If you have not poured the new concrete yet, one thing that I found helps alot with bonding new concrete to old is to hammer drill some holes in to the old concrete and screw in some concrete screws (blue tapcons is what I have used) screwing these in half way gives the new cement something to bond to, you can also get adhesives that you mix with the wet cement and brush on the dry concrete to create a chemical bond as well.
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