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SOG case studies

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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:

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

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