TRANSPORT and ROAD RESEARCH LABORATORY

Department of Transport

TRRL SUPPLEMENTARY REPORT 838

THE USE OF PULVERISED FUEL ASH IN LEAN CONCRETE ROADBASES PART 2 -- PILOT~SCALE TRIALS

by

H M Harding and J F Potter

The views expressed in this Report are not necessarily those of the Department of Transport

Pavement Design and Maintenance Division Highways and Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1985 ISSN 0305--1315

Abstract

1.

2.

3.

4.

CONTENTS

Introduction

Construction of the trials

Structural assessment of the ash-modified lean concrete mixes

Laboratory measurements

4.1 Preparation of concrete specimens

4.2 Density

4.3 Flexural strength and stress-strain behaviour

4.4 Compressive strength

4.5 Elastic modulus

5. In-situ measurements

5.1 Elastic modulus

5.2 Transient deflection

5.3 Transient strain

6. Application to the construction and performance of lean concrete roadbases

6.1 Use of elastic theory

6.2 The onset of cracking

6.3 The effect o f construction traffic on cracking

6.4 Long term performance

7. Conclusions

8. Acknowledgements

9. References

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© CROWN COPYRIGHT 1985 Extracts f rom the text may be reproduced, except for

commercial purposes, provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I 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.

THE USE OF PULVERISED FUEL ASH IN LEAN CONCRETE ROADBASES P A R T 2 - P I L O T - S C A L E T R I A L S

ABSTRACT

Pilot-scale trials were laid to investigate the properties of roller-compacted lean concrete roadbases in which pulverised fuel ash was included as a partial replace- ment for some of the cement. A conventional lean concrete and a cement-rich concrete were included in the trials for comparison. Structural performance was assessed by measuring strain and deflection under a moving wheel and elastic modulus by surface wave propagation. Beam specimens cast in moulds and cut from the trial area were used to determine stress-strain relationships, flexural strength and elastic modulus.

The measured transient deflections and strains in the ash-modified lean concrete pavements were similar to those in the conventional lean concrete; the values of elastic modulus were also similar. Results from the beams showed that in general the ash-modified mixes have a lower early life strength and a higher long term strength than conventional lean concrete. An analytical study suggested that both ash-modified lean concrete with a low early life strength and conventional lean concrete may suffer from microcracking when trafficked by construction vehicles at 7 days old but visible cracking should not occur.

The overall structural behaviour of the pilot-scale trials was sufficiently encouraging to recommend a full-scale trial of ash-modified lean concrete.

1. INTRODUCTION

Lean concrete is used extensively in the United Kingdom as a roadbase material because it is relatively inexpensive

and, being very stiff, reduces the stresses and strains induced by traffic loading in the road foundation. However,

lean concrete cracks because of shrinkage and thermal effects; this often occurs during the initial curing period

before the surfacing is laid. I f the cracks are wide, load transfer by aggregate interlock can be severely impaired and

the combined effect of subsequent thermal and traffic loading may cause upward propagation of the cracks

through the bituminous surfacing. To minimise the likelihood of crack propagation, roads designed for heavy

traffic have a thicker surfacing than is used in conjunction with other types of roadbase as recommended in the

third edition of Road Note 29 published by the Road Research Laboratory (1970).

Thermal cracking depends upon the strength of the concrete, the coefficient o f thermal expansion of the

aggregate and the temperature changes to which the concrete is subjected, particularly during its early life when

its strength is low and when it is not protected by a surfacing. For a given coefficient o f thermal expansion of the

mix, the spacing between cracks increases with the strength of the concrete as described by Deen et al (1980).

Lean concrete with a high early life strength generally develops a few well-spaced but wide cracks that tend to

propagate through the surfacing under the action of traffic. Concrete with low early-life strength can form a

larger number of fine cracks close together but under repeated traffic stresses may not be strong enough to

resist progressive degradation to failure. There is therefore a balance between strength of the concrete and likeli-

hood of crack propagation. The effects of thermal cracking could be minimised by restricting the use of

aggregates with a high coefficient of thermal expansion and/or by limiting the time of year or temperature

conditions when roadbases are laid; these solutions have obvious practical and economic drawbacks. The

Depar tment of Transport (1976) SpecifiCation for Road and Bridge Works attempts to control the crack widths

and spacing by restricting the maximum permissible strength for lean concrete. Another approach and the one

considered in this Report could be to design mixes with a low early life strength but with a high ultimate strength.

