Diffusion vs. Concentration of Chloride Ions in Concrete

This investigation was performed to gain insight and assist in determining the long-term durability of reinforced concrete structures where the external chloride concentrations are different than those typically observed at the permanently immersed, tidal, and splash zones of partially immersed bridges. Insight into the diffusion properties of the concrete at these locations may lead to changes in the state’s policy on corrosion control of reinforced concrete structures. Structures at locations where the external chloride concentrations may have spatial, let alone time, variation are of interest. Additionally, at elevations above the splash zone, the amount of chlorides deposited would be lower, and the concrete close to the surface not saturated. Similarly, the section of structures that extend inland would be subjected to seawater spray and the concrete close to the surface would usually not be water saturated. The performance in regard to chloride penetration of specimens made with three base compositions (the supplementary cementitious materials were: 20% fly ash; 20% fly ash + 8% silica fume; and 50% slag), and water-to-cementitious ratios of 0.35, 0.41, or 0.47 were investigated here. Experiments investigated the diffusion of chloride ions into concrete samples that were exposed in scenarios that simulated the splash, tidal, and immersed portions of a marine structure, with the solution ranging from brackish water to 10% seawater, to seawater. Bulk diffusion experiments were conducted in solutions that contained 0.6%, 3%, and 16.5% NaCl. Rapid migration tests and resistivity measurements were also performed several times over two years, and the non-saturated migration coefficient vs. resistivity values were correlated. The apparent diffusion values from the bulk diffusion tests were correlated to corresponding equivalent resistivity values. Samples exposed for over 18 years, simulating tidal exposure were also part of this investigation. The field component investigated the chloride concentration as a function of elevation. Other experiments were conducted which involved controlling the degree of saturation and exposing a designated surface to finely ground salt. These apparent diffusion results were compared to the apparent diffusion results from specimens in which the chloride transport was due to natural marine atmosphere.