Synopsis
When concrete slabs containing longitudinal circular voids are subjected to transverse bending, a complex stress pattern occurs and the first cracks appear at the inverts of the voids. If sufficient reinforcement is provided in the tension flange, subsequent outer-face cracks appear at about a quarter of the void diameter each side of the first cracks. Previous studies of the cracking behaviour of transverse strips of voided slabs have shown that, due to the predetermined and highly-concentrated type of cracking, very wide and uncontrollable cracks sometimes occur. Since the equations which are currently used for predicting crack widths in solid slabs do not take into consideration the effect of the presence of voids on the crack formation process, a new method is needed for predicting the widths of cracks in voided slabs subjected to transverse bending. This paper presents a suitable method which gives reasonable predictions of the widths of such cracks. The method was evolved from both theoretical and experimental investigations involving tests on fourteen 1: 3·33 scale model transverse strips of practical voided slabs. Consideration is also given to the amounts of reinforcement which are needed (a) to prevent premature yielding of the steel, (b) to achieve a controlled crack pattern, and (c) to limit the widths of the cracks to acceptable levels as specified in BS 5400, the British Standard Code of Practice for concrete bridges.
