3. WHAT IS
CARBONATION???
“ It’s a reaction between the
lime in concrete and the
carbon dioxide from air,
yielding Calcium Carbonate.”
4. Carbonation of
Concrete
A reaction between the lime in concrete
and the carbon dioxide from air, yielding
Calcium Carbonate(CaCO3).
The creation of calcium carbonate
requires three equally important
substances: carbon dioxide (CO2)
calcium(Ca) and water(H2O)
I n carbonation process actual
agent i
s
carbonic acid
5. Carbonation Reaction
1. The first reaction is in the pores where
carbon dioxide (CO2) and water (H2O)
react to form carbonic acid (H2CO3):
CO2 + H2O H2CO3
2. The carbonic acid then reacts with the calcium
phases:
H2CO3 + Ca(OH)2 CaCO3 + 2H2O
6. DETECTING
CARBONATION
Depth of carbonation can be detected
using an indicator.
A chemical such as Phenolphthalein
sprayed on to freshly broken concrete.
Areas remaining alkaline will turn in a
bright purply-pink color.
Carbonated areas of concrete will
remain unchanged in color.
7. Speed of
Carbonation
The speed of the carbonation
process through the concrete mainly
depends on two parameters:
The porosity of theconcrete
The moisture content of theconcrete
8. Factors Affecting the
carbonation
process
Carbonation occurs between the 40-90%
relative humidity
Carbonation is most active at 50-70%
relative humidity.
If the concrete is too dry, (RH<40%) carbon
dioxide cannot be dissolved and no
carbonation can occur.
I f it is too wet (RH>90%) carbon dioxide
cannot enter water and the concrete will not
carbonate.
CO2 cannot pass through the pores filled
with water
9. Factors Affecting the
carbonation
process
Reaction starts at the surface and
slowly penetrates
Poor quality concrete suffers
carbonation earlier and deeper
I f the surface is exposed to salts and
moisture or in salt laden environment,
carbonation may accelerate
reinforcement corrosion.
10. EFFECT OF
CARBONATION
It can cause soft surface, dusting and
color change
It reduces quality concrete
It reduces the concrete ability to
protect reinforcement from corrosion
(in an exposed environment)
It will result in additional shrinkage
in carbonated region.
11. Effects of
Carbonation
I t reduces the quality ofconcrete
Normal pH value of concrete is 12.6-
13.5
Carbonation reduces the pH value to 9
Changes alkaline nature to the acidic
nature
Reduces the ability of concrete to
prevent reinforcement steel bars from
13. Passivation of
Steel
The alkaline conditions that prevail in concrete
(pH > 13.0) favor the formation of a passive layer
on the embedded steel reinforcement.
15. Measurement of
Carbonation
Carbonation can measured in two ways:
First way is to measure the concentration o
f
CO2 absorbed by the concrete specimen
Second way is to carbonate the specimen i
n
(a) natural
(b) laboratory
environment conditions and then
break it and spray a pH indicator to
know the extent of Carbonation.
16. Depth of Carbonation
(Commercial Method)
Depth of carbonation can
be detected using
an indicator.
The phenolphthalein
indicator solution is
applied to a fresh fracture
surface of concrete. If the
indicator turns purple, the
pH is above 8.6. Where the
solution remains colorless,
the pH of the concrete is
below 8.6, suggesting
carbonation. A fully-
carbonated paste has a pH
of about 8.4.
17. Depth of Carbonation
(Laboratory Method)
Using pH indicator
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 5% CO2, 50% Relative Humidity, and
20-22ºC is maintained in a carbonation chamber.
Then sample is broken and is sprayed with a pH indicator.
Popularly a standard solution of 1% phenolphthalein in 70%
ethyl alcohol is used.
18. Depth of Carbonation
(Laboratory Method)
Using pHindicator
noncarbonated
with pH
In the
region
above 9.2,
values
the
phenolphthalein
indicator turns
red; and
purple-
in the
carbonated portion with
9.2, the
remained
pH less than
solution
colorless.
19. Corrosion of Reinforcement due
to
Carbonation
The high alkalinity of cement paste,approx.
pH13 passivates the steel surface and
protects it against oxidation(corrosion).
The presence of chlorides, carbonation, acid
attack or combination of all these, reduce pH
of concrete and the reinforcing steel starts to
corrode.
Carbonation reduces its pH value to 8
Due to the acidic behavior of concrete
passive layer of oxides destructed
21. Bicarbonatio
n
Occasionally concrete may suffer from the s
o
called bi-carbonation process. 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. Contrary to
normal carbonation, bi-carbonation results in an
increase in porosity making the concrete soft.
23. Prevention from
Carbonation
A s carbonation is dependent on the
porosity of concrete. Decrease in
porosity will decrease the
carbonation.
B y decreasing water/cement ratio
dense concrete is produced with less
porosity
Chances of the diffusion on
CO2
becomes less
