Slab Thickness Over 2" 25psi Insulation?

[ICT_Kevin]

I'm preparing to do a new garage and my plan was to put R10 of foam insulation under the floor. I'd like to do radiant heat under a 12 x 12 shop area in one corner. The 60 psi Foamular 600 listed on Menards' site here (Wichita, KS) is not available right now according to two stores. Same for the 40 psi foam. If I use 25 psi foam, what slab thickness would you recommend? Would I need more thickness at the 1/2" pex area?

If you know where to find equations to calculate appropriate slab thickness or foam strength, I'm up to using it.

Thank You,
Kevin

 

[Buff's Place]

Kevin, I cant answer your question, but give Comfort Systems in Wichita a call ((316) 243-9260) I picked there brains when I had my system installed.

 

[bigman68]

400 and 600 Board are a specialty product and wont be handled by your local big box stores, a local concrete supply company in your area should have some, here are 2 I found:

Carter-Waters (316) 942-6712 or Midway Wholesale (316) 944-3999

Give them a try, I am sure they will have something in stock.

 

[Flexia]

4in concrete over 25psi board is fine

 

[ICT_Kevin]

Thank you all for good leads.

I found a good lead past midnight too, "Foamular Expanded Polystyrene Insulation For Cold Storage Applications". It has stress and deflection equations for the slab and foam. The name may be useful if the link moves.
FoamularXPSForColdStorageApplications

The calculations agree 4" is adequate if I don't apply a huge point load. I will still try the concrete places or Comfort Systems to find 40 or 60. I was having a hard time finding anywhere to look. I like things that last and don't need re-done.

Kevin

 

[wssix99]

Any commercial concrete/masonry supply place should be able to get you the foam board you are after.

I just poured a garage like you are describing. I did some "math" for a floor to support a 2 post lift. The calculations are very complex - so much so, most of the technical engineering guides I found don't even get in to it. Making some very conservative assumptions, I recall finding that 25 psi was adequate, but I went with 40 to get an additional factor of safety.

Without a lift, you can calculate the pressures like this:
(Weight of vehicle) / (number of tires) / (tire contact patch size) = surface pressure on slab

So, if you have a 4000 lb vehicle with 4 tires and 20 sq. in. contact patches, the slab will see 50 psi. (Actual contact patches should be larger and the actual loads less.)

That surface pressure radiates out approximately at 45* angles to the base, so if you divide the figure above by 2X the slab thickness, that should approximate the pressure on the foam. So, in the example above at a 4" slab, you'd have 4-7 psi at the foam. (depending on how conservative you make that calculation)

^ That is for a non-moving load. As the car rolls along the pavement, there are additional stresses created. (This is why you'll see the really high psi boards advertised for roads, etc.) The faster the movement, the higher the stress. Given that my personal garage has a very low speed limit, I ignored these effects.

A lift will put similar surface loads on the floor as a car just sitting there, but if you have a two post lift with an off-center load, that might stress out the base a little more. (The assumptions I made around these effects got me close to 25 psi with a full load on the lift.)


BTW - One thing I learned using this foam board was that it was hard to get the proper concrete thickness. With the irregularity of the stone base, the foam bridged the high spots - so the marks that our concrete guys put on the walls prior to the floor ended up being too shallow. (We had to put stakes in the middle of the floor to properly measure the concrete thickness once the boards were down.) I expect the slab might settle a bit as I load the floor and the foam compresses and contours to the base.

 

[Highbeam]

"That surface pressure radiates out approximately at 45* angles to the base, so if you divide the figure above by 2X the slab thickness, that should approximate the pressure on the foam. So, in the example above at a 4" slab, you'd have 4-7 psi at the foam. (depending on how conservative you make that calculation)"

Distributing the load at a 45 degree angle is what you would do for a flexible pavement or even gravel. With a rigid pavement like concrete, the distributed load area is MUCH greater.

25 psi foam is way overkill.

 

[wssix99]

Distributing the load at a 45 degree angle is what you would do for a flexible pavement or even gravel. With a rigid pavement like concrete, the distributed load area is MUCH greater.

25 psi foam is way overkill.

Agreed. My 45* angle is conservative. Have you ever seen simple resources that would give a precise pressure/stress distribution for a situation like this?

I recall doing some foundation pressure calculations in the past (some nightmare-ish process involving Mohr's circle) - but the engineering time and cost to be precise on something like this would be more $$$ than over-designing the slab and insulation.

 

[Norm01]

This may be obvious but there are 144 sq. inches in a square foot, so 25 pounds per square inch = 3600 pounds per square foot. That's a huge capable load. As others have said, 25psi foam board is fine.

