Repair and Structural Performance of Initially Cracked RC Slabs

Construction

Construction and Building Materials 19 (2005) 595–603

and Building

MATERIALSwww.elsevier.com/locate/conbuildmat

Repair and structural performance of initially crackedreinforced concrete slabs

Waleed A. Thanoon, M.S. Jaafar *, M. Razali A. Kadir, J. Noorzaei

Department of Civil Engineering, Universiti Putra Malaysia, UPM, Serdang, Selangor, 43400 Darul Ehsan, Malaysia

Received 29 November 2002; received in revised form 10 September 2004; accepted 28 January 2005

Available online 21 March 2005

Abstract

Crack is one of the most common defects observed in reinforced concrete slabs and beams. Major cracks in concrete structures

may occur due to overloading, corrosion of reinforcement or differential settlement of support. To restore the structural capacity of

the distressed elements, retrofitting and/or strengthening are needed. There are different techniques available for retrofitting and

strengthening of different reinforced concrete structural elements reported in the literature. This paper investigates the structural

behaviour of cracked reinforced concrete one-way slab, which is repaired using different techniques.

Five different techniques are used for the purpose of repair in the cracked concrete slab namely; cement grout, epoxy injection, fer-

rocement layer, carbon fibre strip and section enlargement. The slabs were loaded to failure stage and the structural response of each

slab specimens have been predicted in terms of deflection, variation of strain in concrete and steel, collapse loads and the failuremodes.

The efficiency of different repair and strengthening techniques and their effects on the structural behaviour of cracked one-way

reinforced concrete slab had been analyzed. It was observed that the type of repair technique used will affect the load carrying capac-

ity of the slab and will lead to a redistribution of the strains and hence stresses in both concrete and steel reinforcement. All repair

techniques are found to be able to restore or enhance the structural capacity of cracked concrete slabs.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Repair and strengthening; Distressed concrete slab; Ferrocement; CFRP; Epoxy injection; Grouting; Section enlargement

1. Introduction

In practice, situations arise where existing concrete

structures or some of their components may, for a vari-

ety of reasons, be found to be inadequate and in need of

repair and/or strengthening. The inadequacy may be due

to mechanical damage, functional changes, overstress

due to temperature changes, or corrosion of reinforce-

ment. A common feature of a number of different causes

of deterioration is that there is a reduction of the alka-linity of the concrete, which allows oxidation of the rein-

forcing steel to take place. This oxidation process leads

to cracking of the concrete and possible spalling of the

cover to the reinforcement.

0950-0618/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2005.01.011

* Corresponding author. Tel.: +603 89466377; fax: +603 86567129.

E-mail address: [email protected] (M.S. Jaafar).

Bridges are one of the concrete structures, which nor-

mally exhibit severe distress due to their exposure toharsh environment. Different repair techniques have

been successfully developed to strengthen a given struc-

ture or part of it to restore its serviceability and strength.

It is also prudent to consider durability aspect when re-

pair or strengthening is carried out. With the advance-

ment of new materials technology, which have

superior mechanical properties and excellent resistance

to electrochemical corrosion, many effective repairsand strengthening techniques have been developed.

The final selection of a suitable and most effective meth-

od generally depends on simplicity, speed of application,

structural performance and total cost.

Studies have shown that fibre reinforced plates (FRP)

increase the strength of flexural members significantly.

Carbon fibre reinforced polymer has a high strength to

mailto:[email protected]

Fig. 1. Test set up.

596 W.A. Thanoon et al. / Construction and Building Materials 19 (2005) 595–603

weight ratio, favourable fatigue behaviour and excellent

resistance to electrochemical corrosion to make it prac-

tically suited for structural application [1]. A study con-

ducted by Alfarabi et al. [2] showed that, although the

FRP increase the failure loads, most of the beams

strengthened by FRP started the failure at the curtail-ment zones of the plates. The epoxy used to laminate

the plate at the soffit of flexural members only failed

at loads much higher than the required level [3]. Similar

study also found that the failure modes for repaired

structures may change from ductile to brittle [4]. The

probability of this change depends largely on the

percentage of FRP being used, the location of FRP

and the presence of shear reinforcement in the existingstructures.

