Punching Shear Response of Concrete Slabs Strengthened with Ultrahigh-Performance Fiber-Reinforced Concrete Using Finite-Element Methods

Many times, existing RC flat slabs must be retrofitted to enhance their structural performance and service life. Among other retrofit techniques, concrete flat slabs can be strengthened with a top thin layer of ultrahigh-performance fiber-reinforced concrete (UHPFRC). Since UHPFRC is a relatively expensive material compared to conventional concrete, an optimized investigation regarding the location and thickness of the UHPFRC layer may be necessary. This study examines the punching shear performance of isolated RC flat slabs retrofitted with various layouts of UHPFRC. Two optimal retrofitting configurations are proposed and evaluated using three-dimensional (3D) nonlinear finite-element analysis, where both regular concrete and UHPFRC are simulated using a combined damaged plasticity-based model. The model is verified by analyzing previously tested slab-column connections from the literature. Then the predictive capability of the model is further verified by conducting parametric studies to investigate the effect of varying the thickness and area of the UHPFRC layer on the punching shear performance of the slabs. The maximum shear resistance, deformation, and crack propagation patterns of the composite slabs with different thicknesses, areas, and orientations of the UHPFRC layer are examined. The increase in thickness of the UHPFRC layer increases the punching shear capacity. However, the displacement at the maximum resistance decreases when the thickness of the UHPFRC layer increases. The use of a UHPFRC layer only on the critical areas of a slab can be more effective and economical since the amount of UHPFRC is reduced and the ductility of the slab is increased. (c) 2020 American Society of Civil Engineers.