30-11-2012, 05:59 PM
CARBONATION OF CONCRETE
Carbonation Of Concrete.ppt (Size: 6.73 MB / Downloads: 2,196)
What is concrete?
It is the mixture of sand, coarse aggregate ,fine aggregate and cement along with 25-50% Ca(OH)2.
What is Carbonation of concrete?
“ It’s a reaction between the lime in concrete{Ca(OH)2}and the carbon dioxide(CO2 )from air, yielding Calcium Carbonate.”
How does CO2 reacts?
It is basically a non-reactive gas.
The actual reactive agent is carbonic acid.
Where does carbonic acid come from?
C02+H20 =H2CO3
Moisture in air reacts with carbon dioxide
to produce carbonic acid.
Rate of Carbonation
Carbonation is most active at around 50% relative humidity (RH 40-90%).
If the concrete is too dry, (RH<40%) carbon dioxide cannot dissolve and no carbonation can occur.
If it is too wet (RH>90%) carbon dioxide cannot enter water and the concrete will not carbonate.
Reaction starts at the concrete surface and slowly penetrate into the core.
Poor quality concrete suffers carbonation earlier and deeper
If the surface is exposed to salts and moisture or in salt laden environment, carbonation may accelerate reinforcement corrosion.
EFFECTS OF CARBONATION
Because CaCO3,occupies a greater volume than Ca(OH)2which it replaces, the porosity of carbonated concrete is reduced.
Water released by Ca(OH)2 on carbonation may aid the hydration of hitherto anhydrate cement.
increased surface hardness,
increased strength at the surface,
reduced surface permeability,
reduced moisture movement
increased resistance to those forms of attacks( controlled by permeability),
It also accelerates chloride-induced corrosion of reinforcement.
Bi-carbonation - What is that?
Bi-carbonation may occur in concrete with very high water to cement ratio due to formation of hydrogen carbonate ions at pH lower than 10.
Bi-carbonation results in an increase in porosity making the concrete soft and friable.
Bi-carbonation may be recognized by the presence of large "pop-corn" like calcite crystals and the highly porous paste.
Methods to Measure Carbonation
First way is to measure the concentration of CO2 absorbed by the concrete specimen.
Second way is to carbonate the specimen in (a) natural or (b) laboratory environment conditions and then break it and spray a pH indicator to know the extent of Carbonation.
In this method, first concrete specimen is kept in an open environment for a number of years or in Carbonation Chamber for a number of months. Generally, conditions of 70% CO2, 50% Relative Humidity, and 20-22ºC is maintained in a carbonation chamber, fig. 1.4(a). Then sample is broken and is sprayed with a pH indicator. Popularly a standard solution of 1% phenolphthalein in 70% ethyl alcohol is used. In the noncarbonated region with pH values above 9.2, the phenolphthalein indicator turns purple-red; and in the carbonated portion with pH less than 9.2, the solution remained colorless.