Pre-made foundation?

[sharp21]

So I have just been given 2 concrete slabs in excellent condition. One is 6'x5'x4" & the other is 6'x2'x10".
Looking at the plans, if I put the 2 together it will be just a few inches smaller than what the plan calls for, but large enough to build the 42" oven on top of.
My plan is to dig out the hole & pack sand into it until the slabs are both level & even with the ground. I realize there is a difference in width between the 2, but it shouldnt be too hard to match them.
Any major problems with doing it this way as opposed to pouring a foundation?
S.

[dbhansen]

I have no personal experience with this type of foundation, but I would worry about the Canadian frost heaving them in different ways, or if they were to settle at different rates, leading to an uneven foundation. Can you join them together somehow, maybe with rebar drilled into both sides? Do they have rebar in them already (i.e., are you sure they're strong enough)? At a minimum, you might want to put a thick layer of gravel under the sand and a layer of rigid insulation under the whole thing to help minimize the heaving. Here are a few related links:


Ground Freezing and Frost Heaving - Canadian Bldg Digest
Thread (Using Existing 4" pad)
Shallow foundations

[TDVT]

I wouldn't use sand, that will hold moisture. A bed of washed stone (3/4"+ size, a couple feet deep) with drains in the bottom running to daylight (if you have a slope) or a dry well would be a safer bet. If there's no water underneath, it can't freeze.

A horizontal layer of rigid foam insulation around the slab (buried a few inches deep) would be good too.

[sharp21]

I think a layer of stone, covered with plastic, then sand to level is the way forward.
I could also drill a pile or two down to fill with concrete
Thanks for those links!
S.