Pouring concrete over rocks

Concrete slabs crack when the thickness varies without rebar.

There is a recommended ratio of area of concrete to area of rebar. This is NOT structural steel, but rather a ratio of reinforcing steel to concrete and is often called “temperature steel” and is represented by the symbol Pe.

If the slab can vary from 6” deep to 12” deep, as you say, the deeper slab should have twice the reinforcement in BOTH directions.

Looking at the form work, it appears the slab could be as thin as 1”. If so, this is obviously a problem and will crack. There should be a minimum of 4” thick concrete and it should have rebar extending through this portion of the slab. (4” is minimum because you need 1 1/2” cover over and under the rebar.)

I've been removing some concrete this weekend that I poured years ago over rocks (but without the large boulders). I made the mix a bit more liquid and used a rebar to push the mix into all cracks/spaces by hand, rocks become aggregate. It was as strong as anything for domestic use. Don't build a nuclear bunker if it is only for a car park. BTW, what is this slab for?

Sorry, I'm a little late but for what its worth, my honest opinion it would of been better to add more rock in low spot mid left and jack hammer or chip/saw on the right and do the same for upper left corner.. potentially build up forms to higher grade if needed, looks like a porch or look out decking of sorts.

I too am having concrete issues but of different type, not to mention new to this site.. hope all is well ..

A vibrator will get excellent penetration of regular concrete into the broken rock. Adding more water to concrete mix reduces the strength. I poured a car park addition to a drive way using broken concrete from a sidewalk . The concrete minimum thickness was 2 " or less . It was in fine shape 10 years later when I moved . Of course I doubt it would be strong enough for moving traffic.