An observation-based model for corrosion of concrete sewers under aggressive conditions

Development of rational mathematical models for prediction of the likely present and future internal corrosion of reinforced concrete sewers requires understanding of the important physico-chemical processes, preferably based on field evidence. Samples of new and 70 year old pre-corroded concretes were exposed for up to 31 months in an aggressive sewer environment at 26 °C, 98% relative humidity and 79 ppm H2S concentration (averages). During the initial months of exposure the pH of the new concrete surfaces reduced rapidly however little corrosion loss was observed during this period. Subsequently pH reduced further, mass loss commenced and new concrete losses reached around 24 mm after 2 years. During the same period the old concrete corroded at an approximately constant rate. The presence of the corrosion product layer had negligible influence on corrosion losses. A bilinear corrosion loss model is proposed for practical applications.