Reinforcement Corrosion in Concrete Exposed to the North Sea for More Than 60 Years

Of the some 1,000 almost identical, precast, reinforced concrete elements forming the 1.5-km-long, more than 60-year-old promenade railing immediately adjacent to the North Sea at Arbroath, Scotland, more than 90% still appear to be in remarkably good condition with little evidence of external rust staining or reinforcement corrosion. However, the elements that have been replaced, some only about 15 years ago, mostly show severe reinforcement corrosion. Concrete quality, density, and cover to the reinforcement are generally similar, but concrete permeability does not correlate well with reinforcement corrosion. Detailed investigations show some of the original, apparently sound concrete elements to have severe highly localized reinforcement corrosion, in some cases with a complete loss of steel. This was accompanied by limited amounts of mainly black corrosion product that x-ray diffraction (XRD) analysis showed to have a strong presence of magnetite. The ends of some reinforcement bars showed tunnelling corrosion, a phenomenon not previously reported for reinforcement. It is proposed that the observations show that the long-term corrosion of reinforcement in high-quality concretes involves processes more complex than simply oxidation of the steel under chloride conditions.