EFFECT OF EPOXY REPAIRING ON RC BEAM SHEAR STRENGTHENED WITH

SIDE BONDED CFRP SHEET STRIPS

Feras Al zoubi, Professor Li ZhengLiang (Department of Civil Engineering, Chongqing University, Chongqing 400045, P.R.China)

E-mail: feraschina@yahoo.com ABSTRACT Externally bonded CFRP is an effective to upgraded and strengthen exiting structures whereas epoxy resins usually used as a treatment in a cracked structure. The focus of this study is on the behavior and effectiveness of externally-sides-bonded CFRP sheet strips for shear strengthening of RC cracked beams with epoxy repairing. This study included test four rectangular simply supported RC beams deficient in shear capacity and identical in dimensions, the four beams are 1700 mm in length with supports of 150 mm apart, one control beam without strengthened and pre-cracked, second beam left without pre-cracked and the two others beams were pre-cracked until a diagonal shear crack became visually clear, in one beam from the two was used the epoxy to repair the main shear crack then three beams strengthened with using externally bonded Carbon Fiber Reinforced Polymer to upgrade their shear capacity. All the beams were strengthened with side-bonded CFRPs strips and horizontal anchored strips were used. Results show the feasibility of using CFRPS with epoxy repairing to restore or increase the load-carrying capacity in the shear of cracked RC beams. The failure mode of all the CFRP-strengthened beams is debonding of CFRP vertical strips. Two prediction models available in ACI-440 and European Code were compared with the experimental results. KEYWORDS CFRP, RC beams, strengthening, epoxy. INTRODUCTION Past earthquakes, in many countries around world, they have caused substantial damage to, or collapse of, many buildings. Also, many existing RC members are found to be deficient in shear strength and need to be strengthened. Deficiencies occur for several reasons, including insufficient shear reinforcement or reduction in steel area because of corrosion, increased service load, and construction defects. The use of Carbon Fiber Reinforced Polymer (CFRP) in strengthening reinforced concrete (RC) structures has become as increasingly popular retrofit technique. Because the FRP composite materials have a lot advantages over traditional construction materials such as steel, wood and concrete when they are used as materials for rehabilitations and strengthening the constructions. Incentives for the use of FRP over traditional materials include the material’s high stiffness-to-weight and strength-to-weight ratios, excellent corrosion resistance to environmental agents and constructability. This field of study (shear strengthen), has captured the attention of many researchers (Khalifa and Nanni. 2002; Chen JF and Teng JG 2003b; Triantafillou 1998; Zhichao Zhang et al. 2005; Anders and Bjorn 2005; Fears Alzoubi and Li Zhengling 2007). Crack repairing by injecting epoxy is a system for welding cracks back together. This welding restores the original strength and loading originally designed into the concrete. Epoxy injection restores the structural qualities the concrete design intended. In other word under most conditions it makes the concrete as good as new. The purpose of this paper is to provide experimental data on the response of damaged RC beams strengthened in shear using unidirectional CFRP sheet strips. It should be mentioned that, there is few or no researches have been done on the combination of externally bonded carbon fibre reinforced polymer and epoxy repairing on concrete structures. But researches on the effect of CFRP and epoxy repairing have been done separately. Therefore, the second objective of this project seeks to investigate the combined effects of CFRP sheet strips bonded two-sides and epoxy repairing on the RC beams. EXPERIMENTAL PROGRAM

Test Specimens All the beams to be tested are designed to fail in shear. Four RC beams had a total span 1700 mm and rectangular cross- section of 150 mm wide and 300 mm deep were cast in the wood forms at the concrete

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laboratory of Civil Engineering of Chongqing University. All RC beams made from the same batch of a ready-mix plain concrete using traditional fabrication and curing techniques. The average concrete strength was determined to be 28 MPa. The longitudinal reinforcement consisted of two-20-mm diameters and one-16-mm diameter high-yield strength steel bars in the bottom and two -10- mm in the top. The web reinforcement consisted of 6.5-mm-diameter closed stirrups, spacing 300mm centre throughout the span of the beam as shown in Fig.1. The carbon fiber used in this experiment were in the form of dry unidirectional flexible sheets and supplied in a roll of 100-mm width. The carbon fiber sheets were bonded to the concrete surface using epoxy adhesive, a two component has mixed together a 1:0.4 to make the adhesive material. E-44 epoxy reins adhesive, which used to fill up the diagonal tensional crack only in B4 before bonding CFRP to the beam surface. It consists of two components, which will be mixed at a 1:2 ratio by volume before use. It is low viscously and high strength also has a pot life 25 minute. The properties of the material presented in Tab. 1. The control beam (B1) was kept without strengthening, others three beams (B2, B3 and B4) were strengthened with one ply CFRP sheets (vertical strips) having perpendicular fiber directions 90 degree to the beams axis and the longitudinal anchored strips were bonded on the two sides of beams, with fiber direction parallel to the axis beam , As shown in (Figure .1)