Toong and Li [5] investigated the effect of using car-

bon fibre reinforced polymers (CFRP) plates to

strengthen one-way spanning slab to increase the flex-

ural capacity with particular emphasis on the cracking

behaviour at working load. All the CFRP strengthened

specimens exhibited large increase in load carrying

capacity ranging from 60% to 140%.Ferrocement is a type of thin composite material

made of cement mortar reinforced with uniformly dis-

tributed layers of continuous, relatively small diameter

wire meshes. The use of ferrocement proper in repair

was first introduced by Romualdi [6] and Iron [7] in

the early 1980s mainly as relining membranes for the re-

pair of liquid retaining structures, such as pools, sewer

lines, tunnels, etc. Investigation into the use of ferroce-ment as strengthening components for the repair and

strengthening of reinforced concrete beams was re-

viewed by Paramasivam et al. [8]. In general, the dam-

aged concrete and reinforcement (if also damaged)

were removed and replaced with ferrocement, with or

without any changes in overall dimensions of the beam.

The beams were tested under static or cyclic loading [6]

conditions. The strengthened beams were reported to ex-hibit improved cracking resistance, flexural stiffness and

the ultimate loads compared to the original beams.

These improvements; however, depend on the full com-

posite action between the ferrocement layers.

Al-Kubaisy and Zamin [9] presented the flexural

behaviour of reinforced concrete slabs strengthened with

ferrocement tension zone cover. Twelve simply sup-

ported (500 mm2) reinforced concrete slabs were testedunder flexural load. The effect of the percentage of wire

mesh reinforcement in the ferrocement layer, thickness

of the ferrocement layer and the type of connection be-

tween the ferrocement layer and reinforced concrete slab

on the ultimate flexural load, first crack load, crack

width and spacing and load–deflection relationship were

considered.

Other technical methods used for repair of reinforcedconcrete structures are epoxy injection and cement gro-

uting techniques. These techniques are widely used to

treat cracking problem in concrete. The procedure used

is well established in the literature [10–12].

This paper presents a study on the effects of different

repair techniques on the structural response of one way

reinforced concrete slab. The techniques include:

(a) carbon fibre reinforced polymers (CFRP) strip;

(b) cement grout, i.e., SikaGrout214;

(c) epoxy injection, i.e., Sikadur52;

(d) ferrocement cover and;

(e) section enlargement.

These techniques had been selected for their potential to

either increase the structural capacity of members or torestore the original capacity of the sections. Further-

more, this study focuses on the serviceability, strength

and ductility performance for each of the repair tech-

niques to ascertain their potential application in cracked

reinforced concrete slabs.

2. Experimental procedure and repair technique

In order to investigate the effect of various repair

techniques on the structural response of one way slab,

a total of six full scale one-way reinforced concrete slabs

having a dimension of 2.5 m long · 1.0 m wide and

0.15 m thick are cast, cured and tested. The steel rein-

forcement consists of five 10 mm diameter high-yield de-

formed bars with a characteristic strength of 460 N/mm2. The 28-day cube compressive strength, fcu of the

concrete used is 30 N/mm2. All specimens are tested un-

der two-line load located in the middle third of the slab

specimen as shown in Fig. 1. Initially, all slabs are

loaded to 2/3 of their expected ultimate load capacity

or after the development of cracks in the specimens (ini-

tial load ranges between 34 and 40 kN), except for the

control slab, which is loaded until failure. Subse-quently, the load has been released and the specimens

W.A. Thanoon et al. / Construction and Building Materials 19 (2005) 595–603 597

are removed from the testing frame so that repairs may

be carried out.

The resulting cracks in different concrete slab speci-

mens were repaired using each of the techniques men-

tioned above. The specimens are then re-tested to

failure after allowing a suitable curing period. The re-sponse of each specimen in terms of deflection, stiffness,

cracking load, ultimate load, and failure pattern are

analyzed. The repair techniques applied on cracked

slabs are shown in Fig. 2.

