sidewalk - using old concrete pieces in new mixture

[siteseer]

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?

"Usenet is like a herd of performing elephants with diarrhea -- massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind boggling amounts of excrement when you least expect it."

-Gene Spafford, 1992

[JTM]

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

[Ron]

I have never used concrete pieces, but I poured a garage floor about

30 years ago. I mixed the whole floor by hand in a wooden tub with a hoe. I made the floor in 6 foot squares, one at a time. In the base of each square, there are all the rocks that were laying around the property. I left a space between each rock, and allowed at least one inch above the rock to the finished floor. What I did were place the rocks in the hole, and hose them real well to remove dirt. That also settles the mud under the rocks. The rocks varied from one to 12 inches in size.

This floor has NEVER cracked except along the lines dividing the sections, and they have all remained level on top. This saved lots of concrete and mixing. Farmers used to always do this, so it's not new.

[claudia]

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.