Pulverised fuel ash (PFA) is a by-product o f coal-fired power stations and consists of lightweight spherical

particles with pozzolanic properties. Dunstan (1978) has shown that its addition to lean concrete modifies the

rate of gain in strength during curing by reacting with lime from the hydrating cement to form additional

cementit ious material, thus significantly improving the final strength of the concrete. The use of PFA in concrete

mixes reduces water demand and less cement is required to achieve the same ultimate strength; at the same time

it forms an effective filler. The lower cement content reduces the rate of gain in strength of a mix during its early

life; ash-modified lean concrete roadbases could therefore, in principle, be designed to have a low early life

strength, to produce fine, closely spaced cracking with good aggregate interlock, and a high in-service strength to

ensure good performance under traffic with less likelihood of cracks propagating through the surfacing.

Franklin, Gibbs and Sherwood (1982) have already explored the potential of ash-modified lean concrete by

laboratory studies o f the effect of varying the ash and cement contents on the development of strength with age

and by pilot-scale trials to determine the ease of laying selected mixes. This Report deals with the structural

performance of the materials laid in the pilot-scale trials.

2. CONSTRUCTION OF THE TRIALS

The trial mixes were laid under cover in a test-pit 17 metres long by 5 metres wide. The pit had a concrete

surround and the mixes were laid directly on a 275mm granular sub-base on top of the naturally occurring sandy

gravel Bagshot Beds. The California Bearing Ratio of the soil was on average 8 per cent measured with a cone

assessment penetrometer, as recommended by Black (1979), before the concrete mixes were laid. The pit was

divided by a wooden board into two strips 2½m wide. A total of fourteen mixes in three separate sets of trials

were compared for ease of laying and for compaction: a detailed description of the mixes and the compaction

procedure has been given in the earlier Report by Franklin, Gibbs and Sherwood (1982).

The PFA used was a selected ash with properties within the limits set by British Standard BS 3892 (1982)

and, to make an accurate comparison of the mixes, sufficient quantity was purchased from one batch to complete

the work described in this Report. Ordinary Portland cement was used which had standard cube compressive

strengths of 22.5 MPa and 43.0 MPa at 3 days and 28 days respectively. The aggregate was a Thames Valley

uncrushed flint gravel, from Laleham in Middlesex, with properties given in Table 1.

TABLE 1

Oven-dried relative densities and water absorptions of the Laleham flint gravel aggregate

Aggregate size Oven-dried relative density (mm)

4 0 - 2 0

20 - . 10

10-5

<5

2.55

2.52

• 2.45

2.70

Water absorption (% of dry mass)

0.8

1.3

2.6

• ~' " " 0.24

2

Each mix was batched and mixed for 2 minutes in a 0.5 cubic metre capacity paddle-mixer. The mixed

materials were transported the short distance from the mixer to the test-pit by lorry where they were tipped on

to a banking board, spread by hand and screeded off to the surcharge estimated to give the required compacted

thickness. Compaction was achieved using a combination of two rollers; a small pedestrian operated Stothert and

Pitt vibratory roller, type Vibroll D75, having a mass of 733 kg/m width of vibrating roll, and a Stothert and Pitt

type 42R 4-Ton Tandem roller, having a mass of 2718 kg/m width of vibrating roll. Full details of the rolling

procedures are given in the earlier Report by Franklin, Gibbs and'Sherwood (1982) which also showed that

satisfactory compaction of concrete with PFA had been achieved, even with the lighter roller, in layers up to

300mm thick.

3. STRUCTURAL ASSESSMENT OF THE ASH-MODI FlED LEAN CONCRETE MIXES

The potential of several ash-modified lean concrete mixes as an improved r0adbase material was assessed by direct

comparison with a conventional lean concrete control mix. Also included in the trial was a cement-rich mix, to

determine if the addition of extra cement would improve compactability in a similar manner to PFA. The cement-

rich mix was basically a pavement quality concrete but with a water content low enough for compaction by roller.

The structural properties of the four ash-modified mixes considered most likely to form the basis of a practical

roadbase material w~ere compared with these controls. Table 2 gives details of these mixes from which laboratory

specimens were manufactured. In each trial laboratory measurements were made of flexural strength and equivalent

cube compressive strength according to British Standard BS 1881: Part 4 (1970), dynamic elastic modulus accord-

ing to British Standard BS1881: Part 5 (1970) and the stress-strain behaviour under flexural loading. Tests were also

carried out on the trial slabs; these included the use of wave propagation techniques to follow changes in elastic

modulus during curing and the measurement of transient surface deflection on twelve of the slabs. Tensile strain

was measured at the bot tom of one ash-modified lean concrete roadbase and the lean concrete control under a

moving wheel load.

