Seismic performance of RC columns strengthened by steel tube-confined ultra-high performance concrete

To address the seismic retrofitting demand for existing reinforced concrete (RC) bridge piers in high-intensity seismic regions, a novel strengthening technique using steel tube-confined ultra-high performance concrete (UHPC) was proposed. To investigate the seismic performance of the newly strengthened piers, quasi-static tests were conducted on five specimens, with strengthening type and initial axial load ratio as parameters. Failure modes, hysteretic curves, skeleton curves, and seismic performance indices were measured. Then, based on the experimental results, a finite element model was built to accurately simulate the quasi-static behavior of the strengthened columns. Subsequently, parametric analyses were carried out using finite element simulations, considering the UHPC layer thickness, UHPC strength, and steel tube thickness. Research results show that after strengthening with steel tube-confined UHPC, the plastic hinge region of the RC columns was reduced and concentrated near the cut in the steel tube at the column base. Compared with the UHPC section enlargement method, the steel tube-confined UHPC strengthening technique provides greater improvement in displacement ductility coefficient, cumulative hysteretic energy dissipation, and initial stiffness, along with a more significant reduction in residual deformation under the same conditions. As the initial axial load ratio increases within the range of 0 - 0.3, the failure mode, bearing capacity, and energy dissipation capacity remain largely unchanged. However, displacement ductility and initial stiffness decrease, and residual displacement tends to increase. Increasing the UHPC strength in the strengthening layer significantly improves the energy dissipation capacity. Increasing the thickness of the steel tube in the strengthening layer significantly reduces the residual displacement. Increasing the UHPC thickness in the strengthening layer enhances bearing capacity, displacement ductility, cumulative hysteretic energy dissipation, and initial stiffness, while also markedly reducing residual displacement. These findings are expected to provide a theoretical basis for the application of steel tube-confined UHPC in the preventive seismic strengthening of existing RC bridge piers.