slab talk: Gravel layers and "thickened" slabs

Working on the slab, here's the progression for today.  Below you'll find a picture of the recycled concrete base layer.  Pretty big chunks.  We'll use that to bring the base to a few inches of the desired gravel level.   

You can really only get as close to the level desired as the size of the rock.  so if you're using 2-4, it's pretty hard to get closer than 4".  So the next layer is 2"  clear, which is much smaller and more uniform.  Using that I can get within 2" of the desired level.  (yes, you can do a lot of work to get it smoothed down and really pound it into the ground, but that's a lot of labor.  )

Here's the same area, now with layer of 2" rock on it.   It's within an inch of the desired grade.  I'm working hard on getting the grade right because concrete is very expensive -- about $80/yard, delivered.  Crushed stone is $13/yard.  An extra 1 inch of depth would cost me $640 -- for every inch! 
The next step will be to put an even smaller crushed rock in the center of the slab, raising the center of the gravel 2" or so. 

What the picture above shows you is two slabs.  For this particular project, I don't need the entire slab to be 6" thick; I need the edges of the slab to be strong, strong enough that they can support 10,000lbs of ecology blocks, and I need the edges not to crack under the weight of the tractor, that weighs 12,000lbs.  But the center of the slab won't need to be that strong -- 4" of reinforced concrete with a well-compacted base should be just fine.

So my first step is to level the entire site with the gravel, and then I'll add 2" of height in the center of the slab, and by doing so reduce the total amount of concrete that I need by close to 9 yards -- a whole concrete truck.  Doing the math it reduces my cost about $650, net.

You can do this sort of thing if your grade is very good.  When I'm done I'll be within 1/4" tolerance on my grade.  If I wasn't sure about my grade, I'd go thicker just on general principle.

Too many people grade with concrete.  Sure, it works, but it's a terribly expensive way to fill a hole.  Gravel is pretty easy to work with, and much cheaper.  

Just a note:  For each size of gravel laid I'm carefully spreading it, measuring it with the laser level, and then running the compactor on it.  So every inch of this gravel will have had at least 3 compactor runs over it before I'm done.