Concrete 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