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Damaged RC beams repaired by bonding of CFRP laminates
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 1301–1310
www.elsevier.com/locate/conbuildmat
Damaged RC beams repaired by bonding of CFRP laminates Omrane Benjeddou a, Mongi Ben Ouezdou a
a,*
, Aouicha Bedday
b
Civil Engineering Laboratory, National Engineering School of Tunis, BP 37, Tunis-Belve´de`re, 1002, Tunisia b Center for Testing and Technical Construction, El Ourdia 1009, Tunisia Received 20 May 2005; received in revised form 20 January 2006; accepted 24 January 2006 Available online 22 August 2006
Abstract This paper summarizes the results of experimental studies on damaged reinforced concrete beams repaired by external bonding of carbon fiber reinforced polymer (CFRP) composite laminates to the tensile face of the beam. Two sets of beams were tested in this study: control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP laminates by varying different parameters (damage degree, CFRP laminate width, concrete strength class). All beams were tested in four-point bending over a span of 1800 mm. The tests were carried out under displacement control. The most investigated parameter in this experimental study is damage degree (ratio between pre-cracked load and load capacity of control beam). Repairing damaged RC beams with externally bonded CFRP laminates were successful for different degrees of damage. The observed failure modes were peeling off and interfacial debonding. These failure modes depend only on the laminate width. The results indicate that the load capacity and the rigidity of repaired beams were significantly higher then those of control beam for all tested damage degrees. The authors remarked that for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Finally, the contribution of CFRP laminates on the load capacity and rigidity of repaired RC beams is significant for any concrete strength class. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon fiber; Concrete repair; Flexural behavior; Damaged beams
1. Introduction Many reinforced concrete (RC) structures are damaged. Most of them are suffering from various deteriorations: cracks, concrete spalling, and large deflection, etc. Many factors are at the origin of these deteriorations, such as ageing, corrosion of steel, earthquake, environmental effects and accidental impacts on the structure. Nowadays, it is necessary to find repair techniques suitable in terms of low costs and fast processing time. Externally bonded fibers reinforced polymers (FRP) has emerged as a new structural strengthening technology in response to the increasing need for repair and strengthening of reinforced concrete structures, because of their high *
Corresponding author. Tel.: +216 71 874 700; fax: +216 71 872 729. E-mail address: [email protected] (M. Ben Ouezdou).
0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.01.008
tensile strength, lightweight, resistance to corrosion, high durability, and ease of installation. The FRP reinforcement has shown to be applicable to the strengthening of structural members or repairing of damaged structures of many types of RC structures, such as columns, beams, slabs, tunnels and silos. The FRP can be used to improve flexural and shear capacities, provide confinement and ductility to compression structural members. The FRP is characterized by high strength fibers embedded in polymer resin. The most common type of FRP in industry is made with carbon, aramid or glass fibers. Repairing beam structures by externally bonded FRP composites consists in adhering FRP laminates at the tensile face of the beam. Among these types of FRP, the application of carbon fiber reinforced polymer (CFRP) to strengthen and repair the concrete beams has received the most attention from the research community. Nanni [1], in his work, showed
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that the composite strengthening systems have become very competitive with traditional strengthening techniques using steel. Beams retrofitted with CFRP have been investigated primarily for their strength enhancement. Saadatmanesh and Ehsani [2], Ritchie et al. [3], and Meier and Kaiser [4] showed the effectiveness of using composite laminates as an external reinforcement for strengthening RC beams. Limam et al. [5] proved in their experimental study that the externally bonded CFRP plates can be efficiently used to strengthen two-way RC slabs. Thanoon et al. [6] tested five different techniques to repair cracked one-way RC slabs and they proved that the CFRP repair technique showed superior structural performance in terms of strength. Li and Ghebreyesus [7] used E-glass fiber-reinforced ultraviolet (UV) curing vinyl ester to repair damaged beams (precracked). This technique is very fast but it needs a special material to cure the repairs by an UV light source exposure. The most published work on damaged RC beams repaired by FRP dealt with repairing these structures damaged by corrosion effect [8– 11]. On field level, Haritos and Hira [12], in their works, used CFRP laminates and CFRP sheets to repair bridge slabs. They proved that the CFRP provide the repaired bridge slabs with high mechanical performances. Meier and Kaiser [4], Meier et al. [13], Ramana et al. [14] and Thomsen et al. [15] worked on carbon fibers laminates and found similar results. Khalifa and Nanni [16] proved in their experimental study that the contribution of externally bonded CFRP to the shear capacity was significant. Faza and Ganga [17] reported an increase of 200% in strength when CFRP laminates are wrapped around beams. Gao et al. [18] proved that an increase in the thickness of resin resulted in a transition of failure mechanism from interfacial debonding along the CFRP-concrete interface to concrete cover separation starting from the end of CFRP laminate in the concrete. In this paper, the authors study the effectiveness of the CFRP laminates on the flexural behavior and their contribution on the load capacity and on the rigidity of damaged reinforced concrete beams then repaired with these laminates. Carbon fibers laminates were bonded in the tensile face of damaged beams. The most investigated parameter in this work is the damage degree (the ratio between the load applied to the beam causing its pre-cracking and the load capacity of the control beam), which has been taken as 0%, 80%, 90% and 100%. The authors varied also the width of the laminates (50 mm and 100 mm) in order to observe the effectiveness of the CFRP laminates geometry on the mechanical behavior of the repaired beams. Finally, the authors used two types of concrete to cast the beams: an ordinary concrete (C38: having an average compressive strength of 38 MPa) usually used on the RC structures and a low resistant concrete (C21: having an average compressive strength of 21 MPa) corresponding to a weak concrete. Then, failure modes observed during this work are presented.
2. Experimental study 2.1. Experimental program Eight types of RC beams have been cast. One beam from each type of concrete is considered as a control beam, without carbon fibers laminates, (beam CB1 (concrete C21) and beam CB2 (concrete C38)). The value of the load carrying capacity of control beam is known (F); to obtain the damage degree (D), a fixed load value (=F Æ D) is then applied to the beam. One beam is reinforced by bonding a carbon fibers laminates in his tensile face by using an epoxy resin (beam RB1). The damage degree of this beam is D = 0%, since this beam is not pre-cracked (case of reinforced beam). The remaining beams were damaged with a fixed damage degree and then repaired by bonding carbon fibers laminates in their tensile face by using an epoxy resin. Three sets of beams were tested in this study, two control beams, the beam RB1 (not damaged but reinforced directly), and four damaged and then repaired beams with different amounts of carbon fibers reinforcement by changing the damage degree, the width of laminate, and the concrete strength class. The length of all laminates used to repair the damaged beams is 1700 mm. Finally, all beams were tested under fourpoint bending. The details of the experimental program were described in Table 1. 2.2. Test beams design All test beams had the same overall cross-sectional dimensions, internal longitudinal reinforcement and stirrup arrangement. The beam geometry and dimensions are presented in Fig. 1. The beams were 120 mm wide, 150 mm high and 2000 mm long. The span of the beam (1800 mm) is limited by the testing machine configuration. The beams were reinforced with steel bars (10 mm of diameter) on the tensile side and steel bars (8 mm of diameter) on the compressive side. The stirrups (6 mm of diameter) were included at the 10 mm centre-to-centre space. All beams were overdesigned in shear to avoid conventional shear failure.
