Mechanical Properties of Nano-TiO₂-Modified Concrete Under Freeze-Thaw Environment

This study investigated the freeze-thaw resistance of ordinary and nano-TiO₂-modified concrete (NTC) based on mass loss, ultrasonic velocity, compressive strength, and fracture toughness. The compressive behavior and internal damage evolution were further analyzed using particle flow code in two dimensions (PFC^2D) simulations. The results show that, although neither material exhibited structural collapse after freeze-thaw cycling, visible surface damage was observed, particularly in ordinary concrete. After 100 cycles, NTC showed a 52.17% reduction in mass loss and a 37.31% increase in ultrasonic velocity compared to ordinary concrete. Compressive strength of ordinary concrete decreased by 24.28 MPa (from 41.53 MPa to 17.25 MPa), while that of NTC decreased by only 13.37 MPa, demonstrating that the incorporation of nano-TiO₂ effectively improves the compressive performance of concrete under freeze-thaw conditions. Fracture toughness after 100 cycles decreased by 89.7% in ordinary concrete and 80.9% in NTC, suggesting that while nano-TiO₂ mitigates damage, its effect on maintaining fracture load-carrying capacity remains limited. The PFC^2D simulations were consistent with the experimental results, effectively capturing peak compressive behavior and validating the model's applicability for freeze-thaw degradation analysis.