floor base question

[backintheday]

Hi all,
I'm in the planning stages of a shop build and am curious if anybody has done this? I was thinking about putting down 2" of rigid foam for the slab insulation then a few inchs of sand on top of the insulation then the wire mesh and pex with a 6" slab. My thought is it will assist in the curing of the concrete and provide a bit more thermal mass. Will I be able to drive on the rigid foam to spread the sand around without breaking it?

Thanks all

 

[wssix99]

My thought is it will assist in the curing of the concrete and provide a bit more thermal mass.

It won't help with the curing and I'd think that the concrete would give you better thermal properties since its mostly rock. One of the key factors is where you intend to put your vapor barrier. If you have sand between that and the concrete, there is a school of thought that the sand will allow excess moisture to escape as the concrete cures, leading to less curling of the slab - but that's really a problem for heavy duty industrial floors. You won't have this problem in a garage.

IMO - Going away from the norms on a garage floor will just confuse your concrete guys and could lead to problems instead of solving ones that don't really impact you.

few inchs of sand on top of the insulation then the wire mesh and pex with a 6" slab.

What is the size of the slab and what are you looking to do with it? How far are your saw cuts going to be spaced? Will you be putting a lift on it? You may want to look more carefully at the reinforcing.

Will I be able to drive on the rigid foam to spread the sand around without breaking it?

Most certainly, no. Even if you get a high strength foam that can take the weight of the car, there will be voids under the foam, which will cause it to get totally mangled.

 

[backintheday]

Thanks for the reply.
A concrete guy who pours pads for oil rigs told me that when concrete can release it's moisture equally out of the top and bottom it cures stronger. He also said he doesn't think I need to put insulation down and he won't pour on foam because it will eventually crack. I'm not a concrete guy so I have no idea if this is true. If it was true there would be a lot of guys on this forum complaining about cracked slabs.

FYI I'm doing an ICF foundation.

The slab will be a 40x60. I will have a lift and only plan on doing normal shop type work ( metal fab, build and maintain cars and trucks ) but with a chance a semi could drive on it.

Can't remember the cut spacing but I thought about doing 3 different pours so the slab will have 2 "engineered cracks" if that makes sense, then cut intersecting lines in each section.

Your thought on going away from the norm is a good one. I'm a big fan of the KISS method.....

 

[rburke65]

There has to be hundreds of threads on this subject if you do a search. Advise to KISS. Do you plan to heat the floor? Even if you don't right now you might want to consider at least to put in the pex tubing before the pour.......it will give you the option later......if so, then yes insulate the hell out of it.

 

[backintheday]

There has to be hundreds of threads on this subject if you do a search. Advise to KISS. Do you plan to heat the floor? Even if you don't right now you might want to consider at least to put in the pex tubing before the pour.......it will give you the option later......if so, then yes insulate the hell out of it.

I know there are a lot of threads on the insulation of the slab but what I am curious about is what everybody thinks about putting a few inches of sand on top of the insulation then wire mess and pex on top of the sand.

 

[Leaflessshadetree]

I plan on putting the sand down first and compacting it. Then the vapor barrier, then the insulation. Then I'll put my pex and reinforcement.
As far as curing out the top or bottom, the vapor barrier and foam should slow the curing out the bottom. After the concrete sets up I'll keep the surface wet to slow the curing on top.
When I pour concrete on sand (no vapor barrier, no insulation) I wet the sand down first so that it doesn't "****" the water from the concrete.

 

[wssix99]

A concrete guy who pours pads for oil rigs told me that when concrete can release it's moisture equally out of the top and bottom it cures stronger.

This is true, but for a residential garage, the effect is negligible. By going outside the norm, you will probably throw your concrete contractor off and could end up with a weaker slab as a result.

If you want more information on this, you can purchase The American Concrete Institute's document titled "Design of Slabs-on-Ground," which details this effect. In there you will find that the location of the vapor barrier is also critical.

