Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post Reply
JMarsh4087
WUFI User
WUFI User
Posts: 7
Joined: Tue Nov 26, 2019 4:49 am -1100

Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by JMarsh4087 » Thu Dec 19, 2019 5:19 am -1100

In this article https://www.buildingscience.com/sites/d ... trofit.pdf a metric called "critical saturation" is referred to. This is the amount of material saturation that will result in structural damage of the material.
Is there any reference out there that contains these metrics for typical materials for cross-reference with WUFI's water content and temperature analysis?

Thomas
WUFI Administrator
WUFI Administrator
Posts: 287
Joined: Sun Jun 19, 2005 10:33 pm -1100

Re: Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by Thomas » Thu Dec 19, 2019 7:58 am -1100

Dear JMarsh4087,

unfortunately, there are not many materials for which data on frost-resistance are available.

Please see the help file topic "Appendix" > "Assessment Criteria" (or the respective forum post) for a short discussion of currently available guidelines.

We are involved in a project aiming at providing more frost resistance data for a variety of materials, because these are indeed important assessment criteria for WUFI results, but at the moment the project is only in its starting phase...

Kind regards,
Thomas

JMarsh4087
WUFI User
WUFI User
Posts: 7
Joined: Tue Nov 26, 2019 4:49 am -1100

Re: Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by JMarsh4087 » Thu Dec 19, 2019 9:00 am -1100

That's really unfortunate because it's the main thing we want to analyze on our current project. In your link to the FAQ, it says
"Frost-resistant materials usually can endure freezing water contents up to at least free saturation without suffering frost damage."
What is "free saturation", and where can I understand which materials are "frost-resistant"?

"WTA Guideline on interior insulation recommends that frost-susceptible materials should not exceed a saturation degree of 30%"
Does that mean that a frost-susceptible material should not exceed 30% RH?

Thomas
WUFI Administrator
WUFI Administrator
Posts: 287
Joined: Sun Jun 19, 2005 10:33 pm -1100

Re: Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by Thomas » Fri Dec 20, 2019 3:56 am -1100

"WTA Guideline on interior insulation recommends that frost-susceptible materials should not exceed a saturation degree of 30%"
Does that mean that a frost-susceptible material should not exceed 30% RH?
The explanation is in the parentheses following that sentence:
"...should not exceed a saturation degree of 30% (i.e. 30% of the maximum water content wmax). "

The maximum water content corresponds to the situation where all pores accessible to water are completely filled with water, with no enclosed air bubbles.
"Frost-resistant materials usually can endure freezing water contents up to at least free saturation without suffering frost damage."
What is "free saturation", and where can I understand which materials are "frost-resistant"?
Free saturation is the water content resulting from the capillary absorption of water. Put a specimen into contact with water and let it take up water until the capillary absorption stops. The result is the free saturation of this material. It is usually less than the maximum water content because during the capillary absorption a certain amount of air bubbles remains trapped in blind pores etc.

I would understand the above sentence to say: If you know from experience that a material is frost-resistant, then free saturation can be used as a criterion to assess the frost damage risk in a given exposure situation, based on the water contents resulting from a WUFI calculation.

Regards,
Thomas

JMarsh4087
WUFI User
WUFI User
Posts: 7
Joined: Tue Nov 26, 2019 4:49 am -1100

Re: Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by JMarsh4087 » Fri Dec 20, 2019 6:05 am -1100

So RH% does not equal wmax?
And wmax requires a laboratory test to find; there's no reference information?

Now assume I somehow know wmax is 85%, if WUFI says a region of my material is 100% RH and under freezing temperature, that could cause damage because wmax has been exceeded?

Thomas
WUFI Administrator
WUFI Administrator
Posts: 287
Joined: Sun Jun 19, 2005 10:33 pm -1100

Re: Resource to Predict the Critical Saturation of a Material that will Result in Freeze-Thaw Damage?

Post by Thomas » Tue Jan 07, 2020 6:58 am -1100

JMarsh4087 wrote:
Fri Dec 20, 2019 6:05 am -1100
So RH% does not equal wmax?
No, it does not. The relative humidity measures the amount of vapor in the air (usually in units of percent of the saturation vapor content), the water content measures the amount of liquid water in the pore system (usually in kg/m³), and the maximum water content is the water content which results if the entire pore volume of the material is completely filled with water.
And wmax requires a laboratory test to find; there's no reference information?
It's not very difficult to measure in the laboratory: submerge the specimen until all the enclosed air bubbles have dissolved (this will take a while), the resulting water content is the maximum water content.

If you know the open porosity of the material, you can estimate the maximum water content as porosity * 1000 kg/m3 (where the 1000 kg/m3 are the density of water). For example, if the open porosity of your material is 0.23, then the estimated maximum water content is 230 kg/m3.
Now assume I somehow know wmax is 85%, if WUFI says a region of my material is 100% RH and under freezing temperature, that could cause damage because wmax has been exceeded?
One criterion of the WTA Guideline is
The WTA Guideline on interior insulation recommends that frost-susceptible materials should not exceed a saturation degree of 30% (i.e. 30% of the maximum water content wmax). This criterion includes a safety factor 2.
So for the above-mentioned material with wmax = 230 kg/m3 this Guideline says that at water contents below about 69 kg/m3 the material is safe under freezing conditions (with a large safety margin).

The WTA Guideline goes on to say
Higher degrees of saturation are admissible if the relative humidity of the pore air remains below 95%.
Here reference is made to the relative humidity of the pore air. For the material used in the example above, the Baumberger Sandstone, consulting its moisture storage function shows that in this case a relative humidity of the pore air of 95% corresponds to an equilibrium water content of 57.8 kg/m3. This is less than the 69 kg/m3 found above, so the second criterion does not allow a higher water content than the first criterion.

If the moisture storage function of the Baumberger Sandstone gave, say, 100 kg/m3 as the equilibrium water content at RH = 95% in the pore air, then the second criterion would override the first criterion and you could assume that the material is safe up to a water content of about 100 kg/m3.

Kind regards,
Thomas

Post Reply