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DETERIORATION AND REPAIR OF
ABOVE GROUND CONCRETE WATER TANKS
IN ONTARIO, CANADA

REPORT TO
ONTARIO MINISTRY OF THE ENVIRONMENT

September, 1987

Colder Associates

W.M. Slater

& Associates Inc.

DETERIORATION AND REPAIR OF ABOVE GROUND
CONCRETE WATER TANKS IN ONTARIO, CANADA

AUTHORS

R. GRIEVE W.M. SLATER

Colder Associates W.M. Slater & Associates Inc.

L. ROTHENBURG

Colder Associates

(i)

APPLIED RESEARCH GROUP MEMBERS

G. Aldworth - MacLaren Engineers

T.I. Campbell - Queen's University

R. Grieve - Colder Associates

K. MacKenzie - Dalhousie Materials

W. Pery - W. Pery Engineering (Deceased)

L. Rothenburg - Colder Associates

W.M. Slater - W.M. Slater & Associates Inc. (Chairman)

R. Staton - MacLaren Engineers

R. Crawford - Ministry of the Environment

P. Rostern- - Ministry of the Environment

M. Toza - Ministry of the Environment

O. Wigle - Ministry of the Environment

REPORTS PREPARED UNDER THE APPLIED RESEARCH PROGRAMME FOR
THE MINISTRY OF THE ENVIRONMENT

Evaluation of Waterproof Coatings for Concrete \\ater Tanks.

Mackenzie, K., (Dalhousie Materials), Slater, W.M., (W.M. Slater & Associates
Inc.) and McCrenerc, P., (Knox Martin Kretch) (Editing). Preliminary, 1985.

Freeze Protection for Above Ground Concrete Uater Tanks in Cold Regions.

Aldworth, C, Staton, R., (MacLaren Engineers), and Slater, W.M.,
(W.M. Slater & Associates Inc.). Preliminary, 1985.

Temperature Monitoring, Ontario Concrete Water Tanks.

Crieve, R. (Colder .Associates). May, 1984 and February, 1986.

Ice Loading in Elevated Water Tanks.

Campbell, T.I. and Kong, W.L., (Queen's LTnivcrsity). .April, 1986.

(ii)

FOREWORD

This report began as a study of the
problems associated with the
deterioration of some of the 53 above
ground concrete water storage tanks
built in Ontario during the period 1956
to 1980. It was initiated and funded by
the Ministry of the Environment (MOE)
as part of the concrete water tank
rehabilitation programme which was
supervised by it's Project Engineering
Branch

Concrete tanks are structurally
straightforward systems but, being
exposed to severe environmental
conditions, appear to suffer a rate of
deterioration which has greatly reduced
the expected life of the structures. The
damage ranges from heavy surface
spalling and cracking to delamination
and eventual failure of the structure.
The study showed that the prime factors
identified as determining the rate of
concrete structure deterioration were the
number of freeze-thaw cycles,
temperature amplitudes and frequencies,
concrete permeability, hydrostatic
pressure, location, the effect of steel
reinforcement embedments, and internal
ice formations.

Since construction defects, as well as
the prime factors listed above, have a
dominant affect on the accelerated rate
of deterioration of concrete water tanks
in Ontario, remedial solutions such as
repair of joints and honeycombing,
applying waterproof coatings, insulation,
and replacement by steel tanks, were
proposed.

The study was limited to addressing
already established problems existing in
the 53 pre-1981 structures. Many of the
concrete tanks had site specific
problems, so basic research into such
areas, for example, as the design of
special concrete mixes to improve new

concrete tank service lives to, say, fifty
years was not carried out.

The objective of the study was to be
directed towards seeking rehabilitation
solutions for existing structures in order
to achieve a life expectancy of at least
25 more years. The applied research
programme, therefore, was focussed
mainly in the direction of repair, rather
than on basic concrete research.

In spite of the emphasis in the study to
seek remedial solutions to the various
problems associated with the rapid
deterioration and failure of concrete
water tanks in Ontario, a study of the
mechanisms which caused some of the
observed rapid failures was considered to
be most important in seeking repair
solutions. Numerous field observations
of apparently unique and hitherto
unreported and undocumented types of
failure in reinforced and prestressed
concrete water tanks, required some
attempt to find a scientific explanation
for the causes of the problems.

The mechanisms of concrete dilation and
delaminations, as well as the effect of
internal ice, air entrainment, thermal
differential strains and strain rates, and
other deterioration factors are described
in the report, but no laboratory or proof
tests have been carried out to date.

Since the various factors and
mechanisms may act concurrently, and
have not been described in technical
literature, it is recommended in the
report that basic research be carried out
in these areas in the future to try to
quantify the deterioration identified.

A brief section in the report gives the
principal results of the temperature
monitoring of three concrete tanks, one
uninsulated and two insulated, from
three separate locations in central,
south-west, and north-west Ontario.

(iii)

The main objective of the monitoring
programme was to measure the thermal
history of an uninsulated tank, and by
comparison, measure the effectiveness of
insulation systems developed as possible
engineering solutions to the concrete
tank deterioration problem by reducing
both the number and temperature
amplitude characteristics of freeze-thaw
cycles on the tank walls. The detailed
graphical data is presented in a separate
report prepared for the MOE by Colder
Associates in February 1986, entitled
"Temperature Monitoring, Ontario
Concrete Water Tanks". Useful
information such as the number and
frequency of freeze-thaw cycles,
differences between exposed solar and
shaded quadrants, temperature
differentials and rate of temperature
change, temperature external to and
within the concrete walls, etc. has been
reported and plotted in that report.
Full analysis of this data and its general
aspects is not within the scope of this
report.

The report discusses the various methods
used to repair different types of
concrete tanks and gives
recommendations for assessment and
analysis of repair systems.

Although corrosion of metals is not a
serious problem in concrete water tanks,
some corrosion of tendons and other
metallic components has occurred. The
report gives examples of this type of
deterioration, and the remedial methods
used in the rehabilitation programme.

Interim guidelines have been prepared
for the design and construction of new
concrete water tanks in Ontario based
on the experience gained during the
rehabilitation programme, and from the
applied research carried out. The
guidelines recommend that internal
waterproofing and external insulation be
used as the primary protection against
the deterioration of above ground
concrete water tanks in Ontario.

The main conclusion of the report is
that, without adequate protection of
permeable concrete from direct contact
with water and elimination of cyclic
freezing in the tank wall, above ground
concrete water tanks will continue to
deteriorate rapidly.

(iv)
ACKNOWLEDGEMENTS

The work described in this report is the result of the efforts of many individuals
who assisted with various tasks from tank inspection and testing through to review
and drafting of the report.

The members of the Applied Research Group wish to acknowledge the help,
encouragement, and support offered by the following Ministry of the Environment
personnel: L. Benoit, R.K. Brown, R.G. Crawford, J.C.F. MacDonald (retired),
W.C. Ramsden, P. Rostern, M. Toza, O. Wigle, and T. Wright.

The authors and the Ministry also wish to thank persons from the Ministry of
Transportation and Communications, Ontario Hydro and from various consulting
engineering firms who reviewed draft reports, and offered their advice. In addition
they wish to thank and acknowledge Wyllie and Ufnal and Knox Martin Kretch for
the use of some of their inspection and photographic data.

A special thanks to the typing, drafting and office staff of Golder Associates. In
particular Philomena D'Souza, Mike Wright, Jim Alexander, and Vic Milligan who
greatly assisted in the production of this report.

(V)

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 General

1.2 Description

1.3 Applied Research

1.4 Rehabilitation Programme

1.5 Above Ground Water Tank Types

1.6 Performance Rating

2.0 DETERIORATION OF ABOVE GROUND CONCRETE WATER TANKS 2-

2.1 General 2 -

2.2 Observations 2 -

2.2.1 External Wall Delamination 2 -

.1 Post-Tensioned Unbonded (PTU) 2 -

.2 Post-Tensioned Bonded (PTB) 2 -

.3 Post-Tensioned Wire Wound Gunite

Protected (G) 2-1

.4 Reinforced Concrete Standpipes (RC) 2-2

.5 Conclusions 2-5

2.2.2 Internal Wall Delamination 2-5

2.2.3 Localized Spalling 2-6

2.2.4 Jack-Rod Spalls 2-7

2.2.5 Cover Coat Shrinkage and Cracking 2-8

2.2.6 Quality of Concrete in Water Tanks 2-9

2.2.7 Summary of Observations 2 -10

3.0 TEMPERATURE MONITORING 3-1

3.1 General 3-1

3.2 Temperature Instrumentation 3-1

3.3 Test Results 3-2

3.4 Freezing in Concrete 3-4

3.5 Seasonal Wall Conditions 3-4

3.5.1 Thermal Conductivities 3-5

3.5.2 Autumn - Winter Condition 3-5

3.5.3 Winter - Spring Condition 3-5

3.5.4 Insulated Tank Winter Profile 3-6

3.6 Daily Thermal Cycle 3-6

3.7 Critical Ambient Temperatures 3-7

4.0 INVESTIGATION OF THE FREEZE-THAW FAILURE

MECHANISM IN CONCRETE 4-1

4.1 General 4-1

4.2 Literature Review 4-2

4.3 Some Factors Affecting the Freeze-Thaw Durability Of

Concrete 4-2

4.3.1 Saturation of Concrete in Water Tanks 4-3

4.3.2 Hydrostatic Pressure and Evaporation 4-3

(vi)

Table of Contents (contd)

4.3.3 Air Voids 4-4

4.3.4 Affect of Prestress on Permeability 4-4

4.4 Action of Freezing Temperature on Concrete 4-5

4.4.1 Summary 4-9

4.5 Standard Freeze-Thaw Tests 4-9

4.6 Rate of Re-Saturating Dilated Concrete 4-10

4.7 Stresses in Concrete Due to Frost Induced Expansion .... 4-11

4.7.1 Tensile Stresses in Tank Walls 4 -12

4.7.2 Model of Tensile Stress Accumulation 4 -14

4.7.3 Distribution of Stresses 4 -17

4.7.4 Summary 4 -18

4.8 Spot Saturation 4-19

4.8.1 Jack-Rod Spalls 4-20

4.9 Hydraulic Pressure "Sandwich" 4-21

4.10 Conclusions and Recommendations 4 -22

5.0 REPAIR AND REHABILITATION OF CONCRETE WATER TANKS

IN ONTARIO 5 -

5.1 General 5 -

5.2 Design of Repairs 5 -

5.2.1General 5 -

5.2.2 Structural Evaluation 5 -

.1 Loading 5 -

.2 Analysis 5-3

5.3 Repair Methods Developed 5-4

5.4 Tank Repair Methods 5-4

5.4.1 Condition Surveys 5-4

5.4.2 Surface Preparation 5-5

5.4.3 Delaminations and Spalls 5-5

5.4.4 Crack Repair 5-6

5.4.5 Waterproofing 5-6

5.5 Typical Repair Systems for Various Concrete Tank Types . . 5-7

5.5.1 Concrete Tank Types 5-7

5.5.2 RC-S Type Tanks 5-7

5.5.3 G-S Type Tanks 5-9

5.5.4 PTU-S Type Tanks 5-12

5.5.5 PTB-S Type Tanks 5-12

5.5.6 RC-E Type Tanks 5-12

5.5.7 G-E Type Tanks 5-13

5.5.8 PTU-E Type Tanks 5-15

5.5.9 PTB-E Type Tanks 5-16

5.5.10 G-G Type Tanks 5-17

5.5.11 RC-G Type Tanks 5-17

5.6 Quality Assurance and Measurement 5 -18

6.0 DETERIORATION OF METALS IN CONCRETE WATER TANKS . 6-1

6.1 Introduction 6-1

6.1.1 Role of Metals In Concrete Tanks 6-1

6.1.2 Deterioration Sequence 6-1

(vii)
Table of Contents (contd)

6.1.3 Importance of Construction Process 6-1

6.1.4 Summary 6-1

6.2 Corrosion of Steel Wall Reinforcement 6-2

6.2.1 Need for Reinforcement 6-2

6.2.2 Types of Reinforcement 6-2

6.2.3 Detection of Corrosion 6-2

6.2.4 Protection of Steel by Quality Concrete 6-3

6.3 Observation and Repairs 6-3

6.3.1 Reinforced Concrete Tanks (Type RC) 6-3

6.3.2 Post-tensioned Bonded Tanks (Type PTB) 6-4

.1 Description 6-4

.2 Problems 6-4

.3 Repair 6-5

6.3.3 Post-tensioned Unbonded Tanks(Type PTU) 6-5

.1 Description 6-5

.2 Problems 6-6

.3 Repair 6-6

6.3.4 Gunite Protected Tanks (G Type Tanks) 6-7

.1 Description 6-7

.2 Problems 6-8

.3 Repair 6-9

6.4 Deterioration Of Metal Components 6-1

6.4.1 Steel Access Tubes in Elevated Tanks 6-1

.1 Problem 6-1

.2 Repair 6-1

6.4.2 Aluminum Ladders in Water 6-1

.1 Problem 6-1

.2 Repair 6-1

6.4.3 Recommendations 6-1

6.5 Metal Appurtenances on Concrete Tanks 6 -12

6.5.1 Description 6 -12

6.5.2 Observations 6 -12

6.6 Summary and Conclusions 6 -12

6.7 Recommendations 6-13

7.0 CONCLUSIONS 7-

7.1 Introduction 7 -

7.2 Concrete Deterioration Mechanisms 7 -

7.2.1 General 7 -

7.2.2 Internal Ice 7 -

7.2.3 Freeze-Thaw With Pressurized Water 7 -

7.2.4 Rate of Deterioration 7-2

7.2.5 Freezing in Wall Voids 7-2

7.3 Expansion Joints 7-2

7.4 Corrosion of Prestressing Steel 7-2

7.5 Repair Methods 7-2

7.5.1 General 7-2

7.5.2 Bonded Waterproofing Coatings 7-2

(viii)

Table of Contents (contd)

7.5.3 Steel Liners 7-3

7.5.4 Plastic Liners 7-3

7.5.5 External Post-Tensioning 7-3

7.6 Freeze Protection 7-4

7.6.1 General 7-4

7.6.2 Insulation and Cladding Systems 7-4

7.6.3 Mixing and Heating Systems 7-4

7.6.4 Air Gap Heating 7-4

7.7 Tank Types Not Recommended 7-4

8.0 RECOMMENDATIONS 8

8.1 Introduction 8

8.2 Design & Construction of New Concrete Water Tanks .... 8

8.2.1 Codes 8

8.2.2 Interim Guidelines 8

8.3 Maximum Head 8-2

8.4 Technology Transfer 8-2

8.5 Durability 8-2

8.6 Further Applied Research 8-2

9.0 REFERENCES 9-1

10.0 GENERAL REFERENCES 10-1

11.0 APPENDIX A

GUIDELINE RECOMMENDATIONS OF MINIMUM REQUIREMENTS
FOR THE DESIGN AND CONSTRUCTION OF NEW ABOVE GROUND

CONCRETE WATER TANKS IN ONTARIO II -1

11.1 Introduction 11-1

11.2 Scope 11-1

11.3 References 11-1

11.4 Design 11-2

11.4.1 Design Philosophy 11-2

11.4.2 Tank Design Requirements 11-2

.1 Insulation 11-2

.2 Ice prevention 11-2

.3 Inspection 11-2

.4 Mixing/Heating 11-2

.5 Prestressed Construction 11-2

.6 Prestress Design 11-2

.7 Concrete Wall Thickness 11-3

.8 Waterproofing 11-3

.9 Coatings 11-3

.10 Liners 11-3

11.5 Construction 11-3

11.5.1 Concrete Quality 11-3

11.5.2 Slip Forming 11-3

11.5.3 Jump Forming 11-4

(ix)

Table of Contents (contd)

11.5.4 Vertical Waterstops. . . .

11.5.5 Concrete Joint Preparation

11.5.6 Concrete Curing

11.6 Quality Assurance And Tank Performance Testing

11.6.1 Leakage Testing

11.6.2 Final Inspection of Structure

11.6.3 Heat Loss and Ice Prevention

11.7 Security And Safety

11.7.1 Security Fence

11.7.2 Safety

11.8 Miscellaneous And Appurtenances

- 4

- 5

12.0 APPENDIX B

DILATION EXPANSION OF SPHERICAL REGION WITHIN

A LARGER SPHERE 12-1

13.0 APPENDIX C

TERMS OF REFERENCE 13-1

(X)

LIST OF PHOTOGRAPHS

Photo 1- 1 Collapsed tank (DUNNVILLE) 1-1

Photo 1- 2 Tank implosion (SOUTHAMPTON) 1-2

Photo 1- 3 Detailed view of imploded section (SOUTHAMPTON). ... 1-2

Photo 2- 1 Typical deterioration at bottom of reinforced concrete

'standpipe(\VATFORD) 2-2

Photo 2- 2 External wall delamination (WATFORD) 2-2

Photo 2- 3 External wall delamination (CAMLACHIE) 2-3

Photo 2- 4 Fracture at reinforcing steel (CAMLACHIE) 2-3

Photo 2- 5 Core section showing fracture at external and internal

steel (CAMLACHIE) 2-3

Photo 2- 6 Core through tank wall (ARKONA) 2-5

Photo 2- 7 Close-up of expanded steel concrete interface

(ARKONA) 2-5

Photo 2- 8 External delamination in wire wound post-tensioned

tank (WOODVILLE) 2-5

Photo 2- 9 Internal delamination (ALVINSTON) 2-6

Photo 2-10 Localised spalling (BADEN) 2-6

Photo 2-11 Deterioration caused by construction glove (BADEN) .... 2-6
Photo 2-12 Damage caused by water seepage at water-stop

(VAL CARON) 2-6

Photo 2-13 Coating deterioration and a jack-rod

spall (WOODVILLE) 2-7

Photo 2-14 Jack-rod spall at threaded coupling (BADEN) 2-7

Photo 2-15 Intact spall recovered from bottom of tank

(WOODVILLE) 2-7

Photo 2-16 Typical horizontal cracks and leachate stains

occurring at a prestressed standpipe (WOODVILLE) .... 2-8
Photo 2-17 Typical cracks and leachate stains occurring at a

prestressed ground tank (VAL CARON) 2-8

Photo 2-18 Deterioration of reinforced concrete standpipe

(ALVINSTON) 2-9

Photo 4- 1 Dilation of concrete and subsequent debonding of reinforcing

steel 4-17

Photo 5- 1 Internal wall ice at bottom of tank after demolition in

late spring 5-2

Photo 5- 2 Surfacing tank wall prior to application of coating

(HESPELER) 5-6

Photo 5- 3 Trowelling latex modified mortar (HESPELER) 5-6

Photo 5- 4 Completed surfacing prior to coating application

(HESPELER) 5-7

Photo 5- 5 Installation of post-tensioning anchors.

(ALVINSTON) 5-8

(xi)

List of photographs (cont'd)

Photo 5- 6 Installation of external post-tensioning tendons

(ALVINSTON) 5-8

Photo 5- 7 Typical horizontal cracking of a G-S type tank

(L'ORIGNAL) 5-9

Photo 5- 8 Ring support structure for insulation and cladding

(BADEN) 5-9

Photo 5- 9 Standpipe prior to repair (BADEN) 5-10

Photo 5-10 Standpipe after insulation (BADEN) 5-10

Photo 5-11 Standpipe after insulation (WOODVILLE) 5-10

Photo 5-12 Standpipe prior to repair (HESPELER) 5-11

Photo 5-13 Standpipe after insulation (HESPELER) 5-11

Photo 5-14 Applying coat of MMA to exterior of standpipe

(BADEN) 5-11

Photo 5-15 External post tensioning (GLENCOE) 5-12

Photo 5-16 Steel liner being installed (BRECHIN) 5-12

Photo 5-17 Tank after rehabilitation including strengthening by

post-tensioning, installation of new steel liner,

and insulation and cladding (BRECHIN) 5-13

Photo 5-18 Internal maintenance inspection of liner paint system

after one year in service (BRECHIN) 5 -13

Photo 5-19 Vertical cracking at base of wall due to ineffective

prestressing.(CHELMSFORD) 5-13

Photo 5-20 External post-tensioning added to compensate for

lack of prestressing in wall.(AMHERSTBURG) 5-14

Photo 5-21 Deteriorated internal thrust ring.(CHELMSFORD) 5-14

Photo 5-22 Repair of thrust ring (CHELMSFORD) 5-14

Photo 5-23 Completed repair (CHELMSFORD) 5-14

Photo 5-24 Tank after leakproofing and before insulation.

(BRIGDEN) 5-15

Photo 5-25 Steel support system for insulation and cladding.

(BRIGDEN) 5-15

Photo 5-26 Completed rchabilitaion of tank (BRIGDEN) 5-16

Photo 5-27 Delaminated exterior at source of leak after

removal of fractured concrete. (CASSELMAN) 5 -16

Photo 5-28 Delaminated wall.(CASSELMAN) 5-17

Photo 5-29 Reservoir before repair (PRESTON) 5-17

Photo 5-30 Reservoir after repair (PRESTON) 5-17

Photo 5-31 Tank demolition - cutting hole at base.(CALLANDER) ... 5-18

Photo 5-32 Standpipe toppled (CALLANDER) 5-18

Photo 5-33 Reinforced concrete tank wall after toppling.

(CALLANDER) 5-18

Photo 6- 1 Poorly protected post-tensioning anchorage allowing

water to enter. (BRIGDEN) 6-6

Photo 6- 2 Detailed view of strand showing corroded wires.

(BRIGDEN) 6-6

Photo 6- 3 Typical longitudinal fracture of one of the wires

of a strand (BRIGDEN) 6-7

Photo 6- 4 Start and progression of a typical fracture.

(BRIGDEN) 6-7

(xii)

List of photographs (cont'd)

Photo 6- 5 Delaminatcd cover coat exposing broken prestressing

wires (AMHERSTBURG) 6-9

Photo 6- 6 Corroded prestressing wires under delaminated cover

coat (CHELMSFORD) 6-9

Photo 6- 7 Corroded access tube (PRESCOTT) 6-10

Photo 6- 8 Corroded roof truss (HESPELER) 6-10

Photo 6- 9 Completed roof repair (HESPELER) 6-10

Photo 6-10 Aluminium ladder exhibiting severe pitting corrosion

(BRIGDEN) 6-11

Photo 6-11 Corroded steel manway cover and bolts (WATFORD) .... 6-12

(xiii)
LIST OF FIGURES

Figure 1.1 Categories of Ontario concrete tanks 1-3

Figure 2.1 Delamination survey of standpipe (CAMLACHIE) .... 2-4
Figure 2.2 Jacl<-rod spalls 2-8

Figure 3.1 Location details of temperature instrumentation (ROCKWOOD) 3-1

Figure 3.2 1983 autumn data at north side of an uninsulated tank

(ROCKWOOD) 3-2

Figure 3.3 1983 autumn data at south side of an uninsulated tank

(ROCKWOOD) 3-2

Figure 3.4 1984 winter data on north side of an uninsulated tank

(ROCKWOOD) 3-2

Figure 3.5 1984 winter data on the south side of an uninsulated

tank (ROCKWOOD) 3-2

Figure 3.6 1984 winter data at an insulated tank in Southern

Ontario (ALVINSTON) 3-3

Figure 3.7 1985 winter data at an insulated tank in Northern

Ontario (EAR FALLS) 3-3

Figure 3.8 Typical seasonal wall conditions 3-5

Figure 3.9 Typical thermal profile after insulating 3-6

Figure 3.10 Daily temperature cycles showing continuous freezing

at the north side of the tank 3-6

Figure 3.11 Daily temperature cycle showing daily freeze-thaw

cycles at the south side of the tank 3-6

Figure 4.1 Theoretical saturation zone and seepage discharge .... 4-3

Figure 4.2 Relationship between bleed water paths, water pressure

flow, and stressing force in water tank walls 4-5

Figure 4.3 Permeability as a function of confining stress (ref. 16) . . 4-5

Figure 4.4 Typical experimental length change results from

thermomechanical measurement (ref. 18) 4-6

Figure 4.5 Correlation between degree of saturation and resistance

to freezing and thawing (ref. 14) 4-6

Figure 4.6 Relationship between temperature and dilation for a

plain and air entrained concrete at various levels of

saturation (ref. 19) 4-7

Figure 4.7 Relationship between irreversible expansion and

freezing expansion of frost resistant and non-frost

resistant bricks (ref. 18) 4-7

Figure 4.8 Maximum dilations v's residual expansions after one

freeze - thaw cycle for "fully" saturated specimens

(ref. 22) 4-8

Figure 4.9 Pore size enlargement after a single freeze - thaw

cycle 4-8

Figure 4.10 Schematic diagram of a freeze - thaw cycle 4-9

Figure 4.11 Typical ASTM freeze-thaw results 4 -10

Figure 4.12 Effect of concrete quality and water pressure on re-
saturation time 4-11

Figure 4.13 Theoretical model of tank stress development 4 -12

Figure 4.14 Distribution of stress in tank wall 4 -13

(xiv)
List of Figures (cont'd)

Figure 4.15 Permeability and tensile stress for 30m (100 ft.)

head 4-15

Figure 4.16 Permeability and tensile stress for 15m (50 ft.)

head 4-15

Figure 4.17 Five year accumulated tensile stress related to tank

diameter for 30m (100 ft. )head 4-16

Figure 4.18 Five year accumulated tensile stress related to tank

diameter for 15m (50 ft.) head 4-16

Figure 4.19 Distribution of stresses in tank wall section under

increasing linear water pressure 4 -17

Figure 4.20 Illustration of model used to examine debonding of

reinforcing steel 4 -18

Figure 4.21 Distribution of stresses in post-tensioned tank wall

section 4-18

Figure 4.22 Formation of jack-rod spalls 4 -19

Figure 4.23 Schematic development of thawed zone in wall 4 -20

Figure 4.24 Nature of wall damage at location of unfrozen zone ... 4-21

Figure 5.1 The 10 concrete tank types in Ontario 5-7

Figure 6.1 Bonded tendons

Figure 6.2 Unbonded monostrand

Figure 6.3 Flaws leading to corrosion of wire wrapped circular

pipes and tanks

Figure 12.1 Rupture of a brittle sphere due to dilation

LIST OF TABLES

TABLE 1/1 Number of tanks of each type constructed 1-3

TABLE 1/2 Tank performance rating by category 1-3

TABLE 2/1 Defect rating 2-1

TABLE 2/2 Summary of typical water tank core test results 2 -10

TABLE 4/1 Typical values of permeability of concrete used in dams. 4-5

TABLE 6/1 Types of steel reinforcement 6-2

6 -

-2

1.0

INTRODUCTION

1.1 General

In recent years it has become evident
that concrete water tanks located in
various regions of Ontario suffer
distress.

