Plain & Reinforced

Concrete-1
CE-314
Carbonation of Concrete
Prof. Dr. Zahid Ahmad Siddiqi
SYMBOLS USED IN
CEMENT CHEMISTRY
C = CaO (Calcium)
S = SiO
2
(Silicate)
A = Al
2
O
3
(Aluminate)
F = Fe
2
O
3
(Ferrite)
H = H
2
O (Water of Hydration, Hydrate)
Prof. Dr. Zahid Ahmad Siddiqi
MAIN COMPOUNDS IN PORTLAND
CEMENT
9.1 C
4
AF Tetracalcium
aluminoferrite
4.
10.8 C
3
A Tricalcium aluminate 3.
16.6 C
2
S Dicalcium silicate 2.
54.1 C
3
S Tricalcium silicate 1.
Typical
Percentages
Abbreviation Compound S.
No.
C
3
S with impurities is known as alite.
C
2
S with impurities is known as belite.
Prof. Dr. Zahid Ahmad Siddiqi
Out of these cement components, C
3
A is
undesirable. It does not contribute in
providing strength.
The C
3
A component of hardened paste can
be attacked by sulfates giving calcium
sulfoaluminate (called ettringite).
This material expands considerably
compared with the volume of the reactants
and may cause disintegration of the concrete.
But C
3
A is required in the manufacture of
cement as it helps in the combination of lime
and silica.
Prof. Dr. Zahid Ahmad Siddiqi
APPROXIMATE COMPOSITION OF
PORTLAND CEMENT
3.5 Others
2 1 - 3 SO
3
1 0.2 - 1.3 Alkalis (K
2
O and
Na
2
O)
1.5 0.1 - 4.0 MgO
3 0.5 - 6.0 Fe
2
O
3
6 3 - 8 Al
2
O
3
20 17 - 25 SiO
2
63 60 - 67 CaO
Typical Percentage Percentage Content
Range
Compound
Prof. Dr. Zahid Ahmad Siddiqi
HYDRATION
Hydration of C
3
S is much quicker than C
2
S.
2 C
3
S + 6H C
3
S
2
H
3
+ 3Ca(OH)
2
C
3
S
2
H
3
is named CSH compound and is a
microcrystalline hydrate.
Ca(OH)
2
is crystalline lime.
2 C
2
S + 4H C
3
S
2
H
3
+ Ca(OH)
2
C
3
A + 6H C
3
AH
6
(Tricalcium aluminate hydrate)
The above reaction is very quick and may lead to flash
set.
Prof. Dr. Zahid Ahmad Siddiqi
SULFATE ATTACK
Sulfate attack is caused by exposure of hardened
concrete to external sources of sodium, calcium or
magnesium sulfates usually coming from the
ground water.
These sulfates react with Ca(OH)
2
to produce
calcium sulfate (gypsum) and with hydrated C
3
A to
form ettringite.
Both of these products have a volume significantly
more than the reactants.
Magnesium sulfate is the most dangerous sulfate
as it causes more decomposition of cement
products and resulting magnesium silicate has no
binding properties.
Prof. Dr. Zahid Ahmad Siddiqi
When the concrete becomes disintegrated and
porous, water takes away the remaining Ca(OH)
2
to
the surface by evaporation.
The Ca(OH)
2
combines with CO
2
to form calcium
carbonate.
This formation of whitish powder on the surface is
called efflorescence.
The leaching of Ca(OH)
2
from the concrete mass
further increases the porosity of concrete
enhancing the rate of the attack.
In case of sulfate attack, efflorescence fist appears
near the edges and corners, which is then followed
by cracking and spalling of concrete.
Crystallization of other salts also causes
efflorescence.
Prof. Dr. Zahid Ahmad Siddiqi
Gypsum is added to the cement clinker in
order to prevent flash set. In the fresh
state of concrete, gypsum quickly reacts
with C
3
A to form ettringite. The
corresponding increase in volume is not
harmful as the concrete is not in solid state
at this stage. Hence, C
3
A content for
further sulfate attack is reduced.
