Floating slab for a cold detached garage

Here are some questions for you garage builders, doing such in cold climates.
The garage is a slightly oversized 2 car one, detached, unheated. 22 feet depth, 28 feet width.
Can a floating slab work?
We are thinking of 24-deep thickened edge, rebar along bottoms of thicked edges, WWF reinforcing for the slab.
Dig out the whole thing to the depth of edge depth plus another 4 inches. Place 4 inches of compacted crusher run in entire area, then build up another 20 inches of crusher run to make the high hat center, then mesh and bar, then pour.
Sound OK? Too much overkill in fill at center area?
How would you sawcut the slab to control cracking?
Replies
I'm not a builder, but floating slabs certainly can work in cold climates. I believe most garages in Minneapolis are built that way; my 24x40 unheated, detached garage is on a slab and it's been fine for ten years now (and it replaced a 12x24 garage on a slab that was built in 1948, the slab in that old garage was still in good shape as well when it was removed.)
I don't recall the exact depth of the edges but it's a lot less than 24", it's more like 12" or 16". It has some rebar in the thickened edges, and mesh in the rest of the floor. They didn't cut or form any joints in the slab at all, after a couple years it developed one crack through the middle but there was no shifting or spreading at all.
The integrity of a floating slab garage has as much to do with the soil beneath the garage slab and the way it was placed and compacted as it does with the way it was built. We have built at least a hundred garages like that and it gets down to -30 here in the winter.
You don't need 24" deep edges. Our edges are all 12". The main problem you have with this type of construction is frost heaving on soils that aren't sandy. If you have heavy soils or a situation that traps water in a "bowl" under the slab, the slab will heave and usually crack.
If you have sandy soils that don't hold water, you won't have a problem. In those situations where the soils are heavy or wet, we use frost walls and let the interior slab float inside the frost walls.
In those situations where the soils are heavy or wet, we use frost walls and let the interior slab float inside the frost walls.
would you describe what you mean when you say 'frost walls'?
thanks -
"there's enough for everyone"
I'll chime in here, David. Frostwalls are regular foundation walls bearing on footings, the footings being down below depth of frost.
We consider our frostline at 4 feet here, thus frostwalls are built with 4' forms sitting on top of the footings. Top of wall is about 6" above finished grade.
Gene
Most of the floating slab garages that I had seen in Ia were similar to what you described. Actually the 24" edges are more then I'm used to seeing but a little overkill isnt all that bad!
Doug