2.5 m

Applied L

Epoxy

0.15 m

0.05 m

0.15 m

15 mm

132 mm

50 mm

(e) Section enlarg

Epoxy (Sikadur52)

Epoxy (Sikadur30)

Inlet Port

Fine crack

Reinforced steel bar

(c) Ferrocement layer – S4.

50mm

30 mm

Grout

Fine crack

Before Repair

Reinforced steel bar

(a) Grout pouring – S2.

Fig. 2. Five different repair techniques ap

2.1. Grout pouring technique using SikaGrout 214 (Slab

S2)

At the end of first stage of loading (34 kN), two flex-

ural cracks of 0.88 mm width each were observed at the

middle third of the slab. The resulted crack paths havebeen enlarged by 50 mm in width and 30 mm in depth

to expose the main steel reinforcement. The exposed

reinforcement and concrete surfaces were cleaned using

steel brush, water jet and compressed air before grout is

oad

5Y 10 4R 10 + 5 R 8

1 m

5 Y 104R 10 + 5 R 8

ement – S6.

0.833 m

Ordinary reinforcementWiremesh

Mid-span of concrete slab

Patch with cement mortar

Skeletal steel

(b) Epoxy injection – S3.

0.15 m

1 m

50 mm

1.2 mm

2 m

2.5 m

Carbon Fibre Reinforced Polymer (CFRP)

(d) CFRP strip – S5.

plied on cracked reinforced slabs.

Fig. 3. Cracks repaired using grout pouring technique.


poured into the enlarged extents, see Fig. 3. The grout

used is SikaGrout214 of a density 2.2 kg/L. It is anon-shrink premixed high strength cementitious grout.

The mixing ratio used is 25 kg of grout to 4.4 kg of

water. The specimen has been tested after allowing 7

days for curing.

2.2. Epoxy injection technique (slab S3)

Similar to slab specimens S2, two flexural cracks of0.65 mm width each were observed at the middle third

of the slab under a total load of 37.4 kN. In this repair

method, injection nipples are installed along the crack

path at 200 mm centers as shown in Fig. 4. Sikadur30

was used to fix the injection nipples in position as well

as to seal the surface of the cracks. The epoxy used to

Fig. 5. Cracks repaired usin

(a) Crack pattern before repair.

Fig. 4. Cracks repaired using epoxy injection technique.

fill the crack is Sikadur52, which is low viscosity epoxy,

free flowing and fast curing injection resin based on 2-

component solvent free epoxy resin. It has a density of

1.1 kg/L, tensile strength of 25 N/mm2 (7 days) and a

compressive strength of 40 MPa at 20 �C within 24 h.

The viscosity of Sikadur52 is equal to 290 and 130 cpsat a temperature of 20 and 30 �C, respectively. The crackwidth limits for Sikadur52 is between 0.2 and 5 mm.

Sikadur52 was injected from one end of the crack until

the material exudes from the next nipple. The injection

process was repeated until the whole crack is filled with

the epoxy material.

2.3. Ferrocement layer (slab S4)

The crack pattern observed at the end of initial load

stage (38 kN) is shown in Fig. 5(a). The maximum crack

width found at this load was equal to 0.75 mm. In this

technique, a 30 mm depth concrete from the bottom of

the slab was removed using a concrete chisel and ham-

mer. This concrete layer has been removed only from

the middle third portion of the slab (with dimensionsof 850 mm · 850 mm) as shown in Fig. 5(b).

Two layers of 12.5 mm2 opening galvanized welded

wire mesh of 1.25 mm diameter and a layer of skeletal

steel (5R6) are fixed with the original reinforcement of

the slab after the concrete surface was roughened as

shown in Fig. 5b. Cement mortar (cement to sand ratio

is 1:2 with w/c ratio equal to 0.5) is applied and cured for

28 days.

