Carbonation

Introduction to Carbonation. Detecting Carbonation. Factors affecting Carbonation. Measuring carbonation.

1. PROPERTIES OF CONCRETEPROPERTIES OF CONCRETE

2. CARBONATIONCARBONATION

3. WHAT IS CARBONATION???WHAT IS CARBONATION???
“ It’s a reaction between the lime
in concrete and the carbon
dioxide from air, yielding Calcium
Carbonate.”

4. Carbonation of ConcreteCarbonation 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)
In carbonation process actual agent is
carbonic acid

5. Carbonation ReactionCarbonation 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 CARBONATIONDETECTING 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 CarbonationSpeed of Carbonation
The speed of the carbonation process
through the concrete mainly depends on
two parameters:
The porosity of the concrete
The moisture content of the concrete

8. Factors Affecting the carbonation Factors Affecting the carbonation
processprocess
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.
If 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 Factors Affecting the carbonation
process process
Reaction starts at the surface and slowly
penetrates
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.

10. EFFECT OF CARBONATIONEFFECT 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 CarbonationEffects of Carbonation
It reduces the quality of concrete
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 rusting

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

13. Passivation of SteelPassivation 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 Corrosion of Steel due to
CarbonationCarbonation

15. Measurement of CarbonationMeasurement of Carbonation
Carbonation can measured in two ways:
First way is to measure the concentration of
CO2 absorbed by the concrete specimen
Second way is to carbonate the specimen in
(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 Depth of Carbonation
(Commercial Method)(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 Depth of Carbonation
(Laboratory Method) (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% CO
2
, 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 Depth of Carbonation
(Laboratory Method)(Laboratory Method)
Using pH indicator
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.

19. Corrosion of Reinforcement due to Corrosion of Reinforcement due to
Carbonation 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 ConcretePassivity of Concrete

21. BicarbonationBicarbonation
Occasionally concrete may suffer from the so
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. BicarbonationBicarbonation
Bi-carbonation may be recognized by the
presence of large "pop-corn" like calcite
crystals and the highly porous paste.

23. Prevention from CarbonationPrevention from Carbonation
As carbonation is dependent on the
porosity of concrete. Decrease in
porosity will decrease the carbonation.
By decreasing water/cement ratio dense
concrete is produced with less porosity
Chances of the diffusion on CO2
becomes less