Separated thermal–mechanical damage modelling of concrete at elevated temperatures: formulation, ABAQUS implementation, and fire performance prediction

Predicting the response of concrete to fire remains difficult because of the complex, coupled thermal and mechanical damage that develops at elevated temperatures, which existing constitutive models often treat in a lumped or inconsistent manner. This study develops a thermodynamically consistent damage–plasticity model in which thermal and mechanical damage evolve through independent, energy-based laws, coupled with temperature-dependent plasticity and a full strain decomposition that includes thermal expansion and transient creep. The model is implemented as an ABAQUS UMAT subroutine to enable analysis from the material to the structural scale. Model predictions were validated against published experimental data over the range 20–600 °C. At the material scale, the predicted compressive strength decreases from 39 MPa at ambient temperature to 30 MPa (77%) at 400 °C and 14 MPa (36%) at 600 °C, while the thermal damage variable reaches 0.36 and 0.75, respectively. At the structural scale, the predicted peak capacity of reinforced concrete beams decreases from 470 to 360 and 175 kN over the same range, consistent with test data. A fire-protection design study further shows that, for the specific column geometry and loading condition analysed, a 20 mm coating provides the most favourable balance between capacity retention and material cost for achieving an R120 rating. The proposed framework offers a physically based tool for assessing the high-temperature performance of concrete.