For carbon dioxide attack, carbon dioxide dissolves in water to
form a weak acid called carbonic acid. It would dissolve the cement matrix.
However, the amount of carbon dioxide from the atmosphere is usually not
sufficient to cause harm to concrete structures until additional source of
carbon dioxide is available (e.g. decaying vegetable matter).
Carbonation is the process of converting alkaline hydroxides in
concrete to carbonates by reaction with carbon dioxide. The significance of
carbonation lies in the reduction of pH of pore water in concrete structure
from 12-13 to 8-9 so that it drops the protection to steel reinforcement. The
process takes place at concrete surface and spreads inwards. The passive nature
play an important role in steel corrosion as it prevents corrosion even in the
presence of water and oxygen. This passive nature is derived from a stable and
thin layer of iron oxide formed at the surface of steel reinforcement. However,
if the pH of concrete is dropped, this passive oxide layer becomes unstable and
corrosion may start once water and oxygen supply is available