Constitutive models of ultra-high performance concrete (UHPC) under true tri-axial compression and an analysis-oriented model for FRP-confined UHPC

The confinement mechanism of ultra-high performance concrete (UHPC) has not been fully explored yet, and there is no analysis-oriented model available for FRP-confined UHPC. This study aims to fill this research gap by implementing new constitutive models of tri-axial compression behavior of UHPC in an existing analysis-oriented FRP-confined concrete model. In the first part, the test findings of tri-axial compression tests on 22 UHPC cubes with different fiber lengths and confining stresses were briefly summarized. In the second part, assessments were performed for four relevant axial stress-strain models using the test results. Generally, the four models yielded overestimations for the axial strains at the peak stresses by at least 30%, and the predicted post-peak decline curves misaligned with the test curves. Inspired by the assessment findings, a new axial stress-strain model was proposed using Liao et al.'s model as the basic form, and replacing the post-peak decline curve with a power-law function to capture the steep descending trend. The new axial stress-strain model showed excellent performance in predicting the peak axial stresses and corresponding strains (average error at around 5% and 9%, respectively), and depicting the tri-axial compressive stress-strain curves. With a newly calibrated dilation equation, the proposed axial stress-strain model was applied for the development of an analysis-oriented model for FRPconfined UHPC, which showed corroborated results with the verification tests in axial stress-strain curves, as well as the ultimate axial strengths and corresponding axial strains (average error at 6.5% and 13.3%, respectively).