4. LABORATORY MEASUREMENTS

Dynamic elastic modulus, stress-strain behaviour in flexure and equivalent cube compressive strength were

measured on samples prepared in moulds at the time of construction and cured under water at 20°C according to

British Standard BS 1881 : Part 3 (1970). Measurements were normally made at 1, 4 and 26 weeks to enable the

early growth in strength and elastic modulus to be determined. To assess how closely these samples represented

the in-situ behaviour of the air-cured roadbases, they were compared with beams sawn from the trial area and

tested when 26 weeks old.

4.1 Preparation of concrete specimens

Twelve beams 152mm square by 71 l m m long were cast from material taken from four different lorry loads of

each mix after delivery to the test site. The concrete was compacted in the moulds using a vibrating hammer

following the method recommended by Williams (1961) and then covered by damp matting and polyethylene

sheeting for 48 hours. The beams were then taken out of the moulds and stored under water until required for

testing. The beams sawn from the slabs were removed from the trial area 28 days after it was laid. They were

taken from between the wheelpaths of the lorry used for in-situ testing in order to avoid regions where traffic-

induced cracking could have occurred and also to avoid the edges of the pit where compaction may have been

lower. These beams were then stored under water until required according to British Standard 1881: Part 3

(1970).

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beam was just out of the water. This face was dried and a strain gauge bonded to it using special adhesive. For

flexural testing the beams were installed in the testing equipment with the strain gauged face opposite the point

of loading.

4.2 Density

The density of the beams was determined by weighing them and measuring their dimensions. Table 3

compares the densities of the moulded and sawn beams and shows that in general the compaction achieved in the

trial was close to that achieved in the moulds. However the density of the conventional lean concrete was lower

than that of the ash-modified lean concretes and of the cement-rich mix.

TABLE 3

Density of beams

Mix notation

KLC

KL3A

Type of concrete

Lean concrete

Ash-modified

Origin of beams

Moulded.

Sawn

Moulded

Mean saturated bulk density

Mg/m 3

2.334

2.318

2.380

Sawn 2.377

KL3B Ash-modified Moulded 2.391

KL3C Ash-modified Moulded 2.367

Moulded 2.391

Sawn

Moulded

Sawn

KL3D

KRR

Ash-modified

Cement-rich

2.390

2.409

2.411

4.3 Flexural strength and stress-strain behaviour

• Flexural strength is a good measure of the structural quality of the concrete. The beams were tested in four-

point bending using "equipment described by Galloway and Raithby (1973) in which the load was applied to

sustain a constant rate of increase in stress until the beams failed. The flexural strength was then calculated from

a knowledge of the load applied at failure and simple bend!ng theory. Table 4 compares the flexural strengths

measured on beams from different mixes at 1, 4 and 26 weeks.

At one week the flexural strength of three of the four ash-modified mixes tested was appreciably less than

the control lean concrete mix (designated KLC) but by four Weeks their strengths were similar and by 26 weeks

the ash-modified mixes were much stronger. The flexural strength of the ash-modified mixes increased, on

average, by 220 per cent between 1 and 26 weeks, compared with a 60 per cent increase for the lean concrete

control mix. The cement-rich mix (KRR) was substantially stronger than the others at all ages. For comparison a

5

typical pavement quality concrete mix containing the same proportion of cement as the cement-rich mix (KRR),

but with a higher water content, had a flexural strength of 3.65N/ram 2 at 26 weeks as reported by Galloway,

Harding and Raithby (1979a). The strengths of the ash-modified mixes increased steadily to reach about the same

level at 26 weeks as the pavement quality concrete with its considerably higher cement content. These results

should have important implications on the in-service structural performance of ash-modified lean concrete

roadbases.

During flexural loading the stress-strain behaviour of the different mixes was measured. Fig 1 shows typical

relationships between the applied tensile stress at the underside of the beam, and the tensile strain measured by

the resistance foil strain gauges. At low levels of stress the relationship is linear but as the stress increases, the

strain increases more rapidly until the beam fails. Abeles and Hu (1971) suggested that this deviation from linearity

is likely to be caused by the formation and development of microcracks at the underside of the beam.

The stress-strain behaviour and the flexural strength of the beams sawn from the trials were measured at an

age of 26 weeks. Table 4 shows that the flexural strength was, on average, 82 per cent of the strength of the

moulded beams. This difference may be attributed to the different initial curing conditions because the specimens

sawn from the covered trial bays can be regarded as having been air cured for the first four weeks. When the beams

are cured throughout under water there may be sufficient moisture present to allow the PFA to react fully with

the hydrating cement; this may not be so in air curing or partial air curing. The difference in flexural strength is

in close agreement with earlier work on pavement quality concrete cured under different moisture conditions

reported by Galloway, Harding and Raithby (1979b).

