What is the corrosion of steel and why it is a concern? Why does steel in concrete corrode and how can it be prevented?
ASTM terminology defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of material and its properties”. For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of original steel and none of its good mechanical properties. Corrosion also produces pits or holes in the surfaces of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
Reinforced concrete uses steel to provide the tensile properties that are needed in structural concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural stresses due to traffic, winds, dead loads and thermal cycling. However when reinforcement corrodes, the formation of rust leads to a loss of bond between steel and the concrete and subsequent delamination. If left unchecked, the integrity of the structure can be affected. Reduction in the cross-sectional area of steel reduces its strength capacity.
Steel in concrete us usually a non-corroding, passive condition. However, steel-reinforced concrete is often used in severe environments where sea water or deicing salts are present. When chloride moves into concrete, it disrupts the passive layer protecting the steel, causing it to rust and pit. (One reason why calcium chloride should never be used in steel reinforced concrete).
Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar, the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than the chloride-induced corrosion.
The first defense against corrosion of steel is concrete is quality concrete and sufficient concrete cover over the reinforcing bars. Quality concrete has a water-to-cement ratio that is low enough to slow down the penetration of chloride salts and the development of carbonation. The W/C ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration.
Another ingredient of good quality concrete is air entrainment. (As long as the surface is NOT hard trowelled). It is necessary to protect the concrete from freezing and thawing damage. Air entrainment also reduces bleeding and the corresponding increased permeability due to the bleed channels.
Adequate cover over reinforcing steel is also an important factor. Chloride penetration and carbonation will occur in the outer surface of even low permeability concrete. Increasing the cover will delay the onset of corrosion. For example, the time for chloride ions to reach a steel rebar at 2 inches from the surface is four times that with a 1” cover. A 1.5” cover is recommended as a minimum for most structures and increases it to 2” of cover for protecting from deicing salts.