Punching shear performance of reinforced flat slabs considering in-plane restraints under and after a fire

Abstract

Two full-scale test specimens, a reinforced concrete reference slab (denoted as S1) and a polypropylene fibre (PPF) concrete slab (S2), with a PPF length of 3 mm and PPF dosage of 1.0 kg/m³, were designed to investigate the effects of the in-plane restraints and PPF admixture on the punching shear response of flat slabs under and after a fire. The results indicated that both S1 and S2 showed a clear trend toward punching shear failure at approximately 210 min after ignition, but S2 had smaller crack widths and a greater limit displacement owing to the PPF admixture. Because of the restrained thermal expansion, many radial cracks first appeared around the punching shear cones on the top surfaces of S1 and S2 and then gradually extended across nearly the entire top surfaces of both test slabs, including their four corners. Meanwhile, both test slabs experienced cracks, which expanded and then retracted under a fire owing to in-plane restraints. In the residual load-carrying capacity tests, both S1 and S2 experienced larger ultimate destructive loads than the unrestrained slabs; thus, the in-plane restraints significantly improved their residual bearing capacities. In addition, S2 experienced a lower residual load and larger plastic deformation owing to the PPF admixture, which exhibited certain ductile failure characteristics.