4.4 Compressive strength

The equivalent cube compressive strength was measured using the broken halves of the beams that had failed

in flexure. The change in compressive strength with age of the moulded beams is given also in Table 4and can be

seen to follow a similar pattern to the development of flexural strength. At an age of 1 week the ash-modified

mixes were generally weaker than the lean concrete control mix but at 4 weeks they were noticeably stronger,

and by 26 weeks they were at least 50 per cent stronger. The compressive strength of the ash-modified mixes

improved by approximately 250 per cent between 1 and 26 weeks compared with a 75 per cent improvement for

the lean concrete control. The Department of Transport (1976) specification requires the compressive strength of

lean concrete cubes to be not less than 7N/mm 2 and not more than 14N/mm 2 at 7 days and not less than

10N/mm 2 and not more than 20N/mm 2 at 28 days. Table 4 shows that the strengths of the lean concrete control

mix are within these ranges.

At an age of 26 weeks the compressive strength of the sawn beams was about 80 per cent of the strength of

the moulded beams; a result similar to that noted for flexural s!rength. Clearly any difference in strength or other

properties between concrete cured in the field and concrete samples cured under standard laboratory conditions

is important in assessing the performance of roadbases from tests on samples cured in the laboratory.

During construction of the trials standard 152mm cubes were made from the different mixes for compressive

strength measurements. The results are given in Table 2. These values agree closely with the equivalent cube com-

pressive strength given in Table 4 for the ash-modified mixes and the cement-rich mix, but are higher for the lean

concrete control. This difference may be attributed to the difference in density between the beams and the cubes,

the density of the beams being on average 98.9 per cent of the value for the standard cubes.

6

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The compressive strengths of all but one of the ash-modified mixes tested had strengths within the range

necessary to meet the requirements of the Department of Transport (1976) Specification for Road and Bridge

Works: the ash-modified mix KL3D was stronger than the specification allows at both 7 and 28 days. The

cement-rich mix KRR also was stronger than the specification allows.

4.5 Elastic modulus

The dynamic Young's modulus of the beams was measured using the standard electrodynamic resonance

method recommended by the British Standards Institution BS 1881: Part 5 (1970) at ages of 1,4 and 26 weeks.

Table 5 shows that the elastic modulus of the mixes increased with age. It shows also that the values of elastic

modulus of all the ash-modified mixes as measure.d by the resonance method were slightly higher than the lean

concrete control mix and that the modulus of the cement-rich mix was considerably higher.

5. IN-SITU MEASUREMENTS

The structural response of the in-situ ash-modified lean concretes was compared with that of the lean concrete

control and the cement-rich mix by measuring the transient vertical deflection of the pavements under a slowly

moving lorry wheel as described by Kennedy, Fevre and Clark (1978); Kennedy a:~d Lister (1978) have shown that

the transient deflection is a good indicator of structural condition. This assessment was supplemented on one

ash-modified mix (KL3A) and on the lean concrete control (KLC) by the measurement of the transient tensile

strains induced at the underside of the slabs by a loaded lorry travelling along the surface. Another measure of

the elastic modulus of the concrete was obtained using wave-propagation techniques.

5.1 Elastic modulus

Wave propagation was used to monitor changes in the elastic modulus of the roadbases during the first 4

weeks of curing. The technique described by Jones (1963), consists of determining the wave velocity of the

output signal from a vibrator placed on the road and excited at selected frequencies. The relationship between

wave velocity and frequency provides information about the elastic modulus and thickness of the layers of the

road. Using this method Young's modulus was determined 24 hours after the concretes were laid and then again

on several occasions until 28 days, when the trial sections were removed. These results therefore complement the

measurements made on the beams.

The change in Young's modulus during curing for the ash-modified concrete KL3A, the lean concrete

control and the cement-rich mix is shown in Fig 2. The lean concrete control (KLC) and the ash-modified mix

(KL3A) show a similar pattern with Young's modulus increasing steadily during the first few days of curing

before levelling out to a fairly constant value; the other ash-modified mixes exhibited similar behaviour with the

lower strength mixes generally having a lower early life modulus. Fig 2 and Table 5 shows that the values of

Young's modulus measured by wave propagation on the'concrete slabs were generally slightly lower than those

determined by the electrodynamic resonance method.

