Utilizing Full UHPC Compressive Strength in Steel Reinforced UHPC Beams

Abstract

Steel reinforced ultra-high performance concrete (R/UHPC) flexural members commonly fail by fracture of the steel reinforcement after crack localization rather than crushing of the cement-based matrix as expected in traditional reinforced concrete. When failing after crack localization, R/UHPC specimens show low drift capacity and the high composite compressive strength is not utilized. In an effort to develop design approaches that might fully utilize the high compressive strength of UHPC and guarantee a minimum drift capacity, this study investigates an R/UHPC flexural element failing by crushing. Four-point bending tests are performed on two R/UHPC beams that are designed to fail in one case after crack localization and in the other after UHPC crushing. Experimental measurements include load, mid-span deflection, UHPC surface strain, and reinforcement strain. Surface strains are used to characterize compressive zone behavior for an ultimate strength prediction. Test results demonstrate that the R/UHPC specimen that failed after crushing had a larger drift capacity than the R/UHPC specimen that failed by fracture of the reinforcement after crack localization. The maximum compressive strain in the UHPC at crushing was measured to be 0.0065, at which point the reinforcement had significantly strain hardened. The observed compressive zone behavior and reinforcement behavior are incorporated into a new proposed strength prediction method.