Freezer Slab Insulation - compressive strength

[treopolis]

I have spec'ed freezer slab insulation a couple times in the past and have talked to DOW about how they come up with their numbers.

There are two type of insulation - 30 psi and 45 psi
These numbers are the crushing strength at yield or a 10% deflection (whichever comes first). And using a safety factor of 5 that DOW provides they work out to allowables of 864 psf and 1296 psf, respectively. Which sounds like a lot for a uniform slab, but the problem I am running into is when you have a forklift or point load. Even a moderate forklift wheel will compress around 1600 psf when loaded. And that is already taking into consideration the 45 degree load path angle through the concrete slab above.

What is the appropriate design method for this insulation?

 

[treopolis]

An additional issue that may be a solution to this problem.

The 45 degree angle is the commonly accepted as the failure shear angle in concrete. Does this angle slacken as the load becomes less than the failure load? ie if a load of 1,000-lbs fails the slab, does a load of 500-lbs produce a 2:1 or shallower shear angle, thereby giving you more contact area beneath the slab.

 

[dik]

Dow also has/had a Hi60. Minnesota used to use the Dow material under their highways. Unless you have high point loads, the lower strength material will usually suffice and the Hi60 can accommodate HS25 type loads with proper design.

If the area is subjected to high freezing, then you should look at heating the soil. Insulation only goes so far.

Dik

 

[treopolis]

We have a freezer, that will be kept below freezing. We try to get an R-Value of 15 or greater out of our insulation. We have had success with it as far as freeze-thaw. I am just worried about the high point loads. The client wants to use the 30 psi, or an even cheaper 25 psi. We want to steer them towards to 45 psi, but want a design method to fall back on.

 

[JLNJ]

Maybe you can look at the system as a slab/beam on compressible fill and get a feel for how the slab distrubutes the point load over the foam. You might find that the contact area is quite a bit more than the 45 degree angle assumption yields.

 

[treopolis]

JLNJ-

After talking with co-workers about it, that is what we decided to do. I used the criteria from the insulation cut-sheet of 10% deformation (@ 30 psi) to determine the actual deflection. I then used this deflection and my applied point load to figure out what beam length would result in that deflection. Then I used the length of beam to determine my area (conservative - actually acting as a two way slab). Then I used that area and my applied load to determine the applied force to compare to the 30 psi with the safety factor of 5.

 

[dougantholz]

Designing of a slab over a uniform support is not about compressive strength of the base, it is about the subgrade modulus. WRI, CRSI & The Army Corp of Engineers all have methods for evaluating and designing slabs on grade. If you can get a subgrade modulus of 100-125 lbs/cu inch you should be fine with a forklift on a 5 1/2" un-reinforced slab - but you should design this becuase there are a number of factors to consider.

 

[treopolis]

But freezer slab insulation does not provide a subgrade modulus. The spec for freezer slab insulation only provides compressive strength and safety factors. The subgrade modulus of what is below the insulation may be 100-125 pci, but it doesnt matter if I cant verify that my freezer insulation is over 100 pci.

 

[concretedoc]

If you're an ACI member, you can get a free download of the article on this subject by Tarr and Nussbaum (Oct. 2002). They recommend using plate bearing test data (ASTM D 1196) on the insulation because estimating the subgrade modulus for the material based on compressive strength data from ASTM D 1621 is unreliable. Ask the insulation suppliers if they can provide plate-bearing-test data. Some might be able to do that. Tarr and Nussbaum also found that the subgrade modulus for the insulating material decreased markedly as the thickness of the material increased. A threefold increase in thickness resulted in a threefold decrease in the modulus in one case.

 

[dougantholz]

Treopolis, you can get it from Dow if you push them. I have seen it for Freezermate (TM) and it depends on how thick it is piled and how many layers there are - but it exists.