5.2 Transient deflection

The vertical transient deflection of all the trial concretes under slowly moving lorry wheels was measured

with a deflection beam at five positions on each slab at 1 and 4 weeks after construction. The average deflections

were low, less than 13 x 10 .̀ 2 mm, and consistent with pavements having substantially uncracked lean concrete

roadbases. To compare the deflections of the ash-modified lean concretes with those of the lean concrete control

and the cement-rich mix it was necessary to consider the effect of the thickness of the concrete. Fig 3 shows the

strong correlation between deflection and the thickness determined from cores that is reasonably independent of

mix. Comparison of Fig 3a and 3b shows a reduction in deflection level with time that can be explained reasonably

well by the increase in Young's modulus of the different mixes interpreted through elastic theory.

TABLE 5

Modulus of elasticity results

Concrete Mix

KLC

KL2

KL4

KL3A

KL3B

KL3C

KL3D

KRR

Measured in-situ by Measured by the electrodynamic method wave p~opagation

Modulus of elasticity Origin

of beams

Moulded

Sawn

Age (weeks)

1

4

26

26

Mean GPa

33.7

37.8

41.5

41.0

No. of results

4

4

4

C of V %

5.8

1.7

1.9

3.1

NO BEAMS MADE

Moulded

Sawn

Moulded

Moulded

Moulded

Sawn

Moulded

Sawn

l

4

26

26

1

4

26

1

4

26

1

4

26

26

1

4

26

26

36.2

39.3

48.7

45.4

35.0

40.7

47.2

33.9

38.3

42.2

40.2

42.7

46.4

43.8

48.8

51.8

53.6

52.3

4

4

4

3.7

1 3 . 2

4.1

4.1

2.2

1.8

0.8

3.9

4.7

2.6

1.5

2.5

1.6

1.3

1.5

1.2

0.9

1.6

Age (weeks)

1

4"

1

4

1

4

1

4

Modulus of elasticity

GPa

30.5

37.0

26.5

33.0

34.5

41.5

32.0

40.0

30.5

34.0

33.5

35.5

34.0

34.0

46.5

48.0

9

5.3 Transient strain

The longitudinal and transverse strains generated by a two-axle lorry were measured at the underside of the

lean concrete control mix (KLC) and the ash-modified mix KI_3A. Mix KL3A was chosen for comparison because

it had performed well in earlier laboratory studies reported by Franklin, Gibbs and Sherwood (1982). The two

mixes were laid side by side in the first trial and gauges described by Potter (1972) were installed during construc-

tion in positions so that the nearside wheels of the lorry would pass directly over those in the control section

when the offside wheels were directly above the gauges in the ash-modified section. In each section four gauges

were installed to measure longitudinal strain and four to measure transverse strain; one gauge, measuring transverse

strain in the ash-modified mix, was damaged during construction.

The rear wheels of the lorry were fitted with twin tyres and carried a load of 4770 kg on each dual wheel.

Measurements were made when the concretes were 1 and 4 weeks old and on both occasions strains were recorded

during 20 passes of the lorry travelling at about 10 km/h. Average values for each gauge are given in Table 6 and

show that the longitudinal strain was larger than the transverse strain at the same age; this is typical under twin-

tyred wheels. The reduction in strain with age is consistent with the increasing Young's modulus shown in Fig 2.

The differences between gauge outputs could be a combination of variability in thickness, elastic modulus and

foundation support. The lean concrete control section was on average slightly thicker than the ash-modified

concrete, hence the slightly lower average strain, but when thickness was taken into consideration there was no

significant difference between the strains.

TABLE 6

Transient strains measured at the bottom of the concrete

Transverse strain

Longitudinal strain

Tensile strain (/am/m)

Ash-modified lean Lean concrete control Gauge

concrete KL3A at age KLC at age

1 week 4 weeks weeks

Average

Average

22 26 19

22.3

37 38 37 31.5

35.9

D

17 20 15

17.3

26 28.5 26.5 24.5

26.4

1 week 4

21 22 23 18.5

21.1

33 31.5 32 32

32.1

16 16 18 14

16

27 25 25.5 27

26.1

10

6. APPLICATION TO THE CONSTRUCTION AND PERFORMANCE OF LEAN CONCRETE ROADBASES

6.1 Use of elastic theory

Computer programs developed by Thrower (1968) (1971) based on linear elastic theory were used to

calculate dynamic strains generated at the bot tom of the roadbases by the rear wheels of the test lorry for

comparison with strains measured in the trials. The values of Young's modulus for the roadbases and the sub-base

were derived from the wave propagation tests and the modulus of the soil subgrade from a correlation with the

cone penetrometer measurements. Table 7 shows that the measured larger longitudinal strains were predicted

very closely. This implies that linear elastic theory could justifiably be used to calculate the maximum tensile

strains generated at the bot tom of uncracked cement-bound roadbases by construction traffic laying the

surfacing.