2.4. Carbon fibre reinforced polymers strip (slab S5)



width found at this load was equal to 0.6 mm. A 50 mm

wide and 2 m long CFRP strip having 1.2 mm thickness

has been externally bonded to the tension face of thereinforced concrete slab using Sikadur30 epoxy adhesive

(bonding agent). The carbon fibre strip has been kept at

g ferrocement cover.

(b) Wire mesh used for repair.

(a) Crack pattern before repair. (b) CFRP laminate in position.

Fig. 6. Strengthening the slab with CFRP laminate.


Table 1

Characteristic of Sikadur30 epoxy

Characteristics Guide values

Sag flow 3–5 mm at 35 �CCompressive strength 75–100 N/mm2

Tensile strength 20–30 N/mm2

Shear strength 15–20 N/mm2

E-modulus (static) 8000–16,000 N/mm2

Shrinkage 0.04–0.08%

Glass transition point 50–70 �C

the central part of the slab. The choice of CFRP was

based on theoretical analysis using strain compatibilitymethod and assuming both steel and CFRP yielded at

the same time with the concrete compressive strain

reaches 0.0035. The analytical calculation shows that

the expected failure load of the strengthened slab with

CFRP is equal to 106 kN (double the capacity of the

slab).

The concrete surface where the CFRP strip will be lo-

cated was roughened and cleaned using compressed airand water jet. Sikadur30 epoxy adhesive was next ap-

plied on the roughened concrete surface (2–3 mm thick).

The CFRP was then fixed on the Sikadur30 adhesive

layer as shown in Fig. 6(b). The tensile strength of the

carbon fibre strip is 2800 N/mm2, its modulus of elastic-

ity is 165,000 N/mm2 and the density is 1.5 kg/L. The

main characteristic of the Sikadur30 epoxy is presented

in Table 1.

2.5. Section enlargement (slab S6)



width found at this load was equal to 0.65 mm. In this

repair technique, the bottom of the cracked slab is rein-

forced with additional 50 mm thick concrete layer rein-

(a) Crack pattern before repair.

Fig. 7. Strengthening by enla

forced with additional steel reinforcement. The

strengthened slab was designed to fail at ultimate load

of 104 kN.

The bottom surface of the slab was roughened and a

number of holes have been drilled to a depth equal to

the effective depth of the slab. After cleaning the dust,

48 pieces of R10 steel bars (act as shear connectors) of

155 mm long were inserted in the holes and fixed in po-sition using Sikadur30 epoxy adhesive as shown in Fig.

7(b). Additional flexural steel reinforcement 5R10 and

5R8 was next fixed to the shear connectors. Similar con-

crete mix is used to cast the additional 50 mm concrete

layer and kept for 28 days for curing before retesting

the specimen.

(b) Roughened surface and steel provided.

rging the slab section.

130.93 kN

101.41 kN

57.12 kN52.2 kN

66.96 kN

57.12 kN

0

20

40

60

80

100

120

140

160

S1 S2 S3 S4 S5 S6

Slab

Load

(kN

)

(+ 17.23 % )

(- 8.61 % )(0 % )

(+ 77.39 % )

(+ 129.22 % )

ControlSlab

Fig. 9. Ultimate loads for slabs 1–6.


3. Results and discussions

3.1. Cracking and ultimate loads

The initial cracking loads for different reinforced con-

crete slab specimens are shown in Fig. 8 along with thecontrol slab (S1). All the repaired concrete slabs exhibit

higher cracking load compared to the control slab, ex-

cept S5. The repaired specimens using grout pouring,

epoxy injection and section enlargement techniques

show 35% increase in the cracking loads compared to

the control slab. While the use of ferrocement layer in-

crease the cracking load by 17.8%. The strengthening

of the slab by using the CFRP at its soffit, withoutrepairing the initial cracks, improves the crack width

only. In this specimen, new cracks are developed at

slightly lower load compared to the original slab.

