Construction World - Indian Edition | May 2009
Anti Carbonation Coating
BASF adopts test method for determining Carbon dioxide
diffusion rate and consequently the requirements and factors
to be considered for ‘Anti Carbonation Coating’.
C arbonation is a term widely used in beverage industry.
It refers to the impregnation of Carbon dioxide into a fluid,
common examples being soft drinks, beer, soda, etc. But, in
construction industry, carbonation is referred to as the process
of chemical weathering by which minerals like lime, soda,
and potassium and other basic oxides are changed to carbonate
by action of carbon dioxide and water.
The reaction of the process is:
CO2 + H2O ------> H2CO3
H2CO3 + CaO ------> CaCO3 + H2O
Traditionally concrete is reinforced with steel to increase
tensile strength. Concrete protects steel from corrosion by
forming a “passive layer” because of ‘very
high alkalinity induced by cement and other minerals. But
many concrete struct-ures are exposed to atmospheric CO2 emitted
from various sources. In presence of moisture/water and as
described in the above reaction, atmospheric CO2 diffuses
slowly through the concrete and the process of carbonation
is initiated. Due to this process, the pH of concrete slowly
turns acidic and destroys the passive layer protecting the
reinforcing steel bars. Once the passive layer is destroyed,
rusting of steel bars begins.
The following section describe the test method developed by
BASF along with certain standards to determine the carbon
dioxide transmission rate and the limit values for a coating
to be called ‘Anti carbonation coating’ Excerpt
/ Test Method for determining CO2 transmission rate The following
test method describes how to determine the CO2 transmission
The CO2 transmission rate or CO2 diffusion flux (i) is a measure
of the amount of CO2 diffusing in unit time through unit area
of a coating.
i = ----------- (g/m2.d )
A . DT
By Fick’s First Law, the carbon dioxide permeability
* is calculated using the CO2 transmission rate. It is the
rate of CO2 transmission through unit area of coating of unit
thickness induced by unit partial pressure difference between
two specific surfaces.
* = ----------- ( kg/m.h.Pa ) (1)
s = thickness of the sample
DP = p1 - p2, the partial pressure difference between the
The reciprocal of permeability is called the diffusion resistance.
The diffusion resistance number µ is the quotient of
the diffusion resistance of the sample and of air. This number
indicates how many times a coating is impermeable to carbon
dioxide than a static air layer under the same conditions.
µ = -------- = ------- (2)
1/ *L *
* = permeability of CO2 in the coating
*L = permeability of CO2 in air
The diffusion-equivalent air layer thickness sd [m] is the
thickness of a static air layer that possesses, under the
same conditions, the same CO2 permeability as the coating.
Thus diffusion-equivalent air layer thickness Sd is calculated
from the CO2 diffusion flux.
Factors to be considered
Requirements for water-based anti car-bonation coatings
Impermeable to carbon dioxide diffusion
Impermeable to water
Permeable to water-vapour
Hairline crack bridging
Good adhesion to substrate
Resistance to weathering
Easy to handle and work….water based system