 hours in an open cylindrical tin dish 5£
centimeters (about 2 1 inches) in diameter, with vertical sides measuring approx-
imately 3£ centimeters (about \\ inches) in depth, the loss in weight shall not
exceed 2.0 per cent of the original weight of the sample,
ART. 1] ASPHALT 279
Penetration of Residue: The penetration of the residue when tested as de-
scribed in paragraph 4 with a standard No. 2 needle under a load of 100 grams for
5 seconds at 25° C. (77° F.), shall be not less than one half the penetration of the
original material tested under the same condition.
7. Bitumen Soluble in Bisulphide: Its bitumen as determined by its solu-
bility in chemically pure carbon disulphide at room temperature, shall be not
less than 99.5 per cent.
8. Bitumen Soluble in Tetrachloride : It shall be soluble in chemically pure
carbon tetrachloride at room temperature, to the extent of not less than 99.5
per cent of its bitumen as determined by paragraph 7.
9. Bitumen Soluble in Naphtha: It shall be soluble in 86 to 88° Baume
paraffin naphtha, of which at least 85.0 per cent distills between 35° and 65° C.
(95° and 149° F.), to the extent of not less than 75.0 per cent nor more than 85.0
per cent of its bitumen as determined by paragraph 7.
10. Fixed Carbon: It shall yield not less than 8.0 per cent nor more than 12.0
per cent of fixed carbon.
539. Asphalt Binder for Bituminous Concrete. The American
Society of Municipal Improvements on October 12, 1916, adopted
restricted specifications for four kinds of asphalt any one of which is
acceptable as a binder for bituminous concrete. The full specifi-
cations for asphalt made of Gilsonite and asphaltic oil follow. The
specifications for the four other materials are in exactly the same
form. Table 32, page 280, gives the essential elements of the speci-
fications of all five asphalts.
540. Gilsonite and Asphalt Oil. 1. Foam: The asphalt cement shall be
homogeneous, free from water, and shall not foam when heated to 177° C.
(350° F.).
2. Flash Point: It shall show a flash point of not less than 205° C. (400° F.)
3. Specific Gravity: Its specific gravity at a temperature of 25° C. (77° F.)
shall be not less than 0.970 nor more than 1.000.
4. Penetration: When tested with a standard No. 2 needle by means of a
Dow penetrometer (or other penetrometer giving the same results as the Dow
machine), it shall show penetrations within the following limits for the conditions
stated, the penetrations being expressed in hundredths of a centimeter: 100-gram
load, 5 seconds, at 25° C. (77° F.), from 75 to 90; 200-gram load, 1 minute, at
4° C. (39° F.), not less than 35; 50-gram load, 5 seconds, at 46° C. (115° F.)
net more than 250.
5. Melting Point: Its melting point as determined by the cube method shall
be not less than 55° C. (131° F.).
6. Evaporation: When 50 grams of the material is maintained at a uniform
temperature of 163° C. (325° F.) for 5 hours in an open cylindrical tin dish, 5£
centimeters (about 2j inches) in diameter, with vertical sides measuring approxi-
mately 3 \ centimeters (about \\ inches) in depth, the loss in weight shall not
exceed 1.0 per cent of the original weight of the sample.
Penetration of Residue: The penetration of the residue, when tested as
280
BITUMINOUS fcoAD MATERIALS
[CHAP, viii
described in paragraph 4 with a standard No. 2 needle under a load of 100 grams
for 5 seconds at 25° C. (77° F.) shall be not less than one half the penetration of
the original material tested under the same conditions.
7. Bitumen Soluble in Bisulphide: Its bitumen as determined by its solu-
bility in chemically pure carbon disulphide at room temperature shall not be
less than 99.5 per cent.
8. Bitumen Soluble in Tetrachloride : It shall be soluble in chemically pure
carbon tetrachloride at room temperature to the extent of not less than 99.5
per cent of its bitumen as determined by paragraph 7.
9. Bitumen Soluble in Naphtha: It shall be soluble in 86 to 88° Baume par-
affin naphtha, at least 85 per cent of which distills between 40 and 55° C. (104°
and 131° F.), to the extent of not less than 70.0 per cent nor more than 80.0
per cent of its bitumen as determined by paragraph 7.
10. Fixed Carbon: It shall yield not less than 8.0 per cent nor more than
12.0 per cent of fixed carbon.
TABLE 32
COMPARISON OP SPECIFICATIONS FOR ASPHALT CEMENTS FOR BITUMINOUS
CONCRETE
Standards of American Society of Municipal Improvements, Adopted October 14, 1915
1
1
1
2
a
4
6
6
7
8
9
10
Item.
KINDS OF ASPHALT.
Gilsonite
and Asphalt
Oil
Texas Oil
Asphalt.
California
Oil Asphalt.
Mexican
Oil Asphalt.
Bermudez
Asphalt.
Shall not foam at
Flash point, not less than.
Specific gravity at 25° C. .
Penetration — 100 grs. . . .
— 200 grams. .
Melting point by cube
method
Viscosity by N. Y. float
apparatus
177° C.
205° C.
0.97-1.000
75-90
>35
>55°C.
177° C.
205° C.
1.000-1.030
90-100
>30
>50° C.
177° C.
205° C.
1.030-1.040
70-90
>10
>45° C.
177° C.
205° C.
1 . 025-1 . 050
85-95
>20
> 50° C.
177° C.
165° C.
1.040-1.060
140-160
>40
120-180 sec
<3.0%
93-98%
98.5%
75-85%
11-15%
Evaporation, loss after 5
hrs
<i.o%
99.5%
99.5%
70-80%
8-12%
<i.o%
99.5%
99.5%
72-78%
11-15%
<2.0%
99.5%
99 . 5%
80-88%
10-14%
<2.0%
99.5%
99.5%
67-77%
12-18%
Bitumen soluble in disul-
phide
Bitumen soluble in tetra-
chloride
Bitumen soluble in naph-
tha
Fixed carbon
541. Asphalt for Seal Coat for Bituminous Concrete. The
American Society of Municipal Improvements adopted restricted
specifications for three kinds of asphalt for the seal coat of bitumi-
nous concrete pavements in which tar is the cementing material of
the concrete. These specifications are of the same general form as
those in § 538 and § 540. The essential elements of the specifi-
cations are shown in Table 33.
ART. 1]
ASPHALT
281
TABLE 33
SPECIFICATIONS FOR ASPHALT FOR SEAL COAT FOR TAB-CONCRETE PAVEMENTS
Standards of American Society of Munisipal Improvements, Adopted October 14, 1915
Ref.
No.
Items.
KINDS os ASPHALT.
Gilsonite
and
Asphalt Oil.
Texas Oil
Asphalt.
Mexican
Oil Asphalt.
1
2
3
4
5
6
7
8
9
10
1 '
177° C.
205° C.
1 . 025-1 . 050
85-95
>20
50° C.
<2.0%
>50%
99.5%
99.5%
70-80%
8-12
177° C.
205° C.
1.030-1.045
60-70
>18
60° C
<1.0%
>50%
99.5%
99.5%
70-80%
12-16
177° C.
205° C.
1.025-1.055
60-70
>16
55° C.
<1.0%
>50%
99.5%
99.5%
67-77%
13-18
200 grams
Distillation, loss after 5 hours •.-.•••,
Penetration of residue, per cent of original
Bitumen soluble in tetrachloride
Bitumen, per cent of total soluble in naphtha. . .
542. Asphalt for Sheet Asphalt Pavements. The following are
the specifications of the American Society of Municipal Improve-
ments, adopted October 14, 1915, for asphalt cement for sheet asphalt
and asphalt concrete pavements:
1. Homogeneous: The asphalt cement shall be thoroughly homogeneous.
2. Penetration: It shall have a penetration at 77° F. of from 30 to 55 for
heavy traffic streets, and 55 to 85 for light traffic streets, depending upon the sand
and asphalt used and the local climatic conditions.
3. Flash Point: It shall not flash below 350° F. when tested hi a closed oil
tester.
4. Evaporation: When 20 grams of the asphalt cement are heated for 5
hours at 325° F. in a tin box 2\ inches in diameter and f of an inch deep, after
the manner officially prescribed, the loss shall not exceed 5 per cent by weight;
and the penetration, at 77° F., of the residue left after such heating must not be
less than one half the penetration, at 77° F., of the original sample before heating.
5. Ductility: Either the asphalt cement or its pure bitumen when made
into a briquette in the Dow mold shall have at 50 penetration at 77° F., a duc-
tility of not less than 30 centimeters, when the two ends of the briquette are
pulled apart at the uniform rate of 5 centimeters per minute.
When the asphalt cement as used has a penetration other than 50 at 77° F.,
an increased ductility of 2 centimeters will be required for every 5 points in pen-
etration above 50; and a corresponding allowance will be made for a penetration
below 50.
543. There are two marked differences between the preceding
specifications and those for asphalt binder for bituminous macadam
(§ 538) and those for asphalt binder for bituminous concrete (§ 540).
In the first place, the preceding specifications are briefer, having
only five items while the others have ten; but the longer specifica-
282 BITUMINOUS ROAD MATERIALS [CHAP. VIII
tions contain several items of only minor importance. For example,
the foam test determines only the presence of water, but determines
nothing concerning the quality of the asphalt. Again, the specific
gravity test is of value in identifying the asphalt, but is of no value
in determining its quality. Further, the melting point indicates
consistency, which is more accurately determined by the penetration
test. On the other hand, notice that the specifications of § 542 alone
contain a test for ductility, which is the sole test to determine the
cementing value of the asphalt.
In the second place, the specifications of § 542 are the general or
blanket form, while those of § 538 and § 540 are restricted or special
form. For a discussion of the merits of the two forms, see § 532.
544. Asphalt Filler for Block Pavements. It is important that
the asphalt used as a filler in block pavements shall be affected as
little as possible by temperature changes; and therefore the manu-
facturers have prepared a material especially for this purpose,
partly by refining the asphalt and partly by hardening it by oxidation,
i. e., by passing air through it. The following are the usual speci-
fications for an asphalt filler for brick, stone-block, and wood-block
pavements. *
The asphalt paving cement shall be obtained by the distillation of an as-
phaltic petroleum at a temperature not exceeding 700° F., and shall comply
with the following requirements:
1. It shall be homogeneous.
2. The melting point shall not be less than 130 nor more than 145° F.
3. The solubility in carbon tetrachloride shall not be less than 98| per
cent.
* 4. The penetration at 77° F. shall not be less than 60 nor more than 100;
and the penetration at 100° F. shall not exceed three times the penetration at
77° F. The contractor before beginning work shall obtain from the engineer a
statement in writing as to the penetration desired, and a variation not greater
than ten points either way from this penetration will be permitted.
5. The ductility at 77° F. shall not be less than 40 centimeters, the rate of
elongation being five centimeters per minute.
6. It shall not lose more than 3 per cent by volatilization when maintained
at a temperature of 325° F. for 5 hours; nor shall the penetration of the residue
after such heating be less than one half the original penetration.
7. The asphalt filler shall be used on the work at a temperature of not
less than 275° F.; and shall at no time be heated above 350° F.
8. It shall be delivered where directed by the engineer in time to allow for
examination and analysis.
* Specifications for Stone Block Paving, adopted by American Society of Municipal Im-
provements, 1916.
ART. 2]
PETROLEUM
283
545. COST. The cost of all materials is abnormal at present
owing to the Great European War; but the asphalt market is further
disturbed by the unsettled political conditions in Mexico, which
make it difficult to obtain Mexican petroleum for fluxing. The
prices of solid refined asphalt for January, 1917, which prices
obtained for the year 1917, f.o.b. Maurer, N. J., in tank cars, were
about as follows :
KINDS OP ASPHALT.
Price per Ton
(2000 Lb.)
Per Cent
Bitumen.
Value for
100 Per Cent
Bitumen.
Bermudez
$27.00
95.0
$25 65
Mexican
Residual
15.00
16 00*
95.5
14.32
Texas
18.00
98.8
17 78
Trinidad
19 00
99 5
18 90
* Varies $3.00 to $4.00 either way in different parts of the country,
advanced 100 per cent since 1914.
The average price has
If asphalt is bought in barrels or drums, the cost is usually 1.5
to 2 cents per gallon more than above, with sometimes a little rebate
on the returned barrels.
Liquid asphalt is now (1917) 7 cents per gallon f.o.b. Maurer, N. J.
546. For current prices of asphalts, consult the price lists in the
technical journals.
ART. 2. PETROLEUM
547. CLASSIFICATION. There are two types of crude petro-
leum— one giving a paraffin residue, and the other an asphalt
residue. The petroleums from Pennsylvania, Ohio, Indiana and
Illinois have a paraffin residue or base; while those from California
and Mexico and some from Texas have an asphaltic base. The
oils from Kentucky, Louisiana, and some from Texas have a mixed
paraffin and asphalt base, and are usually called semi-asphaltic oils.
The oils having a paraffin base are more or less greasy, and have
no binding qualities, but rather a lubricating effect; and are useful
only as a temporary dust layer. The oils having an asphaltic or
bituminous base are more valuable for roads, since the bituminous
base binds the particles of the road together, even after the more
volatile portions of the oil have evaporated.
Crude petroleum as it comes from the well is an oily liquid, vary-
ing in color from greenish brown to nearly black, and varying in
specific gravity from 0.73 to 0.97. In the refining process the more
284 BITUMINOUS ROAD MATERIALS [CHAP. VIII
volatile and more valuable constituents, as benzine, gasoline, kero-
sene, and lubricating oils, are driven off by heat. It is the residue
that is used in oiling roads and as a flux for softening the solid native
asphalts (§ 528-29). The character of the residue varies with the
crude petroleum and with the process of refining, and its value for
road purposes depends upon its specific gravity and the amount of
bitumen it contains.
548. METHODS OF REFINING. There are two methods of
refining petroleum to produce materials for use on roads. One
method is that of ordinary distillation by the use of external heat,
usually steam. The distillation is carried on until only the solid or
semi-solid portion remains. This method is not usually applied
to paraffin petroleums on account of the high temperatures necessary
and the resulting decomposition.
The second method consists in blowing atmospheric air through
petroleum heated to a temperature below that required for distilla-
tion. The air produces oxidation and condensation of the lighter
hydrocarbons, and the oil gradually thickens. This method is
usually applied to oils having a paraffin base; and the residue is
known as blown oil, or sometimes as blown-oil asphalt, although it is
usually paraffin or at most a semi-asphalt mixture.
549. SHIPPING THE OIL. Road oil is usually shipped in tank
cars holding either 4000 to 6000 gallons or 8000 to 10,000 gallons, the
latter being much more common. Oil may be had in barrels; but it
is then more expensive, and is also much more difficult to handle.
The tank cars are equipped with steam heating coils, so the material
may be heated in the tank by attaching a steam pipe or hose.
There are a number of simple pumps on the market that will
pump either hot or cold oil. The ordinary thresher's water-tank
pump may be used for pumping cold oil; but not hot oil, as it will
soon burn out the valves. Fig. 92, page 285, shows the method of
pumping road oil from the dome of a tank car by means of a hand
diaphragm pump; and a thresher's water-tank pump may be used
in the same way. However, this method is nearly obsolete, since
state and county highway departments usually own outfits made
particularly for this purpose.
Power-driven diaphragm and rotary pumps are the forms gen-
erally used; and are attached to the outlet in the bottom of the tank
car. These pumps are driven by an independent gasoline engine or
by steam from a steam road-roller, which is sometimes needed at the
tank car to supply steam to heat the oil. Fig. 93 shows the method
ART. 2]
PETROLEUM
285
of transferring road oil from the tank car to the distributing
wagon by means of a gasoline-driven rotary pump.
550. ASPHALTIC CONTENT OF ROAD OILS. Road oils are fre-
quently referred to as containing a certain per cent of asphalt;
and the oil refineries always classify road oils according to their
asphaltic content — see §561.
This " asphalt " is not a definite compound that can be deter-
mined by chemical analysis. To determine the " asphalt " in a road
oil, the oil is heated in a closed oven to a temperature of 400° F.,
which drives off the light or volatile constituents and leaves a semi-
FIG. 92. — UNLOADING OIL WITH DIAPHRAGM
PUMP.
FIG. 93. — UNLOADING OIL WITH ROTARY
PUMP.
solid or solid residue. This residue is called 'asphalt, although
it may contain many different bitumens. If the residue is paraffin,
it is useful as a dust layer, but worthless as a road binder. Since road
oil is so cheap, it is not likely that the residue contains any adulter-
ation. The amount of residue in a road oil gives no indication of its
value for use on a road, since the residue may be wholly paraffin, or
semi-asphalt, or asphalt. However, the per cent of " asphalt •" does
give some indication as to the viscosity of the oil, i. e., as to the
amount of body it contains. The method employed and equipment
used in determining the amount of " asphalt " vary considerably,
and are not usually stated; hence the result is quite indefinite. No
reliance should be placed upon such a loose term; but the oil should
be bought to conform to definite specifications.
The degree of hardness of this residue is measured by the depth
of penetration of a No. 2 needle under a load of 100 grams in 5 seconds.
It is sometimes specified that the road oil shall contain a certain per
cent of "asphalt" having a stated penetration. For example:
" The oil shall contain 90 per cent of 'asphalt ' having a penetration
of 80."
286 BITUMINOUS ROAD MATERIALS [CHAP. VIII
551. SPECIFICATIONS FOR ROAD OIL. Road oils are usually
bought without any specifications as to their composition and also
without inspection or analysis. This is very unfortunate, since the
residue may be paraffin, which is a lubricant rather than a binder;
and also since the residue, even though a bitumen, may have been
burned in the refining process until it possesses little or no binding
qualities.
Specifications differ considerably according to the purpose for
which the oil is to be used or the conditions under which it is applied ;
and practice has not yet established a standard for any particular
purpose or condition. Below are the specifications of several grades
of road oil that have been successfully used.
552. For Park Drives. The following are the specifications of
the light oil used as a dust layer (§ 330), in Washington, D. C., on
gravel park drives.*
1. The oil shall be a viscous fluid product, free from water, and showing
some degree of adhesiveness when rubbed between the fingers.
2. It shall have a specific gravity of not less than 0.940 at 25° C.
3. It shall be soluble in carbon disulphide at air temperature to at least 99
per cent; and shall not contain over 0.2 per cent of insoluble inorganic matter.
4. It shall contain not less than 3 per cent nor more than 10 per cent of
bitumen insoluble in 86° paraffin naphtha at air temperature.
5. When 240° c.c. of the oil is heated in an Engler viscosimeter to 50° C.
and maintained at this temperature for at least 3 minutes, the first 50 c.c. shall
flow through the aperture in not less than 10 minutes nor more than 20 minutes.
6. When 20 grams of the material is heated for 5 hours hi a cylindrical tin
dish, approximately 2| inches in diameter by 1 inch high, at a constant tem-
perature of 163° C., the loss in weight by volatilization shall not exceed 20 per
cent. The residue should be decidedly sticky.
7. Its fixed carbon shall not be less than 3.5 per cent.
553. For Earth Roads. The two following specifications have
been adopted by the Illinois Highway Department,! and have prac-
tically been adopted by nearly all State Highway Departments for
the surface treatment of earth roads. The oils are to be tested
according to the method described in Bulletin No. 314, U. S. Depart-
ment of Agriculture, December 10, 1915.
554. For Loam and Clay. The following light oil should be ap-
plied cold, for a first application on loam or clay. For subsequent
applications use the oil as described in the next section.
* Paper by Col. Spencer Cosby, U. S. Army, in charge of Buildings and Grounds, Washing-
ton, D. C., presented before Section D of the American Association for Advancement of Science,
December 29, 1911.
f Private letter from TV, W, Marr, Chief Highway Engineer, under date of July 18, 1917,
ART. 2] PETROLEUM 287
1. The oil shall be homogeneous and free from water.
2. Specific Gravity 25° C. (77° F.) not less than 0.890
3. Bitumen soluble in disulphide not less than 09.5%
4. Residue of 100 penetration * 40 to 60%
5. Specific viscosity at 40° C. (104° F.) 10.0 to 25.0
When it is desired to apply the oil hot, oils somewhat more viscous and having
a specific viscosity at 40° C. (104° F.) up to 50 will be acceptable, provided
these oils conform to the requirements of the specifications in all other respects.
An oil having a specific viscosity at 40° C. (104° F.) of more than 25.0 will not be
accepted, unless it is to be applied hot.
555. For Sandy Soil. The following oil should be applied cold,
and is suitable for a dust layer on a sandy earth road, for a second
application on a loam or clay road, and also for a surface application
on a water-bound gravel or macadam road.
1. The oil shall be homogeneous and free from water.
2. Specific gravity at 25° C. (77° F.) not less than 0.910
3. Specific viscosity at 40° C. (104° F.) 10.0 to 25.0
4. Loss at 163° C. (325° F.) for 5 hours not over 25%
5. Bitumen soluble in disulphide not less than 99.5%
6. Bitumen insoluble in 86° B. naphtha not less than 5%
7. Fixed carbon not less than 4.0%
8. Specific viscosity at 40° C. (104° F.) 10.0 to 25.0
When it is desired to apply the oil hot, oils somewhat more viscous and
having a specific viscosity of 40° C. (104° F.) up to 50 will be acceptable, pro-
vided these oils conform to the requirements of the specifications in all other
respects. An oil having a specific viscosity at 40° C. (104° F.) of more than 25.0
will not be accepted unless it is to be applied hot.
556. For Water-bound Gravel or Macadam. The three following
specifications have been adopted by the Illinois Highway Department f
for a dust layer and a protective coating on water-bound gravel or
macadam roads, and practical^ the same have been adopted by
most of the State Highway Departments. The oils are to be tested
according to the methods described in Bulletin No. 314, U. S. Depart-
ment of Agriculture, December 10, 1915.
The light oil (§ 557) is preferable for the first application or where
the road is somewhat dusty, since it penetrates better than the
heavier oils; but for subsequent applications a heavier oil (§ 558 or
§ 559) is preferable. If the gravel or macadam is very clean, the
heavier oil may be used for a first application.
* Am. Soc. Test. Mat. Standard Test, D-5-16.
t Private letter from W. W. Marr, Chief Highway Engineer, under date of July 18, 1917.
288 BITUMINOUS ROAD MATERIALS [CHAP. VIII
557. Light Oil. The following oil is to be applied cold.
1. The oil shall be homogeneous and free from water.
2. Specific gravity 25° C. (77° F.) 0.920 to 0.970
3. Loss at 163° C. (325° F.) for 5 hours 20.0% to 30.0%
4. Bitumen soluble in disulphide not less than 99.5%
5. Bitumen insoluble in 86° B. naphtha 5.0 to 20.0%
6. Fixed carbon 4.0% to 10.0%
7. Specific viscosity at 25° C. (77° F.) 30.0 to 70.0
558. Medium Oil. The following oil need not be applied hot
except when the temperature of the air is below 80° F.
1. The oil shall be homogeneous, free from water, and shall not foam when
heated to 100° C. (212° F.).
2. Specific gravity 25° C. (77° F 0.960 to 1.010
3. Flash point not less than 100° C. (212° F.)
4. Float test at 32° C. (90° F.) 30 to 90 seconds
5. Loss at 163° C. (325° F.) for 5 hours not over 15.0%
6. Float test of residue at 50° C. (122° F.) 90 to 180 seconds
7. Bitumen soluble in disulphide not less than 99.5%
8. Bitumen insoluble in 86° B. naphtha 7.0 to 20.0%
9. Fixed carbon 5.0% to 10.0%
10. Specific viscosity at 100° C. (212° F.) 5.0 to 15.0
559. Heavy Oil. The following oil should be applied hot.
1. The oil shall be homogeneous, free from water, and shall not foam when
heated to 150° C. (302° F.)
2. Specific gravity 25° C. (77° F.) not less than 0.980
3. Flash point not less than 150° C. (302° F.)
4. Float test at 50° C. (122° F.) ..100 to 200 seconds
5. Loss at 163° C. (325° F.) for 5 hours, not over 5.0%
6. Float test of residue at 50° C. (122° F.) 120 to 240 seconds
7. Bitumen soluble in disulphide not less than 99.5%
8. Bitumen insoluble in 86° B. naphtha 10.0 to 25.0%
9. Fixed carbon ' 7.0% to 15.0%
10. Specific viscosity at 100° C. (212° F.) 30.0 to 70.0
560. COST. The price of road oils varies greatly, partly because
of the natural variation with locality, but chiefly because a large
proportion of road oils is sold without specifications or inspection.
The demand for oil for road purposes has increased so rapidly in
recent years that the price has advanced more rapidly than most
construction materials.
561. The following are the market quotations in Engineering
News-Record, July 5, 1917. The prices are for road oil in tank cars
(8000 gallons minimum capacity) f.o.b. places named.
ART. 3] TAR 289
New York City, 40-50 per cent asphalt 6| cts. per gal.
60-70 per cent asphalt 7 cts. per gal.
dust layer 71 cts. per gal.
binder 8| cts. per gal.
St. Louis, asphalt 5 cts. per gal.
Dallas, 40-50 per cent asphalt 6 cts. per gal.
60-70 per cent asphalt 7£ cts. per gal.
San Francisco, 75-79 per cent asphalt (barrel = 42 gal.) $1.83 per bbl.
ART. 3. TAR
563. The tar used in road work is obtained as a by-product in
the destructive distillation of bituminous coal in the manufacture
of illuminating gas or in the production of coke, as well as in the
decomposition of petroleum.
564. DEFINITIONS. Coal Tar. Tar produced from the destruc-
tive distillation of coal.
Coke-oven Tar. A by-product in the manufacture of coke.
Gas-house Tar. A by-product in the manufacture of illuminating
gas from coal.
Oil-gas Tar. A by-product in the manufacture of illuminating
gas from petroleum.
Pitch. The solid residue produced by the evaporation or dis-
tillation of tar.
Refined Tar. A tar freed from water by evaporation or dis-
tillation, which process is continued until the tar is of the desired
consistency. When all the water is driven off, it is called Dehydrated
Tar. Refined tar is also produced by fluxing the tar residuum with a
tar distillate, in which case the product is called Cut-back Tar.
Water-gas Tar. A by-product in the manufacture of carbureted
water-gas from petroleum.
565. CHARACTERISTICS OF TAR. Most of the tar used in road
work is coal tar, either coke-oven or gas-house tar. In some partic-
ulars the characteristics of tars overlap; but the following table shows
their chief differences:
CHARACTERISTICS.
Water, per cent
KIND OF TAR:
COKE-OVEN. GAS-HOUSE.
22 2.9
Light oil up to 200° C
34 40
Creosote oil, per cent
14.5 8.6
Naphthalene, crude, per cent
67 74
Anthracene crude per cent
27 3 17 4
Pitch, per cent. ... ....
. ., . 44 3 58.4
Free carbon, ner cent . .
5-8 15-25
290 BITUMINOUS ROAD MATERIALS [CHAP. VIII
The quality of gas-house tar depends upon the temperature at
which the distillation takes place. The distillation usually takes
place at a high temperature; and consequently the tar contains less
of the heavy oils and more of the solid bitumen and more free carbon,
and is not desirable for road work, because of an excess of free carbon
and of the lack of the heavy oils.
Coke-oven tar is usually formed at a lower temperature, and
hence contains more of the heavy oils and less free carbon; and is
therefore usually more suitable for road work than gas-house tar.
Water-gas tar is lighter than coal tar, contains a larger percentage
of heavy oils, and a less percentage of pitch. It is usually low in free
carbon, and does not contain ammonia. Since water-gas tars con-
tain comparatively small proportions of pitch, they are not as suitable
for a road binder as coal-gas or coke-oven tars; but since they con-
tain a larger percentage of the heavier oils, they are desirable materials
for use as dust layers.
566. Crude tar is refined by driving off the lighter oils. The
residue may be liquid or solid according to the temperature to which
the distillation was carried and the extent to which the heavy oils
have been removed. Sometimes the distillation is carried only far
enough to drive off the water and the lighter oils. Such a product is
known as dehydrated tar; and it is more suitable for road work
than crude tar, since it contains no water or ammonia.
567. SHIPPING TAR. Tar is shipped in barrels or metal drums
or in tank cars; and is unloaded and distributed the same as asphalt
and oil — see § 504 and § 549.
568. SPECIFICATIONS FOR TAR. Practice has not established
standard specifications; and consequently there are a great number
in use, which differ according to the source or character of the tar
and also according to the opinion of the one writing the specifica-
tions. Only an expert road engineer and chemist should attempt
to prepare specifications; and then great care is necessary, since a
limitation in one particular may affect the limits of some other
factor. The producers of bituminous materials make a variety of
grades of material, which are sold under different trade names
(see § 578).
Below are the specifications for materials that have been suc-
cessfully used for different kinds of work by good authorities.
569. For Bituminous Surfaces. The two following specifications
have been adopted for refined tar for bituminous surfaces on water-
bound gravel or macadam roads (Chapter IX), and on bituminous
ART. 3] TAR 291
bound roads (Chapter X) by the Illinois Highway Department,*
and are practically the same as those adopted by most State Highway
Departments. The tests are to be made as described in Bulle-
tin 314, U. S. Department of Agriculture, December 10, 1915.
For the first treatment of a road the light tar of § 570 is to be pre-
ferred; and for subsequent application the heavier tar of § 571 is
better.
570. Hot Application. The following tar should be applied
hot.
1 . The tar shall be homogeneous and free from water.
2. Specific gravity 25° C. (77° F.) 1.120 to 1.200
3. Specific viscosity at 40° C. (104° F.) 4.0 to 12.0
4. Total distillate by weight :f
to 170° C. (338° F.) not over 5.0%
to 300° C. (572° F.) not over 35.0%
5. Specific gravity of total distillate 25° C. (77° F.) not less than 1.010
6. Melting point of residue not over 65° C. (149° F.)
7. Bitumen soluble in disulphide 88.0 to 96.0%
8. Inorganic matter (ash) not over 0.5%
571. Cold Application. The following tar should be applied
cold.
1. The tar shall be homogeneous and free from water.
2. Specific gravity 25° C. (77° F.) 1.180 to 1.250
3. Float test 32° C. (90° F.) 90 to 150 seconds
4. Total distillate by weight:f to 180° C. (338° F.) not over 1.0%
to 300° C. (572° F.) not over 25.0%
5. Specific gravity of total distillate, 25° C. (77° F.) not less than 1.030
6. Melting point of residue not over 75° C. (167° F.)
7. Bitumen soluble in disulphide 78.0 to 88.0%
8. Inorganic matter (ash) not over 0.5%
572. For Bituminous Macadam. The American Society of
Municipal Improvements on October 12, 1916, adopted standard
specifications for two grades of tar for bituminous macadam roads
(Art. 1, Chapter X), which materials are optional with each other
and also with any of the four kinds of asphalt described in § 537-38
and Table 31, page 278. The specifications in full for water-gas tar
are given in § 573; and Table 34 shows the essential features of
both tars.
* Private letter from W. W. Marr, Chief Highway Engineer, under date of July 18, 1917,
t Amer. Soc, Test, Mat. Standard Test, D-20-16.
292
BITUMINOUS ROAD MATERIALS
[CHAP, viii
573. Water-gas Tar.* 1. Foam: Refined water-gas tar shall be homogene-
ous, free from water, and shall not foam when heated to 121° C. (250° F.).
2. Specific Gravity: The specific gravity at a temperature of 25° C. (77° F.)
shall be not less than 1.150 nor more than 1.200.
3. Viscosity: When tested by means of the New York Testing Laboratory
Float Apparatus, the float shall not sink in water maintained at 50° C (122° F.)
in less than 120 nor more than 150 seconds.
4. Bitumen Soluble in Bisulphide: The bitumen as determined by its solu-
bility in chemically pure carbon disulphide at room temperature, shall be not less
than 95.0 per cent; and the material insoluble in carbon disulphide shall not show
more than 0.2 per cent ash upon ignition.
5. Distillation: When distilled according to the tentative method recom-
mended by Committee D-4 of the American Society for Testing Materials in
1911, it shall yield not more than 0.5 per cent distillate at a temperature
lower than 170° C. (338° F.); not more than 12.0 per cent shall distill below
270° C. (518° F.); and not more than 25.0 per cent shall distill below 300° C.
(572° F.).
6. Distillate, specific gravity of: The total distillate from the test made in
accordance with paragraph 5 shall have a specific gravity at a temperature of
25° C. (77° F.) of not less than 0.980 nor more than 1.020.
7. Distillate, melting point of: The melting point, as determined in water
by the cube method, of the pitch residue remaining after distillation to 300° C.
(572° F.) in accordance with the test described in paragraph 5, shall be not more
than 75° C. (167° F.)
TABLE 34
COMPARISON OF SPECIFICATIONS FOR TARS FOR BITUMINOUS MACADAM
Standards of American Society of Municipal Improvements, Adopted October 12, 1916
Ref.
No.
Items.
Water-gas Tar.
Coal Tar.
1
Shall not foam at
121° C.
121° C
?,
Specific gravity at 25° C
1 150-1 200
1 180-1 300
3
Viscosity by N. Y. float apparatus
120-150 sec.
150-180 sec
4
5
6
7
Bitumen soluble in disulphide, not less than . .
Distillation, yield to 170° C., not more than.
Distillation, yield to 270° C., not more than.
Distillation, yield to 300° C., not more than. .
Distillate, total, specific gravity of
Residue, melting point of, not more than ....
95.0%
0.5%
12.0%
25.0%
0.98-1.020
75° C.
80.0-95.0
0.5%
10.0%
20.0%
1.020
75° C.
574. For Bituminous Concrete. The American Society of
Municipal Improvements on October 12, 1916, adopted standard
specifications for two grades of tar suitable for the binder of bitumi-
nous concrete (see Art. 2 of Chapter X), which materials are optional
to each other and also with any one of the five grades of asphalt
* Specifications for Broken Roads with Bituminous Surface, adopted by American Society
of Municipal Improvements, October 12, 1916, p. 24-25,
ART. 3]
TAR
293
described in § 540, and Table 32, page 280. The specification in
full for one of the tars is given in § 575; and the essential features of
both are shown in Table 35.
575. Water-gas Tar. 1. Foam: The refined tar shall be homogeneous, free
from water, and shall not foam when heated to 150° C. (302° F.).
2. Specific Gravity: Its specific gravity at a temperature of 25° C. (77° F.)
shall not be less than 1.160 nor more than 1.200.
3. Viscosity: When tested by means of the New York Testing Laboratory
Float Apparatus, the float shall not sink in water maintained at. 50° C. (122° F.)
in less than 140 seconds nor more than 170 seconds.
4. Bitumen Soluble in Disulphide : The bitumen as determined by its solu-
bility in chemically pure carbon disulphide at room temperature shall be not less
than 95.0 per cent; and the material insoluble in carbon disulphide shall show
nor more than 0.2 per cent ash upon ignition.
5. Distillation: When 'distilled according to the tentative method recom-
mended by Committee D-4 of the American Society for Testing Materials in
1911, it shall yield no distillate at a temperature lower than 170° C (338° F.);
not more than 7.0 per cent by weight shall distill below 270° C. (518° F.); and
not more than 20.0 per cent by weight shall distill below 300° C. (572° F.).
6. Distillate, specific gravity of: The total distillate from the test made in
accordance with paragraph 5 shall have a specific gravity at a temperature of
25° C. (77° F.) of not less than 1.000 nor more than 1.020.
7. Distillate, melting point of: The melting point, as determined in water by
the cube method, of the pitch residue remaining after distillation to 300° C.
(572° F.), in accordance with the test described m paragraph 5, shall be not
more than 75° C. (167° F.).
TABLE 35
COMPARISON OF SPECIFICATIONS FOR TARS FOR BITUMINOUS CONCRETE
Standards of American Society of Municipal Improvements, October 14, 1916
Ref.
No.
Items.
Water-gas Tar.
Coal Tar.
1
2
3
4
5
6
7
Shall not foam at
150° C.
1.160-1.200
140-170 sec.
95.0%
0.0
7.0%
20.0%
1.00-1.020
75° C.
150° C.
1.200-1.300
140-170 sec.
75.0-90.0%
0.0
10.0%
20.0%
1.030
75° C.
Specific gravity at 25° C
Viscosity by N. Y. float apparatus
Bitumen soluble in disulphide, not less than. .
Distillation, yield to 170° C., not more than. .
270° C., not more than..
300 °C., not more than..
Distillate total, specific gravity of.
Distillate, residue, melting point of, not more
than
576. For Joint Filler of Block Pavements.* The following are the
specifications for a tar suitable for the joint filler of brick, stone-
* P. P. Sharpies, Manager and Chief Chemist, Tarvia Department, Barrett Manufacturing
Co., forwarded for this use under date of July 13, 1917.
294 BITUMINOUS ROAD MATERIALS [CHAP. VIII
block, or wood-block pavements. The specifications may need a
slight variation for the extremes of northern or southern portions of
this country.
677. 1. Pitch. The pitch shall be straight-run residue from the distillation of
coal tar.
2. Specific Gravity: The specific gravity at 78° F. shall not be less than 1.24
nor more than 1.32.
3. Melting Point: The melting point shall not be lower than 115° F. nor
higher than 150° F. For mastic filler the melting point shall be 115 to 135° F.;
for brick and stone-block 125 to 140° F.; and for wood-block, 140 to 150° F.
The contractor before beginning work on any contract shall obtain from the
Chief Engineer in writing a statement as to the 'melting point desired for that
particular contract, and a variation of 5° F. either way from this value will be
permitted; but the melting point must be within the limits indicated above.
The melting point should be higher, the steeper the grade. For grades
above 10 per cent, in a warm climate, the melting point should be 140° to
150° F.
4. Free Carbon: The free carbon shall not be less than 22 per cent nor more
than 37 per cent.
5. Distillation: The specific gravity of the distillate to 670° F. shall be not
less than 1.07 at 140° F. compared with water at the same temperature.
578. Trade Names. A trade name is in a sense a specification
for a material; and hence the following definitions of well-known
trade names for tar products are appropriate here :
Tarvia A. A refined coal tar for hot surface application to macadam roads
for preserving them and laying dust. Tarvia A in distinction from Tarvia B
forms a perceptible blanket on the surface; and is therefore limited for successful
use to roads receiving either wholly automobile traffic or a high percentage of
such traffic. It has been largely used in park work in the neighborhood of large
cities.
Tarvia B. A refined coal tar for cold surface application as a dust layer
and road preservative. Primarily for use on macadam roads, but also applicable
to gravel and other hard-surfaced roads.
Tarvia KP. A refined coal-tar binder cut back to permit its use cold in
making patches and in other maintenance work on bituminous surfaced and
bituminous bound roads.
Tarvia MF. A refined coal tar prepared for use as a mastic with sand in
filling the joints of brick, stone-block, and lug wood-block pavements.
Tarvia X. A refined coal tar prepared for use as a binder for bituminous
macadam roads. Modifications are made to permit its use in bituminous con-
crete.
Tarvia XC. A Tarvia X prepared for use in patching and maintaining the
joints in concrete roads.
579. COST OF ROAD TAR. Cost data are always difficult to
handle in printed matter, since the record is liable to be out of date
ART. 3] TAR 295
before it is presented to the public; and this seems to be specially
true of tar, particularly at the time this paragraph is written.
The cost of road tars meeting the preceding specification, in
the Middle and Eastern States where the conditions are more uniform
than in other parts of the country, range from 8 to 13 cents per gallon
f.o.b. siding at destination. The lighter materials suitable for cold
application cost 1 or 2 cents per gallon less than those applied hot.
580. For more recent data consult the price reports in the current
technical journals.
CHAPTER IX
BITUMINOUS SURFACES FOR ROADS
582. Before the advent of motor-driven vehicles gravel and mac-
adam roads gave good service; but the coming of the automobile
introduced new conditions that made necessary a radical change in
the construction of a gravel or macadam road having any considerable
proportion of motor-driven traffic. The low-hung swift-moving
automobile, more than horse-drawn vehicles, throws the stone dust
into the air and thus permits it to be blown away, and besides the
rubber tires, unlike steel tires and horse shoes, do not make any
stone dust to replace that blown away. Therefore gravel and mac-
adam roads rapidly deteriorate under any considerable motor-driven
traffic. This state of affairs led to the introduction, in substantially
the past ten years, of several new forms of road construction in
which the binding power of clay or stone dust is replaced by that of a
bituminous material like tar.
There are two general types of such construction, viz.: one in
which a superficial coating of bituminous material is laid upon a
gravel, macadam or concrete road, or even upon a brick or stone-
block pavement; and the other in which the bituminous material is
employed as a binder for the upper stratum of the road. The super-
ficial layer is called a Protective Coating or a Bituminous Carpet,
according to its thickness and construction. This type of con-
struction will be considered in this chapter.
When the second form of construction is employed the road is
known as either a Bituminous-Macadam or a Bituminous-Concrete
Road, according to the details of the construction, which types of
construction will be considered in the next chapter.
583. KINDS OF BITUMINOUS SURFACES. The bituminous sur-
face may consist either of a thin bituminous film or of a compara-
tively thick mat composed of successive layers of bituminous
296
ART. 1] PROTECTIVE COATING 297
material and screenings, sand or gravel. The former is usually called
a Protective Coating, and the latter a Bituminous Carpet.
ART. 1. PROTECTIVE COATING
584. A light oil is sometimes applied to an earth road, a gravel
road, or a water-bound macadam road to lay the dust. It is not
expected that the oil will have any binding power; and frequent
applications are necessary for effectiveness. But when a water-
bound gravel or macadam road is required to carry only a small
proportion of motor-driven traffic, it is sometimes possible to pro-
tect the surface with a thin bituminous coating which will resist
the action of both the horse-drawn and motor-driven traffic, and thus
prolong the life of the road surface.
685. THE BITUMINOUS MATERIAL. The bituminous material
should be fluid at ordinary temperatures in order that it may be
applied cold and spread uniformly. It should contain a small amount
of volatile oils which will evaporate and leave a cementitious film
on the surface. A light refined tar which is fluid at ordinary tem-
peratures (§ 571-72) is generally used. An asphaltic oil containing
from 40 to 50 per cent of asphalt gives fair results. Oils are cheap
and readily applied ; but are not entirely satisfactory for bituminous
coatings for the following reasons: 1. Most petroleum products, even
those having an asphaltic base, while in a fluid state act to a certain
extent as a lubricant. 2. Both medium and heavy asphaltic oils
require considerable time to set up; and therefore, if the road is
opened to travel before the oil has set, more or less movement of the
coating will take place, and it will become wavy and full of bumps.
The amount of tar or asphaltic oil should rarely exceed 0.2 of a
gallon per square yard, and an excessive amount is specially to be
avoided.
586. The field for this form of surface is comparatively limited,
and the effect of such a coating is only temporary; but this treatment
is often a valuable means of carrying an old gravel or macadam road
along until a better form of treatment can be given. It is more
expensive and more permanent than an oil dust-layer (§ 329-31);
but is cheaper and less permanent than a bituminous carpet (§ 588).
298 BITUMINOUS SURFACES FOR ROADS [CHAP. IX
ART. 2. BITUMINOUS CARPET
688. When the proportion of motor-driven traffic on a water-
bound gravel or macadam road becomes considerable (see Table 26,
page 177), it is more economical to protect the road surface with a
bituminous carpet or blanket than continually to add screenings or
gravel to supply binding material. In some cases the mat or carpet
is added to prevent the road from being denuded of binder, and in
other cases the carpet protects the stone itself from excessive wear,
which is particularly important on a road built of soft limestone.
The bituminous carpet not only protects the road but eliminates
practically all dust.
In consideration of the large mileage of water-bound gravel and
macadam roads built before the advent of the automobile, this
method of treating such roads is very important. Under some con-
ditions it is still economical to build new water-bound gravel and
macadam roads, and cover them with a bituminous carpet; although
owing to the difficulties of maintaining a bituminous carpet, it is
usually wiser to build a bituminous-macadam or a bituminous-
concrete road.
589. THE BITUMINOUS MATERIAL. Either refined tar (§ 568)
or asphaltic oil (§ 551) may be used. The particular grade of tar
or oil to be used depends upon the condition of the road and the
amount and character of the travel. If the road has begun to ravel
and most of the stones have been swept bare of binding material, a
refined tar like that in § 571, or a heavy oil like that in § 559 should
be used. If some bonding material remains on the road surface and
the large stones are not entirely exposed, a medium oil like that in
§ 558 would be better. If the surface is tightly bound and hard to
sweep free from dust and fine material, a tar product like that of
§ 570 or a light oil like that of § 557 should be selected.
690. CLEANING ROAD SURFACE. The road should be swept
with a revolving power-broom and then with a hand-broom until
the surface is entirely free from dust and fine particles. The bitumi-
nous material adheres better if the road is sprinkled before treatment,
but it should be allowed to dry before the bituminous material is
applied. Unquestionably water on the road when the bituminous
material is applied is harmful ; but the sprinkling washes off the dust
and therefore is beneficial, provided the road is dry when the bitumi-
nous material is applied.
ART. 2]
BITUMINOUS CARPET
299
Fig. 94 shows the method of cleaning an old macadam road pre-
paratory to applying the bituminous surface.
FIG. 94. — SWEEPING AN OLD MACADAM ROAD BEFOKE APPLYING THE BITUMINOUS SURFACE.
591. APPLYING BITUMINOUS MATERIAL. The binder may be
applied either by hand or by machine. In the hand method, ordi-
nary garden watering pots or special pouring cans are used, being filled
from a large supply tank that is driven along beside the work. It is
very difficult to apply the bituminous material evenly with a hand
pouring car ; ar?d it is necessary immediately to follow the applica-
tion with a brush broom and sweep the surplus oil ahead. This
method of applying the material is very slow and expensive, and is
now seldom used except for small jobs and for patch work.
There are many different types of machines for distributing the
bituminous material, but in outward appearance they do not differ
greatly from the oil distributor shown in Fig. 39 and 40, page 137.
There are a number of hand-drawn cart gravity-distributors.
Some horse-drawn distributors have gravity feed ; but the mechanical
feed or pressure distributor is the more common, since it secures
a more uniform distribution and permits more accurate regulation of
the amount applied. Some of the distributors have their own
heating device; but some are made for spreading cold oil, or depend
upon an auxiliary heater. Some distributors deliver the bituminous
material in small streams, and others in a fan-like sheet, while
still others deliver it in a fine spray. It is claimed that the spray
applies the material more uniformly, and that it strikes the surface
of the road with enough force to penetrate all interstices and to blow
300 BITUMINOUS SURFACES FOR ROADS [CHAP. IX
away all dust, and thus secures a good union with the stone of the
road.
Fig. 95 shows a distributing tank which is drawn with horses or
behind a road roller. The special features of this distributor are:
(1) the bituminous material is heated by steam from a road
roller, and hence can not be burned; (2) the material is dis-
charged by air pressure in the tank, and hence there are no pumps
to become clogged; and (3) the first cost of the machine is low.
FIG. 95. — HORSE-DRAWN PRESSURE DISTRIBUTOR.
The same features are embodied in a self-contained automobile-
truck distributor, which permits work at a greater distance from the
central heating plant.
Fig. 96 shows a motor-driven distributor. The distributing
head is in three sections, either outside one of which may be
turned up and put out of use. With both arms in use the total
spread is 16 feet. Fig. 97 shows a spray in use making a patch.
Views 1 and 5, Fig. 100, page 311, show a distributor spraying
tar.
592. The bituminous material may be delivered in railroad
tank-cars or in barrels or metal drums. The first method is objection-
able owing to the difficulty of having enough road surface ready
to receive 8,000 to 10,000 gallons of tar. When the binder is delivered
in barrels or drums, it is heated in a large kettle while the tank wagon
is distributing a load.
ART. 2]
BITUMINOUS CARPET
301
The heating should be done so as to heat evenly the entire mass,
and the heating should be under positive control at all times. The
tar should be heated to 93° C. (200° F.) and not above 121° C.
FIG. 96. — AUTOMOBILE PRESSURE DISTRIBUTOR.
(250° F.). Any material heated beyond 121° C. should not be used.
The distributing wagon should be supplied with one or more ther-
FIG. 97. — SPRAYING DISTRIBUTOR MAKING A PATCH.
mometers to insure that the temperature of the tar when applied is
between the above limits. It is unwise to apply tar when the air
or road is below 50° F. (10° C.).
593. The bituminous material is applied at the rate of J to J
gallon per square yard, in either one or two treatments. The carpet
should be uniform in thickness, as otherwise the thin places will cut
302 BITUMINOUS SURFACES FOR ROADS [CHAP. IX
through, and the thick portions will bunch if soft, and crack if a
hard material is used. The thinner the carpet that can carry the
traffic the better.
594. After the bituminous material has had a few hours to pene-
trate the surface of the road, and after it has set up a little, stone
screenings or pea gravel is added at the rate of about 1 cubic yard
to every 100 to 150 square yards of road surface. The size of the
screenings or pea gravel should be such as will pass a f-inch screen
and be caught on a ^-inch. The screenings should be from hard
stone, and should be free from, dust and fine material. The harder
the stone the smaller may be the screenings. The screenings are
spread by hand with shovels or with a revolving-disk mechanical
spreader. After being spread, particularly if the work is done by
hand, the screenings should be carefully broomed to secure a uniform
thickness of not over f of an inch.
The purpose of the screenings is to keep the bituminous material
from being picked up on the wheels of vehicles, to make the surface
less slippery, and to increase the wearing qualities of the road.
595. After the screenings have been spread a few hours, it is
advantageous to roll with a roller, preferably the tandem type,
weighing between 8 and 15 tons; but the rolling is not vital.
596. VALUE OF BITUMINOUS CARPETS. Bituminous carpets on
old water-bound macadam roads have been of great value in enabling
such roads to carry a considerable amount of motor-driven traffic;
and under some conditions it has been economical to build a new
water-bound macadam road and cover it with a bituminous mat or
carpet. Such a surface will usually last from 6 months to 2 years
depending upon the amount and kind of travel. Bituminous sur-
faces have not been as successful on gravel as on macadam, perhaps
because the former are more difficult to clean; but with care a fair
degree of success can be insured on gravel.
597. Many attempts have been made to add a bituminous sur-
face to a portland-cement concrete road, but with widely varying
degrees of success. The concrete road ordinarily has a large amount
of travel, and therefore usually has too many steel-tired horse-drawn
vehicles for a bituminous carpet. It seems to be agreed that the
following conditions are important: 1. The concrete itself must be
good. 2. The concrete surface should be roughened by wear before
the bituminous coating is applied. 3. The surface of the concrete
must be warm, dry, and clean when the bituminous material is
applied. 4. A preliminary priming or paint coat of thin tar is
ART. 2] BITUMINOUS CARPET 303
advantageous. 5. Two thin coats of the carpet material are better
than a single thick one. 6. A J-inch coating can not stand up under
much horse-drawn traffic. If there is much horse-drawn traffic, it
may be necessary to make the coating 1 to 1^ inches thick by applying
several successive layers of tar and screenings. Possibly a bitumi-
nous material will yet be made that will be more suitable for
such use.
The advantages of a bituminous surface on a concrete road are:
1. It protects the concrete from wear. 2. It reduces the noise from
the impact of horses' shoes and steel-tired wheels. 3. It removes the
glare of the light-colored concrete. 4. It hides the black blotches
made in rilling the cracks and joints.
598. MAINTENANCE OF BITUMEN CARPETS. The work of
maintenance consists in patching the carpet where it wears through
or peels up, and in removing bunches where the carpet has crawled.
The patching is easily done by following the method employed in
the original construction; but care should be taken that the spot
to be covered is clean, dry and warm.
It is not easy to remove the bunches. If the surface is soft, a
scraping grader (§ 155) will sometimes smooth the surface without
peeling up the carpet; but the work must be done during warm
weather and immediately after a rain. The bunches may be re-
moved by hand with a shovel that is kept hot while in use; but the
shovel will not last long. A sharp chisel-like cutting tool if made of
heavy metal will stand heating better than a shovel, and will remove
the bunches. The bunch can be softened by building a small fire of
twigs over it, or by pouring kerosene over it; but this practice is
likely to ruin the material for some distance around the bunch.
There are surface heaters, i. e., a hood having a gasoline flame under
it, which are used for removing sheet asphalt (Fig. 161, page 451),
which can be employed for removing these bunches; but the process
is slow and expensive, and the flame is likely to damage the material
which is not removed.
599. COST OF BITUMINOUS CARPET. Before the recent dis-
turbance of prices by the Great European War, the cost of oils or
tars for bituminous carpets varied according to the grade of the
material from 4 to 16 cents per gallon, but usually from 6 to 8 cents.
In some states the total cost of a bituminous carpet has been as low
as 3 cents per square yard, while in others it has been as high as 15
or 20 cents.
Below are the details of the cost of applying a light bituminous
304 BITUMINOUS SURFACES FOR ROADS [CHAP. IX
carpet to a gravel road and to a water-bound macadam road by the
Illinois Highway Department in 1915.
600. Gravel Road. The cost at Cairo, Illinois, of applying 0.5
gallon of cold oil (Aztec liquid asphalt) containing 60 to 65 per cent
of asphalt, and 0.006 ton of torpedo gravel, stone chips, and sand
per square yard, to a gravel road Ij miles long, the average haul
being 0.5 mile and the rate of pay for laborers being 15 cents per
hour and for teams 40 cents, was as follows: *
COST
ITEMS. Cts. per
Sq. Yd.
Oil, 8184 gallons at 4.7 cents f .o.b. siding 2 . 34
Torpedo gravel at 59 cents per cubic yard, f.o.b. siding 0.24
Heating and applying oil, demurrage, etc 0.32
Hauling gravel 0.5 mile and spreading 0.31
Sweeping and cleaning old road 0 . 034
Freight on equipment 0 . 54
Superintendence, engineering, and inspection 0.21
Total, exclusive of depreciation, over-head expense, and profits ...... 3 . 99
601. Macadam Road. The cost of applying a bituminous carpet
consisting of 0.33 gallon of Trinidad B asphalt and 0.016 ton of
torpedo gravel per square yard, the average haul being 1£ miles and
the pay of laborers being 25 cents per hour and of teams 50 cents, was
as follows:!
COST
ITEMS. Cts. per
Sq. Yd.
Field superintendence 0 . 35
Bituminous material @ 7.7 cts. f.o.b. siding 2 . 39
Torpedo sand @, 1.825 per ton f.o.b. siding and stone chips @ $.140 2.67
Hauling gravel and chips, 1£ miles 0 . 84
Spreading gravel and chips 0 . 56
Sweeping and cleaning old road 0 . 05
Heating and applying material, demurrage, etc 1 . 04
Freight and equipment 0.15
Repairs to equipment 0 . 07
Incidental expense 0 . 22
Patching holes and repairing culverts 0 . 27
Total, exclusive of engineering, inspection and rent of equipment 8.61
602. State Reports. The annual reports of many of the State
Highway Departments give detailed data of the cost of bituminous
* Illinois Highways, December, 1915, p. 168; or Engineering Record, Vol. 73 (1916), p. 806,
j- Illinois Highways, December, 1915, p. 171,
ART. 2] BITUMINOUS CARPET 305
road surfaces. For example, the 1915 report of the New York
Commissioner of Highways, pages 177-94, shows the kind and quan-
tity of bituminous material used, its cost, the amount applied per
square yard, the area covered, the cost of labor, and the total cost
for each road in each county treated in that year — a total of 1800
miles.
CHAPTER X
BITUMINOUS MACADAM AND BITUMINOUS CONCRETE
ROADS
604. A bituminous-macadam road consists of two or more courses
of broken stone, the wearing course of which is bound with bituminous
cement applied on the surface. Formerly this form of construction
was usually called bituminous macadam by the penetration method,
but sometimes simply penetration macadam.
A bituminous-concrete road consists of one or more courses of
broken stone, the wearing course of which is bound with bitumi-
nous cement mixed with the stone before it is placed. Formerly this
type of construction was usually called bituminous macadam by the
mixing method, but sometimes simply mixed macadam.
Since in both of these types of construction the binder may be
either tar or asphalt, it would be appropriate and more definite to
use the specific terms asphalt macadam or concrete, and tar mac-
adam or concrete; and further it would not be inappropriate to use
the terms native-asphalt and residuum-asphalt macadam or concrete ;
and likewise coke-oven tar macadam or concrete, and water-gas tar
macadam or «oncrete. For a distinction between bituminous con-
crete and asphalt concrete, see § 891.
The present use of the terms bituminous macadam and bitumi-
nous concrete is based upon the analogy between the method of
construction of these roads and that of macadam and concrete,
respectively.
605. The two methods of road construction considered in this
chapter have come rapidly into use since about 1910.
ART. 1. BITUMINOUS MACADAM ROADS
606. The drainage and the preparation of the subgrade is sub-
stantially the same as for the forms of roads already discussed.
607. FOUNDATION. The foundation is often an old gravel or
macadam road, usually the latter; and sometimes, though less
306
ART. l] BITUMINOUS MACADAM ROADS 307
frequently, a new gravel or macadam road is constructed for the
purpose; and occasionally a portland-cement concrete foundation
is used.
If the foundation is an old water-bound macadam road, the sur-
face should be swept with a machine broom. All fine material
that is caked upon the surface and is not removed with the machine
broom should be loosened by hand, and then the surface should be
swept perfectly clean with a hand broom. The coarse stone should
be exposed, so the bituminous binder may adhere well.
If the foundation is a new macadam road, it should be con-
structed as described for water-bound macadam roads (Chapter VI).
Under the same conditions, the total thickness of the road, including
the bituminous wearing course, may be a little less than that of a
water-bound macadam road, as the bituminous top will not wear as
rapidly as the water-bound, since the former is usually built where
motor-driven traffic predominates and rubber tires have but little
effect upon the bituminous top. The first and second courses of stone
are laid, rolled, filled, sprinkled, and again rolled as described for
water-bound macadam, except that the second course is not flushed,
i. e., is not filled so much as to form a film over the surface. It is
essential that there shall be no voids in this course to absorb the
bituminous binder. The stone should be clean and dry when the
binder is applied.
608. WIDTH. For a discussion of matters relating to the width
and position of the improved wheelway, see § 95-97.
609. MAXIMUM GRADE. For a discussion of maximum grades,
see § 79-85; and for recommended values for the maximum grade, see
Table 15, page 57.
610. THE CROWN. The crown for bituminous macadam should
be less than for water-bound, f of an inch to the foot being enough.
See Table 16, page 66.
611. WEARING COAT. The wearing coat consists of a layer of
1^-inch to 2^-inch stone and two applications of asphaltic cement
or refined tar, each of which is followed by a layer of f -inch to ^-inch
screenings. If the stone is soft, the size of the screenings may be a
little greater than stated above; and if hard, a little less.
The layer of stone should be evenly spread to such a depth that
after rolling it will have a thickness of 2£ inches, and should then be
rolled dry until the fragments have become firmly keyed together so
that the stones will not move ahead of the roller or so that they
can not be moved by the thrust of a man's heel. If the foundation
308 BITUMINOUS MACADAM AND CONCRETE ROADS [CHAP. X
is new macadam (§ 607), this course should be spread and rolled
while the top course of the foundation is still moist and soft; but
should not be rolled so much as to force the slush of the foundation
more than half way up into the voids of the course being laid. Fig.
98 shows the rolling of the layer of stone of the wearing coat in
progress. It is important that this course be not rolled so much as
to prevent the penetration of the bituminous binder; and .on the
other hand, the course should not be so open as to require too much
FIG. 98. ROLLING OF LATEK OF STONE FOR WEARING COAT.
binder to fill the voids. If the stone is hard, it may be necessary
after the rolling is partly completed, to fill the voids by applying a
coat of gravel, see Fig. 99.
612. BITUMINOUS BINDER. The bituminous binder may be
either asphaltic cement or refined tar. The asphaltic cement should
meet some one of the specifications in § 537-38; and the tar should
meet one of two specifications in § 572-73. Owing to its greater
cementing value asphalt is better for a road to be subjected to heavy
horse-drawn loads than tar.
The asphaltic cement when applied should have a temperature of
135 to 177° C. (275 to 350° F.); and the tar of 93 to 121° C. (200 to
250° F.).
The amount of bituminous cement for the first application should
be just sufficient to penetrate the third course and fill all of the voids;
and usually this will require about 1 gallon per square yard per inch
ABT. 1] BITUMINOUS MACADAM HO ADS 309
of thickness of the upper course; and for the second application from
-J to | gallons per square yard. An excess of binder is not only
expensive, but causes the wearing course to creep and form waves.
613. Applying the Binder. For small jobs or where it is difficult
to operate tank wagons, the bituminous material is shipped in barrels
or drums, heated in open kettles, and applied by hand from pouring
cans. For the best results, the binder should be hauled in tank
wagons provided with a heater, one or more thermometers, and a
FIG. 99. FILLING LAYER OF STONE WITH GRAVEL.
pump for distributing the binder under pressure in the form of a
spray. The tank wagon must have wheels with tires so wide as
not to make an appreciable rut in the surface of the road. The
spreading must be done so as to secure absolute uniformity. The
area covered with a barrel- or a wagon-load should be measured, and
the rate of application computed. After the binder has been applied,
the surface should be uniformly black, but the spaces between the
stones should show. The temperature of the stone or the air should
not be less than 50° F. (10° C.) during the application.
After the first coat of binder is applied, a layer of £-inch to f-inch
stone screenings, not over f of an inch thick, is spread over the sur-
face; and then the road is rolled. If there is an excess of uncemented
screenings after the rolling, they should be removed with a push
broom, for an excess will cause the seal coat (the last coat of binder)
through lack of adhesion to the first coat, to peel off.
310 BITUMINOUS MACADAM AND CONCRETE ROADS [CHAP. X
After applying and rolling the screenings, a seal coat of -J to f
gallons of binder per square yard is spread by the same means as for
the first coat; and the utmost care should be taken to put on the
material uniformly.
Finally, the road receives a coat of screenings of hard screenings
or pea gravel, not over f of an inch in thickness; and is again rolled.
The rolling is continued until a smooth surface is produced. The
road is now ready for travel.
The eight views in Fig. 100 show the various steps in construct-
ing the wearing cost of a bituminous macadam road.
614. ANOTHER TYPE OF BITUMINOUS MACADAM. The wear-
ing course of the preceding type of bituminous macadam road could
be appropriately described as a course of stone grouted with bitumi-
nous cement. The Massachusetts Highway Commission and others
sometimes use a mixture of tar and sand for the grout instead of a
neat bituminous cement. It is claimed that the sand materially
stiffens and strengthens the road crust and decreases the oxidation
of the tar.
Fig. 101, page 312, shows two views of the construction of a tar-
sand macadam road built by the Massachusetts Highway Com-
mission.
615. CHARACTERISTICS OF BITUMINOUS MACADAM. This form
of construction is adapted to roads having a moderate amount of
travel with not many heavy horse-drawn loads on narrow tires. It
has a pleasing appearance, and is well adapted to both horse-drawn
and motor-driven traffic. The surface seems to deteriorate more
rapidly where considerable quantities of mud are tracked on it. In
warm weather, particularly if an excess of binder was used, there i§ a
tendency for the surface to creep and develop undulations.
There have been a considerable number of failures of bituminous
macadam roads, apparently because of the neglect to observe the
proper methods of construction, — perhaps through lack of knowl-
edge, since the type is comparatively new.
616. COST, This form of construction usually costs about 15
to 20 cents per square yard more than good water-bound macadam
(§ 388-93). There has not yet been sufficient experience to deter-
mine the cost of maintenance or the ultimate life of the bituminous
layer. ' •, '•• -j
617. MAINTENANCE. Bituminous macadam is peculiarly resist-
ant so long as it is intact; but when once broken, due to defective
materials or workmanship, or to wear of travel, or to the opening of a
ART. 1] BITUMINOUS MACADAM ROADS
311
1. Spraying Tar on Layer of Stone. 2. Spreading f-inch Stone on First Coat of Tar
3. Sweeping Screenings to Secure Uniform
Distribution.
4. Rolling after Spreading Screenings.
5. Spraying Seal .Coat of, Tar. 6. Putting Screenings on Seal Coat.
7. Final Rolling of Road. 8. Finished Road.
FIG. 100.— CONSTRUCTING WEARING COAT OP TAR-MACADAM ROAD.
312 BITUMINOUS MACADAM AND CONCRETE ROADS [CHAP. X
trench, it disintegrates rapidly. Under these circumstances the
defective portion should be cut out, the sides and bottom of the hole
coated with bituminous cement, and the hole filled with stone and
cement well mixed and solidly tamped into place. It is well to leave
1. Roller, Tank Wagon and Mixer. ' 2. Pouring Tarvia-Sand Grout.
FIG. 101. — CONSTRUCTING TAR-SAND MACADAM ROAD.
the patch a little high, so it will not be low when finally consolidated
by travel.
If the surface of the road becomes dry and lifeless, a new seal
coat should be applied at once. This will usually occur with a tar
binder in two or three years, depending upon the amount and char-
acter of the travel; and with an asphalt binder, in three or four
years. Applying a new seal coat is peculiarly a case in which a
stitch in time saves nine.
ART. 2. BITUMINOUS CONCRETE ROADS
619. In the form of construction considered in this article, the
wearing course consists of a layer of crushed stone bound together
by either tar or asphalt. The stone and the binder are usually
heated before being mixed, and are laid while hot.
The distinction between bituminous concrete as considered in
this article and asphaltic concrete as considered in connection with
asphalt pavements (Art. 2, Chapter XVI), is that in the latter the
binder is always asphalt, and the aggregate is accurately graded so
as to secure a minimum of binder and also a maximum stability.
One form of construction gradually shades into the other, and it is
impossible to draw a definite line between them. In the accurate
gradation of the aggregates asphalt concrete is closely related to sheet
pavements, and hence the two are considered together in Chapter
XVI.
ART. 2] BITUMINOUS CONCRETE ROADS 313
620. All that was said in § 114-28 concerning drainage, sub-
grade, and cro.wn applies also to bituminous concrete roads.
621. THE AGGREGATE. Gravel may be used, but only for light
traffic. Broken stone is generally used because of the better bond
thus secured. The broken stone should be hard and of compact
texture and uniform grain, be free from adhering fine material, and
preferably should have rough surfaces and sharp angles.
Bituminous concrete is sometimes laid with crusher-run stone;
but since the stone is not so uniform, the result is not so good as
where graded stone is used.
For the best results the aggregate should be carefully graded.
" Practice has demonstrated that a mineral aggregate which will
comply with the following sieve analysis, using screens having cir-
cular openings, will produce satisfactory results: All the material
shall pass a 1^-inch screen; not more than 10 per cent nor less than
1 per cent shall be retained on a 1-inch screen; and not more than
10 per cent nor less than 3 per cent shall pass a ^-inch screen."*
For a more full discussion of the grading of the mineral aggregate
for bituminous concrete, see Art. 2, Chapter XVI — Asphalt Pave-
ments.
622. THE BINDER. The bituminous cement used in the mixing
may be either tar or asphalt cement; but the seal coat should always
be asphalt cement (§ 541). The tar should conform to the specifi-
cations in § 574-75 ; and the asphalt cement should meet the require-
ments stated in § 539-40.
The quantity of bituminous cement to be used in the mix will
depend on the gradation of the broken stone and the character of
the bituminous cement, the climatic conditions, etc. For an aggre-
gate graded as in the preceding section, the mixture should contain
between 5 and 8 per cent by weight of bitumen.
623. MIXING THE CONCRETE. There are two types of mixing,
cold and hot.
624. Cold Mixing. This method is employed only with a special
grade of tar, and the concrete is usually mixed by hand. This
method is not very common, and great care is necessary in using it.
The stone must be perfectly dry, the weather must not be too cool,
and there should be a considerable period of warm weather imme-
diately following the completion of the road. This form of con-
struction is suitable for light traffic; but not for heavy traffic, either
horse-drawn or motor-driven.
* Report of Committee of Amer. Soc. of Civil Engineers, Proc. Vol. 42 (1916), p. 1626.
314 BITUMINOUS MACADAM AND CONCRETE ROADS [CHAP. X
625. Hot Mixing. Usually the stone and binder are heated
separately and then mixed in a machine mixer.
Bituminous concrete can be mixed in an ordinary hydraulic-
cement concrete mixer; but mixers specially designed for mixing it
are much more satisfactory. There are various such mixers on the
market. Some mixers are heated internally by an open flame, and
others externally by a flame or steam. On account of the danger of
burning the cement, the flame in the mixing chamber should not
be used, except perhaps on small jobs and in repair work; and even
then the flame should never be allowed in the mixer after the
bituminous material has been added.
The heating device should be easily regulated, so that there
will be no danger of burning the binder. The most common cause
of failure in bituminous concrete roads is the burning of the binder.
A burned batch will not usually show when laid, but after a few
months will reveal itself.
626. The bituminous cement, if asphalt, is usually heated to
about 135° to 177° C. (275° to 350° F.); and if tar, from 93° to
135° C. (200° to 275° F.). The stone for the asphalt mixture is
heated to about 150° C. (302° F.), and that for the tar mixture to
about 100° C. (212° F.). Any asphalt or tar that is heated more
than stated above should not be used. No tar should be heated in a
kettle containing any asphalt, and likewise no asphalt should be
heated in a kettle containing any tar; and any mixtures resulting
from this cause should be rejected.
When discharged, mixtures of asphalt cement and broken stone
should have a temperature of not less than 93° nor more than 149° C.
(200-300° F.). When discharged, mixtures of refined tar and broken
stone should have a temperature of not more than 121° C. nor less
than 66° C. (250-150° F.).
The mixer should be designed and operated so as to produce a
thoroughly coated and uniform mixture without any segregation
of the stone and the cement.
Bituminous concrete should not be mixed or laid when the tem-
perature of the air in the shade is less than 50° F. (10° C.).
627. LAYING THE CONCRETE. If not mixed upon the street,
the concrete should be hauled in canvas-covered wagons or trucks;
and should be delivered at a temperature of at least 66° C. (150° F.).
The hot mixture should be dumped upon platforms, shoveled into
place with hot shovels, immediately raked to a uniform thickness,
and then thoroughly compacted by rolling. The roller should be of
ART. 2] BITUMINOUS CONCRETE ROADS 315
the self-propelled tandem type weighing from 10 to 12 tons, and
giving a compression under the rear roll of 250 to 350 lb. per
linear inch. The rolling should continue until all roller marks dis-
appear.
After rolling, the wearing course should have a uniform thickness.
The experience with this type of construction is somewhat limited,
but apparently a thickness of 1J or 2 inches is sufficient to stand the
heaviest mixed traffic; and apparently a greater . thickness is unwise,
since it has a tendency to creep and form bunches. The surface
should be free from depressions and irregularities exceeding f of an
inch under a 4-foot straight edge laid longitudinally.
628. SEAL COAT. As soon as possible after the completion of
the rolling, and while the surface is dry and clean, a seal coat of hot
asphalt cement should be applied with a hand-drawn distributor,
and be spread with a squeegee. The asphalt cement should meet
the requirements in § 541; and should be applied at a temperature
of not less than 135° nor more than 177° C. (275-350° F.) at a rate
of J to 1 gallon per square yard.
As soon as possible after the application of the seal coat, and
not more than 20 minutes thereafter, a thin uniform layer of stone
chips (f- to ^-inch or £- to ^-inch) should be spread and thoroughly
rolled with the tandem roller described in § 378.
Fig. 102, page 316, shows six views of the construction of a bitu-
minous concrete road built in Pennsylvania. The maximum size of
the aggregate in this case was that passing a |-inch mesh; and hence
the concrete could be leveled off by striking with a template — see
view 3. Views 5 and 6 are included partly to fill out the plate and
partly to show the two forms of distributors.
629. MAINTENANCE. See § 617, page 310.
630. COST. Under similar conditions, the cost of this type
of road is usually about 20 to 25 cents more thaji that of water-
bound macadam (§ 388-93).
631. COMPARISON OF BITUMINOUS MACADAM AND BITUMINOUS
CONCRETE. Bituminous macadam is the more common, there being
seven or eight times as much in use as bituminous concrete.
The advantages of bituminous macadam are: 1. It is compara-
tively cheap. 2. It requires no expensive machinery. 3. It is
easily and quickly laid. 4. The cost for labor is comparatively
low. The disadvantages are: 1. Considerable care is required to
prepare properly the upper course of the foundation to receive the
bituminous macadam. 2. There is difficulty in securing a uniform
316
BITUMINOUS MACADAM AND CONCRETE ROADS [CHAP. X
distribution of the binder through the wearing coat. 3. It is prac-
tically necessary to use an excess binder, and hence the surface
may creep under traffic. 4. The quality of the binder must be sac-
1. Loading End of Mixers.
2. Discharging End of Mixers.
3. Striking Tar-Concrete.
4. Rolling Tar-Concrete.
5. Applying Seal Coat. 6. Applying Seal Coat.
FIG. 102.— CONSTRUCTION OF TAB-CONCRETE ROAD
rificed to the requirements of the method of its application. 5.
The method is not applicable in cold or damp weather.
632. The advantages of bituminous concrete are: 1. It permits a
perfectly uniform distribution of the binder. 2. It permits the use
ART. 2] BITUMINOUS CONCRETE ROADS 317
of a suitable quality and quantity of binder. 3. It can be laid in
comparatively cold weather. The disadvantages are: 1. The cost of
the labor is comparatively high. 2. It is slow in application. 3. A
considerable quantity of expensive machinery is required. 4. The
total cost is somewhat greater.
PART II
STREET PAVEMENTS
633. Good pavements are necessary to the highest development
of the commercial, sanitary and esthetic life of a city. The large
proportion of people now dwelling in cities makes the subject of pave-
ments an important one; and the fact that the urban population is
increasing much more rapidly than the rural, and also the fact that
the public is awakening to the necessity of ameliorating the condi-
tion of life in the city, will make pavements of increasing concern
in the future.
CHAPTER XI
PAVEMENT ECONOMICS AND PAVEMENT
ADMINISTRATION
ART. 1. PAVEMENT ECONOMICS
634. BENEFITS OF PAVEMENTS. The effect of pavements upon
city life is so important and so far reaching that no enumeration
is likely to include all of the benefits; but nevertheless it will be
of advantage, particularly in discussing the proper distribution of
their cost, to enumerate some of the more important of the benefits
resulting from the construction of pavements. Briefly the principal
advantages are:
1. Good pavements lessen the tractive power required, and
decrease the cost of transportation. See § 4-9 for a discussion of the
cost of transportation.
2. Good pavements increase fire protection by facilitating the
transportation of the fire apparatus.
3. Pavements establish a permanent grade, which is an important
matter when other improvements are to be made,
318
ART. 1] PAVEMENT ECONOMICS
4. Pavements improve the appearance of the street by giving a
uniform surface instead of the irregular one of an unpaved street.
5. Pavements increase cleanliness, since the pavement is less
dusty in a dry time and less muddy in a wet time than an unpaved
street, and since they are easily cleaned.
6. Pavements increase healthfulness by removing holes rilled
with mud and filth.
7. Pavements permit pleasure driving at all seasons, and facili-
tate social intercourse.
8. Pavements allow the use of bicycles, which furnish to many
cheap transportation and healthful recreation.
635. In discussions of this subject it is customary to include the
enhanced value of the adjacent property as one of the advantages of
a pavement; but the increase in the value of the property is simply a
measure of the benefits enumerated above, and hence should not
again be included.
The first three benefits above may be regarded as financial
advantages and the last four as sanitary and esthetic. It is im-
possible to compute even approximately the financial, much less
the sanitary and esthetic, value of good pavements; but it is safe
to say that they are an absolute necessity to both the business
and residency of the larger cities and also for business districts of
the smaller cities, and that on residence streets of small cities good
pavements add greatly to the health, comfort and pleasure of life.
636. INVESTMENT IN PAVEMENTS. The table on page 320
was compiled from statistics published by the U. S. Census
Bureau, and shows the total areas and cost of the different kinds of
pavements in 1909 in the 158 cities having a population of over
30,000.* The areas are probably reasonably accurate, but the unit
prices are only approximate owing to failures to state what was
included in the cost, i. e., whether or not it included grading, foun-
dation, curb, gutter, etc.
From this table it appears that the pavements in these cities
have cost $695,936,294; and as the total population is 25,603,949,
the pavements have cost $27.10 per capita. The area of pavements
per capita varies greatly in the different cities, being practically
independent of the size and location of the city; but the average
seems to agree fairly well with the area of pavements in a number of
very much smaller cities investigated by the author. Therefore, it
* General statistics of cities for 1909, Bureau of Census, Washington, D. C., 1913.
320 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
will be assumed that the above average is representative of the entire
country. According to the U. S. Census Report there were 41,717,-
853 people dwelling in cities of 8000 population or over in 1916.
Therefore the investment in pavement in these cities amounts to
$1,046,320,000. Measured by the money invested, street pave-
ments, except steam railroads, are probably the most important of
any single class of engineering construction.
INVESTMENT IN PAVEMENTS
Asphalt— sheet 83,227,011 sq. yd. at $2 . 75 =$238,874,281
block 5,418,666 " 2.75= 14,901,331
Bithulitic . . . : 4,000,872 " 2.25= 9,001,962
Brick 53,870,578 " 2 . 25 = 120,208,805
Cobblestone 9,083,397 " 0.80= 7,166,718
Concrete, portland-cement 445,478 " 1 . 20 = 434,576
Gravel— water-bound 43,634,491 " 0 . 20 = 8,726,898
bituminous bound 4,674,605 " 0.40= 1,869,842
Macadam— water-bound 107,998,789 " 0 . 75 = 80,998,082
tar-bound 3,008,919 " 1 . 00 = 3,008,919
portland-cement grouted. 303,069 " 1.00= 303,069
Stone block 51,414,901 " 3 . 50 = 179,950,183
Wood block— creosoted 2,936,047 " 3 . 00 = 8,808,141
untreated 10,724,370 " 2.00= 21,448,740
Other kinds 4,367,708 " .20= 873,541
Total 385,409,889 =$675,936,294
637. Data for the year 1899 similar to the above were presented
in former editions of this treatise; and apparently from 1899 to 1909
the area of pavements increased 38 per cent, and the cost 71 per cent.
The increase in area is due to the increase of pavements in each city
and to the increase in the number of cities from 129 to 158. The
increased cost is due chiefly to the increase in the area of pavements,
but partly to the increase in the quality of the pavements and partly
to the increased cost of labor and materials. The quality of pave-
ments has increased greatly in the last few years. For example,
formerly stone-block pavement consisted of roughly dressed blocks
laid on a sand or gravel subgrade with wide joints filled with sand or
pebbles; while now most stone-block pavements consist of accurately
dressed blocks laid on a concrete foundation with close joints filled
with bituminous or hydraulic cement. A corresponding improve-
ment has taken place in most other forms of pavements. However,
the cost of pavements has not usually increased proportionally,
owing to improvements in methods of doing the work. Some of
ART. 2] PAVEMENT ADMINISTRATION 321
these improvements are: The cutting of granite blocks largely by
machinery instead of wholly by hand; the use of the 4-wheeled
scraper and the steam shovel in preparing the subgrade; improve-
ments in the methods of handling and delivering the sand and gravel
for the foundation; the mixing of the concrete for foundations by
machinery instead of by hand; improvements in the methods of
handling the pavement materials, etc.
638. According to Bulletin No. 100 of the 1890 census, the
average annual expenditure for pavement construction and repairs-
in the cities of the United States having a population of 10,000 or
over, was $1.72 per capita, being $1.54 in the cities having more than
100,000 population and $2.04 in cities from 10,000 to 100,000. No
later data seem to have been collected. If the same rate of expense
obtained in 1909, the total annual expenditure for pavements in
cities of 8,000 or more population was $85,104,420. In some smaller
cities the average normal expenditure for pavements is four to five
times the average just stated.
The first cost of pavement and also the annual cost is of such
magnitude that merely as a financial question, street pavements
deserve careful attention and systematic study,
ART. 2. PAVEMENT ADMINISTRATION
640. IMPORTANCE OF PROBLEM. The importance of pave-
ments as an element in municipal finance seems not to be fully appre-
ciated, and this subject has not received from municipal engineers
and city officials the attention and study its importance merits.
Whether measured by their influence upon the commercial, sanitary
or esthetic life of the city, or by the amount of money invested in
them, street pavements belong in the first rank of importance in
municipal affairs.
641. Present Conditions. The following quotation * shows the
surprising attitude of the public and municipal officials toward this
important subject.
" One would suppose that a subject of the magnitude and impor-
tance of pavements would be a matter of the most critical investiga-
tion and scientific research; but it is safe to say that in no other
branch of civil engineering is there expended so large an amount of
* John W. Alvord, C.E., in "The Street Paving Problem of Chicago." A Report to the
Commercial Club of Chicago, 1904.
322 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
money in so unsystematic a manner, and generally with such unsat-
isfactory results.
" Pavements are primarily designed to accommodate travel;
but scarcely any one in this country thinks of investigating the
travel of a city systematically and thoroughly before proceeding
to lay pavements.
"Pavements are financial investments; yet few city officials
before proceeding to raise the necessary capital, undertake to com-
pile data from which to compute the cost of maintenance and the
length of life or depreciation.
" Street pavements are by far the most expensive single improve-
ment that the municipality undertakes; yet in hardly any of the
cities of this country are there suitable laws, proper organization, or
sufficient public spirit adequately to care for the investment after it
is once made.
" The improvement of streets is a legitimate method of adorning
our cities; yet no one thinks of consulting recognized authorities on
good taste in such matters, except in boulevards and parks.
" Pavements have been a necessity of civilization since Rome was
mistress of the world; but cities are still experimenting with the sub-
ject without general and well-defined policies. Community after
community repeats the fundamental experiments, and copies without
reflection or study what it sees being done elsewhere.
" The managers of the railways of the country know to a cent the
cost and the comparative utility of every bolt and scrap of iron that
enters into their road ; and they can tell to several places of decimals
of a cent the cost of moving a ton of freight or the cost of transporting
a passenger a single mile. But the city officials in this country that
can make more than a rough guess of these matters in connection
with the enormously greater travel of cities, can be counted on the
fingers of one's two hands."
642. Causes of Present Conditions. The present anomalous con-
ditions are due to the following causes :*
1. " The administration of American cities change every few
years; and seldom do officers of the municipality have the ambition
or opportunity to become thorough masters of the broader require-
ments of the problems with which they are confronted.
"2. In a republican state the tax-payer is expected to have
a deciding vote in the expenditure of the public moneys, especially
* John W. Alvord, C. E., The Street Paving Problem of Chicago. A Report to the Com-
mercial Club of Chicago. 1904,
ART. 2] PAVEMENT ADMINISTRATION 323
those raised by local taxation. As a result, advancement proceeds
no faster than the education of the whole mass of tax-payers.
" 3. Different kinds of street pavement rise or fall in public
estimation with an undue amount of popular fluctuation. This is
because there is no pavement that is perfect for all classes of con-
ditions; and the pavement that comes the nearest to meeting one
set of requirements may be the furthest away from another set of
requirements. The public having selected a pavement, perhaps ill
adapted to a particular environment, and finding it lacking in im-
portant particulars, is apt to thoughtlessly, and perhaps pettishly,
condemn it in toto when such a sweeping verdict is not warranted.
Even city officials in charge of such matters do not always inves-
tigate carefully enough the causes which make for failure, and
allow personal impressions to take the place of carefully investigated
facts.
" 4. The engineers of this country, up to within a few years,
have not generally interested themselves in the subject of street
paving, because they have not been given very good opportunity
to properly study the question. Finding the tax-payer a self-
appointed and sometimes exacting authority, they have been obliged
more or less to abandon the field, so far as its broader questions are
concerned, and accept his dictum. The specialist on street paving
has been long recognized abroad as an important factor in municipal
progress; and of recent years, he is beginning to appear in the United
States. Seldom, however, does an average city call for his services;
and almost never do municipalities appoint commissions of such
men with ample funds to make an exhaustive study of the street-
paving problem in its broader requirements.
"5. The natural distrust for municipal authorities is the normal
condition of mind of the American tax-payer. This is the inevitable
result of a system that generally produces mediocre results. And
nowhere are mediocre results more apparent and unhappy than in
.the work of street improvements. As a result, the average city
administration, however honest its intentions, feels that it is without
moral support. It does not initiate broad policies, or spend public
moneys for investigation and research; but it gropes its way in the
darkness of chance, and plays its cards like an opportunist, post-
pones all possible trouble to its successors, and blames all deficiencies
onto its predecessors, while ever pleading for revenue for new experi-
ments. A distrustful public generally refuses to cooperate in legis-
lation tending to increase taxation for the future, until absolutely
324 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
assured of wisdom and economy in the present. The public is at
times a harsh judge, and does, not easily overlook glaring imperfec-
tions or fully analyze deficient results.
" 6. The usual method in this country of assessing the cost of
improvements to the abutting property has tended to give undue
prominence to the property owner, as the representative of the public
in deciding upon and paying for street paving. As a matter of fact,
the abutting property owner is an agent. Whenever possible he passes
on to the rest of the community, in the form of increased valuation
and rental of his property, the cost he is assessed. Often he recoups
himself for his outlay many times over; and yet ordinarily he regards
himself as a public benefactor, and as such claims the right to outline
street policies which usually lead to his own pecuniary advantage
rather than to that of the public.
" 7. The method of assessing the first cost of pavements on the
property owner, and then maintaining the pavement out of the public
fund, has resulted in the majority of cases in there being little, if
any, maintenance. No municipality has ever had an adequate
1 general fund.' The general fund is the common prey of all the
more novel municipal projects and ambitions; and the common-
place uses to which it might be put will always be unduly curtailed.
" 8. There is no general public appreciation of the vital necessity
of maintaining pavements after they are once laid; and as a conse-
quence there has been no cooperation on the part of the public in
framing legislation and raising revenues for this purpose. In this
country, the practice has been generally to build pavements at high
first cost, and allow them to wear out with a minimum of repair.
" 9. Finally, in this country the street-paving problem is every-
where regarded as a local or neighborhood problem. The general
public has not yet come to regard it as a national problem, or even
entirely a municipal problem; and hence the lack of appreciation
of the broader municipal requirements, and the insufficiency of study
of its fundamental principles. To this cause may be assigned the
chaotic condition of the art and incoherence of the data now existing
and the absence of any general principles which should govern the
subject as a whole."
643. Remedy of Present Conditions. The present unfortunate
conditions could be largely remedied by making the investigations
and by carrying out the policies mentioned below.
1. The first requirement for a comprehensive plan for street
pavements should be a study of the traffic conditions of the entire
ART. 2] PAVEMENT ADMINISTRATION 325
city, which should include a census of the origin, amount, character,
direction, and density (the amount per foot of width of street or
pavement) of the travel on representative streets in all parts of the
city.* Such a census should be repeated at regular intervals so that
the growth and tendency of the traffic may be known with as much
certainty as vital statistics or the census of population, since only by
so doing can a basis be found for sound present policies or for fore-
casting future necessities. It is not possible to formulate any
scientific and adequate plan for street pavements without knowing
the present and prospective use to be made of the pavements. Un-
fortunately, but few such census data have been obtained for any
American city — see § 34.
2. The streets should be classified as to the amount and char-
acter of the travel, the width of pavement, the depth of foundation,
the kind of wearing surface, the amount necessary for maintenance,
etc. The highway departments of the several states have recently
quite generally classified the rural roads according to the amount of
travel, and have specified certain types of road surfaces for the dif-
ferent classes of roads — for example, see Table 26, page 177.
3. Careful records should be kept of the cost of repairs on differ-
ent kinds of pavements under different traffic conditions; and an
inventory should be made at stated intervals to determine whether
or not a particular pavement is gradually deteriorating, which would
serve as a rough check upon the sufficiency of the annual repairs.
4. Careful records should be kept of the cost of cleaning different
kinds of pavements under different traffic conditions. The cost of
cleaning is a part of the cost of maintenance; and unless the annual
cost of maintenance is known, it is impossible to compare accurately
the total cost of different kinds of pavements.
5. Observations should be made to determine the tractive force
required to draw loads over different types of pavements. There
are tables of tractive resistance for different road and pavements
surfaces, — for example, see Table 7, page 20; — but such data are
quite general and do not represent with sufficient accuracy the pave-
ments for a particular city which are built under certain specifications
and of materials more or less local. The railways of this country
spend large sums to determine the tractive resistance on tracks of
different types and under different conditions; and the results of
such observations are of great importance in maintaining an economic
* For a discussion of travel census for rural roads, see § 29-33.
326 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
relation between the cost of the up-keep of the track and the cost of
drawing trains over the track. Corresponding data for street pave-
ments would be helpful in determining when the condition of any
pavement renders it unfit for further profitable use.
6. The public should be educated as to the financial, sanitary, and
esthetic importance of street pavements; and also as to the fact that
the construction and maintenance of pavements should be left entirely
to those who have made a careful study of such work.
7. The laws should vest the power to determine when a street
should be paved and to select the kind of pavement in either an
elective board of long tenure so that its members may have an oppor-
tunity to acquire knowledge of paving materials and policies, or in a
commission of real experts appointed for that purpose.
644. APPORTIONMENT OF THE COST. There is much discussion
as to who in equity should bear the cost of the pavement. There
are three distinct views.
1. A few claim that as they own neither a horse nor a vehicle and
do not use the pavement, they should not be required to pay for it.
Although a resident may not travel upon the pavement, it is used
by those who serve him; and a pavement confers other benefits
besides those relating to transportation. It is entirely impracticable
to distribute the expense according to the use made of the pavement.
2. Others claim that the pavement is for the benefit of the general
public of the city at large, and hence the abutting property should
pay no more than that in other parts of the city. This claim ignores
the fact that the abutting property secures a distinct benefit for which
it should be required to pay. Laying at least part of the cost upon
the abutting property tends to discourage a demand for lavish
expenditures for unnecessary improvements, that possibly might be
insisted upon if the city contributed the entire cost.
3. Many hold that the benefits accrue only to the abutting prop-
erty, and that therefore the owner of the abutting property should
bear the entire cost. This claim disregards the fact that the pave-
ment is for the use of the general public, and benefits all the people
and all those having business interests in the city. An improvement
in any part of the city is an indirect benefit to the city as a whole.
