REINFORCEMENT CORROSION AND THE DURABILITY OF CONCRETE BRIDGES.

Corrosion can arise in most parts of a bridge substructure or superstructure and usually results from the use of de-icing salts. Substructure corrosion is often associated with leaking joints, faulty drainage or salt spray from traffic. General corrosion is found on substructures and although this is a relatively slow process, producing only small reductions in bar cross-section, it causes substantial disruption of the concrete cover. Localized corrosion has occurred on unwaterproofed decks and is much more rapid than general corrosion; it leads to substantial local reductions in bar cross-section. This form of corrosion is not easy to detect visually because it is not always associated with concrete disruption or rust staining and, in any event, the deck is covered with asphalt surfacing. Several site investigation tests are available. When used together they can usually locate corroding reinforcement, determine its type and causes and estimate its rate, thus providing the information required to specify repairs. The threshold concentration of chlorides in concrete bridges of 0.35% has not proved a reliable basis for predicting corrosion. Likely explanations are that chloride contamination is associated with relatively high concentrations of the more-damaging free chloride ion. Localized anodes also sometimes protect the surrounding steel from effects of high chloride levels. The repair of chloride-contaminated bridges is difficult. Repair of corroding areas is generally effective, but the development of anodes in the concrete adjacent to the repair can lead to further damage. Cathodic protection appears to be the most promising method of overcoming this problem, although further development is needed before it can be applied effectively. (Author/TRRL)