I have a post-frame shed, 12x16. there are posts at 6' on the short sides and every 4' on the long side. posts are set ~18" below grade, on a 2" cap of cement over a few inches of well compacted gravel, holes were all about 27" or so, 2x6 girder runs level at the top (very level all the way around) around base.
I have ~5.5" to grade from bottom of 2x6 girder to top of board, and the I've got posts set in concrete, at 24" from the top of that board so I want to fill that in with concrete, I could put gravel in, say 2" and 3.5" of concrete over that.
Its about 3.75 cu yards of concrete, or like $400 It will cost $100 for gravel to fill in 2", but my concrete is still only a little cheaper, and it will be a thinner slab.
I could put gravel in, say 2" and 3.5" of concrete over that or just full 5.5" of concrete, or I could do lot of gravel and a 2" top coat over that with a bunch of steel and fibers,
Its just a shed, so I'm not too worried about it, I'll put steel mesh in and probably fiberglass to control cracks and run a groover down the center.
Point is, can I get away without using gravel and just lay plastic down and pour a slab, or is the clay soil going to destroy it if I don't have a couple inches of gravel. I am not, going to dig this down deeper, or pour higher that the girder.
Its 5.5" of space, with a few low spots I can rake out. It much faster to do a one day pour so I can put my books back in and my tools to keep them dry, so full 5.5" concrete is good and fast, but a couple inches of gravel would be nice, but is a total extra set of work for little gain
(Yes, I should have done this in a different order--t was not an option at the time.)