 

[Highbeam]

Agreed. My 45* angle is conservative. Have you ever seen simple resources that would give a precise pressure/stress distribution for a situation like this?

I recall doing some foundation pressure calculations in the past (some nightmare-ish process involving Mohr's circle) - but the engineering time and cost to be precise on something like this would be more $$$ than over-designing the slab and insulation.

Mohr's circle gives me nightmares. In this situation, with no particular special high load, and no particular desire to use a super thin slab, the standard methods are the best way to go. That is, pay a little more for a 5" slab or 25 psi foam or both if you are worried.

Even the highway engineers don't try and calculate this stuff to the millimeter and instead they overbuild for the sake of simplicity and longevity. Concrete is relatively cheap.

 

[ICT_Kevin]

I got my 25 psi this evening. The Owens Corning math said I'd get 4% more out of my slab with 60 psi foam than 25 psi. The slab was closer to limits than the foam. Going from 4" to 5" thick slab was worth 40% strength increase, and it's $500 for my 1080 sft project rather than $1000 to get better foam. A lift looked fine as long as I don't get a 12,000 lb unit. With a 10' ceiling, no point in one that big.

Big stack of foam:

 

[26 Flatrod]

I just poured a garage like you are describing. I did some "math" for a floor to support a 2 post lift.

BTW - One thing I learned using this foam board was that it was hard to get the proper concrete thickness. With the irregularity of the stone base, the foam bridged the high spots - so the marks that our concrete guys put on the walls prior to the floor ended up being too shallow. (We had to put stakes in the middle of the floor to properly measure the concrete thickness once the boards were down.) I expect the slab might settle a bit as I load the floor and the foam compresses and contours to the base.

This is all great information. My contractor is getting ready to layout/pour my slab as well. I plan to have a 2 post lift and in-floor heat. Bend-pak said 4" min for the lift so I am going with 6" (especially with the tubing in the floor). Also the edges of the slab will be 12" thick to hold the weight of the building

The contractors are insulating the entire slab but not under the 12" thick perimeter, they say they are concerned about the insulation compressing in that area causing the edges of the slab to sag over time. Is this a concern? if so should the insulation be removed under the 2 post lift as well?

 

[Lootenny]

That sounds like good advice. I would think you would want to mark out where the lift posts would be, and have the radiant tubing go around and not under that area at least. I was under the impression that you would want to have footings under the lift posts, too, but perhaps that was for the bigger ones.

 

[wssix99]

if so should the insulation be removed under the 2 post lift as well?

I assume you have the slab engineered and will have reinforcing bar in it? If so, there shouldn't be any concern. The slab will be designed to take that.

if so should the insulation be removed under the 2 post lift as well?

Definitely not. You want the same thickness, same reinforcing, same insulation, etc. under the whole slab. Changing the uniformity is (in most cases) a recipe for cracking. (You don't need to worry about your radiant tubing in this regard because its non-structural - and I'd also expect it to be on the bottom of the slab in your application.)

I was under the impression that you would want to have footings under the lift posts, too, but perhaps that was for the bigger ones.

This actually makes the lift and floor weaker. For cars and light trucks, putting a car on a lift might even stress the concrete out less than having it just parked on the slab. (The lifts have large plates that spread out the bearing force - so a footer under them would be worthless in that regard.) A 2 post lift creates bending forces in the slab. To counteract this, a uniform, contiguous slab is the way to go. (A footer would disrupt this system and properly keying/pinning it in to the slab is expensive - and will probably not be as strong as a plain slab.)

 

[Highbeam]

The footings were designed (flatrod's) and required a 12" wide thickened edge to act as a footer. The 12" width was surely chosen to correspond with the weight of the building and the bearing capacity of the soil beneath. Your concrete guys don't realize that the foam is as strong as that soil underneath so the foam will no sooner yield than the soil.

Wssix is right. However he assumed that you are having a slab engineered. I assume that you are not having it engineered and as such you should stick to a continuous slab plan that at least meets the lift maker's spec. That is, built it at least that well and do the whole slab the same.

25 psi foam is the same as 3600 psf soil and that is very high strength soil. Double what most earth is rated for.

 

[86turbodsl]

25 psi means pounds per square inch. One square inch of 4" thick concrete weighs less than a pound. You could park a tank on that 25psi foam and it wouldn't hurt it. Under the concrete of course.

 

[ICT_Kevin]

I'm only gunning for a 4-post, so loads are simpler.

I'd say the manufacturer's recommendations are probably enough for the 2-post, but I'm too chicken to bet my life on it without someone doing the actual math.