Cyclic response of precast concrete columns connected with cast-in-place UHPC in plastic hinge region

Ultra high performance concrete (UHPC) is a promising material for lap splice connection of steel bars in reinforced concrete (RC) precast structures given its high compressive strength, its high toughness, and its high bond strength with steel bars. This paper focuses on the cyclic response of bottom frame columns connected by UHPC in the plastic hinge region. The effect of lap length of longitudinal bars in UHPC (6d, 8d and 12d, d = diameter of longitudinal bar), axial load ratio and stirrup ratio of the UHPC connection region on the global response of the columns were presented and analyzed, and a cast-in-site RC column was prepared for comparison. The behavior of the UHPC connected columns was flexural dominated, but columns with 6d and 8d lap length failed due to the gradual bonding degradation between UHPC and longitudinal bars, and the column with 12d lap length failed due to UHPC crushing. It was demonstrated that a 8d lap length could achieve equivalent lateral bearing capacity and ductility to the control RC column, but the lap length should be greater than 8d when applying UHPC connection technology in seismic regions to ensure the energy dissipation. Column with lower axial load ratio (0.17) exhibited lower ductility because shear sliding occurred between the interface of UHPC and footing at the postpeak stage. The UHPC connected specimens exhibited higher resilience and better damage resistance in earthquake because the ratios of the residual displacements to the corresponding peak displacements were between 17 and 39%. Besides, a flexural capacity model for the columns was developed and the results predicted by the model agreed well with the experimental results.