TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 81

REPAIR OF CRACKED REINFORCED CONCRETE:

ASSESSMENT OF INJECTION METHODS

By A J J CALDER

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

Bridges Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU

ISSN 0266-5247

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.

CONTENTS

Page

Abstract 1

1. Introduction 1

2. General description of experiment 2

3. Test Equipment 4

3.1 The reinforced concrete slab 4

3.2 The test rig 4

4. Method 5

4.1 Loading 5

4.2 Repair methods 6

4.2.1 Epoxy repairs 7

4.2.2 Liquid silicate repairs 8

4.2.3 Polyester and methyl methacrylate repairs 9

5. Results and discussion 11

5.1 Long term performance 11

5.2 Assessment of repair methods 12

5.3 Loading tests on repaired slabs 13

6. Conclusions 17

7. References 18

8. Acknowledgements 18

© CROWN COPYRIGHT 1986 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged

REPAIR OF CRACKED REINFORCED CONCRETE: ASSESSMENT OF INJECTION M E T H O D S

A B S T R A C T

Chloride deicing salts are used for winter maintenance on highway bridges, and are known to penetrate concrete and cause corrosion of the reinforcement. Cracks, which provide easy routes for chlorides to reach the steel, might be sealed with reactive resins or with liquid silicate solution. The object of this work was to assess the structural implications of repair by injection and its effectiveness in preventing ingress of deicing salts into the concrete. Concrete slabs were cracked, held under a constant load and repaired using various injection techniques. The structural performance of slabs after repair was then determined. Other slabs were ponded with salt solution and after various periods of exposure were broken up to assess the effectiveness of the repair methods. This Report outlines the design, manufacture, stressing and repair of the concrete slabs. It assesses the effectiveness of the injection methods, details the loading tests and the short and long term structural performance of the slabs. A second Report will describe the exposure of the slabs and their assessment for corrosion. The best overall structural performance was provided by the slabs repaired with epoxy resin. The performance of the slabs repaired with polyester and methyl methacrylate resins may have been impaired by poor penetration, although the results of slant shear tests suggest that, at best, they are not structurally as effective as epoxy resin.

1 INTRODUCTION

Concrete highway bridges are often exposed to aggressive environments. Chloride deicing salts are used for winter maintenance and are known to penetrate concrete and cause corrosion of the steel reinforcement (Cavallier and Vassie, 1981). Beeby (1978 and 1983) and Manning et al (1985) suggest that cracks will exacerbate the problem by providing an easy route for chlorides to reach the reinforcement and so shorten the time to the onset of corrosion. These authors also argue that in the long term, crack width will have little effect on the subsequent corrosion rates. The British Standard on Steel, Concrete and Composite Bridges (1978) limits crack widths according to exposure. It appears that sealing cracks before exposure to deicing salts should provide some protection against corrosion. Cracks might be sealed by injecting with reactive resins (Concrete Society (1982 and 1984)) or with liquid silicate solution. These methods should not be used in cases where the reinforcement is corroding or where the cracking has been caused by corrosion.

Injecting cracks with reactive resins has structural implications because the resins bond the faces of the cracks together and can therefore transmit tensile and compressive forces.

Hewlett and Wills (1973) reviewed crack injection and explained some of the underlying principles. Guidelines for the use of the method have been given by the Louisiana State Highway Research and Development Section (Anon 1975). Resins chosen for repair should bond well to moist concrete, and should have a viscosity and pot life suitable to penetrate the range of crack widths and depths to be injected. Epoxy resins are most commonly used because of their tolerance to moisture. For example, Hewlett and Morgan (1982) injected beams that had been previously loaded so that the reinforcement had yielded. They showed that the performance of repaired beams is at least as good as that of the original beams; these rseults confirmed the earlier work of Chung (1975). The use of methyl methacrylate and polyester resins for sealing cracks is described in Concrete Society Report No. 22 (1982) and by Tyrer and Jones (1985), but little has been published about the behaviour of these resins in cracks under load. Jaber et al (1975) showed that the performance of beams injected with methyl methacrylate decreased because of poor bonding as the moisture levels in the concrete increased. Howdyshell (1972) reports that polyester did not perform well as a structural material in composite concrete/polyester beams, and Shaw (1982) has argued that polyesters give lower bond strengths than epoxies and they do not cure reliably in moist conditions.

The object of this work was to assess the effectiveness of various crack injection systems for protecting reinforcing steel from the ingress of deicing salt and to assess their effect on structural performance. Cracked reinforced concrete slabs, repaired under toad with an epoxy resin, a liquid silicate solution, a polyester resin, and a methyl methacrylate resin were ponded with sodium chloride solution and exposed for up to three years. After exposure, some slabs were cut up and the reinforcement assessed for corrosion. Flexural performance of the repaired slabs was assessed duirng the course of the experiment.

This Report outlines the design, manufacture, stressing and repair of the concrete slabs. It assesses the repair methods, details the loading tests and the short and long term structural performance of the slabs. The properties of the resins used depend upon their formulat ion, and these were not necessarily typical. The results given in this Report can only apply to the particular materials used. A second

Report wil l describe the exposure of the slabs and their assessment for corrosion.

