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The other key factor in concrete deterioration is carbonation. When concrete is first poured, its high alkalinity serves as a passivating barrier, protecting the rebar from corrosion. However, as time goes on and the concrete is exposed to the environment, carbonation takes place, reducing the alkalinity and breaking down the passivating barrier. In time, carbonation will reach the depth of the rebar and corrosion cells will develop.

While chloride penetration and carbonation work closely together to deteriorate concrete, other aspects — such as the quality of the concrete and design of the structure — play a pivotal role. The bottom line: Even if the best available concrete repair mortar is used (i.e., high-density and dimensionally stable), carbonation and chloride penetration are working virtually 24/7 to undermine any repair effort.

A 5-step method to repair and prevent corrosion

What then is the best method known today to address corroding concrete? Short of an "engineered" approach that provides for sacrificial anodic or active cathodic corrosion protection, the basic approach involves the following repair methods as defined by the American Concrete Institute (ACI). As stated earlier, any repair necessitates the involvement of an engineer who examines the structure and supervises material selection, repair methods and repair application.

ACI prescribes the following procedures:

1. Remove deteriorated concrete and profile the surface according to specifications of the repair material's manufacturer. This includes the removal of any carbonated or chloride-impregnated concrete.

Note: There are methods available today that will remove chlorides from existing in-place concrete. These methods should be evaluated and considered by building owners and engineers. Such methods may be appropriate depending on the extent of the damage and of the desired repair.

2. Clean rebar with an appropriate method to remove all corrosion, and repair appropriately to ensure the integrity of the completed repair.
3. Coat the exposed rebar with a high-performance anti-corrosion and bonding agent. To improve bonding of the repair mortar to existing concrete, coat all areas of the repair with the same bonding agent.
4. Select an appropriate repair mortar with the required performance and application characteristics.
5. While this completes the defined ACI repair method, MAPEI recommends going one critical step further to solidify the repair's longevity by an exponential factor — ultimately decreasing lifecycle costs to the owner. For this fifth step, coat the entire structure — both the original concrete as well as the repaired area — with a cementitious, flexible waterproofing and protective coating.

5th step proves successful