Cement paste has a pH of about 13 which provides a protective layer (passive coating) to the steel 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 assessed 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 most commonly carried out by spraying the indicator on freshly exposed surfaces of concrete broken from the structure or on split cores. Alternatively, the powder from drill holes can be sprayed or allowed to fall on indicator-impregnated paper.

Hence there may be corrosion in the zone ahead of the front defined by the indicator. In general the change in pH occurs in this zone which is only a few millimetres ahead and the phenolphthalein method provides a good indication of the location of the depassivation front.

The test is covered by BS EN 14630, Products and systems for the protection and repair of concrete structures. Test methods. Determination of carbonation depth in hardened concrete by the phenolphthalein method.

Also see the fingertip on Phenolphthalein