TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment Department of Transport

TRRL LABORATORY REPORT 947

CRACK CONTROL IN CONCRETE BEAMS BY SURFACE REINFORCEMENT WiTH GLASS FIBRES

by

K D Raithby

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

Bridge Design Division Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1980 ISSN 0305-1293

Abstract

1.

2.

3.

CONTENTS

Introduction

Mechanism of crack suppression by glass fibre reinforcement

Tests on reinforced concrete beams with grc surface reinforcement

3.1 Description of test beams

3.2 Mix proportions and nominal strength properties

3.3 Method of test

4. Results of tests

4.1 Ultimate failure loads

4.2 Deflections

4.3 Strains and crack development

5. Discussion of results

5.1 Deflections

5.2 Ultimate strength

5.3 Crack development

5.3.1 Cracks in concrete

5.3.2 Cracks in grc

6. Effectiveness of grc in controlling cracks in the concrete

7. Conclusions

8. Acknowledgements

9. References

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© CROWN COPYRIGHT 1980 Extracts from 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.

CRACK CONTROL IN CONCRETE BEAMS BY SURFACE REINFORCEMENT WITH GLASS FIBRES

ABSTRACT

Tests were made on composite reinforced concrete beams incorporating a surface layer of glass reinforced cement (grc) to investigate the effect of local surface reinforcement on serviceability behaviour. The grc was in the form of 6 mm thick shallow channel sections moulded into the tension surface of the test beams, which were 3.5m long and were tested in four-point bending on a simply supported span of 3m. On beams which had been kept dry, the grc surfacing had a significant effect in reducing the size of cracks in the concrete at the design working load but had no effect on deflections; no visible cracks appeared on the surface of the grc until the steel had yielded, at a load well above the design ultimate. Some beams were subjected to artificial ageing before test, by immersing them in hot water to represent natural exposure of the grc for periods of up to 30 years. On these beams embrittlement of the grc surfacing rendered it virtually ineffective in controlling cracks in the concrete. The observed behaviour compares well with predictions made from a knowledge of the relevant material properties. Considerable improvements in performance could be expected from newer develop- ments in fibre cement composite technology and by using deeper sections for the surface reinforcement. A logical development, which is likely to have economic advantages, is to use suitable sections of rigid fibre rein- forced cement sheet as permanent formwork for reinforced concrete members.

1. INTRODUCTION

Fibre reinforced materials are sometimes used as permanent fofmwork for placing in situ concrete on

bridge decks. Glass reinforced cement in the form of stiffened panels has been used for this purpose on

several major bridges, for example the Kenfig viaduct on the M4 in South Wales and the Marron bridge

on the A66 in Cumbria. Advantages of such materials are that they are easily handled, ate resistant to

impact damage, are not affected by corrosion and bond well to the concrete. Although not usually taken

into account in the design, the surface skin provided by such permanent formwork can have a beneficial

effect on the resistance of the concrete to cracking.

The concept of using a thin layer of fibre reinforced cement sheet acting compositely with conventional

reinforced concrete to improve its serviceability performance has been developed by N J Dave at the

University of Salford 1,2. Tests on a variety of reinforced and pre-stressed beams and slabs have demonstrated

the efficacy of such materials as asbestos cement in reducing deflections and crack widths under working

loads when the tension zone of the concrete incorporates a surface layer of fibre reinforced cement sheet

(frc). Health hazards associated with the use of asbestos have stimulated the development of frc sheet

materials based on other types of fibre, such as polypropylene and glass. The most readily available alternative

to asbestos cement in sheet form is glass reinforced cement (grc) which is made from chopped strands of

alkali-resistant glass fibre in a matrix of cement paste or cement mortar.

Although grc has properties which are suitable in the short term, with prolonged exposure to moisture

it becomes brittle and therefore is likely to be less effective in controlling the onset and development of

cracking in the adjacent concrete. This Report describes a series of tests in which composite beams

incorporating grc surfacing were subjected to accelerated ageing to determine what changes in overall

structural performance might be expected due to prolonged exposure to the atmosphere.

2. MECHANISM OF CRACK SUPPRESSION BY GLASS FIBRE REINFORCEMENT

It is well known that concrete is weak in tension and develops visible cracks when the tensile strain teaches

about 0.01 per cent (100 microstrain). In a conventional reinforced concrete member subjected to tensile

or flexural loading a well defined pattern of cracking develops in which the crack spacing on the surface

depends mainly on the effective concrete cover 3. Once the crack pattern is established, further increase of

load results in a widening of the individual cracks by an amount roughly proportional to the strain in the

steel reinforcement at the cracked section. For a normal structural grade concrete with high yield deformed

bar reinforcement, the crack spacing might be of the order of 100 mm and the crack width would probably

reach about 0.2 mm at the onset of yielding of the steel.

