TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment Department of Transport

TRRL LABORATORY REPORT 864

EFFECTS OF MOISTURE CHANGES ON FLE×URAL AND FATIGUE STRENGTH OF CONCRETE

by

J W Galloway CEng MIMechE, H M Harding CEng MIERE and K D Raithby BSc CEng MRAeS

Any views expressed in this Report are not necessarily those of the Department of the Environment or of the Department of Transport

Pavement Design Division, Highways Department Bridge Design Division, Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1979 ISSN 0305-1293

CONTENTS

Abstract

Part 1 - Moisture condition at the time of test for water-cured concrete

1.1 Introduction

1.2 Design and preparation of concrete specimens

1.3 Test procedure

1.4 Test results

1.4.1 Moisture loss during drying

1.4.2 Flexural strength test results

1.4.3 Fatigue tests

1.4.4 Dynamic modulus of elasticity

1.4.5 Equivalent cube tests

1.5 Summary and conclusions - Part 1

Part 2 - Effects of different methods of curing

2.1 Introduction

2.2 Test procedure

2.2.1 Preparation of test specimens

2.2.2 Curing coladitions

2.2.3 ,Density and length changes

2.2.4 Flexural strength and fatigue tests

2.2.5 Subsidiary tests

2.3 Results

2.3.1 Density and length changes

2.3.2 Flexural strength test results

2.3.3 Fatigue performance

2.3.4 Modulus of elasticity and stress-strain curves

2.3.5 Equivalent cube tests

2.4 Conclusions - Part 2

3. Acknowledgements

4. References

© CROWN COPYRIGHT 1979 Extracts from the text may be reproduced, except for

commercial purposes, provided the source is acknowledged

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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.

EFFECTS OF MOISTURE CHANGES ON THE FLEXURAL AND FATIGUE STRENGTH OF CONCRETE

ABSTRACT

Results are given of flexural strength and fatigue tests on small unrein- forced beams of pavement quality concrete to study the effects of various moisture conditions. Subsidiary tests included dynamic modulus of elast- icity and equivalent cube crushing tests on the fractured specimens. The Report is in two par t s : -

Part 1 Effects of moisture condition at the time of test. Beams were cured under water for 26 weeks and then tested either wet, surface-dry or oven-dried.

The highest flexural and fatigue strengths were obtained with oven- dried specimens and the lowest with specimens that were air-dried for seven days. There was no correlation between these results and the corresponding equivalent cube strengths, where the effects of air-drying were reversed.

Part 2 Method of curing. Beams were cured for 26 weeks under various combinations of immersion in water, air-drying, fog room and surface coatings.

Immersion in water for one to four weeks followed by air-drying gave the highest flexural and fatigue strengths. Specimens which were air-cured for the whole period were significantly weaker. There was little correlation between compressive strength and flexural strength related to curing con- ditions. The importance of considering moisture states that may occur in practical structures under service conditions is emphasised, as is the need for standardising moisture conditions for research tests.

PART 1 - MOISTURE CONDITION AT THE TIME OF TEST FOR WATER-CURED CONCRETE

1.1 Introduction

A re.view of published information on the fatigue properties of plain concrete in 19681 commented on the difficulty of drawing broad conclusions because of the diversity of types of specimen tested and the variety of test methods used. A comprehensive programme of fatigue tests on plain concrete was then started to inv- estigate the effects of certain time-dependent parameters on flexural strength and fatigue performance to pro- vide a basis for the assessment of the long-term behaviour of concrete structures subjected to live loads. (Some effects of rate of loading are covered'in reference 2.) The work was originally aimed at producing information relating to the structural performance of unreinforced concrete pavements but much of it is applicable to the assessment of probable cracking behaviour of concrete bridges as part of the design procedure for serviceability limit states. Both pavements and bridges experience many millions of 10ad applications during a design life of 50 to 120 years and there is a need for consistent fatigue data on the basic materials used

1

The present Report deals with some effects on strength and fatigue performance of changes in moisture

conditions up to the time of testing. In practice concrete may experience a wide variety of moisture conditions,

ranging from fully saturated in the case of submerged structures, to almost fully dried in some buildings. In the United Kingdom for example, the climatic conditions are such that pavement slabs are likely to remain

wetter than 85 per cent humidity for most of their lives except near the upper surface, whilst bridge structures

may be subject to wider variations of wetting and drying. Many of the published fatigue test results for con- crete have been based on tests carried out on air-dried specimens produced by a variety of curing methods,

with little or no information given as to the moisture state of the concrete at the time of test or its history up to the time of testing.

In this Report, Part 1 covers tests to assess the importance of moisture condition with particular reference to the standardisation of conditions for a programme of fatigue tests which would cover a number of other

test variables and would be spread over a time span of several years. Part 2 covers a separate investigation into

the effects of the method of curing, which may vary very considerably between different research laboratories.

The interpretation of fatigue test data in terms of the long term performance of highway structures is not covered here; this requires results of full scale experiments which are still in progress.

1.2 Design and preparation of concrete specimens

The moisture content investigation was confined to one particular concrete mix (PQ1) containing

uncrushed Thames Valley flint aggregate. This had a 28 day mean cube strength of 44.8 N/ram 2 and an

indirect tensile strength of 3.5 N/mm 2. Details of the mix design and method of preparation of the test

beams are given in reference 2. Quality control tests on the various batches of concrete used were made by

indirect tensile cylinder splitting tests at 28 days.

All beams were cured under water 3 for 26 weeks before being tested. After removal from the curing

tank randomly selected groups of test beams were further conditioned in four different ways:

(i) Series 1. Drying out was prevented by sealing the beams in polyethylene bags containing some free '

water. The beams remained in the sealed bags while they were tested.

(ii) Series 2. The beams were allowed to dry for one week at room temperature in the test laboratory

before being tested and are described as 'surface-dry'.

(iii) Series 3. Test beams were oven-dried for one week at 105°C, allowed to cool in the oven and then

sealed in polyethylene bags for test.

(iv) Series 4. In a limited investigation, some beams which had been oven-dried as for Series 3 were soaked

in water for three weeks before being sealed in polyethylene and tested wet.

