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Re: SOG case studies

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Andrew:

For number 1, the two most likely culprits would be shrinkage cracks and the fact that you have part of the new slab siting on top of a hard surface (i.e. the new slab).  Even if the soil is well compacted/"good", you are likely to get differential movement cracks due to part of the slab being on another slab and part of it being on soil.  It is not clear from your description where the cracks are relative to the part of the new slab that is on top of the old slab, so I cannot say how much of a factor this may or may not be.  But, putting new slabs on grade on tops of "hard points" like old slabs on grade or foundation will typically result in some differential movement cracking somewhat near those "hard points".

For number 2, it sounds like shrinkage cracking to me.  From your description, it sounds like there is only essentially one new crack (i.e. kind of a series of cracks that nominally kind are all the same crack) that runs parallel to the newly cut joint the full length of the driveway at about 6 to 12 inches from the newly cut joint.  Am I understanding that correctly?  If so, then it would seem like shrinkage cracking.  My guess would be that it is a primarily function of that is where there was a weakness in the new layer.  It could also be a function of how well the new layer bonded with the old concrete at various locations.  It is possible that depending on how the new joint was cut, maybe more "dust" accumulated on one side of the new joint and if the contractor did not prep the surface adequately, the "dust" could have weaken the bond where the crack formed meaning the new layer did not have "strength" of the old concrete "fully" behind it.  Just a thought...

Regards,

Scott
On Oct 26, 2010, at 6:57 PM, Andrew Kester wrote:

> Listers, I am looking at way too many SOGs these days. Projects are in
> Florida. Feel my pain and give me your two cents, forensic jobs, not
> my designs:
> 
> (MORE IMPORTANT)
> 1. A 20 year old house with SOG, section of the slab removed in the
> center of the house, and a new slab put back that extends out a little
> as part of an addition and greater remodel project, and about 1/3 of
> the slab was placed over an existing slab where there was an original
> step in the home. Dimensions are irregular but approximately
> rectangular, 21ftx27ft. I don't know anything about the mix, and not
> sure about WWF.
> 
> Owner tells me that the eng/arch dwgs called out for dowels at about
> 2ft oc into the existing slab, so the long edges are now tied into the
> new slab about halfway. The cracks began to form within a month and
> have progressed slightly but not much in the next 2-3 months. They
> range from hairline up to 1/8", and only at the center of the slab was
> a section of crack observed to have slight vertical displacement. Over
> this section of the crack a level rocked back and forth indicating and
> a slight slope to one side. NO JOINTS.
> 
> The orientation of the cracks is difficult to describe but the main
> ones are diagonal and oriented in the long direction, to me, textbook
> shrinkage cracking. I think it is shrinkage cracking all day long
> because it was doweled into the existing slab, and there are no
> joints. There were exposed CMU jambs at the edges of the slab taking a
> significant point load from the roof and there was no cracking around
> them, or cracks in the CMU.
> 
> The X factor, which I would not like to get into, is there were soil
> problems at the site before, but these were treated with chemical foam
> injection, a fairly common practice in this area to densify loose
> sandy soils and improve their bearing capacity with a high success
> rate. I drilled about 8 holes in the slab along cracks and in other
> areas and did CPTs (hand cone penetrometer) and the soils were dense
> to very dense, good for a SOG. I have no reason to think the soil is
> an issue. Also, almost no vertical displacement along the cracks. And
> the slab is the only load on the soil, about 50psf compared to the
> usually safe assumption of 2000+ psf bearing capacity. No reason for
> this thing to settle and the pattern of cracks do not match.
> 
> I think the slab cracked due to shrinkage, and then some slight
> settlement along one of the cracks may have occurred...
> 
> Soil tests do not indicate much in terms of clay at the site.
> 
> 
> 
> 
> 2. A 15 year typical driveway SOG, contractor sawcut a new joint
> (about an inch deep) down the middle of it (lengthwise) to "help" with
> crack control, then put a coating over it similar to pool deck
> coatings. New hairline cracks have since appeared almost the entire
> length of the driveway parallel to this new sawcut joint,
> approximately 6-12" offset of the CJ. There is no vertical
> displacement between crack edges or other evidence of settlement.
> 
> My bet is it is shrinkage related, not curing but temperature/humidity
> related, just cannot figure out why it followed the CJ but offset
> several inches? I would understand if it cracked within an inch or two
> because of the now weakened plane.
> 
> I am OK if I can rule out settlement (which I have), but out of
> curiosity I would like to know why the cracks formed as they did. I am
> OK if it is "because that is where the concrete wanted to crack."
> 
> 
> Thanks in advance, sorry it was so long.
> 
> Andrew Kester, PE
> Florida
> 
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