I removed an old sunken sidewalk, and am getting ready to form for the
new sidewalk. I was wondering how much of the old stuff I can mix
into the new mixture before it compromises it, and if that's a thing
that I should consider at all. It'll save me 1) money for the volume
of concrete that I'll need to buy, and 2) the trouble of hauling the
old chunks away. The sidewalk was partially laid upon an even older
sidewalk under it, and the two layers did not fuse - I slid the new
one off the old one when I discovered it. If I throw my chunks in the
mix, will they fuse properly to the new porridge that I'll lay, or am
I asking for a world of cracks by mixing the old and new? I'll be
busting the old pieces into chunks baseball size or smaller (most of
it is chips now anyway), so there'll be no slabs or anything
outrageously big. My father poured a couple of slabs where he threw a
few old chunks in the mix, but that was minimal, and wasn't a heavily
traveled part.

By the way, I'm pouring over the old sidewalk like the previous people
did. The part that sank was the part that didn't have old sidewalk
under it, so why remove a terrific base? But can I prep it with
something so my new walk bonds to it, or is that a worry that I should
forget about?
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