He also said he doesn't think I need to put insulation down and he won't pour on foam because it will eventually crack.

This is not true - if you use the correct type of foam. You'll want at least 40 psi foam and can upgrade to 60 psi if you want a little extra insurance. The insulation companies even make 100 psi foam for airplanes. (So you can point that out to your friend.)

http://commercial.owenscorning.com/products/foam/selection-guide/

The slab will be a 40x60. I will have a lift and only plan on doing normal shop type work ( metal fab, build and maintain cars and trucks ) but with a chance a semi could drive on it.

Can't remember the cut spacing but I thought about doing 3 different pours so the slab will have 2 "engineered cracks" if that makes sense, then cut intersecting lines in each section.

The separations would be done with expansion joints, just like you see in sidewalks. Then those sections will be cut in to control joints, which create a pathway to control the shrinkage cracks, which will happen.

Typically, those are 10-12 feet apart in an un-reinforced slab. If you want them further apart (to keep them away from your lift posts) you will need to add reinforcement - at least to the section between expansion joints where the lift will be placed. (In between the expansion joints - assuming you don't have any columns or special equipment foundations, you can have different reinforcing and control joint layouts.)

 

[volleyball]

There is zero reason for sand in your project. Compact the earth, Lay down a few inches of gravel. Plastic on to then the foam. I would tie the pex to the mesh putting it closer to the surface so you get the benefit of the heat.
After the pour, keep it damp on top and the bottom will take care of itself.
After the footing and before you backfill, put a sheet of foam vertically to act as a thermal break. It will help tremendously with heaving and cracking under your slab.

 

[1grnlwn]

There is zero reason for sand in your project. Compact the earth, Lay down a few inches of gravel. Plastic on to then the foam. I would tie the pex to the mesh putting it closer to the surface so you get the benefit of the heat.
After the pour, keep it damp on top and the bottom will take care of itself.
After the footing and before you backfill, put a sheet of foam vertically to act as a thermal break. It will help tremendously with heaving and cracking under your slab.

X2

Must have insulation if going with floor heat. If the only thermodynamics law you know is heat rises you will get Burned.(pun intended) You mentioned possible semi traffic? You may want to scrap the rewire and go with rebar and stands. Mine is 6" slab with 3/8" rebar on 12" grid. Heat works better if pex is in center of cross-section. Most rewire ends up on the bottom and if you rely on concrete guy to position wire and pex in center of slab, this could be problematic. I chose not to sawcut my slab, too much invested to take a chance. If you really think you need thermal mass, some will leave a square area in center with no insulation. Me personally, I want efficiency and do not want to heat the ground.

 

[1grnlwn]

Oh, I forgot. Do homework on concrete finisher. I did mine in two pours a year apart. I was really not happy with either guy I had. First one did a good job with the slab but was shady and tried to up the cost at the last minute. Second guy had tons of fancy equipment was there in 2 days and did a crappy job finishing. Neither guy has even given me a receipt?

 

[wssix99]

^ Yea. I had a great finisher but they didn't pay attention to details. They showed up with the wrong size rebar, the wrong grade of rebar, (I had to re-calculate the spacing for the different size/grade on the fly) didn't put the reinforcing on the proper chairs, didn't do the expansion joint layout correctly, and ended up starting a pour of a 4" slab when I paid (and needed) for a 5".

Fortunately, I was there in anticipation that they would F it all up, I had a slab rebar calculator on my computer, and caught the thickness mistake during the pour. The big error on the thickness was that they laser leveled the gravel base before putting down the foam. With the irregularities of the gravel base, the 60 psi rigid foam was a little higher in some spots, which led to the discrepancy when they started pouring to their witness marks made during the gravel compaction.

If you raise the radiant tubes up in to the slab, you'll want to make sure you have enough cover on top of them and the reinforcing. With a 6" slab, you can probably pull it off, but with rebar in a 5" slab, I had to put the tubing at the bottom.