Numerical investigation of non-shear-reinforced UHPC hybrid flat slabs subject to punching shear

Flat slabs are commonly used in the construction of multi-storey buildings due to their ease of placement and installation, relatively short construction time, minimal structural floor depth and economic credentials. However, flat slabs are vulnerable to punching shear, a failure mode that typically occurs without any warning at slab-column connections. While various methods exist to improve punching shear capacity, the use of ultra-high performance concrete (UHPC) offers a highly buildable alternative to use of shear reinforcement in normal strength concrete (NC). However, UHPC is an expensive material and recent research has focussed on its partial use in column zones only, with NC elsewhere i.e. hybrid NC/UHPC. In this paper, a non-linear finite element (FE) model using ABAQUS was developed to simulate the punching shear performance of hybrid NC/UHPC flat slabs without shear reinforcement. The FE model was validated against experimental results from the literature. The validated model was then used to investigate the influence of slab thickness, column size, UHPC area and UHPC compressive strength. The results show that punching shear capacity of the slabs increases with the area of the UHPC zone. Additionally, the ratio of concrete compressive strength of UHPC to that of NC has a significant effect on the behaviour of the hybrid slabs which should be accounted for in future design guides. In order to understand the performance of existing design guides and codes in this context, a comparison between FE results and code predictions is also presented. The results suggest that the code models including ACI 318-14, Eurocode2 and Model Code 2010 cannot accurately estimate the punching shear capacity of reinforced UHPC slabs and hybrid NC/UHPC slabs. A modification to the existing ACI equation is proposed which yields accurate predictions for reinforced UHPC and hybrid slabs.