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Old 07-11-2011, 12:31 PM   #1
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Slab Cracked Along Perimeter


Hi,

We have what appears to be a half inch wide crack in the slab about 2 inches from the inside of the wall(close to where it would join the foundation). There are pictures attached to this post. The crack continues around the perimeter of all of the outside walls of the building. Does this sound like the slab has contracted, or does it sound like the walls were constructed too soon after pooring the slab? Or something else? Please check out the pictures.

What would be required to repair such a crack?
thanks for your insight.
KNRE
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Slab Cracked Along Perimeter-img_0832.jpg   Slab Cracked Along Perimeter-img_0833.jpg   Slab Cracked Along Perimeter-img_0834.jpg   Slab Cracked Along Perimeter-img_0831.jpg  

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Old 07-11-2011, 12:51 PM   #2
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Slab Cracked Along Perimeter


The slab settled from the sub-base not being properly compacted prior to pouring. To fix proper you should remove the portions of slab that have irregularly settled and pour a new slab up to the proper level.

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Old 07-11-2011, 04:05 PM   #3
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Slab Cracked Along Perimeter


I cannot tell from the photo if there is any difference in elevation across the crack. It does not look like it, but please measure the elevation difference by placing a straight edge across the crack and measuring the difference.

That crack looks too straight to be from settlement. As odd as it sounds, that looks like a home made control joint, maybe cut with a circular saw, and not perfectly straight. If there is only a small difference in elevation across the crack, that would tend to confirm the home made control joint theory.
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Old 07-11-2011, 07:48 PM   #4
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Slab Cracked Along Perimeter


ag & dan, is it possible ffe is the same as top of footer ? ? ? photo suggests the floor was poured against the footer's side, NOT against the foundation wall on top of the footer,,, that might be an explanation for such a ratty/ragged const jnt.

we cut control jnts all the time w/7" circle saws & elec demo saws mounted on roller carts
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Old 07-11-2011, 08:21 PM   #5
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Slab Cracked Along Perimeter


If this is a slab on grade house then the slab was a separate pour from the foundation and the crack is just where the slab meets the foundation wall. There is no way to avoid this "crack". If this is a basement slab where the slab was poured over the footing then it is a crack from the slab settling because of inadequate compaction and/or not installing wire mesh properly.
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Old 07-12-2011, 03:56 AM   #6
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Slab Cracked Along Perimeter


wire mesh only serves 2 purposes: 1, to provide conc some initial aid in resisting tension cracking; & 2, hold the broken pieces of conc together altho its possible some well intentioned jabonies did use wire, it could very well be at the btm of the slab,,, the only 1 who got any benefit from that is the guy who sold the mesh wouldn't a slab-on-grade normally be a monopour/unipour ? realize those guys got paid & they're gone but i like my theory the best
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Old 07-12-2011, 05:07 PM   #7
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Slab Cracked Along Perimeter


What kind of engineer or specialist would be the one to inspect a crack in the concrete slab?
thanks!
KNRE
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Old 07-12-2011, 08:50 PM   #8
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Slab Cracked Along Perimeter


I agree that mesh doesn't add any substantial strength to the concrete but it does tend to hold it together and reduce cracking as a slab settles. This becomes especially important in a basement slab as the masons tend to pour a loose (weak) mix so that they can push it around easier. If this is a basement and the slab was poured extending over the spread footing, then it is likely that settling of the soil beneath the slab has caused the slab to drop resulting in a crack above the edge of the footing. The remedy is to remove your wood framed walls, break up the slab, compact the soil and pour again. You are not likely to do that for a crack or small difference in surface heights. You are going to fill the crack or level the floor with a concrete leveling product. Neither really solves the problem.

If this is a slab foundation then depending on what part of the country you are in will somewhat dictate if you can do a monolithic pour. Depending on your soil bearing capacity and the load imposed a spread footing is often required with the foundation being formed above. Any underground mechanicals are then roughed in and the slab poured. Additionally, if KNRE is up north (I may have missed the geographic location but I didn't see it), the slab is poured separately for a thermal break, typically with foam in between the foundation wall and slab.

KNRE: Are the photos of a basement floor or a slab construction at grade level? What part of the country are you in?
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Old 07-13-2011, 12:11 PM   #9
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Slab Cracked Along Perimeter


I'd be willing to bet that it's a control or construction joint, OR, if it's a basement, the floor is poured less than 2" thick over the footing.
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Old 07-13-2011, 12:28 PM   #10
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Slab Cracked Along Perimeter


In Reply to The Camper:
This is a 3400 foot commerical building located in northern Nevada. The building is one-story, and built on soil with varying amounts of sand, silt and clay. There is no basement. It appears to be a slab on grade construction but I need to confirm whether or not there is a crawl space.
Thanks for all your input.
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Old 07-13-2011, 12:31 PM   #11
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Slab Cracked Along Perimeter


The part of concrete directly along the wall is your foundation wall. They used the wall itself as the pour stop for the floor. Poor soil preparation and compaction has caused the slab to settle. As said before you can either use self-leveling compound to infill or remove and replace the slab making sure to prep the sub-base proper the second time around.

If there is a crawl space below this and the crack has a significant vertical change, then call a structural engineer to review for potential structural defficiencies. If there is a cral space and the crack has no vertical change then is from the floor deflection fighting against the rigid wall and causing a crack at the fulcrum point.

Last edited by AGWhitehouse; 07-13-2011 at 12:34 PM.
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Old 07-13-2011, 12:46 PM   #12
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Quote:
Originally Posted by AGWhitehouse View Post
The part of concrete directly along the wall is your foundation wall. They used the wall itself as the pour stop for the floor.

I agree with this part, it's simply a construction joint, with the foundation wall/weight bearing curb on the outside, and the interior floor on the inside, obviously poured at separate times.

Poor soil preparation and compaction has caused the slab to settle.

Maybe my eyes or reading comprehension are going, but I have no idea where you keep coming up this this assumption that there's any settlement.......

As said before you can either use self-leveling compound to infill or remove and replace the slab making sure to prep the sub-base proper the second time around.

If there is a crawl space below this and the crack has a significant vertical change, then call a structural engineer to review for potential structural defficiencies. If there is a cral space and the crack has no vertical change then is from the floor deflection fighting against the rigid wall and causing a crack at the fulcrum point.

I'd be willing to bet that it's simply a slab on grade. I can see zero benefit from a suspended concrete floor over a crawlspace. No reason to spend all of that money w/o adding usable square footage to the building.
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Old 07-13-2011, 01:16 PM   #13
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Slab Cracked Along Perimeter


"but I have no idea where you keep coming up this this assumption that there's any settlement......."

The OP doesn't actually say whether there is definately a vertical change between the two sides of the crack. but from the pictures I feel there may be one (1/2" at most). if there is a vertical difference then there is settlement. If they are level then there is no settlement.

I made the assumption based of my experience with this scenario. More times than not I find there are settlement issues because contractor's are rushed to get slabs down (owner driven schedules) and the sub-base compaction suffers for it.

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