Adding 6 ft of garage to existing slab on grade

[rborloz]

We're building a new 10x20 garage using approximately 4 feet of the slab currently in the living room. We're planning on attaching a new 6ft wide slab with foundation to the side of the existing foundation using expoxied rebar as you had described in one of your other responses. The joint will end up almost in the middle of the garage. The existing slab is 46 years old. The existing foundation wall is 24in high. The ground is flat and has not been disturbed for at least 30 years. Can you think of any problem we might encounter? What should we use for fill under the new slab? What kind of spacing and or pattern should the epoxied rebars be placed? What size and depth?

[Rich]

I would attach the 2 foundations together but I don't think I would attach the 2 slabs together - would probably be better with an expansion joint there. Use 1/2" expansion material against the existing slab (and around the perimeter against the foundation wall) and pour up to it. The problem I've seen with tying open slabs together with rebar is you'll typically get cracking occuring in the new slab right above each piece of rebar as it contracts and expands during curing.
Foundation rebar spacing should be 2 #4 at the top middle and bottom and these should be continuous (with 12" overlap minimum) around the stem wall.
As far as backfill goes - you can use structural fill or some compactable material with 0 clay in it.. or use pea gravel as long as it's not too deep.