Intermediate crack-induced debonding in RC beams and slabs

A variety of debonding failure modes have been observed in tests on reinforced concrete (RC) beams strengthened by bonding fibre reinforced polymer (FRP) plates to their tension face. These can be broadly classified into two types: (a) those associated with high interfacial stresses near the ends of the bonded plate; and (b) those induced by a flexural or flexural-shear crack away from the plate ends. The first type of failures has been extensively studied, leading to many strength models. By contrast, the second type of failures has received much less attention and no strength model appears to have been developed for it. This paper is, therefore, concerned with the second type of failures induced by a flexural or flexural-shear crack, which is referred to as intermediate crack-induced debonding. The mechanism of this type of debonding failures is first examined briefly, highlighting the similarity between such debonding failures and those in simple shear tests of FRP-to-concrete bonded joints used for determining bond strengths. Based on this similarity, the recent Chen and Teng bond strength model is combined with a simple section analysis for predicting the strength of beams and slabs, which fail by intermediate crack-induced debonding. It is shown that through a simple modification of the Chen and Teng bond strength model, the proposed debonding strength model provides a good first approximation to the strengths of such beams. A design procedure for the flexural strengthening of beams incorporating the proposed debonding strength model is finally proposed.