Site, I have poured and finished many sidewalks, new and pour-overs,
and I've never thrown away that old, busted up concrete. Purists in
here will jump all over me, but remember, this is a sidewalk, not the
Bust up that concrete in place. While aggregate should technically be
no more than 2/3 thickness of the pour, remember that when you're done
this old concrete is going to be at least 2 to 3 inches beneath
screed-off level in any case. That old concrete can be large chunks,
expecially if there are 2 or three inch gaps in between for fresh
concrete. A better bond will be achieved if you take these old chunks
and turn them over. Make sure the rough surface you're exposing to the
new pour is hosed very clean. Scrup off dirt if you have to.
Form your walk and hose all old concrete clean. See to it that none of
it is above two inches from your form tops.
For added strength to prevent any future cracking, lay down old chain
link or quarter inch, three-eights rebar when fresh concrete is poured
just barely above old in-place concrete. Then shovel in the rest of
the mud to form level, screed and finish.
The key here is in making sure the old concrete left in place is very
clean and exposed on its rough sides to the new pour.