Punching and post-punching performance of slab-column joints strengthened with combined post-tensioning and partially applied high-strength concrete

Post-tensioned (PT) systems are widely recognised as an effective method for preventing progressive collapse of concrete flat plate structures. Adopting full-depth high-strength concrete (HSC) around columns has also shown to significantly improve the punching shear capacity of the slab-column joints. Combining PT and HSC measures has the potential to significantly enhance the collapse-resistant capacities of flat plate structures, an area that currently lacks systematic research and practical applications. Given the superior performance of both measures, this study experimentally and numerically investigates the punching and post-punching capacities, failure modes, and load-transfer mechanisms of three hybrid PT-HSC slab-column joints with different HSC strengths and one joint fully cast with normal-strength concrete (NSC). All test specimens had in-plane constraints, with a constant PT force and a fixed dimension of HSC strengthened zone. Test results demonstrated that applying full-depth HSC around the column effectively enhanced the punching shear strength of the specimens. Sustained load transfer in PT tendons and effective confinement by HSC also facilitated a large-deformation suspension mechanism, steadily restoring post-punching strength. Based on experimentally validated numerical models, parametric studies were performed considering five additional HSC strengths and three additional dimensions of HSC zones. The numerical results indicated that both parameters significantly influenced the punching and post-punching failure modes and load transfer mechanisms, providing a useful reference for practical applications. Additionally, numerical and analytical strength predictions were also conducted. Compared to the numerical predictions, the modified Eurocode resulted in an 89 % accurate punching strength on average, and the adjusted analytical equations yielded an average of 98 % accuracy in post-punching strength.