A properly designed and constructed concrete is initially water-tight and the reinforcement steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density. Concrete also gives steel within it a chemical protection. Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area. Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value.

However, age takes it toll. Like human beings, Reinforcement Cement Concrete (RCC) structures are also mortals. Depending on the quality of design and construction, there will be an initial period in which no corrosion will occur as the external moisture or chloride is unable to reach the steel causing corrosion. This initial period will also depend on the environment in which the structure is constructed.

Due to wetting and drying cycles, heating and cooling cycles, loading and unloading cycles, cyclic loading , leaching of lime and most importantly additions and alterations done on the structures, isolated cracks, voids, entrapped air and large capillary pores get interconnected and external moisture and chlorides find their way to reinforcement steel and corrosion starts. Corrosion process continues till such time large, cracks develop and spalling of concrete occurs.

Besides moisture and chloride ingress problems, carbonation is also one of the principle causes of corrosion. Concrete when produced is highly alkaline having a pH value between 12.5 to 13.5. Alkaline environment around the steel passivates corrosion process. Due to carbon dioxide and humidity present in the environment the exposed surface of concrete loses its alkalinity due to formation of carbonic acid. This formation gradually penetrates into the concrete mass and is called carbonation of concrete. When carbonation takes place beyond the concrete cover given to reinforcement steel, the environment around the steel loses its alkalinity (dropped to less than pH value 9). The mitigation of corrosion no longer takes place due to chemical protection.

It is observed that lower grades of concrete have shown much deeper carbonation, than higher grades of concrete, for a similar period of time. Grades lower than N20 can carbonate beyond 25 mm to 35 mm within a matter of 20 years. If quality of concrete in the cover region is poor it can take place much faster.

Slender sections like canopies, parapets slabs projecting on the building exterior show greater evidence of deterioration than other structural members due to above reasons.

Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place.

Why Does Steel in Concrete Corrode?Perhaps the greatest threat to the long-term durability of reinforced concrete is corrosion of the steel (primarily composed of iron, or Fe) which expands due to the greater volume of the corrosion byproducts and then cracks the concrete and debonds from the concrete matrix. Billions of dollars in damage occurs each year due to this simple reaction, and our apparent inability to stop it. So what causes this reaction? 1) All metals, except gold and platinum, are unstable and will corrode. Reinforcing steel is made primarily of iron, which is highly corrosive. There are several things that can lead to corrosion of steel in concrete, with the most common being chloridesuch as the chloride in table salt (sodium chloride). These chlorides come from sea water, deicing salts, chlorides used in the mix as an accelerator, or salty aggregate or water. Another common culprit that can cause corrosion is carbonation caused by atmospheric CO2 penetrating the concrete.

2) Corrosion is an electrochemical reaction (a corrosion cell) with positive charges flowing through the moist concrete and negative charges (electrons) flowing through the reinforcing steel. The cathode is a spot where water and oxygen have gotten through the concrete to the steelperhaps at a crack. The anode is where the steel corrodes and forms corrosion byproducts that can crack the concrete.

3) Good sound concrete has a pH in the pore solutions of 13.0 to 13.5, which is much more alkaline than something like Drano. In this environment, the steel forms a thin layer on its surface that we call a passivating layer. This layer protects the rebar and prevents the anode from forming so no corrosion can occur.

4) When chloride ions or carbonation penetrate the concrete and get down to the reinforcing steel, they lower the pH and attack the passivating layer. If there is also moisture and oxygen (at the cathode) then the steel begins to corrode. The corrosion byproducts have no tensile strength, reducing the strength of the reinforcing steel, and have greater volume than the original steel, causing cracks.

5) Chlorides and carbonation will penetrate the concreteits only a question of time. But with good concrete cover (2 inches) and dense (low permeability) concrete, that could take centuries. And if the concrete is covered and dry, then it cant corrode because the ionic migration of positive charges cant take place.