In 1981 a study was undertaken on
behalf of the Ministry of the Environ-
ment by W.M. Slater & Associates Inc.
to identify the nature and extent of the
tank deterioration.

The study revealed (Ref. 1 and 2) that a
large number of the water tanks
inspected had deteriorated significantly
over a short period of time. Although it
was expected that the life of a water
tank should be in excess of 50 years,
most of the tanks studied were less than
9 years old, with an average age of
about 6 years. It was noted that two
water tanks had structurally failed.

Photo 1-1 Collapsed tank (DUNNVILLE)

The main conclusion of the study was
that, in general, the rate of deterior-
ation was unexpectedly high and, if not
arrested, would lead to structural failure

of the tanks. Based on this potentially
dangerous situation, it was recommended
that a repair programme be initiated
immediately.

1.2 Description

During the preliminary surveys and
subsequent remedial work to the tanks
it was found that little data was
available on the types of materials used,
construction records or test results of
materials. It was, therefore, decided
that a central system of acquiring data
and providing transfer of technical
knowledge gained was essential to the
successful outcome of the repair
programme.

1.3 Applied Research

Concurrently with the repair programme,
a programme of applied research was
initiated in order to provide an under-
standing of the factors leading to water
tank deterioration so that current
remedial measures and the design of
structures in the future would have the
advantage of this knowledge. Goldcr
Associates undertook the examination of
the physical mechanisms of tank
deterioration and collection of the
information obtained from the remedial
works. Other studies carried out under
the Applied Research Programme were
directed towards waterproof coatings,
freeze protection (both of the concrete
tank and the stored water), ice loadings,
and temperature monitoring,
(see page (i))

1.4 Rehabilitation Programme

An important part of the tank rehabil-
itation programme in the period 1981 to
1984 was to record any noted tank
defects, establish causes where possible.

I - 1

obtain samples of defects and subsequent
repair materials, and record and monitor
the quality and effectiveness of repair
methods, materials and applications.
Prime consultant for the programme was
W.M. Slater & Associates Inc. Colder
Associates provided an overview of the
materials used and specialized testing.
Design and resident inspection for each
repair was carried out by various
consultants who were responsible for the
supervision and management of the

Photo 1-2 Tank implosion.
(SOUTHAMPTON)

To provide a degree of uniformity in
inspections, a daily inspection sheet was
developed by Colder Associates to be
completed by each resident field
inspector recording observations on
ambient conditions, type of work being
undertaken by the contractor, visual
estimate of quality, and samples ob-
tained. As the remedial programme
proceeded, general patterns of
deterioration with respect to tank types,
methods of construction, construction
defects and the like became evident.

^4r

Photo 1-3 Detailed view of imploded
section. (SOUTHAMPTON)

It is the purpose of this report to
illustrate the typical defects occurring in
concrete water tanks in Ontario and to
present a mathematical model of the
mechanisms of deterioration observed
which are described in sections 4 and
12. The report summarizes the
conclusions reached from the applied
research and proposes reasons for the
rapid rate of deterioration resulting in a
service life of under 10 years for many
concrete water tanks in Ontario. It
describes the repair methods developed
for the various defects observed,
proposes recommendations to improve the
performance and service life of existing
tanks, and makes recommendations for
the design and construction of future
concrete water tanks in Ontario.

1.5 Abo>c Ground NVater Tank Types

Ontario concrete water tanks incorporate
manv different and indi\ idual design

features; however, there are essentially
three different categories, namely:

• standpipes

• elevated tanks

• ground tanks

Standpipes are cylindrical structures up
to 46 m (150 ft.) high and 7 to 9 m (25
to 30 ft.) in diameter. Large storage
capacity and high internal water
pressure in the lower portion of
standpipes are distinct features of this
category of tanks. (See Figure 1.1).

Elevated tanks ha\e a smaller storage
capacity but are capable of providing
high operating head with relatively low
internal head, appro.ximately 10 m (30
ft.). Water pressures on the walls of
ground and elevated tanks are about half
the pressure present in standpipes.

Ground tanks ha\e low operating head,
about 10 m (30 ft.), but have a large
capacity due to their diameter, up to 30
m (100 ft.). These tanks are constructed
at the ground level and can be classified

TABLE 1/1
Number of tanks of each type constructed

STA.-JDPiFES I SI

GROUND TANKS (Gl

Tyce

Ahsreviaced

Construction

Nunber of

^"•"-*-

Oesionation

HetHOG Descripcion

TanJts

.c-s

aeinforceô concrete-
standpipe

G-S

Posc-tensioned wire wound
gunite protected -
standpipe

?ost-tensioned unbonded -
standoioe

PTB-S

Post-tensioned bonded -
standpipe

RC-E

Reinforced concrete -
elevated

G-E

Post-tensioned wire wound
çunite protected - elevated

PTU-E

Post-tensioned unbonded -
elevated

PTB-E

elevated

G-G

Post-tensioned wire wound
gunite protected - ground

RC-G

Reinforced concrete - ground

rc... ' .. 1

as low head tanks. Within each
category of above ground tanks built in
Ontario, there are three main structural
types as listed in Table 1/1. (see also
fig. 5.1 page 5-7)

1.6 Performance Rating

The performance rating of different
types of above ground tanks were
determined in the survey of tanks
carried out by W.M. Slater & Associates
Inc. (Ref. 2). Table 1/2 presents the
average rating of tanks by category
based on data presented in that report.

It can be seen that standpipes were
given the poorest performance rating.
The performance rating of tanks within
the same category varies and data can
be found in Ref. I where the rating
system developed by W.M. Slater &
Associates Inc. is described.

TABLE 1/2
Tank performance rating by category (1981)

Figure 1.1 Categories of Ontario
concrete tanks

Tank

Perforr

ance Ratma i

Rank

Category

N'o.

(0-

) Scale»

Ground

8.0

Elevated

Standpipe

6.9
5.9

sound tank .

1 - 3

2.0

DETERIORATION OF ABO\ E GROUND CONCRETE WATER TANKS

2.1

General

There were 11 main defects revealed in
the survey of concrete water tanks.
These are summarized in Ref. 1 and arc
listed in Table 2/1 below, in order of
diminishing priority and structural
importance, but not necessarily cost.

TABLE 2/1
DEFECT RATING

1. Wall delamination.

2. Vertical cracks in wall.

3. Wall/floor joint leaks and wall

deterioration.

4. Vertical voids in shotcretc.

5. Spalls caused by jack-rods left in

walls.

6. Cover coat delamination and

debonding from prestressing wires.

7. Waterproof coating delamination

and debonding.

8. Cover coat shrinkage cracking.

9. Cold joints and horizontal cracks.

10. Corrosion of prestressing wires.

11. Corrosion of post-tensioning

tendons.

Between 1976 and 1986, approximately 50
concr.ete water tanks were inspected in
detail and were partially or fully
repaired, including the demolition and
replacement of 1 1 concrete tanks by
steel tanks. During this process, it was
found that each tank has its own
deterioration peculiarities; however,
several types of defects can be
associated with individual tank types.

2.2

Observations

2.2.1 External Wall Delamination

Vertical wall delamination was
considered to be the most serious

concrete defect found in concrete water
tanks, and varied in form according to
the type of construction used and
whether the structure was prestressed
or not.

E.xamples of dclaminations associated
with the various types of construction
are described below:

.1 Post-Tensioned Unbonded (PTU)

Two standpipes which failed at
DUNNVILLE and SOUTHAMPTON,
exhibited wall delamination on the
centre lines of tendons which
apparently filled with water and then
froze causing splitting of the wall.
The standpipes were re-built in steel.

.2 Post-Tensioned Bonded (PTB)

Investigations of walls at leaks revealed
serious delamination of the walls at
CASSELMAN, RED LAKE and PICKLE
LAKE elevated tanks. The
dclaminations were on the centre lines
of the post-tensioning ducts and
occurred mainly in the bottom of the
tanks. At PICKLE LAKE, dclaminations
occurred in the presence of major
horizontal cracks caused by ice thrust.
In some cases, the ducts were observed
to be ungrouted at the delamination
locations.

.3 Post-Tensioned Wire Wound Gunite
Protected (G)

Many isolated dclaminations were noted
during repairs to the FENELON FALLS,
BADEN, and other similar wire wound
tanks. These external dclaminations
could also be associated with internal
jack-rod spalls, a defect discussed in
greater detail later in this report. At
VAL CARON, a large diameter wire
wound post-tensioned tank with a

gunite cover coat, it w'as found that
small external delaminations were
associated with leakage through
form-ties.

Photo 2-1 Typical deterioration at
bottom of reinforced concrete stand pipe
(WATFORD)

At AMHERSTBURG and CHELMSFORD
major external delaminations had
occurred at locations where there were
defects in the waterstop between the
wall and floor of the tanks, as well as
at other minor locations.

At WOODVILLE, a post-tensioncd wire
wound gunite standpipe, a series of
delaminations occurred around the tank
at the level of the manhole. Further
detailed inspection revealed that, due to
the presence of the manhole, the
post-tensioning wires had been omitted
from this region of the tank (sec Photo
2-8).

.4 Reinforced Concrete Standpipes
(RC)

The areas of delaminations found in
reinforced concrete standpipes were
much greater than in the tanks
constructed of prestressed concrete.

Photo 2-1 and Photo 2-2 show the
south face of the WATFORD water
tower and illustrate typical
deterioration of the lower section of a
tank. Delamination occurred at the
level of the reinforcing steel with little
or no corrosion of the steel.

Similar delamination was found during
repair of the ALVINSTON tank where
large areas of external and internal
delamination generally occurred on the
side of southern exposure. One
interesting observation from this tank
was that an internal epoxy resin
coating had been applied to the lower
6m (20 ft.) of the tank and all major
delamination had occurred above this
level.

•* >*

Photo 2-2 External wall delamination
(WATFORD)

A third reinforced concrete standpipe
also exhibited extensive wall
delamination (CAMLACHIE) and was
eventually replaced by a new tank.
Photos 2-3 and 2-4 show the major
spalled areas. As with the other
reinforced concrete tanks, no corrosion
of the reinforcing steel was observed.

2 - 2

Photo 2-3 External wall delaminalion
(CAMLACHIE)

Cores taken through the tank walls
indicated fracture of the concrete at
both the external and internal
reinforcing steel (Photo 2-5).

The CAMLACHIE lank shown in Photo
2-3 was monitored during the winter of
1982-83 prior to intended repair. It
was observed that the majority of
fracturing occurred during the spring of
1983, sometimes with explosive force
accompanied by noise and vibrations of
the structure.

Li > t ■ ■

^:^.

I '^Vl

^^H

^^m ^H

1 '

/.' MM

w-jâR

''^^^

Photo 2-4 Fiaciure at reinforcing steel
(CAMLACHIE)

Photo 2-5 Core section showing
fracture at external and internal steel
(CAMLACHIE)

After the detailed tank condition survey
(Figure 2.1) taken during December
1982 and after the subsequent damage
of the 1982-83 winter, a rehabilitation
design was completed. It was decided,
however, to replace the tank with a
steel structure.

Because of the severity of
delaminations found in most reinforced
concrete standpipes and the high cost
of repairs, strengthening, insulation and
cladding, the following additional
standpipes and elevated tanks were, or
will be demolished and replaced by
steel tanks - ARKONA, CALLANDER,
MARKDALE, PITTSBURGH, WEST
LORNE and WYOMING.

ARKONA tank, a reinforced concrete
standpipe was inspected
internally in summer 1985 after
dewatering. Delaminations similar to
the CAMLACHIE and WATFORD tanks
were found at the bottom of the
southern exposure. Cores taken in

2 - 3

W N

CLOCKWISE DISTANCE /«OUND INTERIOR OF
STANDPIPE FROM t MANHOLE

LEGEND

A CORE LOCATIONS, DEC.2, 1982.

T^ BY COLDER ASSOCIATES
_4- CORE LOCATIONS, NOV. 23, 1982,

^ BY OTHERS

LOCATIONS OF HOLLOW AREAS
DETERMINED BY HAMMER
SOUNDINGS

Figure 2.1 Delaminalion survey of slandpipe (CAMLACHIE)

these regions confirmed that the tank
wall had delaminatcd at the plane of the
external steel. In addition the concrete
surrounding the steel was almost totally
disintegrated.

In an effort to obtain a sound core for
compressive strength testing, the
ARKONA tank was sounded throughout
its lower circumference. An area was
selected at the east exposure just
outside of a location considered to be
delaminatcd. Due to the presence of a
waterstop, the core was extracted in two
sections. The waterstop was found to
be defective at that location, however
both core segments were found to be

sound. It was noted that the inside
hoop steel was well bonded to the
concrete, but the outside hoop steel
was completely debonded from the
surrounding concrete and was free to
move because of considerable
enlargement of the interface
surrounding the reinforcing steel.
Detailed examination of this interface
revealed that the dcbonding was not
uniform around the reinforcing steel.
The concrete interface towards the
inside of the tank wall mated perfectly
with the steel but there was a gap of
about 2 mm surrounding the section of
reinforcing steel facing the outside
tank wall.

2 - 4

Due to the geometry of the gap the
more usual eauscs of debonding such as
lack of cleanliness of the reinforcing
steel or excessive bleed water during
concrete placement were considered
unlikely. Since serious delamination of
the tank wall had been revealed in other
cores it was considered possible that the
condition of this core was a result of
the deteriorating mechanism and was
representative of a tank wall shortly
before delamination (sec Photos 2-6 and
2-7).

Photo 2-6 Core through tank wall
(ARKONA)

£?

r>«.'. 4P> "vF*

Photo 2-7 Close-up of expanded steel
concrete interface (ARKONA)

.5 Conclusions

Based on the above observations it was
concluded that, in general, where the
internal water under pressure was
allowed to penetrate to the external

surface, external delamination was
likely; furthermore, reinforced concrete
standpipcs particularly in the southern
lower portions of the tank, appeared to
be more susceptible to external
delamination than other tank types
including elevated prestressed concrete
tanks. In comparing standpipcs, it
appeared that post-tensioning reduced
the likelihood of external delamination
except where internal spalling had
reduced the wall thickness. It was
concluded that the physical factors
involved were the availability of water,
pressure, and southern exposure, with
radial compression being a mitigating
factor.

Photo 2-8 External delamination in wire
wound post-tensioned tank
(WOODVILLE)

2.2.2 Internal Wall Delamination

Several different categories of internal
delamination were observed. The
categories noted were extensive
delamination to the depth of the
internal reinforcing steel, localized
spalling where the depth of cover to
the reinforcing steel was shallow,
internal surface delamination, conical
spalls associated with jack-rod
couplings, and at locations adjacent to
vertical unbonded post-tensioning
tendons.

As with external delamination, major
areas of deep internal delamination
were invariably associated with

2 - 5

Photo 2-9 Internal delaminnlion
(ALVINSTON)

reinforced concrete standpipes. Again,
little or no corrosion of the reinforcing
steel was noted (Photo 2-9).

2.2.3 Localized Spalling

Localized spalling had occurred at many
tanks and could not be associated with
any tank type in particular, however, it
was noted that in the majority of cases
the localized spalling was associated
with a shallow depth of cover to the
reinforcing steel or with individual
aggregate particles (Photo 2-10).

Photo 2-10 Localized spalling (BADEN)

One interesting defect was found at the
BADEN tank where two construction
gloves had been accidentally buried in
the tank wall at separate locations. In
both cases deterioration was extensive

and deep around the gloves (Photo
2-11). In another tank (VAL CARON),
localized spalling occurred at vertical
joints and could be associated with
leakage at the water-stop (see Photo
2-I2V

Photo 2-11 Deterioration caused by
construction glove (BADEN)

In the majority of tanks, the internal
coating had deteriorated away from the
wall (Photo 2-13). Examination of core
sections revealed that in most cases the
surface of the concrete was fractured
to a depth of between 5 mm and 15 mm
In some cases the deterioration was so
extensive that the remnants of the
coating could only be detected by
microscopic examination.

Photo 2-12 Damage caused hy water
seepage at water-stop (VAL CARON)

2 - 6

Deterioration of this nature did not
appear to be associated with tank type
but seemed to be a function of coating
type and quality (sec report to Ministry
of the Environment (1985) titled
"Evaluation of Waterproof Coatings for
Concrete Water Tanks" where this
subject is discussed more fully).

Photo 2-13 C<;fi// ./ a

jack-rod spall (WOODl ILLE)

I.IA Jack-Rod Spalls

Some tanks had been constructed using a
slip form method. Hollow pipe rods,
known as jack-rods, each approximately
3 m in length are used to raise the slip
form. As construction proceeds,
additional lengths are added using
threaded coupling rods. This results in
hollow tubes at approximately 3 m
centres extending the full height of the
tanks and interrupted at each joint by
the solid rods.

In the tanks inspected to date, little
attempt was made to seal these
jack-rods; the effectiveness of the seal
between the roof and the tank walls
was apparently relied on to prevent the
ingress of water. Inspection has shown
that this assumption is untrue and that,
in most cases, water was able to enter
the uppermost jack-rod section. This
water leaks out at the threaded
coupling, saturating the surrounding
concrete.

Observations revealed that these tanks
experienced internal spalls and that the
spalls invariably occurred exactly at the
coupling between the individual rods
(Photo 2-14). The spalls were always
conical in shape, and although a small
amount of totally disintegrated concrete
was usually present at the apex, the
spalled concrete was normally intact
without any sign of disintegration
(Photo 2-15). In general, as many as
20 to 30 of these spalls could be
present in the tanks constructed using
this system (see Figure 2.2).

Photo 2-15 Inlact spall recovered from
bottom of tank (WOODVILLE)

Photo 2-14 Jack-rod spall ai thrccutcd
coupling (BADEN)

2 - 7

2.2.5 Co>er Coat Shrinkage and
Cracking

On tanks where a concrete cover coat
had been applied to the prcstrcssing
wires, external delamination was not a
serious problem, however, the majority
of these tanks had many cracks
principally in the horizontal direction,
(sec Photo 2-16 and Photo 2-17).

Photo 2-16 Typical horizontal cracks and
leachate stains occurring at a
prestressed stand pipe (WOODl'ILLE)

Figu

ELEVATION

re 2.2 Jack-rod spalls

In most instances these external
horizontal cracks could be traced
through to the inside wall. Core
sections taken through this type of
defect sometimes indicated that
freeze-thaw damage was present at a
microscopic level while in other
instances the crack had no
deterioration. The general rule
observed was that where leaks or
leachate stains were present on the
outside, then internally fractured walls
were likclv.

Photo 2-17 Typical cracks and leachate
stains occurring at a prestressed ground
tank (VAL CARON)

2 - 8

2.2.6 Qualit\ of Concrclc in N\atcr
Tanks

.As stated in the Portland Cement
Association's publication, "Design and
Control of Concrete Mixtures", one of
the greatest advances in concrete
technology was the development in the
mid-1930's of air-entrained concrete.
The principal reason for using
intentionally entrained air is to improve
concrete's resistance to freezing and
thawing. However, there are additional
beneficial effects of entrained air in
both fresh and hardened concrete.
Known benefits are improved workability,
resistance to de-icers, and sulphate
resistance. The latter two factors may
be explained by the reduction in water
demand for a given cement content due
to the improved workability and which
leads to a reduction in the water cement
ratio and, hence, produces a less
permeable cement matrix.

However, the application of "gunite" is
not ideal for air entrainment and, as,
shown in Table 1/1 a number of tanks
have, in the past, used this method.

Unfortunately, original quality control
data obtained during the construction of
the tanks utilizing this method are not
readily available; however, it is
understood that normal quality control
tests were made and were consistent
with the Ministry and other relevant
standards specified at that time.
Typically, for normal concrete placement
methods, the requirements were for a 28
day cylinder strength in excess of 3000
psi and, where normal concrete
placement methods were utilized, an
entrained air content of 5 to 7 percent
was specified. Table 2/2 gives details of
typical concrete quality obtained by core
sampling several of the water tanks
during remedial works. The results of
tests for compressive strength indicate
that, in general, the compressive
strength of the concrete considerably
exceeded the required standards of the

day. As stated above, some concrete
tanks were constructed using air
entrained concrete. Other tanks due to
the concrete application method, were
not air entrained. Although in most
cases the air entrained concrete
contained less air than desirable,
nevertheless, most of the examples
given are within the limits of
"acceptable quality" as judged by the
Ministry of Transportation and
Communications Bridge Deck Rehabilita-
tion manual (Ref. 3). Thus, it can be
stated that the presence and pattern of
deterioration was not consistent with
the lack, or otherwise, of air
entrainment. Furthermore, it was
considered that, since most of the
concrete standpipes exhibited external
deterioration at the bottom of the
tanks it would be a remarkable
coincidence if all poor quality concrete
was placed in the lower sections of
these tanks and all durable concrete
was placed in the uppermost sections.

^^^^^Kr ^^^^^^^^^H

VSE'"'^ u ' ^^^^^1

fiÊwi'' ' '' ^^^^^^^H

ÏÏm

mi^i

Photo 2-18 Deterioration of reinforced
concrete standpipe ( ALVINSTON)

2 - 9

TABLE 2/2

Summary of typical water tank core test results

Tank Name

Compressive

Air

Spacing

Remarks

Strength

Content

Factor

(MPa)

(%)

(mm)

Design

Tested

Arkona

28.0

45.7

3.0

0.281

Cast Concrete

Rockwood

35.0

84.5

Shotcrete

61.6

Odessa

35.0

26.6

4.1

0.342

Cast Concrete

2.5

0.214

Woodville

32.0

78.6

Cast Concrete

61.4

Amherstburg

32.0

41.4

1.2

0.124

Shotcrete

Hespeler

32.0

88.0

Shotcrete

2.2.7 Summary of Obser\ations

Based on the results of these surveys
the following generalizations can be
made with respect to the walls of

concrete tanks.

1. All tanks exhibiting general
external deterioration have an
ineffective internal coating.

2. Tanks exhibiting isolated external
deterioration have an internal defect
which allows seepage of water toward
the exterior tank surface.

3. High pressure tanks exhibit more
deterioration than low pressure tanks.

4. Reinforced concrete standpipes
exhibit the most severe external
deterioration.

5. External deterioration tends to be
oriented towards the south exposure
(solar quadrant).

6. There is little galvanic corrosion
of reinforcing steel in areas of
exterior and interior wall
delamination.

7. Atmospheric corrosion of
prestressing wires has occurred
where the concrete cover coat has
delaminated and separated from the
wall.

Apart from the last two points relating
to corrosion, it can be seen that
freezing and thawing is the consistent
underlying phenomenon related to
deterioration of concrete water tanks in
Ontario. In some cases, defects which
may be relatively harmless in other
structures, significantly alter the
durability of the tank. In other cases,
structural types and construction
methods appear to have a significant
bearing on the useful life of the tank.

To help explain these observations,
published research on the freeze-thaw
durability of concrete was examined,
the temperature histories of tanks were
monitored and a conceptual model
describing progressive freeze-thaw
deterioration was developed. These
subjects are discussed in the following
sections.

2 - 10

3.0

TEMPERATURE MONITORING

3.1 General

Although most of the parameters
required for this study were available in
published research, the literature did not
reveal data of adequate detail to assist
in providing an understanding of the
climatic influence on water tanks.
Consequently in January 1983, as part of
the on-going applied research, the
Ministry of the Environment initiated
the installation of a temperature
monitoring system at ROCKWOOD, an
uninsulated concrete water tank. A
similar system was installed at
ALVINSTON, the first concrete tank to
be insulated as part of the tank
rehabilitation programme.

Figure 3.1 Location details of
temperature instrumentation( ROC KWOOD )

The temperature monitoring sensors were
installed in January 1983 prior to
completion of the ALVINSTON insulation.
To study the effects of a northern

environment, a monitoring system was
installed during a 1984 rehabilitation to
EAR FALLS, a tank with the first
combined heating, circulating and
insulation systems installed for freeze
protection.

3.2 Temperature Instrumentation

The ROCKWOOD temperature monitoring
system consisted of a total of 55
temperature sensors and was designed
such that temperature variations with
respect to height and solar exposure
could be analyzed. The assembly
consisted of four main cables at each
cardinal point. Each cable had an array
of sensors at the 20 m (60 ft.), 10 m (30
ft.), and 3 m (10 ft.) levels. The two
uppermost arrays consisted of five
separate sensors embedded into the tank
wall such that the inside sensor was in
the water, three sensors were evenly
spaced inside the tank wall, and one
sensor was exposed to the atmosphere.
The lower array (10 ft. level), consisting
of four sensors, did not have the
external sensor (see Figure 3.1).

The system was linked to data logging
and computer equipment which was set
to monitor the temperature of the
sensors at two hour intervals.

The instrumentation at EAR FALLS and
ALVINSTON was of similar design, but
with some of the sensors placed in the
air gap between the insulation and the
outside concrete to check actual heat
losses and accuracy of heat-loss
calculations (see Figures 3.2, 3.3, 3.4 and
3.5).

Figure 3.2 1983 autumn data at north
side of an uninsulated tank
(ROCKWOOD)

Figure 3.3 1983 autumn data at south
side of an uninsulated tank
(ROCKWOOD)

3.3 Test Results

Figures 3.2 to 3.7 illustrate typical data
from each of the instrumentation
systems.

Figures 3.2 and 3.3 are graphical records
of typical Autumn data comparing the
north with the south aspects of the
ROCKWOOD tank. The water supply is
obtained from an underground source
and has a steady inlet temperature of
approximately 7°C. Figures 3.4 and 3.5
are graphs of typical data obtained at
these locations during the winter period.
Analysis of the data revealed the
following points.

Figure 3.4 1984 winter data on north
side of an uninsulated tank
(ROCKWOOD)

Figure 3.5 1984 winter data on the south
side of an uninsulated tank
(ROCKWOOD)

• Temperatures in the North exposure
are almost continuously below freezing
throughout January. Freeze-thaw
cycling occurs almost daily in the South
exposure during the same period.

