A Unified Damage-Plasticity Constitutive Framework for Freeze-Thaw-Damaged Concrete Under Monotonic and Cyclic...

To provide a unified description of the monotonic and cyclic compressive responses of concrete after freeze-thaw exposure, a one-dimensional phenomenological damage-plasticity constitutive framework is proposed. Freeze-thaw-induced pre-damage is quantified by the degradation of the initial static stiffness. A Mander-type equation is employed to describe the monotonic envelope, while residual strain is introduced to characterize plastic deformation. A signed stiffness variable is defined to distinguish pre-peak compaction from post-peak mechanical damage, and the unloading and reloading paths are represented by piecewise power-law functions for the pre-peak and post-peak regimes. The model is evaluated using 36 monotonic compression curves of recycled coarse aggregate self-compacting concrete subjected to sulfate freeze-thaw cycles and cyclic compression data for ordinary concrete subjected to seawater freeze-thaw cycles. After independent identification of the envelope-shape parameters, the monotonic responses yield an average R² of 0.978 and an average NRMSE of 0.041, whereas the complete cyclic responses yield an average R² of 0.941 and an average NRMSE of 0.066. Because the model parameters are identified separately using data from each freeze-thaw exposure level, these accuracy measures characterize parameter calibration and response reconstruction rather than independent prediction of untested exposure states. Freeze-thaw exposure causes substantially greater degradation of the initial static elastic modulus than reduction in peak stress, indicating that stiffness, strength, peak strain, and envelope shape should be treated as state variables at different hierarchical levels. The cyclic unloading stiffness exhibits both pre-peak enhancement and post-peak degradation, confirming the necessity of separately representing compaction and mechanical damage. The proposed model provides a unified representation of the monotonic envelope, residual deformation, and principal cyclic hysteretic characteristics of freeze-thaw-damaged concrete.