Modeling Chloride Transport in Concrete Under the Coupling of Extreme Saltwater Intrusion and Short-Term Dry–Wet Alternation

Estuarine ship-lock concrete is exposed to chloride concentrations approaching 3.5% and operation-induced drying–wetting cycles as short as 80 min, conditions that are not adequately represented by conventional marine tidal models. This study experimentally investigated chloride ingress under six drying–wetting ratios (D:W = 0:1, 1:9, 3:7, 1:1, 7:3, and 9:1) and compared an 80 min cycle with a conventional 24 h tidal cycle. A modified Fick-based empirical model incorporating exposure time and relative elevation Er, as a proxy for the drying–wetting ratio, was developed. High-frequency cycling produced a surface convection zone approximately 2 mm deep, and the peak chloride concentration was approximately 1.3 times that under the 24 h cycle. Both the surface chloride concentration and apparent diffusion coefficient varied nonmonotonically with the drying–wetting ratio and reached their global maxima at D:W = 7:3 (Er = 0.7). Validation against the held-out 100 d measurements showed that most prediction errors were within ±25%. These results quantify chloride transport under extreme saltwater intrusion and rapid operation-controlled cycling and provide a basis for durability assessment and service-life prediction of estuarine ship-lock concrete structures.