Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates. The conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage.

CO2 by itself is not reactive. In the presence of moisture, CO2 changes into dilute carbonic acid which attacks the concrete and also reduces alkalinity of concrete.

Air contains CO2. The concentration of CO2 in rural air may be about 0.03 per cent by volume. In large cities the content may go up to 0.3 per cent or exceptionally it may go up to even 1.0 per cent. In the tunnel, if not well ventilated the intensity may be much higher.

The pH value of pore water in the hardened concrete is generally between 12.5 to 13.5 depending upon the alkali content of cement. The high alkalinity forms a thin passivating layer around steel reinforcement and protect it from action of oxygen and water. As long as steel is placed in a highly alkaline condition, it is not going to corrode. Such condition is known as passivation.

In actual practice CO2 present in atmosphere in smaller or greater concentration, permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete. The pH value of pore water in the hardened cement paste which was around 13 will be reduced to around 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3 In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion.

The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel.

The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel.

Rate of Carbonation:

The rate of carbonation depends on the following factors.

1. The level of pore water i.e., relative humidity.
2. Grade of concrete
3. Permeability of concrete
4. Whether the concrete is protected or not
5. depth of cover
6. Time

It is interesting to know that if pore is filled with water the diffusion of CO2 is very slow. But whatever CO2 is diffused into the concrete, is readily formed into dilute carbonic acid reduces the alkalinity. On the other hand if the pores are rather dry, that is at low relative humidity the CO2 remains in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary to carbonate the concrete.

The highest rate of carbonation occurs at a relative humidity of between 50 and 70 percent.The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that stronger concrete is much denser with lower W/C ratio. It again indicates that the permeability of the concrete, particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO2 does not take place faster, as in the case of more permeable concrete with higher W/C ratio. Depth of cover plays an important role in protecting the steel from carbonation.

Measurement of depth of carbonation:

A common and simple method for establishing the extent of carbonation is to treat the freshly broken surface of concrete with a solution of phenophthalein in diluted alcohol. If the Ca(OH) is unaffected by CO2 the colour turns out to be pink. If the concrete is carbonated it will remain uncloured. It should be noted that the pink colour indicates that enough Ca(OH)2 is present but it may have been carbonated to a lesser extent. The colour pink will show even up to a pH value of about 9.5.

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1. Concrete Shrinkage
2. Action of sewage on concrete
3. Concrete Compaction
4. Joints in Concrete
5. Concrete curing