FRP debonding from a concrete substrate: Some recent findings against conventional belief

For beams strengthened with FRP plates, many existing theories and concepts related to debonding failure are challenged by recent experimental observations in our laboratory. For debonding initiated by stress concentrations at the plate end, ultimate failure is always preceded by the formation of a major crack in the concrete member, causing interfacial stresses to change significantly from the elastic distribution. Existing elastic models are therefore not applicable to failure prediction. For debonding initiated from a flexural crack near mid-span, fracture mechanics based models indicate that the plate stress at failure is inversely proportional to the square root of the thickness. Test results from beams of various sizes and retrofitted with plates of different thickness show a different trend. To delay debonding failure, bonding of U-shape FRP ‘stirrups’ to the end of the FRP plate has been proposed. Test results indicate that ‘stirrups’ applied away from the plate end can indeed be more effective under some practical situations.