TABLE 7

Comparison of measured and predicted strain

Mix

Lean concrete control KLC

Ash-modified lean concrete KL3A

Longitudinal tensile strain at bo t tom of slab (pm/m)

1 week old 4 weeks old

Measured Predicted Measured Predicted

32.1

35.9

32.5

36.0

26.1

26.0

28.0

30.2

6.2 The onset of cracking

In section 4.3 it was suggested that microcracking could occur when the flexural stress-strain relationship

deviated from linear. The results of the laboratory flexural tests were therefore examined and the levels of tensile

stress and strain at which microcracking may begin were derived for all the measurements on the different

concrete mixes. The results are given in Table 8 and show that the stress and strain levels increase with age and

depend on the concrete mix. Table 8 shows also that these values compare closely with stresses calculated on the

basis of the results of laboratory work by Galloway, Harding and Raithby (1979a) who demonstrated that if the

applied cyclic stress was less than about 68 per cent of the flexuraI strength, the fatigue life of concrete was likely

to be far in excess of 10 million loading cycles.

6.3 The effect of construction traffic on cracking

The Department of Transport (1976) Specification for Road and Bridge Works does not permit newly

constructed lean concrete roadbases to be trafficked until 7 days after they are laid. By this time it is considered

that the lean concrete is strong enough to withstand the stresses induced by construction traffic without

appreciable cracking. The present work has shown that the addition of pulverised fuel ash to lean concrete

modifies the rate of gain in strength of the mix during curing and it has been suggested that a mix with a low

early life strength and a high in-service strength would be advantageous in increasing the frequency and reducing

the width of transverse cracks induced by thermal and shrinkage effects. However a mix with these properties

may not have gained sufficient strength within 7 days to prevent appreciable cracking by construction traffic.

11

TABLE 8

Deduced onset of microcracking

Concrete mix

KLC

KL3A

KL3B

KL3C

KL3D

KRR

Age (weeks)

1 4

26

1 4

26

1 4

26

1 4

26

1 4

26

1 4

26

Deduced onset of microcracking

From flexural stress-strain relationship

Stress (N/mm 2 )

0.9 1.3 1.8

0.8 1,6 3.4

1.0 1.6 2.6

0.8 1.3 2.8

1.2 1.6 2.6

3.2 3.4 3.4

Strain ~ m / m )

33 45 54

29 48 85

38 48 66

31 42 84

37 46 66

67 76 74

At 68 per cent of the flexural

strength

Stress (N/mm 2 )

1.16 1.52 1.86

0.84 1.55 2.94

0.89 1.55 2.74

0.77 1.42 2.4!

1.23 1.86 2.62

3.16 3.30 3.68

To assess the likelihood-of construction traffic causing microcracking in the roadbase the maximum tensile

strains generated by construction traffic were calculated using computer programs by Thrower (1968) (1971) and

compared with the strain levels taken from Table 8 at which microcracking may start. Fig 4 shows the relation-

ships between roadbase thickness and the calculated strains at the bottom of the roadbase induced by a 5 ton

wheel load: the slab thicknesses correspond to designs of between about 1 and 20 million standard axles when

constructed on a subgrade of 5 per cent CBR. Results are presented for the ash-modified lean concrete KL3A,

the conventional lean concrete .and the cement-rich concrete when they were 1 week and 4 weeks old. The three

mixes all show reductions in strain level with time that are broadly in proportion to the increase in Young's

modulus. Also shown in Fig 4 are the strain levels taken from Table 8 at which microcracking may occur when

the concretes are 7 days old. These show that construction traffic could produce microcracking in both the ash-

modified lean concrete and the conventional lean concrete but the formation of microcracks within cement rich

concrete is unlikely.

In practice the importance of microcracking during construction is not known. However if it does occur as

suggested by this analysis the continuing pozzalanic action of ash-modified lean concrete should give the concrete

a greater capacity for autogenous healing than a conventional material and this could mean that the ash-modified

1 2

concrete might be substantially uncracked when in service. It is also not known whether in the road ash-modified

lean concretes are able to sustain the greater long term gain in strength observed in the laboratory tests if initially

micro-cracked by construction traffic. The importance of microcracking occurring during construction could be

assessed in a full-scale trial by trafficking some test sections at 7 days old and then comparing their subsequent

visible crack development with that from identical but untrafficked test sections.

6.4 Long term performance

Deflection measurements are a general indicator of long term performance and in these trials the deflections

measured at 7 days and at 28 days on the ash-modified lean concrete are similar to those observed on the conven-

tional lean concrete. This suggests that the materials have a similar long term performance potential which is

supported by the measurement of similar levels of strain induced at the bot tom of the roadbases by the lorry.

