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I have a hole in the concrete slab of my foundation approximately 4' x 6'. The slab was originally poured directly on the dirt. The slab is approximately 4" thick with rough edges. The hole is towards the middle of the slab.

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I have a few questions:

1) Is there any point in putting gravel in the hole, for drainage, before pouring new concrete? If so, about how much? 4"?

2) Considering that the edges are rough, do I still need to undercut them with a chisel? Or will the new concrete lock onto the edges well enough as is?

3) Should I match the thickness of the existing slab for the new section (4"), or is there another more preferred thickness?

4) Would it be best to create some sort of metal reinforcing structure, or would fiber enhanced concrete do the trick well enough?

Thanks in advance for any help or insights you are able to provide.

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    May want to look for treasure first? Or was this where the coal boiler/furnace was when they poured the concrete around it? – Ecnerwal Feb 13 '18 at 2:40
  • @Ecnerwal. Yes, this was where the coal furnace was. I thought about digging around, but I thought it best to maintain the compactness of the dirt. – laertiades Feb 13 '18 at 2:51
  • Does your basement ever flood? If so, this might be a good place to put a sump. – Mike Baranczak Feb 13 '18 at 15:34
  • @MikeBaranczak No. Flooding is not a problem. I thought about putting a wine cellar there, but there are other places in the basement which would work better. – laertiades Feb 13 '18 at 16:50
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Probably low-stress, may not need anything much, but in the "better safe than sorry" mode I'd drill half-inch holes around the edges and stick reinforcing rod into them anyway. Steel is cheap compared to the rest of the job (labor and concrete cost.) That also deals with the "lock in" aspect better than chiseling.

You won't change the drainage under the rest of the slab by adding stone here, so don't bother.

You won't change the thickness of the rest of the slab either, so unless you have a heavy machine you're mounting on this particular spot that needs a sturdy foundation, 4" should be plenty.

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