Floating slab in Garage question

Old 09-28-15, 10:52 AM
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Floating slab in Garage question

I would like some thoughts and suggestions on my floating slab situation

for my 26 x32 garage.

First, yes, I messed up. When we poured the concrete, I did not antisipate

the extreme pressures placed on the form and the bottom kicked out. It is

like this on the 2 sides and the back.

Because I live in central IL., I went 36" deep with my trench footing. My

concern is the "wedge" shape of the interior of the foundation.

I have not yet poured the floor and am worried about the lift and settling

due to freezing of the ground. My concern would be when it settles (frost

goes out) due to the wedge shape, the slab will exert force against the

foundation and crack it. Or, if it doesn't crack the foundation, the ground

drop away and there is a void below the concrete floor and the gravel,

causing the floor to crack.
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I know that in our area, outside slabs will raise approx 1" in the winter.

my garage will not be heated.I don't know how much actual lift will happen.

What are my best options for the floor?
Old 09-28-15, 12:45 PM
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First things first--all concrete will crack, especially a slab the size of yours. You can control the location of cracking by installing an adequate number of control joints in the slab, either by grooving the fluid concrete or sawcutting immediately after initial set. Sealing the cracked joints to keep water from saturating the gravel base will minimize the amount of frost-heave occurring.

If an adequate bond-breaker (usually in the form of 1/2"-thick expansion felt) is installed completely around the perimeter, against the wedges of the stemwalls before pouring, I don't suspect there will be any significant problems. Presuming the stemwall concrete is reasonably sound and adequately reinforced, of course.
Old 09-30-15, 09:36 AM
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What BridgeMan45 said. Also, I definitely would install concrete remesh wire in the floor before pouring the concrete.

Shop Steel Wire Remesh (Common: 5-ft x 150-ft; Actual: 4.9166-ft x 148.5-ft) at Lowes.com

Lay it flat and tie the edges together with steel bailing wire. When pouring the concrete, gently lift the wire approx 1" off the ground to get it up into the concrete. Don't pull it too far up near the surface of the finished floor. It will make the floor a lot stronger if it's near the bottom of the floor. Use a hoe or bend a piece of steel rod to use as a hook to pull the wire up into the concrete mix.
Old 09-30-15, 10:15 PM
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A better way of using steel mesh reinforcement in poured concrete (other than lifting it after the concrete has partially covered it) is to use dobies or bar chairs, which are small spacers that lift the mesh off the compacted grade before the pour. There are usually more than enough things going on during a concrete pour to have to worry about getting all of the mesh properly lifted. Many concrete placements I've observed over the years have the mesh left entirely on grade, completely buried, where it's doing absolutely no good--a big waste of money. Or worse yet, projecting up to the top surface and protruding above it, as once it's pulled too high up, you'll never get it back down.

Also, steel tie wire is an acceptable way of tying reinforcing mesh or rebar, as opposed to bailing wire.

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