In excuse of this method of payment, it is sometimes claimed that,
although the pavement confers a general benefit, the inequality
will be compensated when all the streets are paved. The answer
is that all the streets may never be paved, and besides traffic natu-
rally concentrates on certain lines and nearly ignores certain others,
ART. 2] PAVEMENT ADMINISTRATION 327
and therefore some pavements will require much more care and
expense than others. Further, there should be no objection to letting
every property holder pay a part of his ultimate share as the work
progresses, instead of paying it in a lump sum when the street in
front of his own property is paved. The second or third view or a
combination of them usually obtains (§ 645). Table 36, page 328,
shows the method of apportioning the expense in fifty American
cities.*
The practice is slightly different for the grading, the original
paving, and the re-paving. All of the cost of grading in 54 per cent
of these cities is paid by the abutting property, in 32 per cent all
by the city, and in 14 per cent part by each in varying proportions.
The cost of the original 'paving in 62 per cent of the cities is charged
entirely to the private owner, in 22 per cent entirely to the city, and
in 16 per cent it is divided between the two. The cost of re-paving
in 42 per cent of the cities is paid wholly by the property, in 40
per cent wholly from the general tax, and in 18 per cent it is divided
between the two. In some cities a street in an addition or sub-
division is not accepted by the municipal authorities until it has been
graded, and hence it is done at the expense of the abutting property;
but on the other hand, some cities are willing to bear a part of the
cost of the street improvement, and therefore pay for the grading.
Only one quarter of the above cities pay the major part of the cost
of the original paving, while 40 per cent pay the major part of the
cost of re-paving. It is the custom, where there is a car track on the
street, to require the railroad to pave an 8-foot strip for each track,
the remainder being divided between the abutting property and the
city at large in the same proportion as on the streets where there is
no track. In some cities intersecting streets are regarded as municipal
property, and the cost of paving the intersection is assessed against
the street, i. e., against the city; but in others the cost of paving the
street intersections is included in the charge against the abutting
property. In most cities lots owned by the municipality pay the
same proportion of the cost of the street improvements as private
property, although usually special authority is required thus to assess
municipal property.
Table 36 also shows that as a rule the eastern and southern
cities pay a larger proportion of the cost of pavements than do the
western. This difference in practice is probably due chiefly to the
* From an article on Theory and Practice of Special Assessments by J. L. VanOrnum, in
Traus. Amer. Soc. of Civil Engineers, Vol. 38, p. 336-422,
328 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
TABLE 36
APPORTIONMENT OF COST OF. PAVEMENTS IN FIFTY CITIES
Ref
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
LOCALITY.
Grading,
Per Cent
Paid by
Original
Paving,
Per Cent
Paid by
Re-paving;
Per Cent
Paid by
State.
City.
Prop-
erty.
City.
Prop-
erty.
City.
Prop-
erty.
City.
50
100
100
50
ioo
100
100
100
50
100
50
50
100
100
67
a
"56"
67
50
100
100
100
100
100
75
50
33
ioo'
100
50
33
50
"d"
c
50
100
"b"
' 'SO
67
50
100
100
'ioo'
'75'
ioo
100
100
100
100
100
ioo
ioo
50
100
'ioo
100
98
100
100
100
50
ioo
ioo
ioo
50
ioo
100
'ioo'
100
50
33
50
d"
100
'ioo'
25
100
100
100
100
100
c
c
100
ioo
d
50
'ioo"
"2c"
c
'ioo'
50
100
100
100
100
ioo'
Arkansas
Califormia
Little Rock
San Francisco
Hartford
Dist. of Columbia . . .
Delaware
Florida
Georgia
New Haven
Wilmington
Jacksonville ....
50.
Atlanta
Illinois
Indiana
Augusta
50
100
100
Peoria
Indianapolis
Burlington
d
100
100
' '25'
100
'ioo
100
ioo
50
100
Iowa
Kansas
Topeka
Kentucky
Louisville
100
75
'ioo
100
"25
100
"ioo
100
100
c
c
Louisiana
Maine
Maryland
New Orleans , . . . .
Portland
Baltimore.
100
ioo'
100
100
100
100
100
ioo
100
100
100
100
100
100
100
98
100
100
100
100
100
ioo'
'166'
ioo'
100
Massachusetts
Michigan
Minnesota
Missouri
Lowell
Springfield. ...
Worcester
100
100
Detroit
Minneapolis
St. Paul
100
100
"56'
ioo
100
100
100
100
100
100
100
98
100
100
100
Kansas City
Nebraska
New Hampshire
New Jersey
New York
Ohio
St. Louis
100
d '
Omaha
Manchester
Newark
Paterson
d '
" 2c'
c
'166'
ioo
100
50c
100
c
Albany
Brooklyn. .
Buffalo
New York
Rochester
"2c"
c
'ioo
100
100
100
c
Syracuse
Cincinnati. . .
Oregon
Dayton
Portland
Harrisburg
3ennsylvania
Ihode Island
South Carolina
South Dakota
Tennessee
Utah
Virginia
Washington
Wisconsin
Philadelphia
Scranton
Providence
100
100
ioo'
'56'
Charleston
Sioux Falls
Nashville
Salt Lake
Richmond. . . .
Seattle
ioo
100
Milwaukee
a. 1 sq. yd. for each front foot; city remainder
6. 3f sq. ft. " " " " ; ••
c. City pays for street intersections.
d, City does not pay for street intersections,
ART. 2] PAVEMENT ADMINISTRATION 329
limited revenues of new cities and to the many demands upon the
general tax for the numerous and varied necessities of rapidly growing
municipalities; consequently the cost of pavements, improvements
having a definite local benefit, has been charged to the abutting prop-
erty. It is equitable and just that the cost should be borne jointly
by the private property and the city at large, since then the cost falls
upon both interests which directly profit by the improvement, and
neither receives a substantial benefit without sharing in its cost.
Ordinarily the proportion of the expense to be borne by the
municipality and by the private property is determined wholly by
financial considerations or usage, and is made uniform over the
entire city; while equity and justice demand that a distinction should
be made depending upon the character of the traffic. The interests
of the general public in a street vary greatly between a residence
street, a business street, and a general thoroughfare. To pave the
first the public should pay only a small share, say, 20 or 30 per cent;
for the second, say, 40 or 50 per cent; and for the third 60 or 75 per
cent. Some such variation in the proportion to be borne by the two
interests finds further justification in the fact that if the street
becomes a general thoroughfare, some of the benefits enumerated in
§ 635 as accruing to the abutting property may be nullified by the
noise and dirt.
646. SPECIAL ASSESSMENTS.* The proportion of the cost of
a pavement paid by the private property is usually collected as
a special assessment, which has been defined as " a compulsory
contribution paid once and for all to defray the cost of a special
improvement to property, undertaken in the public interest, and
levied by the government in proportion to the special benefits
accruing to the property owner. " Special assessments differ from
taxes, both general and special, in that the former are based upon a
direct and measurable benefit conferred upon the contributor,
which is the measure of his liability to be taxed ; while taxes are levied
for the maintenance of the institutions and interests of the govern-
ment, without reference to the particular benefits conferred, according
to the ability of the contributor to pay. The construction of pave-
ments to be paid for by special assessment must be done under the
direction of the public officials.
* For an interesting and instructive clscussion of the history and theory of special assess-
ments, see Special Assessments by Victor Rosewater — Vol. 2, No. 3 of Studies in History
Economics and Public Law. 152 p., 6X9 inches, Columbia College, New York, 1893. See
also the article referred to in the foot note on page 327.
330 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
In a general way it may be said that there are two distinct
methods of apportioning the amount to be paid by the private
property; viz.: (1) according to the frontage, and (2) according to
the area.
647. Frontage Rule. By far the more common method of appor-
tioning the assessments is pro rata according to the frontage upon the
improvement. This method is often designated as the front-foot
rule. Of the forty-five cities in Table 36, page 328, which assess the
private property for street improvements, thirty-eight or 84 per cent
follow the frontage rule, three use a combination of frontage and
area, one uses area alone, one value alone, and in two of the cities
the method employed is left to the judgment of the assessing board.
Ordinarily the frontage is an equitable basis upon which to
distribute the cost; but under some circumstances a rigid appor-
tionment according to frontage gives anomalous results. For
example, if most of the lots have their shorter side on the improve-
ment and one has its longer side thus placed, the frontage rule will
give inequality — particularly if the latter lot is very narrow. This
condition frequently occurs — for example where the most of the
lots front upon the street to be paved, while some front upon an
intersecting street. In this case, it is customary to extend the
assessment to the middle of the block; that is, assess the lots between
the pavement and the center of the block, in which case it becomes
a difficult matter to determine the equitable portion for each of these
lots. A rigid adherence to the frontage rule sometimes works
injustice near the intersection of two streets cutting each other at an
acute angle. However, no method can be devised that may not
require modification to fit unusual conditions.
648. Area Rule. In a few cities, 7 per cent of those in Table 36,
page 328, the cost of street improvement is distributed in proportion
to the area of the abutting lots; but usually the area is used in com-
bination with the frontage. Thus in Brooklyn, N. Y., 60 per cent
of the cost is distributed in proportion to the frontage and 40 per cent
according to the area. An amendment to the charter of St. Louis
proposes to charge 25 per cent of the cost of the pavement according
to the frontage and 75 per cent according to the area. The area rule
finds its greatest justification on curved streets.
649. Corner lots are usually the cause of irritation and objection
under either the frontage or the area rule, and the method of assess-
ing them differs materially in different cities. In some cases each
margin is considered a front on its proper street, without any modi-
ART. 2] PAVEMENT ADMINISTRATION 331
ification in the rate. of assessment; in a few cases under the area
rule, an additional per cent is imposed upon the corner lot for the
pavement of either street ; but usually the corner is assessed according
to frontage at a less pro rata than the inside lots, since it may be
assessed on both streets.
650. Terms of Payment. There are various methods of pay-
ing the assessment. 1. The entire amount may become a lien
upon the property as soon as the work is completed, to be collected
(a) by the contractor, or (6) by the city acting only as collecting
agent for the contractor, or (c) by the city, which also becomes
responsible to the contractor for the payment of the money. 2,
The amount may be divided into equal annual installments, usually
five or ten, with interest on deferred payments, to be collected
(a) by the contractor, or (6) by the city, the contractor receiving
special paving-district bonds, or (c) by the city, the contractor
receiving general city-bonds. 3. The city may raise a paving fund
by general tax or by selling bonds, and pay for improvements as made
independent of the collection of the special assessments. The second
method is the more common. The first is objectionable because the
amount becomes immediately due; and the third is objectionable on
account of the difficulty of making the assessments and collections
keep pace with each other, and also because of a tendency to produce
extravagance.
651. Legality of Levy. Special assessments can be levied only
under explicit authority of the law. The different states have
very complete and explicit statutes governing special assessments;
and the courts always hold that any material departure from the
prescribed procedure invalidates the assessment.
652. GUARANTEEING PAVEMENTS. It is a common custom
to require the contractor to guarantee the pavement for a term
of years, which guarantee is supported either by an indemnifying
bond or by a portion of the cost of the pavement retained by the
municipality until the expiration of the specified period. In some
cases the guarantee is an agreement that if time shall reveal that
the materials or the method of construction are not according to
the contract, the contractor shall make the defect good; but in
other cases, the so-called guarantee is virtually a contract to main-
tain the pavement for the specified period and to turn it over in
good condition at the end of that time.
Apparently the guarantee originated in this country with the
introduction of sheet asphalt pavements. The material was new,
332 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
the method of laying it was untried, and hence no city would run
the risk of paying for an unknown and uncertain pavement; con-
sequently the contractor agreed to guarantee the pavement for a
period of years. At present most cities continue to exact a guarantee
for asphalt pavement, ranging from five to fifteen years, on the
ground that the method of testing the material and the manner of
laying it are too little understood by engineers to insure good and
durable work without a guarantee. At the beginning of the use of
brick as a paving material a guarantee was sometimes demanded;
but at present it is as a rule not required with this material.
653. The requirement of a guarantee of the pavement is justi-
fiable when the material to be used is new and there is little or no
opportunity for the engineering department to acquire the knowl-
edge necessary for an effective inspection of the work; but as a
rule a guarantee, particularly for a long time, is unwise for the
following reasons: 1. The contractor has no control over the street
after the pavement is completed; and it is difficult to discriminate
between defects due to improper material and the effects of ordinary
wear, which may differ materially on different streets. It is also
difficult to discriminate between defective workmanship and damages
due to causes for which the contractor is in nowise responsible, as,
for example, fires, escape of illuminating gas, settlement of trenches
made after the completion of the pavement, etc. 2. It is difficult
to enforce the guarantee clause if, on the one hand, the engineering
department inspects the material and accepts the workmanship;
and, on the other hand, if a representative of the city does not inspect
* the work there is liability that the streets may be needlessly obstructed
and the public greatly inconvenienced by a bungling experiment by
the contractor. The difficulty of enforcing a guarantee is much less
in a large city where there is more work to be had and where the
contractor desires to protect his reputation with a view to securing
contracts in the future, than in a small city having but little work;
and the difficulty is still further increased if the law requires that the
contract shall be let to the lowest responsible bidder — as is usually
the case.
The contractor objects to the guarantee, not without justice,
on the following grounds: 1. The specifications are prepared by
the engineering department of the city, and as the quality of the
material and the method of construction is prescribed by the city
and subject to the approval of its representatives, the contractor
should not be held responsible for the result. However, the suffi-
ART. 2] PAVEMENT ADMINISTRATION 333
cient answer to this objection is that the contractor accepts the
specifications when he enters into contract, and is therefore right-
fully bound by them. 2. The expense is needless and excessive,
whether an indemnifying bond is required or a per cent of the con-
tract price is retained, which expense in the long run adds to the
cost of the pavement. It is more expensive to the contractor if
the city retains a per cent of the contract price, since a portion of
his capital is then tied up, which in turn drives out the small con-
tractor, decreases competition, and tends to increase the cost of
the pavement. On the other hand, the interests paying for the
pavement are better protected if the city retains a per cent than if
an indemnifying bond is accepted, since in the former case the city
has the money in hand with which to make the needed repairs in
case the contractor fails to do so; but the proper care of such de-
ferred payments adds materially to the labor and responsibility of
municipal administration.
The contributing property holders and citizens favor the guar-
antee as a defense against incompetent or dishonest city officials
and employees. The guarantee is also sometimes defended on the
ground that it is the cheapest method of securing good work, since
it is impossible at reasonable cost for the engineering department
to inspect all stages of the preparation of the material or to acquire
the knowledge necessary for an effective supervision of the con-
struction; but in general this claim is not true. It is neither credit-
able to the engineering profession nor economical to the municipali-
ties to leave all exact knowledge of paving matters in the hands of
the paving contractors.
654. The proper length of the guarantee period is a matter
about which there is considerable difference of opinion. For asphalt
pavement a guarantee for five years is quite common, although some-
times a fifteen-year guarantee is required. With stone block, brick
and most other forms of pavements nine months, or at most a year,
is sufficient to reveal any serious defect of material or workmanship,
and therefore a long guarantee is not necessary.
655. Maintenance by Contract. As stated above it is common to
require a so-called guarantee which is virtually a contract for main-
tenance for the specified period. Maintenance by contract is justi-
fiable if the engineering department of the city does not possess,
or can not reasonably be expected to obtain, the information neces-
sary in repairing the pavement; but as a rule maintenance by con-
tract is undesirable, for four reasons: 1. The contractor has no con-
334 PAVEMENT ECONOMICS AND ADMINISTRATION [CHAP. XI
trol over the streets, and the repairs required are dependent upon the
restrictions against opening the pavements and also upon the reg-
ulations for keeping the streets clean. 2. It is difficult to specify
beforehand the amount and the nature of the repairs that may be
required by the ordinary use of the pavements, particularly as the
opening of new streets or the paving of others may materially alter
the amount or nature of the traffic on any particular pavement.
3. It is impossible to determine accurately the condition of the pave-
ment at the end of the contract period. 4. With a new and untried
material it is impossible to determine what is a reasonable expense
for maintenance.
A contract for maintenance is sometimes defended by the prop-
erty holders on the ground that thereby some one is secured who is
admittedly responsible for the condition of the pavement and who
is more amenable for neglect than are city officials. However, if
the city officials can not be trusted to repair the pavements directly,
it is doubtful whether they may reasonably be expected to super-
vise the repairs to be made by the contractor. The choice between
maintenance by contract and by municipal authorities directly
will usually depend upon the local conditions.
The pavements of Paris, France, were formerly maintained
by contract, but are now maintained by the city directly.
656. TEARING UP PAVEMENTS. The most serious cause of
the destruction of pavements is the frequency with which they are
torn up for the introduction or repair of underground pipes, conduits,
etc. No pavement has been introduced, and probably none ever
will be, which is not seriously injured by being torn up. With care
and intelligence a pavement may be replaced in nearly its former
condition; but it almost never is so replaced", and under the condi-
tions which such work is done, it is almost impossible to get it so
replaced. The only remedy for the frequent disturbance of pave-
ments is the construction of a subway in which to place pipes,
wires, etc. ; but it is doubtful if any such remedy would be lasting for
the streets are continually being put to new uses. Formerly it was
thought sufficient to provide for water and gas pipes and sewers;
while now conduits are required for telegraph, telephone, and electric
light wires; and street-car tracks are constructed on the surface,
above the surface, and below the surface; and in some cities space is
required for pneumatic tubes, and pipes for distributing heat, com-
pressed air, cold and hot water, etc.
The only thing that can be done is to reduce the opening of the
ART. 2J PAVEMENT ADMINISTRATION 335
pavements absolutely to a minimum, and then to take the utmost
care to see that as little damage as possible is done in making the
opening and that the pavement is restored in the best way possi-
ble. A few years ago in New York City a quarter of a mile of trench
was opened for each mile of pavement, and in addition there was
an opening for each 35 linear feet of street. The year stated was about
an average for those immediately before and after. In Chicago
in 1902 200,000 square yards of pavements were taken up by public
service companies, which is equal to about 10 miles of pavement 36
feet wide; or in other words, the equivalent of one sixth of all the
new pavements laid in the city in that year was torn up by the public
utilities corporations. This did not include the pavements taken
up by the city itself to lay water pipes, sewers, etc.
The amount of money spent in digging up the streets is a con-
siderable item, not counting the interference with travel and busi-
ness; but the expense, being distributed among various interests,
is not usually sufficient to cause any one company to re-construct
its system. It is probable that the interests of the public are fre-
quently sacrificed to the interests of the private companies using
the streets — usually without paying for the privilege.
Under the best municipal administrations of Europe neither
corporations nor individuals are permitted to disturb the pave-
ments. All removals and restorations are done by the city's own
employees, upon the deposit, by the parties who require the streets
to be opened, of a sufficient sum to cover the expense of each piece
of paving done, at a fixed price per yard according to the kind of
pavement. Moreover, interference with the pavements is of rare
occurrence, for the companies having pipes underground are require
thoroughly to examine and reinstate their mains and services con-
currently with the paving of a street, due notice of the execution of
which is given by the city.
657. Nearly all cities have ordinances governing the opening of
pavements, which differ greatly in character and severity; but gen-
erally the result is unsatisfactory owing to the real difficulties of the
case, or to inefficient administration, or to unexpected emergencies.
There is also great variation as to the method of doing the back-
filling, replacing the foundation, and re-laying the pavement, and
also as to who shall do the work, — whether city departments, private
parties, public utility company, or contractor; and again none is
satisfactory. This is a serious unsolved problem in American
municipal administration.
CHAPTER XII
STREET DESIGN
660. From the point of view of future needs — commercial, sani-
tary, and esthetic, — it is unfortunate that cities grow up by successive
additions under the stimulus of private greed and real estate spec-
ulation, without any comprehensive or well considered street plan.
In some instances — notably Paris, London, and Boston, — vast sums
have been spent to correct what might have been prevented in the
original plan of the streets.* In most cities transformation — slow
and expensive, if it comes at all — is the only remedy; but a mended
article is never as good as one originally well made, f
Unfortunately there are few cities in this country having adequate
regulations governing suburban development. Municipal authori-
ties should regulate the street plan of subdivisions and additions so
as to secure a harmonious whole, and particularly with a view of
making the streets continuous and to afford suitable channels of
communication. Where such regulations do not exist, streets will
be laid out in such a way as best to develop the particular property,
regardless of the interests of the public. Washington City, which
has the best street plan of any American city, has been disfigured
by ill-planned additions, although at present stringent rules govern
the width and the arrangement of the streets of additions and sub-
divisions.
661. STREET PLAN. Since an engineer is occasionally called
upon to plan a city, and often to lay out additions to cities and vil-
lages, the various street plans for a city will be considered. In
planning the streets of a city three objects should be kept in mind;
* For example, Paris spent $14,000,000 in improving the Rue de Rivoli, and London $33,000,-
000 on the Strand Improvement.
t For an elaborate and abundantly illustrated treatise from the view point of an engineer,
of many of the things considered in this chapter, see The Planning of Jthe Modern City, Nelson
P. Lewis; John Wiley & Sons, New York, 1916.
336
STEEET PLAN
viz.: (1) the subdivision of the area in such a manner as to give
the maximum efficiency for business or residence purposes; (2)
sufficient accommodation for the pedestrian and vehicle travel
on the streets; (3) good drainage; and (4) easy communication
between the different parts of the city.
662. Size of Lots. Owners in subdividing property are anxious
to make as many lots as possible; and in some other respects small
lots are to be preferred. It is desirable to make the lots of such a
size that few of them will be subdivided, as clearness of identity in
transferring or assessing the lot is maintained by always referring to
the original number. A frontage of 25 feet seems the best. This
width is suitable for business, purposes, and for residence streets
two or more lots will give proper grounds. Business lots are some-
times made only 18 or 20 feet wide, but 25 feet is by far the more,
common.
Lots are seldom less than 100, nor more than 180, feet deep;
and usually vary from 100 to 150 feet. A lot more than 150 feet
deep is objectionable, because of the temptation to build unsightly
residences fronting on the alley and because of the usual indifference
to keeping a deep lot in good sanitary condition.
663. Size of Blocks. With a rectangular system of streets,
the blocks are preferably long and narrow; since the distance re-
quired between streets in one direction is only that necessary to
give the proper depth of lots, while in the other direction the streets
need be only close enough to provide convenient channels for the
traffic.
For convenience, especially in business districts, it is best to
have an alley run lengthwise through the block. The alley varies
from 10 to 30 feet, but is usually from 16 to 20 feet.
The above depth of lot and width of alley makes the width of
the block 220 to 330 feet. The length of the block will depend upon
the requirements for traffic perpendicular to the principal streets.
Sizes of blocks vary much in any particular city, and still more
between different cities. The following are the dimensions of typical
blocks in several cities: Boston, 220X400 feet, and 100X550 feet;
New York, 200X900 feet, and 200X400 feet; Philadelphia, 400X550
feet, and 500X800 feet; Washington, 400X600 feet, and 300X800
feet; Montreal, 250X750 feet; Chicago, 300X350 feet, and 300X500
feet.
Fig. 103, page 338, illustrates the advantages to be derived from
a careful study of the best size of blocks and of the most advan-
STREET DESIGN
[CHAP, xil
tageous arrangement of streets. The left-hand side of the diagram
shows the typical arrangement of streets and blocks in the residence
district of New York City, the shaded portions representing the
usual buildings. The right-hand side shows a much superior arrange-
ment.* The three center blocks of the present plan comprise an
area of 720X800 feet, and contain 480,000 square feet of building
area and 96,000 square feet of streets, and in the corresponding area
of the proposed plan, there are 481,000 square feet of building area
FIG. 103. — IMPROVED ARRANGEMENT OF STREETS AND BLOCKS.
and 94,200 square feet of streets; therefore the two plans give sub-
stantially the same area for buildings and for streets. In the first
case the length of streets is 1600 feet, in the second 1520 feet;
therefore the two plans have practically equal light and air. The
proposed arrangement is the better in the following particulars:
1, number of corner sites; 2, accessibility of rear entrances for delivery
of provisions, coal, etc., and the removal of garbage, ashes, etc., and
in case of fire; 3, removal from the street of dangerous and cramped
* Proposed by Mr. J. F. Harder, in Municipal Affairs, Vol. 2, p. 41-44 Reform Club
New York City, 1898.
STREET PLAN 33Q
cellar entrances; 4, removal from the main or primary streets of the
loading and unloading of trucks; and 5, increased transportation
facilities in a direction perpendicular to the length of the original
blocks.
664. Location of Streets. In planning a system of streets there
are two objects that should be carefully considered, viz. : the drain-
age and easy communication between the different sections of the
city. Not infrequently these elements have been overlooked or
neglected. The surface drainage, the sewerage and the travel must
follow the general slope of the land; and therefore if there is much
irregularity of contour in the site, a location of the streets with
reference to the contours will afford at once the best drainage and
the easiest communication between different parts of the city. If
the site is nearly level, the relationship between the slope of the land
and the direction of the streets is comparatively unimportant; but
the arrangement of the street plan to afford the greatest facilities for
communication between the different parts of the city is still an
important matter. Therefore the conclusion is that on a site of
irregular contour the streets should be located with reference chiefly
to the topography, and on a level site primarily to secure the most
direct and easiest intercommunication.
665. Location with Reference to Topography. Unfortunately in
this country our very desirable rectangular system of public land
survey has frequently led to the adoption of a very undesirable rect-
angular system of streets which, though convenient for dividing
property into the greatest number of rectangular lots upon which
can be built the greatest number of rectangular buildings, has little
else to recommend it. Surface drainage sewerage and travel should
follow the slope of the country, and any attempt to deviate from this
becomes a serious question in the building of a city upon any but
nearly level ground. The streets are of necessity the drainage lines
of the city and should be placed in the natural valleys, and the failure
so to locate the streets in many cities where the land is very irregular
in contour has led to great expense in the construction of the streets
and of a system of storm-water sewers.
The upper half of Fig. 104, page 340, shows an actual case of a
system of rectangular streets located without any reference to the
topography; and the lower half of the same diagram shows a pro-
posed arrangement * that would save much expense in grading the
* By W. D. Elder in Proc. Michigan Engineering Society, 1898, p. 52.
340
STREET DESIGN
[CHAP. XII
streets and at the same time give a quick entrance into the center
of the city, and also give long easy grades from the heart of the
city to the higher outlying district.
Center of
City
FIG. 104. — LOCATION OF STREETS WITH REFERENCE TO CONTOUBS.
STREET PLAN 341
666. The original rectangular street system of San Francisco
was laid out without much attention to the resulting street grades,
some of which are 55 per cent. As rapidly as possible these excessive
grades are being reduced. Recently $30,000 was spent to reduce the
grade from 29 to 16 per cent through one block. The cost was
something like 15 per cent of the value of the abutting property.
This extreme case involved several unique but expensive features.*
667. Location with Reference to Directness of Communication.
There are three distinct general plans for city streets with refer-
ence to directness and ease of communication.
668. One consists of a system of parallel streets crossing a similar
system at right angles. This is often called the checker-board
system, but more properly the rectangular system, since the blocks
are not necessarily squares. This arrangement gives the maximum
area for blocks, and also furnishes blocks of the best form for sub-
division into lots. The rectangular system is the most common,
and has its most marked exemplification in Philadelphia.
669. A second arrangement of streets consists of the rectangular
system with occasional diagonal streets along the lines of maximum
travel. This system was employed by L 'Enfant in planning the city
of Washington. Fig. 105, page 342, shows a portion of that city. To
a limited degree, the same plan was adopted in laying out the city
of Indianapolis, which has four broad diagonal avenues converging
to a circular park in the center. These two are the only cities of
any importance in which this system was adopted in advance of
building. This system is usually, but somewhat improperly, called
the diagonal system.
The chief advantage of the diagonal street is the economy due
to the saving of distance by traversing the hypothenuse instead of
the two sides of a right triangle. In Rome, london, Paris, and
in numerous other smaller places in Europe, whole districts have
been razed to make way for new streets to serve as arteries for in-
creased traffic.
A second, and by no means an unimportant, advantage of the
combination of the diagonal and the rectangular system is the open
squares and spaces so grateful to the eye and of no little sanitary
value in compactly built cities. New York City has recently been
spending a million dollars a year to create such spaces by purchas-
ing land and demolishing the buildings.
* Engineering News, Vol. 75 (1916), p. 12-13.
342
STREET DESIGN
[CHAP, xii
vJuJt _ l'/3l_JL_mgl _ II _ lOSfLll
'J
omaaaacnciaajannoanssaa acrna
ODflO
anaczinao
WIDTH OF STREETS • 343
Although the diagonal avenue occupies ground that might
otherwise be used for building purposes, there is a compensating
advantage in the greater length of street front obtained.* In many
cases the total cost of cutting diagonal streets through built-up
districts has been paid by the increased value of the property on and
near the street thus opened up.
670. The third arrangement of city streets is the ring or concen-
tric plan, which is very popular in Europe. The most noted example
is Vienna with its Ring-strasse (ring street) within and its Gtirtel-
strasse (girdle street) without. The former is 187 feet wide and
encircles the public buildings and the leading houses of business and
amusement. The enclosed network of streets intersect the Ring-
strasse at forty points, and outward from it extend fifteen main
radial avenues.
671. WIDTH OF STREETS. The width of city streets is important
on account of its influence upon the ease with which traffic may be
conducted and also because of its effect upon the health and com-
fort of the people by determining the amount of light and air which
may penetrate into thickly built-up districts. The streets of nearly
all large cities are too narrow, being crowded and dark. A more
liberal policy in planning streets would probably be of pecuniary
advantage, since there is usually an enhanced financial value due
to wide streets. A lot 100 feet deep on a street 80 feet wide is usually
more valuable than a lot 110 feet deep on a street 60 feet wide;
that is to say, within reasonable limits land is usually more val-
uable in the street than on the rear of the lot. Wide streets are
especially needed where they are bordered by high buildings or are
to carry street-railway lines, f
In order properly to accommodate the traffic in business dis-
tricts of cities of considerable size, a street should have a width
of 100 to 140 feet, the whole of it being used for roadway and side-
walks; while residence streets in a city of considerable size, where
the houses are set out to the property line and stand close together
should have a width of 60 to 80 feet. Although it is advantageous
to have a wide street, it is not necessary, nor even desirable that
the whole width be paved; the central portion may be paved, a
* For a discussion of this phase of the subject, see an article by L. M. Haupt in Jour. Franklin
Inst., Vol. 103, p. 252.
t For an elaborate and instructive article on this subject see a paper by John Nolen before
the 1911 National Conference on City Planning, Engineering and Contracting, Vol. 35 (1911),
p. 621-622.
344 STREET DESIGN [CHAP. XII
strip on either side being reserved for grass plats. The width of
the pavement should be adjusted to the amount of travel, which
varies greatly accordingly as the street is a business street, a thor-
oughfare, or an unfrequented residence street.
The width of the streets in different cities varies greatly. In
the older places in New England and the Central States, many of
the streets are only 30 to 40 feet wide; but in the West a street is
seldom less than 60 to 66 feet wide. In both regions the princi-
pal streets are often 80 to 100 feet wide, and in many of the larger
cities the boulevards and great avenues are 150 to 180 feet. The
main avenues in Washington are 160 feet wide, in New York 135,
and in Boston, 180 feet.
At present the. regulations governing the width and the arrange-
ments of additions and subdivisions of Washington, a city which
has the best street plan of any in America (see § 669) are: " No
new street can be located less than 90 feet in width, and the lead-
ing avenues must be at least 120 feet wide. Intermediate streets
60 feet wide, called places, are allowed within blocks; but full-
width streets must be located not more than 600 feet apart."
672. AREA OF STREETS. The proportion of the area of the
city devoted to streets varies greatly, particularly between the
older and the newer cities. The following is the per cent of street
area in a few extreme cases of American cities : *
MINIMUM STREET AREA MAXIMUM STREET AREA
1. Taunton, Mass 3.20 per cent Duluth, Minn 86.7 per cent
2. Worcester, Mass. ... 5.43 " " Dallas, Tex 78.3 " "
3. Binghamton, N. Y. . 7.55 " " Denver, Colo 73.9 " "
4. Philadelphia, Pa. ... 8.42 " " Indianapolis, Ind 56.4 " "
5. Boston, Mass 8.76 " " Washington, D. C 43.5 " "
6. Lowell, Mass 8.92 " " Davenport, la 42. 1 " "
7. Fall River, Mass 9.17 " " Evansville, Ind 40.8 " "
The area devoted to streets and alleys in a few of the principal
cities of the world is as follows :
AREA OF STREETS AND ALLEYS
1. Washington 54 per cent
2. Vienna 35 « «
3. New York City 35 « «
4. Philadelphia 29 " "
5. Boston 26 " "
6. Berlin 26 " "
7. Paris .25 " "
* Census Bulletin No. 100— July 22, 1891,— p. 16.
WIDTH OF PAVEMENTS 345
673. WIDTH OF PAVEMENTS. It is wise to make the streets of
residence districts of liberal width for sanitary and esthetic reasons;
and also because the future of the street can not be certainly fore-
seen,— the residence street may become a business street, or an
unfrequented street a thoroughfare. However, it is not necessary
that the whole width should be devoted to wheelways and side-
walks, particularly in small cities. A grass plat between the side-
walk and the pavement, in which shade trees are set (§ 696), adds
to the beauty of the street and to the comfort of the residents by
removing the houses farther from the noise and dust of the pave-
ment. The grass plat or parking also affords an excellent place in
which to place water and gas pipes, telephone and electric-light
conduits, etc. In large cities where the street front is built up solid
with houses of several stories, it may be necessary to dispense with
the grass plat, and to devote the entire street to sidewalks and
roadway.
It is universally admitted that pavements are desirable; but
often, owing to the unwillingness of at least some of the people to
pay for them, it is difficult to secure them. Except for the cost,
the wider the pavement the better; but length is more valuable than
width. An excessive width is a needless expense, and delays or
prevents the getting of any pavement at all; hence one help toward
securing pavements is to make the pavement only wide enough to
accommodate the traffic. Not infrequently the pavements of
suburban and residence streets are needlessly wide. A narrow
pavement not only costs less to construct, but also costs less to
clean and maintain; while the cost of maintenance depends chiefly
(or, with a pavement not subject to natural decay, wholly) upon
the amount of traffic, and hence is nearly (or entirely) independent
of the width.
674. Without Car Track. A width of 18 feet affords sufficient
room for a vehicle to pass when another is standing on each side
of the pavement — a rare occurrence; — and therefore it appears
that a pavement 18 feet wide is sufficient for the less frequented
residence streets. The only objection to a very narrow pavement
is the difficulty of turning a vehicle in such a street. The serious-
ness of this objection depends upon the construction of the vehicle.
Many delivery wagons, express wagons, etc., may be turned on an
18-foot pavement. If occasionally a vehicle is compelled to go to
the corner turn, to or even to drive around the block, the incon-
venience is not very serious, and is so infrequent as not to justify
346 STREET DESIGN [CHAP. XII
any considerable expense to prevent it. A width of 20 to 24 feet is
probably sufficient for a majority of residence and suburban streets.
When a residence street is an artery of travel, it may be necessary
to make the pavement wider than stated above. In a number
of cities, there has been a marked tendency in recent years to reduce
the width of pavements on residence streets.
Thirty feet affords sufficient room for two vehicles to pass each
other where two others are standing at the curb; and therefore
this width of pavement is ample for business streets in small places.
On a narrow business street it may be necessary to curtail the width
of the pavement to prevent the sidewalk space from being unduly
encroached upon.
In many of the cities the width of the pavement is uniformly
a fractional part of the total width of the street, regardless of the
needs of traffic. In many cities, both American and European,
the pavement is three fifths or 60 per cent of the width of the street.
In New York City and Brooklyn the rule seems to be to make the
pavement half the width of the street. In Washington City there
is no hard-and-fast rule, but the following is the usual relation:
on streets 60 feet wide or less, the pavement is 25 feet or 40 per cent
of the width of the street; on streets from 60 to 90 feet wide, the
pavement is 25 to 35 feet, or 40 per cent; and for streets 130 to 160
feet wide, the pavement is 40 to 50 feet, or 30 per cent.
675. With Car Track. For a residence street containing a car
track, the minimum width permissible is 28 feet, which will allow
a car to pass with a vehicle on each side of the track. In Brooklyn
a great many streets only 34 feet wide between curbs contain a
double line of street-car tracks, which leaves a space of only 9£
feet between the track and the curb. This is astonishingly small,
but seems to do fairly well.
On a business street containing a car track, it is wise to make
the pavement wide enough to permit a vehicle to pass between
the car and another vehicle standing at the curb. This will require
about 48 feet. If the street is too narrow to permit this width
of pavement and also the proper width of sidewalks, only one track
should be allowed in the street; if a double track is necessary
the cars should be required to make the return trip by another
street.
At Rochester, N. Y., the car tracks on residence streets are
located on the parking at the side of the street. This is an unusual
arrangement, but it possesses some advantages. 1. It separates
STREET GRADES 347
the vehicle and car traffic, and prevents mutual interference. 2.
It permits a narrower pavement. 3. It prevents disturbance
of the pavement to repair the car track. 4. It lessens the danger
of a passenger's being struck by another car or a vehicle in leaving
a car. The objection to this arrangement is that it interferes with
the grade of the driveways to private grounds.
676. STREET GRADES. The fixing of street grades is one of
the most important functions of municipal engineering, since the
grade system of the streets is the foundation of all municipal engi-
neering matters. The grades should be established before the
sewer system is planned; and if they are established before the
property is improved the problem is comparatively simple, since
they may be laid chiefly with reference to obtaining within proper
limits of cost desirable gradients for the street. But when build-
ings have been erected, sidewalks constructed and trees planted, it is
often extremely difficult to secure grades which will harmonize the
various and conflicting interests.
677. Elements Governing Grades. The grades necessarily
depend mainly upon the topography of the site; but in general the
determination of the proper grade for a street requires the consid-
eration of the following elements: (1) the drainage, (2) the cost
of earthwork, (3) the accommodation of travel, (4) the effect
upon the abutting property, and (5) the general appearance of the
street.
678. Drainage. The streets are the natural drainage channels
of the city; the lots must drain into them, and the house must
drain into the sewers placed in the streets. When no storm-water
sewers are to be constructed, the grades become very important,
since the streets must provide for the surface drainage of the city,
and particular consideration must be given to relative grades and
gutter capacities in order to prevent the excessive concentration
of storm water at the lower levels and to provide for its proper
distribution and disposal.
679. Cost of Earthwork. Not infrequently the cost of making
the excavations and embankments is given undue weight. The
balancing of cuts and fills is often properly a controlling element
in country road construction, but it should have relatively little
weight in determining the grades of city streets. The expense
for earthwork is incurred once for all, and a few hundred dollars
more or less is usually unimportant in comparison with the expense
of maintaining the street surface and the drainage system, and
348 STREET DESIGN [CHAP XH
the cost of conducting traffic over the grades, and also in com-
parison with a better general appearance of the street.
680. Accommodation of Travel. The question often is whether
or not to secure ease of traction at the expense of increased cost of
construction. The discussion in Chapter II, § 65-86, sheds a little
light and only a little, as to the proper method of answering this
question. Apparently engineers are inclined to overestimate the
disadvantage to travel of a slight grade. Practical experience has
demonstrated that there is not much difference in effect upon the
cost of transportation between level roads and those having grades
of 2 or 3 per cent unless such grades are very long or have an unusu-
ally smooth surface.
681. Effect upon Abutting Property. The private interests of
the property holder should be carefully considered; although it
is frequently impossible to establish proper grades without injury
to the adjoining property. The general question is how far private
interests should be sacrificed to the general good. It is better that
the city or the other residents on the street should pay the owner
damages than that lasting detriment should be done to the appear-
ance of the street or to the traffic.
682. General Appearance. Some attention should be paid to
the appearance of a longitudinal view of the pavement. It is desir-
able that the longitudinal grade be not changed so frequently as
to give the street a wavy appearance. Further, the transverse
grades at street intersections and on side hills should be so arranged
as not to produce a confused appearance in looking along the street.
The grades of the streets, both longitudinal and transverse, have a
material effect upon the general appearance and beauty of the city.
683. Maximum Grade. In a general way the principles gov-
erning the determination of the permissible maximum grade of a
city street are the same as for a country road, i. e., it is a question
between the cost of operation on the one hand and the cost of con-
struction and maintenance on the other, except that for a country
road the cost of construction is chiefly the cost of moving the earth,
while for a city street the cost of construction should also include
the effect upon abutting property of high embankments or deep
excavations, and except further that usually in the city heavy
loads can take a circuitous route and avoid the maximum grade
entirely. In determining the maximum grade for a street, the
fact should not be overlooked that the smoother the pavement the
more serious is a steep grade.
STREET GRADES 349
684. In the Borough of Manhattan, New York City, are some
business streets having grades as steep as 6 per cent, and a num-
ber of residence streets have 10 per cent grades, and some have
grades of 12, 15 and 18 per cent. Brooklyn, N. Y., has 4 per cent
grades on business streets and 12 on residence ones. A number
of cities have maximum grades on paved streets of 20 per cent — for
example, Worcester, Mass.. Syracuse, N. Y., Borough of Rich-
mond, New York City, and Pittsburg, Pa. Burlington, Iowa, has
an 80-foot street with a 24 per cent grade up which is laid a zigzag
brick pavement 18 feet wide having a maximum grade of 14 J per
cent with a minimum radius of the inside curb of 16 feet. San
Francisco has some extremely steep street grades, for one example
see § 666.
For a discussion of the maximum grade for each kind of pave-
ment, see the heading Maximum Grades in the chapter treating that
particular pavement.
It is usually considered that a grade steeper than 15 per cent
is impracticable and dangerous even for light traffic; and there-
fore if this grade can not be obtained, the street should be divided
into two parts separated by a terrace or stone wall, each portion
being entered only at its intersection with the cross street — see Fig.
118, page 358. A 10 per cent grade is usually considered prohibitive
for heavy loads; and 5 or 6 per cent is considered the limit on busi-
ness streets.
685. The selection of the proper pavement for the maximum
grade is a matter of great importance. For the recommendations
of a committee of the American Society of Civil Engineers concerning
maximum permissible grades, see Table 15, page 57. It is usually
held that sheet asphalt should not be laid on grades steeper than 2 to 3
per cent, although it has often been laid on 6 or 7 per cent grades,
and in one instance on a 17 per cent grade (see § 887). Brick, or
hard sandstone, or granite may be used upon the maximum grade.
The sandstone and the granite blocks should be narrow and should
be of a quality that does not wear smooth. It has been recom-
mended to chamfer the corners of rectangular stone or wood blocks
when laid upon steep grades, to give the horses a good foot-hold;
but it is at least doubtful whether the benefit of a good footing is
not neutralized by the increased tractive resistance. The joints
should be filled with tar or hydraulic cement.
686. Minimum Grade. The street surface should have enough
longitudinal slope to drain its surface well. With a smooth and
350
STREET DESIGN
[CHAP. XII
impenetrable pavement no ruts will be formed, and hence the
determination of the minimum permissible grade is mainly a
question of the grade of the gutter. If the drainage is carried
away by underground storm-water sewers, the street may be
perfectly level longitudinally, since the necessary grade for the
gutters may be obtained by making them deeper as they approach
the inlet to the sewer. For a further discussion of this phase of the
subject, see Grade of Gutter — § 711.
If it is inexpedient to vary the depth of the gutter (§ 710) or to
increase the grade by constructing additional inlets and catch basins,
it is necessary to secure the proper slope for the gutter by inserting
a summit in the street solely for drainage purposes — usually referred
to as an accommodation summit. However, it is undesirable that
there should be frequent changes in the grade, as they give the
pavement an unpleasant wavy appearance when one looks along
the street.
687. Elevations at Street Intersections. One of the most impor-
tant parts of the establishment of a system of street grades is the
arrangement of the grades
W/fo at street intersections. It
is a common practice to
establish only the eleva-
tion of the intersection of
the center lines of the
streets; but this often re-
sults in much confusion in
determining the elevation
for the curb at the corner,
particularly where the two
streets have considerably
different grades. For ex-
ample, in Fig. 106, assum-
ing (for the present at
least) that the curb is to
be at the same elevation
as the center of the street opposite, the elevation of the corner of
the curb, D, as computed from the grade of CB is 90.20 feet;
(< 2?'
Curb
..L
Center Line ofSfreef
FIG. 106. — ELEVATION OF CURB AT CORNER.
while the elevation of the same point as computed from the grade
of BA is 91.20 feet— a difference of 1.0 foot. To obviate this
source of confusion, the elevation of each corner of the curb and
also of the intersections of the center lines should be established,
STREET GRADES
351
•10-
A similar confusion occurs in attempting to compute the elevation
of the corner of the property, from the grade of the corner of the
curb. For example, in Fig. 107, assuming that the grade of the top
of the curb is the same as that of the center of the street, and assum-
ing that the sidewalk has a downward
slope away from the property of 0.24
inch per foot (2 per cent), and also
assuming that the grade of the corner
of the curb, D, has been established
as 80.00, then the elevation of the
corner of the property, G, as com-
puted from the grade of the curb. DE
80.30 feet, while the elevation of
IS
Q0.3O
6O.IO
Curb
FIG. 107. — ELEVATION AT CORNER OF
PROPERTY.
the same point computed from the
grade of the curb DF is 80.80 feet.
Some engineers advocate estab-
lishing the elevation of the corner of
the property and the determination of
the grades of the curb and of the street therefrom; while others
advocate establishing the elevation of the corner of the curb and from
that determining the elevation of the corner of the property and also
of the center of the street intersection. To be legal the elevation
must be fixed by ordinance. The courts hold that the " elevation '*
is the top of the pavement in the center of the street; therefore
it is necessary to establish by ordinance the elevation of the center
of the street intersection. Further, to prevent misapprehension and
error in computing the elevation of the corners of the curbs, and also
to save the labor of computing them anew each time a lot is to be
surveyed, it is wise to establish also the elevation of the corner of
the curb. The ordinance should distinctly state the method
to be employed in computing the auxiliary elevations of the
sidewalk and of the corner of the property. Often the grades are
established for only one street without due consideration of the
intersecting street; and then when the second street is improved,
the result is confusion, disputes, and sometimes suits for damages.
688. When the rate of grade of both streets is small, it is desir-
able that the entire street intersection from property line to prop-
erty line should be level, a condition which permits the continuation
of the section of each roadway until they intersect, makes the top
of the curb at the four corners of the same elevation, and also allows
the sidewalks at the corners to be level. That is to say, in Fig. 108,
352
STREET DESIGN
[CHAP. XII
the four points marked b and all the points marked a are in the
same horizontal plane. Each street has its full crown on the line
bb, and consequently there is a slight rise from b to c.
Where either or both streets have much inclination, it may not
be wise to flatten out the intersection, and thereby increase the
grade on the remainder of the street. Under these conditions,
the best arrangement of the intersection is a matter requiring
careful study and is one upon which there is much diversity of opin-
ion. If steep grades are continued across intersections, they intro-
duce side slopes in the streets thus crossed, which are troublesome
and possibly dangerous — particularly to vehicles turning the upper
corners. Such intersections are also objectionable on account of
the difficulty of properly caring
for the storm water. In resi-
dence districts it is usual to make
the intersection " level from curb
to curb"; that is, in Fig. lb8, the
four points marked b are in the
same horizontal plane. The
level places serve as breathing
places, and lessen the danger of
collision at the intersection.
However, if the street has a con-
siderable grade, a level intersec-
tion appears to have a decided
pitch toward the hill, which
gives the street an unpleasing
appearance; and therefore under these conditions, it is better to
apply, even in residence districts, the principle of the succeeding
paragraph and give the intersection a moderate inclination down
hill. If the intersection has only enough inclination to seem level,
the general appearance of a series of such intersections is pleasing
having the effect of a succession of terraces.
The following rule * for adjusting the grades at street inter-
sections is frequently employed and apparently is the most com-
plete of any that has been proposed. " In the business section all
the street grades of 3 per cent or less should be continued unbroken
over the intersection; and streets having a steeper grade than 3
Fio. 108. — ELEVATIONS AT LEVEL STREET
INTERSECTION.
* Proposed by Messrs. Rudolf Hering and Andrew Rosewater for the streets of Dulutht
Minn., in a report dated March 7, 1890. Engineering News, Vol. 25, p. 148-49; Engineering
Record, Vol. 22, p. 53.
STREET GRADES
353
FIG. 109. — ELEVATIONS AT INCLINED STREET
INTERSECTION.
per cent should have an intersection of 3 per cent between curb lines.
The grade of the curb between the other curb line and the property
line should in no case be greater than 8 per cent. The elevation at the
corner of the property should be
determined by adding to each of
the elevations of the curb opposite
the corner, the rise of the sidewalk
and taking the mean." Fig. 109
shows the elevations of a street
intersection adjusted according to
the above rules, assuming the
transverse slope of the sidewalk to
be 2 per cent (practically £ inch
per foot — the usual value).
The difficulty of adjusting ele-
vations at an intersection is con-
siderably increased if the two
streets do not intersect at right
angles. It is impossible to formulate any general rule, since each
case must be decided according to the local conditions. Close
observation and good judgment are required to secure a reasonably
satisfactory adjustment.
689. Notice that if either street has a grade and is carried past
the intersection nominally unchanged, the area between the four
curb corners and that immediately adjacent will be a warped surface.
For example, in Fig. 110, if the street S
has a descent as indicated and the street
W is level, and the unchanged crowns of
the street intersect at C, the area marked
w must be raised to carry the upper side
of the street W over the intersection, and
the portions marked v must be raised to
carry the street S over the lower side of
the street W. If the grade of either street
is small this adjustment can be made by
" warping in " or ''boning in " the surface
for a short distance.
Curves at Grade Intersection. It is frequently
should be carried straight through from
FIG. 110.— A WARPED STREET
INTERSECTION.
690. Vertical
claimed that the
grade
street intersection to street intersection, i. e., that the grade should
not be broken in the block. Apparently the reason for this practice
354
STREET DESIGN
[CHAP, xii
is the claim that a break of grade between streets is unsightly. As
usually put in, the angle of intersection is simply rounded off a
little by eye; and if the change of grade is considerable, the appear-
ance is not good. A change of grade in the block is nowise different
from a change at the street intersection, except that the former is
a little more conspicuous. For both appearance and the comfort
of the travel, wherever there is considerable change of grade, the
two grade lines should be connected by a vertical curve; and if
this is properly done, a break of grade in the block or elsewhere is
unobjectionable. A vertical curve should be inserted at a change
of grade either of the pavement or of the curb.
By breaking grade in the block, it is possible to fit the grade
line more closely to the natural surface, and thereby to decrease the
cost of construction, to lessen the damage to abutting property, and
to improve the general appearance of the street.
691. A parabola is the best form for a vertical curve and is
most easily put in. In Fig. Ill, AB and AC represent two grade
Fio. 111. — VERTICAL CURVE.
lines meeting in the apex A, joined by the vertical parabola B C,
which is tangent to the straight grade line at B and C. The curve
may be located by measuring ordinates vertically below the points
1, 2, 3, etc. The tangent distances A B and A C are equal. D E
is equal to the rise in half the length of the curve, i. e., from B to
A] and D C is equal to the fall in the second half, i. e., from A to C.
If n represents the number of equidistant points to be established
on the curve (including the second tangent point, C), then the
ordinate at the first point
x =
• The ordinate at
any other point is equal to x times the square of the number of
equal divisions between B and that point; that is, the ordinate
from 2 is 4z, from 3 is 9x, from 4 is 16x, etc. In actual work, the
STREET GRADES 355
grade elevation of the points 1, 2, 3, etc., are to be worked out in
the usual manner; from these elevations subtract the ordinates as
computed above, and the remainder is the grade elevation of the
respective points on the parabola B C. The agreement of the eleva-
tion of the last point on the curve, 6 in Fig. Ill, with the point C
on the tangent, checks the work of computing the elevations.
If the second tangent, A C, is level, D C in the above value
for x is 0; and if the second tangent has an up grade, D C is minus,
and the numerator = D E — D C. If the first tangent is level
D E = 0; and if the first tangent has a down grade, D E is minus,
and the numerator = D C — D E. The principles deduced for
Fig. Ill are equally true, if that diagram be turned upside down.
To secure the best results, there should be 15 feet of curve for
each 1 per cent of change of grade, although 10 feet for each 1 per
cent will give fair results. Long vertical curves make a graceful
street. The effect of any proposed curve in lowering (or raising)
the apex can be judged of beforehand by remembering that the
distance from the apex A, Fig. Ill, to the curve is equal to half of
the difference in elevation between A and the mean of the elevations
of B and C.
692. CROWN OF PAVEMENT. The only reason for crowning a
pavement, i. e., for making the center higher than the sides, is to
afford surface drainage; and therefore the proper crown to be given
to pavements will be considered under the head of Street Drainage
—see Chapter XIII.
To make intelligible the discussion of the succeeding section,
it is necessary to state here that in general the surface of the pave-
ment consists either of two planes meeting at or near the center, or
of a flat convex curve, usually the latter; and for present purposes
it is sufficient to say that the average transverse slope is usually
between 1 and 3 per cent (see § 720-24). The smoother the pave-
ment and the better the construction, the less should be the crown.
693. CROSS SECTION OF SIDE-HILL STREETS. The arrange-
ment of the cross section of a street upon a side hill is a matter
requiring good judgment, that needless damage may not be done to
the abutting property or that the general appearance of the street
may not be uselessly sacrificed. In solving this problem no fixed
rules can be laid down; but each case must be treated by itself,
taking into account the local conditions. Fig. 112 shows the normal
arrangement for a residence street on level ground; both footways
are at the same elevation, the slope of the parking is the same on
356 STREET DESIGN [CHAP. XII
the two sides, the tops of the curbs are at the same level, the gutters
are of the same depth, and the surface of the street rises equally from
each side to the center. The normal section for a business street
66ft
soft --------
FIG. 112. — CROSS SECTION OF STREET ON LEVEL GROUND
would be the same except that the sidewalk would occupy all of the
space between the curb and the building line. On a side-hill street
the above conditions can not always be realized; and various expe-
dients must be resorted to, depending upon the difference in eleva-
tion of the two sides of the street. The following are some of the
common expedients.
1. If the difference is not very great, the curbs may be set at
the same level, and one sidewalk may be placed higher than the other,
the grade of the parking being different on the two sides. On a
business street, where there is no parking, the slope of the footway
may be different on the two sides. With sidewalks consisting of
stone slabs, cement, or asphalt, a slope of at least i of an inch per
foot (1 in 96) is required for drainage; and a slope of more than f
of an inch per foot (1 in 32) is dangerous when covered with ice or
snow.
2. A slight difference of level may be overcome by raising the
curb, i. e., by increasing the depth of the gutter, on the high side,
and lowering the curb on the low side, the crown of the pavement
remaining symmetrical about the longitudinal center line. Fig. 113
tfo//r i m Roadway
- . - -L^ -.--.-
FIG. 113. — CROSS SECTION OF SIDE-HILL STREET.
shows an actual section of a street arranged on this plan.* Except
under extreme conditions, the curb should not show more than 10
inches because of the difficulty of stepping to or from the pavement,
nor less than three inches because of the danger of its being over-
flowed when the gutter is full of melting snow.
* Trans. Amer. Soc. of Civil Engineers, Vol. 42, p. 5.
CROSS SECTION OF SIDE-HILL STREETS 357
Sometimes a double curb is employed with a horizontal tread
about 1 foot wide between the two risers. The combined con-
crete curb and gutter (§ 737) lends itself most readily to this form
of construction. Fig. 114 shows such an arrangement.* The objec-
Walk
fr- i* ~%r- 20; _ ^. _ zo> nW- »z; _ -4
Fia. 114. — DOUBLE CURB FOB SIDE-HILL STHEET.
tions to the double curb are: 1, its cost; 2, the difficulty of keeping
the step neat and sanitary; and 3, it lessens the width available for
roadway and sidewalk. In practice these objections have not proved
to be serious. Instead of the double curb, it has been proposed to
place the second step at the area line or property line, to which
arrangement, particularly on a business street, the owner is liable to
object.
3. A slight difference may also be overcome by making the upper
side of the pavement nearly level and giving the lower half the normal
slope.
4. The crown may be moved toward the high side of the street,
the profile for each side being determined in the usual way; that is,
the surface of the pavement may be two planes meeting at the
ffoacttYay *
66._.
Fio. 115. — CROSS SECTION OF STREET ON A SIDE HILL.
crown with the intersection rounded off a little, or it may be two
arcs of a circle or a parabola tangent to a horizontal line at the
high point (see § 717 and § 718). Fig. 115 is an actual example
of this method of solution.* If the longitudinal grade is consid-
FIG. 116. — CROSS SECTION OF STREET ON A SIDE HILL.
erable, as it usually is under such circumstances, there is no objec-
tion to the upper side of the street's being exactly level transversely.
The extreme of this solution is to make the surface of the pave-
ment a right line from the upper to the lower side — see Fig. 116.
* Trans. Amer. Soc. of Civil Engineers, Vol. 42, p. 5.
358
STREET DESIGN
[CHAP. XII
This arrangement has been objected to on account of its throw-
ing all of the drainage to one side of the street; but this is not a
serious objection, particularly if there is a considerable longitudinal
grade, as usually there is.
5. Where there is a considerable difference of elevation on a
residence street, it is sometimes wise to place the footway next
to the curb, and to allow the slope of the parking to unite with that
of the property — see Fig. 117.
FIG. 117. — CROSS SECTION OP SIDE-HILL STREET.
6. When any or all of the above solutions fail, it may be neces-
sary to terrace the street and to construct an upper and a lower
roadway as shown in Fig. 118. For an example of the application
of this method of treatment and some other interesting features,
see § 666.
694. When the street contains one or more street-car tracks,
the problem of arranging a cross section on the side of a hill is still
more complicated. It is necessary that the two sides of a track
shall be at least nearly on the same level; but it is not necessary
that the two tracks shall be at the same elevation. A difference in
elevation of f of an inch between rails of the same track and of 3
inches between adjoining tracks is permissible.
Fia. 118. — CROSS SECTION OF SIDE-HILL STREET.
696. STREET TREES. It is always desirable both for the shade
and for the appearance to have residence streets lined with trees on
each side. Although trees in the streets have an important sanitary
and aesthetic value, opinions differ regarding the proper responsi-
bility for them. One view vests all right and title to the tree in the
owner of the property before which it stands; and the other asserts
that the trees belong to the city at large, and that the individual
STREET TREES 359
has no more right to the tree in front of his property than has any
other citizen. In the first case, the planting of the tree, its kind,
position, and care depend upon the public spirit of the property
holder; and as a result the street presents a motley, straggling
appearance often with no trees where they are most needed for the
best general effect. Without some degree of public control, it is
impossible even to approximate the best results of tree planting;
but fortunately the number of cities in which the street trees are
under the control of the municipal authorities is rapidly increasing.
In planning a system of streets, the location of the trees should
be definitely provided for. They should be located in the grass
plats between the sidewalk and the edge of the pavement, and
at a sufficient distance from both the sidewalk and the pavement
that there will be no danger of the roots lifting either. The trees
should be spaced in the row so as to permit each when fully grown
to spread to its natural dimensions, which usually requires a space,
center to center, of 25 to 40 feet. Not infrequently trees are planted
much too close — particularly in the fertile and originally treeless
prairies of the Mississippi Valley; — and crowd each other and pre-
vent a symmetrical growth. In planting trees, it is well to alternate
those of rapid growth with those which mature more slowly; and
then as the latter increase in size and demand more room, the former
having served their temporary purpose, can be removed. Increased
stateliness, impressivene'ss, and charm is secured if the trees, at least
the permanent ones, on any one thoroughfare are of one variety.
Different streets can have different kinds of trees, since in nearly all
cities there is a large number of suitable varieties available.
697. In most states there are one or more cities that have ob-
tained— either officially or by volunteer civic-improvement societies
— valuable experience as to the varieties best suited to the environ-
ment, from whom data can doubtless be obtained by those desiring
information concerning the kind of trees to plant in the streets of
any particular city.
698. The following are the requirements for a street tree adopted
by a commission of experts for Washington City.* " 1. A some-
what compact stateliness and symmetry of growth, as distinguished
from a low spreading or pendant form, so that the stem may reach
a sufficient height to allow free circulation of air below the branches.
2. An ample supply of expansive foliage of bright early spring
* Proc? Amer, SPQ, Municipal Improvements, Vol. 5, p. 97,
360 STKEET DESIGN [CHAP. XII
verdure, and rich in the variety of colors and tints assumed during
autumn. 3. Healthiness, so far as being exempt from constitu-
tional diseases, as well as from maladies frequently engendered by
peculiarities of soil and atmosphere impurities. 4. Cleanliness,
characterized by a persistency of foliage during the summer, freedom
from falling flowers, and exemption from the attacks of noxious
insects. 5. It should be easily transplanted, of moderately vigorous
growth, and not inclined to throw up shoots from the root or lower
portion of the stem. A tree of extremely rapid growth is generally
short-lived. 6. The branches should be elastic rather than brittle,
that they may withstand heavy storms; and lastly, there should be
no offensive odor from foliage or flowers."
Of course, no tree planted amid the artificial conditions found
in a large city will fully meet such rigid requirements. In 1872,
at the commencement of systematic tree planting, the above com-
mission recommended the following list of trees. The Silver Maple
(Acer dasycarpum), the American Linden (Tilia americana), the
European Sycamore Maple (Acer pseudo-platanus) and the Amer-
ican Elm (Ulmus americana) are thought to fill all the above require-
ments when not subjected to the attacks of insects. The Tulip
Tree (Liriodendron tulipfera), Sugar Maple (Acer saccharinum) ,
Sweet Gum (Liquidamber styraciflua), and the Red Maple (Acer
rubrum) are the most beautiful of trees, their only drawback being
that of not growing freely after transplanting. The Norway Maple
(Acer platanoides), the Negundo (Acer negundo), and the American
Ash (Fraximus americana) are recommended for certain places. The
Button-woods or Planes (Platanus occidentalis and Platanus orien-
talis) are rapid growing, and for wide avenues are effective trees.
As a result of twenty-five years' experience, the trees are ranked
as follows: " Silver Maple, Norway Maple, and Eastern Plane side
by side in the first rank; then the Ginkgo, and Western Plane;
and last American Linden, Oak, and Sugar Maple."
CHAPTER XIII
STREET DRAINAGE
701. The thorough drainage of a street involves four elements:
(1) the surface drainage, (2) the gutters, (3) the catch basins, and
(4) the underdrainage. They will be considered in the reverse order.
702. UNDERDRAINAGE. The underdrainage of a street is the
first step toward paving it. Without thorough subdrainage a pave-
ment is likely to settle here and there, forming unsightly depressions
on the surface, and possibly breaking through. The subsoil may be
drained by one or more lines of porous tile as described in § 114-24;
but as a rule the surface and underground waters are both collected
in the same drain, and therefore it is advisable to lay a line of tile
at each side of the street or to construct a larger conduit under the
center of the street. Since the pavement is practically impervious
to water, a third line of tile under the middle of the pavement is
unnecessary, however wet and retentive the soil originally.
If there is a grass plat between the pavement and sidewalk, as
is usual on residence streets, the tile should be laid under the outer
edge of the parking or grass plat; and if there is no parking, the tile
should be laid under the gutter. The deeper the tile the better the
drainage and the less the liability of its becoming choked with tree
roots. The tile should not be too small since it is to carry both under-
ground and surface water — the latter from a smooth and impervious
pavement.
The formula for size of tile for the drainage of earth roads
(§ 119) is worthless for pavements, since in cities a large propor-
tion of the rain falls upon impervious roofs, pavements, sidewalks,
etc., and nearly all speedily reaches the storm- water sewers. This
subject has been very carefully studied in connection with the
design of sewers, and the reader is referred to treatises on that sub-
ject, for further information concerning the size of drains or storm-
water sewers required.
361
362 STREET DRAINAGE [CHAP. XIII
703. CATCH BASINS. The catch basin is a pit to receive the
drainage from the surface of the street, in which is deposited the
sand and other solid matter, and from which the water is discharged
into the sewer or storm-water drain. A catch basin should fulfill
the following conditions: (1) The inlet should offer the least possible
obstruction to travel, should have sufficient capacity to pass speedily
all the water reaching it, and should not easily be choked by leaves,
paper, straw, etc. (2) The capacity below the outlet should be
sufficient to retain all sand and road detritus, and thus prevent it
from reaching the sewer; and will depend upon the area drained and
the intervals between cleanings. (3) The water level should be low
enough to prevent freezing. (4) The construction should be such
that the pit may be easily cleaned out. (5) The pipe connecting
the basin with the sewer should have sufficient capacity, and should
be so constructed as to be easily freed of any obstruction. (6)
It is desirable that the outlet should be trapped so as to prevent
floating debris from reaching the sewer. (7) If the catch basin
discharges into a sewer which also carries house sewage, the end of
the outlet pipe should be trapped to prevent the escape of air from
the sewer to the street through the catch basin.
704. The Construction. Catch basins are usually built of brick
masonry, and plastered on the inside, at least up to the water line.
Fig. 119, page 363, the standard of Champaign, 111., is a good form.
The opening of the inlet is protected by six half-inch iron rods.
The several parts of the cast iron top are f and \ inch thick; and
the total weight of the castings is 162 pounds. The pit requires
1,000 brick. The total cost of the catch basin when laid in 1 to 3
cement mortar is $17.00 to $19.00, including castings, excavation,
and the vitrified elbow.
Fig. 120, page 364, shows the standard catch basin of Providence,
R. I.* This form differs from that shown in Fig. 119 in the form
of the inlet and of the trap for the outlet. The latter is made of
iron cast in a single piece, and is somewhat complicated in form, but
a careful study of the two views shown in Fig. 120 will make the
construction reasonably clear. The seal in Fig. 120 is better than
that in Fig. 119; but the latter is used only with storm-water sewers
and for such use this trap is sufficient. Not infrequently, how-
ever, the outlet of the catch basin is left untrapped; and some-
times an inlet is connected to a sewer without the intervention
* By courtesy of Otis F. Clapp, City Engineer
CATCH BASINS
363
of either a catch basin or a trap. This practice is likely to clog the
sewer.
Fig. 121, page 365, is the standard for Milwaukee, Wis.* This
diagram is presented to show (1) the form of the inlet, (2) the method
Plcmof Casting
Fia. 119. — CHAMPAIGN CATCH BASIN.
of preventing floating debris from entering the outlet, and (3) the
method of ventilating the sewer.
Fig. 122, page 365, shows the standard form in St. Pancras
Vestry, London, England, f
In England many earthenware catch basins or " gully pits "
* By courtesy of C. J. Poetsch, City Engineer.
t From a special report by William Nisbet Blair, Vestry Engineer.
364
STREET DRAINAGE
[CHAP. XIII
are used. Some of these forms are quite complicated. American
engineers object to earthenware pits on account of (1) their limited
//////// /X/r/ae s fane ///
\ — 5'ilO'- —
R.AN' WITHOUT MANHOLE FRAME
Fia. 120. — PROVIDENCE CATCH BABIN.
size, (2) their great cost, and (3) their liability to be broken by the
weight and jar of the street traffic.
705. Location. The catch basin is usually placed near the curb
with the cover in the sidewalk or the parking. It is objectionable
to have the cover in the sidewalk, since (1) the cover itself is some-
thing of an obstruction to travel and is dangerous when it wears
smooth or is covered with snow, (2) the clearing of the pit seriously
interferes with the convenient use of the footway, and (3) in empty-
ing the pit the sludge is likely to be spilled on the footway, and at
best the odor is offensive. In some cities these objections are elim-
inated by placing the inlet at the curb line and conducting the drain-
age to a catch basin near the center of the street, one basin serving
CATCH BASINS
365
for two or more inlets. Notice that the catch basin shov/n in Fig.
122 cleans out in the gutter.
pavement...
FIG. 121. — MILWAUKEE CATCH BASIN.
It is customary to place a catch basin at the corner of the curb.
For additional objections to this location, see § 712.
The number and capacity of catch basins will depend upon the
Curb
WaferLine
Outlet
P/an
FIG. 122. — LONDON CATCH BASIN.
area drained, the amount of rain, the grade of the gutter, etc. On
streets having light or level longitudinal grades, catch basins may
be constructed at intervals along the gutter as the circumstances
require.
366 STREET DRAINAGE [CHAP. XIII
706. Form of Cover. When a catch basin or sewer manhole
is located in a pavement, the shape and the surface of the cover
require attention. The upper surface of the cover and also of its
frame should be covered with projections to afford a good foothold
and to prevent it from wearing slippery. The best form for the
frame depends upon the material of the pavement. For macadam
and asphalt the round frame is best, since it offers least obstruction
to travel; the next best form is a square frame set diagonally to the
line of travel. For a pavement rrade of bricks or stone blocks, the
frame set with its sides parallel to the length of the street is beet,
because the bricks or blocks can be most closely fitted against this
form. In Europe and in many American cities, it is customary to
use only a square form, and to set it diagonally in macadam and
asphalt pavements, and square in stone block and brick.
Often water-gate or stop-box covers are round in plan and have
a convex surface, although the convex surface is very oljectionable.
The better form is a cover round in plan with a flat recessed top
set flush with the pavement. Preferably the portion below the
ground should be provided with a cast screw for adjusting the height.,
This form may be had of dealers in street-drainage goods.
707. The Inlet. In a general way, there are stone and cast-iron
inlets. The former consist either of an opening between a store
cover and a stone floor, or a slot through the stone curb (see Fig. 120,
page 364). This form is usually entirely open, but it is sometin es
barred with one or two horizontal iron rods.
There is a great variety of cast iron inlets on the market, which
may be classified as being straight or curved, and also as having a
vertical or a horizontal opening. Fig. 123 shows an unprotected
straight vertical inlet. Sometimes the opening is protected by
FIG. 123. FIG. 124.
one or more horizontal or vertical rods. The latter are the
better, as they offer greater protection against the entrance of
debris — particularly sticks and boards. Fig. 124 shows a vertical
front curved for a corner, having vertical bars. Fig. 125 and 126
are two styles of a form having both a vertical and a horizontal
CATCH BASINS
367
opening. Notice that Fig. 122, page 365, has only a horizontal open-
ing. A horizontal opening is not so good as a vertical one, since the
former is easily stopped by a few leaves, and the accumulation of
FIG. 125.
FIG. 126.
water makes the stoppage more complete; while the barred vertical
opening is less easily obstructed, and as the water rises it can pour
over the obstruction already formed.
708. Inlet without Catch Basin. It is sometimes desirable to
connect two or more inlets to one catch basin — for example, see
§ 713. There are various forms of such inlets on the market and
many cities have their own special designs. Fig. 127, page 368,
shows the form of inlet used in such a case at Omaha, Nebraska.
The entrance A is reduced by cast ribs to three openings 6X9
inches at the top and 4f X 2 inches openings at the bottom. The
section B is rectangular in plan at both top and bottom. The
section C is rectangular at the top and circular at the bottom, and
fits into the hub of a vitrified elbow. Fig. 128, page 368, shows an-
other form of curb inlet without catch basin. Fig. 129, page 369,
shows a commercial form of inlet, which has an adjustable curb.
It is made to fit various sizes of outlet pipe.
709. GUTTERS. The Material. Ordinarily the surface of the
pavement adjacent to the curb serves as a channel to convey the
drainage to the nearest inlet, i. e., the gutter is formed of the same
material as the pavement. With an asphalt or macadam pave-
ment, it is customary to lay brick or stone blocks in the gutters —
with asphalt to prevent its deterioration from being continually
covered with mud and water, and with water-bound macadam to
prevent flowing water from disintegrating it.
A combined concrete curb and gutter (§ 737) is frequently used,
particularly with asphalt, brick, or macadam on residence streets.
A concrete gutter is objectionable on a macadamized street, on
account of the crushed stone's wearing below the edge of the gutter,
368
STREET DRAINAGE
[CHAP. XIII
a condition which interferes with the drainage; but if the macadam
surface is reasonably well cared for, this objection is not serious. A
FIG. 127. — OMAHA INLET WITHOUT CATCH BASIN.
concrete gutter has been objected to for any pavement owing to
the liability of a rut to form along its outer edge. In practice neither
of these objections has proved to be serious. A concrete gutter is
more efficient and looks better than one of any other available
material except asphalt (see last paragraph of § 710).
FIG. 128. — CURB INLET, CHAMPAIGN, ILL.
Usually the gutter is formed by continuing the ordinary slope
of the pavement until it intersects the curb; but occasionally the
CATCH BASINS
369
outer edge of the pavement is given an upward inclination, thus form-
ing a flat V-shaped channel a little way from the curb. This con-
struction makes an excellent channel for the water, but prevents
the driving of a carriage close enough to the curb to allow people
to step in or out easily.
In some cases the curb is set and the gutter formed before the
pavement is laid, in which case the curb and gutter are constructed
Fio. 129. — COMMERCIAL INLET WITHOUT CATCH BASIN.
as they would have been if the street were to be paved, — the gutter
being composed of stone blocks, bricks, or concrete (§ 709). Some-
times a street is macadamized or graveled when it is not desired to
incur the expense of setting a curb, in which case the gutter is built
of cobble stones, or stone blocks, or bricks, in the form of a very flat
V with the side next the property much the steeper.
710. Depth. Where a curb is used, the gutter should not be so
deep as to present a high step for pedestrians, nor so shallow as to
be in danger of being overflowed. Not infrequently gutters are made
needlessly deep. It is easier to keep a curb in line with a shallow gut-
ter than a deep one. On streets having a considerable longitudinal
grade the gutter can have a uniform depth, inlets being inserted
to draw off the surplus water; but on streets having nearly level
grades, the gutter must increase in depth as the inlet is approached.
This can be done easily with a stone curb, but not so easily with a
combination concrete curb and gutter (§ 737), since the latter is
usually made in moulds having a uniform cross section; and there-
370 STKEET DRAINAGE [CHAP. XIII
fore with a concrete curb and gutter, it may be necessary to put a
summit in the pavement to secure proper drainage of the gutters.
Except in extreme cases, the gutter should not be deeper than 9 inches
nor shallower than 3 inches; and ordinarily it should not be more
than 8 nor less than 4 inches — usually it is 5 or 6 inches.
It may be necessary to modify the preceding rules when one
side of the street is higher than the other (see § 693). In localities
where there is a good deal of snow, the gutter must be deeper than
stated above, for shallow gutters readily become clogged with snow
and slush. In some northern cities, the snow is habitually allowed
to pack upon the surface of the street to a depth of 6 or more inches,
in which places the depth of the curb must be extremely deep to
prevent the melting snow and water from filling the gutter and
flowing over the sidewalk into the basements.
711. Grade. For most materials with which gutters are paved,
it is improbable that the grade will be so steep as to do serious harm.
Crushed stone and gravel are exceptions to this rule, however, and
these materials must not be laid on too steep a grade. They may be
used on a 2 per cent grade provided the volume of water is not too
great.
The minimum grade permissible in the gutter will depend chiefly
upon the material with which it is paved, but somewhat upon the
cost of catch basins. Almost any grade can be obtained by estab-
lishing catch basins close together and raising the gutter half way
between them. In a number of cities the minimum grade of gutters
paved with granite blocks, bricks, rectangular wood blocks, or mac-
adam is 1 in 300 or 400. Except under very favorable circumstances,
a slope of 1 in 200 (£ of 1 per cent) should be regarded as the minimum.
Asphalt decays if continually wet, and therefore the condition
governing the minimum permissible grade is different for that than
or Other materials. With a slope of less than 1 per cent, the gutter
will not keep itself clean, consequently the asphalt will decay owing
to the action of mud and water; and hence asphalt should not
be laid in a gutter having a fall of less than 1 in 100. If this fall can
not be obtained, a concrete gutter should be used, or the gutter
should be paved with vitrified brick or carefully dressed granite
blocks.
712. Drainage at Street Intersection. In most cities it is cus-
tomary to construct catch basins at the corner of the curb, using
an inlet with a curved face. This practice is very objectionable.
If the walk across the street is elevated above the pavement, it
GUTTERS
371
is necessary either to carry the water under the walk in a pipe, or to
stop the cross walk within a short distance of the curb to leave a
channel for the water. The latter method is necessary where there
is much water. Frequently this channel is left open at the top, and
sometimes it is covered with a cast iron plate with one edge resting
in a rabbet in the curb and the opposite one in a head stone or false
curb set at the end of the cross walk. The covered gutter is much
better than the open one, although the cast plates are frequently
struck by wheels and broken, and often get displaced. This solution
of the problem is further objectionable since a wheel in turning the
corner must surmount the first raised cross walk, then descend to
the bottom cf the gutter, and finally climb over the second cross walk.
The face of the inlet usually has a depth of 8 to 12 inches below the
top of the curb; and hence if the sidewalks are wide or the parking
is narrow, the shock to a vehicle going around such a corner is con-
siderable.
If the cross walk is not elevated, the step from the curb to the
bottom of the gutter is uncomfortably high, and besides pedestrians
are compelled to cross the gutter where there is the most water.
713. A much better arrangement than either of the above is
to place an inlet at each side of the corner. Each inlet may have
its own catch basin, or the two
Curh
,-^Cafch basin
Parking
Wa/H
Private
Property
may connect with a single pit
by means of tile or vitrified pipe
underground. Fig. 130, page
374, shows such an arrange-
ment. Instead of this plan,
the two inlets at each of the
four corners of the street in-
tersection may be connected
with a single catch basin
placed in the middle of the
intersection or in other suit-
able location. The inlet not
connected directly with a
catch basin can be made by inserting the hub of a curved vitrified
pipe in the bottom of a cast inlet-box (see Fig. 127, 128, and 129).
The advantage of the method shown in Fig. 130 is that it allows
the intersection to be paved almost level with the top of the curb,
and hence there is no obstruction to either pedestrian or vehicular
travel. The only objection to it is the expense for either the extra
FIG.* 130. — INLETS AT STREET CORNER.
372
STREET DRAINAGE
[CHAP, xin
catch basin or the extra inlet and connecting pipe, but the advan-
tage is well worth this comparatively small cost.
714. Where there are no storrn-water sewers, the gutter is some-
times carried across the street intersection. This is objectionable
at any season, and particularly so when the gutter is filled with
snow or ice. If the gutter is deep or the grade is steep, the water
may be carried under the intersection by a shallow culvert with
cast iron top, or better in a cast iron pipe; but if the gutter is shallow
or the grade nearly level, the road surface should be raised a little
to give room for a cast iron storm-water drain under the road-
way. The elevated intersection may be a slight obstruction to
travel, but it is preferable to two open gutters.
715. Elevated Foot-way Crossing. To aid pedestrians in cross-
ing the water in the gutter, it was formerly the practice to raise the
pavement in the line of the crossing so that the surface of the foot-way
J.B..
SECTION A-B
SECTION C-D
(ot Hie centre)
SECTION E-F
PIG. 131. — ELEVATED BRICK CROSSING.
was level from the crown of the carriage-way pavement to the
top of the curb, and leave a channel next to the curb which was
either left open or bridged with a cast iron plate. Fig. 131
shows the details of an elevated brick crossing. Notice that
Fig. 131 has a limestone curb and a brick gutter. Fig. 132 and 133
show the gutter at the end of an elevated brick crossing when a con-
SURFACE DRAINAGE
373
crete curb and gutter is employed. The chief difference between
Fig. 132 and 133 is in the form of the false curb or head stone on the
side of the gutter toward the center of the street. The difference
in the merits of the two methods is mainly in the cost, Fig. 133
usually being slightly the cheaper. In both cases there is a drop of
1 inch in the width of the cast iron bridge plate. Of course, the
crossing could be carried level from gutter to gutter, or more drop
could be put into the gutter plate.
It has always been recognized that as far as the use of the car-
riage-way pavement is concerned, an elevated crossing is undesir-
15'xi'CI. Gutter Plate
FIG. 132. — GUTTER FOR ELEVATED BRICK
CROSSING WITH CONCRETE FALSE CURB.
FIG. 133. — GUTTER FOR ELEVATED BRICK
CROSSING WITH LIMESTONE FALSE CURB.
able, particularly where the pavement is used by a large number of
vehicles or where there is considerable rapid travel; but since
the introduction of the automobile, the elevated crossing is very
undesirable. The elevated crossing is a serious obstruction also to
vehicles rounding the corner; and besides the cast iron crossing
plates are easily displaced and are frequently broken. The elevated
crossing should not be used.
716. SURFACE DRAINAGE. The drainage of the surface of the
pavement is provided for by making the center of the pavement
higher than the sides. The principle governing the amount of crown
for pavements is somewhat different from that of earth, gravel, or
water-bound macadam roads. First, a hard, smooth and practically
impervious pavement needs no crown for the drainage of the surface;
and on such a pavement, the only advantage of a transverse slope is to
drain shallow depressions due to faulty construction, wear, or a set-
tlement of the foundation, and to aid the rain in washing the pave-
ments. Second, the surface of the pavement has no tendency to
wash; and hence the crown need not be increased on a grade as in
374 STREET DRAINAGE [CHAP. XIII
the case of earth roads. The less the crown the better for travel,
and the more uniformly will the travel be distributed over the pave-
ment, although a slight crown is inappreciable in either of these
respects. Therefore pavements require only crown enough to drain
depressions of the surface due to faulty construction, to wear, or
to settlement of the foundation; and the crown may decrease as
the grade increases.
717. Crown. There has been much discussion as to the best form
of the surface of a pavement. Some claim that it should be a con-
tinuous curve, while others contend that it should consist of two planes
meeting in the center. The curved profile is defective in that it gives
too little inclination near the middle, the result being that the pave-
ment wears hollow in the center and permits water to stand there.
To overcome this objection some engineers raise the center of the
pavement ^ or -f of an inch above the curved cross section. The
objection to the two planes is that the sides wear hollow and hold
water. An advantage of the curved profile is that the center of the
street, which is the part especially devoted to travel, is nearly flat;
while the sides, which have the greater inclination, are occupied by
teams standing at the curb. Another advantage of the curved
profile is that it gives a deeper gutter, which confines the storm
water to a smaller portion of the street and reduces the interfer-
ence with pedestrian travel.
It is sometimes claimed that the curved form will support the
greater load, because of its arch action; but the arch action of a
pavement is entirely inappreciable, owing to the flatness of the arch,
to the imperfect fit of the so-called arch stones, and to the insta-
bility of the abutments or curbs.
The surface is usually a continuous curve — generally a parabola.
718. To Lay out a Parabolic Crown. In Fig. 134 the curved line
C B represents the surface of the finished pavement. C is the center
of the pavement; and C D = A B = the amount of the crown.
FIG. 134. — PARABOLIC CROWNED PAVEMENT.
To find the distance from the line A C down to the curved line C B,
divide the half width of roadway, A C, into any number of equal
SURFACE DRAINAGE
375
parts, say n, and designate the distance from the point 1 on A C
vertically down to B C by x; then by the principles of the parabola,
, and the distance from point 2 down to the road surface
x =
n2
is 22 x or 4 x, and the distance from 3 is 32 x or 9 x. In practice a
string with knots in it to represent the points of division of A C is
stretched from the top of the curbs, and then the ordinates computed
as above are measured with a pocket rule.
719. When construction begins, it is wise to give the one in
charge of the work a drawing somewhat like Fig. 135, showing
Sub-grade
F
Concrete
FIG. 135. — METHOD OF SHOWING CROWN OF PAVEMENT.
the relation between the top of the curbs and the cross section of the
subgrade, the top of the concrete, and the top of the finished pave-
ment. Such a drawing prevents misunderstandings and disputes.
Notice that the curves in Fig. 135 are not exact parabolas, the
ordinates at 4 and 12 being | inch too long; but this is sufficiently
exact, since it is not possible to secure mathematical precision in this
class of work.
720. Rules for Amount of Crown. The practice of different
cities is not at all uniform as to the amount of crown. Numerous
empirical formulas have been proposed for the crown of pavements;
but there is not much harmony between them.*
721. Washington Formula. Since 1894 the Engineering Depart-
ment of the District of Columbia has employed the following
formula: C = w(100 - 4p) -5- (6300 + 50p2), in which C = the
For a list of many such formulas, see Engineering News, Vol. 63 (1910), p. 516-18.
376 STREET DRAINAGE [CHAP. XIII
crown in feet, w = the distance between the curbs in feet, and p
is the longitudinal grade of the pavement in percentage. Notice
that the crown decreases as the longitudinal grade increases.
722. Rosewater Formulas. Apparently the formula proposed in
1902 by Andrew Rosewater, then City Engineer of Omaha, is most
frequently used. The latest Rosewater formula is as follows : "The
crown for asphalt is: C = w (100 — 4p) -^ 5,000, in which the nomen-
clature is as in the section next above. For brick, stone block, or
wood block, the crown is five sixths of that for saphalt." A formula
for crown formerly used in Omaha gave a less crown than the above
rule for brick, stone block, and wood block, and a much less crown
for asphalt. The former formula is :
for brick, stone block, and wood block, C = (20 - p) ^ 1,600
for sheet asphalt C = (9 - p) + 600.
The latter formulas are still used by many cities.
Notice that in the preceding rules the crown is decreased as the
steepness of the longitudinal grade increases, which is proper. Also
notice that according to these rules, the crown of sheet asphalt is
more than that of the other kinds of pavements mentioned, which is
contrary to the practice of many cities. Considering only the smooth-
ness of the surface, it appears that asphalt should have the least
crown; but considering only the fact that asphalt rots when con-
tinually wet, it appears that asphalt should have a large crown.
723. The above rules for crown must be modified somewhat
when the two sides of the street are not at the same elevation —
see § 693, page 357.
724. Recommendations of A.8.C.E. Committee. For the crown
recommended for various road and pavement surfaces by a
committee of the American Society of Civil Engineers in 1917, see
Table 16, page 65.
725. Dished Pavements. The early pavements in this country
and at present those in some cities in Europe and South America,
slope from both sides towards the center. In this form the most
valuable part of the street is devoted to drainage purposes, and it is
difficult to carry the water to an intersecting street.* The pave-
ments of alleys usually slope to the center. This form is better for
* The single-gutter street pavement was ably advocated by W. G. Kirchoff in a paper before
the Wisconsin League of Municipalities — See Engineering and Contracting, Vol. 44 (1915),
p. 190-91.
SURFACE DRAINAGE 377
alleys than a gutter at each side, since it keeps the storm water from
flowing along the side of buildings and possibly interfering with light
areas, cellar stairways, etc., and it also carries the water over the
sidewalk with less annoyance to pedestrian travel.
CHAPTER XIV
CURBS AND GUTTERS
728. CURB. A curb is a plank or slab of stone set at the edge
of the roadway to protect the sidewalk or tree space and to form
the side of the gutter. Curbs are not usually set except where the
street is paved, but they greatly improve the appearance of an
unpaved street and protect the grass plats at the side of the street,
particularly during the muddy season.
Curbs were formerly made of natural stone, but concrete curbs,
usually combined concrete curb and gutter, are increasing very
rapidly in recent years — chiefly because of the decrease in the price
of portland cement. Natural stone is used now only in the vicinity
of quarries of suitable stone. Granite is the best natural stone, but
it is usually very expensive. Limestone and sandstone are fre-
quently used, but they are generally too easily chipped or broken.
Concrete unless made with unusual care or protected by steel on
the edge, is too friable for a business street where heavy loads fre-
quently back up against the curb.
729. Stone Curb. Granite curbs are obtained in large quantities
in several states, notably Maine, New Hampshire, Massachusetts,
Connecticut, New Jersey, Pennsylvania, Georgia, Wisconsin, Mis-
souri, South Dakota, California. Husdon River bluestone, a variety
of sandstone commercially known as bluestone, is much used for
curbs, on account of its hardness, durability, and great transverse
strength. It Is evenly bedded, splits with a smooth surface, and is
found in large quantities in the counties of the state of New York
adjoining the Hudson River from Albany to New York City. The
sandstones most used for curbs are the following : A gray stone from
Berea, Ohio; a brownish red stone, known as Medina sandstone,
obtained in the State of New York on the shore of Lake Ontario; a
gray, yellow, brown or red stone from Potsdam, N. Y. ; a metamor-
phic sandstone from Sioux Falls, South Dakota, known as Sioux
378
STONE CURBS 379
Falls quartzite; and a light-pink stone from Sandstone, Minn.,
known as Kettle River sandstone.
730. The thickness should be sufficient to give strength to resist
the blows of wheels and to prevent the frost in the earth back of the
curb from breaking it off at the top of the gutter. The curb is
usually 4 to 6 inches, depending upon the quality of the stone and
the locality. The depth must be sufficient to prevent the thrust of
the earth behind the curb from overturning it, and is usually 18 to
24 inches. If the sections are too short, it is difficult to keep them
in place and the general appearance is not good; and if they are too
long, it is difficult to handle and set them, and nearly impossible to
get a firm bearing on the bottom. They usually vary from 3 to 8
feet in length.
The exposed face of the curb should be bush-hammered or axed;
and where the sidewalk extends to the curb, the back also should
be smoothly dressed so the sidewalk may. fit closely against the curb.
The upper face should be cut to a slight bevel with the front face,
say i-inch to the foot, so that when the face of the curb is set with a
little inclination backward, the top face will be level or slope down-
ward and to the front a trifle. The pavement slopes toward the
gutter, and therefore a wagon wheel inclines toward the curb; hence
the curb is set leaning back a little to prevent a wheel from striking
the face when running at the inner edge of the gutter and also to
secure increased stability. The curb is usually cut with a square
corner at the outer upper edge; but it would be better if this corner
were rounded off slightly, say to a radius equal to one third of the
thickness .of the curb, to decrease the tendency to chip. The ends
of the sections should be smoothly dressed to the exposed depth, and
the part not exposed should be knocked off so as to permit the dressed
ends to come into close contact. The ends should fit closely for
appearance and to prevent the earth, particularly if sand, from run-
ning from behind the curb between the sections into the gutter, or
to prevent the sand cushion of a brick pavement from running from
under the bricks into these cracks and possibly through them into
holes behind the curb. In a number of European cities, notably
Brussels, the curb is cut with a tongue in one piece which fits into a
groove in the next piece, to aid in keeping the curb to line.
The curb should be set with a uniform batter, in a straight line,
and on a regular grade. To fulfill these conditions requires careful
work in the first place, and to prevent the curb from subsequently
getting displaced requires proper design and thorough workman-
380 CURBS AND GUTTERS [CHAP. XIV
ship. The trench in which the curb is to be set should be dug 4 to
6 inches below the base of the curb to allow for a layer of gravel on
which to set the stone; and the width of the trench should be at
least three times the thickness of the curb to allow room for ram-
ming the earth around the stone. The bottom of the trench should
be made smooth and be thoroughly consolidated by ramming, and
the gravel also should be compacted. Where gravel is expensive,
it is dispensed with, the curb being set upon brick or stone. In
filling the trench, the earth should be thoroughly rammed in layers
not more than 4 inches thick. Where gravel is plentiful, it is some-
times specified that the trench shall be filled with gravel to 8 or 10
inches from the top.
In the past there has been so much trouble in keeping curbs in
line, that within recent years there has been a general tendency to
set the curb in a bed of concrete — particularly when concrete is
used for the foundation of the pavement. A 6-inch layer of con-
crete is deposited in the trench and the curb set upon it, after which
the trench is filled with concrete on the street side up to the base of
the proposed pavement and on the back side nearly up to the top of
the curb. When set in concrete, the curb does not need to be as
deep as otherwise, since the concrete then practically becomes a
part of the curb.
731. Owing to the difficulty of keeping stone curbs in line or
rather owing to the expense of setting them so they will certainly
stay in line, stone curbs are becoming much less common than
formerly. They are being replaced by concrete curbs or more fre-
quently by combined concrete curb and gutter (§ 737).
732. Cost. In most localities, split sandstone or limestone curb-
ing 4 to 6 inches thick can be had for 30 to 40 cents per square foot
f.o.b. cars at the destination; and often sawed stone can be had at
about the same price. The additional cost of a bush-hammered or
axed surface will vary with the hardness of the stone and the degree
of the finish, and curved sections will cost 30 to 50 per cent more than
straight pieces. Hudson River bluestone (sandstone) curbing 5
inches thick costs about 30 cents per square foot. Granite curb
costs from 40 to 50 cents per square foot, depending upon locality
and thickness.
For more definite information, see the price reports in current
technical journals.
733. Concrete Curb. In some sections where suitable stone for
curbing is not readily available, curbs have been made of portland
CONCRETE CURBS
381
cement- concrete. Owing to the decreasing price of cement, this
form of curb is coming into more common use. It is usually made
about 6 inches thick and 18 or 20 inches deep. If well made, it does
excellently for residence streets.
For suggestions concerning the construction of concrete curb,
see § 738-17.
734. The exposed corner of a concrete curb, particularly on a
business street, is sometimes protected by a steel angle or special
form which is anchored to the body of the concrete by lugs or a
special stem. Several of these forms are very efficient; but as they
are patented nothing more will be said here. For particulars con-
sult the advertising pages of technical journals.
735. Cost. Table 37, page 382, shows the cost, as determined
by 38 time studies, of the concrete curb shown in Fig. 136. It was
laid in sections 6 feet long with thin metal
partitions between. At first a 1 : 2 : 5 mix-
ture was used for the body, but later a
1 : 2i : 6; and the facing was 1 : 1| : 1|.
The proportions were accurately measured by
volumes. The concrete was mixed by hand
on a wood platform in the middle of the street.
As soon as the concrete had set sufficiently,
the front forms were removed and the face
of the concrete was scrubbed with steel
brushes when the concrete had set hard
enough to require them, but usually with
stiff-bristle brushes.
736. GUTTERS. Incidentally the construction of the gutter
has already been considered in § 709-11, which see.
Concrete*
FIG. 136. — CROSS SECTION
OF CURB.
Grave/
Macadam
Cinders*
Fio. 137. — ST. Louis CONCRETE GUTTER OR PARK DRIVE.
Fig. 137 shows a concrete gutter used in St. Louis, Mo., for park
drives,
382
CURBS AND GUTTERS
[CHAP, xiv
737. COMBINED CONCRETE CURB AND GUTTER. In recent
years the construction of combined concrete curb and gutter built
in place has become very general in the smaller cities, and in resi-
TABLE 37
COST OF CONCRETE CURB *
Superintendence and Over-head Expenses not Included
6
£
1*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Item
COST CTS. PER
LIN. FT.
Aver,
per
Amt.
of
Total.
FEET COMPLETED
PER MAN-HOUR.
Min.
Max.
Aver.
Min.
Max.
Aver.
Labor:
Foreman @ 37|c. per hr.
Water boy @ lOc.
Trenching @ 18c.
Placing forms @ 35c.
Removing forms @ 25c.
Concrete @ 18c. "
Facing @ 18c. "
Finishing @ 35c. "
Scrubbing @ 35c. "
Back-filling @ 18c.
Total for labor
1.1
0.4
2.2
1.0
0.5
2.6
0.9
1.8
0.4
0.3
3.9
1.3
9.4
10.4
5.2
9.5
2.9
10.7
3.4
2.0
2.2
0.6
4.9
4.2
1.8
4.3
1.7
4.8
1.7
1.3
4.7
1.3
10.5
9.0
3.8
9.2
3.6
10.3
3.6
2.8
1.9
2.5
2.0
1.2
6.5
3.7
5.0
5.9
9.6
26.4
38.0
6.8
20.4
20.4
48.1
51.0
'«v
4.1
7.2
7.6
3.2
10.6
8.3
11.4
14.1
11.2
6.2
1.2
6.0
0.4
1.0
1.5
16.3
58.7
8.9
1.5
7.7
0.7
1.0
2.0
21.8
27.5
7.6
1.4
6.9
0.6
1.0
1.8
58.8
16.2
3.0
14.7
1.3
2.1
3.9
Materials:
Cement @ $1.20 per bbl.
Sand @ 0.75 per cu. yd.. ....
Gravel @ 1.90 " " "
Facing @ 1.75 " " "
Lumber, 1^-inch spruce
Water, waste, etc.
Total for materials
19.3
41.2
Grand Total
27.5
80.5
46.8
100.0
* Engineering and Contracting, Vol. 43 (1915), p. 61.
dence districts of larger cities. Such construction is cheap, dur-
able, efficient, and good in appearance. It is very popular with
brick and asphalt where the grades are very flat, and is often used
with a crushed-stone pavement. Fig. 138 shows the cross section
of the usual form. Notice that the face of the curb is battered.
This is important, since the pavement is crowned, and therefore the
plane of a steel wagon-tire is inclined. Consequently if the face of
the curb is vertical, the tire will strike it at its upper edge; but if the
face of the curb makes an angle with the pavement greater than 90°,
the tire will strike the bottom of the curb and do much less damage,
COMBINED CURB AND GUTTER
383
To prevent damage by the striking of steel tires, the curbs to private
drives, particularly narrow ones, usually have a convex face.
Fig. 139 shows the form of combined concrete curb and gutter
employed in St. Louis, Mo.
>sgsys
S-£-«%^K::^P
*:;Concrefe^ -•
•i>a.v.-.- . .. -o.-|Vo:
*'-:\*S?:*\'-\**:*:
,3to^&''*^WM
,//Gra\se/ or Cinders *'
'///////////// ////////
Fio. 138. — STANDARD CONCRETE CURB AND GUTTER.
738. Foundation. A trench is excavated 4 to 6 inches wider
than the base of the concrete, and a layer of cinders or gravel 4 to
8 inches thick (usually 6 inches) is laid, flooded with water, and
then thoroughly tamped. Upon this foundation is erected the
forms in which the concrete is to be laid.
3-0"
FIG. 139. — ST. Louis CONCRETE CURB AND GUTTER FOR PARK DRIVE.
739. The Forms. There are two general methods of constructing
these forms: 1. Some contractors lay alternate sections in boxes
about 6 feet long, and subsequently place boards against the sections
first laid and construct the remaining sections. This plan is more
expensive and does not secure as good alignment as the method
described below. 2. A continuous line of plank is set for the back of
the curb and another for the front of the gutter. These planks are
384 CURBS AND GUTTERS [CHAP. XIV
kept in place by stakes on both sides. Partitions are inserted so as
to divide the mass into sections 6 or 8 feet long.
Two forms of partitions are in common use. Sometimes these
partitions are plank Ij or 2 inches thick, in which case the sections
are laid alternately, the partitions being removed before the second
series of blocks are formed. In other cases, the partitions are made
of steel i or A incn thick, and are left in position until the blocks
are practically finished. There is but little choice in construction
between the two forms of partitions, except that it is difficult to
withdraw the steel partitions without chipping the surface — see § 745.
740. The form for the front of the curb is made by setting a
plank Ij or 2 inches thick against the front of the upper part of the
partitions and clamping it to the plank at the back of the curb with
steel screw-clamps. The lower edge of this plank is rounded to
make the curve between the face of the curb and the top of the
gutter.
The concrete for the base of the gutter is deposited and tamped,
and then the mortar for the face of the gutter is applied — all before
the form for the front of the curb is clamped into place.
741. Mixing and Laying. For information concerning materials
proportioning, and mixing, see Art. 1, Chapter VII, — Concrete
Roads.*
The proportions of the concrete will depend upon the gradation
of the aggregates (§ 418-24), and upon whether the surface is to
consist of a richer mixture than the body of the concrete or whether
the surface is to be finished integral with the body. Usually a
1:2:4 or a 1:3:4 mixture is used for the body, and a 1 : 1^ or
1 : 2 mixture for the facing mortar.
742. The upper face of the gutter slab is finished by adding a
1-inch coat of rich mortar and tamping and troweling it, exactly
as in concrete sidewalk construction. It is important that the sur-
face of the concrete be free from mud or dust when the topping is
deposited; and this is a condition quite difficult to secure, because
the curb and gutter is built in a narrow trench, often a considerable
distance below the surface, and usually with the excavated material
piled near. It is also important that the facing be deposited before
the concrete has begun to set.
* For a full discussion of the method of testing the cement and proportioning the concrete
and of the relative merits of gravel and broken-stone concrete, together with tables of quanti-
ties, strength, cost, etc., see A Treatise on Masonry Construction by Ira O. Baker, 10th edition,
pp. 745, 6 X9 inches, John Wiley & Sons, New York City.
COMBINED CURB AND GUTTER 385
743. There are three distinct ways of finishing the exposed face
of the curb :
1. The surface is faced with a rich mortar built integral with the
body. This may be obtained in either of three ways, of which the
first is most likely to secure a firm union between the backing and
the facing, a. A layer of rich mortar is deposited against the lower
third or half of the front form, and then the concrete backing is
deposited and both are well tamped; and the process is repeated
once or twice until the form is full, when the top surface of the curb
is finished by adding a 1-inch layer of facing mortar. 6. A 1-
inch plank is inserted inside of the front form, and the concrete
for the body is deposited and tamped; then the 1-inch plank is care-
fully removed, and a rich facing mortar is tamped into the vacant
space, c. Instead of the 1-inch plank as above, use a steel plate
i or -fg of an inch thick having vertical 1-inch angles riveted at
intervals along its length. This plate is inserted behind the front
form, the concrete is deposited, and then the facing mortar is depos-
ited between the front form and the steel plate; next the steel plate
is removed, and the facing mortar and the concrete are thoroughly
tamped.
2. The second method of finishing the face of the curb is to mix
the concrete rather wet, and spade the face, i. e., after the concrete
is deposited force a flat spade or its equivalent vertically against the
back of the front form, and then push the handle away from the
form to an angle of 20° or 30° and withdraw the spade. If properly
done, this forces the' larger stones away from the face and allows
enough mortar to flow out against the form to give a solid face and
permit a good finish of the surface. This makes the strongest face
of any of the methods; but it is not popular with contractors, since
to get a solid face it is necessary to mix the concrete so wet as to
greatly delay the finishing of the face, which is objectionable for
several reasons.
3. Sometimes a rich face is obtained by plastering the surface
after the forms are removed. But this method necessitates using
dry concrete in the backing, so as to remove the forms early, and
consequently is likely to give a weak and porous surface upon which
to apply the mortar. It is nearly impossible by this method to
secure a firm union between the plastering and the backing; and the
plastering is nearly certain to be knocked off by passing wheels
(particularly during the excavation of the roadway when the passing
wheels are heavy and the concrete is weak) and to be spalled off by
386 CURBS AND GUTTERS [CHAP. XIV
the pressure due to the temperature expansion of the curb. Although
this method is sometimes used, it should never be permitted.
744. Finishing the Surface. There is a difference of opinion as
to whether the surface should be considered finished when it has been
troweled, or whether it should be afterwards brushed with a slightly
wet brush. An ordinary flat paint brush, with extra heavy bristles,
cut off about 1 inch below the wood portion, may be used for this
purpose. The objections to the trowel-finished surface are that the
trowel marks show more or less, and that the surface has a glaze or
shine clearly indicating that the stone is artificial; while the brush
finish has a uniform dull surface similar to a smoothly dressed natural
stone. The objections to the brush-finished surface are that the
brush leaves a porous surface that is not so durable as a trowel-
finished one, which objection has considerable force if the surface is
not first thoroughly troweled and if the brush is not used lightly.
The less the troweling and the more the brushing, the more rapidly
the* surf ace can be finished; and hence it is difficult when brushing is
permitted to prevent the slighting of the work. Both methods of
finishing are employed by competent engineers; but the trowel
finish is more common.
Recently a method of finishing by drawing a template over the
curb and gutter has been introduced. The few trials made seem to
show that this method is a little less expensive than finishing with a
trowel, but that it gives a better general appearance and a better
alignment, particularly at the joints.
745. Expansion Joints. Concrete curbs 'and also combined
curbs and gutters should be built in sections 6 or 7 feet long with
open joints to allow for expansion. If adequate space for expansion
is not provided, the compression due to expansion is likely to crush
and splinter the curb at the joints, and split off the mortar face or
the plastering. Many such failures occur. For a discussion of a
similar problem, see Contraction Joints in the Chapter on Concrete
Roads (§ 465-68). Sometimes no provision is made for expansion,
the curb or curb and gutter being made continuous with false joints
marked at intervals. Sometimes the curb or curb and gutter is
built in alternate sections without any expansion space between
adjoining sections, the joints being simply a line of weakness which
serves to prevent an unsightly crack if a section is displaced by frost
or by the lateral pressure of the earth. The curb and gutter should
be a practically permanent asset, and hence the prevention of its
destruction by temperature changes is an important matter. One of
COMBINED CURB AND GUTTER 387
three means may be employed to prevent failures at the joints of
curbs and gutters due to expansion.
1. The gutter flag and the curb are cut into short sections after
being laid, much as a sidewalk slab is separated into short sections.
The expectation is that the open joint will afford sufficient space for
expansion; but it is likely to be filled on the face during the finishing
of the face of the curb or gutter, which is particularly bad if a mortar
face is used or if the exposed surface is plastered. Generally the
open joint is reasonably successful for a time; but is likely to become
filled with dirt and cease to be effective, and besides the open joint in
the curb permits the earth behind the curb to escape, or with brick
or block pavements the open joint in the gutter permits the sand
cushion to escape.
2. Instead of cutting the curb and gutter as described in the
preceding paragraph, it is much more common to insert steel dia-
phragms or partitions in the forms at intervals of 6 or 7 feet, which
are withdrawn as the face of the curb and gutter is finished. Thfese
partitions are usually £ or -£$ inch thick. This joint is more efficient
than the one described above, and is easier to construct; but it
is open to substantially all of the objections to the preceding one.
3. Occasionally partitions consisting of one or two thicknesses
of tar paper or one thickness of tar or asphalt felt are inserted in the
forms at intervals of 6 or 7 feet; and after the face of the curb and
gutter has been finished the paper or felt is cut off with a sharp knife
a little below the surface of the concrete.
746. Whatever the method of constructing the expansion joint, it
should be in a vertical plane perpendicular to the face of the curb, or
one section may push past the other.
With any form of expansion joint, there is danger that the expan-
sion of the straight curb will dislocate the curved curb at the corner
of the block and at the alley return. This can be prevented by
making an extra wide expansion joint near the corner. This joint
may be | to 1 inch wide according to the length of the block and the
temperature when the curb is constructed; and it should be filled
with tar pitch or asphalt. A number of proprietary compounds are
upon the market for this purpose, made in sheets of different thick-
nesses.
747. Curbs are frequently damaged by being pushed over or
broken by the expansion of cement walks. The remedy is to insert
an expansion joint between the end of the walk and the back of the
curb, or better in the joint in the walk one section back from the curb.
388 CUEBS AND GUTTERS [CHAP. XIV
The expansion joint may be filled with tar paper or felt; or a thin
board may be inserted during the construction, and after the con-
crete has set the board is withdrawn and the space is filled with tar
pitch. The joints must occasionally be re-filled — preferably once
each year.
748. Cost. The cost will depend upon the price of labor and
materials, and upon the proportions of the mortar of the face and
the concrete of the body. The amount of cement required will vary
a little with the percentage of voids, but will depend chiefly upon the
proportions of the mortar and the concrete.
The following data are for laying more than a mile of combined
curb and gutter of the form shown in Fig. 138, page 383. The pro-
portions of the facing mortar was 1:2; and that of the concrete
1:3:4 washed gravel. A barrel of portland cement made enough
1 : 2 mortar for the facing on 33 linear feet. The length of finished
curb and gutter laid with a barrel of cement was 16 feet, with a varia-
tion of 1 per cent either way on different days. A yard of sand and
pebbles laid 18 lineal feet. The loss of gravel in transportation and
handling, and the shrinkage in tamping was nearly uniformly 20
per cent. The average length of curb and gutter completed, includ-
ing straight work and curved returns at streets and alleys, and also
including excavation, per man per hour was 0.333 foot. The trench
was excavated before the roadway was excavated. The concrete
was mixed in a batch mixer which discharged directly into the
trench. The two finishers received 60 cents per hour, the three men
setting face forms 35 cents, and the remainder 25 cents. The work-
ing force of 18 men when constructing forms and laying curb and
gutter was divided as follows:
1 foreman and finisher,
1 finisher,
2 men setting face forms,
3 men setting back forms,
2 men wheeling and tamping cinders,
2 men running concrete mixer,
2 men feeding concrete mixer,
2 men mixing face mortar by hand,
1 man wheeling facing mortar,
1 man spreading facing mortar,
1 boy carrying water, etc.
The contract price in 1916 for the curb and gutter, including
excavation and back filling, was 50 cents per lineal foot.
COMBINED CURB AND GUTTER
389
749. Double Curb and Gutter. Fig. 140 * shows the details of
the form of the concrete double curb and gutter referred to in Fig.
114, page 357.
timfe r '//I////,///,//////////////,
^ l///6fortcr6v36«/ ffoc* ''/////,
v y.\\\\\\v .\\\\\\\\\\\\\\\ w^ \ \\\\\\ \
Fio. 140. — DOUBLE CURB AND GUTTER.
750. Curb and Gutter at Private Driveway. Fig. 141, page 390,
shows the arrangement of the combined concrete curb and gutter
at a driveway to a gate or a building. The radius of the curve at
the corner of the curb is too small, as a radius of 4 or 5 feet would be
better.
751. Merits of Concrete Curb and Gutter. The advantages
of the combined concrete curb and gutter are: 1. It is usually
cheaper, particularly if account be taken of the fact that the gutter
occupies space that otherwise would be paved. 2. The alignment
of the curb is better and more permanent. 3. The appearance
is better. 4. Usually the concrete is more durable than a natural
stone of equal cost. 5. The gutter is smooth, and easily cleaned.
A concrete curb is suitable only for residence streets, but is more
durable for a business street than soft sandstone or limestone.
752. Other Forms of Curbs. About 1889 there was con-
structed on two streets at Washington, D. C., a concrete curb and
gutter having at the inner lower edge of the curb a 4X 4-inch con-
duit for telegraph and telephone wires, with hand holes about 50
feet apart. The experiment was not considered successful, and the
conduit was never used for wires.
From time to time advertisements appear of burned clay curbs;
but none have been seen which are not so thin as to be easily broken,
and so constructed by sections fitted together as to be unstable.
753. RADIUS OF CURB AT STREET CORNER. As far as vehicular
traffic is concerned, the larger the radius of the curb the better;
* Trans. Amer. Soc. of Civil Engineers, Vol. 42, p. 7.
390
CURBS AND GUTTERS
[CHAP, xiv
but when the gutter is carried to a corner inlet (§ 705), it is incon-
venient to construct or cover the gutter if the curved curb intersects
the sidewalk, i. e., if the radius of the curved curb is too great. If
the pavement has the minimum width, say 18 or 20 feet, the curves
of the corner curbs should be made large so that a vehicle may be
turned around at the street intersection.
_i
J
ttel
--.**!._
PLAN
^^Sf^^^&^^^'a^^^f^S^^^i^^^.
^\£^0^°^^£0±t°'~^-~-Q<&^^<g&?2^<j&£>'g=
2,2s'5!ǤSS3^gSC C.ndera_ .-0gggg.V^^g^
CLtiSfiFa^tiife^/^^iagjfi^j&aA^^^^
Section A-B
Section C-D-
Fia. 141. — CONCRETE CURB AND GUTTER AT PRIVATE DRIVEWAY.
Formerly, when curbs usually were natural stone, the cost of
curved sections was considerably more than that of straight pieces ;
and hence the tendency was to keep the radius as small as possible.
But now that concrete curb is very common, curved curbs cost only
a little more than straight ones. Formerly, when most of the vehic-
ular traffic was horse-drawn, the chief objection to a short radius was
the wear due to wheels striking the curb at the corner; but now on
streets having any considerable motor-driven traffic, a corner curb
RADIUS OF CURB AT CORNER 391
having a short radius makes it nearly impossible for a motorist to
maintain a reasonable rate of speed in turning the corner and at the
same time keep his machine on the right side of the street.
The radius varies from 2 to 12 feet, usually from 6 to 8 feet. The
curb with a 2- oot radius should not be used at a 1, or at least only at
driveways to private grounds. A radius of 10 to 12 feet is usually
satisfactory; but on boulevards or streets where there is consider-
able automobile travel, a radius of 16 or even 20 feet is desirable.
Several cities have recently spent considerable money to increase
the radius of the corner curbs, particularly on main traveled streets
and boulevards, to the satisfaction and safety of automobilists.
754. COMBINED CURB AND WALK. In Chicago a concrete
walk about 1 foot wide has been constructed along the curb in front
of a large apartment building, so as to permit vehicles to stop any-
where along the curb to discharge or receive passengers. Such a
walk has been found to be a great convenience.
CHAPTER XV
FOUNDATIONS FOR PAVEMENTS AND STREET-RAILWAY
TRACKS
757. The term foundation is sometimes applied to the natural
soil upon which an artificial structure rests, and sometimes to the
lower portion of the structure itself. The term will be employed
here in the latter sense, and the soil under the foundation of the
pavement will be referred to as the subgrade.
The foundation of a pavement, as of all other structures, is an
important element, although it is more frequently neglected in pave-
ments than in other structures.
One of the most perplexing problems in connection with pave-
ments is the construction and maintenance of a pavement adjoining
a street-railway track that is durable and does not interfere unduly
with travel. Part of the difficulties are due to the foundation and
part to the construction of the pavement adjacent to the rails. The
former will be considered in Art. 3 of this Chapter, and the latter in
connection with the discussion of the different kinds of pavements.
ART. 1. PREPARATION OF THE SUBGRADE
758. Whatever the form of the pavement or of its foundation, it
must rest upon the soil; and s'nce the chief office of the pavement
and of its foundation is to distribute the concentrated load of the
wheel over an area so great that the natural soil will be able to
support it, it is important to ncrease, as much as practicable, the
bearing power of the soil by drainage and by rolling, and thereby to
decrease the thickness of pavement required.
759. DRAINAGE. The method of draining the subgrade of a
pavement is substantially the same as that of underdraining an
earth road — see § 114. The subgrade of a pavement requires under-
drainage fully as much as does an earth road, notwithstanding
the fact that the former has an impervious roof. The purpose
392
ART. 1] PREPARATION OF THE SUBGRADE 393
of the underdrainage is to prevent the surface of saturation from
rising so high as to soften the subgrade. Unless the subsoil is very
open and porous, it is economical to lay a tile under each edge of
the pavement, 2 or 3 feet below the surface of the subgrade. This
tile may empty into the surface-water catch basins (§ 703).
760. EARTHWORK. The machinery employed in making exca-
vations and embankments for pavements is practically the same
as that used in constructing earth roads — see § 148-57.
In making embankments great care should be taken to com-
pact them solid — see Shrinkage of Earthwork (§ 140), Settlement
of Embankments (§ 141), Rol ing Embankments (§ 143), and Sta-
bility of Embankment (§ 146). For data on the Cost of Earth-
work, see § 164-87.
The excavation for pavements is made by plowing and then
removing the earth either with a drag or a wheel scraper (§ 150, 154),
or by loading it into wagons or carts with hand-shovels. The
subgrade, even though on'y a comparatively thin layer is to be
removed, has recently been excavated and loaded into wagons with
a steam shovel usually of the revolving type; and more recently
the excavation has been made with the four-wheel scraper (§ 154)
drawn by a steam engine while being loaded. It is usual to specify
that no plowing shall be allowed within 2 inches of the subgrade, to
prevent the soil below the subgrade from being loosened. If the
subgrade is thoroughly rolled, as described later, plowing a little
below the finished surface is not a serious matter; but if the sub-
grade is not subsequently well rolled, the loosening of the soil below
the finished surface is very objectionable, since the foundation will
then have an uneven hardness.
The subgrade is often finished
with pick and shovel, but the
work can be done much more
economically with the scraping
grader (§ 155) or with the sur-
face grader, Fig. 142. The former
makes a more uniform surface,
and is usually more economical;
although the latter is an effective FIG. 142.— SURFACE GRADER.
implement. In either case the
loosened earth must be hauled away with scrapers or wagons.
762. A considerable part of the excavation is often done before
the curb is set, but the curb is always set before the subgrade is
394 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
finished. The exact position of the subgrade is determined by
stretching a string transversely across the street from curb to curb
and measuring ordinates similar to those shown in the upper dia-
gram of Fig. 135, page 375. Some contractors pick narrow trenches
down to the subgrade at short intervals transversely across the
street; while others drive stakes with their tops a specified dis-
tance, say 4 or 6 inches, above subgrade, and provide the work-
men with a stick of this length with which to measure down from
the top of the stake to the subgrade. The former method must
be employed when the scraping grader is used. The passage
of the grader fills the trench with loose earth, but it is easy
to see the relative position of the surface and the bottom of the
trench.
763. ROLLING THE SUBGRADE. The finished subgrade should
be thoroughly rolled to consolidate the surface and also to discover
any soft places — particularly over trenches that have not been solidly
filled. If the roller reveals a low place, it should be filled with earth
and be rolled again. The roller, whatever its weight, should be
passed over the subgrade more than once, since the successive pas-
sages have something of a kneading action and add to the solidity
of the soil. Several passes with a light roller give better results than
a few passes with a heavy one. It is well to specify both the weight
of the roller and the number of times it is to pass over the road-bed.
For some hints applicable in rolling the subgrade, see § 369.
Formerly a horse roller was sometimes used for this purpose;
but a steam or rather a self-propelled roller (§ 378) is much better
because it is heavier, and, still more important, because with it the
street can be rolled transversely. The street is full of trenches made
often just before the pavement is laid, in connecting the houses with
the sewer, the water, and the gas; and as these trenches run both
longitudinally and transversely, it is necessary to run the roller in
both directions if the trenches are certain to be solidly filled.
Unless the back-filling of a trench has been unusually well tamped,
a roller run transversely over a trench will leave a depression. In
most soils, the back-filling will not of itself settle into its former
solidity, however long it is left to the action of traffic and. to the forces
of nature; and whatever the foundation of the pavement, the heavier
traffic is nearly certain to cause a settlement over these same trenches,
unless the subgrade is well rolled. Traffic consolidates only a thin
layer near the surface which is usually removed when the pave-
ment is constructed. Ordinarily, if the subgrade is rolled both longi-
ART. 2] PREPARATION OF THE SUBGRADE 395
tudinally and transversely with a roller weighing 10 or 12 tons,
there will be no settlement of the pavement.
In rolling, if a depression is produced over a trench, it should
be filled and then again rolled. If the depression is of considerable
depth, it shows that the trench was badly filled or was very deep,
or both; and therefore it is wise to re-consolidate the trench. One
way of doing this is to make numerous openings through the crust
and keep the depression filled with water until the earth in the
bottom of the trench has become thoroughly soaked; and then
after the ground has dried out below, the roller should again be passed
over the surface. The surest way to prevent settlement over trenches
is to pack the soil solidly when the trench is first filled. For- a dis-
cussion of various methods of back-filling, see § 764.
Insufficient tamping in filling trenches or inefficient rolling
of trenches is a very common defect in pavement construction,
nearly every block presenting one or more such depressions. One
of the purposes of a guarantee of the pavement (§ 652) is to secure
a thorough consolidation of the soil in the trenches.
764. FILLING TRENCHES. The back-filling of trenches opened
to lay water and gas pipes, to make house connection to sewers,
etc., so that the road surface shall be restored to its former level
and remain so, is a matter of importance on both paved and un-
paved roads— particularly the former. The failure to re-fill the
trenches properly is a source of annoyance to those who use the
unpaved road and of damage to the pavement. It is frequently
asserted by those having opportunity for knowing, that the dam-
age to pavements through lack of care in re-filling trenches and
re-placing the pavements is greater than the wear due to traffic.
No kind of municipal work should be more rigorously inspected
than the filling of a trench over which a pavement is to be laid.
The nearly universal result of a neglect in this respect is that a
pavement built at great expense is disfigured or damaged by settle-
ment, the repair of which will cost many times as much as it would
have cost properly to fill the trench originally.
The principal cause of failure is lack of care; but sometimes it
is due to a mistake as to the proper method to be employed. A
discriminating judgment is required to determine the proper method,
and intelligence and faithfulness are necessary in carrying it out.
There are several distinct methods used in consolidating the back-
filling of trenches.
765. Natural Settlement. A common practice of those having
396 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
occasion to make excavations in unpaved streets is to cast back
loosely the material taken out, heaping it into an unsightly and
annoying ridge over the trench and trusting to travel and the ele-
ments to restore the surface to its original level. In nearly pure
sand such a ridge may in time settle to the original level, although
the damage due to the temporary ridge will generally be much
more than the cost of properly filling the trench in the beginning;
but as a rule loam or clay loosely put back will not attain a sufficient
degree of compactness to make it a safe support for a macadam or
other form of pavement. The surface may become very compact
and hard; and yet after the removal of a foot or more of soil,
ordinarily necessitated by the construction of the pavement, it will
be found that the earth in the trench will settle considerably under a
roller run transversely over the trench. Even though the surface
may support the roller, it is highly probable that ultimately a trench
which has been loosely filled will settle and cause a depression in the
pavement. This is proved by the numerous depressions in pave-
ments, and also by the fact that when trenches loosely filled are
opened years afterwards, it is very common to find open cavities.
The promptness with which natural settlement takes place depends
upon the climatic conditions and the underdrainage. It is never
safe to depend upon natural settlement to secure the proper com-
pacting of the soil in trenches over which a pavement is to be laid,
however long the time allowed for the settlement, and much less
the few weeks often specified.
766. Flooding. Where the water can be had cheaply, it is a
common practice to attempt to consolidate the earth in the trench
by flooding or puddling it. If the soil is sand or gravel and is so
pervious that the trench will drain out rapidly, thorough flushing
will compact the material so that no trouble will be experienced
with settlement; but the flushing must be done thoroughly. It
is not sufficient to fill the trench nearly full of loose material, and
then turn on a gentle stream of water until the trench is full; for
trenches thus filled are certain to settle later. The sand or gravel
should be added in layers not more than 8 or 10 inches thick, and
each layer should be flushed with a stream of water having force
enough to wash the finer particles into the voids between the larger
ones. Substantially the same result may be accomplished by
shoveling the sand or gravel into water 8 or 10 inches deep; but
this method will not be effective, if the trench is filled with a scraper
or a scraping grader.
ART. 2] PREPARATION OF THE SUBGRADE 397
However, wherever flushing is effective, tamping would be
equally as good and would probably be less expensive, if the cost
of the water be considered. As a rule attempting to consolidate
trenches by flooding is bad practice.
Neither of the preceding methods of using water should be
employed with clay or clayey soils, since flushing prevents rather
than assists the consolidation of such soils. In other words, flush-
ing or puddling is useful only with soils which water readily breaks
down. If clay is flooded or is deposited in water, the trench is filled
with a watery mud that will shrink very much as it dries out and will
always be loose and porous. It is well known that a stiff-mud
brick which has been moulded under exceedingly heavy pressure
will shrink in drying 5 per cent, and with some clays 10 per cent;
and of course the thin clay mud in a flooded trench will shrink very
much more than this.
767. Tamping. Except in the case of comparatively clean
sand and gravel, back-filling can be thoroughly done only by tamp-
ing; and to make this method successful it is necessary (1) that
the material shall be moist enough to be plastic, but neither too wet
nor too dry, (2) that it shall be deposited in layers not more than
3 or 4 inches thick, and (3) that each layer shall be thoroughly tamped.
To secure thorough tamping the relative numbers of tampers and
shovelers is sometimes specified; but this alone is ineffectual since
there is a natural tendency for the tampers to work less energetically
than the shovelers, and besides more labor is required to tamp the
soil around the pipe than higher up.
The amount of ramming required will vary with the character
and condition of the soil. Clay and hard pan should be moistened
before being tamped, while clean sand or clean gravel may be tamped
dry. The tamping can be most effectively done with a compara-
tively small light rammer or tamper, since the effect of the blow
is transmitted to a greater depth, while a broad heavy rammer
consolidates the surface only. A tamper weighing 5 or 6 Ib. is better
than one weighing 20 or 25 Ib., the lighter one being lifted higher
and giving less fatigue than the heavy one. It is important to
remember that any amount of ramming will affect only a compara-
tively thin layer.
Obviously back-filling should not be attempted when the mate-
rial is frozen, since subsequent settlement is then sure to take place.
768. To prevent disturbing the surface of a pavement, plumbers,
gas fitters, etc., are sometimes given permission to tunnel under
398 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
the pavement to make their connections. This practice is never
justifiable on account both of the excessive cost and of the impos-
sibility of effectively filling the tunnel, owing to the limited space
in which the work must be done. In nearly every case a depression
occurs sooner or later over the tunnel.
769. Replacing All the Material. The result to be obtained
in filling a trench is that the material in the trench shall have the
same compactness as the soil around it; and therefore some con-
tend that the only proper way is to put back all the material taken
out. In a majority of cases this procedure will secure reason-
ably good results; but under certain conditions it will fail. For
example, the water pipe or sewer may occupy a large proportion
of the volume of the trench, and consequently of necessity there
will be a considerable excess of earth. Again, putting back all
the earth does not insure the restoration of the original surface
nor certainly prevent subsequent settlement. It has been shown
that soil when taken from its natural place and compacted in an
embankment will shrink from 8 to 15 per cent (see § 140), and
will probably subsequently settle 2 or 3 per cent and possibly 10 to
25 per cent (see § 141). Consequently with a deep trench con-
taining a small pipe, it is possible to tamp the earth back so solidly
as not to have enough to restore the surface; or it is possible to
put all the soil back by tamping the lower portion of the trench
solidly and the upper portion loosely, and still considerable settle-
ment take place. Therefore the specification to re-place all of the
material, must have a careful and intelligent supervision to insure
good results.
In the past it has not been the custom to fill trenches in such
a manner as to prevent settlement; and therefore if the best results
are to be insisted upon, the specifications should clearly reveal that
fact, for contractors in bidding on work do so on the understanding
that the work is to be done in at least approximately the usual
manner, and any attempt to have it done in any better way, which
was not clearly understood from the beginning, is likely to cause
friction and irritation, and possibly finally to result in failure.
770. Re-filling with Sand or Concrete. On account of the dif-
ficulty of getting trenches in clay or loam filled so that there will
be no settlement, it has been proposed to require the trench to be
filled with clean sand or gravel. It is not known that this method
has ever been tried. It would probably be effective, but usually
its cost would be prohibitive.
ART. 2] THE CONSTRUCTION 399
In at least a few cases trenches have been filled with a fair qual-
ity of hydraulic cement concrete. The expense for the concrete
was not justifiable, since it was much greater than that required
thoroughly to tamp the back-filling.
Sometimes municipal authorities are lax in inspecting the filling
of trenches, owing to the belief that the concrete foundation will
hold up the pavement even though the material in the. trench may
settle; but this is bad practice, since the ordinary thickness of con-
crete is not designed to act as a bridge, and besides if it is thick
enough to bear up over trenches it is needlessly thick elsewhere.
With the usual thickness of concrete foundations, a depression is
almost certain to occur if the material in the trench settles; and
hence the only safe rule is to have the trenches completelv and
compactly filled.
ART. 2. THE CONSTRUCTION
772. In some cases a pavement has been laid directly upon the
natural soil; but this is possible only with brick, stone-block, or
wood-block pavements laid upon clean sand or gravel. This prac-
tice is wise only with light travel. Formerly in Cleveland, Ohio,
many brick pavements were laid directly upon the native sand.
Stone-block and brick pavements were formerly laid upon a layer
of gravel or broken stone ; but the decline in price of hydraulic cement
has made it economical to substitute concrete for the layer of gravel
or broken stone, owing to the labor and care required to secure a
bed of uniform density and smooth surface.
A layer of portland cement concrete is now the nearly universal
foundation for street pavements.
773. PORTLAND-CEMENT CONCRETE FOUNDATION. This is
by far the most common foundation for pavements. The advan-
tages of such a foundation are : 1 . It gives a smooth uniform surface
upon which to lay the pavement. 2. It prevents the surface water
from percolating to the subgrade. 3. By its thickness and resistance
to flexure, it distributes the concentrated load over a considerable
area of the subgrade. 4. Concrete acts as a bridge to support the
pavement in case of a settlement of the subgrade. 5. Being imper-
vious to water and a non-conductor of heat, concrete protects water
and gas pipes from freezing.
774. The Materials. For a discussion of the cement, the sand,
and the gravel or broken stone as ingredients for concrete, see Art. 1
of Chapter VII.
400 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
775. Thickness. The thickness of the concrete varies from 4 to
8 inches, but is usually 6 inches. There is considerable diversity of
opinion as to the sufficiency of a 6-inch concrete foundation, par-
ticularly since the introduction of motor trucks. Examples are
frequently cited of the failure of a concrete foundation, particularly
where a motor truck or heavily loaded wagon has broken through a
pavement; and the conclusion is drawn that the concrete slab was
too thin. However, making the foundation thicker is not neces-
sarily the economical remedy. The foundation may have failed
for one or more of the following reasons : 1. Insufficient rolling of the
subgrade. 2. Insufficient consolidation of back-filling in trenches.
3. The use of natural cement in the concrete, which is weaker than
Portland cement and lacks uniformity. 4. Improper proportions,
insufficient mixing, or inadequate curing of the concrete. As a rule
insufficient attention is given to each of these items. 5. Passage of
loads over the concrete before it had sufficiently set. 6. Vibrations
due to weak construction of street railway track, which shatter the
concrete and allow water to get under the foundation which upon
freezing still further cracks the concrete.
On the other hand, many examples can be cited where a 4-inch
concrete base has successfully carried a heavy traffic. It is prob-
able that a well-constructed slab 4 inches thick laid on a well-con-
solidated subgrade is stronger than a foundation of poor concrete
8 inches thick laid upon an insufficiently rolled subgrade.*
776. In view of the rapid introduction of the motor truck and the
consequent crushing of some pavement foundations, it is probable
that concrete pavement foundations should be improved in quality
or increased in thickness — or perhaps both. The question of im-
proving the quality depends upon the relative cost of materials and
labor; and the advisability of increasing the thickness can be deter-
mined only by a discriminating study of the experience with a par-
ticular thickness.
777. The thickness of concrete roads (§ 447) gives some indica-
tion as to the required thickness of concrete pavement foundations,
although the former are ordinarily more carefully constructed than
the latter. The thickness of concrete roads is usually about 6 inches,
and is seldom more than 6 inches at the sides and 8 inches at the
center, the excess thickness at the center being to provide for reduc-
* For a discussion of this subject, pro and con, see Engineering News, VoJ 72 (1914) p
176, 367, 558, and 1033; and Vol. 75 (1916), p. 1097,
ART. 2] THE CONSTRUCTION 401
tion by wear. The wearing coat adds thickness to the pavement,
which distributes the wheel-load over a greater area of the sub-
grade, and some kinds of wearing surfaces also give additional beam
strength to the pavement as a whole. For example, the binder and
wearing coat of a sheet asphalt pavement adds 2J or 3 inches in
thickness, and gives considerable additional beam strength. Again,
a portland-cement grouted brick wearing-coat has been found to
give so much additional beam strength that the total thickness of
the pavement has been greatly reduced in recent years (see § 1028-
30).
778. It has been proposed to limit motor trucks to certain streets,
rather than build all pavement foundations heavy enough to carry
such loads. There would be some justice in such a requirement, but
the enforcement of it would be difficult.
779. The Proportions. For a discussion of the theory of pro-
portioning concrete, see § 417-24, Chapter VII — Concrete Roads.
The proportions of the concrete for a pavement foundation is
usually determined arbitrarily without much, if any, reference to the
gradation of the coarse and fine aggregate. The proportions and
sizes of the aggregate specified by a number of important cities
whose specifications happened to be at hand, are as follows :
PROPORTIONS SAND STONE
1| :2J 1" to fine J" to 1"
H : 3 I to fine I to l\
2:3 I to fine i to 1 J
2i : 4 I to fine I to 1£
3:5 I to fine £ to 2
3:6 i to fine I to 2
The last proportions seem to be much the most used. It may be
that with the aggregates ordinarily employed in each case, the
proportions specified will give a good concrete; but the quality
of the concrete can not be foretold from the above specifications.
To secure the best results the proportions should be determined, or
at least tested, by a sieve analysis (see § 422) ; and to make the spe-
cifications really significant both the proportions and the gradation
of the aggregates should be stated.
For the proportions used in concrete roads see § 444. However,
it is not customary to use as rich a mixture in concrete pavement-
foundations as in concrete wearing-surfaces; and under ordinary
conditions, it is not necessary.
402 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
780. Mixing. All that is said in § 451-58 concerning the mixing
of concrete for concrete roads, applies to concrete for pavement
foundations.
781. Placing. If the subgrade has been rutted up by ordinary
travel or in the delivery of paving materials, the surface should
be restored; and if the surface has been much disturbed, the sub-
grade should be again rolled. The ridges thrown up at the sides of a
wheel track may materially weaken the concrete foundation. Under
ordinary circumstances the subgrade should be sprinkled just before
the concrete is laid. This will prevent the dry subgrade from
absorbing moisture from the concrete, and will also prevent its drying
out too fast.
It is important that the surface of the concrete shall conform to
the required grade and crown. The thickness may be indicated by
grade stakes set every 4 or 5 feet. Some engineers require the con-
crete to be struck off with a template which may run upon the curbs
or upon screeds carefully placed for that purpose.
The edge of the concrete should form a straight line from curb to
to curb perpendicular to the line of the street.
782. No contraction joints are provided in concrete pavement
foundations as in concrete roads, since the latter are not as much
exposed to temperature changes as the former.
783. Finishing. The concrete should be tamped to consolidate
it. The wetter the concrete, the less the tamping needed; and
usually there is very little tamping. Many engineers claim that the
concrete is ordinarily unduly weak because it is mixed unduly wet.
If the concrete is not mixed too wet, the proper tamping or ramming
of the concrete will consolidate it and fill the voids, and add mate-
rially to its strength. Until recently it was the usual custom to
finish the concrete foundation by light tamping; and often the sur-
face was unduly rough. In defense of this practice it was claimed
that the rough concrete prevented the shifting of an asphalt pave-
ment; and that with brick, stone-block, and wood-block pavements
the roughness of the concrete did no harm, as the cushion layer
gave a smooth surface upon which to place the pavement. In
neither case is the claim valid. A sheet asphalt pavement upon
such a foundation will creep and form waves or humps because of
the difference in compression due to its unequal thickness ; and it has
been conclusively established that the thinner the cushion course the
better for any brick pavement (see § 971) or block pavement (see
§ 1096). The utmost roughening of the surface of a concrete base of
ART. 2] THE CONSTRUCTION 403
sheet asphalt pavement should be that produced by a slight raking
while the concrete is fresh; and for all other kinds of pavements,
the smoother the finish the better.
Sometimes the surface of the concrete is grouted, that is, a rich
mortar is poured upon the surface and swept over it to level up any
depression and to fill up any honeycombing. Sometimes the surface
is broomed without the pouring on of any grout, the surplus mortar
being swept from one part of the surface to level up depressions and
to fill up, or rather hide, honeycombing. The term slushing is
sometimes applied to each of these processes. No such method of
finishing the surface should ever be permitted; although in extreme
cases concrete made of fine stone in the stated proportions may be
used to level up depressions.
Some engineers claim that the surface of a concrete base should be
floated to secure a uniform smooth surface upon which to lay asphalt-
block or wood-block pavements. With a brick pavement the same
result is accomplished in another way (see § 982).
Not infrequently loose stones are left on the upper surface of the
concrete foundation while laying the binder course of a sheet asphalt
pavement or the cushion course of a brick or block pavement. Such
a practice is inexcusable, since the labor to remove such stones is
slight, and since they have a seriously destructive effect upon the
wearing coat.
784. Curing. In building concrete roads, it is nearly universal
after the concrete is laid to protect it during curing by covering it
with canvas, or damp earth, or a sheet of water (see § 464); but in
constructing concrete pavement-foundations, it is quite unusual
to take any such precautions, and consequently the concrete is
frequently seriously damaged by drying out too rapidly in hot or
windy weather, or by exposure to low temperature.
The period during which the concrete base should be allowed
to harden will depend upon the weather conditions and the kind
of pavement to be laid upon it, or rather upon the method of delivering
the subsequent paving materials. Teaming over the concrete in
building the remainder of the pavement should never be permitted
in less than 10 to 15 days, depending upon the weather; and the
pavement should not be open to heavy loads in less than 15 to 21
days from the time the concrete foundation was laid. The pressure
to shorten this time is often very great, particularly on a business
street.
785. Cost of Concrete Foundations. Materials. The cost of
404 FOUNDATIONS FOR PAVEMENTS [CHAP. XV.
materials varies with the locality and the conditions of the markets
(see § 425-27) ; and hence it is unwise to cite examples except as in
§ 790. When the prices are known, estimates may be easily pre-
pared by the use of Table 28, page 237.
786. Labor. Formerly concrete for pavement foundations was
mixed by hand; but in recent years it is almost always mixed by
machine.
787. Hand Mixing: The following data on the labor-cost of
hand-mixing are out of date as to the method of mixing and also as
to the cost of labor; but as the price of labor is stated, these data may
be useful in making estimates when hand mixing is to be employed.
In a small western city the average cost to the contractor of
mixing and laying a thickness of 6 inches of concrete during two
years was about 7 cents per square yard, for 1 part cement, 2 parts
sand, and 4 parts broken stone, turned six times exclusive of casting
into place. With gravel instead of broken stone the cost was about
6 cents per square yard; and with four turnings instead of six, the
cost was about half a cent less than the prices above. All the mixing
was done with shovels. The wages of common laborers was $1.50
for 10 hours.
In a large western city the average cost to various contractors
of mixing and laying a thickness of 6 inches of concrete was 5J
cents per square yard. The mixing was done with hoes, the specifi-
cations requiring that the concrete should be mixed until each
particle of the stone was completely covered with mortar. The
wages of common laborers was $1.50 for 10 hours.
778. The following example * gives the distribution of the labor
of laying a 6-inch concrete pavement foundation, in hours per square
yard:
ITEMS. HOURS
PER SQ. YD.
4 men filling barrows with sand and stone 0.15
10 men wheeling, mixing, and shoveling to place (3 or 4 steps) 0 . 37
2 men ramming 0 . 07
1 water boy, equivalent in common labor 0.01
1 foreman, equivalent in common labor . 0 . 06
Total hours per square yard 0 67
The sand and stone were dumped in the street upon boards, and
were hauled in wheel-barrows about 40 feet to the mixing boards.
* Engineering News, Vol. 46 (1901), p. 424.
ART. 2] THE CONSTRUCTION 405
The mortar was turned three, and the stone three or four times.
Two gangs under separate foremen worked side by side in the same
street.
The same correspondent gives another example which required
0.56 hour per sq. yd., in which case the mortar was turned only
once and the stone twice, water being used in abundance.
The cost of labor in mixing and laying concrete is often 8 or 9
cents a square yard. For the most economical work the 'sand and
stone should be deposited in ridges on the subgrade near the middle
of the street; and if they are piled on the parking, the cost will be
considerably greater than above.
789. For data on cost of hand-mixed concrete for concrete roads,
see § 374.
790. Total Cost. The total cost of a concrete pavement-foun-
dation laid in a city in the Central States in 1916 was as follows:
ITEMS Cu. YD. SQ. YD.
SUBGRADE :
Rough grading,— 1,504 cu. yd $0.298
Surfacing and rolling 3,380 sq. yd., and cleaning up 0 . 090 $0 . 040
Miscellaneous expense 0.036 0.009
Total, exclusive of superintendence, depreciation on
machinery, administration $0.424
CONCRETE BASE:
Cement at $1.48 per bbl. on job, net $1 . 365 $0. 227
Sand at $1.40 per cu. yd. on job 0 . 731 0 . 122
Gravel at $1.50 per cu. yd. on job 0 . 934 0 . 156
Coal and water 0.035 0.006
Labor 0.336 0.056
Miscellaneous expense 0.038 0.017
Total, exclusive of superintendence, 'depreciation on
machinery, administration $3 . 439 $0 . 584
The pavement was 34 feet wide. The proportions of the con-
crete were 1:3:5; and the thickness was 6 inches. The concrete
was mixed in a one-bag mixer. The loss on bags was 1 per cent.
The wages of common labor was 20 cents per hour; the engine runner
on the concrete mixer, 30 cents per hour; and a team, wagon, and
driver, 50 cents per hour.
791. OLD MACADAM FOUNDATION. It not infrequently hap-
pens that a high-class pavement is to replace a water-bound gravel or
macadam surface, in which case it may be economical to use the old
pavement as a foundation for the new, This form of foundation has
406 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
been discussed in connection with the construction of concrete roads.
The possibility of utilizing an old gravel or macadam road as a foun-
dation occurs more frequently with a narrow concrete or brick rural
road than with a comparatively wide street pavement. For a con-
sideration of the difficulties encountered and of the methods to be
emplo3red, see § 437.
792. BITUMINOUS CONCRETE FOUNDATION. Bituminous-
cement concrete has some advantages over hydraulic-cement con-
crete for pavement foundations. 1. The bituminous concrete does
not require any time for curing and hardening; and consequently
the wearing coat may be laid as soon as the foundation is completed,
which may be a decided advantage on a busy thoroughfare. 2.
The bituminous concrete is more flexible than hydraulic concrete,
and hence is not so likely to crack. 3. If the wearing surface of a
pavement is made with a bituminous cement, a bituminous concrete
foundation is advantageous, since then the whole pavement can be
made with one kind of equipment and organization. 4. A bitumin-
ous wearing coat will adhere better to a bituminous concrete base
than to a hydraulic concrete base. 5. The use of a bituminous
concrete base makes unnecessary the binder course of a sheet asphalt
pavement; but on the other hand, with a bituminous concrete base
it is practically impossible to remove the bituminous wearing coat
without materially damaging the foundation. However, it is claimed
that by the use of a surface heater (Fig. 161, page 450), repairs can
be made in the asphalt wearing coat without damage to the bitumin-
ous foundation. For a further discussion of a bituminous concrete
foundation for sheet or monolithic asphalt pavements, see § 806-08.
793. The question of economy depends upon the local prices of
bituminous and hydraulic cement. At present the price of bitumi-
nous cement is substantially 1 cent per pound, while that of portland
cement is about $2.40 for 376 pounds or about 0.66 cent per pound.
The specific gravity of bituminous cement (a paste) is about 1, while
that of hydraulic cement paste is about 2; and hence the prices per
unit of volume are about 1 to 1.3, or in other words, at present prices
the hydraulic cement concrete is about 30 per cent the more expen-
sive. Or to put it another way, when portland cement costs more
than $1.88 per barrel, there is a possibility that bituminous concrete
may be the cheaper. There has not been sufficient experience with
bituminous concrete to determine with any considerable accuracy the
cost of mixing and laying it.
794. Bituminous concrete pavement foundations were used in a
ART. 3] FOUNDATIONS OF TRACKS 407
number of cities in this country from about 1880 to 1895, owing prob-
ably to the high price of hydraulic cement, particularly portland
cement; and some of these foundations are still giving satisfaction.
Tar concrete has been used in England for pavement foundations
for many years.
795. The strength of a bituminous concrete foundation will
depend upon the kind and quality of the bituminous cement used;
and no such foundation is as strong as one made with equal care of
Portland cement.
ART. 3. FOUNDATIONS OF STREET-RAILWAY TRACKS
796. One of the most common failures of pavements is adjacent
to the rails of a street-car track; and is often due to the defective
foundation of the track. In a general way these failures are due to
the vertical vibrations of the rails, which pounds the foundation
to pieces and also breaks the bond between the rail and the pave-
ment, thus permitting water to enter which on freezing heaves the
pavement. The vibrations of the rail may be due to the deflection of
the rail or to the compression of the ties and foundation or to both.
The design and usually also the construction of the foundation
of the track is a function of the engineer of the street railway, while
the construction and maintenance of the pavement adjoining the
track is under the direction of the city engineer; but to secure
reasonably satisfactory results requires the co-operation of both
interests.*
797. FOUNDATION. The foundation may be of gravel, or
broken stone or concrete. With gravel ballast it is practically
impossible to prevent the rail from working up and down owing to
the movement of the ballast and the difficulty of tamping the ties
uniformly. Broken stone gives better results than gravel, but is far
from satisfactory. Gravel or broken-stone ballast is reasonably
satisfactory for railways in the open country upon a private right-
of-way; but the case is very different on a city street where all have
equal rights and which must be paved and maintained for general
public use. It is generally conceded that the track, at least in streets
having any considerable amount of heavy traffic, should rest upon a
concrete foundation.
* For an instructive article on the relations of the two interests involved and a discussion
thereon by several engineers, see "A Suggested Change of Policy for Maintaining the Pave-
ment adjoining the Railway," by N. S. Sprague, Chief Engineer of Bureau of Engineering,
Pittsburgh, Pa., in Proc. Amer. Soc. Municipal Improvements, 1915, p. 271-77 and 277-82.
408
FOUNDATIONS FOK PAVEMENTS
[CHAP, xv
There is great variety in the form of the concrete foundation. It
may be a slab of uniform thickness extending under the track, or the
thickness may be increased longitudinally under each rail or trans-
versely under each tie. An important advantage of the first form of
construction over either of the others is that it eliminates trenches in
the subgrade, which can not be rolled and which are likely to be par-
tially filled with loose earth by the breaking down of the edges of the
trench by workmen. With the trench construction the depth of the
concrete is not likely to be uniform, and is likely to be laid on loose
earth instead of a compacted subgrade.
798. Examples. Fig. 143 shows the two types of track founda-
tions recommended by the Committee on Way of the American
?to9"of/.3:G concrete
i^vf-^-y— -'
bottom of tie- depend/ng //X
a? tffcjcv^ erflowng base.
>-"-H A/fistoaffieVhsnto an
pro** surfcce atow, end under draw anf in &*%%& **^***
Tree &- for a// so/te except very neavy cfay, fyr dense traffic,
and for cars a/} /o J& fans we/gbf:
. \
-°^^L v - J- - f^^-t ^tT,^
Pedt/ey or tinders-
center //ne of douto/e f/vc/t
Always provide yurface and under dnr/ns.
~YPeC:~ ror fmwwfo-Fefatn>/WfOfy0nrfvrtcerfa/n rt?0d? ground,
fbr densest traffic, and for fiear/esf cars.
Fia. 143. — TRACK FOUNDATIONS FOR PAVED STREETS.
Electric Railway Engineering Association.* Notice that in both
examples the rail is the T pattern; but as far as shown in Fig.
143, there is no difference between the T and the grooved rail.
For track foundations with a grooved rail, see Fig. 159, page 442,
Fig. 160, page, 443, and Fig. 196, page 540; and for two track-
* Proceedings Amer. Elect. Ry. Eng'g Assoc., 1915, p. 471.
ART. 3]
FOUNDATIONS OF TRACKS
409
foundations with T rails, see Fig. 197, page 540. Notice that the
lower half of the last shows a concrete beam under each rail.
For a cross section of a street-railway track in a sheet asphalt
pavement, see Fig. 159 and 160, page 442-43; in a brick pavement,
see Fig. 196 and 197, page 540; and in a stone-block pavement, see
Fig. 218, page 587.
799. THE TIES. The ties are either wood or steel, the former
being the more common. Wood ties have a long life when embedded
in concrete, especially if treated by some preservative process;
.V*. ft :- • ;'« °' :' •' ' ' ': » •' • r .'• r * '. : Q./.' >.'•• • * • ' •' j
Tie
f
I •
FIG. 144. — STEEL-TIE STREET-RAILWAY TRACK.
and there is an increasing' use of treated wood ties. For illustrations
showing track with wood ties, see Fig. 143.
Steel ties have been used to only a comparatively small extent.
For a cross section of the track with steel tie used in Chicago, see
Fig. 144. The construction shown in Fig. 144 has been found
to be objectionable on account of the difficulty of paving around
and over the tie-rod between the rails. Concrete ties have been
used in street-railway track only a little, if at all.
800. THE RAILS. The rails are one of three types, viz.: the
flat-top, the grooved top, and the T section. The flat-top rail, which
has been almost abandoned, was very destructive of the pavement,
as steel-tired vehicles frequently ran with one wheel on one of the
railway rails, the result being that the wheel on the pavement wore a
rut. Where the flat-top rail is in use, it is not uncommon to find at
least two ruts on each side of each track — one made by a broad-gage
wagon running with its left wheels on the left rail and its right wheel
to the right of the right rail, one made by any wagon running with its
left wheel on the right rail ; and if the wagons are of two different gages,
the last position will cause two narrow ruts close together or one wide
one. For illustrations of track construction using the T section, see
Fig. 143, page 408, and Fig. 197, page 540; and for illustrations
410 FOUNDATIONS FOR PAVEMENTS [CHAP. XV
showing a grooved top, see Fig. 159 and 160, page 442-43, and Fig.
196, page 540.
801. THE PAVING. In selecting the material for the pavement
adjacent to the rails, consideration should be given to the amount
and character of the vehicular traffic and to the relative cost and life
of the several classes of pavements. The preference of the members
of the American Electric Railway P]ngineering Association is as
follows: granite-block, Medina sandstone-block, creosoted wood-
block, vitrified brick, asphalt-block, sheet asphalt, bituminous
concrete, bituminous macadam, water-bound macadam. The rail-
way company usually prefers granite block because of its durability,
and possibly also because its roughness deters vehicles from using
the railway area.
The precautions to be taken in laying the different paving mate-
rials adjacent to a street-railway track will be considered in the sub-
sequent chapter treating the respective kind of pavement.
For cross sections of a sheet asphalt pavement adjacent to a
street railway track, see Fig. 159 and 160, page 442-43; for a cross
section of a brick pavement and a street-railway track, see Fig.
196 and 197, page 540.
CHAPTER XVI
ASPHALT PAVEMENTS
802. For a discussion of asphalt, its sources, its characteristics,
the methods of testing it, and specifications for asphalt for different
purposes, see Art. 1 of Chapter VIII — Bituminous Road Materials.
Asphalt is used in three forms of pavements, viz. : sheet asphalt,
asphalt concrete, and asphalt block. The wearing coat of the first
consists essentially of an asphalt mortar, i. e., a mixture of sand and
asphalt cement; and that of the second is an asphalt concrete, i. e.,
a mixture of broken stone and asphalt cement. Both of these forms
of asphalt pavements are mixed and laid hot. The wearing coat of
an asphalt block pavement consists of blocks of asphalt concrete
moulded hot and laid when cold. The first form is by far the most
common; but the second, of which bitulithic pavement is a form,
although a comparatively recent development, is quite widely
used (§ 636).
Each of these forms will be treated in a separate article.
ART. 1. SHEET ASPHALT PAVEMENTS
803. A sheet or monolithic asphalt pavement consists primarily
of (1) a wearing coat I? to 2 inches thick composed of asphalt
paving cement mixed with sand; (2) a binder course 1 to 1J inches
FIG. 145. — Two FORMS OF SHEET ASPHALT PAVEMENTS.
thick, composed of broken stone and asphalt cement; and (3) a
foundation of hydraulic-cement concrete — see Fig. 145.
In this country when the term asphalt pavement is used the above
form is usually intended. The term sheet or monolithic pavement is
411
412 ASPHALT PAVEMENTS [CHAP. XVI
not distinctive, since rock asphalt also is laid as a continuous sheet;
but no confusion is likely to result, since in this country the term
sheet is commonly used to distinguish the monolithic form from the
asphalt-block pavement, and since in Europe only one form of asphalt
pavement is used, monolithic natural rock. In centra-distinction
to a pavement made of natural asphalt limestone or sandstone, the
above pavement could with some propriety be called an artificial
asphalt pavement, or the wearing coat could with still more propriety
be called an artificial asphalt paving compound; but the dis-
tinction is not important, since the sheet asphalt pavement is laid
almost exclusively in this country and the rock asphalt almost
exclusively in Europe.
804. HISTORICAL. The first artificial sheet asphalt pavement
in this country was laid in Newark, N. J., in front of the city hall in
1870. In 1873 a small piece was laid on Fifth Avenue, New York
City, opposite the Worth Monument. A few other experimental
sections were laid; but the first test on a large scale was in 1876 on
Pennsylvania Avenue, in Washington, D. C. Preceding 1882, out-
side of Washington, D. C., there were not more than half a dozen
streets in this country paved with any form of asphalt; but since
that date, asphalt pavements have increased rapidly, and now hun-
dreds of miles of it are in use on the streets of American cities. The
following statistics show the rapid growth of this industry: In this
country in 1880 there were 300,000 square yards of sheet asphalt
pavements; in 1885, 1,800,000; in 1890, 8,100,000; in 1895, 21,500,-
000; in 1900, 38,000,000; in 1909, 83,227,000. In Europe in
1900, the latest data available, there were only about 3,000,000
square yards of asphalt pavements of all kinds.
In 1909 in the United States according to the data on page 320,
about one fifth of all pavements were sheet asphalt; and if water-
bound gravel and macadam be excluded, about one third of all
durable pavements were sheet asphalt.
Asphalt pavements can be adapted to a wide range of tempera-
ture, and are in extensive use from Winnepeg to Panama — from the
far north to the tropics.
805. THE FOUNDATION. For a description of the method of
preparing the subgrade, see Art. 1 of Chapter XV,— Pavement
Foundations.
Since the sheet-asphalt wearing surface has little or no power in
itself to act as a bridge, it is essential that it be placed upon a firm
foundation; and consequently it is nearly always placed upon a bed
ART. 1] SHEET ASPHALT PAVEMENTS 413
of hydraulic-cement concrete, which formerly was sometimes made
with natural cement but is now always made with portland cement.
For heavy city traffic, the concrete is usually 6 inches thick; but
for light traffic, it is sometimes only 4 inches thick. For a discussion
of the proper thickness of a portland-cement concrete foundation
and the method of laying it, see § 773-84.
It is necessary that the concrete be thoroughly dry before the
asphalt mixture is laid upon it, as the generation of steam caused
by placing the hot material upon a damp foundation will produce
blistering and possibly disintegration of the wearing coat. This is a
matter that needs close attention in laying an asphalt pavement.
To dry the foundation after a rain or during damp weather, fine hot
sand is sometimes spread over the concrete and then swept off; but
this method is expensive and not very effective, and besides there is
liability that enough sand will be left upon the foundation to inter-
fere with the adhesion of the asphalt.
806. Bituminous Concrete Foundation. It is claimed, with
apparent justification, that asphalt pavements usually fail because
of a defective foundation rather than because of inherent defects
in the surface coat or of the wearing away of its materials; and some
claim that better results would be obtained by the use of a bitumi-
nous concrete foundation instead of a hydraulic concrete base.
For a general discussion of a bituminous concrete base, see
§ 792-95.
807. It is claimed that the bituminous concrete base is superior
to the portland-cement concrete foundation in four particulars, as
follows :
1. There is a lack of frictional resistance between the wearing coat
and the portland-cement foundation, which gives rise to one of the
most common failures of sheet asphalt pavements. The pressure and
impact of wheels upon the surface of the pavement produce a horizon-
tal component which causes the asphalt to creep and form waves or
humps, which make the pavement uncomfortable in use particularly
by automobiles, and these humps are difficult to remove. A slight
roughening of the top of the concrete base is insufficient to resist this
lateral movement; and an excessive roughening is harmful rather
than otherwise, since a difference in thickness of the asphalt sheet
causes a difference in compression and a consequent lateral move-
ment. It is claimed that the asphalt wearing coat will unite more
firmly with a bituminous concrete foundation than with a hydraulic
concrete base.
414 ASPHALT PAVEMENTS [CHAP. XVI
2. There is a lack of adhesion between the asphalt mixture
and the portland-cement concrete, which gives rise to a second com-
mon form of failure, since there is insufficient adhesion between the
asphalt and the concrete to resist the action of water at the surface
of contact of the two materials. If the concrete foundation is not
very dense, water will be drawn to the top of it by capillary action ;
and the effect of water on the under side of the asphalt tends to dis-
integrate the bond between the asphalt and the hydraulic concrete.
Further, since the asphalt sheet prevents evaporation, the upper
surface of a hydraulic concrete foundation is usually moist; and
hence any frost action tends still further to destroy the bond be-
tween the two materials. It is claimed that with equal care and
equally suitable proportions, bituminous concrete will be more
waterproof than hydraulic concrete; but the experience with
bituminous concrete under modern methods of preparing, mixing
and laying has been insufficient to establish such a conclusion.
3. A third objection urged against a hydraulic-concrete founda-
tion is that cracks in the asphalt sheet are caused by temperature
and setting cracks in the foundation. This is probably true; but
the wise remedy is to properly cure the concrete (§ 784).
4. Another argument for the superiority of a bituminous base over
a hydraulic one is that the former is more elastic ; and hence absorbs
the effect of the impact of traffic, and prevents the lateral flow of
the wearing coat. This effect can not be very great; and it is of
doubtful value, since the chief advantage of a monolithic asphalt
wearing surface is its inherent stability.
808. On the other hand, the portland-cement concrete base
possesses the following points of superiority over a bituminous
concrete base.
1. A portland-cement concrete base is stronger, and hence will
better distribute concentrated loads, will better bridge over soft
spots in the subgrade, and will better resist the tendency to crack
due to unequal settlement.
2. Ordinarily it is cheaper.
3. There is less difficulty in making repairs in the asphalt sur-
face.
809. Other Foundations. A sheet asphalt wearing surface has
been laid on old cobble-stone, brick, and stone-block pavements; but
with varying success. The conditions necessary for success seem
to be: 1. The old pavement must be firm and solid. 2. The old
pavement must have a fairly uniform surface so that the asphalt
ART. 1] SHEET ASPHALT PAVEMENTS 4l5
coat will have a nearly uniform thickness, say not less than 1 or Ij
inches nor more than 2 or 2J inches. 3. The old pavement must be
perfectly clean and absolutely dry. 4. Of course, the asphalt wearing
course must be made of suitable material, have appropriate con-
sistency, and be properly applied.
Apparently, failures in laying sheet asphalt on an old pavement
have been more common than successes.
810. BINDER COURSE. In the past there has been considerable
trouble in getting an asphalt wearing coat to adhere to a hydraulic-
cement foundation, and various expedients have been tried; but
at present one of two methods is always employed, viz.: either
apply an asphalt paint coat to the foundation, or lay a binder
course.
811. Paint Coat. A paint coat consists of an asphalt cement
fluxed with naphtha or benzine, which is applied with a brush or
squeegee to the portland-cement concrete. It is essential that the
concrete shall have a fairly smooth surface; that the base shall be
perfectly dry; that the paint coat shall be bright and glossy, but not
sticky; and that it shall be kept clean until the wearing coat is
applied.
This method of construction is applicable only to a light-traffic
street or road where low first cost is necessary. The method has
been used to a considerable extent by cities in California.*
812. Kinds of Binder Course. The binder course is a layer
about 1^ inches thick of broken stone cemented together with asphal-
tic paving cement (§ 818) and rolled in place while hot. It is often
called simply the binder; but this is likely to cause confusion, since
the term binder usually refers to the cementing material in a water-
bound gravel or macadam road, or in a bituminous macadam or
concrete, etc.
There are two forms of binder course, the open and the closed.
The former does not contain as much cement, i. e., is not as rich,
as the latter; and its aggregate is not as carefully graded. The
open binder is used to secure a cheaper pavement; and will not
endure under medium or heavy traffic. The closed binder has
greater inherent stability, and hence is preferable for medium or
heavy traffic.
813. Specifications for Open Binder. Broken Stone. The broken
stone should be clean, and have a compact texture and uniform grain.
* California Highways, January, 1915.
416 ASPHALT PAVEMENTS [CHAP. XVI
For medium or heavy traffic the stone should be strong and break
with sharp edges and corners.
The broken stone should have the following gradation on screens
having circular opening: " All of the material shall pass a If -inch
screen; and not more than 10 per cent, nor less than 1 per cent,
shall be retained on a 1-inch screen; and not more than 10 per cent,
nor less than 3 per cent, shall pass a j-inch screen."4
814. Sand. It is not customary to use sand in an open binder;
and it is often specified that the stone shall not contain more than a
certain amount of fine material. However, the more fine material
(sand or screenings) in the binder the more compact and more
desirable it is, provided the fine material is not more than enough
to fill the voids. But the greater the amount of fine material, the
more the bitumen required to coat all the particles; and conse-
quently the more expensive the binder. For the latter reason, it is
not customary to use much fine material in an open binder; and
usually no fine material is used except that in the stone.
815. Asphalt Cement. The asphalt cement is of the grade
stated in § 542. The amount of cement should be sufficient to coat
the fragments of stone and bind them together reasonably well ; and
will depend upon the kind and gradation of the stone and the rich-
ness desired. With trap or hard limestone graded as above, only
3 or 4 per cent of pure bitumen is used; and with a soft limestone
4 or 5 per cent is common. There should not be so much cement
that it will run off the stone, but there should be enough to give a
bright glossy coat to the stone.
After being rolled the surface of the binder course will be porous
or open, and hence the name given to it.
816. Specifications for Closed Binder. Broken Stone. The
quality of the broken stone should be the same as for open binder
(§ 815).
The gradation of the broken stone should be as follows, as deter-
mined with screens having circular openings: " 95 per cent of the
binder aggregate shall pass a screen having circular openings equal
to three quarters of the thickness of the binder course to be laid;
and the smallest dimensions of the remaining 5 per cent shall not
exceed the thickness of the binder course. The aggregate shall have
the following composition: 20 to 50 per cent shall pass a |-inch
* Report of Committee of the Amer. Soc. of C. E., Proc. Vol. 42 (1916), p. 1626; or page 5
of Specifications for Sheet Asphalt Pavements of Amer. Soc. of Municipal Improvements,
adopted October 14, 1915.
ART. 1] SHEET ASPHALT PAVEMENTS 417
screen and be retained on a 10-mesh screen; and 15 to 35 per cent
shall pass a 10-mesh screen."*
817. Sand. The chief difference between an open and a closed
binder is the greater density and stability of the latter. A closed
binder should contain enough screenings or sand to fill the voids in
the coarse material; and the gradation of the coarse and fine aggre-
gate should be such as to give a minimum percentage of voids, and
consequently require a minimum amount of asphalt cement. Ap-
parently not much attention has been given to the quantity or
gradation of the fine material, i. e., sand and stone screenings,
for the binder course; and certainly no specifications for the fine
material have been published. The quantities in Table 38, page
418, are from actual practice in one of the largest cities in this
country.
818. Asphalt Cement. The object in using a closed binder
course is to secure maximum stability, and hence enough asphalt
cement must be used to coat all the fragments of the aggregate
and fill all the voids. If the aggregate contains considerable fine
sand and dust, the stability will be greater but more asphalt cement
will be required. With very carefully graded aggregate 3.5 per cent
of asphalt cement will give a very stable mixture; but if the gra-
dation is not so good 7 per cent may be required. There should be
enough asphalt cement to give stability; but an excess may be very
harmful, as it will likely collect in pools in the truck while being taken
to the street and appear in spots in the binder course, from which it
will be drawn up on a hot day into the wearing coat and soften
it. A uniformly distributed excess is less dangerous on a light-
traffic street than on a heavy-traffic one, since in the former the
wearing surface is likely to lose its volatile matter and crack, while a
rich binder will slowly enrich the wearing surface.
The asphalt cement should be softer than that in the wearing
coat, because the binder is more open than the wearing coat, and
hence more of the lighter oils are volatilized in the mixing, and also
because the softer cement makes a mixture less liable to rupture.
In ordinary practice the cement for the binder has a penetration
20 or more greater than that for the wearing coat.
819. Amount of Bitumen in Binder. To illustrate the method
of determining the per cent of bitumen in a particular mixture,
* Report of Committee of the Amer. Soc. of C. E., Proc. Vol. 42 (1916), p. 1629; or Speci-
fications for Sheet Asphalt Pavement* of Amer. Soc. of Municipal Improvements, adopted
October 14, 1915.
418
ASPHALT PAVEMENTS
CHAP. XVI
assume that the composition of the asphalt cement and the binder
course are as stated in Table 38. The pure bitumen in the bitumi-
TABLE 38
COMPOSITION OF BINDER COURSE
ASPHALT CEMENT
MATERIAL FOR BINDER COURSE
BITU-
MEN IN
BINDER
Ingredients
Per
Cent
Ingredients
Lb.
Per
Cent
Mexican asphalt. . .
Trinidad asphalt. . .
Indian flux
Total
40
40
20
Asphalt cement
Sand
122
438
1 190
7
25
68
5.8%
Broken stone .
One batch or boxful
100
1 750
100
nous materials is as follows: Mexican asphalt, 99.6 per cent;
Trinidad asphalt, 56.0 per cent; and Indian flux, 99.6 per cent.
Then the bitumen in the asphalt cement is :
Mexican asphalt 40 X 99.6 = 39.84%
Trinidad asphalt 40 X 56.0 = 22.40%
Indian flux 20 X 99.6 = 19.92%
Total Bitumen in Asphalt Cement =82 . 16%
Total Bitumen in Binder Course =82. 16 X 7= 5.8 %
It is impossible from the above computations to determine
whether or not the stated amount of bitumen will fill the voids in
the mineral matter. That could be determined accurately only by
direct test, but could be determined approximately from a knowl-
edge of the gradation of the mineral matter and a comparison of it
with the composition of standard binder mixtures. The method of
determining the amount of bitumen and the gradation of the mineral
matter necessary to give a binder course of maximum stability is
exactly the same as for the wearing course (§ 825 et seq.), and hence
will not be discussed here.
820. Mixing Binder Course. The aggregate and the asphalt
cement should be heated separately, the exact temperature of each
depending mainly upon the character of the asphalt cement.
The cement is usually heated to a temperature between 120° C.
(250° F.) and 177° C. (350° F.) ; and the aggregate is heated between
107° C. (225° F.) and 177° C. (350° F.).
The aggregate and the cement should be thoroughly mixed by
ART. 1] SHEET ASPHALT PAVEMENTS /419
machinery until a homogeneous mixture is obtained in which all
the particles of the aggregate are covered with cement.
The mixing is done in a box in which revolves two axles each
carrying a series of oblique paddles set spirally — see Fig. 146.
FIG. 146. — MACHINE FOB MIXING ASPHALT BINDER.
The mixture is discharged through a sliding door in the bot-
tom of the box. The capacity of mixers vary from 1000 to 2000
pounds.
Fig. 147, page 420, shows a complete semi-portable asphalt mix-
ing plant. On the left is the boiler; and on the right is the elevator
for the fine and coarse aggregate, and the drum for drying them.
Behind the boiler is the asphalt-heating tank or kettle; and behind
the sand-drying drum is the mixer with sand-storage bins above it.
Fig. 148, page 421, shows a portable asphalt-mixing plant. Sim-
ilar plants are mounted upon one and sometimes upon two steam-
railroad cars. In the larger cities are fixed plants which consist of
quite a group of buildings.
821. Laying Binder Course. The binder course should be trans-
ported to the street in wagons or trucks covered with canvas or tar-
paulin; and when delivered should have a temperature between 93°
and 163° C. (200° and 325° F.), the temperature between these
limits being regulated according to the temperature of the atmosphere
and the ease with which the binder course can be spread. The
temperature of the binder on the street should be no greater than is
necessary to permit the mixture to be easily spread.
The stone should be covered with a bright glossy coat of asphalt
420
ASPHALT PAVEMENTS
[CHAP, xvi
FIG. 147. — SEMI-PORTABLE ASPHALT MIXING PLANT.
ART. 1]
SHEET ASPHALT PAVEMENTS
421
cement, as otherwise it will have no coherence. On the other hand,
there should be no excess of cement, as is shown by its running
from the bottom of the truck or by too great richness of the bottom
of the load. If the stone was heated too hot, the cement may run
off the stone at the top of the load and accumulate at the bottom, in
which case the surface of the load will appear dull and dead.
On arrival on the street the mixture should be dumped upon the
foundation at such distance from where it is to be spread as to
FIG. 148. — PORTABLE ASPHALT MIXING PLANT.
require that all of the material shall be moved from where dumped.
This is necessary to secure a uniform thickness after rolling, since
the portion at the bottom of the pile is considerably compressed by
the fall and the weight of the incumbent mass, and hence if it is not
moved the binder course after being rolled will be thicker at this
point than elsewhere.
The binder is first roughly shoveled into place, and then is leveled
off with rakes or shovels. An open binder may be leveled with hot
iron rakes having long tines; but if a closed binder is employed, the
spreading should be done with a shovel or the back of a rake, as the
use of the tines will bring the larger stones to the surface and produce
422
ASPHALT PAVEMENTS
[CHAP, xvi
segregation. Fig. 149 shows the spreading of the binder course.
Notice that the binder is dumped on a steel plate from which
it is shoveled into place.
The binder course may be allowed to cool somewhat before being
rolled, for if it is too hot when rolled it is likely to stick to the roller
and also be pushed out of place. The rolling should be done with a
self-propelled tandem roller weighing 5 to 7 tons,- giving a pressure
Fia. 149. — SPREADING THE BINDER COURSE.
of not less tnan 20U pounds per lineal inch of tread. The object of
the rolling is a kneading action as well as a compression, and hence
many passes of a light roller are better than a few passes of a heavy
roller.
After rolling the surface should be of uniform density; and par-
ticularly there should be no spots containing an excess of asphalt
cement, since on a hot day the excess is likely to be drawn up into
the wearing coat and soften it.
822. After the rolling of the binder course is completed, the
wearing coat (§ 825) should be applied at once, while the binder is
clean and hot; or it should at least be added during the same day for
fear the binder may become dirty and dusty. This is necessary to
secure the maximum bond between the binder and the wearing coat;
and if this is not done, the binder course should be protected from
mud and excessive dust.
823. Thickness of Binder Course. The proper thickness of the
ART. 1] SHEET ASPHALT PAVEMENTS 423
binder course depends upon the amount of the traffic. For light
traffic the thickness is usually 1 inch after being rolled and for medium
traffic is li inches, while for very heavy traffic it is sometimes 2
inches. For data on the thickness of binder course in various cities,
see Table 46, page 453.
The compression by rolling is usually about 40 per cent. The
area that a given weight of the binder course will cover will depend
upon the hardness and gradation of the stone, upon the consistency
and temperature of the asphaltic cement, and upon the degree of
compression.
824. The thickness to which the compressed mixture should be
spread to give the specified thickness can be determined either by
computation or by trial.
1. To compute the area to be covered by a given weight, first
determine the specific gravity of the compressed binder course, and
then compute the area that a given weight should cover to give the
required thickness after rolling.
2. Lay a given weight on a known area, roll it, and then measure
the thickness by probing at several points with a dull pointed awl,
being careful that the awl penetrates to the foundation. After a
few trials the exact area to be covered by a stated weight to give the
desired thickness, can be accurately found.
825. WEARING COAT. The wearing coat consists of sand, and
fine mineral dust or filler, and asphalt cement. The sand and filler
are often referred to as the mineral aggregate.
826. The Sand. The sand is a very important element in a
sheet asphalt pavement, since it constitutes at least three fourths
of the wearing coat. The sand should consist of hard and durable
grains. It should be free from vegetable matter, and should not
contain much clay or loam; although a small amount of these, if not
closely adhering to the grains, will act as a filler and do no harm.
However, a small amount of clayey material adhering to the grains
is highly objectionable, since in passing through the heating drum or
dryer it is likely to be burned onto the grains to such an extent as
not to be removed in the mixer and consequently the bitumen will
not adhere well to the sand.
The gradation of the sand, i. e., the relative proportions of grains
of different sizes, is the chief characteristic to be considered; occa-
sionally, however, a sand is found which for some unknown reason,
perhaps the shape of the grains or character of the surface, proves
to be unsuitable for sheet asphalt pavements.
424
ASPHALT PAVEMENTS
[CHAP, xvi
827. Gradation of Sand. Until comparatively recently but little
attention was given to the gradation of the sand; but it is now
known that this is one of the most important elements in the
construction of a sheet asphalt pavement. Table 39 shows the
grading of the. sand used for the wearing coat of sheet asphalt pave-
ments in a number of cities before the importance of this element was
appreciated. The last line of the table shows the grading now be-
lieved to be the best attainable. The data in this table are mainly
interesting as showing a possible reason for the unsatisfactory service
of some sheet asphalt pavements.
TABLE 39
FORMER GRADING OP SAND FOR SHEET ASPHALT PAVEMENTS*
o*
*
*S
tf
City.
PER CENT PASSING SIEVE No.
Total
Per
Cent.
200
100
80
50
40
30
20
10
1
2
3
4
5
6
7
8
Boston, 1899:
6
31
1
10
2
2
32
0
2
2
0
14
2
17
14.5
0.0
13
39
6
68
15
1
33
1
25
19
1
26
4
40
14.5
17.0
14
21
10
15
17
4
13
2
29
19
1
14
22
30
14.5
17.0
31
8
41
5
52
53
18
36
36
41
48
38
28
10
26.2
30.0
20
1
19
2
9
25
3
32
4
12
46
6
19
1
12.3
13.0
10
15'
2
10
1
17
3
3
3
2
10
1
9.0
10.0
4
5'
"2"
3
2
3
Y
2
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
Buffalo:
bank, fine
lake coarse
Chicago, 1896:
fine
medium
Louisville :
river.
bar
Milwaukee :
coarse beach
9
1
2
1
l6"
1
5.6
8.0
3
2
0
5
3.4
5.0
White Fish Bay
Omaha
St. Louis, 1897:
coarse.
fine.
river, coarse
fine. . .
Richardson's Ideal :f
mineral aggregate
sand proper
* Richardson's Modern Sheet Asphalt Pavement, p. 85.
t Ibid., p. 332.
The ideal grading for sand for a sheet asphalt was obtained by
analyzing pavements that had given the best service; and has been
abundantly tested in practice for more than twenty years. Unfor-
tunately it is not often that a natural sand can be found which
approximates the ideal grading; and therefore an artificial mixture
must be prepared by screening the sand into several lots excluding
one or more of the lots, and then remixing the remainder, or by com-
ART. 1]
SHEET ASPHALT PAVEMENTS
425
bining portions of different sands. Unless the available natural
sand is nearly ideal, the securing of the best grading will entail con-
siderable expense; and in this case it may be wiser to accept a grading
only approximating the ideal. Table 40 shows the ideal grading for
heavy traffic and also two permissible gradings.
TABLE 40
STANDARD GRADINGS FOR SAND FOR SHEET ASPHALT PAVEMENTS
Grade of
of Sand.
Passing
Sieve No.
RICHARDSON'S GRADINGS.*
Forrest's Per-
missible Grad-
ing, t
Ideal for Heavy Traffic.
Permissible for
Light Traffic.
Dust
200
100
80
50
40
30
20
10
8
00.0
17.0%
17.0
00.0
26.0%
30.0
13.0
43.0%
30.0%
0.0
00.0
20 to 30%
not over 40%
20 to 30%
not over 10%
Fine »
Medium
Total 34%
30.0%
13.0
Coarse
Total 43%
10.0%
8.0
5.0
Very coarse
Total 23%
0.0
Total 100 100
100
100
* Richardson's Asphalt Construction for Pavements and Highways, 1913, p. 29
t C. N. Forrest, Chief Chemist, Barber Asphalt Paving Co., in private letter to the author
dated August 17, 1917.
Notice that the aggregate for heavy traffic is finer than that for
light traffic. The reason for this is as follows: Large pieces of
aggregate will be fractured sooner or later by the passage over them
of heavy loads; and when this occurs there are two surfaces which
are not cemented together. This condition permits a movement and
a grinding action, and also allows the entrance of water; and thus
two extremely destructive agents are set to work.
Table 41, page 426, shows the grading of sands used recently by
the Barber Asphalt Paving Co., in pavements in various cities.
These data are instructive as showing the degree of agreement of the
gradings of the best available sands with the standards stated in
426
ASPHALT PAVEMENTS
[CHAP, xvi
Table 40, and are also a valuable guide in selecting a sand for use
in a pavement.
TABLE 41
AVERAGE GRADING OF SANDS RECENTLY USED IN SHEET ASPHALT PAVEMENTS *
City.
PER CENT PASSING SIEVE No.
.23
1 o
PH£
Fine Sand.
Medium Sand.
Coarse Sand.
100
80
Total
50
40
Total
30
20
4.0
4.2
2.5
8.9
5.5
5.3
5.3
5.5
8.0
10
Total
Boston
13.2
16.7
20.6
28.3
16.4
14.6
9.2
20.5
17.0
15.8
12.5
21.9
19.2
11.0
13.3
17.2
19.2
17.0
29.0
29.3
42.5
45.5
27.4
27.9
26.4
39.7
34.0
42.0
34.7
43.6
23.4
35.6
37.5
51.1
37.1
30.0
15.8
9.7
6.4
8.9
17.8
17.3
7.9
8.2
13.0
57.8
44.4
50.0
32.3
53.4
54.8
59.0
45.3
43.0
6.6
5.5
2.5
8.9
9.6
9.3
5.3
6.8
10.0
2.6
9.7
2.5
4.4
4.1
2.7
4.0
2.7
5.0
13.2
19.4
7.5
22.2
19.2
17.3
14.6
15.0
23.0
7
Buffalo • • • •
Chicago ....
Kansas City •
Louisvills
NGW York
Omaha
St Louis
Richardson's Ideal
* Compiled from Richardson's Modern Sheet Asphalt Pavement, p. 331.
828. The Filler. The filler is fine mineral matter mixed with the
sand to fill the voids, and thus reduce the amount of asphalt required,
and also to make the wearing coat more waterproof and less plastic.
Further, the use of a filler permits the use of a softer asphalt cement
and thus makes the wearing coat less liable to internal displacement
in summer and less brittle in winter.
The filler is usually either pulverized limestone or portland cement,
generally the former. Portland cement is preferable for heavy traffic
or where the asphalt surface is subject to the action of water. The
heavier the filler per unit of volume the better, since this usually
indicates greater density; and the denser the filler, the denser and
more stable the wearing coat. The valuable part of the filler is
the impalpable dust which is much finer than the particles just
passing a 200-mesh sieve. " A good filler should contain at least
60 per cent by weight of actual dust, and preferably 70 per cent."
The amount of material in the wearing coat passing a 200-mesh
sieve varies from 10 to 20 per cent, according to the grading of the
sand, but usually from 12 to 16 — see § 835. Less filler is required
with Trinidad asphalt than other kinds, since it naturally contains
about 44 per cent of finely divided mineral matter.
829. ASPHALT CEMENT. The method of preparing the asphalt
cement has been described in § 530; and specifications for it are
found in § 542.
AET. 1] SHEET ASPHALT PAVEMENTS 427
The amount of asphalt cement in the wearing coat should be
sufficient to coat every particle of mineral matter and fill all the voids,
but should not be enough to make the mixture too susceptible to
pressure and temperature changes. If too much asphalt cement is
used, the sand grains will be readily displaced among themselves, and
the wearing coat will push out of place. If too little cement is used,
the surface will crack and is liable to be displaced because of lack
of solidity. The finer the mineral aggregate, the greater the amount
of cement required for stability. Too much dust in the aggregate
causes the mixture to be mushy.
830. Amount of Cement. The amount of asphalt cement to be
used in any particular case depends upon the gradation of the sand
and the filler, and to some extent is a matter of judgment and experi-
ence; but there are four tests that are guides in determining the
best proportions of sand, filler, and cement for the wearing coat.
These tests are the paper-pat test, the impact test, and the deter-
mination of the density and the absorptive power of the compressed
mixture.
831. Paper-pat Test. This test is made as follows: Secure a
sheet of manila paper having a smooth surface, crease it down the
middle, and lay it opened out on a smooth firm wood surface, not
stone or metal, which would cool the mixture too rapidly. With a
wooden paddle having a blade 3 or 4 inches wide and about J inch
thick, tapering to an edge, take a paddleful of the hot mixture, being
careful to get a representative sample. Note the temperature of
the mixture. Drop the mixture sidewise from the paddle on to the
paper, and fold the paper over the mixture. With a block of wood
press the surface of the mixture down until it is flat, and then strike
it five or six blows with the block until the pat is about half an inch
thick. Open the paper, and the stain upon the paper indicates the
amount of bitumen in the mixture. Fig. 150-53, page 428-31, show
four progressive characteristic stains.* Fig. 150 indicates a mixture
in which there is a considerable deficiency of bitumen, Fig. 151 a
slight deficiency of bitumen, Fig. 152 a mixture having the proper
amount, and Fig. 153 an excess of bitumen.
In interpreting the character of the stain consideration must be
given to the temperature of the. sample and to the kind of asphalt.
The sample must be taken and the stain made when the temperature
of the mixture is such that the asphalt is quite liquid. If the mix-
* By courtesy of D. T. Pierce, Executive Assistant, Barber Asphalt Paving Co.
428
ASPHALT PAVEMENTS
[CHAP, xvi
FIG. 150. — LIGHT STAIN. ,
ART. 1]
SHEET ASPHALT PAVEMENTS
429
Fio. 151. — MEDIUM STAIN.
430
ASPHALT PAVEMENTS
[CHAP xvi
FIG. 152. — STRONG STAIN,
ART. 1]
SHEET ASPHALT PAVEMENTS
431
Fio. 153. — HEAVY STAIN.
432 ASPHALT PAVEMENTS [CHAP. XVI
ture is too cold, the test is of no value; and if the mixture is too hot,
the stain will be stronger than for the proper temperature. Further,
the test is more valuable for Trinidad asphalt than for other kinds of
asphalt, since the latter are more susceptible to temperature changes.
The appearance of the surface of the hot pat is nearly as instruc-
tive as that of the stain on the paper. If the mixture is unbalanced
in any way, the surface will have a greasy appearance, which may
be due to an excess of either bitumen or filler; but the cause of the
greasiness can be determined only by trial.
832. Density. Another method of testing the correctness of the
proportions of the wearing coat is to determine the density, or spe-
cific gravity, of the compressed mixture. A cylindrical test speci-
men 1J inch in diameter and about 1 inch high, is moulded while hot
under standard pressure. The specific gravity of the specimen is
then determined either by weighing it in air and in water, or by
weighing it in air and measuring its volume.
The following example illustrates the method of making this test.*
The customary mixture in parts by weight is :
Sand 75 per cent
Dust or filler 10 " "
Trinidad asphalt cement 15 " "
Total 100 " "
The specific gravity of the sand is 2.65, the limestone dust 2.60,
and the asphalt cement 1.25. The volumes of the materials in the
mixture are :
Sand 75 -f- 2.65=28.30 units of volume = 64.10%
Limestone dust 10-^-2.60=3.85 " " " = 8.72
Asphalt cement.. 15 -r 1 .25 = 12.00 " " " =27.18
Total 44.15 " " " =100.00%
If the mass is compressed so as to exclude all the entrained air,
i. e., so that all the voids in the mineral aggregate are filled with
asphalt cement, then the specific gravity of the mixture would be:
Sand 64. 1% X 2.66 = 1.699
Limestone dust 8 . 7% X 2 . 60 = .226
Asphalt cement 27.2% X 1.25= .340
Ultimate specific gravity . . . . ' = 2 . 265
The specific gravity of the specimen should be at least 2.20, and
is usually not over 2.22. If portland cement is used as a filler instead
of limestone dust, the specific gravity will be about 0.02 higher,
* Richardaon's Modern Asphalt Pavement, p. 581,
ART. 1]
SHEET ASPHALT PAVEMENTS
433
since the specific gravity of portland cement is about 3.10. The
greater the proportion of asphalt, the less the specific gravity.
If the specific gravity of the specimen is 2.22, the voids are:
(2.265 - 2.22) -T- 2.265 = 2 per cent.
Pavements having the standard grading, after being rolled, have
within 1 or 2 per cent of the ultimate specific gravity; but some
pavements laid without regard to the best gradation, have specific
gravities as low as 1.90, and have not given reasonably satisfactory
service.
833. Absorptive Power. The denser the mixture and the larger
the percentage of bitumen it contains, the more resistant it will be
to the action of water; hence a determination of the absorptive power
of the compressed mixture gives valuable information concerning
the correctness of the proposed proportions, particularly if the pave-
ment is to be laid in a humid atmosphere.
The absorptive power is determined by moulding a cylindrical
specimen as in determining the density (§ 832), weighing it, suspend-
ing it in water, and then weighing it at intervals. The gain in
weight is the water absorbed; and the absorption per unit of area
may then be computed. Table 42 shows the results with two mix-
tures, the first containing coarse sand and too little filler, and the
second being Richardson's ideal mixture.
TABLE 42
ABSORPTION OP CYLINDERS OP WEARING COAT *
Pounds per Square Yard
Interval
WASHINGTON MIXTURE,
1893
IDEAL MIXTURE, 1904.
Trinidad
Asphalt.
Bermudez
Asphalt.
Trinidad
Asphalt.
Bermudez
Asphalt.
Seven days
0.314
0.434
0.502
0.063
0.194
0.306
0.080
0.093
0.107
0.094
0.093
0.104
Fourteen days
Twenty-eicht days
834. Impact Test. A cylindrical test piece is moulded as in
making the density test (§ 832), and then it is subjected to suc-
cessive blows of the dropping weight of the impact machine, f The
dropping weight or hammer weighs 2 kilograms (4.40 lb.); and the
height of fall is 1 centimeter for the first blow, and an increase of 1
centimeter for each successive blow until the test piece fails. The
* Richardson's Modern Asphalt Pavements, p. 468.
t Bulletin No. 44, Office of Public Roads, U. S. Department of Agriculture, June 10, 1912,
p. 9-11.
434
ASPHALT PAVEMENTS
[CHAP xvi
number of blows required to produce rupture is assumed to repre-
sent the toughness of the specimen. The number of blows required
to produce failure will depend upon the consistency of the asphalt
cement and upon the temperature of the specimen at the time of
testing. The best mixtures at a temperature of 78° F. require from
20 to 30 blows to produce rupture.
835. Proportion from Practice. Table 43 shows the grading of
the wearing coat of sheet asphalt pavements laid in 1916 and 1917
by different contractors in a number of cities,* and also Richard-
son's ideal proportions f and Forrest's permissible composition. J
TABLE 43
PROPORTIONS FOR SHEET ASPHALT PAVEMENTS
Laid in 1916 and 1917
Ref.
No.
LOCALITY.
Bitu-
men,
per
Cent.
PER CENT PASSING SIEVE No.
State.
City.
200
100
80
50
40
10.0
11.0
.0
10.1
12.1
4.3
12.0
10.0
ast
sible
30
8.0
11.0
5.3
13.1
2.9
10.0
8.0
15-
20
5.0
9.0
12.3
5.9
10.4
3.1
7.0
5.0
25
1*
6.0
10.0
7.6
5.8
5.1
2.0
3.0
4
2~0
1.0
1.5
1.5
1.8
2.9
1
2
3
4
5
?
8
9
Massachusetts. . . .
New York
Boston
Roxbury. . . .
New York . .
Kingston. . .
Hamilton. . .
Newark ....
Sumter
11.4
10.1
10.4
10.5
10.2
10.1
10.0
10.0
9-11
12.6
12.9
17.1
11.6
16.1
10.3
11.0
10.0
10-12
17.0
12.0
13
9.4
7.7
14.2
13.0
10.0
15
7.0
6.0
.6
7.4
3.9
16.2
10.0
20.0
-25
21.0
17.0
47
30.7
18.9
30.9
25.0
24.0
le
pos
North Carolina. . . .
Ohio
South Carolina. . . .
Richardson's Ideal,
Forrest's Permissib!
it least
e
836. The specifications for sheet asphalt pavements adopted
by the American Society of Municipal Improvements on October 14,
1915, contain the following requirements for the composition of the
wearing coat:
Bitumen I 9.5 to 13.5%
Passing 200 mesh . . .not less than 10%
total not less than 25%
total 15 to 50%
total 10 to 35%
Passing 80 mesh 10 to 35%
Passing 50 mesh 4 to 35%
Passing 40 mesh 4 to 25%
Passing 30 mesh 4 to 20%
Passing 20 mesh 4 to 12%
Passing 10 mesh 2 to 8% J
Passing 8 mesh 0 to 5%
" The minimum amount of bitumen shall be used only in mixtures con-
taining the minimum passing the 80 mesh; and the percentage of bitumen
must increase as the amount passing the 80 mesh increases."
* By courtesy of D. T. Pierce, Executive Assistant, Barber Asphalt Paving Co., in letter to
the author under date of Sept. 14, 1917.
t Richardson's Moderu Sheet Asphalt Pavement, p. 326.
J C. N. Forrest, Chief Chemist, Barber Asphalt Paving Co., in letter to [the author dated
August 17, 1917.
ART. 1]
SHEET ASPHALT PAVEMENTS
435
837. Percentage of Bitumen. Notice that Table 43 shows the
percentage of bitumen; while the wearing coat is a mixture of asphalt
cement, filler and sand. To compute the percentage of bitumen
in the wearing coat proceed as follows: Assume that the asphalt
cement and the wearing coat have the compositions stated in
Table 44.
TABLE 44
COMPOSITION OF WEARING COAT
ASPHALT CEMENT
WEARING (
?OAT
Ingredients
Per
Cent.
Ingredients
Lb.
Per Cent.
Mexican asphalt
Trinidad asphalt
40
40
Asphalt cement
Limestone dust.
285
300
14.2
15 0
Indian flux
20
Sand.
1 415
70 8
Total
100
Total . .
2000
100 0
The bitumen in the bituminous materials is as follows: Mexican
asphalt, 99.6 per cent; Trinidad asphalt, 56.0 per cent; and Indian
flux, 99.6 per cent. Then the pure bitumen in the asphaltic cement
may be computed as follows:
Mexican asphalt 40 X 99.6 = 39.84 per cent
Trinidad asphalt 40 X 56.0 = 22.40 " "
Indian flux. . . .20 X 99.6 = 19.92 " "
Total bitumen in asphalt cement =82 . 16
Hence the total bitumen in the wearing coat is 82.16X14.02 = 11.7
per cent.
838. Mixing the Wearing Coat. The sand and the asphalt
cement should be heated separately. The sand should have a tem-
perature between 275 and 400° F. (135-205° C.), and the asphalt
cement from 250 to 350° F. (121-177° C.). The exact tempera-
ture in any case depends upon the asphalt used; and the temper-
ature between these limits is to be adopted to suit the particular
asphalt. A temperature which is appropriate for one asphalt
may harden another too much; or a temperature which makes one
asphalt so fluid that it separates from the aggregate, may make an-
other asphalt so stiff that it can not be properly spread and rolled.
The sand, filler and asphalt cement for each batch or mixerful should
be carefully and separately weighed, and then be dumped into the
mixer.
436 ASPHALT PAVEMENTS [CHAP. XVI
The mixing is done in a machine like that shown in Fig. 146,
page 419. The mixing should be very thorough, and be continued
until the mass is homogeneous and each particle of aggregate is
covered with asphalt. The mixing usually requires 1 to 1J minutes.
839. Laying the Wearing Coat. The mixture for the wearing
coat should be brought to the street in wagons or trucks covered
with canvas, at a temperature of 230 to 350° F. (110-177° C.).
The temperature within the above limits is regulated according to
the kind of asphalt, the temperature of the air, and the ease with
which the mixture is spread.
The top of the binding course should be perfectly dry when
the wearing coat is laid, to prevent the top course from being sep-
arated from the course below by the formation of steam. Asphalt
should not be laid in cold weather, since the paving mixture may
become chilled between the mixing plant and the street, and par-
ticularly when it comes in contact with the cold foundation.
The mixture should be dumped outside of the area on which
it is to be spread, so that it shall all be moved in being put into
place and thus secure an even distribution of the material. This is
very important, since great care must be exercised to prevent de-
pressions or elevations in the finished surfaces, as the impact due to
such spots is likely to cause the wearing coat to be pushed out of
place. The mixture is thrown into place with hot shovels after
which it is uniformly spread with hot rakes. The depth to which
the mixture is to be spread is regulated by chalk lines on the curb,
by the length of the teeth of the rake, and sometimes by rods sup-
ported on feet of a length sufficient to bring the top of the rod to
the level of the uncompacted mixture. The compression in rolling
varies with the richness of the mixture, the leaner mixtures compress-
ing most; and is usually from three tenths to four tenths.
Fig. 154 shows the spreading of the wearing course of a pave-
ment on Fifth Avenue, New York City.
840. Immediately after being spread the wearing coat should
be composed by rolling or tamping. Tamping irons are used
around man-hole covers, near curbs, etc., where the roller can
not conveniently be used. Fig. 155 shows two forms of asphalt
tampers. The left-hand one is 8 inches in diameter and weighs
about 30 lb.; and the right-hand one has a face 2JX5 inches and
weighs about 18 lb. Hot smoothing irons, Fig. 156, page 438, are
employed to finish the gutters, angles, edges, and all joints or junc-
tures where one day's work joins that of another. The tampers
ART. 1]
SHEET ASPHALT PAVEMENTS
437
and smoothing irons are heated in a metal basket which is moved
forward on wheels.
Formerly the first rolling was. done with a light hand roller with
FIG. 154. — SPREADING THE WEARING COAT OP AN ASPHALT PAVEMENT.
a very long handle. Fig. 157, page 438, shows a form of hand roller
formerly used. The hand roller has been abandoned in favor of a
light self-propelling tandem roller (Fig. 71, page 213). The use of
Fio. 155. — TAMPERS FOR ASPHALT PAVEMENTS.
hot smoothing irons and hot rollers are objectionable since it is
impossible always to have them of such a temperature as not to injure
the pavement; and since, if the mixture is delivered at the proper
438
ASPHALT PAVEMENTS
[CHAP, xvi
temperature, and the raking and spreading is done expeditiously,
they are unnecessary. Experience shows that the surface of pave-
ments upon which hot smoothing irons were used scales and flakes
ofi more than a surface laid without hot tools.
Fio. 156. — ASPHALT SMOOTHING IBONS.
Fia. 157. — HAND ASPHALT-ROLLER WITH FIRE POT.
841. Rolling. Immediately after being spread the wearing coat
should be rolled. It is important that the rolling should closely
follow the spreading, so that the material shall not cool before the
final compression is obtained. The state of the weather is an ele-
ment to be considered; for if a strong wind be blowing, the material,
ART. 1] SHEET ASPHALT PAVEMENTS 439
spread over a broad surface only 2 or 3 inches thick, will cool much
more rapidly than on a calm day when the temperature is consid-
erably lower.
It is usually specified that the rolling shall be finished with a
roller giving a compression of at least 200 Ib. per lineal inch. But
to secure the best results the rolling should be begun with a 2J-ton
tandem roller, and be followed with a 5-ton roller, and be com-
pleted with an 8-ton roller. The first gives a compression of about
60 Ib. per linear inch of face under the front roll and about 125 under
the driving drum; the second 200, and the third 280, under the
driving drum. Usually only a 5-ton and an 8-ton roller are used,
and sometimes only an 8- or 10-ton. The attempts to do all the roll-
ing with a single 8- or 10-ton roller is very objectionable, since the
roller is too heavy for the hot material, and hence the rolling is
delayed until the mixture is too cold to compact well. Further,
the maximum compression can not be produced by pressure alone,
but requires somewhat of a kneading action; and hence several
passages of a light roller are better than fewer passages of a heavier
roller. On the other hand, the heavier roller is needed at the end
of the rolling to secure the greatest possible compression. Many
loaded-wagon tires give a greater pressure per inch of face than the
heaviest asphalt roller; and^therefore if the rolling is not done with
a reasonably heavy roller and is not long continued, the traffic may
make indentations on the surface and possibly seriously push the
wearing coat out of place.
The lubricating effect of the warm asphalt aids in the com-
pression, so that under the roller the grains of sand are wedged
together and the finer particles worked into the voids, until the mix-
ture occupies less space than the mineral aggregate alone could pos-
sibly be made to occupy. This is proved "by the fact that if all the
bitumen be extracted from a fragment of good pavement of known
volume, it is found to be quite impossible to reduce the dry sand
obtained to as small a volume 'as it occupied in the pavement.
If the asphalt mixture adheres to"the roller, the face of the roller
may be slightly moistened with a mixture of kerosene and water.
Sometimes water is sprayed on the roller; but the use of an excessive
amount of water should not be allowed. The sticking can be pre-
vented by sprinkling or dusting portland cement on the pavement.
If the street is wide enough, the pavement should be rolled
transversely as well as longitudinally; and if this is not possible, the
roller should run as obliquely as possible, so that any little inequality
440
ASPHALT PAVEMENTS
CHAP. XVI
which might be caused by the roller's moving lengthwise may be
taken out by the cross action. The rolling should be kept up until
the heaviest roller leaves no mark, a result which usually requires
at least 5 hours for each one thousand square yards of surface.
New York City specifies that the rolling shall be continued until
the wearing coat has a stated specific gravity, viz., 2.10 for a pave-
ment laid between April 1st and December 1st, and not less than
2.05 for a pavement laid between December 1 and April 1. For
data on laboratory tests of density, see § 832.
As soon as the rolling is completed, the pavement may be thrown
open. Traffic, if not of too heavy vehicles, is an advantage to a newly
laid asphalt pavement, since the pressure of the wheels aids in con-
solidating the wearing coat and in closing the surface, a result which
helps to retain the volatile oils and prevents the entrance of water.
Asphalt pavements in unfrequented streets do not wear so well as
those under a moderately heavy traffic.
Fig. 158 shows two rollers rolling the wearing surface of an
asphalt pavement on Fifth Avenue, New York City.
Fio. 158. — ROLLING TEE WEAKING COAT OF AN ASPHALT PAVEMENT.
842. In spreading and raking the wearing coat there is a ten-
dency for the workmen to step on the uncompressed mixture. If
the foot-print is filled by raking material into it, this part will con-
ART. 1] SHEET ASPHALT PAVEMENTS 441
tain more material, and hence when rolled it will not be brought down
even with the adjoining portion and a hump in the finished pavement
will result. The bump of passing wheels against this hump and the
impact due to the drop of the wheel after having passed the hump
are nearly certain to cause a gradually increasing movement of the
wearing coat. Therefore stepping on the uncompressed wearing
coat should be absolutely prohibited; and if it does take place, the
foot-print should be thoroughly obliterated by raking or at least the
track should not be filled with loose material.
For much the same reasons as in the preceding paragraph, all
compressed lumps of the wearing coat should be broken up with the
rake. Lumps in the uncompressed material make humps in the
finished surface; and the rebound of wheels in passing such humps
causes displacements of the wearing coat and starts waves.
843. Thickness of Wearing Coat. The wearing coat is usually
1J or 2 inches thick, the former for light traffic and the latter for
heavy. It has been established that if the wearing coat is more than
2 inches thick, there is danger of its flowing under travel, i. e., work-
ing into humps and waves.
For data on the thickness employed in various cities, see Table 46,
page 453.
844. The area that should be covered by a given weight of mate-
rial can be determined in either of the two methods described for
laying the binder course — see § 824. The method of determining
the area by computation is more appropriate for the wearing coat
than for the binder course, since often the specific gravity of the
former is determined in fixing proper proportions of the ingredients,
and also since the specific gravity of the wearing coat is not likely to
vary as much as that of the binder course.
If the thickness of the wearing coat is determined by probing,
the test should be made with a putty knife rather than an awl, as in
probing the binder course, so that the knife edge will be arrested by
the stones of the binder course.
845. Some engineers specify that when completed the top of
the wearing coat shall be J inch above the top surface of the gutter
flag, to allow for further compression by traffic without bringing the
surface of the asphalt below the top of the gutter flag. This is of
doubtful wisdom, since it constructs a shoulder to eliminate the pos-
sibility of one being formed by travel.
846. ASPHALT ADJACENT TO TRACK. It is difficult to lay and
maintain sheet asphalt next to the rails of a street-car track. It is
442 ASPHALT PAVEMENTS [CHAP. XVI
well known that many more failures of pavements occur on streets
having car tracks than on those without tracks, and that most of
these failures are adjacent and parallel to the rails. Part of the dif-
ficulties is due to the foundation, the ties, and the rails; and these
have already been considered in Art. 3 of Chapter XV — foundations
of Street-Railway Tracks. Part of the difficulties with sheet asphalt
is in getting a good union between the asphalt and the rail. The
hot asphalt should be compressed thoroughly under and around the
head and flange of the rail; and a good union can not be obtained if
the rail is cold, since the asphalt will become chilled, and then can
not be compressed and will not adhere to the rail. The surface of
the asphalt should be laid even with the top of the rail. If it is laid
lower, the rail will be an obstruction to vehicular travel, and vehicle
wheels will follow the rail and make a rut next to the rail; and if
the surface of the asphalt is laid higher than the top of the rail,
steel-tired wheels will break down the edge of the pavement.
Fig. 159 shows the method of laying sheet asphalt adjacent
to railroad rails adopted in Hartford, Conn.*
Notice in Fig. 159 that the asphalt is in contact with the rail.
It is troublesome to maintain the connection between the rail and
the asphalt because the deflection of the rail will break the bond
-tt
r/4^*w&^/r':£?**^W'^
FIG. 159. — STANDARD PRACTICE IN HARTFORD, CONN.
and permit water to penetrate to the open binder where it freezes
and lifts the wearing coat, and this allows the process to be repeated
upon a larger scale. The only preventive is to apply a thick
coat of soft or rather elastic asphaltic cement to the sides of the rail
before laying either the binder course or the wearing coat; and after
the pavement is in service, the only remedy is to fill the crack adja-
cent to the rail frequently during freezing weather.
Notice that Fig. 159 is for a grooved rail; but the same form of
construction would apply equally well with a T rail.
* Engineering News, Vol. 73 (1915), p. 888.
ART. 1] SHEET ASPHALT PAVEMENTS 443
The Baltimore (Md.) Pavement Commission has recently
adopted the method of laying sheet asphalt pavements adjacent
to street-railway tracks shown in Fig. 160.* Notice that a vitri-
fied paving block intervenes between the asphalt and the rail.
The advantage of this construction is that the vitrified blocks can
be put into place before the laying of the asphalt is begun. The
extreme form of the construction shown in Fig. 160 is that in which
the whole area between the ends of the ties is paved with vitrified
blocks — see Fig. 196, page 540, which also is a standard in Balti-
more.
3"Rail
Fia. 160.— STANDARD PRACTICE IN BALTIMORE, MD.
847. CAUSES OF FAILURE. The construction of an asphalt
pavement involves greater care in selecting and combining the in-
gredients than most other kinds of pavements. Most other forms
of pavements are constructed of a natural or artificial surfacing
material which is prepared and inspected (at least in part) before
being brought on to the street and which needs only to be laid, while
the important parts of a sheet asphalt pavement must be fabricated
in place on the street; and hence greater care is required in the
laying.
Unfortunately the custom has been to contract with asphalt
paving companies to lay asphalt pavements and to guarantee them
for a term of years, and consequently the municipalities have as a
rule made little or no investigation of the materials used nor of the
methods employed in laying the pavement. The result is that there
are but few, if any, public records showing the history of the pave-
ment; and therefore it is often impossible to determine the cause
of either failure or success. The causes of failure, exclusive of those
due to faulty foundation and street-railway track, may be grouped
under the following heads. Unsuitable material, improper manip-
ulation, and deterioration in use.
848. Unsuitable Material. The sand should be clean, hard,
and properly graded; the filler should be hard and properly graded;
* Engineering News, Vol. 73 (1915), p. 884.
444 ASPHALT PAVEMENTS [CHAP. XVI
the asphalt cement should meet the usual specifications, particularly
as to consistency or penetration. Each of these items has already
been discussed; and the best means of preventing failure under
this head is to observe the standard specifications.
849. Improper Manipulation. Even though the materials may
be the best, there is an abundant opportunity for failure through
improper manipulation in heating and mixing the materials.
850. Burned Asphalt. The asphalt may have been damaged by
over-heating or " burning." The burning of the asphalt causes the
surface of the pavement to disintegrate in spots during cold weather;
and may be revealed by a brittleness and a tendency to crack while
being rolled. Excessive heat converts the petroline, or cementi-
tious constituent of asphalt, into asphaltine, which is devoid of
cementing properties, and by so much reduces the cementing quality
— the vital element — of the asphalt. This over-heating may take
place during the refining (§ 492), or during the fluxing (§ 530), or
in mixing the asphaltic cement and the sand (§ 838).
Sometimes the kettle is mounted within brick walls directly
over a fire which comes in contact with only a comparatively small
part of the heating surface, in which case it is highly improbable that
the firing will be done so evenly and slowly as not to burn at least
part of the asphalt. The fire should not be allowed to come in
direct contact with the melting kettle or tank, thereby guaranteeing
that no portion of the asphalt can be burned. When the asphalt has
been badly burned, it will be revealed by a brittleness during rolling;
but there is no way of determining a lesser degree of burning, although
it still may be sufficient to cause a serious defect which will finally
develop into cracks and rotten spots. Therefore the inspector
should insist upon a method of melting that will insure an unburned
product. It is usually specified that the asphalt shall be heated
by steam.
The over-heating of the asphalt may be produced also by over-
heating the sand (§ 838). Every precaution should be used to
have each batch of sand heated uniformly throughout, and its
temperature should be taken before mixing it with the asphalt.
As a further check, the temperature of each load of paving
compound sent to the street should be taken and recorded at the
mixing plant.
851. Improper Consistency. The paving cement may have been
mixed too hard or too soft. If the cement is too hard, the pave-
ment will have, a tendency to crack during cold weather; and if it
ART. 1] SHEET ASPHALT PAVEMENTS 445
is too soft, it will push out of place and form rolls or waves under
traffic.
852. Insufficient Bitumen. The wearing coat may not have
contained sufficient cementing material (§ 830). Within me limits
imposed by the proper softness and haraness of the pavement, the
greater the per cent of asphalt the greater the life of the pavement;
and consequently contractors in laying a pavement under a long-
time guarantee generally use the maximum amount of asphaltic
cement, but when the maintenance period is short they generally use
the minimum. In fluxing, the tendency is for the bitumen to rise
and the mineral impurities to settle; and consequently if the tank is
worked too low, there is a likelihood that the last material taken
from the tank will contain too small a proportion of bitumen and
too large a proportion of sediment or mineral matter. This can be
prevented by careful inspection and by frequently taking samples
and analyzing them.
853. Inadequate Mixing. The ingredients of the wearing coat
may not have been sufficiently mixed. It is important that each
grain of sand shall be entirely surrounded by the cementing mate-
rial, so that no two pieces shall come into actual contact. If the
mixing is not well done, the pavement will disintegrate in spots.
854. Rich Binder. If an excess of asphalt is used in the binder
course, it is likely to work to the surface of that course and then
being absorbed by the wearing coat cause it to disintegrate. This
cause of failure manifests itself by irregular blotches on the surface
of the pavement.
855. Cement Chilled. The mixture for the wearing coat may
become chilled while being transported from the mixing plant to the
street. To prevent this possibility, the temperature of each load
should be taken just before it is laid. The material may also become
chilled by a delay in tamping and rolling, or by attempting to work
during too cold weather or during the prevalence of a high wind.
A batch of chilled mixture will cause a weak spot in the pavement.
856. Separation of Cement and Sand. If the distance from the
plant to the street is long or there is unusual delay, some of the
asphaltic cement may work down to the bottom of the load, and
when the material is dumped there will be both rich and lean spots
— both of which are equally objectionable. The rich spots will
have a tendency to roll or crowd toward the gutter; and the lean
spots will have a tendency to disintegrate under traffic.
857. Damp or Dirty Foundation. The wearing coat may have
446 ASPHALT PAVEMENTS [CHAP. XVI
been laid on a dirty or damp foundation, and therefore have been
prevented from uniting firmly with the foundation. This con-
dition will be revealed by a tendency of the pavement to roll or push
out of place while sound and firm on the surface.
858. Inadequate Compression. The wearing coat may not have
received sufficient compression. The surface must be thoroughly
compacted — particularly in the gutters — to keep out rain water
and the acids and oxygen dissolved in it. The effect of oxidation
is gradually to destroy the cementing power of the bitumen.
859. Deterioration in Service. All materials in nature are under-
going changes due to the action of the elements, and asphalt pave-
ments are no exception. The following are some of the principal
causes leading to the gradual deterioration of such pavements.
860. Ordinary Wear. The pavement may decrease in thickness
due to loss of material by the abrasion of hoofs and wheels; but
since the surface is smooth and somewhat elastic, the loss by wear
is almost imperceptible. In some cases the pavement decreases in
thickness with use, but the decrease is due to consolidation rather
than to loss of material.
861. Natural Decay. All asphalts gradually lose their cement-
ing power with age by volatilization, evaporation, and oxidation.
The pavement is peculiarly exposed to the action of the sun's heat,
and to the combined action of rain water, acids, oxygen, and frost.
The greater the cementing power of the asphalt originally and the
softer the cement, the longer the pavement will resist the influence
of volatilization and evaporation; and the more nearly the voids of
the sand are filled with cement and the more firmly the pavement
is consolidated, the longer it will resist the action of water, acids,
oxygen, and frost. The general decay of the asphalt will be indi-
cated by a tendency of cracks to form during cold weather (§ 866),
particularly during a sudden and extreme drop in the temperature.
862. Weak Foundation. A weak or improperly prepared founda-
tion by unequal settlement or settlement in spots will cause cracks
and depressions in the surface which under traffic will speedily enlarge
and cause the pavement soon to break up.
863. Porous Foundation. A porous foundation permits the
ground water to rise, by capillary action and possibly also by hydro-
static pressure, to the underside of the wearing coat, where by
freezing it may break the bond between the top layer and the base,
and thus permit the wearing coat to be pushed out of place and
broken. This effect has been known to occur with a concrete foun-
ART. 1] SHEET ASPHALT PAVEMENTS 447
dation ; but it is not likely to occur with good concrete. If a section
of pavement disintegrating from this cause be examined, there will
be found a layer of perfectly sound and good material at the surface,
while the lower side of the wearing coat will show evidence of being
disintegrated by water — that is, the sand will appear clean and
white in spots as though there had been insufficient asphalt cement
to cover it. The concrete base under the affected spot will generally
be found to be damp or even wet. This defect may be prevented
by underdraining the soil.
864. Leaky Joints. Lack of a water-tight joint between the
asphalt surface and the curb, the gutter, man-hole covers, crossings,
street-car rails, etc., may permit the water to enter the lower and
less compact part of the wearing coat, where by its solvent action
and also by freezing it may do material damage. It is nearly impos-
sible to keep these joints tight, particularly adjacent to the street-
car rails. The damage often extends a considerable distance from
the place where the water enters.
865. Illuminating Gas. Ordinary illuminating gas, escaping
from leaky pipes under the pavement, is absorbed by the pavement,
and causes the disintegration of the asphalt. There is but one way
to stop the disintegration of a pavement from this cause, and that is
to stop the leak of gas.
Pavements affected by illuminating gas first give signs of their
disintegration by a slight depression over the affected spot, later
fine cracks appear parallel to the line of the street, and finally the
surface coat begins to crown.
866. Cracks. Long irregular cracks in the wearing surface
frequently occur during cold weather. They usually start at the
gutter or man-hole frame, and gradually extend across the street.
They are often found at the joint between an old and a new pave-
ment or at the joint made between one day's work and another.
These cracks are due to the contraction of the wearing surface, and
should not be confounded with cracks due to the failure of the
foundation. Usually these cracks do not occur until the pavement
is two or three years old; at least they are most likely to occur in
an old pavement — one in which the asphalt has lost part of its
cementing power by age. These cracks appear sooner and increase
more rapidly on a street having only a light traffic. When the
pavement is subjected to a continuous traffic, the asphalt surface
which is more or less plastic at all temperatures, is kept from crack-
ing by the constant kneading action of the traffic. Again, when an
448 ASPHALT PAVEMENTS [CHAP. XVI
asphalt surface has but little or no traffic, it becomes more porous
owing to expansion and contraction from heat and cold without the
compression due to traffic, and as a consequence is materially weak-
ened. If cracks occur on a street having a fair amount of traffic,
it is evident that the paving mixture is at fault — either there was
not enough bitumen or the asphalt cement was too hard.
Some engineers leave expansion joints, i. e., cut the wearing
coat through, at intervals to prevent these irregular contraction
cracks. Such a procedure is of doubtful propriety, since the pave-
ment if properly constructed will not crack in several years under
the most adverse conditions, and then only at long intervals and
generally at some old joint; and if the pavement is improperly
made, the expansion joint will have only a slight tendency to pre-
vent these irregular cracks. The principle of the expansion joint is
not applicable to materials with no structural strength, like asphalt
mixtures. These joints are not only useless, but really detrimental
to a pavement. They are only another form of the defect they are
intended to remedy, for they are crevices which retain mud and
water which tend to rot the asphalt, and the edges of the joints are
easily broken down by traffic which also widens the crack.
867. Shifting under Traffic. The surface coat sometimes flows
under traffic, i. e., pushes lengthwise of the street into waves or
crowds toward the gutter. This defect occurs in pavements having
too soft a wearing surface, or where there is a defective bond either
between the base and the binder, or between the binder and the
wearing surface. This is a defect that is impossible to guard against
entirely on a street having very heavy traffic, and especially where
the traffic is confined to a narrow section of the street; but this
defect is inexcusable on streets having only moderately heavy traffic.
This flowing is commonly caused by the surface of the hydraulic
concrete base under the pavement being too smooth, which is the
case where gravel concrete is used or where a stone-and-gravel con-
crete is so rich that its surface is covered with mortar that was
brought to the top by ramming. Unless the binder and the surface
mixtures are made very hard, a condition which makes the pave-
ment likely to crack, the wearing coat will slide on such a base if
there is much traffic. Pavements often roll from a defect in the
binder — either because it was too rich in asphaltic cement, or because
it was dirty when the wearing surface was laid.
868. Damage by Bonfires. Another cause of damage to asphalt
pavements is the building of fires upon them, Of course this ouglrc
ABT. 1] SHEET ASPHALT PAVEMENTS 449
never to occur, but even in the best regulated municipalities it does
sometimes happen.
869. METHODS OF REPAIRING. The repairs necessitated in
the maintenance of an asphalt pavement may be classified as follows :
(1) those due to a settlement of the subgrade; (2) those due to a
disintegration of the pavement in spots; (3) those due to the forma-
tion of waves; (4) those due to the formation of cracks; (5) the
painting of the gutter; and (6) the remedying of defects next to the
street-car rails, crossing stones, man-hole covers, etc.
870. Settlement of Subgrade. The majority of repairs are neces-
sitated by the settlement of the foundation over trenches. To repair
these defects, it is necessary to remove the wearing coat, the binder,
and the foundation; and then, after having consolidated the material
in the trench (see § 764), to re-lay the pavement much as in the original
construction. The edges of the binder course and also of the wearing
coat should be thoroughly covered with a thin coat of asphaltic
cement to secure a perfect union of the old and the new material.
Both the binder course and the wearing coat should be thoroughly
tamped or rolled. Owing to the difficulty of fully consolidating the
patch, it is left a trifle high to prevent a possible depression.
871. Disintegration. If the wearing coat disintegrates in spots,
or forms " macaroons/7 from any of the causes described in § 848-65,
the affected part must generally be cut out, since it is usually affected
to its full depth. If the binder course is the cause of the deteriora-
tion (see § 812), it also must be cut out. The new material is to be
laid as described in the preceding paragraph. If the disintegration
does not extend to the full depth of the wearing coat, the repair may
be made by " skimming," as described in the succeeding paragraph.
872. Formation of Waves or Humps. If the wearing coat has
shifted under the traffic so as to form waves, i. e., until it is thicker
in some parts than others, or if the wearing coat has crowded towards
the gutter, it may be necessary to melt off a portion of the high part,
and also to re-surface the thin part. This is called skimming. The
asphalt is melted off either with an open grate on low wheels in which
coke is burned; or with a special heater having a tank for gasoline,
a hood over the burner, and an asbestos mat to protect the adjacent
pavement. Fig. 161 shows one of several forms of surface heaters
in common use. The surface is heated until the affected portion
can be raked off; and then new material is added to bring the pave-
ment to its proper thickness.
" Whenever the surface-heater or skimming method is employed,
450
ASPHALT PAVEMENTS
[CHAP, xvi
all defective surface shall be removed before replacing it with new
material. In all cases the old surface shall be removed to a depth
of not less than one quarter inch; and the new surface must, when
compressed, be not less than one half inch in thickness. The heat
shall be applied in such a manner as not to injure the remaining
FIG. 161. — SURFACE HEATER FOR REPAIRING ASPHALT PAVEMENTS.
pavement. All burnt and loose material shall at once be com-
pletely removed; and, while the remaining portion of the old pave-
ment is still warm, the new material shall be placed. The new
and freshly prepared wearing coat shall be laid in strict accordance
with the specifications for the original pavement." *
873. Cracks. When cracks have formed in the wearing coat,
all the loose material is cut off, the crack is cleaned out, and hot
asphaltic cement is poured in.
874. Painting Gutters. Owing to the disintegrating effect of
water, asphalt gutters usually require comparatively frequent
repairs either by painting with asphalt rich in bitumen, or by skim-
ming (§ 872), or by removing the wearing coat and re-laying it,
using an asphalt richer in bitumen than that in the remainder of
the pavement.
875. Recording Repairs. The present practice is to make the
repairs to asphalt pavements by contract with a guarantee of the
work for a number of years; therefore it is important that a record
should be kept of the area and location of the several patches and
also of the date when each was made. This is done by dividing
the pavement into imaginary squares, say, 10 feet on a side; and
* Specifications of Amer. Soc. jol Municipal Improvements for Sheet Asphalt Paving,
approved Oct. 14, 1915, p. 12.
ART. 1] SHEET ASPHALT PAVEMENTS 451
then when a patch is to be made, one or more of these squares should
be located by chalk marks on the pavement, and the boundary
of the patch should be sketched in a cross-ruled note-book. The
records of the individual patches are afterwards platted upon a
single sheet to see that a subsequent patch does not overlap one for
which the guarantee has not expired.
876. Using Old Materials. In some cities it is customary to
permit the re-use of the old asphalt; but this is of doubtful widsom,
since usually the repair is required by the inferiority of the old
material, and since it is likely to be over-heated in being removed.
If the asphalt is not damaged, and is cut out with an axe, it may be
used again, provided (1) the pieces are kept clean, (2) it is re-heated
slowly and carefully, and (3) new asphalt is added to flux the old.
It is difficult to melt old material without burning it, and it is also
difficult to secure a uniform mixture with it.
877. COST OF CONSTRUCTION OF SHEET ASPHALT PAVEMENTS.
Asphalt pavements are comparatively expensive, since the tools
and machinery employed in mixing and laying the asphalt are
costly and subject to large depreciation whether idle or in use,
and also since the business requires a considerable proportion of
skilled labor. One of the peculiarities of the business is the dis-
proportionate amount of capital invested in the plant compared
with the business done, often an expensive plant being maintained
in a city for one or more years without laying any pavement or at
most only a small amount. Or a portable plant is moved to a small
city for a comparatively small amount of pavement. Another
peculiarity is that the working season is short, extending only from,
say, the first of May to the first of November; and as expert superin-
tendents and foremen are indispensable, it is necessary to employ
this skilled labor by the year.
878. In connection with data on the cost of construction of a
pavement, it should not be overlooked that the cost of the pavement
proper is not usually the total cost which the property holder must
pay for the improvement of the street when it is paved. Usually
the improvement of the street includes four items, viz. : (1) excava-
tion for the pavement, (2) the construction of curbs and gutters,
(3) laying drains and building catch basins, man-holes, etc., and (4)
the pavement itself. 1. Under ordinary conditions the excavation,
exclusive of surfacing and rolling the subgrade, will cost 10 to 15
cents per square yard. 2. Combined concrete curbs and gutters
(§ 737), will usually cost 30 to 35 cents per square yard of pavement
452 ASPHALT PAVEMENTS [CHAP. XVI
3. The drainage will usually cost 10 to 15 cents per square yard
of pavement. These three items may add 50 to 60 cents per square
yard to the cost of the pavement proper.
879. Estimated Cost. The estimate of the cost of laying an
asphalt pavement shown in Table 45 was prepared for this volume
by a man of acknowledged ability and unquestioned integrity, who
has had 20 to 25 years' experience as an inspecting and consulting
engineer of asphalt paving in various cities.* The estimate is for a
city in which 100,000 square yards are laid in one year.
The table is chiefly interesting as showing the items that go to
make up the expenses which are separate and distinct from that for
materials and labor. Of course, these expenses would be less or
more per square yard, if the area of pavements laid was greater or
less than the amount stated. The estimate is for first-class work
under average conditions prevailing in 1916.
880. Actual Cost. Table 46, page 454, shows the contract price
for constructing sheet asphalt pavements in thirty-three cities.
881. COST OF MAINTENANCE. The cost of maintenance will
vary with the original quality of the pavement, its age, the amount
and nature of the traffic, the width of the street, the presence or
absence of street-car tracks, the frequency with which the pavement
is cleaned and sprinkled, the climate, etc.
In nearly all American cities there is a serious lack of data con-
cerning the cost of maintaining pavements; and this lack is more
serious for sheet asphalt pavements than other forms, since this type
involves more variables. A few cities attempt to keep a record of
the cost of maintaining pavements; but such records are often so
incomplete and so incorrectly compiled as to be valueless. Some of
the reasons for the dearth and incompleteness of the data on the
cost of maintaining sheet asphalt pavements are as follows:
1. The pavement is usually built under a long-time guarantee,
and the city pays comparatively little attention to the quality of
the materials used and the methods of construction employed;
consequently there is no satisfactory record of the quality of the
pavement. In some cases the date when the pavement was laid or
re-surfaced is unknown ; and in many cities no adequate records are
kept of the location of repairs or patches. In recent years cities
are improving in this respect; but the usual absence of such records
*A. W. Dow, for a number of years Inspector of Asphalt and Asphalt Paving for the Dis-
trict of Columbia, and for several years past a consulting asphalt chemist and asphalt paving
engineer in New York City.
ART. 1] SHEET ASPHALT PAVEMENTS 453
TABLE 45
ESTIMATED COST OP SHEET ASPHALT PAVEMENT
Plant and Capital Charges:
Interest on cost of fixed plant, — 5% on $13,500 $675.00
Interest on cost of rollers, tools, etc., —5% of $3,000 150 . 00
Taxes,— 1% of $10,000 100.00
Insurance,^% of $10,000 400.00
Depreciation,— 8% of $16,500 1,320.00
Rental or interest on real estate, — 5% of $4,000 200.00
Interest for 6 months on working capital, — 5% of $6,000. . . 150.00
Current repairs 500 . 00
Watchman for 1 year 400 . 00
Total for 100,000 square yards $3,895.00
Total for 1 square yard .039
Local Management and Clerical Expenses:
Rent of office 1 year $400.00
Telephone, light, water, etc 100 . 00
Salary of superintendent, — 1 year 2,000 . 00
Cashier in charge of office, — 1 year 1,200 . 00
Clerks, timekeepers, etc., — 6 months 700.00
Proportionate part of winter pay roll 750 . 00
Total for 100,000 square yards $5,150. 00
Total for 1 square yard .051
General Officers and Offices:
Laboratory and general expenses . 030
Expense Securing Contracts:
Agent's commission, legal and traveling expenses, etc . 050
Material and Labor per Square Yard:
Subgrade, — 0.25 cubic yard, grading, rolling, etc 0. 125
Foundation,— 6 inches of concrete (1 P. C.: 3 S.; 6 B. S.).. .700
Binder — 1 inch complete . 170
Wearing surface — 2 inches:
22 Ib. of asphalt cement at $20 per ton .220
0.083 cubic yard sand at $1.20 .100
16 Ib. pulverized limestone at $3.50 per ton . 030
Fuel used at plant 020
Oil, waste and sundries . 002
Labor at plant . 060
Hauling material to street . 030
Laying and rolling . 050
Total for materials and labor $1 . 507
Cost of Guaranty:
5 years at 2| cents per year 1 . 00
Total cost of pavement, per square yard $1 . 777
454
ASPHALT PAVEMENTS
[CHAP, xvi
TABLE 46
COST OF CONSTRUCTION OF SHEET ASPHALT PAVEMENTS *
Laid in 1916
Ref.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
LOCALITY.
Amount
Laid'
Sq. Yd.
CONCRETE
BASE.
BINDER
COURSE.
WEARING
COAT.
Cost of Base, Binder
and Wearing Coat,
per Sq. Yd.
State.
City.
Thickness.
Proportions.
Thickness.
+3 «
II
r
Thickness.
Per Cent
Bitumen.
California
Connecticut. . .
Dist. of Col . ". '. .
Illinois
Indiana
Long Beach. . .
San Diego. . . .
Santa Monica.
Hartford
New Haven.. .
Washington. . .
Chicago
11 972
12043
34910
32 195
92742
154 076
1 248 000
36059
45 112
24046
18 702
107 506
131 489
11 000
113 200
47 747
9257
12408
21300
23 130
25000
7407
8000
2351
13 790
43 357
25000
74 134
66727
10585
146 173
6450
20851
4'
5'
5'
6'
6'
6
6
5
6
6
6
5
5
5
4
6
4-6
6
6
5
6
6
4
6
6
6
6
6
5
5
5
5
5
1 3 6
135
1 3 6
2 3
3 6
3 7
3 6
3 6
3 6
3 5
7G
5
3 6
2i :4
2 4
3 6
3 6
3 6
3 6
3 6
3 6
3 6
3 6
3 6
3 6
3J :6
3 6
3 6
2* :5
3 6
3 6
2* :6
1:3 5
1"
1"
1"
1*
li
li
11
li
1
T
i*
1
fr
9
li
li
-jr
l
5^-8'
4-6
5.5
6
"e"
"s"
5-8
5-8
;'
/
!
) •
\
1
\
\
\ .
;.';;:
$1.56
1.50
1.22
1.58
1.84
1.66
1.89
1.81
1.70
1.56
1.81
1.66
1.68
2.10
1.26
1.74
1.37
2.04
2.14
1.90
1.60
1.68
1.73
2.25
1.99
1.99
1.98
2.15
1.90
1.80
1.40
1.86
1.99
10-12
"ii
10.5
11
Moline .
Ft. Wayne
Goshen
South Bend. . .
Vincennes. . . .
Mason City. . .
Sioux Citv. . . .
Iowa
Kansas
Kentucky
Newton
Louisville
Baltimore. . . .
Detroit
. .^. .
16^13
9-is
Michigan
Minnesota
Nebraska
New Jersey ....
New York
Duluth
Omaha
Elizabeth
Syracuse
Durham
Cincinnati. . . .
Lakewood . . .
Toledo
Xenia
North Carolina.
Ohio
Pennsylvania. .
Texas
Utah
Washington. . . .
West Virginia . .
Wisconsin
Pittsburg
Beaumont. . .
Ogden
Seattle
Charleston. . . .
Racine
in the past makes it impossible to make comparisons extending over
any considerable length of time.
2. There are almost no data as to the amount, nature and dis-
tribution of the travel on city pavements (§ 34); and without such
data it is impossible to determine the service obtained from a pave-
ment or to make any accurate comparisons as to the cost of its
maintenance.
3. Many of the records that have been kept fail to discriminate
between pavements on streets with and without street-car tracks;
and even if they do state the presence of a street-car track, they
often fail to state the method of computing the area maintained.
4. Some cities include in the cost of maintenance the expense of
*Municipal Engineering, Vol. 52 (1917), p. 63-65.
ART. 1] SHEET ASPHALT PAVEMENTS 455
repairing cuts made by plumbers, gas-fitters, electricians, etc., which
has nothing to do with the cost of maintenance proper, i. e., with the
durability or wear of the pavement.
5. In some cities the repairs are made by contract, and in some
by the city's force. In some of the former cases, the amount of
repairs is so small and other conditions are such as to eliminate com-
petitive bidding; and hence the cost is abnormally high and con-
sequently valueless. Sometimes when the repairs are made by the
city's force, the record does not include all the elements of the cost.
For example, the following items of cost are often omitted: (a)
Interest on the cost of the plant and equipment employed in making
the repairs; (6) interest on working capital; (c) a charge for depre-
ciation of plant and equipment; (d) a charge for supervision and
office expense; and (e) material and labor supplied by other municipal
departments.
6. Almost no records of the cost of maintenance state anything
as to the condition of the pavement at the beginning or the end of
the period considered. It is probably impossible accurately to
make such an inventory; but the bearing of such an inventory upon
the results should at least be considered. The failure to consider
this phase of the subject is the same as though a merchant should
attempt to compute his annual profits without an inventory at the
beginning and the end of his fiscal year. See §1232.
882. Obviously it is practically impossible in any city to segre-
gate the cost of maintenance according to the quality and the age
of the pavement, the amount of travel, and the presence or absence
of car tracks; but a considerable improvement upon the present
practice of most cities is entirely feasible and very desirable.
Owing to the dearth of accurate and definite data, it will not be
possible to give much reliable or valuable information on the cost of
maintenance of sheet asphalt pavements. Further, since few
contractors are equipped especially for making repairs, and since the
public has no knowledge of the cost of the work to the contractor,
the only data submitted will be those obtained with municipal repair
plants.
In this connection it is not wise to consider pavements laid before
about 1895, since before that time the importance of a proper
gradation of the mineral aggregate was not understood, and con-
sequently such pavements are likely to be much inferior to the bast
pavements laid later.
883. Municipal Repair Plant. A sheet asphalt pavement when
456 ASPHALT PAVEMENTS [CHAP. XVI
constructed in accordance with good practice is a reasonably satis-
factory and economical form of pavement; but it usually requires
repairs at an earlier period in its life than most other pavements,
and the total cost of maintenance during its useful life is also some-
what greater. It is a high-grade pavement, and therefore should
have more careful and skilful attention than most other pavements.
For these reasons the repair of a sheet-asphalt pavement when
needed is a vitally important matter. In the early history of asphalt
pavements the repairs were made by the contractor whose chief
business was to build new pavements; but in recent years many
cities have established municipal asphalt repair plants. The main
conditions leading to this change in practice were as follows :
1. Usually there was little or no competition among contractors
for the contract to repair asphalt pavements. The equipment and
skill required in laying sheet asphalt pavements is so great as usually
to limit the number of contractors doing this kind of work; and
while competition with other forms of pavements is likely to keep
the first cost of asphalt pavements within reasonable limits, there
are so few asphalt-paving contractors that usually there is no com-
petition for the maintenance of such pavements. With a municipally
owned repair plant the city virtually becomes a competitor of the
ordinary contractor.
2. The plant and the other equipment for pavement construction
is not suitable for pavement repairs. The construction plant and
equipment is designed for turning out large quantities of material,
while a repair plant should be designed for turning out small quan-
tities.
3. With a municipal repair plant old material may be used in
repairing pavements that have about reached the end of their eco-
nomical life, and thus utilize the old material without injuring the
pavement.
4. Since the repair plant is small, repairs may be made con-
tinually and when needed, instead of being allowed to accumulate
as usual under the contract method until enough repairs are called
for to warrant either starting up a large plant or diverting the plant
from new construction to repair work.
884. The saving through the municipal repair plant has usually
been quite considerable. Table 47 shows the results for Washing-
ton, D. C., and is fairly representative of the results obtained in
other cities. For a more detailed comparison for the experience
of Brooklyn, N. Y., showing a more marked saving, see Engineering
ART. 1]
SHEET ASPHALT PAVEMENTS
457
and Contracting, Vol. 38 (1912), page 68; and for somewhat similar
data for Niagara Falls, N. Y., see Engineering Record, Vol. 69 (1914),
page 256.
TABLE 47
COST OF REPAIRS OF SHEET ASPHALT PAVEMENTS IN WASHINGTON, D. C.*
Including coal-tar surface, and excluding all pavements under guaranty.
REPAIRED BY CONTRACT.
REPAIRED By MUNICIPAL
PLANT.
Year.
Cents per
Sq. Yd.
Year.
Cents per
Sq. Yd.
1908
3.8
1913
2.0
1909
2.3
1914
1.9
1910
2.6
1915
1.9
1911
2.2
1916
1.8
1912
2-4J
885. Cost of Repairs. In Brooklyn with Municipal Plant. Table
48 shows the detailed cost of making repairs to sheet asphalt
pavements in Brooklyn, N. Y., with a municipal repair plant.
TABLE 48
COST OF MAINTENANCE OF SHEET ASPHALT PAVEMENTS
In Brooklyn, N. Y., in 1911, with Municipal Repair Plant f
COST IN PLACE PER CUBIC FOOT OF
UNCOMPRESSED MIXTURE.
Items of Expense.
Repairs.
Repaying.
Wearing
Coat.
Binder
Course.
Wearing
Coat.
Binder
Course.
Supervision and fixed charges
Supplies, repairs, etc
$0.025
0.067
0.178
0.046
0.182
0.060
$0.024
0.057
0.097
0.043
0.174
0.057
$0.025
0.059
0.178
0.046
0.213
0.080
$0.024
0.057
0.097
0.043
0.203
0.076
Materials
Plant labor
Street labor
Trucking
Total
$0.558
$0.452
$0.601
$0.500
886. In Buffalo by Contract. Buffalo, N. Y., has long been noted
for the extent of its sheet asphalt pavements, and also for the
accuracy and completeness of its records concerning the cost of
* Private letter from Capt. J. J. Loving, Corps of Engineers, U. S. A., Assistant to the Engi-
neer Commissioner of the District of Columbia.
t Engineering and Contracting, Vol. 38 (1912), p. 68.
458
ASPHALT PAVEMENTS
[CHAP, xvi
repairs of pavements. The annual reports of the Bureau of Engi-
neering contain voluminous statistics on the cost of construction
and repair of all kinds of pavements. For example, the report for
the fiscal year ending June 30, 1916, contains 179 pages of tabular
matter showing the width, length, area, original cost, and the cost
of repairs for each year after the expiration of the guaranty, of
all the sheet asphalt pavements in the city. The report also
gives a summary of similar data for several years back; and in
addition contains several pages of data concerning the amount and
age of all pavements, by whom built, the kind of materials
used, etc.
The annual cost of repairs in Buffalo from 1902 to 1916 for
substantially 3,000,000 sq. yd., varied from 1.30 to 8.27 cents per
sq. yd., the average for the fourteen years being 5.06 cents per
sq. yd.
The cost of maintaining 2,369,191 sq. yd. was 4.45 cents per sq. yd.
per year; and the cost of repairs was 6.46 cents per sq. yd. per year.
The average life of sheet asphalt pavements replaced between 1878
and 1906, was 20.51 years, being 21.77 years for streets without car
track and 18.36 for streets having car tracks.
Table 49 gives some of the details concerning the cost of repairs
of sheet asphalt pavement for the year ending June 30, 1916.
TABLE 49
DATA ON REPAIRS OF SHEET ASPHALT PAVEMENTS
Buffalo, N. Y., 1915-16, by contract *
REF
No.
ITEMS.
CONTRACT
PRICE.
AVERAGE
QUANTITY
PER SQ. YD.
COST
PER
SQ. YD.
1
2
3
July 1 to December 31, 1915:
Wearing coat, per cubic foot
Binder course (open), per cubic foot. . . .
Asphalt cement, per gallon
SO. 37
0.17
0 18
1.3673
0.7276
0 1205
$0.5059
0.1237
0 0217
4
Labor, per square yard. . . .
0 51
0 51
5
Total
$1 1613
6
January 1 to June 30, 1916:
Wearing coat, per cubic foot
0 33
1 3843
0 4^68
7
8
Binder course (open), per cubic foot ....
Asphalt cement, per gallon
0.17
0 18
0.8570
0 1204
0.1456
0 0217
9
Labor, per square yard
0 47
0 47
10
Total
$1 0941
* Annual Report Bureau of Engineering, 1915-16, p. 68.
ART. 1]
SHEET ASPHALT PAVEMENTS
459
Fig. 162 shows the cost of repairs of sheet asphalt pave-
ments at different ages. The pavements laid before 1898 were
ZO
25
5 10 15
Age of Paremenf-
FIG. 162. — COST OF REPAIRS OF SHEET ASPHALT PAVEMENTS IN BUFFALO.
on a 5-year guaranty, and those after on a 10-year guaranty. The
A%
curve in Fig. 162 is that for -— -, in which A is the age of the pave-
1«5
ment in years.
887. MAXIMUM GRADES FOR ASPHALT PAVEMENTS. Until
within a few years, it has been assumed that the maximum per-
missible grade for a sheet asphalt pavement was 2 or 2^ per cent;
but experience has shown that this limit is too low. It is now gen-
erally conceded that sheet asphalt may be laid on grades of 5 or
6 per cent, particularly in residence streets — where a clean, smooth,
noiseless pavement is specially desirable, and where there is usually
no great amount of travel. With a 5 or 6 per cent grade, there
may be a few days each year when the pavement is icy and too
slippery for either comfortable or safe use. In New York City, on
a street having a 6 per cent grade paved with asphalt on the sides
460 ASPHALT PAVEMENTS [CHAP. XVI
and granite in the center, the traffic as a rule seeks the asphalt
rather than take the granite; and in the same city traffic has de-
serted one street having a 5 per cent grade paved with granite for
another having a 6 per cent grade paved with asphalt. A number
of cities have sheet asphalt pavements upon a 7 per cent grade, as,
for example, Peoria, 111., Grand Rapids, Mich., Syracuse, N. Y.,
Troy, N. Y.; and Omaha, Neb., and St. Joseph, Mo., have asphalt
pavements on an 8 per cent grade. Scranton, Pa., has a short piece
of asphalt on a 13 per cent grade; San Francisco, Cal., a piece on a
16 per cent grade; and Pittsburg, Pa., one on a 17 per cent grade.
A committee of the American Society of Civil Engineers recom-
mends 5 per cent as the permissible maximum grade for sheet asphalt
—see Table 15, page 57.
888. CROWN FOR SHEET ASPHALT PAVEMENTS. The special
committee of the American Society of Civil Engineers recommends
that the crown shall be between £ and t of an inch per foot of the
half width, see Table 16, page 65.
889. MERITS AND DEFECTS OF SHEET ASPHALT PAVEMENTS.
The advantages possessed by monolithic asphalt pavements con-
structed as described above are: (1) they produce neither dust nor
mud; (2) they are comparatively noiseless, except for the clicking
of the horses' shoes; (3) they do not absorb or retain noxious liquids,
but facilitate their prompt discharge into the gutters and storm-
water sewers; (4) they reduce the force of traction to a moderate
amount (see Table 8, page 21); and (5) they afford a reasonably
good foothold for horses.
The defects of sheet asphalt pavements are: 1, the first cost is
comparatively great; 2, the cost of maintenance is large; and 3,
such pavements are generally considered too smooth for steep grades.
For a discussion of the relative merits of the different pavements,
see Chapter XX.
890. SPECIFICATIONS FOR SHEET ASPHALT PAVEMENTS. The
American Society for Municipal Improvements on October 14,
1915, adopted Specifications for Sheet Asphalt Paving, printed
copies of which may be had of the Secretary of the Society for a
nominal sum. The specifications cover only the selection, prepara-
tion, and laying of the materials for the binder course and the
wearing coat. These specifications are of the so-called blanket type,
that is, they contain general requirements for the asphalt and
asphalt cement which are intended to include all the different
kinds of asphalt.
ART. 2] ASPHALT-CONCRETE PAVEMENTS 461
For a statement of the objections to this form of specification,
see § 532; and for alternate restricted specifications for asphalt for
other kinds of pavements, see § 534-41.
ART. 2. ASPHALT-CONCRETE PAVEMENTS
891. An asphalt-concrete pavement consists of a foundation of
either bituminous or hydraulic-cement concrete, and a wearing coat
of asphalt concrete. There are two differences between asphalt
concrete and the bituminous concrete discussed in Chapter X, —
Bituminous Macadam and Bituminous Concrete Roads. 1. Asphalt
concrete is made with asphaltic cement, while the bituminous con-
crete may be made with either asphalt or tar. 2. Bituminous con-
crete is made with only the care usually employed in making
hydraulic cement concrete, while asphaltic concrete is usually pro-
portioned, mixed and laid with approximately as much care as sheet
asphalt pavements.
The difference between a sheet asphalt pavement and an asphalt-
concrete pavement is in the maximum size of the aggregate. In
the former all of the aggregate will pass a No. 8 sieve, while the latter
may contain 1^-inch stone. The ordinary sheet asphalt pavement
could with some propriety be called a sand asphalt pavement or an
asphalt mortar pavement, to distinguish it from an asphalt con-
crete pavement.
892. DEFINITIONS. There are several types of asphalt-con-
crete pavements, the best known of which are : Bitulithic, Warrenite,
Amiesite, and asphalt concrete.
893. Bitulithic Pavement. This is a patented form of pave-
ment in which the wearing coat is composed of bitumen and mineral
aggregates ranging from 3 inches down to an impalpable powder.
The bituminous mixture is usually mixed in a non-portable plant.
The aggregate is chiefly crushed stone; and the aggregate of the
seal coat is stone chips.
Six patents (No. 727,505 to 727,512) were issued to Frederick F.
Warren, Newton, Mass., between May 16, 1901, and April 14, 1902,
for preparing asphalt for paving purposes and for closely related
forms of asphalt-pavement construction. Of the eighty-two speci-
fications in these patents, sixty-five relate to the proportions and
method of laying the pavement. Apparently the intention is to
cover such gradations of the ingredients as will secure maximum
density and maximum stability. The density of a sheet asphalt
462 ASPHALT PAVEMENTS [CHAP. XVI
pavement made with limestone filler is 2.20 to 2.22 (§ 832); but the
density of a bitulithic pavement made of the same materials may be
2.28, and if the bitulithic is made of trap the density may be 2.50
or even more. This type of pavement is designed for city streets
and it has been largely used for this purpose (see page 320).
894. There has been much controversy and considerable litigation
as to the scope and meaning of these patents, and it is impossible
to state in a single series all the gradations included. Two gradings
actually employed in laying bitulithic pavements are shown in
the following table.
Bitun
Minei
u
it
tt
n
it
tt
tt
tt
tt
it
GRADINGS OF WEARING COAT OF
ien
BITULITHIC PAVEMEJ
No. 1.
7.6%
STS*
No. 2.
7.02%
4.58
3.99
2.76
7.88
1.27
2.39
2.13
3.77
4.85
12.75
46.61
•al aggregate
tt
it
tt
tt
tt
it
it
n
tt
tt
Total
passing
n
tt
tt
it
it
it
tt
tt
it
•i
200-mesh . .
4 9
100-mesh.
4 6
80-mesh
3 2
50-mesh.
7 3
40-mesh.
3 1
30-mesh
24
20-mesh.
2 2
10-mesh
5.1
j-inch ,
91
i-inch.
19 3
1^-inch
31 2
100 0%
100.00%
* Agg's Construction of Roads and Pavements, p. 308.
895. Warrenite Pavement. This type of pavement is covered
by the Warren patents (§ 893); and is especially designed for rural
roads. The mixing is usually done in a semi-portable plant; and
the ingredients are not proportioned with as much care as for bitu-
lithic pavements (§ 893). Sand is largely used in the body of the
wearing course and wholly for the seal coat.
896. Amiesite Pavement. This is a proprietory mixture of
asphalt cement, sand, and broken stone up to lj inches in diameter.
It is mixed at a plant, and is shipped in cars to the city where it is
to be laid. During transit the mixture cements into a solid mass,
and must be heated before it can be shoveled from the car. To
heat it, holes are dug into the mass and steam is blown into them
and permeates the whole mass and softens it. This type of pavement
is laid without a concrete base. The mixture is laid cold, usually in
two courses, the lower being 2J or 3 inches thick, and the wearing
coat 1 or \\ inch.
ART. 2]
ASPHALT-CONCRETE PAVEMENTS
463
897. Topeka Mixture. An asphalt pavement somewhat like
the patented bitulithic pavement was laid in Topeka and Emporia,
Kansas; and as a result of a suit for infringement of patent No.
727,505, the U. S. Circuit Court in 1910 decided that the following
gradation did not infringe said patent:
Bitumen 7 to 11 per cent
Mineral aggregate passing 200-mesh 5 to 11 "
40-mesh 18 " 30 "
" " " 10-mesh 25 " 55 "
4-mesh 8 " 22 "
" " " 2-mesh not over 10
The sieves are to be used in the order named.
Notice that the aggregate is mostly sand, and J-inch and J-inch
stone. Notice also that a wide variation in the grading is possible
under the above specifications; and consequently many somewhat
different gradings have been designated as Topeka mixture. Since
1911 many thousands of square yards of roads and pavements have
been laid under the above so-called Topeka specifications.
898. The American Society of Municipal Improvements recom-
mends the following grading,* which the Warren Brothers Co. has
agreed does not infringe its patents. f
Bitumen 7 to 9 per cent
Mineral matter passing 200-mesh 7
" " " 80-mesh 10
" " " 40-mesh 10
20-mesh 10
" " " 8-mesh 10
" " " 4-mesh 15
2-mesh.. . 5
10
20
25
25
20
20
10
899. Asphalt paving blocks (see Art. 4 of this chapter) have long
been made of a mixture substantially the same as the so-called
Topeka mixture.
900. Stone-filled Sheet-Asphalt Pavement. The so-called To-
peka mixture is frequently described as the wearing-coat mixture
for a sheet asphalt pavement to which has been added j-inch and
^-inch broken stone; and is sometimes referred to as a stone-filled
asphalt mixture, and also as a fine asphaltic concrete.
Mr. Clifford Richardson, a recognized authority on asphalt
paving, in Engineering Record, Vol. 65 (1912), page 718, and
again in Vol. 70 (1914), page 634, shows that for the best
* Proceedings, 1915, p. 415.
t Ibid., p. 423.
464
ASPHALT PAVEMENTS
[CHAP, xvi
results possible within the limits of the above decree, the finer
part of the mixture should conform to the standard grada-
tion for sheet asphalt (§ 827) and that as much as J-inch and J-inch
broken stone should be added as the ruling of the court will
permit. He states that the two gradings in Table 50 have given
satisfaction. The one on Riverside Drive was laid in 1913; and
the one in Rochester has been in use since about 1902. Mixtures
of this type have been laid extensively in the last few years.
TABLE 50
COMPOSITION OF STONE-FILLED SHEET ASPHALT PAVEMENT
(Best Topeka Mixture)
Ingredients.
RIVERSIDE DRIVE,
N. Y. CITY.
ROCHESTER, N. Y.
Total
Mixture.
Finer
Portion.
Total
Mixture.
Finer
Portion.
Bitumen
8.9%
11.9
14.5
18.6
18.9
19.1
8.1
11.1%
16.5
20.1
25.9
26.4
8.9%
12.3
10.8
24.2
16.2
21.5
5.4
H.1%
17.1
15.0
33.7
22.7
Mineral matter passing
u (
it i
(i i
tf t
« t
Total
200-mesh
80-mesh.
40-mesh
10-mesh.
4-mesh
2-mesh
100.0%
100.0%
100.0%
99.6%
901. Asphalt-concrete Pavement. An asphalt-concrete pave-
ment, in a narrower sense than the way in which that term is used in
the heading of this article, is a pavement in which the gradation of
the aggregates is not as carefully made as in the four forms just
mentioned. It is essentially a bituminous concrete (Art. 2, Chapter
X), in which the bituminous cement is asphalt.
The standard specifications of the American Society of Municipal
Improvements for asphalt concrete, provide a wearing surface of
asphalt cement and broken stone, and require the following gradation
of the broken stone: " All of the broken stone shall pass a IJ-inch
screen; not more than 10 per cent, nor less than 1 per cent, shall be
retained upon a 1-inch screen; and not more than 10 per cent, nor
less than 3 per cent, shall pass a J-inch screen. "
902. MIXING AND LAYING ASPHALT CONCRETE. The method
of mixing and laying is substantially the same whatever the gra-
dation of the aggregate. With the most careful grading the aggre-
gate is usually heated and then separated into three sizes, not includ-
ART. 2] ASPHALT-CONCRETE PAVEMENTS 465
ing the dust or filler. Each size is stored in a separate bin. The
predetermined weight of each size is drawn from the bin into a box
resting upon a scale platform. The proper amount of asphalt cement
is determined by weight. The mineral matter and the cement are
heated separately to the proper temperatures, and are then put into
a mixer somewhat like that shown in Fig. 146, page 419. The lim-
iting temperatures are substantially the same as for the wearing
coat of sheet asphalt pavements (§ 838).
An asphalt concrete pavement is usually laid without any binder
course, as the wearing course has sufficient stability to prevent its
flow or displacement under travel.
The mixture is hauled to the street, dumped, spread and raked,
much the same as for the wearing coat of a sheet asphalt pavement
(§ 839^0).
The rolling of an asphalt concrete pavement is a very important
matter. The weight of the roller and the temperature at which
the concrete is rolled depends upon the type of the mixture. The
roller should be as heavy as possible without displacing the paving
material. The large aggregate gives the mixture comparatively
great stability, and hence it does not squeeze out under the roller
as does the sand- or sheet-asphalt mixture; and therefore it is best
to begin rolling as soon as the mixture is spread. The rolling is
usually done with a tandem roller weighing 8, 10, or 15 tons; or
with a roller of the three-wheel type weighing 10 tons. The rolling
should be continued until the roller no longer leaves a mark upon
the surface. The early and heavy rolling aids in securing a firm
union between the asphalt concrete and the base or foundation.
903. After the rolling is completed, a coating of hot asphalt
cement is applied to the surface of the pavement at the rate of about
J to \ gallon per square yard, which is immediately covered with a
layer of hot J- to f-inch stone chips at the rate of about 25 Ib. per
square yard. The pavement is then again rolled until the chips are
firmly bedded in the cement, and until the surface is dense and
waterproof. This seal coat is an essential feature of an asphalt-
concrete pavement, since except the best bitulithic the body of the
asphalt concrete is not waterproof.
904. COST OF ASPHALT-CONCRETE PAVEMENTS. Topeka
Mixture. Table 51, page 466, is an accurate analysis of the cost to the
contractor of laying 18,000 square yards of Topeka asphalt-concrete
pavement in Albany, Oregon, in 1916. The pavement consisted
of a 3§-inch asphalt-concrete base, and a IJ-inch top of Topeka
466
ASPHALT PAVEMENTS
[CHAP, xvi
mixture. The base and top were spread and rolled separately. For
similar data, giving almost the same results, for another contractor
in the same city the preceding year, see Engineering News, Vol. 73
(1915), page 1038.
905. Various Cities. Table 52, page 467, shows the composition
and cost of the several types of patented asphalt-concrete pavements
in 33 cities. Table 52 was compiled from the statistics from 142
cities, and contains all the places for which the data were complete.
906. MERITS AND DEFECTS OF ASPHALT-CONCRETE PAVE-
MENTS. The merits and defects claimed for asphalt-concrete pave-
ments are: 1. An asphalt concrete pavement, in the broader mean-
ing of that term, is cheaper than the sheet asphalt pavement, since
the use of the coarser material reduces the voids and also the surface
TABLE 51
DETAILED COST OF TOPEKA ASPHALT PAVEMENT *
Average for one week's run
Items.
Si-inches
Base.
li-inches
Top.
5-inches.
Total.
MATERIALS:
Gravel @ $0 . 86 per cubic yard
Sand, fine @ $1.50 per cubic yard
coarse @ $1.80 per cubic yard
Asphalt @ $12 per ton
$0.062
O.C26
0.024
0 078
$0.013
0.032
0.058
0 089
$0.075
0.058
0.082
0 167
Fuel oil @ $1.25 per bbl
Wood @ $4 per cord.
0.016
005
0.008
0 002
0.024
0 007
Coal
005
0 003
0 008
Total for materials
.216
205
421
LABOR:
Plant
0 052
0 031
0 083
Street.
0 044
0 032
0 076
Teams
0 013
0 008
0 0021
Total for labor
0 109
0 071
0 180
GENERAL EXPENSE:
Mixing plant, interest, depreciation, repairs
0 10
Roller and small tools
0 05
Overhead.
0 08
Profits . . .
0 08
Total general expense
31
Grand total
96
COST PER SQUARE YARD.
* Engineering News, Vol. 76 (1916), p. 103.
ART. 2]
ASPHALT-CONCRETE PAVEMENTS
467
to be covered with cement, and hence less bitumen is required. The
difference in bitumen is from 1 to 3 per cent. 2. An asphalt-con-
crete pavement is cheaper than a sheet asphalt one, since the former
is laid in a single course. 3. An asphalt-concrete pavement is less
slippery than a sheet asphalt pavement, owing to the use of the stone
fragments.
But, on the other hand, an asphalt-concrete pavement will not
endure under heavy travel, particularly horse-drawn steel-tired
traffic, as well as a sheet asphalt pavement, since sooner or later
the larger stones will be fractured and leave two uncemented sur-
faces which will permit motion, wear and disintegration.
TABLE 52
COST OF PATENTED ASPHALT-CONCRETE PAVEMENTS *
Laid in 1916
Ref.
No.
LOCALITY.
Area
Laid,
Sq. Yd.
CONCRETE BASE
Thick-
ness of
Wear-
ing
Coat.
Cost of
base
and top
Sq. Yd.
State
City.
Thick-
ness.
Propor-
tions.
BITULITHIC PAVEMENT
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
an
California
Tucson
67219
104 672
95682
15870
47431
13660
27600
20004
4626
1 864
35893
18512
7472
89319
8 147
8680
17132
25000
35712
10914
4 112
42734
11 880
4941
IVEMENT
23900
9801
18830
16644
80000
5
4-6
5
5
4
5
5
6
5
6
4
6
6
5
6
4
6
5
5
6
6
6
5
5
6
6
4
5
4
1:3:6
1:3:6
1:2 :4
1:3:6
1:3:5
1:3:5
1:3:5
1:2:4
1:3:5
1:2:4
1:3:6
1:3:6
1:3:6
1:3:6
1:3:6
1:3:6
1:3:6
1:2:4
1 :6
1:3:6
1:3:6
1 :2§ :5
1:3:6
1:3:5
1:3:6
stone
1:3:5
1 : 2J : 5
1:3:6
2i
2
2
2
2
2
2
2
I"
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
?,
Los Angeles
Richmond
Santa Monica
Creston
Knoxville
Iowa
•i
Mt. Pleasant
Eveleth
St. Cloud
Virginia
Billings
Minnesota
New Jersey
Harrison
Irvington
New Brunswick . . .
Binghamton
Herkimer
New Rochelle
Bismarck. :
Fargo
<> •
New York
North r^akota. .'.'...
Ohio
Cincinnati
Lakewood
Sioux Falls
South Dakota
Texas
San Antonio
Sheridan
Wyoming
WARRENITE PJ
Berkeley
Connecticut
Montana
New York
North Carolina.. .
Winsted
Great Falls . .
Elmira
Raleieh. .
$2.35
2.16
1.58
2.16
1.40
1.89
1.98
1.89
2.50
2.15
2.44
1.91
2.29
2.40
2.65
2.30
2.18
1.50
2.38
2.14
2.57
2.20
2.10
.30
2.12
1.40
1.35
1.90
2.25
1.52
AMIESITE PAVEMENT
31
Connecticut
Danbury
8000
macadam
2
1 05
32
New York
New Rochelle
19 297
2
1 29
33
Salamanca
12215
stone
2i
1 65
* Municipal Engineering, Vol. 52 (1917), p. 248-49.
468
ASPHALT PAVEMENTS
[CHAP, xvi
Table 53 shows the composition and cost of the non-patented
asphalt-concrete pavements in 32 cities.
TABLE 53
COST OF NON-PATENTED ASPHALT PAVEMENTS *
Laid in 1916
LOCALITY.
CONCRETE BASE.
«*-
Cost of
Ref.
No.
State.
City.
Amount
Laid,
Sq. Yd.
hickness.
Propor-
tions.
hickness o
Binder
Course..
hickness o
Wearing
Coat.
Binder,
and
Wear-
ing
Coat
M
H
H
TOPEKA MIXTURE
1
9
California
Berkeley
Fort Dodge. . . .
36660
40000
5'
6
3
Kansas
Great Bend. . . .
41800
4
4
5
Michigan
Topeka
Ludington
64 120
700
5
6
6
Missouri
Springfield
7073
4
7
8
9
Nebraska
North Carolina. . . .
Texas. .
Norfolk
Raleigh
Taylor . .
63000
70000
130 000
6
4
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Alabama
ASPHAL
Gadsden
LaGrange
Oak Park
South Bend. . . .
Cedar Rapids . .
Webster City . .
Emporia
Manhattan ....
Louis ille
Grand Rapids. .
St. Paul
Greenwood ....
Beatrice
Trenton
Batavia
Niagara Falls. .
Toledo
r CONCRE
23 i?i '
185 241
37444
35000
16622
9875
39930
20092
14 110
91 690
8930
36100
23334
1800
4560
12 174
18313
62000
26512
32 126
126917
12200
39593
96 690
22475
TE
5'
6
6
5
4
5
4
5
6
5
5
4
5
5
5
6
6
5
4
5
5
6
4
4
5
5
Illinois
Indiana
Iowa
Kansas
Michigan
Minnesota
Mississippi
Nebraska
New Jersey
New York
Ohio
Oregon
South Carolina. . . .
South Dakota
Tennessee
Texas
Portland
Greenville
Mitchell
Chattanooga. . .
Houston
Salt Lake
N. Yakima. . ..
Charleston
Fond du Lac. . .
Utah
Washington
West Virginia
Wisconsin
1
: 3 : 6
2"
$1 18
1
3 :5
2
1.57
1
1
21 :5
21 :5
2 '
2
1.26
1.35
1
6
2
1.05
1
3 : 7
2
1.22
1
5G
2
1.51
1
3 :6
2
.63
1
2 :4
2
1.40
2 : 5
2"
2"
1.36
3 :6
2
1.36
3 : 6
. . „ .
2
1.55
7G
. . „ .
1.45
3 :5
1.49
3 : 6
*!
1.65
2* :5
2
1.27
5
2
1.22
3 I 6
2
1.68
31 :7
21 :5
2
2
1.65
1.60
3 :5
2
1.48
3 :6
2
1.57
3 : 6
2
1.75
21 :5
"2 '
seal
1.75
3 :6
2
2.40
31 :6
. „ . .
2
1.85
3 :6
11
1.30
3 : 6
. 2
1.30
51
2
1.80
3 :6
2
1.46
3 :6
2
1.80
3 :6
2
1.75
4 : 6
2
1.17
21 :5
2
1.78
3 : 5
2
1.53
* Municipal Engineering, Vol. 52 (1917), p. 193-95.
907. SPECIFICATIONS. The American Society of Municipal
Improvements on October 14, 1915, adopted complete " Specifica-
tions for Bituminous Paving," in which the binding material was the
asphalt specified therein; but in 1916 the Society amended the gen-
eral specifications by eliminating the special requirements for asphalt,
and stating that the asphalt, the flux, and the asphalt cement should
conform to the requirements for these materials given in the Spe-
cifications for Asphalt Paving ( § 542) . Printed copies of the amended
ART. 3] ROCK ASPHALT PAVEMENTS 469
specifications for " Bituminous Asphalt Concrete Paving " (Asphalt
Concrete Pavements) may be had of the Secretary of that Society
for a nominal sum.
With the above Specifications are printed also specifications for
the wearing coat of Bitulithic Pavements.
908. The above Society also publish Specifications for Bituminous
Concrete Pavements in which the mineral aggregate is the com-
plete product of a stone crusher and in which the binding material
may be any one of the asphalts described in § 539-40 or either of
the two tars described in § 574-75.
ART. 3. ROCK ASPHALT PAVEMENTS
910. A rock asphalt pavement is made by crushing asphaltic
limestone or sandstone, and laying it while hot upon a concrete
foundation. In Europe this is the common form; and when the
term asphalt pavement is used there, this kind is intended.
Rock asphalt pavements have been laid only to a comparatively
small extent in America. Rock asphalt pavements have been used
in a small way in California for many years, and San Francisco,
Los Angeles, and other cities have several miles of such pavements.
Apparently both asphaltic limestones and sandstones are used in
California; but the most of the so-called rock asphalts used for
paving purposes are asphaltic limestones.
A bituminous limestone to be suitable for paving purposes
should be as coarse-grained as possible, should contain between 9
and 10 per cent of bitumen soluble in carbon bisulphide, and should
contain very little matter volatile below 400° F. Often one or more
natural rocks are mixed to secure the proper proportion of bitumen;
and sometimes a natural asphalt is added to the natural rock to
increase the proportion of bitumen.
911. CONSTRUCTION. The asphaltic rock is quarried, and
then crushed to about egg size by toothed rollers. These pieces
are first reduced to powder and then sifted to uniform fineness.
The powder is dropped through a hopper into a revolving cylinder
like a coffee roaster, which is about 6J feet in diameter, and is sur-
rounded by a chamber the air in which is heated by a movable
furnace placed just below it. The cylinder itself revolves and,
since it is provided with blades arranged in screw form, the pow-
dered rock is well mixed with hot air and is thus thoroughly heated
to a temperature between 300° and 350° F. Specifications fre-
470 ASPHALT PAVEMENTS [CHAP. XVI
quently permit the rock asphalt to be heated to but 200° to 250° F.
When the powder & hot enough, the furnace is removed from under
the heater and a cart replaces it, into which the asphalt powder is
discharged and hauled to the street. The powder will retain its
heat for several hours and so admits of being carted long distances
without losing its heat, thus doing away with the necessity of having
roasters at the point where the surface is to be laid, as was at one
time the practice. For the best results, the mixture should be
delivered upon the street at a temperature of not less than 250° F.,
although specifications sometimes permit a temperature of but
190° F.
The heated powder is spread upon the concrete base to a uniform
thickness about 40 per cent greater than that required for the fin-
ished pavement. This must be done with great care in order that
the material, which while hot has a great tendency to consolidate,
may not be denser in one spot that another. The material is com-
pacted by rolling in much the same way as is described for the arti-
ficial asphaltic paving compound, except that as a rule the natural
rock asphalt is not consolidated to so great an extent as is customary
in laying- the artificial mixture. The evidence of this is that a rock
asphalt pavement will continue to shrink in thickness under traffic
for a year or two; while the artificial mixture shrinks but little, if
any, after completion.
912, The general appearance of the completed pavement is
much the same as that of the pavement made of the artificial mix-
ture, except that the European rock pavements are lighter in color.
The claim is that European natural rock asphalt pavements are
more slippery and less susceptible to changes in temperature than
are American artificial asphalt pavements.
Not infrequently the term rock asphalt pavement is inappro-
priately applied to a pavement made of an artificial mixture of sand
and of asphalt extracted from a natural rock.
ART. 4. ASPHALT-BLOCK PAVEMENTS
913. There are two general forms of asphalt pavements, the
sheet or monolithic and the block. Three forms of the first have been
fully described in the three preceding articles. The asphalt-block
pavement is constructed by first molding rectangular blocks com-
posed of asphaltic cement and crushed stone, and then placing these
blocks side by side upon a gravel or concrete foundation. In 1909
ART. 4]
ASPHALT-BLOCK PAVEMENTS
471
there were in this country about 5,500,000 square yards of asphalt
block pavement, see page 320.
Fig. 163 shows a perspective view and cross section of a carriage
way and foot way paved with asphalt blocks.
Fia. 163. — ASPHALT-BLOCK PAVEMENT.
914. THE BLOCKS. At first crushed limestone was used, but
now the blocks are made with crushed trap, granite, or gneiss.
The asphaltic cement is mixed substantially as for sheet pavements.
The proportions employed in making the blocks vary slightly with
the climate, and considerably with the fineness of the crushed stone;
but are about as follows:
Asphaltic cement 6 ,5 to 8 per cent
Limestone dust 10 " 15 " "
Crushed stone 67 " 78 " "
Since the blocks contain larger fragments than sheet pavements,
they contain a smaller per cent of voids, and hence can be made
with a slightly smaller per cent of bitumen.
The ingredients are mixed and heated about as for sheet pave-
ments, and are then molded while hot under heavy pressure. For-
merly the blocks were made 5 X 12 X 4 inches deep; and later they
were 4 X 12 X 3 inches deep, and also 5 X 12 X 3 inches deep;
but the blocks are usually 5 inches wide, 12 inches long, and 2, 2J
or 3 inches deep according to the traffic conditions. Tiles are made
now 8X8X2 inches deep; and also with a hexagonal top surface
having the same area as the square tile. The blocks are used for
carriage ways, and the tiles for foot ways.
The blocks are usually moulded under a pressure of about 2
472
ASPHALT PAVEMENTS
[CHAP, xvi
tons per square inch; and must be manufactured at such a temper-
ature that the materials will press together in a mass having a spe-
cific gravity of not less than 2.5 if made of trap, and of not less
than 2.35 if made of limestone.
915. The blocks are laid on a sand cushion or on a half-inch
mortar bed on a portland concrete foundation — recently the latter
is more common. After the blocks are laid, the surface of the pave-
ment is covered with clean, fine sand or hard fine stone screenings,
which are swept into the joints. The joints are not usually filled
with a bituminous cement, as the blocks are malleable, and there-
fore travel soon seals the joints and makes the pavement prac-
tically water-tight.
916. COST. Table 54 shows the cost of asphalt block pave-
ments in several representative cities.
TABLE 54
COST OP ASPHALT BLOCK PAVEMENTS *
Laid in 1912
LOCALITY.
Amount
Laid
in 1912
Sq. Yd.
CONCRETE BASE.
Depth of
of
Blocks.
Averate
Cost in-
cluding
Base and
Grading
per Sq. Yd.
State.
City.
Thick-
ness.
Propor-
tions.
New York
Jamestown
Niagara Falls
Port Chester
Rye
Rye
13408
56619
7 117
9800
18 500
4800
3800
5083
19000
51800
6000
5992
13000
6 185
5"
5
?
4
2
6
5J
1 2i :5
136
1 3 6
' ' 2>V ' '
I1
3
2
2*
2|
S2.941
3.201
2.132
2.56
2.20
2.65
1.94
2.251
2.08
1.73
2.10
2.95
3.42
2.65
New Jersey
Ohio
Michigan . .
W. New York
Ashland
1 3 5
1 2J : 5
1 3 5
1 3 4
Highland Park....
Mt. Clemens
Savannah
Georgia
Way Cross
Kingston
Toronto
"2J"
Canada
4
4
136
1 3 7
Regina
1 Includes curbs.
2 Does not include curbs.
* Engineering and Contracting, Vol. 39 (1913), p. 373.
917. MERITS AND DEFECTS. The advantages claimed for a
pavement of asphalt blocks over a continuous asphalt sheet are:
1. It is less slippery, owing partly to the joints and partly to the
roughening of the surface due to the use of a hard crushed stone.
2. It can be laid like any block pavement, and at the same time has
almost the continuity of a sheet asphalt pavement. 3. It can be
laid in cities where there is no asphalt plant. 4. It can be repaired
by removal of individual units by common labor without expensive
ART. 4] ASPHALT-BLOCK PAVEMENTS 473
plant and expert labor. 5. Having numerous joints, it is free from
irregular and unsightly contraction cracks.
The disadvantages of an asphalt block pavement in comparison
with a continuous asphalt sheet are: 1. Its first cost is more, since
the wearing coat of the block pavement is 2 inches or more thick,
while that of the continuous sheet is at most only 2 inches. 2.
The edges of the blocks chip off, and the pavement wears rough.
3. It is slightly more noisy. 4. Owing to its numerous joints,
it is less sanitary. 5. It is more expensive to clean. 6. Unless
bedded with unusual care, the blocks have a tendency to crack.
7. It is less durable, particularly under heavy or even moderate
traffic. 8. The blocks are not rigid, and do not hold their shape
as well as other paving blocks; and hence the pavement is not as
easy to repair as other block pavements.
CHAPTER XVII
BRICK PAVEMENTS
920. A brick pavement consists of brick set on edge on a suit-
able foundation — either concrete, gravel, or native soil. Brick
pavements have been used in Holland for perhaps a century, and to
a much less extent and for a shorter period in northern England.
Brick pavements were first used in the United States in 1870 at
Charleston, W. Va., a place having a population of 12,000. An
experiment was tried with a short section — less than a block; —
and in 1873 a block on the principal business street was laid with a
good quality of building brick, and remained in service until 1909 —
36 years. A block of brick pavement, laid in 1875 on a leading
business street of Bloomington, Illinois, a place of 20,484 popula-
tion in 1890, though constructed of an inferior building brick made
of a superior clay, continued in service for 20 years.
At present brick is the chief paving material employed in most of
the smaller cities of the Mississippi Valley, and it is used exten-
sively in many of the larger cities in that territory. In all parts of
this country, the use of brick for residence streets and light traffic
business streets is rapidly increasing. For the relative amount of
brick pavements in use, see page 320. Notice that in yardage of
what may be called durable pavements, brick ranks second. There
are in this country nearly two hundred plants devoted to the man-
ufacture of paving brick, some having annual outputs of 60,000,000
to 100,000,000 bricks.
921. The term brick pavements will be used in this chapter as
including the brick wearing coat of both rural roads and city streets.
The discussion will primarily be made with reference to city pave-
ments, since in extent they exceed rural roads; but later brick-paved
rural roads will be considered.
474
ART. 1] THE BRICK 475
ART. 1. THE BRICK
922. A paving brick is simply a brick which, owing to careful
selection of the clay and to skilful manufacture, is so hard and
tough that it will resist the crushing and the abrading action of
the travel.
923. THE CLAY. Three distinct classes of clay may be em-
ployed in the manufacture of paving brick: surface clays, impure
fire clays, and shales. Surface clays are almost exclusively used for
the manufacture of building bricks; and are not ordinarily suitable
for making paving bricks, since it is practically impossible to burn
them hard enough without their losing their shape. On account
of its infusibility, pure fire clay is unsuitable for making paving
brick, the brick being expensive to burn and lacking density, hard-
ness, and strength; but quite impure fire clay makes a fair quality
of paving brick, although the process of manufacture is compara-
tively expensive. Bricks made from impure fire clay are usually
light in color, varying from cream to buff, and ordinarily are quite
porous, absorbing from 2.5 to 5.0 per cent of water. Most paving
bricks are made from shale, — an impure, hard, laminated clay
which has been subjected to great pressure by the superincumbent
earth strata. Shale is widely distributed, and usually makes a
better and cheaper paving brick than either surface or fire clay,
although some fire clays make excellent paving bricks.
