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Laminate debonding process of FRP-strengthened beams

Despite the significant enhancement in service and ultimate conditions that can be achieved by bonding a fibre-reinforced polymer (FRP) laminate to a beam, the existing experimental research has shown the appearance of some types of brittle failures that involve the laminate debonding, before the design load is reached and a classical failure mode (concrete crushing or FRP rupture) occurs. The laminate debonding is generally initiated from within the concrete substrate between the externally bonded laminate and the internal reinforcement. The debonding initiation point can be found either at the laminate end if debonding is due to a high stress concentration at the cut-off point, or along the span when debonding is caused by the effect of bending moments and/or shear forces. The design procedure to obtain the laminate area to strengthen a reinforced concrete element should avoid these premature peeling failures. Therefore, there is a need to understand the mechanics of the laminate debonding process in order to prevent it. The propagation process of an interfacial crack can be described through the evolution of different stages by using non-linear fracture mechanics (NLFM) theory and assuming a bilinear constitutive law for the interface between the concrete and the laminate. For each stage, it is possible to obtain the interfacial shear stress distribution and the transferred force between the laminate and the support. Since the reliability of the FRP reinforcement depends mainly on a proper stress transfer between concrete and laminate through the interface, the transferred force should be limited to a maximum value in order to prevent peeling failure. This paper provides this limit value for the transferred force along the interface. In addition, the stress distributions are obtained for a particular case of a beam in a three-point bending configuration.