R E P O RT O N

DR .AYUB ELAHI
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DEPARTMENT OF CIVIL
ENGINEERING

C A R BO N AT I O N O F C O N C R E T E
WHAT IS CONCRETE?
It is the mixture of cement, coarse aggregate and fine aggregate along with sand that can be mounded to
any shape while it is in fresh state. the properties of the concrete is influenced by many factors and results
in failure like segregation,bleeding,freeze and thaw action etc.carbontion is also one of these effects that
cause a very dreadful damage to its strength and life.

CARBONATION OF CONCRETE

INTRODUCTION
Generally concrete is prepared on the assumption that it does not react with air in the atmosphere but this
was really not the case.
In reality the air contains CO2, which in the presence of moisture reacts with hydrated cement, the actual
agent is carbonic acid because gaseous CO 2 is not reactive.

WHAT IS CARBONATION
Its a reaction between the lime in concrete and the carbon dioxide from air, yielding Calcium Carbonate.

DESCRIPTION

The action of CO2 takes even at small concentrations such as are present in rural air where the CO 2
content is about 0.03% by volume. it may rise up to 0.1% in an unventilated labortary;in large cities it is
on 0.3%.the rate of carbonation of concrete increases with an increase in the concentrations of
CO2,espacially at high water/cement ratios.
Of the hydrates in the cement paste ,the one which reacts with CO 2 most readily is Ca(Oh)2.CACO3 will
be the product of reaction, but the hydrates are also decomposed and hydrated silica, alumina and ferric
oxide being produced. When Ca (Oh)2 becomes depleted ,for instance by secondary reaction with
pozzolanic silica, the carbonation of calcium silicate hydrate C-S-H is also possible.

The process of carbonation includes;

1. Diffusion of CO2 in the gaseous phase into the concrete pores,
2. Its dissolution in the aqueous film of these pores,
3. The dissolution of solid Ca(OH)2in the water of the pores,
4. The diffusion of dissolved Ca(OH)2in pore water and its reaction with the dissolved CO2,
Final result of several steps through which the calcium carbonate is formed may simply be described by
the following reaction which is assumed to be irreversible
Ca2+(aq) + 2(OH)(aq) + CO2(aq) CaCO3(s) + H2O
However, it is now accepted [4] that the other hydration products (calcium silicate hydrate or CSH) and
even the residual unhydrated cement compounds which are present in all concrete (namely tricalcium
silicate (C3S) and dicalcium silicate (C2S) also react with CO 2 as shown in the following reactions:
CSH + 3CO2 (3CaCO3 2SiO2 3H20)
C3S + 3CO2 + H20 SiO2 H20 + 3CaC03
C2S + 2CO2 + H20 SiO2 H20 + 2CaC03

HOW TO RECOGNIZE CARBONATION?

Carbonation may be recognized in the field by the presence of a discoloured zone in the surface of the
concrete. The colour may vary from light gray and difficult to recognize to strong orange and easy to
recognize. Carbonation can be visualized by using phenolphthalein.

In the optical microscope carbonation is recognized by the presence of calcite crystals and the absence of
calcium hydroxide, ettringite and un-hydrated cement grains. Porosity is unchanged or lower in the
carbonated zone.

FACTORS AFFECTING CONCRETE CARBONATION

The rate of carbonation depends on porosity (for CO2to Diffuse) & moisture content of the concrete (for
dissolution of solid Ca (OH)2). The diffusivity of CO2 depends upon the pore system of hardened
concrete and the exposure condition. The pore system of concrete depends upon the type and the content
of binder, water/binder ratio, and the degree of hydration. Thus, the main factors affecting concrete
carbonation are:
Pore system of Hardened Concrete which in turn depends upon w/c ratio, type of binder, and
1.
degree of hydration,
Relative humidity (for dissolution of Ca(OH) 2),
2.
3.

The concentration of CO2.

Optimal conditions for carbonation occur at a RH of 50% (range 4090%).
o
o

If RH <40%, CO2 cannot dissolve,

If RH >90%, diffusion of carbon dioxide will be inhibited by the water that has filled the
pores and hence CO2 cannot enter the concrete.

The most dangerous range of relative humidity for carbonation is 40% to 80%, since the
carbonation reaction calls for the presence of water, while under higher atmospheric
humidity the diffusion of carbon dioxide will be inhibited by the water that has filled the
pores.

RATE OF CARBONATION
Carbonation occurs progressively from the outside of concrete exposed to co 2,but does at decreasing rate.
the rate is slow because co2 has to diffuse through the pore system, including the already carbonated
surface zone of concrete. the diffusion will be much faster in case of water as co2 can diffuse4 times
faster in it in magnitude as compared to air. this shows that the rate of carbonation depends on moisture
content of concrete which varies with the distance from its surface.
The highest rate of carbonation occurs at a relative humidity of between 50 and 70 percent.
In the presence of steady hygrometric conditions, the depth of carbonation increases in proportion to the
square root of time, thus the following relation can be derived;

D=kt0.5
Where k=carbonation coefficient in mm/year 0.5,
T=time of exposure in years.

