Cement paste has a pH of about 13 which provides a protective layer (passive coating) to the steel and protects reinforcement against corrosion. Loss of passivity occurs at about pH 11. Carbonation of the concrete, caused by carbon dioxide in the atmosphere, has the effect of reducing the pH.
Carbonation depth is generally assessed on site using a solution of phenolphthalein indicator that appears pink in contact with alkaline concrete with pH values in excess of 9 and colourless at lower levels of pH. The test is covered by BS EN 14630: Products and systems for the protection and repair of concrete structures and Test Methods: Determination of carbonation depth in hardened concrete by the phenolphthalein method.