The formation of cracks is dangerous for protection against corrosion. Once concrete cracks, the external depassivating agents can penetrate deep into concrete and set off the process of corrosion. Cracks running transversely to the reinforcement are less harmful than the longitudinal cracks along the reinforcement.
Thus in order to induce the process of corrosion and to keep it going, at least one of the following conditions must exist in any RC structure.
Chloride ion concentration in excess of the threshold value at the interface of the reinforcement and concrete or sufficient advancement of the carbonation front to destroy the passivity of the ferric oxide surface layer on the reinforcement.
Adequate moisture in the concrete of facilitate the movement of chloride ions and provide a conduction path between the anodic and the cathodic areas on the steel.
Sufficient oxygen supply to the cathodic areas in order to maintain such areas in a depolarized condition.
Difference in electrochemical potentials at the surface of the reinforcement.
Low values of electrical resistivity of concrete.
Relative humidity in the range 50-70%.
Higher ambient temperature.