Influences of Time, Temperature, and Humidity on Chloride Diffusivity: Mesoscopic Numerical Research

Chloride-induced corrosion of reinforcing steel bars has a big impact on the performance of reinforced concrete (RC) structures subjected to saline environments, and it is essential to understand how chloride ions ingress in concrete. In this study, first, an experiment of chloride ingress in specimens under one-dimensional and two-dimensional diffusion was conducted in a climate chamber, and profiles of chloride concentration along the depth were obtained. Then, a mesoscopic model that considers environmental factors, i.e., temperature, relative humidity, and time of exposure, was developed to investigate chloride diffusivity in concrete. Concrete is treated as a three-phase composite: cement paste, aggregates, and interfacial transition zones (ITZs). It is assumed that chloride diffusivity can take place only in the cement paste and ITZ, whereas the aggregate is considered impermeable. Influence of ITZ thickness, i.e., 0, 50, and 80 μm, on chloride diffusivity in concrete is examined. Chloride concentrations, which are simulated with the mesoscopic model with consideration of environmental factors, are compared with previous test data. Finally, time to corrosion initiation of RC structures is predicted based on the developed mesoscopic model considering the distribution of aggregates, and the influence of environmental factors on the chloride concentration of the corrosion initiation point is investigated. It is found that ITZs have a significant effect on chloride diffusivity in concrete. However, different thicknesses of the ITZ, i.e., 50 and 80 μm, have a small impact on the chloride diffusivity in concrete; hence, an ITZ thickness 80 μm is recommended for efficiency. The simulation results with consideration of the environmental factors are in good agreement with the test data, and the corrosion initiation point on the surface of the steel embedded in concrete varies because of the distribution of aggregates. Corrosion initiation time decreases with increases in the water-to-cement (w/c) ratios but increases with increases in the thickness of cover, temperature, and humidity. An increase in either temperature or relative humidity can lead to a significant increase in the chloride concentration of the corrosion initiation point on the steel.