Enhancing punching shear resistance of RC flat slabs using post-installed SHCC/UHPC-filled holes

This study investigates the structural performance of reinforced concrete (RC) slab-column zones incorporating designed holes within the punching zone, subsequently strengthened using strain-hardening cementitious composite (SHCC) and ultra-high-performance concrete (UHPC). The primary objective was to evaluate the influence of the number and configuration of the holes, as well as the type of strengthening material, on the punching shear performance of the slabs. In addition, the effect of reinforcing each SHCC/UHPC-filled hole with steel bars was examined. An experimental program was carried out on eight slabs tested to failure under concentrated loading. All slabs had identical geometrical dimensions of 1000 × 1000 mm², with a thickness of 120 mm. The main parameters investigated in this study included the number of holes, the geometrical configuration of the holes, and the type of filling concrete (SHCC and UHPC). The recorded response was assessed in terms of cracking behaviour, ultimate punching capacity, load-deflection response, and failure mode. The results indicated that the investigated parameters improved the punching shear performance of the flat slabs by as much as 59% and increased the elastic spring stiffness by approximately 77%, while also contributing significantly to the absorbed energy and cracking load. Nonlinear three-dimensional finite element models of the test specimens were also developed in Abaqus and validated against the experimental results. The developed model was then employed for a parametric study. Both numerical and experimental results confirmed that the enhancement was more pronounced when the hole arrangement was favourably configured within the punching zone, especially when combined with internal steel reinforcement. The numerical models closely predicted the performance of the tested slabs with an error of less than 6%. An analytical formula was then proposed to estimate the ultimate capacity of slabs strengthened with the studied parameters. The proposed formulation was derived by considering the reduced punching contribution of the NC slab after deducting the influence of the holes, together with the additional contribution provided by the SHCC/UHPC filling material.