Ultra-high-performance concrete hollow columns confined with high-strength steel

Reinforced concrete bridge columns with hollow cross sections may be a viable solution for long-span bridges. Meanwhile, several concerns are raised about the seismic behaviour of existing hollow-section bridge columns, such as excessive concrete spalling, longitudinal rebar buckling, and undesirable shear failures. Ultrahigh-performance concrete (UHPC) has the potential to address these seismic performance issues. However, a major impediment to the use of UHPC is the need to use additional confinement due to higher compression strength. This paper focused on using high strength steel (HSS) reinforcement to overcome this impediment. The efficiency of using HSS confinement to prevent flexural ductility loss for UHPC columns was investigated using an analytical flexure model. Material-scale tests were conducted to develop constitutive nonlinear stress-strain relationships of UHPC under compression and tension. These constitutive models were used as input in the analytical flexure model. Parametric study based on the analytical model was conducted to understand the effects of UHPC on the column's axial force and moment interaction diagram and concrete spalling mitigation. UHPC outperforms NC in maintaining tension-controlled failure for columns with large longitudinal ratios and under high axial force. Concrete spalling can be effectively avoided by using UHPC. The effects of HSS confinement on the column's performance in terms of confined concrete property and flexural ductility were also studied. The results showed that HSS confinement can improve the flexural ductility of UHPC for high axial loads and high longitudinal reinforcement ratios, making UHPC suitable for hollow columns. The applicability of ACI 318-14 code provisions on confinement to UHPC was also evaluated.