Recent design codes 4,5 restrict permissible crack widths at serviceability limit states, the permissible

width depending on the severity of the environment. Although there is now some doubt about the validity

of the concept of allowable crack widths insofar as liability to corrosion of internal reinforcement is concerned 6,

any move towards reducing the susceptibility to cracking of concrete in tension is likely to have beneficial results.

By a proper choice of materials, the use of surface reinforcement intimately bonded to the concrete

offers the prospect of providing a surface layer which will accommodate large strains without fracture. I f

there is no slip between the surfacing and the underlying concrete it should then be possible theoretically to

replace isolated wide cracks in the concrete by a larger number of finer cracks for a given level of effective

strain. The tests reported here were carried out to study the effectiveness of glass reinforced cement in

achieving this objective, taking account of changes in its properties due to prolonged exposure to moisture.

If grc is subjected to wetting and drying, the strain at failure reduces with time until eventually it falls

to that of the unreinforced matrix 7. Results of tests at the Building Research Establishment 8 have shown

the following changes in direct tension properties of test specimens subjected to natural weathering.

UTS - N/mm 2

Matrix cracking stress (BOP) - N/mm 2

Young's modulus - GN/mm 2

20 years 28 days 1 year 5 years i0 years (estimated)

14--17

9 - 1 0

20--25

11-14

9 - 1 0

20--25

7 - 8

7 - 8

25-32

7--8*

6--8*

27--30*

5 - 7

5 - 7

25-32

* Difficulty in testing because of brittle behaviour.

2

It has been found empirically that similar changes can be produced to an accelerated timescale by

immersing the grc in hot water. The following approximate conditions apply:

One month in water at 60°C is equivalent to 10 years natural weathering,

Three months in water at 60°C is equivalent to 30 years natural weathering.

Changes in strain capacity would be expected to have a significant influence on the effectiveness of surface

reinforcement and this was one of the main points of the tests reported here.

3. TESTS ON REINFORCED CONCRETE BEAMS WITH GRC SURFACE REINFORCEMENT

3.1 Description of test beams

Four-point bending tests were carried out on two types of reinforced concrete beam as shown in

Figure 1. Both types were 3.5m long and over the central portion were reinforced on the tension side only

by two 12 mm diameter deformed bars ('Torbar'). Shear reinforcement was included towards each end of

the beam.

Type R was a conventional reinforced concrete beam with an effective depth to the reinforcement of

214 mm and with an effective cover* of 36 mm. Type G had a thin channel section of glass reinforced

cement cast into the tension surface. In this case, although the effective depth to the internal reinforcement

remained the same, the reinforcing bars were in direct contact with the grc, resulting in a slight reduction in

overall depth. Some preliminary tests had been made on an intermediate type of beam which also had grc

surface reinforcement but had the same overall depth as the conventional reinforced concrete beam. These

tests, however, were inconclusive because the relative levels of the internal reinforcement and the top of the

grc channel tended to induce premature failure. The results of these preliminary tests are not reported here,

although some further mention of them is made in Section 5.1.

The grc contained 5 per cent by weight of alkali-resistant glass fibres and was supplied by the Building

Research Establishment in the form of shallow channel sections, 3.5m long, 50 mm deep and 120 mm wide.

They were made from fiat sheet produced by the spray suction method, cut into strips while wet and draped

over wooden formers before being left to harden. The finished thickness of the grc sheet was nominally

6 mm. After delivery to TRRL the grc channels were kep~ for about nine months in an air conditioned

laboratory (20°C and 65 per cent RH) before use. Each channel section was then put in the bot tom of a

wooden mould and the steel reinforcing cage placed in position on the grc channel. Concrete was placed in

the mould in the usual way, using pokers for compaction. The beams were cast in an unheated building,

demoulded after one day, and then covered with wet sacking and polyethylene sheeting and left for three

months before being tested. After the three months curing period, some of the grc reinforced beams were

subjected to artificial ageing by immersing them for periods of up to three months in water at 60°C. This

was done in a thermostatically controlled tank at Pilkington Bros. Research Laboratories. Samples of the

grc channel sections were also subjected to artificial ageing at the same time. After soaking in hot water

the beams were allowed to dry in the laboratory atmosphere for about 2 weeks before being tested.

* ie distance from centre of reinforcing bar to surface of beam.

3.2 Mix proportions and nominal strength properties

The concrete was made from Thames Valley flint gravel aggregate with a maximum size of 19 mm.

