| Understanding the Different Types of Corrosion that Affect Concrete
When mild steel is used as reinforcing in concrete, a protective oxide layer is naturally formed on the surface of the steel due to the alkalinity of the concrete. As long as this film is maintained, the reinforcement will indefinitely remain in a very passive state. Once corrosion is initiated, it is only a matter of time before the expansive pressures from steel oxidation causes concrete cracking, spalling and delaminations. If the on-going corrosion activity is not addressed, section loss of the reinforcing will occur and significant structure repair or replacement may eventually be required.
In most cases, the protective oxide film on the reinforcing steel is destroyed by the presence of elevated levels of chloride ions. The chloride threshold to initiate corrosion is generally considered to be around 1.0 to 1.4 lbs of water soluble Cl- per cubic yard of concrete (at the level of the steel). This chloride threshold will vary depending upon the pH of the concrete. For example, concrete that has experienced a loss of alkalinity will require fewer chlorides to initiate corrosion. Chloride-induced corrosion can be commonly found in structures exposed to de-icing salts, a marine environment, or certain industrial processes. On some structures, sufficient quantities of chlorides to cause corrosion were introduced in the original construction by the use of chloride-containing admixtures or contaminated aggregates.
The passive oxide film can also be destroyed by the loss of alkalinity in the concrete matrix surrounding the reinforcing steel. The reduction in alkalinity is generally caused by carbonation, a reaction of atmospheric carbon dioxide with calcium hydroxide (in the cement paste) in the presence of water. The result is a reversion of the calcium hydroxide to calcium carbonate (limestone) which has insufficient alkalinity to support the passive oxide layer. The amount of time for the carbonated zone to reach the level of the reinforcing is a function of the amount of concrete cover, concrete porosity, humidity levels, and the level of exposure to carbon dioxide gas. Carbonation-induced corrosion is a particular threat to older building structures.
Patch Accelerated Corrosion
Commonly referred to as “ring anode corrosion” or “halo effect”, patch accelerated corrosion is a phenomenon specific to concrete restoration projects. When repairs are completed on corrosion-damaged structures, abrupt changes in the concrete surrounding the reinforcing steel are created. Typical concrete repair procedures call for removal of the concrete around the full circumference of the reinforcing steel within the repair area, cleaning of corrosion by-products from the steel, and refilling the cavity with new chloride-free, high pH concrete. This procedure leaves the reinforcing steel embedded in adjacent environments with abruptly different corrosion potentials. This difference in corrosion potential (voltage) is the driving force for new corrosion sites to form in the surrounding contaminated concrete. The evidence of this activity is the presence of new concrete spalling adjacent to previously completed patch repairs.