Sulfate attack can be reduced by reducing
the permeability of concrete.
PREVENTION
Prof. Dr. Zahid Ahmad Siddiqi
A low water cement ratio used with full
compaction, either by the use of more
cement or by the use of plasticizers, is very
beneficial.
Special types of cements may be used to
reduce sulfate attack.
The concrete may be protected to come in
contact with ground water containing
sulfate salts. A concentration of sulfur
trioxide (SO
3
) equal to 1000 ppm is
moderately severe while 2000 ppm is very
severe. Further, magnesium sulfate is
more harmful.
Prof. Dr. Zahid Ahmad Siddiqi
The active silica present in aggregates may
react with the alkaline hydroxides derived
from alkalis of cement (Na
2
O and K
2
O) to
produce alkali-silica reaction (ASR).
This reaction produces alkali-silicate gel
that absorbs water and swells exerting
internal pressure on the surrounding
cement paste.
This swelling causes expansion, cracking,
pop-outs and spalling of the concrete.
Swelling of the aggregate particles is the
most harmful phenomenon.
ALKALI-AGGREGATE REACTION
Prof. Dr. Zahid Ahmad Siddiqi
Poorly crystallized silica (SiO
2
) dissolves and
dissociates at high pH (12.5 - 13.5) in alkaline
water.
The soluble dissociated silicic acid reacts in the
porewater with the calcium hydroxide (portlandite)
present in the cement paste to form an expansive
calcium silicate hydrate (CSH).
The size of silica particles determines the speed
of the reaction.
Fine particles (20 to 30 m) produce expansion
within four to eight weeks, whereas larger
particles may take years.
Generally, this effect is developed after about five
years.
Prof. Dr. Zahid Ahmad Siddiqi
The reactive forms of silica are opal
(amorphous), chalcedony (cryptocrystalline
fibrous) and tridymite (crystalline).
Porosity of the aggregate, permeability of
the cement paste, the quantity of the alkalis
in the cement and the availability of water
in the paste affect ASR.
The reaction is more pronounced in
permanently wet conditions and at
temperatures greater than 10 C with
optimum value at 38 C.
Prof. Dr. Zahid Ahmad Siddiqi
Mortar-bar test (ASTM C 227 - 90) is the
most common test to evaluate the quality
of aggregate with respect to ASR.
Aggregates in crushed fine state of a
prescribed grading and cement of
equivalent alkali content not less than 0.6
percent are used to make special cement-
sand mortar bars.
The bars are stored over water at 38 C.
High W/C ratio is used to accelerate the
reaction.
The aggregate is considered harmful if it
expands more than 0.05 percent after 3
months or more than 0.1 percent after 6
months.
Prof. Dr. Zahid Ahmad Siddiqi
A low alkali cement, addition of
pozzolanic materials and use of at least
30 percent of limestone coarse
aggregate is recommended if the use of
active aggregates cannot be avoided.
Prof. Dr. Zahid Ahmad Siddiqi
The conditions required for alkali silica
reaction are threefold:
(1) Aggregate containing an alkali-reactive
constituent (amorphous silica),
(2) Sufficient availability of hydroxyl ions (OH),
and
(3) Sufficient moisture, above 75 % relative
humidity (RH) within the concrete.
This phenomenon is sometimes popularly
referred to as "concrete cancer".
This reaction occurs independently of the
presence of rebars: massive concrete
structures such as dams can be affected.
Prof. Dr. Zahid Ahmad Siddiqi
CARBONATION OF CONCRETE
Carbonation, or neutralisation, is a
chemical reaction between carbon dioxide
in the air with calcium hydroxide and
hydrated calcium silicate (C
3
S
2
H
3
or CSH
compound) in the concrete.
As a result of this reaction, calcium
carbonate (CaCO
3
) is formed in the
concrete.
The creation of calcium carbonate requires
three equally important substances: carbon
dioxide (CO
2
), calcium phases (Ca), and
water (H
2
O).
