Carbonation

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CARBONATION
INTRODUCTION
Carbonation is a slow process.
Carbonation of concrete is associated with loss
of alkalinity of the pore solution.
Carbonation is a process by which is carbon
dioxide in the ambient air penetrates the
concrete and reacts with the hydroxides such
as calcium hydroxide to form carbonates.
In the reaction between CO
2 and calcium
hydroxide, calcium carbonate is formed.

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CO
2 + H
2O Carbonic Acid (Ha
2CO
3)
CaOH
2 + CO
2 CaCO
3 + H
2O
This carbonic acid neutralizes the alkalinity of
the cement matrix in the concrete cover.
In the carbonated concrete the alkalinity
diminishes appreciably and below a certain pH
level the reinforcement is no longer protected
against corrosion.
The rate of carbonation depends on w/c ratio,
type of cement and relative humidity.
Carbonation induced corrosion

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Carbonation reduces alkalinity that protects the reinforcement
bar
Carbonation is highly dependent on the
relative humidity of the concrete.
Carbonation occurs more rapidly at humidity
in range of 50% to 70%.
Below 25% relative humidity, the degree of
carbonation that takes place is considered
insignificant.
Above 75% relative humidity, moisture in the
pores restricts CO
2 penetration.
Rate of carbonation increases with increasing
permeability.

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Carbonation depth increased with reduced free
lime such as through use of pozzolanic ash as
partial cement replacement.
The amount of carbonation is significantly
increased in concrete with
a) a high water-to-cement ratio
b) low cement content
c) short curing period
d) low strength
e) highly permeable or porous paste.

Carbonation initiated deterioration of concrete

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Carbonation induced corrosion
Carbonation-induced corrosion often occurs on
areas of building facades that are exposed to
rainfall, shaded from sunlight, and have low
concrete cover over the reinforcing steel.

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Carbonation Test
A common and simple method to establish the
extent of carbonation is by spraying a freshly
broken concrete with a solution of
phenolphthalein in diluted alcohol indicator.
Uncarbonated area will turn into a bright
purple colour.
Carbonated areas of concrete will remain
unchanged in colour.
Concrete surface before carbonation test

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Phenolphthalein solution sprayed on concrete surface
Carbonated area remain colorless

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