Good Quality Concrete
(pH=12-13) Steel is Passivated 
Carbon Dioxide Enters, pH
Begins to Drop. Steel is not yet affected. 
pH at Steel Drops Below 9.5,
Corrosion Begins. 
Volume Expansion of Rust Causes Cracking and Spalling.
Carbonation of Concrete.
The most common cause of loss of passivating alkalinity is carbonation - a process whereby atmospheric carbon dioxide reacts with the soluble alkaline calcium hydroxide and other cement hydrates in the concrete. These are converted into insoluble calcium carbonate. This process is completely natural. However, the alkalinity of the cement matrix is reduced and its passivating ability is lost progressively from the surface inward. The speed of penetration depends largely upon the permeability of the concrete and its moisture contents.
For carbonation to take place, moisture must be present. The carbonation reaction proceeds most rapidly when the relative humidity is between 50% and 75%. At lower humidities, there is insufficient water in the pores of the concrete for significant quantities of calcium hydroxide to dissolve. Above 75% humidity the situation is reversed and the pores become progressively blocked with water, allowing the calcium hydroxide to freely dissolve but largely preventing the ingress of carbon dioxide.
It is important to remember that cracks, bugholes, honeycombing, day-joints, etc., are all critical areas which may permit the ingress of corrosion promoting substances and effectively reduce the actual thickness of the concrete cover.
Once the concrete in contact with reinforced steel has carbonated, the reinforcing steel is not longer protected. In the presence of moisture and oxygen, corrosion damage is inevitable.
Chlorides in Concrete.
The potential for reinforcement corrosion is greatly enhanced if chlorides are present in the concrete.
Chlorides can enter the concrete through direct or indirect exposure of the structure to deicing salt, exposure in a marine location, use of contaminated original materials, or perhaps leakage from swimming pool.
The concentration of chlorides to promote corrosion of embedded reinforcement is affected by the pH of the concrete. The pH of fresh concrete (12.7 - 13.2) a threshold level of about 7,550 - 8,000 ppm is required to start corrosion on embedded reinforcement but if the pH is lowered to 10.5 - 11.5 (still sufficient to turn phenolphtalein solution purple) the chloride threshold is significantly lower, at below 100 ppm.
In addition to their electrochemical influence on the initiation and rate of corrosion, chlorides also can cause physical damage to the concrete surface. This is due in effect to "thermal shock" resulting from accelerated freeze-thaw cycles. Progressive surface spalling can occur, reducing or eliminating cover to steel reinforcement.
