# Pour concrete over sunken slab (floor)?



## CopperClad (Jun 22, 2012)

Yes, please post a photo and we will help !


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## joecaption (Nov 30, 2011)

Sagging like that is a sure sign they used the wrong material to back fill and or did not compact it properly before the pore.
Add more concrete and it is most likly to keep sagging and fail again.
Were the new concrete is thinest is where a new layer is going to start to fail first, because new concrete does not like to stick to old.

The only correct way I know of to fix this is to cut out the failed part, figure out what went wrong with the back filing, fix that then do a whole new 4" thick pore.


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## ThomasC (Jun 25, 2012)

@CopperClad: I'll take some pictures later, but I don't have the carpet fully removed yet (and can't do that for the next 1-2 weeks or so), so I can just show you a picture of the crack, and the extent of sinking in the "worst" corner.

@joecaption: If I pour over and it settles again, do you know how quick that would happen? Are we talking about "right after pouring", or more like "maybe 10 years down the line"? Just want to get a feel for it... Also, why cut and replace? Seems drastic. What's wrong with mud/slab jacking?

Thanks,

Thomas


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## Canarywood1 (May 5, 2012)

ThomasC said:


> Hi!
> 
> We've bought our first house at the beginning of this year, and are planning to change the flooring, from carpet to laminate.
> The previous home owners had an addition built to the house about 10 years ago, but something went wrong when pouring the slab, as the right half of the addition is sunken from the left part about 1-2in. They mentioned this to us, too. When I started peeking under the carpet today, I realized that the slab is basically broken in half along the room, and along the edge of the wall. The actual foundation (so, the "rim") doens't have any issues, but just the "plate" on top (sorry, I'm German, I'm sure I'm not using the right words here, hope you know what I mean ).
> ...


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## Daniel Holzman (Mar 10, 2009)

As previously noted, the slab (English term for plate) has settled almost certainly because of improper preparation of the subgrade. Permanent repair requires removal of the concrete, removal of the unacceptable subgrade material, replacement of the subgrade material with suitable material (gravel, structural fill), compaction of the subgrade, then placement of new concrete. This is of course likely to be expensive, as it is a lot of work to fix.

If you don't care to spend the money to fix the problem, there is nothing wrong with placing more concrete over the old concrete, understanding that the new, thicker slab will almost certainly settle and crack again. Then there will be more concrete to remove, should someone eventually want to correctly fix the problem. But that is the only real drawback.

Mudjacking the slab is an option, but since the subgrade is almost certainly unacceptable material, mudjacking is not necessarily a permanent solution either, and could cost more than placing new concrete for a similar life expectancy.


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## ThomasC (Jun 25, 2012)

Is there any guideline on how quick it would crack again? Really not looking for any promises here, but just some (very rough) experience values. If it would crack pretty much instantly, then I would feel more assured in case I pour it and it doesn't crack within a week or so. If it takes 10 years to crack again, I wouldn't worry too much about it. If it falls down a foot 4 weeks after pouring and right after I installed the laminate, that would suck


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## ThomasC (Jun 25, 2012)

Note sure if this helps much, but here's some picture I just took with my cell phone. 

This is the corner where it's sunken the most. You can see that the plate has broken away from the actual foundation on the right.









Hard to see from the angle, but the drop is just a bit short of an inch









This is about 6.5ft to the left of the previous pictures, that's the crack down the middle of the room. The drop there is about the same as in the far corner, but someone has even it out gradually with...something (spackle?):









Here's a wall that is between the first two pictures and the last one. The crack from above is on the right side of the doorway of which you can see the left side in this picture:









Note how the wall basically "hovers", there's a gap of maybe 1/4-1/2 in. below it. The wall isn't holding anything up, so I thought I'd shim it as much as possible before pouring the concrete (or having someone take care of it otherwise, haven't decided yet).

