Time-Dependent and Stress-Dependent Chloride Diffusivity of Concrete Subjected to Sustained Compressive Loading

This paper studies the chloride diffusivity of concrete subjected to long-term exposure in a simulated marine environment with sustained compressive loading for up to 224 days. A series of concrete prisms were loaded under five different compressive stress levels (0, 0.2, 0.3, 0.5, and $0.7f_c$) and were subjected to different numbers (4, 8, 12, and 16) of wet/dry seawater exposure cycles. At the end of each exposure period, the concrete specimens were analyzed to determine their chloride penetration profile, the apparent chloride diffusion coefficient $D_a$ of the concrete, and the surface chloride content $C_s$. It was found that the $D_a$ and $C_s$ values are both time and stress dependent and that these two dependencies have complex interactions. The value of $D_a$ decreases with time due to the hydration of the cement matrix, whereas the value of $C_s$ increases over time. The effects of compressive stress on the values of $D_a$ and $C_s$ strongly depend upon the applied stress level, which has a threshold value at approximately 30% of the ultimate compressive strength. Below this threshold, the value $C_s$ remains constant; after this threshold, $C_s$ increases linearly as the applied stress increases. Below the threshold, the value of $D_a$ decreases marginally as the stress level increases. However, above the threshold, $D_a$ increases rapidly with the stress level as a result of microcracking. Through regression of the chloride penetration profiles, empirical models are proposed to quantify the dependence of $D_a$ and $C_s$ on the exposure time and stress level. The validity of these models is evaluated through comparisons with test results.