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# Weight of Concrete

Member Posts: 615
all for the input. It is clear to me that the HO can use 25 psi rated blueboard since the concrete will only weigh .78 lbs per sq inch and the only load on top will be people walking around. Wish I'd taken up engineering when I was young and in college, instead of beer drinking and girl chasing. ( although maybe not) :)))WW

• Member Posts: 615
How much weight

does concrete put of styrofoam per square inch. I have a customer who wants to put a 9 inch thick slab in a passive solar setting but will only put 3/4 pressure treated plwood on the bottom for insulation. Made we wonder, how many inches before we start to compact Extruded polystyrene??? WW.

• Member Posts: 345
Concrete

The weight of concrete is about 150 lbs. per cubic foot.
• Member Posts: 608
ck Dow Corning

Their website has the compression strengths of "blue board".
As I recall, 1" holds 45 PSF. Foil/Double Bubble/Poly holds 60 PSF. Much easier to use, less expensive, higher R value to boot ! Tell your customer that 3/4" plywood has an R value of 1. AND, it will rot if it ever gets wet. That won't be very strong then. Celotex is even worse and is not approved for that use. Be careful.
• Member Posts: 615
So if

there are 1728 cu inches per cubic ft (12 x 12 x 12= 1728) then 150 lbs divided by 1728 would be .087 Lbs per cu inch. Then a 9 inches thick piece of concrete, 9 x .087=.78 lbs of weight per square inch on top of the styrofoam. That doesn't seem like a lot. Am I looking at this correctly?? WW

• Member Posts: 615
If a cubic foot

weighs 150 lbs then a square foot of 9 inch slab would weigh 112 lbs per square foot, since it is 3/4 of a foot thick and .75 x 150 lbs = 112. Too heavy for blue board. I guess that's why he's going with pressure treated on top of the 1/2 steel which is underneath for support. There is a basement underneath the slab. WW

• Member Posts: 608

That doesn't sound right. I've pored 6" on blue board rated for 45 PSF w/o any compression. I think the weight issue may hinge more on the load ON the concrete, not the load OF the concrete. I had a grade beam 14" thick x 24"wide carrying 57 PSF (architect's calc's) and was instructed to design for the 57. No problems.
• Member Posts: 608
suspended slab?

• Member Posts: 781
Duh

If this is a pre-stressed suspended slab, and there is no insulation below, what do you think the temperature will be in the basement? I have run into this, and the slab was 14" thick. No insulation whatsoever. Why do you worry about compression if it is a pre-stressed suspended slab? Insulate below.
• Member Posts: 21,866
Becareful with foam ratings

The stuff sold at home centers, etc is often 15 psi stuff. Owans 250 Foamular is 25 psi (hense the 25 psi rating) They also make a Fomular 150 typically used in non bearing applications. Often used when filling against a basement wall (vertical applications)

Same with Dow of GreenBoard brands. Thew rating psi is generally in the product number.

For heavy equipment shops I use Dow High Load 40 or 60. Keep in mind the soil below may not be up to the psi loading. On commercial jobs insist on a soil compaction test. Doesn't do any good to use high psi foam on uncompacted, or weak sub bases:)

Generally 25 psi foam is adequate for residential slabs and garages.

This is how he explained it. IF the truck was equally loaded on all 4 wheels (rarely the case) I would have a 7500 lb load on the slab below the tires 12" X 12" footprint. The loads are calculated using a 45% angle. So the load on the foam was really spread out like this picture from JLC explains.

Clear as mud?!!
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 608
obviously

of course, you're right... I was asking if it was suspended or on grade as that wasn't made clear.
• Member Posts: 21,866
Wayne

make sure you aren't confusing psi with psf. The foamboards are listed in psi rating.

Owens Corning FoamulaR 250 (pink board) provides a minimum of 25 pounds per square inch of compressive strength. That is an ASTM rating, not a manufactures number.

I'm not sure the bubble products are listed to an ASTM standard, regarding weight bearing ability. Ask them for documantation.

hot rod
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 345

1 Ft. x 1 Ft. x .75 Ft. (9") = .75 cubic feet x 150 lbs. per CF = 112.5 lbs.
• Member Posts: 424
Is the foil/double bubble/poly

rated for use under a slab? It its true that the pink or blue board, high density foam is psi and not psf, as I was assuming too, then the foam board at 45-55 psi is way higher than the bubble wrap at 60 psf. I think I am going to look at a sheet of foam closer, as it seems I remember it to be rated at psf, but could be wrong.
I would think the slightest bit of moisture would about totally void the insulating value of the bubble wrap.

Steve
• Member Posts: 608
PSF or PSI

Sorry, long week & short-handed must've made me foggy. Dow board (blue) is 25 PSI, not PSF. Double bubble is 60 PSI, not PSF. I checked specs to be sure. Foil side down, poly side up as concrete will "eat" the alum. foil. Serves as a vapor barrier also, when taped w/poly tape they provide for that purpose. Water permeability is 0. The site w/the info is www.fifoil.com . I'll likely never use board again. I take 1-1/2 hours to completely insulate and seal 1500 sq.ft. slab w/one helper. Of course, he IS my best helper! At 14 he has the eye for quality and the troubleshooter's analytical mind. Sorry, can't help myself, just hope he wants to get into it and enjoys the biz as much as I do.
Also, it makes everybody else's jobs easier if anything like closet flanges need to be accounted for. Bubble is 5/16 thick.
• Member Posts: 35
psi to psf

25psi = 3600 pounds per square foot.

psi x 144 = pounds per square foot.

The load of a tire presses directly down on the concrete and onto the insulation. From the bottom of the slab is where the 45 degree pressure agle is. The insulation is receiving the brunt of the load and is why I never use underslab insulation where the vehicle load is trucks or commercial traffic.

These are businesses and the added operating cost is minimial versus a broken slab.

• Member Posts: 21,866
25 psi foam

would carry 3600 psf. So an equally loaded vehicle with a weight of 14,000, if the tire foot print is 1 square foot. That should handle most residential or light truck loads. My 1 ton truck tips the scales at 9700 lbs, biased to the rear I imagine, although it rides on 6 tires. Putting 60%, 5820lbs to the rear on 4 tires. Still within the 25 psi foam rating.

I used a 60 psi on the commercial shop slab which would be 8600 psf. There is also a 80 and 100 psi High Load foam available,should handle most shop applications. I wonder what foam would be used under an airport runway? I know the slabs are pretty thick. Can't imagine any application that would see more point loads than a runway or airliner hanger.

One thing that confuses me, foam is something like 90% air. The high density 60 psi stuff I used is real heavy, almost feels like wood, weight wise. I imagine the 100 psi foam is heavier still! How can it still be rated at the same R-value as 25psi foam if there is less air, and more plastic?
Isn't the air bubble the key to the R-value?

hot rod
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 35
true -

The typical slabs I see here are 4" thick residential with the wire (if they use it - somehow they think fiberglass is identical in purpose) at the bottom.

When I decided to do my driveway - asphalt - a buddy who does concrete work tried to talk me into a minimum of 5" for residential. He really likes 6".

In commercial applications I use a constant circulation control - which ultimately heats the ground below until the downward loss is nearly zero. Right? It's the same principal as a warehouse or other slab on grade.

I'll use insulation in a residential design since 95% of the time it's an on/off strategy. "It's going to snow tonight - turn it on before bed."
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