ROAD RESEARCH LABORATORY

Ministry of Transport

RRL REPORT LR 247

CREEP OF CONCRETE PIPES UNDER LOAD

by

J.. W.. Grainger

Road Research Laboratory Crowthorne, Berksh ire

1969

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on ! st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Results obtained in the pilot experiment at Christchurch

3. Tests on pipe rings

4. Results

5. Conclusions

• P a g e

1

" 2

• 2

3

•

5

6. Discussion of results 6

7. Acknowledgements

8. References 6

" ~ . , ~ i ~ • ~ . " . . ~ ' .

CROWN COPYRIGHT 1969 Extracts from the text may be reproduced

provided the source is acknowledged

CREEP OF CONCRETE PIPES UNDER LOAD

ABSTRACT

When 36-in (914-mm) diameter Class I concrete pipes were buried

4 ft. (1.22 m) below the surface of a representat ive road consisting

of 4 in (102 mm) of bitumen macadam on 9 in (230 ram) of hardcore

on 3 in (76 ram) of clinker, the circumferential tensi le strains

measured in the pipes on completion of the road were 33 and 35

microstrains at the top and bottom respect ively. These strains

increased by about 40 microstrains at the top of the pipe and by

30 microstrains at the bottom during the next seven weeks during

which the road was not open to traffic.

During the seven weeks, no change occurred in the strains

recorded in 12-in (305-mm) glazed vitreous clay pipes or in 36-in

(914-mm) cast iron pipes buried to the same depth below the road.

Although it seemed certain that creep of the concrete pipes under

the backfill loading could account for some increase in the circum-

ferential strain, it was thought that the large increases recorded

threw doubt on the readings. An experiment has now been made

to determine the magnitude of the creep occurring in a concrete

pipe under prolonged loading and to see whether it is affected by

the age of the concrete when the pipe is loaded.

In these tests, two 36-in (91~mm) concrete pipe rings were

loaded when the concrete was a month old and two rings were

loaded when the material was seven months old. Loads of 600 Ib

(272 kg)were applied to each ring and held constant for six months.

With the rings loaded when the concrete was a month old, the creep

strain resulting from six months of loading was 56 microstrains and

thus was 9..8 times the elastic strain of 20 microstrains resulting

from initial application of load. With the rings loaded when the

concrete was seven months old, the creep strain resulting from six

months of loading was 15 microstrains and thus approximately equal

to the initial elastic strain of 16 microstrains. The initial elastic

strains were, therefore, smaller than those measured in the pipes

under the road, but the creep strains in the rings loaded when the

concrete was a month old were larger than those recorded in the

road. The results of the tests on the pipe rings thus support those

obtained in the earlier road experiments. They emphasise that

concrete pipes should not be used for instrumental purposes when

studying the increase of backfill pressure under a road. Such tests

are best done with metal pipes which do not exhibit creep under

load.

I. INTRODUCTION

The Ministry of Housing and Local Government's Working Party on the Design and Construction of

Underground Pipe Sewers organised a pilot experiment at the Military Engineering Experimental

Establishment, Christchurch, Hants 1. The objective was to investigate the repetitive loads which,

when applied to the surface of a typical road, cause failure of representative underground sewer

pipes. The Road Research Laboratory was responsible for much of the instrumentation.

After the pipes had been laid in their trenches, and the backfilling and road construction were

completed, seven weeks elapsed before the loading tests began. Ouring this time a 12-in (305-mm)

diameter glazed vitreous clay pipe and 12-in (305-ram) and 36-in (914-mm) diameter east iron pipes

showed only a negligible increase in the circumferential strains measured at the top and bottom of

the pipe, but the strains measured in two 36-in (914-mm) diameter concrete pipes increased consider-

ably. The changes of strain recorded in the concrete pipes tended to throw doubt on the measure-

ments of strain. However, there was the possibility that the increase of strain was due to creep of

the concrete under the loading provided by the backfilling and the road construction. An examination

of the literature on creep in concrete indicated that creep might be more severe in new concrete

pipes than in those which have been stored for some time before use. To obtain ,information on these

points, an experiment has been made at the Road Research Laboratory to determine the magnitude of

the creep likely in a concrete pipe and to see whether it is affected by the age of the pipe when the

load is first applied.

