Carbonation of concrete is a process by which Carbon di Oxide from the air penetrates into the concrete and reacts with calcium hydro-oxide to form calcium carbonates. Conversion of Ca(OH)2 into CaCO3 by 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 reinforcement and also reduces alkalinity of concrete. Carbonation test is also a type of non destructive test.
Air contains CO2. In actual practice it is present in the atmosphere in smaller or greater concentation, 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. Such a low pH value, destroys the protective layer and results in corrosion of steel.
Carbonation is one of the reason responsible for corrosion of reinforcement.
Test procedure
This test is carried out to determine the depth of concrete affected due to combined attack of atmospheric carbon dioxide and moisture. It cause a reduction in level of alkalinity of concrete.
- A spray of 0.2% solution of phenolphthalein is use as pH indicator.
- The change of color of concrete to pink indicates that the concrete is in good health.
- where no change of color takes place, it suggestive of carbonation-affected concrete.
- To conduct this test, we drill a hole on the concrete surface of different depths upto cover concrete thickness
- Then we remove the dust and spray phenolphthalein with physician‟s injection.
Rate of Carbonation
The rate of carbonation depends on the following factors:
- The level of pore water
- Grade of concrete
- Permeability of concrete
- depth of cover
- Time
Note : The highest rate of carbonation occurs at a relative humidity of between 50 and 70 percent.
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