Table 1 Various parameters for different beams used in the experimental program Beam reference
Concrete strength fc28 (MPa)
Designation
Damage degree (%)
Width of laminate (mm)
CB1 CB2 RB1 RB2 RB3 RB4 RB5 RB6
21 38 21 21 21 21 21 38
Control beam Control beam Reinforced Repaired Repaired Repaired Repaired Repaired
– –
– – 100 100 100 100 50 100
0 80 90 100 90 90
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
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Fig. 1. Details of the test beams.
2.3. Material characterization Concrete and mild steel bars were used in preparation of beam specimens. Unidirectional carbon fibers laminates were used with an epoxy adhesive for strengthening. The details of these materials are briefly discussed here. 2.3.1. Concrete Two types of concrete have been caste: a C21 concrete having an average compressive strength of 21 MPa and a C38 concrete having an average compressive strength of 38 MPa. These values have been measured experimentally through compressive strength tests at 28 days. In the rest of this work, the authors used only this notation: C21 and C38. Ordinary Portland cement (CPA42.5), locally available sand and gravels were used for making concrete. The maximum size of coarse aggregate used was 16 mm and the fine aggregates were coarse in nature (fineness modulus = 3.56). The details of the concrete characteristics are given in Table 2. The elastic modulus (Young’s moduTable 2 Mechanical properties test materials Materials
Elastic modulus (GPa)
Concrete
C21 C38
Steel bars
High yield steel Mild steel
Carbon fibers laminates Epoxy resin
Compressive strength (MPa)
30 36
21 38
200 200
400 235
165 12.8
Tensile strength (MPa) 1.86 2.88 400 235 2800 4
lus), E, has been determined through the French concrete code [19] pffiffiffiffi E ¼ 11; 000 3 fc where fc is the compressive strength at 28 days. The tensile strength, ft, has been also determined by the French concrete code [19] ft ¼ 0:6 þ 0:006f c : 2.3.2. Steel For all concrete test beams, standard deformed reinforcement steel bars with a characteristic strength of 400 MPa and an elastic modulus of 200 GPa, as given by the manufacturer, (Table 2) were used for the longitudinal reinforcement: two high yield steel bars of 10 mm diameter were used in the tensile face and two high yield steel bars of 8 mm diameter were used in the compressive face. Mild steel bars of 6 mm diameter (stirrups) with a characteristic strength of 235 MPa and an elastic modulus of 200 GPa, as given by the manufacturer, were used for the shear reinforcement (Table 2). The steel bars design appears in Fig. 1. 2.3.3. Carbon fibers laminates and epoxy adhesive Twenty-eight days after concrete hardening, six beams were damaged at different damage degree. Then, these beams were repaired using unidirectional carbon fibers laminates ‘‘SIKA CARBODUR LAMELLE’’ (Fig. 1). Five beams were repaired with S1210 having 100 mm width and one beam was repaired with S1205 having 50 mm width. The carbon fibers laminates were bonded by using the epoxy resin SIKADUR 30 COLLE. The thickness of the
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two types of laminate is 1.2 mm. The mechanical properties of the carbon fibers laminates and the epoxy resin appear in Table 2.
bon fibers laminate has been bonded on the tensile face of the damaged beam and the load has been applied to the failure of beam.
2.4. Test setup
2.5. Repairing technique
All beams were tested in four-point bending under displacement control at the rate of 1 mm/min on a universal testing machine (UTM) with a maximum load capacity of 500 kN. The load–displacement data was automatically recorded through a data logger. The beam supports consisted of a pin support and a roller support at the two ends. The outer loading span was 1800 mm and the inner loading span was 600 mm. The test setup, the various monitoring devices, and their location along the beam are presented in Fig. 2. The damaged beams were precracked also by this machine. The load, necessary to attain the fixed damage degree, has been applied. Then, the car-
The steps of loading–unloading are illustrated in Fig. 3. After loading to the desired damage degree, the beams were unloaded. The observed damages are as the following: At 0% of damage degree, the beam is reinforced by the laminates without pre-cracking. At 80% of damage degree, the beam is still in the elastic range: no cracking. At 90% of damage degree, two cracks appeared at each side and the beam have a curvature of about (1.8 mm). The beam reached the plastic range.
Fig. 2. (a) Four-point bending setup and (b) schematic drawing.
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
At 100% of damage degree, more cracks appeared and the beam reach a curvature of 10.5 mm.
Loading Unloading Bonding of laminate Repaired beam Failure
40 35
Load [ KN]
30 25 20 15 10 5 0 0
2
4
6
8
10
1305
12
Deflection [mm] Fig. 3. Details of the test on the repaired beam.
14
All cracks and curvature were left as found and the carbon fibers laminates were bonded to the beams. The bonding process is as the following: all loose particles of concrete surface at the tensile side of the beam were chiseled out by using a chisel. Then the surface was roughened with wire brush before cleaning it with air blower to remove all dust particles. Also, it was ensured that no moisture was visible on the surface. The resin was applied on the prepared concrete surface and laminate surface. Then, the carbon fibers laminates were attached starting at one end and applying enough pressure to press out any excess epoxy from the sides of the laminate. Excess epoxy was removed from sides of the laminate. Finally, the last step was to load the repaired beams until failure (Fig. 3).
Fig. 4. (a) Failure of the control beam and (b) schematic drawing.
