Re: Driveway Base: is 12\" Crushed Stone Overkill?
First, provide a good subgrade under your stone. Cut out and replace anything that is not sound clay. Grade your clay subgrade and proof roll it with a loaded dumptruck. Any places that move need to either be undercut and replaced with aggregate, or compacted more, or both. Put your effort into providing a good subgrade.
Pay particular attention to proof rolling that old trench. You may need to undercut that.
I generally place concrete pavements on a 6-inch compacted crushed aggregate base. Use State DOT road stone. Concrete pavement isn't as sensitive to base thickness as is asphalt, because it is a rigid pavement. The base does allow you to have a clean, compact surface. Compact the base to 95%, if you have a way of measuring it. Watch the moisture content in the aggregate, some compact well in only a narrow range of moisture levels. Proof roll your base with a loaded dump truck.
If you insist on the 12 inch base, place it in two lifts and compact and proofroll each lift.
Depending on your drainage, you might consider a porous base, using open graded stone. This will allow you to put drain tile along each edge and extract any water that gets into the base course. You'll definitely want to use geofabric with a porous base, and it's not a bad idea with a normal base on clay.
Watch your drainage. You want to get the water away from your road. Put in roadside ditches and drain them to somewhere, if possible. This will do more than anything else to keep water out of your base. A saturated base is probably the biggest cause of road failure.
Put some cross grade on your slab if you can, to get the water off the road. You can either crown it or slope to one side. I generally call for a 2% (1/4 inch / ft) cross slope on roads. You can go as flat as 0.5% slope on concrete, if you can control the grades that accurately. Anything flatter, and you will have birdbaths.
4,000 or 5,000 psi mud with air entrainment is what you want. I concur with not allowing the addition of extra water to help with placement. That weakens the mix. A five inch slab is generally adequate for cars. A seven inch slab will generally carry semis. I like the thickened edge slab, as the edges are the weak bit. You might add an inch to your edges.
You might consider welded wire mesh instead of rebar. You may be able to get the same weight of metal if you pick the right mesh. The biggest value of reinforcing is to eliminate shrinkage cracks. It doesn't do much for the structural strength of a slab, as the slab is pretty thin. But, as you say, it holds the pieces together.
Be sure you put in control joints as soon as you physically can. The joints should be spaced at maximum intervals of thickness/24 (two feet per inch of thickness) with the panels as square as practical. If you're going to saw cut the joints, the saw should be on the slab as soon as the concrete will cut without ravelling. If you wait until tomorrow to saw cut, much of the cracking has already occurred. Tooled joints, of course, are put in while the concrete is still soft, and are fine if they are deep enough. I usually think about cuts t/4 in depth.
Visqueen the surface or apply curing compound to ****** moisture loss. Concrete cures by chemical hydration, not by drying. It needs to be kept wet. Once it dries out, it stops hydrating.
Concrete reaches about 2/3 of design strength in 7 days and full design strength in 28 days.