Figure 1. Shown the longitudinal and web reinforcement with dimension, strengthening scheme of the RC beam

and test set-up of specimens

Table 1. Representative properties of materials used

Preparing and Curing (1) Both sides of the beams, the concrete surface were prepared using mechanical abrasion until the layer of

laitance was removed. (2) And then the surface of the beams was cleaned using compressed air to remove any looses particles. (3) The fibers sheets were measured and cut to the required length prior to installing on the surface. (4) Adhesive resin was used for bonding the CFRP sheet to the concrete, after adhesive was mixed; it was

applied on the concrete surface using a roller in the designated position. (5) The sheets were then placed onto both sides of the beam and gently pressed into the adhesive; a trowel was

used to remove any air bubbles. A ribbed roller was rolled in the fiber direction to facilitate impregnation by separating fibers.

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(6) The sheet was then put on with a second layer of adhesive resin and the excessive resin was removed by using a roller.

(7) The main shear crack of B3 was sealed on the surface by using a quick-hardening resin paste. Four plastic nipples (needles) were installed to inject epoxy into the crack. Required hole was constructed on the crack to attach nipples.

(8) After couple of days the shear crack was examined by inserting water to check whether if crack remained unsealed. The crack was found sealed during injecting water. Later E-44 epoxy was inserted through the injection, the procedure started from the lowest nipple and as soon as the resin was leaked from the mouth of the next nipple the procedure was discontinued, the mouth was sealed, and the same process were repeated for the next nipple. Care was taken during the operation of injecting epoxy in the crack. After one day, when the epoxy was hardened, the resin pasted was removed from the surface with an emery wheel .After the application of epoxy it was left for 3 days to set. Later repaired beam was shear strengthened by side faces CFRP strips according to the same produces followed to beams above.

Test Procedure The beams were instrumented with a linear variable differential transformer (LVDT) at mid-span to monitor deflection. Strain gauges were bonded on middle height internal shear reinforcement and external shear reinforcement to measure deformations. The beams, spanning 1500-mm-, were subjected to a four-point flexural test. An automatic data acquisition system was used to monitor loading as well as mid-span deflection and deformation in the internal and external reinforcement. The load was applied stepwise to the beam by means a testing machine with 5000 KN capacity and was measured by a load cell as show in photographic in (Figure 1). At the conclusion of each step, cracks were sketched (in the preloading stage), observed (in the reloading stage) and related to a measured mid-span deflection and applied load. The first stage preloaded the RC beams to level of damage and then removed the load, also including test the control beam B1. The beams B2, B3, B4 were preloaded to 0%, 90% and 90 % ultimate carrying load of the control beam and then fully unloaded, respectively. Where the B3 and B4 were preloaded until a diagonal shear crack became visually clear as shown in (Figure .2).

Figure .2 Main shear crack is preloaded stage

The second stage was rehabilitated the RC damaged -beams and then reloaded the strengthened damaged RC beams until the failure. Figure .3 shows the two stages of loading (preloading and reloading) for B3 repair with (CFRP) and B4 repair with (CFRP + Epoxy).

Figure 3. Mid-span deflections versus applied load in two stages for B3 and B4

B3 B4

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RESULTS AND ANALYSIS A summary of the test results of four beams under two-point loading is shown in Table 2. The load deflections curves of four beams are depicted in Figure 4. Strength Beam B4 has been found to exhibit the highest loading capacity with an ultimate load of 240 KN, which gets a 85 KN increase as compared with the 155 KN loading capacity of unstrengthened beam B1. Beam B2 and beam B3 fall behind beam B4 with an increase of 80 KN and 70 KN, respectively. The shear capacity of the strengthened beam was about 48-54% higher than the unstrengthened beam. As described earlier, The Beam B3 and the beam B4 were pre-cracked to the same level of damage before strengthening. There is epoxy filled main shear crack for beam B4 before bonding of the CFRP strips, while the B3 directly strengthened by CFRP strips without repair the shear crack. Comparing the curves for two beams in Figure 4, it can be clearly seen that the stiffness for beam B4 is higher seen than for beam B3. This means that repairing the high-damaged beam in shear by CFRP strips and epoxy resin can restore or even increase the beam stiffness degraded during preloading stage (before repair). For this reason, the Epoxy injection is strongly recommended for shear repair with externally bonded CFRP sheet. It can be clearly noticed that the beams B2 and B3 they give some improvement in ductility as compared to control beam. Beam B3 shown more ductility from the beginning the test as compared with the original CFRP strengthened beam B2 and control beam. While repaired beam B4 give little bit improvement over control beam as shown in Figure 4 it is still not as good as the original beam with CFRP reinforcement. The beam B4 stiffness is higher than any strengthened beam in this experiment. The load capacity of damaged beam B4 was increased over the capacity beams B2 and B3. The failure mode B4 was similar to failure mode (B2, B3), but the performance of the beam B4 during reloading stage process was completely incomparable to the beams (B2, B3).

Table 2. Experiment results

Beam designation load at ultimate

(KN)

Deflection at ultimate

(mm)

CFRP shear capacity

(KN) Failure mode

B1 155 0.408 ---------- Diagonal shear failure

B2 235 0.61 40 Strip delamination

B3 225 0.695 35 Strip delamination

B4 240 0.405 42.5 Strip delamination

Figure 4. Comparison of load –deflections curves of beams

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

pped

oad

()

B1B2B3B4

Deflection (mm)

App

lied

Load

(KN

)

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Failure Model The first beam tested (B1) was a control beam without strengthening. As expected, it was a typical shear failure (diagonal shear failure). All the strengthened beams have the same model of failure, brittle-shear failure and the same behaviour during the load processing approximately. As the load increased, more flexural and shear cracks were appeared on both sides of beam. The cracks were observed on the pre-cracked beams earlier than the non-precracked beam .The shear cracks continued to grow until the load reached 210 KN. when the load was approximately 1.4 times or more of the failure load in the control beam. A clicking or popping sound was occasionally emitted from the beam. The occurrence of clicking increased in frequency as the beam was loaded closer to the maximum load bearing capacity. The Loud popping sound and CFRP delamination were observed as the beam started to fail, and sudden failure occurred due to debonding of the first vertical strip and second vertical strip over the diagonal shear crack. This failure accompanied with debonding of top a longitudinal strip, it started peeling off from point load or top of a diagonal shear crack and extended to little bit far after the end of second vertical strips on the side of failure the beam as shown in Figure.5. For CFRP strain in the strengthened non- predamaged beam B2, the carbon fibers strain increased slowly until the first diagonal shear formed, after that, diagonal shear formed carbon strain increased rapidly. The carbon fibers strain in B4 has similar behaviour to B2, approximately. The strengthened pre-damaged beam B3 behaved somewhat differently, whereas the composite stressed from the beginning of loading (Figure 6). The maximum local CFRP vertical strain measured at failure in B2, B3 and B4 were 0.005298 mm/mm, 0.00467mm/mm 0.00446 mm/mm, respectively. Almost 27 % to 22 % of the ultimate strain of CFRP, this indicated that the CFRP did not reach its ultimate strength in all our cases. Strain in steel stirrups was measured at the middle of the hight, and the same locations with strain of CFRP Strips approximately. It was observed that the steel stirrups of the strengthened beams yielded when the applied load was about 92% to ultimate load (Figure 6).

Figure 5. Model failure of the beam

Figure 6. Applied loads versus vertical CFRP and steel strain for beams B2, B3 and B4 PREDICTION OF EXPERIMENT RESULTS Prediction Models The shear capacity of RC beams strengthened using externally bonded CFRP shear may be computed by Eq (1).

V=VC +VS + Vf (1) Assuming that the shear cracks are inclined 45 degree to longitudinal axis of beam and the angle between the principal directions of the fibres of the FRP reinforcement and longitudinal axis of beam 90 degree. According to ACI-440.2R-02, Vf for FRP sheet can be calculated by the following equation (2):

B2 B4 B2 B3

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0

10

20

30

40

50 ACI-440 European Code Experiment

fsfd

fEfefAfv )...(ε= (2)

fA : Area of FRP shear reinforcement fff wtA .2= ft is the thickness of the FRP reinforcement and fw is

the width of the FRP reinforcement. feε : Effective strain level in FRP reinforcement. fE : Modulus of

elasticity of FRP. Effective strain in CFRP reinforcement is computed depended on the model of failure. Therefore, the effective strain due to debonding CFRP sheet from the concrete surface can by computed by the following equation(3):

fdfEft

eLfdcffe .58.0).(

).2.(66.0).(2176,0 −=ε (3)

Cf : Compressive stress of concrete (N/mm2), Le: effective bond length; 58.0)./(23300 ff EtLe = . fd : The

effective depth of FRP shears reinforcement (mm), fE : modulus of elasticity of FRP (N/mm2). According to fib European Code Vf for FRP sheet can be calculated by Eq (4).

fsd

fEdfefAfv )..,.(9.0 ε= (4)

The effective strain due to peeling-off for CFRP side strips calculated by Eq (5, 6)

31056.0].

65.0)([65.0 −×=

ffEcmf

fe ρε (5)

cmf : Mean compressive strength of concrete in Map, fρ : FRP reinforcement ratio= )//()/2( ffwf swbt for

strips.

f

fekdfe γ

εε =, (6)

For CFRP debonding failure 8.0=K , safety factor fγ = 1.3.

Note: the two codes are able to expect the shear capacity of strengthened non-damaged beam and without consider the anchorage vertical strips. Comparison with Experiment Figure 7. shows the comparison between the computed and experimental values of shear contribution of the CFRP sheet to the shear capacity of beam. It is seen that the two codes could not estimate the experiment values for the beams. But in the other hand, the two codes have predicted values smaller than the experimental observation. Thus, the two codes are suitable for expected the shear capacity of the strengthened beam with CFRPS (horizontal and vertical strips) as what have achieved in this experiment. A number of design proposals for the shear strength of CFRP sheet strengthened RC beams (literature review paper, Fears and Li Zhengliang 2007). They used to compare with test results Figure 7. It is clearly from these models proposals; they were not able to predict the experimental results with good accuracy or approval values.

Figure 7. Comparison between the computed and experiment values

B2 B3 B4 B2 B3 B4

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CONCLUSION AND RECOMMENDATIONS China is facing a growing need for effective means of rehabilitation and strengthening of RC structures. Because of their outstanding mechanical, physical, and chemical properties, in addition to simplicity and effectiveness, advanced composite materials show promise in this area. Results of test perform in the present study demonstrated the feasibility of using CFRP sheet bonded side strips to restore or increase the load capacity in shear of damaged RC beams. In particular, the following conclusion can be draw:

1. The test results confirm that the strengthened technique of CFRP sheet strips system is applicable and can increase the shear capacity of damaged beam.

2. According to the observations, the dominate failure mode of CFRP strips strengthened beam was debonding strips, it is strongly recommended using longitudinal anchored strips in additional to vertical CFRP strips.

3. For practical use it can be use the ACI-440 code to predict the shear capacity of damaged beam strengthened with bonded two sides CFRP sheet vertical and horizontal strips.

The main goal of this research was conducted to find the effect bonded -sides CFRP strips on shear strength of damaged RC beams repaired with epoxy.

4. From the result obtained after testing, it can be concluded that for the shear strengthening of damaged RC beam externally bonded CFRP sheets in combination of epoxy repairing is a useful technique. Crack repairing by epoxy can restore the strength of the damaged RC beam whereas CFRP sheets strips sides-bonded increases the stiffness and decrease the deflections of the beam. As a result improves the ultimate shear strength capacity of the damaged beams.

5. The main shear crack from the failure of B4 repaired beam is newly developed crack. Previously developed main shear crack which is repaired by means of injection epoxy, remain unchanged throughout the whole test. Based on this observation, it is recommended that the shear repair of the beam should be carried out prior to development cracks whenever it is applicable.

6. Epoxy injection in the shear cracks is strongly recommended for pre-cracked RC beams deficiencies in shear before shear strengthening by using the CFRP sheets bonded–sides strips.

It can be concluded that, the best way to repair or strengthen a damaged RC beam, is using the CFRP sheets and restore the cracks by epoxy. Also, the quality of workmanships during installation of CFRP and injection the epoxy is play important a role for obtained good results. References Anders C & Bjorn T. (2005). “Experimental study of strengthening FOR increased shear bearing capacity”,

Journal of Composites for Construction, 9(6) December: 488-496. ACI Committee 440.2R-02. Guide for the Design and Construction of Externally Bonded FRP Systems for

Strengthening Concrete Structures, ACI, Farmington Hills, MI, USA. 2002. Chen JF, Teng JG. (2003B). “Shear capacity of FRP strengthened RC beams: FRP debonding”. Construction

and Building Materials, 17(1): 27–41. Fib CBE-FIB. Externally Bonded FRP Reinforcement for RC Structures, bulletin 14 .July.2001. Feras Alzoubi & Li Zhengliang. (2007). “Overview shear strengthening of RC beam with externally bonded

FRP composites”, Journal of Applied Sciences, 7(8):1093-1106. Feras Alzoubi, Zhang Qi ,Li Zhengliang.( 2007). “Experimental study on shear strengthening of pre-damaged

RC beams with CFRP sheet strips. Journal of Chongqing University-Eg- Ed, 6(4):305-310. Khalifa A & Nanni A.(2002). “Rehabilitation of rectangular simply supported RC beams with shear deficiencies

using CFRP composites”, Construction and Building Material, 16(3): 135–146. Triantaf`illou, T., C. (1998). “Shear strengthening of reinforced concrete beams using epoxy-bonded FRP

composites ”, ACI Structural Journal , 95(2) March-April: 107-115. Zhichao Zhang et al (2005). “Shear strengthening of reinforced concrete beams using carbon-Fiber reinforced

polymer laminates”, Journal of Composites for Construction, 9(2) April: 158-169.

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