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. The time required for carbonation can be estimated knowing the concrete grade and using the following equation:
t=(d/k)2
where,
t is the time for carbonation,
d is the concrete cover,
k is the permeability
Typical permeability values are shown in Table 1. 
The significance of carbonation is that the usual protection of the reinforcing steel generally present in concrete due to the alkaline conditions caused by hydrated cement paste is neutralized by carbonation. Thus, if the entire concrete cover over the reinforcing steel is carbonated, corrosion of the steel would occur if moisture and oxygen could reach the steel.
EQUIPMENTS
If there is a need to physically measure the extent of carbonation it can be determined easily 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.
PROCEDURE
The 1% phenolthalein solution is made by dissolving 1gm of phenolthalein in 90 cc of ethanol. The solution is then made up to 100 cc by adding distilled water. On freshly extracted cores the core is 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.
Another formula, which can be used to estimate the depth of carbonation, utilizes the age of the building, the water-to-cement ratio and a constant, which varies depending on the surface coating on the concrete.
y=7.2XC2/(R2(4.6x-1.76)2)
where,
y is age of building in years,
x is water-to-cement ratio,
C is carbonation depth,
R is a constant (R= αβ).
R varies depending on the surface coating on the concrete (β) and whether the concrete has been in external or internal service (α). This formula is contained in the Japanese Construction Ministry publication “Engineering for improving the durability of reinforced concrete structures.” α is 1.7 for indoor concrete and 1.0 for outdoor concrete. β values are shown in Table 2.
RANGE AND LIMITATIONS
The phenolphthalein test is a simple and cheap method of determining the depth of carbonation in concrete and provides information on the risk of reinforcement corrosion taking place. The only limitation is the minor amount of damage done to the concrete surface by drilling or coring.