The different classes of clay so shade by imperceptible degrees
one into the other that it is impossible sharply to discriminate be-
tween them. Surface clays are soft and unconsolidated, and are
found at or near the natural surface. Shales are dense and rock-like;
but are easily reduced to powder, and are readily worked into a
plastic mass when mixed with water. Shale is often incorrectly
called soapstone, from which it differs in nearly every respect.
Shale is also frequently, but erroneously, called slate, from which
it differs only slightly in origin and composition; but slate, unlike
shale, can not be rendered plastic by mixing it with water. The
only method of distinguishing between shale and impure fire clay,
except by a kiln test, is the fact that shale gives a conchoidal frac-
ture while fire clay does not.
924. Chemical Composition. It is not wise to enter into any
extended consideration of the chemical composition of brick clays,
since the subject is very complicated, and since the engineer is
476 BRICK PAVEMENTS [CHAP. XVII
interested only in the physical properties of the finished product
and should not attempt to prescribe the materials or to limit the
methods employed by the manufacturer.
925. Physical Properties. A chemical analysis of a clay may
furnish sufficient evidence upon which to condemn it for brick-
making purposes, but never enough for its indorsement. The
following physical properties are important factors in determining
the value of a brick clay: 1, its plasticity; 2, the amount of water
required to make a plastic mass; 3, the amount of shrinkage, both
in burning and in drying; 4, the rapidity of drying and also of burn-
ing; 5, the temperature of incipient and complete vitrification;
6, the density before and after burning; and 7, the strength of the
burned brick.
926. MANUFACTURE OF THE BRICK. Soft, homogeneous clay
may be run through rollers, to crush the lumps, and from the crusher
it may go directly to the moulding machine; but it is usually desirable
to run it first through a pug mill, where it is mixed and worked
with water into a plastic mass. Hard clays and shales are usually
reduced to a powder in a dry pan, which consists of two heavy
rollers or wheels running in a revolving pan having a perforated
bottom. It is important to have the clay finely pulverized, because
it will then fuse at a lower temperature, and also because fineness
is necessary to the production of ari even and close-grained texture
which conduces to make the brick tough and impervious. The
powdered clay is screened and then tempered with water in the pug
mill or a wet pan. Fire clays are sometimes both crushed and
tempered in a wet pan, which is similar to a dry pan except that
the bottom is water tight. The wet pan gives better results than
the pug mill, as the clay can be retained in the pan until it is thor-
oughly tempered, but as it requires a large plant, and takes more
labor and power, it is not usually employed in making paving brick.
The more thoroughly the clay is worked or tempered, the more
uniform and better will be the brick.
927. Moulding. Paving brick are now made by the stiff-mud
process. The moulding is done by an auger machine which forces
the tempered clay or stiff mud through a die, thus giving a contin-
uous bar of compressed clay. Fig. 164 shows an auger brick-
moulding machine. At the right is the auger machine, in the
middle the bar of clay, and at the left the cutting table or the
machine for cutting the bar into bricks.
Instead of an auger producing a continuous stream of clay,
AET. 1]
THE BRICK
477
reciprocating plungers are sometimes employed, which give an inter-
mittent bar. The auger machine is the cheapest, and is almost uni-
versally used.
928. The weak point of the stiff-mud process is the laminations
that must inevitably result from pushing a stream of clay through
a fixed die. The friction of the sides of the die will cause differential
speeds in the flow of the clay, and these variations must necessarily
result in laminations in the clay bar. If the air has been expelled
FIG. 164. — AUGER BRICK-MOULDING MACHINE.
from the clay by the pug mill, these lines can be largely closed up
again by a properly shaped die, and a first-class brick will result in
which the laminations will be inconspicuous and of no importance;
but if the air has not been expelled, or if the tapering former and
the die are not properly designed, there will be a number of concentric
lines that divide the cross section of the brick into a series of shells
or concentric cylinders which greatly weaken the brick. These
laminations vary with the character and the condition of the clay;
and as a rule, the more plastic the clay the more prominent the
laminations.
929. Cutting the Brick. Formerly the size of the die was such
as to give a bar of clay about 4J by 2J inches, which the automatic
machine cut into lengths of about 9J inches by forcing a wire through
it, thus producing what is called an end-cut brick. But now the
die is usually a little greater than 8J by 4 inches, the excess depend-
ing upon the shrinkage of the clay in drying and burning, and the
bar is cut into sections 3J inches thick, thus producing a side-cut
brick. Substantially all paving brick are now side-cut.
478
BRICK PAVEMENTS
[CHAP, xvii
930. Kinds of Brick. The first brick made especially for paving
purposes was a square-edged side-cut brick with plane sides; but
now several kinds or forms are in use.
931. Re-pressed Brick. A re-pressed brick is one that after
being moulded is subjected to a heavy pressure. The re-pressing
makes the brick more symmetrical in form and of better appearance.
In the early history of paving brick industry, it was claimed that
one of the advantages of re-pressing was that it enabled the man-
ufacturer to form grooves in the faces of the brick which facilitated
the introduction of the joint filler and also increased the power of
the filler to hold the brick in place in the pavement. Fig. 165
shows three forms of grooved paving brick.
FIG. 165. — OLD-STYLE GROOVED PAVING BRICKS.
The above form of brick proved undesirable, since the joints
between the bricks when set or laid in the pavement were so narrow
that it was difficult to get the filler into the joints. The demand
next was for a brick having projections on the side which would
automatically space the joints so they could be easily and com-
pletely filled by the filler. In answer to this demand manufacturers
formed lugs, or buttons, or raised letters on the side of a re-
pressed brick, which served to keep the bricks a uniform distance
apart and thus made joints into which the filler could easily be
poured or swept. Fig. 166 shows re-pressed paving bricks which are
much used.
932. Even though re-pressed under a great total pressure, the re-
pressing does not increase the density of the brick. In fact the re-
ART. 1] THE BRICK 479
press invariably increases the volume of the brick. The reasons
for this are: 1. The box in which the brick is re-pressed must be
slightly larger than the die-moulded clay block, so that the block
can be easily dropped in; and hence re-pressing compresses the
block in one direction, but expands it in the other two. 2. For
FIG. 166. — MODERN RE-PRESSED PAVING BRICKS.
practical reasons the re-pressed ' block must have rounded edges;
and the forming of the round edges disrupts the original structure
and doubtless opens up some of the laminations. However, in
1916, a few manufacturers began to make re-pressed brick with
square edges. The practice seems to have been abandoned. 3.
The formation of the lugs or buttons, or the making of the raised or
sunken letters breaks the original bond of the clay and opens up the
laminations.
Experiments show that bricks not re-pressed are stronger and
freer from structural defects than re-pressed brick. The cost of
re-pressing is about 2 cents per square yard of pavement. There
is little or no advantage to compensate for the decreased strength
and increased cost due to re-pressing.
933. A marked disadvantage of a re-pressed brick is that it has
rounded edges, which makes it impossible to maintain the joint
level full of filler. The filler chips out of such a joint much more
easily than a joint between square-edged bricks. Re-pressed paving
bricks were almost exclusively used for a quarter of a century.
934. Wire-cut Lug Brick. In 1910 a method of cutting the bar
of clay into bricks was introduced which gives a square-edged brick
having on one of its faces four lugs integral with the body of the
brick, and also having a groove adjacent to each lug. The lugs
project TQ of an inch, which insures a joint at least Y& of an inch
wide; but in 1918 the lugs are to project only £ of an inch, which
will give a minimum joint of J inch. The brick also has a double
bevel or bulge of ^ of an inch at each end, which insures an end
480
BRICK PAVEMENTS
[CHAP, xvii
joint at least & of an inch wide at the top and bottom of the end
joint. Fig. 167 shows this form of brick; and Fig. 168 shows such
brick laid in the pavement.
FIG. 167. — WIRE-CUT LUG PAVING BRICK.
FIG. 168. — WIRE-CUT LUG BRICK IN PAVEMENT.
Fig. 169, page 481, shows the machine for cutting the bar of clay
into wire-cut lug bricks. The clay is cut by forcing a wire horizon-
tally through the bar, the wire being guided by narrow slots in
plates above and below the clay. The slots on one side of the brick
are wavy and form the lugs; while the slots on the other side are
straight and form a plane surface.
The advantages of the wire-cut lug brick are: 1. There are no
new laminations. 2. The lugs space the bricks when laid in the
pavement so as to make joints of uniform width and the beveled ends
AET. 1]
THE BRICK
481
and the grooves on the vertical faces make it easy to fill the joints
completely. 3. The square edge of the brick makes a joint that
holds the filler better than the round-edge of the re-pressed brick,
since the filler does not feather out at its wearing surface. 4. The
wire-cut face is rougher than the smooth face of the re-pressed bricks,
and therefore the joint filler adheres better.*
FIG. 169. — MACHINE FOR CUTTING WIEE-CUT LOG BRICK.
On the other hand, the wire-cut lug bricks require considerably
more filler than the re-pressed brick.
The wire-cut lug brick is patented; but many manufacturers
are licensed to make it, and it is sold in unrestricted competition.
Something like three fourths of all the paving bricks used east of
the Mississippi river are of this type. Many manufacturers make
both the wire-cut lug and the re-pressed paving brick.
935. Vertical-fiber Brick. The vertical-fiber brick is one cut
from a die-moulded bar of clay by wires traveling in straight par-
allel slots, which is laid in the pavement with the wire-cut face up,
and hence the wear comes upon the end of the laminations or fibers.
Lugs are formed on one side of the brick by notches or grooves in
one side of the die. Apparently this form of brick is made only by
members of the Western Paving Brick Manufacturers Association,
and promoted by it.
The advantages officially claimed for this type of brick are: 1.
* For experimental data proving this, see Engineering Record, Vol. 69 (1914), p. 607.
482 BRICK PAVEMENTS [CHAP. XVII
It is not patented, and therefore can be made by any manufacturer
without paying royalty. 2. The depth of the brick as laid in the
pavement can easily be changed by simply changing the spacing of
the cutting wires. 3. The brick can be set in the kiln so that all
kiln marks come upon the vertical surfaces, and hence leaves the
bedding and wearing faces free from such marks and makes a smoother
pavement. 4. In the vertical-fiber brick the wear comes upon the
end of the laminations, instead of on the sides as in other kinds.
5. The wire-cut surface gives a good foothold for horses. 6. If a
bituminous filler is used, the wire-cut surface aids in retaining a
carpet of bituminous material on the surface.
Of the above claims 1, 2, 3, and 4 must be admitted as being true;
but there is a great difference of opinion as to the importance of the
advantage claimed. Claim 5 is of doubtful value, since all brick
pavements afford a satisfactory foothold for horses. Claim 6 is a
disadvantage rather than an advantage, since the smoother the sur-
face of a brick pavement the better, and since at best a thin film of
bituminous cement can not endure long on a brick pavement (see
§ 579). It is a reversal of good practice to place the rougher face
horizontal and the smoother vertical.
The weight given to the above claims seems to be largely a matter
of locality. The wire-cut lug brick is favored by at least most of the
members of the National Paving Brick Manufacturers Association,
while the vertical-fiber brick seems to be preferred by the members
of the Western Paving Brick Manufacturers Association. The ter-
ritory of the former is east of the Mississippi river, and that of the
latter west of that river. However, as only about 4 per cent of the
paving bricks made in the United States are manufactured between
the Mississippi river and the Rocky Mountains, this difference of
opinion is not important. Many re-pressed and wire-cut lug bricks
are used west of the Mississippi river.
936. Hill-side Brick. To afford a better foothold for horses on
steep grades, a special hillside brick is made. There are two forms:
1. A brick laid with its long dimension across the street and hav-
ing one edge each of its top and bottom edges chamfered, which gives
a series of continuous parallel grooves running transversely across
the road or street. These bricks are usually re-pressed, the cham-
fered corners being produced by filling up opposite edges of the
mould.
2. A brick laid with its long dimension lengthwise of the road
or street, and having one or more transverse grooves on each of
ART. 1]
THE BRICK
483
its two edges, thus producing a series of non-continuous parallel
grooves across the road. These brick are die-moulded side-cut,
the grooves being produced by
metal lugs on the sides of the die.
Fig. 170 shows a wire-cut lug hill-
side paving brick; and Fig. 171
shows such brick in the pave-
ment before rolling and before
the application of the joint filler.
Fig. 171 is a street in Toronto,
Canada, on a 6 per cent grade.
Sometimes a strip of hill-side
bricks is laid on each side of the
street with a strip of ordinary
paving bricks in the center.
937. After being moulded, or
after being re-pressed, the bricks
, FIG 170. — WIRE-CUT LUG HILL-SIDE PAVING
are placed on trucks or cars, BRICK..
and conveyed to the dry house.
A paving brick immediately after being moulded contains 20 to 30
per cent of water; and hence thorough drying greatly facilitates
the burning of the brick.
FIG. 171. — HILL-SIDE BRICK IN A PAVEMENT.
938. Burning. Paving bricks are usually burned in down-draft
brick-ovens having fire pockets or furnaces built in their outer walls.
484 BRICK PAVEMENTS [CHAP. XVII
The bottoms of the kilns are perforated to allow the gases to pass
through the flues, which are beneath the floor, and which lead to the
chimney. The fire passes up from the furnaces into the kiln, then
down through the brick to be burned to the flues, and thence to the
chimney. The burning is the most critical step in the manufacture
of paving brick. At first the heat is applied slowly in order to drive
off the remaining water without cracking the brick. A low heat is
continued until the smoke passing off shows no further signs of steam
or " water-smoke," after which the fires are gradually increased
until the temperature throughout the kiln is sufficient to vitrify the
brick. Most shales vitrify at from 1,500° to 2,000° F.; but impure
fire-clays require from 1,800° to 2,300° F. From seven to ten days
are required to raise the entire kiln to the vitrifying temperature.
There has been much discussion as to the meaning of the term
vitrification as applied to brick making. Literally speaking, to
vitrify means to render glassy; but as applied to clay working,
vitrification has come to mean incipient fusion of the particles of
the clay into a new chemical compound. The degree of vitrification
increases with the temperature, and the logical end of the process
is complete fusion. A clay is partially vitrified if its constituents
have begun to unite by heat into a compound silicate, even though
it may not have a glassy fracture. The physical peculiarities which
mark vitrification in a burned clay are the conchoidal fracture, the
absence of pores, and the blending of the ingredients into one mass.
Cracks, fissures, and cavities may be found, but porosity must not
exist in a well vitrified brick; and the original particles must have
begun to cohere by the bond of heat instead of the bond of plas-
ticity. Within limits which are different for different clays, the
degree of vitrification in a burned clay is measured by its ability to
absorb water. A lightly burned brick will greedily absorb water,
and the greater the degree of vitrification the less the water absorbed,
a fully vitrified brick absorbing absolutely no water.
After the bricks have been vitrified entirely through, the kiln is
tightly closed and allowed to cool very slowly. Rapid cooling
renders the brick brittle; but by slow cooling they are annealed and
rendered tough. Slow cooling is the secret of toughness, and the
slower the cooling the tougher the brick. The annealing is fre-
quently unduly hurried, much to the detriment of the toughness
of the brick. The kiln is often cooled in three to five days, when
seven to ten would materially improve the product and usually
would be worth the extra cost.
ART. 1] THE BRICK 485
With the utmost care a considerable per cent of the contents of
the kiln are unsuitable for paving purposes, because of some being
under-burned and some over-burned. With shale 80 to 90 per cent
of first-class paving brick is a high average, while with impure fire
clay 85 to 90 per cent may be produced.
939. Size of the Brick. Formerly there was considerable differ-
ence of opinion as to the best size for paving brick, some advocating
2±X4X8", others 3iX4X8J", and a few 4X5X12". The first
size is always referred to as a brick, but the last two are sometimes
called paving blocks. The last was never made in any quantity,
and has been entirely abandoned. There is no conventional line by
which to distinguish bricks from blocks. It was often claimed that
one or the other size made the better pavement, but there is no
material difference in the quality of the pavement between the dif-
ferent sizes.
The advantages of the building-brick size are: (1) being smaller
they are more easily vitrified, and therefore a little cheaper to man-
ufacture; and (2) brick unsuitable for use in the pavement can be
more readily disposed of for building purposes, a fact which tends to
cheapen the cost of the brick used in the pavement. The advan-
tages of the block-size to the manufacturer are that there are fewer
pieces to handle; and in the pavement the blocks chip or spall on
the edges less than the bricks, particularly if the filler is not rigid
(see § 1014). The manufacturer of the block sometimes places
building brick in that part of the kiln in which it is difficult to burn
blocks thoroughly (the bottom of a down-draft kiln), a process which
decreases the per cent of blocks unsuitable for paving purposes, and
at least partially eliminates the second advantage of the building-
brick size as above. In the early history of brick paving, bricks
were most in favor; but now the blocks are used almost exclusively,
and usually they are called bricks.
940. Uniformity of size is very desirable to prevent confusion in
buying and bidding, and particularly for convenience in making
repairs. Unfortunately the sizes of building bricks and also of paving
bricks or blocks vary considerably in different parts of the country.
941. The Specifications of the National Paving Brick Manu-
facturers Association, which have been widely adopted by engineers,
prescribe re-pressed and wire-cut lug paving blocks shall be 8J
inches long, 4 inches deep, and 3J inches wide, with the provision
that shallower brick may be used (see § 942). The specifica-
tions of the Western Paving Brick Manufacturers Association seem
486 BRICK PAVEMENTS [CHAP. XVII
to have no standard size for vertical-fiber brick; but seem to make
such brick from 3f to 4f inches wide, from 8 to 9 inches long, and
2|, 3 or 4 inches deep, a depth of 2| inches seeming to be the
most common.
Until 1916 the Specifications for Brick Pavements adopted by
the American Society of Municipal Improvements permitted the
use of either bricks or blocks; but in 1916 the specifications were
amended so as to permit the use of only blocks 8J inches long, 4
inches deep, and 3f inches wide.
The width is always exclusive of lugs or buttons.
942. Thus far in the history of brick pavements, the depth of
the brick has quite uniformly been 4 inches; but some engineers
claim that no brick pavement ever failed through the wear on the
brick, and therefore the depth of the brick should be reduced.
In 1915 a new type of brick pavement was introduced (§ 982),
which would safely permit a reduction in the total thickness of the
pavement. This and relative matters are discussed in § 1028.
943. TESTING THE BRICK. It is important to have a definite
method of testing the qualities of any artificial material, since then
all parties may know exactly the grade called for, and since the
results obtained by different observers with different materials may
be compared. This is particularly true of brick, since the clays
differ greatly in quality, and also since a slight variation in each step
of the manufacture materially affects the result. The object of
testing paving brick is two-fold: (1) to determine whether the mate-
rial is suitable for use in a pavement; and (2) to enable comparisons
to be mad© between different classes of brick.
Several te'sts formerly employed have now been practically
abandoned; but for the sake of completeness these will be briefly
considered.
944. General Appearance. A critical examination of a paving
brick by the experienced eye aided by a hand hammer is a fair method
of determining the relative merits of different bricks of a particular
kind; but unfortunately experience with one make is not of much
value with brick made by a different process or of a different kind of
clay, and further the results by this method of testing admit of no
numerical evaluation or even of being described accurately. It is a
method of selecting or inspecting rather than of testing.
The brick should be reasonably straight; and have flat sides and
square corners; be uniform in size, texture and shape; and be hard,
tough, and evenly burned. If the edges of the bricks are square,
ART. 1] THE BRICK 487
they should be smooth and free from serrations or " ragging," due
to friction in the die. If the edges are rounded, the radius should
not exceed three sixteenths of an inch. Kiln marks or impressions
from the over-lying brick in the kiln must not be more than three
sixteenths of an inch deep. One face should have not less than two
nor more than four projections, which should be not more than one
fourth nor less than one eighth inch high, nor exceed one half square
inch in area.
When broken the interior of the brick should show a uniform
fracture, be free from lime, and contain no uncrushed or lumpy
material, especially if such material is not united by vitrification
with the remainder of the material. There should be no marked
laminations.
945. Size. The brick should closely conform to the standard
size (§ 941) or to the specified size. The brick from any one man-
ufacturer should be of uniform size; and the brick for any one job
should be of practically the same size. The usual specifications are
as follows: "A brick shall not vary from standard or specified
dimensions more than J an inch in length, nor more than f inch in
width or depth." Sometimes it is also specified that " the bricks
in any one shipment must not vary in width or depth more than
| of an inch."
946. Color. The color is no criterion of the value of a paving
brick, when comparing bricks of various makes; but, in inspecting
bricks from a single factory, the color will usually furnish a fairly
safe guide as to the relative hardness, when the inspector is thor-
oughly acquainted with the particular manufacture. The knowl-
edge gained regarding the relation of color and quality in inspecting
one make of brick, however, can seldom be used with that of another
make from a different locality, as clays vary greatly in kind and
degree of color. The popular belief is that hardness is proportional
to the darkness of the color of the brick, and that light color is prima
facie evidence of softness. As a rule ,the impure fire clays make
excellent paving material, although the bricks are light colored,
usually buff, while shale bricks are reel or brown. For a particular
clay, the color of the bricks indicates the degree of heat they have
received, provided they were burned with the same fuel and under
the same conditions; and ordinarily the higher the heat the darker
the color, and presumably the better the brick. The uniformity
of the color of the interior of the brick is more important than the
color of the exterior.
488 BRICK PAVEMENTS [CHAP. XVII
The color of the outside of the brick is sometimes valueless
owing to the sand employed to prevent sticking in the kiln, or to
the effect of sulphur in the coal used in burning, or to salt glazing.
Salt glazing is a trick occasionally employed to give a dark gloss to
the outside which is very attractive to the superficial observer,
but which is practically worthless, since it is only skin deep and
soon wears off. Salt glazing makes it more difficult to detect soft
brick, and should never be allowed on paving brick.
947. Specific Gravity. In a general way, the more dense a brick
the harder and stronger it is; and consequently early in the history
of brick testing it was believed that a knowledge of the specific.
gravity would be of value in judging of the quality of a paving
brick. It is now known that the specific gravity reveals nothing
not determined by other tests; and further that the density depends
upon the character of the clay, the kind of fuel, etc., and in no way
measures the quality of the product. The specific gravity may be
computed by the formula :
Wa
Specific gravity = >
in which Wa represents the weight of the dry brick in air, Ws the
weight of the saturated brick in air, Wi the weight of the brick
immersed in water. The specific gravity of shale brick ranges
from 2.05 to 2.55, and usually from 2.20 to 2.40; and that of brick
made from impure fire clay ranges from 1.95 to 2.30, and generally
from 2.10 to 2.25.
948. Crushing Strength. The results for the crushing strength
vary more with the details of the method employed than any other
test of paving brick. There is no standard method of making this
test. For experimental data showing the marked effect of the dif-
ferent methods of testing, see the author's Treatise on Masonry
Construction, tenth edition, § 10-17, and § 78-81.
Tests on cubes cut from paving brick show that the best paving
brick have a crushing strength of 10,000 to 20,000 Ib. per square
inch. This is the crushing strength when the load is applied uni-
formly over the surface of the test specimen; but if the pressure is
applied to only a small part of the upper surface of a brick, the
strength will be much greater.* Any brick that is likely to be
* See Baker's Masonry Construction, tenth edition, § 657.
ART. 1] THE BRICK 489
accepted for paving purposes by any of the tests hereafter described,
is in no danger of being crushed by the pressure of the wheel of a
vehicle. For example, the surface of contact between a wheel
having a 1^-inch tire loaded with half a ton is about one half square
inch, which gives a pressure on the brick of only about 2,000 Ib.
per square inch.
If the crushing strength could be easily and accurately found,
it would be of value in determining the relative strength, and hence
would be useful in comparing the quality of different brick; but
owing to the difficulty of making the experiments and to the uncer-
tainty of the results, the test has been abandoned.
949. Absorption Test. In the early days of the paving brick
industry, many of the brick used were so porous and brittle that it
was feared they would be disintegrated by the action of frost; and
consequently the absorption test was employed to eliminate porous
brick. Subsequent tests by repeatedly freezing and thawing paving
bricks showed that any brick which was likely to be accepted for
paving purposes would not be appreciably injured by the action
of frost. There are probably two elements that prevent frost from
seriously injuring even a soft paving brick; viz.: (1) the cushion-
ing effect of the air remaining in the pores of the brick, and (2) the
strength of the brick may be greater than the disrupting effect of
the frost. Alternate freezing and thawing might injure a non-
vitrified brick, which is not only very porous but is also deficient in
strength; but such a brick would be rejected for paving purposes
as the result of a casual inspection. The absorption test is no
longer regarded of importance as measuring the ability of the brick
to resist freezing and thawing.
Different bricks vary widely in their rate of absorption. For
example, one brick absorbed in one day 80 per cent of its total
amount, while another absorbed only 8.7 per cent; and two other
specimens absorbed 71.8 and 19.5 per cent respectively in the same
time. The absorption of whole brick is slightly less than that of
half brick, and the absorption of half brick is considerably less
than that of small chips. For the above reasons and for other
minor ones, results for the absorptive power are likely to be untrust-
worthy.
950. Transverse Strength. This is determined by resting the
brick upon two knife-edges and applying a steady pressure on the
upper side of the brick through a third knife-edge placed midway
between the other two. The results are expressed in terms of the
490 BKICK PAVEMENTS [CHAP. XVII
modulus of rupture, which is computed by the following formula:
3JFJ
~26tP
in which R represents the modulus of rupture in pounds per square
inch, W the breaking load in pounds, I the distance between sup-
ports in inches, b the breadth of the brick in inches, and d the depth
of the brick in inches. The brick may be tested edgewise or flat-
wise, although the former is usually the better method, since then
W is larger. The knife-edges should be rounded transversely to
a radius of about one sixteenth of an inch and longitudinally to a
radius of about 12 inches, to secure better contact and to prevent
the brick from being crushed at the edges. Some authorities rec-
.ommend grinding opposite edges of the brick to parallel planes,
but this is a useless expense. If the brick is warped, the contact
between the brick and the knife-edges can easily be made entirely
satisfactory by placing pieces of metal under the blocks carrying
the lower knife-edges, or by shifting the brick longitudinally, or by
turning it.
The modulus of rupture of bricks that have given excellent
service in a pavement varies from 1,500 to 3,500 Ib. per square inch,
usually from 2,000 to 3,000. Owing to apparently unavoidable
variations in the structure of the brick, it is not possible to attain
closely concordant results in making this test; and with the utmost
care in selecting the brick and in making the tests, the variation
from the mean ranges from 8 to 30 per cent, and on the average is
about 20 per cent.
The cross-breaking test furnishes a means of comparing the
toughness of various kinds of . paving brick. The uniformity of
the results for any particular kind of brick indicates its structural
soundness, freedom from air checks, etc., and shows whether the
material has been properly treated in the various stages of manu-
facture. The transverse strength indicates the resistance of the
brick to cross breaking when laid in the pavement on an unyield-
ing and uneven surface; but this element is not entitled to much
consideration, since brick are seldom thus broken in the pave-
ment, at least not until nearly worn out.
The test is comparatively easy to make, and is a valuable check
upon the rattler test (§951).
951. Rattler Test. This test is made by rolling or tumbling the
bricks in a foundry rattler, i. e., a revolving cast-iron barrel; and it
ART. 1] THE BRICK 491
greatly exceeds in importance all the other tests combined. It
imitates more closely than any other, the impact due to the horse's
hoofs and shoes, and to the bumping of the vehicle wheels, and also
the abrasion due to the slipping of the horse's feet and the sliding
of the wheels. This test could with propriety be called an impact
and abrasion test. The result of the test is jointly dependent upon
the toughness of the brick — its ability to resist shock, — and its
hardness — its ability to resist abrasion.
To make this test of any scientific value, it is necessary to have
some standard method of conducting the experiments. Several
methods of standardizing this test have been proposed. The first
test was made by the author.* Brick that had seen service in a pave-
ment and pieces of well-known natural stones used for paving pur-
poses, together with small pieces of scrap cast iron, were rolled in a
rattler. Shortly after being proposed, this method was quite widely
adopted; but it did not give satisfactory results, chiefly because the
original experiments were made with a rattler having wooden staves,
while subsequent tests were made with rattlers having cast-iron
staves. The method was objectionable on account of the trouble
and expense of preparing the test pieces of natural stone. Later
each of four radical modifications of the test gained prominence in
succession for a time. Finally it was found that seemingly unim-
portant details materially affected the results, as, for example, the
chemical composition of the cast iron in the staves and abrading
material, the stiffness of the staves, the frequency of renewal of the
staves and abrading material, the speed of rotation, the method of
driving the rattler, etc.
952. In 1910 after a very elaborate series of tests, the National
Paving Brick Manufacturers Association proposed specifications
which set forth in great detail the method of constructing and using
the rattler; and in 1915 substantially these specifications were
adopted by the American Society for Testing Materials, and they
have been generally accepted as the standard, f
Fig. 172, page 492, shows the standard rattler and abrasive mate-
rial. The latter consists of two sizes of cast iron spheres, the larger
weighing 7.5 Ib. each and the smaller 0.95 Ib. The total abrasive
* Durability of Paving Brick, by Ira O. Baker, pp. 46, 5" X 8". T. A. Randall & Co.,
Indianapolis, Ind., 1891. Out of Print.
t Proc. Amer. Soc. for Testing Materials, Vol. XV, Year Book 1915, Report of Committee
C-3, pp. 396-407. Copies of complete specifications for the inspection and testing of paving
brick may be had by addressing Secretary Amer. Soc. for Testing Materials, Philadelphia. Pa.,
or Secretary National Paving Brick Mfrs. Assoc., Cleveland, Ohio.
492
BRICK PAVEMENTS
[CHAP, xvii
charge consists of 10 large spheres and 245 to 260 small ones, the
collective weight being as nearly as possible 300 Ib.
FIG. 172. — STANDARD BRICK RATTLER.
In consulting the literature concerning tests of paving brick,
it is necessary to carefully distinguish between the present and the
former standard. The latter gives trie smaller loss.
953. Making the Test. To make the rattler test, the bricks are
thoroughly dried, weighed, placed in the rattler, and turned 1,800
revolutions at a speed of 30 revolutions per minute, and then
weighed. The percentage of loss indicates the quality of the brick.
Fig. 173 shows the brick charge before and after testing. Ten
brick of the so-called block-size constitute a charge.
954. The object of the rattler test is twofold, viz.: (1) to deter-
mine whether the bricks are tough enough for use in a pavement,
and (2) to determine whether the material is uniform in quality.
The first is determined by the average loss of a charge, and the sec-
ond by the uniformity of loss of the several bricks. Uniformity
of wear is an important quality, for a single soft brick may wear
so as to make a hole in the pavement, and then each passing wheel
will rapidly destroy adjacent bricks even though they themselves
are of excellent quality.
To determine the uniformity of wear, the rattler test should be
ART. 1]
THE BRICK
493
so conducted as to find the loss of each brick. This requires the
marking of the bricks so they can be identified after being tested.
FIG. 173. — BRICK CHARGE BEFORE AND AFTER TESTING.
955. Marking the Brick. There are several schemes in use for
marking the several bricks of a charge.
The following method is used by William H. Howell, Engineer
of Streets and Highways, Newark, N. J.* "The holes may be
made with a small cold chisel, after a little experience, in twenty
to twenty-five minutes." Fifteen holes are required.
1. One drill hole on one side.
2. One drill hole on one edge.
3. One drill hole on each side.
4. One drill hole on each edge.
5. One drill hole on one end.
6. One drill hole on each end.
7. Two drill holes on one side.
8. Two drill holes on one edge.
9. One drill hole each on one edge and one end.
10. Blank.
The method shown graphically in Fig. 174, page 494, was pro-
posed by C. A. Baughman, Instructor in Civil Engineering, Iowa
State College. f The holes are made with a small diamond drill,
* Proc. Amer. Soc. of Municipal Improvements, 1911, p. 95.
t Engineering and Contracting, Vol. 44 (1915), p. 470.
4S4
BRICK PAVEMENTS
ICHAP. xvn
and are about one eighth of an inch deep. Eighteen holes are
required.
o
o
o
o
7
0
0
FIG. 174. — BAUGHMAN'S METHOD OF MARKING BRICK.
Fig. 175 shows the method proposed by Mr. B. L. Bowling,
Assistant in Road Laboratory, University of Illinois, for marking
wire-cut lug brick. Brick No. 10 is not marked. Notice that only
nine holes are required. The holes are one fourth of an inch in
diameter, and one fourth of an inch deep; and can be made with a
compressed-air percussion drill in about forty minutes.
J/c/e
Edge
Fio. 175. — BOWLING'S METHOD OP MARKING WIRE-CUT LUG BRICK.
Of course, determining the loss of each brick in the charge re-
quires extra time in marking and weighing; but it is believed that
the additional cost is abundantly justified. It is sometimes claimed
that the brick can not be marked so as to identify them after the
test without weakening them and increasing the loss in the rattler;
ART. 1] THE BRICK 495
but it has been proved that this is not true to an appreciable extent
in any of the three methods of marking mentioned above.
956. Limit of Loss. The standard specifications do not pre-
scribe any limit for the permissible loss; but distinctly state, that
such limit shall be determined by the contracting parties. The
standard specifications give " the following scale of losses to show
what may be expected of tests executed under the foregoing speci-
fications :
For bricks suitable for heavy traffic 22 to 24 per cent.
For bricks suitable for medium traffic 24 to 26 per cent.
For bricks suitable for light traffic 26 to 28 per cent.
"Which of these grades should be specified in any given district and for any
given purpose, is a matter wholly within the province of the buyer; and should be
governed by the kind and amount of traffic to be carried, and the quality of paving
bricks available."
957. The limit that should be specified for the average loss in
any particular case will depend upon the following: (1) the traffic
to be carried, (2) the ordinary quality of the brick available, (3)
the expense to be incurred in culling or selecting the brick, (4) the
size of the brick or block, (5) the form of the edge of the brick, (6)
the minimum dimension of the brick, (7) the uniformity of the loss.
1. The amount and character of the travel may be such as to
make it unwise to specify the highest grade of brick.
2. The locality may be such that the ordinary paving bricks are of
a high quality, and hence no appreciable increase of expense will be
incurred by requiring a high grade of brick, i. e., a low rattler loss.
For example, in a certain year the average loss of all the bricks sub-
mitted to a testing laboratory in an eastern state was 18 per cent,
and several lots of each of three brands gave an average loss of
only 14.3 per cent; but on the other hand, all the bricks submitted
in a year to a laboratory in a western state gave an average loss of
22.56 per cent, with only three charges having a loss less than 17
per cent. In some localities, specifying a small loss may limit
competition and thus increase the price of the bricks.
3. If the bricks available are not uniformly good, and if the
service required of the proposed pavement is severe, it may be wise
to specify a quality which will require careful selection and possibly
include only a comparatively small percentage of the kiln run. Of
course, the last method is expensive, because of the cost of culling,
and also because the better bricks should bear part of the possible
loss on the rejected bricks.
496 BRICK PAVEMENTS [CHAP. XVII
4. The limit to be set for the loss depends upon the size of the
bricks or blocks, i. e., whether the pavement is to be built of bricks
or blocks. However, as but few bricks are now used in pavements,
this phase of the subject is not important. Apparently the relative
loss of bricks and blocks has not been determined with the 1910
standard rattler; but in view of data obtained with the former
standard, some authorities permit a differential of 2 per cent in favor
of brick in comparison with block.
5. The limit varies also with the form of the edge or corner of
the brick. If a brick has square corners, it will lose more in the
rattler than one having rounded corners. The standard re-pressed
brick has corners of a j%-inch radius, and the absent corner represents
about If per cent of the volume. Therefore a square-cornered
brick could lose If per cent in the rattler before being on a par with
a standard round-edge re-pressed brick. For this reason, some
claim that the former should be allowed 1 or 2 per cent greater
rattler loss than the latter. The Illinois Highway Commission
allows the standard wire-cut lug brick a differential of 1 per cent
over the standard re-pressed brick.
6. The limit should depend upon the minimum dimension of the
brick. Since the general use of a concrete foundation for brick
pavements, and particularly since the introduction of the semi-
monolithic and the monolithic construction, there has been a marked
tendency to use a shallower brick. Formerly paving brick were
quite uniformly 4 inches deep; but now, bricks of various depths
are being used, 3J, 3, 2} inches (§ 1028). The prescribed limit of
rattler loss should be less the thinner the brick. The loss in the
rattler is mainly due to the edges being worn or broken off (Fig.
173, page 493). The central portion of a brick loses almost nothing
except at its edges or corners. A brick 3JX4X8J inches has 64
inches of edges or corners, while a brick 3J X3 X8J inches has only
60 inches of edges, or 6J per cent less. Apparently then for this
reason a difference should be made in the limiting loss between a
4-inch and a 3-inch brick. Further, it is claimed that the form of
the rattler test is unjust to a brick thinner than the 3|X4 XSj-inch
standard, since if a thin brick becomes bridged in the rattler it is
much more likely to be broken than a thicker one. The Ohio High-
way Department takes account of these two factors, at least
approximately, as follows: It specifies a loss for standard 4-inch
brick of not more than 22 per cent, and then inserts the following
clause in its standard specifications: " If other than standard sized
ART. 1J THE BRICK 497
blocks are required by the plans or specifications, the average rattler
loss allowed shall be 22 per cent multiplied by the ratio of the volume
of the standard block to the volume of the block specified, less 2
per cent." For another method of testing 3-inch brick, see the
second paragraph of § 960.
7. The limit depends also upon the uniformity of loss of the sev-
eral bricks of a charge. A low rattler loss with a wide range will
probably not give as durable a pavement as a larger average loss
with a narrower range. A single soft or brittle brick will soon wear
below those adjacent to it, and then each passing wheel, particularly
a steel-tired one, in dropping into the depression chips and crushes
the adjoining bricks (however good they are) and tends to destroy
the pavement. The Illinois State Highway Department recog-
nizes this principle, and specifies that the average loss of wire-cut
lug bricks may be 23 per cent provided the individual bricks have
losses between 17 and 27, or 25 per cent provided the individual losses
are between 20 and 28, or 27 per cent provided the individual losses
are between 23 and 29. The degree of uniformity of the rattler loss
depends upon the quality of the bricks, but chiefly upon the care
and skill employed in culling the brick at the kiln. For data on
the degree of uniformity obtained in practice, see § 959.
958. Loss Found in Laboratory. It is presumable that the pav-
ing brick sent to a laboratory to be tested are usually samples pro-
posed for use in a pavement; and hence the average losses are in-
structive as showing the quality of the material available in that
locality, and the range of loss in any charge is evidence of the skill
employed in culling the brick at the kiln. Sometimes the sample
may be sent to obtain for the manufacturer information concerning
some point in manufacture; but usually the manufacturer will
make such tests at home, and hence the samples tested at a public
laboratory may be considered fairly representative. However,
the results obtained in any state or city laboratory will depend
somewhat upon the maximum rattler loss permitted by the official
specifications of that state or city. For example, if one state or city
specifies a lower rattler loss than another state or city, manufac-
turers when shipping brick for work under the former specifications
are likely to select a better quality or cull the bricks more carefully
than when shipping to the other state or city.
In 1916 the Illinois Highway Department tested 59 charges of
re-pressed blocks, which gave an average loss of 20.9 per cent; and
71 charges of wire-cut lug brick, which gave an average loss of 19.9
498 teRicK PAVEMENTS [CHAP, xvn
per cent.* The three smallest losses for re-pressed brick were 16.8,
16.8, and 17.0 per cent; and the three smallest for wire-cut lug brick
were 17.4, 17.4, and 17.5 per cent. The three least ranges in loss of
the individual bricks in a charge were: for re-pressed brick 3.5,
3.8, and 3.8 per cent; and for wire-cut lug brick 4.0, 5.7, and 6.0
per cent. The three greatest ranges in loss of individual bricks
were: for re-pressed bricks 25.1, 27.8, and 31.1 per cent, and for wire-
cut lug bricks 23.9, 24.1, and 26.2 per cent. The last results simply
show that probably some of the samples were not carefully culled,
although it is well known that it is sometimes practically impos-
sible by appearance alone to eliminate all the poor brick.
In 1916, Vermilion County, Illinois, tested 124 charges of blocks
which were practically all wire-cut lug brick from a local plant. The
average loss of all was 18.06 per cent. One charge had a loss be-
tween 14 and 15 per cent, 7 between 15 and 16, 27 between 16 and
17, 31 between 17 and 18, and 25 between 18 and 19. The average
loss of the brick used in 8 miles of rural road was 17J per cent. The
range in the five best lots was 3.5, 3.7, 3.7, 4.1, and 4.3 per cent, and
in the three worst was 21.1, 29.2, and 31.5 per cent.f
The Iowa Engineering Experiment Station in 1916 tested 85
different charges writh an average loss of 22.56 per cent. The three
smallest were: 16.31, 16.78, and 16.80. Twelve charges of re-
pressed brick gave an average loss of 19.44; and 33 charges " that
were not re-pressed" gave an average loss of 24.06 per cent, and
omitting four of the largest the average is 22.33 per cent.t The
kind of brick in* the remainder of the charges is not known; and the
results for individual bricks are not known.
In 1917 the Ohio Highway Commission tested 302 lots of 23
different brands of blocks 4 inches deep, the average loss of all brands
being 21.14 per cent. The three smallest average losses were 17.47,
18.98, and 19.34; and the three largest were 23.95, 25.08, and 27.84.
The range of average losses for the three brands having the smallest
losses were respectively: 3.20 for four lots; 5.96 for 26 lots, and
10.04 for 14 lots. Apparently, no results were obtained for individual
bricks. §
In one year the Maryland Roads Commission tested 19 charges of
six different brands of blocks, the average being 18.7 per cent. Four
* Data from F. L. Roman, Engineer of Tests.
t Data from P. C. McArdle, Superintending Engineer, Danville, 111.
J Data from R. W. Crum, in charge of testing.
§ Data from A. S. Rea, Engineer ofl Tests.
ART. 1J THE BRICK 499
charges of one brand average 14.5 and three charges of another 14.3
per cent. In another year the Commission tested 12 charges of one
brand which averaged 20.83 per cent, one lot was rejected, and the
average for the 11 lots accepted was 20.5 per cent.*
The New York State Highway Commission in the past few years
has tested many paving blocks, in one year making exactly 400
duplicate tests. For 1914-17 the average loss for wire-cut lug
blocks was 21.3 per cent, and for re-pressed 20.9 per cent. The
average loss in a duplicate test usually varied from about 17 to 24
per cent, with a few as low as 15 and a few as high as 27 per cent.f
In consideration of the way in which the samples were obtained it is
not permissible to attempt to draw any conclusions from these data
as to the relative quality of re-pressed and wire-cut lug brick. The
results seem to show a slight improvement in quality from year to
year; and the results from several other laboratories agree with this
conclusion.
959. Loss Allowed in Practice. The classification of losses sug-
gested by the American Society for Testing Materials is stated in
§ 956. These limits have been adopted by many cities. The rat-
tler loss allowed by the Illinois Division of Highways for wire-cut
lug brick is stated in the previous section; and the permissible
loss for round-edged re-pressed brick is 1 per cent less in each case.
Vermilion County, Illinois, in 1916, built 8 miles of monolithic
brick rural roads, and specified that the average loss of a charge
should not exceed 23 per cent, and that no single brick should exceed
27 per cent. The general average loss of the brick used was 17J
per cent.{ For a summary of all the rattler tests made, see the third
paragraph of § 958.
The Ohio Highway Department specifies that the average loss
of standard brick shall not exceed 22 per cent; and that the range
shall not exceed 8 per cent. For brick thinner than the standard, a
differential is allowed as stated in paragraph 6 of § 957.
The New York State Highway Department specifies a maximum
average loss of 24 per cent.
960. Changes in Test Proposed. In the early history of brick
pavements the joints were usually filled with sand, and hence the
wear of the brick in the pavement was due largely to impact; and
therefore a form of rattler test was adopted in which the wear was
* Data from H. G. Shirley, Chief Engineer.
t Data from H. E. Breed, First Deputy Commissioner.
t Engineering Record, Vol. 74 (1916), p. 678.
500 BEICK PAVEMENTS [CHAP. XVII
largely due to impact. But now the joints of brick pavements are
usually filled with grout or at least a comparative hard bituminous
cement; and hence the wear on the brick in the pavement is mainly
abrasion. Therefore some engineers claim that the present standard
rattler test does not reasonably well represent the conditions in the
pavement. Consequently several changes have been proposed in
the method of testing paving brick.
The City of Baltimore has modified the standard rattler test
by leaving out the large balls and replacing them by an equal weight
of the small spheres. This change was made because it was believed
that the 7J-lb. spheres were unduly severe on brick thinner than
the standard paving block. The City of Baltimore allows 3-inch
brick when tested in this way a differential of 1J per cent over a 4-
inch brick. This differential was arrived at by measuring the loss
of the middle inch in a 4-inch brick, somewhat as described in para-
graph 6 of § 957. This introduces a new method of testing, which
is unfortunate since it makes confusion and limits comparisons. It
is unfortunate that a differential for the 3-inch brick was not adopted
for temporary use until the propriety of the present standard rattler-
test for thinner brick could be fully investigated. Baltimore and
the State Highway Departments of both New York and Pennsyl-
vania are making comparative tests of 4-inch and 3-inch brick by this
method; but at present there are insufficient data to warrant any
definite conclusions.
Some competent authorities claim most of the preceding diffi-
culties would be met and more equitable results would be obtained,
if the brick were tested by the standard rattler test using the same
number of bricks regardless of their size, and were then compared
by their absolute loss in weight rather than by the per cent of their
losses.
Attempts have been made to test paving brick by a sand blast;*
but not much progress has been made.
St. Louis has discarded the rattler test, and trusts to comparing
the brick on the street with standard samples previously selected, f
It has not been proved that this method is not subject to more
objections than the rattler test.
361. SERVICE TESTS. The relationship between the loss in
the rattler and the service in the pavement has not been indisput-
* Trans. Am. Soc. Test. Mat., Vol. 16 (1914), Part II, p. 557-64; or an abstract of the same,
Engineering Record, Vol. 70 (1914), p. 215.
t Engineering Record, Vol. 72 (1915), p. 200.
ART. 1] THE BRICK 501
ably established. From time to time several experiments have
been undertaken to determine the relative qualities of different
grades of paving bricks by actual service in the pavement. The
experiment consists in making a standard rattler test of different
grades of paving blocks, and then laying short sections of pavement
with each of the several kinds. For one reason or another, all of
these experiments, except the one mentioned in § 962, have failed
to give a conclusive result.
For example, the first of these experimental sections was laid
in May, 1898, in Detroit, Michigan. Transverse strips of fourteen
kinds of brick were laid in a distance of 222 feet. The blocks were
tested by a former N. B. M. A. standard rattler test. A comparison
between the results of the rattler tests and a general observation of
the effect of three years' wear in the pavement failed to show any
close agreement between the rattler test and service in the pave-
ment. But this test was not conclusive, because it was later found
the rattler test used failed to discriminate between the good and the
bad brick, and it was for this reason abandoned.
962. In 1912-13 the Office of Public Roads and Rural Engineering
of the U. S. Department of Agriculture directed the construction
of an experimental section of road upon an extension of Connecticut
Avenue known as Kensington Road, near Chevy Chase, Maryland.
The improvement was 6,195 feet long, and was divided into six
sections each having a different road surface. Two sections had a
surface of bituminous concrete, two oil-cement concrete, one hy-
draulic-cement concrete, and one (the one nearest Chevy Chase)
vitrified brick. The details of the design and construction of the
several sections are described in the following publications : Circulars
No. 98 and 99 of the Office of Public Roads; Bulletins No. 105
(1914), 257 (1915), and 407 (1916) of the U. S. Department of Agri-
culture. Each year an examination is made of the condition of the
several sections, and a report of the same is published; and doubtless
this practice will be continued for a number of years. A census
of the travel on the road is taken for twenty-four hours on every
thirteenth day.
The following are the particulars for the brick-paved section.
The brick pavement is 18 feet wide, and 980 feet long. The con-
crete base is 6 inches thick, and the proportions are 1:3:7, the
coarse aggegate being gravel (pebbles). The bedding course is 2
inches of sand. Fourteen varieties of paving blocks were laid.
Table 55 shows the character of the blocks. The pavement
502
BRICK PAVEMENTS
CHAP. XVII
was rolled with a 5-ton tandem roller; and then the joints were
filled with a 1 : 1 portland-cement grout. The depth of the bricks
constituting two courses of each variety were measured, and the
location of these courses were recorded, so that in the future these
brick may be taken up and measured, and the amount of wear thus
be determined. The average traffic one-way on half of the road
during 1915 consisted chiefly of 56 horse-drawn wagons and 342
motor-driven cars.
TABLE 55
CHARACTERISTICS OF PAVING BLOCKS ON CHEVY CHASE ROAD
Ref.
No.
Description of Blocks.
Absorp-
tion —
Average
of Five
Tests.
Loss in
Rattler, —
A verage
of Three
Tests.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Shale wire-cut lug, hard burned
1.39%
1.31
0.88
1.65
1.10
1.81
2.29
3.74
2.86
1.56
2.38
4.04
3.73
3.68
21.12%
16.36
25.57*
17.67
22.04*
18.80
27.92f
22.68
22.59f
19.11
37.68f
38.89f
24.31f
31.19
" " " medium hard burned
Shale, re-pressed, well vitrified
hard burned, coarsely ground
" very hard burned.
coarsely ground
" medium hard burned, even wear. . . .
finely ground,
coarsely "
Fire-clay, re-pressed, medium hard burned, coarsely
ground.
Fire-clay, repressed, soft burned, coarsely ground ....
Shale, re-pressed, soft burned, coarsely ground
Fire-clay, re-pressed, soft burned, finely ground
" wire-cut lug, hard burned, laminated
•
* Loss due mainly to chipping.
t Uniform.
Three annual inspections have been made of the above sections
of brick paving and each time the conclusion is that there is no
appreciable difference in wear between the several varieties of brick.
Apparently time enough has not elapsed to justify any trustworthy
conclusion, since the wear has been so slight as to make it impossible
to discover any difference between the different varieties of brick.
It has been asserted that this experiment already proves that a
brick having a large loss in the rattler wears as well as one having
a much smaller loss; but this conclusion is not justifiable, since the
wear on any brick is as yet exceedingly small, and the difference
between different varieties is too small to warrant any conclusion
as to relative wear. Doubtless in due time valuable information
will be obtained as to the relation between the loss in the rattler and
that in actual service.
ART. 2}
CONSTRUCTION
503
ART. 2. CONSTRUCTION
963. Fig. 176, shows the several parts of a brick pavement of the
standard type.
Fio. 176. — SECTION OF BRICK PAVEMENT WITH SAND CUSHION AND CONCBETE FOUNDATION
964. SUBGRADE. An essential feature in the construction of
such a pavement is the proper preparation of the subgrade. It
should be thoroughly underdrained, should be rolled until it is solidly
compacted, and the surface should be smooth and of the correct
crown and grade.
Underdrainage has been fully discussed in § 113-24; and street
drainage has been considered at length in Chapter XIII. The
smoothing and rolling of the subgrade is considered in Art. 1 of
Chapter XV.
965. FOUNDATION. In the evolution of the brick pavement
several types of foundation were used for a time.
966. Abandoned Types. The first brick pavement in this
country, that at Charleston, W. Va., was laid on a foundation of
1-inch tarred boards resting on a layer of 3 or 4 inches of sand, with
a l|-inch sand cushion between the bricks and the boards. This
form was not used to any considerable extent, and has been entirely
abandoned.
During the first ten or fifteen years after the introduction of
brick pavements in the Middle West, the foundation consisted
almost exclusively of a course of brick laid flatwise on a thin bed
504 BRICK PAVEMENTS [CHAP. XVII
of gravel or cinders. Such pavements are generally known as two-
course brick pavements. The layer of cinders or gravel was leveled,
and inferior paving brick were laid flatwise thereon; and then the
joints of the bricks were swept full of sand. The chief defect in this
form of foundation was that the joints of the lower course were not
fully filled, and consequently after the pavement was in service the
sand of the cushion coat (the layer between the two courses of brick)
would work into these joints and permit the bricks in the wearing
course to settle. To cheapen the pavement, broken and chipped
brick were used in the lower course, and the tendency was to place
the larger face uppermost, thus making it nearly impossible to fill
entirely the joints during the time of construction. This form of
foundation was abandoned on account of its cost and inferior
quality.
In some localities where gravel or broken stone was cheap, brick
pavements were laid upon a layer of gravel or broken stone; but the
difficulty and expense of getting such a foundation thoroughly com-
pacted and properly shaped led to the substitution of a concrete
foundation.
In localities where the native soil is clean sand or fine gravel,
brick pavements were constructed directly upon the natural soil.
The subgrade is simply shaped and rolled. Quite a number of cities
in the North, some of which have a considerable traffic, for example
Cleveland, Ohio, and Galesburg, Illinois, and many cities in the
South, lay such pavements on native sand. The sand is usually
simply graded and puddled, the puddling being mainly to keep the
subgrade hard and smooth until the bricks can be laid and the joints
filled. Many southern cities lay brick pavements upon a 1-inch
layer of cement mortar or fine concrete, this bedding course being
used to prevent the sand subgrade from working up into the joints
while the bricks are being rolled. Such foundations are wise only
for light traffic streets, and the decreased cost of portland cement
has led to the increasing use in such localities of a concrete founda-
tion for even light traffic streets.
967. Old Macadam Foundation. Not infrequently a brick pave-
ment replaces a water-bound gravel or macadam surface, in which
case it may be economical to use the old pavement for a founda-
tion for the new. For a consideration of this case, see § 791 and
§437.
968. Bituminous Concrete Foundation. For a discussion of this
type of foundation, see § 792-95.
ART. 2] CONSTRUCTION 505
969. Concrete Foundation. At present a layer of portland-
cement concrete is the almost universal foundation for brick pave-
ments. This form of foundation is fully considered in Art. 2 of
Chapter XV — Pavement Foundations.
970. BEDDING COURSE. The bedding course is a layer of sand
or mortar between the foundation and wearing coat to provide
for slight variations in the surface of the foundation and small
irregularities of size and form of the bricks. There are three dis-
tinct forms of bedding layer, viz.: sand, a dry mixture of sand and
cement, and wet mortar.
971. Sand Bedding Course. The proper thickness of this layer
will depend upon the regularity of the upper face of the concrete
foundation and also upon the uniformity of the bricks in size and
form.
For reasons stated later (§ 977-78), the layer of sand should be as
thin as will afford a good bed for the bricks; and therefore the top
of the concrete foundation should be carefully finished with a surface
parallel to the surface of the pavement. Not infrequently loose
fragments of stone are left on the surface of the concrete, a result
which is very undesirable, since they necessitate a thicker cushion
and at best prevent the bricks from coming to a uniform bearing.
With good workmanship in laying the concrete, there will be no loose
pieces of stone on the surface; and if they do happen to get there,
they should be removed before laying the cushion coat.
The sand for the cushion should preferably be so fine as to be of a
soft, velvety nature and should contain no pebbles of any consider-
able size, or loam, or vegetable matter. The size of pebbles permis-
sible depends upon the thickness of the sand bed. Pebbles will
prevent the brick from having a uniform bearing; the loam is likely
to be washed to the bottom of the layer and cause the brick to settle ;
while the vegetable matter will decay or wash away, and leave the
bricks unsupported. The sand should be dry when it is spread.
Even a small per cent of moisture in the sand adds considerably to
its volume, particularly if it is fine; and hence if the sand when laid
is wet and dry in spots, the cushion will not be of uniform thickness
when dry. The shrinkage of the sand cushion away from the brick
causes depressions which are unsightly, unpleasant to users of the
pavement, and causes the pavement to wear more rapidly. Further,
the shrinkage of the sand cushion away from the brick sometimes
causes an unpleasant noise when vehicles pass rapidly over these
spots (§ 1055).
506
BRICK PAVEMENTS
[CHAP, xvn
972. Spreading the Sand. The spreading of the sand should be
carefully done, so as to secure a uniform thickness and to have its
upper surface exactly parallel to the top of the finished pavement.
After the sand has been distributed approximately to the proper
thickness with a shovel, the surface should be leveled by drawing
over it a template conforming exactly to the curvature of the cross
section of the proposed surface of the pavement.
Fig. 177 shows a common form of template, which was used in
constructing a pavement 33 feet wide. It is trussed to prevent it
FIG. 177. — TEMPLATE FOR STRIKING THE SAND CUSHION.
from sagging at the middle; and is also trussed to prevent it from
deflecting toward either the front or rear. The template is pro-
vided with two rollers at each end which run upon the top face of
the concrete gutter. Some templates are provided with a roller
upon a bent lever, by which the template can be lifted and rolled
back. The length can be varied by means of fish-plates at each end;
and the elevation of the cutting edge can be adjusted by the screw
and hand-wheel at the left.
Practice differs considerably as to the length of the template.
Some contractors make the template the full width of the pavement,
if that is less than about 30 feet, and for a wider pavement make the
template half the width of the street. This form of template must
ART. 2] CONSTRUCTION 507
be made of a 2-inch pine plank of sufficient width to permit of the
cutting of its lower edge to the proper curvature, which may be
determined by the method explained in § 718 (page 374). If the
template is long, it must be braced to prevent bending and sagging;
and it must have a long and substantial handle at each end by which
to draw it forward, and another handle at each end by which to carry
it backward. It is desirable that the template shall have consider-
able weight to keep it from lifting up as it is drawn forward; and
when being drawn forward, the face of it should lean backward a
little to keep it from lifting up. At each end there should be a roller
or a metal runner to carry the template along the top of the curb or
along the edge of the concrete gutter. The roller is more common,
but the runner is better since it eliminates small irregularities in the
top face of the forms, and also since it distributes the weight upon
the forms over a longer length. If the template is to run on top of
the curb, a roller also should be provided to keep it away from the
curb. If the length of the template is equal to half the width of the
street, one end of it may run upon a screed, or wood strip, equal in
thickness to that of the cushion layer, placed in the center of the
street. If there is a car track in the street, one end of the template
may be made to run on the rail.
A long template requires considerable force to draw it forward,
and it is difficult to move backward. Some contractors, therefore,
use a template equal to one quarter of the width of the pavement.
For a pavement 30 to 40 feet wide, screeds made of 2-inch by 4-inch
scantlings are placed at the crown, in the gutters, and also midway
between the crown and the gutter, being bedded on a thin layer
of sand so that their tops conform to the finished surface of the pro-
posed sand cushion. The position of these screeds is determined by
measuring down from a string stretched from curb to curb. The
template may be made of a 1-inch by 6-inch plank, with a 1-inch by
2-inch handle braced by two 1-inch by 2-inch pieces. The edge
should be hollowed out to fit the curved surface of the pavement,
although often this is not done. The middle ordinate for the curved
cutting-edge of the template may be computed by the formula
C d2
m = -jTg-, in which m is the middle ordinate in inches, C the crown
of the pavement in inches, d half the length of the template in feet
and D half the width of the pavement in feet.
After the sand for the cushion layer has been distributed with
shovels, the template should be drawn slowly over it several times,
508 BRICK PAVEMENTS [CHAP. XVII
any depressions that develop being filled by sprinkling sand into them
with a shovel. A considerable quantity of sand should be drawn
along in front of the template, as this aids materially in packing the
bed. It is necessary to draw the template several times to pack the
sand well, particularly if there are wet and dry spots, as the suc-
cessive jarring of the sand grains causes them to settle more closely
together. When the sand cushion is properly packed, it will have a
uniform, smooth, velvety appearance, and will not look rough,
porous, and grainy. No one should be allowed to step on the sand
cushion after it has been spread, nor after it has been rolled.
Formerly, when the concrete for the base was mixed by hand,
the template was pulled forward entirely by men, or sometimes by
one or two horses; but now it is moved forward, at least .for the
first trip, by hitching it to a self-propelling concrete mixer, or better
by passing a rope over a winding drum on the mixer.
973. The surface of the cushion layer is sometimes prepared with
a short lute or scraper without any screeds; but the template and
screeds secure a more uniform surface and also give a greater com-
pression and a more even bed. With hand luting the surface of the
pavement is almost certain to be covered with saucer-like depressions
after it has been rolled. Hand luting should be prohibited except
where the use of the template is impossible, as at street intersections,
around manhole covers, etc.
A considerable part of the difference in tractive resistance between
brick pavements No. 4 and Nos. 5 and 6 of Table 7, page 20, is due
to the difference in the preparation of the sand cushion, the remainder
of the difference being in the rolling of the brick (§991).
974. In adjusting the thickness of the sand cushion adjoining
concrete gutters, manholes, etc., care should be taken that the upper
surface of the brick after being rolled is not below the upper face of
the gutter.
975. After the sand cushion has been struck off with the tem-
plate, it should be rolled with a hand roller about 30 inches long,
24 inches in diameter, and giving a pressure of about 15 Ib. per
linear inch. An ordinary two-section lawn mower with a 12-foot
handle is satisfactory for this work.
A 2-inch layer of sand will compress about J an inch under the
above rolling, and consequently the height of the template should
be adjusted accordingly. To bring the template to the right height,
J-inch strips should be laid upon the curbs and screeds; and then
after striking the sand cushion and rolling it, these strips should be
ART. 2] CONSTRUCTION 509
removed, and the template be again drawn over the sand to test the
surface of the sand bed. If the surface is high in spots, the second
drawing of the template will plane them down; and if there are low
spots, sand should be sprinkled over them, and the template be
drawn again.
The spreading and shaping of the sand cushion is of prime im-
portance in securing an even surface in the finished pavement;
and can be successfully done only by careful and skilful men.
976. At street intersections there are no curbs or gutters to act
as guides for the template, and hence the above method of striking
the sand cushion can not be applied. In such cases the sand cushion
is usually shaped with a hand lute. Stakes about J-inch square
should be driven at close intervals to aid in bringing the top of the
sand cushion to the right elevation.
977. Objections to Sand Cushion. Until comparatively recently
the only bedding for the brick was a layer of sand about 2 inches
thick. The purpose of the sand was to level up the foundation and
to give a good bearing for the brick; and in the early history of
brick pavements probably a thickness of 2 inches was required for
this purpose. However, later experience proved that such a great
thickness was unnecessary and also inadvisable. A better prepara-
tion of the surface of the concrete foundation, the greater uniformity
in paving brick, and the closer inspection of the brick made unneces-
sary so thick a sand cushion.
There are three serious objections to a sand cushion.
1. With a thick cushion, it is nearly impossible to secure uniform
density in the sand layer. The sand is likely to be more moist in
some spots than in others; and when it dries out, it will shrink and
leave the brick unsupported, which will ultimately cause a settle-
ment of the brick and make a depression on the surface of the pave-
ment. Such depressions are saucer-like, and can be seen in many
brick pavements. Such depressions are most apparent in pave-
ments having joints filled with cement grout, since the boundaries
of the depression are indicated by a break of the bond of the joint
filler. Tapping the surface of such a pavement, particularly a
nearly new one, with a hammer will reveal many spots which sound
hollow, showing that the sand cushion has shrunk away from the
brick. Such a spot is likely to become a depression.
2. With a thick sand cushion, the rolling of the pavement is
almost sure to force the sand up into the bottom of the vertical
joints between the bricks, and thus prevent the cementitious joint-
510 BRICK PAVEMENTS [CHAP. XVII
filler from penetrating the full depth of the brick. Examples are on
record in which the sand was thus forced nearly or quite to the top
of the bricks ; and not infrequently sand is forced up half the depth
of the bricks, particularly if the sand of the cushion is fine, as is
usually the case.
This objection to the sand cushion is particularly important if a
rain should wet the cushion before the bricks are rolled; for if the
sand cushion is wet, the bricks can not be rolled adequately without
forcing the sand up into the joints.
3. Both of the above objections apply to any sand cushion, but
particularly to a thick one; and the following objection applies to
any sand cushion, even a thin one. There is danger that the sand
cushion may leak away through cracks into sewers, manholes, etc.,
particularly if the wearing course of brick is not water-tight. Such
flow of sa^id often occurs on steep grades. Cases have been known
in which a pavement having sand-filled joints and being on a 1 per
cent grade, sunk next to the curb an inch in twenty years from this
cause. Even with a water-tight pavement, the sand cushion some-
times leaks into trenches opened through the pavement and left
unfilled for a time. A street-railway track has a tendency to cause
the sand cushion to flow away. The vibration due to passing cars
has a tendency to break the bond of the joint filling near the rails
and make a crack that will let water down to the sand cushion, and
then the water will flow toward the curb and carry the sand with it.
Further, the track will be forced down by the weight of the car and
will spring back when the car has passed, thus pumping the water
in and out, which forces the water through the sand cushion and
tends to displace it.
4. Lately some have claimed that the sand cushion served also
to give elasticity and resiliency to the pavement, and consequently
protected the brick from excessive wear and possible breakage. It
is likely that the sand cushion compresses under use, particularly
if it is not thoroughly compacted before the brick are laid, and also
if the brick are not firmly settled into the sand cushion by rolling;
but under ordinary conditions this compression must be quite small,
and takes place comparatively soon after the pavement is opened to
travel, and hence can not have any appreciable effect upon the
durability of the pavement. Further, little or none of this compres-
sion is due to the elasticity of the sand cushion, and hence the sand
cushion can have little or no effect in absorbing shock. The sand
cushion is a cause of shock rather than an absorber of shock.
ART. 2] CONSTRUCTION 511
978. In the early history of brick pavements a 2-inch cushion
was customary; but later some cities reduced it to 1J inches, some
to 1 inch, and a few to f inch. At present the better practice does
away with any mobile sand cushion.
979. Cement-sand Bedding Course. This form of bedding
consists of a layer of dry cement and sand about J inch thick. The
cement and sand are thoroughly mixed dry in the proportion of
1 : 3 or 1 : 4, and then spread and struck as described for the sand
cushion (§ 972-74). The mixture of cement and sand, after being
spread and struck with a template, is so well compacted and of
such uniform density as not to require rolling; and besides if it is
rolled, even with a light roller, the bed is so hard as to make it nearly
impossible to roll the brick to a smooth surface. After the bricks
for the wearing coat have been set in place upon the mortar bed and
rolled, the bricks should be thoroughly sprinkled, and care should be
taken to see that the water really reaches all parts of the sand-
cement bedding course and converts it into mortar. This can be
tested by taking up an occasional brick. Subsequently the joints
in the wearing course are filled with hydraulic-cement grout, as will
be described later.
There are really two types of this form of construction, viz., one
in which the amount of cement is sufficient to make a mortar of
fair strength, and another in which the amount of cement is suf-
ficient only to prevent the cement-sand bed from leaking away or
shifting under traffic. With the latter form, the bedding course
was usually 1 or 1J inches, and hence considerable cement was
required, even though the mixture was a lean one. Gradually the
bedding course was decreased in thickness and increased in richness.
980. The cement-sand bedding course has two advantages over
the sand cushion: 1. It is rigid; and hence can not compress or
shift under travel, nor leak away. 2. The mortar adheres to the
foundation and also to the bottom of the brick, and binds them
together, thus converting the foundation and wearing course into a
partial monolith. Such a pavement is usually called a semi-mono-
lithic brick pavement.
A bed of sand and cement laid as described above never makes
a really good mortar. In the first place, if the water is applied
sparingly there is no certainty that enough water reaches the mix-
ture to make a mortar; and on the other hand, if water is applied
profusely it may wash the cement out of the sand and destroy the
mixture as a mortar. Again, even though the mixture receives the
512
BRICK PAVEMENTS
CHAP. XVII
proper amount of water, the resulting mortar will be of poor quality
owing to the lack of mixing after the addition of the water.
981. Apparently the first example of this type of pavement
was constructed in Baltimore, Md., in 1906; but the most noted
example is the vehicle entrance to the Pennsylvania Railway
Passenger Station in New York City constructed in 1910. Another
innovation in this pavement was that the bricks were only 2J inches
deep. In 1916, 7,800 vehicles passed over this pavement daily; and
yet Fig. 178 shows that the surface is still practically perfect.
FIG. 178. — SEMI-MONOLITHIC BRICK PAVEMENT — PENNSYLVANIA PASSENGER STATION, NEW
YORK CITY.
This form of construction was justly popular until the true mono-
lithic construction was developed, as described in the next section
below.
982. Mortar Bedding Course. This consists of a layer of cement
mortar on which the brick are set while the mortar is still green. The
chief difference between this form of construction and the cement-
sand type is that the layer of mortar is placed upon the concrete
foundation before it has begun to set; and then the brick are placed,
rolled, and grouted before the cement in either the concrete base of
the mortar bed has taken its initial set. With careful work it is
reasonably certain that the whole construction is really one solid
mass. This form is known as the monolithic brick pavement.
There are two slightly different forms of this type of construction.
In one the concrete foundation is laid in the usual way, and on it a
layer of rather dry cement mortar is spread by means of a template
similar to that used in gaging the thickness of the sand cushion
ART. 2]
CONSTRUCTION
513
(§ 972). This form was first used near Paris, 111., in 1914. In
the other form of construction the concrete foundation is laid about
half an inch thicker than the depth required, and is then tamped
with a tamping template (§ 462) to reduce the thickness to that speci-
fied. The tamping flushes a layer of mortar to the top; and then the
surface of the concrete, or rather the mortar, is struck off with a
cutting template, and the brick are set on the mortar surface. This
form of construction was first used near Danville, 111., in 1915.
When gravel is used for the coarse aggregate of the concrete
foundation, the concrete and the mortar bedding course are struck
off at a single operation with a double template. Fig. 179 shows
this template. The front template is usually a steel I beam which
Fia. 179. — DOUBLE TEMPLATE FOR STRIKING MORTAR BEDDING COURSE.
strikes the surface of the concrete foundation; and the rear template
is usually a steel channel, which is | or Y& of an inch higher than the
front one, strikes the upper surface of the mortar bedding course.
The space between the templates is 2 feet wide, and is kept full of
mortar, which is previously mixed in a small machine mixer.
Fig. 180, page 514, shows the mortar bed after the bricks have
been placed upon it and rolled. The mortar is usually forced up
into the joint i to f of an inch. When a brick is thus removed, the
surface of the mortar should be damp, but there should be no film of
water on the top of the mortar. The monolithic type of construc-
tion has been adopted with great rapidity; but thus far it has been
employed chiefly on rural roads.
The monolithic brick pavement is not adapted to a steep grade,
514
BRICK PAVEMENTS
[CHAP, xvii
owing to the difficulty of keeping the green concrete at the correct
grade and cross section.
FIG. 180. — VIEW OP MORTAR BED AFTER BRICK SURFACE HAD BEEN ROLLED.
983. Comparison of Types. For a comparison of the relative
merits of brick pavements having the preceding forms of bedding
course, see § 1023-33. It is not wise to attempt to discuss this
phase of the subject until the complete construction' of the pave-
ment has been considered.
984. LAYING THE BRICK. Delivery. The brick are usually
placed in piles at the side of the road or pavement before the grading
is done. The brick should be kept clean, and should be handled
so as not to needlessly nick and break them.
Formerly there was considerable discussion as to the relative
merits of (1) delivering the brick along the curb a considerable time
before they are to be laid, or (2) hauling them to the street as they
are laid. In the former case, the brick were transported from the
parking to the men who set them, either in wheelbarrows or by hand
on a board or later in a pair of tongs. In the latter case, the wagon
was hauled to the middle of the street on planks, and the bricks were
carried by hand or with tongs directly from the wagon to the layers.
ART. 2] CONSTRUCTION 515
There were some disadvantages in the last method, but there was
the possibility of saving 2J- to 3 cents per square yard.
Both methods have been superseded, as far as street, i. e., wide,
pavements are concerned, by an improved method of delivering
brick from the parking to the setters. This device consists of a
roller conveyor, — a series of rollers set in an inclined frame down
which the brick roll by their own weight in a stream from the parking
toward the middle of the street, and from which the setters pick
them to set them into place. This is an example of one of the im-
provements that have helped to keep the price of street pavements
down, notwithstanding the advance in cost of materials and labor
(§ 637).
For rural roads, i. e., for narrow pavements, the brick are carried
by hand on a pallet or in a pair of tongs from the side of the pave-
ment to the setter.
985. It is undesirable to use wheelbarrows in transporting the
bricks from the parking to the setters, since the bricks are likely to
be chipped in placing them in the wheelbarrow and in dumping
them out, and further since dumping them is likely to tilt the bricks
already set, which will make the surface of the finished pavement
uneven and rough.
986. If the brick are delivered to the setters in tongs or on a
pallet, the men should be provided with planks to walk upon ; and
they should not be allowed to step upon the bricks before they are
rolled, as it is liable to tilt them and cause the surface of the pave-
ment to be rough after it is rolled. Further, workmen should not
be permitted to track mud upon the brick. When the condition of
the ground is such that mud will be tracked upon the pavement, the
work of laying brick should not be allowed.
987. Direction of Courses. It is customary to lay the brick
with the length perpendicular to the curb, except at street inter-
sections; but there are a few cities in which the brick are laid in
courses making an angle of 45° with the length of the street, with
the idea that the tendency to form ruts would be less if the wheels
crossed the bricks diagonally. There is no advantage in the diagonal
over the square courses; they are more difficult to lay, cutting the
corner of the brick in making the fit next to the curb is wasteful of
material, and the diagonal courses do not give as good foothold to
the horses.
The hill-side brick shown in Fig. 170 and 171, page 483, must be
laid with its length in the direction of the street.
516
BRICK PAVEMENTS
[CHAP, xvii
Occasionally a few courses of brick are laid longitudinally in the
gutter, similar to the practice with stone blocks; but this is unneces-
sary, since the brick pavement is much smoother than the ordinary
stone-block pavement, and besides the running joint where the trans-
verse and the longitudinal sections join is likely to develop into a rut.
988. At street intersections and junctions the bricks should be
laid diagonally — a compromise position between the directions of
the travel on the two streets. Street intersections need special care
in construction, since they are exposed to the traffic of two streets.
Fig. 181 shows the usual arrangement of the courses for a street
Fia. 181. — DOUBLE-DIAGONAL BRICK INTERSECTION.
intersection; and Fig. 182 and Fig. 183 (page 518) show two other
arrangements that have occasionally been used. Slight objections
have been urged against all three plans. The bond in Fig. 181 is
weak along the middle line of each street; Fig. 182 is objectionable
owing to the tendency of ruts to form along the lines running through
the ends of the bricks; and Fig. 183 is defective since traffic around
the corners A and B is parallel to the courses of brick.
ART. 2]
CONSTRUCTION
517
At a street junction only half of the common area should be laid
with diagonal courses. For example, assuming that in Fig. 181
the street enters the lower side of the transverse street but does not
cross it, then the lower half of the intersection would be laid with
courses as in the diagram, while in the upper half the length of the
bricks would be perpendicular to the transverse street.
989. Setting the Brick. In setting the brick the man should
stand on those already laid, and not upon the sand cushion. Under
no consideration should the sand bed be disturbed. The brick should
be set on edge as closely and compactly as possible, each being
FIG. 182. — HERRING-BONE BRICK INTERSECTION.
pressed or rather bumped both endwise and sidewise against those
already laid. The bricks are stronger and more durable than any
material that can be used to fill the joints, and consequently the
thinner the joints the better. The bond should be approximately a
half brick. If the brick were laid without bond, ruts would form
along the continuous end-joints; and therefore the more the bond
the better. No bats should be used, except in making closures; and
518
BRICK PAVEMENTS
[CHAP, xvii
in cutting a brick to close a course, care should be taken to get a
square end and to make as tight a fit as practicable. As far as pos-
sible, the bats should be obtained from chipped and broken brick,
or from misshapen ones rejected in the inspection after the brick
are set. It is usually specified that no bat less than 2J inches long
shall be used. Under this specification, if the space is less than 2J
inches, it is necessary to take up the next brick and chip enough off
to permit the use of a bat more than 2J inches. Fig. 184 shows the
hammer employed in cutting, or rather in breaking, a brick to close
a course.
FIG. 183. — SINGLE-DIAGONAL BRICK INTERSECTION.
In some cities it is required that each four or five courses of brick
shall be driven up from the face by striking with a sledge against a
2" X 4" or 4" X 4" timber resting against the last course; but this
is unnecessary, if each brick when laid is pressed, or rather bumped,
against the side of the course already in position. In any case the
courses should be straight across the street; and if they are not laid
so, they should be straightened by driving up each four or five
courses from the face. Sometimes the bricks in a row are crowded
ART. 2] CONSTRUCTION 519
together endwise by inserting a crowbar at the curb; but this is
unnecessary, provided each brick as it is laid is bumped against
the end of the preceding one.
It is usually specified that nothing smaller than a 2j-inch bat
shall be used in making a closure. The alternative is either to close
up the joints between the ends of the
bricks by prying with a crowbar until a
larger bat can be inserted, or to open up
a few joints until the space at the end of
the course is moderately small. The latter
is undesirable, since it is likely to displace
the brick vertically so as to make the
surface of the pavement rough after it
has been rolled.
Of course, lug brick should be set with FIG. 184— BRICK PAVEB'S
the lugs always in the same direction.
990. Inspecting. After the bricks are laid, the pavement should
be inspected, all soft, broken, and badly misshaped brick being
marked for removal. To reveal the soft brick, it is customary to
sprinkle the pavement heavily with a hose. While the water is
being applied, the soft brick will appear comparatively dry; but after
the sprinkling is stopped, the soft brick will appear to be the wetter.
A brick having only a small piece chipped from the corner or edge
may be turned over. Objectionable brick may be marked with
chalk, a cross or circle indicating a brick to be removed and a single
straight line one to be turned. Rejected brick are removed with
tongs having broad flat noses and long stout handles.
991. Rolling. After all rejected brick have been removed and
the pavement has been swept, it is ready for rolling. The purpose
of the rolling is to settle the bricks uniformly into the bedding course.
A heavy roller is undesirable, at least in the beginning of the rolling,
since the first passage of it tilts the bricks to one side so much that it
is nearly impossible to straighten them up again. Unless the top
faces of the bricks are brought to a plane, the pavement will be rough
and noisy, and will lack durability.
It is important that the rolling shall closely follow the spreading
of the bedding course, so that a shower may not wet it. Whatever
the material of the bedding course, if it is rained upon, the bricks
with unfilled joints should be taken up and a new bedding course be
spread. For much the same reason the filling of the joints (§ 994-
1005) should closely follow the rolling.
520
BEICK PAVEMENTS
[CHAP. XVII
If the bedding course is sand or dry sand and cement, the -roller
should weigh 3 to 5 tons; but if the bedding course is green mortar
(§ 982), the rolling should be done with a hand roller about 30 inches
in diameter, 24 inches long and weighing from 600 to 900 Ib. The
rolling should be continued until the surface of the pavement is
smooth.
The pavement should first be rolled longitudinally, beginning
at the crown and working toward the gutter, taking care that each
return trip of the roller covers exactly the same area as the preceding
trip so that the second passage of the roller may neutralize any
careening of the brick due to the first passage. Pavements that
have been rolled only once or always in one direction, are very much
rougher and more noisy than when properly rolled. If a spot is
skipped on the return passage of the roller, it can be detected by a
casual inspection or by the noise of a passing vehicle. The first
passage of the roller should be made at a slow speed to prevent undue
canting of the brick. After the pavement has been rolled longi-
tudinally, it should be rolled back and forth transversely, or at least
in both directions at an angle of 45° from curb to curb. If the
pavement is narrow, and particularly if it has a high crown, it may
not be wise to roll it transversely or even diagonally, because of the
time required and also because of the probability of
breaking brick. In this case the longitudinal rolling
should be more thorough.
If the rolling is well done, the sand cushion will
be pushed up between the brick J to f of an inch.
992. There are sometimes places which can not
be reached by the roller, for example around manhole
covers; and in these cases the brick should be settled
to place by ramming. The ramming should be done
with a paver's rammer, Fig. 185, wighing not less
than 50 Ib. The ramming should be done on a 2-inch
plank 10 to 12 inches wide and 6 feet long laid parallel
to the curb. The ramming should be continued until
the pavement has a good surface at the proper ele-
vation.
993. After the rolling is completed, the joints
snould be inspected; and if the bedding course has
been forced up between the bricks more than \ an
inch, the bricks should be taken up and relaid.
It is usually specified that after the final rolling the surface of
ART. 2]
CONSTRUCTION
521
the pavement shall be tested with a 10-foot straight edge laid
parallel to the length of the street, and any depression exceeding
| of an inch shall be taken up and re-laid.
Fig. 186 shows a monolithic brick pavement after it has been
rolled and before the application of the filler. The two-section
hand roller used in rolling the bricks stands in the foreground.
FIG. 186. — MONOLITHIC BRICK PAVEMENT ROLLED AND READY FOR THE FILLER.
994. JOINT FILLER. The joints should be filled (1) to keep the
brick in the proper position, (2) to lessen the chipping of the edges of
the brick, and (3) to prevent water from penetrating to the cushion
coat and to the foundation. Three forms of filler are in common
use, viz.: sand, hydraulic-cement grout, and bituminous cement,
i. e., asphalt or tar; and recently a mixture of tar and sand has been
used.
995. Sand Filler. Sand was the first filler employed for brick
pavements, and in the Middle West is even yet largely used. The
sand should be fine and dry, and be worked into the joints by sweep-
ing it over the pavement, which also should be dry. Although the
sand is nominally swept into the joints, it is usually simply spread
upon the surface and left to be worked in by travel, which is unde-
sirable since the joints are then partially filled with manure and
street dirt. The sand can be swept into the joints effectively and
economically with a revolving machine sweeper. After the joints
522 BKICK PAVEMENTS [CHAP. XVII
have been filled, the surface of the pavement is covered with a layer
of sand J to J inch thick, which is left on for a few weeks after the
street is thrown open to travel, to secure the thorough working of the
sand into every joint.
The cost of sweeping the pavement and filling the joints with sand
is 0.2 to 0.3 cent per square yard, and the cost of a |-inch layer of
sand at $1.08 per cubic yard is 1.5 cents per square yard. To cover
waste and contingencies, the sand joint-filler is usually estimated at
2 cents per square yard.
The advantages of a sand filler are: 1. It is cheap. 2. The
pavement may be thrown open to traffic as soon as the bricks are
laid. 3. The pavement may be taken up easily and without break-
age of the brick. 4. It is practically water tight, particularly
after being in service a short time. Whenever a brick pavement
having a sand filler is opened, the sides of the brick are always found
dry and clean a little distance below the wearing surface.
The disadvantages of a sand filler are: 1. It does not protect the
edges of the brick from chipping. 2. It may wash out from the crown
toward the gutter. 3. It is removed from the top of the joints by the
street sweeper — either the broom or the pneumatic, — and by auto-
mobiles.
996. Cement Grout. The grout should be composed of part
Portland cement and 1 or 1J parts sand. If the sand is well graded,
1 part of cement will fill the voids of 1J parts of sand, and give a
filler of maximum strength; but if the sand is not well graded, the
grout should be 1 : 1. The cement should meet the requirements of
the standard specifications. The sand should not contain more than
1 per cent by weight of clay or loam, and should contain such gra-
dation of sizes that all will pass a 20 standard sieve and all be re-
tained on a 100 standard sieve. Some contractors place the sand in
bags when unloading it from the car, and distribute the bags of sand
with an equal number of bags of cement at proper intervals along
the pavement, which secures the proper proportion of ingredients
and prevents loss of materials and time in measuring. The sand
and cement should be mixed dry until the mass is homogeneous and
of a uniform color. The cement and sand should be mixed dry so
that the cement will not ball up when the water is added. For the
best results a considerable amount of this mixture should be pre-
pared at one time, but not more than will be used up in two hours.
To prepare the grout, a small batch of the mixed sand and cement,
preferably not more than 2 cubic feet, should be placed in a suitable
ART. 2]
CONSTRUCTION
523
box or machine, and water should be added to make a grout that will
freely flow to the bottom of the joints without separation. It is
important that the water be added slowly and that the grout be
mixed thoroughly. For the best results, the dry mortar should first
be reduced to a uniform and plastic mortar, and then more water
should be added, while the mass is mixed vigorously, until the desired
consistency is attained.
997. Mixing Box. The mixing may be done in a box made for
the purpose, which should be 3J to 4 feet long, 27 to 30 inches wide,
and 14 inches deep, and should have legs of different lengths, so that
the mixture will readily flow to one corner of the box, which should
be 8 to 10 inches above the pavement.*
The grout should be removed from the box to the pavement
with a scoop shovel, and not by overturning the box upon the pave-
ment; since by the last process the sand, cement, and water are
separated, and are deposited in different portions of the pavement.
While the box is being emptied, the grout should be constantly
stirred to prevent a separation of the sand from the cement.
A mortar box should be provided for each 10 or 12 feet of width
of pavement.
998. Mixing Machine. Recently a small mechanical mixer
FIG. 187. — GROUT-MIXING MACHINE.
been introduced for preparing the grout. Fig. 187 and 188
show such a machine. In using the machine, it is important that
* The National Paving Brick Manufacturers Association publish complete specifications
and working drawings for such a box, which may be had gratuitously of the Secretary, Cleve-
land, Ohio,
524
BRICK PAVEMENTS
[CHAP, xvii
the mixing be not hurried; and that the conditions stated in the
second paragraph of § 996 be observed.
Some engineers permit the use of the machine for mixing the first
application of grout, but not for the second, as the latter should be
stiffer. However, if carefully manipulated, the machine will make
as good grout as can be made in a box; and besides the mixing is less
likely to be slighted, and the cost of mixing is less.
FIG. 188. — GROUT-MIXING MACHINE AT WORK.
999. Applying the Grout. Before the grout is applied to the
pavement, the brick should be thoroughly wetted by being gently
sprayed. A strong stream is likely to displace the mortar. The
grout should be applied to the pavement in small quantities, and
should be quickly swept into the joints with an ordinary brush broom.
The strokes of the broom should be mostly lengthwise of the brick
to most effectively get the grout into the joints. It is better that
the joints should be only about half or two thirds filled at the first
application, since then there is a less depth of grout in the joints and
consequently less liability of the separation of the sand, the cement,
and the water.
Fig. 189 shows the process of making the first application of
cement-grout filler using mixing boxes. Notice that the filler is
dipped out of the box and poured upon the pavement with scoop
shovel; and that the grout is spread with brush brooms.
In applying the cement filler it is very important that the grout
shall not bridge across or dam up a joint; and care should be taken
to see that the grout really reaches the bottom of all joints.
ART. 2]
CONSTRUCTION
525
1000. If a grout filler is to be used with a sand cushion, the first
application of filler should be made very thin, so that it will pene-
trate the sand that is pushed up into the joint by the rolling and
convert the sand into mortar. If the sand in the bottom of the joint
is not thus converted into mortar, the bricks are likely to spall on the
top owing to concentrated pressure at the top of the joint, due to the
expansion of the pavement.
In making this very thin grout, it is necessary first to mix the sand
and cement dry, and then to add water gradually and stir the mix-
ture vigorously, until a good stiff mortar is produced, and next to
FIG. 189. — FIRST APPLICATION OF GROUT FILLER.
continue the gradual addition of water and the mixing until a very
thin mortar results. If all the water is added at once, or if the
water is added too rapidly, or the mortar is not thoroughly and con-
tinuously mixed while the water is being added, it is nearly impos-
sible to keep the sand, cement and water from segregating.
1001. After the first application has been carried forward 40 or
50 feet, and after it has settled but before the initial set has begun,
a second application should be made in the same manner as the first,
except that the grout should be somewhat thicker.
f- Fig. 190 shows the process of making the second application of
grout filler, when the mixing bases are used. The grout is dipped
526
BRICK PAVEMENTS
[CHAP, xvii
with a scoop shovel and spread with a squeegee, a wooden scraper
having a rubber edge.
1002. After the second application has settled but before initial
set has begun, all surplus grout on the surface should be forced along
FIG. 190. — SECOND APPLICATION OF GROUT FILLER.
over the pavement with a squeegee. The squeegee should always
move at an angle with the joints, thus leaving them level full.
In making the final finish, the squeegee should be drawn, not
pushed, over the surface; or in other words, the workmen should not
track up the finished surface, for doing so raises little projections on
the surface of the grout which when hardened are very destructive
upon automobile tires. This precaution was not observed in building
the automobile brick race-track at Indianapolis, even though rec-
ommended by the engineer; but after the track was completed, it was
necessary to spend considerable time and money to remove these pro-
jections. Of course, on a public highway these projections will
ultimately be worn off by steel-tired vehicles; but in the meantime
much damage will be done to auto tires, and it costs practically noth-
ing to prevent the projections.
Fig. 191 shows a vertical view of a perfectly grouted brick pave-
ment. Fig. 192 shows a well-grouted Ohio rural brick road.
Fig. 193 shows a sand-cushion cement-grouted brick pavement
on South Sixth Street, Terre Haute, Indiana, when 28 years old.
This is one of the first, if not the first, cement-grouted brick pave-
ART. 2]
CONSTRUCTION
,527
ment; and it is in a remarkably good condition. The view was
taken two blocks from the main business street. When this pave-
FIG. 191. — VERTICAL VIEW OF A PERFECTLY GROUTED BRICK PAVEMENT.
ment was laid Terre Haute had a population of 30,217, and in 1910
it had 58,157. The joint filler and the brick have worn down to-
gether, and the surface is as smooth as a marble mosaic. The top
FIG. 192. — A WELL-GROUTED OHIO RURAL ROAD.
faces of the brick are flat, and the joints are level full of cement grout.
Scarcely a single chipped or broken brick can be found; and the
528 BRICK PAVEMENTS [CHAP. XVII
general wear, in the middle third of the street, has been only about ^
to TG of an inch of depth, with a very few holes | inch deep caused
by soft brick. The brick are not as good as those made at the present
time; but the pavement, particularly for that time, was unusually
well constructed. It was provided with an adequate foundation,
FIG. 193. — SAND-CTTSHION CEMENT-GROUTED BRICK PAVEMENT 28 YEARS OLD.
the brick were well burned, and were carefully and thoroughly rolled,
and the joints' were entirely filled with good portland-cement grout,
and consequently this pavement has worn exceedingly well. Of
course other pavements constructed with as good material and with
the same care would wear equally well.
1003. With hill-side brick (§ 936) the grout should be swept from
the grooves before it sets.
