We have learnt in last post about difference between different processes of fluid ingression into concrete. Diffusion is a process in which vapor or gases is transported through concrete due to differential concentration or concentration gradient but not due to pressure gradient.
Diffusion may permit liquid vapor and gas to transport into concrete mass. When we are concerned about gases, our headache is for
-carbon dioxide
-oxygen
Other gas can be diffused but maximum harm is done by those two gases.
When water is considered notable amount of aggressive ions like chlorides and sulfates can migrate by diffused into pore water. We will discuss about ion ingression into pore water in concrete elaborately in our next posts.
Carbon dioxide do the harm with carbonation of the hydrated cement paste and oxygen take part in the corrosion of steel embedded in reinforced concrete.
There have a relation between diffusivity coefficient and molar mass of gas; this is diffusivity coefficient is inversely proportional to square root of molar mass of respective gas. So using this relation, we can determine diffusivity coefficient of an unknown gas from a known value of another gas (determined by experiments); say carbon dioxide diffuses 1.17 times (theoretically) slower than oxygen due to difference in molar mass.
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