Constitutive laws of softened UHPC in biaxial tension-compression: Experimental study using a planar bi-directional element tester

Ultrahigh-performance concrete (UHPC) has been gradually applied in bridge and building engineering to replace traditional reinforced concrete (RC). More realistic simulations are required for the design of structures. However, a two-dimensional constitutive model for UHPC is lacking. Tsinghua University developed a planar bidirectional element tester (PBET) that can perform tests on various structural types of element specimens. In this study, one nonreinforced UHPC element specimen under uniaxial compression and ten nonreinforced UHPC element specimens under biaxial tension and compressive coupling condition were tested using PBET. The compressive stress-strain relationship was established based on the uniaxial compressive test. By analyzing the test results, the softening effect of the principal tensile strain on the peak compressive stress and strain of UHPC was evaluated. Stress and strain softening coefficients were proposed, and the constitutive law of softened UHPC under compressive stress was modified and verified using the test results, which contributes to the improvement and development for the two-dimensional constitutive law for UHPC.