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| OJIN OF HIGHWAY RESEARCH | |

UNITED STATES DEPARTMENT OF AGRICULTURE

BUREAU OF PUBLIC ROADS

INVESTIGATING EFFECT OF PAVEMENT TYPE ON IMPACT REACTION

U.S. GOVERNMENT PRINTING OFFICE: 1928

PUBLIC ROADS

A JOURNAL OF HIGHWAY RESEARCH
U. S. DEPARTMENT OF AGRICULTURE
BUREAU OF PUBLIC ROADS

CERTIFICATE: By direction of the Secretary of Agriculture, the matter contained herein is published as administrative information and is required
for the proper transaction of the public business

The reports of research published in this magazine are necessarily qualified by the conditions of the tests from which the data are
obtained. Whenever it is deemed possible to do so, generalizations are drawn from the results of the tests; and, unless this is done,
the conclusions formulated must be considered as specifically pertinent only to the described conditions

VOL. 9, NO. 6

AUGUST, 1928

R. E. ROYALL, Editor

TABLE OF CONTENTS
Page
_Effect of Pavement Type on Impact Reaction Kis
Truck Is Big Factor in Fruit Transport 5 ; Peer EDS
The Design of Pavement Concrete by the Water-Cement Ratio Method ' ; ee ag 2A

THE U. S. BUREAU

OF PUBLIC ROADS

Willard Building, Washington, D. C.

REGIONAL HEADQUARTERS
Mark Sheldon Building, San Francisco, Calif.

DISTRICT
DISTRICT No. 1, Oregon, Washington, and Montana.
Box 3900, Portland, Oreg.

DISTRICT No. 2, California, Arizona, and Nevada.
Mark Sheldon Building, San Francisco, Calif.

DISTRICT No. 3, Colorado, New Mexico, and Wyoming.
‘ 301 Customhouse Building, Denver, Colo.
DISTRICT No. 4, Minnesota, North Dakota, South Dakota, and
Wisconsin, 410 Hamm Building, St. Paul, Minn.
DISTRICT No. 5, Iowa, Kansas, Missouri, and Nebraska.
8th Floor, Saunders-Kennedy Bldg., Omaha, Nebr.

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1912 Fort Worth National Bank Building,
Fort Worth, Tex.

OFFICES

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South Chicago Post Office Building, Chicago, IIl.

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South Carolina, and Tennessee.
Box J, Montgomery, Ala.

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shire, New Jersey, New York, Rhode Island, and Vermont.

Federal Building, Troy, N. Y.
DISTRICT No. 10, Delaware, Maryland, North Carolina, Ohio, Penn-

sylvania, Virginia, and West Virginia.
Willard Building, Washington, D. C.
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Goldstein Building, Juneau, Alaska.
DISTRICT No. 12, Idaho and Utah.
Fred J. Kiesel Building, Ogden, Utah.

— eee
ee

Owing to the necessarily limited edition of this publication it will be impossible to distribute it free to any persons or
institutions other than State and county officials actually engaged in planning or constructing public highways, instructors
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Others desiring to obtain Pustic Roaps can do so

by sending 10 cents for a single number or $1 per year to the Superintendent of Documents, U. S. Government Printing

Office, Washington, D. C.

EFFECT OF PAVEMENT TYPE ON IMPACT REACTION

Reported by J. T. THOMPSON, ! Highway Research Specialist, Division of Tests, United States Bureau of Public Roads

HE question has been repeatedly raised as to
whether or not certain types of pavement or of
pavement surfacing possess inherent cushioning

or shock-absorbing properties, by virtue of which these

types function to protect themselves or (when used as

a surfacing) the bases on which they are employed.
In order to add to the rather meager data available

The deflection of this spring is a measure of the decele-
ration. The coil-spring deflection is recorded upon
silicated paper by a brass stylus mounted upon,the
accelerometer plunger.

In order to measure the permanent deformation
produced in bituminous pavements by the impact
machine, a crude but very satisfactory profilometer

on this subject, the Bureau of Public Roads carried out
the investigation described in this report.

shows the character and
location of the types tested.

TESTING APPARATUS DE-
SCRIBED

The testing apparatus
consisted of an impact ma-
chine ? for producing the
necessary impact forces
and an accelerometer * for
determining these forces.
Both have been described
in detail in previous arti-
cles, and only a very brief
description of each will be
given here.

The impact machine
consists of a plunger the
lower end of which is
equipped with the rear
wheel of a conventional
motor truck, carrying, in
this case, dual worn solid
tires. The plunger can be
raised against the action
of a calibrated, conven-
tional truck spring by
cams actuated by a hand-
wheel through a suitable
gear train. When the
plunger, with the truck
wheel, has risen to the de-
sired height of drop, it is
released from the cam, the
truck wheel being forced
down upon the pavement
under the combined influ-
ence of gravity and the
truck spring. The height
of drop of the plunger and
the spring pressure can
both be varied by means
of suitable controls.

was devised.

Table 1

IMPORTANT CORRECTION

By inadvertence a final paragraph was omitted from the
article entitled ‘The effect of the length of the mixing
period on the quality of the concrete mixed in standard
pavers,”’ published in the July number of Public Roads.

In the last paragraph of the article as published it was
stated that—

“Summarizing the situation in the light of the data
collected during this study, the evidence strongly indicates
that where standard 21E and 27E pavers which are in good
condition are used, neither strength nor uniformity of test
results is improved by mixing the concrete over 45 seconds.”

This statement properly summarizes the evidence
obtained in the investigation upon which the article
reported, but it might be inferred from it that the Bureau
of Public Roads advocates reduction of mixing time to
three-quarters of a minute. There was no such intention.
While it is true that the tests quite generally disclosed no
appreciable difference in strength between concrete mixed
for three-quarters of a minute and longer periods up to
three minutes, it is also true that half-minute mixing
failed to produce satisfactory concrete. It is apparent,
therefore, that reduction of the mixing time to three-
quarters of a minute would leave a very narrow margin
of safety to allow for the normal irregularities of practical
operation.

It may also be questioned whether the pace set by so
short a mixing time can be regularly and consistently
maintained by the hauling equipment and other units of
the construction plant. Weighing the results of the
tests against these considerations it was the intention to
suggest in a final paragraph the adoption of a mixing
time of one minute as the minimum time desirable in
practical operation.

The omitted paragraph is as follows:

As three-quarter minute mixing provides an insufficient
factor of safety for practical operations, and as it is doubt-
ful whether construction processes other than the mixing
operation itself can be speeded up sufficiently to take
advantage of the marginal time fraction, it is recom-
mended that a mixing time of one minute be adopted as
the minimum consistent with assurance of reasonable
uniformity and adequate strength of the concrete. The
evidence is strong that thoroughly satisfactory concrete can
be produced by (21E and 27E) pavers in good condition
with a one-minute mixing period.

The editor regrets the omission.

It consisted of a straight piece of wood
about 2 feet long by 3 inches wide, upon which a piece

of silicated paper was fas-
tened with thumb tacks.
Through the spot to be
tested a line was drawn
upon the pavement per-
pendicular to the plane of
the wheel of the impact
machine and broad-head
nails were then driven into
the pavement on this line
and so spaced as to come
under each end of the pro-
file board, supporting it in
a level position. The orig-
inal profile was then re-
corded upon the silicated
paper by means of a piece
of stiff copper wire which
was so bent that when it
was held vertically with
one end of the wire upon
the line drawn upon the
pavement the other end
would make a mark upon
the paper. The section
was then tested, and after
the testing machine had
been moved away the de-
formed surface of the pave-
ment was also recorded
upon the profile board.
The presence of both the
original and final profiles
upon the same piece of
paper made it possible to
determine how much and
in what manner the pave-
ment had deformed.

In addition to the regu-
lar impact-machine tests
made with the apparatus
described in the foregoing,
some supplementary mo-
The

The accelerometer used is of the coil-spring type and
consists of a weight mounted upon a vertical plunger,
both of which are supported by a coil spring. When
the mass to which the instrument is attached is deceler-
ated, the weight and plunger of the accelerometer
are forced downward, compressing the coil spring.

1 Also associate professor of civil engineering, The Johns Hopkins University. _
2 Teller, L. W., Impact Tests on Concrete Pavement Slabs, Public Roads, vol. 5,

No. 2, April, 1924. P
3 Teller, L. W., Accurate Accelerometers Developed by Bureau of Public Roads,
Public Roads ,vol. 5, No. 10, December, 1924.

4136—28 -

tor-truck impact tests were also conducted.
apparatus or equipment used in these tests, as well
as the manner in which the tests were made and the
data secured, are discussed in the latter part of this
report.

UNIFORM PROCEDURE ADOPTED FOR ALL TESTS

It was desired to develop for each type of pavement
tested a characteristic curve showing the relation
between the height of the drop of the wheel and the

113

PUBLIC ROADS

Vol. 9, No 6

TABLE 1.—Type and location of pavements tested

| |
| Num-
Babe ae Description of type Name and location
| tested |
1 | 124 .2-inch Topeka 6-inchili si) Concrete Daseaae es panera one ee eee ee eee ee Connecticut Avenue, Chevy Chase, Md.
2 12 , 2-inch bituminous concrete (District of Columbia specification); 6-inch 1: 3:7 concrete base Do.
3 | 6 | 38-inch combined sheet asphalt and binder; 6-inch 1:3:7 concrete base_.____.____..-..--..-_-- Connecticut Avenue, District of Columbia.
4 | 6 | 3-inch combined sheet asphalt and binder; 7-inch 1:3:6 concrete base._.____________________- Do.
6 3 1-inch sheet asphalt; surface-treated water bound macadain-Dasces -ee wane Gare ge renee ns nee Chevy Chase Circle, District of Columbia.
6 | 3 | Surface-treated waterbound macadam.____-____-_- 8.2221...) Georgetown- Alexandria Road, Arlington County, Va.
3: 6inch 2 32 24:concrete +- ake Se a see ee eke ee ele i an ys EAL Woodbine Street, Chevy Chase, Md.
ie (eed a C6 Vs err eel ean pp eaneene rary mene Rear ee Se gS Cm Die ee, eee Pe Rey mel Sie ey Williams Lane, Chevy Chase, Mad.
BP \ er ot JS s EO oS Se SE a aoe I 2 ee A eee ee nit Fa East Underwood Street, Chevy Chase, Md.
i WAS 3 eee Cos eee pairs Sie al pee ee Beene or a eg A ec NE Foy Boa Actas igs wane othe A | Thornapple Street, Chevy Chase, Md
2a evGO 22 2. SET OS ERE Bee SI Se ee Oe eat Ses ee See are eran at Northampton Street, District of Columbia.
Die ee eS ay Coe Pe ts ees, Sek Ree A et Ld oye As ae anes WRENN ky Chevy Chase Parkway, District of Columbia,
Pleas! co eae erg Nae Ne Se Bene ph Sate Si Sy Sa eee ke eee ee a OS ee ua ye | Nebraska Avenue, District of Columbia.
8 9/3 6-inch 1: 2:3 concrete Columbia Pike, Arlington County, Va.
16 -----d0 ae ee Test road, Arlington Experiment Station, Virginia.
9 2 | 7-inch 1: 2:4 concrete - | Connecticut Avenue, District of Columbia.
10 4 | 7-inch 1:2:3 concrete- | Columbia Pike, Arlington County, Va.
ll 10{5 8-inch 1 :2:3 concrete_ Do.
(ate ea COTE eee ee Test road, Arlington Experiment Station, Virginia.
12 6 10-inch 1:2:3 concrete 0.
13 4 | 214-inch brick; 34-inch sand cushion; 6-inch 1:114:3 concrete base Circular test track, Arlington Experiment Station, Virginia.
14 3 | 21-inch brick; 34-inch 1:4 sand-cement cushion; 6-inch 1: 114 :3 concrete base Do.
15 3:| 4-inch brick; 34-inch sand cushion; 6-inch 1:114:3 concrete base._.._____.___________.__..._. Do.
16 3 4-inch brick; 34-inch 1: 4 sand-cement cushion; 6-inch 1:14%:3 concrete base________________ Do.

