Frost Depth in Concrete Slab?

[SteveHen]

How do I calculate the frost depth in a concrete slab?

I am working in an area where typical frost depth is around 6 feet. But as concrete is a worse insulator than typical soils, is the frost depth greater? how do i calculate this?

We have a 2m thick reinforced concrete slab. Initial ideas were to put surface water drainage pipes within the slab but these would then be above the frost depth.

Should the drainage pipe therefore be placed below the concrete slab? And would we need to provide a layer of insulation board?

We don't want the drainage pipes to freeze, block up and the area to flood.

Thanks,

 

[BigInch]

Without knowing more about what you're actually building here;

My first thought is that you need to be considering sloping the top of concrete to drain to the edge of this slab or putting a roof above it if its rain/melting snow that makes this water you're trying to drain, rather than putting drainage pipes in 2m thick slabs.

But assuming all that is already worked out and you need to drain through the slab using drainage pipes embedded in the slab, then put them in the slab. I would NOT recommend you put them under a 2 meter thick slab. That's a load of 900 psf just from the slab alone, not considering any load on the slab itself. That will take some strong thick walled pipe to keep from collapsing right there. If there is any settlement at all, those pipes below won't drain anyway.

So, you've got pipes in the slab and you need to keep them from freezing. Think about these,

1.) Run heat tracing inside a conduit located immediately next to the embedded drain pipe and keep it above the freezing point.

2.) Wrap insulation around the drain pipe and cover the insulation with an insulation shield.

3.) Use a casing for the drain pipe with insulation injected into the annular space.

4.) Use a casing pipe and circulate warmed air or water or whatever through the casing.





**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Ron]

Why don't you post this in a more relevant forum, such as
"Concrete Engineering" or one of the Geotech, Materials, or Structural forums?

Concrete is a relatively good insulator and air entrained concrete does not generally freeze except superficially at the surface. As long as the concrete is not saturated, you shouldn't have a big issue.

 

[BigInch]

Ron, I thought I understood that the freeze depth is typically higher under an exposed concrete slab relative to the surrounding earth due to the heat transfer coefficient of concrete of 1.5 - 2.0 W/m-ºK, depending on density of the concrete (M-H). Soil is 1 to 1.5 W/m-ºK, depending on moisture content. Wouldn't that indicate that an exposed concrete slab will have approximately the same thermal gradient as the surrounding soil and maybe worse. If the soil is frozen to 6 feet, I'd expect a slab to be frozen to 6 to 9 feet deep, or so.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

sorry, "typically lower"

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SteveHen]

I am under the impression that freeze depth in a concrete slab is greater than in soil as BigInch explained.

But does anyone know what formula can be used to show this?

Berggren equation for frost depth asks for volumetric latent heat which relates to soil properties of dry unit weight of the soil and water content of the soil. I'm unsure if this can be used for concrete?

In which case how can I accurately determine what thickness of polystyrene insulation is required?

heat tracing is obviously expensive so the preference is to provide adequate insulation

 

[BigInch]

Why didn't you ask before? Formula 1 of course.

http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SteveHen]

Is there a formula that relates frost depth through a concrete slab to freezing index?

 

[BigInch]

Why didn't you ask before?

see page 247

http://books.google.es/books?id=DdH...X&oi=book_result&ct=result&resnum=1#PPA247,M1

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SteveHen]

Is there a formula though?

There must be some formula that was used to produce this graph??

 

[BigInch]

Could just ahve been from connecting dots that were measured.

If you want a formula, make one by entering various data points picked off the curve in Excel, trend them and check the option box, "show equation on chart".

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[CarlB]

You can find a discussion of the principles at

http://pavementinteractive.org/index.php?title=Frost_Terminology

If you can get your hands on the computer model "berg-2" by the Alaska DOT you can easily vary soil, concrete, insulation properties to find maximum frost depth.

Billy Connor was the author of Berg-2,

http://www.alaska.edu/uaf/cem/ine/autc/about/director.xml

Also look at discussions/publications regarding "frost shield". Eng-tips:
thread378-217830
thread164-196645