Protection of Metals in Concrete Against Corrosion

Description

The corrosion of metals, especially reinforcing steel, in concrete has received increasing attention in recent years be-cause of its widespread occurrence in certain types of structures and the high cost of repairing the structures. The corrosion of steel reinforcement was first observed in marine structures and chemical manufacturing plants.1-3 Recently, numerous reports of its occurrence in bridge decks, parking structures, and other structures exposed to chlorides have made the problem particularly prominent. Extensive re-search on factors contributing to steel corrosion has in-creased our understanding of the mechanics of corrosion,especially concerning the role of chloride ions.

It is anticipat-ed that the application of the research findings will result infewer instances of corrosion in new reinforced concretestructures and improved methods of repairing corrosion-in-duced damage in existing structures.

For these improve-ments to occur, the research information should bedisseminated to individuals responsible for the design, con-struction, and maintenance of concrete structures. Concrete normally provides reinforcing steel with excel-lent corrosion protection. The high-alkaline environment inconcrete creates a tightly adhering film that passivates thesteel and protects it from corrosion.

Because of concrete’sinherent protective attributes, corrosion of reinforcing steel does not occur in the majority of concrete elements or struc-tures. Corrosion of steel, however, can occur if the concrete does not resist the ingress of corrosion-causing substances, the structure was not properly designed for the service envi-ronment, or the environment is not as anticipated or changesduring the service life of the structure.While several types of metals may corrode under certainconditions when embedded in concrete, the corrosion ofsteel reinforcement is the most common and is of the greatest concern, and, therefore, is the primary subject of this report.Exposure of reinforced concrete to chloride ions is the ma-jor cause of premature corrosion of steel reinforcement.

Cor-rosion can occur, however, in some circumstances in theabsence of chloride ions. For example, carbonation of con-crete reduces concrete’s alkalinity, thereby permitting corro-sion of embedded steel. Carbonation is usually a slowprocess in concretes with a low water-cementitious materialsratio (w/cm). Carbonation-induced corrosion is not as com-mon as corrosion induced by chloride ions