Coupled macro-microscopic properties evolution and deterioration mechanisms of concrete under freeze-thaw action

In cold regions, freeze-thaw action significantly impairs concrete durability through complex multiscale deterioration. To elucidate the underlying mechanisms, this study investigated the effects of freeze-thaw action on the appearance characteristics, mechanical properties, and microstructure of concrete. Thermodynamic theory was applied to analyze the ice-water phase transition and the evolution of freezing stress. Grey relational analysis was employed to quantify the correlation and contribution of microscopic parameters to the deterioration of macroscopic properties. The results show that freeze-thaw action weakened the mortar-aggregate bond and reduced deformation resistance, which compromised concrete integrity and significantly deteriorated appearance characteristics and mechanical properties. Moreover, the freeze-thaw action accelerated the decomposition of hydration products, and promoted pore enlargement and microcrack propagation, ultimately resulting in a loose and porous microstructure. During freeze-thaw cycles, the ice-water phase transition exhibited strong dependence on pore size and pronounced hysteresis. As the freeze-thaw cycles increased, the repeated phase transitions caused a progressive rise in ice content, resulting in freezing stress accumulation and further facilitating microcrack expansion. After 300 freeze-thaw cycles, the freezing stress reached 35.74 MPa, with a maximum microcrack width of 30.10 μm. The freeze-thaw deterioration of concrete involved a synergistic interaction between surface and internal damage. Furthermore, significant correlations were observed between microscopic parameters and the deterioration of macroscopic properties. The deterioration of appearance was primarily governed by pore structure, whereas that of mechanical properties was mainly controlled by the combined effects of pore structure and freezing stress. • Effects of freeze-thaw action on the macro-microscopic properties are analyzed • Process of ice-water phase transition and evolution of freezing stress are revealed • Correlation between microstructure and macroscopic deterioration is proposed