Deep Purple and Rainbow Indicator

When carbon dioxide (CO2) in the air penetrates into concrete, it reacts with the calcium hydroxide (CaOH2) in the cement paste producing calcium carbonate (CaCO3). This reaction is called carbonation and causes the alkalinity of the paste to decrease. When the pH drops below 9, in the presence of moisture and oxygen, the embedded steel will corrode. This measurement of the depth of carbonation is important for Condition Assessment and Corrosion Evaluation of a reinforced concrete structure. 

To measure the pH of the cement paste, a freshly broken piece of concrete or a newly cut core is sprayed with the indicator, and allowed to dry. The approximate pH of the paste is indicated by a color index which in turn indicates the depth of the carbonation front.

Carbonation indicator for concrete.

• To evaluate the cause of corrosion when conducting corrosion surveys,
• To estimate service life where penetration of the carbonation front is critical,
• To monitor the effectiveness of protective coatings, sealers or procedures for re-alkalization of the cover layer,
• To make a rough estimate of concrete strength from the age of concrete and the relative humidity.

Color index of the indicators:

Ordering numbers:

• Technical data-sheet 

More documentation is available upon request.

BS EN 14630:2006 “Determination of carbonation depth in hardened concrete by the phenolphthalein method”