The mix proportions and nominal and achieved strengths are given in Table 1. The method of preparing

and mixing the concrete was broadly as described in reference 9. This involved oven-drying all aggregates

before batching to ensure that the design water/cement ratio was achieved on all test beams, with minimum

variability of concrete strengths throughout. That this was achieved is shown by the extremely low values

of coefficient of variation obtained on 28 day cube and flexural strengths, 2.1 per cent and 4.0 per cent

respectively. Table 1 also includes design and measured strengths for the deformed bar longitudinal rein-

forcement and mean strength values for the grc surfacing.

3.3 Method of test

Each test beam was simply supported on a span of 3m and load was applied to the upper surface at

the middle third points through a single compression jack acting through a steel load-spreading beam to give

four-point bending, as shown in Figure 1.

Vertical deflections were measured at the mid-span position.

Longitudinal strains were measured at a typical cross-section in the constant moment region to

determine strain distributions at various stages of loading, using C and CA type 'portal' gauges with 100 mm

gauge length. Demec points were attached to a vertical face of the beam at 50 mm intervals at the level of

the internal reinforcement and just above the top of the grc channel to enable crack opening displacements

to be measured at these two levels. Load was applied in increments under deflection control until the

beam failed by excessive yielding of the reinforcement.

4. RESULTS OF TESTS

4.1 Ultimate failure loads

Loads at failure for each type of specimen are given in Table 2, for comparison with design loads and

calculated failure loads. Design loads are based on normal reinforced concrete theory, assuming that both

concrete and grc are cracked below the neutral axis and that tensile forces are resisted by the internal steel

reinforcement only. The design ultimate load is calculated from the nominal characteristic strengths of the

concrete and steel (Table 1). The design working load is derived from the design ultimate by arbitrarily

assuming an overall safety factor of 1.7.

The calculated failure loads are based on mean test values for the strength of the concrete and steel

(Table 1) and on typical stress-strain curves and tensile strengths for the grc derived from tests at BRE

(Figure 6). Unaged grc has an effective tensile strength at strains well beyond the yield strain of the steel

reinforcement and is assumed to carry its share of the flexural loading. The failure strain of artificially

aged grc is very much lower and in this condition the grc does not contribute to the flexural strength of

the beam.

4.2 Deflections

Typical load-deflection curves for each of the four types of test beam are given in Figure 2 and the

central deflection of each beam at the design working load is given in Table 2. There is not much difference

between them, showing that the presence of the grc channel had little overall effect on the flexural

stiffness of the beam. 4

4.3 Strains and crack development

Longitudinal strains measured with the portal gauges showed a generally linear distribution over the

depth of each test beam, thus confirming that there was a good bond between the concrete and the grc

right up to failure and that for subsequent analysis of the results it was reasonable to assume full composite

action, with plane sections remaining plane and with a gradual shift of the neutral axis towards the

compression surface. Typical examples of the strain distribution are shown in Figure 3. The non-linearity

in Figure 3(a) is probably due to the influence of a crack developing in the concrete near the lowermost

gauge position.

Crack width measurements derived from Demec readings are illustrated in Figure 4 a t two levels

(a) at the level of the internal reinforcement and (b) just above the level of the top of the grc channel.

These measurements are plotted as apparent crack widths, since the total displacement over the gauge

length included the effect of some strain in visually uncracked concrete and grc as well as the displacement

due to the opening of existing cracks. For ordinary reinforced concrete, errors due to neglecting concrete

strains are negligible. In the case of the beam without grc and the beams with grc which had undergone

artificial ageing, discrete cracks at the level of the reinforcement became visible at a load of 15-18 kN.

With the beams containing unaged grc, however, no cracks were visible in the grc until the steel had yielded,

even though the measured strains were well in excess of the probable cracking strain of the cement mortar

matrix. This point is discussed further in Section 5.3.2. At the higher level, cracks were observed in the

concrete at loads between 15 and 18 kN for all four types of specimen but the crack opening under a given

load was rather less with the unaged grc specimens. Typical crack patterns at the design ultimate load are

illustrated in Figure 5. The development of cracking for each individual test beam is summarised in Table 3.

5. DISCUSSION OF RESULTS

5.1 Deflections

A comparison of the central deflections (Figure 2 and Table 2) shows very little difference between

beams with and without grc surfacing, whether artificially aged or not. This observation apparently conflicts

with the results of comparable tests at Salford University on reinforced concrete beams with asbestos

cement surfacing 2, which indicated that the inclusion of surface reinforcement should result in reduced

deflections up to and beyond the design working load.

The apparent differences in behaviour between the TRRL and Salford tests can be explained by

differences in the stress-strain relationships for grc and asbestos cement. A typical stress-strain curve for

grc is reproduced in Figure 6. This shows a steep initial slope, corresponding to the uncracked sheet,

a well-defined 'bend over point' and a secondary slope which is at least an order of magnitude less than

the initial slope. It is unlikely, therefore, that the grc will have any appreciable influence on the overall

stiffness of the beam once the effective strain in the concrete has exceeded the bend over point at about

300 microstrain. This occurs well below the design working load. With asbestos cement, however, the

slope of the stress-strain curve is quite different 10. The initial slope is maintained up to much higher strains,

so that at the design working load the asbestos cement surfacing would still be expected to have an influence

on the stiffness of the beam.

A further minor difference between the TRRL and Salford tests is that the concrete had rather

different compressive strengths (mean values 57 and 45 N/mm 2 respectively). This would tend to make

5

any contribution of the surfacing to the overall stiffness of the beams less in the case of the TRRL tests,

with the higher strength concrete. (In some preliminary tests at TRRL which involved grc reinforcement

of similar beams with a lower grade of concrete - mean cube strength 46 N/mm 2 - there was some

indication of slightly reduced deflections on the beams containing grc.)

5.2 Ultimate strength

The failure loads agreed closely with those predicted from normal reinforced concrete theory (Table 2).

There was little difference between beams with and without grc, although the beams with unaged grc were

about 10 per cent stronger until the grc fractured. Final failure always occurred by yielding of the steel

reinforcement, which meant that once yield developed, the fracture strain of the grc was exceeded even

for the beamswhich had not been artificially aged. In the latter case this usually resulted in a single

major crack right through the grc channel. With the artificially aged beams several major fractures of

the grc had occurred by this time (see Section 5.3.2 below). Once the grc channel had fractured, each

beam behaved as a normal reinforced concrete beam approaching collapse.

5.3 Crack development

See Table 3 and Figure 4.

5.3.1 Cracks in concrete. For the beam without surface reinforcement the development of flexural

cracks followed the usual sequence, in which a fairly regular pattern of cracks developed in the constant

moment span, with an average spacing of about 80 mm by the time the design ultimate load was reached.

This compares with a value of 83 mm from the empirical formula s m = 2.3 Ce, derived from large scale

beam tests at BRE 11. Here s m = average crack spacing, c e = effective cover as def'med by the distance

from the centre of the bar to the surface.

For the beams with grc surface reinforcement cracks became visible in the concrete at the top of the

upstanding leg of the grc channel section before the design working load was reached. The average crack

spacing was much the same as for the beam without grc, whether the beams had been artificially aged

or not. It was not possible to say how far cracking of the concrete extended downwards behind the grc

surfacing. In all cases cracks extended upwards with increase in load, with a corresponding shift in the

position of the neutral axis at the cracked section. Before artificial ageing, the grc had some effect in

reducing the width of cracks in the concrete, the reduction being about 33 per cent at the design working

load and 26 per cent at the design ultimate load (Figure 4(b)). After artificial ageing (one month or three

months) there was little or no effect.

If it is assumed that there is some local debonding between the concrete and the grc in the vicinity

of wide cracks in the concrete'it may be inferred that such cracks would extend down at least to the level

of the reinforcement. The apparent crack widths of Figure 4(a) may then be taken to approximate to

actual crack widths in concrete hidden by the grc. At the design working load the unaged grc has the effect

of reducing the maximum apparent crack widths at the level of the reinforcement by about 30 per cent

from 0.095 mm to 0.065 mm. This compares with a design value for reinforced concrete of 0.06 mm for

short term loading using the formula 2.3a_ r e 1 given in BS 54005, or 0.075 mm for average crack width

(0.15 mm maximum) given by 2.3c e e I 1 ~. In these two formulae e I is the tensile strain in the reinforcing

steel and acr is the perpendicular distance from the point considered to the surface of the nearest longitudinal

bar. After artificial ageing the grc itself developed visible cracks similar in magnitude and spacing to those

which developed in the reinforced concrete without surface reinforcement.

6

5.3.2 Cracks in gre. For the beams which had not undergone artificial ageing, no cracks were visible

in the grc channel until the steel had yielded at an applied load of about 45 kN, nearly 30 per cent above

the design ultimate load. However some quite large strains had been measured by this time, indicating

that cracking of the cement matrix must have occurred (Figures 6 and 7).

Theoretical predictions of matrix cracking strain for 2-dimensional random dispersions of chopped

glass fibre in a cement paste 12 indicate strains of the order of 0.02 per cent (200 micro strain) for a

composite containing 5 per cent by weight of alkali-resistant fibres, slightly higher than the failure strain

of a cement paste without fibres. For unaged grc the theory of Aveston, Cooper and Kelly 13 predicts

that cracking should be fully developed at strains of about 0.13 per cent, with an average crack spacing of

about 1 -2 mm. Such values are consistent with the typical shape of stress-strain curves obtained in direct

tension tests carried out by BRE on samples cut from grc boards produced by the spray suction technique

(Figure 6). At higher strains, the load is carried entirely by the fibres and final fracture occurs when the

overall strain reaches about 1 per cent. Figure 6 also indicates how the failure strain reduces after exposure

to natural weathering.

These observations may be related to typical strain measurements taken on the test beams.

Figure 7 shows apparent strain, as measured across Demec points in the vicinity of a crack in the concrete,

plotted against applied load. This suggests that before artificial ageing micro-cracking of the grc matrix

probably started at about the same time as discrete cracks began to form in the concrete and that multiple

cracking of the grc then developed, becoming complete at about the design working load. At this stage

the crack spacing in the grc is likely to have been of the order of 2 mm. Further increase in applied load

would then result in opening up of these multiple fine cracks until eventual failure by fibre pull out at

quite high strains. An estimate of the probable crack width at the onset of yielding in the steel reinforcement

can be obtained from Figure 7. Assuming yield at 45 kN applied load, the integrated crack opening

displacements over the 50 mm gauge length ('apparent crack width') were 0.17 mm. With a crack spacing

of about 2 mm this means that the average crack width was only about 0.007 mm, well below the threshold

of visible cracking.

After artificial ageing, cracking of the grc surface was observed at loads below the design working

load and was consistent with fracturing of the grc at strains of less than 0.02 per cent,both for one month and

for three months ageing periods. The resulting cracks had similar width and spacing to those observed on

the beam without surface reinforcement. Direct tension tests were attempted on some samples sawn from

aged and unaged grc sections in order to get some idea of the effect of the artificial ageing on the basic

properties of the grc. Although the testing machine available was not entirely suitable, the results do give

an indication of the extreme brittleness of the grc after soaking in hot water. Based on the mean of about

12 tests in each case, the results were as follows:-

Unaged

UTS - N/mm 2 10.2

Indicated strain at failure - per cent 3.1

1 month at 60°C 3 months at 60°C

6.0 3.7

0.019 0.011

7

J

6. EFFECTIVENESS OF GRC IN CONTROLLING CRACKS IN THE CONCRETE

It may be concluded that initially the grc was effective in controlling cracks in the concrete in the vicinity of

the internal steel reinforcement. At the design ultimate load, crack widths were reduced to values not much

greater than those corresponding to the design working load of beams without surface reinforcement. With

deeper sections of grc surfacing, extending upward to the neutral axis, for example, the surface reinforcement

is likely to be even more effective 2. After periods of artificial ageing ranging from one to three months

immersion in hot water at 60°C (equivalent to 10 to 30 years natural exposure), the effectiveness was

• practically eliminated, however, as the grc became extremely brittle and the strain to failure dropped below

that expected of the unreinforced matrix.

It is arguable that the most important period when cracks need to be controlled is during the early

life of a structure when creep and shrinkage rates are at a maximum and the concrete has not yet reached

its fully developed strength. From this point of view surface reinforcement in the form of grc permanent

shuttering could be quite effective for up to 5 years or even more. It would obviously be preferable,

however, to maintain its effectiveness for a longer period. Recent developments in alkali-resistant glass

fibres 14 should lead to improved performance here; the strain at failure of the new Cem-Fil 2 fibre after

30 years exposure is claimed to be nearly four times that of the original Cem-Fil. Other promising

possibilities include new developments in polyolefin fibres 15.

As far as protecting the internal steel from corrosion is concerned the presence of a cement-rich dense

fibrous surface layer is likely to increase the effectiveness of the concrete cover to the internal reinforcement

and provide some protection against local impact damage. Results of exposure tests on reinforced concrete

at BRE 16 have shown that the susceptibility to corrosion of internal steel is very much reduced by an

increase in cement content and a decrease in permeability, both of which lead to a marked reduction in the

rate of carbonation of the concrete surrounding the steel. Although it remains to be proved conclusively

by long-term exposure tests it is reasonable to suppose that placement of reinforcing bars directly on the

cement-rich fibre reinforced layer, when it is to be used as permanent formwork, will still give adequate

protection for the steel against corrosion. Provided that gross cracking does not develop such protection

may be maintained even when the grc has become embrittled by exposure to moisture. If this is so there

are potential economic benefits to be gained, resulting from a considerable simplification of steel fixing

for the reinforcing cage in practical applications such as deck Slabs for concrete bridges. There is also a

prospect of reducing the amount of reinforcing steel needed for crack control in such structures.

7. CONCLUSIONS

Tests on reinforced concrete beams having a thin layer of glass reinforced cement as surface reinforcement

on the tension side have confirmed that the presence of such a layer can have a significant effect on the

cracking behaviour under load.

Within the first few years of casting the concrete the effectiveness of the grc is such as to reduce the

width of cracks in the concrete and to eliminate yisible surface cracking in the zone covered by the grc,

fight up to the ultimate failure load. After prolonged exposure to natural weathering, the grc becomes

less effective because of a marked reduction in fracture strain. This results in multiple fractures similar

to the cracking which develops in tension zones of ordinary reinforced concrete beams.

