Application of improved hybrid bonded FRP technique to FRP debonding prevention

External bonding of fiber reinforced polymer (FRP) is one of the most efficient techniques for retrofitting and strengthening concrete structures. However, the bond interface is the weakest link in such retrofitted or strengthened concrete members, resulting in premature FRP debonding. Therefore, a newly improved hybrid bonded FRP (IHB-FRP) technique is developed in this paper for strengthening concrete members. Unlike the conventional mechanical fastening method, the mechanical fasteners in the technique do not penetrate the FRP strips and the capping plates serve to apply vertical pressure to the FRP strips. An experimental study on the flexural strength of IHB-FRP technique strengthened beams is conducted. 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. Based on the experimental results, the effects of the steel reinforcement ratio, the number of FRP plies, and the fastener spacing on the ultimate load are evaluated. Finally, a simplified method is proposed for estimating the ultimate bending moment of the IHB-FRP strengthened beam. The validity of the method is verified with the experimental results.