Let me back up a bit. I re-read your original post and I think I have a better understanding of what you are trying to accomplish. So your design consists of a 6" structural slab supported by a pier every 12' throughout the interior and rebar doweled into the perimeter wall?
I'm curious about the reasoning for this approach. A ground supported slab on grade is a much simpler and risk free approach but maybe something about your local soil conditions lead to the need for a structural slab.
My first question is whether this was engineered or did your contractor come up with the plan. I'm not an engineer but I've built hundreds of thousands of square feet of structural slabs and the design seems a little sketchy to me. My experience is with large commercial and industrial projects that are probably over designed so maybe I'm overthinking it but I'd want an engineer to check the design if one hasn't already.
I'd much prefer to see the slab supported on a shelf at the perimeter to provide better support and allow some horizontal movement to and I'd be looking to thicken the slab over the interior piers to resist punching shear. I don't know what the load bearing capacity of a 6" slab would be spanning 12' is but it seems kinda light to me. Finally crack control is an issue. Structural slabs don't have control joints and with your perimeter constrained by the rebar dowels, the slab can't contract and you will get a lot of random cracks. Shrinkage cracks aren't generally a concern in a ground supported slab but that's not what you are building so reinforcing becomes crucial.
Keep in mind that your slab will be supported 100% by the piers and dowels. Even well compacted soil will settle a bit. This isn't an issue for a slab on grade as long as it's relatively uniform. When this occurs under your slab, the load will be transferred to the structural supports. It might be fine but I'd want an engineers stamp to protect my investment. Might be time to tap the brakes.
You are correct, it will be a 6” structural slab supported by a row of piers in the middle of the slab and rebar anchored into the foundation walls every 12” all the way around.
We have expansive clay soils here, which probably means more to you than me. My original design with the beveled foam around the periphery as a heat break is code in Fort Collins, CO, so I printed their code diagram and showed it to him yesterday. He explained they can do that there because they have very different soils than we have here (Kansas City).
To explain the site a little bit, my house was built 30 years ago with an on-grade basement slab floor (walk out basement) and then around the side of the house where my existing side-entrance garage is 9’ of fill were used to bring grade up to the level of the garage floor for the driveway. My existing garage’s foundation wall from the footer to the top of the stem wall is 10’ tall, and the stem wall is only 18” above the top of the slab. They are very tall foundation walls. My garage addition, which is abutting the existing garage, necessitated excavating down 9’ to find virgin dirt for the footers for the foundation walls. It’s a lot of old fill. We discussed hauling away all the dirt inside the foundation and bringing in new for compaction (and how that would be the ideal scenario), but it was ultimately decided that financially such an undertaking wouldn’t be financially feasible given that the addition is 1,350sq. ft. He suggested going the structural slab route mirroring how he has had engineers design them in the past. He did ensure the pier design for my project was appropriate with his engineer, but I don’t have a stamped document verifying that. I do, however, believe him.
Funny you should mention wanting the slab to sit on a ledge all the way around - that is exactly how I drew up my initial foundation detail, but I shared the design here on garagejournal to ask a question about how to achieve a certain spacing and everyone was telling me it was a bad idea and not to do that. So here I am now, rather wishing I had done it!!
I really, really like your idea of thickening the slab some more over the piers. The original plan was to use a 3’ auger attachment on a skid steer to drill down for the piers, then bell out the bottoms of the holes by hand, and pour it all continuously with the slab. As it turned out the be so much fill with big rocks he wasn’t able to auger the holes and had to instead over dig the holes with a mini ex. Then pour footers and use sono tubes for the piers. Given that the sono tube piers are a smaller diameter than augered piers would have been, I believe you are spot on the money with thickening the slab in those areas. Thank you.
We have discussed control joints and he’s leaving that decision up to me. I’d not considered that a structural slab shouldn’t have them. The entire slab will be 2,000sq.ft. (28’x69’), and I am worried about shrinkage cracks, especially due to the 69’ dimension. Do I forgo them and accept that with rebar 12” OC EW they will be fairly small and contained?
And as far as better supporting the slab at the periphery, is there anything than can be done now that the foundation wall has already been poured? Have rebar drilled every 6” at the periphery instead of every 12”? Should I “just” (ha!) increase the overall slab thickness to 7”? I don’t want to add another $2K to my concrete bill, but I also don’t want this slab to fail.