The value of k is often more than 3 or 4mm/year0.5 for low strength concrete. so we can say that
concrete having water/cement ratio of 0.60, a depth of 15mm would be reached after 15 years. but this
expression holds only when the exposure conditions are steady.
If the surface of concrete is exposed to a variable humidity, with periodic wetting, the rate of carbonation
will be slowed down because of a slowing down of diffusion of co2 through saturated pores in the
hardened cement paste. Sheltered parts of a structure undergo carbonation at a faster rate than those
exposed to rain. Small variation in temperature has little effect but a high temperature increases the rate of
carbonation unless drying overshadows the temperature effect.
The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that
stronger concrete is much denser with lower W/C ratio. It indicates that the permeability of the concrete,
particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO 2 does not take
place faster, as in the case of more permeable concrete with higher W/C ratio. Depth of cover plays an
important role in protecting the steel from carbonation.

BI-CARBONATION - WHAT IS THAT?

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

EFFECTS OF CARBONATION:
Adverse effects
Carbonation per does not cause deterioration of concrete but it has important effects. Carbonation
shrinkage is the one. with reference to durability,the importance of carbonation lies in the fact that it
reduces the ph of pore water in hardened Portland cement paste from between 12.6 to 13.5 to a value
about 9.this reducing in ph causes to remove a protective layer formed around steel bars known as
passivity layer, thus resulting in the corrosion and rusting of steel bars as a result of interaction between
oxygen and water.
Carbonation has also very adverse effect on the degree of concrete alkalinity. Carbonation increases
shrinkage of concrete that is fully matured, which can cause additional cracking

 Positive impacts
Because caco2, occupies a greater volume than ca (0h) 2 which it replaces, the porosity of carbonated
concrete is reduced. Also water released by ca (oh)2 on carbonation may aid the hydration of hitherto
unhydrated cement. These changes are beneficial and they result in increased surface hardness, increased
strength at the surface, reduced surface permeability, reduced moisture movement and increased
resistance to those forms of attacks which are controlled by permiability.it also accelerates chlorideinduced corrosion of reinforcement.
CARBONATION AND CONCRETE DURABILITY

Understanding Corrosion
Corrosion is an electrochemical process involving the flow of charges (electrons and ions). At active sites
on the bar, called anodes, iron atoms lose electrons and move into the surrounding concrete as ferrous
ions. This process is called a half-cell oxidation reaction, or the anodic reaction, and is represented as:
(Figure 1.2).

2Fe -> 2Fe2++ 4e- (Eq. 1.3)

2H2O + O2 + 4e- -> 4OH- (Eq. 1.4)
2Fe2++ 4OH- -> 2Fe(OH)2 (Eq. 3.5)
This initial precipitated hydroxide tends to react further with oxygen to form higher oxides. The increases
in volume as the reaction products react further with dissolved oxygen leads to internal stress within the
concrete that may be sufficient to cause cracking and swelling of the concrete cover.

Once this layer is destroyed rusting of iron bars and subsequent expansion of the concrete takes place and
durability of concrete decreases. Hence Carbonation is harmful for reinforced concrete .
Methods to Measure Carbonation
Extent of Carbonation is measured in two ways:
First way is to measure the concentration of CO 2 absorbed by the concrete specimen.
Second way is to carbonate the specimen in (a) natural or (b) laboratory environment conditions
and then break it and spray a pH indicator to know the extent of Carbonation.

1) IR Spectrum Analysis

The carbonation set-up consists of a close loop in which a mixture of air and carbon-dioxide could be
introduced at a certain RH. Due to the carbonation reaction, an amount of CO 2molecules will be
immobilized reducing the concentration of carbon dioxide in the circulating gas mixture. The

CO2 concentration in the gas is measured using an IR absorption device. A pump is used to circulate the
gas while also temperature and RH are measured .

2) Using pH Indicators
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 70% CO 2, 50% Relative
Humidity, and 20-22C are maintained in a carbonation chamber, fig. 1.4(a). Then sample is broken and is
sprayed with a pH indicator. Popularly a standard solution of 1% phenolphthalein in 70% ethyl alcohol is
used. In the noncarbonated region with pH values above 9.2, the phenolphthalein indicator turns purplered; and in the carbonated portion with pH less than 9.2, the solution remained colorless.

PREVENTION
To avoid this unfortunate situation, an indirect portable heating unit(s) such as Ameritech Herman
Nelson's BT400 Series can be used to heat the curing area. The combustion exhaust can be vented to the
outside while the heated air exhaust can cure the concrete. The cost of fuel will increase somewhat, but it
will be infinitesimal compared with the cost of repairing a carbonated surface.

REFERENCES
1.
2.
3.
4.
5.
6.

www.wikipedia.org
http://www.nbmcw.com
http://www.concrete-experts.com
www.understanding-cement.com
www.concrete.org
poc fourth edition by A.M. Neville