Debonding failure along a softening FRP-to-concrete interface between two adjacent cracks in concrete members

A concrete beam can be strengthened by bonding a fibre reinforced polymer (FRP) plate to the tension face, and a common failure mode for such beams involves the debonding of the FRP plate that initiates at a major flexural crack, which is widely referred to as intermediate crack (IC) debonding. To understand IC and other debonding failures, the bond behaviour between FRP and concrete has been studied extensively using simple pull-off tests, in which a plate is bonded to a concrete prism and is subject to tension. However, the behaviour of the FRP-to-concrete interface in a beam can be significantly different from that captured in a pull-off test as, in a beam, whether debonding along the FRP-to-concrete interface occurs at a major flexural crack or not depends on the conditions at this crack as well as at the adjacent crack on the path of the debonding propagation. This paper is therefore concerned with the debonding process of an FRP-to-concrete bonded joint where the FRP plate is subject to tension at both ends, which closely approximates the IC debonding process in a flexurally strengthened RC member. The same problem has been the topic of a previous study by the authors, where a bilinear local bond–slip model was employed for the FRP-to-concrete interface. However, that solution is rather complex and difficult to apply in practice. The aim of this study is to produce a simplified solution by employing the simple linearly softening local bond–slip law for the interface. Results from this simplified analytical solution are compared with those from the previous solution, showing little loss of accuracy in predicting the load–displacement response and the ultimate load. The most significant outcome of the new solution is a simple expression for the ultimate load of the bonded joint which offers the potential for direct practical application. While the emphasis of the paper is on FRP-to-concrete joints, the solution and methodology are applicable to similar joints between other materials such as FRP-to-steel or steel-to-concrete bonded joints.