An improved constitutive model for steel tube-confined ultra-high-strength concrete

Abstract

This study aims to accurately simulate the compressive behavior of ultra-high-strength concrete (UHSC) under various confinement conditions through conventional triaxial and equi-biaxial compressive tests. The triaxial compressive tests revealed that the stress and dilation angle of the concrete varies with changes in axial plastic strain and confining pressure, which is not well-considered in existing mechanical models for UHSC material. To address this limitation, an improved constitutive model based on the concrete plastic damage (CDP) model is proposed in this paper. The improved model incorporates the confining pressure in the flow rule and the hardening/softening rule, in addition to the axial plastic strain. The tensile and compressive meridians of the yield surface are calibrated based on the triaxial and biaxial compressive test results. Furthermore, the study proposes formulas for expressing the stress–strain relation and the lateral-to-axial plastic strain relation of UHSC under different confining pressures. These formulas help generate the hardening/softening rule and the dilation angle evolution of the flow rule, respectively. To verify the proposed constitutive model, the study simulated tests on UHSC-filled steel tube (UHSCFST) stub columns under compression using the finite element method. The simulated results of the peak bearing capacity of the tested UHSCFST stub column specimens are satisfactory, and the interaction between the steel tube and the concrete is reasonably predicted. The proposed improved constitutive model has the potential to accurately simulate the compressive behavior of UHSC under different confinement conditions, which can aid in the design of composite columns made with UHSC materials.