Is this a properly installed sub-base for concrete slabs?

by Bob
(Hugo, MN, USA)

We have a contractor out pouring new concrete slabs for us (12 of them). The reason for the new slabs is some were hollow, or sunk down about 2 to 4 inches from the other slabs, and some where cracked and sagging.

The contractor came out, busted up the 12 slabs, and removed the big chunks leaving baseball size and smaller chunks behind.

Now they are building the forms and pouring crushed gravel inside the forms on top of the old concrete chunks, they did not dig any of the old dirt out. They are going to be using #3 rebar when they pour the new slabs. The slabs are going to be 4 inches thick once completed. Do you think this is the proper way of doing the work and do you think we will have issues in the future with these slabs?

A. I think they should have completely removed all the old concrete. It sounds like the sub-base should have been dug out and new gravel installed. "Why did the old slabs crack and settle?" The depth of the new gravel would have been determined based on what is already there, but 18" minimum of new gravel up to 4' in heavy freeze/thaw areas and if a lot of clay is present in the existing sub-base.

If the sub-base failed to support the old slabs, what makes the contractor think adding some crushed rock on top will support the new slabs long term?