Concrete on top of concrete?

I bust up concrete all of the time and reuse it as large scale aggregate and I've never had a problem. The post you were responding to was a bit misleading, though. To bust up sidewalk concrete into 8" pieces and reuse it in a new sidewalk pour would seem to create a concrete mosaic surface. Maybe that's not what he meant, but that's how I'm reading it. He might mean to excavate down some more, and bury the busted up concrete that way, but they you're dealing with more excavation and have to relocate the dirt.
I bust up the concrete so it's in smaller pieces for a slab, no bigger than half of the depth of the new concrete, place it in the bottom of the form and pour the concrete on top. I do it more for "reuse, recycle" reasons than anything else - I have to dispose of less and it's not a compromise. But you're not getting rid of a section of slab by reusing it in the same size replacement, maybe a third, and for one small section it might not be worth the effort for some people.
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