Hope the pictures help a bit illustrating the issue. I think I'll have someone come in and give me an estimate, and hear some options of what can be done. 
If everything else ends up being too expensive, I think I'll go with the "concrete on top" option and hope for the best.
I'd love to do it myself, but not sure how to go about mixing and distributing the concrete.
I haven't done the math yet, but very, very roughly I'd need somewhere about 10 cubic feet of concrete to cover an area of roughly 6.5' by 19' an inch high (right? It is SO hard to do math in the imperial system when you've grown up with the metric system ).
Is that something I can mix up by myself without running into timing issues in distributing it?


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## Canarywood1 (May 5, 2012)

ThomasC said:


> Is there any guideline on how quick it would crack again? Really not looking for any promises here, but just some (very rough) experience values. If it would crack pretty much instantly, then I would feel more assured in case I pour it and it doesn't crack within a week or so. If it takes 10 years to crack again, I wouldn't worry too much about it. If it falls down a foot 4 weeks after pouring and right after I installed the laminate, that would suck


 
There is no way to tell how long this patch will work,just as there's no way to tell how mud jacking will last,as Daniel states this is not the proper fix,but could possibly last for a number of years,just no way to tell,personally i would think that it should last for some time seeing as how it's an inch to an inch and a half,and that didn't happen overnight,but more than likely was a gradual thing over time.

My guess is from your figures it would probably take about 1 cubic yard of grout to level things up,and your talking 4000 lbs of weight added on top of the slab,not a lot of weight considering the area involved.


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## Duckweather (Mar 26, 2012)

joecaption said:


> Sagging like that is a sure sign they used the wrong material to back fill and or did not compact it properly before the pore.
> Add more concrete and it is most likly to keep sagging and fail again.
> Were the new concrete is thinest is where a new layer is going to start to fail first, because new concrete does not like to stick to old.
> 
> The only correct way I know of to fix this is to cut out the failed part, figure out what went wrong with the back filing, fix that then do a whole new 4" thick pore.


Good call Joe, but splatter some new concrete on some old you don't want it on, and it sticks like super glue. (Note to self, remember to clean up before it sets.


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## ThomasC (Jun 25, 2012)

*Update*

I took out the carpet and pad today, and finally saw what's going on with the slab. I vaguely now remember the seller of the house saying that the problem already occurred when the slab was poured, and I now see what he means.
Basically the slab is broken away from the foundation wall, *everywhere* in the addition. Looks like they either didn't tamp it right, or somehow didn't make a proper connection between the slab and the foundation. Anyway, the slab is basically completely disconnected from the foundation, and has sunken between 1/4in and 15/16in, mostly around 3/4.
We'll get some foundation guys over for an estimate on how to fix this, but I was just curious what you guys think has happened here. How can this be so messed up?

Here's some more pictures, hope you can see the issue. Basically this nice, even "border" around the inside of the room is the foundation wall:


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## wkearney99 (Apr 8, 2009)

I'd worry about leaks. You've now got a gap running around the edge of the foundation. If there's any water issues then the settling is likely to get worse. Just how damp is the basement now (compared to the rest of the house)?

As has been posted already, it certainly looks like whoever prepped the base didn't do a decent job. The weight of the concrete (and anything on the floor) caused it to settle and break the slab. The best solution is get the bad part out of there and put a new one in properly. How complicated this will be depends on just how bad the conditions are under it.


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## ThomasC (Jun 25, 2012)

There's no basement. This is a ranch, and the slab is about 10in above grade. We're nowhere near a river/lake/ocean, so there's really no real chance of water getting into the house unless... well I don't know really 
I'm not really sure about taking it out...I'm worried about all the dirt/breakage with taking it out, and the long curing time and added moisture inside the house during the curing process. Plus hauling off ~450sqft. of foundation somehow...


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## ThomasC (Jun 25, 2012)

Also, not sure if I said that, but this hasn't just happened. We just bought the house, and the sellers knew this (didn't just communicate it in as many clear words to us ). From what I remember them saying, they had issue with the slab right off the bat, so ever since it was poured over 10 years ago.


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## hand drive (Apr 21, 2012)

looks like they poured the foundation part seperate of the slab and the two did not bond well. there would no be such a clean break otherwise, it would most likely crack in a random way across the floor.