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3. Results and discussion 3.1. Failure modes The failure mode of the two-control beams is by steel yielding giving a large deflection of the beam. This failure mode is characterized by the appearance of two first cracks localised under the two loads (Fig. 4), followed by the appearance of micro-cracks between the two large cracks. For all repaired beams, the authors observed two failure modes in the concrete structures: peeling off (Fig. 5) and interfacial debonding (Fig. 6). For all repaired beams, there was no failure of the CFRP laminates. The peeling off was observed for all reinforced beams with the laminate width of 100 mm and the interfacial debonding was observed for all reinforced beams with the laminate width of 50 mm. From these observations, the authors can conclude that the width laminate affects the failure modes of the repaired RC beams. For a larger width, the failure that took place by peeling off is explained by the fact that the contact between the lam-
inate and the concrete is strong enough so that no debonding occurs, while for smaller width this contact does not allow good adherence. Similar observations were noted by Gao et al. [18] but the studied parameter was the laminate thickness. The failure of the control beams was by steel yielding. The elastoplastic behavior is characterized by a large plastic range, showing a significant ductility. But, for all repaired beams, the failure was brittle and was reached suddenly, due to the purely elastic behavior of the CFRP laminates. Thus, the ductility of the repaired beams is less than that of the control beams. This phenomenon has been also observed and studied by Ramana et al. [14]. They noted that, as the degree of strengthening increases, the deflections at ultimate load is reduced, resulting in a reduction of the ductility of strengthened beams. 3.2. Effect of the damage degree of beams The question is: ‘‘is it possible to use the CFRP laminates for any damaged RC beams?’’. In this study, the authors
Fig. 5. (a) A photo showing a repaired beam failure by peeling off and (b) schematic drawing.
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
tested the effect of the damage degree on the load capacity and the rigidity of the repaired RC beams by bonding an externally CFRP laminates. A control beam CB1 and four damaged beams have been tested: RB1 (damage degree: D = 0%), RB2 (D = 80%), RB3 (D = 90%) and RB4 (D = 100%) and then repaired by CFRP laminates having 1700 mm of length and 100 mm of width. The load–deflection responses for the individual beams, with different damage degree, are plotted in Fig. 7, while the summary of the results are presented in Table 3: failure load, deflection at failure, load capacity contribution, rigidity coefficient, rigidity contribution, failure modes. The load capacity contribution is the ratio between the load carrying capacity of the repaired beam and that of the control beam. The rigidity coefficient is the ratio between the elastic limit of the beam and the corresponding deflection. The rigidity contribution is the ratio between the rigidity capacity of the repaired beam and that of the control beam. From the results presented in Table 3 and Fig. 7, the authors observed that all repaired beams have a mechanical behavior, in terms of load capacity and rigidity, higher
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than those of control beam (the load capacity increases by 87% from CB1 to RB1 and by 44% from CB1 to RB4. The rigidity coefficient increases about two times from CB1 to RB1 and by 47% from CB1 to RB4). The authors conclude that, for any damage degree, the CFRP laminates provide the repaired RC beam higher mechanical performances. This observation is illustrated by Fig. 8, which describes the failure load versus damage degree. Furthermore, and even though the beam RB4 was completely damaged (pre-cracked to failure and large deflection: 10 mm), the contribution of CFRP laminate on the load capacity (144%) and on the rigidity are very significant (147%). The authors remarked that RB1 and RB2 have similar load capacity (RB1 = 40.11 kN and RB2 = 37.66 kN) and similar rigidity (RB1 = 4.845 kN/mm and RB2 = 4.878 kN/ mm) because RB1 (D = 0%) was not damaged but was directly reinforced and RB2 (before repairing) has not reached the elastic limit, so the two beams have the same mechanical characteristics before repairing. Finally, the authors noted that the mechanical behavior of the repaired beams changed from elastoplastic to elastic behavior. The
Fig. 6. (a) A photo showing a repaired beam failure by interfacial debonding and (b) schematic drawing.
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O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310 CB1:Control beam RB1:D=0% RB2:D=80% RB3:D=90% RB4:D=100%
45 40 35
Load [KN]
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
22
24
Deflection [mm] Fig. 7. Load versus deflection curves for beams with different damage degree.
Table 3 Summary of the bending test results Beam reference
Failure load (kN)
Deflection at failure (mm)
Load capacity contribution (%)
Rigidity coefficient (kN/mm)
Rigidity contribution (%)
Failure modes
CB1 CB2 RB1 RB2 RB3 RB4 RB5 RB6
21.41 23.92 40.11 37.66 32.10 30.75 30.10 37.37
12.86 13.50 9.02 8.65 9.87 19.50 13.50 10.50
– – 187 176 150 144 140 156
2.352 1.818 4.854 4.878 4.445 3.448 3.225 4.716
– – 206 207 189 147 137 289
Steel yielding Steel yielding Peeling off Peeling off Peeling off Peeling off Interfacial debonding Peeling off
authors conclude that for any damage degree the repairing of RC beams by using CFRP laminates is effective and that the performance of the repaired beam is mainly attributed Damaged beams Control beam
55
to the higher mechanical characteristics of the CFRP laminates. Furthermore, the 80% and the 0% damage degree beams behave likely and they give a higher performance in term of load capacity and rigidity due to the additional contribution of the reinforced concrete. 3.3. Effect of the carbon fibers laminate width
50
Failure Load [KN]
45 40 35 30 25 20 15 10 5 0 0%
80%
90%
Damaged degree Fig. 8. The failure load versus the damage degree.
100%
In order to observe the effect of the width of the carbon fiber laminate on the load capacity and the rigidity of the repaired concrete beams, two similar beams have been damaged and then, these beams were repaired by changing the width of laminates: RB3 (l = 100 mm for about 83% of the total width of the beam) and RB5 (l = 50 mm for about 42% of the total width of the beam). At first, the authors noticed that the width laminate affects the failure modes of the repaired beams. These failure modes change from interfacial debonding to the peeling-off when the width increases from 50 to 100 mm. The results of the two widths are given in Fig. 9, which presents the load–deflection curves. From the results presented in Table 3 and Fig. 9, the loads capacities are:
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
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CB1:Control beam RB3: l=100 mm RB5: l=50 mm
45 40
Load [ KN ]
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
22
Deflection [mm ] Fig. 9. Load versus deflection curves for beams with different laminates width.
3.4. Effect of the concrete strength class
RB3 = 32.10 kN and RB5 = 30.10 kN and The rigidity coefficients are: RB3 = 4.44 kN/mm and RB5 = 3.22 kN/ mm. The authors conclude that for the two widths laminates the mechanical behavior of the repaired beams, in terms of load capacity and rigidity, is higher than that of control beam (load capacity = 21.41 kN and rigidity coefficient = 2.35 kN/mm). Furthermore, the authors may also conclude that the increase of laminate width contributed to a small increase of a load capacity (from 140% for RB5 to 150% for RB3), but contributed also to a significant increase of rigidity (from 137% for RB5 to 189% for RB3). Therefore, for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory in this case. Even, when interfacial debonding occurs; it is possible to proceed then with a further CFRP replacement.