1.3 Test procedure

The main part of the test programme involved four-point bending tests on a simply supported span

of 406mm, the method of loading being as described in reference 2. The beams were mounted in the

machine with the free surface as cast uppermost. Static loading was carried out at a standard loading rate

of 2.67 kN/min (equivalent to a rate of increase of nominal surface tensile stress of approximately

0.017 N/mm2s). Fatigue tests were performed at 20 Hz and constant load amplitude, with a minimum load of nominally zero.

2

On most of the specimens central deflections were measured relative to the supports, using linear

variable differential transformers and on some electrical resistance strain gauges were used to record strains

at the tension surface. Special techniques were required for bonding the 102mm gauge length foil strain

gauges to the surface of wet concrete specimens. During the later stages of the programme it became possible

to use a semi-automatic data logging system for recording dynamic loads, deflections and strains during the

fatigue tests.

Subsidiary tests included the measurement of the dynamic modulus of elasticity before the beams

were loaded, using the standard electrodynamic method 4, and the determination of the equivalent cube

crushing strength 5 after the beams had failed in flexure. For the latter tests a comparison was made between

the crushing strength close to the fracture and as near to the free ends as possible: this was done to minimise

any effects due to local damage which may have occurred during the flexural loading.

1.4 Test results

1.4.1 Moisture loss during drying. The water content of concrete may conveniently be divided into

evaporable (gel and capillary) and non-evaporable (chemically bound) water 6. The former approximates to

that lost when the concrete is oven-dried at 105°C but the latter can only be driven off at much higher

temperatures. Evaporable water content is usually expressed as a percentage of the oven-dry weight; in

this Report, however, it is more convenient to relate loss of moisture to the weight of a fully saturated test

specimen. Moisture gradients are difficult to determine 7 and no satisfactory method was found.

Figure 1 compares the loss of weight of oven-dried beams with beams dried naturally in a controlled

laboratory atmosphere (20°C, 65 per cent RH). In the latter case the beams were monitored over a period

of 5 years. The weight loss at this time had still not reached the value attained by the oven-dried specimens

in 7 days. (It may be noted that the design water content of the concrete used was 7.4 per cent of the total

weight when mixed).

1.4.2 Flexural s t rength t e s t results. Results of the flexural strength tests are summarised in

Table 1, flexural strength being defined as the nominal tensile stress at failure calculated from simple elastic

bending theory.

The mean strength of the beams which had dried for a week in the laboratory before being tested

(Series 2) was 21 per cent lower than those tested wet (Series 1). The loss of strength may be due to surface

tensile stresses resulting from non-uniform shrinkage induced by surface drying. Loss of strength as a

result of partial drying has been reported by Walker and Bloem 8, who showed that there was a relationship

between strength and drying time. For concrete moist-cured for 91 days and then dried at 38°C there was

a strength reduction of up to 40 per cent after 4 days, dropping to 20 per cent after 32 days.

The beams which had been oven-dried (Series 3) had a mean strength 50 per cent higher than the wet

beams. This contrasts with a 40 per cent reduction in flexural strength as a result of oven drying at 1 lO°C

for 24 hours observed by Mills 9 in 14 day wet-cured specimens.

Soaking for three weeks after oven-drying (Series 4) enabled most of the evaporable water to be

replaced but the flexural strength was only partially restored, being midway between the strengths of the

wet beams and the oven-dried. This result conflicts with the findings of Walker and Bloem 8 and Mills 9

3

who found that soaking dried specimens for 48 and 24 hours respectively resulted in flexural strengths lower

than for saturated specimens.

1 .4.3 Fa t igue tes ts . Results of the fatigue tests are given in Table 2. A typical fatigue performance

curve, for beams tested in a wet condition, is given in Figure 2, which illustrates the scatter to be expected

from nominally identical test specimens. In Figure 3 comparative endurance curves are given for each of the

three moisture conditions. These curves are drawn through the geometric mean endurance for each stress

level; for tests which were terminated before failure the number of cycles applied was taken as a conservative

estimate of the life to failure. The effect of the different moisture conditions on fatigue performance follows

quite closely the effects on flexural strength except for the case of the oven-dried specimens which had been

resoaked (Series 4). In this case the mean fatigue life, based on only two test results, was very similar to

that of the wet specimens of Series 1 although the modulus of rupture was significantly greater. There was

some indication of slightly less scatter with the wet specimens.

Both central deflections and surface strains tended to increase with increasing number of load cycles

applied. Typical curves are given in Figures 4 and 5 which show for several individual tests how the deflections

and strains varied with the percentage of the fatigue life used. There was generally a fairly steady increase

throughout the period of testing, possibly due to the onset of microcracking, with a more rapid increase

towards the end of the test. Failure usually occurred when the cyclic strain range reached between 120 and

200 microstrain. As well as this gradual increase in strain (or deflection) amplitude there was a steady increase

in residual strain and deflection, indicating that cumulative creep was taking place under the influence of the

varying unidirectional load. The magnitude of this creep strain reached between 8 and 37 per cent of the

cyclic strain range towards the end of the fatigue test. The highest creep occurred with a surface-dry specimen

having an unusually low elastic modulus. Although limited in number the measurements suggest that strain

and deflection tend to increase at an earlier stage with the surface-dry and oven-dried specimens than with

those which have been prevented from drying.

1.4.4 Dynamic modulus of elasticity. Table 3 gives measured values of dynamic modulus obtained

by the standard electrodynamic method, together with measured densities. Oven-drying resulted in a

reduction in dynamic modulus of about 16 per cent compared with the saturated value; subsequent soaking

in water for three weeks brought this reduction back to about 14 per cent. Corresponding reductions in

density were 5.5 and 0.8 per cent. The relationship between density and dynamic modulus showing the

effects of age and moisture change is illustrated in Figure 6.

1.4~5 Equivalent cube tests. 'Equivalent cube' crushing tests were carried out on most of the flexural

test specimens after they had been broken under static or fatigue loading.

There was no correlation between crushing strength and previous loading history. The results from the

flexural test specimens and the fatigue test specimens were therefore combined and are summarised in

Table 4. Individual results are shown in Figure 7 where the crushing strength near the fracture plane is

plotted against the strength at the ends. These results show a slight but statistically significant reduction in

strength (about 2 per cent) near to the fracture plane, probably as a result of microcracking produced by

the flexural loading.

