Repair a concrete slab opening

[MiamiCuse]

Hi I used a jack hammer to open the 4" concrete slab and made a trench for the plumber to come to redo the underground pipe fittings.

Once they are done I need to pour the concrete slab back.

Question I have is...

(1) As I broke off the old concrete slab, there were a lot of the large chunks of concrete I have as debris. Should I use those to fill in the trench up to the 4" mark below grade, or should I use sand and pack them tight?

(2) There were wire mesh inside the concrete slab that I broke up. The trench is about 8" to 12" wide about 10 feet long. Is there anything I need to do as far as the wire mesh? Should I cut another wire mesh of that size and put it in? The way many of them broke off I am not sure I can tie back to the old ones.

(3) Underneath the concrete slab is a sheet of plastic, is that a moisture barrier or some insect barrier? Now that it's broken what can I do to have the same effect?

Thanks,

MC

[Mike]

I would definitely not use the pieces of concrete. They may settle in odd ways.

The mesh is there for crack control. I doubt that you would need to worry about a crack spreading from new floor to old floor. But someone with more knowledge may know better than I do.

Moisture barrier. I would cut a piece of plastic (could be 4 mil) and place it in your trench. You could try to glue it to the old moisture barrier. I have not seen people do this, but I cannot imagine it would hurt in any case. Use a glue rated for below grade (obviously).

mh

[beecrofter]

A rule of thumb is to use aggregate no bigger than 1/3 the slab thickness.

[RicodJour]

As long as they're not pressing on the pipe, sure use them as fill. Just make sure that whatever you put down is well compacted. Lift and drop a sledgehammer over the entire trench fill then smooth out the surface.

It really doesn't do all of that much. If your slab wasn't cracked to the point that the sides were uneven, it's not going to prevent anything. The subgrade preparation was adequate and the mesh was used as insurance. If it makes you feel better, put it in - it can't hurt. Just won't help if you prepare the trench subgrade properly and place the concrete correctly.

Put some more plastic back. Again, it's not going to do much as there's no real way of sealing poly to poly easily. It's cheap and easy enough to do, but any little gap will allow moisture to migrate.

R

[** Frank **]

I've done that couple of years ago using chunks of concrete from another project - no problems so far. As mention elsewhere, make sure its compacted. I would have used a concrete vibrator for extra insurance if I had one than.

I would drill holes on both sides of the existing slabs for the rebar to pin the new pour in place to keep it even over time.

I would just add a new sheet of plastic like other mention - not much more you could do. Its for moisture barrier more than anything else. I found out for exterior installations if you don't use it, tree suckers will find the smallest cracks to pop out even if its 20' or more under the slab.

[MiamiCuse]

I cannot seal them anyways, as some of the plastic sheets are torn at the edge where the concrete broke off so most of the edges do not have any plastic protruding just along a few spots. So there is no way to have that seal. Unless I loosen the sand under the concrete slab all the way around the edges six inches in or so, then glue a thick 10" strip from the underside onto the existing barrier, then fill in the sand to the bottom of slab, then add in a new piece to glue to the other side of the strip, then fill with concrete. Probably not worth it I guess.

MC

[Rudy]

No..I'd ditch them, cover my new pipe/fittings with sand and refill the trench with 3/4" to 1/2" *minus* aggregate with FINES and compact well before repouring the 4" slab. Plain sand doesnt compact well. Moisten the "old" concrete edge so the new stuff will adhere better.

Dont worry about a 8-12" wide cut

Replace it with some 6 mil vapor barrier and tape the NEW to the OLD with RED Tuck Tape making a good seal. It keeps any moisture below the slab...Below the slab.