Application of fracture mechanics to debonding of FRP from RC members

During the last two decades, externally bonded uni-directional fiber-reinforced polymer (FRP) composites have been widely used for strengthening, repairing, and rehabilitation of reinforced concrete (RC) structural members. The bond characteristics contribute to the effectiveness of the stress transfer achieved between the FRP composite and the concrete substrate. Debonding of the FRP composite reinforcement is the most critical concern in this type of application. Under monotonic and fatigue loading conditions, FRP-concrete shear debonding has been idealized as a Mode-II fracture problem along the bi-material interface. A cohesive material law is used to describe the interfacial stress transfer at the macroscopic level. The area under the entire curve represents the fracture energy and is related to the load-carrying capacity of the interface. In this paper, previous experimental results and literature are discussed to show how the fracture energy can be considered a true fracture parameter. In addition, a simplistic onedimensional numerical analysis of the direct shear test is presented with the intent of pointing out the effect of the fracture parameters related to the cohesive material law on the load carrying capacity. The results are instrumental to discuss the strain limits provided in the ACI 440.2R-08 document.