Concrete

Concrete is a composite material composed of aggregate bonded together with fluid cement which hardens over time. Most use of the term "concrete" refers to Portland cement. Concretes are made with other hydraulic cements, such as cement fondu. However, road surfaces are also a type of concrete, "asphaltic concrete", where the cement material is bitumen.

The fundamental ingredients required to make concrete are cement clinker, water, fine aggregate, coarse aggregate and certain special additives. Cement clinker is essentially a mixture of several anhydrous oxides. For example, standard Portland cement consists mainly of the following compounds, in order of decreasing weight percent: 3CaOSiO₂,2CaOSiO₂,3CaOAl₂O₃, and 4CaOAl₂O₃Fe₂O₃.

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. The scale of this problem has reached alarming proportions in various parts of the world.

The three most common mechanisms of reinforcing steel corrosion damage in concrete are:

Localized breakdown of the passive film by chloride ions
Carbonation, a decrease in pore solution pH, leading to a general breakdown in passivity. Harmful chloride ions usually originate from deicing salts applied in cold climate regions or from marine environments/atmospheres. Carbonation damage is predominantly induced by a reaction of concrete with carbon dioxide (CO₂) in the atmosphere.
Chloride-induced rebar corrosion. Corrosion damage to reinforcing steel is an electrochemical process with anodic and cathodic half-cell reactions. In the absence of chloride ions, the anodic dissolution reaction of iron,

Fe = Fe²⁺ + 2e

is balanced by the cathodic oxygen reduction reaction,

1.2O₂ + H₂O +2e + 2OH^-

Oxygen diffuses to the reinforcing steel surface through the porous concrete, with cracks acting as fast diffusion paths, especially if they are not filled with water. The Fe²⁺ions produced at the anodes combine with the OH^- ions from the cathodic reaction to ultimately produce a stable passive film.