2 G E N E R A L D E S C R I P T I O N OF E X P E R I M E N T

Reinforced concrete slabs were cracked, and held under a nominal ly constant load during repair with materials and methods in fair ly common use. All the slabs were cast during July and August 1981; cracking and repair, in sets of six, took place during 1982 and 1983. One slab from each set was cut up and examined soon after repair; one was load tested to failure, three were exposed to a corrosive environment and one was kept as a control in a non- corrosive environment. The t ime table for the experiment is given in Figure 1.

The slabs were mounted in reaction frames (Plate 1) and loaded in f lexure using hydraulic jacks so that the longitudinal reinforcement was in tension across a series of transverse cracks in the concrete. The aim was to achieve surface crack widths in the range 0.1-0.4 mm with a target mean of 0.2 mm. After stressing, the jacks were removed from the reaction frames and a nominally constant load maintained throughout repair and exposure by springs mounted

at each corner of the slab. Under this applied load, the cracks were injected by specialist repair contractors who used various repair systems. Six slabs were repaired with each of 4 injection materials: epoxy resin (Series A), liquid silicate (Series B), polyester resin (Series C), and methyl methacrylate resin (Series D). The liquid silicate repair material is said to react chemically with the parent concrete to form crystals which seal the crack. Comparisons are being made with uncracked slabs and slabs containing unfilled cracks (Series E). The structural performance of the slabs during the cracking, repair and exposure was assessed by monitoring the curvature and the surface crack widths. The effectiveness of the springs in maintaining a constant load was assessed by monitoring the load in the tie rods of the reaction frames.

A pond constructed over the constant bending section of each slab (Plate 2), was used to retain the salt solution and water. The slabs were first wetted twice weekly for 4 weeks with applications of 400 ml of water. This was followed by 3 periods of conditioning and exposure. Each conditioning period consisted of applying 400 ml of 5 per cent solution and 400 ml of water on Tuesdays and Fridays respectively for 5 weeks. The sodium chloride solution was replaced by 400 ml of water on the control slabs. During the exposure cycles, which

I , c s,,n B D Stress'ng C ~ 1 Repair D D Water added E ~ Conditioning F ! ~ 1 Exposure I

Series A: Epoxy

l O ~ A B C I 1~--2"1 _ _ D E F E F E

o I M , HHti- I . , , I I

10

Series B: Silicate

r N i B C

D E F E F E F

.~ Series C: Polyester ¢0

A B C "~ 1 0 / D E F E "6 F

E 0 Z

Series D: Methyl methacrylate

10 F I ~ A B C DE F =

0/1 . 1 1 Series E: Control slabs

A 1 0 7 - _ _ D E F E F

, , 1981 1982 1983

E

l . , i

I I 1984 1985 1986

Fig.1 T imetable of experiment

i

! 1987

2

Bracing beam

_ ~ . ~.~f Hydraulic jack

Plate 1 The test rig and general arrangement

Neg. no. B14,83

Plate2 A slab during

Neg. no. B169 $5

exposure

lasted for 23, 48 and 72 weeks, 200 ml of water was applied to all the slabs on Tuesdays and Fridays if the slabs were dry. One salted slab from each set was cut up and examined after each cycle to assess the effectiveness of the repair in preventing corrosion of the reinforcement. A more detailed account of the

condit ioning and exposure of the slabs and their assessment will be given in a second Report.

The allocation of slabs between condi t ioning and exposure, load tests to failure and post repair assessment are given in Table 1.

3

TABLE 1

Allocation of slabs

Series

A D

Methyl methacrylate

E Controls (unrepaired)

Item Epoxy Silicate Polyester Uncracked Cracked

Assessment after repair A1 B1 C1 D1 -- --

Load test to failure A2 B2 C2 D2 E4, E5 --

Slabs used for corrosion assessment

Conditioning with water only

3 cycles of exposure -- B3 C3 D3 I 51 I E8 m

Conditioning with sodium chloride solution and water

1 cycle of exposure A3 B4 C4 D4 -- E7

2 cycles of exposure A4 B5 C5 D5 E2 E8

3 cycles of exposure A5, A6 B6 C6 D6 E3 E9

3 TEST E Q U I P M E N T

3.1 T H E R E I N F O R C E D C O N C R E T E S L A B

The design and analysis of the slab is given by Calder (1981) and the method of manufacture by Gowan and Calder (1982).

The test slab shown in Figure 2 was designed to provide a series of injectable transverse cracks in the tension zone with the reinforcement loaded in its elastic range. Each slab was 1.8 m x 0.7 m x 0.1 m and contained four longitudinal 10 mm diameter and three transverse 6 mm diameter high yield reinforcing bars (characteristic strength: 460 N/ram2). The cover was 15 mm. The reinforcement was wire brushed and was degreased to remove any loose rust prior to assembly in the mould. The concrete, which had a characteristic strength of 30 N / m m 2 and a water- cement ratio of 0.47, contained Thames Valley f l int gravel aggregate (maximum size: 10 mm) and zone 3 sand wi th ordinary Portland cement. The mean 28 day compressive strength was 46 N/ram 2.

The slabs were cast in batches of three and cured for 28 days under damp hessian in an open ended shed where they were stored until required. The distr ibution of 28 day compressive strengths for each batch is given in Figure 3. There are significant

differences in strengths between batches despite careful control of the mixing process. The slabs were distributed into sets of 6 for repair so that any bias would be minimised.

Demec pips were fixed along the upper surface of both longitudinal edges of each slab so that the crack widths could be monitored using a 50 mm demec gauge. Pips were also mounted on the upper surface at the outer edges so that the curvature of the slabs could be monitored using a curvature gauge consisting of a dial gauge mounted centrally on a beam which records the mid-point displacement relative to the 2 outer pips.

