Experimental Study On Improved Hybrid Bonded FRP Technique

Fiber reinforced polymer (FRP) is widely used for retrofitting and strengthening reinforced concrete (RC) structures. Externally bonded FRP (EB-FRP) is one of the most popular techniques. However, the EB-FRP strengthened members often fail due to FRP debonding, reducing the strength utilization ratio of FRP and leading to brittle structural failure. To solve the problem and based on the original technique proposed by Yu-Fei Wu of City University of Hong Kong, an improved hybrid bonded FRP (IHB-FRP) technique is developed in this paper. The technique combines EB-FRP and improved mechanical fastening. The pull-out test was conducted first to select an appropriate mechanical fastener for the technique. The slip resistance capacity of the. threaded rod is checked in order to select a threaded rod with better slip resistance capacity for the IHB-FRP technique. Then IHB-FRP strengthened RC beams were tested under three-point bending to verify the effectiveness of the technique. The failure mode, ultimate load, ductility, crack distribution, FRP strain distribution, interfacial shear stress distribution and performance of mechanical fastener were examined. For the application of the technique, a simplified method is proposed for estimating the ultimate bending moment of the IHB-FRP strengthened beam. Based on the experimental results, the effects of the steel reinforcement ratio, the number of FRP plies, and the fastener spacing are evaluated. It is found that all the beams strengthened with the IHB-FRP technique fail due to tensile rupture of FRP strips even for seven plies of FRP strips. Thus, the IHB-FRP technique is highly effective in preventing FRP debonding, and increases FRP strength utilization ratio. The ultimate load may increase by a factor of 11 compared with the RC beam without strengthening. Compared with EB-FRP strengthened beam, the IHB-FRP strengthened beam has a better ductility, a shorter average crack spacing, and a more uniform strain distribution in the FRP strip. The shape of the strain distribution is similar to that of the bending moment. The IHB-FRP technique is effective in increasing the interfacial friction between FRP and concrete. The validity of the simplified method has been verified with the experimental results. The slip between the threaded rod and concrete is little in the three-point bending test, indicating that the selected threaded rod and the torque applied to the nut before loading are acceptable in practical applications.