The presentation of the chloride threshold level for corrosion of steel in concrete

It has been argued that presenting the chloride threshold level as a free chloride content or chloride to hydroxyl concentration ratio in the pore solution of concrete is an improvement over the more commonly used total chloride content. In this review the basis for this hypothesis is examined. Contrary to expectations, an analysis of the literature suggests that, on balance, bound chloride presents a corrosion risk, an effect which may be due to its contribution to the reservoir of available chloride at the steel concrete interface. Furthermore, the soluble hydroxyl concentration in the pore solution, which is largely determined by the presence of alkali metals, is not an adequate measure of the inhibitive properties of the cement. Its most important property appears to be its ability to resist a fall in pH to values below that required to sustain a passive film. Thus, in terms of currently used representations, chloride threshold levels are best presented as total chloride contents expressed relative to the weight of cement. This may be viewed as the total potential aggressive ion content expressed relative to the total potential inhibitor content. It may be possible to improve this by, for example, expressing the total chloride content relative to the alkaline reserves of the concrete, but further work is needed to confirm this hypothesis.