Hi Guys...
I am building a 28X40 garage in central Wisconsin this spring. I plan on pouring a floating slab for it in 3 sections (pours). I plan to make the edges 10" deep and 10"wide with the rest of the floor at 3.5" thick.

There are no building codes in my county that stipulates a foundation for a detached garage... that said. I want to make sure that this is a good foundation. Thoughts...

I plan to tie the sections together with 3/8 bar and put some in the corners of the 10X10 edges. Do you guys think this is OK?

Also... I am digging out all the Black dirt and filling the hole with sand. This will be about 2.5 ft of sand in some spots (deep black dirt) so I plan to use a compactor... What is the best way to compact this volume? A ft at a time? Soak it with water and comapact. Sorry for all the questions..

Thanks...

Hartwa

It is common in my area to use a mixture of gravel and gravel dust for such applications. Called 21A in Virginia because that's state department of highways designation. No compaction really necessary, just run you machine over it a couple times. A little soft when it gets wet but dries out hard like a rock.

What county do you live in? In my county what your are doing is called a thicken esdge slab and it works just fine. However the county just did an about face and said all detached garages must have full footing foundation. DO NOT use sand. Sand never really stops moving and that is not a friend of a slab so do not use it. Harold is correct but I really think you should talk directly to the building inspector about this. You need a permit and he will approve or not approve of everything and this may be a waste of your time if the person who said there is no code was mistaken.

Thanks to both of you for the help. I already talked to the county zoning dept. (chippewa county) and they said there is no regulations on the foundation for a detached garage. They gave me all the stand offs from the property line... that is it. I have to submit the plans and have the site inspected to get the permit.

Will the material guys here be familiar with the 21A mix? What exactly is it? 1/2" gravel and gravel dust at what ratio?

After I remove all the black dirt I will be on clay soil... Should I fill that entire cavity with the 21A mix or just the edges and they sand undernearth the flatwork?

Thanks again for the help...

21A :
Luck Stone - Construction Aggregates

My experience is in roadbuilding and I will acquiesce to the previous posters who didn't mention it but 3.5 inches depth of concrete for vehicle loads is inadequate. Maybe I am mssiing something but our min. for NON-traffic bearing sidewalk was 4 inches and a review of several sites suggested 4 inch minimum and many included wire fabric mesh, which helps control cracking.

Clarification: that should have read "suggested 4 inch minimum for garage slabs and..."

Agree with smearcase about the thickness of the concrete. I only addressed the fill material. I would probably pour about 8".

No, the gravel people in Wisconsin will have no idea what 21A means. That's just a local term. A mixture of gravel not larger than about 1/2" to 3/4" diameter with gravel dust included, meets VA department of highways specification 21A. You want the dust to fill in the voids between the gravel and reduce water retention.

Just ask for a mixture of small gravel and gravel dust. Talk to gravel people and tell them what you are doing. They can tell you whats available in your area and what to use.

Primary point is that the gravel does not require compaction. Soil compaction can be difficult because you have to have the correct moisture content and have to compact it every few inches of fill.

Also suggest that you slope the pad 1/4" per foot toward the door or a drain so melting snow on you vehicles run out the door.