Axial load response of ultra-high-strength concrete columns and high-strength reinforcement

Eight ultra-high-strength concrete (UHSC) square columns were constructed and tested to investigate the effects of yield strength of transverse (f_yh ≈ 550 and 800 MPa [79.8 and 116.0 ksi]) and longitudinal reinforcement (f_yh ≈ 480 and 640 MPa [69.6 and 92.8 ksi]) on axial load response. The UHSC had compressive strengths varying from 185 to 200 MPa (26.8 to 29.0 ksi). The effects of using high-strength transverse reinforcement with two different configurations and amounts were evaluated. For a constant amount of confinement reinforcement, high-strength transverse reinforcement improves the performance of well-confined UHSC columns in terms of strength, post-peak deformability, and toughness, provided that the hoops are detailed with proper end anchorage (135-degree seismic hooks). The use of high-strength transverse reinforcement was also effective in reducing steel congestion for UHSC columns. The effects of using high-strength longitudinal reinforcement in UHSC columns were also studied. It was determined that the combination of high-strength longitudinal reinforcement and UHSC results in yielding of this reinforcement. However, for typical longitudinal reinforcement ratios, the benefit of using high-strength longitudinal steel is limited due to the relatively small contribution of this reinforcement to the strength of UHSC columns and the early cover spalling of UHSC.