M. Tech.

in Civil (Structural) Engineering II Semester 2022-23

Durability Assessment and Structural Strengthening of Reinforced Concrete [DASSRC], CE- 22302

By

Dr. L. K. Mishra
Professor, CED
Department of Civil Engineering
Motilal Nehru National Institute of Technology Allahabad, Prayagraj

Carbonation in Concrete

 Carbonation of concrete is another commonly observed environmentally induced

chemical phenomenon in tropical environment.
 Concrete exposed to ambient environment interacts with air that contains CO2.
 The ambient content of CO2 in pollution free environment is about 0.03% by volume
and can be as high as 1.0 % due to heavy vehicular and industrial pollution.
 The environmental carbon di - oxide reacts with hydrated cement component of
concrete and produces calcium carbonate. In addition to this, the C-S-H phase of
hydrated cement paste is also subjected to carbonation and gets decomposed into
calcium carbonate and an amorphous silica gel with porous structure.
 Depending upon the source of carbon di-oxide, (whether atmospheric or dissolved in
water), different effects on concrete are observed. In conventional concrete
terminology the formation of carbonates due to ingress of atmospheric CO 2 is defined
as carbonation.
 Infact all the alkali hydroxides including NaOH, KOH and Mg(OH) 2 are converted in
to carbonates due the action of CO2. However, this conversion depends upon
concentration of diffusing CO2.

Chemistry of Carbonation

 The atmospheric CO2 dissolves in water available in the capillaries and pores of the
hardened concrete and forms carbonic acid.
 This carbonic acid reacts primarily with Ca(OH)2 and calcium silicate hydrates to
form calcium carbonate.
 Carbonates of magnesium, sodium and potassium are also formed due to higher
concentration of CO2. Set of reactions associated with the formation of carbonates of
magnesium and alkali elements represent secondary carbonation.
Primary Carbonation

CO2 + H2O H2CO3

H2CO3 + Ca(OH) 2 CaCO3 + H2O

H2CO3 + CaO. SiO2 . H 2O CaCO3 + SiO2 + 2 H2O

Secondary Carbonation

H2CO3 + Mg(OH) 2 / NaOH / KOH MgCO3 / Na2CO3 / K2CO3 + H2O

 It is estimated that the above reactions will result in about 12.0 % increase in volume.
This moderate increase in volume would densify the hardened concrete and reduce
permeability of concrete.
 Calcium carbonate so produced may occur in three modifications viz. calcite, vaterite
and aragonite. Accordingly, carbonation results in significant changes in the phase
composition and pore structure. Carbonation is generally accomplished with reduction
in capillarity porosity and reduction in permeability of cover concrete
 Carbonation has favourable effect on reduction of permeability for OPC, however,
slag cement showed drastic increase.
 The process of carbonation leads to extensive decomposition of hydration products of
cement including the chloride bearing hydration products.
 This ultimately leads to reduction in the chloride binding capacity of the calcium –
silicate hydrate phase.
 Consequently the process of carbonation releases formerly immobilized chloride ions
and increase their concentration in the concrete pore water. This accelerates the
chloride induced corrosion of embedded steel reinforcement in the concrete resulting
in eventual loss in the functional life of the structural element.
 The relative humidity of the range 50-70% is suitable for carbonation reaction in
concrete. However, optimum RH depends upon local climate as different optimum
values have been proposed by various researchers.
 Extent of carbonation in a concrete matrix depends upon concrete mix, methods of
production including curing and exposure conditions.
  The alarming effect of carbonation on concrete is to reduce pH value of the concrete.
The pH value of Ca(OH)2 in pore water is 12.5.
o Alkali metal hydroxide make it nearer to 13.5. Saturated solution of Ca(OH)2
has pH value of 9.4
o Hence carbonation results in lowering of pH value below 9.4 due to depletion
of Ca(OH)2 in pore solution.
o This reduction in pH value of concrete pore solution ultimately results in loss
of the protective layer of oxides surrounding the reinforcing steel and
protecting it from associated corrosion due to permeating oxygen.
o This represents a stage of initiation of reinforcement corrosion in RC elements
which would govern the life of the structure.
o Much more reduction in pH value is observed in severe carbonation conditions
due to formation of secondary carbonates.
Carbonation Assessment

The extent of carbonation can be identified using standard indicator solutions for pH
measurement. The indicators solutions for pH measurement in a carbonated concrete are
given in the following Table .