Prof. Dr. Zahid Ahmad Siddiqi
Carbon dioxide (CO
2
) is present in the
surrounding air, calcium phases (mainly Ca(OH)
2
and CSH) are present in the con-crete, and water
(H
2
O) is present in the pores of the concrete.
Cement paste contains 25-50 percentage by
weight calcium hydroxide (Ca(OH)
2
) along with
some potassium hydroxide and sodium hydroxide,
which mean that the pH of the fresh cement paste
is at least 12.5 to 13.5.
As a result of carbonation, the pH value of pore
water in the hardened cement paste is reduced to
around 9.0.
When all the Ca(OH)
2
has become carbonated,
the pH value will reduce up to about 8.3 and up to
7.0 in extreme cases.
Prof. Dr. Zahid Ahmad Siddiqi
Carbonic acid is a solution that is formed
when atmospheric carbon dioxide dissolves
in water (rainwater is essentially carbonic
acid).
When the calcium compounds react with
the carbonic acid, the calcium of each of
them forms calcium carbonate, the
combined water is released as "free water,"
and the rest of the compounds become
silica and alumina gels.
The exception is the sulfoaluminates, which
decompose to form calcium sulfate
dihydrate (gypsum) and calcium
carboaluminate hydrates.
Prof. Dr. Zahid Ahmad Siddiqi
The first reaction is in the pores where
carbon di-oxide (CO
2
) from the atmosphere
and water (H
2
O) react to form carbonic
acid (H
2
CO
3
):
CO
2
+ H
2
O H
2
CO
3
The carbonic acid then reacts with the
calcium phases:
H
2
CO
3
+ Ca(OH)
2
CaCO
3
+ 2H
2
O
The above two reactions can be combined
in one equation for simplicity as follows:
Ca(OH)
2
+ CO
2
CaCO
3
+ H
2
O
Prof. Dr. Zahid Ahmad Siddiqi
Once the Ca(OH)
2
has converted and is
missing from the cement paste, hydrated
CSH (Calcium Silicate Hydrate -
CaOSiO
2
H
2
O) will liberate CaO which will
then also carbonate:
H
2
CO
3
+ CaO CaCO
3
+ H
2
O
When these reactions take place the pH
value will start falling.
Prof. Dr. Zahid Ahmad Siddiqi
The carbonation process requires the
presence of water because CO
2
dissolves
in water forming H
2
CO
3
.
If the concrete is too dry (RH <40%) CO
2
cannot dissolve and no carbonation occurs.
If on the other hand it is too wet (RH >90%)
CO
2
cannot enter the concrete and the
concrete will not carbonate.
Optimal conditions for carbonation occur at
a RH of 50% (range 40-90%).
It is interesting to know that if pore is filled
with water the diffusion of CO
2
is very slow.
Prof. Dr. Zahid Ahmad Siddiqi
Whatever CO
2
is diffused into the concrete, is
readily formed into dilute carbonic acid
reduces the alkalinity.
On the other hand if the pores are rather dry,
that is at low relative humidity the CO
2
remains
in gaseous form and does not react with
hydrated cement.
The moisture penetration from external source
is necessary to carbonate the concrete.
The concentration of CO
2
in rural air may be
about 0.03 per cent by volume.
In large cities the content may go up to 0.3 per
cent or exceptionally it may go up to even 1.0
per cent.
In the tunnel, if not well ventilated the intensity
may be much higher.
Prof. Dr. Zahid Ahmad Siddiqi
The carbonation starts at the concrete
surface and slowly penetrates inside.
If the concrete is cracked due to any other
reason, CO
2
quickly reaches deep inside and
carbonation rate increases.
This can occur so fast that in minutes the
paste at the surface is very thinly, but
completely, carbonated.
The water released by the chemical reactions
continues both the formation of carbonic acid
and the carbonation process.
Prof. Dr. Zahid Ahmad Siddiqi
Fully carbonated paste in the concrete surface. Carbonated
paste appears orange-brown in crossed polarized light,
Taken from web.
