A three-parameter model for debonding of FRP from concrete substrate

For concrete beams retrofitted with bonded fiber reinforced plastic (FRP) composite plate, failure can occur due to debonding of the plate from the concrete surface. To predict the failure load in design, a proper model for FRP debonding is required. It is now commonly accepted that the debonding zone can be separated into two parts: a process zone within which the shear stress is decreasing with interfacial sliding, and a stress-free zone where the debonded surfaces have completely separated. Based on this physical understanding, the complete debonding process can be analysed once the shear stress vs shear displacement (tau-delta) relationship is known. In previous work, the tau-delta relation is often assumed to be bilinear. However, recent experimental results indicate that debonding is associated with a sharp drop in interfacial shear resistance, followed by more gradual decrease with further sliding. For such a situation, the simplest tau-delta relationship will involve three parameters: the maximum shear stress for debonding to initiate, the maximum residual stress right after debonding occurs, and a parameter governing the reduction of shear stress with sliding. In this paper, a FRP debonding model based on these three parameters is developed. The applicability of the model is verified through comparison with experimental results. Simulations are then carried out to study the effect of various parameters on the debonding process. Implications to retrofit design are discussed.