Mesoscopic simulation of the thermodynamic behavior of concrete-encased structural steel composites under freeze–thaw cycles

A thermo–hydro–mechanical coupled model is developed to simulate the heat transfer and pore water crystallization–melting processes in concrete-encased structural steel composites (commonly referred to as steel-reinforced concrete) subjected to freeze–thaw cycles. A random polygonal mesoscopic model is established, explicitly representing aggregates, mortar, reinforcement, encased steel, and three interfacial transition zones: aggregate–mortar, reinforcement–mortar, and steel–mortar. Accelerated freeze–thaw experiments are conducted on steel-reinforced concrete specimens, and corresponding numerical simulations are performed to validate the proposed thermo–hydro–mechanical framework. Based on the validated model, a mesoscopic parametric analysis is carried out to investigate the effects of mortar permeability, aggregate gradation, aggregate volume fraction, steel ratio, encased steel shape, and thermal gradients on the thermodynamic response of steel-reinforced concrete under freeze–thaw action. The results indicate that mortar permeability, aggregate volume fraction, steel ratio, and temperature gradients are the dominant factors influencing frost resistance, whereas aggregate gradation and steel section shape play secondary roles. The proposed simulation framework effectively captures multifield interactions and crystallization pressure evolution, offering valuable insights for durability assessment and design optimization of steel-reinforced concrete structures in cold regions.