Reinforced concrete is a composite material. Its structural performance is realized only when concrete and steel act in unison during the service life of the structure. The comprehensive and tensile loads are carried by concrete and steel respectively. Steel protects concrete from cracking under tensile loads and concrete protects steel from corrosion by providing an alkaline environment around it.
So long as this happens, the reinforced concrete structures perform satisfactorily. Even though the concrete can be a very strong material, it is also subject to deterioration. Concrete can be porous so that chemicals can penetrate the pores and attack the paste.
The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion or rebar contributes to the deterioration of concrete.