Influence of Cyclic Drying-Wetting and Carbonation on Oxygen Diffusivity of Cementitious Materials: Interpretation from the Perspective of Microstructure

The oxygen diffusion coefficient of coastal concrete structures under cyclic drying-wetting and carbonation is one of the most crucial factors in its durability performance. In this work, the oxygen diffusion coefficients of cement mortar and concrete after 28-day cyclic drying-wetting cycles and carbonation were tested. The microstructure development of mortar and concrete after drying-wetting cycles and carbonation was characterized by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) methods. It was found that porosity and critical pore diameter have strong correlation with oxygen diffusion coefficient on plain mortar and concrete. After drying-wetting cycles, the refined porosity and critical pore diameter did not show obvious correlation with the oxygen diffusion coefficient. Nevertheless, the reduction of the oxygen diffusion coefficient after drying-wetting cycles was proportional to the change of porosity and critical pore diameter. A theoretical model based on Fick's law was derived to calculate the oxygen diffusion coefficient of the carbonated area. It was found that with the development of carbonation duration, the oxygen diffusion coefficient in carbonated area decreased. The influence of porosity on oxygen diffusion coefficient after carbonation is similar with plain cementitious materials, while the oxygen diffusion coefficient in the carbonated area is less influenced by the critical pore diameter change. It can be concluded that the retarding effect on oxygen diffusion coefficient by drying-wetting cycles and carbonation can delay the corrosion rate for reinforced concrete structures.