Interface debonding failure in beams strengthened with externally bonded GFRP

Although strengthened concrete members with Fiber Reinforced Polymer Plastic (FRP) are well-known to improve both strength and stiffness of constructions, relatively little is known about their premature failure, and specially, the development of interface debonding when flexural macro cracks at the mid-point of a strengthened beam reach a critical state and an interface crack propagates to the end-block. Herein, an experimental study using GFRP-strengthened beams is performed, in which the effects of length, thickness, and plate width on interface debonding failure were examined. According as strengthening length is increased, failure pattern changed from rip-off to interface debonding, and there was comparatively smaller effect in which failure pattern by the strengthening amount changes. From the comparison of anchoring schemes in end-block of FRP, U-wrap anchoring is more effectively resistant to shear stress than fiber anchor. As flexural cracks developed near the mid-point of the beams, horizontal interface debonding cracks between the concrete and GFRP propagated to the end-blocks of the plates after the tensile rebars yielded. In this study, a semi-empirical equation is developed to predict debonding loads, based on the test results. The proposed theory, which is based on nonlinear analysis and the critical flexural crack width, predicted the debonding failure loads of the specimens relatively well, and can be used for the analysis and design of GFRP-strengthened RC beams.