Effect of existing cracks on shear failure behaviour of reinforced concrete members

Abstract

The effect of existing cracks on shear failure behaviour of reinforced concrete (RC) members was investigated in this paper. The authors conducted the experiments on shear response of pre-cracked RC members with various quantities of stirrups. The objective of the study is to identify the effect of pre-cracks on concrete that resists shear through diagonal tension, as in beams without stirrups, and through compressive struts, as in beams with large amounts of stirrups. In the former case, it is found that pre-cracks delay diagonal tension failure, thus elevating the shear capacity. The apparent increase in concrete tension capacity arises from the crack arrest mechanism. The propagation of a diagonal shear crack is arrested by the pre-crack. A considerable increase in shear capacity is obtained, and the failure mode is changed from brittle shear to flexural yielding. On the other hand, in the case of a beam containing large amounts of stirrups, pre-cracks decrease the web-crushing capacity. Pre-cracks are supposed apparently to weaken the concrete struts by two mechanisms: a reduction in the effective contact area of crack surface and a reduction in the compressive strength of concrete near pre-crack owing to micro-fracturing damages. In the case of beams with a fair amount of stirrups, the failure is governed by yielding of stirrups rather than crushing of web concrete, hence the web-crushing mode is not relevant. The increase in diagonal tension capacity of concrete therefore leads to the increase in the overall shear capacity.