Observations about the corrosion of reinforcement in marine environments

The corrosion of reinforcement bars and prestressing strands in concrete structures such as bridges and coastal and harbour facilities is a significant maintenance and renewal issue. Mostly such structures are considered to have useful lives of 2-3 decades, but, surprisingly perhaps, there are a considerable number of structures still in existence with much longer useful lives, some showing little evidence of reinforcement corrosion despite being either exposed directly to hostile marine environments or very likely to have been made with seawater as mixing water. A small number of the latter, and a recent review of the performance of many (>300) of such structures is given. That review did not support the conventional thinking that modern cements were responsible for the poorer longterm durability of modern structures. Instead it was proposed, on the basis of electrochemical arguments and observed experience, that the properties of the aggregates is a key factor. To test this hypothesis a series of exposure tests were commenced several years ago in which seawater was used as mixing water and with different aggregates, including those rich in calcium-carbonate, which the conventional wisdom suggests lowers the concrete pore solution sufficiently to initiate reinforcement corrosion. Preliminary results from these tests are described and it is shown that calcium carbonate concretes, even with seawater, do not lead to early corrosion initiation. Results for the effect of higher cement content, water cement ratio and the presence or absence of salt water in the concrete mix also are presented.