Some experts have concluded that chloride ions, including chlorides present in the initial mix or from seawater and road deicing salts, are responsible for corroding the reinforcing bars which, in turn, spall off the covering concrete. Those who are calling for very tough restriction on the amounts of chloride ions are basing their opinions on corrosion problems experienced with two classes of structures: coastal/marine structures and bridge decks. Both share many characteristics: they are subjected to a full range of weather extremes; they are exposed for prolonged periods of time to virtually unlimited amounts of chlorides (deicing salts or seawater); they are water-saturated for long periods; and, during the periods of saturation and heavy chloride exposure, they undergo thousands of loading-unloading deflections.
The chloride critics generally have ignored the fact that these similarities of conditions and structural performance clearly place marine structures and bridge decks in a class apart from all other reinforced concrete structures.
Relatively rapid stress reversals constantly open and close fine cracks in the concrete. This opening and closing subjects the rebars to the two elements that cause corrosion air and water. Since a heavy concentration of chlorides, from deicing salts or seawater, exists at the surface, chlorides are pumped downward to the reinforcing steel along with air and water. When air and water come in contact with steel, the corrosion process begins. If chloride ions then are added to the water, the ongoing corrosion process can speed up. But, if either air and/or water are removed, the corrosion will stop. Air, water and steel are necessary to the corrosion process, but chloride ions are not. Air and water reaching the reinforcing steel causes the corrosion; chlorides may accelerate the process but they do not cause it. Therefore, the real cause of rebar corrosion is concrete that, because of cracking, high porosity, high permeability or insufficient cover, fails to protect the steel from air and water.