In this study, the authors tested the effect of the concrete strength class on the load capacity and the rigidity of the repaired RC beams by bonding an externally CFRP laminates. Four beams have been tested: CB1 and RB3 were cast by using C21 (corresponding to a weak concrete) and CB2 and RB6 were cast by using C38 (an ordinary concrete usually used in RC structures). CB1 and CB2 were considered as control beams and RB3 and RB6 were damaged (damage degree: D = 90%) and then repaired by CFRP laminates having 1700 mm of length and 100 mm of width. Fig. 10 presents the load–deflection curves for these two concrete strength classes. From the results presented in Table 3 and Fig. 10, the authors observed that for the two types of concrete, the
CB1:control beam
laminate width = 100 mm
CB2:control beam RB3:Fc28=21MPa
45
RB6:Fc28=38MPa
40
Load [KN]
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
Deflection [mm] Fig. 10. Load versus deflection curves for different concrete class.
22.5
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mechanical behavior of the two repaired beams, in terms of load capacity and rigidity, is higher than those of control beams (For C21, the load capacity increases from CB1 to RB3 by 50% and rigidity coefficient increases from CB1 to RB3 by 89%; for C38 the load capacity increases from CB2 to RB6 by 56% and rigidity coefficient increases from CB2 to RB6 by 189%). The authors remarked that the two repaired beams have the same rigidity even though they have a different concrete strength. Then, the authors conclude that the rigidity of the repaired beams is affected only by CFRP laminate. Furthermore, the difference between the load capacity of the two beams (RB3 and RB6) is due to the increase of the concrete strength. For any concrete class, the CFRP added about a half of the load capacity. Finally, the CFRP laminates have more influence on the mechanical behavior of repaired RC beams than the effect of the concrete and steel properties. 4. Conclusion In the present study, the mechanical behavior of the composite repaired concrete structures was investigated through standard laboratory tests. Two sets of beams were tested in this study; control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP reinforcement by changing the width of CFRP laminate, the damage degree and the concrete strength class. Throughout the results from all experimental works, several conclusions can be drawn as follows: 1. The mechanical performance of the repaired RC beams is highly increased by using the CFRP laminates. Therefore, this technique is effective to at least restore the mechanical performance of cracked or damaged RC beams. 2. The laminate width affects the failure modes of the repaired beams. These failure modes change from interfacial debonding to the peeling-off when the width increases from 50 mm to 100 mm. 3. The mechanical behavior of the repaired beams changed from elastoplastic to elastic behavior. The ductility performance of the repaired beams is less than that of the control beams. 4. The rigidity of the repaired beams is due only to the contribution of CFRP laminate. 5. The overall result is that, for any damage degree, the CFRP laminate contribute to higher mechanical performances of the repaired damaged RC beams. For all repaired beams, there was no failure of the CFRP laminates. 6. For a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Even, when interfacial debonding occurs; it is possible then to proceed with a further CFRP replacement. 7. For any concrete class, the CFRP laminates added about a half of the load capacity.
Acknowledgements The authors thank the Center for Testing and Technical Construction ( CETEC) and ‘‘Sika Tunisienne’’ for their assistance in the experimental work. References [1] Nanni A. Composites: coming on strong. Concrete Constr 1999;44: 120–4. [2] Saadatmanesh H, Ehsani MR. Reinforced concrete beams strengthened with GFRP plates–I. Experimental study. ASCE J Str Engng 1991;117(11):3417–33. [3] Ritchie PA, David AT, Le-Wu L, Guy MC. External reinforcement of concrete beams using fiber reinforced plastics. ACI Str J 1991;88(4):490–500. [4] Meier U, Kaiser HP. Strengthening of structures with CFRP laminates. In: Proceedings on advanced composite materials in civil engineering structures, MT. Div./ASCE/Las Vegas, 1991; p. 224– 32. [5] Limam O, Foret G, Ehrlacher A. RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach. Compos Struct 2003;60(4):467–71. [6] Thanoon WA, Jaafar MS, Razali M, Kadir A, Noorzaei J. Repair and structural performance of initially cracked reinforced concrete slabs. Constr Build Mater 2005;19(8):595–603. [7] Li G, Ghebreyesus A. Fast repair of damaged RC beams using UV curing FRP composites. Compos Struct 2006;72(1):105–10. [8] Bonacci JF, Maalej M. Externally bonded Fiber-Reinforced Polymer for rehabilitation of corrosion damaged concrete beams. ACI Struc J 2000;97(5):703–11. [9] Sherwood EG, Soudki K. Rehabilitation of corrosion damaged concrete beams with CFRP laminates – a pilot study. Comp Part B: Engng 2000;31(6–7):453–9. [10] El Maaddawy T, Soudki K. Carbon-Fiber-Reinforced Polymer repair to extend service life of corroded reinforced concrete beams. ASCE J Compos for Constr 2005;9(2):187–94. [11] Badawi M, Soudki K. Control of corrosion-induced damage in reinforced concrete beams using carbon fiber-reinforced polymer laminates. ASCE J Compos for Constr 2005;9(2):195–201. [12] Haritos N, Hira A. Repair and strengthening of RC flat slab bridges using CFRPs. Compos Struct 2004;66(1–4):555–62. [13] Meier U, Deuring M, Meier H, Schwegler G. Strengthening of structures with CFRP laminates: research and applications in Switzerland. In: Neale KW, Labossiere P, editors. Advanced Composite Materials in Bridges and Structures. Canadian Society for Civil Engineering; 1992. p. 243–51. [14] Ramana VPV, Kanta T, Mortonb SE, Duttac PK, Mukherjee A, Desaia YM. Behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening. Compos Part B: Eng 2000;31(6–7):461–70. [15] Thomsen H, Spacone E, Linkatanyu S, Camata G. Failure Mode Analyses of reinforced concrete beams strengthened in flexure with externally bonded fiber-reinforced polymers. Compos Constr 2004;7(2):123–31. [16] Khalifa A, Nanni A. Rehabilitation of Rectangular Simply Supported RC Beams with Shear Deficiencies Using CFRP Composites. Constr Build Mater 2002;16(3):135–46. [17] Faza SS, Ganga R. Fiber composite wrap for rehabilitation of concrete structures. In: Proceedings of the materials engineering conference 804, ASCE. 1994; p. 1135–9. [18] Gao B, Kim JK, Christopher KY. Experimental study on RC beams with FRP strips bonded with rubber-modified resins. Comp Sci Tech 2004;64(16):2557–64. [19] BAEL. Re`gles techniques de conception et de calcul des ouvrages et constructions en be´ton arme´ suivant la me´thode des e´tats limites. French Concrete Code Eyrolles ed. 1992 (in French).
