FLEXURAL STRENGTH OF RC BEAMS WITH MULTIPLE LAYERS OF CFRP SHEET

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 12, December 2017, pp , Article ID: IJCIET_08_12_016 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed FLEXURAL STRENGTH OF RC BEAMS WITH MULTIPLE LAYERS OF CFRP SHEET Nadzirah Musa, Bashar S Mohammed, and Mohd Shahir Liew Civil and Environmental Engineering Department, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia Parnam Singh RNC Technology (M) Sdn Bhd, 47650, Subang Jaya, Selangor, Malaysia ABSTRACT Carbon Fiber Reinforced Polymer (CFRP) composite sheets has been universally used in the construction industry for the last twenty years as a common technique for strengthening, rehabilitation and retrofitting purpose of the RC structures. Five RC beams with different layers of CFRP sheets were prepared and put under four-point load. The aims of this research are to investigate the efficiency of multiple layers of CFRP and how different number of layers of CFRP will influence the RC beam behavior. Besides, to identify the optimum number of CFRP sheets should be used to strengthen the RC beams. It was proven that the load carrying capacity of the strengthened RC beams were increased compared to the control beams (B1). However, the outcomes from the load-deflection curve shows that the optimum number of CFRP sheets layer was three layers with load increment by 14.63%. This is because when the beam was strengthened by four layers of CFRP sheets, it shows the lowest load increment by 2.23%. The failure mode experienced by the strengthened beams was concrete cover separation, which is categorized as shear failure. Keywords: RC Beam, Strengthening, CFRP, External bonding, Flexural behavior Cite this Article: Nadzirah Musa, Bashar S Mohammed, Mohd Shahir Liew and Parnam Singh, Flexural Strength of Rc Beams with Multiple Layers of Cfrp Sheet, International Journal of Civil Engineering and Technology, 8(12), 2017, pp INTRODUCTION Strengthening and repairing of Reinforced Concrete (RC) beams in shear and flexural with externally bonded CFRP composite sheets has been universally used in the construction industry for the last twenty years. CFRP materials can be considered as a common system used both for post-planned and pre-planned building. The main reasons of using CFRP in this industry are because of its high strength to weight ratio, high stiffness, light weight, flexible and easy to install, non-magnetic properties, high editor@iaeme.com

2 Structural concepts instruction model for architecture students regarding motivation and creativity promotion tensile strength and non-corrosive as compared to other materials [7,8,12,13]. Researchers have shown that the use of CFRP contribute to higher stiffness, ductility and strength of the RC structures. Alferjani et al. [1] have reported that FRP in civil engineering applications can be classified into three categories as follows: applications for pre-planned construction (design), repair and rehabilitation applications, and architectural applications. There are three different types of CFRP composites, which are: solid bar, inflexible plate, and textile or fabric sheet. Using of FRP bars in concrete structure has been widely accepted to substitute the use of steel reinforcement in the design stage of concrete structure [15]. While FRP plate and fabric types normally are being used in structural strengthening and retrofit purposes. FRP plate is used to strengthen the concrete structure by placing the plate on straight surface. While, FRP fabrics is flexible and can be used to wrap the structural RC member and it is available in continuous sheets that can be easily cut to fit any geometry [16]. Many studies have been carried out and proven repeatedly to strengthen existing RC structure using CFRP either by externally bonded reinforcement (EBR) or near surface mounted (NSM) technique. The approach of RC beam flexural strengthening by placing CFRP plates and sheets at the underneath of the beam via epoxy adhesives had denoted an improvement in the load carrying capacity and beam s stiffness [12]. For NSM technique, CFRP material is placed inside the concrete cover, which to have high protection against outside contact, wear and vandalism actions, as well as from the effects of warmth and fire [10]. From previous research by Fathelbab et al. [9] have reported that the use of CFRP sheets for strengthening could enhance the load capacity of RC structure as well as the RC structure toughness. In his study, it was proven that the load capacity increment is in the range of 79.8% and 107.7%. The efficiency of CFRP in the purpose of RC beam s strengthening relies on few factors, including the stiffness of CFRP plies, quantity of the thermosetting resin, compressive strength, number of layer of CFRP sheet, wrapping scheme, and fiber orientation angle of the FRP. Bsisu et al. [3] have suggested the using of multiple layers of CFRP is better than one layer with the desired thickness to achieve the desired strength required for strengthening the concrete. In his eleven beams experimented, with different width and number of CFRP sheets plies, they give variant contribution for each parameter. It has been shown that by using multiple layers of wide CFRP sheets give result in a high development for the beam strength and capacity, but will reduce the ductility of concrete beams. However, very limited and scarce studies have been done to determine the effectiveness of using multi CFRP layers in strengthening of RC beams. Therefore, the aims of this research are to investigate the efficiency of multiple layers of CFRP and to identify the optimum number of CFRP sheets layers in order to strengthen and restore the loss of the RC beams load capacity. 2. METHODOLOGY 2.1. Geometry of the beams and Test Setup A total of five RC beams were tested. All specimens with 1.4 m effective span, 230 mm in height and 100 mm in width. The main reinforcement comprised of two 10 mm deformed bars and two 8 mm plane hanger bars. Shear reinforcement comprised of 8 mm plain bars stirrup with 50 mm spacing in shear region as shown in Figure 1. The distance between two point loads was maintained to be 400 mm, while the span between point load and beams support was set to be 500 mm for all beams as shown in Figure editor@iaeme.com