Provided that cement-bound roadbases remain uncracked, their high values of elastic modulus are large

enough to reduce the vehicle induced stresses and strains in the sub-base and subgrade to levels at which failure

of the foundation should not occur. However the tensile stress generated at the bot tom of cement-bound road-

bases by traffic increases as the modulus of the Concrete increases relative to that of the road foundation. It is

therefore important to ensure that the in, service strength of the concrete can withstand the thermal stresses and

repeated traffic stresses without cracking. The greater long term gain in strength of ash-modified lean concrete

coupled with its capacity for self healing should enable this roadbase material to withstand higher levels of stress

and strain. The in-service deflections should also be reduced when compared with those of conventional lean

concrete.

7. CONCLUSIONS

The addition of pulverised fuel ash to lean concrete roadbase modifies the rate of gain of flexural and compressive

strength of the concrete but makes little difference to the rate of gain in elastic modulus during curing. The

structural behaviour of the trial lengths of ash-modified lean concrete mixes assessed by measurement of transient

deflection and transient strain af7 and 28 days old was the same as conventional lean concrete.

Ash-modified lean concretes with early life strengths that are less than conventional lean concrete could

suffer from microcracking caused by construction traffic allowed o n the roadbase after 7 days, but visible cracking

should not occur. A theoretical analysis suggests that construction traffic could also cause microcracking in

conventional lean concrete roadbases but not in cement-rich concrete.

The ease of compaction and structural behaviour of ash-modified lean concrete are sufficiently encouraging

to warrant a full-scale trial to study construction techniques and to evaluate the long-term structural performance.

8. ACKNOWLEDGEMENTS

The work described in this Report forms part of the research programme of the Pavement Design and Maintenance Division (Head of Division: Mr J Porter) of the Highways & Structures Department of TRRL. The authors thank

Mr A R Halliday for his assistance in the experimental work.

13

9. REFERENCES

ABELES, P W and C H HU. (1971) Flexural microcracking in unreinforced concrete beams. ACI Journal, 1971, 68. (10), 770-787.

BLACK, W P M. (1979). The strength of clay subgrades: its measurement by a penetrometer. Department of the Environment Department o f Transport, TRRL Report LR 901: Crowthorne, (Transport and Road Research Laboratory).

BRITISH STANDARDS INSTITUTION. (1970) Methods of testing concrete. Part 3. Methods of making and

curing test specimens. BS 1881: Part 3: 1970. London, (British Standards Institution).

BRITISH STANDARDS INSTITUTION. (1970) Methods of testing concrete. Part 4. Methods of testing concrete for strength. BS 1881: Part 4: 1970. London, (British Standards Institution).

BRITISH STANDARDS INSTITUTION. (1970) Methods of testing concrete. Part 5. Methods of testing

hardened concrete for other than strength. BS 1881: Part 5: 1970. London, (British Standards Institution).

BRITISH STANDARDS INSTITUTION. (1982) Pulverised-fuel ash. Part 1. Specification for pulverised-fuel ash

for use as a cementitious component in structural concrete. BS 3892: Part 1. 1982 London (British Standards Institution).

DEEN, R C, J H HAVENS, A S RAHAL and W V AZEVEDO. (1980) Cracking in concrete pavements.

J. Transp. Engng. Div. Am Soc. Civ. Engrs., 1980, 106, (March), 155-169.

DEPARTMENT OF TRANSPORT. (1976) Specification for Road and Bridge Works. London, (H M Stationery Office).

DUNSTAN, M R H. (1978) Rolled concrete with particular reference to its use as a hearting material in concrete dams. Kirkwood Dodds Travel Fellowship 1977. London, (The Concrete Society).

FRANKLIN, R E, W E GIBBS and P T SHERWOOD. (1982) The use of pulverised fuel ash in lean concrete road-

bases, Part 1, Laboratory Studies. Department of the Environment Department of Transport, TRRL Report SR 736: Crowthorne, (Transport and Road Research Laboratory).

GALLOWAY, J W, H M HARDING and K D RAITHBY. (1979a) Effects of age on flexural strength, fatigue and

compressive strength of concrete. Department o f the Environment Department o f Transport, TRRL Report LR 865: Crowthorne, (Transport and Road Research Laboratory).

GALLOWAY, J W, H M HARDING and K D RAITHBY. (1979b) Effects of moisture changes on flexural and

fatigue strengths of concrete. Department o f the Environment Department of Transport, TRRL Report LR 864: Crowthorne, (Transport and Road Research Laboratory).

