fracture energy of the concrete–FRP interface in strengthened beams

•Debonding of FRP plates from concrete beams is a tensile fracture of concrete.•The steel bars limit the extent to which the fracture process zone can develop.•The fracture energy is independent of the length of the existing crack.•Fracture energy value obtained from small test specimens should be used in analyses.•The use of this fracture energy in the debonding model predicts accurate results. To determine the load at which FRPs debond from concrete beams using global-energy-balance-based fracture mechanics concepts, the single most important parameter is the fracture energy of the concrete–FRP interface, which is easy to define but difficult to determine. Debonding propagates in the narrow zone of concrete, between the FRP and the (tension) steel reinforcement bars in the beam, and the presence of nearby steel bars prevents the fracture process zone, which in concrete is normally extensive, from developing fully. The paper presents a detailed discussion of the mechanism of the FRP debonding, and shows that the initiation of debonding can be regarded as a Mode I (tensile) fracture in concrete, despite being loaded primarily in shear. It is shown that the incorporation of this fracture energy in the debonding model developed by the authors, details of which are presented elsewhere, gives predictions that match the test results reported in the literature.