MATERIALS
Construction and Building Materials 21 (2007) 1301–1310
www.elsevier.com/locate/conbuildmat
Damaged RC beams repaired by bonding of CFRP laminates Omrane Benjeddou a, Mongi Ben Ouezdou a
a,*
, Aouicha Bedday
b
Civil Engineering Laboratory, National Engineering School of Tunis, BP 37, Tunis-Belve´de`re, 1002, Tunisia b Center for Testing and Technical Construction, El Ourdia 1009, Tunisia Received 20 May 2005; received in revised form 20 January 2006; accepted 24 January 2006 Available online 22 August 2006
Abstract This paper summarizes the results of experimental studies on damaged reinforced concrete beams repaired by external bonding of carbon fiber reinforced polymer (CFRP) composite laminates to the tensile face of the beam. Two sets of beams were tested in this study: control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP laminates by varying different parameters (damage degree, CFRP laminate width, concrete strength class). All beams were tested in four-point bending over a span of 1800 mm. The tests were carried out under displacement control. The most investigated parameter in this experimental study is damage degree (ratio between pre-cracked load and load capacity of control beam). Repairing damaged RC beams with externally bonded CFRP laminates were successful for different degrees of damage. The observed failure modes were peeling off and interfacial debonding. These failure modes depend only on the laminate width. The results indicate that the load capacity and the rigidity of repaired beams were significantly higher then those of control beam for all tested damage degrees. The authors remarked that for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Finally, the contribution of CFRP laminates on the load capacity and rigidity of repaired RC beams is significant for any concrete strength class. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon fiber; Concrete repair; Flexural behavior; Damaged beams
1. Introduction Many reinforced concrete (RC) structures are damaged. Most of them are suffering from various deteriorations: cracks, concrete spalling, and large deflection, etc. Many factors are at the origin of these deteriorations, such as ageing, corrosion of steel, earthquake, environmental effects and accidental impacts on the structure. Nowadays, it is necessary to find repair techniques suitable in terms of low costs and fast processing time. Externally bonded fibers reinforced polymers (FRP) has emerged as a new structural strengthening technology in response to the increasing need for repair and strengthening of reinforced concrete structures, because of their high *
Corresponding author. Tel.: +216 71 874 700; fax: +216 71 872 729. E-mail address: [email protected] (M. Ben Ouezdou).
0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.01.008
tensile strength, lightweight, resistance to corrosion, high durability, and ease of installation. The FRP reinforcement has shown to be applicable to the strengthening of structural members or repairing of damaged structures of many types of RC structures, such as columns, beams, slabs, tunnels and silos. The FRP can be used to improve flexural and shear capacities, provide confinement and ductility to compression structural members. The FRP is characterized by high strength fibers embedded in polymer resin. The most common type of FRP in industry is made with carbon, aramid or glass fibers. Repairing beam structures by externally bonded FRP composites consists in adhering FRP laminates at the tensile face of the beam. Among these types of FRP, the application of carbon fiber reinforced polymer (CFRP) to strengthen and repair the concrete beams has received the most attention from the research community. Nanni [1], in his work, showed
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that the composite strengthening systems have become very competitive with traditional strengthening techniques using steel. Beams retrofitted with CFRP have been investigated primarily for their strength enhancement. Saadatmanesh and Ehsani [2], Ritchie et al. [3], and Meier and Kaiser [4] showed the effectiveness of using composite laminates as an external reinforcement for strengthening RC beams. Limam et al. [5] proved in their experimental study that the externally bonded CFRP plates can be efficiently used to strengthen two-way RC slabs. Thanoon et al. [6] tested five different techniques to repair cracked one-way RC slabs and they proved that the CFRP repair technique showed superior structural performance in terms of strength. Li and Ghebreyesus [7] used E-glass fiber-reinforced ultraviolet (UV) curing vinyl ester to repair damaged beams (precracked). This technique is very fast but it needs a special material to cure the repairs by an UV light source exposure. The most published work on damaged RC beams repaired by FRP dealt with repairing these structures damaged by corrosion effect [8– 11]. On field level, Haritos and Hira [12], in their works, used CFRP laminates and CFRP sheets to repair bridge slabs. They proved that the CFRP provide the repaired bridge slabs with high mechanical performances. Meier and Kaiser [4], Meier et al. [13], Ramana et al. [14] and Thomsen et al. [15] worked on carbon fibers laminates and found similar results. Khalifa and Nanni [16] proved in their experimental study that the contribution of externally bonded CFRP to the shear capacity was significant. Faza and Ganga [17] reported an increase of 200% in strength when CFRP laminates are wrapped around beams. Gao et al. [18] proved that an increase in the thickness of resin resulted in a transition of failure mechanism from interfacial debonding along the CFRP-concrete interface to concrete cover separation starting from the end of CFRP laminate in the concrete. In this paper, the authors study the effectiveness of the CFRP laminates on the flexural behavior and their contribution on the load capacity and on the rigidity of damaged reinforced concrete beams then repaired with these laminates. Carbon fibers laminates were bonded in the tensile face of damaged beams. The most investigated parameter in this work is the damage degree (the ratio between the load applied to the beam causing its pre-cracking and the load capacity of the control beam), which has been taken as 0%, 80%, 90% and 100%. The authors varied also the width of the laminates (50 mm and 100 mm) in order to observe the effectiveness of the CFRP laminates geometry on the mechanical behavior of the repaired beams. Finally, the authors used two types of concrete to cast the beams: an ordinary concrete (C38: having an average compressive strength of 38 MPa) usually used on the RC structures and a low resistant concrete (C21: having an average compressive strength of 21 MPa) corresponding to a weak concrete. Then, failure modes observed during this work are presented.
2. Experimental study 2.1. Experimental program Eight types of RC beams have been cast. One beam from each type of concrete is considered as a control beam, without carbon fibers laminates, (beam CB1 (concrete C21) and beam CB2 (concrete C38)). The value of the load carrying capacity of control beam is known (F); to obtain the damage degree (D), a fixed load value (=F Æ D) is then applied to the beam. One beam is reinforced by bonding a carbon fibers laminates in his tensile face by using an epoxy resin (beam RB1). The damage degree of this beam is D = 0%, since this beam is not pre-cracked (case of reinforced beam). The remaining beams were damaged with a fixed damage degree and then repaired by bonding carbon fibers laminates in their tensile face by using an epoxy resin. Three sets of beams were tested in this study, two control beams, the beam RB1 (not damaged but reinforced directly), and four damaged and then repaired beams with different amounts of carbon fibers reinforcement by changing the damage degree, the width of laminate, and the concrete strength class. The length of all laminates used to repair the damaged beams is 1700 mm. Finally, all beams were tested under fourpoint bending. The details of the experimental program were described in Table 1. 2.2. Test beams design All test beams had the same overall cross-sectional dimensions, internal longitudinal reinforcement and stirrup arrangement. The beam geometry and dimensions are presented in Fig. 1. The beams were 120 mm wide, 150 mm high and 2000 mm long. The span of the beam (1800 mm) is limited by the testing machine configuration. The beams were reinforced with steel bars (10 mm of diameter) on the tensile side and steel bars (8 mm of diameter) on the compressive side. The stirrups (6 mm of diameter) were included at the 10 mm centre-to-centre space. All beams were overdesigned in shear to avoid conventional shear failure.
