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Old 04-16-2011, 02:49 PM   #1
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Crack in new slab


I had a slab poured over a garage floor about 3 1/2 mos. ago.
Details:

The garage is 20'x10' 5"
The floor was raised 5"
Form: 7 sheets of 8'x2'x2" styrofoam bead board was used to reduce weight, rebar drilled into original slab and bent over, #10 remesh.

I noticed today that the slab has a small crack extending almost all the way across the 10' width (see imagaes).

This area will be a living space. I haven't decided what kind of floor I will put down. Should I be worried?

The small image is a crop so that the crack is viewable. The dark gray lines in both slab images were editied in for clarity.

Thanks for any comments.

Mark

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Old 04-16-2011, 03:49 PM   #2
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Crack in new slab


I don't see any control joints in the image, but perhaps they are there, simply off camera. Concrete is prone to cracking, no matter how carefully placed and cured. The control joints are installed to direct the cracking into the joints, rather than occurring in the slab, where they are more visible. If the mix was good, and the curing was done correctly, a small crack such as the one you show should not be a problem.

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Old 04-16-2011, 04:07 PM   #3
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Crack in new slab


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I don't see any control joints in the image, but perhaps they are there, simply off camera.
There are no control joints, and I was not made aware of the necessity. Are they required for any slab in any climate, interior or exterior, regardless of slab size? I was not happy with the job and was unhappy with the contractor who did the job, as his follow-through throughout was poor.

Is their a window of time in which to cut control joints?

Quote:
Concrete is prone to cracking, no matter how carefully placed and cured. The control joints are installed to direct the cracking into the joints, rather than occurring in the slab, where they are more visible. If the mix was good, and the curing was done correctly, a small crack such as the one you show should not be a problem.
I imagine that the mix was OK, as I ordered it directly from Baker Readymix and it was poured from the truck.

As far as the curing process, I can't really say. How is it correctly cured?

Thanks for your interest.

Mark
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Old 04-16-2011, 04:33 PM   #4
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Crack in new slab


As Daniel noted, it really should have had a control joint. Two 10' by 10'5" sections would have been sufficient. The max. aspect ratio for concrete slabs is typically about 3:2, meaning the max. length of a 8' wide section would be 12' long. The closer to square, generally the better. Additionally, no 4" thick (especially 3", which I suspect is the case here) slab should ever be more than 100 sq. feet w/o a control joint.

Oh, one more thought. With the gaps you have between the sheets of foam, it's very likely the 90 degree corner created by the sheet edges became a stress raiser for the concrete directly above. I wouldn't doubt that there's an insulation edge directly under the crack.

The good news is that it will little, if any, negative effect on the floor that covers it with the wire mesh in place.
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Old 04-16-2011, 04:53 PM   #5
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Crack in new slab


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Oh, one more thought. With the gaps you have between the sheets of foam, it's very likely the 90 degree corner created by the sheet edges became a stress raiser for the concrete directly above. I wouldn't doubt that there's an insulation edge directly under the crack.
Hi jomama45. Thanks for your response. I believe that the crack is near the edge of one of the beadboards. I thought about installing enough of the boards to cover the entire floor, but felt it would be structurally weaker.


Quote:
The good news is that it will little, if any, negative effect on the floor that covers it with the wire mesh in place.
Good to hear. Believe it or not, I got 4 estimates from concrete contractors, and all of them but one told me not to install remesh, that adding fiber to the mix would be adequate. The one who agreed that remesh would be good got the job (although there were othe issues).

There is no fiber in the concrete. The new slab is 5" thick minus the 2 " bead board. The old slab was around 3.5-4" thick.

Thanks again,
Mark
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Old 04-16-2011, 05:00 PM   #6
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Crack in new slab


I would expect more cracks to develop over time. Each one corresponding somewhat to the edge of each of the foam boards.

Was the wire (remesh) suspended in the top three inches of the cast or was it allowed to remain laying on the surface of the foam when the mix was poured?
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Old 04-16-2011, 05:06 PM   #7
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Crack in new slab


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I would expect more cracks to develop over time. Each one corresponding somewhat to the edge of each of the foam boards.

Was the wire (remesh) suspended in the top three inches of the cast or was it allowed to remain laying on the surface of the foam when the mix was poured?
That was one of my complaints with this contracotor...he promised to suspend the remesh with a come-along while the pour was done, but instead, just kind of pullled it upward with his concrete 'rake' and let it go at that.

