Construction World - Indian Edition | May 2009

Technology

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 rate.
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.
DPm
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.
i.s
* = ----------- ( kg/m.h.Pa ) (1)
DP
where
s = thickness of the sample
DP = p1 - p2, the partial pressure difference between the two surfaces
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.
1/* *L
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

 




 

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