Table 1 Various parameters for different beams used in the experimental program Beam reference
Concrete strength fc28 (MPa)
Designation
Damage degree (%)
Width of laminate (mm)
CB1 CB2 RB1 RB2 RB3 RB4 RB5 RB6
21 38 21 21 21 21 21 38
Control beam Control beam Reinforced Repaired Repaired Repaired Repaired Repaired
– –
– – 100 100 100 100 50 100
0 80 90 100 90 90
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Fig. 1. Details of the test beams.
2.3. Material characterization Concrete and mild steel bars were used in preparation of beam specimens. Unidirectional carbon fibers laminates were used with an epoxy adhesive for strengthening. The details of these materials are briefly discussed here. 2.3.1. Concrete Two types of concrete have been caste: a C21 concrete having an average compressive strength of 21 MPa and a C38 concrete having an average compressive strength of 38 MPa. These values have been measured experimentally through compressive strength tests at 28 days. In the rest of this work, the authors used only this notation: C21 and C38. Ordinary Portland cement (CPA42.5), locally available sand and gravels were used for making concrete. The maximum size of coarse aggregate used was 16 mm and the fine aggregates were coarse in nature (fineness modulus = 3.56). The details of the concrete characteristics are given in Table 2. The elastic modulus (Young’s moduTable 2 Mechanical properties test materials Materials
Elastic modulus (GPa)
Concrete
C21 C38
Steel bars
High yield steel Mild steel
Carbon fibers laminates Epoxy resin
Compressive strength (MPa)
30 36
21 38
200 200
400 235
165 12.8
Tensile strength (MPa) 1.86 2.88 400 235 2800 4
lus), E, has been determined through the French concrete code [19] pffiffiffiffi E ¼ 11; 000 3 fc where fc is the compressive strength at 28 days. The tensile strength, ft, has been also determined by the French concrete code [19] ft ¼ 0:6 þ 0:006f c : 2.3.2. Steel For all concrete test beams, standard deformed reinforcement steel bars with a characteristic strength of 400 MPa and an elastic modulus of 200 GPa, as given by the manufacturer, (Table 2) were used for the longitudinal reinforcement: two high yield steel bars of 10 mm diameter were used in the tensile face and two high yield steel bars of 8 mm diameter were used in the compressive face. Mild steel bars of 6 mm diameter (stirrups) with a characteristic strength of 235 MPa and an elastic modulus of 200 GPa, as given by the manufacturer, were used for the shear reinforcement (Table 2). The steel bars design appears in Fig. 1. 2.3.3. Carbon fibers laminates and epoxy adhesive Twenty-eight days after concrete hardening, six beams were damaged at different damage degree. Then, these beams were repaired using unidirectional carbon fibers laminates ‘‘SIKA CARBODUR LAMELLE’’ (Fig. 1). Five beams were repaired with S1210 having 100 mm width and one beam was repaired with S1205 having 50 mm width. The carbon fibers laminates were bonded by using the epoxy resin SIKADUR 30 COLLE. The thickness of the
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two types of laminate is 1.2 mm. The mechanical properties of the carbon fibers laminates and the epoxy resin appear in Table 2.
bon fibers laminate has been bonded on the tensile face of the damaged beam and the load has been applied to the failure of beam.
2.4. Test setup
2.5. Repairing technique
All beams were tested in four-point bending under displacement control at the rate of 1 mm/min on a universal testing machine (UTM) with a maximum load capacity of 500 kN. The load–displacement data was automatically recorded through a data logger. The beam supports consisted of a pin support and a roller support at the two ends. The outer loading span was 1800 mm and the inner loading span was 600 mm. The test setup, the various monitoring devices, and their location along the beam are presented in Fig. 2. The damaged beams were precracked also by this machine. The load, necessary to attain the fixed damage degree, has been applied. Then, the car-
The steps of loading–unloading are illustrated in Fig. 3. After loading to the desired damage degree, the beams were unloaded. The observed damages are as the following: At 0% of damage degree, the beam is reinforced by the laminates without pre-cracking. At 80% of damage degree, the beam is still in the elastic range: no cracking. At 90% of damage degree, two cracks appeared at each side and the beam have a curvature of about (1.8 mm). The beam reached the plastic range.
Fig. 2. (a) Four-point bending setup and (b) schematic drawing.
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
At 100% of damage degree, more cracks appeared and the beam reach a curvature of 10.5 mm.
Loading Unloading Bonding of laminate Repaired beam Failure
40 35
Load [ KN]
30 25 20 15 10 5 0 0
2
4
6
8
10
1305
12
Deflection [mm] Fig. 3. Details of the test on the repaired beam.
14
All cracks and curvature were left as found and the carbon fibers laminates were bonded to the beams. The bonding process is as the following: all loose particles of concrete surface at the tensile side of the beam were chiseled out by using a chisel. Then the surface was roughened with wire brush before cleaning it with air blower to remove all dust particles. Also, it was ensured that no moisture was visible on the surface. The resin was applied on the prepared concrete surface and laminate surface. Then, the carbon fibers laminates were attached starting at one end and applying enough pressure to press out any excess epoxy from the sides of the laminate. Excess epoxy was removed from sides of the laminate. Finally, the last step was to load the repaired beams until failure (Fig. 3).
Fig. 4. (a) Failure of the control beam and (b) schematic drawing.