• There is a substantial difference in
daily temperature fluctuation between
North and South ambient temperatures.
This difference is reflected in the
concrete wall temperature. The
amplitude of the ambient temperature
and at 25 mm (1 in), into the wall
surface at the South exposure is
approximately 20°C and 15°C,
respectively. Corresponding amplitudes
for the North exposure are 7°C and 5°C,
respectively. These differences in

3 - 2

amplitude account for the significant
difference in the number of occurrences
of freeze-thaw cycling in the South
exposure when compared to the North
exposure.

• Freezing and thawing regularly takes
place on the inside of the tank wall and
has a considerable influence on the
water temperature close to the tank
wall.

• Temperature records of the tank
water obtained during sub-zero ambient
conditions indicate that the tank water
provides very little buffering effect and
suggests that little water circulation
takes place inside the tank. Sensors at
the centre of the water in the tank
indicate a steady reduction in water
temperature (throughout the period).

• Sensors in the water close to the

tank wall indicate that the water quickly
freezes at that region of the tank
forming an ice ring around the wall of
the tank. It should be noted that for
the period examined, water close to the
North wall froze 4 days prior to the
water at the South wall.

• The sensor in the water at the 20 m
(60 ft.) level showed that an ice cap
forms at the top of the tank to a
considerable depth (greater than 3 m).
Visual observations confirmed the
presence of the ice cap and its presence
was noted throughout the winter period.

]

I •'

'J il*

Figures 3.6 and 3.7 are graphical records
from the insulated ALVINSTON and EAR
FALLS tanks where, in both cases, water
is from a surface source.

Figure 3.6 shows that the insulation has
a significant moderating influence on the
air space where minimum temperatures
are roughly one third of the minimum
external ambient temperatures. It should
be noted that the construction of the

insulation was completed on this tank on
January 12, 1984. (shown as a vertical
line in figure 3-6).

Figure 3.6 1984 winter data at an
insulated tank in Southern Ontario
(ALVINSTON)

Figure 3.7 1985 winter data at an
insulated tank in Northern Ontario
(EAR FALLS)

Figure 3.7 is a graphical record of the
EAR FALLS tank during January 1985.
Again, the data illustrates the significant
moderating influence of the insulation on
the air gap. It was recorded that the
temperature of the tank water is largely
independent and isolated from the
ambient conditions.

(Detailed records from which the above
noted observations were obtained are
given in an Applied Research Report
Titled "Temperature Monitoring of
Ontario Concrete Water Tanks" issued by
the MOE in April 1986).

3.4 Freezing in Concrete

3.5 Seasonal Wall Conditions

Distilled water exposed to atmospheric
conditions will freeze at 0°C. The
freezing point of water, however, is
lowered with reduction in purity and
increasing pressure. In concrete
there is a considerable range of pore
sizes. Research by Helmuth (Ref. 4)
demonstrated that supercooling is more
likely to occur than freezing unless it is
seeded by a catalyst in a process called
nucleation.

The smaller the pore size, the more
difficult it is for nucleation to occur.
For this reason, at any point during
cooling below 0°C, all the water
occupying cavities smaller than a certain
size remain unfrozen. The lower the
temperature the greater becomes the
quantity of frozen cavities. The
formation of ice in the pores of
concrete can occur over a considerable
temperature range. However, research
has demonstrated that for practical
purposes freezing of the pore water
begins at around -2°C and is essentially
complete at around -4°C (ref. 5, and 6).

Another consideration of temperature
reduction in this range occurs when the
concrete is saturated. As each cavity
size is frozen, the passage of water
through it is prevented.

The significance of the above phenomena
is that freezing in the pores of wet
concrete is not a simple event but
occurs throughout a range of
temperatures below 0°C. As stated
above, it can be considered that the
majority of freezing is completed at
-4°C and depending on the degree of
saturation, is accompanied by a
reduction in effective permeability.
When all pores are frozen, therefore, the
effective permeability of the concrete
approaches zero starting on the outside
face of the structure.

The temperature monitoring of the
various tanks noted above has revealed
that significant seasonal wall conditions
can develop depending on such factors
as inlet water temperature, the ratio of
daily water used with respect to tank
volume, tank exposure conditions and
geographical location. Where these
factors are favourable, namely high inlet
temperatures, high tank turnover and
sheltered exposure conditions, (e.g.
ground tank) then ice is less likely to
form on the inside of the wall.
However, these conditions are rarely the
case. For example, at VERNER, an
elevated tank, a 2 m (6 ft.) ring of ice
was observed around the perimeter of
the inside of the wall. At ROCKWOOD,
an unbonded tendon standpipe, a 1 m (3
ft.) ring of ice was observed. Divers
measured the thickness of ice occurring
during January, 1984, at CASSELMAN, an
elevated tank, and reported the thick-
ness of ice to be 300 mm (12 in.) around
the walls and 150 mm (2 in.) at the
bottom of the tank. Tests at PORQUIS
JUNCTION, a reinforced concrete tank,
revealed that due to a low daily
requirement compared to volume, the
effective tank volume was reduced by
75% by the massive formation of ice
inside the tank. These observations and
many others have shown that ice cap,
wall ice and floor ice, often estimated at
several hundred tons, regularly form on
the inside of most tanks during winter
and are often present until the middle
of May.

The formation of ice on the internal
walls of tanks has several disadvantages
such as deterioration of the internal
coating due to physical movement of the
ice, prevention of internal tank
inspection for a large portion of the
year due to the danger of falling ice
and a reduction in the operating volume
of the tank. However, the formation of
ice on the internal wall can also produce
significant changes in the thermal

3 - 4

profile of the tank wall which can ha\c
serious consequences on its durability
and is, therefore, worthwhile
considering in detail.

3.5.1 Thermal Conducti\ities

To estimate the thermal profile of the
various wall conditions the following
thermal conductivities were used.

Steel
43.0

W/°C-^2

Concrete (Wet)

1.81 \\/oç-m2

• Ice

2.16 \V/°C-

• Water

0.60 W/°C-'"2

• Air

0.024 w/oc-""^

• Slyrofoam

0.029 w/°C-"'2

3.5.2 Autumn - NMnter Condition

Figure 3.8a is an example of an average
temperature which regularly occurs
between autumn and mid-winter when
the minimum 24 hour ambient
temperature is -10°C. During this
period, ice has not yet formed on the
inside of the wall and therefore the
tank water maintains the inside of the
wall above zero (typically 2°C). Thermal
calculations demonstrate that for this
condition the maximum depth of total
freezing (-4°C) is about half the wall
thickness.

3.5.3 Winter - Spring Condition

Figure 3.8b is an example of an average
temperature profile which can occur
after the development of a ring of ice
on the inside surface of the tank and
typically occurs between mid-winter and

early spring. In this case, a significant
thickness of ice has formed on the
inside surface of the concrete. The
presence of this ice prevents any
buffering effect from the internal water

a) Ice free condition (autumn - winter)

h) Ice ring present inside the tank

Figure 3.8 Typical seasonal wall
conditions

and, therefore, the entire tank wall can
be frozen to a point where freeze-thaw
cycling can occur throughout the
thickness of the wall.

The temperature data for January 1984
obtained from ROCKWOOD illustrates the
change of temperature profiles as the
ice ring is formed and is given on
Figure 3.5. The graph shows that the

water close to the south wall began to
freeze around January 17th. Prior to
this, the temperature of the outside of
the wall was cycling between -10°C and
+5°C and the inside face was
consistently above zero. After January
17th, the temperature of the water fell
below zero and ice formed. At that
point, the entire wall temperature was
consistently below zero and temperature
cycles occurring in the wall are clearly
seen.

Figure 3.9 Typical thermal profile after
insulating

3.5.4 Insulated Tank Winter Profile

A typical thermal profile through the
wall of an insulated tank is given in
Figure 3.9 which illustrates the
beneficial effect of the insulation when
the ambient temperature is -10°C. At
that ambient temperature the air gap
temperature is approximately -2°C and
the water in the smaller pores remains
unfrozen in the outer layer of the
concrete. Field inspections made during
winter conditions revealed that internal
ice formations were essentially
eliminated, thus improving the thermal
profile through the tank wall and
preventing deterioration of the internal
coating by freeze-thaw cycling on the
inside wall. In addition it should be
noted that the insulation reduces the

amplitude of the daily thermal cycle
considerably - a significant additional
benefit.

3.6 Daily Thermal Cycle

Figures 3.10 and 3.11 are graphs giving
details of the thermal history of the
ROCKWOOD tank between January 14
and January 17, 1984. They illustrate
typical thermal cycles occurring
approximately 50 mm (2 in.) away from
the outside of the tank wall (ambient)
and approximately 25 mm (1 in.) into
the concrete surface and demonstrate
the substantial thermal difference
between the North and South exposures.

Figure 3.10 Daily temperature cycles
showing continuous freezing at the
North side of the tank

Figure 3.11 Daily temperature cycle
showing daily freeze-thaw cycles at the
South side of the tank

3 - 6

At the North exposure, the maximum
ambient temperature adjacent to the wall
tends to be consistently below zero.
Daily ambient temperature fluctuations
are about 7°C, with corresponding
concrete temperature changes of about
4°C to 5°C, and is similar to shaded
ambient conditions. At the South
exposure the temperature fluctuation is
significantly different. It can be seen
that the daily amplitude of ambient
temperature change is about 20°C, with
a corresponding amplitude of about 15°C
inside the concrete, from +7°C to -8°C.

3.7 Critical Ambient Temperatures

Examination of the detailed temperature
history of the North and South faces of
an uninsulated tank has demonstrated
that the above noted differences are
consistent throughout the year. During
all seasons, the South exposure receives
considerable additional solar energy
which increases the daily temperature
amplitude when compared to the North
exposure. However, the data
demonstrates that under certain ambient
temperature conditions, it is this
additional solar energy received during
the winter which greatly increases the
number of freeze-thaw cycles at the
South exposure.

Based on the data, the daily amplitude
on the South face of the wall during
winter is about 20°C and daily freezing
and thawing cycles can occur where the
shaded ambient conditions are between
-5°C and -15°C and clear sunny
conditions prevail.

At the North face of the structure, the
corresponding daily temperature

amplitude is about 7°C; consequently,
the critical range of shaded ambient
conditions for freeze-thaw cycling to
occur is between -7°C and -5°C; this
may occur infrequently.

The temperature monitoring of an
uninsulated tank has revealed that, there
is a difference in the daily temperature
amplitude between the South and North
exposures which accounts for the
difference in the number of freeze-thaw
cycles experienced by concrete at the
South exposure. It can be seen
therefore that the majority of
freeze-thaw cycles is a result of solar
radiation and occurs when the
background ambient temperature is
within a critical range below zero, and
solar radiation thaws the concrete. In
Southern Ontario it appears that the
critical temperature is between -5°C and
-15°C at the South exposure and
between -5°C and -7°C at the North
exposure.

Comparing Southern Ontario with other
regions, it is likely that these basic
environmental conditions are less
prevalent in either more Northerly or
more Southerly climates where the
shaded ambient conditions are likely to
be either above or below this critical
range. Considering these conditions for
other climates, it is also likely that even
where ambient conditions are within this
critical range, reduced solar radiation
due to the prevalence of cloud cover
will reduce the amplitude of the daily
cycle and consequently reduce the
critical temperature range. This aspect
of the data may help explain why
concrete deterioration is so prevalent in
Southern Ontario.

3 - 7

4.0 INVESTIGATION OF THE FREEZE-THAW FAILURE MECHANISM IN CONCRETE

4.1

General

There can be several causes of
premature deterioration of concrete,
some chemical in nature or some as a
result of severe climatic conditions.
With the exception of the presence of
chlorides, most chemical agents have to
be present in significant concentrations
in order to produce rapid deterioration.
Since water tanks are being used to
store drinking water and elaborate
precautions are made to exclude
concentrated chemicals, it was inferred
early in the study that the deterioration
observed was not chemical in nature.

Many concrete structures which are
subjected to extremes of weathering
exposure will deteriorate unless
protected. Structures frequently exposed
to saturation by water, followed by
cycles of freezing and thawing,
deteriorate more rapidly than others.
Such structures include concrete
pavements, bridge decks, curbs and
gutters, spillway floors and drainage
channels.

Concrete water tanks may incorporate
some system to prevent water from
contact with concrete. Coating
breakdown or an inadequate coating can
expose the concrete surface to water
saturation. This can lead to severe
deterioration of concrete under sub-zero
temperatures (Ref. 7). The following
quotation from this source provides an
insight into the nature of the problem:

"The effect of freezing on concrete
tanks constitutes a problem of
considerable magnitude. Concrete has
some porosity; and when it is subjected
to high water heads, the water
permeates throughout these pore
structures. It is easy to see the effect
this will have when the excess water is

subjected to freezing temperatures. An
expansion of the water during freezing
will start a slow breakdown of the
concrete. The action is known as
spalling, and unless it is arrested, the
entire tank may end up as an unusable
water holding facility. At best,
expensive repair bills can result."

The survey of water tanks carried out
by W.M. Slater & Associates Inc.
revealed that the protective coating in a
large number of tanks was ineffective.
Exposure of concrete to water under
high hydraulic head combined with
freezing temperatures undoubtedly
creates conditions of dangerous exposure.
Although deterioration of concrete in
these conditions is a distinct possibility,
it is far from evident that conventional
measures such as air entrainment of
concrete will be able to prevent it.

While it is technically possible to
eliminate conditions of severe exposure
by providing an unconditionally
impermeable liner, e.g. an internal steel
shell (Ref. 8) or by providing insulation
to the tanks (Ref. 8), such drastic
protective measures add significantly to
repair or construction costs and should
be considered only after gaining a
detailed knowledge of the deterioration
mechanisms, their rates, and factors
which affect them.

It is the purpose of this study to
explain the mechanisms of concrete
water tank deterioration and to identify
factors which affect the deterioration
rates in order to provide a rational basis
for evaluation of repair and design
alternatives.

4 - 1

4.2 Literature Review

The literature on concrete water tanks
is not extensive and the main emphasis
is on the structural design of tanks,
their construction and repair methods
(Refs. 8, 9, 10, 11, 12). A record of
19th century water tank failures is
contained in the monograph "Stand Pipe
Accidents and Failures" by Prof. W.D.
Pence who, in 1894, collected data on
such occurrences from the earliest
known, to the time of publication. It is
interesting to note that out of 45
accidents with steel standpipes presented
in the book "23 were total wrecks, 14
were slightly damaged and 8 were
slightly injured. As far as determined,
the cause of the accident was in 22
cases due to water; in 11 cases water
and ice: 11 were reported due to the
wind, while a number of cases were from
failure of foundation". Several more
cases were reported in Ref. 9 which
dates back to 1910 and from which the
above quotation of Prof. Pence's work
was taken. Since tanks in that era were
not constructed using concrete, most of
the accidents described are not directly
relevant to modern concrete tanks.
However, the sentiments expressed can
be associated with current concerns and
provide echoes from the past.

The history of modern concrete water
tanks dates back to 1908, when the first
U.S. patent on prestressed concrete
tanks was taken. An interesting history
of the evolution of modern water tanks
is described in Ref. 7. This recent
monograph is apparently one of the few
publications which clearly states that
concrete water tanks are problem
structures in freezing conditions.

Review of modern literature revealed
that much of the research on
freeze-thaw deterioration of concrete
relates to studies of the material aspects
of this phenomena. A very significant
portion of this work is either directly
related to road structures like pavements

and bridge decks or is derived from
observations on the performance of
these structures. Little research has
been reported for the freeze-thaw
durability of concrete under constant
hydrostatic loading. To our knowledge,
above ground concrete water tanks have
not previously been a subject of a study
concerned with deterioration mechanisms
under freeze-thaw conditions.

4.3 Some Factors Affecting the
Freeze-Thaw Durability Of
Concrete

The following remarks were made by
T.C. Powers in 1966 when introducing
his paper on "Freezing Effect in
Concrete". These appropriately state the
dilemma when considering the action of
freezing and thawing under hydraulic
pressure.

"Specimens of concrete kept continually
wet on all surfaces by spraying or
immersion usually become damaged or
destroyed when they are cooled well
below the normal freezing point of
water; if the period of soaking is long,
nearly all concretes so exposed, air
entrained or not. cannot withstand
freezing. On the other hand, concrete
structures having at least one surface
exposed to air continually show
extremely various behaviour, from total
failure, usually localized, to apparent
immunity to freezing effects" (Ref. 13).

Examined from this point of view, the
concrete tank represents long term
immersion; however, one face is also
continually exposed to the air. It
appears, therefore, that several factors
are involved in the durability of
concrete water tanks and not just the
simple expedient of adding appropriate
air entrainment.

4 - 2

4.3.1 Saturation of Concrete in Neater
Tanks

(i) Unsaturated Concrete

Saturation of concrete can occur in
natural conditions of weathering
exposure (Ref. 15). It is normally
expected, however, that the external
concrete walls of a structure will not
become saturated. The major difference
between water tanks and other types of
structures is that without an appropriate
waterproofing system, the applied
hydraulic pressure will force water
through the pores of the concrete, and
result in saturation of the concrete
walls.

The rate at which water will flow
through concrete is a function of its
porosity which depends on the size,
distribution and continuity of the pores.
Research has shown that these factors
are a function of mix design, hydration,
curing, construction techniques and the
like.

(ii) Saturated Concrete

Calculations based on D'Arcy's law have
shown that in the lower portion of a
typical high pressure head tank,
saturation may be achieved between 2
and 4 years and is illustrated in Figure
4.1. This rate of saturation will of
course be increased by the presence of
normal defects such as horizontal and
vertical cracks.

It is only after the concrete becomes
saturated to a critical level, above 90%,
that the action of freezing and thawing,
which was previously harmless, starts its
destructive action. It can be
demonstrated that even the highest
quality of concrete will achieve this
state under the pressure conditions
prevailing in a typical concrete
standpipe.

Figure 4.1 Theoretical saturation zone
and seepage discharge

4.3.2 Hydrostatic Pressure and
Evaporation

As stated elsewhere, entrained air
bubbles cannot effectively protect
saturated concrete from the development
of freezing and thawing induced pore
pressures. Where one face of the
concrete is exposed to air, then, under
low hydraulic pressure, the evaporation
process can reduce the level of
saturation and, therefore, may reduce
the freeze thaw deterioration.

For a typical concrete water tank
structure the factors which determine
the rate of water entering the pores of
the concrete are the permeability of an
internal coating, the presence of cracks
and fissures in the wall and the pressure
of the water acting on the wall. This
last factor indicates that, if the other
factors are uniform throughout the
height of the tank, the degree of
saturation (or perhaps more accurately
the depth of saturation within the wall)
increases with hydrostatic pressure and
is at a maximum at the bottom of the
tank. Thus, it is likely that the effects
of evaporation are negated by the
presence of a sufficiently high
hydrostatic pressure at the bottom of
high head tanks.

4.3.3 Air Voids

From the point of view of saturation,
the presence of air voids represents an
increase in porosity, and therefore, in
theory, air entrained concrete should
saturate more readily. However, as
Verbeck (Ref. 14) points out in his
discussion on concrete porosity, the
presence of air voids has other
beneficial factors such as increased
workability and reduced segregation,
which tend to improve the overall
homogeneity and, consequently, decrease
the permeability of the concrete
compared to its non-air entrained
counterpart.

The main function of entrained air
bubbles in the cement paste is to
prevent the development of internal
hydraulic pressure. This pressure can be
produced by the physical increase in the
volume of ice compared to water.
However, it can also result from the
development of osmotic pressure due to
the presence of alkali in the concrete.

The water entering the pore space of
the concrete due to external hydraulic
pressure will contain a considerable
quantity of dissolved alkali. As the
water in the larger pores drops below
zero some of it freezes. Since it is only
the water that freezes, the effect is to
increase the concentration of the alkali
at that pore space. At other regions,
the space is smaller and therefore has
not yet begun to freeze. This water,
containing dissolved alkali of low
concentration will tend to move to the
zone of highly concentrated alkali
contained in the larger, partially frozen
pores, creating osmotic pressure.

The presence of entrained air greatly
reduces the hydraulic pressures
generated by either source. In the case
of hydraulic pressure generated by
physical expansion of the ice the air
bubbles act as relieving reservoirs.
Osmotic pressure is also relieved by the

air bubbles since any water containing
dissolved salts and exuding into the
bubble will immediately freeze. If the
bubble is not full, then, although the
dissolved alkali is concentrated by the
effect of freezing, no osmotic pressure
can occur due to the discontinuity of
the fluid.

Based on the details of these two
mechanisms, namely, hydraulic pressure
and osmosis, it can be seen that in
addition to the importance normally
given to the quality, size, and spacing of
the entrained air, it is equally important
that the air bubbles are not filled or
even partially filled with moisture since
the more moisture there is in the air
bubble the less protection it can impart
to the cement paste surrounding it.

As pointed out by T.C. Powers, another
consideration with respect to air
entrainment is the fact that some 60% to
70% of the concrete mass cannot be air
entrained, namely the aggregate
particles. Research has indicated that
concrete, provided it is made with
saturated aggregates and maintained at a
high level of saturation prior to test,
immediately fails a standard freeze-thaw
test.(Ref. 13, and 17).

Under high hydrostatic pressure
conditions, the presence of air voids has
an effect on the permeability of the
concrete, however, when these voids are
filled with water they cannot prevent
the development of pore pressures during
freeze-thaw cycles and may aggravate
the condition.

4.3.4 Affect of prestress on permeability

In addition to the factors previously
described, placement conditions and the
presence of stress affect the
permeability (see Figure 4.2). Mills (Ref.
16) found that the permeability of
concrete was greater in a direction
parallel to bleeding and that the

4 - 4

Table 4/1

Typical values of permeability of

concrete used in dams

Cûmcnt Lx

ntent

Water/cement

Permeabil

ity

kc|/m'

lb/yd'

ratio

10"" m/s

156

263

0.69

151

254

0.74

138

235

0.75

223

376

0.46

Figure 4.2 Relationship between bleed
water paths, water pressure flow and
stressing force in water tank walls.

FLOW PARALLEL
TO BLEEDING

FLOW TRANSVERSE
TO BLEEDING

LATERAL STRESS, Cfs , MPo

Figure 4.3 Permeability as a function of
confining stress (ref. 16)

presence of lateral stress reduced
permeability (see Figure 4.3). In concrete
water tanks, therefore, water flows in
the least permeable or horizontal
direction and circumferential prestressing
reduces the permeability. Typical values
of permeability are given in Table 4/1.
(ref. 19)

4.4

Action of Freezing Temperature on
Concrete

The durability of concrete structures
exposed to moisture and freezing
temperatures has been a major concern
to engineers for a long period of time.

Although microscopic mechanisms of
frost action arc not entirely understood,
the major features of concrete behaviour
under freezing temperatures are well
established.

• Dry concrete is not affected by frost.

• Wet, highly saturated concrete
expands when it freezes.

The expansion of concrete under
sub-zero temperatures is frequently
attributed to an increase in specific
volume of water during ice formation
(Ref. 17). Dilation of concrete is then
attributed to high porewater pressures
which can be created when excess water
is forced out of freezing areas causing
disruption of the internal structure and
overall expansion of concrete. From the
point of view of this mechanism, the
degree of pore saturation is a major
factor affecting concrete behaviour
under freezing conditions.

Brittle porous materials such as
concrete, rock and bricks all appear to
exhibit similar behaviour under cyclic
freezing and thawing in a saturated
condition. Figure 4.4 is an example of
laboratory freezing and thawing of clay
bricks where the dimensional changes
which can occur in these types of
materials under one cycle of saturated
freeze-thaw conditions are shown as a
solid line. (Ref. 18).

4 - 5

As the specimen is cooled to 0°C,
normal contraction occurs. At a
temperature of approximately -4°C, the
specimen undergoes rapid expansion. At
around -12°C to -15°C, the maximum
expansion is reached and normal thermal
contraction is reinstated. During the
thawing cycle the reverse pattern is
observed. The specimen expands at a
slightly greater rate than the rate of
contraction until it reaches a maximum
at 0°C. At that point, ice within the
specimen melts and thawing shrinkage
occurs. Depending on such factors as
permeability and pore size, the amount
of shrinkage is normally less than the
amount of expansion and consequently
produces a residual or non-elastic
expansion after each cycle.

Also plotted on the right hand axis of
figure 4.4 is the time history during
freezing and thawing. The dashed line
(time v's temperature) shows that the
entire cycle is completed within 1 hour
and that both during the freezing cycle,
and also, during the thawing cycle, a
delay occurs at -4°C and 0°C
respectively, indicating that heat is

0. u
0. 10
0.08
0.06
0 04
0.02

IfMPERAIURE,

being absorbed or given off and
indicates that the water is changing
state at these temperatures.

Figure 4.5 illustrates that concrete with
a low degree of pore saturation is
immune to cyclic freezing whereas
similar concretes which have a high
degree of pore saturation become highly
susceptible to frost damage. The critical
saturation level at which dilation occurs
appears to be about 85% or above.

Apart from the degree of saturation, the
magnitude of dilation depends on a
variety of factors such as strength of
concrete, permeability of concrete,
presence of air entrainment and type of
aggregate. Figure 4.6 illustrates the
influence of some primary factors on the
magnitudes of dilation.

oo-oog-

-T-1

-T-i

n \\

I o

si^Avo

60 6S TO 7S 80 as 90 9S 100

DECREE OF SATURATION, %

Figure 4.4 Typical experimental length
change results from thermomechanical
measurement (ref. 18)

Figure 4.5 Correlation between degree of
saturation and resistance to freezing and
thawing (ref. 14)

Research has established a relationship
between the magnitude of residual
expansion and the total expansion
occurring during a freezing cycle.
Figure 4.7 illustrates this relationship
for bricks and indicates a high degree of
correlation for both durable and
non-durable bricks.

