Strengthening of Existing RC Two-Way Slabs using New Combined FRP fabric/rod Technique

Journal of Rehabilitation in Civil Engineering 3-2 (2015) 30-44 journal homepage: http://civiljournal.semnan.ac.ir/ Strengthening of Existing RC Two-Way Slabs using New Combined FRP fabric/rod Technique P. Behzard 1* , M. K. Sharbatdar 2 and A. Kheyroddin 3 1. Ph.D. In Structural Engineering, Faculty of Civil Engineering, Semnan University, Semnan, Iran. 2. Associate Professor, Faculty of Civil Engineering, Semnan University, Semnan, Iran. 3. Professor, Faculty of Civil Engineering, Semnan University, Semnan, Iran. Corresponding author: ARTICLE INFO Article history: Received: 07 June 2015 Accepted: 27 February 2016 Keywords: Combined FRP technique, NSM GFRP rods, EB CFRP fabrics, RC two-way slabs, Finite element. ABSTRACT This study presents the results of an experimental program to investigate the effectiveness of an innovative combined FRP technique using combination of externally bonded (EB) FRP fabrics and near surface mounted (NSM) FRP rods for flexural strengthening of existing reinforced concrete (RC) two-way slabs with low clear cover thickness. Three fullscale RC slabs (1500×1500×120 mm) were tested under monotonic four-point bending. One slab was kept unstrengthened as the control specimen, one slab was strengthened using NSM GFRP rods, and the other one slab was strengthened using combination of EB CFRP fabrics and NSM GFRP rods. The load-deflection responses, strain measurements, and failure modes of the tested slabs were studied and discussed. The behavior of the slab strengthened with this technique was compared to the behavior of the slab strengthened with GFRP rods. The test results confirmed the feasibility and efficacy of this technique in improving the flexural behavior of RC two-way slabs. Strengthened slabs showed an increase in flexural capacity between 250 and 394% over the control specimen. The slab strengthened using this technique showed higher ductility compared to the slab strengthened using GFRP rods. A 3D nonlinear numerical model was also developed using the finite element (FE) method to predict the flexural behavior of the tested slabs. A good agreement between experimental and numerical results was observed. 1. Introduction Fiber reinforced polymer (FRP) reinforcements have been shown to be a proper option for strengthening of reinforced concrete (RC) structures. The main advantages of FRP materials for strengthening over traditional methods are P. Behzard et al./ Journal of Rehabilitation in Civil Engineering 3-2 (2015) 30-44 their high tensile strength to weight ratio, high fatigue strength, non-corroding, and high chemical resistance [1,2]. The near surface mounted (NSM) method is one of the latest strengthening techniques for RC structures. In this technique, grooves are cut into the surface of the concrete members; FRP reinforcements are inserted into the grooves and then bonded using epoxy or cement-based adhesive [3]. When compared to the externally bonded (EB) technique, the NSM technique assures a higher anchoring capacity to the FRP reinforcing material. Recently, De Lorenzis and Teng [4] provided a critical review of existing research on strengthening of structural members using the NSM FRP reinforcement technique. Over the past few years, extensive researches have been conducted on the NSM FRP reinforcement strengthening technique [5,6], and the results have shown the effectiveness of this technique in improving the flexural and shear behavior of RC structural members. However, most of the studies have focused on the strengthening of RC beams [7–10] or slab strips [11–15]. There are little experimental or analytical studies in the behavior of RC two-way slabs flexural strengthened with NSM FRP reinforcements. The flexural capacity of RC two-way slabs can be increased by bonding the FRP rods or strips into grooves cut perpendicularly on the tension face. For two-way slabs, the NSM strengthening technique using the FRP reinforcement must be performed along two perpendicular directions. Foret and Limam [16] investigated experimentally the flexural behavior of RC two-way slabs with two strengthening techniques: NSM CFRP rods and EB CFRP strips. The experimental study consisted of three full-scale strengthened slabs in addition to one un-strengthened slab as the control specimen. It was concluded that the NSM technique improves the bearing capacity of RC two-way slabs. A more ductile behavior was also observed 31 comparing to the EB technique. It was shown that, the specimen strengthened using EB CFRP strips experienced the premature debonding failure of the strips and carried approximately half the load carried by the specimen strengthened using NSM CFRP rods. However, it should be mentioned that conventional NSM technique can only be applied for slabs with sufficient clear cover thickness, because the grooves in one direction must be cut deeper than those in another direction. Therefore, this NSM strengthening technique is often limited, because the sufficient clear cover thickness is not available in most existing RC slabs. Furthermore, conventional EB technique is often limited in most slabs as well, due to the premature debonding failure of the EB FRP reinforcements from the concrete surface at low strain levels. To overcome these drawbacks, an innovative combined FRP technique for strengthening of existing RC two-way slabs through combination of EB CFRP fabrics and NSM GFRP rods is proposed in this study. 2. Research significance This study investigates the feasibility and efficacy of an innovative combined FRP technique using combination of EB CFRP fabrics and NSM GFRP rods to increase the flexural capacity of existing RC two-way slabs with low clear cover thickness. To the authors’ knowledge, the technique introduced in this study is the first investigation in the technical literature on the use of combined FRP technique for strengthening of the RC two-way slabs. An experimental program of four-point bending tests is carried out on the RC two-way slabs with a limited amount of internal steel reinforcements. To assess the effectiveness of this technique, a comparison is carried out between the behavior of the slab strengthened according to this technique and the behavior of the slab strengthened with conventional GFRP rods. A 3D nonlinear finite element (FE) simulation is also developed using 32 P. Behzard et al./ Journal of Rehabilitation in Civil Engineering 3-2 (2015) 30-44 the FE code ANSYS [17] to predict the behavior of flexural strengthened slabs. The FE numerical predicted load-central deflection responses and failure modes are compared with that of the measured experimental results. 3. Experimental program 3.1. Material properties Table 1 summarizes the material properties of the concrete, steel bars, FRP reinforcements, and epoxy used for test specimens. The average 28-day compressive strength of concrete from compressive cylinder tests was 24.82 MPa. The steel reinforcements were 10 mm diamet (...truncated)