3.2 THE TEST RIG Each slab in its reaction frame was loaded in 4-point bending using jacks and load cells mounted on crossheads at each end of the slab: this is illustrated in Plate 1. The crossheads were connected to the reaction frames by stainless steel tie rods, some of which are fitted with full bridge electrical resistance strain gauges to monitor the load during the experiment. After cracking, the jacks and crossheads

L r°ang~ ~ rdien ~ la rb~ r s6 ml O m g / ~ ~

/ / ~ ~ / 7 ~ ~ T ~ °ncrete

I ~ / f / / / . / ~ / _ Reinforcement

J //Y-Y-7-fX n 100 ~ 15 cover millimetres

i_ .j i- -I

F i g . 2 R e i n f o r c e d c o n c r e t e s l a b

"5

-Q E 3

3 -

2

1

0 40

I I

I I I I I

5o Mean compressive strength (Nmm -2)

Fig.3 Distribution of results of compressive strength tests o n c o n t r o l cubes

I

were removed from the rigs and the load was maintained by a stack of spring washers at each corner of the slabs: see Plate 2.

4 M E T H O D

The test slabs were brought into the laboratory (20°C and 65 per cent RH) in sets of 6 as required and were kept there for at least 7 days before being mounted in the test rigs ready for loading.

4.1 LOADING A detailed account of the procedure has been given by Calder (1985).

The aim of the loading was to produce a series of transverse cracks having surface widths in the range

of 0 .1 -0 .4 mm in the tension face of the concrete; this is at the lower end of the range injected in practice. The procedure adopted enabled the flexural performance of each slab to be determined at the various stages of the test. The method is il lustrated in Figure 4 wh ich is a plot of applied moment against curvature. The slope of this graph gives the flexural stiffness determined for each slab at various stages during the experiment.

Z E

o E

c~ {:z <

15

10

0 I I o o.10

Testing of °lraiginal

~_ ~/ / Testing of / A / /'~'~ repaired / / / o A o ' I /, o" I l J I J

0.02 0.04 0.06 0.08

Curvature (m -1) Fig.4 Loading diagram for slab

The initial loading of each slab was carried out in two stages; OABO' and O'B, Figure 4. The load was first increased in increments to give an applied moment of 8 kNm (B), and this produced about 7 or 8 tranverse cracks in the surface of the concrete. The slope of the linear section, OA of the moment-

curvature graph gave the uncracked stiffness of the slabs. Initial cracking of the concrete took place in the non-linear part of the curve (AB). The load was then released (BO') and reapplied (O'B), the gradient of O'B giving the cracked stiffness of the slabs. The load was then transferred to the springs by t ightening nuts on the tie rods onto the spring caps. The jacks were then removed from the rig. Af ter the initial loading test, the concrete crept under the action of the nominally constant load applied by the springs. The creep is represented by BB'. Photographs were taken of the top surface of each slab to record the crack pattern.

A load test to failure was carried out on one slab from each series soon after repair. The jacks were remounted onto the test rigs, the load transferred onto the jacks and released (B'O") . The slab was then reloaded up to the repair point (O"B') . The slope O"B ' gave the repaired stiffness. The load was then released again before the slab was reloaded in in increments to failure (O"B'C) determined by yield of the reinforcing bar. The reloading slope, O"B ' , gives a second estimate of the repaired stiffness of

the slab. As the load was increased beyond the point B' cracking of the concrete recurred which was partially associated with the repaired cracks. After the test the top surface of each slab was photographed and the position of the new cracks relative to the original cracks was estimated to within _+2 mm.

4.2 REPAIR M E T H O D S Specialist contractors repaired the slabs using their own materials and methods. The epoxy repairs were carried out in June 1982, the silicate repairs in September 1983, the polyester and methyl methacrylate repairs in November 1982. The physical properties of the resins from the data sheets supplied by the repair specialists are given in Table 2. Twenty four slabs were repaired, six for each material.

The repairs to the slabs were carried out a minimum of 6 weeks after stressing. The results of the measurements of curvature, crack width and applied load confirmed that the initial creep of the slabs had taken place before repair.

TABLE 2

Physical properties of the resins

Epoxy

Property Resin Hardener Polyester Methyl methacrylate

Viscosity (cp) 700-900 15-20 80 10-15

Gel Time (mins) 50-60 at 20oc

Flash point (°C) 120 114 11

Cure t ime (hours) 48

Specif ic Gravity 1.12-1.16 0.91-0.95 1.0 0.988 (mixed) 1.18 (cured)

Compressive Strength (N /mm 2) 76 65 130

Tensile Strength (N /mm 2) 15

Shear Strength (N/ ram 2) 15-17 32

Flexural Strength ( N / m m 2) 100

Adhesion to concrete (N/ ram 2) >10

Volumetr ic Shrinkage (%) 5 0.12

Coeff. of expansion ( x10 -6 ) / °C 35 17.5

Young 's Modulus ( kN /mm 2) 25

No data are available for the silicate solution, (most of t'he properties tabulated were not applicable).

6

The shear bond strength of the materials used for crack injection was measured using the slant shear test described by Tabor (1978). Plaques containing a single diagonal crack were injected at the same time as the slabs, then tested after 7 days and compared with plain uncracked specimens.

