Fundamental Principle
Carbonation of
concrete occurs when the atmospheric carbon dioxide, in the presence of
moisture, reacts with hydrated cement minerals to produce carbonates. The process
of carbonation is also called depassivation . Carbonation penetrates below the
exposed concrete surface extremely slowly. The time required for carbonation
can be calculated knowing the concrete grade and using the following equation:
Where,
Typical permeability values are shown in Table 1
Table
1. Permeability Values versus Concrete Grade
Concrete Grade
|
Permeability
|
15
|
17
|
20
|
10
|
25
|
6
|
30
|
5
|
35
|
4
|
40
|
3.5
|
The significance
of carbonation is that the usual protection of the reinforcing steel present in
concrete due to the alkaline conditions caused by hydrated cement paste is
neutralized by carbonation. Thus, if the entire concrete cover over the steel
reinforcement is carbonated, corrosion of the steel would occur if moisture and
oxygen could reach the steel.
Equipment
If there is a
need to measure the extent of carbonation it can be determined easily by
spraying a freshly exposed concrete surface with a 1% phenolphthalein solution.
The depth at which Pink color stops is the carbonation depth.
Procedure
To make 1% phenolthalein solution dissolve 1 gm
of phenolthalein in 90 cc of ethanol. Then distilled water is added to it to
make the solution up to 100 cc. If the test is to be done on freshly extracted
cores, the core is first sprayed with phenolphthalein solution. Then depth of
the colored layer (the carbonated layer) from the external surface is measured
to the nearest mm at 4 or 8 positions, and the average is taken. If it is to be done in a drilled hole, first remove
the dust from the hole using an air brush. Then the depth of the uncolored
layer is measured at 4 or 8positions and the average is taken. If the concrete
still retains its alkaline characteristic its color will change to purple. If
carbonation has taken place the pH may get changed to 7 (i.e. neutral
condition) and there will be no color change. Another formula, which can be
used to estimate the carbonation depth, using the age of the building, the
water-cement ratio and a constant, which varies depending on the surface
coating on the concrete is obtained from the equation given below:
Where,
Y is age of
building in years
X water-to-cement
ratio
C is carbonation
depth
R is a constant
(R=αβ)
R varies depending
on the surface coating of the concrete (β) and whether the concrete has been in
external or internal service (α). (α) is 1.7 for indoor concrete and 1.0 for
outdoor concrete. (β)Values are shown in Table 2
Table
2. Values of β
Finished condition
|
Indoor
|
Outdoor
|
no layer
|
1.70
|
1.00
|
plaster
|
0.79
|
|
mortar + plaster
|
0.41
|
|
mortar
|
0.29
|
0.28
|
mortar + paint
|
0.15
|
|
tiles
|
0.21
|
0.07
|
paint
|
0.57
|
0.80
|
Therefore the carbonation depth is given by:
Range and Limitations
The
phenolphthalein test is a simple and cheap method used for determining the
depth of carbonation in concrete. It provides information on the risk of corrosion
to the concrete reinforcement. The only limitation is the small amount of
damage occurring in the concrete surface by drilling or coring.
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