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Thread: Stability of once jackhammered slabs?

  1. #1
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    Stability of once jackhammered slabs?

    How stable (generally speaking) are floor slabs that have been jackhammered up to replace sewer lines and then repaired? Is it best not to re-do the floor with ceramic tile and instead pick a material that's easier to re-re-do if he floor settles -such a vinyl tile, sheet vinyl, or laminate?

    In particular, I have the case of 1958 vintage slab that's about 5 inches thick with steel mesh reinforcement. Obviously one should compact the earth that's replaced before pouring concrete over it. As to using reinforcement in the repair, I haven't found examples of people doing that. The soil being excavated is moist - probably from the sewer leak and/or from a previous owner irrigating a planter along the front wall of the house by flooding it.

  2. #2
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    When I've seen professionals do the repair, it is every bit as stable and strong as the original floor.
    I have a friend that put a two post car lift in his garage. The concrete guys cut a big "H" in the floor about 12" deep. Drilled into the side of existing slab, and used some type of a rebar tie to join the two slabs.Layed down new rebar and hardware cloth, located the base supports for the columns, and poured. It was beautiful when they got done.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  3. #3
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    I would backfill with dry sand and compact in lifts as the hole is filled. Compacting can be as simple as a hand tamper, or even a 2x4 if space is narrow. Being careful not to disturb your repaired plumbing, of course. Original wet earth does not make for a stable base after the fact.

    Rebar needs to be drilled in on either side of the cut. Don't skimp here, it's what will hold the slabs in the same plane. Every 12" minimum, if you wanted to go every 10", 8" or even 6" more power to you. Old school, we'd drill an appropriate sized hole and pound rebar in with a big hammer. New school, the hole gets drilled slightly oversize and the rebar epoxied into place. Since the replacement concrete will be trapped between existing slabs I would think the beat it in method would be fine. Tie both sides together and also tie runs lengthwise. When completed, if it's strong enough to walk on it'll be strong enough to hold the new concrete in place, even if the backfill settles out from under.

    When the concrete is placed use as dry of mix as you can work. That will help cut down on curing shrinkage. If your trench has cut edges it will be much easier to tool. If your trench has the jagged edges left from the jackhammer it'll be a chore. The new concrete has to be worked thoroughly into the nooks and crannies of the existing. Back up bit, I would apply a bonding agent to the existing concrete before pouring, too. Pour the replacement level with the existing slab. It will likely shrink slightly when cured, then you use a self-leveling compound to make the whole affair nice and smooth.

    If redoing tile, or even if placing tile on a new, unbroken concrete slab, an uncoupling membrane should be used. If cracks develop in the concrete along the repair an uncoupling membrane will allow the tile to move independently of the concrete and hopefully not develop the same crack. There are a number of different membranes on the market. A local tile/flooring store would be a good place to see what's available in your area.

  4. #4
    Quote Originally Posted by Tim Bueler View Post
    I would backfill with dry sand and compact in lifts as the hole is filled. Compacting can be as simple as a hand tamper, or even a 2x4 if space is narrow. Being careful not to disturb your repaired plumbing, of course. Original wet earth does not make for a stable base after the fact.

    Rebar needs to be drilled in on either side of the cut. Don't skimp here, it's what will hold the slabs in the same plane. Every 12" minimum, if you wanted to go every 10", 8" or even 6" more power to you. Old school, we'd drill an appropriate sized hole and pound rebar in with a big hammer. New school, the hole gets drilled slightly oversize and the rebar epoxied into place. Since the replacement concrete will be trapped between existing slabs I would think the beat it in method would be fine. Tie both sides together and also tie runs lengthwise. When completed, if it's strong enough to walk on it'll be strong enough to hold the new concrete in place, even if the backfill settles out from under.

    When the concrete is placed use as dry of mix as you can work. That will help cut down on curing shrinkage. If your trench has cut edges it will be much easier to tool. If your trench has the jagged edges left from the jackhammer it'll be a chore. The new concrete has to be worked thoroughly into the nooks and crannies of the existing. Back up bit, I would apply a bonding agent to the existing concrete before pouring, too. Pour the replacement level with the existing slab. It will likely shrink slightly when cured, then you use a self-leveling compound to make the whole affair nice and smooth.

    If redoing tile, or even if placing tile on a new, unbroken concrete slab, an uncoupling membrane should be used. If cracks develop in the concrete along the repair an uncoupling membrane will allow the tile to move independently of the concrete and hopefully not develop the same crack. There are a number of different membranes on the market. A local tile/flooring store would be a good place to see what's available in your area.
    Wow, good information and suggestions. Thanks for posting.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #5
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    Quote Originally Posted by Tim Bueler View Post
    Rebar needs to be drilled in on either side of the cut..
    Your other recommendations are clear, but In my case, the slab is only 4 to 5 inches thick and drilling into it to insert rebar will weaken or break up the old slab.

  6. #6
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    That won't be an issue if you're doing 1/2" rebar. It only needs to be drilled in a couple of inches.

  7. #7
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    Quote Originally Posted by Stephen Tashiro View Post
    Your other recommendations are clear, but In my case, the slab is only 4 to 5 inches thick and drilling into it to insert rebar will weaken or break up the old slab.
    When I say drill in on either side of the cut what I mean is on opposing sides of the trench, and the ends, centered on the thickness of the slab. As Frank said just a couple inches is enough. Once the repair is completed the rebar connections will be subject to shear forces only, not tension, so you don't need withdrawal strength. Even #3 bar (3/8") is probably sufficient if your trench isn't too wide, like under 3'-ish. If drilling in a rebar every foot makes your slab break you've got bigger problems than just a open trench in your concrete floor. That is also assuming all loose concrete leftover from jackhammering has been removed back to solid material (and a solid base). We usually cut the concrete before removing with a jackhammer so the surrounding slab isn't compromised. It sounds like you're past that point so just make sure all concrete you're drilling into, and the base it's sitting on, is stable.

    Now, that said, I spent my career in a marine environment. I have seen concrete blown out because water infiltration caused the rebar to rust, which makes it expand. I've also seen blown out concrete from this type of repair when water infiltrated and froze. You'll also see this a lot in regions where deicers are used to keep highways from freezing, expansion joints on bridges are especially vulnerable. It can happen, but the only times I personally have seen it is in some fairly harsh environments.

    However (I know, blah, blah, blah)! If you're really worried about compromising your existing slab, drill the holes oversize and install the rebar with epoxy. Now-a-days there are many wonderful epoxies and mixing/injecting nozzles that weren't available back when.

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