3 Javad Tabatabaei, Mehdi Mahmoudi Kamelabadi and Mahyar Javidruzi Figure 1 Configuration of the beams design (dimension in mm) One beam, B1 was designed without any additional CFRP sheet attached and considered as control beam. While the remaining of the beams B2, B3, B4, and B5 were strengthened with one, two, three and four layers of CFRP sheets, respectively with different anchorage length at the center of the beam s soffit for flexural strengthening as summarized in Table 1. Beam Table 1 CFRP Configuration No of Layers CFRP Sheets Distance of CFRP between support (mm) B1 - - B B B B The load was applied to the simply supported beams using hydraulic jack and the load values were controlled using the load cell placed beneath the hydraulic cylinder with a capacity of 500 kn. The concentrated load was transmitted to the RC beam through a spreader beam and acted as two point load support. The load increment was set to be 0.1 kn/s until failure. All beams were instrumented with strain gauge of different length of 84 mm, 5 mm and 60 mm to measure the strain values on the concrete, steel reinforcement bar, and CFRP sheets, respectively. In order to measure the deflection, one transducer (LVDTs) was also used and put at the center of the beam s span as shown in Figure 2. Figure 2 Configuration of experiment set-up editor@iaeme.com

4 Structural concepts instruction model for architecture students regarding motivation and creativity promotion 2.2. Materials All RC beams were casted and designed with average compressive strength of MPa after 28 days. While the measured yield strength of the 10 mm deformed steel reinforcement bar was MPa. The CFRP sheets, comprised of 100 mm width and thickness of mm for each layer were externally bonded to the concrete beam s soffit by using two part of epoxy adhesive (Sikadur 330) with the mix proportion of 4:1 ratio and cured at room temperature. The mechanical properties of materials are given in Table 2. Materials Table 2 Mechanical properties of material f y (MPA) ε y (%) f u (MPA) ε u (%) E (GPa) Steel (10 mm) Concrete CFRP a (t f = 0.129mm) Source: Product Data Sheet SikaWrap 230 C 3.0 RESULTS AND DISCUSSIONS 3.1 Load-deflection curve and failure mode Load-deflection behavior for all beams and experimental results are drawn in Figure 3 and tabulated in Table 3. The load carrying capacity of the strengthened beams have been increased when externally bonded of CFRP sheets have been added. The increment load in the range of 2.23% and 14.63% Figure 3 Load-deflection behavior The tested beams showed two different failure modes. For the control beam (B1), the failure was by concrete crushing at the mid-span of the beam after the steel has been yielded as shown in Figure 4. While the failure for the strengthened RC beams B2, B3, B4, and B5 were by concrete cover separation, due to crack formation at the end of CFRP sheets from the tension face of the concrete as shown in Figure 5-8. The initial crack pattern for strengthened beams seems similar with control beam which at the moment constant zone. However, when editor@iaeme.com

5 Javad Tabatabaei, Mehdi Mahmoudi Kamelabadi and Mahyar Javidruzi the steel yielded, all loads were distributed to the CFRP sheets at the underneath of the beams, which leading to the concrete cover separation failure mode as shown in Figure 9. This failure mode is categorized as shear failure [11]. Based on the results shown in Table 3, the ultimate load of the strengthened beams B2, B3, and B4 increased consistently as the number of layers of CFRP sheets was increased by 6.27%, 10.15% and 14.63%, respectively as compared to beam B1 due to restraining effect of CFRP sheet to the concrete. However, for beam B5, the load carrying capacity shows the lowest load increment only by 2.23% as compared to beam B1 and decreased by 10.81% as compared to beam B4. This result shows that the optimum number of CFRP sheets to increase the strength capacity of the RC beam is only up to three layers, otherwise the effect of CFRP sheet in flexural strengthening the RC beams will not be affected. However, the use of externally bonded of CFRP sheets decreased the beam s ductility as the number of CFRP layers increased as shown in Table 3. Table 3 Summary of load-deflection result Beam P y (kn) Experimental result Δ y (mm) P u (kn) Δ u (mm) Δ f (mm) Analytical results Ultimate Load (kn) Ductility Index B B B B B Note: Py = Load at yield; Pu = Ultimate Load; Δy = Deflection at yield; Δu = Deflection at ultimate failure; Δf = Deflection at final failure Mode Failure Flexural at mid span Concrete cover separation Concrete cover separation Concrete cover separation Concrete cover separation Figure 4 Concrete crushing after tension steel yielded failure mode (B1) editor@iaeme.com

