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.