8

Whilst there would be benefits during the first few years of service from using grc as permanent

formwork in practical structural applications, it would obviously be much better to use fibre reinforced

materials which retained their extensibility over a longer time scale. There are good prospects of achieving

this by using some of the new developments in fibre reinforced cement composites. Among these are grc

based on improved alkali-resistant glass fibres, recently introduced, and fibre reinforced sheets based on

fibrillated polypropylene film.

Improved long-term effectiveness of surface reinforcement could lead to a reduction in the amount

of reinforcing steel needed for crack control in reinforced concrete structures, with consequent economic

advantages.

8. ACKNOWLEDGEMENTS

The work described formed part of the research programme of the Bridge Design Division (Division Head:

Dr G P Tilly) of the Structures Department of TRRL. All the testing was done by Mr J W Grainger who

also carried out a preliminary analysis of the results; his help in preparing this Report is gratefully .

acknowledged. Thanks are due to Mr J F Ryder of the Building Research Establishment for the supply of

grc channel section and to Messrs D A Warrior and B A Proctor of Pilkington Bros. for the use of their

facilities for the artificial ageing tests.

9. REFERENCES

. DAVE, N J. Fibre reinforced cement (frc) composite concrete construction - a new approach.

RILEM Symposium on Fibre Reinforced Cement and Concrete. London, 1975. Fol 2, p 615.

. DAVE, N J and J D PENNINGTON. Fibre reinforced cement composite concrete construction

employing asbestos cement as surface reinforcement. Conference on fibre reinforced materials:

design and engineering applications. Institution of Civil Engineers. London, 1977. p109.

. BROMS, B B. Crack widthand crack spacing in reinforced concrete members. Journal o f the

American Concrete Institute, 1965 (62), No. 10, p 1237.

4. BRITISH STANDARDS INSTITUTION. The structural use of concrete CP110, 1972.

. BRITISH STANDARDS INSTITUTION. Steel, concrete and composite bridges. Part 4 Code of

Practice for design of concrete bridges. BS 5400, 1978.

. BEEBY, A W. Corrosion of reinforcing steel in concrete and its relation to cracking. The Structural

Engineer, March, 1978, Vol. 56a, No. 3, p 77.

7. BUILDING RESEARCH STATION. A study of the properties of Cem-Fil OPC composites.

Building Research Establishment Current Paper CP38/76. 1976.

. BUILDING RESEARCH STATION. Properties of grc: 10-year results. Building Research Establishment

Information Paper IP36/79. November, 1979.

.

10.

11.

12.

13.

14.

15.

16.

FRANKLIN, R E and T M J KING. Relations between compressive and indirect tensile strengths of

concrete. Department o f the Environment, RRL Report LR 412. Crowthorne, 1971 (Road Research Laboratory).

ALLEN, H G. Tensile properties of seven asbestos cements. Composites. June, 1971, p 98.

ILLSTON, J M and R F STEVENS. Long-term cracking in reinforced concrete beams. Proceedings o f the Institution o f Civil Engineers, Vol. 53. December, 1972, p 445.

ALI, M A, A J MAJUMDAR and B SINGH. Properties of glass fibre cement - the effect of fibre

length and content. Journal o f Materials Science, Vol. 10, 1975, p 1732.

AVESTON, J, G A COOPER and A KELLY. Single and multiple fractures. NPL Conference on the

properties of fibre composites. IPC Science and Technology Pres& 1971, p 15.

FIBREGLASS LTD. Cem-FIL 2 alkali resistant glass fibre. Cem-FIL Product Leaflet. October, 1979.

HANNANT, D J and J J ZONSVELD. Polyol~fin fibrous networks in cement matrices for low cost

sheeting. Royal Society discussion meeting. New fibres and their composites. May, 1978.

GRIMER, F J. The durability of steel embedded in lightweight concrete. Building Research Station Current Papers. Engineering Series 49, 1967.

10

TABLE 1

Material properties

Concrete

Reinforcing steel

Grc

Mix proportions by weight:-

19-10 mm aggregate 2.02

10 - 5 mm aggregate 0.96

< 5 mm aggregate 1.83

cement 1.00

water/cement ratio 0.46

Slump - mm

28 day cube strength - N/mm 2

28 day flexural strength - N/mm 2

12 mm dia Torbar yield stress - N/mm 2

Sand/cement mortar with

25% by weight of sand and

5% by weight of chopped Cem-FIL

AR glass fibre, sprayed and dewatered

Tensile strength at 28 days - N/mm 2

after 5 yrs weathering

after 10 yrs weathering

Strain at failure at 28 days

after 5 yrs weathering

after 10 yrs weathering

Characteristic design value

60-180

30

460

Mean test value

96

57

6

489

16 t

7t 7t

0.85% t

O.os t

t BRE data

11

TABLE 2

Overall behaviour of test beams

Beam No.

