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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"siteseer" wrote in message
...
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?

Baseball sized aggregate? This seems too big for a good bond. Concrete
typically shrinks on curring, but old concrete has already done its shrinking.
This creates shear stresses at the bond area which is why pouring concrete on an
existing slab results in a poor bond. As the new concrete shrinks it constantly
breaks any bond that is trying to form between the layers.

If you really want to use the old concrete I would break it to no more than golf
ball sized chunks and would use remesh (in any case).

Regards,

John

siteseer wrote in message . ..

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
Hoover Dam.

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.