2. RESULTS OBTAINED IN THE PILOT EXPERIHENT AT CHRISTCHURCH

Strain-measuring equipment was attached to two 36-in (914-mm) diameter concrete pipes which were

between four and five months old when tested. Electrical resistance strain gauges of the foil type,

2 in (51 mm) long and of 120 ohms resistance were cemented along the top (the crown) and bottom

(the base) of each pipe to measure the circumferential strains in the inner surface at right angles

to the pipe axis. There were five gauges along the crown and five along the base of each pipe.

Along each line of gauges one gauge was attached close to each end of the pipe and the remaining

three gauges were equally spaced along the pipe. All of these gauges recorded tensile circum-

ferential strains. To compensate for temperature effects, each of the " l ive" gauges attached to the

pipe was electrically connected with a matching "dummy" gauge attached to a block of similar

material placed within the pipe. Full details of the instruments were given in the report describing

the pilot experiments 1.

2

The resul ts of the measurements of tensile strain mad~ when the road construction was

finished and after an interval of seven weeks are summarised in Table 1. This table shows that the

creep at the crown of the pipe was 40-42 microstrains (average of the three readings taken on the

central gauges) while that at the base was 27-30 microstrains. There was variation in the initial

values of the tensile strain, but the average value for the central portion of the pipe was 33 micro-

strains along the crown and 35 microstrains along the base. In calculating this latter average, the

very large value of 102 microstrains was omitted.

3. TESTS ON PIPE RINGS

In the tes ts described in the present report, strain and deflection measurements were made on four

concrete pipe rings each 36 in (914 ram) diameter, and 24 in (610 mm) long. The rings were made at

the same works as the pipes used in the pilot experiments. They were considered to be of standard

Class I quality and did not contain steel reinforcement. The average wall thickness was 2.40 in

(61 ram). The pipes were made under controlled conditions to ensure uniformity of the concrete mix

and of the manufacturing technique, and to make them comparable in quality with the pipes used

in the pilot experiments. The rings were allowed to cure for a fortnight in the maker's stockyard and

were then Iransported to the Laboratory and stored on their sides in a temperature- and humidity-

controlled building. The loading tests were made in this building but the control was not so good

as anticipated owing to severa l mechanical failures of the newly installed air-conditloning plant

during the prolonged loading tes ts .

One month after manufacture, two of the rings were set in position as shown in Fig. 1 and dial

gauges reading to 0.001 mm were installed to measure changes of the vert ical diameter under load.

Electrical res is tance strain gauges of the foil type (2 in (51 ram) long and 120 ohms resis tance) were

attached to the inner surface of the pipe ring to measure the internal circumferential strains at the

crown, base and the two sides of each ring. As in the pilot experiments, each strain gauge was

connected electr ical ly with a "dummy" gauge cemented to a piece of concrete placed within the pipe

ring to provide temperature compensation. All of the strain gauges were covered with a layer of

cotton wool to minimise temperature flectuations. After the rings were instrumented, they were

left for two days to minimise the effects of any strains result ing from the weight of the pipe material.

It was estimated that a load of 600 lb (272 kg) applied to the top of each pipe would produce

a circumferential strain of the order of 20 microstrains. The load was applied to each r ing by hand-

loading with 56-1b (25.4-kg) weights - see Fig. 1. This was, in fact, the greatest load which could

be applied to the top of the pipe without difficulty and without constructing special loading apparatus.

Measurements on the resis tance strain gauges showed that tensile strains of about 20 microstrains

were developed at the crown and base of each of the two rings loaded one month after the date of

manufacture, and compressive strains of similar magnitude were recorded at the sides. These

values of strain compare reasonably well with the average values of 33 and 35 microstrains obtained

respectively at the crown and base of the concrete pipes in the pilot experiments at the completion

of the road construction.

3

Although the resistance strain gauges were satisfactory for recording the strain resulting from

the first application of the load of 600 lb in the tests on the pipe rings, they were found unsuitable

[or watching the progress of creep over long periods of time. They gave erratic readings due, it is

thought, to air currents which caused local temperature fluctuations around them. Because of this,

the long term behaviour of each pipe ring was determined by measuring the changes of the vertical

diameter by means of the dial gan.ge. Where strains were required for comparison with those

developed in the pilot experiment, they were deduced from the initial measurements of strain and the

diametral changes of the pipe rings.

The deflection gauges were read at intervals over a period of six months from the time of

applying the 600-1b (272 kg) load to each pipe ring. At the end of this loading period, each ring was

unloaded and the readings of deflection were continued for a further two months in order to watch any

recovery which might occur. When the loads were removed from the first two rings, the other two

rings were subjected to similar loading and the series of measurements was repeated. The results

obtained are shown in Fig. 2 and summarised in Table 2.