4 - 6

2 —

—

LEVEL OF ^

100%/

SATURATION ^

\ /se

■

W/C = 0. 60

1 1 i

PLAIN MIXES
111 1

100%

W/C : 0.60

NOMINAL 8% AIR
ENTRAINED MIXES

A I \

20 15 10 5 0 -5 -10 -15 -20 20 15

TEMPERATURE , °C

10 5 0-5 -10 -15 -20

TEMPERATURE ,°C

Figure 4.6 Relationship between temperature and dilation for a plain and air
entrained concrete at various levels of saturation ( ref. 19 j

O HOH-FBOST RESISTANT

- ^fiS^ *^

FREEZING EXPANSION, %

Figure 4.7 Relationship between
irreversible expansion and freezing
expansion of frost resistant and non-
frost resistant bricks (ref. 18)

Investigations of this nature have also

Investigations of this nature have also
been done for concrete. Maclnnis and
Whiting (Ref. 20) established that a
similar pattern of dilation occurs during
freezing of saturated concrete. A total
of 112 samples cut from concrete paving
slabs together with laboratory prepared
specimens were tested. The entrained
air contents of these samples ranged
between 0 and 6.5%. The saturation
procedure was one hour of applied
vacuum (75 cm mercury), in a dry
condition; one hour of vacuum in a
submerged condition, and three days of
saturation under atmospheric pressure.
The samples were subsequently
conditioned into 5 sets representing
varying degrees of saturation, i.e., 100%,
88%, 72%, 47% and 30%, and subjected to
5 freeze- thaw cycles. Length change
measurements were made during these
cycles.

4 - 7

Figure 4.8 shows a typical length change
pattern for a fully saturated specimen
and indicates a similar behaviour pattern
to that for saturated brick samples;
dilation commences at about -4°C and on
completion of the cycle a residual
expansion remains. Maclnnis reports
that no dilation occurred on the other
sets of specimens with a reduced degree
of saturation. For the saturated
specimens, the average dilation per cycle
was 2.2 X 10"* for the air-entrained
concrete and 1.8 x 10"^ for plain
concrete.

Some investigators have carried out
research on the physical changes that
occur at a microscopic level during the
freezing and thawing cycle. Using
hardened pastes of plaster-of-paris,
Marusin (Ref. 23) demonstrated that a
single freeze-thaw cycle was
accompanied by expansion (dilation), an
increase of water absorption, a decrease
in strength and an increase in the
average pore size as shown on
Figure 4.9

This research by Maclnnis corroborates
other researchers who report that there
is a critical degree of saturation
required for dilation to occur.
Additionally, it suggests that under the
saturation procedure used the entrained
air voids may be filled, rendering them
ineffective against frost action. It is
argued that this is also likely to occur
under hydrostatic pressure; this would be
consistent with the deterioration of
concrete water tanks in their lower
section regardless of the presence or
magnitude of entrained air.

^1-

RESIDUAL EXPANSION— X 10 (IN/IN)

Figure 4.8 Maximum dilations v's residual
expansions after one freeze - thaw cycle
for "fully" saturated specimens (ref. 22)

11
II

h
II

// _

Figure 4.9 Pore size enlargement after a
single freeze - thaw cycle

A schematic diagram of the sequence of
events is given in Figure 4.10 and
demonstrates that the residual expansion
results in a reduction in the degree of
saturation. If no additional water is
introduced into the concrete it seems
reasonable to assume that there will be
little additional dilation with further
freeze-thaw cycles since the concrete
now has a reduced degree of saturation.
Small additional expansion may occur
until the concrete is below the critical
saturation level. However if more water
is introduced into the concrete, its
original degree of saturation may be
achieved and therefore another
freeze-thaw cycle will produce more
dilation. From Figure 4.10 it can be
seen that the volume of water required
to restore the original level of
saturation is equal to the volume of
residual expansion.

4 - 8

Figure 4.10 Schematic diagram of a
freeze - thaw cycle

With respect to damage caused by
freezing and thawing, this concept
distinguishes between effective and
non-effective cycles. Some freeze-thaw
cycles will not be effective in
contributing to damage if concrete
previously dilated is not re-saturated
prior to the onset of the next freezing
cycle. Although research has
demonstrated that freezing and thawing
expansion can occur in one hour (Figure
4.4) the governing factor in the
continued effectiveness of freeze-thaw
cycles to cause damage is the rate of
water supply during the thaw period.

4.4.1 Summary

From the data on the physical changes
occurring during a single freeze-thaw
cycle of saturated concrete, it can be
concluded that

• Freezing expansion occurs at
approximately -4°C.

• Freezing expansion ceases at
approximately -15°C.

• Additional expansion occurs during
the thawing cycle.

• The entire cycle can occur within a
period of 1 hour.

• A residual expansion remains after
completion of one cycle and is
directly related to the total
expansion.

• There is a critical degree of
saturation at approximately 90% above
which significant dilation occurs.

• A freezing and thawing cycle
permanently enlarges the size of the
individual pores.

4.5 Standard Freeze-Thaw Tests

To illustrate the failure mechanism
further, it is worthwhile considering the
two standard ASTM procedures for
testing the durability of concrete under
freezing and thawing cycles, and their
results.

In the two Standard ASTM procedures
(designations C671 and C666) saturated
concrete is subjected to freeze-thaw
cycles.

The major difference between the two
tests is the frequency of freezing and
thawing. In the ASTM procedure, C671,
concrete is kept in water for two weeks
between cycles, while in procedure C666
concrete is not allowed to "rest" and
freeze- thaw cycles follow one another
within a 2 to 4 hour period. Although
damage is assessed differently in these
procedures, in the slow cycle test
method where specimens are allowed to
"rest" in water, the concrete is generally
considered damaged after far fewer
cycles than those in the rapid method;
typically the comparative ranges arc 1 to
10 cycles and 40 to 300 cycles,
respectively (Figure 4.11a and b).

The point is that the "rest" in water is
detrimental because additional water can
enter the pores at this stage. This is
precisely the condition of the saturated
concrete walls of a water tank subjected
to freezing and thawing.

4 - 9

BER OF CYCLES

a) Relative dynamic modulus (ASTM
C666)

NUMBERS OF CYCLES

b) Dilation - cycle (ASTM C67 1 )

Figure 4.11 Typical ASTM frceze-thaw
results

The tests described reflect two possible
types of concrete exposure to frost and
water:

• conditions in which the time between
freeze-thaw cycles is insufficient to
re-saturate concrete (ASTM designation
C666 - "Resistance to Rapid Freezing
and Thawing")

• conditions in which re-saturation of
concrete occurs between freeze-thaw
cycles (ASTM designation C671 -
"Critical Dilation of Concrete Specimens
Subjected to Freezing")

In both cases, concrete can be damaged,
but in the condition of re-saturation,
damage occurs in a smaller number of
cycles.

The reason for the drastic effect of
"rest" periods in water is that after
reducing its degree of saturation in a
single freezing cycle the concrete then
absorbs more water and becomes
re-saturated. The initial loss of
saturation occurs due to dilation which
is microscopically translated into an
increase in average pore size.

For the next cycle to be as damaging as
the previous one, this extra volume of
pores must be filled with water. Two
weeks "rest" between cycles appears to
be sufficient to re-saturate normal
concrete under atmospheric pressure but
the time is dependent on the coefficient
of permeability.

This evidence of the difference between
standard test methods supports the
concept that re-saturation of the
concrete during thawing is the critical
factor which affects the rate and extent
of damage due to frost action,
permeabilities being constant.

It can be understood that thawing during
winter conditions will occur more
frequently in the walls of water tanks
between the south-east and south-west
quadrants where the direct rays of the
winter sun are sufficiently warm to thaw
the concrete after freezing at night.
This concept suggests that if
re-saturation is possible during a thaw,
then rapid and severe damage could
occur. It is therefore important to
examine the time taken to re-saturate
dilated concrete so that the rate of
deterioration of concrete tanks can be
estimated.

4.6 Rate of Re-Saturating Dilated
Concrete

The re-saturation period is the time
daring which the volume of water
re-fills that portion of the concrete
pores equal to the volume increase due
to the residual expansion.

To provide a simple model, the rate of
water influx into concrete pores under a
given head was assessed using D'Arcy
flow assumptions. Figure 4.12 illustrates
re-saturation rates for typical
permeabilities of good to excellent
quality concrete (2 to 8 x 10"^'^ m/s)

built with good quality concrete will not
re-saturate daily and therefore will be
less affected by freeze-thaw cycles.

Damage to concrete during freezing and
thawing results in general deterioration
of its mechanical properties i.e.
reduction of strength and stiffness.
This degradation is conventionally
viewed as ultimately leading to spalling,
frequently observed on the outer layers
of the affected region. The phenomenon
of surface spalling, easily detectable
visually, has masked other mechanical
and more serious consequences of
freezing and thawing in concrete water
tanks in Ontario which cannot be so
readily observed.

The next section examines development
of internal stresses related to
differential movements created by
dilation of concrete.

4.7

Figure 4.12 Effect of concrete quality
and water pressure on re-saturation tirrte

It can be seen that at a pressure head
of 33 m (100 ft.) excellent quality
concrete takes approximately 4 days to
re-saturate but good quality concrete
can re-saturate within 1 dav.

Stresses in Concrete Due to Frost
Induced Expansion

After each freezing cycle a saturated
specimen dilates resulting in a residual
expansion. Continuation of this process
results in breakdown of the matrix and
general disintegration of the concrete as
the pore walls expand and ultimately
fail.

This illustrates the reason for the
divergence in the rate of deterioration
with respect to concrete quality.
However, it is clear that concrete with
permeabilities below the illustrated range
will deteriorate at the same rate for the
same pressure head since they can all be
re-saturated within the period prior to
the next freezing cycle. Conversely
concrete with lower permeability will
deteriorate at a slower rate. It should
be noted that the permeabilities used for
this illustration are representative of
good to excellent quality concrete.

Another point illustrated in the graph is
that tanks with a low pressure head and

In practice however, a concrete member
can be fully saturated and only frozen
to a limited depth. In this
circumstance, part of the concrete
member is dilating while the remainder
is unaffected by frost. Under this
condition, the dilating concrete is
restrained and will produce reaction
forces, the magnitude being dependent
on the geometry of its constraints and
on the magnitude of unconstrained
dilation.

4-11

DILATING ZONE

(t-T)

Û - GAP WHICH WOULD BECREATED WITHOUT
RESTRAINING ACTION OF INTERNAL SHELL

P - TENSILE STRESS REQUIRED TO CLOSE GAP

Figure 4.13 Theoretical model of lank stress development

In mechanical terms, the resulting
stresses are of the same nature as
thermal stresses induced by temperature
increase. An analogous situation occurs
when a thick epoxy resin topping is
applied over a concrete base. On
heating and cooling, the epoxy resin,
having a different thermal coefficient
compared to concrete expands and
contracts more, resulting in reaction
forces which break the bond between
the two layers. Since the magnitude of
concrete dilation is roughly equivalent to
a thermal expansion in the range of
10°C - 30°C, stresses induced by
freezing and thawing cycles can be
significant.

Due to the permanent and irreversible
nature of the non- elastic volumetric
changes caused by frost action, stresses

created in constrained concrete will not
be relieved immediately after the cycle
(as is the case in a temperature cycle).
Some slow relief due to creep may be
expected, but if freeze-thaw cycles
follow closely one after another and the
time between cycles is sufficient to
permit re-saturation of the concrete,
stress accumulation will occur eventually
reaching the rupture limit of concrete.
Thus, frost related stresses can be much
more dangerous than temperature related
stresses.

4.7.1 Tensile Stresses in Tank Walls

Stresses in dilating concrete can be
induced not only by the presence of
localized external constraints but also
due to the restraining action of
different parts of a structure.

p
e E

0020

T
R -

0 08

/ i

0 06 \

0. 0 1 5

/ y^

~^^\ \

1 / 1_

-0 04 \ \

0.010

/ / X^"^

~~^^\\\

I // i

- 0 02 \ \ \

0.00 5

^^"^

d (X)

a) Maximum radial stress v's
position of dilating zone

b) Distribution of radial stress
along the wall

Figure 4.14 Distribution of stress in tank wall

Consider a situation when tiie outside
part of a concrete tank wall dilates due
to freezing, while the internal part of
the wall is not affected by frost (Figure
4.13). The dilating outer shell will tend
to increase in diameter and pull away
from the non-dilating part of the wall.
This tendency to differential movement
will result in tensile stress occurring in
the entire wall. The tensile stresses
developed can be calculated as follows:

radius to wall thickness and depth of
freezing zone are given in Figure 4.14a
and indicate the effect of tank radius on
the development of the induced stresses.
Figure 4.14b illustrates that the
maximum stress occurs at the interface
of the dilating and non-dilating zones
when this interface is at the centre of
the wall. In this case, since t = 0.5T,
tensile stress (P) can be calculated as
follows:

eE t (t

R T

where e

strain due to
dilatancy

Young's modulus
of concrete

wall thickness

tank radius

thickness of
dilating concrete

<^(x)

radial stress

c7(.x) maximum

Using this simplified formula, with
typical parameters for good quality
concrete, stress occurring during one
freeze- thaw cycle of a saturated
concrete tank with freezing to half the
wall thickness is as follows:-

Calculations for various ratios of tank
radius to wall thickness and depth of

4 - 13

Strain due

to dilatancy (e) 1 x lO"*

"*i'oung's, modulus

of concrete (E) 28 GPa

(4x10^ psi)

Tank radius (R) . .

Wall thickness (T) .

Average modulus of
rupture of concrete.

. 5m (15 ft.)
. 25 cm (10 in.)

245 kPa (35 psi)

Induced tensile stress

due to single cycle ... 35 kPa (5 psi)

The apparent modest stress induced in
one cycle of stress application can
nevertheless accumulate successively
under repeated cycles of freezing and
thawing until the modulus of rupture is
reached. The mechanism attains a
maximum rate under the following
conditions:-

• re-saturation occurs in the period
prior to the next freezing cycle; and

and February of each year. With this
information, together with published data
and the previously given formula for
tensile stress development, a theoretical
model of stress accumulation was
constructed.

4.7.2 Model of Tensile Stress
Accumulation

To assess the rate of tensile stress
build-up under cycles of freezing and
thawing, a mathematical model
incorporating the following
environmental assumptions was
developed.

i) Cycles of freezing and thawing follow
daily for two months every year, i.e. 60
daily cycles of freezing and thawing per
annum.

ii) In each cycle, frost penetrates to the
middle of the tank wall.

iii) Concrete is exposed to water under
pressure continuously applied to the
inside face of the tank wall.

• insufficient time is available between
freezing cycles to permit stress
relaxations due to creep.

Compare the laboratory induced
freeze-thaw cycle (Figure 4.4) with the
records from the tank monitoring study.
They are quite similar. It can be seen
that typically, on a daily basis, the
temperature of the south side of the
tank is reduced below -4°C, the
temperature at which dilation initiates,
then continues to reduce towards -15°C
the temperature at which maximum
freezing dilation occurs. It peaks above
0°C where thawing and residual
expansion is accomplished.

Based on the temperature data from the
ROCKWOOD experiment, it seems
reasonable to assume that cycles of
alternate freezing and thawing would
occur on a daily basis during January

iv) Dilation of concrete takes place only
when concrete becomes re-saturated in
the period between cycles, i.e. cycles
occurring in the period when
re-saturation is in progress are
"inactive".

v) Tensile stress relaxation occurs
continuously. Data on stress relaxation
in concrete are taken from Ref. 21.

Computations were performed using
direct computer simulation of time
events. The model assumed that the
tank wall was initially saturated and
that dilation would occur in the first
cycle. Using D'Arcy's law, the time
taken to re-saturate the additional
volume of pores created by previous
expansion was calculated. If
re-saturation was completed in the
period between two consecutive cycles
then further dilation was considered to

4 - 14

occur causing extra tensile stress in the
tank wall.

If the time between cycles was
insufficient to re-saturate the concrete
then effective freezing cycles were
omitted until such time as the water in
the concrete could be replenished.

Figure 4.15 Permeability and tensile
stress for 30m (100 ft.) head

PRESSURE HEAD 15m

1501. UNO TANK RADIUS 9m (30fl

isoo

PEBMEABILirr

/v. /\ ...0- m/,.<

?5!!5:r;

!/ '

Figure 4.16 Permeability and tensile
stress for 15m (50 ft.) head

The resultant stress was accumulated for
a period of 60 cycles. At that point it
was assumed that no further freezing
and thawing cycles would occur and the
accumulated stresses would be gradually

reduced due to creep for the remaining
part of that year. This process was
continued until the concrete had built up
sufficient tensile stress known to result
in fracture of the concrete.

The results of the simulation are given
in Figures 4.15 and 4.16 for various
concrete permeabilities at pressure heads
of 30 m (100 ft.) and 15 m (50 ft.),
respectively, and illustrate their
influence over a five year period. In all
cases the following parameters were
used:

strain due to dilation

1 X 10-*

wall thickness 25 cm

(10 in)

tank diameter.

. 9 m
(30 ft.)

The overall trend of the graphs
demonstrate the steady accumulation of
stresses through the winter period and
their partial relaxation during the rest
of the year due to creep resulting in a
"ratchet" effect. Also illustrated is the
dramatic influence of the internal water
pressure on stress accumulation, since at
high pressure heads, practically every
frceze-thaw cycle produces dilation and
associated stress. In low internal
pressure tanks, the stress accumulation
is not nearly so acute because the
re-saturation rate is less than the rate
of freezing and thawing.

These effects relate to the geometry of
the tank, however, the graphs also
illustrate that the permeability of the
concrete is one of the most important
governing factors. It should be noted
that as previously described the
permeabilities were deliberately selected
as representing good to excellent quality
concrete.

It can be seen that radial tensile stress
in high pressure standpipes can
accumulate dangerously over a five year

4 - 15

period to a level of tensile stress which
would fracture concrete.

It should be noted that due to the
internal water pressure, this radial
tensile stress is superimposed on a
compressive stress of about 140 kPa
(20psi) which exists in the middle of a
tank wall. The accumulation of tensile

PRESSURE HEAD 100

PET»<EABIUrf

Figure 4.17 Five year accumulated tensile
stress related to tank diameter for 30m
(100 ft.) head.

PRESSURE

HE,0

50 ft.

>ooo--^

:::

-^Ti;;^

— ____

PERMEABILITY

—

— -

—

•'•'O""-'-'

Figure 4.18 Five year accumulated tensile
stress related to tank diameter for 15m
(50 ft.) head.

stress with time will surpass this
compressive stress and the net tensile
stress can reach a level which is
sufficient to cause the fracture of
concrete.

Perhaps one surprising feature of the
model is the influence of the tank radius
on the accumulation of stress. For
example, under a 30m (100 ft.) head of
water an accumulated stress of 2.1 MPa
(300 psi) would occur in five years in a
10 m (30 ft.) diameter tank whereas only
1.1 MPa (160 psi) tensile stress would
accumulate under the same head in a
tank of 20 m (60 ft.) diameter. The
relationship between tank diameter and
tensile stress for various permeabilities
is illustrated in Figures 4.17 and 4.18.

This particular influence, perhaps masked
by other factors, has not been noted in
practice, however, the model
corroborates many of the observations
previously mentioned. For example the
high pressure head of a standpipe results
in rapid deterioration. Also,
construction defects which allow the
water access to the external surface
effectively amounts to a reduction in the
wall thickness and hence reduced flow
path resulting in faster re-saturation.
Under these circumstances a localized
section of a low pressure tank could
behave like a high pressure tank since
the reduced flow path allows the
remaining concrete to re-saturate more
rapidly. Another feature of the model is
that all tanks will suffer some stress
accumulation regardless of the quality of
the concrete because the permeability of
good quality concrete is insufficient by
itself to prevent re-saturation,
particularly under high pressure heads.
This feature also highlights the
importance of the internal coating,
which, if sufficiently impermeable, will
reduce the re-saturation rates.

One major difference which has been
observed in the field is that reinforced
concrete standpipes deteriorate by
dclamination more rapidly than
prestressed concrete standpipes. To
understand the possible reasons for this
it is necessary to examine the
distribution of the stresses as they
accumulate in the wall of a tank.

4 - 16

4.7.3 Distribution of Stresses

Figure 4.19 illustrates typical forces
acting on a reinforced concrete wall
when there is stress accumulation due to
dilation of saturated concrete. In the
example given, half the wall thickness is
subjected to freezing and thawing and is
restrained by the remaining half of the
wall section.

REINFORCED CONCRETE

Figure 4.19 Distribution of stresses in
tank wall section under increasing linear
water pressure.

As can be seen there is a compressive
stress acting on the wall due to the
hydrostatic pressure on the internal face
of the wall. This pressure is maximum
at the internal concrete surface and is
zero at the external surface. Due to
the restraint provided by the internal
half of the wall section, reaction forces
are developed between the dilating and
non-dilating zones. The distribution of
stress throughout the wall due to this
reaction can be calculated as described
previously.

These effects represent a general picture
of stress accumulation and do not
consider the presence of reinforcing
steel in the area of tensile stress
accumulation. The presence of
reinforcing steel results in local stress
re-distribution which may eventually lead
to tensile stress concentrations if the
bond between the reinforcing steel and
the concrete is destroyed.

The example on Figure 4.20 is designed
to illustrate the possibility of reinforcing
steel debonding when saturated concrete
is dilating due to cyclical freezing and
thawing. Tensile stresses in the
concrete-steel interface are created by
the tendency of the dilating material to
move away from the reinforcement.
Concrete-steel bond will restrain the
movement of dilating concrete and
therefore tensile stresses will develop.

Calculations show that the interface
tensile stress induced in one freezing
cycle is 1.2 MPa (170 psi). This stress
is based on a dilatant strain of 1.0 x
10"^. .\ strain twice this value will
result in debonding of the reinforcement
in a single freeze-thaw cycle, (see
appendix B)

This phenomenon has been observed in
cores taken in the walls of reinforced
concrete tanks as shown in Photo 4-1.
The debonding of the reinforcing steel
will effectively create cylindrical
openings in concrete and will induce
stress concentrations in the matrix
between the reinforcing steel.
Depending on the diameter of the steel
and the spacing between bars, the stress
concentration can be between 2 to 4
times the general level of stress.

Photo 4-1 Dilation of concrete and
subsequent debonding of reinforcing steel

I-Vc

ItVc l-t2ii (It Vclll-7Vc)

: TENSILE STRESS AT THE INTERFACE

= LINEAR STRAIN DUE TO DILATANCY

= young's modulus of CONCRETE

= POISSON'S RATIO OF CONCRETE

Figure 4.20 Illustration of model used to examine debonding of reinforcing steel

Consequently, although stress
accumulation may be greater near the
centre of the wall, fracture at the level
of the reinforcing steel is more likely.

POST TENSIONEO CONCRETE
TANK'WALL

Figure 4.21 Distribution of stresses in
post-tensioned tank wall section

Figure 4.21 is a diagram of a similar
situation in a post-tensioned tank. In
this case, significant radial compressive
stress caused by post-tensioning offsets
the magnitude of tensile stresses
accumulating due to freezing and
thawing. The post-tensioned tank
therefore would require considerably
more freeze-thaw cycles to accumulate

sufficient net tension to fracture
concrete at the centre.

The above theoretical models appear to
support the field observations that
reinforced concrete standpipes are more
susceptible to delamination due to
freezing and thawing than are
post-tensioned tanks where the radial
compression of the post-tensioning wires
or tendons reduce or eliminate the
tendency to cause tensile fracture.
Post-tensioning may also be beneficial
since it produces a lateral stress which,
as previously mentioned, reduces the
permeability of the concrete (Ref. 16).

4.7.4 Summary

In the above analyses, concrete tank
walls were assumed to be uniformly
saturated and subjected to uniform
freezing temperatures. This rather
simplistic set of conditions appears to be
adequate to explain general trends in the
deterioration of concrete tanks as
follows:-

• Standpipes deteriorate more rapidly
than elevated or ground tanks since the
internal pressure in standpipes is
sufficient to re-saturate concrete on a
daily basis.

4 - li

-RESTRAINT

DILATING
SATURATED
ZONE

INTERNAL
SURFACE -

> u

-V

INTERNAL
SURFACE-

FRACTURE INITIATION
( DILATANT STRAIN ^ 3 x lO"'
ie 3 CYCLES

UNSTABLE ACCUMULATED
FRACTURE GROWTH

DILATANT STRAIN l^i: lOx lO""* )

Figure 4.22 Formation of Jack-rod spalls

• Post-tcnsioned tanks do not
externally delaminate to the same extent
as reinforced concrete standpipcs due to
the presence of radial compression.

• Under high internal pressure
conditions, normal high quality concrete
may not be sufficiently impermeable to
eliminate effective freeze-thaw cycles by
substantially reducing the rate of
re-saturation.

• Defects and cracking in tanks can
reduce the effective wall thickness and
allow a rate of re-saturation which may
induce delamination even in low pressure
tanks.

4.8 Spot Saturation

In the previous analyses, concrete tank
walls were assumed to be uniformly
saturated and subjected to uniform
freezing temperatures. This rather
idealized set of conditions is adequate to
analyse general trends in tensile stress
build-up under freezing cycles, but is
unable to predict random patterns of
concrete deterioration accompanied by
spalling.

In actual service conditions, concrete
tank walls may not be saturated in all
areas, due to, for example, the partial
presence of protective coating, local
variations in quality, permeability, and
porosity or non uniform exposure to

external moisture. In practice,
therefore, spot saturation is the rule
rather than the exception.

The presence of a saturated zone
dilating under freezing conditions can
result in a considerable internal stress
due to the restraining action of the
non-saturated and, therefore,
non-dilating area.

Depending on the geometry of the
dilating zone and its proximity to the
concrete surface, dilation can be
accompanied by tensile stresses resulting
in spalls. Due to an almost unlimited
variety of geometrical possibilities and
the difficulties in estimating the
resulting stresses, the precise
mechanisms of spalling cannot be readily
identified.

One case of increased restraint that
merits detailed explanation, is the case
where a regular pattern of internal
spalling invariably occurs at the coupled
joints between jack-rod sections.

4.8.1 Jack-Rod Spalls

As previously described, jack-rod spalls
occur where the hollow rods used for
slip forming have not been filled with
grout. Inspection showed that typically
20 to 30 spalls occurred in each of the
tanks constructed using this method and
they invariably occurred at the jack-rod
junction (see Figure 4.22).
An attempt has been made to explain
spalls by ice formation within the hollow
jack-rods. Although this mechanism
could account for some observed vertical
splitting along the length of the tube, it
could not explain the regular occurrence
of spalls at the couplings.

