Hydrated cement carbon dioxide calcium carbonate

Good-quality dense concrete may only show car-bonation to a depth of 5-10 mm after 50 years, whereas a low-strength permeable concrete may carbonate to a depth of 25 mm within 10 years. If reinforcement is not correctly located with sufficient cover it corrodes causing expansion, spalling and rust staining. The depth of carbonation can be determined by testing a core sample for alkalinity using phenolphthalein chemical indicator, which turns pink in contact with the uncarbonated alkaline concrete. Where steel reinforcement has become exposed due to carbonation and rusting, it may be coated with a rust-inhibiting cement and the cover restored with polymer-modified mortar, which may contain fibre reinforcement. Additional protection against further attack can be achieved by the final application of an anti-carbonation coating, which acts as a barrier to carbon dioxide. Thermosetting polymers such as polyurethane and chlorinated rubber, also certain acrylic-based polymers, give some protection against carbonation.

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