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Corrosion-inhibiting admixtures, such as calcium nitrite, have been available for many years. They are added to the concrete at the time of mixing. They react with ferrous ions on the surface of the reinforcement to form a passive layer which is capable of remaining stable even when chlorides are present up to a certain concentration.
More recently, products known as migrating corrosion inhibitors (MCIs) have been developed which use a similar principle but which are applied to the surface of the concrete and migrate through the concrete to attach to the surface of the reinforcement. On reaching the reinforcement they form a stable layer on the surface, protecting the reinforcement from attack.
MCIs can be used on concrete structures where the reinforcement has started to corrode but there is little physical damage. They may also be used in conjunction with surface coatings to give added assurance of protection. Surfaces to be treated must be clean and dry so they absorb the inhibitor. Application of the inhibitor itself is straightforward and any method such as brush, roller or low-pressure hand spray can be used provided that the surface is fully saturated. Between three and five coats may be required depending on the absorbency of the substrate.
MCIs are relatively new and therefore do not have a long track record.
Acknowledgement: The Concrete Society
Other references:Concrete Society Current Practice Sheet 139, ´Corrosion inhibitors´, Concrete, June 2004, pp 8 & 9.
(Note that Concrete Society Current Practice Sheets can be downloaded from the Members area of the Concrete Society web site.)
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