Yuki SATO

Development of a model for predicting mass transport in concrete considering the external environment

Fuminori NAKAMURA

Salt damage to concrete structures occurs due to the “external” transport of salt from the ocean to the atmosphere via wind and waves, and the “internal” transport of chloride ions that adhere to and penetrate the concrete surface. The accumulation of chloride ions leads to reinforcement steel corrosion, cracking, delamination, and structural capacity loss. Predicting chloride ion concentration in concrete is crucial for preventing salt damage.
To understand chloride migration, exposure experiments were conducted on concrete surfaces and surface layers to evaluate the effects of salt and rainfall. A cylindrical specimen (100 mm in diameter and 200 mm in height) was used. Samples were taken at 2-mm intervals between 20 and 30 mm from the exposed surface. Results showed that salinity was highest in the first layer and remained stable beyond it. Even after multiple exposures, salt did not immediately penetrate. However, when moisture was applied, chloride ions migrated up to 10 mm into the concrete. These results indicate that water and salt permeate into the interior in a short time, and that the process of salt migration differs depending on the rainfall.
These findings suggest that chloride ion migration can be more accurately modeled by incorporating changes in boundary conditions. To improve prediction, external and internal salt permeation models were integrated. Chloride concentrations were calculated based on external salinity, rainfall, and temperature variations. The model results were validated by comparing them with measurements from concrete exposed to coastal environments, showing a strong correlation.
Therefore, the combination of the analysis method for airborne salinity, the penetration model for adhered salts, and the analysis method for salt migration in concrete developed in previous studies can reproduce the long-term salt penetration behavior in real structures.