Computationally Efficient 2D Mesoscopic Coupled THM Simulation of Freeze–Thaw Damage in Steel Fibre-Reinforced Concrete

Understanding the coupled thermo–hydro–mechanical (THM) processes governing freeze–thaw (F–T) damage in steel fibre-reinforced concrete (SFRC) is essential for improving the durability of cold-region infrastructure. Fully three-dimensional mesoscopic THM simulations are often computationally prohibitive, limiting systematic mechanism identification and parameter sensitivity analyses. This study develops a computationally efficient two-dimensional (2D) mesoscopic coupled THM framework that explicitly represents mortar, aggregates, steel fibres, and interfacial transition zones (ITZs), and accounts for heat transfer, moisture transport, ice-water phase transition, and stress development. Comparisons with the present F–T experiments show that the model captures the measured temperature histories at the specimen centre (MAE = 2.48 °C; RMSE = 3.19 °C) and predicts the evolution of pore pressure and associated stress development. Parametric analyses indicate that mortar permeability governs pore-pressure diffusion and stress relaxation: higher permeability promotes pressure dissipation and reduces tensile-stress build-up, whereas lower permeability favours pressure accumulation and localised cracking. The water–cement ratio ( w / c ) regulates pressure build-up by altering porosity and the amount of freezable water. Fibre geometry primarily affects local crack deflection and bridging, while its influence on the global pore-pressure response remains limited in the present simulations. The proposed framework enables THM-based mechanistic interpretation of F–T deterioration and provides an efficient tool for trend-level durability assessment and mixture-design optimisation of SFRC in cold regions, while quantitative prediction of absolute peak pore pressures and localised stresses still requires full three-dimensional simulations or direct experimental validation.