The observed pattern of spalls is
attributed to saturation of concrete in
the area of the jack-rod couplings. It
is reasonable to assume that water would
leak from the couplings and would
create a zone of saturated concrete

which would dilate on freezing. The
concrete outside the zone would provide
a restraint to dilation and tensile
stresses outside of the dilating zone
would be developed, (see Figure 4.22).

a) Thermal profile produced by frozen
internal wall section

{h)Tliawed zone trapped between two
frozen zones

Figure 4.23 Schematic development of
thawed zone in wall

Calculations show that a dilating zone of
about 25 mm radius (1 in.) will initiate
local fracture when the dilating strain is

4 - 20

^ FROZEN :ONÇ

1 \ TRANSITION ZONE

Figure 4.24 Nature of wall damage at location of unfrozen zone

about equal to the limiting tensile strain

of concrete (3 x 10 ^). Accumulated

dilatancy of this order of magnitude can

be reached in several freeze-thaw cycles

(typically 3 cycles where the dilatancy

strain is 1 x 10 ^). Subsequent dilation

cycles will result in fracture propagation

to the concrete surface. Calculations

show that this would occur when an

-4
accumulated dilatancy of about 10 x 10

has been achieved in the area near the

coupling.

4.9 Hydraulic Pressure "Sandwich"

Another mechanism can be envisaged
which could result in the generation of
considerable hydraulic pressure and,
therefore, produce the types of
deterioration witnessed in many of the
concrete water tanks. This mechanism
requires the formation of an ice ring

4 - 21

inside the tank which allows the
concrete wall to freeze throughout its
thickness. As previously discussed, this
usually occurs during mid-winter to
spring in the majority of tanks. Since
this mechanism may well be additive to
the previously discussed dilation
mechanism, it may help explain why an
increase in the extent of deterioration is
often more noticeable during spring than
during the autumn.

As stated in Section 3.4, when saturated
concrete is frozen, one of the main
effects is to reduce the effective
permeability. Subsequent freezing and
thawing cycles occurring at the exterior
of the tank wall can push a zone of
unfrozen water ahead of the freezing
front which may be accommodated until
it reaches the frozen layer. At that
point, due to the reduced permeability, a
hydraulic pressure may be generated
which could be sufficient to rupture the
concrete matrix. This mechanism has
been duplicated in the laboratory by
Adkins.'(ref. 25)

Figures 4.23a and b, are sketches
representing thermal conditions which
can result in an unfrozen centre zone.
The sketch in Figure 4.23a indicates that
the ambient temperature has been below
zero for a considerable period resulting
in the formation of interior ice in the
tank and frozen concrete throughout the
thickness of the wall. This is followed
by a warm period which partially thaws
the outer part of the wall. The cycle is
completed by the ambient temperature
returning to below freezing (in this case
-10°C) resulting in a thawed centre
region, an inward moving freezing front,
and an impermeable interior wall section.

The increasing hydraulic pressure of the
pore water trapped in the decreasing
space between two frozen layers, can
cause horizontal tensile forces in a tank
wall. The location of the unfrozen
water layer will determine the plane of
failure and dclamination splitting which

will be manifested as external or
internal scaling or dclamination.

Based on this mechanism, the type of
visible damage occurring depends on the
extent of the thawing period compared
to the freezing period, since that
determines the location of the unfrozen
central section. Figure 4.24 illustrates
the nature of the damage which could
occur depending on the position and
geometry of the unfrozen section at the
onset of the freezing cycle.

4.10 Conclusions and Recommendations

Based on the review of historical and
current practices, field observations, and
the concepts and mathematical models
described, the following conclusions were
presented to the task force and the
Ministry.

• The detailed surveys confirmed the
initial work of W.M. Slater & Associates
Inc. that the deterioration of the
Province's concrete water tanks was
widespread.

• Tank inspections showed that where
internal coatings existed they could not
provide the required reduction in
permeability to prevent saturation and
consequent deterioration due to freezing
and thawing cycles.

• Due to their high internal hydraulic
pressure, standpipe structures, and in
particular reinforced concrete standpipes,
are prone to deterioration by
dclamination.

• Small defects which are perhaps
harmless in other types of structures,
allow ingress of water into tank walls
and can initiate dclamination even in
elevated and ground tanks subjected to
freezing conditions.

4 - 22

From the abo\c conclusions it was
recommended that:

1. Methods of providing insulation to
inhibit the effect of frceze-thaw cycles
should be studied.

2. Waterproofing barrier systems for
potable water storage tanks should be
studied.

3. Condition surveys and quality control
procedures should be initiated as they
are of the utmost importance during
tank rehabilitation. They provide
detailed information on the repairs
required and ensure that necessary
quality of materials and workmanship is
carried out.

These recommendations were accepted
and acted upon by the Ministry. Item 1
and 2 form the basis of other reports
referenced on page (i). Condition
surveys, repair methods and quality
control are discussed in the next
sections.

5.0 REPAIR AND REHABILITATION OF CONCRETE WATER TANKS IN ONTARIO

5.1 General

Repair is defined as the restoration of
defects in a structure to the intended
original state. Rehabilitation is defined
as repair and upgrading to a new desired
state, such as designing for increased
seismic forces, and insulation and
waterproofing requirements. The tank
repair and rehabilitation programme was
more complex and difficult than the
investigation and diagnostic stage,
because of the following factors:-

• Exposure to height (safety)

• Harsh environment

failure. The main causes of failure were
an appreciation of the limitations of the
materials used, the need for careful
surface preparation and the provision of
proper environmental conditions.
Leakproofing cold joints and deteriorated
concrete at cracks formed during slip
forming is difficult and costly; using
bonded coatings alone is not always
adequate without removal and
replacement of the deteriorated concrete
before surfacing and coating. Complete
reconstruction of the bottom of walls
has been necessary at locations where

water has flowed past or over
water-stops and where it has frozen on

the inside.

• Limited construction season

• Limitation of suitable contractors,
specialists, materials

• Need for temporary storage and
pumping to ensure continued
operation of a municipality's water
supply

• Limitations of condition survey
techniques

• Project management of short term
contracts

• Short lead time for the development
of methods of repair

• Need for development of efficient and
safe work stages

Many of the tanks inspected in the
period 1981 to 1982 had already been
unsuccessfully repaired, some of them
several times before the rehabilitation
programme started. Reasons for the
ineffective repairs arose from the lack
of understanding of the real causes of

5.2 Design of Repairs

5.2.1 General

Remedial measures can be put into four
categories as follows:-

• Upgrading of structural integrity
where necessary;

• Repair of deteriorated concrete;

• Containment of water (leakage);

• Prevention of saturation and freezing.

It should be noted that the last item
regarding waterproof coatings and ice
protection is not fully discussed in this

report since it forms the basis of other
reports.

5.2.2 Structural Evaluation

.1 Loading

Loads may be placed in two categories,
namely applied loads and environmental

loads as follows:

Applied Loads

Environmental
Loads

• water pressure (a)

• post-tensioning (a)

• wind (b)

• seismic (b)

• thermal (b)

• shrinkage (b)

• creep (b)

• ice (c)

The loads may be described as a)
well-defined b) defined, or c) ill-defined.
All these factors, if not handled with
care, result in stress accumulations
causing cracking of concrete,
deterioration of waterproofing and
severe leakage rendering the tank
unserviceable in a relatively short time.

Water pressure is directly related to the
head of water in the tank, while the
specified prestress is applied at the
construction stage. Thus both these
loadings are well- defined. Wind and
seismic loads at specified locations in
the Province of Ontario are defined in
relevant codes (Ref. 26) and thus the
levels of such loads for design purposes
are defined.

Present design approaches seem to treat
environmental loading on water tanks in
a somewhat cursory manner. Some
aspects are either underestimated or are
not considered at all. A substantial
amount of work carried out in New
Zealand (Ref. 27) on thermal effects
shows that these are generally
underestimated due to the neglect of
direct solar radiation. Thermal gradients
up to 30°C have been recorded.
Shrinkage effects are analogous to
thermal effects and can be treated using
an equivalent temperature gradient (ETG)
approach. The ETG is a function of the
wall thickness and the boundary
conditions of the walls. Creep is
generally beneficial in that it tends to
reduce displacement induced stress.
However, it also reduces the effective
prestress in a prestressed tank. While
such loadings are defined in codes, the

levels to be considered in design may be
underestimated.

Ice loading in tanks is not considered in
North American approaches but is
considered in some other countries (Refs.
28, and 29). Most past work on ice
pressures has been related to ice sheets
on lakes and reservoirs and little
information on its effects in water tanks
is available. Evidence of ice formations
in tanks in Ontario exists (Ref. 2).
These ice formations may be in the form
of a plate near the top of the tank, a
cylinder on the inside walls, a slab built
up at the domed floor of a tank or in
the cracks in the wall of the tank.

Photo 5-1 Internal wall ice at bottom of
tank after demolition in late spring

Loading from ice formations is most
critical when the ice heats up, since the
coefficient of thermal expansion of ice
is approximately five times that of
concrete. Work in Finland (Ref. 28)
shows, that the rate of heating is a

5 - 2

significant parameter, and that pressures
of up to 250 kPa (35 psi) can be
generated in a tank by a plate of ice
which warms from -20°C to 0°C in four
hours. Cracking of an ice plate due to
differential temperature and subsequent
freezing forming ice in the resultant
cracks can develop significant pressures
700 kPa (100 psi) as shown by work in
Sweden (Ref. 29).

Environmental loading on elevated water
tanks has been studied at Queen's
University with particular emphasis on
the effects of ice. This work has been
reported by T.I. Campbell and W.L. Kong
in "Ice Loading in Elevated Water Tanks"
dated April 1986.

.2 Analysis

Methods of analysis may be classified as
(a) simple, (b) adequate, and (c) refined.
In a simple analysis a tank may be
modelled as a thin-walled cylinder, while
in an adequate analysis use can be made
of charts (Refs. 12, 30, 31 and 32) for
the computation of the stress resultants.
A refined analysis can be made using
classical approaches (Ref. 33) or a finite
element model (Ref. 34). All methods
have their use but should be used with
caution.

Reinforced concrete tanks have usually
been designed by taking care of the
straight hoop forces with the horizontal
ring reinforcing. This arrangement, for
water load only, would be suitable if the
wall at the bottom of the tank were as
free to expand, as it is for most of the
tank's height.

However, in many cases the tank wall is
rigidly connected to a heavy slab or
foundation mat. This restraining effect
completely changes the stress pattern at
this location. The hoop forces are
reduced to near zero and their role is
replaced by significant vertical bending
moments and horizontal shear forces.
These stresses rapidly taper off further

up the wall and in most cases, for all
practical purposes, can be neglected
except in the vicinity of the wall slab
junction where their influence is
significant. The problem can further be
complicated by inadequate vertical
reinforcing.

In uninsulated water tanks in Ontario
with large height to diameter ratios and
cold surface water sources, ice formation
inside the tanks is a common occurrence.
The influence of ice pressure, especially
at rigidly restrained structural modes
must be investigated.

Additional problems can be expected due
to temperature differences through the
concrete wall thickness. These differ-
ences can be rather large, considering
the cold water inside in combination
with warm outside air together with
solar radiation. The significant bending
moments in both the vertical and
horizontal direction have to be added to
the other effects already discussed.

Fast filling of a new tank with cold
water during a hot summer may cause
thermal shock and cracking.

Post-tensioning is applied on the empty
tank, causing larger than water load
stresses in reverse. In places of
restrained freedom of movement these
stresses can crack the concrete
before the tank is filled up with water.
Design analysis will very seldom deal
with this potential problem.

In some elevated tanks the post-
tensioning has been applied in stages,
while the structural system of the tank
was changing during the course of the
construction. For example, the
cylindrical wall of the tank and the
tension ring is constructed first and
partially post-tensioned, followed by
building of the domed bottom and roof
and the completion of the remaining
post-tensioning.

5 - 3

It has been noted in cases investigated,
that considerable temporary bending
moments are created in locations without
sufficient reinforcing to take care of
them. The cracks later appearing at
such places might have developed in
these early stages.

A finite element model shows that a
standpipe behaves like a thin-walled
cylinder except in the region of the
wall-floor intersection where steep
moment gradients occur in the wall.
Thus, refined methods of analysis need
only be carried out in this region
provided relevant boundary conditions
are incorporated into such a model.
Generally, charts are not readily
applicable to standpipes since most
charts cover only tanks with floors at
grade level and having aspect ratios
outside those of a standpipe. However,
they may be used provided the designer
has a proper understanding of the
overall behaviour of this type of tank.
The classical approach (Ref. 33) is
applicable only to tanks with radial
symmetry. Thermal loading which is not
symmetrical due to effects of solar
radiation, wind etc, can only be analyzed
by a refined model. However, the cost
of a refined model can be high and such
models should be used with
discrimination.

5.3 Repair Methods Developed

Repair methods were developed, and
suitable materials evaluated, in the
following main areas:

Structural

• Vertical crack control

• Replacement of corroded
prestressing steel

• Concrete spalls

• Delaminated prcstressed wall
removal

Leakproofing

• Steel Liners

• Epoxy injection

• Caulking
Waterproofing

• Internal coatings (non-toxic,
odourless, tasteless)

• External corrosion protection for
prestressing wires

• Surfacing materials and
procedures

External stage post-tensioning hardware
and methods have been used for
replacement of corroded prestressing
wires in the wall and may be used for
vertical crack control.

Ice prevention systems such as
insulation, circulation mixing, and
heating are now being used in
conjunction with structural repairs in
order to correct cracking caused by the
expansion force from internal ice
formations, or where these formations
are excessive inside the tanks.

5.4 Tank Repair Methods

5.4.1 Condition Surveys

Prior to designing a repair or
rehabilitation programme it is essential

that a condition survey is undertaken to
determine the nature and extent of

deterioration. The main components of
the survey are visual inspection and
exploratory inspection. Record

photographs should be obtained at all
stages of the survey. The methods used

are mapping, light jack hammering,
coring and laboratory analysis.

5 - 4

The first aspect of visual inspection is
to determine the location of external
leaks, spalls and cracks. It has been
found that this is best accomplished
when the tank is filled. These defects
should be noted on a plan indicating the
cardinal points and if possible a point of
reference such as the manhole.

The next stages are accomplished after
emptying the tank and consist of
external and internal delamination
mapping and are accomplished using
hammer sounding.

It is important that potentially defective
areas are thoroughly inspected in detail
to examine the depth of degradation.
Some of the tank repairs completed
required further remedial works within
the maintenance period, which sometimes
could be attributed to lack of detailed
inspection at these particular locations.

Based on current experience, the
presence of the following possible
defects should be verified in addition to
general defects observed:

• All internal coatings should be
examined for depth of fracturing on the
inside concrete surface - obtain cores
and examine by microscopy.

• Reinforced concrete standpipes should
be examined for depth of external and
internal delamination - cores obtained
from outside and inside the tank.

• Gunited tanks should be examined for
presence of loosely compacted concrete
and delamination between layers (from
the shotcrete process). Take cores in
suspect areas.

• Post-tensioned, unbonded tanks should
be examined for the presence of water
leakage into the tendon ducts - look for
cracks, water leakage and possible
corrosion.

• All tanks should be examined for the
presence of vertical and horizontal
cracks indicating possible structural
weakness caused by ice formation or
thermal movement or insufficient
prestress.

• All construction joints should be
examined for water infiltration and
possible deterioration through the
section.

• Obtain external and internal cores to
the depth of the waterstop, particularly
where leakage is suspected. Verify in
areas not showing leakage.

• Determine if jack-rods have been
used in construction and verify if they
are completely filled with grout.

• Locate presence of possible voids in
tank wall created by improper
construction methods.

5.4.2 Surface Preparation

The first step in all coating
rehabilitation work is to remove all

unsound concrete using light chipping
hammers and sand or grit blasting.
Water jetting was successful in some
cases in removing the internal coating
and fractured internal concrete surface.

If reinforcing steel is present then it
can be cleaned using mechanical wire
brushing and sandblasting.

5.4.3 Delaminations and Spalls

Where the concrete is removed to a
depth greater than 75 mm, welded wire
fabric reinforcing is included to provide
mechanical bond. This is accomplished
by drilling eye inserts and installing ring
fasteners. The formwork is then placed
over the patched area and grout poured
in an entry port. Since only small
quantities of materials are required,
proprietary brands of pre-mixed grout
containing bonding agents are normally
used.

5 - 5

At locations where the depth of removal
is less than 75 mm, a latex bonder is
first applied and a stiff mortar is
trowelled in.

Curing of all of the above repairs is
achieved using wet burlap over the
patched area.

5.4.4 Crack Repair

Prior to the determination of the method
of crack repair, an assessment of the
overall pattern of cracking has to be
made. In some tanks vertical and
horizontal cracks were somewhat isolated
and cores indicated sound concrete
within the vicinity of the crack. In
such cases the procedure used was to
rout out the crack and fill it with a
sand filled epoxy mortar. In some
tanks, parts of the inside surface are
occasionally highly fissured. Dealing
with each crack separately was judged
to be uneconomical and therefore the
approach taken was to trowel a surface
layer of epoxy mortar over the fissured
area. This is usually done in
conjunction with the provision of an
overall surface layer as described in
detail in the report on coatings
evaluation.

Photo 5-2 Surfacing tank wall prior to
application of coating. (HESPELER)

One difficult area to design repairs for
is at joints. Typically, joints occur

between the floor and wall, occasionally
there are also vertical joints.
Examination of these joints has shown
that, in many cases, the water has
penetrated the waterstop and that
concrete deterioration is common at
these locations.

5.4.5 Waterproofing

As discussed in previous sections of the
report, it is important that the water is
contained within the structure and is
not allowed to be in close proximity to
the external surface, as may occur due
to improper design or defective
workmanship.

Photo 5-3 Trowelling latex modified
mortar. (HESPELER)

Both bonded coatings and steel liners
have been used for waterproofing.

5 - 6

Where coatings have been applied, it was
found difficult to provide an intact film
over the rough surface of the tank wail.
Consequently, the internal walls of tanks
undergoing this treatment were surfaced
with a layer of either modified latex or
epoxy mortar.

Photo 5-4 Completed surjocuig prior lo
coating application. (HESPELER)

5.5

Typical Repair Systein.s for \'arious
Concrete Tank Types

5.5.1 Concrete Tank Types

The different categories, designations
and construction methods used in
building concrete tanks in Ontario are
given in Figure 1.1 and Table 1/1.
These are summarized in Figure 5.1. The
various construction methods,

structural types and forms of
prestressing resulted in a variety of
defects and forms of deterioration
requiring individual repair systems to be
adopted for each tank type. These arc
described in the following pages with
respect to each of the tank types
repaired in the rehabilitation programme
to the end of 1986.

caccsc TAiT^S

Figure 5.1 The 10 Concrete tank types
in Ontario (see Table 1/1 page 1-3 for
description and abbreviations)

5.5.2 RC-S Type Tanks

Two (2) standpipes of this type, namely
WATFORD and ALVINSTON were
repaired, and five (5) have been or will
be replaced with new steel standpipes
since the repair cost of the badly
dclaminated tanks were equal or greater
than the cost of a new steel standpipc.

Leaking active vertical (hoop tension)
cracks at 300 mm(l ft.) approximate
spacing, apparently static horizontal
cracks and defective construction joints
at 600 mm (2 ft.) lifts, had contributed
to rapid saturation with resultant wall
delamination and massive external
spalling in these tanks. The remedial
methods were to remove and repair the
delaminated concrete, in one case.

Photo 5-5 Installation of post-tensioning
anchors. Note leaks at construction
joints. (ALVINSTON)

by casting on a new 200 mm (8 in.)
thick exterior reinforced concrete wall
bonded to the old, and closing or
controlling the vertical cracks from
opening by external post-tensioning.
Internal waterproofing was carried out
using a bonded epoxy coating.

One standpipe (ALVINSTON) was
supplied from a surface water source.
The temperature of the inlet water was
found to be marginally above freezing
during winter, resulting in considerable
ice formation in late winter and possible
damage due to ice thrust forces.
Consequently, as part of the remedial
works the tank was the first in Ontario
to be insulated and clad with
pre-finished steel. Additionally, as part
of the applied research programme the
performance of the ALVINSTON tank
was monitored with temperature
transducers hooked up to a data logger
and computer.

Photo 5-6 Installation of external post-
tensioning tendons. (ALVINSTON)

5 -

5.5.3 G-S Type Tanks

Eight (8) standpipcs of this type have
been repaired or replaced up to the end
of 1986, namely BADEN, CHESLEY,
FENELON FALLS, GRAVENHURST,
HESPELER, L'ORIGNAL, WINGHAM and
WOODVILLE.

Photo 5-7 Typical horizontal cracking
a G-S type tank. (L'ORIGNAL)

These types of tanks usually exhibited
external leakage with incipient and
actual wire corrosion at locations where
the cover coat had spalled. The
majority of these defects were attributed
to the presence of internal jack-rod
spalls. Additionally, wall damage and
horizontal micro- cracking caused by slip
forming and poor concrete placement
was present at some of the areas where
the cover coat had spalled.

Repairs were effected by removing
deteriorated internal coatings and

Photo 5-8 Ring support structure for
insulation and cladding. (BADEN)

locating and grouting up the jack-rods
in the wall. In later repairs these rods
were more precisely located using
radiographic methods. Significant extra
costs were incurred due to the variety
and extent of wall preparation required
after removal of old (mainly cementitious
based) coatings. Bonded epoxy coatings
applied in 2 or 3 coats to a total
thickness of 15 to 35 mils were used as
the waterproof coating on 6 standpipes.
In some cases a surfacing mortar was
applied before the epoxy waterproofing
to fill bugholes and provide a smooth
surface for the coatings.

Initial problems were encountered in
applying effective waterproof epoxy
coatings in the first rehabilitation
contracts. These problems were

condensation of moisture on the walls,
pinholes in the coating, and failure to
remove or seal and leakproof weak,
porous, and micro-cracked substrates.
Condensation was prevented by
introducing environmental controls and
dry heat. Pinholes were eliminated by
using near 100 per cent solids epoxies
(solvent free) and using either acrylic
modified cement or epoxy mortars as a
parging or surfacing material to fill all
surface holes, and provide a smooth
surface layer for the coating. A
positive air pressure was applied in some
tanks.

4 , *•; ^^

Photo 5-9 Standpipe prior to repair
(BADEN)

Concrete deterioration of the walls in
this type of tank is sometimes difficult
to identify due to the presence of the
"confinement" of the prcstressing
compressive force. This compressive
force limits typical external delamination
found in reinforced concrete standpipcs
but can result in a highly microfractured
concrete section. For example, in one
of the repaired tanks, a 5 m high band

Photo 5-10 Standpipe after insulation
(BADEN)

;;^-' ^OOOVl^

Photo 5-11 Standpipe after insulation
(WOODVILLE)

^SPELER

Photo 5-12 Stand pipe prior to repair
(HESPELER)

of microfractured concrete was not
identified as a particular problem during
surface preparation. The sub-strata,
after scabbling and sand blasting
appeared to be of suitable quality for
epoxy coating and this was completed.
Later, in service, small pin-hole leaks
formed in the coating and allowed water
into wall micro-fractures. Several
months later leachate stains were
discovered on the outside of the tank.
The source of the problem was identified
only after extensive laboratory analysis
of core samples, and remedial solutions
were developed.

It is probable that the initial cracks,
having a vertical spacing of between 50
and 250 mm were caused by problems
with the slip forming process and that
subsequent cyclic freezing produced the
fractures within the matrix.

Photo 5-13 Staudpipe after insulation
(HESPELER)

Photo 5-14 Applying coat of MMA to
exterior of stand pipe (BADEN)

5.5.4 PTU-S Type Tanks

GLENCOE standpipe, which is a twin to
one which failed at DUNNVILLE, has
been repaired by adding external post-
tensioning, grouting all the jack-rods
left in the wall after slipforming was
completed (located using radiography)
and internally coating with a bonded
epoxy. It was later insulated and clad.
Two (2) failed standpipes were replaced
with new steel standpipes.

Photo 5-15 External post tensioning
(GLENCOE)

5.5.5

PTB-S Type Tanks

One large standpipe of this type in
Northern Ontario, EAR FALLS, leaked at
the construction joints at the top of
each jump-formed lift. The
rehabilitation method adopted was to
install a steel liner, grout between the
liner and existing concrete wall to

provide a leakproof system, install
exterior insulation and cladding and
provide a mixing and heat boosting
system.

5.5.6 RC-E Type Tanks

One tank (BRECHIN) was repaired by
taking the roof off and installing a
fabricated internal steel liner. The gap
between the concrete wall and the liner
was grouted with portland cement grout
to provide corrosion protection to the
back face of the steel. The inside face
of the steel was coated with vinyl paint.
External post-tensioning was added to
control vertical cracking and insulation
and cladding, for freeze protection.
Another tank of this type was
demolished and replaced with a steel
standpipe (PITTSBURGH).

Photo 5-16 i'/ct/ liner being installed.
Note external post-tensioning (BRECHIN)

Bi-

Photo 5-17 Tank after rehabilitation
including strengthening by post-
tensioning. installation of new steel
liner, and insulation and cladding.
(BRECHIN)

5.5.7 G-E Type Tanks

The only two (2) elevated tanks of this
type built with gunite (shotcrctc) walls,
namely, AMHERSTBURG, and
CHELMSFORD, have required extensive
repairs.

ÏI

Photo 5-18 Internal maintenance
inspection of liner paint system after
one year in service (BRECHIN)

Photo 5-19 Vertical cracking at base of
wall due to ineffective prestressing.
This was caused by inadequate provision
for inward movement between internal
thrust ring and wall.(CHELMSFORD)

Emergency strengthening repairs were
required to keep them in service because
of concrete delamination, prestressing
wire corrosion and breakage, resulting in
concern for public safety. The leakage
leading to the wire corrosion in one
case was due to the presence of
shotcrete rebound and improper water-
stop installation resulting in serious wall
delamination 40 mm (1 1/2 inch) deep.

Both tanks required major wall repairs -
in one case an 2.5 m x 1.8 m (8 ft. x 6
ft.) section was cut out of the
prestressed wall. The walls of both
tanks were strengthened with external
post-tensioning and their horizontal
floor thrust blocks and rings were
structurally upgraded to resist earth-
quake forces.

Internal waterproofing was carried out
in both tanks using Tapecrete latex
modified cement slurry with fabric
reinforcement. Additionally, a
mechanically anchored partial liner of
PVC coated nylon fabric was installed in
one tank, together with back-up drains
to substitute for the defective waterstop.
Some wall leaks re-appeared in one of
the tanks.

5 - 13

Photo 5-20 External post-lensioning
added to compensate for lack of
prestressing in wall. (AMHERSTBURG)

At one of the tanks many vertical voids
which occurred between the original 6
mm (1/4 inch) thick cover coat, wires,
and reinforcing steel were filled.
Subsequently a 20 mm (3/4 inch) thick
latex gunite was applied over the wires
as corrosion protection. In the other
tank a white MMA exterior coating was
applied over the thin cover coat to give
the wires added corrosion protection.