GALLOWAY, J W and K D RAITHBY. (1973) Effects of rate of loading on flexural strength and fatigue performance of concrete. Department of the Environment, TRRL Report LR 547: Crowthorne, (Transport and Road Research Laboratory).

14

JONES, R. (1963) FoUow'_mg changes in the properties of roadbases and sub-bases by the surface wave

propagation method. Cir. Eng., London, 58 (682), 613,615,617: (683), 777--80.

KENNEDY, C K, P FEVRE and C S CLARKE. (1978) Pavement deflection: equipment for measurement in the

United Kingdom. Department of the Environment Department of Transport, TRRL Report LR 834:

Crowthorne, (Transport and Road Research Laboratory).

KENNEDY, C K and N W LISTER. (1978) Prediction of pavement performance and the design of overlays.

Department of the Environment Department of Transport, TRRL Report LR 833: Crowthorne, (Transport and

Road Research Laboratory.

POTTER, J F. (1972) Stresses and strains generated in road structures. Proceedings of the Joint British

Committee for Stress Analysis Conference on the recording and interpretation of engineering measurement.

London, (The Institute of Marine Engineers), pp 63-70.

ROAD RESEARCH LABORATORY. (1970) A guide to the structural design of pavements for new roads.

Department of the Environment, Road Note No 29 (third edition). London (I-I M Stationery Office).

THROWER, E N. (1968) Calculation of stresses and displacements in a layered structure. Ministry of Transport, RRL Report LR 160. Crowthorne, (Road Research Laboratory).

THROWER, E N. (1971) Calculation of stress, strains and displacements in a layered elastic structure, Part II.

Department of the Environment, RRL Report LR 373. Crowthorne, (Road Research Laboratory).

WILLIAMS, R I T. (1961) A laboratory investigation of methods of compacting test cubes of dry lean concrete.

Cement and Concrete Association, Technical Report TRA/322, London.

15

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P R I N T E D IN E N G L A N D

ABSTRACT

The use of pulverised fuel ash in lean concrete roadbases. Part 2 -- pi lot~cale trials: H M HARDING and J F POTTER: Department of Transport, TRRL Supplementary Report 838: Crowthorne, 1985 (Transport and Road Research Laboratory). Pilot-scale trials were laid to investigate the properties of roller-compacted lean concrete roadbases in which pulverised fuel ash was included as a partial replacement for some of the cement. A conventional lean concrete and a cement-rich concrete were included in the trials for comparison. Structural performance was assessed by measuring strain and deflection under a moving wheel and elastic modulus by surface wave propagation. Beam specimens cast in moulds and cut from the trial area were used to determine stress-strain relationships, flexural strength and elastic modulus.

The measured transient deflections and strains in the ash-modified lean concrete pavements were similar to those in the conventional lean concrete; the values of elastic modulus were also similar. Results from the beams showed that in general the ash-modified mixes have a lower early life strength and a higher long term strength than conventional lean concrete. An analytical study suggested that both ash-modified lean concrete with a low early life strength and conventional lean concrete may suffer from microcracking when trafficked by construction vehicles at 7 days old but visible cracking should not occur.

The overall structural behaviour of the pilot-scale trials was sufficiently encouraging to recommend a full- scale trial of ash-modified lean concrete.

ISSN 0305-1315

ABSTRACT

The use of pulverised fuel ash in lean concrete roadbases. Part 2 - - pilot-scale trials: H M HARDING and J F POTTER: Department of Transport, TRRL Supplementary Report 838: Crowthorne, 1985 (Transport and Road Research Laboratory). Pilot-scale trials were laid to investigate the properties of roller-compacted lean concrete roadbases in which pulverised fuel ash was included as a partial replacement for some of the cement. A conventional lean concrete and a cement-rich concrete were included in the trials for comparison. Structural performance was assessed by measuring strain and deflection under a moving wheel and elastic modulus by surface wave propagation. Beam specimens cast in moulds and cut from the trial area were used to determine stress-strain relationships, flexural strength and elastic modulus.

The measured transient deflections and strains in the ash-modified lean concrete pavements were similar to those in the conventional lean concrete; the values of elastic modulus were also similar. Results frorfi the beams showed that in general the ash-modified mixes have a lower early life strength and a higher long term strength than conventional lean concrete. An analytical study suggested that both ash-modified lean concrete with a low early life strength and conventional lean concrete may suffer from microcracking when trafficked by construction vehicles at 7 days old but visible cracking should not occur.

The overall structural behaviour of the pilot-scale trials was sufficiently encouraging to recommend a full- scale trial of ash-modified lean concrete.

ISSN 0305-1315