Carbonation can be defined as the reaction of carbon dioxide CaO2 with the hydrated cement. The carbon dioxide gas is present in the atmosphere with varying percentage from rural to urban and highly industrialized area. In rural area air, the amount of CO2 is found 0.03% by volume, 0.1% or even more in urban areas and more than 0.3% in large and industrialized cities. In the presence of moisture carbon dioxide (CO2) forms carbonic acid, which reacts with calcium hydroxide Ca(OH)2 forming calcium carbonate (CaCO3). In the process other cement compounds are also decomposed. The process of carbonation causes contraction in the concrete, resulting in a small shrinkage. This contraction is known as carbonation shrinkage.
The pH values of pore water in the hardened concrete usually vary between 12.5 to 13.5 depending upon the alkali content in the cement. The high alkalinity of cement forms a thin protective layer around the steel reinforcement and protects it from the action of water and oxygen. As long as steel is in the high alkaline zone, it is not going to be corroded. Such condition is known as passivation.
In actual practice carbon dioxide present in atmosphere in smaller or greater concentration, diffuses or permeates into the concrete and reacts with calcium hydroxides, forming carbonates, and reducing the PH value of pore water from about 13.0 to about 9.0. When all the calcium hydroxide Ca(OH)2 has been carbonated, the pH value reduces to about 8.3. In such a low pH value the protective layer gets dissolved or destroyed and the steel is exposed to corrosion.
Carbonation proceeds from surface of the concrete inwards, but its rates of travel are very slow.
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The actual rate of carbonation depends on the following factors:
i. Permeability of the concrete
ii. Its moisture content
iii. Quantity of CO2
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iv. Relative humidity of ambient medium
v. Grade of concrete
vi. Depth of cover
vii. Time
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viii. Whether the concrete is protected or unprotected
As the permeability of concrete is a function of w/c ratio and it’s curing, concrete with high w/c ratio and inadequately cured will have more carbonation, i.e., the depth of carbonation will be more.
It has been observed that if pores are filled with water, the rate of diffusion of CO2 is very slow. But whatever amount of CO2 is diffused, it is converted into dilute carbonic acid readily and the alkalinity of the concrete is reduced. On the other hand, if pores are dry, i.e. the relative humidity is low the carbon dioxide remains unchanged in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary for carbonation of concrete.
The highest rate of carbonation takes place at a 50 to 70% relative humidity. The rate of carbonation depth is slower in case of stronger concrete as it is less permeable and much denser with low w/c ratio. Further the permeability of the skin concrete is much less for low w/c ratio and the diffusion of carbon dioxide takes place at a very slow rate than the more permeable concrete of high w/c ratio. Fig.17.4 and Table 17.9 show the depth of carbonation of different grades of concrete.
Now it is well established that for longer durability of concrete it needs protection. The Long span bridge girders, industrial structures, fly-overs and chimney etc. need protective coating. It has been observed that in case of protective concrete the depth of carbonation remained constant at 4.0 mm from 3 years to 25 years age, where as in case of un-protective concrete the depth of carbonation continuously increased more or less at a constant rate from 4.0 mm to 11.5 mm in 3 years to 25 years age as shown in Fig. 17.5.
The depth of cover plays an important role in protecting the steel from carbonation. Table 17.10 shows the relation between w/c, depth of cover and time in years for carbonation to reach the reinforcement.
Measurement of Carbonation:
For establishing the extent of carbonation, following simple and most common method may be used.
Method:
A freshly broken concrete surface is treated with a solution of phenolphthalein in dilute alcohol. If there is no reaction of calcium hydroxide Ca(OH)2 with carbon dioxide CO2, the colour of the concrete will turn pink i.e. if the colour turns to be pink, there is no carbonation of concrete. In case there is no charge in the colour of concrete, the carbonation of concrete has taken place. The pink colour indicates that there is enough amount of Ca(OH)2, but it might have been carbonated to a lesser extent. The pink colour will be there even upto a pH value of about 9.5.