1

2

3

4

5

6

7

Type (see Figure 1)

R (no grc)

G O

G O

G 1

G 1

G 3

G 3

Design working load

kN

20.5

20.5

20.5

Design ultimate load

kN

35

35

35

20.5 35

20.5 35

20.5 35

20.5 35

Load at failure kN

Calculated Test

45 45.0

49.5 48.7

49.5 49.1

45 37.8 t

45 37.6 t

45 43.5

45 43.7

Central deflection at design

working load mm

5.0

4.7

4.7

5.2

6.0

4 . 5

5.0

t Test terminated before complete yielding of steel reinforcement to avoid local shear failure at supports.

12

oO

.J oo

~ I~1 ' o I ~.~

t ~

0

,-,,,i

o oo N 2 ~o o~

o . ~ = x~ ~ i I ~ ,'~ , ~

~ ~ ~_~, ,

o ' ~ ~ , o . . . . . ~ o o

. I ~° i

.~ ~ ~ I

o

N ~

+ - . ~ -

° I oo ~ oo

o 0 o o , 0

I ~ - ~ ~

z

I

"F.

L ) . . . . . .

6 Z

0

0

r ~

0

, .o

o

- t - -

z

II II

13

129 Torbar

2, I I

30 ~ - - ~ 3 0

Type R

1T F P4

io_o / / / / ' ~ ( /

6mm thick GRC

Go= No artificial ageing G 1 = Immersed in water at 60°C for 1 month G3= Immersed in water at 60°C for 3 months

Type G (a) BEAM SECTIONS

1000 I

1000 -=' = 1000

Appliedjir load I

l

I1~ II//lllllllllllll'~lll~'i'j*'~'q~Al~t I ' I ~ Crack width rl-' " - ~

Strain gauged measurements (DEMEC) section

Fig. 1

(b) METHOD OF LOADING AND INSTRUMENTATION FOR TYPE G BEAMS

All dimensions in mm

DETAILS OF TEST BEAMS AND LOADING ARRANGEMENT

50

40

30

"O

O

._~ O. O.

20

10

• W

° " ~ l i , N ©

No GRC GRC unaged GRC aged 1 month at 60°(3 GRC aged 3 months at 60°(3

ol m m m 0 5 10 15

Calculated failure loads:-

With unaged GRC

With aged GRC or no GRC

Design ultimate load

Design working load

20

Central deflection (mm)

Fig. 2 TYPICAL LOAD-DEFLECTION CURVES

A

E E

v

C.}

~ L

o

E o

N - -

¢ . ) ¢--

E3

=

O A

Strain (per cent)

--0.10 0 0.10 0.20 O

50

100

150

200

250

(a) TYPE R -- NO GRC

: A t onset of cracking in concrete

O A t design working load

"- A t design ult imate load 1 A

E E

v

== Q _

o

E o

N - -

C -

Strain (per cent)

- 0 .10 0 0

5 0 -

100 -

150 -

200

226

25O

0.10 !

0.20 I

; [

\ :'

(b) TYPE G o - UNAGED

A

E E

v

¢. ) ~ 3

t

Q . o

E o

4 - -

G~ { . ) t "

° ~

E3

Strain (per cent)

- 0 . 1 0 0 0.10 0.20

Gauge positions 50 as for (b)

100 -

150 -

200 -

226

(c) TYPE G1 - AFTER 1 MONTH IN HOT WATER A T 60°C

- 0 0

-~ 50

E

.~= 100 Q .

9 E 2 "- 150

t -

a

200

226

Strain (per cent)

10 0 0.10 0.20 ' '

Gauge positions as for (b)

w

(d) TYPE G 3 - AFTER 3 MONTHS IN HOT WATER A T 60°C

Fig. 3 T Y P I C A L M E A S U R E D S T R A I N D I S T R I B U T I O N S

.~°

0 - - ~ " 0

= - o ~ v ._o

c- t-

O • " ~

0 0

c, t- O ._o ~

E E ~"

n" ~ n -

'- E ," E b • ~-- 0 .~-- 0 ~

~ E ~ E -~

5 ~ 7 ~ of- "~ "- ~ '~" c'~ ~

0 -~ "~ z N ~

~ z

.~g

" I , ~.~. I ~ . ~ . . . ~ " 0 0 0 0 0 0

(N>I) peol Pa!lddV

x DEMEC gauge points I

500

~( X X /X ~ X X X /

! , /x X ( X X X X

Type R - No GRC Beam

X X X

X X X

15 x:x xr.x xlx, xlx:x (/////// l / lX//x /S///~

Type G - Before artificial ageing

Loading ), ~ )oint

Type G - After

x~x ~~ ,~/x/x /kA//b~A/Jl,

month in hot water at 60°C ._J.

j xJ xSx xlX x Type G - After 3,months in hot water at 60°C

..i=.d Fig. 5 TYPICAL PATTERNS OF VISIBLE SURFACE CRACKING AT

D-ESIGN ULTIMATE LOAD

15

E E z

==

10

Ultimate tensile strain after natural weathering:-

2 years

6 months

5 years L !

