Experimental Study on Intermediate Crack Debonding in FRP-Strengthened RC Flexural Members

Abstract

A commonly observed failure mode of reinforced concrete (RC) flexural members strengthened with a fibre reinforced polymer (FRP) plate bonded to their tension face involves debonding that initiates at a major flexural or flexural shear crack and propagates towards a plate end, which has often been referred to as intermediate crack (IC) debonding. This paper first presents the results of IC debonding tests conducted on eighteen cantilever slabs in which IC debonding initiated at or near the fixed end, and four simply-supported slabs in three-point bending where IC debonding initiated at the load position. A number of factors are covered by these tests, including the type of FRP material, the width of FRP strip, the length of FRP strip, the slab width, the concrete strength, the longitudinal steel ratio, the concrete cover, and pre-cracking of concrete. The test results are next used to assess the accuracy of four available strength models that can be used to design against IC debonding. Of these four models, Teng et al's (2003) model is shown to provide safe predictions of the test results in most cases but can lead to overly conservative results; the scatter of the predictions is also large. The other three models from recent design guidelines (JSCE, 2001, fib 2001, ACI 440 2002) are not sufficiently safe for use in design. This assessment clearly illustrates the need for much further research to develop a more accurate IC debonding strength model.