CD06-002 INVESTIGATION OF BENDING BEHAVIOR OF REINFORCED CONCRETE BEAMS STRENGTHENED WITH CFRP SHEETS A.R. Mardookhpour 1, H. Jamasbi 2 1 Ph.D Department of Civil Engineering, Islamic Azad University of Lahijan. Iran 2 M.Sc. Department of Civil Engineering, Islamic Azad University of Lahijan. Iran ABSTRACT The effect of FRP (Fiber Reinforced Polymers) sheets on bending strength of beams is one of the advantages of utilizing carbon fibers in concrete structures. By utilizing FRP sheets, reinforcing bar ratio which is used as longitudinal tensile reinforcements would increase in specimens and bending strength would be improved. In this research study, by testing 12 concrete beam specimens with known dimensions with 3 different reinforcing bar ratios the effect of FRP in flexural behavior strength, displacements, ultimate load and stiffness of the concrete beams have been investigated. The results show that in addition to increased strength, failure may occur with high adequate ductility in reinforced concrete beams. Keywords: FRP sheets, bending strength, concrete beam, bar ratio 1. INTRODUCTION Retrofitting and strengthening of a constructed structure are currently very significant in modern civil engineering. One of the modern methods in strengthening concrete structures is utilizing fiber reinforced polymers (FRP) bonded to concrete beams as strips made of carbon fibers [3]. This method has several advantages over traditional ones, especially increasing high strength, decreasing beam weight and creating durability of concrete structures. Based on experimental results obtained by Teng et al, Bonacci, Maalej and Feo [1, 8], the most common failure mode is derived from debonding of FRP plate or ripping of the concrete cover. In addition, some premature failures are generally associated with reduction in deformability of the strengthened tensile members.[ 2]. Numerous experiments have been carried out to determine failure mode and behavior of concrete beams. [5]. Based on existing studies typical failure modes observed in experiments is shown in Figure.1. [4,8]. These failure modes are type (1), type (2), type (3) and type (4) as the following schematic representation [8].
884 / Investigation of Bending Behavior of Reinforced. Figure 1. (a) - failure type -1- Figure 1. (b) failure type -2- Figure 1. (c) failure type 3- Figure 1. (d) failure type 4 (a) Figure 1. (e) failure type 4 (b) Figure 1. (f) failure type 4 (c) Figure 1. failure modes of concrete beams
3 rd International Conference on Concrete & Development / 885 According to Sebastian and Teng the corrosion of tensile longitudinal steel bars, changing of reinforcing bar ratio and shear forces may increase the probability of these types of failures [6 ]. The ratio of reinforcing bar of beams affects the above patterns and bending behavior and the width of cracks.[8]. The influence of FRP, bond around tensile longitudinal bars, on flexural strengthening of reinforced concrete beams and the ductility of beams are investigated in this paper by the test results of 12 beam specimens strengthened by carbon fiber reinforced polymers (CFRP). 2. EXPERIMENTAL SET UP In order to perform research on the materials some tests carried out have been introduced. 2.1. Materials For the beam specimens the compressive strength is 240 MPa. The concrete mixture proportions are shown in Table 1. Table 1: Concrete mixture design (kg / m 3 ) Coarse aggregate sand cement water 750 1000 300 160 *-maximum size of aggregate is 12 mm Also different sizes of tensile bars which have been used in beams are 8,10,12,16 and 20mm.The yield and ultimate strength of bars is indicated in Table 2. Table 2: Characteristics of reinforcing bars Diameter 8 10 12 16 20 Yield stress (MPa) 350 365 400 420 450 Ultimate stress (MPa) 460 570 575 585 590 Mechanical properties of CFRP sheets are presented in Table 3. Table 3: Mechanical properties of CFRP sheet Layer thickness (mm) Ultimate strain Tensile strength ( MPa ) Modulus of elasticity (GPa ) 0.170 0.0160 3750 230 Also the stress strain relationship is sketched in Figure 2. The adhesive used for binding the CFRP sheet on the concrete surface is handmixed epoxy and the air between concrete surface and CFRP sheet is removed. The adhesive curing time is 6 days according to instructions of the manufacturer.[9,11]
886 / Investigation of Bending Behavior of Reinforced. Figure 2. Stress-strain diagram of CFRP sheets 2.2. Experiments on Specimens 12 concrete beam specimens with dimensions according to Figure 3 are manufactured. The reinforcing bar ratios are 20%, 40% and 70% of the tensile reinforcement balanced ratio. Figure 3 (a): concrete bream in tests The dimensions and details of reinforced specimens are shown in Table 4. Table 4: Details of constructed beams
3 rd International Conference on Concrete & Development / 887 As shown in Table 4, seven specimens are strengthened by CFRP sheets. Also three specimens are kept as control specimens without strengthening. After loading the deflection of the specimens the strains at the mid- spans are measured by gauge. Also, the strain of concrete at the level of the tensile and compressive reinforcing bars and the strain of CFRP sheets at the mid- span of beam are measured by gauge according to Figure 4: Figure 4. Measuring instruments The output data are recorded by a computer. 2.3. Results and Discussion The control specimens B1, B5 and B9 failed after straining of tensile bars in a very ductile manner. In B2 failure occurred due to fracture of CFRP sheet but the beam carried a higher load than B1.Specimens B3 and B4 failed in Type three due to high shear and normal stresses at the ends of the CFRP sheets due to debonding of CFRP. B6, B7 and B8 failed due to fracture of CFRP sheets (Type 2) after yielding of reinforcing bars. Specimens B10 failed in Type 2 due to fracture of CFRP sheets around the mid-span after yielding reinforced bars. Specimens B11 and B12 failed in Types (4-b) and (4-c).In both of them debonding of CFRP sheet started due to shearing cracks. Compared to other specimens more shearing cracks with closer spacing occurred in B11 and B12. Figureure 6(a)-(c) shows the load versus mid-span displacement relationship of beams. According to these Figureures, at earlier stages, before flexural cracking the load-displacement curves are close to each other. With increasing the load, the strengthened specimens exhibited larger stiffness. After yielding of reinforcing bars, the strength and stiffness of the strengthened specimens were larger compared to the control specimens. In specimens B11 and B12, the load displacement curves continued without dropping and failure was initiated due to separation of FRP. However, the specimens failed due to crushing of concrete with adequate ductility as indicated in Figureure 6 (c). As shown in Figureure 6, by increasing the load, the strengthened specimens demonstrate larger stiffness. After yielding tensile bars the strength and stiffness of specimens reinforced by CFRP are larger compared to the control specimens. Also, after failure of CFRP the load- displacement curves of strengthened drops.
