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I've got a situation similar to this one (slab finishing/patching around roughed-in plumbing remodel), but in my situation the finishing job poses a couple extra problems:

1) The trench routes under a floor-to-ceiling, built-in wall cabinet, so there is about 1.5 to 2 feet of trench I can't access vertically.

I'm worried about getting a good meld between the concrete I fill underneath the built-in and the bit of slab that remains below it. Obviously there is a reasonable pocket I need to fill, and I can't think of a great way to get the concrete under it so that it's well-packed.

2) The area (small laundry/utility room) is carpeted, and currently there's a surgically-cut area of carpet pulled away from the trench.

I've read concrete "bleeds" out some water before its ready to be finished/smoothed. How long should I let it sweat before I smooth it, and is it worth my while to attempt protecting the carpet beyond the trench? The carpet should ultimately be replaced, so my biggest concern is getting a level patch, but I'm wondering where will the bleeding water go?

Am I over-thinking this, or is it as simple as mix some 'crete, dump it in the trench, and smooth with a short length of 2x4?!?

The trench going under the built-in. There are at most, 2 or 2.5 inches between the slab and the pea gravel there.

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Top-down view of the trench showing the carpet. I'm worried, what happens to the water the concrete will bleed out?

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  • I don't understand this statement "... there is about 1.5 to 2 feet of trench I can't access vertically." Your photo shows an irregular (but closed) opening in a slab. How did the concrete get removed from UNDER the built-in, such that it needs to be replaced?
    – HerrBag
    Jun 15, 2013 at 18:45
  • Those guys used shovels, and I think a crow bar to dredge under that area horizontally. They only needed to feed the PVC pipe through it, so it wasn't a huge deal to them---but I'm realizing now it complicates finishing the job. I've since added the pea gravel, so it's hard to tell from the photo how far the cavity extends under the built-in. :/
    – elrobis
    Jun 15, 2013 at 19:06

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You can mix a pourable patch for this repair, but I don't recommend it. That's when you would experience bleed water. You just wait for it to reabsorb after floating and before finishing.

You would be better served with a moldable but not wet mix that is used for deck mud. As such, it will pack and smooth down with a wood float, but won't run or bleed.

After floating, let it rest 10-15 min or until a thumb press has some resistance (as opposed to sinking). Then use steel trowels for a smooth finish.

Be sure to dampen the edges of the cut slab and spritz the gravel (check for an intact vapor barrier under the gravel, you might add one if missing).

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    I would delay any concrete UNDER the built in, until such time as you have access. The 'curb' you build at the perimeter, and under the lip, will suffice until then.
    – HerrBag
    Jun 15, 2013 at 19:26
  • Fair-enough---thanks for the tips. So it sounds like the most important detail is merely avoiding a soupy concrete mix, in favor of a more oatmeal-like mix?
    – elrobis
    Jun 15, 2013 at 19:29
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    yes. 'orange' size ball can be squeezed together, but won't wring water
    – HerrBag
    Jun 15, 2013 at 19:36

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