The durability of reinforced concrete structures in marine environments

Much concern exists about the apparently short durability of modern reinforced concrete structures exposed in marine atmospheric and tidal environments. Modern practice specifies high quality low permeability concretes in an effort to reduce the inward diffusion of chlorides, considered critical in the initiation of reinforcement corrosion in marine environments. However, for such environments a review of more than 300 cases reported in the literature found many reinforced concrete structures survived for long periods of time, despite poor quality concrete, low cover and very high chloride levels next to the reinforcement. Mostly these structures showed little or no external evidence of reinforcement corrosion. The available data shows a degree of correlation between long-term immunity from reinforcement corrosion and the composition of the concrete and its aggregates. It is shown that alkaline materials such as calcium carbonates and non-reactive dolomite add alkalinity to the concrete and that this is related to long term durability. While calcium carbonate usually is associated in the reinforcement corrosion literature with 'carbonation' which reduces pH levels and thus allows initiation of reinforcement corrosion, this not supported by corrosion science principles and by careful field observations. It is noted also that some reinforced concrete structures can have very aggressive localized corrosion without obvious exterior signs. This has serious practical implications.