4.2.1 Epoxy repairs The method used, illustrated in Plate 3, was broadly in accordance with the method given by the Louisiana Department of Highway Research and Development (Anon 1975). The resin and the hardener were stored in separate containers within the injection unit, electrically heated and then pumped separately into the mixing head. The larger cracks were first surface sealed with polyester putty. Short lengths of clear polythene tube were set in the putty, each with an open end directly over the crack. The tubes were spaced at intervals up to 50 ram. These were then connected in turn to the output nozzle of the mixing head starting with a tube closest to the edge of a slab. Resin was pumped into the crack at a constant pressure until visible in the

adjacent tube. The pressure was then released, the nozzle disconnected and the tube sealed wi th a plastic bung. The nozzle was then connected to the tube adjacent to the sealed tube and the process repeated for each tube along the length of the crack until the crack was completely filled wi th resin.

In the method used for injecting finer cracks, the tubes were replaced by areas of exposed concrete at intervals along the seal (Plate 4). The injection head, f i t ted wi th a rubber grommet, was pressed onto the concrete directly over the crack. The procedure for work ing along the crack was similar to that employed in fi l l ing the larger cracks. In some cases cracks were fil led by a combinat ion of the two methods. The contractors would not have fi l led these fine cracks in normal practice.

When the resin had cured, the polyester put ty was scraped from the surface of the concrete after being softened by b lowing hot air over the surface. Cracks which had been judged by the contractors to be too small to be injected were sealed with f ive coats of an epoxy sealer painted onto the concrete surface.

• ~ 1 4 o " •

Neg. no. CR473/82/1

Plate 3 Epoxy repair method

I

I I Neg. no. CR466/82/11

Plate 4 Method of injecting fine cracks with epoxy resin

4 .2 .2 L iqu id s i l i c a t e repa i rs

The si l icate repair sys tem is d i f fe ren t in pr inc ip le f rom the o the r sys tems. A so lu t ion of so lub le si l icate was p u m p e d into the cracks, where , acco rd ing to the suppl ier , it w o u l d react w i t h the ca lc ium hydrox ide in the conc re te to f o r m crystals w h i c h seal the crack. It is not c la imed tha t these crystals p rov ide any s t ruc tura l s t reng th .

A series of 10 m m d iame te r holes were dr i l led 50 60 m m d o w n the cracks in the concre te at abou t 90 m m spac ing. No p recau t ions were taken to p reven t the cracks be ing b locked w i t h the dust f rom the dr i l l ings, or to sur face seal the cracks b e t w e e n the holes. Specia l nozzles were sealed into the holes and c o n n e c t e d via a man i fo ld to a manua l hydraul ic p u m p . Each nozzle had its o w n valve. In pract ice up to 5 nozzles we re connec ted to the p u m p at any one t ime and these we re inserted in ad jacent holes a long a crack; see Plate 5. L iquid w a s in jected th rough all the nozzles s imu l taneous ly .

Al l the cracks on each slab were first f lushed wi th saturated l ime solut ion to ensure that there was suff ic ient l ime present to react w i th the sil icate solut ion. The l ime was pumped into the cracks f rom the holes unti l it became visible at the surface of the concrete along the line of the crack. The nozzles were then moved to the next set of holes and the process repeated. Af ter f lushing all the cracks on the slab, the process was repeated using the sil icate solut ion. However , in this case the crack was sealed by rubbing cement powde r into it when the solution appeared on the surface of the concrete. The cement set very quickly and sealed the crack.

When the inject ion was f inished, the dri l led holes were fi l led in layers w i th ordinary Port land cement powder fo l lowed by a mix ture of sil icate solut ion and cement . More cement powder was rubbed into the cracks on the surface of the concrete and then brushed w i th sil icate solut ion. The repaired slabs were left uncovered to cure in an open ended shed for about 3 months before the cond i t ion ing and exposure.

/ /

/ Z

Plate 5 Silicate repair method

Neg. no. CR589,'83,'3

4.2.3 Polyester and methyl methacrylate repairs

The polyester and methyl methacrylate repairs were carried out at the same time and by the same method. Tyrer and Jones (1985) describe the method, which involved drawing resin into the cracks under vacuum. The concrete surface 20 mm each side of the crack was first grit blasted to expose the aggregate. Any loose grit was removed from the cracks by compressed air. The surface of the concrete, which was damp as a result of the grit blasting operation, was gently dried using a blow torch. Steel plates with short tubes welded to their upper surface (Plate 6) were fixed to the concrete using polyester putty. Generally three plates were attached to the concrete along the length of a single crack spanning the full width of the slab. Small circular nipples were also used along with the plates on the larger cracks and also on the vertical faces of the slabs. The exposed cracks between the plates and around the edges of the plates and nipples were sealed with polyester putty. Nitrogen was then blown through the cracks. The surface of the concrete was wetted with soap solution so that any leaks in the seals would be visible as bubbles.

Both the polyester and methyl methacrylate resins and their hardeners were batched by weight and mixed in a polythene beaker which was placed in a pressure pot. The resin was drawn under vacuum into one crack at a t ime (Plate 7). A tube, with one end immersed in the resin in the pressure pot, was connected to one of the plates or nipples on a crack, and the other plates or nipples were connected to the vacuum pump and a vacuum established. Nitrogen from a cylinder at a pressure of about 1 atmosphere was then applied to the pressure pot so that resin was forced from the pot, along the tube and drawn into the crack. (A bubble was formed in the tube from the pressure pot to moni tor the f low.) When resin appeared in the vacuum tube from the adjacent plate, the latter was clamped. The pressure and vacuum were maintained in the remaining tubes until the bubble in the tube from the pressure pot stopped moving, at which point, the supply tube was clamped. The supply tube was then cut off behind the clamp and connected to the next vacant plate, and the process cont inued until the crack was completely filled.

