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Old 02-02-2010, 08:17 AM   #1
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slab construction

Looking to pour a 20 x 24 foot, 5-6 inch slab for a family room. I want it to be dry and warm. Should I insulate and if so where and how much? What type of vapor barrier? Live in Northern Indiana. Footers will be 3 ft deep.Also rebar or screen or both and how much?
Thanks for recomendations
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Old 02-02-2010, 06:20 PM   #2
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Based on what I read, we're planning on putting down 2" of high-density polystyrene board with a 6 mil poly vapor barrier. I plan on taping the joints in both, although that's probably overkill. We're in a relatively moderate climate (central Missouri), so I don't believe there is any need for more than that. I believe my contractor will be using welded wire mesh in the concrete, along with fibers.

Of course you'll want a bed of compacted washed stone under the insulation to act as a capillary break.

There's also some controversy about putting 2" of sand on top of the insulation to absorb moisture from the concrete. I'm convinced that it's not necessary. Ideally you'll keep the concrete surface wet for at least a week while the concrete gains about 80% of it's ultimate strength.

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Old 02-02-2010, 06:49 PM   #3
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The 6mil poly vapor barrier is code mandatory. Everything else is optional if you're pouring over undisturbed soil, unless local requirements exist. I'd insulate the footing and the perimeter of the slab and would personally use rebar. If it were mine I'd put it at 24" centers each way. Also plan to put down a few inches of gravel.
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Old 02-02-2010, 09:11 PM   #4
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Find out your heating degree days and compare to one of these:

Don't use sand:

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Old 02-02-2010, 10:24 PM   #5
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The construction I see around here uses unopened 90lb bags of concrete under the slab foundations before pouring, and rebar. You can't post-tension with mesh.
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Old 02-04-2010, 02:41 PM   #6
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here's a typical spec. 4' 3000 psi concrete w/ 6"x6"(9") 10 gauge wwf(welded wire fabric) w/ trowel finish on 4-6" porous fill on 6mil polyethylene on undisturbed earth saw cut to 1/3" depth same day @ max 120 sq ft and fill joints with one part polysulfide caulk. keep surface damp (but not submerged) for 7 days.

i'd insulate the outside of the foundation wall with 2" blueboard rigid insulation down to the footing to prevent the conduction of cold into the block and down the footing. if you're gonna lose some heat, it's better to allow that heat to be able to keep your footing warm in the coldest of winter to prevent heaving. don't use polystyrene, the white board they make coffe cups out of, it becomes water logged in wet applications and the R-value is then lost. depending upon the size of the slab, i guess you could insulate under the slab, but it's above and beyond the call of duty...the slab is going to feel cold no matter what unless you plan on a radiant heat integrated slab....cha-ching. i'd go with extra porous fill( run of bank, pea gravel, and nonexpansive draining material).

as for dry......always an issue for green concrete. the vapor barrier over the earth is a good start but here's the thing. there is a lot of moisture in 3000 psi concrete and some makes it way through the bottom of the slab as it cures and it can collect under the slab. there are two schools of thought. one is place the gravel then cover it with the poly sheet. or the one i prefer, place the poly sheet on the earth and then cover it with the gravel. this second method IMHO does three things better than the first method. 1 once covered with gravel you can walk on it without tearing it. 2 it creates a space below the slabe that the excess water can move away from the slab creating a humid environment for the concrete to cure properly without being supersaturated with water therefore weakening the concrete and creating larger voids and a more permeable slab. and 3. the wet bottom and potential for a dry top create the possibility of a cupped slab. you wouldn't think it could happen, but a green slab of concrete can act just like a wooden board the is dried in the sun on the top and wet on the bottom, it bends!

the interesting thing about concrete is that people always talk about The Expansion Joint or the need to place that expansion material at the walls for expansion. well, the only real use for that stuff is to give the finishers a place to screed to. concrete does NOT expand more than when it is installed, it's full of extra water. once cured and dried, it has shrunk as far as it's going to go, from there it will expand and contract ever so slightly and un-noticeably. this is why we saw cut same day while the concrete is firm but workable. as concrete ages it slowly dries and out and gets harder and harder. the cracking of concrete totally a function of shrinkage (part to do with thickness) so we must tell it where to crack. 120 square feet is good or 10' by 12' area, personally i like them closer together. once this is done and set the polysulfide caulk is a self leveling non hardening stuff exactly designed for this purpose. done properly, you won't even see the this brings up something else, what is going over the slab as a finish? if any....personally i love polished concrete as a finish surface. if you ever want to put tile down, you'll want to place a grout joint over this saw cut and not a whole or part of a tile, guaranteed to crack right there since this is where we told the slab to releave it's shrinking.

as for rebar....only if you plan on driving a fork-truck or skid steer, not to mention 5"-6" is overkill for a living room floor....spend the money on the compacting the earth and gravel under the slab. and sawcut sawcut sawcut! if you'd like you could use 6 gauge welded wire fabric but that's expensive and for high traffic industrial slabs. technically the fiber reinforcment is un-necessary as well.
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Old 02-04-2010, 04:55 PM   #7
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I place the poly directly under the slab and on top of the compacted base.
The dry gravel sucks the moisture out of the fresh pour and it drys from the bottom which, I am told, not the right way t do it.
We also don't poly un-occupied spaces, like sheds or garages, but I am using a different code system.
An expansion joint is a good idea in case of settling, especially when pouring next to an existing structure.
Rebar OR mesh OR fibre mesh. Pick one. The steel/mesh is for separation in the slab, not structural strength.
Insulation on the outside wall is good, and the first 2 feet inside the footing horizontally.
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