4. RESULTS

The Fig. 2 shows that all of the rings gave similar curves relating diametral change with time.

There was an elastic deflection when the load was applied, a gradual reduction of the ring diameter

during the six months under load, elastic recovery when the load was removed, a very small

additional recovery during the two months following load removal, and a permanent set. The elastic

deflection resulting from application of the load was approximately equal to the recovery accompany-

ing its removal, and this occurred even with the two rings which were tested a month after manu-

facture and at a time where their' strer~gth and elastic modulus would be increasing with age to a

significant extent.

Because the elastic modulus of concrete increases with age during the first few months of its

life, the elastic deflections resulting from applying the 600-1b load to the rings which were seven

months old were smaller than those of the rings loaded a month after manufacture, the difference

being about 25 per cent.

The diameter of the rings loaded a month after manufacture diminished on the average by

208prn during the six months under load. This creep was nearly four times that which occurred

with the rings which were seven months old when tested (56/zm). In the pilot experiments, creep

was recorded over a period of seven weeks, so, for reasons of comparison, deflections at a cor-

responding time after application of load have been read off the curves given in Fig. 2. During the

seven weeks of loading, the rings loaded a month after manufacture crept by 77 per cent of the

amount recorded during the six months under load, whereas those loaded seven months after manu-

• facture crept by only 52 per cent of the six months' loading value.

The circumferential strains in the rings would be expected to vary in proportion to the change

in vertical diameter. Measurements on the first pair of pipes showed that a reduction in pipe

4

diameter of 75#m accompanied circumferential tensile strains at the top and bottom of the pipe of

20 mierostrains. The ratio between hoop strain and diametral change has been used to convert the

remaining diametral changes to approximate strains in order to compare the results with those

obtained in the pilot experiment. Strains estimated in this way are given in Table 3 below:-

TABLE 3

Estimated strains developed in 36-in diameter concrete pipe rings loaded by 600 lb.

Elastic strain resulting from application

of load

Creep strain resulting from maintenance of

load for six months

Total strain at end of six months under

load (= elastic strain + creep)

Elastic recovery when load was removed

Residual strain immediately after removal

of load

Additional recovery during interval of

two months after load removal

Creep strain resulting from maintenance of

load for seven weeks

Total strain after seven weeks under load

Circumferential tensile strain (in micro-

strains) at the crown or base of pipe rings

loaded by 600 lb at an age of

1 month

20

56

76

21

55

1.6

43

63

7 months

16

15

31

17

14

X,3

8

24

In the pilot experiment at Christchurch, the initial strain within the concrete pipes was about

36 microstrains and increased to 65 microstrains at the base of the pipe and to 75 mierostrains at the

crown during the seven weeks which elapsed between completion of the road and the start of the

loading tests. These initial strains were somewhat greater than those recorded with the pipe rings,

but the creep produced by seven weeks of loading was comparable with that produced in the pipe

rings loaded a month after manufacture.

S. CONCLUSIONS

(1) Concrete pipes exhibit creep under load even when the initial elastic strains are fairly small.

(2) Less creep occurs with pipes which are some months old when loaded than with pipes which

are loaded soon after manufacture.

(3) The creep strains recorded in the tests on the pipe rings were comparable with those recorded

in the pilot experiment and there is, therefore, no reason to doubt the results obtained at

Christchurch.

(4) The results indicate that it would be unwise to employ concrete pipes in field-scale experi-

ments designed to study the increase of backfill pressure and resultant strain under prolonged

trafficking of a road. Such tests are best done with metallic pipes which neither creep under

load nor shrink and swell under moisture changes.

6. DISCUSSION OF RESULTS

The results show that greater amounts of creep under load occur with new concrete pipes than with

those which are of reasonable age when loaded. However, storage for as long as six or seven months

prior to loading does not eliminate the risk of creep altogether. The storage period is likely to be a

complicating factor in any field-scale experiments on concrete pipes and also in the application of

the resultant data to practice. It is certainly unlikely that pipe manufacturers will wish to store a

considerable number of concrete pipes for long periods in order to minimise the effects of creep.

Because of the loading arrangement employed, it was not possible to add greater loads than

600 lb (272 kg) to each pipe ring and thus determine the load causing failure or examine the

relationship between creep and applied load. Such an investigation appears desirable and should

be included in the research programme being prepared by the Working Party.

7. ACKNOWLEDGEMENTS

This work was carried out in the Structural Properties Section under the direction of Dr. A. C.