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3. Results and discussion 3.1. Failure modes The failure mode of the two-control beams is by steel yielding giving a large deflection of the beam. This failure mode is characterized by the appearance of two first cracks localised under the two loads (Fig. 4), followed by the appearance of micro-cracks between the two large cracks. For all repaired beams, the authors observed two failure modes in the concrete structures: peeling off (Fig. 5) and interfacial debonding (Fig. 6). For all repaired beams, there was no failure of the CFRP laminates. The peeling off was observed for all reinforced beams with the laminate width of 100 mm and the interfacial debonding was observed for all reinforced beams with the laminate width of 50 mm. From these observations, the authors can conclude that the width laminate affects the failure modes of the repaired RC beams. For a larger width, the failure that took place by peeling off is explained by the fact that the contact between the lam-
inate and the concrete is strong enough so that no debonding occurs, while for smaller width this contact does not allow good adherence. Similar observations were noted by Gao et al. [18] but the studied parameter was the laminate thickness. The failure of the control beams was by steel yielding. The elastoplastic behavior is characterized by a large plastic range, showing a significant ductility. But, for all repaired beams, the failure was brittle and was reached suddenly, due to the purely elastic behavior of the CFRP laminates. Thus, the ductility of the repaired beams is less than that of the control beams. This phenomenon has been also observed and studied by Ramana et al. [14]. They noted that, as the degree of strengthening increases, the deflections at ultimate load is reduced, resulting in a reduction of the ductility of strengthened beams. 3.2. Effect of the damage degree of beams The question is: ‘‘is it possible to use the CFRP laminates for any damaged RC beams?’’. In this study, the authors
Fig. 5. (a) A photo showing a repaired beam failure by peeling off and (b) schematic drawing.
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
tested the effect of the damage degree on the load capacity and the rigidity of the repaired RC beams by bonding an externally CFRP laminates. A control beam CB1 and four damaged beams have been tested: RB1 (damage degree: D = 0%), RB2 (D = 80%), RB3 (D = 90%) and RB4 (D = 100%) and then repaired by CFRP laminates having 1700 mm of length and 100 mm of width. The load–deflection responses for the individual beams, with different damage degree, are plotted in Fig. 7, while the summary of the results are presented in Table 3: failure load, deflection at failure, load capacity contribution, rigidity coefficient, rigidity contribution, failure modes. The load capacity contribution is the ratio between the load carrying capacity of the repaired beam and that of the control beam. The rigidity coefficient is the ratio between the elastic limit of the beam and the corresponding deflection. The rigidity contribution is the ratio between the rigidity capacity of the repaired beam and that of the control beam. From the results presented in Table 3 and Fig. 7, the authors observed that all repaired beams have a mechanical behavior, in terms of load capacity and rigidity, higher
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than those of control beam (the load capacity increases by 87% from CB1 to RB1 and by 44% from CB1 to RB4. The rigidity coefficient increases about two times from CB1 to RB1 and by 47% from CB1 to RB4). The authors conclude that, for any damage degree, the CFRP laminates provide the repaired RC beam higher mechanical performances. This observation is illustrated by Fig. 8, which describes the failure load versus damage degree. Furthermore, and even though the beam RB4 was completely damaged (pre-cracked to failure and large deflection: 10 mm), the contribution of CFRP laminate on the load capacity (144%) and on the rigidity are very significant (147%). The authors remarked that RB1 and RB2 have similar load capacity (RB1 = 40.11 kN and RB2 = 37.66 kN) and similar rigidity (RB1 = 4.845 kN/mm and RB2 = 4.878 kN/ mm) because RB1 (D = 0%) was not damaged but was directly reinforced and RB2 (before repairing) has not reached the elastic limit, so the two beams have the same mechanical characteristics before repairing. Finally, the authors noted that the mechanical behavior of the repaired beams changed from elastoplastic to elastic behavior. The
Fig. 6. (a) A photo showing a repaired beam failure by interfacial debonding and (b) schematic drawing.
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O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310 CB1:Control beam RB1:D=0% RB2:D=80% RB3:D=90% RB4:D=100%
45 40 35
Load [KN]
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
22
24
Deflection [mm] Fig. 7. Load versus deflection curves for beams with different damage degree.
Table 3 Summary of the bending test results Beam reference
Failure load (kN)
Deflection at failure (mm)
Load capacity contribution (%)
Rigidity coefficient (kN/mm)
Rigidity contribution (%)
Failure modes
CB1 CB2 RB1 RB2 RB3 RB4 RB5 RB6
21.41 23.92 40.11 37.66 32.10 30.75 30.10 37.37
12.86 13.50 9.02 8.65 9.87 19.50 13.50 10.50
– – 187 176 150 144 140 156
2.352 1.818 4.854 4.878 4.445 3.448 3.225 4.716
– – 206 207 189 147 137 289
Steel yielding Steel yielding Peeling off Peeling off Peeling off Peeling off Interfacial debonding Peeling off
authors conclude that for any damage degree the repairing of RC beams by using CFRP laminates is effective and that the performance of the repaired beam is mainly attributed Damaged beams Control beam
55
to the higher mechanical characteristics of the CFRP laminates. Furthermore, the 80% and the 0% damage degree beams behave likely and they give a higher performance in term of load capacity and rigidity due to the additional contribution of the reinforced concrete. 3.3. Effect of the carbon fibers laminate width
50
Failure Load [KN]
45 40 35 30 25 20 15 10 5 0 0%
80%
90%
Damaged degree Fig. 8. The failure load versus the damage degree.
100%
In order to observe the effect of the width of the carbon fiber laminate on the load capacity and the rigidity of the repaired concrete beams, two similar beams have been damaged and then, these beams were repaired by changing the width of laminates: RB3 (l = 100 mm for about 83% of the total width of the beam) and RB5 (l = 50 mm for about 42% of the total width of the beam). At first, the authors noticed that the width laminate affects the failure modes of the repaired beams. These failure modes change from interfacial debonding to the peeling-off when the width increases from 50 to 100 mm. The results of the two widths are given in Fig. 9, which presents the load–deflection curves. From the results presented in Table 3 and Fig. 9, the loads capacities are:
O. Benjeddou et al. / Construction and Building Materials 21 (2007) 1301–1310
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CB1:Control beam RB3: l=100 mm RB5: l=50 mm
45 40
Load [ KN ]
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
22
Deflection [mm ] Fig. 9. Load versus deflection curves for beams with different laminates width.
3.4. Effect of the concrete strength class
RB3 = 32.10 kN and RB5 = 30.10 kN and The rigidity coefficients are: RB3 = 4.44 kN/mm and RB5 = 3.22 kN/ mm. The authors conclude that for the two widths laminates the mechanical behavior of the repaired beams, in terms of load capacity and rigidity, is higher than that of control beam (load capacity = 21.41 kN and rigidity coefficient = 2.35 kN/mm). Furthermore, the authors may also conclude that the increase of laminate width contributed to a small increase of a load capacity (from 140% for RB5 to 150% for RB3), but contributed also to a significant increase of rigidity (from 137% for RB5 to 189% for RB3). Therefore, for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory in this case. Even, when interfacial debonding occurs; it is possible to proceed then with a further CFRP replacement.
