[merkr]

I want to build a 10x12 shed like a house for a mini-gym (2x6 framing, insulated, gypsum, flooring, etc).

Is a 4" (3.5" actual) at 3000 psi with 3/8 rebar good enough?
Do I need some compacted gravel at the bottom? Or thicker 6" (5.5" actual) better?

I'm in California, my dirt quality is here: https://soilseries.sc.egov.usda.gov/OSD_Docs/V/VISTA.html

The location photo is attached below.

 

[F250]

I building an 8x10 and have been researching the issue for my own NC location. Everything I read says that a 4" slab is typically adequate for up to 1500 square feet. Given that you're using a heavier wall structure, footing will be a good idea.

Also, since you've talked about potential use as a weight room, that might push you to a slightly thicker floor.

Regarding the gravel, everything I've read says that you should always install a gravel bed under a remote slab.

When it comes to concrete, though, I'm a net-reading novice. More knowledgeable folks will come along and chime in soon enough.

 

[Nealtw]

IF you have removed all organic material from the area, and if you have scraped it down to undisturbed soil it will be fine but if you have levelled it with loose dirt it will need to be compacted.
The hill on the right side will be a water problem so I would gig out wider there so you can put a drain line around that corner and catch and direct water to a lower area.

I would also raise a concrete wall in that back corners to counter the dirt that will wash down against the building.

A 7" high wall can be framed up and poured at the same time as the deck.

I think gravel is a good idea and sheet poly under the slab.

 

[merkr]

Yes, I already planned for more excavation around that hill and a retaining wall so there is enough space to have a brick walkway. My focus today was primarily the concrete base. Thanks for the input!

 

[lug-nuts]

You do not need to go thicker than 4" on the slab. Every driveway across this country that is concrete is formed with 2x4's and unless you plan on parking a Semi on it 4" is plenty thick. A typical concrete 6 bag mix will yield about 4000 pounds per square inch of compressive strength after 28 day's. I doubt you will be placing more than 4000 pounds of anything in a single square inch anywhere on your slab.

Yes, compact, and then compact again. Also that's a big 10-4 on the gravel, compact that too.

 

[Missouri Bound]

4" is good for the concrete. Consider a vapor barrier and insulation under the slab.
It will help with any moisture that could come through the concrete and be very beneficial if you choose to heat or cool the area. Granted it won't take much for a 10 X 12 shed.