Carbonation is a reaction between lime in concrete and carbon dioxide from air that forms calcium carbonate, reducing concrete's pH. It occurs between 40-90% relative humidity and depends on concrete porosity and moisture content. Carbonation depth can be detected using pH indicators. It reduces concrete quality and protection of reinforcing steel, potentially leading to corrosion. Higher quality concrete with low water-cement ratios can help prevent carbonation.

1. PROPERTIES OF CONCRETE

2. CARBONATIO
N

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

12. Effects of
Carbonation
Initiates the corrosion process of
steel bars by demolishing the
protective oxide layer around steel
bars

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.

14. Corrosion of Steel due to
Carbonation

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

20. Passivity of
Concrete

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.

22. Bicarbonatio
n
Bi-carbonation may be recognized by the
presence of large "pop-corn" like calcite
crystals and the highly porous paste.

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