New sliding aluminum hatches with a
folding access ladder, to allow lifting
above winter ice, were added in one
tank. In the other tank, a plastic sky
dome was installed replacing an area of
poor concrete and acted both as a repair
and to allow more light into the tank.

Both tanks were insulated and clad in
1986 and 1987, respectively.

Photo 5-21 Deteriorated internal thrust
ring. Note thrust block against wall
stopping movement. (CHELMSFORD)

Photo 5-22 Repair of thrust rin^
(CHELMSFORD)

Photo 5-23 Completed repair. Note
external tensioning at base of tank.
(CHELMSFORD)

5 - 14

Photo 5-24 Tank after leakproofing and
before insitlationf BRIGDEN )

5.5.8 PTU-E Type Tanks

One (1) elevated tank of this type, with
hoop and vertical unbonded tendons, has
been repaired. The BRIGDEN tank
exhibited leakage at the floor, the
wall/floor joint, and some wall dampness,
which was repaired in two stages. An
interior flexible joint sealant was
installed by cutting a groove in the
floor and bonding the sealant to the
wall. The entire circumferential band
surrounding the floor joint was given
two coats of epoxy to waterproof the
joint.

The first stage was completed prior to
the winter of 1983, and included
temporary repairs to the damp buttress
recesses holding the unbonded tendons
and to leaks in the tank roof.

Photo 5-25 Steel support system for
insulation and cladding. (BRIGDEN)

During the second stage, completed the
following spring, poor quality concrete
mortar was removed from all the post-
tensioning anchor recess pockets, which
revealed a number of slack anchorages
due to failure of five of the unbonded
prestressing tendons. Detailed
investigation demonstrated that the
strands had broken as a result of stress
corrosion. The corrosion was caused by
water entering the anchorages through
the porous mortar and into plastic tubes
sheathing the strands which were
inadequately protected by corrosion
inhibiting grease. The inhibitors in the
grease may have been rendered inactive
or the grease itself may have been
displaced by the infiltrating water. (See
Section 6 for more details). The failed
strands were removed, replaced with new
regreased strands, and the recess
pockets were filled with a dense
non-shrink reinforced mortar.

5 - 15

Photo 5-26 Completed rehabilitation of
tank. (BRIGDEN)

During the leakage test for the stage 2
repairs, dampness was again observed at
the wall/floor joint which had been
sealed during the previous stage 1
repair. This included removal and
replacement of areas of deteriorated
surface mortar.

Investigation revealed that although the
surface mortar was "sound", deteriorated
concrete existed underneath and close to
the waterstop situated at the centre of
the wall, and was hidden by the mortar.
All unsound material was removed,
except adjacent to the vertical post-
tensioning dead end anchorages, and
further repairs were carried out to
correct the original fault of a poorly
installed waterstop. The bottom of the
wall was re-constructed using epoxy pea
gravel and sand mortar.

It was considered that the inferior
material at the centre of the wall was
the end product of an improperly
installed waterstop which allowed the

mortar to saturate and whose structure
had subsequently been destroyed by
freezing and thawing.

This tank was insulated and clad as the
final stage of rehabilitation.

One other tank of this type was
demolished and replaced by a new steel
tank (VERNER).

Photo 5-27 Delaminated exterior at
source of leak after removal of fractured
concrete. (CASSELMAN)

5.5.9 PTB-E Type Tanks

Eight (8) tanks of this type were
constructed. The 1981 study indicated
that these tank types had one of the
highest performance ratings, exhibiting
the least amount of visible deterioration.
Consequently, their repair was scheduled
for the latter stages of the programme.
After the 1981 external inspection, three
(3) tanks developed serious deterioration
problems and were repaired earlier than
anticipated.

In 1984/85 winter the CASSELMAN tank
suddenly developed a small wall leak
adjacent to an ungrouted post-tensioning
duct. Detailed investigation revealed
that the wall was delaminated at that
region of the tank. The remedial
solution adopted for this tank was to
demolish the walls and roof of the tank,
and to construct a new steel tank on
the existing base.

...MP-.. mh ' \l
Photo 5-28 Dclaminaled wall. Inspection
revealed ungroiited post-tensioning duels
at this region. (CASSELMAN)

The PICKLE LAKE tank was noted to be
badly cracked and delaminated by
internal ice forces after the 1982/83
winter. The remedial solution designed
for this tank was to construct a steel
liner inside the existing concrete tank
and to insulate the space between the
steel and concrete walls.

Although RED LAKE exhibited little
external deterioration, the 1985 internal
inspection revealed considerable
delamination of the inside walls of the
tank. A remedial solution similar to the
PICKLE LAKE repair system was selected
for this tank.

The remaining five (5) tanks of this type
have exhibited only minor deterioration.
The rehabilitation solutions adopted for
these tanks have included the application
of cementitious or epoxy coating to the
inside concrete wall, and the installation
of external insulation and cladding.

5.5.10 G-G Type Tanks

Two (2) wire wound gunite ground tanks,
namely BARRY'S BAY and ORILLIA,
were repaired because of excessive
leakage - one through the floor, the
other through the walls. One of the

Aiiîy-i- ■* 'i?^ "^

m.,..

Photo 5-29 Reservoir before repair
(PRESTON)

Photo 5-30 Reservoir after repair
(PRESTON)

tanks which had extensive floor cracking
was sealed and Tapecrete was applied
over the floor area and around the
entire region of the wall/floor joint.
The other tank, a large municipal tank,
was waterproofed with the Tapecrete
fabric and slurry system.

The smaller of these tanks, BARRY'S
BAY, exhibits an ice formation in late
winter and will be insulated and clad.

5.5.11 RC-G Type Tanks

CAMPBELLFORD tank was internally
coated using a cementitious slurry and
fabric method and was insulated and
clad.

5 - 17

? s;»-'

Photo 5-31 Tank demolition - culling
hole at base. Note ice still inside empty
slandpipe. (CALLANDER)

Photo 5-32 Standpipe toppled.
(CALLANDER)

Photo 5-33 Reinforced concrete tank
wall after toppling. Note no corrosion of
steel and delamination of concrete from
steel. (CALLANDER)

5.6 Quality Assurance and Measurement

As stated at the beginning of this
report, field observations soon indicated
that the Ontario water tanks were
located in a very severe environment.
Small defects in the tanks, perhaps
insignificant in other types of
structures, have resulted in rapid
deterioration of the tank or at least a
significant part of the tank. It was,
therefore, considered that quality
assurance of the remedial works was of
the utmost importance to assure success
of the repairs. Resident inspection of
the remedial work was completed on a
100 per cent basis. The inspection was
supplemented by routine testing and
occasional specialist advice and testing.

During the repair programme, several
quality assurance issues arose. These
related to humidity, concrete wall
temperature, surface preparation, coating
thickness and bond strength
measurement.

In order to check that sand or
grit-blasted surfaces were prepared to
the desired roughness, test patches were
prepared and compared with a selected
grade of sandpaper with grit size (ALO
80). This is reported in the coatings
report.

Sites were issued with sling
psychromcters and rotary bi-metal
thermometers. It was found that the
rotary thermometers were inadequate to
measure air or wall temperatures with
sufficient accuracy. However, a rapid
response surface electronic thermometer
proved successful.

Coating thickness was initially measured
using wet thickness combs and gross
volumetric measurement. Where coatings
had solvents or were applied on rough
substrates, the wet thickness gauges
proved to be inadequate. Additionally,
on rough surfaces, although measurement
of the consumption of gross quantity of

5 - 11

materials accurately reflected the
average thickness, microscopic analysis
of core samples showed that in many
areas the coating was extremely thin.
Consequently, a non-metallic coating
thickness gauge was introduced. This
test essentially involves scratching the
coating down to the concrete interface
with a cutter of known grooving angle.
Measurement of the dimensions of the
groove, and hence coating thickness are
obtained using an inbuilt measuring
microscope. It was found that
measurements of the minimum coating
thickness (the specified measurement)
could be determined rapidly by the site
inspector.

Measurement of bond strength cither of
the cementitious based repair materials
or the coating itself was an important
consideration, and therefore, during the
course of the 1983 programme, test
methods were developed which could be
used to assist the repair programme. A
modification to the Lok-test apparatus
was constructed. The Lok-test is
essentially a hydraulic ram which exerts
a pull normal to the test surface. To
test the repair materials, a 50 mm
(2 in.) diameter diamond core drill was
used to make a cut extending beyond
the repair material. A steel disc was
attached to the surface using rapid
setting epoxy resin and was pulled in
direct tension using the modified
Lok-test apparatus.

To test the bond strength of the coating
to the sub-strata, a similar technique
was used. However, for this test, the
initial core cut was not required.

Time domain reflectometry was used to
monitor the grout and water levels.
This technique uses guided electrical
pulses and is sensitive to the medium
surrounding the wire. Wires were
positioned to the height of the liner at
cardinal points and connected to an
oscilloscope at ground level. Using a
series of switches it was possible to
monitor the grout and water level by
observation of the oscilloscope at ground
level. This system also enabled control
of pumping rate and sampling for
percent bleeding and compressive
strength to be accomplished at one
location.

The above practical techniques were
found useful to avoid some problems
encountered in the tank repair
programme. High humidity and low wall
temperatures have initiated the forma-
tion of condensation, with resultant
blistering of the coating, after one year
of service. Cold wall temperatures have
made the application of epoxy resin
coatings difficult due to a substantial
increase in viscosity at low
temperatures. Rough surfaces have
produced coatings with an uneven profile
with the consequent necessity to apply
additional coats. The systematic
application of some of the above
techniques, therefore, cannot be
overemphasized and will result in
benefits to both the contractor, and the
consultant in addition to assuring an
acceptable repair.

Additional quality assurance procedures
were required during the grouting of
steel liners. Due to the use of thin
liners it was necessary to avoid high
fluid pressures occurring in the grout
which could buckle the liner. One
procedure used, was to balance the fluid
pressure by filling the tank with water
at a similar rate to the rate of grouting.

5 - 19

6.0

DETERIORATION OF METALS IN CONCRETE WATER TANKS

6.1

Introduction

6.1.1 Role of Metals In Concrete Tanks

The main part of the report is focussed
on the deterioration of concrete as a
material, and on the reasons for the
need to rehabilitate defective reinforced
and prestressed concrete water tank
structures. Metals, however, especially
steel, provide important and necessary
structural components of the concrete
tanks. These metals, as explained in
this section, can corrode, and in some
cases, critically weaken the structure.
In addition to the steel reinforcing bars
and post-tensioning tendons cast in the
concrete wall itself, prestressing wires
or strands are also wound around the
walls and protected with gunite
shotcrete. This steel reinforcement
provides the principal structural tensile
reinforcement for concrete tanks;
however, many other important concrete
tank components, vital to operations, are
fabricated from metals, mainly steel and
aluminum.

Some of these other metal components
are access manways and covers, roof
beams, decks, hatches and covers,
elevated tank floor beams, air vents,
internal and external ladders and
landings, safety rails, inlet and outlet
pipes and valves, floor drains and
covers, overflow pipes and supports, air-
craft lights, and more recently, elements
of freeze protection systems such as
mixing units, temperature sensors, and
roof hoists. All these tank
appurtenances are subject to
deterioration and must be maintained in
good and safe working condition in
order that a concrete water tank can be
operated satisfactorily, inspected, and
maintained efficiently and safely.

6.1.2 Deterioration Sequence

An important point which has resulted
from the inspection of many tanks
during the Ministry rehabilitation
programme is that the inspections of
leakage and concrete deterioration
have generally, incidentally, led to the
discovery of the corrosion and
deterioration of metal components in
concrete tanks. Corrosion has been
observed in critical components in tanks
in service for less than 8 years. Since
these components are often vital for the
safe operation of the tank, it is
important that the problem is rectified
as soon as possible. In a planned
maintenance programme, the condition of
all metal components should be inspected
in a separate scheduled programme.

6.1.3 Importance of Construction
Process

Another important factor which must not
be ignored, is the process by which
post-tensioning is installed in the
concrete walls. The steel tendons are
placed in steel or plastic tubes so they
can stretch during tensioning. Water
entering through defects in these hollow
tubes can freeze and expand, cracking
the concrete section. In some cases,
this can lead to sudden leakage problems
and even tank failure. This is neither a
direct concrete material, nor a metal
deterioration problem, but a problem
resulting from the construction process
itself.

6.1.4 Summary

On a cost comparison basis, corrosion of
steel and deterioration of other metals
has not been as serious a problem in
concrete tanks in Ontario to date, as
has been the problem of concrete
deterioration in the freezing
environment. Recent observations,

6 - 1

however, indicate that the corrosion
problem is increasing and cannot be
ignored in a rehabilitation and mainten-
ance programme.

TABLE 6/1
Types of steel reinforcement

6.2

Corrosion of Steel Wall
Reinforcement

6.2.1 Need for Reinforcement

Steel reinforcement provides the tensile
strength required at ultimate and service
loads to resist the applied loads from
the retained water, wind and earthquake,
as well as environmental loads, in all
concrete water tanks. Concrete, being a
brittle material, cracks at a very low
tensile strain. Sufficient steel
reinforcement, therefore, must cross an
incipient or actual crack to resist the
load and control the crack width.
Alternatively, sufficient prestress must
be supplied by post-tensioning tendons
to pre-compress the concrete to balance
and resist the tensile load, and for
leaktight construction, prevent the
concrete from cracking. Concrete and
steel materials must, therefore, be
combined in the construction process to
result in a load resisting structure.

6.2.2 Types of Reinforcement

Four types of steel reinforcement
consisting of two grades - reinforcing
steel and prestressing steel, have been
used in the construction of the 53
concrete tanks in Ontario, and are listed
in Table 6/1.

All 3 types of post-tensioning use high
tensile prestressing wires on either an
individual basis (G), or as strands of 7
wires (PTB and PTU). All prestressed
concrete water tanks, including the G
type, include some ordinary reinforcing
steel, as well as tendons, in the
concrete wall.

No. of

Tank

Description

tanks

designation

built

Reinforcing
bars

PTB

Post-tensioned
tendons (bonded)

PTU

Post-tensioned

tendons

(unbonded)

Post-tensioned
wires, wound and
gunite protected

53 To

6.2.3 Detection of Corrosion

The mechanism of general corrosion of
steel resulting from the development of
an electrical battery with an anode, a
cathode, and an electrolyte, is described
in many texts and will not be described
here, except where special circumstances
pertaining to concrete tanks occur.
Half-cell measurements using equipment
consisting of a copper/copper sulphate
cell can, in some circumstances, detect
the level of corrosion activity of steel
within a concrete wall from electrical
potential readings. "Hot spots" where
electrical potential readings over 350
milli-volts are read can lead to locations
where active steel corrosion within the
concrete may be occurring. However,
this method must be used with caution
to survey the corrosion state of
prestressing wires. In the walls of
prestressed tanks, steel reinforcing bars

6 - 2

in vertical and horizontal directions, as
well as vertical steel jack-rod pipes, may
exist in the wall in addition to the
prestrcssing wires. Readings of
corrosion "hot spots" in the wall can
lead to conclusions that the initial
prestrcssing wires are corroding, when
in fact, investigations by chipping into
the wires and the wall may indicate that
the wires are bright and corrosion free
on the outside, but that the corrosion is
taking place at an unimportant jack-rod
coupling near the inside face of the
wall. It is, therefore, important to
combine half-cell investigations with
visual examination of corrosion at
"hot-spots" by removal of concrete,
before conclusions are made as to what
type of steel is corroding in the wall,
and the actual location and state of the
corrosion.

In most cases of steel corrosion
observed in concrete tanks, except in
the case of unbonded tendons, signs of
corrosion are usually evident, and can be
seen as rust, pits or rust stains on the
concrete surface. Where the concrete is
delaminating, atmospheric corrosion is
sometimes observed after removal of the
concrete.

6.2.4 Protection of Steel by Quality
Concrete

It has been stated previously that
general corrosion of steel reinforcement
in concrete has not been a serious
problem in the concrete tanks inspected
except at locations where there is
leakage and dampness, exposure of the
steel to the atmosphere, or at a visible
concrete delamination, spalling or porous
area of concrete. Good quality concrete
with a maximum water/cement ratio of
about 0.50 and an adequate Portland
cement content, provides a protective
passive alkali environment for steel
reinforcement, prestrcssing wires and
strands. The pH (hydrogen ion level) on
the O (acid) to 14 (alkali) scale, with 7
being neutral, should be a minimum of

12 in concrete in order to provide
permanent protection of steel against
corrosion. The pH value of poor quality
and porous concrete can be reduced by
carbonation (COj), acid rain, and an
aggressive environment so that the
original protective passive property of
the alkali concrete material is lost, and
the corrosion of steel can start.

The widespread corrosion and concrete
delamination caused by chlorides from
de-icing salts as experienced in concrete
bridge decks and parking garages in
recent years, is noticeably absent in
concrete water tanks in Ontario.

6.3 Observation and Repairs

6.3.1 Reinforced Concrete Tanks
(Type RC)

Corrosion of the reinforcement in
reinforced concrete (RC) tanks which
are non-prestressed has been negligible.
Even where external spalling of the
concrete has occurred causing the steel
to be exposed for years, the general
corrosion has not been serious
(WATFORD, CAMLACHIE, ALVINSTON,
etc.).

Examination of reinforcement adjacent to
vertical delaminations 1 mm wide in the
walls of concrete tanks under perma-
nently saturated conditions has revealed
no sign whatsoever of corrosion of the
steel with approximately 30 mm of con-
crete cover (ALVINSTON, CAMLACHIE).

Examination of the reinforcement from a
large elevated reinforced concrete tank
demolished after less than ten years in
service (PITTSBURGH) revealed that
there was no corrosion and that the
steel was in an "as new" condition.

All observation confirms that a
protective cover of about 25 mm of good
concrete provides satisfactory corrosion
protection for reinforcement during the

6 - 3

normal expected service life of 30 to 50
years of a concrete water tank
(PRESTON - 30 years old).

6.3.2 Post-tensioned Bonded Tanks
(Type PTB)

.1 Description

Post-tensioned bonded tendons in
Ontario concrete water tanks consist of
a single 16 mm diameter 7 wire
prestressing strand placed in 30 mm
diameter corrugated ducts or sheaths
manufactured from plain bright steel
strip.

After tensioning the strands, protection
against corrosion is carried out by
injecting a neat portland cement grout,
containing an expansion admixture, into
the ducts with a pressure pump, so they
are filled with alkali material with a
minimum pH of 12.

The grout then hardens, thus effectively
bonding the strands in the corrugated
ducts within the structure like normal
hi-bond reinforcement in concrete
material.

Hoop or circumferential tendons are
generally 180°, (half circle) with steel
anchorages at the ends installed in
external buttresses or internal pockets.
The strands are locked off after
tensioning (and before injection) in
tapered holes in the steel anchorages,
with tapered wedges, and with machined
teeth. Straight vertical tendons are
incorporated in some PTB tanks.

.2 Problems (See Figure 6.1)

Ungrouted ducts

Problems can occur later if the ducts
are not properly filled with portland
cement grout by injection after
tensioning the strands. As described
elsewhere, the empty ducts may fill

with water some years later and freeze
in winter, causing expansion and
cracking of the wall (EAR FALLS,
CASSELMAN).

Figure 6.1 Bonded tendons. Bleed void,
corrosion of unprotected prestressing
steel. Concrete anchorage pocket plug
shrinks (large circle, upper left) and
becomes loose. Poor bond or non-
expansive mortar permit aggressive
materials access to anchorage and
prestressing steel, likewise with
improperly bonded and anchored
exterior-end anchorage (shown at left
end of prestressing steel) protection,
(ref. 35)

This can lead to increased leakage in
the spring followed first by corrosion of
the sheet steel ducts and then corrosion
of the unprotected and vital prestressing
strands themselves. The deterioration
process here, as described elsewhere in
the report, is similar and occurs in 3
stages. First the initiation stage
consists of the steady breakdown of the
waterproofing system followed by
concrete saturation and finally leakage
into the empty ducts. The second active
stage consists of the filling of the ducts
with water, freezing, cracking of the
concrete section and subsequent leakage
of water to the outside. Exposure to
air, together with moisture, initiates the
start of corrosion of the duct and
prestressing strand. The third and final
stage is the loss of steel section and
prestressing force, eventual failure of

6 - 4

the strands or wires and loss of strength
of the tank wall section.

Investigations under the spalled concrete
fill in the vertical tendon anchorage
recesses at the roof level of one
standpipe (MILLBROOK) revealed severe
corrosion of the strands and wedge
anchorages. Some strands had been
completely eaten away down into the
corroded wedges to the point where
anchorage of the strand tendon by grout
bond was essential, because the steel
anchorage had failed. The severity of
the aggressive corrosion into the
anchorage indicated that calcium
chloride, or a similar corrosive material,
may have been used in the mortar to
either speed up hardening of the mortar
or prevent it from freezing.

.3 Repair

Diagnosis of the strand corrosion
problem (a) described above, by
investigating at the leak locations, must
be made early so that the corrosion
process can be stopped before
significant damage is done. This can be
accomplished by filling the ducts with
protective epoxy or cement grout before
the loss of section and the state of
corrosion of the wires of the strands
becomes serious from pitting, or if
stress corrosion occurs.

If strands are seriously corroded or have
failed, it may be necessary to strengthen
the tank with added external post-
tensioning tendons, as well as
leakproofing the tank. In this case the
designer must check that the wall is not
overstressed in the empty tank state by
too much additional prestress.

6.3.3 Post-tensioned Unbonded Tanks
(Type PTU)

.1 Description

Post-tensioned unbonded tendons in
Ontario concrete water tanks consist of

single 13 or 16 mm diameter 7 wire
prestressing strands, coated with
protective water-resistant grease charged
with rust inhibitors, and pushed into
black polyethylene tubes with walls
about 2.5 mm thick. The air space
between the coated strand and the inside
of the tube may be about 2 mm. Steel
anchorages are installed at the ends of
each 180° or 360° hoop tendon to lock
off the strands with tapered wedges with
machine teeth in tapered holes in the
anchorages. Straight vertical tendons
are added in the walls of many PTU
tanks. If either the strand or an
anchorage fails, the total unbonded
tendon fails and is lost to the structure.

Figure 6.2 Unbonded monostrand. The
anchorage plug shrinks and becomes
loose. Poor bond and non-expansive
mortar permit aggressive materials access
to anchorage and prestressing steel.
Strand portion is exposed to concrete
because no physical connection is made
between the sheath and anchorage. At
stressing end. this portion of tendon is
pulled through intimate concrete closure
when stressed. Tie wire between
perpendicular tendons causes local
indentations in sheaths which tend to
shear off when tendons are tensioned.
Reinforcing bar indentation causing hard
point that tends to shear off when
tendon is tensioned.

6 - 5

.2 Problems (Sec Figure 6.2)

Two problems exist with unbonded
tendons, firstly that of corrosion of the
strands which is the subject of this
section; and secondly, the filling of the
air spaces in the plastic tubes with
water under pressure, and freezing,
especially of vertical tendons. This may
contribute to the rupture of the wall
causing complete and sudden failure of
tanks (DUNNVILLE and SOUTHAMPTON)
which is d'^scribed elsewhere in this and
other reports.

Five (5) of the 116 unbonded strand
tendons were found to have failed by
stress corrosion cracking during the
rehabilitation of the seven year old
BRIGDEN elevated concrete water tank.
For a typical failure see Photo Nos 6-1,
6-2, 6-3 and 6-4. The sequence of
events were presumed to be as follows:

• water from external precipitation
penetrated to the tendons through the
poor quality and porous concrete fill in
the anchorage recess pockets.

• stress corrosion cracking across
some wires started at the
non-metallic inclusions in the steel
surface.

• longitudinal brittle fractures in the
wire started at the stress corrosion
crack sites as a result of bending
stresses on the wire due to the wall
curvature.

• tensile brittle fractures of the
remaining uncorroded wires occurred
similar to that expected from an
increased load on the remaining wires.

Photo 6-1 Poorly protected post-
lensioning anchorage allowing water to
enter. (BRIGDEN)

Photo 6-2 Detailed view of strand
showing corroded wires. (BRIGDEN)

.3 Repair

All anchorages were exposed by
removing the covering mortar from the
recesses. Lift-off load tests were
carried out on all but 4 of the 116
strands in the tank with specially
developed tools and post-tensioning jacks
to a load of 70 per cent of the
guaranteed ultimate strength of the

6 - 6

strands which was 17 per cent above the
design load of the strands (60 per cent
ultimate). This action resulted in
obtaining a proof test of the total hoop
strength of the tank at that time and
identifying that all 5 strands failed from
corrosion and were no longer capable of
supplying the proof test force slightly
above the required design strength of
each strand.

Photo 6-3 Typical longitudinal fracture
of one of the wires of a strand. Lower
break shows some elongation which was
not typical of breaks observed. Some
grease is present on the wire.
(BRIGDEN)

Techniques were developed for removing
the 5 strands, either already failed or
failing to meet the proof test load, and
replacing them with new strands coated
with grease in the existing plastic ducts.

Finally the anchorages and recesses were
sand-blasted, mesh reinforcement
fastened with drilled inserts was
installed, and a high quality latex
concrete mortar was used to fill the
recess pockets to seal the ends of all
tendons and prevent further water
leakage into the ducts.

6.3.4 Gunite Protected Tanks
(G Type Tanks)

.1 Description

The sequence of construction of
post-tensioned wire wound (G type)
tanks is different to that of PTB and
PTU types described previously, where
the strand post-tensioning tendons inside
ducts are placed first, then cast into the
concrete walls. After the tank wall is
completed and when the concrete
reaches a specified minimum compression
strength, normally 24 to 28 MPa, the
tendons are tensioned and anchored.
This is carried out in a specified order
of stressing so that the wall is not
cracked by the applied initial
prestressing forces during the
post-tensioning process.

