Pouring slab for existing wood shed.

I would never pour concrete on a hand-tamped pad. Too much investment in the mud, too much time in setup and finishing to risk it. Compaction should be done with a Rammax or Wacker in lifts of 4-8", depending on soil. Vibra-plates are for finishing asphalt or paver blocks, or can be used as a finish for concrete after being compacted and tuned up with a more reliable method. Compaction is everything in concrete work. It determines the future of your project. The exception is "gravel" which is for drainage and compacts easily with a vibra-plate, but aggregate base and fill need to be compacted right.
 
If your not disturbing the ground (digging out and filling back up) then compacting is not as big a deal.
But packing that gravel by hand I would consider almost imposible.
I put in 2 inches of gravel and when it is compacted it's very much like walking on any sidewalk. Until you drag a toe or something.
Rental rates around here are cheap as far as renting one of those machines.
I'm no professional contractor or anything. I'm just crowing about how I did it.

My addition previously (9 years ago) I made the mistake of removing to much soil.
I filled it back to the height I wanted with roadmix. I was told it didn't need compaction. So far so good on that addition also.
 
Keep in mind that it may be on blocks for a reason. In some places any structure with a foundation that's over a certain measurement (in my area it's 12' in any dimension) requires zoning and a building permit, and has to either be to code or have approved engineered plans. Once you secure it to the slab it becomes permanent. It's weird, because you can have as large of a shed as you want as long as it's not on a permanent foundation, sort of a loophole.
You may not be in an area where that will matter, but you don't want to go through all that trouble and the county inspector see you and raise a stink. Look into it beforehand.
-Drew
 
FWIW, I know very little about doing concrete. I am in Wisconsin so we do see real winters here. I poured a deck 35 years ago and acording to the experts I did everything wrong. now it is 35 years later and there are NO cracks at all. I even used it as a parking place for 15 years. I put dowm 4 inches of gravel tamped by stomping it with my size 14 feet. I have rebar around the perimeter and wire mesh across the whole thing. There was no such thing as fiber back then, at least not that I had ever heard of. I poured the whole thing 5-1/2 thick because I did not want to risc any cracks.

Of course YMMV.

As mentioned above, check your building codes, some inspectors get great enjoyment in finding fault with what you do, and a flat out violation, whether you knew you were in violation or not, will really make their week.
 
Much of Florida has a kind of limestone ground base which is basically calcinated shells. It is basically a porous limestone. Not like Louisiana marsh.
I’d do a biopsy of what’s below your brick wall first. See how deep it goes and is it set on a concrete base. Sort of chain wall. Best would be if you can pay a contractor or engineer to inspect it for you and give an opinion.
 
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