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.