General

This standard presents corrosion control guidelines that are applicable to existing atmospherically exposed structures made of concrete conventionally reinforced with carbon steel (ASTM⁽¹⁾ A 615¹). These guidelines may be used to develop specifications involving repair and rehabilitation of steel-reinforced concrete structures. These guidelines should be used primarily when repair or rehabilitation is being implemented because of deterioration resulting from the corrosion of steel reinforcement.

Reinforcing steel is compatible with concrete, not only because of similar thermal expansion characteristics, but also because the highly alkaline portland cement allows a stable, protective oxide film to form on the surface of the encased steel. If the film does not form or is weakened or destroyed so that it does not protect the steel, corrosion can occur. The protective oxide film does not form or is destroyed if (1) the cement paste is not in contact with the reinforcing such as at voids and cracks; (2) alkalinity is lost by reaction with certain gases and liquids; or (3) excessive amounts of chloride or other aggressive ions are present. It has been shown that chloride ion content as low as approximately 0.2 percent by weight of cement (or approximately 0.6 kg/m³ [1 lb/yd³] of concrete, depending on the cement content of the mix) at the steel depth can initiate the corrosion process. If one or more of these conditions are present, and moisture and oxygen are available to the embedded reinforcing steel, an electrochemical cell forms, resulting in corrosion.

Corrosion most commonly proceeds by the formation of an electrochemical cell. This electrochemical cell is composed of four elements: (1) an anode; (2) a cathode; (3) an electrical connection between the two; and (4) an ionic connection provided by an electrolyte (concrete). Direct current (DC) caused by electrochemical potential differences, such as between different metals or the same metal in different environments, flows from the anodic area to the cathodic area through the electrolyte. Corrosion occurs at the anode, where the current leaves the metal. If any one of the elements of the electrochemical cell is eliminated, corrosion can be prevented. Dissimilar metal couples and stray DC can initiate and accelerate corrosion.

The corrosion product of iron occupies several times the volume of the base metal. The expansive pressure exerts a significant tensile force on the surrounding concrete. The resulting cracks propagate either to the surface or to nearby reinforcing steel, resulting in a delamination. Steel sectional losses, which may or may not have structural significance, can generate cracking of the concrete. Bond forces and corrosion both put concrete in tension and are additive. Relatively small amounts of metal loss at the surface of the reinforcing steel can be sufficient to crack the concrete cover and result in the loss of bond and anchorage. Other problems resulting from delamination and spalling include danger from falling concrete, increased corrosion, loss of fireproofing, and safety considerations.

Other forms of corrosion, such as those caused by dissimilar metal couples and DC stray currents, can initiate or accelerate corrosion.

This standard describes various approaches that may be taken with respect to corrosion control of existing structures. The flow chart in Figure 1 delineates the repair or rehabilitation strategy covered in this standard. Because of the complexity of corrosion problems on individual structures, advice from a professional engineer or a corrosion specialist, whose professional qualifications include suitable experience in corrosion control of reinforced concrete structures, should be sought before proceeding with repair and rehabilitation programs. Also, a structural engineer may be required if a decision on structural integrity is needed.

This standard does not cover pretensioned and post-tensioned reinforced concrete.

⁽¹⁾ ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.