Critique of the environmental exposure classification for marine concrete structures in the tidal zone

Preventing chloride-induced steel reinforcement corrosion in marine concrete structures remains a concern for structural engineers and asset managers. Marine structures are typically exposed to high chloride concentrations through direct exposure to sea water, which provides severe conditions in terms of reinforcement corrosion. It is generally understood that the tidal zone is the most severe marine exposure zone as high chloride concentrations and exposure to moisture leads to a shorter time to corrosion initiation and a high corrosion rate. For this reason, the marine tidal zone has been classified as a severe environment in different standards and guidelines e.g. BS EN 206-1 (2013). However it has been observed in several aged marine structures along the west and east coasts of Southern Africa after roughly 45 - 65 years of exposure, that there is usually no or relatively insignificant reinforcement corrosion damage in the tidal zone, despite very high chloride contents. Existing exposure classifications for marine structures are largely based on chloride ingress. In order to develop a full understanding on the severity of exposure, it is imperative other factors that directly affect reinforcement corrosion, such as oxygen availability (which relates to the frequency of wetting and drying) are considered. The quality of the concrete and the cover thickness influence the time it takes for the concrete to dry out to the level of the steel. Hence, if the drying period between two subsequent high tides is short, or if the cover depth and/or quality are high, the concrete at the level of the steel will probably be oxygen-deprived and not facilitate reinforcement corrosion. This paper provides a review of these aspects in order to develop a better understanding of the tidal exposure zone for RC structures.