Replies

1. 
   johnnyd | Jul 23, 2003 06:16pm | #1

   Has alot to do with the compressive strength of the foam versus how much wieght will regularily be on it, and how the slab will be used.  Airplanes are pretty light, right?

   But will you have heavy shop equipment, will you use jacks, engine stands, etc that concentrate alot of weight in a small area.

   After pondering the same thing, I used Dow 150 instead of 250 for a 24X29 garage slab, reinforced with re-bar over the PEX.  Held up well under full wheel barrows. So far so good on the longevity after 1 1/2 months.

   FWIW, neighboring town codes 250 for all garage slab applications.

   
   

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   Edited 7/23/2003 11:24:48 AM ET by johnnyd

1. 
   dtown | Jul 24, 2003 05:52pm | #2

   I like when I log in to start a discussion and it is already going.  I also am pouring a foundation next week.  22x26 monolithic garage with pex and wire mesh.  Both Owens Corning and Dow suggest the 25 psi product for this application.  I have had others on the forum reccomend 15 psi products.   Is that sufficient enough for a garage application?

1. 
   johnnyd | Jul 24, 2003 06:16pm | #3

   I guess if I had been able to easily aquire and transport the 250 before the concrete sub was ready, I would have used it. Not that much difference in cost.  As it turned out, I got the 150 ready and down just in time.

   I was there during the pour, and noticed little if any "crushing" or denting when they 'barrowed full loads over the PEX/re-bar/ 150 rigid sandwich.    When you think about it, as long as the substrate is the right material and compacted properly (in my case compacted crushed rock over sand), and the concrete is the right strength and well re-enforced, what difference will the compressive strength of 150 versus 250 make?  The 250 sure won't save your slab if the substrate settles.

   There's another thread here with opinions favoring the 150.

   Maybe I'm just rationalizing after the fact.  Time will tell.

   One difference in my case is that the slab was poured within a pole building, thus the slab is not holding anything up except itself.  Maybe it would be different if your slab is also acting as the foundation....more reason to go with 250....given the importance of perimeter insulation with a heated slab.

   Edited 7/24/2003 11:20:51 AM ET by johnnyd

1. 
   donpapenburg | Jul 25, 2003 06:39am | #4

   I think Owens Corning also makes 35 psi for the real heavy duty garages. I used 25psi in all of my floors ,the cost differance is only pennies.

2. 
   DennisS | Jul 25, 2003 07:02am | #5

   Maggie -

   At 150lbs/cf for the concrete, your 5" slab weighs slightly in excess of 60lbs/sf. Add a point load of an airplane wheel (you'll have to provide that number - I don't know how much airplanes weigh) the weight of the point load spread over some theoretical area by the concrete slab, and you have the compressive load imposed on the insulation. Other than making sure this is accounted for such that your insulation doesn't fail in compression locally, you should probably make sure there are no ground water problems that would dictate an insulation material that performs adequately under those circumstances. Regardless of the presence of ground water, I always spec at least a 6" lift of 3/8" pea gravel under slabs on grade as a capillary break. The insulation goes over this.

   Your reinforcing bar will be/should be chaired up off the insulation which will also dictate either a compressive strength in the insulation that will resist crushing under the point load of the rebar chairs during slab placement or something to spread the load of the chairs over the insulation. That's not usually a problem with the pink polyiso insulation I ordinarily use.

   By the way, 150lbs/cf is what we use for conc. weight around here. Your mileage may vary so adjust the figures accordingly.

   ...........

   Dennis in Bellevue WA

   [email protected]

1. 
   maggie2142 | Jul 25, 2003 01:36pm | #6

   Many thanks for the replys.

   The insulation I want to use is Perform Guard (anti-bug treated) Expanded Styrene (R_Control EPS), not the Extruded Styrene, (XPS-pink, blue or green made by Owens Corning and others). Like the XPS, the EPS Perform Guard comes in different densities. The 1 lb/cu ft material is about 10 psi (1,440 lb/ square foot). The 2 lb/sq ft type is 25 psi (3,600 lb/sq ft).

   Even if you derate the compressive strength by a factor of 3 or 5 to account for long-term creep, the strength of even the 1 lb/sq ft material is more than enough for a residential garage. Small airplanes weigh less than cars. Plus, since I will have 5" of concrete with rebar 20" OC, the slab will be very stiff compared to the squishy foam underneath, so any point loads will be distributed over a large footprint. So I don't worry about the in-place strength of the foam.

   I would like to use the 10 psi Perform Guard because it costs 1/2 of the 25 psi. ($12 vs. $24 for a 4x8) My only worry is whether my concrete guys will tear it up while placing the slab.

1. 
   dtown | Jul 25, 2003 06:05pm | #7

   Maggie2142,  You and I are on the same train of thought on this one.  I was trying to get the Type 2 EPS product in order to save the dough over the XPS products which are quite expensive.  I never really sat down and did the calculation of the loading.  My slab be a monolithic 10" footing with slab.  The the slab will be 5" with 6x6 WWM throughout.  The WWM will be tied to the footing with 4 foot #4 rebar bent and tied from the WWM horizontally into the footing vertically.  I think the slab will spread the load sufficiently to use the 10 psi product. 

   Can anyone else shed some light here?