Corrosion of Reinforcement

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.

Concrete normally provides excellent protection to reinforcing steel. Notwithstanding this, there are large number of cases in which corrosion of reinforcement has caused damage to concrete structures within a few years from the time of construction. One of the most difficult problems in repairing a reinforced concrete element is to handle corrosion damage. Reinforcement corrosion caused by carbonation is arrested to a great extent through repairs executed in a sound manner. However, the treatment of chloride-induced corrosion is more difficult and more often the problem continues even after extensive repairs have been carried out. It invariably re-occurs in a short period of time. Repairing reinforcement corrosion involves a number of steps, namely, removal of carbonated concrete, cleaning of reinforcement application of protection coat, making good the reduced steel area, applying bond coat and cover replacement. Each step has to be executed with utmost care. When chlorides are present in concrete, it is extremely difficult to protect reinforcing steel from chloride attack particularly in cases where chlorides have entered through materials used in construction and residing in the hardened concrete.

This increase in volume causes high radial bursting stresses around reinforcing bars and result in local radial cracks. These splitting cracks results in the formation of longitudinal cracks parallel to the bar. Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place

 

 

 

It is therefore important that a good physical and chemical bond must exist between reinforcement steel and concrete surrounding it. Due to inadequacy of structural design and /or construction, moisture and chemicals like chlorides penetrate concrete and attack steel. Steel oxidizes and rust is formed. This results in loss of bond between steel and concrete which ultimately weakens the structure.

The best control measure against corrosion is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting.