Carbonation of concrete occurs when the carbon dioxide, in the atmosphere in the presence of moisture, reacts with hydrated cement minerals to produce carbonates, e.g. calcium carbonate.
The carbonation process is also called depassivation. Carbonation penetrates below the exposed surface of concrete extremely slowly. Carbonation is significant as it neutralizes the alkanility present in the concrete. Thus, if it reaches the reinforcements it makes them more susceptible to corrosion.
The extent of carbonation is determined by spraying a freshly exposed surface of the concrete with a 1% phenolphthalein solution. The calcium hydroxide is coloured pink while the carbonated portion is uncoloured.
To measure the carbonation depth hence giving a indication of the present risk of reinforcement corrosion.
Codes are not available but an empherical relation between permeability and concrete cover is known and can be used.
100ml of 1% phenolthalein solution is made. The core is then sprayed with phenolphthalein solution, the depth of the uncoloured layer (the carbonated layer) from the external surface is measured to the nearest mm at 4 or 8 positions, and the average taken. If the test is to be done in a drilled hole, the dust is first removed from the hole using an air brush and again the depth of the uncoloured layer measured at 4 or 8 positions and the average taken. If the concrete still retains its alkaline characteristic the colour of the concrete will change to purple. If carbonation has taken place the pH will have changed to 7 (i.e. neutral condition) and there will be no colour change.
The only limitation is the minor amount of damage done to the concrete surface by drilling or coring