Yu SAKURAI

Coupled Moisture and Chloride Transport Analysis in Concrete under Various Wetting and Drying Conditions

Takumi SHIMOMURA

Chloride attack compromises the durability of concrete structures acting as critical infrastructure. In underground service tunnels accommodating electrical power facilities , the concrete is exposed to a complex environment: drying caused by heat generation from cables, and wetting due to water infiltration. This concurrent wetting and drying environment leads to localized chloride accumulation and severe deterioration, such as concrete spalling and reinforcement corrosion. Conventional analytical models struggle to continuously compute these adjacent boundary conditions because they treat capillary suction and diffusion as separate physical models.
This study aims to develop a new numerical simulation method that uniformly treats wetting and drying  using a single diffusion equation. As a practical engineering approach to the coupled moisture-chloride model, we adopted a diffusion model with saturated water content as a fixed boundary condition, extending it to a two-dimensional analysis. By numerically correcting moisture diffusivity strictly near the wetting surface, this method reproduces moisture penetration via capillary suction  without complicating the computation.
To verify this method, experiments were conducted where one surface of a concrete specimen was subjected to continuous wetting  while the opposite surface was dried, under 20°C (standard) and 60°C (high-temperature) conditions. The results revealed a temperature-dependent reversal in the chloride concentration profile : at 20°C, chloride ions  penetrated deeply with moisture, whereas at 60°C, intense evaporation caused them to concentrate near the surface. The proposed model successfully reproduced the 20°C results, though it revealed limitations in fully expressing intense drying effects at 60°C, thereby clarifying its applicability range.
Finally, two-dimensional numerical simulations successfully reproduced the phenomenon where chloride ions  entering from a wetting section migrate toward an adjacent drying section, concentrating near the boundary due to evaporation. This physically explains the localized chloride-induced corrosion mechanism observed in actual service tunnels. Ultimately, this study establishes a practical method for predicting moisture and chloride ion transport in concrete under complex wetting and drying conditions.