Pore Pressure Evolution and F-T Fatigue of Concrete: A Coupled THM-F Phase-Field Modeling Approach

This study presents a coupled thermo-hydro-mechanical-fatigue (THM-F) model, developed based on variational phase-field fatigue theory, to simulate the freeze-thaw (F-T) damage process in concrete. The fracture phase-field model incorporates the F-T fatigue mechanism driven by energy dissipation during the free energy growth stage. Using microscopic inclusion theory, we derive an evolution model of pore size distribution (PSD) for concrete under F-T cycles by treating pore water as columnar inclusions. Drawing upon pore ice crystal theory, calculation models that account for concrete PSD characteristics are established to determine ice saturation, permeability coefficient, and pore pressure. To enhance computational accuracy, a segmented Gaussian integration strategy based on aperture levels is employed. The pore pressure estimation model is applied to assess the frost resistance of concrete with varying air-entraining agent contents, confirming that optimal air-entrainment significantly improves pore structure and lowers the overall freezing point of pore ice. The derived permeability coefficient and pore pressure estimation models are integrated into the THM-F coupled framework, which employs a staggered iterative solution scheme for efficient simulation. Mesoscale numerical examples of concrete demonstrate that the proposed THM-F model effectively captures structural degradation and accurately tracks the procession of F-T-induced fatigue cracks. Validations against experimental measurements, including temperature variations, stress-strain curves, and strain history, shows excellent agreement, underscoring the model’s accuracy and applicability. This study provides a robust theoretical and computational framework for quantitative analysis of coupled F-T-fatigue damage in concrete.