Three-Dimensional Plastic Damage Model for Concrete under Nonisothermal Conditions

The mechanical properties of structural concrete are greatly influenced by both lateral confinement and elevated temperature. Nevertheless, it remains challenging to establish a constitutive formulation that can simultaneously capture thermal strain development, irreversible deformation, and stiffness deterioration in concrete subjected to multiaxial loading during fire exposure. Hence, this paper proposes a three-dimensional nonisothermal plastic damage model for concrete. Specifically, a capped yield surface that evolves with temperature is used to define the plastic behavior, and a nonassociated flow rule is adopted to govern the plastic strain increment. The evolution of plasticity and stiffness degradation is further governed by hardening and damage formulations that depend on both temperature and confinement, with regularization introduced to reduce mesh sensitivity during strain-softening. Moreover, a stress triaxiality measure is embedded in the load-induced strain formulation to capture the contribution of lateral confinement during heating. For numerical implementation, the model is implemented in Abaqus version 2021 through a UMAT subroutine, and the stress update is carried out by an explicit scheme that couples adaptive Runge-Kutta-Dormand-Prince sub-stepping with stress correction to maintain the accuracy of the updated stress state. Its predictive capability is assessed against 26 material-level tests and five concrete-filled steel composite column tests under ambient-temperature and fire conditions. The comparisons indicated that the model captures key experimental features, including confinement-induced strength enhancement, thermal deterioration, thermal strain evolution, and stiffness degradation, thereby supporting its application to nonlinear analysis of structural concrete under ambient and fire scenarios.