1004. After the joints have thus been filled, and after the grout
has set so that a coating of sand or earth will not absorb moisture
from the joint filler, a half inch of fine sand should be spread over the
entire surface of the pavement; and if the weather is very hot or
dry, the sand should be sprinkled at intervals for two or three days,
to insure that the cement does not lose by vaporization the water
necessary for chemical combination in setting. Some engineers
prefer hay or straw instead of sand or loam, since they can be moved
ahead and used a second time.
Travel should be kept off the pavement from seven to ten days,
or at least until the cement has fully set, and it is much better if
ART. 2] CONSTRUCTION 529
travel is kept off longer. Some engineers specify three weeks in
warm weather, and longer for cold weather. If the cement filler is
disturbed before it is firmly set, it is practically no better than sand.
If the cement filler is put in as described above and allowed to set
firmly before travel is admitted, the filler will wear no faster than the
best paving blocks and will prevent spalling and chipping of the
bricks at the edges and corners.
1005. To separate the grouted section from the ungrouted por-
tion, a row of metal strips yr by 6 by 36 inches should be inserted in
a transverse joint of the pavement. By this means the grouting will
end in a transverse joint. These metal strips should be removed
when the grout has become stiff, but before initial set.
1006. Cost. The amount of grout required will vary with the
openness of the joints, and also with the quantity of sand of the
cushion course that works up into the lower part of the joints while
the bricks are being rolled.
" If a 1 : 1 portland-cement grout is used, the area filled with
one barrel of cement will be as follows : With 4-inch brick and a sand
cushion, 32 square yards of re-pressed brick, and 24 square yards
of wire-cut lug brick; and with 4-inch brick on a f-inch mortar bed,
30 square yards of re-pressed brick, and 22 square yards of wire-cut
lug brick."* It is quite common to estimate f of a barrel of cement
per square yard for 1 : 1 grout. Of course, the cost of the cement
for the filler will vary with the market price of cement. The grout
will require about 0.2 cubic foot of sand per square yard of pavement;
and its cost at $1.00 per cubic yard, will be about 0.7 cent per square
yard of pavement. The cost of applying the grout filter will
vary considerably with the details of doing the work, i. e., the number
of applications, and whether the mixing is done in a box or a machine;
but the cost will usually be 2 or 3 cents per square yard. The total
cost of grout filler was formerly 8 to 10 cents per square yard, but in
1917 was usually 10 to 12 cents per square yard.
1007. Merits. The advantage of the cement filler is that it pro-
tects the edges of the bricks from chipping, and thus adds to the
durability of the pavement. When the joints are filled with sand,
the edges of the brick chip off, the upper faces wear round, the pave-
ment becomes rough, and the impact of the wheels in jolting over the
surface tends to destroy the brick; while with a good cement filler,
the edges do not chip, the whole surface of the pavement is a smooth
* H. E. Bilger, Road Engineer, Illinois Highway Commission, in Engineering and Contract-
ing, Vol. 46 (1916), p. 502.
530 BRICK PAVEMENTS [CHAP. XVII
mosaic over which the wheels roll without jolt or jar, and conse-
quently the life of the pavement is materially increased. Fig. 193,
page 528, shows a grout-filled pavement 28 years old upon which there
have been practically no repairs. See the last paragraph of § 1002
for a description of its present condition.
An objection to the cement filler is that it does not take up the
expansion of the pavement due to increase of temperature, and that
consequently the pavement is likely to rise from the foundation and
give out a rumbling noise as vehicles go over it. This rumbling
can be eliminated by inserting longitudinal expansion joints as
described in § 1017.
Another objection to the cement filler is that in making repairs
it is difficult to remove the bricks without breaking many, and it is
difficult to clean the bricks so that they may be used again. This
is an advantage, if it will in any degree prevent the tearing up of the
pavement; and at best this objection ought not to have much weight
against durable construction.
A third objection is that the street can not be used while the
cement is setting. Often the cement is not allowed to set fully
before throwing the street open to travel, and consequently the chief
advantage of the rigid filler is lost. The semi-monolithic and the
monolithic types of construction are free from this objection (§ 979
and § 982).
1008. Bituminous Filler. Both asphalt and tar are used as a
filler for the joints of a brick pavement.
i 1009. Asphalt Filler. The various producers and refiners of
asphalt -prepare a grade of asphalt particularly for use as a filler for
brick, stone-block, and wood-block pavements. For the specifica-
tions of such, see § 544.
1010. Tar Filler. For the specifications of a tar suitable for a
joint filler for brick pavements, see § 576-77.
1011. Applying Bituminous Filler. The bricks should be dry,
and the bituminous filler should be hot enough to flow freely and
adhere to the brick. The asphalt fillers should be applied at a tem-
perature of 350 to 450° F., and the tar fillers between 300 and 350°
F. If either filler must be heated hotter than this to make it pour
freely, then it will be so hard as to chip out of the joint in cold weather;
and if it can be poured much colder than this, it will be so soft as to
run out of the joints in hot weather. However, manufacturers vary
the temperature of pouring to fit extreme climates.
The bituminous filler is poured into the joints through the point
ART. 2] CONSTRUCTION 531
of a cone-shaped pouring can. The point of the can has a cast iron
tip with an opening in it about J inch in diameter. The tip is opened
and closed by a valve, which is operated by a handle projecting at the
top of the can. The cast iron tip is placed in a joint, the valve is
opened, and the can is drawn along as the joint becomes filled. A
helper fills the can as it is emptied.
As soon as the joints in a short section of the pavement have
been filled, and while the bituminous cement is still soft, a light
layer of sand should be spread over the pavement, but only enough
to prevent the cement from sticking to passing wheels. In cold
weather the sand should be heated so as to bond rqfidily with the
pitch.
Particular care should be taken in applying the filler around man-
holes, at the gutter, etc., to prevent leakage of water into the sub*
grade.
1012. A pouring can, or rather tank, having multiple spouts
has recently been put upon the market. The tank is mounted upon
wheels, and somewhat flexible spouts project below. The tips of
the spouts are placed in the joints, and the tank is drawn along by
hand as the joints are filled.
Contractors claim that it is materially more difficult to fill the
joints of a pavement made of wire-cut lugs than of re-pressed brick,
as with the former it is more difficult to keep the tip of the pouring
can in the joint. Recently bituminous filler has been successfully
applied with a squeegee. The only objection to this method is that
the cement may be chilled by contact with the brick and fail to
penetrate to the full depth of the joint.
1013. Cost. The cost depends upon the locality, the closeness
of the joints, and the amount of bituminous material left upon the
surface of the pavement. A tar filler usually costs 8 to 10 cents per
gallon and asphalt about 10 to 12; and 1 to If gallons is generally
required for a square yard of pavement. The labor of heating and
pouring is usually about 5 to 7 cents per square yard. The total
cost of a tar filler is therefore about 13 to 15 cents per square yard,
and of asphalt about 15 to 19 cents.
1014. Merits and Defects. A bituminous filler is superior to
sand in that it makes a perfectly water-tight pavement, and better
protects the edges of the bricks. Bituminous filler is preferable
to cement grout in that the pavement can be opened to travel as
soon as it is laid; but bituminous filler does not protect the edges of
the brick as well as grout.
532
BRICK PAVEMENTS
[CHAP, xvii
1015. Tar-sand Filler. Recently a mixture of tar and sand,
usually called tar mastic or pitch mastic, has been employed as a
joint filler for brick pavements. The tar pitch should conform to the
specifications in § 576-77. The filler is made of pitch and as much
fine clean sand as the pitch tar will carry, usually about 1:1; but
in no case should the volume of the sand exceed that of the tar.
The coarser the sand, the smaller the proportion of it should be used.
The sand when mixed with the tar should be at a temperature be-
tween 300° and 400° F. ; and the tar shall be heated to 250° to 325°.
The mixing is most easily done with a hoe in a wheelbarrow or a
concrete bugg}i The mastic is poured on the pavement and pushed
into the joints with a squeegee.
Fig. 194 shows the method of applying tar-mastic filler. The
smoke indicates that the sand was too hot.
This filler has been used for brick pavements in a few cases with
every evidence of success; but the experience is too limited in both
FIG. 194. — APPLYING TAR-SAND FILLER.
extent and time to establish the merits of the method. A tar-sand
filler protects the edges of the bricks better and is less susceptible
to temperature changes than tar alone; and the only question is
ART. 2] CONSTRUCTION 533
whether or not the tar-sand filler can be made so as to flow satis-
factorily into the joints of a brick pavement. Since the joints of
stone-block pavements (Chapter XVIII) are wider than those of
brick pavements, such a filler is more needed and can be more sat-
isfactorily applied to the former than to the latter. For specifica-
tions for a tar-sand filler, and for further details concerning its use,
see § 110.
1016. EXPANSION JOINTS. Expansion joints may be either
longitudinal or transverse.
1017. Longitudinal Joints. With a sand filler (§ 995) there is
little or no need for expansion joints, since the sand in the joints
will yield enough to compensate for the expansion or contraction
due to changes of temperature. For much the same reason, expan-
sion joints are not necessary with a bituminous filler (§ 1008). But
with a rigid grout filler and sand bedding-course, it is necessary to
construct longitudinal expansion joints next to the curb or gutter on
each side, to provide for the expansion and contraction due to changes
of temperature in the wearing course. To be perfectly safe the
expansion joint should extend to the bottom of the concrete base;
although often it reaches only to the top of the concrete base. The
omission of longitudinal expansion joints is likely to cause the ex-
pansion of the wearing course to lift the brick from the bedding
course and to cause the pavement to give out a deafening noise
when a heavy-laden steel-tired vehicle goes over it at any consid-
erable speed (§ 1027).
The longitudinal expansion joint may be made by placing a
\- to 1-inch board on edge against the curbs; and then after the
bricks are set withdraw the plank and fill the space with tar or
asphalt. A close examination of Fig. 171, page 483, will show such
a plank in position with wedges between it and the curb to facilitate
the removal of the board. An objection to the poured expansion
joint is that it is liable to get blocked by a pebble or a brick spall
getting into the space before the bituminous material is poured.
Instead of using the plank as described above, a much better way is
to place pre-moulded strips of mastic next to the curbs before laying
the brick. There are several forms of these strips on the market.
The strips are usually f of an inch thick for a pavement 20 to 30
feet wide, and proportionally thicker for wider pavements. The
strips are comparatively short, and should fit closely end to end;
and should extend the full depth of the brick, and should be stiff
enough to stand alone in place until the bricks are placed against
534 BRICK PAVEMENTS [CHAP. XVII
them. The material should be pliable at 32° F., and should not
melt or flow at 125° F.
A true monolithic pavement needs no longitudinal expansion
joints, although they are sometimes provided.
1018. Transverse Joints. Transverse expansion joints are not
needed with either a sand or a bituminous filler. Opinions differ
as to the need of such joints with grout filler; but the best practice
seems to be to omit them.
There are two forms of transverse expansion joints in use. In
one method two or three or four of the transverse joints between the
courses of brick are filled with bituminous cement. In the other
method a J-inch plank is inserted between courses of brick at inter-
vals of 25 to 50 feet; and then after the brick are laid and grouted,
the plank is removed and the space is filled with bituminous cement.
Or, instead of the plank, a pre-moulded sheet of mastic (§ 1017) is
used.
1019. There are several objections to 'transverse contraction
joints in a pavement having a grout filler.
1. The expansion joint is weaker than other joints, and hence
the weight of passing wheels is likely to break the bond of a brick
next to the joint, and then the bond of one brick after another fails
in succession.
2. The contraction joint concentrates all of the shortening of a
section of the pavement at one line, and opens the contraction joint
so as to permit water to enter the sand cushion. The water acts as
a lubricant and causes the sand to shift, and often permits a brick
to settle; and then the impact of a passing wheel breaks the bond of
another brick, and the defect gradually extends. The water may
freeze and lift the pavenent, which rarely returns to its former
position, for the game reason that the continued action of frost lifts
loose stones to the top of the ground. If there are no contraction
joints, the contraction is likely to open many narrow cracks which
are less harmful than a few wider ones.
3. The filler in the expansion joint becomes more rigid at the
top than at the bottom, partly by vaporization and oxidation, and
partly by the pounding in of street dirt; and consequently the
expansion of the pavement concentrates pressure at the top of the
joint, and the adjoining brick are spalled. This roughens the pave-
ment and increases the effect of impact, which breaks the bond and
causes the sand cushion to shift. If there are no transverse expan-
sion joints, the expansion simply produces compression in the pave-
ART. 2]
CONSTRUCTION
535
ment and does no harm. This conclusion is in harmony with expe-
rience with concrete pavements (§ 466-68), namely, that trans-
verse expansion joints are not only not needed, but are a positive
detriment.
1020. Fig. 195 shows two examples of failures due primarily to
a contraction joint. In the left-hand view the wheel-track crosses
the joint near the middle of the picture; and doubtless the damage
FIQ. 195. — FAILURES AT TRANSVERSE CoNTRAcriONLJoiNTS.
started at this point and gradually progressed in all directions, for
each of the three reasons explained in § 1019. In the right-hand
view the contraction joint slopes up and to the right across the
picture. For some reason the damage is more to the right of the
joint than to the left, perhaps because of defective grouting (§ 1059).
1021. Expansion Joints at Anchors. An expansion joint | to
f of an inch wide should be provided around nlanhole covers, water
boxes, etc., which might act as anchors to prevent the expansion of
the pavement. Frequent examples are seen where the pavement
buckles at such points owing to the lack of adequate provision for
expansion.
1022. HEADERS. A header is a wood or stone or concrete curb
or protection placed at the end of the pavement or at an alley and
street intersection, to protect the edge of the pavement from vehicle
wheels bumping against it in getting on the pavement. A wood
plank 2 to 4 inches thick, held in position by posts, is sometimes
used; but stone or concrete are more durable, and are not much
higher in first cost. A 4-inch hard limestone or a 6-inch concrete
slab is usual.
536 BRICK PAVEMENTS [CHAP. XVII
With a monolithic brick pavement the header is not absolutely
necessary, as the brick will stand a good deal of bumping without
being dislodged; but even in this case the use of a substantial header
is true economy.
1023. COMPARISON OF TYPES OF BRICK PAVEMENTS. Brick
pavements differ chiefly as to the nature of the bedding course and
the character of the joint filler. The different types will be com-
pared as to durability, smoothness, noisiness, thickness, time in
construction, and cost.
1024. Durability. The durability depends chiefly upon the ma-
terial of the joint filler. The merits of the several joint fillers have
already been considered; and hence little need be said here. How-
ever, it may be repeated that a cement-grout filler protects the
edges of the brick best, and that such a filler makes the most durable
pavement.
1025. Smoothness. It has been abundantly proved by expe-
rience in the field that it is easier to get a smooth surface with a
mortar bedding-course than with a sand cushion. Smoothness
promotes durability; and besides the smoother the pavement the
less noisy it is.
1026. A grout or a bituminous filler is not retained in the joints
of re-pressed brick as well as of those not re-pressed, since the former
have a rounded edge while the latter have a square edge. With a
rounded edge, if the joint is filled level full, the filler feathers out at
its edges and is easily crumbled off; and then the next wheel drops
into the depression, and breaks out more of the filler. Soon the
filler is broken out to a considerable depth, and then the joint be-
comes a groove into which each passing wheel drops with a bump that
disintegrates the edge of the brick. With a square-edged brick the
joint filler wears away only as fast as the face of the brick.
1027. Noisiness. A brick pavement may produce noise from
two causes. One of these is the roughness of the surface, which has
just been considered in the preceding paragraph.
The second occurs only with a sand bedding-course and grout-
filled joints, and is due to the fact that the wearing course is sepa-
rated from the bedding course, which causes the pavement to give
out a rumbling or roar when a steel-tired wheel goes over it. This
separation may be due to the drying out of the sand cushion in spots,
which causes it to shrink away from the brick wearing coat. Many
brick pavements rumble from this cause. When this occurs the
pavement gives out a rumbling when a steel-tired wheel goes over
ART. 2] CONSTRUCTION 537
one of these hollow spots. Sometimes a high temperature lifts the
whole wearing coat up from the bedding course, when the noise is
very marked. Something like the same result occurs in cold weather,
possibly owing to the expansive action of freezing water in the soil
behind the curb crowding the curbs inward and thus lifting the
wearing coat up from the bedding course. If each curb of a 40-foot
pavement is forced inward ^V of an inch, the crown of the pavement
will be lifted from the foundation more than an inch. This result
will occur only when the subsoil outside of the curbs freezes while it
is at least nearly saturated with water.
Bgth the semi-monolithic and the monolithic types of brick pave-
ment are free from any rumbling noise.
1028. Thickness. Nominally there are three forms of bedding
courses; but really there are only two, viz.: sand, and cement
mortar. A brick pavement has thickness primarily to enable it
to distribute the concentrated load of a wheel over sufficient area
of the subgrade to enable the native soil to support the load. The
pavement distributes the load mainly, if not wholly, by its strength
as a beam, which enables it to bridge over any soft spot and also to
resist the lifting action of frost in the subgrade.
The thickness <3f a brick pavement having a sand cushion is
about as follows: foundation, 6 inches; sand cushion, 2 inches;
and wearing coat, 4 inches, — a total of 12 inches; but considered
as a beam, the effective thickness of such a pavement is only that
of the concrete base, i. e., 6 inches. The thickness of a pavement
having a cement mortar bedding-course is as follows: foundation,
6 inches; bedding course, 1 inch, and wearing coat, 4 inches, — a total
of 11 inches; but considered as a beam, the effective thickness of such
a pavement is 1 1 inches. The strength of a beam varies as the square
of its depth, and therefore the relative beam strength of the two
pavements as above is as 36 to 121; or the monolithic pavement
considered as a beam is 3.35 times the stronger.* Even though
there may not be a perfect union between the foundation and the
wearing coat, the above ratio is nearly correct, for the bedding
course is nearly at the center of the beam and hence there is little
or no longitudinal shear upon it, and hence the pavement acts nearly
as a solid beam. Laboratory experiments show that a well-grouted
layer of brick has as great transverse strength as a concrete slab
of equal depth.
* For data on the strength of slabs of monolithic brick pavements, see Engineering Record,
Vol. 73 (1916), p. 86; and Engineering News-Record, Vol. 79 (1917), p. 820-23.
538 BRICK PAVEMENTS [CHAP. XVII
Since the practicability of laying the brick in a bed of cement
mortar has been demonstrated, it has often been proposed to reduce
the total depth of a brick pavement having a mortar bedding-course
and cement-filled joints. In support of the possibility of making
thinner pavements when the wearing coat is cement-grouted, it is
often cited that 4-inch concrete roads in California give at least fair
satisfaction; and that many, perhaps most, concrete roads in the
Mississippi Valley are only 6 inches thick. Some engineers have
reduced the thickness of the concrete foundation, and others have
reduced the thickness of the wearing coat.
The extreme of the former practice is perhaps in Stockton -Town-
ship, Vermilion County, Illinois, in which in 1916 6J miles of rural
brick roads were constructed with a brick wearing coat 4 inches
thick and a concrete base only 1 inch thick. Another striking
example is in Polo, Illinois, where in 1917 4-inch bricks were laid on
2 inches of concrete. The purpose of either the 1-inch or 2-inch
concrete in the above examples is not to act as a foundation to sup-
port either the brick or the load upon the pavement, but to make a
smooth surface on which to set the brick and also to prevent the grout
filler from penetrating the subgrade. In both of the above examples
the subsoil is clay or loam.
An example of the practice of reducing the depth of the brick is
to be found in many cities in the Mississippi Valley west of the river,
in which many pavements were built between 1912 and 1917 using
vertical-fiber brick 2J inches deep. Another example of the use of
2j-inch brick is the driveway entrance to the Pennsylvania Passenger
Station in New York City (§ 981). The wear on a grout-filled brick
pavement is very small (see Fig. 193, page 528) ; and hence a brick
2| inches deep will last nearly as long as a brick 4 inches deep.
An example of reducing the thickness of both the foundation and
the wearing coat, is over 50 miles of rural brick roads in Florida
consisting of 3J-inch grouted brick laid on puddled native sand;
and similar pavements are laid on the streets of several southern
cities. Some of these pavements have been in service two years,
and have carried 10-ton motor trucks without any signs of distress.
Of -course, these pavements are not subjected to frost action.
1029. It is impossible to compute or otherwise determine in
general the permissible minimum thickness for any particular form
of construction, since the required thickness varies greatly with the
character of the subsoil and the climate; but it is certain that under
conditions where a sand-cushion brick pavement gave fairly satis-
ART. 2] CONSTRUCTION 539
factory service, a thinner pavement may be used if it is built mono-
lithic. Only considerable experience will determine the safe and not
extravagant thickness of a pavement.
1030. Of course, the cost will vary with the thickness; and
whether it is cheaper to diminish the thickness of the concrete base
or that of the brick-wearing course will depend upon conditions.
There are localities where it is economical to decrease the depth of
the brick, as for example where bricks are expensive and materials
for concrete are cheap; and on the other hand, there may be con-
ditions under which it is wiser to decrease the thickness of the con-
crete.
1031. Time under Construction. One of the most important
advantages of the monolithic type of brick pavement is the length
of time required for construction. All parts of it (the concrete
foundation, the bedding course, and the grout filler) are constructed
and seasoned simultaneously. With a concrete foundation, sand
cushion, and grout-filled joints, the concrete foundation should
be allowed to set for about 20 days before the sand cushion is spread
and the brick set (see § 464) ; and another 20 days should be allowed
for the cement grout to harden. This is one reason why a grout
filler is not used more frequently.
1032. Cost. The cost of construction of the monolithic type is
10 to 12 cents per square yard less than that of the sand-cushion
grout-filled type. The reasons for this difference of cost is as fol-
lows: 1. All parts of the work are done at substantially the same
time, and hence so much care is not required in protecting and caring
for the work while the cement sets. 2. A lighter roller is used in
rolling the brick. 3. Less sand is used for the bedding course.
4. There is less risk of having to take up and re-lay brick on account
of the sand cushion having been rained on. 5. It is possible to use
either a thinner concrete foundation or a shallower brick. 6. For
a rural road the monolithic construction does away with the need
of a curb or edging.
1033. Conclusion. In all points the monolithic pavement is
superior to any other type of brick pavement.
1034. PAVEMENT ADJACENT TO TRACK. It is exceedingly
difficult to construct any pavement adjacent to a street-railway
track that will not need frequent and extensive repairs. A large
part of the difficulty is due to the foundation of the track, which
subject has been considered in Art. 3, of Chapter XV — Foundations
for Street-railway Tracks. Another difficulty is in keeping a water-
540
BRICK PAVEMENTS
[CHAP, xvn
tight joint between the head of the rail and the pavement. Fig. 196
shows the standard practice of laying brick in the track area when
9"Raif ^Mortarbect
;:;^te^.fc^g I ye^g^f!±fS %m$jj$M$$j§
FIG. 196. — STANDARD PRACTICE IN BALTIMORE, MD.
a grooved rail is used; and Fig. 197 shows the corresponding
arrangement when a T rail is used. Most brick manufacturers
f /£ "Pawng Sand
3andand ^
Cement
"* 7-0" Wh,fc Oak Tie
a^g^vlJJ^J,]. -ImU ,\ ,\*L \ \ -Ml • I -LI L^L
///
, brich-
Fia. 197. — STANDARD PRACTICE OF FORT WAYNE AND WABASH VALLEY TRACTION Co.
make the " bull-nose J> brick for placing next to the inside of the
rail as shown in the upper view in Fig. 197. Notice in the lower
part of Fig. 197 that the paving between the rails is level with
the top of the rails; but often it is made level a little below the top
of the rail. Notice also that the lower half of Fig. 197 has a longitu-
dinal concrete beam under each rail.
Bricks are much used for paving the railway area, particularly
between the rails, because of their low first cost and of the ease with
which they can be laid. *|
It is practically impossible to maintain a brick pavement with a
sand cushion adjoining a railway track, since the cushion will shift
under travel and since water will leak into the cushion and freeze.
1035. MAXIMUM PERMISSIBLE GRADE. The Committee of
the American Society of Civil Engineers recommends 12 per cent
as the permissible maximum grade for a brick pavement with a
ART. 2] CONSTRUCTION 541
bituminous filler, and 6 per cent for a grout filler— see Table 15,
page 58. The report impliedly recommends the use of a plain brick
and bituminous filler on steep grades; but this is not in accordance
with accepted good practice. The best practice employs grout on all
grades; and uses plain brick on grades up to 5 or 6 per cent, and
hill-side brick (§ 936) for grades up to 10 or 12 per cent. Hill-side
brick and grout filler have given fair satisfaction on grades as high
as 15 and 18 per cent.
1036. STREETS PAVED WITH BRICK. The preceding discussion
of brick pavements has been without special reference to either city
streets or country roads. A few differences resulting from the dif-
ferent locations of the pavement remain to be considered.
The chief difference in the construction of a brick pavement on a
city street and on a rural road is due to the fact that usually the
paved portion is wider on the former than the latter. This neces-
sitates either the use of a longer template in striking the concrete
foundation and the bedding course, or the placing of screeds and the
use of a shorter template. The construction of the concrete founda-
tion has been discussed in Chapters VII and XIV. The template
employed in striking the concrete is described in § 460, and that used
for the bedding course in § 972.*
The bricks are usually transported from the parking to the setter
either by hand with pallets or tongs, or by a roller gravity conveyor
—usually the latter.
Longitudinal expansion joints are always required for street
pavements.
1037. ROADS PAVED WITH BRICK. The foundation for brick
pavements on rural roads more frequently than on city streets is an
old macadam road (§ 437).
Formerly brick roads usually had a sand cushion, but recently
the semi-monolithic or monolithic construction is ordinarily em-
ployed— generally the latter. Fig. 198 shows two typical views of
the construction of a monolithic brick road. The left-hand view
shows the bricks, the fine* and coarse* aggregate, and the cement
delivered ready for work, and also the side forms in place. Notice
that the materials have been transported to the job on an industrial
railway. The right-hand view shows work in progress. In the lat-
ter notice the steel side-forms, and the tamping template. The
double template (§ 982) is shown between the tamping template
j* For an illustrated account of the laying of a monolithic brick pavement 33 feet wide on a
city street, see Engineering News, Vol. 76 (1916), p. 978-79.
542
BRICK PAVEMENTS
[CHAP, xvii
and the concrete mixer, and is weighted down with bags of
cement.
Formerly the sand-cushion brick' road was built with an inde-
pendent curb (§ 729-34), or with an integral curb or an edging
(§ 771). Fig. 199 shows the concrete foundation for a sand-cushion
FIG. 198. — CONSTRUCTION OF MONOLITHIC BRICK ROAD.
FIG. 199. — CONCRETE FOUNDATION WITH EDGING.
brick road with edging, in process of construction. The concrete is
being spread to grade with shovels and smoothed with the back of a
shovel. The surface on the concrete slab is not first-class; but is
good for the method used, and is good enough in consideration that a
sand cushion is later to be used. The combined curb and gutter
AET. 2]
CONSTRUCTION
543
(of the shallow V type) is completed on the far side, and the form for
the integral edging is in position in the foreground of the left side.
The trussed scantling or template is being used in testing the crown
or elevation of the concrete, the ends of the projecting strip indicating
the height the concrete should have.
Fig. 200 shows four views of a semi-monolithic brick road
with edging, in process of construction. In view 1, notice the
low spot in the sand-cement bed. In view 2, notice that the men
are walking on the ungrouted brick, which is objectionable, but less
so with a cement-sand bed than with a sand-cushion (§971). In
1. Striking the Sand-cement Bed.
2. Laying the Bricks.
3. Sprinkling the Brick. 4. Second Application of Grout.
FIG. 200. — CONSTRUCTION OF MONOLITHIC BRICK ROAD WITH EDGING.
view 3, the stream of water is so solid or heavy as to wash out the
cement in the bedding layer, although the laborer says the stream
does no harm as he always plays upon the center of a brick. View 4
shows the mush-like consistency of the last coat of grout, which is
dangerously near being too thick to run into the joints well.
1038. Since the introduction of the monolithic brick pavement,
the brick wearing-coat for rural roads is laid without any curb or
edging (§ 471), experience having shown that the bricks at the edge
of the pavement are not dislodged by traffic.
544 BRICK PAVEMENTS [CHAP. XVII
The edge or corner of a monolithic brick road is rather rough and
ragged, and very destructive of automobile tires in turning off and
onto the pavement, if the earth is not kept well filled up against the
pavement. It is nearly impossible to keep the earth shoulders full,
and gravel or broken-stone shoulders are seldom used; and hence
this ragged edge is an objection to omitting the concrete edging.
This objection could be eliminated by laying a bull-nose brick (see
§ 1034) at the end of a course; but it is not known that this has ever
been done. The outer corner of the concrete edging can be readily
rounded off with an edging tool, although it is not often so done.
1039. COST OF BRICK PAVEMENTS. The cost will vary with the
locality and the details of construction, and consequently any gen-
eral statement of cost will be only approximately true for any par-
ticular case.
The grading is usually done by the cubic yard; and the cost
varies with the character of the soil, the depth to be removed, the
length of haul, etc. The cost of grading ranges from 15 to 50 cents
per cubic yard; but in easy soil and moderate cuts, it generally
varies from 25 to 35 cents. It usually costs 3 to 5 cents a square
yard to dress off the subgrade after it has been graded with drag
or wheel scrapers, and to throw the material into wagons.
The cost of rolling the subgrade will depend upon whether it is
rolled longitudinally only, or both longitudinally and transversely.
With a self-propelled roller the cost of rolling, both transversely and
longitudinally, will be about 0.6 cent a square yard, exclusive of
interest, storage, and depreciation of the roller.
The cost of the concrete foundation will vary with the price of
cement, the proximity of broken stone or gravel, the character of the
concrete, etc. Ordinarily the materials for a 6-inch course will cost
50 to 60 cents per square yard, and the labor 6 to 8 cents per square
yard.
The price of bricks will vary with their size, the locality, and the
freight rate. Previous to 1916 there was no uniformity in size,
common sizes for the wearing face being 3| by 8J inches, 3f by 8,
and 3 by 9; and some brands requiring 42 for a square yard, some 40,
and some 38. Since the beginning of 1916 there has been a vigorous
attempt to have all paving bricks of standard size, or rather to have
the wearing face 3J by 8i inches, of which 40 make a square yard of
pavement. In 1915, before the disturbance of prices due to the
Great European War, the average price at the plant for standard
blocks 4 inches deep was about $15.00 per thousand, or 60 cents per
ART. 2] CONSTRUCTION 545
square yard; but in 1917 for various reasons the price was practically
50 per cent more. There is no difference in price between wire-cut
lug and re-pressed bricks. In estimating the freight it may be
helpful to know that a brick 2£ X 4 X 8J inches weighs about 7
lb., and a block 3J X 4 X 8J inches about 9.75 Ib. In estimating
freight, the fact should not be overlooked that for one reason or
another a considerable number of bricks are rejected. With careful
grading at the kiln the broken and rejected brick is likely to be 1 to 2
per cent.
In the early history of brick paving it was customary for the
contractor to buy brick by the thousand; but the contractor claimed
that the manufacturer did not cull the brick sufficiently carefully
at the kiln, and consequently the rejections on the job were unduly
great. For a time it became customary to buy the brick f.o.b.
destination at a stated price per square yard in place in the pave-
ment; but under this plan, the manufacturer claimed that the con-
tractor did not use proper care in handling the bricks, did not keep
them clean, used good brick instead of chipped or broken brick in
making closures, and left good bricks along the finished pavement.
At present it is customary to sell the brick by the thousand f.o.b.
destination. The usual price is $25.00 per thousand f.o.b. destina-
tion.
The cost of hauling and piling on the side of the street is about
$1.50 per thousand for a haul of 1 mile, of which sum about half is
the cost of loading and unloading, and half the cost of team and
driver; but this cost for team and driver necessitates the use of three
wagons with each team.
The cost of setting blocks of which 40 make a square yard varies
from 4 to 6 cents per square yard.
The cost of turning the chipped brick and replacing the rejected
ones will depend mainly upon the severity of the inspection and upon
the degree of care employed in culling the brick before they are laid.
In a particular case, 80 hours were required to turn the chipped
blocks and to replace the rejected blocks with good ones, in 1,633
square yards of pavement, or, say, 1 hour for each 20 square yards.
The blocks were 3X4X9 inches, and about 2 per cent were
turned and about 2 per cent were rejected.
For data concerning the cost of sand filler, see § 995; for cost of
cement filler, see § 1006; and for cost of bituminous filler, see § 1013.
Examples of the actual cost of brick pavements, are given in
§ 1041-48.
546
BRICK PAVEMENTS
CHAP. XVII
1040. In this connection it should not be overlooked that the
cost of the pavement proper is usually not the only cost of improving
the street when it is paved. For details see § 878.
1041. Examples of Cost. In the following examples an attempt
has been made to present the data in such detail as to make clear
the form of construction; but unfortunately it is not always possible
to present information concerning important economic conditions, as
freight rates, the condition of the wagon roads over which material
is hauled, efficiency of labor, etc.
1042. Sand-cushion Asphalt-filled Brick Pavement. Table 56
gives the details of laying a brick pavement. The excavation was
done with a Maney 4- wheel scraper (§ 154) and a 20-H.P. tractor.
Wages were as follows: Common labor 20 cents per hour; brick
setters, 55 and 40 cents; engine runner for concrete mixer, 30
cents; team, wagon, and driver, 50 cents per hour.
TABLE 56
COST OF SAND-CUSHION ASPHALT-FILLED BRICK PAVEMENT
In Central Illinois in 1916
ITEMS.
COST PER SQ. YD.
Partial.
Total.
SUBGRADE :
Rough grading at 29.8 cts. per cubic yard $0 . 137
Surfacing and rolling with 10-ton roller .040 $0.217
CONCRETE FOUNDATION, 6 inches of 1 : 3 : 5 :
Labor at 20 cts. per hour 056
Cement at $1.48 per barrel (net) on job 227
Gravel at $1.40 per cubic yard on job . 156
Sand at $1.50 per cubic yard on job 122
Coal and water 006 . 568
SAND CUSHION, 1| inches thick 0. 640
BRICK, wire-cut lug, 3|X4X8| inches:
Purchase price, f.o.b. destination 807
Handling in car 027
Hauling to street— average 2,200 feet 020
Laying, contract price .050 .904
JOINT FILLER:
Asphalt, 12 Ib. per square yard 095
Labor, heating and pouring 060 . 155
MISCELLANEOUS EXPENSE .052
Total cost, exclusive of administration, tools and profits $1 . 960
AET. 2]
CONSTRUCTION
547
1043. 3-inch Brick Pavement. Table 57 shows the cost of a
brick pavement having a 4-inch concrete foundation, IJ-inch sand
cushion, 3-inch vertical-fiber brick wearing coat, and asphalt joint-
filler.
TABLE 57
COST OF 3-iNCH BRICK PAVEMENT IN FALLS CITY, NEB., IN 1914*
COST PE
R SQ. YD.
Partial.
Total.
SUBGRADE I
Rough grading
$0 03
Surfacing and rolling
015
$0 045
CONCRETE FOUNDATION, 4 inches thick:
Cement at $1.63 per barrel
0 185
Stone at $2.25 per cubic yard
232
Sand at $1 10 per cubic yard.
060
]Vlixing and placing
050
527
SAND CUSHION, 1^ inches, at $1.10 per cubic yard
.046
BRICK, vertical-fiber, 3 inches deep, f.o.b destination. . . .
Unloading and hauling
0.70
.05
Preparing cushion and setting brick
04
Rolling
006
796
JOINT FILLER:
Bituminous material . .
0 12
ADolvinff
03
150
INCIDENTAL EXPENSES
017
Total, exclusive of administration, depreciation, and profits.
$1.581
* Engineering News, Vol. 73 (1915), p. 223.
1044. Brick Roads in New York. Table 58, page 548, shows the
average cost of a great number of brick roads built in New York
by the State Highway Commission in 1912 and 1913.* The roads
were 15 feet wide, had 5-inch concrete base, 6-inch concrete edging,
sand cushion, and grout filler. The cost below includes engineering
expenses.
1045. Monolithic Pavement. Table 59, page 548, shows the labor
and materials required for a monolithic brick pavement 33 feet wide,
with 4-inch brick, and 1-inch special expansion joints every 82 feet.
Table 60, page 548, shows the labor cost of this job.
* Engineering News, Vol. 70 (1913), p. 1149.
548
BRICK PAVEMENTS
[CHAP. XVII
TABLE 58
AVERAGE COST OF BRICK ROADS IN NEW YORK IN 1912 AND 1913
TOTAL COST.
ITEMS.
Per Cent.
Per Mile.
Excavation
9.0
$2200
Drainage structures.
2.8
700
Subgrade and foundation
25.9
6300
Brick wearing-course and edging
60.1
14700
Minor expenses
2.0
500
Total .
100 0
$24 400
TABLE 59
AMOUNT OF LABOR AND MATERIALS FOR MONOLITHIC BRICK PAVEMENT*
Central Illinois, 1916
Labor.
Hours per
Sq. Yd.
Materials.
Cu. Ft. Per
Sq. Yd.
BASE AND BED:
laborers
engine runner
0.2305
.0154
CONCRETE BASE, 4 inches :
1 : 6 cement
gravel
0.5
3.000
sub-foreman
0138
BEDDING MORTAR ^-inch *
BRICK SETTING:
1 : 2 cement
.094
laborers
1772
sand
188
setters
0227
GROUT 1:1*
filling joints.
0887
cement.
144
spreading sand
0048
sand
144
OVERHEAD :
timekeeper
0171
TOP COVERING:
sand
188
foreman.
0171
water boy
.0171
TABLE 60
COST OF LABOR FOR MONOLITHIC BRICK PAVEMENT*
Central Illinois, 1916
'm ITEMS.
COST
PER. SQ. YD.
Concrete base, 4 inches of 1 : 6 gravel
$0 0649
Setting brick, 3| X4 X8| inches
0525
Filling joints with 1 : 1 grout
0244
Spreading top covering of sand
0011
Total cost of labor
$0 1429
*" Engineering News, Vol. 76 (1916), p. 1219."
AET. 2] CONSTRUCTION 549
1046. Brick on Old Macadam. Table 61 shows the cost of
laying a new brick pavement on an old macadam base by the Street
Department of Carlisle, Pa. The macadam was spiked with a 13-ton
3-wheel self-propelled roller, excavated to subgrade, surfaced with
hand picks, and rolled. The bricks were rolled with a 5-ton roller
drawn by 12 men.
TABLE 61
COST OF NEW BRICK PAVEMENT ON OLD MACADAM FOUNDATION*
ITEMS.
COST
PER SQ. YD.
Grading and rolling subgrade
$0 1126
Placing 5 inches of concrete in pipe trenches
0848
Cushion course, — limestone screenings
0543
Brick f.o.b. destination
8600
Unloading and hauling brick.
0777
Laving brick
0376
Rolling brick by manual labor
0069
Grouting joints
0845
Expansion joints, longitudinal and transverse
0353
Top coating of sand .
0047
Total
$1 3587
* Engineering News, Vol. 72 (1914), p. 1263.
1047. Semi-monolithic Brick on Old Macadam. Table 62, page
550, shows the cost of laying a semi-monolithic brick pavement on
an old macadam road. A 2-inch dry 1 : 4 mixture of cement and
sand was used for the bedding course. The water cost nothing
except the piping. The self-propelling roller used on the subgrade
was loaned, and no charge therefor is included. Common labor
received 20 cents per hour, and team and driver $6.00 per day.
1048. Cost in Various Cities. Table 63 and 64, page 550 and
551, shows the cost and several details of 3-inch and 4-inch brick
pavements in various cities. A few cities use 3j-inch brick, and a
few 2 J-inch ; but none of these are given here.
1049. MERITS OF BRICK PAVEMENTS. Bricks as. a paving
material have some attractive features. 1. They may be had in
small units of practically uniform size. 2. They may be had in
large or small quantities. 3. They may be laid rapidly without
special expert labor. 4. When ailing pipes or other causes neces-
sitate the disturbance of the pavement, ordinary tools and intelli-
gence can restore the original surface. 5. Brick pavements give a
good foothold for horses. 6. They do not wear slippery, 7. They
550
BRICK PAVEMENTS
[CHAP, xvn
TABLE 62
COST OF SEMI-MONOLITHIC BRICK PAVEMENT ON MACADAM*
Alton, Illinois, 1915
ITEMS.
COST PEK SQ. YD.
Partial.
Total.
LABOR :
Grading, setting forms, building barricade $0 . 084
Placing mortar bed and laying brick 087
Mixing and applying 1 : 1 grout filler 027 $0. 198
BRICK: rattler loss 25% 560
MORTAR BED:
Sand— 0.083 ton at $0.82 068
Cement— 0.079 barrel at $1.35 105 0.173
GROUT FILLER:
Sand— 0.008 ton at $0.82 007
Cement— 0.036 barrel at $1.35 047 .054
MATERIALS FOR FORMS:
Lumber at $20 per M— salvage at 67% 009
Template 008
Stakes and nails 002 0.020
MISCELLANEOUS:
Depreciation on small tooh and water line, teaming, etc 0 . 038
CLEANING UP . 017
Total, exclusive of administration and profits $1 .060
* Engineering Record, Vol. 73 (1916), p. 414.
TABLE 63
COST OF 3-lNCH BRICK PAVEMENTS IN VARIOUS CITIES IN 1916 *
LOCALITY.
State.
City.
Sq. Yd
Laid in
1916.
FOUNDATION.
Thick-
ness,
inches.
Propor-
tions.
BED COURSE.
Thick-
ness,
inches.
Kind, t Kind. J
FILLER.
Propor-
tions.
Cost
Per
Sq.Yd.
Kansas.
Missouri . . . .
Nebraska. . . .
Oregon
Texas
Arkansas City
Ottawa
Parsons
Salina
Topeka
Sedalia
Fremont
Lincoln
Astoria
Houston
San Antonio . .
26 132
14000
6208
27461
46000
12 345
20809
"3418
17500
41 571
: 5
$1.81
1.57
1.63
1.92§
1.77H
1.66
2.04
1.95
2.70||
2.45
2.50
* Municipal Engineering, Vol. 52 (1917), p. 128-30.
t S = sand; S-C = sand-cement; M = mortar,
t A = asphalt; G = grout.
§ $2.15 with 4-inch re-pressed brick.
f $1.97 with 4-inch re-pressed brick.
|| 2f-inch brick.
ART. 2]
CONSTRUCTION
551
TABLE 64
COST OF 4-lNCH BRICK PAVEMENTS IN VARIOUS CITIES IN 1916
LOCALITY
Sq.Yd.
Laid in
1916.
FOUNDATION.
BED COURSE
FILLER.
Cost
Per
Sq.Yd
$2.52
1.65
1.81
1.72
1.76
1.63
1.60
1.30
1.79
2.03
1.60
2.25
1.80
1.81
2.77
2.14
1.90
1.94
2.50
2.08
2.16
1.60
1.74
1.82
2.04
1.80
2.10
1.95
2.55
1.99
2.15
State.
City.
Thick-
ness.
Propor-
tions.
Thick-
ness.
Kind.f
Kind.J
Propor-
tions.
California. . .
Illinois
Los Angeles. . .
Alton
Champaign. . .
Danville
Galesburg. . . .
Mattoon
Peoria
Crawf ordville .
Ft. Wayne
Muncie
New Castle. . .
Wabash
Leavenworth. .
1 800
51 510
30000
42000
17760
32269
36536
4 108
21005
12000
19408
2575
4 717
59758
13844
71 800
34 512
2572
4310
19947
25000
50 120
34637
22223
63 121
42000
42333
15000
112581
14488
39326
4"
4
6
5
4
5
4-5
4
6
6
5
6
5
6
5
6
5
5
6
5
5
5
6
6
6
5
6
4
5-6
5
5
1:3:6
1:3:6
1:3:5
1:3:5
1:3:6
1 : 4* : 8
1:3:5
1:3:5
1:3:6
1:3:6
1:3:6
1:3:6
1:3:5
1:3:6
1:2:4
1:3:6
1 :3i :7
1:2:5
1:3:6
1 : 6
1:3:6
1:3:5
1:3:5
1:3:6
1 : 3i : 6
1:3:6
1:3:6
1:3:6
1:3:6
1:3:6
1:3:5
.5-2"
1
i!
1
1-2
U
2
2
ii
1*
!J
1
li
!»
2
2
M
.....
S
S-C
s
s-c
s
s
s
s
s
s
s-c
s
s
s
s
s
s
' M'
s
M
s
s
s
M
s
s
s
s-c
s
s
G
G
A
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
" G'
G
A
G
G
G
G
A
G
i i
i i
1 1
1 1
Indiana
Kansas
Kentucky. . .
Mass
Michigan. . . .
New York . . .
Ohio
S. Carolina. .
Washington.
Wisconsin. .
i i
1 2
1 1
1 1
1 1
i i
1 2
Fitchburg. . . .
Detroit
Grand Rapids .
Port Huron. . .
Amsterdam. . .
Jamestown. . .
Poughkeepsie .
Canton
Findlay
Lakewood . . .
Toledo
Warren
1 1
1 1
"i i1
i i
i i
i i
i ii
Youngstown .
Greenwood . .
Seattle
Beloit
Racine
* Municipal Engineering, Vol. 52 (1917), p. 128-30.
t S = sand; S-C = sand-cement; M = mortar,
t A = asphalt; G = grout.
are adapted to all grades. 8. They have low tractive resistance,
particularly if the joints are filled with cement grout. 9. They are
not specially noisy when properly laid. 10. Brick pavements yield
no mud or dust. 11. They are easily cleaned. 12. If the joints
are filled with sand, they are only slightly absorbent; and if filled
with hydraulic or bituminous cement, they are non-absorbent. 13.
Brick pavements have a pleasing appearance. 14. They are very
durable, particularly if the joints are filled with cement grout. 15.
They are cheap in consideration of their small cost of maintenance
and long life.
1050. SPECIFICATIONS. The American Society for Testing
Materials publish specifications for the standard rattler, the standard
rattler test, and paving brick, copies of all of which can be had for a
nominal sum of the Secretary. The American Society of Municipal
Improvements publish complete specifications for brick street pave-
ments having sand bedding course, and grout or tar or asphalt filler,
552 BRICK PAVEMENTS CHAP. XVII
copies of which may be had for a nominal sum of the Secretary.
The National Paving* Brick Manufacturers Association publish com-
plete specifications for all varieties of brick paving for both streets
and roads, using wire-cut lug or re-pressed brick, copies of which
may be had gratuitously of the members of the Association and of the
Secretary, Brotherhood Building, Cleveland, Ohio. The Western
Paving Brick Manufacturers Association publish specifications for
all varieties of brick paving including the use of vertical-fiber brick,
copies of which may be had gratuitously of members of the Asso-
ciation, or the Secretary, D wight Building, Kansas City, Missouri.
The Dunn Wire-cut Lug Brick Co., Conneaut, Ohio, an organization
to promote the use of wire-cut lug brick by securing good and eco-
nomical brick pavements, publish a complete set of specifications
for wire-cut lug brick-paving, which may be had gratuitously.
Many of the State Highway Commissions publish complete speci-
fications for brick paving, which can doubtless be had gratuitously
by residents of the respective states.
ART. 3. MAINTENANCE
1052. The maintenance of pavements has never received atten-
tion in proportion to either its economic importance or the comfort
of the user. This is particularly true of brick pavements, partly
because they are comparatively new, and partly because the
material not being subject to decay the need of maintenance has
been over-looked. There has been no comprehensive diagnosis of
the diseases of brick pavements, nor have any effective remedies
been developed.
1053. REPAIRS OF BRICK PAVEMENTS. The more common
matters that need attention in the maintenance of brick pavements
are: (1) holes due to soft brick, (2) depressions due to shrinkage or
flow of the sand cushion, (3) depressions due to sinking of founda-
tion, (4) depressions due to settlement of trenches, (5) defects at
transverse expansion joints, (6) defective grouting, (7) bulges,
(8) longitudinal cracks, (9) re-laying pavements in patches and cuts,
and (10) re-surf acing, worn pavements.
1054. Soft Brick. A soft brick in the pavement wears away and
makes a hole. Each wheel, particularly a steel-tired one, dropping
into the hole crushes and chips the adjoining bricks even though
they are hard, and the hole gradually increases in size and depth.
When such a brick shows itself it should be cut out and be replaced
ART. 3] MAINTENANCE 553
with a good one. The defective brick must be cut out with a chisel
and hammer, the joints adjacent to it must be cleaned, and the bed-
ding course must be removed. If the bedding course is sand, it
must be carefully compacted ; and if it is mortar, a new bed must be
laid. After the new brick has been placed and firmly settled into the
bedding course, and found to conform to the general surface of the
pavement, the joints should be filled. If the joint filler is grout,
the new brick should be dampened before the grouting is done, and
care must be taken to see that the sand cushion has not pushed up
into the joint; and after the joint is filled, the spot should be bar-
ricaded until the cement has set.
The patch should be barricaded for at least a day or two. Theo-
retically, this time is far too short for the cement grout to gain its
full strength; but experience seems to show that a day or two is
reasonably safe. The explanation of this anomalous result is that
the spot being small, there is not much probability that a wheel
carrying a maximum load will go over the spot until the cement
grout has gained sufficient strength to hold it. Obviously then,
the denser the traffic or the heavier the loads, the longer the barri-
cades should remain.
If the brick are not quite uniform in quality, there may be so
many soft bricks as to make it impracticable to remove each soft one,
in which case the pavement must continue in service until it is re-
surfaced as described in § 1063, 1068, or 1069, or until it is replaced
with a new pavement. The possibility of this condition arising is
the reason for making the rattler test in such a way as to secure
information as to the uniformity of the brick; and is also a reason
for carefully inspecting the bricks after they are laid.
1055. Shrinkage of Sand Cushion. A common defect of brick
pavements having a sand cushion is the shrinking of the cushion
away from the brick, due probably to the sand being wet when the
bedding course was spread. If the joints are filled with sand, the
first evidence of the shrinkage of the cushion is a shallow saucer-like
depression of the surface of the pavement. The remedy is to take
up the low spot and re-lay the sand cushion. If the- spot is not
repaired, the depression is likely to be enlarged by the impact of
passing wheels and the flow of the sand cushion. The flow of the
sand cushion sometimes gives rise to broad shallow ruts, particularly
on a street having a street-car track, since then the travel is con-
centrated on a narrow strip.
If the joints between the bricks are filled with cement grout, the
554 BRICK PAVEMENTS [CHAP. XVH
first evidence that the sand cushion has shrunk away from the
brick is a rumbling noise when a steel-tired wheel goes over the
spot. Tapping the surface of such a pavement with a hammer will
reveal many such hollow spots. Such spots finally break down by
the 'shearing of the joint filler. After this is done, water enters
through the broken joints, and freezing lifts the pavement; and
sometimes breaks other joints, and perhaps tilts a portion of the
pavement and thereby roughens the surface. The examination of
any sand-cushion pavement after it has been in service a year or
two, will show a number of such breaks. The only remedy for this
defect is to cut out the low spot and re-lay it. Many old pave-
ments have so many such breaks that it is impracticable to repair
them. The adequate preventive is not to use a sand cushion, or at
least not a thick one. For a discussion of the method of resurfac-
ing a brick pavement, see § 1063, 1068, or 1069.
1056. Settlement of Foundation. Sometimes a round depression
is due to the settlement of a spot of the subgrade. It can not always
be determined from a surface examination whether the defect is due
to the shrinkage of the sand cushion or the settlement of the sub-
grade. To determine the cause, remove the wearing coat and the
bedding course, and examine the top of the foundation for a crack
near the edge of the depression. If the foundation is broken, it
should be taken out, and the cause of the settlement be sought for.
Perhaps it is due to lack of consolidation at the time of construction,
or perhaps it is due to a leaking pipe or a spring. If it is due to the
first, the remedy is thorough tamping; if to a leaky pipe, the remedy
is obvious; and if to a spring, then adequate drainage must be pro-
vided. After the causes have been removed, the pavement must be
replaced as described in § 1061.
1057. Settlement of Trench. The settlement of the back-filling
of a trench is one of the most common and most serious defect in
pavements. The settlement over a longitudinal trench is more
noticeable but less damaging than that over a transverse trench.
The prevention during construction is discussed in § 764-70.
When a. pavement sinks over a trench, it is difficult to eliminate
the defect. The first step is to remove in succession the wearing
coat, the bedding course, and the foundation. The second step is
to consolidate the back-filling; but this is not easily done. During
construction, if the trench is not too deep and if it was not back-
filled under very unfavorable conditions, the back-filling can usually
be consolidated fairly well by rolling (§ 763) ; but this is impracticable
ART. 3] MAINTENANCE 555
after the pavement is laid. Under some conditions flooding (§ 766)
may be worth trying. The most feasible, way is to dig out the trench
and replace the material with adequate tamping (§ 767-69). The
usual method is simply to fill the trench, generally a little more than
full, and re-lay the pavement, allowing it to be a little high with the
expectation that it will settle to place. But sometimes the pave-
ment does not settle, and sometimes it settles too much; and then
in either case, another trial is needed to secure a good surface.
Sometimes an attempt is made to remedy such a defect by laying
a thicker concrete foundation over the trench and allowing the slab
to extend laterally beyond the trench; but usually such a remedy
is not effective except perhaps where the trench is comparatively
narrow and where the banks of the trench are fairly solid. A radical
remedy would be to dig out the trench and fill it with sand, or gravel,
or a very lean concrete (§ 770). The difficulty of removing this
defect is a reason in favor of good original construction. .
For a discussion of the method of replacing the brick in making
such a repair, see § 1061.
1058. Transverse Contraction Joints. Transverse contraction
joints are a common cause of deterioration in a brick pavement
having grout filler. The action of the joint in damaging the adja-
cent pavement has been explained in § 1019-20, and illustrated in
Fig. 195, page 535. The only remedy in such cases is to remove
the expansion joint and re-lay the pavement. For some hints con-
cerning the method of re-laying the pavement, see § 1061.
1059. Defective Grouting. Defective grouting is a cause of
serious deterioration in a brick pavement. In a pavement with a.
rigid filler, it is essential that the filler extend from top to bottom of
the joints, as otherwise the pressure due to the expansion of the pave-
ment will be concentrated on one portion of the joint, which will
crush the filler and break the bond, and cause the pavement to wear
as though it were sand-filled ; or the pressure may cause the brick jto
spall or crush. There are two possible reasons for such defects: ,1.
The sand of the cushion course may have pushed up into the bottom
of the joint when the bricks are rolled; and consequently the filler
at the bottom of the joint will not be as rigid as that at the top. 2.
The grout, particularly that of the second coat, may have been so
thick as to bridge the joint and fill only the top of the joint. Fig. 201,
page 556, shows a spot in which the grouting was defective. Notice
that a rut has formed at the right of the defective spot, that a brick
at the top of the spot is badly shattered, and that several bricks have
556 BKICK PAVEMENTS [CHAP. XVH
spalled. These defective spots are usually small in the beginning,
but they gradually enlarge owing to the increased effect of impact
and also to the effect of the water that penetrates such spots.
FIG. 201. — DEFECTIVE GROUTING.
These spots may be cut out by hand with chisel and hammer,
or with a pneumatic chisel or chipping hammer as shown in Fig. 202.
PIG. 202. — PNEUMATIC CHISEL CUTTING OUT DEFECT.
In Baltimore the cost of toothing out a brick pavement having
grout filler (see Fig. 204, page 559) was 18 cents per lineal foot by
hand, and 2.76 cents per lineal foot with a pneumatic chisel.*
* Municipal Engineering, Vol. 52 (1917), p. 6.
ART. 3] MAINTENANCE 557
In cutting out such a spot no half brick should be left, as other-
wise there will not be sufficient bond between adjacent bricks. The
joint-filler should be thoroughly cleaned from the edges of the toothed
bricks, care being taken not to break the bond of the remaining
bricks. The bedding course should be adjusted so as to bring the
tops of the new bricks to the proper elevation. Care should be
taken that the brick to be used in filling the patch are of exactly the
same size as the old ones. The brick should be laid, tamped, and
grouted as in the original construction.
1060. Bulge. A bulge is a buckling or heaving of the wearing
coat of a brick pavement due to its expansion. Sometimes the bulge
is simply a wave, sometimes a ridge having a crack at its crest,
and occasionally an explosion or " blow-up " in which a number
of bricks are thrown into the air with considerable force. Fig. 203
FIG. 203. — Two BULGES IN BRICK PAVEMENT.
is a view of two upheavals. The right-hand example is a mild up-
heaval, and the left-hand one is a moderate explosion. A bulge
usually occurs at a crowned foot-way crossing or a street intersec-
tion. Notice that both of the bulges shown in Fig. 203 occurred at a
crowned foot-way crossing.
If a main street is paved and its crown is carried through the
intersection with another street and later the latter is paved, the
pressure due to the expansion of the pavement of the cross street
against the abutments of the crown of the main street may cause
the crown of the arch to rise with or without an explosion. The
mildest form of this phenomena is simply lifting the pavement from
the sand cushion, in which case the pavement will give out a rumbling
sound as a wheel passes over it, and it will come back to place when
the pavement cools. Or the pavement may be cracked near the
558 BRICK PAVEMENTS [CHAP. XVII
middle of the main street, in which case the pavement must be re-
laid along the crack, since the pavement will not of itself return to
its former position. Upheavals or cracks from this cause can be
prevented by putting in an expansion joint where the pavement of
the cross street abuts against that of the main street.
Sometimes a lateral pressure on the pavement will lift the crown
from the sand cushion, as is shown by a rumbling when a vehicle
goes over it; and in extreme cases the pavement will crack near the
middle of the street. This lateral thrust may be due to an inadequate
or obstructed expansion joint next to the curbs. A poured longi-
tudinal expansion joint may become obstructed by pebbles or brick
spalls dropping into the space or by the sand cushion's running into
it, before the mastic is poured. The crown of the pavement may be
lifted also by the expansion of freezing water in the soil behind the
curbs.
An upheaval may occur in a level stretch of pavement due to
defective grouting. If only the top of the transverse joints are
rilled, the pressure of the expansion is concentrated at the top of
the joints; and if the joints over a considerable area are in like con-
dition, they may all fail at once — usually with a loud report and a
general upheaval of the affected area.
1061. Re-laying Pavement. If a brick pavement is to be main-
tained in a fair condition, it will frequently be necessary to re-lay
the pavement over patches and also over openings made to lay or
repair sewers, water or gas pipes, electric conduits, etc., for with the
utmost care and foresight many such openings will be made (§ 656-
57). The typical case is re-laying a pavement over a trench.
In making the cut in the pavement alternate brick will some-
times be broken in the middle, thus leaving three bricks with their
ends in line, which would prevent a good bond of the new pavement
with the old; and therefore these broken bricks must be " toothed
out " so that only whole brick remain. This may be done by hand
with a stout long-handled chisel and hammer, or with the pneu-
matic chisel (Fig. 202, page 556). After the broken brick are
" toothed out," the cut will have the general appearance shown in
Fig. 204. Next the joint-filler should be chiseled off from the brick
that are " toothed out," care being taken not to break the bond of
these bricks with those adjoining.
Before re-laying the concrete foundation, the soil in the trench
should be thoroughly compacted; and particularly the soil that has
run out into the trench from under the edge of the foundation, should
ART. 3] MAINTENANCE 559
be replaced and rammed laterally to give a firm bearing. The con-
crete should then be laid and tamped as in the original construction.
If the bedding course is sand, great care is necessary in packing or
ramming it under the edge of the undisturbed pavement.
As far as they are available, the old brick should be cleaned and
used, for they will match the others in size and color. If new
brick are used, they should be of exactly the same width as the old
Fia. 204. — PATCH PROPERLY TOOTHED OUT.
ones so that the courses will match. The brick should be bedded
in the sand or mortar bed-course so their upper faces conform to the
surface of the pavement; and then the joints should be filled. If
grout is used, two or three applications should be added as described
in § 999-1004; and after the grout has taken an initial set, the patch
should be covered with sand or straw which is kept wet for 3 or 4
days. In this connection, see the latter portion of the second para-
graph of § 1054.
With care in making a re-placement, the patch can be made so
that the original surface and strength of the pavement is fully re-
stored; and if the work is well done, the patch will hardly be visible.
Fig. 205, page 560, shows such a patch over a trench in a pavement
in dleveland, Ohio. This patch is visible only because of the differ^
560 BRICK PAVEMENTS [CHAP. XVII
ence in color of the old and the new brick. In that city are many
such repairs which are scarcely visible.
Part of the reasons for this good work is that the back-filling of
the trench is inspected and the pavement is re-laid by employees of
the division of street repairs.
1062. Cracks. The only cracks requiring consideration are lon-
gitudinal ones, which are usually near the center of the road. The
transverse cracks are usually narrower, and are less harmful. A
FIG. 205. — BRICK PAVEMENT RE-LAID OVER A TRENCH.
longitudinal crack is likely to develop into a rut. A longitudinal
crack may be due to any one of several causes. 1. It may be due
to the heaving action of frost under the edges of the foundation. 2.
It may be due to the settlement of the edges of the foundation when
the frost is out under the edges of the foundation and not under the
center. 3. It may be due to the shrinkage of the soil under the
edge of the foundation due to the soil's drying out. 4. It may be due
to expansion as explained in the last two paragraphs of § 1060.
It is sometimes impossible to explain the cause of such cracks.
They seem to be more common in the North than in the Sou.th;
and hence it is concluded that frost is a common cause. They seem
ART. 3] MAINTENANCE 561
to be less frequent the flatter the crown of the pavement; and they
seem to be less frequent with a flat than with a crowned subgrade.
The only practicable thing that can be done with a longitudinal
crack is to clean and fill it with a bituminous cement as described for
concrete roads — see § 482.
1063. RE- SURFACING. Re-surfacing is a method of radical repairs.
There are many sand-filled pavements so badly worn and so full of
depressions as to be of but little value as a pavement, which may
be used as a foundation for a new wearing surface. There are three
methods that may be employed in re-surfacing such pavements,
viz.: (1) covering the brick with a bituminous surface; (2) turning
the brick upside down and re-laying them; and (3) laying a new
monolithic surface on the old pavement.
The bituminous surface may consist of either asphalt or tar.
1064. Asphalt Top. The asphaltic surface may be either asphalt
concrete or a binder course and a wearing coat similar to that of a
sheet asphalt pavement. When the work is well done in every par-
ticular, the result is very satisfactory; but there have been many
failures, probably owing to the failure to meet one or more of the
conditions necessary for success. There are several essential con-
ditions to be fulfilled. 1. The original pavement must be absolutely
rigid, and not show any vibration or settlement when a heavy load
goes over it. The lack of rigidity in the original pavement or in
portions of it which are re-laid preparatory to re-surfacing it, is one
of the most common causes of failure of a bituminous top. 2.
The surface of the old pavement must be leveled up by filling the
depressions with concrete, so that the asphalt will be of nearly uni-
form thickness, as otherwise it will creep and loosen from the brick.
Depressions deeper than 2 inches should be filled with hydraulic
concrete; but depressions less than 2 inches deep may be filled with
the mixture for the asphalt binder course (§ 812-24). 3. The asphalt
must be of good quality and proper consistency. 4. The pavement
must be perfectly clean. The dirt must not only be removed from
the surface but also from the cracks for at least J an inch. This
can be done with wire brooms, but it requires great care. The dirt
can be removed more effectively with a fire hose than by sweeping.
5. The brick should be perfectly dry when the asphalt is applied.
6. The old brick should not be cold when the asphalt is applied. If
a surface heater (Fig. 161, page 450) is available, it is wise to warm
the old brick before applying the asphalt. 7. Apply a paint course
of asphaltic cement thinned with naphtha, at the rate of not more
562 BRICK PAVEMENTS [CHAP. XVII
than a gallon to the square yard. Dust or dampness or cold will
prevent the paint coat from adhering perfectly. 8. As soon as the
naphtha has fully evaporated from the paint coat, and while it is
still perfectly clean, the sheet asphalt should be laid and rolled.
The sheet asphalt may be either a binder course and a wearing
coat of a total thickness of 2 inches, or a wearing coat alone having a
thickness of 2 inches. The binder course and also the wearing coat
are to be mixed and laid as described for the corresponding opera-
tions for a sheet asphalt pavement (§ 812-24 and § 825-45, respec-
tively). Experience has shown that if it is possible, the asphalt
surface should not be less than 2 inches thick at -any point, as it is
likely to creep and form humps. This is due to the fact that in rolling
the binder course or the wearing coat, the wide roller will be sup-
ported on the high points and not compress the asphalt in the holes;
and later narrow tires will compress the asphalt in the holes and
make a depression, and wheels dropping into these depressions will
displace the asphalt and loosen it from the bricks.
1065. One of the most difficult questions encountered in plan-
ning to add a new surface to an old pavement is the matter of drain-
age. The curb or gutter that was only deep enough for the original
pavement will be too shallow if a new 2- or 3-inch surface is put on
top of the old. This difficulty is not as serious on streets having
considerable longitudinal grade as on nearly level streets. On the
latter, either of two things may be done,* viz.: 1. Take up the old
brick and the foundation next to the curb (or next to the combined
curb and gutter) for a width of 3 or 4 feet, and lay a new concrete
foundation at such a height that when the old brick are re-laid and
the asphalt is placed thereon, the gutter will be of suitable depth
(or the top of the asphalt will be even with the top of the concrete
gutter). 2. Take up the old brick for 3 or 4 feet next to the curb
(or next to the combined curb and gutter) and lay concrete in this
space making its upper surface next to the gutter of such a height
that the asphalt when laid thereon will given sufficient depth of
gutter (or will come even with the top of the concrete gutter), and
making the top of its edge next to the undisturbed brick level with
the top of the brick. Of course, both of these methods increase the
transverse slope of the pavement near the curb, but usually this is
not serious.
At street intersections that are not to receive an asphalt top, a
* Thomas H. Brannan, Superintendent Asphalt Streets, Columbus, Ohio, in Proc. Amer.
Soc. Municip. Improvements, 1916, p. 107-8.
ART. 3] MAINTENANCE 563
somewhat similar adjustment is necessary where the new asphalt
top meets the pavement of the side street.
1066. Tar Top. Tar has not been used for this purpose as much
as asphalt, but it has been employed enough to show that it can be
made to give reasonably satisfactory results. The method of apply-
ing the tar is as follows: 1. The brick surface should be rigid, clean,
dry, and warm as described in items 1, 4, 5, and 6 of § 1064. 2.
All depressions more than 1 inch deep should be filled with hydraulic
concrete. 3. All depressions less than 1 inch deep should be thor-
oughly painted with tar, and then be filled with J- to ^-inch broken
stone perfectly free from dust. 4. The tar is then applied sub-
stantially as described for bituminous carpets (§ 591-93), except that
a greater quantity is applied, depending upon the degree the bricks
are worn. Enough should be applied to have a layer of about f of
an inch thick on the face of the brick. The tar should be well
brushed or rubbed into the joints with a wire broom. 5. As^soon
as the tar is rubbed into the joints, a half-inch layer of stone chips
is applied as described in § 594-95.
When the surface is finished, it will have the appearance of a
bithulithic pavement (§ 893), and will give good service for several
years, depending upon the character and amount of traffic. When
worn through, the surface can be renewed by the addition of a new
coat of tar and screenings.
Water injures a tar surface, and hence this treatment is more
permanent the better the drainage of the surface.
1067. Turning the Bricks. In Champaign, Illinois, in 1916, two
experiments were tried of laying a monolithic brick pavement on the
existing concrete base, turning and using the old bricks. The old
pavement was laid on a 2-inch sand cushion, and the joints were
filled with sand. The brick were badly worn.
The repair work was carried out as follows: 1. The concrete base
was cleaned, and on it was laid a 2-inch layer of 1 : 3 : 2 gravel con-
crete for a bedding course. 2. The brick were thoroughly cleaned
with wire brushes, and laid on the bedding course before the con-
crete had set. 3. The brick were rolled and grouted in the usual
way (§991 and §996-1002). 4. The street was barricaded for 15
days.
In one section, the brick were very badly worn, and varied in
depth from 2J to 4 inches. About 10 per cent had to be replaced
with new ones. No attempt was made to size them, and it was
difficult to get a smooth surface. If the brick had been sorted, and
564 BRICK PAVEMENTS [CHAP. XVII
the most worn ones placed next to the curb and the least worn at the
crown, it is believed a better surface would have been obtained with
less labor. On the other section, the brick were not worn so much,
and a good surface was obtained with much less trouble. The sur-
face of this section compares favorably with that of a new mono-
lithic pavement. The cost was 80 cents per square yard, which
could doubtless be reduced with greater experience,
1068. MONOLITHIC BRICK TOP. In 1917 near Danville, 111., a
new method of re-surfacing an old brick pavement was tried.* The
pavement was on a suburban road which carries a dense and heavy
traffic; and after considering asphalt, tar, and concrete as materials
for the new surface, a monolithic brick surface with a thin concrete
base was adopted. The concrete was one part of cement to four
parts of fine, well-graded gravel; and its thickness varied from 1 to 5
inches according to the depth of the holes in the old pavement. Part
of the new brick were 4 and part 3 inches deep. The concrete,
the bedding course, and the brick were laid as described under mono-
lithic pavements (§ 969, § 982, and § 996-1002, respectively). Fig.
206 shows two views of this work in progress. The cost was $1.65
per square yard; and the saving was substantially the cost of a
new concrete base.
<&.
fet.
• ^
FIG. 206. — PUTTING A BRICK TOP ON AN OLD BRICK ROAD.
Obviously this method of re-surfacing would be inapplicable to a
city street, on account of the difficulty of maintaining proper drainage.
1069. COST OF MAINTENANCE. As stated in § 868, the City
of Buffalo, N. Y., is noted for the completeness of its records of the
cost of construction and maintenance of pavements. The following
is a summary for the cost of maintenance of brick pavements for
- v * Harlan H. Edwards, Engineering News-Record, Vol. 79 (1917), p. 830-32.
ART. 3] MAINTENANCE 565
the year ending June 30, 1916. Of the twenty brick pavements over
twenty years old, two short streets have required no repairs; and
the repairs on the other eighteen streets cost from 0.22 to 4.92 cents
per square yard per year, all but six costing less than 0.85 cent per
square yard per year. Of the fifty brick-paved streets from ten to
twenty years old and out of guaranty, twenty have required no
repairs; and the repairs on the other thirty have ranged from 0.04
to 1.58 cents per square yard per year, only three of these costing
more than an average of 1 cent per square yard per year.* The
average cost of repairs during 1915-16 on 231,355 square yards was
2.9 cents per square yard.f
* Report of Dept. of Public Works— Bureau of Engineering, 1915-16, p. 481-511.
t Ibid., p. 70.
CHAPTER XVIII
STONE-BLOCK PAVEMENTS
1073. Stone-block pavements rank third in area among the
permanent pavements, being exceeded by sheet asphalt and brick
— see the tabular statement on page 320.
1074. CLASSIFICATION. The earliest pavements of ancient
times consisted of irregular shaped blocks of stone more or less
accurately fitted together. The form and size of the blocks have
varied greatly from time to time, a fact which has given rise to
different classes of pavements. A few of these will be briefly
described.
1075. Roman Roads. The Roman roads so frequently referred
to by modern writers are the earliest examples of stone-block pave-
ments. The details of construction varied somewhat, but as a
rule they were about as follows: The foundation was laid in a trench
about 3 feet deep, with no attempt at underdrainage. The base
was formed of one or sometimes two courses of large flat stones
laid in lime mortar, and was usually about 15 inches thick. Upon
this was laid a 9-inch course of small fragments of stone imbedded
in lime mortar, the intention of this course apparently being to bind
together the tops of the large stones in the course below. Next
was laid a 6-inch layer of concrete, apparently to make a smooth
bed to receive the stones of the top course. The wearing surface
consisted of closely-jointed, irregular-shaped stones, about 6 inches
thick. The total thickness of the road was about 36 inches. In and
near the cities, the top course was formed of irregular blocks of
basalt, porphyry, or lava, which had a top area of 4 or 5 square feet
and a thickness of 12 to 15 inches. These blocks were dressed
and fitted together with extreme accuracy, and were imbedded
in cement. These ancient pavements have aptly been described
as " masonry walls laid on their sides."
The Romans seem to have located their roads in straight lines,
566
CLASSIFICATION
567
running them toward prominent land-marks without much regard
to the topography or to natural obstacles. They were wasteful of
materials and labor, which, however, cost nothing but the lives
of captives who were forced to build these roads for the armies of
their captors. The results were roads which are remarkable chiefly
for their cost, and which were inferior to modern pavements costing
only one eighth to one quarter as much. The durability of these
roads does not seem so remarkable when it is remembered that the
traffic was light, and consisted mostly of footmen, unshod horses,
and ox-carts having wooden wheels, and also that probably the
surface of the road was kept covered with earth two or three inches
deep.
1076. Cobble-stone Pavement. A cobble-stone pavement con-
sists of cobble stones or small bowlders placed side by side upon a
bed of sand or upon the natural soil. The stones, usually somewhat
kidney-shaped, are selected with some relation to size, set on end
side by side in holes dug in the sand or unconsolidated native soil
by a laying tool, one end of which serves as a scoop and the other as a
hammer to settle the stones in place, and lastly sand or fine gravel
is spread over the surface to fill the spaces between the stones. Fig.
207 shows a transverse section of a cobble-stone pavement; and
Fig. 208, page 568, shows the only tool used in laying it.
FIG. 207. — SECTION OF COBBLE-STONE PAVEMENT.
The earliest pavements in many of the older cities, both American
and European, were of this type; and until about the beginning of
this century on account of their comparatively low first cost were
568 STONE-BLOCK PAVEMENTS [CHAP. XVIII
quite common. In 1884, 93 per cent of all the pavements in
Philadelphia were made of cobble stones; but in 1901 less than 6 per
cent were of this kind. In 1902 Baltimore
had 321 miles of cobble-stone pavement,
— more than any other city in the United
States, — over 90 per cent of the pave-
ments being cobble stones. In Septem-
ber, 1901, New York City still had 229
FIG. 208.-COBBLE-8TONE miles of streets paveci with cobble stones,
HAMMER
but nearly all of them have been replaced
with better pavements. Since the introduction of asphalt, brick, and
bituminous pavements, and since the decrease in the cost of stone-
block pavement by the introduction of improved methods of quarry-
ing and manufacture, there is no excuse for the construction of
cobble-stone pavements, and little excuse for their continuance. The
construction of such pavements has been practically abandoned, and
in some cities it has been prohibited by law — like theft and murder.
1077. Rubble Paving. In some cities having no cobble stones
but having comparatively plenty of even bedded sandstone or
limestone, the streets were paved by laying rough rubble stones
flatwise, the stones being 4 to 6 inches thick and having a top sur-
face of 4 to 6 square feet. The irregular joints between the stones
were filled with spalls. The blocks chipped on the edges, wore round
on top, and got out of place, thus making an exceedingly rough
pavement.
1078. Belgian-block Pavement. This is a stone-block pave-
ment made of blocks nearly cubical in form, from 5 to 7 inches on a
side. For a time this form of pavement was very common in both
Europe and America. The abjections to the Belgian pavement are:
1, On account of the size and form of the blocks, it is difficult to
keep them in place; 2, the blocks are of such a form as to give a poor
foothold to horses; and 3, there is always a considerable length of
joints parallel to the line of travel, which causes ruts to form in the
pavement. Belgian blocks have usually been laid with their sides
perpendicular and parallel to the sides of the street; but if a square
block is to be used, it should be laid in courses diagonal to the street,
so that no joints shall be parallel to the line of travel, a method which
would add some extra expense. The Belgian block has been dis-
carded in this country for the oblong block.
1079. Oblong Block Pavement. At present practically the
only stone paving-blocks employed are about 3J to 4J inches wide,
ART. 1] THE STONE 569
8 to 12 inches long, and nominally 4 or 5 inches deep. They are laid
on a concrete base with their longest dimension perpendicular to
the line of the street. This is the form of stone-block pavement
that will be considered in detail in this chapter.
1080. Durax Pavement. This form consists of granite cubes
from 2| to 4 inches on a side. This form of pavement will be con-
sidered only briefly — see § 1117.
ART. 1. THE STONE
1081. As stone-block pavements are employed only where the
travel is heavy, the material of which the blocks are made should
be hard enough to resist the abrasive action of the travel, and tough
enough to prevent being broken by the impact of loaded wheels.
The hardest stones will not necessarily give the best results in the
pavement, since a very hard stone usually wears smooth and becomes
slippery, and the edges of the block chip off and the upper face
becomes rounded, thus making the pavement very rough. A hard
stone may be necessary under a heavy traffic; but under medium
traffic a softer stone may give more satisfactory results.
The stone could be tested to determine its strength and dura-
bility much as paving bricks are tested, but it is not known that any
such tests have been made. An examination of a stone as to its
structure, the closeness of its grain, its homogeneity, etc., may
assist in forming an opinion as to its value for use in a pavement;
but in the present state of our knowledge, a service test in the pave-
ment is the only certain guide.
Granite, trap, sandstone, and limestone have been used for
paving blocks.
Granite paving blocks are much the most com neon, and ordinarily
the term granite-block pavement is employed as being synonymous
with stone-block pavement.
1082. GRANITE. This is a massive, unstratified, granular rock
composed essentially of quartz and feldspar; but almost always
containing other components, such as mica, hornblende, and tour-
maline in varying proportions. The quartz and the feldspar are
called essential ingredients, since their presence is necessary to
form a granite; while the other constituents are called accessories,
since they merely determine the variety of the granite. The term
granite is popularly applied to any feldspathic granular rock, and
includes gneiss, syenite, and porphyry, or any crystalline rock
570 STONE-BLOCK PAVEMENTS [CHAP. XVIII
whose uses are the same as granite. Gneiss is a rock of granitic
composition that has a decided banding or parallel arrangement
of its mineral constituents. Syenite is a granitic rock containing
no quartz. Porphyry is popularly any fine-grained compact rock
having large crystals scattered throughout its mass.
Granite varies in texture from very fine and homogeneous to
coarse porphyritic rocks in which the individual grains are an inch
or more in length. The color may be red, dark mottled, light to
dark gray, or almost black. The durability is closely related to the
accessory minerals present; and although granite is popularly
regarded as the hardest and most durable stone, there are some nota-
ble exceptions. A quartoze granite, one in which quartz predom-
inates, is too brittle for paving purposes; a feldspathic granite, one
containing an excess of feldspar, is too easily decomposed; and a
micaceous granite, one containing considerable mica in parallel
laminas, is too easily split for use in paving blocks. Gneiss is usually
too much stratified to make a good paving material. Syenite is
one of the gest materials for paving blocks, and usually the darker
the color the better the stone.
The crushing strength of granite usually lies between 15,000 and
20,000 Ib. per square inch. It. is customary to specify that the
granite shall have a toughness of not less than 9 (§ 342), and a French
coefficient of wear of not less than 11 (§ 343).
A most important property possessed by all granitic rocks is
that of splitting in three planes at right angles to each other, so
that paving blocks may readily be formed with nearly plane faces
and square corners. So far as discovered, this valuable property is
possessed only by the granitic and trappean rocks. This property
is called rift or cleavage, and was caused by pressure before the rock
was consolidated. The fine-grained granites possess the most perfect
rift, and it decreases as the size of the grains increase, so that a coarse-
grained variety is likely to require considerable dressing to bring the
face of the block to a plane surface.
1083. Granite paving-blocks are produced in large quantities
in Wisconsin, Maine, New Hampshire, Massachusetts, North Car-
olina, Georgia, and Minnesota. The order in the above list is that
of the number of blocks produced in 1916, the first two states pro-
ducing more than all the others. In recent years the production of
granite paving-blocks has greatly fallen off, apparently more than
one half, probably owing to the substitution of asphalt and brick
for stone blocks for paving purposes; but the quality has greatly
ART. 1] THE STONE 571
improved, partly in response to a demand for smoother block-
pavements; and partly by using smaller blocks, which can be cut
more accurately; and partly by abandoning the use of the hardest
granites and using the softer and finer-grained varieties, which split
more easily and regularly and make a better wearing surface.*
1084. TRAP. This is a popular term applied to any dark-
colored, massive, igneous rock. Owing to the difficulty of making
them, trap is not much used for paving blocks.
1085. SANDSTONE. Sandstones are rocks made up of grains of
sand which are cemented together by siliceous, ferruginous, calca-
reous, or argillaceous material. The texture of the stone varies
according to the sizes of the sand grains, of which there are all gra-
dations from those that are so fine as to be barely discernible to those
that are very coarse. The hardness, strength, and durability of the
stone is dependent upon the character of the cementing material.
Only the harder and tougher sandstones, generally those in which the
cementing material is siliceous, are used for paving. Sandstone
paving-blocks are common in the Lake and Western cities. The
principal quarries from which sandstone paving-blocks are obtained
will be briefly described.
1086. Medina Sandstone. This stone is found in the state of
New York, extending from Oneida and Oswego counties on the
east along the shores of Lake Ontario westerly to the Niagara river.
It is generally a deep brownish red in color, though sometimes light
and yellowish, and in a few localities gray. The stone is evenly
bedded, and the beds are divided into blocks by systems of vertical
joints, generally at right angles to each other, an arrangement which
greatly facilitates the work of quarrying. It absorbs 2J to 3^ per
cent of water, but it is not materially affected by alternate freezing
and thawing.
This stone is much used for paving in the Lake cities, where it
is often preferred to granite, since it does not wear slippery.
1087. Potsdam Sandstone. This formation is worked at a
number of places in the state of New York, the largest quarries being
near Potsdam. That quarried at Potsdam is hard and compact,
evenly grained, and reddish in color. It is largely used as a building
stone and to a considerable extent for pavements.
1088. Colorado Sandstone. In Boulder County, Colorado, are
several deposits of sandstone that furnish stone for paving purposes.
* For an interesting and elaborately illustrated article on the Manufacture of Granite Paving
Blocks, see Engineering News, Vol. 73 (1915), p. 376-81.
572 STONE-BLOCK PAVEMENTS [CHAP. XVIII
It splits easily, and breaks readily at right angles, so that it is formed
into flagging, curb stones, and paving blocks without difficulty.
It is hard and tough, and wears well in a pavement. It is never slip-
pery; and after a little wear forms a smooth and pleasing pave-
ment, very similar to one made of Medina stone.
1089. Sioux Falls Quartzite. This is a metamorphic sandstone
quarried at Sioux Falls, South Dakota. The stone is almost pure
silica with only enough iron oxide to give it color, which varies
from light pink to jasper red. It is very close grained, and will
take a polish almost like glass. It is said to be the hardest stone
in this country. Its crushing strength is about 25,000 Ib. per square
inch. It possesses a remarkably good rift and grain, although
not so perfect as that of granite. It is used considerably as a paving
material, being shipped as far east as Chicago; but it wears smooth
with a glassy surface.
1090. Kettle River Sandstone. This is a fine-grained, light-
pink sandstone, found in large quantities at Sandstone, Minn.,
about a hundred miles north of Minneapolis, which has been used
for paving purposes in Wisconsin and Minnesota. The stone wears
flat, does not polish, and approaches granite in its resistance to
crushing.
1091. LIMESTONES. These differ greatly in structure, from a
light friable variety highly charged with fossils to a hard compact
rock denser and heavier than granite. The thin bedded varieties
are easily broken into paving blocks. Although some varieties of
limestone are very dense and strong, it wears unevenly when used as
a paving material, and the blocks are speedily shivered by traffic
and split by frost, owing to the fact that the lamination is vertical,
ART. 2. CONSTRUCTION
1093. Fig. 209 shows a transverse section of the better form of
stone-block pavements.
1094. FOUNDATION. The method of preparing the subgrade
has already been discussed — see Art. 1, Chapter XV. Formerly
the foundation always consisted of a bed of sand upon the natural
soil (§ 966), but at present it is nearly always a layer of concrete
(Art. 2, Chapter XV). Stone-block paving is laid only on streets
subject to heavy travel.
1095. BEDDING COURSE. On the concrete foundation must
be spread some material to even up the surface of the concrete and
ART. 2]
CONSTRUCTION
573
to give a good bed for the blocks. The smoother the surface of the
concrete and the less the variation in the depth of the blocks, the
thinner can be the cushion coat. Sand has generally been used for
the bedding course, but recently cement mortar has been employed
tentatively.
,vc«l:y>:alS^ypfee?:.5a:.^g^Q^^^j
FIG. 209.— SECTION OF STONE-BLOCK PAVEMENT.
1096. Sand Cushion. The sand should be. fine, clean and dry.
The finer the sand the better. It should contain no pebbles
greater than J inch in diameter. The sand should be clean so as to
compress uniformly; and it should be dry so it will not shrink away
from the blocks in drying out (§ 1055).
In spreading the sand cushion for brick pavements, great care is
taken to secure a bed of uniform thickness and density (§ 972), so
that when the pavement is rolled, the surface will be smooth; but
stone-blocks are not as uniform in size as bricks, and hence they must
be settled to place by ramming each individual block, and therefore
it is not necessary to spread the sand for stone-blocks with as much
care as for bricks. However, for the best results, it is wise to spread
the sand with a shovel as uniformly as possible, and then rake it to
loosen up any spots that have been consolidated by throwing down
a shovelful of sand and to level it off and secure a layer of uniform
thickness and density. After the sand is leveled off it should not be
stepped upon; and in laying the blocks the men should stand upon
those already placed. However, these precautions are seldom taken;
and usually the blocks are deposited on the sand cushion and the
man who sets them stands upon the sand cushion while at work.
574 STONE-BLOCK PAVEMENTS [CHAP. XVIII
The thickness of the bed should vary with the accuracy of the
dressing of the blocks. If the more inaccurately dressed blocks
(paragraph 1, § 1100) are employed, a depth of 2 inches may not be too
much; but if the most accurately dressed blocks (paragraph 2,
§ 1100) are used and if the top of the concrete bed is reasonably
smooth, the sand cushion need not be more than 1 inch. The sand
cushion should be no thicker than is necessary to give a good bed for
the blocks.
1097. For a statement of the objections 'to a sand cushion for
brick pavements, see § 977-78. These objections apply with nearly
equal force to stone-block pavements.
1098. Mortar Bedding Course. In view of the success of the
monolithic brick pavement, particularly for rural roads (§ 979-82),
it has been proposed to lay granite blocks in a mortar bedding-course;
but there has been only a little experience with this form of con-
struction. The mortar bedding course could be a dry mixture of
cement and sand (§ 979) on a concrete base partially set, or a coat
of green mortar on a concrete base which has not taken initial set
(§ 982).
Some claim that with a monolithic stone-block pavement on
city streets it would be too difficult to make openings to lay or repair
pipes, conduits, etc.; but this might be an advantage, if such a
pavement would cause greater care in laying the pipes in the begin-
ning.
1099. THE BLOCKS. The blocks should be made of sound and
durable stone, free from seams, and should be of uniform hardness,
since the pavement will wear unevenly if hard and soft blocks are
laid together. For the appearance of the pavement, it is desirable
that blocks of only one color be laid together.
Fig. 210 shows four stages in the manufacture of modern
granite paving-blocks.
1100. Dressing. The blocks should be split and dressed so as
to have as nearly as possible plane rectangular faces and square
corners. The more regular the blocks the thinner the joints, and
consequently the smoother and more durable the pavement. In a
general way there may be said to be three standards in dressing stone
paving-blocks.
1. Formerly the blocks were roughly dressed; and would lay
v/ith joints f to 1 inch wide or perhaps more, and would show de-
pressions of 1 inch under a 3-foot straight edge laid parallel to the
curb. The joints were filled with pea gravel and sand or tar.
ART. 2] CONSTRUCTION 575
2. Recently there has been a demand for a less noisy and more
sanitary pavement, and hence the blocks have been more accurately
dressed, and the joints have been filled with bituminous cement and
sand or portland-cement grout. In this case the blocks are dressed
to conform to specifications about as follows: " The blocks shall be
FIG. 210. — FOUR VIEWS OP THE MANUFACTURE or GRANITE PAVING-BLOCKS.
approximately rectangular on top and sides, and uniform in width.
They shall be so cut that the joints between individual blocks when
laid shall average not more than f of an inch. The head of the block
shall have no depression greater than J inch from a straight edge
laid in any direction and parallel to the general surface of the block."*
Fig. 211, page 576, shows the two types of pavements described
above.
* Specifications, Borough of Manhattan, New York City, 1917,
576 STONE-BLOCK PAVEMENTS [CHAP. XVIII
3. It is very difficult to meet the above specification even when
the best splitting granites are available, and it is practically impos-
sible with the harder granites; and further, if the joints are filled
with cement grout, there is little need to require joints as thin as
Old style in foreground; new style in background.
FIG. 211. — OLD AND NEW TYPES OF STONE BLOCK PAVEMENTS.
f inch. Therefore some good authorities specify that the blocks shall
be dressed so as to lay side joints not more than f of an inch wide,
end joints not more than \ inch, and that the top shall not depart
more than \ of an inch.fyom a true plane.
1101. Re-cutting, formerly the blocks were made larger,' especi-
ally deeper, than is now considered good practice; and besides the
blocks were not dressed as accurately as is the custom at present.
Consequently there are many stone-block pavements, particularly in
the older cities, that are very rough and composed of comparatively
large blocks; and hence in recent years many of these old blocks
have been taken up, re-cut, and re-laid. Different methods are
employed in breaking the old blocks depending upon their size.
For example, blocks 12 X 8 X 4 inches may be broken into four
new ones 6X4X4 inches. But blocks either much smaller or
much larger receive different treatment. For example, with a
smaller block, sometimes a new block is taken from the end, and two
new ones are made from the remainder of the old block by dividing
the depth; and for larger blocks sometimes four new blocks may be
taken side by side successively from the end.* The blocks are
* For a liberally illustrated account of the method of re-cutting granite paving-blocks, see
Engineering News, Vol. 73 (1915), p. 1020-23,
ART. 2] CONSTRUCTION 577
usually dressed to conform to the specifications in paragraph 2 or 3
of § 1100. The depth of the re-cut or napped blocks is usually less
than that of the old ones; and therefore the re-cut blocks generally
make a greater area of pavement than the original ones, the excess
sometimes being nearly 100 per cent. The cost of taking up and
re-cutting ranges from J to f of the cost of new blocks. *
1102. Size of Blocks. For an ideal pavement the blocks should
be of one size; but if it were necessary to cut the blocks to exact
dimension, the expense would be unreasonably great. It is con-
sidered good practice to allow variations in length from 8 to 12 inches,
in width from 3J to 4J inches, and in depth from 4J to 5J inches.