1 year

28 days

Theoretical strains in unaged GRC (Ref. 12) F

Matrix cracking

t ~ I I

Development

o~ mult i~e~ 0.2 crackin~l I

1000 2000

Crack opening

0.4 0.6 I I I I I

3000 4000 5000 6000 7000

Strain (microstrain)

0.8 (per cent) I J

8000

Fig. 6 STRESS-STRAIN CURVES FOR GRC IN TENSION AFTER NATURAL WEATHERING (From Ref. 7)

± I Beam with unaged GRC

~ Beam with fully aged GRC (3 months at 60°C)

A

z

o -o

Q. <

40

30

20

10

/ /

/ /

Design ultimate load

1st Crack seen in aged GRC

Approx. fracture strain of I ful ly aged GRC

Approx. cracking strain of GRC

- - Design working load

Development of multiple cracking in GRC

1 ,~t Crack opening in GRC (unaged)

Approx. fracture strain of unaged GRC

o l l i I , , I I I 0 0.2 0.4 0.6 0.8 1.0

Apparent strain (per cent)

Fig. 7 MEASURED STRAINS IN GRC AT LEVEL OF REINFORCEMENT

(1112) Dd0536380 1,400 7/80 HPLtdSo ' ton G1915 PRINTED IN ENGLAND

ABSTRACT

Crack control in concrete beams by surface reinforcement with glass fibres: K D RAITHBY: Department of the Environment Department of Transport, TRRL Laboratory Report 947: Crowthorne, 1980 (Transport and Road Research Laboratory). Tests were made on com- posite reinforced concrete beams incorporating a surface layer of glass reinforced cement (grc) to investigate the effect of local surface reinforcement on serviceability behaviour. The grc was in the form of 6 mm thick shallow channel sections moulded into the tension surface of the test beams, which were 3.5m long and were tested in four-point bending on a simply supported span of 3m. On beams which had been kept dry, the grc surfacing had a significant effect in reducing the size of cracks in the concrete at the design working load but had no effect on deflections; no visible cracks appeared on the surface of the grc until the steel had yielded, at a load well above the design ultimate. Some beams were subjected to artificial ageing before test, by immersing them in hot water to represent natural exposure of the grc for periods of up to 30 years. On these beams embrittlement of the grc surfacing rendered it virtually ineffective in controlling cracks in the concrete. The observed behaviour compares well with predictions made from a knowledge of the relevant material properties. Considerable improvements in performance could be expected from newer developments in fibre cement composite technology and by using deeper sections for the surface reinforcement. A logical development, which is likely to have economic advantages, is to use suitable sections of rigid fibre reinforced cement sheet as permanent formwork for reinforced concrete members.

ISSN 0305-1293

ABSTRACT

Crack control in concrete beams by surface reinforcement with glass fibres: K D RAITHBY: Department of the Environment Department of Transport, TRRL Laboratory Report 947: Crowthorne, 1980 (Transport and Road Research Laboratory). Tests were made on com- posite reinforced concrete beams incorporating a surface layer of glass reinforced cement (grc) to investigate the effect of local surface reinforcement on serviceability behaviour. The grc was in the form of 6 mm thick shallow channel sections moulded into the tension surface of the test beams, which were 3.5m long and were tested in four-point bending on a simply supported span of 3m. On beams which had been kept dry, the grc surfacing had a significant effect in reducing the size of cracks in the concrete at the design working load but had no effect on deflections; no visible cracks appeared on the surface of the grc until the steel had yielded, at a load well above the design ultimate. Some beams were subjected to artificial ageing before test, by immersing them in hot water to represent natural exposure of the grc for periods of up to 30 years. On these beams embrittlement of the grc surfacing rendered it virtually ineffective in controlling cracks in the concrete. The observed behaviour compares well with predictions made from a knowledge of the relevant material properties. Considerable improvements in performance could be expected from newer developments in fibre cement composite technology and by using deeper sections for the surface reinforcement. A logical development, which is likely to have economic advantages, is to use suitable sections of rigid fibre reinforced cement sheet as permanent formwork for reinforced concrete members.

ISSN 0305-1293