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Carbonation depth

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.

Also see the fingertip on Phenolphthalein

Acknowledgement: The Concrete Society

Other references:Concrete Society, Current Practice Sheet 131, ´Measuring the depth of carbonation´, CONCRETE, January 2003.
(Note that copies of Concrete Society Current Practice Sheets can be downloaded from the Members Area of the web site.)

Building Research Establishment, IP 6/81, ´Carbonation of concrete made with dense natural aggregates´, 1981.

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TR54 Diagnosis of deterioration in concrete structures- identification of defects, evaluation an

TR60 Electrochemical tests for reinforcement corrosion

TG2 Guide to testing and monitoring the durability of concrete structures

Carbonation