The mean compressive strengths of the surface-dry and oven-dried beams were 12 per cent and 16 per

cent respectively greater than that of the saturated concrete. The strength of the dried and soaked concrete

4

(Series 4) was 18 per cent lower. This may have been due to strain gradients induced by differential expansion

as water was re-absorbed into the pore structure of the concrete and to weakening of cohesion between

mortar and aggregate, as suggested by Mills 9. The oven-dried specimens suffered a much more explosive

type of failure than the others.

The differences in equivalent cube strength for the different moisture conditions are not consistent

with the differences observed in flexural strength. It may be concluded therefore that the equivalent cube

test does not give any indication of the variation of flexural strength with different moisture conditions.

1.5 Summary and conclusions - ,Dart 1

The results of all the Part 1 tests are summarised in Table 5 which gives the mean values obtained for

each of the four moisture states considered. In Table 6 mean values of dynamic modulus of elasticity,

flexural strength, equivalent cube strength and fatigue strength for the various moisture conditions are

expressed as a proportion of the corresponding yalues for the saturated specimens, which were kept wet

throughout the test period.

The main conclusions are as follows:

1. The flexural strength of the concrete used (uncrushed flint aggregate) was greatly influenced by the

moisture condition at the time of testing. Specimens which were allowed to dry in the laboratory air for

7 days before being tested were about 20 per cent weaker than similar specimens which were kept wet.

Oven-dried specimens were about 50 per cent stronger than the wet ones; subsequent soaking in water

reduced this gain in strength to 25 per cent.

2. Flexural fatigue performance generally followed the same trend, the best fatigue resistance being shown

by the oven-dried beams. The lowest scatter in fatigue test results occurred with the fully saturated beams.

3. Differences in equivalent cube strength showed no correlation with differences in flexural strength or

fatigue performance. In the case of the surface-dry beams, for example, the mean equivalent cube strength

was 12 per cent greater whilst the mean flexural strength was 21 per cent less than the corresponding values

for saturated specimens.

4. The sensitivity of flexural, fatigue and compressive strengths to the moisture conditions at the time of

test underlines the importance of considering relevant moisture conditions when using strength data to assess

the performance of real structures.

PART 2 - EFFECTS OF DIFFERENT METHODS OF CURING

2.1 Introduction

Various methods of curing concrete test specimens have been used by different research workers and

it is not always made clear in published papers what method has been used. The most common for laboratory

test specimens are immersion in water, curing in high humidity air (fog room) or curing in air at normal

room temperature and humidity.

5

In Part 1 it was shown that the moisture condition at the time of test can have a significant effect on

the strength, stiffness and fatigue performance of small beams which have been cured by immersion in

water for 26 weeks. A further investigation was therefore conducted to see to what extent the method of

curing might affect such properties. Broadly three types of cure were used:

(i) Various combinations of immersion and air curing

(ii) Fog room

(iii) Use of coatings.

The type of concrete and method of test were as described in Part 1 but additional information was

obtained from measurements of density and length changes during the curing period.

2.2 Test procedure

2.2.1 Preparation of test specimens. Six batches of concrete PQ1 were prepared over a period of

eight weeks, each batch comprising twenty 508mm x 102ram x 102mm beams and five 152mm x 102mm

diameter control cylinders. After being demoulded on the day after casting two beams from each batch

were cured under each of the conditions defined below. The control cylinders were all cured under water

for 28 days and were used for indirect tensile tests to check the quality of the concrete from each batch.

2.2.2 Curing,conditions. All beams were cured for 26 weeks at a controlled temperature of 20°C

-+ 1°C. Twelve beams (two from each batch) were cured by each of the following methods.

(i) (ii) (iii) (iv) (v) (vi) (vi0 (viii) (ix) (x)

Immersed in water for the full 26 weeks.

13 weeks in water and 13 weeks in air at 65 per cent relative humidity.

4 weeks in water and 22 weeks in air at 65 per cent relative humidity.

1 week in water and 25 weeks in air at 65 per cent relative humidity.

26 weeks in air at 65 per cent relative humidity.

26 weeks in air at variable humidity.

26 weeks in a fog room at 95 per cent relative humidity.

Sealed in paraffin wax and kept at 65 per cent relative humidity.

Sealed in a polyethylene bag and kept at 65 per cent relative humidity.

Sealed with a coating of sodium silicate and kept at 65 per cent relative humidity.

At the end of the curing period each beam was sealed in a polyethylene bag before being tested, the

specimens cured wholly under water being sealed with a little free water in the bag.

2 .2 .3 Density and length changes. All beams were weighed immediately after demoulding and

thereafter at weekly intervals throughout the curing period for all conditions except (vi), (vii), (viii) and

(ix). Each beam was weighed again at the end of the curing period. (In the case of the wax coated beams,

the wax was first scraped from the surface).

For all conditions except (vi), (vii), (viii) and (ix) changes in length of each beam were measured

at weekly intervals throughout the curing period, using an invar reference rod system similar to that specified in BS 1881.

2.2.4 Flexural strength and fatigue tests. For each curing condition one beam from each of the six

batches was tested statically and one in fatigue, strain gauge and deflection measurements being made on

representative specimens. Particular care was taken to minimise the surface area exposed to the atmosphere

while the strain gauges were attached.

The fatigue tests were all at one load level, 4.23 + 4.21 kN. This gave a nominal cyclic tensile stress of

approximately 1.61 + 1.61 N/mm 2 but the actual values varied slightly due to small dimensional differences

between individual beams. In some of the fatigue tests dynamic stress-strain curves for individual load cycles

were derived from continuous trace records of load and strain on an ultra violet oscillograph. A digital

data recording system was used to record maximum and minimum values of load and strain.

Where specimens remained unbroken after more than 5 x 106 cycles, cyclic loading was stopped and

their residual flexural strength was determined about one week later.

2 .2.5 Subs id iary tests. As before ,the dynamic modulus of elasticity was measured before the beams

were tested and equivalent cube crushing tests were made on the broken beams after the flexural tests.

2.3 Results

2.3.1 Density and lengtil changes. Densities of the test beams measured at the end of the curing

period are included in Table 7, and in Figure 8 the variation in density with the time of immersion in water

is shown, expressed as a percentage of the mean density of specimens immersed for the full 26 weeks. As

they were exposed to the air, moisture was lost by evaporation and there is a nearly linear relationship

between density and time of immersion over the 26 week curing period.