6 Structural concepts instruction model for architecture students regarding motivation and creativity promotion Figure 5 Crack pattern at final load (B2) Figure 6 Crack pattern at final load (B3) Figure 7 Crack pattern at final load (B4) Figure 8 Crack pattern at final load (B5) editor@iaeme.com

7 Javad Tabatabaei, Mehdi Mahmoudi Kamelabadi and Mahyar Javidruzi Figure 9 Concrete cover separation failure mode (B2, B3, B4, and B5) 3.2. Relationship between load and strain The strains were measured for tension steel, compression of the reinforced concrete and CFRP sheets to determine the ability of the material over load applied. Figure 10 shows the load strain curves of mid span of CFRP sheet, top concrete and steel for all beams. The steel strains, top concrete strains and CFRP strains are originally almost similar at loads before the concrete s first crack. After the beams reach yield point which is 2187μm/m, the strains of steel were 3315μm/m, 2781μm/m, 2708μm/m, 2601μm/m and 5061μm/m for B1, B2, B3, B4, and B5 respectively. As plotted in the graph below, the strain in the steel seems exceeded both strain for CFRP and to concrete, except for beam B1 without CFRP because there is no strengthening material to prevent the beam from being crushed at the centre of the beam s span. At failure load, the strain of CFRP for beam B5 shows the lowest value compared to beam B2 with one layer of CFRP shows the highest value of strain, which are 579μm/m and 3936μm/m respectively editor@iaeme.com

8 Structural concepts instruction model for architecture students regarding motivation and creativity promotion Figure 10 Load-strain responses of CFRP, top concrete and steel for B1, B2, B3, B4, and B5 Meanwhile, the strain values at the left and right end of beam s soffit were plotted to compare the strain between each beams with different layers of CFRP as shown in Figure 11 and Figure 12 respectively. It shows that B1, without CFRP sheets attached underneath the beam has maximum value of strain at the left end and minimum value of strain at the right end of the beam. The strain values at the left end of the beam seem increased consistently as the load applied increased. However, the strain values at the right end of the beam seem decreased when the beam reached yield point as shown in Figure 12 except for the control beam B1. Figure 11 Load-strain responses at left end of beam s soffit Figure 12 Load-strain responses at right end of beam s soffit editor@iaeme.com

9 Javad Tabatabaei, Mehdi Mahmoudi Kamelabadi and Mahyar Javidruzi 4. THEORETICAL MODEL In order to develop a design approach for the flexural members strengthened with CFRP by externally bonded system, the model of the moment capacity specified in the ACI 440.2R-08 was adopted. The moment capacity of the flexural strengthening member was expressed in the following form: = +ѱ [h ] (1) where is the nominal bending moment, and are the cross-sectional areas of the longitudinal steel reinforcement and FRP respectively, f s is the ultimate steel tensile stress, is the effective ultimate tensile stress of the FRP, h is the total depth of the beam, is the effective depth of the steel reinforcement, c is the position of the neutral axis, and ѱ =0.85 is a safety factor, and is an parameter given in Section of ACI Building Code [2]. The values of the load carrying capacity of the beams estimated by ACI Building Code [2] was correlated to the experimental values as tabulated in Table 3. It shows that the values of experimental result were lower than the analytical one, except for control beam (B1). This result due to maximum strength of CFRP, which is 4000 MPa was calculated in the analytical approach. Besides, one other possible reasons for the difference was due to the anchorage length of the CFRP was neglected in the Equation (1), which will affect the total strength of the externally bonded of CFRP sheets. 5. CONCLUSIONS The following conclusions are summarized based on the results obtained: 1. The load carrying capacity of strengthened RC beams was increased up to 14.63% from the control beam. However, if more than three layers of CFRP sheets were added, the load carrying capacity of the beams will reduced. It shows that the optimum number of CFRP layer used for strengthening purpose limit to 3 layers. 2. The use of externally bonded CFRP sheets will decreased the beam s ductility as the number of CFRP sheets increased. 3. At failure load, the strain of CFRP for beam B5 with four layers of CFRP shows the lowest value as compared to beam B2 with one layer of CFRP shows the highest value of strain, which are 579μm/m and 3936μm/m respectively. 7. ACKNOWLEDGEMENTS The author would like to thanks to all Technologist of Concrete Laboratory, Universiti Teknologi PETRONAS for the contributions during the course of this work to conduct all experiments. REFERENCES [1] Alferjani, M.B.S., et al., Use of carbon fiber reinforced polymer laminate for strengthening reinforced concrete beams in shear: a review. Int. Refereed J. Eng. Sci.(IRJES), (2): p [2] American Concrete, I., Building code requirements for structural concrete (ACI ) and commentary (ACI 318R-05). 2004: American Concrete Inst. [3] Bsisu, K.A.-D., S. Srgand, and R. Ball, The Effect of Width, Multiple Layers and Strength of FRP Sheets on Strength and Ductility of Strengthened Reinforced Concrete Beams in Flexure. Jordan Journal of Civil Engineering (No 1) editor@iaeme.com

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