Diffusion vs. concentration of chloride ions in concrete.
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Diffusion vs. concentration of chloride ions in concrete.

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  • Abstract:
    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.

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