Concern of the maximum water content

[WEI LI]

Hi experts,

I know the maximum water content is determined by the porosity of the material in WUFI. MWC=1000*porosity (kg/m^3)
However, I am not sure if this is an assumption in the numerical calculation or if it has physical meaning?
Take PIR Board as an example, the free saturation WC is 2.15 kg/m^3, but the porosity is 0.99, hence the MWC is 999kg/m^3.
This is a huge gap.
BTW, also not sure if the 0.99 porosity is an assumption or is a truth? 0.99 means the material is almost totally made out of air?

Thanks for considering my questions.
Wei

[Thomas]

I know the maximum water content is determined by the porosity of the material in WUFI. MWC=1000*porosity (kg/m^3)
However, I am not sure if this is an assumption in the numerical calculation or if it has physical meaning?

Hi WEI LI,
the maximum water content has a clear and definite physical meaning: it is the water content when the pore spaces of the material are completely filled with water.

Free saturation, on the other hand, is defined (for porous hygroscopic materials) as the water content which is reached when the material is allowed to take up liquid water by capillary absorption (imbibition experiment, water absorption experiment). During this process air bubbles are usually trapped in the pore spaces so that free saturation is usually less than maximum saturation. In general, free saturation is well reproducible for a given material, so it can be used as a material parameter which determines how much water the material can take up when exposed to liquid water (in WUFI: exposed to rain).

Take PIR Board as an example, the free saturation WC is 2.15 kg/m^3, but the porosity is 0.99, hence the MWC is 999kg/m^3.
This is a huge gap.

Yes.

Apparently this material refuses to take up much water voluntarily by capillary forces, hence the low free saturation.

BTW, also not sure if the 0.99 porosity is an assumption or is a truth? 0.99 means the material is almost totally made out of air?

In the case of this PIR dataset it is probably just an estimate, but it cannot be far from the truth. The PIR board is a polymeric foam, so it is indeed mostly air or some filling gas.

Kind regards,
Thomas