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Corrosion-inhibiting admixtures increase the passivation of reinforcement and other embedded steel. This can inhibit corrosion when passivation would otherwise have been lost as a result of chloride ingress or carbonation. They are added to concrete during production and are referred to as ‘integral’ corrosion-inhibitors. So-called ‘migratory corrosion-inhibitors’ applied to hardened concrete are not admixtures.
Corrosion-inhibiting admixtures can significantly reduce maintenance costs of reinforced concrete structures throughout a typical service life of 30 to 40 years. Although corrosion-inhibitors can raise the corrosion threshold, they are not an alternative to impermeable durable concrete.
Corrosion-inhibiting admixtures have little effect on strength at either early or later ages. The function of this type of admixture is to enhance the long term durability of reinforced concrete by enhancing passivation to the cathodic and/or anodic areas of embedded steel. Structures built in the 1970s with calcium nitrite still contain the original level of nitrite and show no signs of deterioration. The addition of corrosion-inhibiting admixtures based on calcium nitrite or amino alcohol does not affect the long-term durability of the base concrete but can offer extended life to the overall structure.
Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of corrosion-inhibiting admixtures in concrete on the rates of carbonation and chloride ingress.
Note: Not all admixture types are covered by EN 934. BS 8443:2005 Specification for establishing the suitability of special purpose concrete admixtures, covers the following; underwater concrete admixtures, shrinkage reducing admixtures, corrosion inhibiting admixtures, pumping aids, segregation reducing admixtures, foaming admixtures, semi-dry concrete admixtures.
Acknowledgement: The Concrete Society
Other references:Concrete Society Current Practice Sheet 139, ´Corrosion inhibitors´, Concrete, June 2004, pp 8 & 9.
(Note that copies of Concrete Society Current Practice Sheets can be downloaded from the Members Area of the Concrete Society web site.)
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