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Broke the concrete in my basement to run new pipes for the basement bathroom. The plumber put sand under the pipes to support them at the correct angle. I asked him not to glue the vertical sections as I'd probably have to disassemble them to build the wall there.

My question is now... What type of cement do I use to fill it back in? Should I hammer drill into the sides of the cutout and put a few pieces of rebar in? Can I just fill it up with sand and level it?

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Also to the answers you've gotten I'd advise putting some Foam around the pipes where they will penetrate the slab. Can remember what it's called but is usually 50mm wide and 4-5mm thick. It is to prevent the pipe snapping if you have any movement in the house.(especially when all the penetrations are in 50mm pipe :-( ) – UNECS Apr 25 '12 at 21:08
If you are going to put tiles, panels or vinyl over the floor you can fill the gaps with weak concrete(more sand than concrete)and screed it to the rest of the floor. It also helps in the future to break the concrete apart if a serious blockage or leakage occurs but it strong enough to support good weight. If you are not covering it any attempt to mask it will not work as cracks will appear near the joints because concrete always works(expands and contracts to heat and moisture). – ppumkin Apr 26 '12 at 16:00

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Can you just fill it with sand and level it? Yes, I suppose so, though it is not a good idea. This would probably be viewed as a problem when you decide to sell this house. It would be a tripping point, so dangerous. It might also be an avenue for radon gas to get into your basement.

Do you need to have rebar into the sides? I doubt that. There is no load being carried here. No real need for serious strength.

Just pick up a few bags of cement from the home store, mix it up as indicated in a wheelbarrow, and trowel it in. Make it reasonably flat, but you need not be a perfectionist if this is simply a storage area. Just try to minimize any trip points.

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When my post was edited, a bit of context was lost.. these pipes are roughed in for the basement bathroom, and I still need to build a subfloor over the pipes and walls around them. So far I assume I need to remove a bit of sand from the sides of the pipes so that I can have the concrete deep enough. I was thinking of filling the whole thing to about 1/4" from the top and using self-leveling cement so that my floor could be well supported - any opinion on that? – kavisiegel Apr 26 '12 at 17:03
I'm not sure that 1/4 inch of cement on a bed of sand will be stable. Step on it, and you will get cracks. – woodchips Apr 26 '12 at 21:20
I meant fill with cement close to the top, not sand! My bad not specifying. I actually already poured 600lbs of cement in there, just giving it a few days to cure before I put the self-leveling stuff over it. I hope the surface is close enough to flat that the floor joists don't teeter and crack grout lines. – kavisiegel Apr 29 '12 at 22:07
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You can just use bag cement(quickcreate) to fill it in. Rebar is not necessary. Just make sure you compact the fill and that you have a min of 2-3" of concrete. And don't forget to duct tape any stubbed out openings.

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The rebar is only required if the concrete is going to be put in tension -- Such as a spanning application. Slabs on compacted earth are pure compression. – Chris Cudmore Apr 25 '12 at 20:07

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