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## ThomasC (Jun 25, 2012)

Yeah, that's what I thought. I'm not really sure how it's done in the US (I'm German), but from what I remember we pour the foundation and the slab together...


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## Canarywood1 (May 5, 2012)

The slab was never attached to the foundation wall,those were two seperate pours,it's how it's done,the slab subase was not properly installed and that's why you have the slab sinking,proper fix is tear out and repour,that is after a new base is put in.


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## ThomasC (Jun 25, 2012)

Im not sure I understand how this is usually done. Is there e.g. rebar in there? Does the rebarvconnect the foundation to the slab? If not, why not? Shouldnt that prevent it from sagging? When I tear it out, and theres no rebar connecting to the foundation, how do I know the new soab doesnt do just what the old did?
Just trying to understand...


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## wkearney99 (Apr 8, 2009)

One way to couple the slab to the foundation is to drill into the foundation and use epoxy to attach rebar into it. Then couple those protruding pieces to the rest of the new slab's rebar grid.


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## Canarywood1 (May 5, 2012)

ThomasC said:


> Im not sure I understand how this is usually done. Is there e.g. rebar in there? Does the rebarvconnect the foundation to the slab? If not, why not? Shouldnt that prevent it from sagging? When I tear it out, and theres no rebar connecting to the foundation, how do I know the new soab doesnt do just what the old did?
> Just trying to understand...


No there is no rebar in the slab,wire mesh is used in place of rebar,and the slab is not connected to the foundation,it's a floating slab.

Have you thought of putting a plywood sub floor on sleepers down,rather than tear out the existing floor?


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## ThomasC (Jun 25, 2012)

Yeah, I wondered about the plywood option, but wasn't really sure how I can get the plywood level. If I could do that, it'd be the best solution in my opinion, since it doesn't add much weight to the slab.
Would I just put some shims directly under the boards? How do you usually attach the plywood to the concrete, glue it? Nail it?
What would also be good for that solution is that the addition where the problem is has domed ceilings, about 10-12ft high, so raising the floor wouldn't really be an issue space-wise. Only problem is that the rest of the house would be slightly lower then, though.
Or I could make an actual raised floor with 2x4 or something, and make a step between the main house and the addition. I might run into issue with the baseboard heating then, though...
Hm... lots to consider  I have two companies coming in next week to look at it and give an estimate, so we'll see what they come up with.


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## Canarywood1 (May 5, 2012)

ThomasC said:


> Yeah, I wondered about the plywood option, but wasn't really sure how I can get the plywood level. If I could do that, it'd be the best solution in my opinion, since it doesn't add much weight to the slab.
> Would I just put some shims directly under the boards? How do you usually attach the plywood to the concrete, glue it? Nail it?
> What would also be good for that solution is that the addition where the problem is has domed ceilings, about 10-12ft high, so raising the floor wouldn't really be an issue space-wise. Only problem is that the rest of the house would be slightly lower then, though.
> Or I could make an actual raised floor with 2x4 or something, and make a step between the main house and the addition. I might run into issue with the baseboard heating then, though...
> Hm... lots to consider  I have two companies coming in next week to look at it and give an estimate, so we'll see what they come up with.


The plywood i've seen installed was pressure treated and was installed on top of 3/4" 1"x4" sleepers which you could shim to level before you lay the plywood,then you would have a nice level surface to install whatever kind if floor you wish.

Forgot to say you should have a vapor barrier down before the sleepers,and you can use a ram set to attach the sleepers,then use screws to attach the plywood or liquid nails and screws, plywood should be tongue and groove.


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## willchester (Apr 5, 2017)

ThomasC--I'm having the same problem! What did you end up deciding to do? What would you recommend now?


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## Erico (Oct 24, 2016)

willchester said:


> ThomasC--I'm having the same problem! What did you end up deciding to do? What would you recommend now?


Seeing as though that member hasn't posted in 5 years, you probably won't hear back.


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