SHowinG Deraits or Impact Macuine, But Wirxout
SPECIAL EQUIPMENT USED IN THESE TESTS

total impact reaction developed between the wheel and
the pavement. It was decided to define “height of
drop”’ as the distance through which the wheel dropped
from the moment it was tripped off the cams until its
rubber tire first came in contact with the pavement
surface.

In accordance’ with this definition, the following
technique was employed for testing bituminous-topped
pavements. The impact machine was first leveled
carefully so that the plunger would be truly vertical,
and then the plunger was raised on the cams until it
was just at the point of trip-off. A deflection of 1
inch was then produced in the truck spring of the
impact machine, and this deflection exerted a pressure
of 1,500 pounds upon the plunger. The entire frame
of the impact machine, carrying with it the cams
upon which the plunger still rested in trip-off posi-
tion, was next lowered (by means of four screws at its
corners) until the tires on the wheel just made con-
tact with the pavement surface. This condition was
called ‘“‘contact.’’ The relative position of the plunger
and impact-machine frame was then determined by
careful measurement.

All of these operations were carried out with the
plunger supported upon the cams at trip-off position,
the wheel not being permitted to rest upon the pave-
ment until the first impact was delivered and then
for only as brief periods as possible.

The impact machine and of course its cams, with
the plunger still resting upon them in trip-off position,
was next raised to the first height of drop—i. e., one-
third inch. In order that the truck spring always be
deflected 1 inch at the instant the wheel came in contact
with the pavement, regardless of the height of drop,
it was necessary that the ends of the spring and the
surface of the pavement bear a fixed relation to each
other. However, since the ends of the spring are con-
nected to the frame of the impact machine, every time
the frame was raised it was necessary to adjust down-
ward the ends of the spring by an amount equal to the
change in elevation of the frame, so as to maintain this
fixed relation.

Having completed the setting of the machine for the
one-third inch height of drop, the pavement was struck
three blows from this height, an accelerometer record be-
ing obtained for each blow. ‘The new relative position
of the plunger and frame at contact was then determined.
The difference between this position and that formerly
recorded represented the depth of any permanent defor-
mation which had occurred in the pavement surface.

In preparing for the second height of drop (two-thirds
inch) the machine was raised an additional third of an
inch by means of the corner screws and the spring ad-
justment compensated for the increased height of drop.
Then the machine was lowered by means of the corner
screws an amount exactly equal to the permanent pave-
ment deformation. This compensation was usually
small and seldom amounted to more than 0.04 inch for
three blows, even at the maximum height of drop em-
ployed, 124 inches. For subsequent heights of drop the
procedure was exactly the same as that just described.

Upon rigid pavements without bituminous surfacing
the procedure was similar to that just described, except
that no compensation was made for permanent pave-
ment deformation.

August, 1928

PUBLIC ROADS

115

DATA ANALYZED TO GIVE RELATION BETWEEN HEIGHT OF DROP
AND IMPACT REACTION

In, preparing the following data the accelerometer
records were first accurately measured. These readings
were then multiplied by the calibration constant of the
accelerometer, 2,514 feet per second? per inch of
record, to obtain the deceleration which, when multi-
plied by the mass of the wheel-plunger combination,
1646
39.9 9 1D
To this was added the weight of the plunger, 1,646
pounds, and the residual spring pressure at the moment
of maximum impact and resulting maximum tire
deflection.

The foregoing operation may be expressed by the
formula

gave the impact force due to the striking mass.

F= M(at+g)+P
where
F= total impact force.
M=mass of the wheel-plunger combination.
a= deceleration recorded by the accelerometer.
g= acceleration due to gravity.
P=residual spring pressure.

In evaluating P for the different heights of drop a
curve (fig. 1) was obtained experimentally between
height of drop and tire deflection. The recorded tire
deflections deducted from the 1-inch spring deflection
at contact gave the residual spring deflection. The
pressure corresponding to this residual spring deflection
was secured from static load-deflection data for the
same spring and plotted against height of drop. This

1 CURVE OF RESIDUAL SPRING DEFLECTION (ONE INCH SPRING DEFLECTION
LESS LOSS IN SPRING DEFLECTION DUE TO TIRE DEFLECTION)
2 TIRE DEFLECTION CURVE

INCHES

CURVE OF RESIDUAL SPRING PRESSURE

1200

800

Pa 2x

400

ane

100

RESIDUAL SPRING PRESSURE- POUNDS

1.33 1.66 2.00

ie) 33

.66
HEIGHT OF DROP ABOVE CONTACT POSITION -INCHES

Fig. 1.—Curves DrvELOPED IN Evatuatine P

SHOWING DeETAILS OF ACCELEROMETER MoUNTED ON

Truck. Tuis Picturs was TAKEN CONNECTION

witH ANOTHER INVESTIGATION

Sy,

curve was used in evaluating P. Table 2 shows

tabulation of all of the data collected.

CHECK TESTS INDICATE A SATISFACTORY DEGREE OF ACCURACY

At the beginning of the tests it was realized that the
data would be worthless unless the settings of the
machine could be so controlled as to be able to duplicate
impact phenomena at a given test section for successive
set ups. Accordingly, as a preliminary step, the
machine was set up over a trial section of concrete at
the Arlington Experiment Station and impact forces
produced and measured for each height of drop. The
machine was then replaced upon its chassis, towed
away a few feet, and then moved back to the trial
section and the test data redetermined. It was found
for the same section that the impacts produced by the
machine for identical settings could be duplicated and
measured with a satisfactory degree of consistency.

Throughout the tests numerous check determinations
were made, and near the end of the experiment the
machine was brought back to the Arlington Experiment
Station and a check test made upon the trial section
referred to above. <A close check was obtained, indi-
cating that the performance of neither the machine nor
the accelerometer had changed during the three
months’ period of testing.

A study of the data obtained shows that the indi-
vidual tests are to be regarded as very satisfactory. <A

116

PUBLIC ROADS

Vol. 9, No. 6

plotting of any of the tabulated impact reactions against TABLE

corresponding heights of drop will give smooth, con-
sistent curves.

A general idea of the scope of the experiment may
be had by referring to Figure 2, which shows the
characteristic curve for each type tested. This dia-
gram is much too confusing to be of use in drawing
conclusions. It was considerably simplified by aver-
aging into one curve all values for similar types whose
maximum reactions lay within a zone 1,000 pounds

Taste 2.—Tabulation of impact reactions expressed in thousand
pounds

TYPE 1.—2-INCH TOPEKA; 6-INCH 1:3:7 CONCRETE BASE,
CONNECTICUT AVENUE, MARYLAND

Test No. Av-
Height an % er-
pace oe
inches) vy or
1 2 3 4 5 6 7 8 9 LOG ela a a2 type
OFSS| Petey lO etl. Ol LO tims Olek zed eller | mt allel 2: Olds toes Uae amiediLaG
266\2 16.6) LS. VL77 Lhe8 Tons sO Tee 8) 7" 188. 0) 182) 17k s.6
1.00} 23.2} 24.1) 28.5] 22.9) 23.8) 25.3) 23. 5) 24.9) 24.3) 23. 5) 24.3) 23.5) 23.9
1.33} 28.8] 28.9} 30.3] 27.1) 26.7} 31.7] 30.0} 30.6) 30.8) 30.2) 29.9) 26.8) 29.3
1.66] 33.3] 34.7] 35.3} 31.8) 32.4) 37.2) 35.2) 36.1) 35.9] 35.8) 35.8] 33.2) 34.7

TY PE 2.—2-INCH BITUMINOUS CONCRETE (DISTRICT OF COLUMBIA
SPECIFICATION); 6-INCH 1:3:7 CONCRETE BASE, CONNECTICUT
AVENUE, MARYLAND

12.3 11.6
18. 8) 18. 0)
25. 1) 24 9
31. 5) 32.1
37. 1| 37.8

ABE 5)
19. 1
25.3
31.5
37.5

| |
12.4) 11.5] 10.1) 11.0
18.2| 16.8) 15.3| 16.4
24.7, 23.1) 21.4) 21.6
30. 4] 29. 7| 27.0] 25.7
35.9 34.3) 31.6) 31.2

|
10. 2)
16.2
22. 6,
29. 4
33. 7|

0. 33
- 66
1. 00
1.33
1. 66

11.3
18.3
24.0
30. 2
35. 6

Wwhrrre
COS ie
aonowmncu

TYPE 3.—3-INCH COMBINED SHEET ASPHALT AND BINDER; 6-INCH
1:3:7 CONCRETE BASE, CONNECTICUT AVENUE, DISTRICT OF
COLUMBIA

| \
OHSS PLL Ti Oe LS: 0 AD Oe BEZe Gly DS ees eee os ee ee pe ee eae 12.1
66) AST TBS ASsO 20s 91> 19. BiG; Oi eee ee ee ee 19. 0
1. OO 2b.) ©2658) 26 cee es One 20517 | 6227 9] a eae as ae ee | eee ee ae een
LUBSlr (Oz. 2) NOL IS MUOle Ol rOke Lie Oost MO) Ul ge eel ects rem eee eee ee ee 31.8
INGE S 3819), w38. dl 88s dl ANS TIER BSr Ol S4eG | eee oie sale meee | se 25. ee (ee | 38. 2
| |

TYPE 4.—3-INCH COMBINED SHEET ASPHALT AND BINDER; 7-INCH
Basco ee, BASE, CONNECTICUT AVENUE, DISTRICT OF
sUMBIA

| | |

12. 8) 13. 2| 13.0) 12.6) 12.9) 13.
19. 8) rise | 20.0) 19.2} 20.6} 20.

