Turkish Code (TS500-2000) imposes the min. reinf. ratio as 0.01 but if the amount of reinf. required for the cross section is increased 30% this limit can be reduced to 0.005.
As far as I know and implement, take this limit as 5'..
and also after concrete touch the wet surface, do not allow the concrete spread more than 10'(diameter!)..
THe main idea for keeping this thickness is to prevent the reinforcement from moisture, water or some undesired chemicals/salts in soil. Even ona crushed stone layer this thickness should be kept at least 3", I think..Bcs crushed stone is a porous media that does not keep water or other...