Photo 6-4 Start and progression of a

typical fracture. Fracture starts at the
surface of the wire then progresses
longitudinally going deeper into the wire
as it progresses. Note corrosion of wire
near the bottom of picture. An example
of the second type of fracture
(transverse) can be seen in the wire on
the right of the strand. (BRIGDEN)

In the G type tanks, the wire tendons
are tensioned as they are pulled through
a die and wound continuously around the
already constructed concrete tank wall
after it reaches the required minimum
strength, usually 31.5 MPa. The walls of

6 - 7

ground and elevated G type tanks are
normally constructed of shotcrete
concrete (gunite) mortar. All standpipes
in Ontario, except one, were constructed
using slipformed concrete. This method
uses vertical jack-rod pipes, coupled
every 3 m (10 feet) height in the walls,
on which the forms are continuously
raised by hydraulic jacks during the
concreting process. The jack-rod pipes
left in the walls have caused serious
concrete deterioration problems in many
tanks because of internal spalling at the
coupling points due to the pipes filling
with water and then freezing, causing
explosive forces on the concrete. This
deterioration is described elsewhere in
this report.

The hoop post-tensioning of G type
concrete tanks is carried out using No. 8
gauge high tensile prestressing wire of
4.1 mm diameter drawn through a die to
3.6 mm. The initial wire stress is 980
MPa reducing to a final effective stress
of 735 MPa after all losses. After the
post-tensioning of each layer of wires is
completed, they are covered with a thin
2-3 mm wash layer of gunited on
concrete mortar (shotcrete). The
number of layers of prestressing wires
placed on top of previous layers is a
function of the hoop force from the
wires required to resist the applied
water load, and any other design loads
on the tank. A maximum of five (5)
designed layers has been observed.
Additional layers to the designed number
have been applied in some tanks
(L'ORIGNAL and FENELON FALLS)
because of problems during post-tension-
ing, or with the quality of the wire.

After the completion of winding on the
final exterior layer of wire, the wires
are protected with a final coat of the
pneumatically applied concrete gunite
mortar. This cover coat thickness is
usually specified as either a minimum of
20 or 25 mm.

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V' ■■■■ ■'.'-.

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■ Cover ..
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Figure 6.3 Flaws leading to corrosion of
wire wrapped circular pipes and tanks.

A) sand pockets or voids which permit
the passage of electrolytes and oxygen.

B) uncontrolled structural cracks,
permitting the environment access to the
prestressing steel.

C) Bundled wires or strands which can
result in a continuous void.

D) Delaminated cover coat which is
generally connected with a
circumferential crack exposing the backs
of the prestressing steel.

E) Inadequate cover coat which is less
than 15 mm or pervious, permitting
access of environmental contaminants.

F) Cracks or honeycomb in cylinder core
giving access of contaminated material
to prestressing steel.

G) Electrical connection of prestressing
steel to other metal components.

Note that electrolyte has to be present
for corrosion to occur

.2 Problems (See Figure 6.3)

Two elevated G type tanks were built in
Ontario, at AMHERSTBURG, and
CHELMSFORD. Both these tanks
developed severe wire corrosion problems
at the bottom of the tank walls for

6 - 8

Photo 6-5 Delaminated cover coat
exposing broken prestressing wires
(AMHERSTBURG)

heights of 2 m above the floor slab and
required major repairs, including
strengthening by external
post-tensioning, after about ten years in
service.

Seven broken wires were found at the
AMHERSTBURG tank and a further
number badly corroded in an area 2 m x
2 m adjacent to a major leak at a wall
delamination or split and where the
cover coat had fallen off, exposing the
wires, (see Photo No's 6-5, 6-6) The
original cover coat was only about 5 mm
thick over the wires.

A similar area of corroded wires, to that
described above, was located at
CHELMSFORD, when a large area of
de-bonded cover coat which had bulged
75 mm outward, was removed. The
de-bonding had exposed the wires to the
elements probably soon after the tank
was filled, judging by the severe state
of the corrosion, and other observations.
Vertical cracks in the tank indicated
that strengthening was required.

A number of broken or severely
corroded wires have been found in three
(3) G standpipes, generally at locations
of local leakage and where the thin

Photo 6-6 Corroded prestressing wires
under delaminated cover coat
(CHELMSFORD)

cover coat has de-bonded and fallen off
the vertical face exposing the wires.

Figure No. 6.3 shows flaws leading to
corrosion of wires of wire wrapped tanks
(after Schupack).

.3 Repair

Two external post-tensioning procedures
have been used for strengthening tanks
with corroded or failed circumferential
prestressing wires, both using special
anchorages developed for tanks.

The first method was to install greased
strands in black polyethylene tubes
around the tank. The second method
was to place bare strands around the
tank with the stressing anchorages near
the broken wire. This assures that the
maximum force is applied near the
break in wires, because of the
steel/concrete friction loss around the
tank. The strand is then protected from
corrosion by spraying on a cover of
gunite.

Care must be exercised in the design,
specifications, and the application of
post-tensioning not to overstrcss the
tank wall so that it cracks in the empty
state.

6 - 9

obtained in 2 coats (with some touch up)
enhancing the appearance of the repair,
which is important in the repair of
concrete tanks because of their high
profile, (see photos 5-23 and 5-30)

Photo 6-7 Corroded access lube
(PRESCOTT)

Corrosion of prestressing wires in G
type tanks from exposure to external
moisture and air through a cover coat
which is too thin ( much less than the
recommended 20 mm minimum), presents
a difficult maintenance problem. If
corrosion is general, widespread and well
advanced, or a number of broken wires
exist, strengthening by adding external
post-tensioning may be possible. If the
corrosion is minor and sporadic, extra
protection to the wires by adding
surface coatings can be carried out. An
extra gunite layer, preferably a 20 mm
additional thickness of Portland cement
mortar, modified with latex, or a methyl
methacrylate (MMA) coating, which is
less costly, have been used. The MMA
coating has been developed so that an
attractive solid white gloss finish can be

Photo 6-8 Corroded roof truss
(HESPELER)

Photo 6-9 Completed rooj repair
(HESPELER)

6.4 Deterioration Of Metal

Components

6.4.1 Steel Access Tubes in
Elevated Tanks

.1 Problem

Three (3) elevated concrete tanks in
Ontario, AMHERSTBURG, CHELMSFORD
and BRIGDEN, are constructed with
internal steel tubes containing access
ladders to the roof, or into the tank,
and provide support for the roof. These
steel tubes are primary tank components
and must be protected from corrosion so
that their strength and leaktightness are
maintained, or the tanks will fail to
retain water.

Photo 6-10 Aluminium ladder exhibiting
severe pitting corrosion (BRIGDEN)

.2 Repair

Severe corrosion and pitting of the
central steel access tube in contact with
the water occurred in the BRIGDEN tank
after six years in service. Protection
was carried out by removing the rust
and preparing the steel by grit blasting
and re-painting with a five coat vinyl
paint system as specified for new
Ministry steel water tanks.

6.4.2 Aluminum Ladders in Water

.1 Problem

Severe, deep, and widely distributed
pitting of the internal aluminum ladder
below the water line was observed in
the BRIGDEN tank described above, with
a central corroded steel access tube.

Investigation of the corrosion showed
that the pits were initiated on the
surface of the aluminum because of the
central presence of iron on the surface.
Iron was present in the water because
of the corroding central steel tube.
Small particles of iron set up localized
galvanic cells which resulted in pitting
corrosion over 10 mm deep in places.

.2 Repair

As it was virtually impossible to
completely stop the advanced pitting
corrosion in the aluminum it was
recommended that the ladder be
replaced. It was decided to replace the
ladder with one fabricated from steel
and hot-dip galvanized for protection.

6.4.3 Recommendations

• There have been reports of certain
types of aluminum corroding in
chlorinated water.

• If aluminum is used, it is
recommended that it be an aluminum
alloy such as Alclad 6061-T4 or T6
temper..

• Some authorities are now specifying
fiberglass rather than metal ladders in
water treatment and sewage plants.

6.5

Metal Appurtenances on
Concrete Tanks

6.5.1 Description

A description of miscellaneous metal
components in concrete tanks is given in
paragraph 6.1.1 of this section. These
metal components include main structural
members, manhole covers, access ladders,
safety lights and piping, and they are
fixed to the concrete tank with metal
fastenings.

»i^-*

Photo 6-11 Corroded steel manway cover
and bolls. (WATFORD)

6.5.2 Observations

• In some cases, the main component
may be in satisfactory condition but the
fastenings, cast or drilled in the
concrete, such as inserts, are corroded,

often because they have not been
protected satisfactorily against corrosion
or because they are different to or are
an inferior metal to the appurtenance.
In the case of ladders and landings, this
corrosion of fastenings can lead to
unsafe conditions.

• Working or critical parts of
appurtenances such as hatch and manway
hinges, threaded retaining bolts for
manways, keyholes in locks, etc., are
often badly corroded causing delays in
entry for inspections and maintenance
work, and costly replacement if threaded
studs cannot be removed, for instance,
(see Photo 6-11)

• Steel mesh screens in cylindrical
aluminum air vents installed to prevent
entry of insects and birds into the tanks
are almost invariably severely corroded
because of the bi-metallic contact.

6.6 Summary and Conclusions

• The two most common metals used in
concrete water tank construction are
steel and aluminum. Steel is installed
inside the concrete in the form of
reinforcing bars and as post-tensioning
tendons. Appurtenances on tanks such
as external and internal ladders,
landings, hatches, vents etc. are
normally aluminum or galvanized steel.

• Exposed metals on concrete tanks
must be protected and maintained, in
order to arrest or prevent deterioration.

• Examples of severe corrosion of steel
reinforcement are rare, but have
occurred where the steel was exposed.

• Hi-tech methods for the detection of
corrosion "hot-spots", such as half cell

6 - 12

voltage measuring, should be validated
by visual examination. The reason for
this visual examination is because steel
pipe jack-rods may be in close proximity
to each other or touching in the wall.
Severe corrosion activity in a jack-rod
pipe, for example, may be read
inaccurately as being in the prcstressing
wires, which is erroneous.

• Dense, high quality concrete provides
good protection for steel reinforcement
in concrete water tanks, providing the
cover exceeds 25 mm. The cement
content must be sufficiently high in
order to maintain a pH value of 12 or
more. This alkali environment protects
the steel from corroding.

• Corrosion of post-tensioning wires
and strands will occur if the high tensile
prestressing steel is unprotected by
grease containing corrosion inhibitors
(PTU tanks), by Portland cement grout
injected into the ducts (PTB tanks), or
by a minimum of 20 mm thickness of
high cement content gunite cover coat
(G tanks) and is exposed to the
atmosphere and external or leakage
water from the tank.

• Stress-corrosion can cause brittle
tensile failures in unbonded prestressing
strands without rust signs, because the
steel fails inside a plastic tube.

• Detection of corrosion of
post-tensioning tendons is sometimes
difficult because they are hidden from
view. Start inspections near leakage
points.

• Concrete tanks weakened by corroded
internal or wirewound tendons may be
strengthened by external
post-tensioning.

• Steel access tubes in concrete tanks
may be primary structural elements and
must be inspected and maintained in a
corrosion free condition.

• Severe pitting corrosion of aluminum
has been observed in a concrete tank
with a corroding steel access tube.

• Many instances of the corrosion of
tank appurtenances have been observed,
especially fastenings and vital working
parts, which need costly replacement
when not maintained.

6.7

Recommendations

• The metal parts of concrete tanks
often form primary structural elements
and must be inspected and maintained on
a regular basis.

• Post-tensioning tendons exposed to
external water or tank leakage are

vulnerable to weakening and failure due
to corrosion and must be maintained and
protected from this environment and all
leakage or infiltration of water to
prestressing steels must be stopped.

• Aluminum ladders and fittings should
not be used in chlorinated water, unless
the metal is proven to be a corrosion
resistant type alloy in that environment.

6 - 13

7.0

CONCLUSIONS

7.1

Introduction

7.2

Concrete Deterioration
Mechanisms

The principal conclusions in this report
relevant to the deterioration of concrete
tanks, result from the five year study of
the deterioration of 53 uninsulated
concrete tanks in Ontario built since
1956, but mainly since 1970. The study
was initiated by the Ministry of the
Environment in 1981 and includes the
period up to the end of 1986. It
incorporates findings from the applied
research programme and references
listed, as well as from investigations,
inspections, repairs, and rehabilitation of
the tanks. Eleven tanks have been, or
will be replaced. The total
rehabilitation programme in this five
year period has cost over $15 million,
including engineering, research,
development, remedial
repairs, waterproofing, replacement,
insulation and cladding, freeze
protection, construction contracts, and
temporary storage.

The study was started in 1981 because
of the sudden failure of two new
municipally owned concrete standpipes 45
m high, the first in 1976 at
DUNNVILLE, the second in 1980 at
SOUTHHAMPTON and because of reports
from various sources at that time of
widespread deterioration and leakage of
many of the other concrete tanks in
Ontario with as little as 5 years service.
This led to general concern for the
condition of the tanks, their service life,
and for the safety of the public. No
above ground concrete water tanks have
been constructed in Ontario since 1980.

7.2.1 General

A major factor in the extent of
deterioration observed in above ground
concrete water tanks in Ontario is the
type of construction used, e.g. bonded or
unbonded post-tensioning tendons, jack-
rods left in the tank wall, inadequate
gunite cover coat, cold joints produced
during jump forming etc. This,
combined with a cold region environment
has resulted in catastrophic reductions
in expected service life.

7.2.2 Internal Ice

Cold region environments can result in
detrimental internal ice formations in
uninsulated water tanks with low water
turnover. Ice formations inside concrete
tanks can cause significant hoop
pressures on vertical walls subjected to
rapid temperature rises. Pressures of 0.7
MPa have been measured in ice caps due
to expansion caused by a rise in
temperature. These pressures are
capable of splitting a concrete tank wall.
Tanks with upward sloping walls reduce
ice pressure effects.

7.2.3

Freeze-Thaw With Pressurized
Water

Freeze-thaw action on permeable
concrete, saturated by water under
pressure, can result in rapid failure of
the concrete microstructure. Studies of
this failure mechanism show that dilation
or expansion of the water-filled micro
pores occur on freezing at about -4°C.
Part of this expansion on thawing has
been demonstrated in this report to be
non-elastic and irreversible. At the
start of the thaw cycle, the ice may
expand further, before melting. The
greater expanded volume of the pores in

7 - 1

the microstructure may then be filled
with additional water under pressure
which on freezing causes a further
expansion.- The accumulation of this
ratchet effect of permanent residual
dilation and associated incremental
strains, results in the initiation of the
failure of the matrix microstructure
when the tensile strength of concrete is
reached.

7.2.4 Rate of Deterioration

Observations of concrete deterioration
indicate that the rate of deterioration in
concrete water tanks is related primarily
to the water pressure, freeze-thaw
temperature amplitudes and frequencies,
concrete permeability and the orientation
of the wall section to the sun. The
latter observation corroborates the
postulation that it is temperature
amplitudes and frequency which
determine the rate and extent of
deterioration. Additionally, temperature
monitoring demonstrated that the
amplitude and frequency of temperature
changes were similar throughout the
height of the tank. Since the
deterioration was consistently at the
lower portions of the tank this suggests
that the rate of freezing in a single
cycle is not as significant as previously
considered. Dilation occurring in densely
reinforced wall sections has resulted in
progressive delaminations in reinforced
concrete standpipes in cold regions. Air
entrainment bubbles or voids, while
reducing damage in other building
structures, may contribute to damage
under freeze-thaw conditions in high
pressure water tanks.

7.2.5 Freezing in \\all Voids

Water under pressure, freezing in wall
voids and cracks, can result in confined
ice pressures measured as high as
21 MPa producing spalling and
delamination of concrete water retaining
structures. Voids can occur under
horizontal reinforcing, in ungrouted

metal post-tensioning ducts, in unbonded
tendons, and inside hollow coupled
vertical jackrod pipes left in the wall
after slipforming. Deterioration of the
concrete is accelerated at leakage points.

7.3 Expansion Joints

Poor installation of plastic or rubber
waterstops in expansion joints can result
in leakage and costly repairs.
Waterstops can act as dams in the wall
to permeating water; concrete
deterioration due to freeze-thaw action
then often occurs on the interior face
where the concrete is saturated by water
under pressure.

7.4 Corrosion of Prestressing Steel

Losses of the protective gunite cover
coat from wire wound prestressing wires
has resulted in general corrosion and
wire failure causing loss of
circumferential prestress. This prestress
loss results in vertical cracking of the
tank wall. Poorly filled external
post-tensioning anchorage recesses have
permitted water to enter horizontal
unbonded post- tensioning tendons
resulting in stress-corrosion failure of
strands. Metal ducts accidentally
flattened during concrete placing has
prevented the insertion of
post-tensioning tendons. Lack of grout
in ducts has allowed water to enter and
freeze, causing cracking and
delamination of the concrete wall, and
general corrosion of the prestressing
steel.

7.5 Repair Methods

7.5.1 General

To be effective, the cause of the
problems must be analyzed thoroughly
before the repair is designed. Inspection
and repair techniques expose people to
the dangers of heights and high winds.
Repairs are difficult, require specialist
contractors and inspectors, and must be

7 - 2

carried out within Ontario's limited
construction season.

7.5.2 Bonded Waterproofing Coatings

One method of preventing water freezing
in the saturated permeable concrete and
voids is to apply a bonded waterproofing
coating on the inside of the concrete
wall. The most effective coating
materials used to date have been 100
percent solids epo.xies which are non-
toxic, tasteless and odourless. The
epoxy coating has been applied in 3
coats on a dry grit-blasted surface.
Good bond (2Mpa minimum) is essential.
Strict environmental control during
epoxy application and curing is essential.
A surfacing subcoat 3 mm thick must be
used on rough concrete to avoid pinholes
in the epoxy. Latex and epoxy mortars
have proved to be good surfacing
materials with adequate bond. It is
difficult to completely prevent pinholes
from forming in epoxy coatings without
pressurizing the tank slightly during
epoxy application. Water-filled blisters
have been observed in epoxy coatings
during the one year warranty
inspections. These can be repaired
satisfactorily, however in some cases the
blisters were not repaired where it was
considered they were stable.
Any caulking used, must be durable
under water pressure and resistant to
chlorine during tank disinfection
operations.

7.5.3 Steel Liners

Interior steel liners have been used
successfully. Two methods of
constructing steel liners inside concrete
tanks have been used. One method
employs bolts fastened to the concrete
wall to provide temporary support of the
steel plates, prior to welding. The other
method is to construct a freestanding
steel tank inside the concrete tank
without using any mechanical
connections to the concrete wall. A
Portland cement grout having a pH

greater than 11.5 provides protection of
the steel against corrosion on the
unpainted outside face. The steel liner
face in contact with the water is
protected against corrosion by applying a
bonded coating.

Where large roofs such as domes exist,
access holes have been cut so that the
steel liner plates can be loaded inside.

In three elevated concrete water tanks,
insulation has been placed between the
steel liner and the inside of the existing
wall, before grouting. External
insulation and cladding has also been
used on one other elevated tank and one
standpipe where interior steel liners
were installed.

7.5.4 Plastic Liners

A partial plastic liner has been installed
as a cut-off over a defective wall floor
expansion joint. The liner was a tough
woven nylon fabric coated with PVC,
and has performed satisfactorily for 5
years. Recent inspection has revealed
that the liner has now become somewhat
stiffer and some holes have had to be
repaired.

Full height hypalon and polythene liners
have been investigated, but to date have
not been considered a practical or
economical solution.

7.5.5 External Post-Tensioning

External post-tensioning has been
carried out to control vertical hoop
cracks in reinforced concrete standpipes,
and to strengthen tanks initially
post-tensioned by the wire winding
process and where corrosion failure of
the wires had taken place. Special
anchorages were developed for single
and twin strand tendons wound around
circular tanks. Stage post-tensioning of
the hoop tendons is necessary to control
cracking of thin walls during the
application of prestress.

7 - 3

On one project, water entering tubes has
caused 3 percent tendon failure after
seven years exposure to the elements.
The failure from corrosion was
apparently because of an inadequate
original coating of protective grease on
the strands.

7.6

Freeze Protection

7.6.1 General

Freeze protection systems have been
developed and installed in concrete tanks
in cold regions to attempt to eliminate
internal ice formations, reduce the
possibility of freeze-thaw cycling in the
concrete, and to prevent freezing from
taking place in the walls of standpipes
constructed using unbonded tendons.
Temperature sensors in the water and
walls have been used to compare actual
performance against heat loss
predictions.

7.6.2 Insulation and Cladding Systems

7.6.3 Mixing and Heating Systems

In extreme cases, external booster water
heaters and internal eductor jet mixing
units activated by external pumps are
installed in the system.

7.6.4 Air Gap Heating

An air gap booster heating or guard ring
system, external to the tank, has been
developed to prevent concrete wall
surface temperatures of existing tanks
constructed with unbonded
post-tensioning tendons, or where leaks
and wall defects have been difficult to
repair, from falling below freezing.

7.7 Tank Types Not Recommended

Two types of above ground water tank
construction are not recommended in
freezing environments. These are
prestressed concrete tanks
post-tensioned with unbonded tendons,
and reinforced concrete standpipes.

A system developed to insulate circular
tanks consists of rigid Styrofoam SM
sheets and corrugated pre-painted metal
cladding to form a polygonal external
wall separated from the concrete tank
wall by an air gap. The system is
removable so that regular exterior
inspections of the concrete exterior may
be performed.

Measured performance of the external
insulation system with an air gap shows
a dampening down of local temperature
fluctuations in the wall and a reduction
factor of about 3 for the ratio of
ambient to air gap temperature (e.g. at a
temperature of -30°C the air gap
temperature is -10°C). Differences
between the theoretical temperatures
determined from heat loss calculations
and the actual recorded temperatures are
attributed to air leakage in the system;
however, harmful freeze-thaw cycling
and internal ice formations have been
eliminated.

7 - 4

8.0

RECOMMENDATIONS

8.1

Introduction

8.2.2 Interim Guidelines

Although no above ground concrete
water tanks have been constructed in
Ontario since 1980, there may be reasons
in the future for them to be construct-
ed, such as the need for competition
with steel because of the future high
cost or shortage of steel. These recom-
mendations are directed at describing
how to try to build concrete water tanks
in Ontario in the future, without under-
going the problems experienced in the
past.

8.2

Design & Construction of New
Concrete Water Tanks

8.2.1 Codes

Codes and guidelines for the design and
construction of above ground water
tanks in Ontario and Canada generally
do not specifically address the important
freeze-thaw deterioration problems desc-
ribed in this report and do not include
specifications to ensure a satisfactory
performance for the design service life
of the structure in the Ontario environ-
ment. Such codes should be developed.
General guidelines, recommended to be
followed where applicable, are as
follows:-

CAN3-A23.3-M84

Design of Concrete Structures for
Buildings.

CSA S474 Part 4

CSA Preliminary Standard for the
Design, Construction and Installa-
tion of Fixed Offshore Production
Structures.

ACI Committee 344

Report on Recommendations for
Design and Construction of Cir-
cular Prestressed Concrete Tanks.

Interim guideline recommendations of
minimum requirements for the design and
construction of new above ground water
tanks in Ontario (prepared by W. M.
Slater & Associates Inc.) have been
included as Appendix 'A' in this report.

The main requirements in the guidelines
are as follows:-

• Design and construct for a service
life of 50 years with minimum main-
tenance.

• Concrete permeability shall not
exceed 0.25 x 10"^^ m/s, as measured
first by test mixes and confirmed by
cores taken from the actual tank.

• Concrete tanks shall be leakproofed
by the installation of a liner or the
interior shall be coated with an
approved waterproof coating. A one year
extended warranty against future leakage
shall be required after a satisfactory
leakage test.

• Concrete water tanks shall be
biaxially post-tensioned with external or
grouted tendons to give a reserve comp-
ression of 1.5 MPa in horizontal and
vertical directions after consideration of
all applied and environmental loads.

• Concrete water tank structures, and
the stored water, shall be protected
from freezing by insulation, air gap
heating, or other methods. No internal
ice formations will be permitted. Carry
out heat loss calculations and assess the
requirement for either a passive or an
active ice prevention system.

- 1

8.3

Maximum Head

The maximum head of new concrete
ground or elevated tanks should be
12 m. No standpipes shall be
constructed.

8.4 Technology Transfer

The principal conclusions and recomm-
endations in the tank applied research
reports should be made available to the
engineering profession, consultants, and
operations personnel, by means of
reports, seminars, papers, etc. so that
repairs underway, and future concrete
tank design and construction, and
maintenance, might benefit from the
work.

8.5 Durability

Further development is required in des-
ign, construction, and materials to opt-
imize the economy of a new breed of
insulated concrete tanks in the following
areas:

• concrete mix designs and methodology
to result in minimum construction
defects and a minimum coefficient of
permeability.

• design for better concrete placement

• improved wall/floor joint details

• development of a thin (3 mm thick)
stainless steel liner (no maintenance)
waterproofing system for concrete tanks
using normal concrete mixes.

8.6 Further Applied Research

• The models presented in chapter 4 of
this report should be verified under
laboratory conditions. Development of a
laboratory model will enable greater
understanding of the rate of concrete
deterioration under varying hydrostatic
pressures and saturated freeze-thaw
conditions. It will also permit

preventative measures such as concrete
permeability, liners, coatings, penetrating
sealers and imperfections in these types
of barriers to be evaluated accurately.

• The potential to passively maintain
the tank inlet water at temperatures
greater than 2°C should be investigated.
Possible sources of heat gain would be
the source water and the feeder pipe
system. Seasonal fluctuations at these
locations should be monitored to opt-
imize system design and minimize the
energy costs accrued during the service
life of the tank.

• Cost effective and less disruptive
methods of internal inspection of water
tanks should be developed in order

to allow regular inspection and minimize
maintenance costs. Remotely operated
underwater vehicles equipped with video
cameras may allow inspections to be
carried out without having to empty
tanks and disinfect after the inspection
has been completed.

• Early warning of any undesirable or
potentially dangerous conditions should

be made available to operators. Where
possible these should be remotely sensed
and controlled to avoid the necessity of
manual inspection during severe winter

conditions. Currently this may require
the operator to climb 45 m on external

ladders at temperatures below -25°C.
Continuous monitoring of water inlet and

outlet, concrete wall and the
temperature of other significant loca-
tions as well as viewing internal ice and
wall coating conditions etc. could be
made available in the water plant by
installing equipment developed from
monitoring technology already used in

the tank rehabilitation programme.