9

I :~__ .i ~' .f~.: ,;.~:,.-~.. :I i

Neg. no. CR859/82/7

Plate 6 Plates glued to the surface of the concrete used for injecting polyester and methyl methacrylate resins

Neg. no. CR855/82/10

Polyester and methyl methacrylate repair method Plate 7

10

When the resin had cured, the plates were knocked off the concrete and most of the polyester putty removed with a hammer and chisel. The surface was then cleared with an angle grinder. This method of removing the polyester from the concrete tended to pull pieces of aggregate from the surface and leave a rough finish to the concrete. Finally the cracks were pointed by priming wi th polyester or methyl methacrylate and sealed with polyester putty.

5 RESULTS AND DISCUSSION

I E

~3

5.1 LONG TERM PERFORMANCE The long term flexural performance of the slabs under constant applied load was monitored throughout the life of the experiment by measuring the curvature, tie rod loads and crack widths. The measurements were made first on a weekly basis and then less frequently. Typical results for the epoxy and silicate repaired slabs are given in Figures 5 and

.~ ;_-

-I II

0.04

0.03

0.02

0.01

0

0.25

0.20

0.15

0.10

0.05

0

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I I I I I I I I I I I I = J l

0 200 400 600 800 1 0 0 0 1200

II

1 II

II I I I I I I

0 Jj 200 400 600 800 1 0 0 0 1200

10 -I II

z v

8 - ( 3 O

F-

6

2 il 0 I I 1 I 1 I

200 400 600 800 1 0 0 0 1200

Days after first loading

Fig.5 Typical variation in c u r v a t u r e , c r a c k w i d t h a n d t ie rod load o f e p o x y repa i red slab

6, where curvature, tie rod loads and crack widths are plotted against time. In all cases the crack widths and curvature increased because of creep of the concrete during the first few weeks after stressing; consequently the springs relaxed and so reduced the load. Af ter the initial creep had taken place, the measured curvatures, crack widths and loads remained approximately constant for the remainder of the experiment. The repairs to the slabs were carried out a minimum of 6 weeks after stressing. In all six s!abs repaired w i th epoxy resin there is evidence of a slight decrease in the measured crack widths during the cure of the resin immediately after repair. This may have been caused by shrinkage of the resin which pulled the sides of the cracks together. Crack widths in the six slabs repaired wi th liquid silicate increased immediately after the repair suggesting that there may be an expansive reaction. Data on the crack width of the polyester and methyl methacrylate repaired slabs is very l imited, but it appears that the curvatures provide no evidence of expansion or shrinkage after repair.

0.04

f 0.03

I ii 0.02

0.01

o I , i l , , , , , i = i

0 200 400 600 800 1000 1200

- i il E

L)

0.25

0.20

0.15

0.10

0.05

0

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- I ii I I

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600 800 1000 1200

1o I II

v

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10 o

6

4 I li

2 I li 0 I

0 200 I I 1 I I

400 600 800 1000 1200 Days after first loading

F i g . 6 T y p i c a l v a r i a t i o n in c u r v a t u r e , c r a c k w i d t h a n d t ie r o d l o a d o f s i l i ca te r e p a i r e d s lab

11

5.2 A S S E S S M E N T OF REPAIR M E T H O D S

The distribution of crack widths at the time of repair is given in Figure 7, and the range and mean for each slab in Figure 8. With the exception of slabs repaired with epoxy resin, an attempt was made to inject all the cracks regardless of width. The cracks judged by the contractors to be too fine to inject wi th epoxy resin were surface sealed. Figure 9 il lustrates the distribution of crack widths that were injected or surface sealed with epoxy resin. Seventy two per cent of the cracks having widths of less than 0.15 mm were surface sealed.

8 0

7 0

6 0

E

5o 2

~ 40

. Q E 3 0

z

20

10 I I, 0 0.1 0.2 0.3 0.4 0.5

Crack width (mm)

Fig.7 Distribution of surface crack widths prior to repair

After repair and exposure, the slabs were sawn up for examination as shown in Figure 10. The penetration of the resin down the cracks at each intersection with the sawn faces was measured using a crack microscope, and the results of the data obtained from 3 slabs from each series are summarised in Figure 11. There are no data available from the slabs repaired with silicate solution because the products of the reaction, if present, were indistinguishable from the parent concrete. The penetration of the epoxy was much better than that of the polyester and methyl methacrylate resins, and the polyester penetrated to a greater depth than the methyl methacrylate. The distribution of penetration of the resins into cracks with widths up to 0.25 ram, and between 0.25 and 0.50 mm are given in Figure 12. As the distribution of the two ranges of crack width were similar for all three resins it appears that surface crack width had little effect on the penetration.