0. 33

. 66
1. 00
1, 33
1, 66

27.4) 26.5) 26.7) 27.9) 27.
33. 7} 33.6) 34.0) 34.4) 35.
40.1) 40.1) 40.3) 41. 1 42. ¢

TYPE 5.—1-INCH SHEET ASPHALT ON SURFACE-TREATED, WATER-
a MERON er hic ee BASE, CHEVY CHASE CIRCLE, DISTRICT OF
A

0.33 aa 06 10.11 Pee ee ee [crea piaed Sse ee | ed ieee 9.6

466) T4119; Silt 459mm soiree lem ae ees ese Re ea se eee 13.9

1200) 2007) e17eal AO. Aires igen on I ok 2a oie ee Nee a oan aoe 19,2

1,99) $24. B) 21,7) OB dio st a ae FOREARCS ko ae Cea Waa | 23.8

TYPE 6—SURFACE-TREATED, WATERBOUND MACADAM, GEORGE-
TOWN-ALEXANDRIA ROAD, ARLINGTON COUNTY, VA.

2.—Tabulation of impact reactions expressed in thousand

pounds—Continued

TYPE 7.—6-INCH 1:2:4 CONCRETE

Test No.
]

1 fa eae Pha As 80% ele ao Sie 10 an rare
Height age
of drop fn
ones Under- type

Woodbine Street, | Williams Lane, wie eau iad
Maryland Maryland ’ See
Mary- Maryland
land ;
Opes medidas: 9/216 11.3) TD) “LORS LSS 4 LS eOy SiO esl eo eee ens
66) 17.6) L720) 172) 18) Wie 72) 18l0l Loi 1759) 18.4 SOO eee pee
1.00} 23.8] 22.7) 24.4] 25.0] 24.8] 24.7] 24.8] 23.9] 24.2] 24.8] 24.6/_____]_-___
133i 30, 1h Q8iem) © 80.8] sold, SB2eLi- Sl6/3i55) s02 LE) 2054 S20 Ol sc ke ee es
1.66) 36.4] 34.9} 37.3! 38.1) 38.4! 38.3] 37.5) 35. 5] 35.8] 39.1) 38.5 --__|___-.
| Chevy .
Northamp- 4-.| Nebraska
ton Street, | aah cas Avenue,
District of pistrirt of | District of
Columbia Columbin Columbia | |
| | or & as:
ONS3)) 1229) I2 4S Sai eas o6 | Pelsea Is iGleo eee Naoko eels ee ee ee 11.8
66. 20:8) 1928) -19. 8) 20. LF LOs4 20n 7S as Ss a | eee |e eee eee 18.7
100K 2625i) 26.3) 26511) 26.6) 27201926. 8| oe a eee ae re ee ee ee 20. 1
LS8l— B88) -B8.8l W327 Vas. Ol oa. 4) ae igi ee ee ea ene eee eee Olek
1.66) 3827} 238.6) 39:4)5 8058) “3858 39.41 22 Sees | se ee ee eee 38. 0:
TYPE 8.—6-INCH 1:2:3 CONCRETE
| Columbia Pike, Test road, Arlington Experi-
Virginia ment Station, Virginia
| iincae eel ho
0.33] 12.0) 12.8) 12:2) 11/8) 12.9 13. 1] 13.70 12.41 12) 4 ee a ee ee 12. 6.
- 66) 19.7) 19.5) 18.8) 18.7) 19.6} 19.4) 20.3) 20.8) 20. 7|_--.- Fasenosifo=? a 19.7
1.00} 25.8] 26.7! 26.8) 26.4) 26.7) 25.6) 27. 0 28:5 Li 2 7Slesaee ee eric = 26. 8.
USS 1S 4e2i B43) 3 He2l 382i ASS. LiLo Li 4a 41 on | oo eee pee ee 33.9
1.66) 40.6] 41.8) 42.1) 39.9) 39.1) 38. 5 40. 8 AT 6) 4053'S) ee 40. 6.

TYPE 9.—7-INCH 1:2:4 CONCRETE, CONNECTICUT AVENUE,

DISTRICT OF COLUMBIA

|
O33) 14.3) VIB eQ ee chee = Se eee = es ra ee ae ree ee Hetete
BOON 20. oleae ON Fiera ae | eee oral ete cea Ste See 8 2 eee eS ee | eee 20. 5s
1: 00h 27% SRB acy ale. ewe tee Ramee alia ones Fai (os Se es 27.1
P33!) (8332) 435..6|-o2e o2te eee allan on aS ce | ec 34.4
166} (89:9) €40) Slo02 see een |e theese lose =| eek eee eel eae see een ees 40. 3.
TYPE 10.—7-INCH 1:2:33 CONCRETE, COLUMBIA PIKE, VIRGINIA
| ll
€.33) 412, SNe 13d). 12: he 11, Sees a ene eee tc val cee Seem eae 12.5
= O6\) LOCSH MSS Oli TOS ee OuA eer Soe eee eee | eee 12 Sows Se oe ale eee 19.3
1.00) 25.9} 2973) 26. 7l- -26.8/-c ceo cascleocn toon. acer pean Was 0 26. 6
133) 32: 9)S 35.3235: 0] 83s Oe oe ae ee ee Pe ee here ee) ee ee 34.3
1.661 0-41.50 64d D4 jh) “40. Sif esc eu se oS Se ae ee Ss eee 41.1
TYPE 11.—8-INCH 1:2:33 CONCRETE
ae . . . | Test road, Arlington Experiment Sta-
Columbia Pike, Virginia tion, Virginia
0.33) T1912) 7) 1280 1S. Sh S1258d 353i) La Sa a2. Gee eee ee 13. O
+66) 20.0) 19.0) 20.4) 20:0) 19. 6) 19.5) 2058! 19) 41 1929) 20;5/ee ese 19.9
100) 2716)" 262-7) | 27, 9N 27 5ie (2743) QEn9 25735 Zones Oo eel eee 26.9
L338). 35.3) (84. Liss S8e7 85. J), 8816) 88. 7\d2. 4182241030. 6234s. | eee ers 34. 0
1.66) 41.6) 42.9) 41. “ 42.0} 39.1) 39.8) 38. 38. 7| 41. 2| 40; 3\_- oe 40.5

TYPE 12.—10-INCH 1:2:33 CONCRETE, TEST ROAD, ARLINGTON EX-

PERIMENT STATION, VIRGINIA

0. 33 ats

| 4) 11.1 Fi een eee eee ee eee allen | Ue || 4 ee 11.8
OBI LO:f7i P18. Ol S072. GIES oa ee oy | ee gs ee | eee ear jo Sa Soe 17.4
100} p21: G]y 222/91). 230| se. el eeriero aieieal a Bron fines Mime | ae es 22.6
133 206) mee! Ol 20: O/C Ee See ee ee a bee ae loaee’ ecdeet eee een. 28. 2
ih 31.7| 33.6 | Pe URE Sede eee 33. 2

0. 33

- 66
1. 00
1, 33

1. 66

10. 2
18.9
26. 8

40. 8

13.0
20. 4
27. 2|
33. :

12.9
19. 6
27.5
33.8
41.5

40. 8

————— '

ie Os

August, 1928

PUBLIC ROADS

TaBLE 2.—Tabulation of impact reactions expressed in thousand
pounds—Continued

TYPE 13—24%-BRICK ON %-INCH SAND CUSHION; 6-INCH 1:1¥4:3:
CONCRETE BASE, CIRCULAR TEST TRACK, ARLINGTON EXPERI-
MENT STATION, VIRGINIA

TY PE 14.—2144-INCH BRICK ON 34-INCH 1:4 SAND-CEMENT CUSHION;
6-INCH 1:144:33 CONCRETE BASE, CIRCULAR TEST TRACK, ARLING-
TON EXPERIMENT STATION, VIRGINIA

TYPE 15.—4-INCH BRICK ON %-INCH SAND CUSHION; 6-INCH 1:114:3
CONCRETE BASE, CIRCULAR TEST TRACK, ARLINGTON EXPERI-
MENT STATION, VIRGINIA

10. 9| iH

0.33 |) ate: a aa Hitel oe slciaee Ambac ack bee 9.9
Beaieira7 tio Tesla (Ce ee eo i RO aes 17.8
1.00) 24.9) 26.0) 28.4) 022222 o eel ae aa A eee 24. 8
1,33] 31.2 32.5] 30.7|------|------ SPS ES 0 ear a a ekere 2s ets 31.5

5) 6

1.66| 38. 2 39.

TYPE 16—4-INCH BRICK ON %-INCH 1:4 SAND-CEMENT CUSHION
6-IN CH 1:144:33 CONCRETE BASE, CIRCULAR TEST TRACK, ARLING-
TON EXPERIMENT STATION, VIRGINIA

Ossian, G2bp 10,9 Atel, A ese 3 Sa a ee es 0.6
ReOimUG am iTS. IES Geeta]. sche Ck oe eee Saas Raa Ire age (hey
POON Cao AIR OK A On Olon weal eee eae Ae eee Sate | 25.1
SSPE aoe lipeS4h Othe ee [ee eee ee Nb. OS ee ee eae Le ATs aa | 32.6
G61 R33: Mls, 40: 1) 0 40, Glare ol ee ele we ee eee Sere eee | 39.6

wide (in the case of brick sections this zone was 2,000
pounds wide), the values for each height of drop being
averaged and the average curve drawn through these
points. The simplified curves are shown in Figure 3
and will be used later in making certain comparisons.
In order to still further clarify the presentation of the
data, special limited groupings of types directly com-
parable are made for purposes of study. These will be
found in Figures 4, 5, 8, 9, and 13.

BITUMINOUS PAVEMENTS ON NONRIGID BASES SHOW CUSHION-
ING PROPERTIES

In considering the data, certain questions naturally
arose, and these have been set forth below, the per-
tinent data discussed, and, wherever possible, con-
clusions drawn.

1. Are bituminous-surfaced types (sheet asphalt,
Topeka, bituminous concrete, etc., laid upon rigid or
nonrigid bases) capable of cushioning or absorbing the
shock of impact forces? Referring to either Figure 2
or 3, 1t is indicated that surface-treated and partially
penetrated water-bound macadam bases are distinctly
cushioning to motor-truck impact, since for identical
impact conditions these types offer reactions which are
appreciably lower than those obtained for the rigid
types.