In this study, the authors tested the effect of the concrete strength class on the load capacity and the rigidity of the repaired RC beams by bonding an externally CFRP laminates. Four beams have been tested: CB1 and RB3 were cast by using C21 (corresponding to a weak concrete) and CB2 and RB6 were cast by using C38 (an ordinary concrete usually used in RC structures). CB1 and CB2 were considered as control beams and RB3 and RB6 were damaged (damage degree: D = 90%) and then repaired by CFRP laminates having 1700 mm of length and 100 mm of width. Fig. 10 presents the load–deflection curves for these two concrete strength classes. From the results presented in Table 3 and Fig. 10, the authors observed that for the two types of concrete, the
CB1:control beam
laminate width = 100 mm
CB2:control beam RB3:Fc28=21MPa
45
RB6:Fc28=38MPa
40
Load [KN]
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
18
20
Deflection [mm] Fig. 10. Load versus deflection curves for different concrete class.
22.5
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mechanical behavior of the two repaired beams, in terms of load capacity and rigidity, is higher than those of control beams (For C21, the load capacity increases from CB1 to RB3 by 50% and rigidity coefficient increases from CB1 to RB3 by 89%; for C38 the load capacity increases from CB2 to RB6 by 56% and rigidity coefficient increases from CB2 to RB6 by 189%). The authors remarked that the two repaired beams have the same rigidity even though they have a different concrete strength. Then, the authors conclude that the rigidity of the repaired beams is affected only by CFRP laminate. Furthermore, the difference between the load capacity of the two beams (RB3 and RB6) is due to the increase of the concrete strength. For any concrete class, the CFRP added about a half of the load capacity. Finally, the CFRP laminates have more influence on the mechanical behavior of repaired RC beams than the effect of the concrete and steel properties. 4. Conclusion In the present study, the mechanical behavior of the composite repaired concrete structures was investigated through standard laboratory tests. Two sets of beams were tested in this study; control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP reinforcement by changing the width of CFRP laminate, the damage degree and the concrete strength class. Throughout the results from all experimental works, several conclusions can be drawn as follows: 1. The mechanical performance of the repaired RC beams is highly increased by using the CFRP laminates. Therefore, this technique is effective to at least restore the mechanical performance of cracked or damaged RC beams. 2. The laminate width affects the failure modes of the repaired beams. These failure modes change from interfacial debonding to the peeling-off when the width increases from 50 mm to 100 mm. 3. The mechanical behavior of the repaired beams changed from elastoplastic to elastic behavior. The ductility performance of the repaired beams is less than that of the control beams. 4. The rigidity of the repaired beams is due only to the contribution of CFRP laminate. 5. The overall result is that, for any damage degree, the CFRP laminate contribute to higher mechanical performances of the repaired damaged RC beams. For all repaired beams, there was no failure of the CFRP laminates. 6. For a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Even, when interfacial debonding occurs; it is possible then to proceed with a further CFRP replacement. 7. For any concrete class, the CFRP laminates added about a half of the load capacity.
Acknowledgements The authors thank the Center for Testing and Technical Construction ( CETEC) and ‘‘Sika Tunisienne’’ for their assistance in the experimental work. References [1] Nanni A. Composites: coming on strong. Concrete Constr 1999;44: 120–4. [2] Saadatmanesh H, Ehsani MR. Reinforced concrete beams strengthened with GFRP plates–I. Experimental study. ASCE J Str Engng 1991;117(11):3417–33. [3] Ritchie PA, David AT, Le-Wu L, Guy MC. External reinforcement of concrete beams using fiber reinforced plastics. ACI Str J 1991;88(4):490–500. [4] Meier U, Kaiser HP. Strengthening of structures with CFRP laminates. In: Proceedings on advanced composite materials in civil engineering structures, MT. Div./ASCE/Las Vegas, 1991; p. 224– 32. [5] Limam O, Foret G, Ehrlacher A. RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach. Compos Struct 2003;60(4):467–71. [6] Thanoon WA, Jaafar MS, Razali M, Kadir A, Noorzaei J. Repair and structural performance of initially cracked reinforced concrete slabs. Constr Build Mater 2005;19(8):595–603. [7] Li G, Ghebreyesus A. Fast repair of damaged RC beams using UV curing FRP composites. Compos Struct 2006;72(1):105–10. [8] Bonacci JF, Maalej M. Externally bonded Fiber-Reinforced Polymer for rehabilitation of corrosion damaged concrete beams. ACI Struc J 2000;97(5):703–11. [9] Sherwood EG, Soudki K. Rehabilitation of corrosion damaged concrete beams with CFRP laminates – a pilot study. Comp Part B: Engng 2000;31(6–7):453–9. [10] El Maaddawy T, Soudki K. Carbon-Fiber-Reinforced Polymer repair to extend service life of corroded reinforced concrete beams. ASCE J Compos for Constr 2005;9(2):187–94. [11] Badawi M, Soudki K. Control of corrosion-induced damage in reinforced concrete beams using carbon fiber-reinforced polymer laminates. ASCE J Compos for Constr 2005;9(2):195–201. [12] Haritos N, Hira A. Repair and strengthening of RC flat slab bridges using CFRPs. Compos Struct 2004;66(1–4):555–62. [13] Meier U, Deuring M, Meier H, Schwegler G. Strengthening of structures with CFRP laminates: research and applications in Switzerland. In: Neale KW, Labossiere P, editors. Advanced Composite Materials in Bridges and Structures. Canadian Society for Civil Engineering; 1992. p. 243–51. [14] Ramana VPV, Kanta T, Mortonb SE, Duttac PK, Mukherjee A, Desaia YM. Behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening. Compos Part B: Eng 2000;31(6–7):461–70. [15] Thomsen H, Spacone E, Linkatanyu S, Camata G. Failure Mode Analyses of reinforced concrete beams strengthened in flexure with externally bonded fiber-reinforced polymers. Compos Constr 2004;7(2):123–31. [16] Khalifa A, Nanni A. Rehabilitation of Rectangular Simply Supported RC Beams with Shear Deficiencies Using CFRP Composites. Constr Build Mater 2002;16(3):135–46. [17] Faza SS, Ganga R. Fiber composite wrap for rehabilitation of concrete structures. In: Proceedings of the materials engineering conference 804, ASCE. 1994; p. 1135–9. [18] Gao B, Kim JK, Christopher KY. Experimental study on RC beams with FRP strips bonded with rubber-modified resins. Comp Sci Tech 2004;64(16):2557–64. [19] BAEL. Re`gles techniques de conception et de calcul des ouvrages et constructions en be´ton arme´ suivant la me´thode des e´tats limites. French Concrete Code Eyrolles ed. 1992 (in French).