Durability of Rebars in Concrete
R. N. Swamy
Appears on pages(s):
carbonation; chlorides; coatings; concrete construction; corrosion resistance; corrosion tests; cover; mechanical properties; durability; reinforced concrete; reinforcing steels; specifications; Structural Research
Corrosion of reinforcing steel is now recognized as the most critical limit state affecting the durability and long-term stability of concrete structures. Although concrete is inherently alkaline and provides a natural protection to any embedded steel in it, thereby insuring its chemical passivity, concrete structures deteriorate for a variety of reasons. Steel corrosion is the most insidious and destructive form of damage, and once it starts, it is almost impossible to stop the process (unless such remedial measures as cathodic protection systems are applied), until eventually the safety, stability, and design life of the structure are all drastically reduced in time. The primary causes of steel corrosion are inadequate cover to steel, carbonation, neutralization due to atmospheric pollutants, and/or chloride penetration. The quality of concrete, its pore structure, and permeability characteristics are thus major factors controlling steel corrosion. However, it is inherent in the nature of concrete construction that there is no single method of corrosion protection that may be presumed to insure satisfactory serviceability throughout the life of a structure. This paper briefly reviews the factors that influence concrete deterioration and loss of alkalinity. It is shown that long-term durability of concrete structures can only be insured by a dual approach--developing a durable concrete, and providing a protective coating to the steel. Such methods of steel protection as cement-slurry coating, epoxy coating, and galvanizing are discussed. Extensive test results are presented on the corrosion resistance of plain, epoxy-coated, and galvanized bars when exposed to a marine environment, and chloride intrusion in cracked members. Cracking, cover, and concrete quality are identified as the major parameters influencing steel corrosion, but cover to steel is the most critical factor in preserving the electrochemical stability of steel. The paper shows that both epoxy-coated reinforcing bars and galvanized reinforcing bars can provide excellent resistance to chloride-induced corrosion.