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Old 01-23-2014, 12:02 AM   #1
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cantilevered concrete slab. Arrrrggg


Not sure what to do about this unexpected find while prepping for a simple tile job in our sun room. It's actually an old covered entrance where horse drawn carriages could pull up. There is a slab floor I think is original but might have been added later. The walls were later enclosed with wood french doors around 3 walls. The 4th wall is the side of the house. House was built in 1925 and has a brick exterior. The slab was poured on a raised load of red clay that was brought in. But, there is a gap of about 3 to 4 feet between where the clay ends and the edge of the house, leaving an open space. The slab just hangs over this area with no support. Why would it have been done like that? To protect the exterior wall of the house? Where the slab meets the house it is broken in several places leaving up to six inch wide open gaps a few feet long. These gaps are primarily in front of 2 original granite steps into the house. I'm guessing the carpet layers might have broken it while installing tackless strips but not sure. They handled the problem by bending some metal and screwing it to the slab and steps so they would have something to attach their strips to. Possible it was broken earlier.
I'm going to need to fill those gaps but not sure how to do that with out doing some kind of pier system or something to supply support. It's only a foot or so from the slab to the ground and not a lot of room to work. Plus I would need to cut back the slab further to gain access. I may also need to pour leveling compound in some areas if not over the complete slab. Slab is 12 x 16. Any suggestions on how to deal with this would be welcome before I give up and just re carpet the way it was. It's also worrisome that there is no support under any of the slab next to the house for 3 or 4 feet. Hope that all made sense. Thanks

Shot of the problem area after carpet and lino tiles taken up. The sheet metal covered this gap and was screwed to the floor and the granite steps
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Last edited by Mike in Arkansas; 01-23-2014 at 12:05 AM.
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Old 01-23-2014, 05:43 AM   #2
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cantilevered concrete slab. Arrrrggg


From what I am gathering from your post is you have voids under the slab that is an enclosed porch. The voids probably were not there originaly. Ground settling will do this.

This is a problem we are facing in the factory where I work. Several years ago we had a piece of equipment fall thru the floor. Turns out there was a void that settled under the floor and the weight and vibration of the machine caused the concrete to fail. We had a company come in that uses ground penetrating radar/sonar to detect the other voids. Once we had them located we called in a grout pumping company and had them fill the voids. An interesting side not to this is that the shop floor was quieter when this was done as the slabs were not acting as drum heads and amplifying the sound.

Anyway, you may be able to find a local company in your area to fill these voids under the slab.

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Old 01-23-2014, 06:07 AM   #3
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cantilevered concrete slab. Arrrrggg


'red clay' is an expansive soil - if the moisture content was high when placed, its shrunk thru dry times,,, that + its very difficult to compact properly,,, you could pour grout into voids to fill them & return floor desired elevation,,, they did that because they didn't know no better ( its arkansas, fergawdsake )

i'd reach down in there & see how much is unsupported,,, then you'll have a better idea of how to fix it,,, conc does have flexural strength so grout & some foam-in-a-can may be the way to go

we sometimes inject foam under slabs to restore base integrity, fill voids, & raise slabs - even highway slabs
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Old 01-23-2014, 06:33 AM   #4
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cantilevered concrete slab. Arrrrggg


See if you can find someone local that does "mud jacking".
You local concrete supplier may have a phone # for one.
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Old 01-23-2014, 07:58 AM   #5
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cantilevered concrete slab. Arrrrggg


Quote:
Originally Posted by joecaption View Post
See if you can find someone local that does "mud jacking".
You local concrete supplier may have a phone # for one.
It's also called "slab jacking" (at least around here). Had to be done where I used to work.
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Old 01-23-2014, 08:36 AM   #6
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cantilevered concrete slab. Arrrrggg


Thanks but the clay pulling away is not the problem. Sorry for not being more clear. The problem is that their isn't anything under the slab for 3 or 4 feet out from the house edge. I can see an access hole into this area broken out in the house crawlspace. Why they needed access to this area I don't know. Nor do I know if it was originally built this way or if someone later broke through the crawlspace wall and dug it out. The edge of the clay where it ends is pretty straight and even. Very neat looking in fact. The void under the slab is 16 feet long, 3 to 4 feet wide and 1 to 2 feet deep.
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Old 01-23-2014, 09:59 AM   #7
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cantilevered concrete slab. Arrrrggg


That's where the Shine was stored.
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Old 01-23-2014, 09:39 PM   #8
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cantilevered concrete slab. Arrrrggg


The easiest way to do this may be to fill in the void with flowable fill which is basically a very low strength concrete. After this is poured and have been allowed to cure you could then pour a small section of higher strength concrete to level up the slab. It's obviously would be much easier than trying to configure some type of support structure to pour the new concrete on. You're only going to need about 5 yd. or so of the material so the cost would not be prohibitive.

Here is some information on Flowable Fill: http://www.cement.org/cement-concret...ength-material

Last edited by Msradell; 01-23-2014 at 10:03 PM. Reason: Added information about Flowable Fill
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Old 01-23-2014, 11:45 PM   #9
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cantilevered concrete slab. Arrrrggg


Interesting. Thanks for the info. I was considering some kind of pack material but would have had to break back the slab for that. Never heard of it before, would any concrete supplier be able to furnish it?
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Old 01-24-2014, 05:07 AM   #10
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cantilevered concrete slab. Arrrrggg


that's larger area than 1st described OR i saw/read it wrong yep, flowable fill but it doesn't move that far sideways even when vibrated & to overvibrate would be terribly wrong.

i like the idea of f/f but i'd core 3" holes in the slab & pump directly thru them,,, kenrich [ no $ interest ] makes a manual grout pump - there may be someone in your area who does this work - or maybe has a pump,,, still & all, 5cy thru a manual pump 1" hose will take come time.

any conc plant can supply f/f - its just lo-strength conc w/o rock,,, trailer-mtd pump's my suggestion,,, then again, 1 works w/what's avail

when you run the #'s, it might be less expensive/better to drop the existing conc into the void, add more fill, & replace the floor

good luck

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