A VIEW POINT OF CRACK PROPAGATION AND FAILURE MODES OF FRP STRENGTHENED FLEXURAL RC ELEMENTS

Abstract

This study is focused on certain aspects of structural performance of FRP strengthened RC elements in bending. These structural aspects relate to (a) section load carrying capacity, (b) the failure mode and (c) the manner of crack propagation. The study sets out to verify a definitive link between points (b) and (c) and their combined influence on the manner of section deformation until failure. The additional focus of study is to understand the way that the presence of FRP composites changes the extent and intensity of cracking together with the new failure mechanisms and geometry. The parametric changes in this research have been: (i) beam geometry and loading configuration such as shear span, mechanical spans (load spacing) and (ii) the strengthening arrangement, e.g. FRP type, fibre content and FRP geometry. It was found that the total tension reinforcement (steel and FRP) had a significant influence on structural behaviour. It also determined whether the section reached the full intended deformation capacity, or whether premature failure was triggered. Finally it was established that there were three limiting cases of total reinforcement ratio, which dictated the deformation and failure characteristic: (1) under-strengthened members where section behaves in a ductile manner similar to conventional flexural RC elements, (2) balanced strengthening, where strength and deformability optimise at approximately the same total reinforcement level and (3) over-strengthened sections which have an unpredictable and brittle nature, frequently brought about by premature failure. The failure modes and crack patterns often have a distinct geometry, dissimilar to that of conventional RC beams.