A marked tendency for the pavement to deflect over
a considerable area and then spring back was observed
in the case of both of the nonrigid types tested. This

4136—28——2

TOTAL IMPACT REACTION - THOUSAND POUNDS

HEIGHT OF DROP~ INCHES

1.—2-inch Topeka; 6-inch 1: 3 : 7 concrete base.
2.—2-inch bituminous concrete, District of Columbia specifica
tion; 6-inch 1: 3: 7 concrete base.
3.—3-inch combined sheet asphalt and binder; 6-inch 1:3:7
concrete base.
4.—3-inch combined sheet asphalt and binder; 7-inch 1: 3:6
concrete base.
aparece sheet asphalt; surface-treated water bound macadam
ase.
6.—Surface-treated water bound macadam.
7.—6-inch 1: 2:4 concrete.
8.—6-inch 1: 2:3 concrete.
9.—/7-inch 1: 2: 4 concrete.
10.—7-inch 1: 2: 3 concrete.
11.—8-inch 1: 2: 3 concrete.
12.—10-inch 1: 2:3 concrete.
13.—2%-inch brick; 34-inch sand cushion; 6-inch 1:
crete base.
14.—214-inch brick ; 34-inch sand-cement cushion; 6-inch1:114:3
concrete base.
ere brick ; 84-inch sand cushion; 6-inch 1: 144 : 3 concrete
ase.
16.—4-inch brick; 34-inch sand-cement cushion; 6-inch 1: 144: 3
concrete base.

1% :3 cons

Fig. 2.—CHARACTERISTIC CURVES FOR Hacu Type or PAvVE-
MENT TESTED

deflection amounted to at least one-half inch directly
under the impact wheel and was noticeable even at a dis-
tance of 2 or 3 feet from the center of impact. The de-
flected area seemed to resume its original position after
removal of the load except at the point where the impact
wheel had struck; here it was permanently deformed.
In the case of the rigid types, however, the deflections
were obviously very small, so small, indeed, as not to
be apparent to the eye, although they could be felt.
The lowering of impact reactions in the case of the
nonrigid types is attributed to this characteristic,
which leaves the observer with the same impression he
acquired when he had learned to ease the shock of a
hard-thrown ball by moving his hands in the direction

118

PUBLIC ROADS

Vol. 9, No. 6

TOTAL IMPACT REACTION - THOUSAND POUNDS

HEIGHT OF DROP - INCHES

1 and 2.—2-inch Topeka and 2-inch bituminous concrete;
6-inch 1: 3: 7 concrete base.
3.—3-inch combined sheet asphalt and binder; 6-inch
1: 3:7 concrete base.
4.—3-inch combined sheet asphalt and binder; 7-inch
1: 3:6 concrete base.
5.—1-inch sheet asphalt; surface-treated water-bound
macadam base.
6.—Surface-treated water-bound macadam.
7.—6-inch 1:2 : 4 concrete.
Sito 12— 6.758, andlOlinch). 13.2503 concrete /-inehi ia2ea4,
concrete.
13 to 16.—21% and 4 inch brick on sand and sand-cement cush-
ion; 6-inch 1: 14% : 3 concrete base.

Fic. 3.—SIMPLIFIED CHARACTERISTIC CURVES DERIVED BY
GROUPING INTO ONE CuRVE ALL CURVES, FOR SIMILAR
Types, WHosE Maximum Reactions Lay WITHIN A ZONE
1,000 Pounps Wipr, Excerpt ror Brick SEcTIONS WHERE
THE ZONE wAs 2,000 Pounps WIDE

of the ball’s motion at the instant of impact rather than
to depend entirely upon the cushioning properties of
the padding in his glove—the impression that cush-
ioning is dependent upon relatively large deflections.
Attention was called to this in a previous article in
Public Roads.‘

Barring the flexible types which have just been dis-
cussed and considering only the rigid types of bitumi-
nous-surfaced pavements, it is not as easy to arrive at
definite conclusions as it was in the former case. Re-
ferring to Figure 3, it will be seen that 2-inch Topeka
and bituminous concrete laid on 6-inch 1:3:7 concrete
base, 3-inch sheet asphalt and binder on 6-inch 1:3:7
concrete base, and 38-inch sheet asphalt and binder on
7-inch 1:3:6 concrete base, occupy lower, middle, and
upper positions, respectively, of the zone covered by
the rigid types. Figures 4 and 5 also show these curves
compared with closely comparable types.

4 Teller, L. W., Impact Tests on Concrete Pavement Slabs, Public Roads, vol. 5,
No. 2, April, 1924.

TOTAL IMPACT REACTION-THOUSAND POUNDS

HEIGHT OF DROP-INCHES

1.—6, 8, and 10 inch 1: 2: 3 concrete and 7-inch 1:2: 4 concrete.

2.—2-inch combined sheet asphalt and binder; 6-inch 1:3: 7
concrete base.

3.—6-inch 1: 2:4 concrete.

4,—2-inch Topeka and 2-inch bituminous concrete; 6-inch
1: 3:7 concrete base.

Fic. 4.—Limitep GROUPING OF CURVES FOR COMPARATIVE
PURPOSES

Whether the reduction in reaction of approximately
10 per cent indicated in ‘Figure 3 for the 2-inch bitu-
minous top on 6-inch 1:3:7 base, as compared with the
uncovered concrete pavement, is really due to cushion-
ing properties of the top is open to considerable doubt.
In the first place, we have no way of knowing whether
or not the concrete base was badly cracked at or near
the point of test. This pavement, Connecticut Avenue,
Chevy Chase, Md., was laid by the Bureau of Public
Roads in 1912 for experimental purposes. In it are
concrete sections without tops laid at the same time as
the bituminous-surfaced sections under discussion and
of the same thickness, namely, 6 inches. These have
no top, but are of a richer mix (1:134:3) and have been
subjected to the same traffic. These concrete sections
are badly cracked, and it is therefore reasonable to
suppose that the concrete base under the 2-inch top is
likewise badly cracked. Such a condition would, of
course, have the effect of lowering the reactions.®

Similar doubt clouds the small reduction shown ‘by
the 3-inch sheet asphalt and binder laid on 6-inch
1:3:7 base as compared with the uncovered concrete

5 Since these tests were completed, in the course of another investigation, several
cores were drilled from the base of the section just described, and it was found that
the base is badly cracked, although in general the alignment of the base surface is
well preserved.

August, 1926

TOTAL IMPACT REACTION-THOUSAND POUNDS

HEIGHT OF DROP-INCHES

3.—3-inch combined sheet asphalt and binder; 7-inch 1: 3:6
concrete base.

9.—/7-inch 1: 2: 4 concrete.

10.—7-inch 1: 2: 3 concrete.

12.—10-inch 1: 2: 3 concrete.

Fic. 5.—Limirep Grovupine or CURVES FOR COMPARATIVE
PURPOSES

pavement. This pavement was laid in 1920 and
shows from numerous surface cracks and sunken spots
evidence of base cracking, although the test sections
were chosen at places where the pavement showed a
minimum of such symptoms.

The 3-inch sheet asphalt and binder on 7-inch 1:3:6
base (fig. 5) was, fortunately, a new pavement, com-
pleted about two weeks before it was tested. It shows
no reduction in reaction whatever as compared with a
7-inch 1:2:4 uncovered slab, for example, which
strengthens the belief that possibly base cracking was
responsible for the reductions discussed above.

Due to the impracticability of observing the condi-
dition of the bases mentioned above, the foregoing
explanations are admittedly not infallible.

In view of the foregoing it might be said that the
data support the conclusion that while the bituminous-
surfaced pavements show, in general, somewhat lower
reactions for the same impact conditions than do the
unsurfaced concrete pavements, these differences are
not consistent and are of no greater magnitude than the
differences found between the various unsurfaced con-
crete pavements tested.

TEMPERATURE EFFECTS CONSIDERED

2. Do the impact reactions offered by bituminous-
surfaced pavements vary with changes in pavement
temperature? It is reasonable to believe that the
behavior of bituminous-surfaced pavements would be
affected by temperature, and the data were analyzed

3INCH COMBINED SHEET ASPHALT AND BINDER ON
7-INCH '1:3:6 CONCRETE BASE

3-INCH COMBINED SHEET ASPHALT AND BINDER ON
6-INCH 1:3:7 CONCRETE BASE

ACT REACTION FOR 12-INCH HEIGHT OF DROP - THOUSAND POUNDS

SERIES NO.Z
# SAME TEST SECTION

& SAME TEST SECTION

SERIES NO.!
SS@ 2-INCH TOPEKA ON 6-NCH 1:3:7 CONC. BASE
~y4 2-INCH BIT.CONC.(DIS.COL.SPEC) ON G-INCH |:3:7 CONC. BASE

TOTA

PAVEMENT TEMPERATURE- DEGREES FAHR.

Fie. 6.—Curves Suowina INFLUENCE oF PAVEMENT
TEMPERATURE UPON Impact REACTION

with this in mind. Referring to Figure 6, it will be
seen that for types employing a bituminous top upon a
rigid base no consistent or marked change occurs, and
the question must be answered in the negative for the
types and range of temperatures studied. Limited
data in the case of one nonrigid type (1-inch sheet
asphalt on surface-treated water-bound macadam)
does, however, show a consistent indication of decrease
in reaction with increased temperature, the impact
reaction being 10 per cent higher at a pavement tem-
perature of 78° F. than at 106°.

3. Do the permanent deformations caused by succes-
siveimpact blows upon a bituminous-surfaced pavement
increase as temperatures increase? By referring to Fig-
ure 7 it will be seen that for the types and temperatures
studied there is evidence of a very small increase in per-
manent deformations for certain types employing a
rigid base, but the evidence is neither marked nor con-
sistent. In the case of one nonrigid type (1-inch sheet
asphalt on surface-treated water-bound macadam base)
there is, however, a marked increase, the deformation
at 106° F. being four times as great as at 78°.

4. Do the reactions of a given bituminous-surfaced
pavement on rigid base increase with additional com-
pacting, such, for example, as would be occasioned by

PERMANENT PAVEMENT DEFORMATION -INCHES

-50

.30

-20

PUBLIC ROADS

Vol. 9, No. 6

3-INCH COMBINED SHEET ASPHALT AND BINDER ON 7-INCH 1:3:6 CONCRETE BASE

3-INCH COMBINED SHEET ASPHALT AND BINDER ON 6-INCH 1[:3:7 CONCRETE BASE

BERERBSS Caen

1-INCH SHEET ASPHALT ON SURFACE TREATED WATERBOUND MACADAM a

@ 2-INCH TOPEKA ON 6-INCH 1:3:7 CONCRETE BASE

xX 2-INCH

DISTRICT OF COLUMBIA SPECIFICATION, BITUMINOUS CONCRETE ON 6-]NCH 1:3:7 CONCRETE BASE

PAVEMENT TEMPERATURE - DEGREES ‘FAHR.

Fie.

repeated impact blows upon the same spot? On several
occasions after the necessary 15 blows of increasing inten-
sity had been struck, the machine was moved about a
foot and blows struck from the maximum height of drop
only. The reactions offered by the second spot differed
in no essential from those corresponding to maximum
height of drop for the first spot. The question must,
therefore, be answered in the negative.