Mean changes in weight due to moisiure movement, expressed as a percentage of the weight of the

specimen when demoulded, are shown for each curing condition in Figure 9. Similarly changes in length,

expressed as expansion or shrinkage relative to the original length, are shown in Figure 10 for some of the

curing conditions.

Figure 9 indicates that after being immersed in water, the specimens gained on average 0.6 per cent

of their original weight in one week, 0.74 per cent in four weeks and thereafter at a reduced rate to 0.93

per cent after 26 weeks. In this time concrete cured in the fog room gained a little over 0.6 per cent. With

specimens exposed to the air for 26 weeks the total loss of evaporable moisture was nearly 3 per cent,

approximately the same as for beams which had been immersed for one week before drying. For the other

water-cured specimens the rate of loss in weight up to 26 weeks became less as the time of immersion

increased, probably due to hydration products rendering the concrete less porous. There was virtually no

difference between constant 65 per cent humidity and variable humidity laboratory conditions, where the

actual humidity ranged from 41 to 75 per cent, with a mean of 57 per cent.

The effectiveness of coatings varied considerably. Specimens cured in sealed polyethylene bags lost

less than 0.2 per cent of moisture in 26 weeks. Wax coating was less effective, the loss being more than one

per cent. Coating with sodium silicate did virtually nothing to stop evaporation. The drying curve up to

8 weeks was the same as for air-dried specimens; thereafter they diverged slightly, the coated specimens

losing slightly less moisture.

7

Most of the expansion following immersion in water took place within the first two or three weeks

(Figure 10). After removal from the water, shrinkage occurred at a considerably lower rate. At 26 weeks

the final shrinkage for air-cured specimens was very similar to that of specimens immersed for one week

and then dried. These values were approximately three times the values for specimens which had been

immersed for 4 and 13 weeks.

The results of the above measurements suggest that early shrinkage and loss of moisture are

considerably delayed by an initial period of immersion in water. In such conditions the products of hydration

during the immersion period are able to restrict subsequent moisture movements. Subsequent gain in strength

enables the concrete to resist differential shrinkage stresses more effectively and this may be expected to

have some influence on the strength properties of the beams.

2.3.2 Flexural s t rength tes t results. Results of all the flexural tests are summarised in Table 8.

The mean strength of the group of specimens tfiat had been cured under water for 26 weeks was 13

per cent higher than the value obtained some three years earlier from identical specimens made from the

same stockpile of materials'for tests reported in Part 1. However, the indirect tensile strength was 5 per

cent lower. Both these differences are statistically significant. The only difference in procedure was the

use of a somewhat larger mixer in the later tests. In view of these differences the more recent results have

been used as a basis of comparison, as these beams were made at about the same time as the rest of the

specimens reported on.

Table 8 also includes the results of residual strength tests made on specimens which had remained

unbroken after fatigue loading of at least 5 x 106 cycles.

The highest flexural strengths were achieved with specimens immersed in water for one week

followed by air curing for 25 weeks. Similar results, not statistically significantly different, were obtained

with beams immersed for 4 weeks, with 22 weeks air drying. Both of these results were about 10 per cent

greater than for beams that had been immersed for 13 or 26 weeks. The weakest beams were those which

had been air-cured throughout, there being no difference between constant humidity and variable humidity.

The strengths were nearly 20 per cent lower than the beams which had remained in water for the full

26 weeks. Of the remainder, the beams cured in the fog room were about 7 per cent less, whilst the

surface coated beams were 10-15 per cent less. Statistical analysis of these results suggests that the above

differences are significant for the air-cured and surface-coated specimens, not quite significant at the 5 per

cent level for the one and four week immersion tests, when compared with 26 weeks immersion.

The most likely reason for the loss of strength with the air-cured specimens is the development of

differential shrinkage strains during the early stages of dry!ng. This would be less marked with beams which

had remained in water for a week or more before being dried.

The flexural strengths of specimens which had been subjected to previous fatigue loading were

generally greater than for the normal specimens (Table 8). The initial fatigue loading varied between

5 x 106 and 12 x 106 cycles at stress levels of between 61 and 81 per cent of the original mean flexural

strength. These results suggest that provided local strains are not high enough to propagate any existing

shrinkage or fatigue cracks the repeated stressing may actually strengthen the concrete, possibly as a result

of local plastic deformation relieving stress concentrations around incipient cracks. Similar effects have

8

been noted under compressive stress 10. Another possible explanation, of course, is that beams that survived

the fatigue loading were stronger than average and residual strengths would therefore be greater than the

original mean strength.

2.3.3 Fatigue performance. Results of the fatigue tests, given in Table 9, are expressed in terms of

the geometric mean life to failure (assuming a log normal life distribution) and the coefficient of variation of

the logarithm of the number of cycles applied. The life of a beam was defined as the maximum number of

load cycles applied in the fatigue tests, including those cases where the test was terminated before failure.

The mean values given in the table therefore represent a slightly conservative estimate of the true life.

For approximately the same cyclic stress levels the fatigue endurance of the various groups followed

a similar trend to the differences observed in flexural strength. The best fatigue performance (with only

two specimens failed) came from specimens which had been immersed in water for one week and then

dried for 25 weeks. This condition also produced the highest flexural strength (Table 8). The shortest

lives came from beams which had remained sealed in polyethylene. These also had relatively low

flexural strength.

If the fatigue loading is expressed in terms of the flexural strength corresponding to the same curing

conditions the results may be plotted for comparison with the results reported in Part 1 (see Figure 11).

This method of plotting is not strictly correct because of the large difference in loading rates between the

static and the fatigue tests 2, but it does form a useful and convenient basis for comparison. Most of the

results shown in Figure 11 fall fairly close to a single curve (tile exceptions being the wax sealed and fog

room cured specimens, both of which .gave rather low values of mean endurance compared with the

average curve). On this basis it should be possible to predict approximately the effect on fatigue

performance of varying the conditions of curing, from a knowledge of the corresponding values of modulus

of rupture.

The variation of maximum tensile strain with number of loading cycles is shown for typical examples

of some of the curing methods in Figure 12. Strains usually showed a gradual increase over most of the

life, with a very rapid increase in the last few hundred cycles. The final strain measurements were recorded

close to failure but it was not possible to record the true failure strain. As in previous tests some of the

increased total strain was due to cumulative creep. The development of dynamic surface strains did not

appear to follow any consistent pattern. In some tests there was a gradual increase throughout the test

whilst in others the maximum strain remained almost constant up to failure. Such differences may be

the result of variations in the rate of propagation of microcracks.