Indicators for Assessing Carbonation ( Bayani, 1972)

Indicator Solution pH ( Range) Colour Change

Nitrazine Yellow 6.6 ( 6.4 - 6.8) Yellow  blue
Phenol Red 7.3 ( 6.4 – 8.2) Yellow  red
Diphenol Purple 7.8 ( 7.0-8.6) Yellow  violet
Cresol Red 7.9 ( 7.0 – 8.8) Yellow  violet/red
α-Naphtholphthalein 8.0 ( 7.3 – 8.7) Yellow  blue
m-cresol purple 8.2 ( 7.4 – 9.0) Yellow  violet
Phenolphthalein 9.0 (8.2 – 9.8) Colourless  magneta
Thymolphthalein 9.9 ( 9.3-10.5) Colourless  blue*
Brilliant Orange 11.3 ( 10.5- 12.0) Yellow  red*
Tropaeolin O 11.9 ( 11.1- 12.7) Yellow  red*
Titan Yellow 12.5 ( 12.0 -13.0) Yellow  red
(*) Not readily visible on powder concrete.
  These indicators show identifiable colour change with change in pH value as
given.
 The indicators can be directly sprayed on the fractured concrete surface.
 The laboratory investigation involves preparation of clear water extract prepared
by crushing a representative concrete specimen, grounding it to powder (size <
0.20mm) and mixing 50gm of it with 200ml of distilled water and then filtering
it. Indicators are then applied to this clear water extract to measure pH value.

Rate of carbonation
Rate of carbonation is controlled by the ingress of CO2 into the concrete pore
system by diffusion. Simplified expression for depth of carbonation can be found
using Fick’s First law of diffusion :
 Depth of carbonation at time t (in min.) =C√ t

Where C = Constant depending upon diffusion coefficient of carbon dioxide

through Concrete carbonated.

The above expression is derived on the basis of following simplifying assumptions:

(i) Diffusion coefficient of CO2 through carbonated concrete is assumed constant

irrespective of the location, structure of the hydrated cement paste, curing effects,
moisture content over the carbonated space, cement composition, degree of hydration
and temperature. However, carbonation of concrete is a diffusion controlled process
and diffusion coefficient for CO2 through concrete and carbonated concrete represents
the controlling factor and reliability in carbonation related prediction.
(ii) The amount of CO2 (in g/m2) required for carbonation of alkaline matter contained in
unit volume of cementitious material is constant. However, this depends on type of
cement, the mix proportion and pozzolanic additions. Also, this amount depends upon
concentration of CO2 since below certain concentration of CO2, carbonation of non-
calcareous constituent of hydrated cement paste does not take place. These deviations
from the basic assumptions led many researchers to recommend modified equations to
model the process of carbonation in concrete.

The following expression has been proposed by Schieβl (Jorg Kropp, 1995) for the progress
of carbonation in hydrated cement concrete taking into account effect of varying moisture
content on diffusion coefficient of CO2 along with associated diffusion of alkalis from interior
section towards the carbonation front. This also attempts to quantify the ultimate carbonation
depth to assess the degree of carbonation.

t = a/b [x – xoo ln (1-x/xoo)]

Where t = time in seconds

a = the amount of CO2 (in g/m2) required for carbonation of

alkaline matter contained in unit volume of cementitious
material

b = factor describing change in diffusion coefficient of CO2 and

CO2 demand of associated diffusion of alkalis from interior

x = depth of carbonation in mm

xoo = final depth of carbonation in mm

The depth of carbonation decreases with increase in characteristic compressive strength of

concrete.

However, the relation is dependent on the type of cement. The concrete made with GGBFS
normally shows greater depth of carbonation compared to other types of cements for a given
characteristic compressive strength.

It has also been observed that for a given grade of concrete, the depth of carbonation reduces
with increase in curing duration.
The cover provided to steel reinforcement in concrete should be in accordance with the final
depth of carbonation.

 Pore structure of cover concrete governs the extent of carbonation in concrete. As the
hydration of cement progresses only in a saturated state of hydrating cement system,
the hydration process tends to cease after termination of curing. Therefore, the
compressive strength of concrete at the end of curing represents structure of the cover
concrete even at higher ages. Although there are evidences of increase in compressive
strength of concrete after the curing is terminated but they are incidental and are not
related to refinement in the pore structure.

Accordingly, carbonation depth is better correlated with the compressive strength of concrete
at the end of the curing and relationships have been derived for this consideration.

Effect of Carbonation on Properties of Concrete

 Carbonation has mixed effect on properties of concrete.

 The environmentally induced microcracks get filled with products of carbonation and
therefore permeability of concrete gets reduced.
 The resistance to ingress of aggressive agent either in solution form or in gaseous
form is increased, hence the sulphate resistance of concrete increases.
 However, due to reduction in pH value of the pore solution below 9.5, and reduction
in chloride binding capacity of products of hydration, the susceptibility of
reinforcement to get corroded increases.
 The carbonation in concrete is not a critical consideration in reference to sulphate
attack.

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