RESULTS ON UNCOVERED CONCRETE DISCUSSED

Considering uncovered concrete sections as a
ee what effect, if any, does the thickness have upon
impact reactions? As the difference in deflection of 6,
8, and 10 inch pavement slabs under a given load is
relatively small, one would not expect material varia-
tions in reaction for concrete pavements within this
range, and the data indicate this to be true. Referring
to Figure 8, it will be noticed that all save one of the
characteristic curves of uncovered concrete types are
bunched, the only one showing a significant difference
being the 6-inch 1:2:4 type. The fact that this one
type. does not line up with the others leads one to the
belief that one can expect as great differences to exist
between the uncovered types as is to be found when
comparing the covered and uncovered types.

7.—CURVES SHOWING INFLUENCE OF PAVEMENT TEMPERATURE UPON PAVEMENT DEFORMATION

BRICK PAVEMENTS SHOW ONLY SLIGHT CUSHIONING

6. Do brick pavements, consisting of a concrete base,
either a sand or sand-cement cushion, and a vitrified
brick wearing surface exhibit any cushioning or shock-
absorbing properties? In Figure 3 it will be seen that
the characteristic curve for such types occupies a
position corresponding closely to that of the average of
all uncovered concrete sections. It seems, therefore,
that this type of pavement does not exhibit cushioning
properties to any significant extent.

Referring to Figure 9, it will be seen that, for a

given thickness of brick, sand and sand-cement cush-

ions yield about the same reactions, but that there is
some indication that the 21-inch brick sections give
rise to somewhat higher impact reactions that do the
4-inch sections. The difference is small, however, and
is not thought to be particularly significant i in view of
the limited number of tests made.

The fact that there appears to be little difference in
the reactions for brick surfaces laid on the two types
of bedding material is, however, of considerable interest
in view of the recent accelerated traffic ‘tests on the
same sections, in which it was found that there was
about twice as much breakage where bricks rested on

August, 1928

PUBLIC ROADS

TOTAL IMPACT REACTION - THOUSAND POUNDS

1.00

HEIGHT OF DROP - INCHES
7.—6-inch 1: 2:4 concrete.
8.—6-inch 1: 2 : 3 concrete.
9.—7-inch 1:2: 4 concrete.
10.—/-inch 1: 2: 3 concrete.
11.—8-inch 1: 2: 3 concrete.
12.—10-inch 1; 2: 3 concrete.
Fia. 8.—LIMItep GROUPING OF CURVES FOR COMPARATIVE

PURPOSES

sand-cement cushions as when they rested upon sand
cushions.°®
SUMMARY OF CONCLUSIONS

1. For the limited data in hand it is indicated that
bituminous pavements of the nonrigid type, such as
surface-treated waterbound macadam, may substan-
tially cushion the effect of impact forces.

2. Bituminous-surfaced pavements, such as sheet
asphalt, Topeka, etc., laid upon concrete bases show,
in general, some indications of cushioning impact forces,
but the magnitude of this cushioning effect appears to
be relatively small, and in some cases there is consid-
erable doubt as to its actual existence. It should also
be pointed out that the differences observed between
reactions for the bituminous-surfaced pavements and
those for the unsurfaced ones are no greater than the
differences found to exist between reactions for the
various sections of unsurfaced concrete pavement.

3. In general, the impact reactions of bituminous-
surfaced pavements on rigid bases fail to show any
marked or consistent change with changes in pavement
temperatures up to 106° F. However, in the case of
one nonrigid type of bituminous pavement (1-inch sheet
asphalt on waterbound macadam base) a consistent
substantial decrease in reaction with increased tem-
peratures was noted.

6 Pauls, J. T., and Teller, L. W., Thin Brick Pavements Studied, Public Roads,
vol. 7, No. 7, September, 1926.

; Vy

23-INCH BRICK, #-INCH SANO-CEMENT CUSHION yA
23-INCH BRICK , 3-INCH SAND CUSHION Af / |

4-INCH BRICK, 3-INCH SAND-CEMENT CUSHION Wb

4-1NCH BRICK , FINCH SAND CUSHION

30
26 +

33 1.00 1.66

TOTAL IMPACT REACTION- THOUSAND POUNDS

HEIGHT OF DROP - INCHES

Fig. 9.—Limirep GROUPING OF CURVES FOR BriIcK SEC-
TIONS FOR COMPARATIVE Purposres. ALL Brick Larp
Upon 6-1ncH 1:14 :3 ConcRETE

4. The data regarding the permanent deformation
due to successive impact forces upon bituminous-sur-
faced pavements on rigid bases show only a small and
not very consistent tendency to increase with increas-
ing temperatures up to 106° F. The limited tests of
nonrigid types, however, show a marked tendency for
these types to suffer greater permanent deformations
as the pavement temperatures increase.

5. The impact reactions of bituminous-surfaced pave-
ments on rigid bases show no tendency to increase as
the section becomes more compact, due to repeated
impact blows.

6. Within the range of thicknesses studied, impact
reactions of uncovered concrete pavements do not ap-
pear to be affected materially or consistently by varia-
tions of slab thickness.

7. Brick types in which the brick wearing surface was
bituminous filled and rested upon a sand or sand-
cement bedding course on a concrete base show no
marked tendency to cushion impact forces.

8. The reactions of brick types employing plain sand
bedding courses are practically the same as those of
types employing sand-cement bedding.

RESULTS WITH IMPACT MACHINE CONFIRMED BY TESTS WITH
MOTOR TRUCK EQUIPPED WITH ACCELEROMETER

It was thought advisable to conduct the main tests
with the impact machine and to supplement them with
tests using actual motor-truck impacts. It was recog-
nized that impacts could be much more closely con-
trolled with the impact machine, but data obtained
with the truck were suggested as a means of making
sure that the factors affecting the impact-machine reac-
tions would also affect actual motor-truck impacts.

T22 PUBLIC ROADS Vol. 9, No. 6

AVERAGE OF ALL TESTS
OF THIS Bie

AVERAGE OF ALL TESTS
ORS TEISS Eee

IMPACT REACTION - THOUSAND POUNDS
IMPACT REACTION-THOUSAND POUNDS

SPEED OF TRUCK-MILES PER HOUR SPEED OF TRUCK -MILES PER HOUR
Fic. 10.—Moror-Truck Impacts on a 2-1ncH ToPEKA Fy, 12—Moror-Truck Impacts on A 6-INcH 1: 2:4
SURFACE ON A 6-INCH 1:3:7 BASE ConcRETE

MOTOR TRUCK IMPACTS
AVERAGE CURVES FOR THREE TYPES TESTED

6-INCH, 1:2:4 CONCRETE
2-INCH, BIT. CONC. ON 64NCH I:3:7 BASE
2-INCH, TOPEKA ON 6-INCH 1:3:7 MEI:

AVERAGE OF ALL TESTS
OF THIS TYPE

IMPACT MACHINE IMPACTS

6-INCH, I:2:4 CONCRETE
2-JNCH, BIT. CONC. ON 6-INCH 1:3:7 BASE
2-INCH, TOPEKA ON 6-INCH 1:3:7 BASE

IMPACT REACTION - THOUSAND POUNDS

IMPACT REACTION- THOUSAND POUNDS

2 6 10 14 18 22 26 30

SPEED OF TRUCK- MILES PER HOUR

aK) 1.00 1.66:

(0) 8 12 | 0 24
: : 6 HEIGHT OF DROP OF IMPACT MACHINE

SPEED OF TRUCK- MILES PER HOUR
Fic. 13.—Limitep Grouping SHowina Moror-TrRucK AND:

Via. 11.—Moror-Truck Impacts on A 2-INcH BrrumMINous Impact-MacHINE REACTIONS FOR THE THREE TYPES FOR
CONCRETE ON A 6-INcH 1:3:7 Basn wuHicH ActuaL Motor-Truck Impacts ARE AVAILABLE

August, 1928

PUBLIC ROADS

Accordingly, when sufficient data had been secured
with the impact machine to indicate that some differ-
ences existed between the characteristic curves for the
first three types tested, namely, 2-inch Topeka on
6-inch 1:3:7 base, 2-inch bituminous concrete on 6-inch
1:3:7 base, and 6-inch 1:2:4 concrete (curves 1, 2, and

fig. 2), it was thought desirable to see if similar
characteristic curves, developed by actual motor-truck
impacts, would show a tendency to differ in the same
general direction. Accordingly, a 2-ton truck with
rear tires comparable to the tire used on the impact-
machine plunger was run over a j7-inch inclined
obstruction in such a manner that one rear wheel would
strike upon the pavement at a section’ previously
tested by the impact machine.

The force of this impact was measured by means of
an accelerometer ° essentially the same as that used in
the impact-machine tests, the instrument being
mounted on one of the rear wheels. The truck was so
loaded that impacts delivered by it at varying speeds
would about cover the range of impacts secured with
the impact machine.

The speed of the truck was determined by means of
stop-watch measurements and checked and controlled
be speedometer observations.

The data obtained have been plotted in Figures 10, 11,
and 12 for the three types tested and the average results

7 The impact-machine wheel was made to strike 3 feet 4 inches from the pavement
edge, the truck wheel 8 feet from the same edge. This was necessary on account of
traffic conditions.

8 For a more detailed description of this apparatus see Public Roads, vol. 7, No. 4,
Motor Truck Impact as Affected by Tires, Other Truck Factors, and Road Rough-
ness, by James A. Buchanan and J. W. Reid.

ACCELEROMETER. THE REAR

Truck ECUIPPED WITH
WHEEL 1s ABoutT TO DRoP FROM THE WEDGE-SHAPED
OBSTRUCTION

for each of these types plotted separately in Figure 13,
upon which have also been plotted the impact-machine
reactions for these types for comparative purposes.

From Figure 13 it will be seen that the motor-truck
impact reaction curves show a definite tendency to
differ in the same general direction as do the char-
acteristic curves obtained with the impact machine and
by about the same general amounts.

TRUCK IS A BIG FACTOR IN FRUIT TRANSPORT’

The marked increase in the use of motor trucks for
hauling farm produce direct from farms to markets is
shown in a survey in New York City which brought
out that from 20 to 30 per cent of the supply of leading
fruits on the New York market is hauled into the city
by motor truck.

For about three months in midseason New York
gets nearly one-third its peach supply, one-fourth its
tomatoes, and one-fifth its apples, by motor truck, and
sometimes during the busy season more than one-half
the New Jersey produce supply moves in trucks. The
survey was made by the Bureau of Agricultural Eco-
nomics of the United States Department of Agriculture
and the New York Food Marketing Research Council
cooperating.

The tendency to change from the horse-drawn wagon
or the railroad car to the motor truck has been going
on for a dozen years. The New York dealers wanted
to know just how much produce was coming by truck
and about 45 of them agreed to give by telephone each
morning the number of packages of each product
received by truck, according to the State of origin.

The investigation was limited to five important lines

of produce—peaches, cantaloupes, tomatoes, apples,
and peppers. The peeer ane started July 20, and con-
tinued through the peak period to October 22. Infor-

mation gathered in this way does not give the complete
total of receipts coming by motor truck into the New
York area. Produce arrives by this means at the
Newark, Harlem, Gansevoort, and Wallabout markets,
us well as the several farmers’ markets, but it appears

1 Digested from article under the same title appearing in the Official Record,
United States Department of Agriculture, August 1, 1928.

that the bulk of New Jersey motor-truck shipments
sues in New York City has been accounted
or.