2.3.4 Modulus of elasticity and stress, strain curves. The results of conventional dynamic modulus

measurements for each curing condition are given in Table 7. The scatter was slightly higher than that

observed in earlier tests, being affected by a fault in the frequency meter. In Figure 8 the mean values of

dynamic modulus are plotted against time of immersion for those specimens which were cured partly in

water and partly in air; this shows a gradual reduction of modulus with drying time. Modulus values

for the silicate-coated and completely air-cured specimens were about 20 per cent less than for the fully

immersed specimens. For the curing conditions where specimens were not exposed to the atmosphere,

wax coating produced the lowest modulus, 8 per cent less than for the wholly water-cured concrete.

9

Typical stress-strain curves from the static loading tests are shown in Figure 13 for various curing

conditions. Some results are included from specimens which had survived more than 5 x 106 cycles of

fatigue loading before being statically loaded to failure. Derived values of tangent modulus of elasticity

at 50 per cent of the failure load are given in Table 7 and in Figure 8 are plotted against immersion time

for the beams which were cured partly in air. This shows a similar trend to the dynamic modulus and indicates

that the stiffness of the concrete increases with time of immersion. Similar observations have been made by

Johnston and Sidwell for direct tension tests on 28 day concrete 11.

Dynamic stress-strain curves from fatigue tests at 20 Hz are illustrated in Figures 14, 15 and 16 for

three different curing conditions. In general these show a gradually decreasing slope throughout the duration

of the test, with an increase in residual (creep) strain. Some values of initial dynamic modulus of elasticity,

derived from strain measurements near the beginning of the fatigue test, are given in Table 7.

2.3.5 Equivalent cube tests. Results of equivalent cube tests from the static and fatigue test specimens

combined are summarised in Table 7.

The compressive strength of specimens cured under water for 26 weeks was about 9 per cent greater

than that for the fog room and polyethylene-wrapped specimens and approximately 30 per cent greater

than the wax-coated and completely air-cured concrete. These differences are attributed to different

rates of loss of moisture, restricting the rate of hydration and the formation of shrinkage cracks. The highest

strength was obtained after 13 weeks in water and 13 weeks in air. Tests on cubes and cylinders by Pomeroy 12

and Sangha et a113 have also shown that after a certain duration of mixed curing the strength tends to fall.

Conditions for highest cube strength did not coincide with conditions for highest flexural strength.

2.4 Conclusions -- Part 2

A comparison of the flexural strength, flexural fatigue performance and 'equivalent cube' strength of

small concrete beams, all cured at 20°C for 26 weeks under different combinations of immersion and air

drying, leads to the following conclusions:

1. Flexural strength was highest when the concrete was first immersed in water for one to four weeks,

followed by air-drying in the laboratory. Immersion for 26 weeks or for 13 weeks followed by air-drying

for 13 weeks gave results that were approximately 10 per cent weaker. Specimens cured wholly in air were 25 per cent weaker.

2. Changes in fatigue performance followed changes in flexural strength fairly closely.

3". Stress-strain curves for static flexural loading confirmed that a reduction in stiffness resulted from

loss of moisture to the atmosphere.

4. Compressive strength based on 'equivalent cube' tests on flexural test specimens after static and

fatigue tests, was highest for specimens which had been immersed for 13 weeks and then dried for 13 weeks.

There was no correlation between the variation of compressive and flexural strengths.

5. The results confirm that the strength, stiffness and fatigue properties are substantially affected by the

extent to which moisture is lost during the curing period. This emphasises the need to define and control the

curing conditions for laboratory tests in relation to moisture conditions which are likely to occur in practical structures. 10

6. For comparative laboratory studies, the only practical way to control the moisture condition at the

time of testing and to avoid moisture gradients is to cure specimens under water and keep them wet through-

out the test period.

3. ACKNOWLEDGEMENTS

The experimental programme was undertaken as part of the work of the former Structural Properties

Division of the Structures Department and the analysis was completed partly in the Pavement Design

Division of the Highways Department (Division Head: Mr N W Lister) and partly in the Bridge Design

Division of the Structures Department (Division Head: Dr G P Tilly).

4. REFERENCES

1. RAITHBY, K D and A C WHIFFIN. Failure of plain concrete under fatigue loading - a review of

current knowledge. Ministry o f Transport, RRL Report LR 231. Crowthome, 1968 (Road Research

Laboratory).

. GALLOWAY, J W and K D RAITHBY. Effects of rate of loading on flexural strength and fatigue

performance of concrete. Department o f the Environment, TRRL Report LR 547. Crowthorne,

1973 (Transport and Road Research Laboratory).

3. BRITISH STANDARDS INSTITUTION. Methods of testing concrete, BS 1881, 1970, London.

Part 3 - Methods of making and curing.test specimens (British Standards Institution).

4. BRITISH STANDARDS INSTITUTION. Methods of testing concrete. BS 1881, 1970, London.

Part 5 - Methods of testing hardened concrete for other than strength (British Standards Institution).

5. BRITISH STANDARDS INSTITUTION. Methods of testing concrete. BS 1881, 1970, London.

Part 4 - Methods of testing concrete for strength (British Standards Institution).

. MONFORE, G E. A review of methods for measuring water content of highway components in

place. Highway Research Record 342. Highway Research Board, 1970.

. HUNDT, J and J BUSCHMANN. Moisture movements in concrete. RILEMMaterials and Structures,

Vol 4, No. 22, 1971.

. WALKER, S and D L BLOEM. Effects of curing and moisture distribution on measured strength

of concrete. Proc. Highway Research Board 36, 1957.

. MILLS, R H. Strength-maturity relationship for concrete which is allowed to dry. RILEM 1st

Symposium on concrete and reinforced concrete in hot countries. Haifa, 1960.

10. BENNETT, E W and N K RAJU. Effect of understressing on the deformation and strength of plain

concrete in compression. Proc. Int. Conf. on Mechanical behaviour o f materials. Vol. 4, Kyoto, 197 I.

