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

Predicting the life of reinforced concrete structures in severe marine environments

It is generally agreed that reinforced concrete structures exposed to harsh marine environments will, within the space of one or two decades, start to show modest or even serious deterioration due to reinforcement corrosion unless special care is taken to prevent or reduce the rate of entry of aggressive chlorides. Usually the life prediction is based on the rate of ingress of chlorides ions. However, careful review of many older reinforced concrete structures shows that some have survived decades despite very high chloride levels and little or no protective measures or special additives. Conversely, there are cases were reinforcement corrosion is evident despite considerable concrete cover and high concrete quality. A review is given of more than 300 cases for which corrosion initiation and corrosion progression took many years to occur. Ccnsiderable differences were found in the time to corrosion initiation and in time to active corrosion. Figure 1 gives an example. A small number of brief case studies are given, together with an extended discussion of the possible reasons why some reinforced concrete structures show much better long-term durability than others. It is argued that long-term durability depends on pH reserves (i.e. alkalinity) and this can be provided by aggregates such as limestone and non-reactive dolomites. This observation is consistent with corrosion science principles. It is concluded also that calcium carbonate by itself (such as caused by 'carbonation') does not lower the concrete pH immediately adjacent to the reinforcement to permit corrosion initiation. The additional leaching of alkalies must occur before corrosion can initiate. These findings potentially have important practical implications for the prediction of the life of reinforced concrete structures. This matter is currently under investigation