Experimental and theoretical investigations of UHPC-NC composite slabs subjected to punching shear-flexural failure

This paper presents an experimental study on the punching shear behavior of concrete flat slabs partially reinforced with ultra-high-performance concrete (UHPC). The effects of the area and depth of UHPC on the failure mode and punching shear capacity are investigated. Test results indicate that the application of full-depth UHPC at the punching shear area changed the failure mode from a brittle punching shear failure to a ductile punching shear-flexural failure, while the application of partial-depth UHPC at the punching shear area caused brittle punching shear failure. By using full-depth UHPC, the first crack strength and punching shear strength of the slabs were increased by up to 65% and 117%, respectively. Partial-depth UHPC did not increase the first crack strength and punching shear strength. According to the testing results, the optimal application of UHPC in the flat slabs is to use full-depth UHPC within the area enclosed by a perimeter located at a distance equal to the slab thickness away from the column face. A new energy dissipation ductility index is proposed to characterize the post-peak load energy dissipation ability. Finally, an analytical model based on yield line theory is proposed to estimate the punching shear strength of the flat slabs. The proposed model reached a good agreement with the testing results while the current ACI model, Chinese model and fib MC2010 model under-estimated the punching shear strength of the test specimens.