Whiffin and the author was assisted by Mr. J. N. Neal.

8. REFERENCES

. HAINSWORTH, I. H., E. P. POTOCKI, J. J.. TROTT and O. C. YOUNG 'Pilot experiments to

determine the loads causing failure of sewer pipes under roads'. Institutions of Civil and

Municipal Engineers, Joint Meeting on 23rd January, 1967.

Printed at the Road Research Laboratory, Crowthorne, Berkshire, England

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TABLE 2

Results of creep tes ts on 36-in diameter concrete pipe rings

Ref.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Reduction of vert ical diameter of

ring due to application of load

Rings 7 months old

when loaded by

600 lb

Rings 1 month

old when loaded

by 6OO lb

Mean Ring Ring Ring Ring

A B A i B 1 1 2 2

Mean

68 81 75 57 61 59

Total reduction of diameter after 236 240 238 85 91 88

7 weeks under load

= (h) - (a) = Reduction of diameter 168 159 163 28 30 29

due to creep under 7 weeks of

loading

Total reduction of diameter at 9.91 275 283 108 121 115

end of 6 months under load

= (d) - (a) = Reduction of diameter 223 194 208 51 60 56

due to creep under 6 months of

loading

Total reduction of diameter 203 200 202 47 52 50

remaining after removal of load

= (d) - (f) = Increase of diameter

due to removal of load

Total reduction of diameter

remaining 2 months after removal

of load

88 75 81 61 69 65

197 194 196 42 47 45

6 6 5 5 = (f) - (h) Recovery 2 months

after load removal

Elast ic deflection on applying load

Creep during 6 months under load

Elast ic recovery when load removed

Additional recovery during 2 months after

load removal

Percentage of creep which occurred during

first 7 weeks after loading = 100 x (c)./(e)

75

208

81

77

59

56

65

52

8 All diametral changes are in pm

Static toad composed of standard 56tb. wts.

Crown of pipe Rubber pads

",~ Dial gauge

Tubu|ar support cemented to .pipe

))

Base of pipe

-4----12 in. channet iron

Fig. 1. HETHOD OF LOADING PIPE RIHGS AND RECORDING DEFLECTION

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ABSTRACT

Creep of concrete pipes under load: J. W. GRAINGER': Mini- stry of Transport, RRL Report LR 247: Crowthome, 1969 (Road Research Laboratory). When 36-in (914-mm) diameter Class I concrete pipes were buried 4 ft. (1.22 m) below the surface of a representative road consis t ing of 4 in (102 ram) of bitumen macadam on 9 in (230 ram) of hardcore on 3 in (76 ram) of clinker, the circumferential tensi le strains measured

in the pipes on completion of the road were 33 and 35 micro- strains at the top and bottom respectively. These strains in- creased by about 40 microstrains at the top of the pipe and by 30 microstrains at the bottom during the next seven weeks during which the road was not open to traffic.

During the seven weeks, no change occurred in the strains recorded in 12-in (305-ram) glazed vitreous clay pipes or in 36-in (914-ram) cast iron pipes buried to the same depth below the road. Although it seemed certain that creep of the concrete pipes under the backfill loading could account for some increase in the circumferential strain, it was thought that the large increases recorded threw doubt on the readings. An experiment has now been made to detenlline the magnitude of the creep occurring in a concrete pipe under prolonged load- ing and to see whether it is affected by the age of the con- crete when the pipe is loaded.

In these tests , two 36-in (914-mm) concrete pipe rings were loaded when the concrete was a month old and two rings were loaded when the material was seven months old. Loads of 600 lb (272 kg) were applied to each ring and held constant for six months. With the rings loaded when the concrete was a month old, the creep strain resulting from six months of load- ing was 56 microstrains and thus was 2.8 times the elast ic strain of 20 microstrains resulting from initial application of load. With the rings loaded when the concrete was seven mon- ths old, the creep strain resulting from six months of loading was 15 microstrains and thus approximately equal to the in- itial elastic strain of 16 microstrains. The initial e last ic

strains were, therefore, smaller than those measured in the pipes under the road, but the creep strains in the rings loaded

when the concrete was a month old were larger than those re- corded in the road. The results of the tes ts on the pipe rings

thus support those obtained in the earlier road experiments.

T h e y emphasise that concrete pipes should not be used for

instrumental purposes when studying the increase of back- fill pressure under a road. Such tests are best done with metal pipes which do not exhibit creep under load.