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Carbonation of concrete Structures
The microstructure of concrete is such that it has capillary pores to the extent of 28%. The extent of pores depends upon quality of concrete and the presence of water at the time of mixing of concrete. Making more dense concrete with less water/cement ratio reduces the amount of pores. These pores are created due to evaporation of excess free water during strengthening of concrete mass. These pores are inter connected and goes inside the concrete mass from surface of concrete structures.
Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete through pores and reacts with calcium hydroxide to form calcium carbonates. It has seen that the conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage.
We shall see another aspect of carbonation, as 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 (i.e. pH value reduces).
Air contains CO2. The concentration of CO2 in rural air may be about 0.03 percent by volume. In large cities the content may go up to 0.3 percent 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 up to 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.
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