Comparison of constitutive relationships of ultra-high performance fiber reinforcement concrete under conventional and true triaxial loads

In recent decades, ultra-high performance fiber-reinforced concrete (UHPFRC) has been intensively examined by numerous scientists due to its superior mechanical qualities and widespread use in construction structures. In-depth study has been undertaken on the uniaxial static and dynamic properties of UHPFRC materials, however, research on their triaxial compression properties is still limited. This study evaluated the mechanical properties of UHPFRC through standard and real triaxial tests. The compressive strength, ductility, stress-strain response, and failure mode of UHPFRC specimens under different stress states are the analyzed and discussed. Studies have shown that under different confining pressure conditions and triaxial stress conditions, the compressive strength of UHPFRC significantly increase (with confining pressure was increased from 0 MPa to 40 MPa in the conventional triaxial test, and the strength increase rates were 15%, 40%, 68%, 76%, 111%, and 125% respectively; with confining pressure was increased from 0 MPa to 60 MPa in the true triaxial test, and the strength increase rates were 7.6%, 38.6%, 48.4%, and 108.8% respectively; with σ 3 increased from 0 MPa to 15 MPa in the biaxial test, the biaxial strength increase rates were 74.7%, 87.0%, 107.9%, and 153.9% respectively.), the stress-strain curve undergoes significant changes, the elastic modulus and ductility significantly enhances, and the failure mode changes. Based on the experimental data from this study, and further referring to the unified strength theory, the constitutive models of UHPFRC under standard and conventional triaxial compression are proposed based on experimental results and literature reviews. In addition, it is believed that the UHPFRC failure criteria described in the literature are relevant to UHPFRC under active or passive restriction. The derived models correspond well with the experimental findings.