12

.4 F Flsl o

I F ~ ] Max value Mean value Min value

Epoxy repairs

A1 A2 A3 A4 A5 A6

E

Silicate repairs

.4

"6 C

B1 B2 B3 B4 B5 B6

r , - , , - 1 .4 _ Polyester repairs ~_~

o C1 C2 C3 C4 C5 C6

Methyl methacrylate repairs ~ ]

o D1 D2 D3 D4 05

Fig.8 Range of crack widths prior to repair

30 --

Injected 20 -- /

Sealed

10 - -

F 0 0.1 0.2 0.3 0.4

Crack width (rnm)

Fig. 9 Distribution of crack widths repaired using epoxy resin

D6

I 0.5

Saw cut Reinforcement

Top surface of concrete slab

. l _ - - J I

I I

J I

' I

Fig.10 Plan of concrete slab showing position of saw cuts

E 20 ==

E

.Q E 0 #

20 --

10 --

Epoxy repairs Slabs: A1 A3 A4

I I 0 10

I I I I - - 20 30 40 50 60 70

y! 0 10

Polyester repairs Slabs: C1 C4 C5

q_ L

L I I l-~, I ' , '

20 30 40 50 60 I

7O

20

10

t i e t t r e airs h~llabse: h~; YDI4 t ; ; p

I I I - -F ' I , t , I - 1 0 10 20 30 40 50 60

Resin penetration (mm)

Fig.11 Distribution of resin penetrations

I 7O

The superior penetration of the epoxy system is unlikely to be attributable to the properties of the materials because epoxy resin was more viscous than the polyester and methyl methacrylate resins: see

Table 2, and probably had a shorter pot life. An explanation for the differences in penetration is therefore sought in the differences in method. The injection points for the epoxy resin were at a spacing approximately equal to the depth of the cracks, and their diameters were small compared with the spacing. In this situation the penetrating front is assumed to be radial (Hewlett and Wills 1973) so that, in principle, the resin reaches the next port at about the same time as the crack is completely filled. In the case of the polyester and methyl methacrylate repairs, a vacuum was applied to one port whi lst resin was injected at the adjacent one. The vacuum may have caused a pressure gradient to be set up within the crack and resin to be sucked preferentially towards the adjacent port, thus giving a show of resin before the crack was completely filled. Furthermore, the distance between the adjacent plates was small and the plates large compared with the crack depth; when the injection started the space between the surface of the concrete and the plate may have filled preferentially with resin which was then drawn the comparatively short distance along the crack to fill the void beneath the next plate. These two factors offer possible explanations for the poor penetration achieved by this system. These factors would have been exacerbated because both the inlet and outlet ports were on the upper surface of the concrete. A subsequent review however, suggests that normal injection procedures were not fol lowed and Farthing (1985), has reported that cracks wider than 1 mm have been successfully injected using a vacuum technique. The differences in the penetration of the polyester and methyl methacrylate resins must be attributed to material properties as the same method of injection was used for both materials, for example, the methyl methacrylate may have had a shorter pot life than polyester.

5 . 3 L O A D I N G T E S T S O N R E P A I R E D S L A B S

The flexural stiffness of each slab was determined at various stages during the experiment. It was calculated by linear regression from the slope of the moment-curvature graph illustrated in Figure 4. OA and 0 'B represent the uncracked and cracked stiffness prior to repair; and O"B ' represents the repaired stiffness measured wi th the repair materials in compression. The results given in Table 3, were variable, but it is clear that the stiffness of cracked slabs was about one quarter of their uncracked value and the stiffness of the repaired slab was about half the uncracked value. Hewlett and Morgan (1982) found that all the uncracked stiffness was regained on cracked beams that had been loaded to point of yield of the reinforcement bars prior to repair. The similarity in stiffness after repair is illustrated by the moment-curvature curves in Figure 13 for slabs loaded to failure.

13

Crack widths: 0 -- 0.25turn

F 0 10 20 30 40 50

Crack widths:

Epoxy repaired slabs: A1 A 3 A4

6

4

I 0 I I • I 60 70 0 10 20

0.26 -- 0.50rnm

30 40 50 I

60 r l

70

t -

.9

"6

J~

E 3 z

8

6

4

2

0 0 10 20 30 40 50 60 70

Polyester repaired slabs: C1 C4 C5

8 m

6 --

4 -

2 --

I 0 0

] F , 7 i | i 1"7 i I

10 20 30 40 50 60 70

Methyl methacrylate repaired

8

- 6

-

1"7 1 ' 7 1 i i o 0 10 20 30 40 50 60 70 0

Resin penetration

slabs: D1 D4 D5

i 7 i i ! - I 1"7 I 10 20 30 40 50 60 70

(mm)

F i g . 1 2 D i s t r i b u t i o n o f resin penet ra t ion for crack widths

(from 0 - 0.25ram and 0.26 - 0.50turn)

The stiffnesses of the slabs after repair were broadly similar despite the differences between the elastic moduli and the penetrat ions achieved by the dif ferent resins. This is surprising but Keer and Emberson (1985) suggest that tensile stiffness of repaired prisms is insensitive to the modulus of elasticity of the resin injected. The results for the silicate repaired slabs suggest that the compressive loads were supported by the mortar plugs formed in the cracks at t ime of repair (section 4.2.2) The results of the slant shear tests in Table 4 show that cracks injected wi th silicate solut ion had no structural strength.

