Carbonation of concrete occurs when carbon dioxide in air enters and reacts with the lime and alkali materials in concrete thereby reducing the pH below 10 at which point the concrete protects the reinforcement.

The depth of carbonation is normally taken at concrete dust sample locations although it can be undertaken elsewhere. When the concrete is sprayed with a phenolphthalein solution the carbonated concrete remains clear and the un-carbonated concrete changes to a magenta colour. It is important that the cover is recorded at the test location to enable the depth of cover to be compared to the depth of carbonation.