Fig. 9 shows the ultimate failure loads for all the slab

specimens. It could be observed that all the repair tech-

niques used in this study are capable of restoring the

ultimate capacity of the defected slab except specimen

S3 where the cracks have been treated by epoxy injec-tion. However, the reduction in strength is only 8.6%

compared with the control slab. The ultimate capacity

of slabs S5 and S6, which are repaired by CFRP and sec-

tion enlargement, respectively, show 77.4% and 130%

higher ultimate load capacities compared to the control

slab. The increase in the ultimate strength for slab spec-

imen S5 is in agreement with the result reported by

Toong and Li [5] even though the ratio of CFRP striparea to the overall cross-section area used in this study

is very small (0.04). Moreover, there is no increase in

the ultimate capacity of the specimen repaired by ferro-

cement cover compared to the control slab which is

matching well with the conclusion reported by Al-Kub-

aisy and Zamin [9].

Both crack and ultimate loads for all slabs also indi-

cate that the repaired structures had a high degree ofintegrity. The main concern that engineers normally

have are related to the ability of the repair material to

integrate and act compositely with the parent materials

37.00 kN

25.00 kN

32.52 kN

37.44 kN37.44 kN

27.60 kN

0

10

20

30

40

50

60

S1 s2 s3 s4 s5 s6Slab

Load

(kN

)

(+ 35.65 %) (+ 35.65 %)

(+ 17.83 %)

(- 9.42 %)

(+ 34.06 %)

Control Slab

Fig. 8. Cracking loads for slabs 1–6.

did not appear to be a problem for any of the repair

techniques being investigated in this study.

3.2. Slab deflections

Fig. 10 shows load–deflection for each of the slabs.

These deflections are recorded at the mid-span. The slab

specimens show almost similar stiffness except specimen

S6, where the deflection has decreased due to the stiff-

ness of the extra concrete layer of concrete that has been

added. The stiffness has increased more significantlycompared to other specimens. Deflection patterns for

the control slab S1 and specimens S2, S3 and S4 showed

that all of them had similar initial stiffness. However,

after two-third of the ultimate load, these specimens ex-

hibit different level of ductility pattern. The maximum

deflections observed in S3 (epoxy injection) and S4 (fer-

rocement layer) are 15% lower compared to the control

slab, while pouring the crack with grout show 20% in-crease in maximum deflection compared to the original

slab.

19.00 mm

22.90 mm

16.31 mm

15.83 mm

38.65 mm

18.83 mm

0

20

40

60

80

100

120

140

0 10 20 30 40 5Deflection (mm)

Load

(kN

)

0

Control Slab - S1Grout Pouring - S2Epoxy Injection - S3Ferrocement - S4CFRP Strip - S5Enlargement - S6

Fig. 10. Load–deflection curves for all slabs.

185

230

195

60

115

360

0

50

100

150

200

250

300

350

400

450

S1 S2 S3 S4 S5 S6

Slabs

Co

ncr

ete

Str

ain

(x

10-6

)

Fig. 12. Concrete compressive strain at 50 kN load level.


On the other hand, specimen S5 (CFRP) and speci-

men S6 (Section enlargement), show different variation

in load–deflection curves compared to all other speci-

mens. The specimen repaired by section enlargement

exhibits higher stiffness and the load–deflection curve

shows much stiffer behaviour. The maximum deflectionobserved is almost the same as observed in the control

slab but occurred at more than twice the ultimate load.

Moreover, non-ductile variation in the load–deflection

curve could be observed in this specimen, which change

the ductile behaviour observed in the control slab. The

specimen reinforced with the carbon fibre strip shows

no change in the initial stiffness compared to the control

slab. With the increase of loading, the stiffness decreasesat a higher rate compared to specimen S1, S2, S3 and S4.

Hence, the CFRP strip has a significant effect on the

stiffness in the advance stage of loading.

3.3. Strain distribution

Fig. 11 shows the variation of the concrete compres-

sive strain at the mid-span at a distance of 25 mm belowthe top fibre of the reinforced concrete slab specimens

versus the applied load. This location however, is very

close to the N.A of the slab section and hence the values

of the strains are very small. It was recorded during the

test that the N.A. is shifting up as the applied load in-

creases and its depth during different loading stages

changed from 40 to 30 mm (approximately) in speci-

mens S1, S2, S3 and S4. For specimens strengthenedwith CFRP and reinforced concrete layer (S5 and S6),

the depth of N.A changed approximately from 70 to

45 mm near failure load.

Fig. 12 represents the concrete strain in different slab

specimens when the applied load is equal to 50 kN (near

by failure load of the control slab). All the slab speci-

mens exhibit lower strain values compared to the control

slab. The decrease in the concrete strain in specimens S2,S3 and S4 has been found ranging between 30% and

539226

257206

177

314

0

20

40

60

80

100

120

140

0 100 200 300 400 500 600Strain (x 10-6)

Load

(kN

)

Control Slab - S1Grout Pouring - S2Epoxy Injection - S3Ferrocement - S4CFRP Strip - S5Enlargement - S6

Fig. 11. Variation of concrete compressive strain for all slabs.

50%, while in S5 and S6 specimens, the reduction in

compressive strain of concrete has been recorded at

65% and 85%, respectively.

For the specimen reinforced with additional rein-

forced concrete layer, it was observed that the strain in

the additional reinforced concrete layer is not compati-

ble with strain in the original slab although initially bothlayers act as a composite section. This is due to provid-

ing insufficient number of shear connectors which leads

to the occurrence of horizontal longitudinal cracks be-

tween the two layers.

3.4. Failure modes and mechanism

Fig. 13 shows the crack pattern of all the strength-ened slab specimens at failure. The tests were stopped

when excessive deflection and/or excessive wide cracks

were observed although the specimens did not com-

pletely collapse. It is clear from the crack patterns in

the slabs S1, S2, S3 and S4 almost similar modes of fail-

ure have been observed. Cracks started at the tension

sides and increased in width and length with the applied

loads. In the control slab the neutral axis location isshifted upwards until the concrete strain reaches its ulti-

mate value. At this stage, the steel reinforcement is

yielded which quickly led to compressive crushing of

concrete. This failure mechanism is a typical ductile fail-

ure observed in under-reinforced concrete sections.

However, in the repaired specimens, the ductility is not

clearly observed as in the control slab.

The failure mechanism in Slab S5, which has beenstrengthened by CFRP strip, is different from other slab

specimens since CFRP laminates is additional reinforce-

ment. The failure is characterized by shearing of

the concrete interface with the CFRP strip (relative

Fig. 13. Crack patterns and modes of failures for strengthened slabs.


slippage) associated with less warning compared to

other slab specimens. The failure was sudden and oc-

curred immediately after the peeling of the CFRP strips.

This is due to insufficient anchorage length of the CFRPstrip. The strain measured before failure in the CFRP

laminate is 60% of its yielding strain which comply with

peeling failure (and not rupture failure) observed in this

specimen. The number of cracks observed at failure is

more but the cracks width are smaller compared to

other specimens.

For S6 slab specimens, in addition to the transverse

cracks, horizontal crack had been formed at the bound-aries between the original slab and the additional con-

crete layer due to shearing.

4. Conclusion

Based on this study, the following conclusions could

be drawn:

(i) The repaired structures had similar or higher

cracking and ultimate loads compared to the con-

trol slab.

(ii) The repairs using grout, epoxy injection and ferro-

cement layers showed behaviours similar to that of

the control slab in terms of strength and ductility

performance. In other words, these repair tech-niques can safely adopt the normal reinforced con-

crete design for concrete slabs.

(iii) CFRP and section enlargement repair techniques

for the cracked slabs showed superior structural

performance in terms of strength. The ductility

performance for these slabs, however, is less than

that of the control slabs.

(iv) It could also be concluded that all repairs tech-niques used are effective to at least restore the

structural performance of cracked reinforced con-

crete slabs.

Acknowledgements

The authors thank Sika Kimia Sdn. Bhd., Malaysiafor providing the CFRP strip and other Sika products.

Moreover, the authors acknowledge Mr. Wong Chee

Wai and Mr. Tan Khong Yee for their contribution in

the experimental work.


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Documents

Repair and Structural Performance of Initially Cracked RC Slabs