When the moment applied was increased beyond the load at repair, the repair materials were in tension

and the slabs cracked again (Figure 13). Table 5 shows that nearly 90 per cent of the new cracks in the slabs repaired wi th the silicate solution and the methyl methacrylate were at repaired cracks. Some 60 per cent of the new cracks in the slab repaired wi th polyester were at the repaired cracks, whereas the value for the epoxy repaired slab was only 30 per cent. These results show that the epoxy resin had penetrated the cracks well enough to support tensile loads and modify the stress distribution in the slabs. The polyester wi th its lower penetration was less effective in modifying the stress distribution. Neither the liquid silicate nor the methyl methacrylate appeared to support tensile stress. The poor performance of the methyl methacrylate can possibly

14

T A B L E 3

Measured stiffness of slabs

Epoxy

Silicate

Polyester

Repair material

Methy l Methacry late

Unrepaired

Slab number

A1

A2

A3

A4

A5

A6

Mean

B1

Uncracked stiffness

kNm 2

1792

1914

1781

1566

1895

Cracked st i f fness

kNm 2

Repaired st i f fness

kNm 2

353 I

454 977, 945 I

379

444

375

1231 431 I I

1697 404 961 , t

1759 458

B2 ' 2155 391 I I

B3 21 58 405 I I

B4 1592 360

806, 805

B5 1898 I

B6 1534 I

Mean 1849

314 I

393 I

387 806 I

497 C1 * I I I

C2 * 400 1000, 841 I I I

C3 * 397 I I I

C4 * 435

C5 * 440

C6 * 469 I I I

Mean * 440 921

445 D1 * I I I

D2 * 443 921, 764 I I I

D3 * 419 r i i

D4 * 409 I I I

D5 * 370 I

D6 * 450 I

Mean * 423 843

E4 1555 358 r i

E5 1442 391 I [

E5 * 401 I I

E6 * 377 I I

E7 * 458 I I

E9 * 466 I

Mean 1499 409

Notes: *No t measured Stiffness of repaired slabs tested to fai lure was measured twice.

15

T A B L E 4

Results of slant shear tests

Repair material

Epoxy

Mean

Polyester

Mean

Methyl methacrylate

Mean

Note: No uncracked prisms

Compressive strength (N/ram 2)

repaired prism

78 77 79 75

77

47 46 41 44

45

63 49 58 65

59

uncracked prism

81 73 86 84

81

76 69 77 76

75

Mode of failure

Failure in concrete close to crack

Failure occurred due to loss of adhesion of the joint

Failure occurred in the joint with some concrete cracking

were tested in conjunction with the methyl methacrylate repairs

be attributed to the poor penetration as it has been shown by Jaber et a l (1975) that methyl methacrylate will bond to cracks in dry concrete and support significant flexural loads.

The above argument is supported by the results of the slant shear tests in Table 4; these show that the epoxy repaired prisms failed at higher compressive strengths (77 N/ram 2) than the prisms repaired with methyl methacrylate (59 N/ram 2) and polyester

(45 N/mm 2) resins. These differences are reflected in the respective failure modes; the epoxy repaired slabs failed in the concrete whereas the methyl methacrylate failed in the crack and the polyester failed due to loss of adhesion between the resin and the concrete. The silicate solution gave no structural strength.

Overall, the results suggest that penetration and modulus affect repairs in tension but not in

TABLE 5

Cracking of slabs loaded to failure

Length of repaired cracks (mm)

Length of failure

Length of failure cracks associated

with repaired Slab number: material cracks (mm) cracks (mm)

62: Epoxy 4177 4089 1244 30

B2: Silicate 5266 4107 3621 88

C2: Polyester 5974 3710 :~:~ 60

D2: Methyl methacrylate 5503 3732 3310 89

Percentage of associated cracking

16

15

10

15

Epoxy repair Slab A2 B

S/I',, 0 0.02 0.04 0.06 0.08

C

0.10

<

10

5

0

15

I Silicate repair Slab B2

0 0.02

S I I I

0.04 0.06 0.08 0.10

10

I / o ' J I I 0 0.08 0.10

15

10

~ Polyester r e p a i r / Slab C2 f C

o"~1 I 0 0.02 0.04 0.06

Methyl methacrylate~ repair Slab D2 m B l 'e

5 ; / / / 8 °

o fo '~ l J o " I I 0 0.02 0.04 0.06

15

! 0.08 0.10

Unfilled cracked C

10 t ~ ~

S r o ' & I I I I 0 0.02 0.04 0.06 0.08 0.10

Curvature (m -1 )

Fig.13 Moment against curvature for slabs load tested to failure

compression. The epoxy resin is more effective in tension than the other two thermosetting resins, and the liquid silicate repair, as expected, had no structural strength.

6 C O N C L U S I O N S

Twenty four reinforced concrete slabs with surface crack widths in the range 0.1 to 0.4 mm were repaired using four different materials and methods. The properties of the resins depend on their formulation and there is no one formulation of each type which is generally typical. The results given in this Report apply only to the particular materials used.

1. The epoxy resin penetrated the cracks to a markedly greater depth than either the polyester or the methyl methacrylate resins. This difference may be the result of the different injection methods employed rather than to differences in the material properties. The penetration of the polyester was better than that of the methyl methacrylate resin. Data for the silicate repaired slabs were not available as the reaction products, if present, were indistinguishable from the parent concrete.

There was no evidence that the penetration was better in wider cracks.

2. The stiffness of the cracked slabs was about a quarter of that of the uncracked slabs. The repairs reinstated only about half of this loss. The stiffnesses of all the repaired slabs were broadly similar. This shows that in compression the stiffness of the slabs after repair is insensitive to the modulus or to the penetration of the repair material.

