Contrary to common belief, concrete itself is a complex composite material. It has low strength when loaded in tension and hence it is common practice to reinforce concrete with steel, for improved tensile mechanical properties. Concrete structures such as bridges, buildings, elevated highways, tunnels, parking garages, offshore oil platforms, piers and dam walls all contain reinforcing steel (rebar). The principal cause of degradation of steel reinforced structures is corrosion damage to the rebar embedded in the concrete.
Iron is unstable in nature, and because reinforcing steel used in precast concrete is made largely of iron, it, too, becomes unstable when exposed to corrosive agents such as salt, carbonation, and even air. Iron, as we commonly recognize it, is not generally found in nature because of its instability. It takes a great deal of energy to produce iron from its ore, and even then it is so unstable that it must be coated to keep it from reverting back to its ore forms (hematite, magnetite, and limonite).
The two most common causes of reinforcement corrosion are (i) localized breakdown of the passive film on the steel by chloride ions and (ii) general breakdown of passivity by neutralization of the concrete, predominantly by reaction with atmospheric carbon dioxide.Sound concrete is an ideal environment for steel but the increased use of deicing salts and the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this material. The scale of this problem has reached alarming proportions in various parts of the world.
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