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The corner of the slab in my garage was removed to bring in plumbing. It's been backfilled with ~3 feet of dirt (slab was 2ft thick). What is the best way to repair this? I'm thinking I'll dig out approx. 1 ft., compact as best I can, and fill with cement, only applying adhesive to slab sides (not the stem wall) and maybe chisel out an undercut on the slab. Anything I should do differently/think about? I've never worked with cement before. Thanks.

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  • Good answer below from @michaelkaras , but I'm dying to know... was that a typo about the slab being 2 feet thick? Or are we just talking about the footer around the perimeter? If the slab itself is way thinner, don't waste time trying to get rebar in it to pin the new concrete -- just drill into the footer on the two outside walls. (6" penetration, 5/8" hole, 1/2" rebar, around 16" long, structural epoxy is our recipe, but you could probably skimp a little on that.) – Aloysius Defenestrate Jan 28 '18 at 2:37
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    Yeah it's about 2 feet thick. Actually looks like two slabs on top of each other, the bottom one being a coarser aggregate. It was put in before I was born, so not sure why. It borders a wetland and this time of year it gets real wet around 3 feet, not sure if that has anything to do with it. But you're saying anchor the patch to the stem wall instead of the slab? I had thought the slab was supposed to "float" inside the stem wall, so wanted to check. Thanks for the reply. – tb510 Feb 1 '18 at 18:00
  • Interesting that the slab would be so thick... in that case, do pin the patch to the slab. – Aloysius Defenestrate Feb 2 '18 at 14:55
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Pour in concrete after the base is known to have been fully compacted and settled. But do not make it a foot thick. If you ever have to work on this plumbing later you would highly regret having to break apart that block.

There should be no reason to have the patch portion be any thicker than the original slab. Also it appears that the original slab was not tied into the main foundation under the walls so you should maintain that with the patch. I see no special reason to chisel out an undercut of the slab. Just make sure that the edges of the slab are clean and free of loose particulate so that the new cement has the ability make somewhat of a bond to the existing slab. Adhesive on the slab edges would be a waste of time/money because the wet cement you pour into there would not stick to it.

If you want to work at it you could bore some holes into the edges of the existing slab and bond some short pieces of small diameter rebar that sticks out into the patch region. That would do more good to keep the patch edges lined up with the slab over time. If you so this however do it carefully so you don't end up just crumbling more of the existing slab in the process.

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