11

11. JOHNSTON, C D and E H SIDWELL. Influence of drying on strength of concrete specimens.

ACI Journal No. 9, September 1969.

12. POMEROY, C D. The effect of curing conditions and cube size on the crushing strength of concrete.

Cement and Concrete Association. Technical Report 42.470, July 1972.

13. SANGHA, C M, J G L MUNDAY, M ERINCER and R K DHIR. Significance of different degrees

of water curing on concrete behaviour. Cement Lime and Gravel. November 1972.

12

TABLE 1

Results of flexural strength tests - Part 1

Test Mean Number Coefficient series Moisture condition flexural strength of of variation

N/mm 2 results %

1 Saturated 4.21 10 7.8

2 Surface-dry 3.31 5 4.8

3 Oven-dried 6.35 9 7.7

4 Oven-dried and then soaked in water 5.24 10 4.5

TABLE 2

Results of fatigue tests - Part 1

Test series

1 (Saturated)

2 (Surface-dry)

3 (Oven-dried)

4 (Oven-dried and soaked in water)

Maximum stress

N/mm 2

3.16

2.69

2.53

2.35

3.22

2.70

2.53

5.64

4.82

4.69

3.68

Number of

results

10

9

11

10

13

10

10

Number of cycles to failure

Geometric mean

70.66 x 103

> 7 0 0 x 103

> 1.48 x 106

> 15.58 x 106

421

77.46 x 103

> 153.53 x 103

2 340

24.9 x 103

>239.2 x 103

1 350

C.V.

% Range

Lowest Highest

4 460 696 x 103 14.3

33 x 103

1 1 . 8 x 10 3

1.7 x 106

60

3 560

2 500

133

2 000

10 x 103

> 5 . 7 x 106

> 18 x 106

> 4 3 x 106

5 010

1.4 x 106

> 5 x 106

10.53 x 103

233 x 103

> 16.5 x 106

4 520 400

c.v. = coefficient of variation of logarithm of endurance > = greater than figure stated because group includes at least one specimen unbroken

11.1

11.2

8.3

24.7

17.0

19.2

16.9

17.3

21.1

23.7

1 3

TABLE 3

Dynamic modulus and density results - Part 1

Test series Condition when tested

3 (Oven-dried)

Number of

results

Dynamic modulus

Mean kN/mm 2

1 Saturated 47 45

(Saturated)

2 Saturated 40 45

(Surface<lry)

Saturated 46 44

Oven-dried

4 (Oven-dried and soaked in water)

12

I

Saturated

Oven-dried

Partially saturated

38

45

36

38.5

C.V.

%

1.5

1.7

1.1

1.7

1.0

'3.1

1.9

Density

Mean c.v. kg/m 3 %

2 330 0.5

2 335 0.5

2 350 0.4

2 220 0.5

2 335 0.4

2 205 0.4

2 315 0.6

TABLE 4

Equivalent cube strength - Part 1

Test series Number of

results

Mean of ends

Compressive strength N/mm 2

1 39 49.74

(Saturated)

2 32 55.65

(Surface.dry)

3 36 57.80

(Oven-dried)

4 (Oven-dried and 11 41.03 soaked in water)

c.v. = coefficient of variation

C°V.

%

4.0

4.9

2.5

2.5

Close to fracture

Number Compressive of strength

results N/mm 2

39 48.55

32 54.26

36 56.55

11 40.67

C.V.

%

4.2

4.6

3.8

2.8

Strength ratio:

fracture ends

0.976

0.975

0.978

0.991

1 4

. . I

I

NI~ ~ ~1 ~

. ~ . - z

~ z

0 0

e~ 0 ° ~

tt'3

e~ 0

e~ 0

0

-t-

0

o ~

~- ~°

0

c'-I

o

E o o

° ~

e~

15

TABLE 7

Effects o f curing condi t ion on density, modulus of elasticity and equivalent cube strength

Curing condi t ion

Modulus of elasticity* Density kN/mm2

Mean kg/m 3

Water 26 weeks 2 335

Water 13, air 13 2 286

Water 4, air 22 2 274

Water 1, air 25 2 256

Air, constant RH 2 241

Air, variable RH 2 246

Fog room 2 340

Wax seal 2 288

Polyethylene 2 313

Sodium silicate 2 248

C.V° % ER E S E F

0.7 45.5 38.0 48.0

0 42.5 33.0 -

0.4 40.5 29.5 -

0.8 39.0 29.0 -

0.6 36.5 27.0 30.5

0.9 36.5 26.5 -

1.0 45.5 34.5 38.0

0.6 42.0 - -

0.8 43.5 36.0 35.0"

0.6 36.5 27.0 -

Equivalent cube strength

Number Mean c.v.

N / m m 2 o f results %

49.09 16 11.9

55.87 23 10.5

51.23 20 12.1

47.22 20 12.1

33.49 22 7.6

36.62 19 10.6

44.68 18 15.6

34.87 22 24.8

44.71 15 9.2

34.10 !9 8.8

E R = dynamic modulus from standard e lectrodynamic tests

E S = tangent modulus at 50 per cent of failure load in static flexural tests

E F = dynamic tangent modulus from fatigue tests at 20 Hz

16

TABLE 8

Flexural strength test results - Part 2

Curing condition

Water 26 weeks

Water 13 weeks, air 13

Water 4 weeks, air 22

Water 1 week, air 25

Air, constant RH

Air, variable RH

Fog room

Wax seal

Polyethylene

Sodium silicate

Normal flexural strength tests

Mean Number strength of N/mm 2 results

4.77 6

4.74 6

5.17 6

5.24 6

3.94 6

3.94 5

4.43 6

4.27 6

4.14 6

4.14 6

C.Vo

%

7.4

11.6

9.9

8.2

6.8

11.6

6.9

10.9

6.0

6.5

Residual strength after initial fatigue loading

Individual results

Number of initial

loading cycles

106

Flexural strength N/mm 2

12.0 5.19

6.5 4.93

8.3 5.23

5.0 5.09

8.4 6.30

5.1 4.21

6.4 5.29

8.7 5.64

5.2 4.41

8.4 5.16

5.1 5.45

6.4 4.93

6.7 4.47

12.0 4.67

Mean residual strength N/mm 2

5.06

5.54

5.05

4.99

4.57

5.2 4.45 4.45

8.7 5.06 5.06

9.5 4.98 4.98

17

TABLE 9

Fatigue test results - Part 2

Curing condition

Maximum cyclic stress

N/mm 2

Percentage of mean fiexural strength

Number of

results

Geometric mean life

cycles

C.V.

