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Carbonation is the reaction of carbon dioxide in the environment with the calcium hydroxide in the cement paste. This reaction produces calcium carbonate and lowers the pH to around 9. At this value the protective oxide layer surrounding the reinforcing steel breaks down and corrosion becomes possible. The reaction of carbon dioxide and calcium hydroxide only occurs in solution and so in very dry concrete carbonation will be slow. In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide and again carbonation will be slow. The most favourable condition for the carbonation reaction is when there is sufficient moisture for the reaction but not enough to act as a barrier. In most structures made with good quality concrete, carbonation will take several (or many) years to reach the level of the reinforcement.Measures for improving the resistance of the concrete to carbonation and approaches to modelling carbonation rates are given in Concrete Society Technical Report 61, Enhancing reinforced concrete durability.
A simple test can be used to determine the depth of carbonation penetration (see Carbonation depth).
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