But I did drill in rebar about every 2 or 3', bent over in the open sections and also bent over the beadboard about in the middle (as seen in the image of the form). Also, rebar drilled into the side of the existing foundation slab (right side in form image), rebar in the side of the original foundation behind camera, wedge anchors on the 'left' side and at the end.

Also, the original garage floor slab was monolithically poured with the foundation.

So, I guess I should have installed beadboard across the entirety of the slab? The question is, given these additional facts, can I expect problems, or is this condition non-problematic?

Last edited by rightit; 04-16-2011 at 05:17 PM.
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Old 04-16-2011, 05:34 PM   #8
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Crack in new slab


The slab isn't going anywhere. I wouldn't expect any major issues. The thing is concrete is concrete and can have a mind of its own. I would expect more cracking but probably nothing more than hairlines.

The rewire will work to a small degree because it is suspended in the mix between the foam slabs. That's probably a good thing.

Quote:
styrofoam bead board was used to reduce weight
To reduce weight or to reduce concrete cost?
Why would you want to reduce the weight, I'm not following that concept in this case.
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Old 04-16-2011, 06:01 PM   #9
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Crack in new slab


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The slab isn't going anywhere. I wouldn't expect any major issues. The thing is concrete is concrete and can have a mind of its own. I would expect more cracking but probably nothing more than hairlines.

The rewire will work to a small degree because it is suspended in the mix between the foam slabs. That's probably a good thing.
Hairlines I can live with.


Quote:
To reduce weight or to reduce concrete cost?
Why would you want to reduce the weight, I'm not following that concept in this case.
To reduce weight. This is a permit job and an engineer assessment and procedural recommendation of the slab was required. He recommended the styrofoam beadboard to reduce the weight of 5" of concrete poured over a 3.5"-4" garage floor slab, although there are pilings in the ground underneath the enitre foundation, including the garage floor slab (we are below sea level here). The reduced cost was a benefit as well, but not the main event.
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Old 04-16-2011, 06:17 PM   #10
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Crack in new slab


Wow, in all my years and all my dreams, I have waited to here of the product that could reduce the weight of whatever put on top of it. You do know you have won the lottery. Please stop complaining about the tiny crack.

There should have been some release points of some kind put into the concrete pour but get over it. Fill the crack with a quality epoxy filler and go on with what you have in mind.

Last edited by user1007; 04-16-2011 at 06:23 PM.
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Old 04-16-2011, 06:29 PM   #11
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Crack in new slab


I wouldn't typically recommend pouring concrete over the low strength white bead insulation, but considering you have direct bearing on the existing floor every 4', I think you'll be fine. They make insulation for direct use under concrete, as well as foam that's pitched for pre-cast plank topping applications.
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Old 04-16-2011, 06:38 PM   #12
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I wouldn't typically recommend pouring concrete over the low strength white bead insulation, but considering you have direct bearing on the existing floor every 4', I think you'll be fine. They make insulation for direct use under concrete, as well as foam that's pitched for pre-cast plank topping applications.
I thought it a bit odd too, but it was what the engineer specified and code enforcement endorsed. Thanks for your opinion and suggestions. Thanks to all others, too! Very nice to have questions answered by those who work in the industry and are so available, knowledgeable and friendly.



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Old 04-16-2011, 07:23 PM   #13
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Crack in new slab


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Very nice to have questions answered by those who work in the industry and are so available, knowledgeable and friendly
I have to admit I'm somewhat puzzled and maybe a little bit lost. I can't argue the concept but I certainly don't understand why reducing the weight would be a necessity. Generally speaking I think concrete comes in at about 3000 pounds per cubic yard. Why would the old slab not support the new addition on top of it spread out over 200 square feet? A quick estimate tells me that the additional concrete would weigh-in at about 10# per square foot. I don't get it.
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Old 04-16-2011, 08:22 PM   #14
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Crack in new slab


The actual concrete weights:

40#'s PSF @ 3" thick

53#'s PSF @ 4" thick

67#'s PSF @ 5" thick.

I'm guessing the engineer was concerned about the original slab settling more with the additional weight on top of it, but being the internet, can't tell if it's warrented concern or not.
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Old 04-16-2011, 08:42 PM   #15
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Crack in new slab


I believe the bottomline is liability. Code enforcement doesn't want it, passes it to the engineer. Engineer doesn't want it, over-compensates. It is a legitimate concern, however, as this region is known for sinkage and is compensated for with code required pilings. That said, the original slab (1964) was fine and had no perceivable problems.

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