of log. life %

Water, 26 weeks 3.13 65.6 6 > 532 350 17.2

Water 13 weeks, air 13 3.19 67.2 6 >2 ,893 700 8.6

Water 4, air 22 3.22 62.2 5 > 697 2 8 0 26.7

60.6 > 3 606 600 Water 1, air 25 3.18 6.1

Air, constant RH ~ 3.18

Air, variable RH 3.18

Fog room 3.15

Wax seal 3.20

Polyethylene 3.17

Sodium silicate 3.17

80.6 6 > 33 479 42.4

80.6 4 > 48 473 50.0

71.1 6 37 076 16.5

74.8 6 > 23 152 34.5

76.5 5 5 229 15.3

76.7 • 6 > 9 454 38.5

Before life signifies that group included at least one specimen that did not fail.

18

All specimens init ial ly cured for 26 weeks in water at 20°C

O . v

E-

O

Oven-dried at 105°C

Air-dried in laboratory

2 -

1

0 1 1 I 0.01 0.1 1 10 100

Time after removal from water (weeks)

1000

Fig, 1 MOISTURE LOSS DUE TO DRYING

3.5

A

E E z

P

E --s

E ¢o

3 .0 "

2.5

2.0

O CX~

[ ]

Max. load Min. load (kN) (kN)

O 8.36 I • 7.11 0.025 [ ] 6.69 ~7 6.27

- -~ Specimens which did not fail

O0 0

- -

0 DD •n

V

[ ]

0 l I I I I 103 104 10 s 106 107

Cycles to failure

Fig. 2 F A T I G U E T E S T R E S U L T S , S A T U R A T E D C O N C R E T E

108

A

"E E

F z

E

E '~

6 I

5 --

4 - -

3 - -

2 -

1 -

0 0.1

. Oven-dried then ~ ~ soaked in water " ~

~ ~ ~ , ~ = ~ C ~ ~ n'dr ied

O - - - D -----_- Saturated

A

Surface-dry

= Some specimens did not fail

I I I I I I I I 1 10 10 2 10 3 10 4 10 5 10 6 10 7

Cycles to fai lure

Fig. 3 M E A N F A T I G U E C U R V E S F O R D I F F E R E N T M O I S T U R E S T A T E S

10 8

0.06

0.05

E E

0.04

E I

X

E 0.03 c- O

= 0.02

D

0.01

Maximum stress Cycles to fai lure

- N/mm 2

Oven-dried 646 300 .-- .... -~ " /

- / 2 ° . . . . ~ ~ ~ ~ J " " 2.73 ~

,- - - - - Z " - ' - " -- ~ 17 900 / 7 2.52

Surface-dry / 2.76

Wet ? 3 088 100

I I I I I I I I I 10 20 30 40 50 60 70 80 90 100

Percent of fatigue life

Fig. 4 C E N T R A L DEFLECTION IN FATIGUE TESTS

200

150

O

e--

.E 100

e -

I---

50

0 0

Cycles to f Max imum stress failure ~ J

/ J - N / m m 2 646 300 . ~ - , s

Oven-dried 4.70 _. - " ~ " -- -- J

- _~ 0 ~ . . . . . -- "" 17 9

2.52

~ ~ . . 2 . / ~ ~

3 088100....,....... j ~ '

T 2 . 7 6 ~ - /

Curve estimated - Wet beyond this point

l I I I I I I I i 10 20 30 40 50 60 70 80 90 i 00

Percent of fatigue life

Fig. 5 S U R F A C E S T R A I N I N F A T I G U E TESTS

A

E E

z

-O O

E c.}

E

E3

47

45 -

43 --

41 --

39 --

37 --

35 2100

I Control specimens immersed in 24_4 ? 26 water at 20UC W J

O Control specimens air-drying at y ~ , 26 20vC, 65 per cent R H • ."

• Mean line for specimens oven- i " ~ ./" dried at 105uC then resoaked / I J' in water at 20°C I ~1V26

Numbers indicate ages of / / specimens in weeks / / /

/s 2 6 8 / 6

/ ~ 30

/ t",~

I I 2200 2300 2400

Density ( kg/m 3)

Fig, 6 RELATIONSHIP BETWEEN DYNAMIC MODULUS AND DENSITY

A

E E

z

,9o O

t -

¢-

60

50

40

. J

/

Modulus Flexural of rupture fatigue tests tests

Saturated ~7 Surface-dry A • Oven-dried 0 • Oven-dried and then soaked rl •

30 i I 30 40 50 60

Strength at ends (N/mm 2 )

Fig. 7 RELATIONSHIP BETWEEN 'EQUIVALENT CUBE' STRENGTH OF ENDS AND CLOSE TO PLANE FRACTURE

100

90

80

,~ Density

'E I I I I I 1 I I

1oo, "6 9 o - g

Tangent modulus ~ 8 0 - ~ . ~ ~ '

70 I I I I I I I I I i I I

110

10(

== 90

• E 80

'~ 110

N 100,

g ~ 90

P 0_ 80

70 -

60 26

Flexural strength

I I I I I I __ I I I I I I

~ , ~ , ~ " ~ = ~ Valent cube s t r e n g t h ~

Immersior I I I I I I I I I I I I

24 22 20 18 16 14 12 10 8 6 4 2 0

I I I I I I I I I I I D / ry ingl 2 4 6 8 10 12 14 16 18 20 22 24 26

Curing times (weeks)

Total curing time 26 weeks in all cases

Fig. 8 VARIATION OF DENSITY, MODULUS OF ELASTICITY AND STRENGTH WITH TIME OF IMMERSION AND

SUBSEQUENT DRYING

8 " o E

i= ._=

+- ' .~

E ~ ~ , ~ o o i= . _ ~

I ._ ~ i qD

I TM

I I I

I /

/ /

/

0

(~l 0 ~'- CXl 0 ~ 0 (:3 d d d d

uo!suedx::l a6e~lu!JqS

4~.BUal u! aSUE40 aBe~.ua3~e d

I I ,! />, I I I ! I I I I

u!eE) sso-I

J.LI6!aM U! aSueqo aBe~uaoJe d

z WJ .J

z

W

-~ r- ~ z _oz

~ W C J

0

~ ~ 0 0

U U.I U . U . W

U .

