Composite model for predicting the punching resistance of R-UHPFRC-RC composite slabs

Adding a thin layer of Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC), with or without steel rebars, over a Reinforced Concrete (RC) slab is an efficient reinforcement method for existing structures. The thin layer of UHPFRC serves as a tensile reinforcement for the RC slab, creating a composite element. A recent experimental campaign showed that the layer of UHPFRC significantly increases the rigidity and the punching shear resistance of a RC slab submitted to a point force. An analytical composite model is developed herein to predict the global bending behavior of the composite slab and the punching shear resistance. A multilinear moment-curvature relation for composite sections is proposed to calculate the global force-rotation behavior of a slab which can then be used in combination with a composite failure criterion to predict the punching shear resistance. The contribution of the concrete section to the punching shear resistance is obtained with existing models from the literature. The UHPFRC layer resists to punching shear by out-of-plane bending over a limited length equal to its height. This mechanism induces tensile stresses perpendicularly to the interface with the concrete. The contribution of the UHPFRC layer to the punching shear resistance thus depends on the tensile strength of concrete. The results of this analytical composite model are in good agreement with the experimental result. A method is also proposed to consider pre-existing deformation of the RC section for a post-installed UHPFRC layer.