- 2

9.0

LIST OF REFERENCES

1) Slater, VV.M., & Associates Inc.
Interim Report on Ontario Concrete
Water Retaining Structures".
Ministry of Environment of Ontario,
Project WTS-001, 1982

2) Slater W.M., "Concrete Water Tanks
in Ontario". Proceedings, Canadian
Society of Civil Engineering 1983
Annual Conference, June 1-3, 1983

3) Ministry of Transportation and
Communications of Ontario.
"Bridge Deck Rehabilitation, Manual",
Part One, Downsview 1983.

4) Helmuth, R.A., "Capillary Size
Restriction on Ice Formation in
Hardened Cement Pastes",
Proceedings, Fourth International
Symposium on the Chemistry of
Cement, Washington, D.C., V.2, 1960.

5) Fagerland, G., "The Significance of
Critical Degrees of Saturation at
Freezing of Porous Brittle
Materials". ACI Publication SP-47,
American Concrete Institute, Detroit,
1975.

6) Beaudoin, J.J. and Maclnnis, C,
"Dimensional Changes of Hydrated
Portland Cement Mortar Due to Slow
Cooling and Warming". ACI
Publication SP-47, American
Concrete Institute, Detroit, 1975.

7) Kays, W.B., "Construction of Liners
for Reservoirs, Tanks, and Pollution
Control Facilities", John Wiley &
Sons, 1977

8) Gray, W.S., "Reinforced Concrete
Water Towers, Silos, and Gantries".
Concrete Publications Limited, 1953

9) Hazlehurst, J.N., "Towers and
Tanks for Water Works", John Wiley
& Sons, 1910

10) Creasy, L.R., "Prestressed Concrete
Cylindrical Tanks", John Wiley &
Sons, 1961

11) Manning, G.P., "Concrete Reservoirs
and Tanks", Concrete
Publications Limited, 1967

12) Gray, W.S., "Reinforced Concrete
Reservoirs and Tanks", Concrete
Publications Limited, 1954

13) Powers, T.C., "Freezing Effects in
Concrete" ACI Publication SP-47
American Concrete Institute,
Detroit, 1975.

14) Verbeck, G., "Significance of Tests
and Properties of Concrete and
Concrete - Making Materials"
Chapter 18. ASTM Publications STP
169B, 1978.

15) Carrier, R.E. and Cady, P.D.,
"Moisture Distribution in Concrete
Bridge Decks and Pavements" ACI
Publication SP-47, American
Concrete Institute, Detroit, 1975.

16) Mills, R.H., "The Permeability of
Concrete for Reactor Containment
Vessels", Report 84-01 University
of Toronto, July, 1983.

17) Powers, T.C., "A Working
Hypothesis for Further Studies of
Frost Resistance of Concrete". ACI
Journal, Proceedings, V. 41, No. 3,
February 1945.

18) "Frost Durability of Clay Bricks -
Evaluation Criteria and Quality
Control" - Proceedings No. 8
CBAC/DBR 1984

19) Neville, A.M., "Properties of
Concrete" Pitman Publishing
Company, 1974.

20) Maclnnis, C. and Whiting, J.D., "The
Frost Resistance of Concrete Subject
to a De-icing Agent", Cement and
Concrete Research, Vol.9, 1979.

21) Neville, A.M., "Creep of Concrete:
Plain, Reinforced and Prestressed",
North-Holland Publishing Co.,
Amsterdam, 1970.

22) Khalil, S.M., Ward, M.A., and
Morgan, D.R., "Freeze-Thaw
Durability of Non Air-Entrained High
Strength Concretes Containing
Superplasticizers", Proceedings, First
International Conference on
Durability of Building Materials and
Components, Ottawa, Canada,
August, 1978.

23) Marusin, S., "The Effect of
Variation in Pore Structure on the
Frost Resistance of Concrete,
Cement and Concrete Research,
Vol. 11, 1981.

24) Litvan, G.G., Maclnnis, C, and
Grattan-Bellow, P.O., "Cooperative
Test Program for Precast Concrete
Paving Elements", Proceedings, First
International Conference on
Durability of Building Materials and
Components, Ottawa, Canada,
August, 1978.

25) Adkins, D.F. Laboratory Duplication
of Surface Scaling. Concrete
International Vol 8 No. 2
February, 1986

26) National Building Code of Canada,
1985.

27) Wood, J.H., Adams, J.R.
"Temperature Gradients in a
Cylindrical Concrete Reservoir",
Proceedings of the 6th Australasian
Conference on the Mechanics of
Structures and Materials,
Christchurch, N.Z. 1977.

28) Tuomioja, M., Jumppanen, P. and
Rechardt, T., "Jaan Lujuudcsta ja
Muodonmuutoksista" (The Strength
and Deformation of Ice),
Rakennustekniikka, 1973:1 pp
233-246.

29) Bergdahl, L., "Thermal Ice Pressure
in Lake Ice Covers", Report A2,
Department of Hydraulics, Chalmers
University of Technology, Sweden,
1978.

30) Reynolds, CE., "Reinforced
Concrete Designer's Handbook",
Concrete Publications Ltd., London,
1961.

31) Ghali, A., "Circular Storage Tanks
and Silos", Spon Ltd., London, 1979.

32) Portland Cement Association,
"Circular Concrete Tanks without
Prestressing".

33) Markus, G.Y., "The Theory and
Computation of Circular
Symmetrical Structures".

34) Hibbitt, Karlsson and Sorensen,
Inc., "ABAQUS - Structural and
Heat Transfer Analysis Program"

35) Schupack, M., 1982. Protecting
Post-tensioning Tendons in
Concrete Structures, Civil
Engineering - ASCE.

9 - 2

10.0

GENERAL REFERENCES

ACI Committee 344. 1970. Design and
construction of circular prestresscd '
concrete structures. ACI Journal,
September, pp/ 657-672.

ACI Monograph No. 3, 1966, "Freezing
and Thawing of Concrete - Mechanisms
and Control", Detroit.

Aldworth, G., Staton, R., (MacLaren
Engineers), and Slater, W.M., (W.M.
Slater & Associates Inc.). Preliminary,
(1985). "Freeze Protection for Above
Ground Concrete Water Tanks in Cold
Regions".

ASCE Civil Engineering. 1981. Structural
failures. New York, December, p. 44.

Beeby, A.W. 1978. Cracking: what are
cracking limits for? Concrete, London,
July, p. 3.

Campbell, T.I., and Kong, W.L., (Queen's
University). April, (1986). "Ice Loading
in Elevated Water Tanks".

Carpenter, C.H. 1982. Constructing and
maintaining distribution storage
structures. AWWA Journal, Vol. 74, No.
11, Denver, Col., pp. 581-583.

Cedolin, L., Poli, D., loro, I. 1983.
Experimental determination of the
stress-strain curve and fracture zone for
concrete in tension. Proceedings of the
International Conference on Constitutive
Laws for Engineering Materials,
University of Arizona, Tucson, Arizona,
pp. 393-398.

Code of practice for concrete structures
for the storage of liquids. 1978. Part I,
design based on "resistance to cracking"
approach, Wellington, N.Z.

Crowley, F.X. 1976. Maintenance
problems and solutions for prestresscd
water tanks. AWWA Journal, Denver,
Co., November, pp. 579-585.

Frederking, R., Sinha, N.K. 1977. Ice
action on wharf at Strathcona Sound.
Proceedings of Fourth International
Conference on Port and Ocean
Engineering under Arctic Conditions,
Memorial University of Newfoundland,
St. John's, pp. 707-717.

Gold, L.W. 1958. Some observations on
the dependence of strain on stress for
ice. Canadian Journal of Physics, Vol.
36, Ottawa, Ontario, pp. 1265-1275.

Gold, L.W. 1960. The cracking activity
in ice during creep. Canadian Journal of
Physics, Vol. 38, No. 9, Ottawa,
Ontario, pp. 1137-1 148.

Gold, L.W. 1965. The initial creep of
columnar-grained ice. Part I: Observed
behaviour. Part II: Analysis. Canadian
Journal of Physics, Vol. 43, Ottawa,
Ontario, pp. 1414-1422 and 1423-1434.

Gold, L.W. 1966. Observations on the
movement of ice at a bridge pier.
Proceedings of a Conference on Ice
Pressures against Structures at Laval
University, Quebec City, pp. 135-141.

Gold, L.W. 1966. Elastic and strength
properties of fresh-water ice.
Proceedings of the Conference on Ice
Pressures against Structures at Laval
University, Quebec City, pp.13-23.

Grieve, R. (Golder Associates). May,
(1984) and February, (1986).
"Temperature Monitoring, Ontario
Concrete Water Tanks".

10 - 1

Harper, W.B. 1982. Inspection, painting,
and maintaining steel water tanks.
AWWA Journal, Vol. 74, No. 11, Denver,
Col., pp. 585-587.

Hassett, R.W. 1980. Design and
construction of water storage facilities.
Strand Associates, Inc., Madison,
Wisconsin.

Hertzberg, L.B., Westerback, A.E. 1976.
Maintenance problems with wire-wound
prestressed concrete tanks. AWWA
Journal, Denver, Col., December, p.44.

Krajcinovic, D., Selvaraj, S. 1983.
Constitutive equations for concrete.
Proceedings of the International
Conference on Constitutive Laws for
Engineering Materials, University of
Arizona, Tucson, Arizona, pp. 399-406.

Krausz, A.S. 1963. The creep of ice in
bending. Canadian Journal of Physics,
Vol. 41, No. 1, Ottawa, Ontario, pp.

167-177.

Krausz, A.S. 1966. Plastic deformation of
fresh-water ice. Proceedings of the
Conference on Ice Pressures against
Structures at Laval University, Quebec
City, pp. 5-12.

Krausz, A.S. 1981. Adfreeze strength of
model piles in ice. Canadian Geotechnical
Journal, Vol. 18, No. 1, pp. 8-16.

Kronen, H., Anderson, J.H. 1982.
Concrete exposed to cryogenic
temperature, Nordisk betong, Norway, pp.
13-17.

Larrabee, R.D., Billington, D.P., Abel,
J.F. 1974. Thermal loading of thin-shell
concrete cooling towers. ASCE Journal,
Structural Division, December.

Mackenzie, K., (Dalhousie Materials),
Slater, W.M., (W.M. Slater & Associates
Inc.) and McGrenere, P., (Knox Martin
Kretch) (Editing). Preliminary, (1985).
"Evaluation of Waterproof Coatings for
Concrete Water Tanks".

Perkins, P.H. 1978. Concrete structures:
repair, waterproofing and protection.
Reprinted. Applied Science Publishers
Ltd., Barking, Essex, England, 302 p.

Pitkanen, A. c. 1980. Roihuvuori water
tower, Helsinki, Finland. Publication
unidentified, 2 p.

Priestley, M.J.N. 1976. Ambient thermal
stresses in circular prestressed concrete
tanks, ACI Journal, October.

RILEM (Reunion internationale des
laboratoires d'essais et de recherches
sur les matériaux et les constructions).
1981. Materials and structures.
Properties of set concrete at early
ages-state-of-the-art report. Vol. 14,
No. 84, Paris, France, pp. 399-449.

Schupack, M. 1981. How to inspect and
evaluate prestressed concrete tanks.
Public Works, September, pp. 88-89.

Schupack, M., Suarez, M. 1981. Some
recent corrosion embrittlement failures
of prestressing systems in the United
States. ACI Journal, Vol. 27, No. 2,
Chicago, Illinois, pp. 38-56.

10 - 2

Slater, W.M. 1975. Stage Timoshcnko, S., Woinowsky-Krieger, S.

post-tensioning-a versatile and economic 1959. Theory of plates and shells,

construction technique. PCI Journal, Vol. McGraw-Hill.

20, No. 1, Chicago, Illinois, pp. 14-27.

Vessey, J.V., Preston, R.L. 1978. A
Slater, W.M. & Associates Inc. 1982. critical review of code requirements for

Study of concrete elevated water tanks. circular prestressed concrete reservoirs.

Part 1, Analysis and evaluation of FIP 8th Congress, London,

defects using a numerical coding system.
Part 2, Interim guideline
recommendations. Report 3 (for review).
Project WTS-001, Toronto Ontario,
111 p.

10 - 3

11.0

APPENDIX A

INTERIM GUIDELINE RECOMMENDATIONS OF MINIMUM REQUIREMENTS
FOR THE DESIGN AND CONSTRUCTION OF NEW ABOVE GROUND CONCRETE
WATER TANKS IN ONTARIO

11.1 Introduction

This report has concluded that under
high pressure, the permeability of normal
structural concrete is insufficient by
itself, to eliminate the deterioration of
the walls of a concrete water tank
indefinitely, in the environmental condi-
tions prevailing in Ontario. It is,
therefore, essential that either the
concrete is prevented from attaining a
high degree of saturation, or that cyclic
freezing and thawing is eliminated.

Based on the experience gained to date,
reduction of severe freezing and thawing
by means of external insulation is the
more positive protection. It is less
dependent on the site and seasonal
conditions, and is less costly than the
application of waterproof coatings and
steel liners. A second, but important
benefit is the elimination of the internal
formation of ice round the tank walls.
In addition, small deviations from
specification do not have radical
consequences to the expected life of the
tank. Since the insulation is external to
the tank structure, it can be rectified
without undue interruption to normal
operations. Conversely, protecting a
tank by means of an internal coating
only, is less assured, since it is difficult
to install, may have a limited life, and
will not prevent the internal formation
of ice. Internal repairs result in costly
delays to normal operations.

The following interim guidelines will
result in concrete water tanks which
have higher initial, but reduced
maintenance costs than have been

experienced. It is the intention of the
guidelines to recommend standards
which will ensure the construction of a
tank which will not deteriorate during
its expected service life of fifty (50)
years (minimum), and should not require
costly maintenance. It is recommended,
therefore, that alternate designs should
be compared on the basis of initial and
maintenance costs.

11.2 Scope

These recommendations are not
"stand-alone" and are limited to the
primary design and construction
requirements of new concrete water
retaining structures only, and exclude
details of appurtenances and the
system, except where noted.

11.3 References

The recommendations are additional to
and take precedence over the
requirements of the following listed
codes, standards, and references:

• Province of Ontario
Building Code. O. Reg. 419/86
August 6, 1986.

• The National Building Code of
Canada (1985).

• Supplement of NBC (1985).

• CSA Standard CAN3-A23.1-M77,
CAN3-A23.2-M77

Concrete Materials and Methods of
Concrete Construction
Methods of Test for Concrete.

11 - 1

CSA Standard CAN3-A23.3-M84.
Design of Concrete Structures
for Buildings.

ACI Standard 318-83 Building Code
Requirements for Reinforced
Concrete.

ACI Committee 344 Report on the
Design and Construction of Circular
Prestressed Concrete Structures
(1970).

ACI Committee 350 Report on
Concrete Sanitary Engineering
Structures (1977).

AWWA Standard for Welded Steel
Tanks for Water Storage
ANSI/AWWA DlOO-79.

Ministry of Environment Standard
Specifications.

11.4 Design

11.4.1 Design Philosophy

In addition to the strength requirements
of the specified codes, standards and
references listed in section 11.1.3 for
Applied Loads, design for the following
conditions shall be carried out:

• Environmental loads (ice, thermal
differential, etc.)

• Durability for the actual freeze-thaw
environment.

• Provide a service life goal with
minimum maintenance, of fifty (50) years
(minimum).

11.4.2 Tank Design Requirements

.1 Insulation

Concrete tanks shall be protected from
freezing and shall be insulated and clad
on the exterior to reduce heat loss and

prevent significant internal ice
formation.

.2 Ice Prevention

• An ice protection system shall be
designed to prevent the tank water
from freezing during the worst winter

conditions anticipated during the
service life of the structure (50 years).

• Actual tank heat loss is dependent
on the quality of the construction and

air tightness of the insulation,
therefore, heat loss calculations shall
be made assuming a minimum tank
water temperature of 2°C, unless more

accurate data is available and based
upon temperature monitoring results of
similar systems and materials.

.3 Inspection

The cladding and insulation shall be
demountable to allow exterior
inspections and maintenance.

.4 Mixing/Heating

• Mixing and booster heating of the
water in some tanks with low turnovers
may be necessary to supplement the
heat loss reduction from the insulation
for short periods during severe winters.

• The provision of heated air in the
air gap between the insulation and the
concrete is an alternative to the above

where it is important that the concrete
surface temperature does not fall below
0°C.

.5 Prestressed Construction

All concrete water tanks shall be of
prestressed construction.

.6 Prestress Design

Concrete water tanks shall be
post-tensioned in the vertical and
horizontal directions with the following

11-2

reserve compression after service loads
are considered:-

o Reserve Compression (Hoop)

The minimum final (after losses) reserve
hoop (circumferential) prestrcss shall be
1.5 MPa (214 psi).

• Reserve Compression (Vertical)

The minimum final (after losses) reserve
vertical prestress shall be .5 MPa (71
psi).

• Post-tensioning

Post-tensioning shall be carried out
using internally bonded (grouted)
tendons, or external tendons and
hardware permanently protected against
corrosion.

.7 Concrete Wall Thickness

The minimum thickness of the concrete
wall shall be 200 mm (8 in.).

.8 Waterproofing

Concrete water tanks shall be maintained
permanently leaktight using impermeable
concrete, bonded waterproof coatings,
steel liners or other liners of
impermeable material proven to be
durable in chlorinated water.

.9 Coatings

• Waterproof coatings shall be 100 per
cent solids epoxies or a modified
cementitious slurry with fabric and shall
be applied to all concrete in contact
with water.

Epoxy coatings shall be applied on dry
concrete in a minimum of 3 coats (each
coat 6 mils minimum) on top of
surfacing mortar when the original
surface exceeds a critical roughness or
exhibits too many surface pin holes.

• The permeability of the concrete for
walls to be waterproofed with bonded
coatings shall not exceed 2.5 x 10"^^

m/s tested at 90 days.

.10 Liners

• Where plain or stainless steel is
used as an internal waterproof liner it

shall be 5 mm (3,16 inch) minimum
thickness. Where plain steel is used, it
shall be protected on the face in
contact with the water by an MOE
approved single component epoxy or
vinyl paint system.

• The space between the liner and the
concrete wall shall be filled with an
MOE pre-approved cementitious grout
having a minimum pH value (hydrogen
ion value) of 12.

11.5 Construction

11.5.1 Concrete Quality

• The concrete mix, wall design, and
construction shall result in a crack free
and leaktight wall. No cold joints will
be permitted without waterstops being
installed.

11.5.2 Slip Forming

• Jack-rod pipes used for jacking up
the forms shall be removed and the
wall void formed by the pipes grouted
up from the bottom to the top, or the
pipe sections shall be completely

grouted individually from bottom to top.

• Locations of the jack-rods shall be
recorded on as-built shop drawings.

• Details of grout vents and methods
of grouting jack-rods shall be approved
by the authority before construction of
the tank.

• Horizontal micro-cracking caused by
binding of the slipform shall be
prevented, or if they occur they shall

11 - 3

be repaired by routing out and tilling
with epoxy mortar.

11.5.3 Jump Forming

• Pour iieights shall not exceed
2.5m (8 ft. 2 inches).

• Form vibrators clamped to the forms
shall be used in addition to internal
vibrators.

• Waterstops shall be installed at the
top (and bottom) of each lift.

11.5.4 Vertical Waterstops

• Only plain, de-greased, mild steel
waterstops shall be used.

• Minimum dimensions shall be 200 mm
(8 ins.) for the base and 150 mm

(6 ins.) high elsewhere, and 6 mm
(1/4 in.) thiclt.

• Vertical joints in steel waterstops
shall be continuously lap welded (fillet).

11.5.5 Concrete Joint Preparation

• Construction joints shall be
sandblasted to remove all laitance,
before placing fresh concrete.

11.5.6 Concrete Curing

11.6.2 Final Inspection of Structure

• An external maintenance inspection
to prove that the tank is leaktight and
sound, and an internal inspection to

prove that the waterproof coating or
liner is satisfactory, shall be made
after the tank has been in continuous

service for a period of between 9 and
12 months.

• Any deficiencies shall be repaired
and the tank re-inspected after a

further equal period in
service.

11.6.3 Heat Loss and Ice Prevention

• External ambient temperatures,
temperatures in the air gap between
the insulation and the wall, and the
tank water temperatures shall be
measured during an extreme cold period
of the first winter of tank operation.

• Verify that the insulation is airtight,
and that the freeze protection system

is functioning satisfactorily, according
to the design and heat loss
calculations.

• Sensors installed in the locations
described above are recommended for
temperature monitoring and control of
water temperature.

• Continuous wet curing and covering
of the concrete with burlap or tarpaulins
shall be carried out for a minimum of 7
days after stripping the forms.

11.6 Quality Assurance And Tank
Performance Testing

11.6.1 Leakage Testing

• A 3 day leakage test shall be carried
out and any leaks or damp spots
repaired and retested, if necessary,
before applying waterproof coatings or
exterior insulation.

• Inspect top surface of water at wall
in late winter to observe that
significant ice formation(s) does not
exist.

11.7 Security And Safety

11.7.1 Security Fence

• Tanks should be enclosed within a
perimeter security fence to prevent
vandalism to the cladding, and
insulation (fire), climbing by
unauthorized persons, etc., and to
provide protection from falling objects.

ice, etc., and during maintenance or
repair operations.

11.7.2 Safety

• New concrete water tanks should be
located at a minimum distance of twice
(x 2) their height from public
thoroughfares or at least three times (.x
3) their height from occupied buildings.
This will increase the safety of the
public during demolition or during a
major disaster causing collapse or
toppling of the tank.

11.8 Miscellaneous .And .Appurtenances

• Aluminum. Permanent aluminum
ladders shall not be used in contact
with chlorinated water. Use
galvanized steel or fibre glass.

• Inlet/Outlet Pipes. Separate inlet
and outlet pipes to provide some
natural mixing of the tank water.

12.0 APPENDIX B

DILATION EXPANSION OF SPHERICAL REGION WITHIN A LARGER SPHERE

A confined saturated spot dilating under
freezing will result in a considerable
internal pressure which can cause
fracture of concrete. The magnitude of
resulting stresses will strongly depend
on geometrical dimensions and the shape
of a saturated zone. A variety of
qualitative effects and magnitudes of
stresses caused by expansion of confined
zones can be investigated by considering
expansion of a spherical region confined
within a sphere of a larger diameter.
Due to the symmetry of the problem, an
analytical solution for stresses and
deformations is possible.

It is physically clear that the expansion
of an inner sphere within a larger
sphere will result in a compressive
lateral stress accompanied by tensile
hoop stresses. Maximum tensile stress
will be at the interface of the dilating
and non-dilating zones. If the tensile
stress is large enough, brittle fracture
will be initiated in concrete. Further
increases in the dilation will cause
fracture propagation. Due to the
symmetry of the problem, the fractured
zone can be assumed to be spherical. In
general, there could be three different
zones (Figure 12.1):

• Dilating zone - radial and hoop
compression (zone A)

• Fracture zone - radial compression,
zero hoop stress (zone B)

• Elastic zone - radial compression,
hoop tension (zone C)

Stresses and displacements in each of
the above zones can be determined by
solving equations of static equilibrium
and geometrical compatibility. The
location of the interface between
fractured and elastic zones can be
found from the condition that the
tensile strain in the elastic zone docs
exceed the limiting tensile strain at
fracture. Figure 12.1 illustrates various
features of the solution.

Pressure at the interface of the dilating
and non-dilating zones is proportional
to dilatant strain and increases with
the thickness of the non-dilating
material covering the dilating region
(Figure 12.1a).

Fracture at the interface is initiated at
a certain dilatancy (Figure 12.1b). The
more confined the dilating zone is, the
larger the dilatancy required to initiate
fracture.

Further increase in dilatancy causes
fracture propagation. When dilatancy
reaches a certain "critical" value, the
process of fracture propagation becomes
unstable and the entire sphere is
ruptured (Figure 12. Ic).

The dilatant strain which causes the
rupture increases with the thickness of
non-dilating material (Figure 12. Id).

12 - 1

A- RADIUS OF DILATING ZONE
B - RADIUS OF ELASTIC ZONE
C- RADIUS OF FRACTURED ZONE
Ed-LINEAR STRAIN DUE TO DILATANCY

et-LiMrriNG tensile strain

Ec- YOUNG'S MODULUS

P- PRESSURE AT INTERFACE OF DILATING
AND FRACTURED ZONES

/" PRESSURE

08
0 7

- / B£FOF?E INITIATION
( OF FRACTURE

-^ 0.6
EcEd

■ Cé^'

Û4

. ' \ — ^

1 1 1 1 1 I J

15
B/A

DILATANT STRAIN
TO INITIATE
FRACTURE

PROFOGATON

B/A = 8

OF FRACTURE

ZONE

/ ^,^ UNSTABLE

INSTABLE "^

B/A = 6

Figure 12.1 Rupiuve of a brittle sphere due to dilation

12 - 2

13.0

APPENDIX C
TERMS OF REFERENCE

The terms of reference included as Appendix 'A' in the agreement between the
Ministry of the Environment and W.M. Slater & Associates Inc. for "The
Investigation of Elevated Concrete Water Retaining Structures" dated June 24, 1981
were as follows:

1) Inspect approximately 30 concrete
water retaining structures in all six
MOB Regions to establish their
condition, define deficiencies,
recommend remedial measures and
develop standard methods for repairs
where warranted.

2) Study available material on record for
each structure inspected and relate
the findings to the condition of the
structure at the time of inspection.

3) Prepare a report on each structure
investigated and inspected including
the method of effecting permanent
repairs together with associated
costs.

4) Make necessary arrangements for
local co-ordination of investigations
including testing of materials and any
other analyses.

5) Prepare design and maintenance
guidelines including standard
methods of repairs and inspections.

6) Record all findings and submit a
"General Study Report"in a format
suitable for general distribution.

7) Identify structures requiring urgent
repairs before the onset of winter
1981 and recommend courses of
action to protect such structures.

8) If required, prepare specifications
for tendering repairs and perform
the supervision of construction
related thereto.

9) Act as expert witness for the
Crown, if required.

Explanatory Note

In response to items 4, 5, 6, 7 and 8 in the above terms of reference, the report on
"Immediate Research Needs" dated October 20, 1982 was prepared. Funds were
subsequently made available to carry out applied research in the areas of structural
effects of freezing, ice (freezing) prevention, and leakproof ing. This report records
the observations made and the applied research carried out in the area of freeze-
thaw failure mechanism. It was later decided to expand the terms of reference to
include sections on the deterioration of metals, and the repair of concrete tanks.

Provincially owned at the time of
construction

13 - 1

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