0

p~

I.O

o

A I:=

.=,=o uJI-

< ~:~

I 0

. . I I11

O}

• , 0 I.l.. ¢0

0

q

I 0

re_mE -~-E

O ~ ~

~'B ~

~-=~_~{~ ® ~ o ~ ~®~

~ - ~ 0 ~

~o~O~ilPmOx® I

0

I 0

I d

aJnJ.dnJ jo snlnpowlssaJJ.s O!lOAO "XelAl

o

cJ >. c.)

JO

E c-

0

0

0

W

I -

lL 0

~ Z -JO

O ~ ~ z

0

i - i ' , -

. J ~

IJ.l I.I.I U n " Z m

~ u . ~ a o M.

W

W

I -

o

0 o o

E o

o U .

" r t

!

I' % L ~ ~

°t.

2 e ~ _

/

o

.m

Q)

.m

Q,)

}._

Q,)

o o o

(9_OL) u!eJ~s el!SUaJ, LunuJ!xelAI

ID

~ 0

m 0

~ O

- %

- - O

o

-d

¢- . _

E

z

; [ <

Z <

(n iJJ .. i

Z ILl I--

_ l , . J

~ Z

, , , n -

r..n z . el. -1- (n Z 0 I - < ..J uJ rr

U.

- r

r c -

, B

~ D e -

0

o ~

t -

O

~ ~ - - r

° °i

f " c " e - E E

"6 • LLI E U

,,n :6~ 0 m O LO O ~ e , -

N

0 g

I I I o ~

(auJW/N) ssaJ:l.s aJq!J, auJaJ),×a pa~,elnole 0 z~

N

m

(zWW/N) ssaJ),S eJq! t awgJ:lXa pe~,elnole 0

g "~_

(~WW/N) ssaJ].s aJqH awaJlxa palelnole 0

0~ \

\ . . i o o ~ " , , ,, ~ / "

• .~

~ ~ o ~

( ~WW/N ) ssaJJ.s aJq,~ aLuaJJ.xa pe~elnole 0

Z

II 0 ~

~ ~o ~

r-- - ~ ~ b=

~ . ~ ~'

' ~ 0 0

(aWW/N) ssaJls aJqH awaJJ.xa pa~elnOle 0

0 0

¢~ i'N

o = o _ °o ~ >,

' - ~ o

~ e

I I I o ° N

(auJuJ/N) ssaJ:l.s aJql~ aLLlaJJ.Xa pa~eln01e O

" ~

i ~ °

/ I . t [ CO ¢'~ ,-- 0

(auJuJ/N) ssaJ~,s aJq!~ auuaJ],xa .pm, eln01e 0

A 04

E E

Z

2

.Q

E

x

"5

Number of loading cycles (Test stopped at 11 988 200 cycles)

560 520 060 3 503 000

11 987 800

0 50 100 150 200

Measured tensile strain (10 -6 )

Fig. 16 DYNAMIC STRESS-STRAIN CURVES (SPECIMEN CURED IN AIR AT 20°C 65 PER CENT RH)

(2600) Dd0536316 1/79 H P L t d S o ' t o n G191S PRINTED IN ENGLAND

ABSTRACT

Effects of moisture cllanges on flexural and fatigue strength of concrete: J W GALLOWAY CEng MIMech E, H M HARDING CEng MIERE and K D RAITHBY BSc CEng MRAeS: Department of the Environment Department of Transport, TRRL Laboratory Report 864: Crowthorne, 1979 (Transport and Road Research Laboratory). Results are given of flexural strength and fatigue tests on small unreinforced beams of pavement quality concrete to study the effects of various moisture conditions. Subsidiary tests included dynamic modulus of elasticity and equivalent cube crushing tests on the fractured specimens. The Report is in two p a r t s : -

Part 1 Effects of moisture condition at the time of test. Beams were cured under water for 26 weeks and then tested either wet, surface-dry or oven-dried.

The highest flexural and fatigue strengths were obtained with oven-dried specimens and the lowest with specimens that were air-dried for seven days. There was no correlation between these results and the corresponding equivalent cube strengths, where the effects of air drying were reversed.

Part 2 Method of curing. Beams were cured for 26 weeks under various combinations of immersion in water, air-drying, fog room and surface coatings.

Immersion in water for one to four weeks followed by air-drying gave the highest flexural and fatigue strengths. Specimens which were air-cured for the whole period were significantly weaker. There was little correlation between compressive strength and flexural strength related to curing conditions. The importance of considering moisture states that may occur in practical structures under service conditions is emphasised, as is the need for standardising moisture conditions for research tests.

ISSN 0 3 0 5 - 1 2 9 3

ABSTRACT

Effects of moisture changes on flexural and fatigue strength of concrete: J W GALLOWAY CEng MIMech E, H M HARDING CEng MIERE and K D RAITHBY BSc CEng MRAeS: Department of the Environment Department of Transport, TRRL Laboratory Report 864: Crowthorne, 1979 (Transport and Road Research Laboratory). Results are given of flexural strength/~nd fatigue tests on small unreinforced beams of pavement quality concrete to study the effects of various moisture conditions. Subsidiary tests included dynamic modulus of elasticity and equivalent cube crushing tests on the fractured specimens. The Report is in two p a r t s : -

Part 1 Effects of moisture condition at the time of test. Beams were cured under water for 26 weeks and then tested either wet, surface-dry or oven-dried.

The highest flexural and fatigue strengths were obtained with oven-dried specimens and the lowest with specimens that were air-dried for seven days. There was no correlation between these results and the corresponding equivalent cube strengths, where the effects of air drying were reversed.

Part 2 Method of curing. Beams were cured for 26 weeks under various combinations of immersion in water, air-drying, fog room and surface coatings.

Immersion in water for one to four weeks followed by air-drying gave the highest flexural find fatigue strengths. Specimens which were air-cured for the whole period were significantly weaker. There was little correlation between compressive strength and flexural strength related to- curing conditions. The importance of considering moisture states that may occur in practical structures under service conditions is emphasised, as is the need for standardising moisture conditions for research tests.

ISSN 0 3 0 5 - 1 2 9 3