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Concrete is the second most widely used material in the world after water. Most of the structures you will see around yourself are made up of concrete such as bridges, car-parks, roads etc. Though, these structures offers wide range of advantages over other construction materials such as high strength. Still, durability of the reinforced concrete structures are of great concern as its performance may get affected by various environmental conditions. Corrosion of reinforcement is the major concern affecting the concrete durability. In fact, 70-90% of the structures fail due to corrosion of steel in concrete structures.

Mechanism of corrosion in reinforced concrete structures
During casting of concrete structures, when the raw materials are mixed, cement reacts with water and hydration of cement occurs. As soon as the hydration starts, a passive layer is formed around the embedded steel in concrete which protects it from corrosion. This layer is of few micrometers in thickness. As long as the steel in surrounded by this layer, it is prevented from corrosion. The breakdown of this film occurs when harmful agents from environment such as chloride ions and carbon dioxide enters into the concrete, penetrates and reaches the steel bar. Thus, the first protection from corrosion is good concrete cover. When corrosion occurs through ingress of chloride ions, it is called as chloride induced corrosion and when it occurs due to carbon dioxide it is called as carbonation.

The general mechanism of corrosion involves anodic and cathodic reactions. These reactions occurs both as the steel surface. At anode, steel looses electron and converted into Ferrous ions (Equation 1). These electrons travel to cathode site on the steel bar and reacts with pore water present in the concrete and oxygen from the environment to form hydroxide ions (Equation 2). Thus, iron oxidation reaction occurs at anode and oxygen reduction occurs at cathode. Finally these hydroxide ions will travel back to the anode and react with ferrous ions to form ferrous hydroxide, which on subsequent oxidation forms hydrated ferric oxide which is called as rust. These rust products have volume 6-10 times of steel and thus due to increase in volume, concrete cracks, spall and delaminate. Thus, for a corrosion reaction five important parameters are: Anode, Cathode, Water, Oxygen and electrical connection between anode and cathode. Absence of any one of these parameters will stop corrosion. For fully saturated concrete, there will be limited oxygen hence corrosion rate will be low. Similarly for completely dry concrete, corrosion rate will be low due to absence of sufficient water.

Anodic Reaction Fe → Fe2+ + 2e−               (Equation 1)

Cathodic Reaction 4e− + O2 + 2H2O → 4OH−             (Equation 2)