Chloride threshold values in concrete - A look back and ahead

Over the last 60 years, extensive research efforts aimed at determining the so-called chloride threshold value in reinforced concrete. The belief that such a threshold exists is the root of all efforts to measure and model chloride ingress into concrete. This paper addresses the usefulness of this established concept by evaluating the experience available for portland cement systems. Additionally, it is critically discussed whether the concept can be applied to modern materials, particularly SCMs. Finally, suggestions for future research are made. It is concluded that the pronounced stochastic nature of the chloride threshold currently permits only corrosion prognoses with large uncertainties. It is shown that even the most sophisticated chloride transport model in concrete will not significantly improve this. Instead of refining mass transport models, future research should thus aim at finally understanding the relevant parameters governing corrosion initiation in concrete. There is strong indication that a number of such parameters are overlooked in the current concept of the chloride threshold value. We believe that as long as initiation of chloride-induced corrosion is not fully understood, it does not make sense to continue applying the (unsuccessful) concept of the chloride threshold value to modern materials.