Experimental investigation on punching shear behaviour of RC-(R)UHPFRC composite flat slabs without transverse reinforcement

Strengthening existing reinforced concrete (RC) flat slabs by casting a thin layer of ultra-high performance fibre-reinforced cementitious composite (UHPFRC) over the top surface constitutes an efficient solution for significant durability and flexural capacity enhancements. Previous research has shown that also the punching shear capacity can be substantially increased. However, the existing experimental evidence is still limited and does not allow a systematic evaluation of the influence of the governing parameters. In the present work, eight slabs without transverse reinforcement were tested up to punching shear failure. The following parameters were studied: the contribution of the UHPFRC overlay with and without reinforcement bars, the shape of the column (square or rectangular), the effect of the reinforcement ratio in the existing slab, the eccentricity of the punching force and the material of the strengthening layer (UHPFRC or ordinary RC). The height of the RC substrate and UHPFRC layer thickness were kept constant at 180 mm and 40 mm, respectively. The results confirm the significant contribution of the UHPFRC layer to the punching shear strength. The experimental failure loads are compared to the predictions provided by a composite failure criterion based on the Critical Shear Crack Theory and taking into account the contribution of the UHPFRC layer. Good agreement was obtained with the model being capable of reproducing the effect of all the studied variables.