During the period of investigation the five products
reported were shipped by motor-truck to New York
from points as distant as Virginia, Maryland, and
Delaware, as well as from the near- -by sections of New
Jersey, Pennsylvania, Long Island (N. Y.), and
Connecticut. Excepting cantaloupes from Virginia,
Maryland, and Delaware, the New Jersey motor- truck
receipts so far exceeded those from other States that
the latter appeared insignificant.

Thirty per cent of all peaches arriving at New York
during the period reported came by motor truck.
Twenty-five per cent of the tomatoes, 20 per cent of
the apples, and 9 per cent of the peppers also arrived
in this way.

Taking a single week during the height of the season
the truck receipts of peaches were 58 per cent of all
peach receipts, apples 78 per cent, tomatoes 52 per
cent, and peppers 16 per cent.

Some of the wholesale dealers do not lke this ten-
dency of change to motor-truck service because they
can not hold the truck operator lable for injury to
produce during the trip to market. Something may
have to be done to provide insurance protection or
direct liability on truck loads. Such protection in
one way or another adds something to the costs of the
trucking service. No other drawbacks from the point
of view of the dealer are mentioned.

It is claimed that motor trucking to market helps
the producers who are outside the old market-gardening

(Continued on page 128

THE “DESIGNVOEVRBAVENMENT: CONGR ERE nese lites

WATER-CEMENT

RATIO METHOD

Reported by F. H. JACKSON, Senior Engineer of Tests, Division of Tests, United States Bureau of Public Roads

tions of cement, sand, and coarse aggregate for

concrete, even though it may in most instances
provide a satisfactory job from the standpoint of
quality, is at best unsound from an economic point of
view. ‘This is true because in order to insure concrete
of the designed strength under conditions which involve
the possible use of a variety of materials it is necessary,
when using fixed proportions, to adjust them on the
basis of the most unfavorable combination possible
under the specification. This is, of course, on the
assumption that within the usual specification limits
variations in such factors as character and grading of
aggregates and quality of cement appreciably affect the
quality of the concrete.

The investigation by the Bureau of Public Roads in
cooperation with the New Jersey State Highway Com-
mission,! as well as the tests now under way at Arling-
ton, indicate that, in so far as character of aggregate
is concerned, such variations may influence to a marked
degree the transverse and tensile strength of the
concrete, even though the crushing strength may be
but slightly affected. Investigations conducted by the
bureau in cooperation with the American Association
of State Highway Officials indicate clearly that varia-
tions in the physical properties of Portland cements,
all meeting the American Society for Testing Materials
specifications, may quite appreciably affect the strength
of the concrete.

From the standpoint of yield, also, it is well known
that, under the present system of proportioning, varia-
tions in yield will occur, due to both type and gradation
of aggregates. Such variations lead to fluctuations in
the cement factor which are frequently the cause of
misunderstandings and arguments between engineers
and contractors. From many standpoints it seems
desirable to so modify our procedure as to take advan-
tage of such variations in aggregates and cement as
normally occur in a given locality, as to produce con-
crete of the required strength at a minimum cost, and
at the same time to provide such methods of handling
and measuring the materials as will insure the produc-
tion of fixed and uniform quantities of concrete. In
this paper an attempt will be made to develop such a
method by utilizing the well-established water-cement
ratio law, and at the same time taking into consideration
the various factors which render it impossible to make
a general application of that law, as has been attempted
by some authorities in the past.

The suggested method of design has already been
tried with success on building construction, and no
originality is claimed by the writer. This method will
be developed, together with a discussion of changes in
methods of securing bids on concrete-paving projects
which, it is believed, will be necessary in order to make
the suggested scheme of design effective.

r ‘HE present method of specifying arbitrary propor-

1 Jackson, F. H., Comparative Tests of Crushed-Stone and Gravel Concrete in
New Jersey, Public Roads, vol. 8, No. 12, February, 1928.

124

WATER-CEMENT RATIO THEORY DISCUSSED

It is now almost universally recognized that there is
a well-defined relation between the strength of concrete
and the water-cement ratio for any given combination
of materials. Many tests have also indicated that this
relation when plotted takes a form which may be ex-

pressed by the general equation S = = This is the gen-

=
eral form of the well-known formula derived by Abrams.”
Values for A and B depend upon the particular com-
bination of materials used, as well as the character of
the stress being investigated. S represents the strength
of the concrete at 28 days and x an exponent, the water-
cement ratio. To use the formula it is necessary to
determine the constants for the particular materials
being investigated, which, of course, must be done
experimentally by testing a series of concrete specimens
made with various water-cement ratios and plotting
the strengths obtained against the corresponding ratios.

Before proceeding to a discussion of the principles
governing the proposed method of designing concrete
it may be well to state that, because of the experimental
work involved, it will be necessary in the application
of this method to have available a well-equipped labo-
ratory with a qualified concrete testing engineer in
charge. The designing of a concrete mixture to be
used in a structure which is guaranteed to meet certain
requirements as to strength, durability, etc., is just as
much a technical operation requiring the services of
a trained personnel as is the designing of the structure
itself.

Furthermore, all attempts which have been made to
design concrete through the application of certain
formulas based only on considerations of grading of
aggregates, such as fineness modulus, grading factor,
surface area, etc., have failed, at least in so far as con-
crete for pavements is concerned, in one important
respect—they do not take into account the character
of the aggregates employed. By character is meant
not only type—that is, crushed stone, gravel, ete.—but
such factors as surface texture, angularity of frag-
ments, etc. These factors affect the quality of the
concrete in two ways—first, by influencing workability,
which in turn controls the ratio of fine to coarse aggre-
eate as well as the relative water content and, second,
through the adhesion or bond which is produced
between the cement and the aggregate surfaces.

The effects of such factors are particularly noticeable
when the concrete is subjected to tensile and flexural
stresses and are therefore of importance to the high-
way engineer. They apply alike to fine and coarse
aggregates and explain why the experimental or trial
method of design must be used. In other words, we
have not yet reached the point where we can entirely
discard actual tests of trial mixtures in favor of mathe-
matical formulas in the design of concrete mixtures.

2 Abrams, D. A., Design of Concrete Mixtures, Bull. 1, Structural Materials
Research Laboratory, Lewis Institute, Chicago, Il.

i
i
;
;

August, 1928 PUBLIC

ROADS 1235

There are, however, certain fundamental principles
underlying all methods of concrete design which must
be thoroughly understood by everyone who intends to
use the so-called trial method, and these will be dis-
cussed briefly before giving the various steps in the
suggested method.

METHOD OF DETERMINING RATIO OF FINE TO COARSE AGGREGATE
DESCRIBED

The first question to decide is the proper ratio in
which to combine the various fine and coarse aggregates
which are available for a given job, giving in each case
due consideration to both workability and economy.
There are four general rules which may be applied to
this particular problem, as follows:

(1) The proportion of sand should be increased as
the sand becomes coarser.

(2) The proportion of sand should be increased as
the maximum size of the coarse aggregate becomes
smaller.

(3) The proportion of sand should be increased as
the percentage of fine material in the coarse aggregate
becomes smaller.

(4) The proportion of sand should be increased as
the percentage of angular fragments in the coarse aggre-
gate becomes larger.

These principles are well known. The average speci-
fication for concrete, however, recognizes them only in
a general way, usually by a clause giving the engineer
the power to slightly change the proportion of fine to
coarse aggregate to secure maximum density. It
should be possible in designing the mix to fix this ratio
much more accurately than is possible under the pres-
ent arbitrary method. The most important point to
remember is that a balance will have to be struck be-
tween a high sanded mix, which, although workable, is
apt to be uneconomical, due to the fact that, for a con-
stant water-cement ratio, more cement will be required
for a given consistency, and a low sanded mix, which,
although economical in so far as cement content is
concerned, is apt to give trouble in placing.

In the writer’s opinion the ideal combination is the
one in'‘which the voids in the coarse aggregate are main-
tained at a minimum, so as to permit the use of the
smallest amount of mortar possible and still have a
workable mix. In order to do this, the grading of the
coarse aggregate must be controlled very carefully
throughout the entire job, and this can best be done
by handling and measuring it in separate sizes. This
method serves also to eliminate segregation, and thus
makes possible the use of a larger maximum size of
coarse aggregate, which is economical from the stand-
point of cement required. In the case of crushed
stone the use of a larger size involves less crushing and
is therefore more economical. Too little attention has
been paid to such details in the past with the result
that, although most of our concrete may be, and prob-
ably is, of satisfactory quality, it has not been designed
so as to make the best use of a closely controlled coarse
aggregate grading, which is the only way maximum
workability can be attained with a minimum of cement.
Just how far we can go in any particular case will
depend, of course, upon the materials available, meth-
ods of finishing to be employed, ete. In general, there
is no reason for using more mortar than is necessary to
secure the proper finish. Under such a condition it
will usually be found that there is enough mortar pres-
ent to fill the voids in the coarse aggregate with a

slight excess, and it is believed that, under our modern
methods of finishing, this will prove sufficient in prac-
tically all cases to secure a dense, homogeneous con-
crete free from honeycomb.

For the usual run of materials for concrete roads the
proper ratio of the volume of fine to course aggregate
will range from a 30:70 ratio for a relatively fine sand
combined with a closely graded easy-working coarse
aggregate to a 40:60 ratio for a coarse sand combined
with a high-void, harsh-working coarse aggregate. As
previously stated, effort should be made when studying
various possible combinations to keep as near the former
ratio as possible, for the sake of economy, always
remembering that the final value to use will depend
entirely upon whether it is possible to secure a satis-
factory finish and a concrete free from honeycomb
with the placing and finishing equipment to be used
on the job.

Using modern methods of handling it is believed
that this condition can be attained frequently with a
lower sand content than was possible with the old
hand-finishing methods. As far as this factor is
concerned, it is necessary to fall back upon judgment
backed by actual observations on the job, rather than
to rely entirely upon set formulas, helpful as they
may be in giving preliminary indications.

As a guide, however, in making such a preliminary
estimate of the proper ratio to use in any specific case,
the values given in Table 1 may be taken. In general,
these ratios are about the same as would be obtained
by the use of the fineness modulus method suggested
by the Portland Cement Association,® except that
in no case is the percentage of fine aggregate less than 25
or more than 45 per cent of the sum of the volumes of the
fine and coarse aggregate measured separately. It will
be observed that these values illustrate the principles
governing the proper ratio of fine to coarse aggregate

TABLE 1.—Approximate ratios, by volume, of fine to coarse aggre-
gate for paving concrete, machine finished

| Fine aggregate—size limits

Coarse aggregate size limits | 7

| 0-No. 4 |0-34-ineh

| 0-No. 16 | 0-No. 8
| | eas 2
No. 4 to 34-inch 35:65 | 37:63 40:60 | 45:55
INos4: topic Sees Ss err are eae 30:70 | 32:68 85:65") 40:60
IN Oa 4 t0:2-1Nn Chics sawn Nee Meee es See | 25:75 | 30:70
|

27:73 | | 35:65

Notrrs.—The above values are based on the use of the usual
type of natural sand combined with a coarse aggregate consisting
essentially of rounded fragments. With coarse aggregate con-
sisting essentially of angular fragments it may be necessary to
increase the percentage of sand slightly over the values above
given.

