A comparison of chloride ion diffusion coefficients derived from concentration gradients and non-steady state accelerated ionic migration.

A method for electrical indication of the ability of concrete to resist chloride ion penetration was first developed in the USA in the late 1970's. The test method (AASHTO T 277) related chloride ion diffusion to the measurement of electrical charge passed through a test specimen in a relatively short period of time. In this paper, chloride concentration profiles obtained from specimens used in the initial development of the method are used to compute apparent chloride diffusion coefficients D-app by application of ionic migration principles. These coefficients are compared to D-app calculated from fitting data obtained from 90-day ponding tests carried out on companion specimens not subjected to application of electrical currents. It is shown that there is a strong linear relationship for conventional concretes between coefficients calculated from chloride concentration gradients produced either by long-term ponding or by accelerated ingress of chloride into the test specimens. Also included in the comparative study are effective diffusion coefficients, D-eff, based on the measurement of specimen resistance and calculated using Einstein's equation. Results indicate that values calculated by any of the techniques yield lower values for diffusion coefficients than those obtained from longer-term ponding. It is shown, however, that rankings of concretes on the basis of diffusion coefficient are similar for both calculated and measured values. Large errors occur in calculated coefficients in cases where anomalies exist in measured chloride gradients. These errors are related to the dependence of calculated diffusion coefficients on chloride concentration ratios. For specialty polymer concretes, mechanisms of penetration of chloride into concrete may be different in the presence of an applied electrical field. These differences may be the result of the short test time in the AASHTO T 277 procedure as well as of the relatively high voltage used. These effects prevent the concrete from reaching the same chloride surface concentration, C-s, as in longer-term (90-day) ponding. Anomalously high D-app valves are computed when the concrete is too resistive or has been surface treated. In these cases, the C-s valves are far greater than those obtained during 90-day ponding.