888 / Investigation of Bending Behavior of Reinforced. Displacement (mm) Figure 6 (a). B1, B2, B3 and B4 Displacement (mm) Figure 6 (b). B5, B6, B7 and B8 Figure 6 (c). B9, B10, B11 and B12
3 rd International Conference on Concrete & Development / 889 The displacement and ultimate strength Pu of concrete beams are shown in Table 5.The increase in strength of beams reinforced by CFRP sheets which varies with the reinforcing bar ratio are also submitted in this table. Table 5: Results of tests 2.4. Comparison Between Experimental Results and Theoretical Predictions According to proposed equations by ISIS Canada, a linear variation over the depth of concrete section and the value of 0.0035 for the maximum concrete strain are being considered.[3] Also ISIS supposes the reduction factors of 0.6, 0.85 and 0.75 for concrete, steel and FRP sheet respectively [3]. The ratios of ultimate test loads to the calculated values supposed by ISIS are given in Table 6. Table 6: Comparison between test results and the value calculated by ISIS Comparing the test results of specimens B2, B6, B7, B8, B10, B11 and B12, ISIS overestimates the ultimate bending strength in the case of strengthened beams with
890 / Investigation of Bending Behavior of Reinforced. small reinforcing bar ratios. According to Table 6 by increasing the reinforcing bar ratio in concrete beams, the ratio of Ptest / PISIS increases. Therefore, the equations proposed by ISIS are more appropriate for concrete beams with high reinforcing bar ratios. [8,10]. 3. CONCLUSION Generally from the test results and calculated values the following conclusion has been obtained: 1. The flexural strength and stiffness of RC beams increases by CFRP. 2. While the reinforcing bars increases, the ratio of the test load to the Load calculated (Ptest / PISIS) increase. 3. With high reinforcing bars near balanced reinforcement ratio failure of the concrete beams occurs in either Type - 4 (b) and 4 (c) with adequate ductility. ACKNOWLEDGMENTS The authors wish to thank the University of Science and Research Islamic Azad University (Tehran) and the Islamic Azad University of Lahijan for their experimental instruments and scientific labors. REFERENCES 1. Bonacci J.F, Maalej.M. Behavior trends of RC beams strengthened with externally bounded FRP. Journal of Composite for Construction. 2001. 2. Esfahani, M.R., et al.flexural behavior of reinforced concrete beams strengthened by CFRP sheets, Engineering structures. 2007. doi: 10.1016/j.engstruct. 3. Feo.l. Modeling of composite / concrete interface of RC beams strengthened with composite laminates. Composite Part B: Engineering. 2000. 4. ISIS Canada strengthening reinforced concrete structures with externally- Bounded fiber reinforced polymers. Design manual. No 4. 2001. 5. Nguyan. D.M., Chan. T.K. Brittle failure and bond development length of CFRP concrete beams. Journal of Composite for Construction.ASCE.2001. 6. Rahimi. H, Hutchinson. A. Concrete beams strengthened with externally bonded FRP plates. Journal of composite for construction. ASCE. 2001. 7. Ross.C.A., Jerome. D.M., Tedesco. JW., Hughes. M.L. Strengthening or reinforced concrete beams with externally bonded composite laminates. ACI structural Journal. (1999).96(2): 212-220. 8. Saadatmanesh. H. Ehsani. M.R. RC beams strengthened with FRP plates. I: Experimental study. Journal of Structural Engineering. 1991. 9. Shahawy. M.A., Arockiassamy. Reinforced concrete rectangular beams strengthened with CFRP laminates. Composites: Part B.1996. 10. Tajari. A.R., esfehani.m.r. Flexural behavior of reinforced concrete beams strengthened by CFRP sheets. Elsevier. 2006. 11. Teng.JG. Smith. ST. Intermediate crack-induced debonding in RC beams and slabs. Construction and building material. 2003.