It is customary to require that the blocks shall be sorted according
to width, and be laid in courses of practically uniform width.
The above specifications are for a block nominally 5 inches deep;
but a few cities use blocks nominally 4 inches deep. When stone
blocks were usually set in the native soil or in a thick sand cushion,
it was customary to make the depth 7 or 8 inches; but when a con-
crete foundation was introduced, the depth was generally reduced to
5 inches. It is probable that with the more accurate cutting
now customary, with a concrete base, a mortar cushion-coat
and a grout joint-filler, a 4-inch granite block meeting the spe-
cifications of § 1100 will be more durable than either a 5-inch block
with a sand cushion and a gravel filler, or a 7- or 8-inch block with a
thick sand foundation and a gravel filler. The reduction of a block
in depth by wear under the heaviest travel is inappreciable, partic-
ularly with a rigid filler. For information concerning the use of
granite blocks less than 4 inches deep, see § 1117-18.
1103. Measuring. Usually the contractor buys the blocks by
the thousand, but gets paid for them by the square yard; and there-
fore it is to his financial advantage to use as many large blocks as
possible. Again, the man who sets the blocks is usually paid by the
square yard; and therefore it is to his financial advantage to make
the joints as wide as he may. It is very undesirable that it should
be to the financial interests of the contractor and of the paver to
secure a poor pavement, i. e., one having large blocks and wide
joints. An excess in the width of the block is more important than
in the length, since it is proportionally a larger matter, and also
since it has a more important influence upon the quality of the pave-
* For an account of the history of re-cutting granite paving-blocks with examples of the
saving in a number of cases, see Proc. Amer. Soc. Municipal Improvements, 1914, p. 321-35,
and p. 336-42.
578 STONE-BLOCK PAVEMENTS [CHAP. XVII 1
ment; and therefore special care should be taken to prevent either
an excessive width of blocks or too thick side-joints. This precau-
tion was more important formerly than at present, since then the
joints were filled, or rather partially filled, with pebbles, and con-
sequently wide joints were more destructive than now; but never-
theless the principle is still worth considering. To identify as far
as possible the interests of the contractor with those of the city, the
following method of measuring a stone-block pavement has been
proposed.*
"The blocks must be substantially smooth and square on all their faces, and
within the limits of the following dimensions: Not less than 3^ inches nor more
than 4^ inches wide across their upper and lower faces; not less than 7 nor
more than 8 inches deep; and not less than 8 nor more than 14 inches long, except
where shorter stones are necessary to fill out courses.
"The sum to be paid per square yard shall be ascertained as follows: The
number of blocks per square yard upon which the bid of the contractor is based
shall be 22 £. The actual average^ number of blocks laid per square yard shall
be determined as follows: The City Engineer shall from time to time, during the
progress of the work, measure the width of 50 to 100 courses, and from this deduce
the average width of a course. The average length of the blocks is hereby fixed
for the purpose of computing the number of blocks laid per square yard, at 12^
inches, f
"For each block or fractional part thereof, that the average number laid per
square yard shall exceed 22|, there shall be added to the contractor's bid per
square yard an amount computed at the rate of 9| cents per block. For each
block or fractional part thereof, that the average number laid per square yard
shall fall short of 22^, there shallibe deducted from the contractor's bid per square
yard an amount computed at the rate of 9| cents per block."
According to this method, if the contractor uses narrow blocks
and thin joints, the price per squard yard is proportionally increased;
but if he uses thick blocks and wide joints, the price per yard is
decreased. To meet the case in which a contractor should buy large
blocks at a considerable reduction, it might be wise to make the
amount per block to be deducted greater than that added. For
convenience in applying the above method, a table is computed
which gives in one column the width of 50 courses and in a second
column the corresponding number of blocks per square yard. Of
course, the number of blocks to a square yard would vary with the
specified dimensions of the blocks and with the width of joints,
* By Horace Andrews, City Engineer of Albany, N. Y., in 1890 in Engineering Record, Vol.
21, p. 314 and 329; Vol. 25, p. 110-11.
t This value was determined by measuring a number of blocks in pavements laid with blocks
of the size stated above.
ART. 2] CONSTRUCTION 579
which latter would vary with the different kinds of stone and even
with the same kind from different quarries, and could be deter-
mined in any particular case only by measuring the combined width
of a number of courses of blocks in the pavement. The normal or
contract number of blocks per square yard should be stated according
to the quality of work desired.
Some cities buy the blocks and contract for laying them, a method
which eliminates the interest of the contractor in using large blocks.
In some cities it is the custom for the contractor to buy the blocks by
the square yard in the pavement, in which case the contractor pays
only for the blocks accepted, and has no financial interest in the size
of the blocks or the thickness of the joints. In Great Britain it is
customary to buy the blocks by weight, a method which eliminates
any interest of the contractor in the size of the blocks.
1104. Some cities require the blocks to be inspected and sorted
to sizes before being piled on the street. The advantages of this
are: (1) After being stacked upon the street it is nearly impossible
to inspect them, since only the outside blocks of the pile can be
seen; (2) when the blocks are being laid, the inspector has enough
to do to watch the quality of the workmanship without having also
to inspect the blocks; (3) removing rejected blocks from the pave-
ment delays the opening of the street; and (4) if the blocks are sorted
before being piled upon the street, different sizes are not so likely
to get into the same course, and therefore the joints will be narrower.
In Cleveland, Ohio, where the specified width of the stone paving-
block was from 3J to 5 inches, the blocks were sorted into three
classes. Class No. 1 included blocks from 3J to 3J inches, Class
No. 2 blocks from 3f to 4J inches, and Class No. 3 blocks from 4J
to 5 inches. Blocks in Class No. 1 were marked with red paint,
blocks in Class No. 2 with blue paint, and those in Class No. 3 with
black paint, so that when the blocks were delivered on the street
each class could be easily recognized and laid by itself.
1105. SETTING THE BLOCKS. In placing the blocks, the work-
man should stand upon the finished work, that the sand cushion
may not be disturbed; but he usually
stands on the sand cushion, the blocks
being piled on the sand bed behind him.
The workman with the pointed end of
the hammer shown in Fig. 212 excavates FlG' ™-~s™™ PAVER'S HAMMEB.
a hole, if need be, into which to set the block.
To secure the proper form to the surface of the pavement, a
580 STONE-BLOCK PAVEMENTS [CHAP. XVIII
chalk line is made upon each curb or a string is stretched in each
gutter to indicate the top of the blocks, and a row of blocks 20 to 25
feet apart is set in the center of the street with their tops to grade
as determined by measuring down from a string stretched from
curb to curb. If the street is wide, one or more rows of blocks
are placed between the curb and the crown. Ordinarily the sur-
face of the pavement is brought to grade between the guide blocks
with the unaided eye; but in the best work, a straight edge or string
is placed parallel to the line of the street on the guide blocks, by which
to grade the surface, and between these lines the blocks are brought
to the surface indicated by a straight edge parallel to the line of
the street resting upon the pavement already completed.
The blocks should be set with their long dimension across the
street, except at street intersections ; and should be placed in straight
rows with as close joints as possible. Each course should be formed
of blocks of uniform width and depth; and the bond should be ap-
proximately half the length of a blocks or at least 3 inches. As the
blocks are of uneven lengths, the securing of the proper bond requires
careful attention. The paver is instructed to secure thin joints, and
consequently has a tendency to set the block with the larger end up ;
but when set in this way the block will surely sink under traffic.
Placing the large end of the block down makes a wide joint, which is
objectionable if the joints are to be filled only with sand and pebbles
(§ 1108), but is no serious objection if the joints are to be filled with
hydraulic-cement grout (see § 1112).
The courses at street intersections are arranged substantially
as in brick pavements (§ 988). The work should progress up grade
and from the gutter towards the crown, so that the blocks may have
no tendency to settle away from each other and thus increase the
width of the joints.
Fig. 213 shows four views of the laying of stone-block paving.
1106. RAMMING THE BLOCKS. After the blocks have been
placed, they should be thoroughly rammed until they come to a
firm bearing. As a rule the workman is more interested in secur-
ing a uniform surface than in bringing the blocks to an unyielding
bearing. Each block should receive at least three hard blows —
one near each end and one in the middle. The rammer employed,
Fig. 214, page 582, weighs from 50 to 90 lb., ordinarily 60 to 75 Ib.
If, after being rammed, a block does not conform to the general
surface of the pavement, it should be lifted out, and sand should
be added to the sand bed or extracted from it to bring the top of the
ART. 2]
CONSTRUCTION
581
582
STONE-BLOCK PAVEMENTS
[CHAP, xvin
FIG. 214. — STONE-BLOCK
RAMMER.
block to the proper elevation. Any imperfect or broken blocks
should be removed and be replaced with perfect ones. Finally
each block should be adjusted so that it
stands perpendicular to the sand bed and
has its top face conforming to the surface
of the pavement. The quality of the pave-
ment depends largely upon the care with
which this adjustment is made.
The ramming is likely to be slighted
unless closely watched. The man who does
the ramming is likely to tap lightly a block
which if thoroughly rammed would be
driven below the general surface of the
pavement; and subsequent travel will force
the stone down and make a depression in
the surface. The important thing is to
have each block equally and sufficiently
rammed to bring it to a solid bearing on
the bedding course and at the same time
bring its top to the proper elevation.
To secure a thorough ramming of the pavement, it is sometimes
specified that there shall be one rammer to each paver, and occa-
sionally one rammer to two pavers. No ramming should be allowed
within 20 or 25 feet of the course last laid, to prevent the tipping
of the block out of the vertical position ; but all the blocks set should
be rammed before work ceases for the day.
Fig. 215 is a near view of a granite-block pavement.
1107. FILLING THE JOINTS. Four materials are in common use
for filling the joints of stone-block pavements, viz.: (1) pea gravel,
(2) tar and sand, (3) asphalt and sand, and (4) cement grout. The
first is used with joints that are f to 1 inch wide, and the others with
joints f to f of an inch wide.
1108. Pea Gravel. Where the joints are J to 1 inch wide, it is
customary to fill them with pea gravel or pea gravel and tar. It
is usually specified that the pea gravel shall pass a J-inch mesh and
be retained on a J-inch mesh. If tar is to be poured upon the peb-
bles, they should not be too small, or they will not permit the tar
to flow freely to the bottom of the joint. Since the joints are wide
and the blocks are roughly cut, the joints are usually partly filled
(say, 1| to 2 inches deep) with pebbles before the blocks are rammed,
to keep them in place during ramming. After the joints have been
ART. 2]
CONSTRUCTION
583
partially filled and the blocks have been rammed, the pebbles in the
joints are tamped with a bar having a chisel-shaped end. The joints
are next swept full of hot pebbles and again tamped.
1109. There are two methods in more or less common use for
completing the filling of the joints.
1. The filling is completed by spreading fine sand over the pave-
ment to a depth of J to 1 inch, and allowing travel to work it into the
Joints in foreground not filled; joints in background filled with tar.
FIG. 215. — NEAR VIEW OF GRANITE-BLOCK PAVING.
joints. Until recently this was the only method employed, and even
yet it is quite common. When filled in this way, the joints are not
impervious; and the filling does not aid much in keeping the blocks
in position.
2. Recently it has become the custom with the better class of
stone-block paving to complete the filling of the joints by pouring
hot tar over the pebbles. The tar is applied in substantially the
same way as in the case of brick pavements — see § 1011-12. The
584 STONE-BLOCK PAVEMENTS [CHAP. XVIII
pebbles should be perfectly dry, for an almost inappreciable amount
of water will cause the tar to foam and will prevent it from adhering
to the pebbles and from forming a water-tight joint. It may be
necessary to dry the pebbles artificially. The tar must not be
applied when the pebbles are very cold. The joints should be
entirely filled with the tar, to secure which it is usually necessary
to pour the joints twice. To keep the contractor from having a
financial interest in not filling the joints entirely full, it is sometimes
specified that there shall be brought upon the ground not less
than a stated number of gallons of paving cement for each square
yard of pavement, and that whatever remains after the completion
of the work is the property of the city.
In some cases it is specified that the pebbles and tar shall be
applied alternately in three stages.
The quantity of tar required to fill the joints varies from 1 to 3|
gallons per square yard, according to the width of the joints, which
varies with the quality of the stone and the workmanship.
The tar in the joints makes the pavement impervious, and
therefore more sanitary. The tar also assists in keeping the blocks
in position, and therefore adds to the durability and smoothness of
the pavement.
Fig. 216 shows the process of filling the joints of a stone-block
pavement with tar. Incidentally this figure also shows the differ-
ence between the new and old types of pavements.
1110. Tar and Sand. When the joints are nominally f of an inch
wide, they are often filled with tar and sand, which is sometimes called
tar-pitch mastic or pitch-sand mastic. The following are the speci-
fications for this form of joint filler adopted by the American Society
of Municipal Improvements. *
"The joint filler shall be the paving pitch hereafter described [see § 576-77],
thoroughly mixed with as much hot dry sand as the pitch will carry; but in no
case shall the volume of the sand exceed the volume of the pitch. The sand shall
be fine and clean, and all of it shall pass a 20-mesh screen. It shall be heated to a
temperature of not less than 300 nor more than 400° F.; and shall be between
these limits when mixed with the paving pitch.
"The paving pitch shall be heated in kettles properly equipped with an
approved thermometer, which shall register the temperature of the pitch.
"The mixture shall be flushed on the surface of the blocks and pushed into
the joints with suitable tools, re-flushing or re-pouring, if necessary, until the
joints remain permanently filled flush with the surface of the pavement. As
little as possible of the mixture shall be left on the surface.
*Specifications for Stone-block Paving, as revised in 1916.
ART. 2]
CONSTRUCTION
585
" In applying the filler care should be taken that the pavers are closely followed
by the filler gang, and in no case shall the paving be left over night, or when work
is stopped, without the filling of the joints being completed. In case rain stops
the filler gang before its work is finished, the joints should be protected by the
use of tarpaulins or other means, to keep out water. Under no circumstances
shall the filler be poured into wet joints."
The tar pitch should comply with the specifications in § 576-77,
except that northern cities, or rather cities that are subject to cool
Modern pavement in foreground; old-style pavement in background.
FIG. 216. — FILLING THE JOINTS WITH TAB.
weather the greater part of the year, should use pitch having a melting
point from 115 to 125° F., and cities which have long-continued hot
weather should specify a melting point from 125 to 135° F.
1111. Asphalt and Sand. The following are the specifications
for this form of filler adopted by the American Society of Municipal
Improvements. *
"The joint filler used shall be the asphalt cement hereafter described [see
§ 544], thoroughly mixed with as much hot, dry sand as the cement will carry;
but in no case shall the volume of the sand exceed the volume of the cement.
The sand shall be fine and clean and all of it shall pass a 20-mesh screen. The
* Specifications for Stone-block Paving, as revised in 1916,
586
STONE-BLOCK PAVEMENTS
[CHAP, xvin
sand shall be heated to a temperature of not less than 300 nor more than 400° F.;
and shall be between these limits when mixed with the asphalt cement.
"The asphalt cement shall be heated in kettles properly equipped with an
approved thermometer, which shall register the temperature of the cement.
"The mixture shall be flushed on the surface of the blocks and pushed into
the joints with suitable tools, re-flushing or re-pouring, if necessary, until the
joints remain permanently filled flush with the surface of the pavement. As
little as possible of the mixture shall be left on the surface."
The specifications for the asphalt cement referred to above are
given in § 544, page 282.
1112. Cement Grout. To secure the smoothest and most durable
stone-block pavement, the joints should be filled with portland-
cement grout, which should be mixed and applied as described in
FIG. 217. — GRANITE-BLOCK PAVEMENT EIGHT YEARS OLD.
§ 996-1005. However, since larger quantities are required for stone
blocks than for bricks, it is usual to permit the grout to be mixed in a
batch machine-mixer.
Fig. 217 shows a grout-filled granite-block pavement at Lowell,
Mass., eight years old.
The portland-cement grout makes the joint impervious, holds the
AET. 2] CONSTRUCTION 587
blocks firmly in position, prevents the edges from chipping and the
top face from wearing round, and adds materially to the smoothness
and durability of the pavement.
1113. EXPANSION JOINTS. If the joints are filled with port-
land-cement grout, an expansion joint of J to 1 inch in width should
be provided next to each curb, constructed as described in § 1017;
and expansion joints should be provided around manhole covers,
water boxes, etc., as described in § 1021.
If the joints are filled with pebbles or bituminous cement, no
longitudinal expansion joints are necessary.
Transverse expansion joints should not be provided, as they are
not needed and are a decided detriment (see § 1018-20).
1114. PAVING ADJACENT TO TRACK. Fig. 218 shows the
standard method employed by the Paving Commission of Baltimore
in laying granite-block pavement next to street-car rails.*
^Bituminous f/7/er
9-Rafl I'Morfarbet" ' «™*"L%!t>' Z-
FIG. 218. — GRANITE-BLOCK PAVING ADJACENT TO TRACK, BALTIMORE.
Granite blocks are much used for paving the railway area, because
of their durability. When granite blocks are laid in the narrow strip
between the rail and some other form of paving, they should be laid
as stretchers or as headers, i. e., without toothing.
In Worcester, Mass., an expansion joint is constructed between
the pavement on the track area and that on the remainder of the
street. The joint extends through the wearing coat and the con-
crete foundation. The joint in the block course is made by nailing
a pre-moulded mastic strip (§ 1017) against the ends of the ties. The
joint is to prevent the rumbling of the grout-filled pavement due to
the passage of a street car; and is effective.!
1115. MAXIMUM GRADE. Stone-block pavements are freely
employed upon grades up to 10 per cent; and if the stone 5s a quality
that does not wear smooth, they may be used upon grades up to
15 per cent.
"Engineering News, Vol. 73 (1915), p. 884.
•\Ibid., News, Vol. 74 (1915), p. 398.
588 STONE-BLOCK PAVEMENTS [CHAP. XVIII
It has been recommended that on steep grades to afford a good
foothold for the horses, (1) the edges of the blocks be chamfered, (2)
that the joints be comparatively wide, and (3) that the joints be filled
to within about an inch of the top with cement mortar. It is not
known that these expedients have ever been employed; but the
probabilities are that wide joints would be equally as effective
without chamfering the blocks, since the edges spall off soon when
the joints are wide and are filled with either gravel or tar. Further,
the accumulation of dirt in the wide joints would probably largely
neutralize their effect. Fig. 219 shows another method that has
been proposed, but it is not known that it has ever been tried.
1116. MERITS AND DEFECTS. The only merit claimed for
stone-block, particularly granite-block, pavement is durability.
The material of the blocks does not decay or wear entirely out.
But if the joints are filled with gravel or a soft filler, the face of the
blocks wear round; and if a thick sand cushion is used, some blocks
settle more than others. The result is that such a pavement becomes
excessively rough and noisy; and if the granite is hard, the pave-
ment is slippery.
FIG. 219. — STONE-BLOCK PAVEMENT ON STEEP GRADE.
However, if the blocks are carefully dressed, are of nearly uniform
size, and laid with thin joints; and if they are laid in a mortar bed-
ding course and the joints are filled with portland-cement grout, the
pavement is very durable and not specially noisy. A granite-block
pavement is the form universally chosen where there are many
heavily laden steel-tired wagons and trucks.
1117. DURAX PAVEMENT. This is a pavement made of cubes
of granite placed upon a concrete foundation. In America and
England it is known as durax pavement, and in Germany as klein-
pflaster. This pavement has been used in Europe since about 1885,
but the first in this country was laid in the Brooklyn Navy Yard in
1913. Since then it has been laid in a number of American cities.
The blocks are approximately cubes having faces 2J to 3J inches
square. They are usually cut to lay approximately f-inch joints,
ART. 2] CONSTRUCTION 589
The cubes are generally machine made, and can be turned out cheaper
per square yard than large hand-made ones; but on the other hand,
they are not usually as accurately cut as the best large blocks. How-
ever, since the durax blocks are nominally cubical, they may be laid
on any one of three sides, which gives a little advantage in fitting
them into place. The blocks may be laid with any form of bedding
course (§ 1095-98), or with any joint filler (§ 1107-12); but appa-
rently they are usually laid on a 1-inch sand cushion, and with
asphalt filler.
The blocks are not usually laid in straight course, but in concen-
tric segments of circles, in what is sometimes called the oyster-shell
FIG. 220. — LAYING A DURAX PAVEMENT.
pattern. Fig. 220 shows the process of laying a durax pavement;
and Fig. 221, page 590, is a close view of such a pavement.*
The advantages claimed for the segmental form of courses are:
1. There are only a few joints parallel to the direction of travel, and
hence the stones wear better than in the ordinary oblong block pave-
ment. This would not be important, if a grout filler is used. 2.
Since there are no continuous transverse joints, opposite wheels of a
vehicle can not drop into a joint at the same time; and hence there is
less jar and less wear on pavement and vehicle. This would not be
an important advantage, if the joints are filled with portland-cement
grout. 3. Since the courses need not be kept straight, the blocks
can be turned so as to give the narrowest joints. This may be an
advantage in placing some of the blocks; but it is a disadvantage in
* Engineering News, Vol. 72 (1914), p. 529.
590
STONE-BLOCK PAVEMENTS
[CHAP, xvin
making closures between different segments. On the whole, the
joints can not be as narrow as with large blocks equally accurately
cut.
1118. The first cost of the blocks for durax pavement is less per
square yard than for an ordinary granite-block pavement; but the
labor of laying is much greater, and the total cost of the small-cube
FIG. 221. — OYSTER-SHELL PATTERN OF DURAX PAVEMENT.
pavement is more than that of the large-block pavement. The
small cubes have been used to re-surface old macadam or other
pavements. One advantage of the small cubes is that they may be
made of the same thickness as a brick or asphalt or wood-block
pavement, and hence a durax surface may replace the old one with-
out disturbing the old foundation or changing the grade of the pave-
ment. It is said that durax pavements are not now being laid
in Europe to any considerable extent, and that the area of durax
pavements in Europe is only about 3 per cent of that of oblong
blocks.
ART. 2] CONSTRUCTION 591
1119. COST. Price of Blocks. The following is the market
quotation for stone paving-blocks for November 1, 1917.*
New York City, Manhattan, standard granite ............. $2 . 50 sq. yd.
other boroughs standard granite ............. 2 . 25
other boroughs 5-inch granite ............... 2 . 55 "
Boston standard granite ............. 2 . 55 "
Chicago, ordinary dressing, standard granite ............. 1 . 80 "
best dressing standard granite ............. 2.25 "
St. Paul standard sandstone .......... 1 . 65 "
Kansas City standard limestone ........... 2 . 15 "
The variation in price is partly due to the difference in freight
and in price of labor, but chiefly to the ease with which the available
material may be dressed. For example, according to data published
by the U. S. Geological Survey, the average price of granite paving
blocks per thousand in 1916 varied from $32 in California and $33 in
Georgia to $62 in Minnesota and $66 in Wisconsin, the first two
having easily worked granites and the last two granites difficult to
work. Ordinarily, to lay a square yard of pavement requires 28 to
31 blocks.
1120. Granite-block Pavement. New York. In New York City
in 1917 the cost of standard granite-block pavement is as follows:
COST
per Sq. Yd.
CONCRETE BASE, 6 inches: materials and labor ....................... $1 . 10
SAND CUSHION, 1 inch: material and labor ............................ 07
WEARING COAT:
29 blocks at 9 cents on street ........................... 2.61
labor laying .......................................... .22
Total for wearing coat ......................................... $2 . 83
JOINT FILLING: 2 gallons of bituminous filler, sand, and labor of applying. .45
Total cost to contractor, exclusive of administration, tools, etc., and
grading ............................. $4.45
With a cement-grout filler in place of the bituminous filler, the
cost is about 20 cents less, or $4.25 per square yard. With a cement-
grout filler and a cement-mortar cushion, the cost is 5 to 10 cents
per square yard less or $4.35 to $4.40 per square yard. In the
Borough of Manhattan with the improved block there specified
and with the difference in working conditions in that Borough, about
40 cents per square yard should be added, making the total cost about
$4.85 per square yard.
^Engineering News-Record, Vol. 79 (1917), Construction News, p. 179.
592 STONE-BLOCK PAVEMENTS [CHAP. XVIII
1121. Chicago. The average cost to the contractor of laying
specially dressed granite blocks (3| to 4 inches wide, 8 to 10 inches
long, 5 inches deep, of which 28 to 31 lay a square yard) at Chicago
in 1917, was about as follows:*
ITEMS-
CONCRETE BASE : 6 inches: materials and labor ....................... $0 . 92
SAND CUSHION:
2 inches of sand at $2.50 per cu. yd .................... 13
labor spreading ................................... -02f
Total cost sand cushion ....................................... $ . 15|
WEARING COAT:
granite blocks f .o.b. Chicago ......................... $2 . 35
hauling to street .................................... 13
carrying to paver ................. ............ ....... 06
laying and ramming ............................... . 19
Total for wearing coat ........................................ $2.73
FILLING JOINTS:
paving gravel at $2.00 per cu. yd ..................... $ .13
labor spreading .................................... .04
tar at 9 cents per gallon .............................. 10
labor applying .................................... .07
Total for joint filler .......................................... . $ .34
Total cost to contractor, exclusive of tools, administration, etc., .
and grading ...................... $4 . 14f
Ordinary granite blocks cost 25 to 30 cents per square yard
less than the special dressed blocks above, the cost of laying is
8 cents per square yard less, and the total of the other items is sub-
stantially as above, thus making the total cost of the ordinary granite-
block pavement on concrete foundation about $3.75 per square yard.
1122. Removing, Re-cutting, and Re-laying. / kiladelphia. The
following data on the cost of removing, re-cutting, and re-laying
granite-block pavement are from experience in Philadelphia, f
Removing old blocks .................................. $0 .035 per sq. j d.
Clipping the old blocks to Hnch joints ................... ,50 " "
Piling and inspecting new blocks ......................... 07 " "
Sand cushion .......................................... 08 " "
Laying and grouting ....... . ............................ 22 " "
Gravel for filling joints ................................. .04 " "
Cost of grout .................................. .09 " "
Total $1.035 " " "
*By courtesy of W. L. Wccden, Field Secretary of Granite Block Producers Association.
tW. H. Connell, Chief of Bureau of Highways (Streets), Philadelphia, in Engineering and
Contracting, Vol. 40 (1913), p. 290.
ART. 2] CONSTRUCTION 593
1123. Schenedady. Table 65, page 594, shows the details of the cost
of re-cutting and re-laying 2,578 square yards of granite blocks in
Schenectady,N. Y.* The old blocks were 12 by 8 by 4 inches; and the
new ones 6 by 4 by 4 inches, and were laid on a new 4-inch green
concrete base with a thin bedding course of 1 : 3 dry cement mortar.
The joints were filled with a 1 : 2 portland cement grout. The day
was 8 hours. The work was done without interrupting travel.
1124. Durax. The following data are from experience in Louis-
ville, Ky., in laying a small area of cubical granite blocks, f The
blocks were 3| to 4 inches on a side, were made in North Caro-
lina, cost $9.30 per ton f.o.b. Louisville, and were guaranteed to lay
7 square yards per ton, and consequently the guaranteed price
was $1.33 per square yard, which was 67 cents per square yard less
than standard-size blocks from the same quarry would have cost.
A man laid 2.2 square yards per hour, whereas of standard blocks
he would have laid 3.3 to 4 square yards per hour.
1125. Medina Block. Buffalo. For somewhat obvious reasons,
the prices in 1917 were quite erratic, and hence it is not wise to cite
them. Table 66, page 595, shows the representative cost of a Medina-
sandstone block pavement in Buffalo, N. Y., in 1916.J
1126. Cleveland. Table 67, page 596, shows the representative
cost of Medina-sandstone pavements in Cleveland, Ohio, under a
5-year guarantee, in 1916.§
1127. Rochester. Table 68, page 597, shows the cost of Medina-
sandstone pavements at Rochester, N. Y., in 1917.
1128. Cost of Grouting. 1 1 Lawrence. The cost at Lawrence,
Mass., of grouting standard granite blocks was as follows: Cement
cost $1.08 per barrel, pea gravel $2.30 per cubic yard, sand $1.00
per cubic yard. To hold the blocks in place while being rammed, the
joints were filled to a depth of 1 inch with pea gravel. The grout
was mixed 1 : 1 in iron boxes (§ 997), and scooped onto the pavement
and broomed. With wages at $2.25, the cost of applying the grout
was 6.4 cents per square yard. The total cost of the grout in place
was 26f cents per square yard.
1129. Lowell. At Lowell, Mass., grouting granite blocks on a
*Chas. A. Mullen, City Engineer, in Municipal Engineering, Vol. 46 (1914), p. 431.
t D. R. Lyman, Chief Engineer of Department of Engineering, in Engineering News, Vol. 72
(1914), p. 948.
t Frank L. Bapst, President German Rock Asphalt Co., which company lays much stone-
block paving in Buffalo.
§ Robert Hoffman, Commissioner and Chief Engineer, Department of Public Works, Cleve-
land. O.
|| Engineering and Contracting, Vol. 44 (1915), p. 350-51.
594 STONE-BLOCK PAVEMENTS [CHAP. XVIII
TABLE 65
COST OF RE-CUTTING AND RE-LAYING GRANITE-BLOCK PAVEMENT
Schenectady, N. Y.
Taking up old pavement and preparing subgrade: Sq. Yd.
Labor removing asphalt surface and concrete base, at $2.25 per day. $0.0445
Team " " " $5. 00 per day. .0156
Labor removing old granite blocks at $2.25 per day 0305
" regulating and preparing subgrade at $2.25 per day 0417
Team hauling materials from subgrade at $5.00 per day 0241
Total $0. 1564
Concrete Foundation — 1 : 3 : 6. 4 inches thick:
Labor mixing and placing, at $2.25 per day $0 . 1192
Cement, delivered ($1.24 per bbl. f.o.b. cars) 1018
Sand, " ($0.25 per ton f.o.b. bank) 0275
Stone, " f-inch at $1.30 per ton f.o.b. track >
H-inch at $1.75 per ton on job f -
Total $0.3320
Bedding Course — 1 : 3 cement mortar:
Labor, mixing and placing, at $2.25 per day $0.0738
Cement, delivered ($1.24 per bbl. f.o.b. cars) 0868
Sand, " (25 cents per ton f.o.b. bank) 0242
Total $0.1848
Re-cutting and re-laying granite blocks:
Labor breaking and dressing, at $5.00 per day $0 . 7385
" sharpening and making tools 0501
Materials for sharpening and dressing tools 0058
Horse and wagon, moving blocks at $4.00 per day 0059
Labor transporting blocks to pavers at $2.25 per day 0396
" setting blocks, at $5.00 per day 1796
" ramming blocks, at $2.25 per day 0065
Total $1.0260
Grout Filler — 1 : 2 portland cement-
Labor mixing and placing, at $2.25 $0.0466
Cement, delivered ($1.24 per bbl. f.o.b. cars) 0707
Sand, " (25 cts. per ton f.o.b. bank) 0106
Total $0. 1279
Over-head charges:
Foreman at $4.00, assistant foreman at $3.50, etc $0.0819
Watchman at $2.25 0552
Total $0.1371
Extras:
Repairs to curbs, sidewalks, sewers, etc $0 . 0430
Total cost of removing, re-cutting and re-laying $2 . 0072
ART. 2] CONSTRUCTION 595
TABLE 66
COST OF MEDINA-BLOCK PAVEMENT IN BUFFALO IN 1916.
CONCRETE FOUNDATION, 1:8:
cement at $2.00 per bbl., f.o.b. Buffalo ............................. $0.37
sand | 32
crushed stone f '
mixing and laying, at 37£ cents per hour ........................... 1. 15
Total for concrete base ........................................ $0 . 84
SAND CUSHION: labor and material ................................. $0 . 15
WEARING COAT:
blocks f.o.b. quarry .......... .................................... $1 . 60
freight Medina to Buffalo ......................................... 18
unloading and hauling, at 75 cents per hour for team and driver. ...... .20
labor laying blocks, at 60 cents per hour ............................ 35
Total for wearing coat ......................................... $2 . 33
FILLING JOINTS:
cement at $2.00 per bbl., f.o.b. Buffalo ............................. $0 . 08
labor applying grout .............................................. 25
Total for filling joints $0 . 33
MISCELLANEOUS:
overhead expenses $0 . 10
indemnity insurance at 3.17% of pay roll 025
discount on City tune warrants, 3% 11
maintenance during 10-year guarantee period 10
paid city for water 02
Total miscellaneous $0 . 355
Total cost of pavement exclusive of excavation $4 . 005
2-inch sand cushion with 1 : 1 grout required 0.295 bag of cement per
square yard ; and the average cost was 24 \ cents per square yard.
1130. Worcester. At Worcester, Mass., a 1 : 1 grout required
0.36 cubic foot of cement per square yard; and the total cost of
grouting was 24 cents per square yard.
1131. Albany. At Albany, N. Y., the cost of grouting standard
granite blocks having side joints not exceeding \ inch, using 1 : 1
grout mixed by machine, cost 13.9 cents per square yard. The cost
of mixing by machine was 1.5 cents per square yard, and by hand
5.25 cents per square yard. The cement required was 0.4 bag per
square yard.
1132. Philadelphia. At Philadelphia, Pa., when each standard
block is " struck in " at the base to secure a close joint, and when
2 inches of pea gravel are deposited in the joints before ramming, a
596 STONE-BLOCK PAVEMENTS [CHAP. XVIII
TABLE 67
COST OF MEDINA-BLOCK PAVEMENT IN CLEVELAND IN 1916.
COST
ITEMS. per Sq. Yd.
CONCRETE BASE, 6 inches of 1 : 3 : 6:
materials and labor* $0 . 98
SLAG CUSHION, 2 inches:
material and labor spreading 08
WEARING COAT, 6 to 6| inches thick:
stone blocks, f .o.b. quarry 1 . 65
freight Medina to Cleveland 54
transporting from car to street, at 90 cents per hour for team 10
laying .45
Total for wearing coat $2 . 74
TAR FILLER : material and labor 35
FOREMAN: supervision .02
Net cost to contractor $4.17
OVERHEAD: administration, depreciation, interest, profits, etc .78
Price bid, exclusive of excavation $4 . 95
Granite blocks cost $2.50 per square yard, f.o.b. cars Cleveland, which is
31 cents per square yard more than Medina blocks.
1 : 1J grout required 0.27 bag of cement per square yard. The total
cost of grouting, using mixing boxes (§ 997) and including contractor's
profits, was from 17 to 20 cents per square yard.
1133. Cost of Tar-sand Filler. At Englewood, N. J., the total
cost of applying a 1 : 1 tar-sand hand-mixed filler on standard
blocks having ^-inch joints in which had previously been deposited
" a small amount of grit," was as follows:!
Pitch, — 1.7 gallons per square yard $0 . 143 per sq. yd.
Labor handling the pitch 02 " " "
Grit Oil " " "
Sand *. 019 " " "
Labor 038 " " "
Total ; $0.231 " " "
" Investigation showed that in every case the filler penetrated
to the bottom of the block. It was also found that the volume of tar
required to fill the joints was less than that smeared over the surface
of the pavement in the two or three successive pourings."
*Sand $1.15 per ton f.o.b. cars Cleveland; broken stone $1.35 per ton f.o.b. cars. Loading
and hauling sand and stone, 30 cents per ton per mile. Labor from 30 to 43 cents per hour,
t Engineering and Contracting, Vol. 47 (1917), p. 134.
ART. 3] MAINTENANCE 597
TABLE 68
COST OF MEDINA-BLOCK PAVEMENTS IN ROCHESTER IN 1917 *
COST
ITEMS. per Sq. Yd.
CONCRETE, 6 inches:
materials and labor $1 . 00
SAND CUSHION, 2 inches:
sand at $1 . 50 per cu. yd. in place 09
WEARING COURSE, 6 inches deep:
blocks f .o.b. quarry 1 . 50
freight to Rochester 07
loading and unloading 03
hauling 1 mile 05
distributing and sorting 03
laying 15
FILLING JOINTS:
0.025 cu. yd. sand at $1.50 ...- 04
1| gallons of tar at 10 cents 23
labor applying filler 09
SUPERINTENDENCE :
foreman at 40 cents per hour for 30 square yards 015
Total cost to contractor exclusive of administration, tools, etc $3.29
1134. Contract Price. Table 69, page 598, shows the contract
price of stone-block pavements in various cities, and incidentally
gives considerable detailed information as to the practice in the
several cities.
ART. 3. MAINTENANCE
1135. There are almost no data concerning either the method or
the cost of maintaining granite-block pavements. Formerly, when
the wide-joint and roughly dressed granite-block pavement was the
only form, little or no attention was given to methods or cost of
pavement maintenance, and this was particularly true of granite-
block pavements. Since the introduction of the better dressed narrow-
joint granite-blocks, there has not been time in which to develop a
system of maintenance nor to determine the cost, particularly as a
modern granite-block pavement is very durable and needs no repairs
for the first few years.
* By courtesy of Walter L. Weeden, Secretary Granite Paving Block Manufacturers' Asso-
ciation.
598
STONE-BLOCK PAVEMENTS
CHAP. XVIII
TABLE 69
CONTRACT PRICE OF STONE-BLOCK PAVEMENTS IN VARIOUS CITIES *
Laid in 1912
LOCALITY.
Amount
Laid in
1912,
sq. yd.
CONCRETE BASE.
Kind
of
Filler.
Guar-
antee,
years.
Total
Thick-
ness,
inches.
Aver-
age
Price i
sq. yd.
State.
City.
Thick-
ness,
inches.
Propor-
tions.
California. . . .
Oakland '.
11 445
6
grout
0
S4.002
San Francisco
21 565
gravel
7-9
3.502
Connecticut
Ansonia.
1 500
6
1 3:6
2. 10
Georgia
LaGrange. . .
5000
tar
0
10
1.75
Louisiana
New Orleans .
4 119
6
1 3:6
grout
3
14
3.95
Maine
Portland. . . .
1 004
grout
2.30
Massachusetts
Lawrence. . . .
30704
6
1 3:6
grout
0
7i
2.72
Leominster . .
2 268
4
1 3:5
pitch
4
2.47
Lowell
22418
6
1 4:11
3
' 'i2' '
3.15
New Bedford.
12310
4
Hassam
grout
10
1.902
Westfield. . . .
6 197
4
1 3.: 6
" l" '
8£
3.30
Minnesota. . . .
Duluth
23 146^
5
1 3:6
grout
5
13
2. 68 2
Minneapolis. .
79004
grout
2.75*
Missouri
Kansas City.
13 248
6
1 3:6
grout
5
13
3.10
New Jersey . .
Newark
75735
5
3.03
New York ....
Rochester. . .
42700
6
1 3:6
grout
5
13
3.18
Troy
4 000
6
1 5
grout
5
3.35
Oregon
Portland*. . . .
3914
6
1 3:6
grout
12
3.45
Pennsylvania.
Scranton. . . .
Wilkesbarre. .
1 688
2 156
"e
i 2: 5
sand •
grout
5.
9
12
2.10
2.70
Washington. . .
Seattle
17436
6
1 3:6
grout
12
3.50
Tacoma
11 002
6
1 3:6
grout
in
2.50
Wisconsin ....
Superior
10078
5
1 3:5
5
13
2.52
Canada
Montreal. . . .
48000
6
1 3:6
5
12
3.65
Ottawa
10041
6
1 3:6
grout
5
13
3.78
Saskatoon . . .
16 662
5
1 1\
grout
5
11
5.10
Vancouver. . .
55359
6
1 2i : P
grout
5
13
4.50
1 Including grading and concrete base. 2 Not including grading.
3 Part grout and part pitch and tar. 4 Kettle River Sandstone.
1136. REPAIRS REQUIRED. The repairs ordinarily required
are re-laying small areas, re-filling the joints, repairing spalling joints,
raising low blocks, repairing where the foundation has settled, and re-
laying over trenches or other openings.
1137. Re-laying. At present the most common work required
in connection with the maintenance of stone-block pavements is
taking up the old blocks, re-cutting, and re-laying them, which is a re-
construction rather than repair or maintenance; and usually the
new pavement is of entirely a different type than the old. This
subject has already been considered in § 1101.
* Engineering and Contracting, Vol. 39 (1913), p. 378-79.
ART. 3] MAINTENANCE 599
1138. Re-filling Joints. If a bituminous filler is used, it may
run out of the top of the joints, particularly near the crown, or be
picked out by the traffic. When this occurs, the joints should be
poured again. With dense traffic this may be required every two or
three years. With a good portland-cement filler, the joints are not
likely to need re-filling; but if it develops that in spots the filler was
poor, the joints should be digged out and again filled.
1139. Spalling Joints. If pea gravel or sand is put into the
joints, there is danger that the grout filler will fill only the top of the
joint; and hence that the pressure due to expansion of the pavement,
being concentrated near the top of the joint, will cause the edge of the
block to spall. Usually there will be no spalling, if the grout pene-
trates 3 inches; but if the penetration is 1 inch or less, the blocks are
likely to spall — generally the first summer after the pavement
is completed, but sometimes not for two or three years, depending
upon the time required for travel to fill the joints opened by the
contraction of the pavement.
The spalling at joints can be prevented by clearing the joints of
sand or gravel to a depth of 3 inches before applying the grout filler.
If spalling occurs in the finished pavement, the blocks should be
taken up, the joints cleaned, the blocks re-laid, and the joints re-
filled.
In effect this is substantially the same defect as that of brick
pavements described in § 1059.
1140. Raising Low Blocks. If a block was not properly
bedded or sufficiently rammed, it may become depressed under
travel, particularly with a thick sand bedding-course; and if so, it
should be taken up, and re-laid. If a grout filler is used, it is prac-
tically impossible to remove a block without destroying it; and this
is reason for special care in bedding and tamping blocks that are to
be grouted.
1141. Settlement of Foundation. If there is a depression on
the surface, it may be due to a settlement of the foundation and
can be corrected substantially as described for brick pavements,
see § 1056.
1142. Settlement of Trench. The effect and the remedy of
the sinking of a stone-block pavement is the same as that of a
brick pavement, see § 1057 and 1061.
1143. COST OF REPAIRS. There are almost no data on the cost
of repairs for stone-block pavements, probably partly because of their
long life and partly because only few repairs are ever made, the pave-
600 STONE-BLOCK PAVEMENTS [CHAP. XVIII
ments usually being allowed to continue in a bad state of repairs
The only data on record seem to be cost of repairs in Buffalo, N. Y
(see § 868), where most of the stone-block pavements are Medina
sandstone (§ 1086). In 1916 the annual cost of repairs on 218,090
square yards was 3.66 cents per square yard.*
*Report of Dept. of Pub. Wks.— Bureau of Engineering, 1915-16, p. 70.
CHAPTER XIX
WOOD-BLOCK PAVEMENTS
1144. KINDS OF PAVEMENTS. There are two forms of wood-
block pavements, viz. : the round-block and the rectangular-block.
1145. Round-block Pavement. Fig. 222 shows the usual form
Fro. 222. — ROUND WOOD-BLOCK PAVEMENT.
of round wood-block pavement. This form is often called a cedar-
block pavement, since the blocks are usually sections of cedar
poles or trees. These blocks are generally placed on a foundation
of planks nailed to scantling which are placed on, or rather in,
sand.
Until about the close of the last century, untreated round wood-
block pavements were laid in considerable quantities in localities
where lumber was cheap; but now they are seldom laid, owing chiefly
to the rapid increase in price of lumber and partly to the introduction
of other cheap forms of pavements. Such pavements are laid now
only where first-cost is the controlling factor, as for example, in new
601
602
WOOD-BLOCK PAVEMENTS
[CHAP, xix
city additions in states where the first pavement is selected and paid
for by the owners of the abutting property, and the cost of mainte-
nance and renewal is paid from the general property tax. In view
of these facts, the round wood-block pavement will not be further
considered in this volume.
1146. Rectangular-block Pavement. Fig. 223 shows the usuel
FIG. 223. — RECTANGULAR WOOD-BLOCK PAVEMENTS.
form of rectangular wood-block pavement. The rectangular block
is usually treated with a preservative when used for paving pur-
poses. This type of pavement will be considered in detail in this
chapter.
1147. HISTORICAL. Wood appears to have been employed as a
paving material first in Russia, where though rudely fashioned it
has been used for some hundreds of years. Wood pavements were
first laid in New York City in 1835-36, and in London in 1839.
The first wood used for pavements was untreated (§1145). The
first pavement in this country made of treated blocks was laid in
Galveston, Texas, in 1874; and remained in service for 29 years.
A treated wood-block pavement was laid in St. Louis, Missouri, in
1882-85; and remained in use until worn out by the traffic. The
first extensive use of treated blocks for pavements was in Indianap-
olis in 1896; and after 21 years these blocks are still in use and are
said to be in a good state of preservation. Although a number of
methods of preserving timber piles, railroad ties, etc., had been used
in this country for several years, it was not until about 1900 that
ART. 1] MATERIALS AND TREATMENT 603
there was any considerable use here of a preservative for wood
paving-blocks.
1148. In 1909 3J per cent of the pavements in the United States
were wood-block (see table on page 320); but in 1900 such pave-
ments constituted 10 per cent of the total (see a table similar to that
on page 320, in the former edition of this volume). This seems to
prove that the percentage of wood-block pavements is rapidly
decreasing. However, the decrease is wholly in the untreated-block.
The percentage of treated-block pavements is rapidly increasing,
although it is still less than 1 per cent of the pavements included
in the table on page 320.
ART. 1. MATERIALS AND TREATMENT
1150. THE TIMBER. Both hard and soft woods have been em-
ployed for making paving blocks. The hard woods were used
untreated, and the soft varieties were treated. At present only
treated timber is used; and of course the softer and cheaper woods
are preferred, since they only can be impregnated with the preserva-
tive. Exceptions to the above statement are two Australian hard
woods, jarrah and karri, that are much used in London without
treatment.
1151. Jarrah and Karri. Jarrah is short grained and free split-
ting, and breaks with a clean fracture and burns with a black ash.
In color it looks nearly like cherry. When seasoned, it has a specific
gravity of 1.01 and absorbs about 10 per cent of water when im-
mersed 48 hours.* Its transverse and crushing strength is about
the same as that of English oak and Indian teak.
Karri is interlocked in the grain and is difficult to split ; it splinters
in breaking and burns with a white ash. It is a little lighter colored
than cherry. When seasoned, it has a specific gravity of 1.12, and
absorbs about 7 per cent of water when immersed 48 hours. Its
transverse strength is a little greater than that of English oak or
Indian teak, and its crushing strength is considerably greater.
For street paving, there is little difference between jarrah and
karri, although for exceptionally heavy traffic karri shows slightly
less wear. Karri shrinks less than jarrah. Both timbers are very
plentiful in Western Australia, the trees growing with large, long
straight bodies without limbs. Jarrah and karri are preferred in
some vestries of London to any other form of wood paving-blocks.
* Most •aoft -woods will absorb 20 to 25 per cent.
604 WOOD-BLOCK PAVEMENTS [CHAP. XIX
1152. Wood for Treated Blocks. The cost of lumber in the
United States in the last few years, even before the Great European
War, has been so great that the first cost of the blocks is an important
consideration. Southern long-leaf yellow pine was almost exclusively
used in early treated wood-block pavements; and is most largely
used at the present time. It makes the most durable blocks of any
timber yet tried. However, it is not entirely satisfactory, for the
hardness which gives it durability against wear also makes it slippery;
and further, it is liable to split when the blocks are taken up to
repair underground work. Nevertheless, long-leaf yellow pine is the
most satisfactory timber for treated wood-block pavements. How-
ever, as this timber is produced in only one section of the country
the transportation charges are likely to be high, and besides the
supply is nearly exhausted; and therefore where traffic conditions will
permit, it is desirable to use a cheaper material.
Some years ago the U. S. Forestry Service, to obtain data as to
the relative value of different species of wood for paving purposes,
laid a great variety of woods under the same conditions on a street in
Baltimore, Maryland, and conducted a similar experiment in Minne-
apolis, Minnesota. The conclusion reached as a result of the exami-
nation of the Baltimore experiment after four years of service, was
that the different varieties could be grouped into classes in the order
of the value for paving purposes as follows: (1) Southern long-leaf
yellow pine; (2) Norway pine, white birch, tamarack, eastern hem-
lock; (3) Western larch; and (4) Douglas fir. The conclusion from
the Minneapolis experiment after eight years of service was sub-
stantially the same.
The 1917 specifications of the American Wood Preservers' Asso-
ciation and also those of the American Society of Municipal Improve-
ments call for Southern pine, which permits the use of either long-
leaf or short-leaf Southern pine.
1153. Specifications for Blocks. Dimensions. The width was
formerly 4 inches; but in recent practice it is sometimes 3 inches.
The length varies widely so as to make available planks of different
widths. The length specified is usually 5 or 6 inches to 10 inches;
and often the average width is specified to prevent the use of too
many short or too many long blocks. The average length is usually
6 or 8 inches. The depth must be enough to give the block stability, —
say, 3 inches. This depth is used where the traffic is light; but
under heavy traffic the depth is 4 inches, and in extreme cases 4J
inches, as for pavements in the Borough of Manhattan, New York
ART. 1] MATERIALS AND TREATMENT 605
City. If a 3-inch block is used its length should not exceed
8 inches.
While it is usual to specify that all the blocks for one job, or at
least in any one city block, shall be of the same width, it is customary
to permit a variation of f inch in the width. A variation of Y& inch
in depth is generally permitted. Sometimes, to prevent a block from
being laid on its side, it is specified that there shall be a difference
between the width and depth of at least J inch.
1154. Quality of Blocks. The blocks should be sawed square and
true. They should be free from large, unsound, loose, or hollow
knots; and should not contain any shakes, checks, or other defects.
The blocks should be free from any blue tinge, which is a sign of
incipient decay.
Specifications usually state the minimum number of annual rings
permitted — some permitting 5 or 6 per linear inch, and others not
less than 8 or 9. The amount of sap wood allowed varies from 10 to
40 per cent. In the early use of treated wood blocks, specifications
were more rigid in limiting the amount of sap wood ; but recent expe-
rience seems to indicate that there is no noticeable difference in wear
between heart and sap wood.
1155. CAUSES OF DECAY. The decay of wood is due to a low
form of plant life called fungi. Air, heat, and moisture are necessary
for the existence of the fungous growth; and without any one of these
the fungi can not live. Since air and heat are present in all climates, it
is necessary to eliminate moisture to preserve the timber from decay.
Seasoning, both air drying and kiln drying, is a method of removing
part of the moisture, and hence is a method of preserving timber
against decay; but any seasoned timber will re-absorb moisture, and
hence seasoning is only an imperfect method of preservation. A
more effective method is to inject into the timber some substance
that will change the organic matter in the wood so it will no longer
serve as food for the fungi.
1156. THE PRESERVATIVE. The preservative performs two
functions, viz.: (1) acts as an antiseptic to prevent decay; and (2)
acts as a waterproofing material to keep out moisture.
It is desirable that the preservative should be stable and remain
in the block as long as possible; since where the travel is light, the
life of the pavement depends upon the resistance of the blocks to
decay. However, if an antiseptic has thoroughly penetrated the
block, it will ordinarily be preserved against decay, even though a
larger proportion of the preservative evaporates or is washed out.
606 WOOD-BLOCK PAVEMENTS [CHAP. XIX
But if the antiseptic evaporates or is washed out, the timber becomes
again susceptible to changes in volume with changes of moisture
content, which is objectionable in paving blocks.
Any material that renders a block waterproof is in itself a fair
preservative, even though it is not an antiseptic. For example,
petroleum is a fair preservative of timber, although it is not anti-
septic. If water could be kept entirely out of the block, there
would be little or no decay; but this is practically impossible, since
no amount of preservative will make wood absolutely Waterproof.
1157. Creosote is a distillate of coal tar, and is usually called
creosote oil, and sometimes dead oil of tar. Creosote oil was the first
material used for preserving paving blocks; and it is still the essen-
tial constituent in all such preservatives.
Creosote oil was not entirely satisfactory as a preservative for
paving blocks. It preserved the wood and prevented decay; but
it gradually evaporated and washed out, and permitted a change in
the moisture content of the blocks, which caused expansion and con-
traction. To remove this objection, coal tar was added to the
creosote to increase the waterproofing qualities of the preservative.
The addition of tar also cheapens the preservative.
Coal tar may contain something like 40 per cent creosote oil, and
hence is in itself a fair antiseptic; but on the other hand; it is harder
to force the viscous tar into the wood than the more fluid creo-
sote oil.
There is a considerable difference of opinion as to the relative
merits of the different creosote preservatives. Some argue that the
preservative should be pure creosote oil, having a specific gravity of
1.03 to 1.07,. and a few claim that it should have a higher specific
gravity but still be free from tar; while others prefer a mixture of
creosoted oil and coal tar having a specific gravity of 1.08 to 1.12.
Most cities now use an oil having a specific gravity of 1.10 to 1.14
and containing a large proportion of coal tar, which is usually re-
quired to be nearly free from carbon. The presence of any consid-
erable amount of carbon is likely to plug the pores of the wood and
prevent the introduction of the preservative.
Some engineers claim that water-gas tar (§ 564) gives satisfactory
results, even though it has no antiseptic properties; but others claim
that experience with this material has been too limited in both extent
and time to warrant its use on a large scale. It is usually cheaper
than a mixture of creosote oil and coal tar.
1158. Two proprietary methods of treating wood paving-blocks
ART. 1] MATERIALS AND TREATMENT 607
were introduced comparatively early. One, called the kreodone
process, consisted in impregnating block under pressure with a
secret proprietary preservative. The other, called the creo-resinate
process, consists in mixing melted resin and formaldehyde with the
creosote, the resin being to waterproof the wood, and the formalde-
hyde is to increase the antiseptic effect of the preservative. This
process was very efficient, but was discontinued on account of the
increase in the cost of resin.
1159. Specifications for Preservatives. The specifications in
§ 1160, § 1161, and § 1162, were prepared by the American Wood-
Preservers' Association,* and have virtually been approved by prac-
tically all of the national engineering societies interested in wood
preservation.!
1160. Creosote Oil. Creosote oil was formerly used for railway
ties, structural timber and wood pavmg-blocks ; but lately, owing to
its scarcity and cost, has not been used much, particularly for paving
blocks. Paving blocks do not require as perfect a preservative as
ties and structural timber, since usually their life is limited by their
resistance to wear rather than to decay. Creosote oil when tested
in accordance with the standard methods of the American Wood
Preservers' Association, should comply with the following require-
ments :
"1. The oil shall be a distillate of coal-gas tar or coke-oven tar.
"2. It shall not contain more than 3 per cent of water.
"3. It shall not contain more than 0.5 per cent of matter insoluble in benzol.
"4. The specific gravity of the oil at 38° C. compared with water at 15.5° C.
shall be not less than 1.03.
"5 The distillate, based on water-free oil, shall be within the following limits:
up to 210° C. not more than 5 per cent; and up to 235° C. not more than 25 per
cent.
"6. The specific 'gravity of the fraction between 235° and 315° C. shall
not be less than 1.03 at 38° C., compared with water at 15.5° C. The specific
gravity of the fraction between 315° and 355° C. shall be not less than 1.10 at
38° C., compared with water at 15.5° C.
"7. The residue above 355° C., if it exceeds 5 per cent, shall have a float-test
of not more than 50 seconds at 70° C.
"8. The oil shall yield not more than 2 per cent coke residue."
1161. Coal-tar Distillate Oil. Coal-tar distillate oil for paving
blocks, when tested in accordance with the standard methods of the
*Procccdings, 1917, p. 307-9.
•\Ibid., p. 41 and 325.
WOOD-BLOCK PAVEMENTS [CHAP. XIX
American Wood Preservers' Association,* shall comply with the fol-
lowing requirements :
"1. The oil shall be a distillate of coal-gas tar or coke-oven tar [§ 564],
"2. It shall not contain more than 3 per cent of water.
"3. It shall not contain more than 0.5 per cent of matter insoluble in benzol.
"4. The specific gravity of the oil at 38° C., compared with water at 15.5° C.,
shall be not less than 1.06.
"5. The distillate, based on water-free oil, shall be within the following
limits: up to 210° C. not more than 5 per cent; and up to 235° C. not more than
15 per cent.
"6. The specific gravity of the fraction between 235° and 315° C. shall be not
less than 1.03 at 38° C., compared with water at 15.5° C. The specific gravity
of the fraction between 315° and 355° C. shall be not less than 1.10 at 38° C.,
compared with water at 15.5° C.
"7. The residue above 355° C., if it exceeds 10 per cent, shall have a float-test
of not more than 50 seconds at 70° C.
"8. The oil shall yield not more than 2 per cent coke residue."
1162. Coal-tar Paving Oil. Coal-tar paving oil for paving blocks,
when tested in accordance with the standard methods of the American
Wood Preservers' Association,* shall comply with the following
requirements :
"1. It shall be a coal-tar product of which at least 65 per cent shall be a dis-
tillate of coal-gas tar or coke-oven tar, and the remainder shall be refined or fil-
tered coal-gas tar or coke-oven tar [§ 564].
"2. It shall not contain more than 3 per cent of water.
"3. It shall not contain more than 3 per cent of matter insoluble in benzol.
"4. The specific gravity of the oil at 38° C., compared with water at 15.5° C.,
shall be not less than 1.07 or more than 1.12.
"5. The distillate, based on water-free oil, shall be within the following limits:
up to 210° C., not more than 5 per cent; and up to 235° C. not more than
25 per cent.
"6. The specific gravity of the fraction between 235° and 315° C. shall be
not less than 1.03 at 38° C., compared with water at 15.5° C. The specific
gravity of the fraction between 315° and 355° C. shall be not less than 1.10
at 38° C., compared with water at 15.5° C.
"7. The residue above 355° C., if it exceeds 35 per cent, shall have a float-test
of not more than 80 seconds at 70° C.
"8. The oil shall yield not more than 10 per cent coke residue."
1163. Water-Gas Tar. Refined water-gas tar shall conform to
the following requirements:!
"1. The specific gravity shall be not less than 1.12 nor more than 1.14 at
38° C., referred to water at the same temperature.
* Proceedings, 1917, p. 309-21.
t Specifications for Creosoted Wood-block Paving, American Society of Municipal Improve--
ments, 1916, p. 5-6.
ART. 1] MATERIALS AND TREATMENT 609
"2. Not more than 2.0 per cent shall be insoluble by hot extraction with
benzol and chloroform.
"3. On distillation when made as hereinafter described,* the distillate,
based on water-free oil, shall be within the following limits: up to 210° C.,
not more than 5.0 per cent; up to 235° C., not more than 15.0 per cent; up to
315° C., not more than 40.0 per cent; and up to 355° C. not less than 25.0 per
cent.
"4. The specific gravity of the total distillate below 355° C. shall not be less
than 1.0 at 38° C., referred to water at the same temperature.
"5. The oil shall not contain more than 2.0 per cent water; and due allow-
ance shall be made for all water and insoluble foreign matter it may contain by
injecting a corresponding additional quantity into the blocks."
1164. TREATMENT OF BLOCKS. There are two methods of
treating the blocks with the preservative, viz.: (1) the open-tank
process, and (2) the pressure process.
1165. Open-tank Process. In this process the blocks are im-
mersed in the preservative from a few minutes to an hour, depending
upon the kind of wood and the degree of penetration desired. This
method is largely used in France. In this country early in the his-
tory of treated paving blocks, it was much used; but it is not now
used.
1166. Pressure Process. The standard specifications for the
treatment of the timbers mentioned in § 1152, except Douglas fir,
are substantially as follows:
"The blocks are placed in a closed cylinder, and subjected to steam at a tem-
perature of 220° to 240° F. for not less than 2 hours nor more than 4 hours.
The steam and moisture are blown out of the cylinder, and then the blocks are
subjected to a vacuum of not less than 22 inches of mercury for at least 1 hour.
While the vacuum is still on, the preservative, heated to a temperature of 180°
to 220° F., is run in until the cylinder is completely filled, care being taken that
no air is admitted. Pressure is then gradually applied at a rate not to exceed
50 Ib. per square inch per hour, and is maintained at 100 to 150 Ib. per square inch
until the wood has absorbed the required amount of preservative. Next a sup-
plemental vacuum of at least 20 inches is applied for at least 30 minutes. If
desired this vacuum may be followed by a short steaming period.
"The timber may be either green or air seasoned; but should preferably be
treated within three months after it is sawed. Green and seasoned timber shall
not be treated together in the same charge.
"After treatment the blocks shall show a satisfactory penetration of the pre-
servative; and in all cases the preservative must be diffused throughout the sap
wood. The surface of the blocks after treatment shall be free from deposit of
objectionable substances; and all blocks that have been materially warped,
checked, or otherwise injured in the process of treatment shall be rejected."
* Accompanying the printed specifications, but not reproduced in this volume.
610 WOOD-BLOCK PAVEMENTS [CHAP. XIX
1167. The following are the reasons for the several steps in the
treatment.
The preliminary steaming softens or liquefies the sap, so it may
later be removed from the pores of the wood. The steaming also
equalizes the moisture content of heart and sapwood, which equalizes,
the resistance to penetration, and thus prepares the wood to receive
the preservative more uniformly. The preliminary steaming is .
applied whether the timber is green or seasoned. In green timber
the sapwood contains more water than the heartwood; and unless
the excess sap is removed, the blocks will contain untreated, or at
least under-treated, sapwood. Therefore, green blocks should be
subjected to steaming and the vacuum process to remove the excess
water from the sapwood. On the other hand, seasoned timber is
more easily treated, i. e., takes the preservative more easily, than
green timber. Sometimes seasoned timber accepts the preservative
so easily that almost no pressure is required to produce the desired
absorption ; and consequently the easily treated portion receives too
much preservative and the portion that is more difficult to treat
receives too little. The sapwood is likely to be more thoroughly
seasoned than the heartwood; and therefore the former may receive
more preservative than the latter, and consequently the latter may
decay because of incomplete penetration of the preservative. The
steaming of seasoned blocks expands them, so that when treated and
laid in the pavement they will have a minimum expansion with the
absorption of moisture. Therefore, seasoned timber should be sub-
jected to steaming to prepare it to receive the preservative and to
decrease expansion when laid in the pavement.
The preliminary vacuum is applied to remove the sap and mois-
ture, which equalizes the resistance to the penetration of the pre-
servative.
The preservative is applied under pressure and for a considerable
time to secure complete penetration; but the pressure is applied
slowly so as not to injure the strength of the wood.
The supplemental or final vacuum is applied to equalize the distribu-
tion of the preservative, but chiefly to remove the excess preservative
in the outer portion of the block and thus decrease bleeding (§ 1211).
The final steaming is applied to soften and remove the excess
preservative from the surface of blocks.
Finally, green and seasoned timber should not be treated together
in the same charge, since they have unequal resistance to the pene-
tration of the preservative.
ART. 1] MATERIALS AND TREATMENT 611
1168. Amount of Preservative. The blocks laid in Indianapolis
in 1896 (§ 1147) were treated by the open-tank process and contained
only about 3 Ib. of creosote oil per cubic foot. In the earlier appli-
cations of the pressure process, the amount was usually 10 to 12 Ib.
per cubic foot; but later the general practice was to inject from 20 to
24 Ib. per cubic foot, which resulted in a greatly increased cost and
an excessive bleeding of the blocks (§ 1211). Recently the tendency
has been to reduce the absorption; and at present the average is
about 16 Ib. of water-free preservative per cubic foot. The standard
specifications of the American Wood Preservers' Association and also
those of the American Society of Municipal Improvements require
16 Ib. per cubic foot.
The amount absorbed is determined from gages on the treat-
ing cylinder and a knowledge of the volume of the charge; and may
be checked by weighing several blocks before and after treatment.
1169. Testing the Blocks after Treatment. Usually the only
test made is to determine the penetration of the preservative. " To
determine this, at least twenty-five blocks shall be selected from
various parts of each charge, and sawn in half at right angles to the
fibers, through the center; and if more than one of these blocks show
untreated sapwood, the charge shall be retreated."
1170. Occasionally a test is made to determine the amount of
water a treated block will absorb. The usual specifications for this
test are: " The treated blocks after being dried in an oven at 100° F.
for 24 hours, and then immersed in clear water for 24 hours, shall not
absorb more than 3J per cent of their dry weight if pine, nor more
than 4J per cent if tamarack." This test is to determine the pos-
sibility of the block's swelling after being laid, by the absorption of
moisture; and therefore is quite important. The absorption will
increase with the time after treatment; and therefore the blocks for
this test sho ild be taken from those about to be laid rather than
from those recently treated.
The results by this test depend more upon the time since treat-
ment and upon the method of storage than upon the method of
treatment; and hence this is not an accurate test of absorbing power.
Further, since no preservative process can make blocks absolutely
waterproof, they should be so laid as to reduce to a minimum the
amount of moisture absorbed (see § 1180).
1171. Care of Blocks after Treatment. The blocks should prefer-
ably be laid in the street as soon as possible after being treated. If
they can not be laid within two days, provision should be made to
612 WOOD-BLOCK PAVEMENTS [CHAP. XIX
prevent them from drying out, by stacking in close piles and covering
them; and if possible, sprinkling them thoroughly at intervals. To
prevent expansion in the pavement through the absorption of water,
the blocks should be well sprinkled about two days before being laid.
ART. 2. CONSTRUCTION
1173. FOUNDATION. The subgrade should be prepared as
described in Art. 1 of Chapter XV, — Pavement Foundations. The
usual foundation is a layer of hydraulic concrete, which should be
constructed as described in Art. 2 of Chapter XV.
The specifications of the American Society of Municipal Improve-
ments state: " At no place shall the surface of the finished concrete
vary more than a half inch from the given grade."
1174. BEDDING COURSE. A bedding course is necessary to
compensate for any unevenness in the top surface of the concrete
foundation and to afford a good bearing for the blocks. Three
forms of bedding course are in common use, which for brevity may
be designated as sand, hydraulic-cement mortar, and bituminous
cement.
1175. Sand Cushion. The sand bedding-course varies in thick-
ness from 1 to 2 inches. The disadvantages of a thick sand cushion
are very much the same for wood blocks as for brick— see § 977.
In recent years, as in brick pavements, there has been a tendency to
reduce the thickness of the sand cushion. The 1916 specifications
of the American Society of Municipal Improvements require a
cushion 1 inch thick, of " sand that will pass a J-inch screen and
contain 10 to 25 per cent of loam or clay." The sand cushion should
be struck with a template to a surface parallel to the grade and con-
tour of the finished pavement; and should then be rolled.
Sometimes, instead of striking the sand cushion with a template,
screeds are laid transversely across the pavement at intervals of 8 or
10 feet, being placed upon a ridge of sand or mortar so as to bring the
top surface of the screed parallel to, and at the right distance below,
the surface of the pavement. The sand is spread between the screeds,
and then struck off to the right depth with a straight edge which
rests upon the screeds and is kept parallel to the curb. This method
requires more labor and does not give as accurate a surface as striking
with a template. The only disadvantage of using a template is
that a new one is required with each change in crown or width,
ART. 2] CONSTRUCTION 613
although this objection is overcome in part by using a template
which is slightly adjustable.
1176. Substantially all of the comments concerning the sand
cushion for brick pavements (§ 971-78) apply also to that for wood-
block pavements. In addition, a sand cushion holds moisture, and
hence increases the absorption of the blocks and adds to the troubles
due to their expansion in the pavement. Formerly the sand cushion
was the most common form of bedding course; but it has now prac-
tically been abandoned.
1177. Dry-mortar Bed. The method of laying wood blocks on a
dry-mortar bed is substantially the same as for laying bricks on a
cement-sand bedding course (see § 979-81). It would be possible
to lay wood blocks upon a wet-mortar bed by either of the processes
employed for brick pavements (see § 982); but it is not known that
it has ever been done.
The 1916 specifications of the American Society of Municipal
Improvements for preparing the dry-mortar bed for wood blocks are
as follows:
"The concrete foundation shall be cleaned and swept; and shall be thor-
oughly dampened immediately in advance of the spreading of the cushion course.
Upon the surface of the foundation thus prepared shall be spread a layer of mortar
not exceeding \ inch in thickness, made of one part portland cement and three
parts of sand. Only sufficient water shall be added to this mixture to insure a
proper setting of the cement, thelntention being to produce a granular mixture
which may be raked or struck by a template to the desired grade. The mortar
shall be [thoroughly mixed, and shall be spread in place upon the foundation by
means of a template immediately in advance of the laying of the blocks."
1178. It is doubtful if the mixture of cement and sand ever gets
enough water to cause it to set fully, since the joints between the
blocks are quite narrow. The only reason for using a granular mix-
ture is that it can be spread and struck easily; but the ordinary
cement mortar containing enough water to insure a complete set,
can be spread and struck without serious trouble. Or, better still,
if the concrete foundation is finished with a slight excess of mortar on
the surface, the wood blocks can be set in the mortar as are the
brick in the monolithic brick pavement (§ 892).
An objection to the mortar bedding course is that the pavement
can not be used until the mortar has set; but if the mortar bedding
course is laid immediately after the concrete foundation is placed, this
objection is eliminated.
A serious objection to the dry-mortar cushion is that not enough
614 WOOD-BLOCK PAVEMENTS [CHAP. XIX
water is used to secure a good quality of mortar. When the dry-
mortar cushion is used for a brick pavement, the mortar is thor-
oughly wet by sprinkling the brick after they are laid; but this
should not be done with wood blocks, since they absorb more water,
and since with wood blocks the joints are usually filled with bitumi-
nous cement (§ 1187), which should be applied only when the blocks
are dry.
1179. Bituminous Bed. If the top of the concrete foundation
has not been' finished to an accurate surface by strkiing with a tem-
plate (§ 461-62), it should be leveled up by spreading a layer of 1 : 2
FIG. 224. — FINISHING MORTAR BEDDING-COURSE WITH A STEEL FLOAT.
cement mortar on it. This mortar should be of such consistency
that it may be easily spread; and should be applied to the surface
of the concrete before initial set of it has begun. The mortar should
be then worked to an accurate surface by means of a long-handled
wood float having upturned ends. When finished the surface should
not show any depressions greater than J inch under a 5-foot straight
edge laid parallel to the curb. Fig. 224 shows the method of
finishing the mortar bedding-course with steel floats; and Fig. 225
shows the method of finishing the bedding course with a wood-
float.
1180. After the concrete base and the mortar coat have set and
hardened, and after the surface has been thoroughly cleaned, and
while it is perfectly dry, a coat of coal-tar pitch is spread upon the
surface, The pitch should meet the specifications of § 576-77
ART. 2]
CONSTRUCTION
615
(page 295) for filler for wood blocks; and should be applied at a
temperature between 250° and 300° F. It should be spread to a
uniform thickness of not more than £ inch, and be finished to a smooth
surface. The blocks should be set directly upon this paint coat
within 30 minutes after it has been applied. If the work is properly
done, the blocks are firmly held in place; and in tearing up such a
pavement it is not unusual to have the pitch pull up a film of the
concrete base. If the surface of the concrete or the mortar coat
has ridges or depressions in it, the blocks are likely to split under
travel. If the pitch coat is thicker than £ inch, it is likely to flow
FIG. 225. — FINISHING MORTAR BEDDING-COURSE WITH A WOOD FLOAT.
and split the blocks. The slipping of the blocks on the pitch coat is
sometimes called " floating."
The chief advantage of the bituminous bedding course is that
the bituminous cement completely seals the pores of the block, and
prevents the absorption of any water that may reach the top of the
concrete base through cracks in the wearing coat. This method
represents the best modern practice, and has recently been adopted
quite generally.
1181. LAYING THE BLOCKS. Upon one of the three bedding
courses described above, the blocks are set with the fiber vertical,
in straight parallel courses, leaving a space at the curb 1 inch in
width for the expansion joint. The blocks are laid "hand tight";
but each eight or ten courses are driven together by laying a 4- by
616
WOOD-BLOCK PAVEMENTS
[CHAP, xix
4-inch scantling against the last course and striking it with an axe or a
sledge. No joint should be more than £ of an inch in width, although
some good authorities permit a width of Y& of an inch. The blocks
should lap at least 2 or 2J inches. Only whole blocks should be used,
except in starting and closing a course. The block used in starting
or closing should have its cut face perpendicular to the top. In
placing the blocks the workman should stand upon the blocks already
placed, and never upon the bedding course.
There is no agreement as to whether the courses shall be per-
pendicular to the curb, or at an angle of 45° or 67J°. It is claimed
that if the courses are oblique to the curb, the wear at the joints, par-
ticularly the transverse ones, will be less than if the courses are per-
pendicular to the curb. But this claim has not been established;
and as it is most convenient and costs about 2 cents per square yard
less to lay the courses at right angles to the curb, this method has
generally been adopted.
FIG. 226.— LAYING WOOD-BLOCK PAVEMENT BETWEEN STREET-CAR RAILS.
1182. Fig. 226 shows the method of laying wood blocks between
the rails of a street-car track on a mop coat of tar over a smooth
ART. 2]
CONSTRUCTION
617
concrete base. Fig. 227 shows the manner of laying wood blocks
between the railway area and the curb. Note the plank next to
the curb to provide space for the longitudinal expansion joint.
The blocks are being laid on a mop coat of tar on a concrete base.
FIG. 227. — LAYING WOOD-BLOCKS ON A MOP COAT OF TAR.
1183. Rolling. After being placed, the blocks are inspected, and
the rejected blocks are removed and replaced with acceptable ones.
The surface should then be swept clean, and be rolled with a tandem
roller weighing from 3 to 6 tons. The roller should begin at the side
of the pavement, run slowly parallel to the center line, and work
inwardly until the center of the road is reached. It should then move
to the opposite side of the pavement and proceed as before. As the
roller passes back and forth, it should overlap its course each time.
After one rolling of the entire surface, the speed of the roller may be
increased and the rolling continued until the blocks are thoroughly
and evenly bedded.
Portions of the pavement inaccessible to the roller should be
thoroughly rammed with a paver's rammer (§ 992) weighing not less
than 50 lb., striking upon a plank not less than 6 feet long, 10 to 12
inches wide, and 2 inches thick. The plank should be laid parallel
to the curb, and moved so that the surface will be equally rammed
and brought to the proper elev.stion.
618 WOOD-BLOCK PAVEMENTS [CHAP. XIX
If the bedding course is green mortar, the rolling should be com-
pleted before the mortar has set.
When the rolling and ramming are completed, the surface of the
pavement should conform so nearly to that indicated on the plans,
that it will nowhere depart more than one fourth of an inch from
properly formed templates or from a 10-foot straight edge applied
parallel to the center line.
1184. JOINT FILLER. The joints between the blocks are filled
with grout, sand, tar pitch, or asphalt.
,1185. Grout. Occasionally the attempt is made to fill the joints
of a wood-block pavement with hydraulic-cement grout; but if the
blocks are set as closely together as they should be, the joints will be
so narrow that it is impossible to get a good grout to fill them. If
the grout is thin enough to flow into narrow joints, it will not con-
tain enough cement to make it of much, if any, value; and besides
it will not adhere well to the sides of treated wood-blocks, and even
the preservative seems to kill the cement.
1186. Sand. Fine clean dry sand is swept into the joints; and
the surface is covered with sand to the depth of | an inch. The
sand is placed upon the surface to insure that every joint is filled
and to permit travel to grind the sand into the surface of the blocks.
After the sand has remained on the pavement for a time, depending
upon the density of the travel, it is swept up and hauled away.
Sand was formerly the most common joint filler, but it has
practically been abandoned. It is cheap and makes a fairly water-
tight pavement; but it has little or no effect in binding the blocks
together to prevent settlement due to shrinkage of the sand cushion
(§ 1055) or other causes (§ 1056-57), or to prevent upheavals
(§ 1060).
1187. Tar Pitch. The tar-pitch filler should conform to the spe-
cifications stated in § 576-77, page 295. It is heated in kettles or
tanks on the street. The temperature should never exceed 325° F.;
and it should be applied to the pavement at a temperature between
250 and 300° F. The hot pitch may be poured into the joints as
described for brick pavements (see the second paragraph of § 1011).
Fig. 228 shows a somewhat antiquated method of applying the filler
to a wood-block pavement. This method is more suitable for a
brick pavement than a wood one, since with the former the joint is
wider, and hence it is easier to follow a joint with the point of the
can; and also since more tar is required, and hence the tar flows
better through the tip of the can.
ART. 2]
CONSTRUCTION
619
FIG. 228. — FILLING THE JOINTS WITH A CONICAL CAN.
Fig. 229 shows a better method of applying the tar joint-filler.
Fio. 229. — APPLYING TAR JOINT-FILLER WITH BUCKET AND SQUEEGEE.
620
WOOD-BLOCK PAVEMENTS
[CHAP, xix
Fig. 230 shows a buggy for transporting and applying pitch joint-
filler.
Fia. 230. — BUGGY FOR APPLYING PITCH JOINT-FILLER.
Fig. 231 shows the finished wood-block pavement.
FIG. 231. — FINISHED WOOD-BLOCK PAVEMENT.
11)88. In filling the joints care should be taken to leave as little
bituminous cement on the surface as possible, since one of the ob-
jections to a treated wood-block pavement is that the preservative
exudes, i. e., " the pavement bleeds," and covers the surface with a
sticky mass (§ 211). If any joint filler is left upon the surface, it
increases^this objection. Further, some object to a bituminous filler,
because it is liable to be forced out of the joints onto the surface of the
ART. 2]
CONSTRUCTION
621
pavement. An advantage of a bituminous filler is that it virtually
surrounds each block with an expansion joint, and hence decreases
the expansion due to the absorption of moisture by the blocks.
Under no circumstances should the pitch be applied when there
is any moisture in the joints; and therefore a pitch filler should not
be used with a cement-sand bedding course (§ 1177).
1189. The specifications of the Ohio Department require that
the lower half of the joints shall be filled with tar pitch as in § 1187,
and the upper half with sand as in § 1186.
1190. OPEN- JOINT CONSTRUCTION. Since a.wood-block pave-
ment made as described above is quite smooth, it is customary to
modify the construction on steep grades so as to give a good foothold
to horses. Such construction is not employed unless the grade is
more than 3 or 4 per cent — see Table 15, page 57.
Formerly a corner was cut out of the upper edge of a block
as shown in Fig. 232 so as to give an open joint J inch wide and
FIG. 232. — RECTANGULAR WOOD-BLOCK PAVEMENT WITH OPEN JOINTS.
1^ inches deep; but at present it is more common and cheaper
to place between adjacent courses a creosoted wood lath 3^ of an inch
thick and 2 inches wide. The joint is poured about half full of
bituminous filler; and then the space above the lath is filled with
hot crushed stone, and the interstices between the stones are filled
with bituminous cement.
Sometimes the upper edge of the face of the blocks is chamfered
at an angle of about 45° to a depth of about f of an inch; and this
space is filled with the ordinary joint filler. This method is not as
622 WOOD-BLOCK PAVEMENTS [cHAP. XIX
satisfactory nor as cheap as the second one described in the preceding
paragraph.
1191. EXPANSION JOINTS. The expansion of wood paving-
blocks due to changes of temperature is not great enough to require
any consideration; but the expansion and contraction due to change
in the moisture content requires attention. The amount of expan-
sion and contraction due to moisture depends somewhat upon the
method of treatment. If the blocks are seasoned before being treated,
and are not steamed before they are impregnated with the preserva-
tive, they are likely to absorb moisture and swell after the preserva-
tive has evaporated. Again, with green timber the steaming and the
vacuum liquefies and removes the sap, and reduces the volume of the
wood so it will be less likely to shrink when laid in the pavement.
Finally, the amount of preservative should be such as to fill the cells
of the wood and cover the fibers, thus making the blocks partially
waterproof. An additional means of reducing the expansion and
contraction due to moisture is to lay the blocks in a paint or mop
coat of bituminous cement (see § 1179-80).
1192. To allow for expansion due to temperature and moisture,
it is customary to construct a longitudinal expansion joint next to
each curb. There are two forms of expansion joints, the poured and
the pre-moulded.
The poured joint is made by placing a f or a 1-inch board next
to the curb, and setting the blocks against it (see Fig. 227, page 617);
and after the blocks are set and the joints are filled, the board is
removed and the space is filled with bituminous joint-filler poured hot
(see Fig. 233). For an illustration of a brick pavement showing a
thin board in place with wedges behind it to facilitate its removal,
see Fig. 171, page 483.
The pre-moulded expansion joint is a sheet of felt or its equivalent
saturated with tar pitch or asphalt. There are several somewhat
similar forms on the market. For a few additional items concern-
ing pre-moulded expansion joints, see the second paragraph of
§ 1017.
1193. Formerly transverse expansion joints were inserted in
wood-block pavements; but they have been discontinued, because
they are not needed, and are a positive detriment. The objections
to the transverse expansion joint for wood-block pavements are sub-
stantially the same as for brick pavements — see § 1018.
Care should be taken to fill the joints around manholes, water-
boxes, etc., to prevent water's reaching the foundation, where it will
ART. 2J
CONSTRUCTION
623
freeze and lift the pavement, or be absorbed by the blocks and cause
them to expand.
1194. CROWN. The surface of a wood-block pavement should be
quite smooth; and therefore the crown should be comparatively
small. A special committee of the American Society of Civil Engi-
neers recommended that the transverse slope be between J and \ of
an inch per foot — see Table 16, page 65.
1195. MAXIMUM PERMISSIBLE GRADE. The maximum per-
missible grade for a close-joint wood-block pavement is 3 or 4 per
cent (see Table 15, page 57); and with the open-joint construction
(§ 1190), the maximum grade may be 6 or 7 per cent.
FIG. 233. — POURING THE LONGITUDINAL EXPANSION JOINT.
1196. PAVING ADJACENT TO TRACK. Wood-blocks are laid
adjacent to the rails of street railway tracks in substantially the same
manner as bricks — see Fig. 196 and 197, page 540.
1197. COST OF CONSTRUCTION. Price of Blocks. Table 70,
page 624, shows the market quotation for treated wood paving-
blocks for November 1, 1917. For more recent quotations, see con-
struction news in current technical journals.
1198. Cost of Pavement. The following estimate was prepared
for this volume by a specialist in wood preservation and wood paving
who has no financial interest in contracting.!
1199. Cost of Blocks. Table 71, page 624, shows the amount of
timber and preservative required. The method of using Table 71 in
t Mr. Walter Buehler, Mem. Amer, Soc,.gf Civil Engra., Chicago.
624
WOOD-BLOCK PAVEMENTS
[CHAP, xix
TABLE 70
MARKET-PRICE FOR TREATED WOOD PAVING-BLOCKS*
LOCALITY.
ABSORPTION,
Ib. per cu. ft.
DEPTH,
inches.
PRICE,
per sq. yd.
New York City
16
3£
$2.00
(( U (I
16
4
2 25
Chicago
16
4
1 85
St Louis
16
31
1 80
u n
16
4
2 03
Kansas City
16
4
2.50
St Paul
16
31
2 00
San Francisco
16
3
2 15
<; «
16
31
2 26
(. ti
16
4
2 57
Seattle
16
4
2 35
TABLE 71
AMOUNT OF LUMBER AND PRESERVATIVE REQUIRED FOR PAVING BLOCKS
LUMBER REQUIRED,
PRESERVATIVE REQUIRED,
Feet, B.M.,perSq. Yd.
Gallons per Sq. Yd.
Depth of
Block,
Inches.
Absorption, Lb. per Cu. Ft.
Net.
Waste.
Total.
10
12
16
3
2.7
2.7
30
2.443
2.945
3.928
3|
31.5
3.15
35
2.862
3.546
4.583
4
36
3.6
40
3.274
3.928
5.238
determining the cost of blocks is as follows: Assume the cost of
lumber at the treating plant at $30 per thousand feet, B. M., which
under normal conditions prevailing a few years ago would not be over
$25. Assume that the preservative meets the specifications of
§ 1161; and that it costs 9 cents per gallon. Assume that the blocks
are to be 3| inches deep, and are to be treated with 16 Ib. per cubic
foot. The cost of the blocks are:
Lumber 35 feet B.M., at $30 $1 .05 per sq. yd.
Preservative 4.583 gallons at 9 cents 414 " " "
Labor, depreciation, interest, etc 240 " " "
Factory cost $1 . 704 " " "
Profits at 15 per cent gross 30 " " "
Factory selling price $2 . 004 " " "
The approximate weight of wood blocks treated with 16 Ib. per
cubic foot is as follows: 3-inch, 130 Ib. per square yard; 3j-inch
150 Ib. per square yard; and 4-inch, 170 Ib. per square yard.
* Engineering News-Record, Vol, 79 (1917), Construction News, p. 180,
AfcT. 2]
CONSTRUCTION
625
1200. Cost of Wearing Coat. Table 72 shows in detail the esti-
mated cost of a 3i-inch wood-block pavement with two forms of bed-
ding course.
TABLE 72
ESTIMATED COST OF 3|-iNCH WOOD-BLOCK PAVEMENT
BEDDING
COURSE.
ITEMS.
Tar Paint
Coat.
Dry Mortar.
Sub-grade see Table 56 page 546
$0 217
$0 217
Concrete Foundation see Table 56, page 546.
568
568
extra finish to surface.
01
Bedding Course 1 '. 4 dry cement and sand . .
18
0 5 gallon of pitch, and labor
.06
Wood Blocks, see § 1199
2 004
2 004
freight, say, 200 miles at 6 cents per 100 blocks
hauling at 60 cents per hour.
.09
05
.09
05
laying at 25 cents per hour.
08*
08*
rolling
01
02
Joint Filler, including longitudinal expansion joint
Top Dressing, purchasing and spreading sand . . . .
.12
02
.12
02
Total cost, exclusive of interest, insurance, deprecia-
tion profit etc
$3 229
$3 349
1201. Example of Cost. Table 73, page 626, shows the cost of
laying 150,000 square yards of creosoted wood-block paving in
Minneapolis by city force, and are given in unusual completeness, and
hence are specially valuable in making estimates.
1202. The following are the details of the cost of laying a wood-
block pavement in Cambridge, Mass., in 1913. f Common labor
was 31 cents per hour. The foundation consisted of 5 inches of
1 : 2 J : 5 concrete; and the bedding course was 1 inch of cement and
sand. The blocks were southern long-leaf yellow pine treated with
20 Ib. of preservative. The joints were filled with 1 : 1 cement
grout. Longitudinal expansion joints were provided at each curb,
and transverse contraction joints at each 30 feet. 4-inch blocks cost
$2.59 per square yard delivered on the street, and $4.11 per square
yard complete in 'the pavement; and 3|-iLch blocks cost $2.29 and
$3.81, respectively.
1203. Contract Price. Table 74, page 627, shows the contract
price of wood-block pavements in various cities, and incidentally
also gives considerable information as to the details of practice of
these cities.
*If there is a street-railway track, add 2 cents.
^Engineering News, Vol. 71 (1914), p. 1131.
626 WOOD-BLOCK PAVEMENTS [CHAP. XIX
TABLE 73
COST OF WOOD-BLOCK PAVEMENT IN MINNEAPOLIS*
ITEMS. PER SQ. YD.
SUBGRADE: grading and shaping $0.2 87
CONCRETE BASE, 1:3:6, 6 inches thick:
cement, $1.12 per bbl., f.o.b. cars 1222
sand $0 . 60 per cu. yd., delivered 0395
stone, $1.00 per cu. yd., f.o.b. quarry, $1.70, delivered ; 2386
labor mixing and placing by hand at 28 cents per hour 1392
hauling cement, plank, etc., at 59 cents per hour 0238
concreting strip H ft. wide between railway ties 0189
Total for concrete base $0. 5822
SAND CUSHION, 1 inch: sand at 60 cents per cubic yard on job $0 . 200
WOOD BLOCKS, Norway pine and tamarack:
i 4-inch, treated with 12 Ib. of creodone creosote 1 .3275
I hauling blocks at 59 cents per hour .0495
laying blocks at 22£° with curb, at 40 cents per hour 0716
Total for blocks $1 .4486
JOINT FILLER, sand and pitch:
sand at 60 cents per cu. yd., on job 0055
pitch at 5.7 cents per gallon 0490
labor at 28 cents per hour 0175
Total for filler $ .0720
HEADERS:
4 X 10-inch plank at cross streets and alleys 0030
MISCELLANEOUS:
materials . 0077
labor 0002
cleaning up 0071
tools.. .0200
Total miscellaneous. . . $ . 0350
Total average cost $2 . 3795
1204. MERITS AND DEFECTS. The merits of a treated wood-
block pavement are: 1. It has a smooth surface, and therefore is a
quiet pavement. It is less noisy than sheet asphalt, brick or stone-
block; and from the standpoint of tenants,, this is an important
advantage. 2. It is a reasonably durable pavement, even under
heavy and dense travel. This conclusion has been established in
many cities in this country and in Europe. 3. The pavement is
easy to clean; and its surface does not grind up and make dirt. 4.
It has a low tractive resistance.
*B. H. Durham, Street Engineer, in Engineering and Contracting, Vol. 35, p. 451.
ART. 2]
CONSTRUCTION
627
TABLE 74
CONTRACT PRICE OF WOOD-BLOCK PAVEMENTS IN VARIOUS CITIES*
Laid in 1912
LOCALITY.
Amount.
Laid in
1912,
Sq. Yd.
CONCRETE BASE.
Kind
of
Filler.
Guar-
antee,
Years.
Total
Thick-
ness,
Inches.
AVER-
AGE
PRICE.
Per
Sq. Yd.»
State.
i
City.
Thick-
ness,
Inches.
Propor-
tions.
Connecticut. . .
Georgia
Illinois
Bridgeport. . .
S. Norwalk. . .
Albany
Granite City .
Quincy
Burlingtor.. .
Louisville. . . .
New Orleans.
Shreveport. . .
Bangor
Springfield . . .
Hibbing
Minneapolis. .
Owatonna. . .
Virginia
10000
3400
10000
10000
9 171
22000
1 578
18401
54000
1 700
12949
44608
130000
29 1843
22854
6
5
5
6
5
6
6
5
6
1 3 6
1 3 5
1 3 6
1 3 5
1 3 6
1 3 6
1 3 6
135
1 3 6
sand
sand
sand
pitch
asphalt
asphalt
sand
pitch
sand
pitch
5
5
9*
9
10*
9*
10i
$3.10
3.19
2.18
2.542
2.66
2.77
2.85*
3.002
2 24
3.88
3.13
2.77
2.43
2.292
2.692
"5"
5
5
3
5
0
5
. „. .
5
Iowa
Kentucky. . . .
10
8
9i
Maine
Mass
Minnesota. . . .
5
"5"
6
1 3 6
136
1 8
1 2 4
asphalt
pitch
pitch
pitch
9J
"9J"
10»
Mississippi. . .
Missouri
Greenwood. . .
Kansas City. .
Miles City. . .
25000
1 021
4500'
6
6
5
1 3 5
1 3 6
1 6
bitu.
sand
asphalt
5
5
3
11
11
9
2.422
2.95
3.202
Montana
New Jersey . . .
Jersey City. . .
11 891
5
1 3 5
sand
9J
3.00
New York....
Plattsburg . . .
Rochester. . . .
1 979
8392
6
6
1 3 6
1 3 6
sand
sand
3
5
10
10*
3.08"
3.32
S. Carolina. . .
Texas
Canada
Charleston. . .
Brownsville. .
Hamilton. . . .
Vancouver. . .
9000
29000
12000*
189 875
4
1 3 5
sand
5
8
2.72»
2.70
2.85
3.202
6
6
1 3 6
1 2J :5
10
10*
pitch
5
1 Including grading and concrete base 2 Exclusive of grading. 3 3-inch block * 20 Ib. per cu. ft.
The defects of a treated wood-block pavement are: 1. It is some-
what slippery, particularly when its surface is moist. In this respect
it is about on a par with sheet asphalt. 2. Its surface is liable in
hot weather to become covered with a sticky mass which adheres to
wheels of vehicles and tracks into houses (§ 1211). 3. It is rather
high in first cost.
1205. SPECIFICATIONS. The American Society of Municipal
Improvements in 1916 adopted complete specifications for Creosoted
Wood-block Paving; and substantially the same specifications have
been adopted by the American Wood Preservers' Association and
* Engineering and Contracting, Vol. 39 (1913), p. 380-81.
628
WOOD-BLOCK PAVEMENTS
[CHAP, xix
other associations interested in wood paving. Copies of these speci-
fications may be had for a nominal sum of the secretary of the first-
mentioned society.
ART. 3. MAINTENANCE
1206. The experience with treated wood-block paving has been
comparatively short, and hence there has not been developed any
general method of maintenance or repairs.
1207. DEFECTS TO BE REMOVED. The principal matters
requiring attention are: removing poor blocks, raising low spots,
re-laying over trenches, lowering bulges, removing exudation.
1208. Removing Poor Blocks. Blocks fail owing to defects in
the timber or to imperfect treatment. The latter do not usually
appear until after the pavement is several years old. Generally,
only a portion of a single block fails, and usually only a few blocks
in each city block. Fig. 234 shows the failure of a single block;
FIG. 234. — FAILURE OF A BLOCK ON WESTMINSTER PLACE, ST. Louis.
and Fig. 235 shows the failure of several blocks. Not infrequently
the failing blocks are in a bunch, indicating that there was prob-
ably something wrong with a single charge. The failure or decay
is ordinarily due to insufficient preservative in either the sap-
ART. 3]
MAINTENANCE
629
wood or the heartwood (see § 1167). At first the hole is small and
shallow, and does no great harm, although it gradually enlarges, par-
ticularly if there is much heavy steel-tired traffic. The defect can be
temporarily cured by filling the hole with bituminous joint-filler
Treated in 1903. Photographed in 1915.
FIG. 235. — FAILURE OF SEVERAL BLOCKS ON WESTMINSTER PLACE, ST. Louis.
or better with mortar or fine concrete made with bituminous cement.
The only permanent remedy is to cut out the defective block and
replace it with a good one, which can be done easily and quickly.
With a little care and attention a new block can be inserted so that
the patch is hardly visible.
1209. Raising Low Spots. Frequently shallow depressions appear
in the pavement. These holes may be due to the settlement of the
foundation (§ 1056), to the settlement of the soil in a trench (§ 1057),
or to the shrinkage or shifting of the sand cushion (§ 1055). Such
holes are objectionable because they are unsightly, particularly when
filled with water; and they hold water which dissolves the preserva-
tive, and also causes the blocks to swell and perhaps buckle. The
sinking of the blocks break the bond of the joint filler, particularly
if it is not bituminous; and may permit water to reach the founda-
tion, which if it freezes may lift the pavement. The remedy is to
take up the spot, remove the cause and re-lay the blocks. For a
630 WOOD-BLOCK PAVEMENTS [CHAP. XIX
discussion of precautions to be taken in re-laying a brick pavement
under similar conditions, many of which are equally applicable in re-
laying wood-block pavements, see § 1061.
1210. Re-laying over Trenches. It is frequently necessary to re-
lay a wood-block pavement over a trench on account of the settle-
ment of the soil in the trench or because a trench is opened to
repair or lay a pipe or sewer. For a discussion of the method of re-
laying a brick pavement over a trench, see § 1061.
1211. Lowering Bulges. A bulge or ridge is sometimes formed
in a wood-block pavement by the expansion due to the absorption of
moisture. If the pavem entis not provided with adequate longitudi-
nal expansion joints, the bulge may be longitudinally along the crown
of the pavement; or a bulge may take place at a raised footway
crossing or at the crown of an intersection pavement (see § 1060).
Usually a bulge can be replaced by removing a few blocks along the
crest of the bulge, pressing the adjoining pavement back to place, and
re-laying the blocks that were removed.
1212. Bleeding. In some cases the preservative fluid oozes out
of the blocks and forms a thick sticky mass on the surface of the
pavement, which is picked up by the wheels of passing vehicles and is
tracked into houses. When this occurs the pavement is said to
bleed. The bleeding may be due to one or more of the following
causes, viz.: 1. The expansion by heat of the air in the pores of the
block may force out the preservative. 2. The absorption of moisture
by one part of a block or one portion of the pavement may cause an
expansion which forces the preservative out at some other point.
3. Too much preservative may have been injected.
Steaming and the vacuum treatment of green blocks decreases the
bleeding by removing the air from the cells and by -reducing the
absorption of preservative in the sapwood; and the steaming of
seasoned blocks reduces bleeding by expanding the blocks to their
maximum size so that when laid they will be less likely to expand
by the absorption of moisture. The bleeding occurs only in hot
weather. The character of the preservative makes little or no dif-
ference in the amount of bleeding, many claims to the contrary not-
withstanding. Apparently a pavement bleeds less under heavy than
under light travel, partly because the traffic seals the pores of the
wood and prevents the oil from escaping, and partly because passing
wheels carry away the sticky materials as rapidly as it oozes out.
1213. The remedy for a bleeding pavement is to sprinkle it with
fine dry sand, and remove the sand after it has absorbed the bitumi-
ART. 3] MAINTENANCE 631
nous material. In extreme cases it may be necessary to apply a
second coat of sand. Usually the worst cases do not bleed after
the first year or two.
1214. COST OF MAINTENANCE. There is an unfortunate
dearth of data concerning the cost of repairs or of maintenance of
any pavement; but the lack is greater for treated wood-block pave-
ments than for any other kind, since the experience with such pave-
ments is comparatively limited (§ 1147), and since most such
pavements have been laid under a 5-year guarantee.
The following examples are from a report by George W. Tillson,
Engineer of the Borough of Brooklyn, New York City, presented at
the Third International Road Congress in London in 1913.*
Wood-blocks treated by the creo-resinate process (§ 1158) were
laid on Tremont Street, Boston, in 1900; and after the pavement
" had been in use 12 years it had cost absolutely nothing for repairs,
and was said to be in such a condition that it would probably remain
intact for 10 years longer. It is stated by the engineer in charge of
the Boston pavements that the same is true of 14,000 square yards
laid at about the same time and in the same way."
" In the Borough of Brooklyn, the first creosoted wood pave-
ment was laid in 1902, without any guarantee, and has cost abso-
lutely nothing for repairs. Pavements that were laid later and have
been out of guarantee from 3 to 4 years, have been kept in repair by
the Borough; and an accurate record kept of their cost. Some of
these pavements have cost absolutely nothing, and the average cost
for the entire area out of guarantee has been 1.05 cents per square
yard per year. Many of these pavements, however, have been
opened for sub-surface work; and the engineer in charge of pave-
ments states that in his opinion practically all of the repairs are due
to settlements over trenches and damage caused by fires, and not to
actual wear and tear of traffic."
" The Borough of Manhattan has three streets which have been
out of guarantee three years, one of heavy traffic, one of medium
traffic, and one of light traffic. The heavy traffic street has cost 7
cents per square yard per year, while the average of all has been
6 cents per square yard per year. But the repairs have been due to
wear and tear only on the heavy traffic street, which is a wholesale
street in the business section. Repairs on the other streets are due to
settlements over trenches, and damage caused by fire; and prac-
tically nothing to wear and tear of traffic."
* Engineering and Contracting, Vol. 40 (1913), p. 7-9.
632 WOOD-BLOCK PAVEMENTS [CHAP. XIX
" The City of Minneapolis, Minn., has 1,000,000 square yards of
wood-block pavements, the first of which was laid in 1902. The
City Engineer states that these pavements have required practically
no repairs, the cost in 1911 being less than -& cent per square yard.
He also states (in 1913) that the street paved in 1902 is in good con-
dition, and looks as if it might last for 10 years longer."
" In St, Louis, Mo., in 1909, the repairs to 50,000 square yards
of wood pavement laid in 1903 cost $2.10; and in 1911 these same
50,000 square yards cost less than A cent per square yard, so that
the total cost of repairing the 50,000 square yards of wood pavement
the first nine years they were laid was 3% cent 'per square yard.
These pavements are all on light traffic streets."
CHAPTER XX
SELECTING THE BEST PAVEMENT
1217. KINDS OF PAVEMENTS. Pavements have been con-
structed of a variety of materials; but the forms discussed in the
preceding chapters — hydraulic concrete, bituminous concrete, asphalt,
brick, stone block, and wood block — are the only ones of importance
now constructed; and it is improbable that any other paving material
of value will be introduced. From time to time notices appear in
the general newspapers of the introduction of some new pavement.
Among the new paving materials of which somewhat laudatory
notices have appeared are compressed hay, devitrified glass, cork, and
rubber. All such novelties are either an attempt of an eccentric
inventor to sell his goods, or a construction to meet limited and
peculiar conditions. For example, it has been stated that rubber
has been tried as a paving material in London; but the facts are that
it has been used only to the extent of 300 or 400 square feet in a hotel
porte cochere.
1218. Table 75, page 634, shows the number of miles and the
percentages of the different kinds of pavements in the 158 cities
having a population of over 30,000 in 1909. These data are the same
as those in the table on page 320.
It is interesting to note that (1) practically one half of all the
pavements in Table 75 are in the 16 cities having a population of
300,000 or over; (2) two thirds of the asphalt pavements are in cities
having a population of 300,000 or over, and that one third of this
amount is in New York City; (3) New York City, Indianapolis and
Minneapolis have more than one half of the creosoted wood-block
pavements; and (4) nearly one half of the water-bound macadam is
in cities having a population of over 300,000.
Table 76, page 635, shows the percentages of the different kinds
of pavements for three different dates in the larger cities of the
United States. These data are interesting as showing the progressive
633
634
SELECTING THE BEST PAVEMENT
[CHAP, xx
TABLE 75
PERCENTAGES OP DIFFERENT KINDS OF PAVEMENTS*
In 1909 in cities having a population of 30,000 or over
Ref.
No.
Kinds of Pavement.
Length, Miles.
Per Cent.
1
Asphalt-sheet
4293
20.4
0
block
261
1 2
3
Bitulithic .
192
0.9
4
Brick
2807
13.4
5
Cobble stone
5361
2 6
«
Concrete — Portland cement
25
0 1
7
G ravel — water-bound
2 556
12 2
Q
bituminous-bound
274
1 3
q
Ivlacadam — water-bound
6325
30 1
10
tar-bound.
142
00 7
11
12
portland-cement grouted
Stone block
16
2596
0.08
12 4
13
Wood block — creosoted
156
0 7
14
untreated
6142
2 9
it;
Other kinds
211
1 0
Total
21 004
100 0
Nearly half in Baltimore.
2 More than half in Chicago.
changes in the percentages of the different forms of pavements.
For example, note the increase in the percentage of asphalt pave-
ments, and the decrease in cobble-stone. Notice that no brick pave-
ments were reported separately in 1890. It is interesting to note
that the percentages of water-bound macadam and stone-block
pavements, the extremes as to durability, remained nearly stationary.
The increase in the total number of miles of pavements, is shown
below.
YEAR.
NUMBER OF CITIES.
CITIES HAVING POPU-
LATION OF OVER.
TOTAL MILES OF
PAVEMENT.
1890
1901
1909
262
135
158
10000
30000
30000
12453
15099
21004
Doubtless if Table 76 were brought up to date, there would
be some material changes. For example, cobble-stone pavements
would practically disappear, portland-cement concrete would greatly
increase, bituminous concrete (other than bitulithic) would appear
in the list, a considerable proportion of the water-bound gravel and
macadam pavements would change to bituminous bound, and un-
treated wood block would nearly disappear.
* Compiled from "General Statistics of Cities for 1909," Bureau of Census, Washington,
D. C.r 1913, p. 154~59.
ART. 1]
THE DATA FOR THE PROBLEM
635
TABLE 76
PERCENTAGES OF DIFFERENT KINDS OF PAVEMENTS AT DIFFERENT DATES
Ref
p
ERCENTAGE8 I
f
No.
18901
1901 2
1909 »
1
Asphalt-sheet {
3.2
13 6
21 6
2
3
block J '
Bitulithic
0 9
Brick
7 9
13 4
K
Cobble-stone
15 1
6 8
2 6
6
Concrete — portland cement.
0 1
7
Gravel — water-bound
31 0
14 7
12 2
g
bituminous-bound
1 3
9
Macadam — water-bound
27 0
30 6
30 1
10
tar-bound.
0 7
11
portland-cement grouted. .
0 08
12
Stone block.
12.1
13.4
12 4
13
Wood-block — creosoted
0.7
14
untreated.
7 8
8 7
2 9
15
Other kinds
2 9
4 3
1 0
Total
100 0
100 0
100 0
1 Compiled from "Social Statistics of Cities, Eleventh U. S. Census, 1890," p. 15-16.
2 Compiled from "Statistics of Cities, Bull. No. 36, U. S. Dept. of Labor, Sept., 1910,"
p. 876-79.
3 Compiled from "General Statistics of Citiea for 1909," U. S. Bureau of Census, Washing-
ton, D. C., 1913, p. 154-55.
ART. 1. THE DATA FOR THE PROBLEM
1219. DURABILITY OF PAVEMENTS. The durability or life of a
pavement is the most important factor in determining which is the
best pavement. The durability of a perishable paving material, as
untreated wood and to some extent macadam and asphalt, depends
upon both the climate and the traffic; but in general the durability
of paving materials depends chiefly upon the amount of the travel,
and consequently the durability of different pavements can be accu-
rately compared only when the nature and the amount of the travel
over each is known. Unfortunately there are very little definite
data as to the amount of travel upon American pavements. Not
infrequently the travel on a particular pavement is referred to as
being " heavy " or " light," but such general terms are practically
worthless in comparing the durability of different kinds of pavements.
1220. Travel Census. Although data on the use made of pave-
ments are of vital importance in attempting to compare the relative
durability of different paving materials, comparatively few obser-
vations have been made concerning the travel upon American pave-
636 SELECTING THE BEST PAVEMENT [CHAP. XX
ments. For a discussion of the causes that have led to this surprising
result, see §640-42 (page 321-24). For a statement of the im-
portance of a travel census in considering the cost of construction
and maintaining a road or pavement, see § 29 (page 25). For a brief
account of some observations made concerning the nature and
amount of the travel on rural roads, see § 30-33 (page 26-28); and
for a brief reference to the few censuses that have been taken of travel
on American streets, see § 34 (page 28).
In some respects the most elaborate census of street travel taken
in this country was that made by the Barber Asphalt Paving Com-
pany in 1885. Table 77, page 637, shows the results. It is not worth
while to describe the methods employed in making the observations
or in computing the results, since the data are very greatly out of
date owing to radical changes in both the character and the amount
of the travel. For example, in St. Louis from 1914 to 1915, the total
travel on certain business streets increased 20 per cent, the motor-
driven traffic increasing 53 per cent and the horse-drawn decreasing
15 per cent.* However, apparently the data in Table 77 are the
most elaborate that have yet been published. Table 77 is instruc-
tive as showing the great variation in the travel on different streets
of any particular city and also of different cities.
1221. Table 78, page 638, gives the travel record of certain streets
in London and Liverpool. The marked difference in the travel on
the pavements of London and on those of New York is due chiefly
to the use of omnibuses in London and street cars in New York City.
This example illustrates the importance of having definite data as
to the amount of travel; and also shows the importance of taking
account of local conditions in attempting to compare the results in
one city with those in another.
1222. It is desirable that engineers in charge of streets should
ascertain by direct observation the amount of tonnage passing over
each pavement, in order that the service per unit of cost of different
pavements may be accurately compared. The only measure of the
durability of a pavement is the amount of travel tonnage it will bear
before it becomes so worn that the cost of replacing it is less than the
expense incurred by its use. It is also desirable that all such observa-
tions should be made in accordance with a standard plan, so the
results from different cities will be comparable (see § 35-38).
1223. Elements Modifying Durability. Although the effect of
* Engineering News, Vol. 76, (1916), p. 832-34.
ART. 1]
THE DATA FOR THE PROBLEM
637
TABLE 77
TRAVEL ON CERTAIN STREETS IN VARIOUS AMERICAN CITIES IN 1885*
Ret.
No.
LOCALITY.
«!
"o'l
JS C
-S g
s
NUMBER OF TONS.
City.
Street.
Total
per Day.
Per
Vehicle.
II *
fc&°
£*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
New York . .
u it
<t a
Philadelphia
«
Chicago ....
It
U
l(
Boston
Broadway, near Pine
Fifth Ave., opp. Worth Monum't.
Wall, corner of Broad.
40
40
27
65
65
26
50
45
36
38
27
32
26
39
26
24
36
50
36
36
36
40
30
70
50
50
60
56
42
38
50
61
36
35
63
60
10 905
3744
2357
9237
6302
1928
7561
6398
2756
2604
5301
5028
3265
2938
1 130
744
3691
3618
2554
942
259
6204
1065
4622
1 688
1455
1289
2613
1505
825
714
4176
2402
977
2967
1 449
1.39
.68
1.00
1.52
1.24
1.06
2.08
1.46
.90
1.11
.99
1.02
.93
.80
.79
.67
1.13
1.23
1.16
.90
.84
1.81
.94
1.02
.87
.88
1.01
.83
1.88
1.47
1.24
1.25
1.05
1.05
.62
.59
273
94
87
142
97
74
151
142
77
69
196
157
126
75
43
32
103
72
70
27
7
155
35
66
34
29
21
47
36
22
14
69
67
28
47
24
Broad, in front of P.R.R. Station
Filbert, in front of City Hall. . . .
Chestnut, corner of Fourth
Wabash, near Lake
Clark, near Madison. .
La Salle, near Locust
Dearborn, opp. Washington P'k .
Devonshire, opp. Post Office ....
Devonshire, near Milk
(t
Kilby, near State
u
ii
St. Louis.. . .
« «
« K
(t a
u «
New Orleans
u u
Washington.
(i
Buffalo. . .
Washington
Arch, near Summer
Court Square
Locust, near Beaumont
Broadway, near Olive
Pine, near Garrison.
Chestnut, near Beaumont
Olive, near Beaumont
Tchoupitoulas, near Poydras. . . .
St. Charles, near Washington . . .
15th, opposite Treasury
9th, between D and E
7th, between D and E
6th, between Pa. Ave. and B. . . .
Main, near Swan
«
Main, near Bouck Ave
«
it
Louisville. . .
u
(I
Omaha
«
Lin wood, near Ferry.
Main, near Glenwood
Main, near 3d
8th, near Walnut
7th, near Jefferson
Douglass, near 15th.
Farnham near 14th
* Trans. Ainer. Soc. of Civil Engra., Vol. 15 (1886), p. 123-38.
638
SELECTING THE BEST PAVEMENT
[CHAP. XX
TABLE 78
TRAVEL ON CERTAIN STREETS IN LONDON AND LIVERPOOL IN 1879*
Ref.
No.
LOCALITY.
PAVEMENT.
NUMBER OF TONS.
City.
Street.
Kind.
a
51
3fe
£
Per Day.
Per
Vehicle.
ii*
*|a
r*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
London. . .
(i
u
n
it
(i
a
K
ti
n
n
((
u
u
Liverpool. .
«
Gracechurch
King William
Poultry
Asphalt
Wood
Asphalt
Wood
Macadam
Wood
Asphalt
Macadam
Granite
Wood
Granite
u
Macadam
Wood
Macadam
Wood
Granite
ti
Wood
a
32
40
22
37
45
57
32
30
37
44
'32
43
52
13507
16484
8330
13596
14380
17076
9260
7588
9358
10658
6 506
8376
9668
1.11
1.06
1.02
.84
1.01
1.01
.98
1.08
.87
.88
i ' 02
1.02
.90
422
412
378
367
322
300
290
253
253
242
216
203
195
186
156
145
93
382
232
231
100
Strand and Fleet. . .
Parliament
Oxford
Cheapside
Leadenhall
Piccadilly
Euston
Brompton
King William
Edgeware
Regent.
King's
Victoria
Sloane
40
5780
.96
(Not named)
U U
Great Howard. . . .
Bold
travel is dependent chiefly upon the number of tons per foot of width,
its influence is modified somewhat by (1) the character of the pave-
ment, (2) the state of repairs, (3) the degree of cleanliness, (4) the
presence or absence of car tracks, (5) the width of pavement, (6)
the character of the travel, and (7) the climate.
1. The durability of a particular kind of pavement will vary
with the details of the method of construction. The foundation
may be more qr less rigid, the materials may differ greatly in dura-
bility, with any form of block pavement the joints may be more or
less open, and the surface may also vary more or less in roughness.
2. The durability will depend upon the care employed in repairing
the pavement. If holes, depressions, or ruts are allowed to remain
for any length of time, whatever the material the pavement will
wear abnormally fast.
3. The degree of cleanness will materially modify the durability
* Trans, Amer. Soc. of Civil Eng'rs, V<?1. 15, (1886), p. 131,
ART. 1] THE DATA FOR THE PROBLEM 639
of a pavement. An imperishable material is benefited by a cov-
ering of detritus, since it serves as a carpet to protect the pave-
ment; and if the covering is heavy enough, the pavement virtually
becomes a foundation and is entirely protected from wear. On the
other hand, the decay of a perishable material, as wood and asphalt,
is hastened by a covering of street dirt which collects moisture and
hastens the decay and disintegration of the pavement.
4. The presence of a street-car track on a street concentrates
traffic at the two sides, thus virtually narrowing the street, and also
causes the travel to go substantially in one track, a result which
is particularly destructive of gravel and macadam roads.
5. The wider a pavement the more evenly will it wear, and conse-
quently the longer it will last. If several irregular lines of travel can
be maintained, the wear will be much more even and the durability
greater than if the vehicles are restricted to practically a single line.
6. The durability of the pavement will vary with the weight per
unit width of tire, the method of shoeing the horses, and the rapidity
of the travel. In Europe, the weight per unit of width of tire is
generally regulated by law, and calks on the horses' shoes are pro-
hibited ; but in America there are no such restrictions. Rapid travel
is more destructive to a block pavement than slow travel.
7. The climate affects the durability of several kinds of pave-
ment. The durability of an untreated wood pavement is affected by
heat and moisture conditions, that of macadam and gravel by mois-
ture and winds, and that of asphalt by moisture, particularly by
street sprinkling.
1224. There are two facts of a somewhat different character
that should not be overlooked in attempting to determine the life of
a pavement.
1. The average wear does not determine the life of a pavement,
since even the most carefully constructed pavements wear so unevenly
as to require re-laying before the wearing coat is entirely worn out.
This is true of sheet asphalt and gravel and macadam (both water-
bound and bituminous), which have a comparatively thin wearing
coat; and is particularly true of pavements made of blocks, as brick,
stone and wood, since the edges of the blocks wear off and leave
the top face rounded, and when the pavement reaches this stage the
wear is much more rapid than previously.
2. In a block pavement the blocks must have a certain depth to
enable them to keep their place; and consequently bricks and shallow
wood blocks can not be worn more than about half-way through.
640
SELECTING THE BEST PAVEMENT
[CHAP. XX
If the blocks are made deeper, the durability of the pavement is
not increased much, if any, since owing to unequal wear the pave-
ment must be re-laid before any considerable depth is worn off.
Asphalt and macadam have some decided economic advantages
over other forms of pavements, since the wearing coat is compara-
tively thin and can be replaced when it is worn out or wears rough,
without proportionally as much loss as when a block pavement is re-
surfaced. A further economic advantage of these pavements is
that when holes begin to form, a patch may be applied and thus the
uniformity of the surface may be preserved and the life of the pave-
ment be extended.
1225. Data on Life of Pavements. Until more complete data
concerning the volume of travel on pavements and the amount of
wear are obtained, it will be impossible to make any reliable estimates
as to the durability of different paving materials. At present the
best that can be done is to accept the estimate of those most com-
petent to give an intelligent opinion.
1226. Asphalt, Brick, and Wood. Table 79 gives the estimated
life of sheet asphalt, brick, and wood-block pavements as reported
to the U. S. Census Bureau. An extensive list was given for asphalt
TABLE 79
ESTIMATED LIFE OF PAVEMENTS*
Ref.
No.
City.
SHEET ASPHALT.
BRICK.
WOOD-BLOCK.
fl*
c 2
fS
W
SS
c'E
o -^
fi
K
i|
si 2
|S
PQ
11
dj -*i
;§•
'S5 Q
tf
i!
o 2
fa
PQ
§i
c'C
v +=
Is
PH
1
2
3
4
5
6
• 7
8
9
10
11
12
13
14
15
16
New York City, N. Y
Boston, Mass
12
10
15
15
14
12
10
14
20
14
25
11
5
"l6"
11
10
25
25
15
11
9
13
"l7"
15
10
10
8
"30"
25
15
20
20
30
18
15
30
20
Cleveland O
Indianapolis, Ind
10
5
11
10
11
15
12
12
10
10
18
12
7
Portland, Ore
Columbus, Ohio.
18
10
12
18
25
25
Toledo, Ohio.
Atlanta Ga
Oakland, Calif
Cambridge, Mass
Springfield, Mass
Holyoke, Mass.
12
15
15
20
12
ii
12
18
"25"
20
10
15
18
20
10
South Bend, Ind
Saginaw, Mich .
Sacramento, Calif
Galveston, Tex
11
> * General Statistics of Cities for 1909, Bureau of the Census, 1913, Washington, D. C.
p. 69-70.
ART. 1] THE DATA FOR THE PROBLEM 641
and brick, but only a brief list for wood-block; and only those in
the former list are presented in Table 79, for which data were
given also for wood-block.
Of course, the life of a pavement will depend upon the specifica-
tions and the details of the construction, which can not be pre-
sented in a tabular statement. Further, the life of a pavement
depends upon the extent and character of the annual repairs. It is
possible to keep some types of pavements, as for example sheet
asphalt, going almost perpetually by patching; while with other
forms, as for example brick, it is not possible to greatly prolong the
life of the pavement by patching.
1227. Granite. The census report from which the data in Table
79 were taken, contained no statistics for the life of stone-block pave-
ments. The following data to some degree supply that lack.
Mr. Samuel Whinery, a competent authority, estimates the life
of granite-block pavements in Boston as follows : *
A, central business streets having a very large volume of the heaviest
travel 13 years
B, secondary business streets with travel of moderate volume and weight . 17 years
C, main city or suburban thoroughfares having a large volume of com-
paratively light travel .16 "
D, suburban or residence streets where the travel is mainly of a local
character and light 20 "
Mr. George W. Tillson, Consulting Engineer, Borough of Brook-
lyn, estimates the life of a granite-block pavement at 25 years, f
The Bureau of Municipal Research of Cincinnati, Ohio, estimates
the life of granite blocks at 25 years.f
1228. Water-bound Macadam. Macadam varies in quality more
widely than any of the preceding forms of pavements, and hence
there is a wider range in its estimated life.
Mr. Whinery estimates the life of trap-rock water-bound mac-
adam for the four classes of streets mentioned in the second para-
graph of § 1227, as follows:
A, 5 years; B, 6 years; C, 7 years; and D, 10 years.
The Bureau of Municipal Research of Cincinnati estimates the
life of limestone water-bound macadam at 8 years, t
* Report to the Finance Committee of Boston, Mass., Engineering and Contracting, Vol. 73
(1910), p. 30-31.
t Paper presented to International Municipal Congress, Chicago, Engineering and Con-
racting, Vol. 36 (1911), p. 405.
t Municipal Engineering, Vol. 42 (1912), p. 459.
642 SELECTING THE BEST PAVEMENT [CHAP. XX
1229. REQUIREMENTS OF AN IDEAL PAVEMENT. The perfect
pavement is an ideal which will never be attained, since some of
the qualities required in a perfect pavement are antagonistic to
each other. For instance, perfect durability would require a pave-
ment without friction, for friction causes wear and ultimately destruc-
tion of the pavement; but without friction there could be no adequate
foothold for horses drawing loads. Again, to be the least injurious
to horses, a pavement should be soft and yielding; but a soft and
yielding pavement is opposed to ease of traction. The conditions to
be fulfilled by the ideal pavement will first be considered; and sub-
sequently an attempt will be made to estimate the degree to which
each kind of pavement approximates the perfect ideal.
A perfect pavement should satisfy the following conditions:
1. It should be low in first cost.
2. It should be durable, i. e., the cost of perpetually maintaining
its surface in good condition should be small.
3. It should have a smooth, hard surface, so as to have a low
tractive resistance.
4. It should afford a good foothold to enable horses to draw
heavy loads, and to prevent them from slipping and falling and
possibly injuring themselves and blocking traffic.
5. It should be smooth, so as to be easily cleaned.
6. It should be comparatively noiseless.
7. It should be impervious, so as to keep in good sanitary condition.
8. It should yield neither dust nor mud.
9. It should be comfortable to those who ride over it.
10. It should not absorb heat excessively.
Each of the ordinary forms of pavements will be considered under
each of the above requirements.
1230. Cost of Construction. The cost of construction of a pave-
ment varies with the specifications, the character of the work, and
the locality. For detailed data on this subject, see the several kinds
of pavements in the preceding chapters.
Since the subject under consideration in this chapter is a com-
parison of the different forms of wearing coats, the cost to be con-
sidered here is that of the pavement exclusive of the expense for
curbs, gutters, catch-basins, etc. To have data for use in sample
computations later in this chapter, the cost of the best grade of each
of the several kinds of pavements is assumed to be as follows:
1. Asphalt, sheet $1 . 75 per sq. yd.
2. Brick 1 .80 " " "
ART. 1] THE DATA FOR THE PROBLEM 643
3. Granite block 3 . 00 per sq. yd.
4. Macadam, water-bound 1 . 25
5. Wood block, creosoted 2.75
1231. Cost of Maintenance. By the cost of maintenance is
meant the expenditure necessary to keep the pavement in practically
as good a condition as when it was new. Unfortunately there is a
greater dearth of exact information under this head than for almost
any other phase of pavement construction. For a general state-
ment of the causes of this lack, see § 640^3 (page 321-26) ; and for a
more detailed explanation why there is little accurate information
on the cost of repairs of asphalt pavements, which reasons substan-
tially apply also to all kinds of pavements, see § 881-82 (page 455-57).
The cost of maintenance consists of two distinct elements, viz.:
(1) the cost of repairing or patching of small areas; and (2) the cost
of renewing or re-surfacing large areas. Each will be considered
separately.
1232. Cost of Repairs. The cost of repairs is here used as mean-
ing the cost of correcting any defects that may have developed
through use. Not infrequently this term is used to include also the
expense of re-laying or restoring the surface of the pavement after
it has been opened to lay or repair sewers, water or gas pipes, etc.
In one sense the cost of restoring the surface after the pavement has
been opened is part of the cost of maintenance; but the cost of
restoring the surface is practically independent of the quality or life
of the pavement, and hence should not be included in comparing the
economic value of the different pavements.
Unfortunately, not many municipalities keep accurate accounts
of the cost of repairing and restoring pavements, and very few
separate the costs of patching and restoring; and hence there are
almost no reliable data on the cost of repairs.
Further, many of the accounts purporting to show the cost of
repairs contain one or both of the two serious errors discussed in the
two succeeding sections.
1233. REPAIRS vs. NEW CONSTRUCTION. In computing the cost
of repairs of roads and pavements, it is common to fail to discrim-
inate between the renewal of an old surface and the substitution
of a new and better surface; or, in other words, it is common to
charge to maintenance an item which should be charged to new
construction (see paragraph 6, § 881, page 455). For example, if
some form of bituminous wearing coat is applied to an old water-
bound macadam road, the new surface is not a renewal of the old one,
644 SELECTING THE BEST PAVEMENT [CHAP. XX
but is the construction of a new pavement surface upon the old one
as a foundation; and consequently the cost of the new construction
should not be regarded as the cost of repairs to the old pavement.
This mistake in almost the exact form of the above example has
often been made in discussions of highway economics. For example,
it is often stated that the cost of road maintenance has recently
taken an enormous leap upward due to the advent of the automo-
bile; while the truth is that the advent of the automobile has caused
the substitution of a more expensive form of road construction, and
little or nothing is known as to the cost of maintenance of the new
form of construction, partly because the methods of both construc-
tion and maintenance have been experimental, and partly because
not enough time has elapsed to secure trustworthy data.
Because of the above error, many estimates of the cost of
annual repairs are radically wrong, as also the resulting conclusions.
Errors of this character have for somewhat obvious reasons been
more common in connection with surfaces suitable for light travel
than for heavy travel.
1234. AGE OF PAVEMENT. Sometimes an error is made by not
duly considering the age of the pavement. During the first part of
the life of a pavement, there should be but few, if any, repairs;
and hence to secure a trustworthy value for the annual cost of repairs,
an average should be taken for a number of years, the exact number
being proportional to the life of each particular pavement.
Obviously, the greater the area of the pavements included and
the greater the number of years the better; but it is not correct to
simply take an average of the annual cost of repairs for each of sev-
eral pavements of different ages and areas. Assume that the areas
and ages of the several pavements are as below; that the total cost
of repairs for each pavement is known ; and that the correct average
annual cost of repairs is desired.*
AREA, Sq. Yd. AGE, Years. YARDS XYEARS.
10 000 16 160 000
8 000 12 96 000
7 000 10 70 000
8 000 8 64 000
33 000 390 000
Multiplying the area by the corresponding age gives the number
of yard-years that have been repaired, i. e., gives the equivalent of
* Engineering and Contracting, Vol. 37 (1912), p. 311,
ART. 1] THE DATA FOR THE PROBLEM 645
the number of yards that have been maintained for 1 year. The
total cost of repairs divided by the total yard-years (390,000) will
give the correct average annual cost of repairs. The weighted average
age of the pavement in the above example is: 390,000 -j- 33,000 =
11.8 years.
1235. DATA ON COST OF REPAIRS. For a few data on the cost
of the different kinds of pavements, see the preceding pages, as
follows: Sheet asphalt, Table 47, page 457, and Fig. 162, page 459;
brick, § 1069, page 564; wood block, § 1214, page 631.
Table 80 shows the best general data available, although it is
not certain that these values, or the ones referred to in the paragraph
above, are free from the two errors mentioned in the two pre-
ceding sections. Table 80 is given primarily to have data for use in
sample computations to be presented later.
TABLE 80
ASSUMED AVERAGE ANNUAL COST OF REPAIRS
Kind of Pavement.
CENTS PER SQ. YD. PER ANNUM.
Heavy Travel.
Light Travel.
Asphalt sheet, — see § 886
5.0
3.0
10.0
2.0
4.0
3.5
2.0
2.0*
2.0
2.0
Brick,— see § 1069
Macadam, water-bound
Stone block, — see § 1142
Wood block, creosoted, — see § 1214
1236. Cost of Renewal The second element in the cost of main-
tenance of a pavement is the cost of renewal. Since the foundation
or base of a modern pavement is not subjected to wear or disinte-
gration, the cost of renewal is only the cost of adding a new wearing
coat; although, not infrequently, the cost of a new concrete base is
erroneously included in the cost of renewal (see § 1233).
The annual cost of renewal is that sum which each year must be
placed at compound interest to accumulate a sum equal to the esti-
mated cost of renewal at the end of the life of the wearing coat of the
pavement. As an example, it will be assumed that the cost of
renewal is desired for a sheet asphalt pavement. It will be assumed
that the pavement costs $1.75 per square yard (§ 1230), and that the
grading and concrete base cost $1.00 and the wearing coat $0.75.
The latter is a little less than the pro rata share according to Table 45,
* Report Dept. pf Public Works, Bureau of Engineering, Buffalo, N. Y., 1915-16, p. 70.
646 SELECTING THE BEST PAVEMENT [CHAP. XX
page 453; but round numbers are sufficient for an example. The
life of the wearing coat will be taken as 15 years, which is probably
too small, notwithstanding the data in Table 79, page 640. It will
be assumed that the life of the concrete base is 30 years, which is
probably too small; and that its removal will cost 20 cents per square
yard. Interest will be assumed at 3J per cent. It will be further
assumed that the removal of the old wearing coat will cost 10 cents
per square yard. Finally, to find the annual sum to be placed at
interest, a sinking fund table is desirable.
From a sinking-fund table it is found that $0.05182 deposited
annually at 3J per cent interest will amount to $1.00 in 15 years;
and that at the same interest $0.01937 will amount to $1.00 in 30
years. Then the annual cost of renewal is as follows :
TTFM ANNUAL COST
Cts. per Sq. Yd.
Concrete base: renewal, $0.75 X 0.01937 1 . 452
removal, 20 cents ^30 0 . 667
Wearing coat: renewal, $1.00 X 0.05182 5. 182
removal, 10 cents -*- 15 0 . 667
Total 7.968
In some cases there is a salvage value to the old wearing coat, in
which case its value after removal divided by the life of the pave-
ment should be subtracted from the annual cost found as above.
1237. Economic Life of a Pavement. When does the cost of
repairs become great enough to justify renewal, that is, what is the
economic life of a pavement? This subject has frequently been dis-
cussed by writers on road and pavement economics, and generally
the method employed has been radically wrong. For an elaborate,
correct, and instructive discussion of this subject, see Gillette's
Hand-book of Cost Data, Second Edition, page 27-34.
1238. Tractive Resistance. Table 8, page 21, gives the tractive
resistance of different pavements, from which it is seen that the
rank of the various pavements according to tractive resistance, in
order beginning with the one offering the least resistance, is about as
follows: portland-concrete, sheet asphalt during cold weather, brick,
best water-bound macadam, asphalt during warm weather, rectan-
gular wood block, good gravel, accurately dressed stone block,
ordinary water-bound macadam, gravel, roughly dressed stone block.
The tractive resistance will vary greatly with the state of repair of
the surface.
Many attempts have been made to compute the financial advan-
ART. 1]
THE DATA FOR THE PROBLEM
647
tage of a decreased tractive resistance; but it is impossible to deter-
mine its value with any degree of accuracy, although it is certain
that the tractive resistance of the pavements of a city are impor-
tant factors in determining the cost of conducting transportation.
Ease of traction is, however, not relatively as important for city
pavements as for country roads, since in the latter ease of traction
is a matter of first importance (see § 4-7), while in the former it is
comparatively unimportant (see § 634). On the other hand, the
cost of transportation per ton-mile is considerably more in the cities
than in the country.
1239. Slipperiness. The method of comparing pavements in
this respect is to determine the distance a horse travels on the dif-
ferent pavements before he falls. The most complete observations
made in the United States to ascertain the prevalence of accidents on
the different pavements were made under the direction of Capt.
F. V. Greene.* The observations were made from 7 a.m. to 7 p.m.
on six consecutive days in October and November, 1885, in ten of the
leading American cities on thirty-three streets having the heaviest
travel for each kind of pavement in the particular city. The number
of horses observed on sheet asphalt pavement were 360,254, on old-
style granite block 376,384, and on wood 70,914; and the number
of miles traveled by the horses while under observation was 41,427
on the asphalt pavements, 34,723 on the granite, and 4,901 on the
wood. A summary of the results is shown in Table 81.
TABLE 81
MILES TRAVELED BY A HORSE ON AMERICAN PAVEMENTS BEFORE AN ACCIDENT
OCCURS
Observations made in 1885
Ref.
No.
Kind of Pavement.
Fall on
Knees.
Fall on
Haunches.
Complete
Fall.
Accident
of Any
Kind.
1
Asphalt, sheet
1 534
2 180
1 647
KQQ
2
( Imnite block — old style
510
5 954
3 472
41 Q
3
Wood1.
408
983
4 QOI
979
1 The kind of wood-block is not stated, and apparently it can not now be determined.
These data are very much out of date, and are not of much value,
since the character of the prevailing forms of construction has
materially changed; but nevertheless Table 81 contains the only
* Trans. Amer. Soc. of Civil Engineers, Vol. 15 (1886), p. 123-28.
648 SELECTING THE BEST PAVEMENT [CHAP. XX
definite data on record for American pavements. No observations
similar to the preceding have been made for brick pavements; but
it is probable that they are less slippery than asphalt, wood-block, or
stone-block. It is certain that modern stone-block pavements, i. e.,
those with comparatively small blocks and narrow joints filled with
bituminous or hydraulic cement, are less slippery than the old-style
stone-block pavement upon which the observation in Table 81 were
made.
1240. The above observations relate to the slipperiness for a
horse; but slipperiness is nearly as important for an automobile as
for a horse. However, no systematic observations have been made
as to the effect of slipperiness of pavements upon automobile
traffic.
1241. Conclusion. It is generalty conceded that wood-block
pavements are the most slippery, sheet asphalt next, brick next, and
then granite.
1242. Investigation in Progress. In 1916 the State Highway
Commission of Massachusetts built ten sections of pavements pri-
marily to determine their relative slipperiness. Incidentally it is
expected that the experiment will demonstrate other things of
interest.*
The test sections were built on Washington Street, Boston, on a
4 per cent grade, and each is 500 feet long. The surfaces of the
several sections are those ordinarily employed for rural and suburban
roads rather than those used on city streets; and consist of two of tar
macadam (§ 604), two of asphalt macadam (§ 604), two of tar con-
crete (§ 604), one of asphalt concrete (§ 604, 891, and 901), one of
hydraulic concrete (Chap. VII), one of tar and sand (§ 614), and one
of Topeka asphalt mixture (§ 897-98). Each of the first three forms
of pavements was laid with a " rough " and a " smooth " surface.
The different materials are subject to the same conditions, and
hence are strictly comparable; and the same materials were used in
the several sections, and hence the methods of construction are
strictly comparable. The only noteworthy conclusions that have
been announced relate to the durability of the several forms of con-
struction; and no decision has been reached as to the relative slip-
periness of the different surfaces.
1243. Ease of Cleaning. The facility with which a pavement
may be cleaned is an important matter both economically and
* Engineering News, Vol, 76 (1916), p. 1162,
ART. 1] THE DAtA *Cfc THE PROBLEM 649
esthetically. Col. Geo. E. Waring, noted for his service as Street
Cleaning Commissioner of New York City, in 1896 estimated that
if all the streets of New York City were paved with asphalt where the
grades would permit, the cost of street cleaning would be reduced
from $1,200,000 to $700,000 per year. At that time New York had
431 miles of pavement of which 94 were asphalt, and the above annual
saving is equal to 3 per cent of the cost of laying asphalt pavements
upon all of the streets not already asphalted.
Sheet asphalt and hydraulic concrete pavements are most easily
cleaned, and next in order are : wood blocks with close joints, asphalt
blocks, brick with joints filled with hydraulic cement, accurately-
dressed stone blocks with cement joints, and old-style stone blocks.
Macadam and gravel are smooth and for this reason are easily
cleaned; but their surfaces, particularly if water-bound, grind up
into powder under dense or heavy travel, and for this reason there is
considerable detritus to be removed, a fact which adds to the expense
of cleaning.
1244. Noiselessness. The noise made by travel upon a pave-
ment has an important effect upon the comfort and health of the
people using the pavement or living adjacent to it. A quiet pavement
is particularly desirable adjacent to office buildings, schools, churches,
hospitals, etc.; and the noise of travel upon a rough pavement
aggravates, if it does not cause nervous disorders.
On sheet asphalt and hydraulic concrete, and well-grouted brick,
the only noise is the sharp click of the horses' shoes; and on asphalt
block and re-pressed brick filled with tar or grout, there is the click
of horse's shoes and a slight rumbling of the wheels passing over the
joints. On well-dressed granite blocks filled with hydraulic cement,
there is a considerable rumbling due to the passage of heavy steel-
tired vehicles; and on the old-style granite block with sand-filled
joints, there is a deafening roar due both to the rumbling of the
wheels and to the blows of the horses' shoes. Upon wood pavements
the horses' feet produce no noticeable noise; while the wheels make a
dull rumbling noise, but not loud enough to be seriously objection-
able. Macadam and gravel are more quiet than wood.
In order of their freedom from noise, pavements rank about as
follows: wood-block having the joints filled with tar or grout,
sheet asphalt, asphalt block, asphalt concrete, hydraulic concrete,
square-edged brick having grouted joints, re-pressed brick having
joints filled with tar or grout, accurately dressed stone blocks having
joints filled with grout, and old-style stone block. Bituminous gravel
650 SELECTING THE BEST PAVEMENT [CHAP. XX
and macadam are nearly as quiet as sheet asphalt, and water-bound
gravel and macadam are not seriously noisy. However, the freedom
from noise on any pavement depends greatly upon the care used in
construction and maintenance.
1245. Healthfulness. The effect of a pavement upon the health
of the residents in its locality will depend upon the tendency of the
materials composing it to decay, and also upon its permeability.
The healthfulness of a pavement was much more important formerly
than at present. The form of pavements that were most unhealth-
ful have gone out of use. These are: cobble-stone and stone-block
pavements having joints filled with pebbles, cylindrical wood-blocks
with sand-filled joints, and stone-block having wide joints filled
with sand or pebbles. The difference in healthfulness of the best
modern pavements is negligible.
1246. Freedom from Dust and Mud. The materials of an
ideal pavement should not grind up and make dust in dry weather
or mud in wet weather. The dust and mud not only add to the
expense of cleaning the pavement, but are a discomfort to those who
use the pavement and to those who live or do business adjacent to it.
1247. Comfort in Use. If the pavement is to be used for pleasure
driving, the comfort of the users must be considered; and therefore
the pavement should have a smooth surface which is free from dust
when it is dry and free from mud when it is wet.
1248. Temperature of Pavements. During hot weather, there
is frequently complaint that one pavement reflects or radiates more
heat than another. Observations made in Washington, D. C., when
the temperature of the air 2 feet above the pavement was 104° F.,
showed the temperature of three pavements to be as follows: sheet
asphalt 140°, asphalt block 122°, and macadam 118°.* Observa-
tions in Boston, when the temperature of the air in the shade was
98° F., gave the temperature of four pavements as follows: wood
block 124|°, granite block 115°, sheet asphalt 113°, and macadam
102J°. The observations are not conclusive as to the relative tem-
peratures of different pavements, but show that there is no very
great difference between the several kinds. The temperature of the
pavement depends upon its color, which varies with the material.
* Proc. Amer. Soc. Municipal Improvements, Vol. 5, p. 161.
ART. 2] THE SOLUTION OF THE PROBLEM 651
Art. 2. THE SOLUTION OF THE PROBLEM
1250. The problem of selecting the best pavement for any partic-
ular case is a local one, not only for each city but also for each of the
various parts into which the city is imperceptibly divided; and
involves so many elements that the nicest balancing of the relative
values for each kind of pavement is required to arrive at a correct
conclusion.
There are two methods that may be employed in deciding which
is the best of several pavements. One method assumes that the
selection should be made upon economic grounds alone, in which case
the best pavement is that for which the total annual expense is a
minimum. The other method assumes that the decision should be
based upon other factors beside the economic features. Each of
these two methods will be considered; and for convenience the first
will be called a Problem in Economics, and the second a Non-economic
Problem.
1251. PROBLEM IN ECONOMICS. As a problem in economics,
the selection of the best pavement consists in finding that form of
pavement, or wearing course, for which the total annual cost is least.
The annual cost consists of (1) interest on the cost of construction,
(2) the annual cost of repairs, (3) the annual cost of renewal, (4)
the annual cost of cleaning, (5) the annual cost of conducting the
transportation the pavement carries, and (6) the annual cost of
sprinkling, — when that is necessary.
The first three items of cost have already been considered in
Art. 1. The last three items of expense have not been considered;
but will now be briefly discussed.
1252. Cost of Cleaning. Some forms of pavements require
sprinkling for economic maintenance, as for example water-bound
macadam; and some types require sprinkling for the comfort of the
users and of those living adjacent to it.
The cost of cleaning depends upon the amount and character of
the travel and also upon the smoothness of the pavement, and more
upon the former than the latter. The difference in cost of cleaning
the different forms of modern pavements is not great. For example,
in 1895-97 in New York City it was determined that the relative
ease of cleaning different pavements was as follows: asphalt, brick
and rectangular wood-block, each 100; granite block, 150; Belgian
block, 160; and cobble-stone, 400, But since the date of that inves-
652 SELECTING THE BEST PAVEMENT [CHAP. XX
tigation, cobble-stone pavements have been practically eliminated,
and Belgian blocks nearly so; while the prevailing type of con-
struction of granite-block pavements has changed so that now
they are practically as smooth as brick pavements. Hence, if the
cost of cleaning is omitted, it will not materially affect the con-
clusion as to the relative economic merits of the different pave-
ments.
1253. Cost of Transportation. At first thought, it does not ap-
pear that the cost of conducting the transportation over a pavement
is part of its annual cost; but really the cost of the transportation is a
part of the cost of operating the pavement, and hence is a part of its
annual cost. The cost of construction is usually paid by the abutting
property holder, and ordinarily he pays also the cost of repairs and
renewals; the city usually pays the cost of cleaning; and the owners
of the horses and wagons and of the motor cars pay the cost of trans-
portation.
Modern pavements are so nearly alike in the smoothness and
hardness of their surfaces that there is no material difference in the
cost of conducting transportation on them. Further, the transpor-
tation is conducted by so many different parties under so many
different conditions, that it is practically impossible to determine
its cost with any degree of accuracy.
Therefore, for lack of the requisite data, it is necessary to omit
the cost of transportation from the summary of the annual cost of
the pavement; but this omission does not materially affect the con-
clusion as to the most economical pavement.
1254. Cost of Sprinkling. Water-bound macadam is about
the only pavement that requires sprinkling, either for maintenance
or for the comfort of users of the pavement or of those living adja-
cent to it. The cost of sprinkling will vary widely with the local
conditions — the character and amount of the travel, the climate,
the material of the road surface, the quality of the construction,
etc.
Since this form of pavement surface is rapidly going out of use,
there are no recent data on this subject. Further, such pavements
are not likely to be constructed to any appreciable extent in the
future. Therefore, this item will be disregarded.
1255. Total Annual Cost. The cost of construction is stated in
§ 1230 (page 642); Table 80 (page 645) shows the cost of repairs;
and § 1236 (page 645) shows the method of computing the cost of
renewals,
ART. 2]
THE SOLUTION OF THE PROBLEM
653
The method of computing the total annual cost of a sheet asphalt
pavement under heavy travel is as follows:
TTFMS ANNUAL COST
Cts. per Sq. Yd.
Interest: concrete base, $1.00 at 3£ per cent 3 . 500
wearing coat, $0.75 at 3£ per cent 2.625
Repairs (Table 80, page 645) 5.000
Renewals (see § 1236, page 645) 7.968
Total interest, repairs, and depreciation 19 . 093
1256. A Common Error. Not infrequently, in computing the
annual cost of a pavement, there is added to the above items an
annual contribution to a sinking fund sufficient to redeem the bonds
issued to pay for the pavement. This is incorrect, since a bond issue
is only a means of deferring the payment of the first cost; and it is
clearly wrong to include both interest on first cost and an annuity
to pay the first cost. If the bond matures at the end of the life
of the pavement, an annuity to redeem the bond is precisely the
same thing as a depreciation fund to renew the pavement; and
hence it is clearly an error to include both in the annual cost.
1257. Comparison of Total Annual Costs. To compare the eco-
nomic value of different pavements, a computation similar to that in
§ 1255 should be made for each pavement; and the one showing the
least annual cost is the most economical. Substantially the above
method was applied to three forms of pavements and four classes
of streets in a certain large city, with results in Table 82. *
TABLE 82
ANNUAL COST OF PAVEMENTS
CLASSES o
p STREETS.
A.
B.
C.
D.
Granite block
$0.511
$0.372
$0.365
$0 294
Sheet asphalt
.521
.324
.283
222
Water-bound macadam
.669
.430
.312
151
Although the error mentioned in § 1233, i. e., not discriminating
between renewal and new construction, was made in computing the
cost of repairs of the macadam, the results are sufficiently correct to
show the method of utilizing such an investigation. The compu-
tations included a charge for cleaning for each form of pavement,. and
* Engineering and Contracting, Vol. 33 (1910), p. 31.
654 SELECTING THE BEST PAVEMENT [CHAP. XX
also an item for sprinkling the macadam. Of the four classes of
streets, A had the heaviest travel and D the least. The results
show that for Class A streets, a granite block pavement is most
economical; for Class B, sheet asphalt; for Class C, sheet asphalt;
and for Class D, macadam.
Of course, the value of an investigation similar to that above
depends upon the values assumed for the life of each of th'e pave-
ments, and also upon the value used for the cost of repairs; but unfor-
tunately there are practically no reliable data for either of these, and
hence this method is not as exact as its form implies. For example,
for any of the four classes of streets in Table 82, the differences in
the annual costs of the three pavements are so small that the con-
clusions might be changed by a change in the assumed life of one
or the other of the pavements.
1258. NON-ECONOMIC PROBLEM. Sometimes the selection of a
pavement is determined by a single factor, as for example, the
proximity of one or more paving materials, and sometimes the
selection can be made from a consideration of only the economic
features (see § 1251-58) ; but usually the selection requires a careful
consideration of all the factors involved. The benefits to be derived
from good pavements are stated in § 634 (page 318-19), from which it
appears that only two of the eight advantages relate directly to
economics. The requirements for an ideal pavement are enumer-
ated in § 1229, from which it appears that only five of the ten items
concern economics.
The decision as to which is the best pavement will often be largely
a matter of judgment; and when this is the case, the engineer should
reach his conclusion by a series of carefully considered steps, and not
by a single haphazard leap. He should weigh all the evidence, and
not base a decision upon a single item, as is too often the case; nor
should he adopt the practice of some other locality without a careful
consideration of the local resources and of the needs of the place in
which the pavement is to be laid, as is frequently done.
Local conditions should always be considered, and hence it is
not possible to lay down any fixed rule as to what material makes the
best pavement; but a careful study of the requirements of the ideal
pavement and of the qualities of the different kinds of pavements
will promote an intelligent selection in any particular case.
1259. Relative Merits of Pavements. It is proposed to compare
different kinds of pavements by assigning percentages to the different
qualities of an ideal pavement, and then with this as a guide to assign
ART. 2]
THE SOLUTION OF THE PROBLEM
655
numerical values to the various qualities of the several kinds of
pavements.
The various qualities of a perfect pavement have been discussed
in § 1229 to § 1248, and these qualities have been grouped in Table
83 under the three heads: (1) economic qualities, (2) sanitary
qualities, and (3) acceptability. Opposite each of these qualities
in the first column of Table 83 is placed a number which is believed
to represent the average relative importance of that particular
quality on a scale of 100.
TABLE 83
RELATIVE VALUES FOR THE DIFFERENT QUALITIES OF VARIOUS PAVEMENTS
±-E!
ICENTAG!
. ASSIGN
ED TO T
HE yUAI
,ITY.
Ref.
No.
Qualities.
Ideal
Pave-
ment.
Sheet
Asphalt.
Brick.
Granite
Block.
Water-
bound
Mac-
adam.
Wood
Block.
I
Economic qualities:
Low first cost
20
11
10
3
18
5
2
5
Low cost of repairs
Ease of traction
20
10
14
9
18
8
20
7
8
6
16
9
4
Good foothold
5
2
4
4
5
2
5
Ease of cleaning
10
10
9
6
3
9
Total
65
46
49
40
40
41
6
Sanitary qualities:
Noiselessness
15
10
7
5
15
14
7
Healthfulness
5
5
4
3
2
4
Total
20
15
11
8
17
18
8
9
10
Acceptability:
Free from dust and mud . . .
Comfortable to use
Non-absorbent of heat
10
3
2
10
3
1
9
1
1
8
1
1
2
3
2
9
2
1
Total
15
14
11
10
7
12
Grand total
100
75
71
58
64
71
The most important matter in preparing Table 83 is the assign-
ment of the numbers for the Ideal Pavement, for the number assigned
to any one quality limits the range of the corresponding assignments
to the different pavements. The assignment of the numbers is
wholly a matter of judgment, and different individuals will differ
greatly as to the relative values to be given to each quality; but the
table is only to show a method whereby the good and the bad qualities
656 SELECTING THE 3EST PAVEMENT [CHAP. XX
of one kind of pavement may be balanced against those of another
kind, and a conclusion may be reached, step by step, which repre-
sents the algebraic sum of the judgment on each item.
Different values should be assigned to the same quality according
to the attendant conditions. If the street is in a manufacturing
district and subject to heavy traffic, ease of traction should be
assigned a comparatively high value, and noise a very low value.
For an office district, quietness is the controlling factor, and should
therefore have a relatively high value. Similarly, for a residence
district with its light driving, healthfulness and freedom from dirt
and dust may be the most important element; for a residence dis-
trict where the property owners can not afford an expensive pavement,
the first cost may determine the kind of pavement; and on a steep
grade slipperiness may out- weigh all other conditions in determining
the kind of pavement to be employed. The application of the
principles is likely to be complicated by the personal interests of the
residents or property-holders, since opinions are likely to differ
according to whether the point of view is that of a tenant, a resident
property-holder, or a non-resident property-holder.
1260. Each quality of a pavement will now be considered, and
the degree of perfection of this quality possessed by each kind of
pavement will be indicated by a numerical value.
1261. First Cost. In § 1230 (page 642) are given assumed values
for the average cost of construction for the best of each kind of pave-
ment. These values are repeated below in the order of their cheapness :
KIND OF PAVEMENT. COST PER SQ. YD. RELATIVE WEIGHT,
1. Macadam, water-bound $1 . 25 18
2. Asphalt, sheet 1 . 75 11
3. Brick 1.80 10
4. Wood block, creosoted 2.75 5
5. Granite block 3.00 3
The last column of the above exhibit shows the relative weights
assigned to the quality of cheapness. Since macadam is the lowest
in first cost, it possesses the quality of cheapness in the highest
degree; and consequently it is given a weight of 18 — nearly the value
assigned to the ideal pavement in Table 83. The weights assigned
to this quality decrease from gravel, the cheapest, to granite block,
the most expensive. The .several weights assigned above to low
first cost are entered opposite this quality in the table.
1262. The first cost of a pavement not infrequently has undue
weight in comparing the relative merits of different kinds of pave-
ART. 2] THE SOLUTION OF THE PROBLEM 657
ments. The pavement which costs the most to construct is not
always the most expensive, nor is the one lowest in the first cost
always the cheapest in the end (see § 1251-57).
A pavement is sometimes selected because of its low first cost,
for other than economic reasons. Often the cost of construction is
charged against the abutting property, while maintenance is paid
for by the whole city; and the result is that many property owners
perfer a cheap pavement because they must pay for it, notwithstand-
ing the fact that the cheaper pavement may cost more for main-
tenance and be dearer in the long run. Again, the property holders
are sometimes really unable to pay for the most economical pave-
ment, and hence a pavement low in first cost is selected as a tem-
porary expedient.
1263. Cost of Repairs. Table 80, page 645, contains the best avail-
able data on the cost of repairs, although they are not very reliable.
The data for heavy travel are re-arranged and transcribed below :
KIND OF PAVEMENT. COST PER SQ. YD. RELATIVE WEIGHT.
1. Stone block 2.0 20
2. Brick 3.0 18
3. Wood block 4.0 16
4. Asphalt 5.0 14
5. Macadam, water-bound 10.0 8
1264. Ease of Traction. Under this head may be included not
only the power required to move loads, but also the consequential
damages to vehicles, since they both vary with the roughness of the
pavement. From a study of the results in Table 8, page 21, remem-
bering that the tractive resistance of the best type of several of the
pavements has decreased since the observation in Table 8 were
made, the weights are assigned to this quality for the different
kinds of pavements, as shown in Table 83.
1265. Foothold. From a study of § 1239, the relative degree of
slipperiness is stated in numbers and entered in Table 83. If the
pavement is to be upon a steep grade, this quality may be a con-
trolling factor.
1266. Ease of Cleaning. The relative ease with which certain
types of pavements may be swept, as determined by the cost of doing
the work in New York City, is as follows: asphalt, 100; brick, 100;
rectangular hard-wood blocks, 100; granite blocks, 150; Belgian
blocks, 160; cobble stones, 400.* For sanitary reasons, New York
* Street Cleaning in New York City in 1895-97, p. 157 — Supplement to Vol. II. of Municipal
Affairs. New York, 1898.
.
658 SELECTING THE BEST PAVEMENT [CHAP. XX
City has spent a million dollars a year for the past few years in sub-
stituting sheet asphalt pavements for stone-block in the congested
tenement districts, chiefly on account of the greater ease with which
the asphalt is kept clean.
The cost of sweeping ordinary stone-block, round wood-block, and
brick with sand filler usually ranges between 40 and 48 cents per 1,000
square yards for each sweeping, and sheet asphalt from 30 to 38
cents, depending upon the thoroughness of doing the work, the fre-
quency of sweepings, the kind of business in the property adjoining,
and the amount of the traffic. The relative weight to be assigned
to this item will vary with the frequency of cleaning.
The estimated weight to be assigned to the several pavements
on account of their ease of cleaning is entered in Table 83.
1267. Value for Other Qualities. From a consideration of the
discussion in § 1244-48, the percentages for the other qualities are
inserted in Table 80.
1268. Conclusion. The totals at the foot of Table 83 represent
the summation of the individual decisions on the several qualities,
and the larger the total the more desirable the pavement. The
particular results in this example may not be applicable to any
locality, and each person will have his own opinion as to the merits
and defects of any particular pavement; but the method of analysis
is applicable to any particular case, and will enable the engineer
intelligently and unerringly to reach the final conclusion to which his
opinion in detail leads. The above method has something of the
mathematical form ; but the fact should not be forgotten that it is
based upon judgment, and that therefore it can not be expected to
give results of a high degree of accuracy.
In practice the application of this method is much less compli-
cated than appears from the above example, for usually proximity
of some natural pavement material or freight rates on others, limits
the choice to a comparatively few kinds of pavements. Further, the
decision as to the kind of pavement to be laid is often influenced by
the fancy or ability of those who pay for it. However, the engineer
should employ a logical process in arriving at his own conclusions,
and thus be in a position to give sound advice upon the funda-
mental principles involved.
1269. Finally, in any important case, it is wise to determine the
best pavement by both the economic and the non-ecomonic method,
so as to check one method against the other.
INDEX
ASP
Asphalt, 267
American, 270
Bermudez, 269
California, 269
cement, 268
preparation, 274
specifications, 275
binder for macadam, 278
bituminous concrete, 279
bituminous surface, 277
filler for block pavements. 282
seal coat, 280
sheet asphalt pavements, 281
characteristics, 268
cost, 283
crude, 267
Gilsonite, 270
liquid, 275
specifications for, 275
petroleum residue, 270
properties of, 270
binding power, 271
chemical stability, 270
freedom from decomposition, 271
resiliency, 271
waterproofness, 271
refined, 267
rock, 268
shipping, 270
sources, 268
tests, see Bituminous materials, tests
Trinidad, 268
Asphalt pavement, 411
amount in U. S., 320
block, 470
composition, 471
cost, 472
merits, 472
concrete, 461, 464
Amiesite, 462
bitulithic, 461
area in U. S., 320
cost of construction, 465
Amiesite, 467
bitulithic, 467
Topeka mixture, 465
Warrenite, 467
definition, 461
laying, 464
merits, 466
mixing, 464
specifications, 468
stone-filled, 463
Topeka mixture, 463
Warrenite, 462
foundation, 412
bituminous, 413
hydraulic, 412
kinds, 411
block, 411, 470
concrete, 411, 461, 464
Amiesite, 462
bitulithic, 461
stone-filled, 463
Topeka mixture, 465
Warrenite, 462
of.
659
ASP
Asphalt pavement, kinds, rock, 469
sheet, 411
rock, 469
construction, 469
sheet, 411
adjacent to track, 441
binder course, 415
bitumen, 417
kind, 415
closed, 415
specifications, 416
open, 415
specifications, 415
paint coat, 415
laying, 419
mixing, 418
rolling, 422
thickness, 422
cause of failure, 443
improper manipulation, 444
burned asphalt, 444
chilled cement, 445
damp foundation, 445
high heat, 444
improper consistency, 444
inadequate compression, 446
inadequate mixing, 445
insufficient bitumen, 445
rich binder, 445
separation of sand and cement, 445
natural causes, 446
bonfires, 448
cracks, 447
decay, 446
illuminating gas, 447
leaky joints, 447
ordinary wear, 446
porous foundations, 446
shifting under traffic, 448
weak foundation, 446
unsuitable materials, 443
asphalt, 416, 417, 426
sand, 416, 417, 423
cost of construction, 451
actual, 454
estimated, 453
cost of maintenance, 454
contract, 457
Buffalo, 458
municipal plant, 455
Brooklyn, 457
crown, 46
foundation, 412
bituminous, 413
hydraulic, 412
other forms, 414
grade, maximum, 459
history, 412
merits, 460
repairing, method of, 449
cracks, 450
disintegration, 449
formation of waves, 449
old material, 451
painting gutters, 450
settlement of subgrade, 449
660
INDEX
ASP— BIT
Asphalt pavement, sheet, repairs, method of
recording, 450
price of, 457, 458
specifications, 460
wearing coat, 423
absorptive power, 433
bitumen, per cent, 435
cement, 426
amount, 427
testing, 427
absorptive power, 433
density, 432
impact, 433
density, 432
filler, 426
impact test, 433
laying, 436
mixing, 435
proportions, 434
rolling, 438
sand, 423
thickness, 441
Bermudez asphalt, 269
Bitulithic pavements, area in U. S., 320
see also Asphalt pavements.
Bitumen, 267
Bituminous concrete roads, 310
aggregate, 311
binder, 311
cost, 315
laying, 312
mixing, 311
seal coat, 315
vs. bituminous macadam, 315
Bituminous macadam roads, 306
applying binder, 309
bituminous binder, 308
characteristics, 310
cost, 310
crown, 307
definition, 306
foundation, 306
maintenance, 310
maximum grade, 307
tar-sand mastic, 310
wearing coat, 307
width, 307
Bituminous materials, 267
definition, 267
tests of, 271
bitumen soluble in disulphide, 273
naphtha, 273
tetrachloride, 272
consistency, 272
float apparatus, 272
penetration, 272
viscosity, 272
distillation, 273
ductility, 274
fixed carbon, 273
flash point, 272
float test, 272
foam test, 271
melting point, 272
paraffin scale, 274
penetration, 272
specific gravity, 271
vaporization, 273
viscosity, 272
Bituminous surface, definition, 296
kinds. 296
carpet, 298
applying material, 299
bituminous material, 298
cleaning surface, 299
cost, 303
on gravel, 304
on macadam, 304
maintenance, 303
BIT— BRI
Bituminous surface, kinds, carpet, value of
302
coating, 297
bituminous material, 297
Blocks, size of city, 337
Brick, 475
chemical composition, 475
clay, 475
hillside, 480
kinds, 478
hillside, 482
re-pressed, 478
vertical fiber, 481
wire-cut lug, 479
manufacture, 476
burning, 483
cutting, 477
moulding, 476
re-pressed, 478
service test, 500
size, 485
specifications, 485
testing, 486
absorption, 489
appearance, 486
color, 487
crushing strength, 488
rattler test, 490
changes proposed, 499
limit of loss, 495, 497, 499
marking brick, 493
specifications, 491
size, 487
specific gravity, 488
transverse strength, 489
vertical fiber, 481
wire-cut lug, 479
Brick pavements, 474
adjacent to track, 539
area in U. S., 320
bedding course, 505
cement and sand, 511
mortar, 512
sand cushion, 505
comparison of types, 536
cost, 539
durability, 536
noisiness, 536
smoothness, 536
thickness, 537
time in construction, 539
construction, 503
bedding course, 505
cement-sand, 511
comparison, 514
mortar, 512
sand, 505
cost, 544
discussion, 544
examples, 546-51
expansion joint, 533
at anchors, 535
longitudinal, 533
transverse, 534
foundation, 503
abandoned type, 503
bituminous concrete, 504
hydraulic concrete, 505
macadam 504
grade, maximum, 540
header, 535
inspecting, 519
joint filler, 521
applying, 524
bituminous, 530
cost, 529
grout, 522
merits, 529
mixing, 523
sand, 521
INDEX
661
BRI— CON
Brick pavements, joint filler, tar-sand, 532
laying brick, 514
delivery, 514
direction of courses, 515
rolling, 519
setting, 517
maintenance, 552
cost, 564
repairs, 552
bulges, 557
contraction joints, 555
cracks, 560
defective grouting, 555
longitudinal cracks, 557
re-laying, 558
re-surfacing, 561
asphalt, 561
brick, 563
tar, 563
settlement of trench, 554
shrinkage of cushion, 553
sinking of foundation, 554
soft brick, 552
transverse joints, 534
turning brick, 563
merits, 549
monolithic, 512
roads, 541
specifications, 551
streets, 541
Brick rattler, 492
specifications, 491
Bridges, 112
Broken stone, see Macadam stone.
Catch basing, 362
construction, 362
cover, 366
examples, 363, 364, 365
inlet, 366
location, 364
Catch-water, 83
Cement, asphalt, 268
hydraulic, 227
Census, travel, 25
see also Travel census.
Chevy Chase experimental brick road, 501
Clay-sand roads, see Sand-clay roads.
Cobble-stone pavements, area in U. S., 320
construction, 567
hammer, 568
Concrete, bituminous, see Bituminous con-
crete.
aggregate, 228
cement, 227
consistency, 248
data for estimates, 235
gravel, 229
ingredients for cu. yd., 236
Fuller's rule, 237
materials, 227
aggregate, 228
cement, 227
gravel, 229
stone, 229
methods of proportioning, 230
mixers, 247
mixing, 246
proportions, 245
theory of, 230
sieve analysis, 231
Concrete curb and gutter, 382
see also Curb.
Concrete pavements, 263
see also Concrete roads.
Concrete roads, portland-cement, 227
characteristics, 263
construction, 238
consistency, 248
cost, 259
CON— DRA
Concrete roads, Portland cement, construc-
tion, curing, 252
finishing, 251
machine, 256
mixing, 246
one vs. two course, 241
placing, 249
proportions, 245
protecting, 252
side forms, 244
striking, 249
thickness, 244
width, 244
contraction joints, 254
cross section, 242
curbs, 258-
data for estimates, 235
drainage, 238
grade, maximum, 243
maintenance, 264
bituminous surface, 265
cost, 265
work required, 264
materials, 227
aggregates, 228
cement, 227
gravel vs. broken stone, 229
one vs. two course, 241
reinforcement, 256
shoulders, 257
specifications, 264
subgrade, 239
super-elevation, 243
template, 250
thickness, 244
width, 244
Connecticut gravel road, 170
Crown, pavements, 374
amount of, 275
laying out, 374
roads, 65
Culverts, 113
Curb, 378
combined, 382
expansion joints, 386
finishing surface, 386
forms, 383
foundation, 383
laying and mixing, 384
concrete, 380
cost, 381
integral, 258
stone, 378
cost, 380
Curves, horizontal, in road, 58
vertical, at grade intersection, 353
Cut-back product, definition, 267
Distance equivalent to 1 ft. of rise and fall, 54
value of saving, 43
vs. rise and fall, 53
Drainage, road, 72
catch-waters, 83
side ditches, 78
surface, 81
crown, 82
side ditches, deep, 82
shallow, 82
street, 361 .
catch basins, 362
constructions, 362
examples, 363
Champaign, 363
Milwaukee, 365
London, 365
Providence, 364
inlet, 366
intersection, 370
commercial, 369
Champaign, 368
662
INDEX
DRA— EAR
Drainage, street, catch basins, intersection,
Omaha, 368
location, 364
crown, 374
dished-payements, 376
foot-crossing, 372
gutter, 367
depth, 369
grade, 370
material, 367
Dynagraph, 19
Earth rpads, 70
artistic treatment, 114
construction, 70
earthwork, see Earthwork,
machinery, see Road-building machinery,
cross section, 71, 82, 83
super-elevation, 71
crown, 65, 81
definition, 70
drainage, 72
surface, 81
side ditches, 78, 82, 83, 84
underdrainage, 72
object, 72
tile, 74
cost, 75
fall, 75
laying, 77
cost, 77
location, 78
size, 76
earthwork, see Earthwork,
embankments, rolling, 89
settlement, 88
stability, 90
grades, 71
improving old, 91
machinery, see Road-building machinery,
maintenance, 115
care of ditches, 124
surface, 117
roadside, 124
trees, 125
cost, 129, 130
dragging, 129
total, 130
destructive agents, 115
equal axles, 117
horse before wheel, 117
narrow tires, 115
dragging, 120
cost of, 121, 129
filling holes, 124
improving old roads, 91
machinery, see Road-building machinery,
preventing dust, 133
removing stones, 124
scraping, cost of, 123
snow, obstruction by, 125
systems, 126
by contract, 129
continuous maintenance, 127
continuous repairs, 127
intermittent repairs, 127
V road-leveler, 123
surface oiling, 133
applying the oil, 135
cost, 136
effect on maintenance, 134
oil, see Oil; also Petroleum,
preparing surface, 135
width, 70
on curves, 71
Earthwork, 83
balancing cuts and fills, 86
computing, 86
cost of, 103
drag-scoop scraper, 104
EAR— GRA
Earthwork, cost of, elevating grader, 104
finishing slopes, 112
profits, 112
scraper, four-wheel, 110
two-wheel, 106
scraping grader, 103
' wagons, 110
embankment, rolling, 89
settling, 88
stability of, 90
overhaul, 89
rolling, 89
setting slope stakes, 86
settlement, 88
shrinkage, 87
Elevating grader, 101
operating, 102
Embankments, 85
cross section, 85
finishing slopes, 112
rolling, 89
settling, 88
stability of, 90
Excavation, 84
cross section, 85
Flux, 267
specifications, 274
Foot-way crossing, 372
Foundation, pavements, 392
bituminous concrete, 406
concrete base, hydraulic cement, 399
cost, 403
curing, 403
finishing, 402
mixing, 402
placing, 402
proportions, 401
thickness, 400
drainage, 392
see also Drainage,
earthwork, 393
see also Earthwork,
filling trenches, 395
flooding, 396
natural settlement, 395
re-filling with sand, 398
replacing material, 398
tamping, 397
macadam, 405
railway track, 407
examples, 408
subgrade, 392
rolling, 394
thickness, 400
French coefficient of wear, 188
French standard macadam road, 198, 199
Grade, effect on load, 50
effect on location, 48, 69
maximum, 54
minimum, 57
Grade resistance, 21
Grader, elevating, 101
scraping, 95
Shuart, 208
Granite pavements, amount in U. S., 320
Granite paving blocks, 574
Gravel, road-building, 150
binder, 151
materials of, 151, 152, 153
characteristics, 158
Buck Hill, 160, 161, 163
Decatur, 160, 161
Lexington, 160, 161, 162
Oaktown, 160, 161, 163
Paducah, 160, 161, 164
Peekskill, 160, 161, 162
Rockford, 160, 161, 164
Rock Hill, 160, 161, 163
INDEX
663
GRA-HOR
Gravel, road-building, characteristics, Ros-
etta, 160, 161, 164
Shaker Prairie, 160, 161, 163
Shark River, 160, 161, 163
Urbana, 159, 160, 161
cherty, 155
composition of, 158
defined, 150
distribution of, 154
durability, 150
exploring for, 156
mineralogical analysis of, 161
requisites for, 150
binder, 151
durability, 150
sizes, 151
screening, 173
sieve analysis, 160
sizes, 151
Gravel pavements, amount in U. S., 320
Gravel roads, 150
bituminous surface, see Bituminrus surface.
Connecticut standard, 170
construction, 165
bottom course, 172
Connecticut Standard, 170
cost, 175
cross section, 169
crown, 166, 355
drainage, 165
forcing gravel into subgrade, 174
forms of construction, 167
comparisons of, 171
surface, 167
trench, 169
hauling gravel, 174
loading gravel, 174
measuring gravel, 174
rolling, 171
specifications, 178
durability, 178
dust palliative, 181
moistening with salts, 182
practice in Washington, D. C., 183
sprinkling with fresh water, 181
light oil, 183, 192
proprietary compounds, 182
sea water, 181
earth track beside, 171
economic value, 176
grade, maximum, 166
maintenance, 178
cost, 180
destructive agents, 178
re-surfacing, 180
sprinkling, 180
Texas standard, 170
tractive resistance, 15, 16, 18, 21
travel on, effect of, 177
width, 166
Guard rails, 113
Guide posts, 114
Gutter, 367, 381
combined, 382
concrete, 382
cost, 388
finishing surface, 386
forms, 383
foundation, 383
laying, 384
depth, 369
expansion joint, 386
grade, 370
material, 367
private driveway, 389
street intersection, 370
Hammer, brick, 519
stone-block, 579
Horse, power of, 22
HOR-PAV
Horse, power of, effect of grade upon, 23
maximum load on grade, 24
Iron ore, binder for gravel roads, 152
Jarrah wood, description of, 603
Karri wood, description of, 603
Labor road tax, 38
Load, effect of grade on, 23
Location of roads, 41
curves, 58
aesthetic value, 61
super-elevation, 60
distance, 42
value of saving, 41
grade, 45
effect of, 47
limiting effect of, 50
maximum, 54
minimum, 57
rise and fall, 47
vs. distance, 53
grade line, 69
placing line, 65
safety at summit, 56
wheelway, position of, 64
width, 61
improved portion, 62
on curves, 64
right-of-way, 61
Lute for sand cushion, 508
Macadam pavement, 634, 635
area in U. S., 320
Macadam road, 185
see also Bituminous Macadam, 306
Water-bound Macadam, 185
Macadam stone, 186
binding power, 186
cementing power, 186
hardness, 186
tests of, 187
abrasion, 188
cementation, 188
hardness, 187
impact, 187
toughness, 187
toughness, 186
Massachusetts standard macadam road, 197,
New Jersey standard macadam road, 197
New York standard macadam road, 197
Oil for roads, cost of, 288
specifications for, 286
earth roads, 287
gravel roads, 287
macadam roads, 287
park drives, 286
Oiling machines, 137
Oil, see Petroleum.
Over-haul, 89
Pavement administration, 321
causes of inefficiency, 322
conditions, 321
importance of problem, 321
remedy, 324
Pavements, apportionment of cost, 326, 328
area of, in U. S., 320
asphalt, see Asphalt pavements,
assessments for, 329
area rule, 330
frontage rule, 330
legality of levy, 331
terms of payment, 331
benefits, 318
664
INDEX
PAY— ROA
Pavements, brick, see Brick pavements,
cobble-stone, see Cobble-stone pavements,
comparisons, 642, 654
cost of construction, 642, 656
cost of maintenance, 643
renewals, 645
repairs, 643, 657
sprinkling, 652
total, 653
transportation, 6fi2
comfort in use, 650
durability, 635
ease of cleaning, 648, 651, 657
freedom from mud and dust, 650
healthfulness, 650
noiselessness, 649
slipperiness, 647
temperature, 650
tractive resistance, 646, 657
concrete, see Concrete pavements,
cross section, 355
side-hill streets, 355
crown, 355
foundation, 392
bituminous concrete, 406
hydraulic concrete, 399
macadam, 405
gravel, see Gravel roads,
guaranteeing, 331
maintenance by contract, 333
investments in, 319
openings, 334
selecting the best, 633
stone-block, see Stone- block pavements,
tearing up, 334
widths of, 345
with car tracks, 345
without car tracks, 3< 6
wood-block, see Wood-t lock pavements.
Pavement foundation, see Foundation.
Paving railway areas, 407. 441, 539, 587, 623
Petroleum, 283
' asphalt content, 285
asphalt residue, 270
classification, 283
cost of, 288
method of refining, 284
shipping, 284
specifications for, 286
Poll tax, 35
Preserving timber, see Wood-block pavement.
Rails, car tracks, 409
Railway rails, 409
Railway ties, 409
Rammer, brick, 520
stone-block, 582
wood-block, 617
Rattler for testing brick, 492
specifications, 491
Retaining walls, 113
Road, artistic treatment, 114
bituminous concrete, see Bituminous con-
crete roads.
bituminous surfaces for, see Bituminous sur-
faces.
earth, see Earth roads.
gravel, see Gravel roads.
hydraulic concrete, see Concrete roads.
macadam, see Bituminous macadam roads.
see Water-bound macadam roads.
sand, see Sand roads.
sand-clay, see Sand-clay roads.
taxes, see Taxes.
Road administration, 30
national, 34
state, 32
unit, 31
Roads, advantages of good, 3
artistic treatment of, 114
ROA- STO
Roads, classification, 34
estimated cost of bad, 10
expenditures for in U. S., 40
improving old, 91
toll, 35
Road-building machinery, 91
drag, see Road drag
elevating grader, 101
roller, 212
tandem, 213
three-wheel, 212
scrapers, 92, 93, 94
scraping grader, 95
Road drag, 117
plank, 118
rules for using, 120
split-log, 118
steel, 118
Road grader, scraping, 95
elevating, 101
Road improvement, financial value of, 11
Rollers, 213
asphalt type, 213
macadam type, 212
tandem type, 213
three-wheel type, 212
Sand roads, 139
drainage, 139
hardening the surface, 139
shade, 139
tractive resistance, 16, 21
Sand-clay roads, 140
clay on sand subgrade, 145
clay, 145
construction, 146
cost, 147
design, 141
maintenance, 148
natural mixtures of sand and clay, 141
construction, 143
tests of, 141
sand on clay subgrade, 143
construction, 145
proportions, 144
sand, 144
thickness, 144
travel census of, 149
Scrapers, 92
drag, 92
Fresno, 93
scoop, 92 '
slip, 92
wheel, 94
four-wheel, 95
two-wheel, 95
Scraping grader, 95
operating, 97
Snow, cost of clearing, 126
obstruction by, 125
State aid, 32
Stone-block hammer, 579
Stone-block pavement, 566
adjacent to track, 586
amount in U. S., 320
classification, 566
Belgian block, 568
cobble-stone, 567
durax, 569
oblong block, 568
Roman, 566
rubble, 568
construction, 572
bedding course, 572
mortar, 574
sand, 573
blocks, 574
dressing, 574
re-cutting, 576
measuring, 577
INDEX
665
STO— STR
Stone-block pavement, construction, blocks,
ramming, 580
setting, 579
size, 577
re-cutting, 576
filling joints, 582
asphalt, 585
gravel, 582
grout, 586
pea gravel, 582
tar and sand, 584
foundation, 572
cost, 591
blocks, 591
Buffalo, 595
Chicago, 592
Cleveland, 597
contract price, 598
durax, 593
grouting, 593
Medina stone, 595, 596, 597
New York, 593
re-cutting and re-laying, 592
Rochester, 597
Schenectady, 573
tar-sand filler, 596
various cities, 598
durax, 588
expansion joint, 587
grade, maximum, 587
granite, 569
hammer, 579
limestone, 572
maintenance, 597
cost, 599
raising blocks, 599
re-filling joints, 599
re-laying, 598
repairs, 598
settlement of trenches, 599
sinking of foundation, 599
spalling joints, 599
Medina sandstone, 571
merits, 588
paver's hammer, 579
paver's rammer, 520, 582
quartzite, 572
sandstone, 571
Colorado, 571
Kettle River, 572
Medina, 571
Potsdam, 571
Sioux Falls, 572
trap, 571
Stone-block rammer, 582
Stone crusher, 203
gyratory, 204
oscillatory, 203
Stone-crushing plant, 205
Street, cross section on side-hill, 355
design, 336
area of streets, 344
blocks, size of, 337
location of streets, 339
directness, 341
topography, 339
an of streets,
plan of streets, 336
blocks, size of, 337
lots, size of, 337
shade trees, 338
width of streets, 343
drainage, see Drainage, street.
grades, 347
elevations at street intersection, 350
maximum, 348
minimum, 349
vertical curves at intersection, 353
location, 339
directness, 341
topography, 339
STR— VEH
Street, pavements, width of, 343
with car tracks, 346
without car tracks, 345
plan of streets, 336
checker-board, 341
C9ncentric, 343
diagonal, 341
trees, 358
vertical curves, 354
width, 343
Swiss standard macadam road, 198
Tar, 289
characteristics, 289
cost of, 295
kinds, 289
shipping, 290
specifications, 289
bituminous concrete, 292
bituminous macadam, 291
bituminous surfaces, 291
filler for block pavements, 294
trade names, 294
tests of, see Bituminous materials, tests of.
trade names, 294
Tax, road, 34
automobile, 40
labor, 38
money, 38
poll, 35
property, 38
toll, 35
Telford road, 185, 189, 191
Template, brick pavements, 505, 513
mortar bedding-course, 513
sand cushion, 505
concrete roads, 249
Texas gravel road, 170
Thank-you-marms, 83
Ties, street-railway, 409
Tile, cost of, 75
drainage, 72
laying, 77
location, 78
one vs. two lines, 77
size of, 76
weight, 75
Tires, width of, effect on traction, 14
Tractive resistance, 12
American experiments, 19
axle friction, 12
data on, 15, 16, 17, 18, 20, 21
diameter of wheel, effect of, 13
French experiments, 17
rolling resistance, 13
speed, effect of, 16
springs, effect of, 17
width of tire, effect of, 14
Transportation, cost of wagon, 6
annual saving, 10
Travel census, 25
American roads, 26
streets, 28
classification of travel, 28
diverting travel, 29
weight of vehicles, 29
French, 26
history, 26
Illinois, 27
Iowa, 28
Massachusetts, 26
weight of vehicles, 30
width of traveled way, 29
width of vehicles, 31
Trees on street, 358
Trinidad asphalt, 268
V road-leveler, 123
Vehicles, weight of, 30
width of, 30
666
INDEX
WAS— WOO
Washington, sprinkling gravel with oil, 183
street plan of, 342
Water-bound macadam, 185
binder, 216
bituminous surface, see Bituminous sur-
faces.
construction, 189
binding, 217
cost, 220
crown, 192
foundation, 189
rolling, 213
setting Teltord, 201
shoulders, 191
shrinkage, 209
size of stone, 204
spreading stone, 207
subgrade, 190, 200
super-elevation, 194
Telford's, 191
thickness, 194
width, 191
wings, 196
crown, 192
crushing stone, 202
grade, permissible, 199
maintenance, 223
cost, 226
• forms of, 189
patching, 225
raveling, 224
rolling, 226
sprinkling, 226
standard, French, 198, 199
Massachusetts, 197, 198
New Jersey, 191, 197
New York, 197
Swiss, 198
super-elevation, 194
thickness, 194
width, 191
wings, 196
Water-breaks, 83
Water-ways, 113
Wheelway, position of, 64
width of, 62
Wings for macadam roads, 196
Wood-block pavement, 601
adjacent to track, 623
area in U. S., 320
blocks, 604
care after treatment, 611
causes of decay, 605
laying, 615
specifications, 604
dimensions, 604
WOO
Wood-block pavement, blocks, specifications,
quality, 605
testing, 611
treatment, 609
construction, 612
bedding course, 612
bituminous, 614
mortar, 613
sand, 612
cost, 623
blocks, 623
examples, 625
various cities, 627
filling joints, 618
grout, 618
sand, 618
tar, 618
foundation, 612
laying blocks, 615
crown, 623
decay, cause of, 605
expansion joints, 622
grade, maximum, 623
history, 602
kinds, 601
rectangular blocks, 602
round blocks, 601
maintenance, 628
bleeding, 630
bulges, 630
cost, 631
low spots, 629
poor blocks, 628
re-laying, 630
merits, 626
open joints, 621
preservative, 605
amount, 611
specifications, 607
creosote oil, 607
coal-tar distillate, 607
coal-tar paving oil, 60S
water-gas tar, 608
rolling, 617
specifications, 604
treatment, 609
open-tank process, 609
pressure process, 609
timber, 603
hemlock, 604
jarrah, 603
karri, 603
larch, 604
pine, 604
tamarack, 604
yellow pine, 603
THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW
~1
AN INITIAL FINE OF 25 CENTS
WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $1.OO ON THE SEVENTH DAY
OVERDUE.
APR 24 13679 2
MAY 2 4 1967
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LD 21-95m-7,'37
382053 Z^
UNIVERSITY OF CAUFORNIA LIBRARY