Numerical Solution for Chloride Transport at the Corners of Square Piles Subjected to Wetting–Drying Cycles

The durability of marine concrete square piles is critically governed by chloride transport at the corners of piles, regions subject to multi-directional erosion and pronounced accumulation under cyclic wetting–drying conditions. In this study, we developed a two-dimensional coupled moisture–chloride convection–diffusion model for a quarter section of the corner of a square pile, incorporating the time-dependent surface chloride concentration and a nonlinear moisture diffusion coefficient. The governing equations were numerically solved using the unconditionally stable alternating direction implicit (ADI) finite-difference method, which effectively overcomes the instability issues inherent in long-term simulations of strongly coupled systems. Model predictions were validated against experimental data from the literature, showing good agreement. Parametric investigations revealed that (1) the effect of moisture–chloride coupling is significant, with a low initial degree of saturation intensifying capillary-driven convection and accelerating early-stage chloride ingress; (2) a higher water-to-cement ratio markedly increases pore connectivity, exacerbating chloride accumulation under bidirectional erosion; and (3) increasing the drying-to-wetting time ratio effectively reduces net chloride buildup by curtailing the total duration of immersion. These findings provide a theoretical foundation for durability design and service-life assessment regarding square pile foundations in marine tidal and splash zones.