A novel damage-based constitutive model for air-entrained concrete under confined loads after freeze–thaw cycles

To address the damage evolution of air-entrained concrete (AEC) under freeze-thaw (F-T) cycles and coupled service loading in cold regions, a novel constitutive model is proposed, combining multi-scale homogenization theory with the pore-crack damage mechanism. The model establishes a quantitative relationship between pore water freezing pressure and crack density evolution. It incorporates two homogenization steps: the first at the micro-level (mortar-crack interactions) and the second at the meso-level (mortar-aggregate interactions). An equivalent elastic modulus degradation formula, accounting for F-T damage, is derived. Additionally, the influence of confined stress is integrated, leading to the development of a comprehensive stress-strain curve for triaxial compression under F-T cycles. Experimental validation was performed using conventional triaxial compression tests on concrete samples with varying porosities (1.1%, 1.7%, and 2.9%). Stress-strain curves after F-T cycling were systematically compared with the model's predictions. The results demonstrate that the model accurately captures the evolution of F-T damage, with the largest prediction errors for elastic modulus, peak stress, and peak strain being less than 15%, 10%, and 17%, respectively. Furthermore, it shows high agreement with experimental data under various porosity levels and confinement stresses (3-6 MPa). This framework provides a theoretical foundation for the design and life expectancy prediction of concrete structures in cold regions, offering deeper insights into the damage progression under the combined effects of F-T cycles and mechanical loading.