Corrosion of the reinforcing steel in a concrete structure only happens when three conditions are met. If any one of the following conditions is not met, then corrosion cannot occur:
1. Loss of passivity
2. Presence of oxygen
3. Sufficient moisture in the concrete.
Principle 7 - preserving or restoring passivity focuses on creating chemical conditions in which the surface of the reinforcement is maintained or returned to a passive condition.
In normal conditions, the reinforcement steel is protected from the alkalinity surrounding the concrete cover. This alkalinity creates a passive film of oxide on the steel surface which protects the steel from corrosion. However, this passive film can be damaged due to the reduction of the alkalinity by carbonation when the carbonation front has reached the reinforcement steel. A break-down also occurs due to chloride attack. In both these instances, the protecting passivation is then lost. Different methods to reinstate (or to preserve) the passivity of the reinforcement are available, including:
Method 7.1 - Increasing cover with additional mortar or concrete
If the reinforcement does not have adequate concrete cover, then by adding cementitious mortar or concrete the chemical attack (e.g. from carbonation or chlorides) on the reinforcement will be reduced.
Method 7.2 - Replacing contaminated or carbonated concrete
Through removing damaged concrete and rebuilding the concrete cover over the reinforcement, the steel is again protected by the alkalinity of its surroundings.
Method 7.3 - Electrochemical re-alkalisation of carbonated concrete
Realkalisation of concrete structures by electrochemical treatment is a process performed by applying an electric current between the embedded reinforcement to an external anode mesh, which is embedded in an electrolytic reservoir, placed temporarily on the concrete surface. This treatment does not prevent the future ingress of carbon dioxide. So to be effective on the long term, it also needs to be combined with appropriate protective coatings that prevent future carbonation and chloride ingress.
Method 7.4 - Re-alkalisation of carbonated concrete by diffusion
There is limited long term experience with this method. It requires the application of a very alkaline coating over the carbonated concrete surface and the realkalisation is achieved by the slow diffusion of the alkali through the carbonated zone. This process takes a very long time and it is very difficult to control the right distribution of the material. After treatment, it is also always recommended to prevent further carbonation by applying a suitable protective coating.
Method 7.5 - Electrochemical chloride extraction
The electrochemical chloride extraction process is very similar in nature to cathodic protection. The process involves the application of an electrical current between the embedded reinforcement and an anode mesh placed at the outer surface of the concrete structure. As a result, the chlorides are driven out toward the surface. Once the treatment is completed, the concrete structure has to be protected with a suitable treatment to prevent the further ingress of chlorides (post treatment).
Methods 7.3-7.5 are all processes, yet once the processes have occurred suitable protective coatings will be required to prevent further deterioration of the concrete structure. Concrete coastings have been covered extensively in Principle 1 & 2
, but can include dependent on site requirements:
The selection of the appropriate method will depend on various parameters such as: the reasons for the loss of passivation (e.g. due to carbonation or chloride attack), the extent of the damage, the specific site conditions, the repair and protection strategy, maintenance possibilities, costs and so forth.