It has been assumed that the concrete will be machine finished.
For hand-finished work the percentage of sand may have to be
increased somewhat.

For an aggregate to be given a certain maximum size, at
least 15 per cent must be retained on the next smaller sieve shown
injthe table. For instance, a sand having 16 per cent retained
on.a No.8 sieve is classed as a 0O-No. 4 sand. For a sand to be
classed as a 0—No. 16 sand, at least 15 per cent must be retained
on a No. 380 sieve.

All coarse aggregates are assumed to be reasonably well
graded from the maximum size to No. 4, with not more than 15
per cent passing the No. 4 sieve.

The use of a O0-No. 16 sand is not recommended, except under
conditions where a coarser sand is not available, on account
of the fact that concrete in which a fine sand is used is in general
not quite so resistant to wear as when a coarse sand is used.

3 Design and Control of Concrete Mixtures, published by the Portland Cement
Association, Chicago, Il.

126 PUBLIC

ROADS Vol. 9, No 6

~ >

given above. The values given in the table do not
represent necessarily the final ratios to use. The best
final values in any case can only be determined by trial,
bearing in mind that the smallest amount of sand
consistent with workability should ordinarily be used.

DETERMINATION OF PROPORTIONS BASED ON LABORATORY TESTS

Having tentatively fixed the proper ratio of fine to
coarse aggregate for each available combination, the
next step is to determine in each case how much cement
and water to use to secure concrete meeting the re-
quirements of the specification. For purposes of dis-
cussion it will be assumed that a minimum flexural
strength (modulus of rupture) is specified. It is recog-
nized that strength is not the only criterion upon which
to judge the quality of concrete. Durability and resist-
ance to wear are of great importance. However, it
must be confessed that at present we are able to talk
only in generalities with regard to durability. We
have no definite requirements which may be set up in
specifications. About all that we can do, assuming
that the aggregates themselves possess the necessary
properties as regards durability and resistance to wear,
is to assume that concrete which will meet the strength
requirements and which is sufficiently workable to be
placed and finished in a satisfactory manner will be as
permanent as it is possible to make it with our present
knowledge of the art.

It will be assumed that the specifications call for a
concrete which when tested under standard laboratory
conditions will have a certain modulus of rupture, say
600 pounds per square inch at 28 days. The problem
is to determine the most economical mixture which will
give this strength and at the same time be sufficiently
workable to place and finish properly on the job. Unfor-
tunately at the present time it is impractical to attempt
to control the quality of the cement to be used on the job
further than to require that it pass the American Society
for Testing Materials requirements. In trial determina-
tions, therefore, it will be necessary to use a cement
which corresponds to about the lowest-strength cement
likely to be used on the job. The use in construction of
any higher-strength cement than this simply serves to
provide an additional factor of safety, in so far as the
cement is concerned. Having selected the cement, it will
now be necessary to fix experimentally the relation be-
tween the water-cement ratio and the flexural strength
for this laboratory cement, using stock aggregates of
known satisfactory quality. The determination of the
strength developed at 28 days with water-cement ratios
0.6, 0.7, 0.8, 0.9, and 1.0 will usually give enough points
from which to develop this relation. Such a relation
for an assumed case is shown in Figure 1. It will be
observed that a ratio of 0.7 gives a strength of approxi-
mately 600 pounds per square inch.

The next step is to make up concrete specimens with
each of the aggregate combinations, using 0.7 water-
cement ratio and the consistency which will be used
on the job. It is important in this experiment to main-
tain the consistency as nearly constant as possible.
With a constant water-cement ratio this will necessitate
variable proportions, depending upon the type and
gradation of the materials. The proper amount of
cement to use in each case must be obtained by trial,
adding small quantities of the aggregates in question to
the cement paste until the proper consistency has been
reached. The predetermined ratio between fine and

MODULUS OF RUPTURE-POUNDS PER SQUARE INCH

WATER - CEMENT RATIO

Fig. 1.—TypicaL ReLAaTiIon BETWEEN WATER-CEMENT RaTIO
AND FLEXURAL STRENGTH OF CONCRETE AT 28 Days

coarse aggregate must be maintained throughout the
operation. From the final quantities used the pro-
portions either by weight or by volume may be readily
calculated. The flow table * is recommended for
determining relative consistency in the laboratory as
being more positive than the slump test. It is, however,
not a test for workability in the strict sense of the
word, nothing having so far been developed to take the :
place of the eye in judging thisimportant characteristic.
According to the original water-cement ratio theory,
concrete specimens made with various aggregates as de-
scribed above should all have substantially the same
strength, because the water-cement ratios are the same
and the mixtures are all workable. We know from
experience, however, that the strengths will probably not
be the same, due to the influence of the character of
ageregate. Let us assume that the following strengths
were actually obtained on six combinations of material:

Modulus Modulus
of rupture of rupture
sees in pounds | e a7 de in pounds

Combination per Combination per

square || square
inch || inch
|

Pe Oe A ea he ed ae Ee C70K |, ears ee See. eee ree 600
Ben | aed ees en eens O25 SIN ic = oe oo eens eee bs ee 580
Cie Oe oe ene | COQ Hise. Shee See ee ee 535

These values are plotted in Figure 1. It is observed
at once that four of the six combinations give strengths
either identical or practically identical with the base
or standard laboratory combination. There are, how-
ever, two outstanding exceptions, one much higher
and one much Jower. These two combinations, A
and F, will be used as the basis for further discussion,
since the same methods, somewhat simplified, can be
applied to combinations B to E.

It is now assumed that had curves been developed
for the relation between water-cement ratio and strength
for these combinations, as was done for the base mix,
the curves would be substantially parallel to the base

44.8. T. M. Standards, vol. 2, 1927, p. 115.

August, 1928 PUBLIC

ROADS 137

curve over the comparatively narrow range in which
we are interested. This may or may not be absolutely
correct, but it is believed that for the range of mixtures
covered by paving concrete it is substantially true.
Granting this, we can omit the actual determination
of this relationship for any of the combinations in
which we are interested and simply draw through the
value which we have plotted a line parallel to the basic
curve. This has been done in Figure 1 for combina-
tions A and F. To determine the water-cement ratio
to use with either of these combinations to obtain a
strength of 600 pounds, simply follow the curve repre-
senting the material either to the right or left, as the
case may be, until it intersects the 600-pound line and
use the corresponding water-cement ratio. Figure 1
shows this to be 0.85 for combination A and 0.60 for
combination F. <A choice between these combina-
tions will depend entirely on which is the cheaper, all
things considered, always assuming that the aggre-
gates in both cases are structurally sound and have
sufficient resistance to wear.

Before the cost can be determined it will be neces-
sary to determine by trial method the proportions re-
quired in these two cases to give the consistency re-
quired at the water-cement ratios indicated—that is,
0.85 for combination A and 0.60 for combination F.
Assume, for purposes of illustration, that the propor-
tions for combination A with a 0.85 water-cement
ratio reduce to 1:2:4 by volume and that the propor-
tions for combination F with a 0.60 water-cement ratio
reduce to 1:1144:3. Which of the two is the cheaper
will, of course, depend almost entirely on the relative
costs of the aggregates delivered on the job.

QUANTITIES OF MATERIAL DETERMINED BY SIMPLE CALCULATION

The next step is to work out for each case the theo-
retical cement factor as well as the quantities of aggre-
gates required to produce a unit volume of concrete,
knowing the specific gravities and weights per cubic
foot of all of the materials. For this purpose a simple
formula proposed by Stanton Walker ® may be used.
This formula gives the number of bags of cement re-
quired to produce 1 cubic yard of concrete, knowing
the proportions as well as the weights per cubic foot
and apparent specific gravities of the materials. It
is based on the assumption that, for plastic mixes, the
volume of concrete produced will be equal to the sum
of the absolute volumes of cement and aggregates plus
the volume of water, and may be expressed as follows:

— 27 ———_
WG ie Weak
0.5+2+ 65 45,1 62.45,

where
C=number of bags of cement per cubic yard of concrete.
0. 5=approximate absolute volume of cement in one bag.
x=water-cement ratio=volume. of water in a one-bag
batch.
Wys=weight in pounds of fine aggregate used in a one-bag
batch.
S;=apparent specific gravity of fine aggregate.
W,=weight in pounds of coarse aggregate used in a one-
bag batch.
S,=apparent specific gravity of coarse aggregate.
62. 4= weight per cubic foot of water.

This formula is sufficiently accurate for comparing
concrete yields in the laboratory. It gives, in general,

5 Walker, Stanton, Estimating Quantities of Materials for Concrete, Bull. 1,
National Sand and Gravel Association, Washington, D.C.

somewhat lower values for cement content than will
be found by trial either in the laboratory or in the
field. It will be necessary, before making final accurate
estimates of quantities for field use, to make actual
determinations of yield on the combination finally
selected.

In the above formula it will be observed that the

quantities
W; Wo
62.49, ""° 62.45,

and

represent, respectively, the absolute volumes of fine
and coarse aggregate used with each bag of cement.
The basis for the formula which is supported by
laboratory determinations is that for given materials
the absolute volumes of the aggregates used control
the volume of concrete which will be obtained with
a given amount of cement. This furnishes one of the
principal arguments for measuring aggregates by
weight instead of by volume, because as long as the
specific gravity of the aggregate remains constant the
weight of aggregate controls the yield irrespective of
void content.

The application of the formula to the problem
under discussion may now be made by continuing the
illustration given above and assuming the following
additional facts relative to the materials:

Combina-| Combina-
tion A tion F

Weight per cubic foot fine aggregate__..............-----..-..- 90 80
Weight per cubic foot coarse aggregate_.__......._-----_--..--- 95 109
Apparent specific gravity of fine aggregate_._......._...-.------ 2. 65 2. 65
Apparent specific gravity of coarse aggregate..-_.....-.-_.----- 2. 55 25.70
Price per ton delivered, fine aggregate__._-...____._--------__- $1.75 $1. 00
Price per ton delivered, coarse aggregate.........--.----------- $2. 00 | $1. 00
IPFice: perbag, COMent asses. ts aha fo. ee eee a a See $0. 60 $0. 60

These values, though assumed, might readily be en-
countered in actual practice, the idea in this case being
that the aggregates represented by combination A are
from sources some distance from the work, so that the
cost of transportation must be considered, whereas
aggregates F are locally available. Any other factor
which might cause differences in price, such as cost of
production, might, of course, just as well have been
assumed.