Prof. Dr. Zahid Ahmad Siddiqi
Carbonated paste along cracks inside a concrete.
The cracks are formed due to alkali silica reaction,
Taken from web, Taken from web.
Prof. Dr. Zahid Ahmad Siddiqi
Weak carbonation of paste at the rim of large
connected voids in zero slump concrete.
Prof. Dr. Zahid Ahmad Siddiqi
A common and simple method for
establishing the extent of carbonation is to
treat the freshly broken surface of concrete
with a solution of phenolphthalein in diluted
alcohol.
Phenolphthalein is a white or pale yellow
crystalline material.
For use as an indicator it is dissolved in a
suitable solvent such as isopropyl alcohol in a
1% solution.
If the Ca(OH)
2
is unaffected by CO
2
the color
turns out to be pink.
Measurement Of Depth Of Carbonation
Prof. Dr. Zahid Ahmad Siddiqi
If the concrete is carbonated it will remain
uncolored. It should be noted that the pink
color indicates that enough Ca(OH)
2
is
present but it may have been carbonated to
a lesser extent.
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
significant carbonation.
A strong, immediate, color change to purple
suggests a pH that is rather higher, perhaps
pH 9 or 10.
Prof. Dr. Zahid Ahmad Siddiqi
In confirmation of this, microscopy -
either optical microscopy using thin-
sections, or scanning electron
microscopy using polished sections -
shows carbonation effects at greater
depths than indicated by
phenolphthalein indicator.
Nevertheless, this test is very useful as
a means of making an initial
assessment - it is quick, easy and
widely used.
Prof. Dr. Zahid Ahmad Siddiqi
Phenolphthalein indicator solution applied to a
fresh fracture surface through a concrete slab, Taken
from web.
Prof. Dr. Zahid Ahmad Siddiqi
The indicator has not changed color
near the top and bottom surfaces,
suggesting that these near-surface
regions are carbonated to a depth of at
least 4 mm from the top surface and 6
mm from the lower surface.
Where the indicator has turned purple -
the centre of the slab - the pH of the
concrete pore fluid remains high (above
8.6, probably nearer 10).
Prof. Dr. Zahid Ahmad Siddiqi
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.
Bi-carbonation
Prof. Dr. Zahid Ahmad Siddiqi
"Pop-corn" like calcite crystals present in
carbonated paste. The concrete is suffering from
bi-carbonation Taken from web.
Prof. Dr. Zahid Ahmad Siddiqi
Negative Effects Of Carbonation
Very early carbonation, when concrete is
still plastic or semi-plastic (before setting),
results in a "carbonation shell" around the
cement particles that "seals" the particles
and keeps them from hydrating.
This carbonation results in dusty and weak
surfaces and can occur when unvented
heaters create high carbon dioxide (and
carbon monoxide) environments.
Prof. Dr. Zahid Ahmad Siddiqi
The carbonation process also causes bound
chlorides to be released, which produces a
higher concentration of soluble chloride
immediately in front of the carbonation zone.
This may cause chloride attack on steel
reinforcement.
Prof. Dr. Zahid Ahmad Siddiqi
Craze cracking at concrete surfaces is enhanced
(particularly at high water-cement ratios) because
of the addition of carbonation shrinkage with the
normal drying shrinkage.
The conversion of Ca(OH)
2
into CaCO
3
by the
action of CO
2
results in a small shrinkage.
Since the permeability of concrete is governed by
the water/cement ratio and the effectiveness of
curing, concrete with a high water/cement ratio
and with inadequately curing will be more prone to
carbonation.
The carbonation shrinkage close to 50 % humidity
can be as high as the drying shrinkage ( =
0.0009).
Prof. Dr. Zahid Ahmad Siddiqi
If the carbonation front reaches embedded
steel, the steel can corrode. Good concrete
design and construction requires steel to be
located deeply enough that the carbonation
front will not reach it during a structure's
expected lifetime.