3. When the applied moment was increased beyond the moment at repair, the slabs recracked. Only 30 per cent of the new cracks were associated with cracks repaired with epoxy resin, some 60 per cent were associated with cracks repaired with polyester resin and almost 90 per cent with cracks repaired with methyl methacrylate resin and liquid silicate. Thus the bond and penetration of epoxy resin was sufficient to modify the stress distribution in the slab. The polyester had less penetration and was less effective. Neither the liquid silicate nor the methyl methacrylate sustained much tensile load, the latter because of poor penetration.

4. The overall structural performance of the slabs repaired with epoxy resin was best. The performance of the slabs repaired with polyester and methyl methacrylate resin may have been impaired by poor penetration, although the results of the slant shear tests suggest that, at best, they are not structurally as effective as epoxy resin. The tests confirm that the liquid silicate repairs had no structural strength.

The corrosion of the slabs which have been subjected to conditioning with salt solution and exposure will be discussed in a second Report.

17

7 REFERENCES

ANON. (1975). Structural repair of concrete cracks by injection of epoxy resin adhesive. Louisiana Department of Highway Research and Development Section.

BEEBY, A W (1978). Cracking and corrosion. CIRIA/UEG, Cement and Concrete Association, Department of Energy. Concrete in the Oceans Technical Report No 1. Slough: Cement and Concrete Association.

BEEBY, A W (1983). Cracking, cover and corrosion of reinforcement. Concrete International. Vol 5, No 2, February, pp 35-40.

BRITISH Methods BS 1881.

STANDARDS INSTITUTION (1970). of testing concrete. British Standard London: British Standards Institution.

BRITISH STANDARDS INSTITUTION (1972). The structural use of concrete. Code of Practice, CPI lO. London: British Standards Institution.

BRITISH STANDARDS INSTITUTION (1978). Steel, concrete and composite bridges. British Standard BS 5400. London: British Standards Institution.

CALDER, A J J (1981). Design of test slabs for repair of concrete structures. TRRL Working Paper WP/BD 8/81. Crowthorne: Transport and Road Research Laboratory (unpublished).

CALDER, A J J (1985). Loading equipment and stressing procedure for evaluation of repairs to cracked concrete slabs. TRRL Working Paper B/91/85. Crowthorne: Transport and Road Research Laboratory (unpublished).

CAVALIER, P G and VASSlE, P R (1981). Investigation and repair of reinforcement corrosion in a bridge deck. Proc. Inst. Civ. Engs. Part 1. August, pp 461-480.

CHUNG, H W (1975). Repaired reinforced concrete beams. American Concrete Institute Journal. Vol 72, No 5, May, pp ~3 -234 .

CONCRETE SOCIETY (1982). Non structural cracks in concrete: report of Working Party. Concrete Society Technical Report No 22. London: The Concrete Society.

CONCRETE SOCIETY (1984). Repair of concrete damaged by reinforcement corrosion: report of Working Party. Concrete Society Technical Report No 26. London: The Concrete Society.

FARTHING, D W (1985). Design and construction of concrete aircraft pavements with particular reference to plastic shrinkage cracking. 3rd Int. Conf. on Concrete Pavement Design and Rehabilitation. Purdue University, Indiana. April.

GOWAN, N R and CALDER, A J J (1982). Manufacture of slabs for evaluating repairs to concrete structures. TRRL Working Paper WP BD 29/82. Crowthorne: Transport and Road Research Laboratory (unpublished).

HEWLETT, P C and WILLS, A J (1973). A fundamental look at structural repair by injection using synthetic resins. Symposium: Resin and concrete. University of Newcastle on Tyne. April.

HEWLETT, P C and MORGAN, J G D (1982). Static and cyclic response to reinforced concrete beams repaired by resin injection. Magazine of Concrete Research. Vol 34, No 118, March, pp 5-17.

HOWDYSHELL, P A (1972). Creep characteristics of polyester concrete. Construction Engineering Research Laboratory. Technical Report M-23, Champaign, Illinois: Construction Engineering Research Laboratory.

JABER M M, FOWLER, D M and PAUL D R, (1975). Repair of concrete with polymers. University of Texas. Research Report 114-3,

KEER, J G and EMBERSON, N (1985). Resin-injected reinforced concrete members under load: a theoretical and experimental study. University of Surrey.

MANNING, D Ge t al. (1985). Debate: Crack width, cover and corrosion. Concrete International. Vol 82, No 2, May, pp 20-35.

SHAW, J D N (1982). A review of resins used in construction. International Journal of Adhesion and Adhesives. Vol 2, No 2, April, pp 77-83.

TABOR, L J (1978). The evaluation of resin systems for concrete repair. Magazine of Concrete Research. Vol 30, No 105, December, pp 221-225.

TYRER, R G and JONES, N (1985). A resin injection technique for fine cracks in concrete. Proc. Second Int. Conf. on Structural faults and repair. Edinburgh, April, pp 233-236. Engineering Technics Press.

8 ACKNOWLEDGEMENTS

The work described in this Report was carried out in the Bridges Division (Division Head Dr G P Tilly) of the Highways and Structures Department of TRRL.

The author wishes to thank the specialist repair contractors who repaired the slabs.

18 Printed in the UK for HMSO by Hobbs the Printers of Southampton (1951/86) Dd8222661 9/86 G426