Applying the data given in the table to the formula
and remembering that a water-cement ratio of 0.85 and
proportions of 1:2:4 have been selected for combina-
tion A and corresponding values of 0.60 and 1:144:3
for combination B, the cost of materials for each com-
bination is obtained as follows:

ESTIMATE FOR COMBINATION A

27
ai LY 130 x 380 =5.6 bags of cement
; ‘ 62.42.65 62.42.65
Weight of fine aggregate per cubic yard of concrete=22 OR
=(0.505 tons.
Weight of coarse aggregate per cubic yard of concrete
_5-6X380_, o¢
=~2,000 =1.065 tons.
COSTS
Oementy yo. 0: Dagsnau. 0.0 0 sw ee ele eee ee $3. 36
Hine acaresace..O! 505 One yG) a lie ee ee ee ee eer . 88
Coarse aggregate, 1.065 tons, at $2.___..-......_-2_.- 2. 13
Cost of materials per cubic yard of concrete___--- 6. 37

128 PUBLIC

Vol. 9, No. 6

ESTIMATE FOR COMBINATION F

C cat —_—— 9907
120 327
0.5-+0.60-+gy gen gst areenz0

: 7.16120
Weight of fine aggregate per cubic yard of concrete= “5 569,
=>
=(.429 ton.
Weight of coarse aggregate per cubic yard of concrete
_ 7.16 X 327

2,000 -=1,.18 tons.
COSTS
Gement.7.16 bags, .at-$0,000. aan eee ene eee $4. 30
Hineragcregate; 05429 oni pentyl eee eee one eee . 43
Goarse aggregate, 1:1/ tons; at. $l ese 2 a ee ee eee 1.18
Cost of materials per cubic yard of concrete_____- 5. 91

These values are not given as typical of the relative
cost of concrete using imported or local aggregates, but
only to illustrate a method whereby reliable informa-
tion as to comparative costs may be obtained, as well
as to show that the most expensive concrete is not
necessarily the one containing the most cement.

There are other factors in addition to actual cost
which must be considered when comparing sources of
aggregate supply. For instance, the selection of ma-
terials represented by combination F in the above
illustration would only be justified on the basis of an
adequate supply of material equal in quality and of
the same grading as the sample upon which the design
is based. This, in turn, involves not only a thorough
inspection of the source as to the extent and uniform-
ity of the deposit but also presupposes adequate plant
equipment for producing the aggregates. It is wasted
effort to go to the trouble of designing a mix based
on an examination of samples submitted for test and
then find that it is either impossible or impractical to
produce materials equal to the samples for the actual
job. In the past most of the attempts to design con-
crete mixtures for paving work using local materials
have failed because proper emphasis was not placed
on the importance of uniformity of the material sup-
ply. Uniform concrete may be obtained in no other
way.

EFFECT ON SPECIFICATIONS DISCUSSED

In using the proposed method of designing concrete
mixtures in actual construction, it will be necessary
to change the present method of specifying arbitrary
proportions to a specification based on a certain re-
quired minimum strength. Such a method of speci-
fying has recently been suggested by J. T. Voshell,
district engineer, Bureau of Public Roads, which would
also involve a ue in the method of pagans: un

plication of the proposed method of design can be
worked out as follows:

Each bidder, instead of specifying a price per square
yard for concrete in place, would be required to sub-
mit separate bids for all materials which he is prepared
to furnish, together with a separate bid price per square
yard for mixing, placing, finishing, and curing the con-
crete in accordance with the requirements of the speci-
fication. After receipt of proposals the engineer will
examine all of the sources proposed, first, with the
view to eliminating any which do not comply with
the basic requirements of the specifications, and, sec-
ond, in order to determine which of the materials pro-
posed will produce concrete of the required quality at
the lowest cost, using a procedure similar to that out-
lined above. The award should be made to the con-
tractor who can supply the materials and mix and place
the concrete at the lowest total price per square yard.

With this method of procedure the responsibility for
selecting the materials and adjusting the mix to secure
concrete of the desired quality, as well as the respon-
sibility of seeing that the production of the concrete is
carried out in accordance with the specifications, rests
solely with the engineer. This is where it belongs,
unless we are prepared to go to the other extreme and
specify the quality of the finished concrete and allow
the contractor to use any materials and methods of
production he desires so long as he fulfills this require-
ment. The writer believes that we should adhere
strictly to one course or the other and not attempt to
control every step in the process of construction and
still hold the contractor responsible for the result. ;

In applying the method of design described in this
paper the selection of aggregates and proportions to be
employed should be based on laboratory tests under
controlled conditions so as to insure that when the
pavement is constructed using the same aggregates
and proportions and in accordance with the detailed
specifications governing mixing, placing, and curing a
structure of satisfactory quality will result. It is
believed that under such circumstances specifications
should not contain provisions as to the strength of the
finished concrete, and strength tests, if made, should
be for the guidance of the engineer only.

If, on the other hand, it is desired to specify the
quality of finished product only, the same technical
procedure for designing mixtures may still be employed,
only in this case it becomes a method to be applied
by the contractor instead of the engineer, because
under such a specification the contractor must deter-
mine for himself the materials to be used and the pro-
portion in which to combine them in order to make an
intelligent bid.

TRUCK IS A BIG FACTOR IN FRUIT TRANSPORT

(Continued from page 123)

region, giving them the advantages of prompt delivery
wherever the load is wanted and without delay,
rehandling, or extra charges.

Many of the dealers have their own trucks which they
take to the producing section and do their buying direct.

This plan suits the grower rather well because he sells for
cash and need have no more trouble about the matter.

As for the condition of the trucked produce, there is
some discussion about the effect on soft fruits, yet it is
claimed that strawberries trucked to market hold their
condition better and longer than those shipped in iced
cars, although the iced berries will look better for oe
first four or five hours.

pire

ROAD PUBLICATIONS OF BUREAU OF PUBLIC ROADS

Applicants are urgently requested to ask only for those publications in
which they are particularly interested. The Department can not under-
take to supply complete sets nor to send free more than one copy of any
publication to any one person. The editions of some of the publications
are necessarily limited, and when the Department’s free supply is
exhausted and no funds are available for procuring additional copies,
applicants are referred to the Superintendent of Documents, Govern-
ment Printing Office, this city, who has them for sale at a nominal price,
under the law of January 12, 1895. Those publications in this list. the
Department supply of which is exhausted, can only be secured by pur-
chase from the Superintendent of Documents, who is not authorized
to furnish publications free.

ANNUAL REPORTS
Report of the Chief of the Bureau of Public Roads, 1924.
Report of the Chief of the Bureau of Public Roads, 1925.
Report of the Chief of the Bureau of Public Roads, 1927.

DEPARTMENT BULLETINS

No. 105D. Progress Report of Experiments in Dust Prevention
and Road Preservation, 1913.

*136D. Highway Bonds. 20c.

220D. Road Models.

257D. Progress Report of Experiments in Dust Prevention
and Road Preservation, 1914.

*314D. Methods for the Examination of Bituminous Road
Materials. 10c.

*347D. Methods for the Determination of the Physical
Properties of Road-Building Rock. 10c.

*370D. The Results of Physical Tests of Road-Building
Rock. 15c.

386D. Public Road Mileage and Revenues in the Middle
Atlantic States, 1914.

387D. Public Road Mileage and Revenues in the Southern
States, 1914.

388D. Public Road Mileage and Revenues in the New
England States, 1914.

390D. Public Road Mileage and Revenues in the United
States, 1914. A Summary.

407D. Progress Reports of Experiments in Dust Prevention
and Road Preservation, 1915.

*463D. Earth, Sand-clay, and Gravel Roads. 15c.

*532D. The Expansion and Contraction of Concrete and
Concrete Roads. 10c.

*537D. The Results of Physical Tests of Road-Building
Rock in 1916, Including all Compression Tests.
oC.

*583D. Reports on Experimental Convict Road Camp,
Fulton County, Ga. 25c.

*660D. Highway Cost Keeping. 10c.

*670D. The Results of Physical Tests of Road-Building
Rock in 1916 and 1917. 5c.

*691D. Typical Specifications for Bituminous Road Mate-
rials. 10c.

*724D. Drainage Methods and Foundations for County
Roads. 20c.

*1077D. Portland Cement Concrete Roads. 15c.

1259D. Standard Specifications for Steel Highway Bridges,

adopted by the American Association of State
Highway Officials and approved by the Secretary
of Agriculture for use in connection with Federal-
aid road work.

1279D. Rural Highway Mileage, Income, and Expendi-
tures, 1921 and 1922.

DEPARTMENT BULLETINS—Continued

No. 1486D. Highway Bridge Location.
DEPARTMENT CIRCULARS
No. 94C. T. N. T. as a Blasting Explosive.

331C. Standard Specifications for Corrugated Metal Pipe
Culverts.

TECHNICAL BULLETIN
. 55. Highway Bridge Surveys.
MISCELLANEOUS CIRCULARS

62M. Standards Governing Plans, Specifications, Con-
tract Forms, and Estimates for Federal Aid
Highway Projects.
93M. Direct Production Costs of Broken Stone.
*105M. Federal Legislation Providing for Federal Aid in
Highway Construction and the Construction of
National Forest Roads and Trails. 5c.

FARMERS’ BULLETINS

No. *338F. Macadam Roads. 5c.

SEPARATE REPRINTS FROM THE YEARBOOK

*739Y. Federal Aid to Highways, 1917.
*849Y. Roads. 5c.
914Y. Highways and Highway Transportation.
937Y. Miscellaneous Agricultural Statistics.

No. 5c.

TRANSPORTATION SURVEY REPORTS

Report of a Survey of Transportation on the State Highway
System of Connecticut.

Report of a Survey of Transportation on the State Highway
System of Ohio.

Report of a Survey of Transportation on the State Highways of
Vermont.

Report of a Survey of Transportation on the State Highways of
New Hampshire.

REPRINTS FROM THE JOURNAL OF AGRICULTURAL RESEARCH

Vol. 5, No. 17, D- 2. Effect of Controllable Variables upon
the Penetration Test for Asphalts and
Asphalt Cements.

Relation Between Properties of Hard-
ness and Toughness of Road-Build-
ing Rock.

. A New Penetration Needle for Use in
Testing Bituminous Materials.

Tests of Three Large-Sized Reinforced-
Concrete Slabs Under Concentrated
Loading.

5. Tests of a Large-Sized Reinforced-Con-

crete Slab Subjected to Eccentric
Concentrated Loads.

Vol. 5, No. 19, D- 3.

Vol.
Vol.

5, No.
6, No.

24,
GE D=as:

Vol. 11, No. 10,

* Department supply exhausted.

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Full text of "Public Roads: A Journal of Highway Research, Vol. 9, No. 6"

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