More importantly, it reduces the alkalinity of
the concrete which leads to the corrosion of
the reinforcing steel.
The increased volume of the resulting
corroded steel results in internal stresses,
spalling and delamination, and the ultimate
reduction of the structure's capacity.
Prof. Dr. Zahid Ahmad Siddiqi
Of course, oxygen and moisture are the other
components required for corrosion of embedded
steel.
Steel undergoes oxidation or rusting in acidic
environments.
The rusting is prevented in basic environments
and the phenomenon is called passivation of
steel.
The steel remains passive for the pH values
above 9.5.
Concrete cover to the reinforcement protects the
steel from corrosion by not allowing the
carbonation front to reach the reinforcement.
Prof. Dr. Zahid Ahmad Siddiqi
Positive Effects Of Carbonation
Normal carbonation results in a decrease of the
porosity making the carbonated paste stronger.
Carbonation usually strengthens concrete surfaces,
increases wear resistance, modulus of elasticity,
surface hardness and resistance to frost and sulfate
attack.
Further, it makes the concrete less permeable.
As an exception, sometimes carbonation may
actually decrease the surface strength in concrete
with a very high water-cement ratio because the
carbonation can result in segregation of the calcium
carbonate and other carbonation products (silica
and alumina gels)--a phenomenon known as
"bicarbonation."
Prof. Dr. Zahid Ahmad Siddiqi
A useful result of carbonation is the
progressive, but self-limiting, carbonation
phenomenon.
This may allow carbonation to be used to
identify the time of cracking (such as
cracking caused by earthquakes) by
comparing the depth of carbonation
cracks to the depth at the surface.
Estimating the time of cracking from
carbonation depth, however, should be
left to experts since there are many
factors to consider and erroneous
opinions may be expected.
Prof. Dr. Zahid Ahmad Siddiqi
Rate of Carbonation
The rate of carbonation depends on the
following factors:
1. The level of pore water i.e., relative humidity.
2. Grade of concrete
3. Permeability/porosity of concrete
4. Protection to the concrete
5. Depth of cover
6. Age of concrete
Prof. Dr. Zahid Ahmad Siddiqi
The highest rate of carbonation occurs at a
relative humidity of between 50 and 70 percent.
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 again 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.
The carbonation process has an ongoing need for
carbon dioxide (CO
2
) from the atmosphere.
Prof. Dr. Zahid Ahmad Siddiqi
For car-bonation to spread, fresh carbon
dioxide from the surface needs to be supplied
continuously deeper and deeper into the
concrete.
Low porosity and per-meability will decrease
the ingress speed of carbon dioxide (CO
2
)
from the atmosphere, thereby delaying the
ingress of the carbonation.
Depth of cover plays an important role in
protecting the steel from carbonation.
Prof. Dr. Zahid Ahmad Siddiqi
Within a few hours, or a day or two at
most, the surface of fresh concrete will
have reacted with CO
2
from the air.
Gradually, the process penetrates deeper
into the concrete at a rate proportional to
the square root of time.
After a year or so it may typically have
reached a depth of perhaps 1 mm for
dense concrete of low permeability made
with a low water/cement ratio, or up to 5
mm or more for more porous and
permeable concrete made using a high
water/cement ratio.
Prof. Dr. Zahid Ahmad Siddiqi
Cracks in the concrete allow easy access of
carbon dioxide through the concrete cover
and greater rate of carbonation is obtained.
The active coefficient of carbon dioxide
diffusion in a concrete crack 0.2 mm wide is
about 1000 times higher than in average-
quality crack-free concrete.
Prof. Dr. Zahid Ahmad Siddiqi
The rate of carbonation is mainly influenced
by the permeability and the calcium content
of the concrete as well as the ambient
atmospheric conditions: amount of carbon
dioxide, relative humidity, and temperature.
Concrete carbonates more rapidly in a hot
climate than in a moderate climate.
Prof. Dr. Zahid Ahmad Siddiqi
Taken from web
Prof. Dr. Zahid Ahmad Siddiqi
Concluded