Some results on punching shear in plain and fibre-reinforced micro-concrete slabs

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Abstract
Punching shear, caused by a concentrated load, induces a complex crack pattern and different failure modes in concrete slabs. In order to study slab behaviour under punching, a series of circular slabs supported along the partially restrained boundary was cast and tested, with various amounts of hooked-end fibres. Although the fibre contribution is certainly affected by the fibre aspect ratio L/D, only a single type of medium-length hooked-end fibres was adopted, as a reasonable compromise among fibre length, slab thickness and maximum aggregate size. Three different volume contents were adopted and, whenever possible, comparisons with plain concrete slabs were carried out. In order to assess the role of the geometry of the punching tip, two different shapes were considered: hemispherical and flat. In fact, different resistant mechanisms are activated and their role depends on the tip shape: bending, an arch action within the slab, punching and, if the tip dimension is small enough, crushing too. The experimental investigation carried out highlights the role ofthe different mechanisms in order to understand the nature of the failure and post-peak behaviour, in which the fibre contribution is expected to be more efficient.

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