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Re: SOG Cracks

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Andrew-
 
Is your <1" is a differential movement across a joint or just differences in slab elevation across 10ft?
 
Settlement along foundation walls above uncompacted fill is a common problem locally although 1" sounds like a bit much.  I will more often see voids under slabs in this condition with the slab cantilevering to the wall.
 
FWIW, I have very sandy soil under my garage and my slab (and walls too) has settled about 2" over 20 years.  Cracks were much more noticeable after my driveway was paved last year - lots of vibration from the roller definitely worsened the situation.   An excavator working about 100ft away a few years ago also caused some settlement and cracking when he was pounding the ground with his bucket.
 
I don't suppose you can blame any of the issues on frost heave or settlement...
 
SOG's up here usually get fibers, occassional WWR, sometimes nothing.
 
Jim Wilson
Stroudsburg, PA


From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
To: seaint(--nospam--at)seaint.org
Sent: Wed, October 21, 2009 10:38:21 PM
Subject: RE: SOG Cracks

Differential settlement CAN be an issue.  But more likely is shrinkage and differential curling.  Clay soils with a sustained load would be more prone to differential settlement as opposed to sandy soils. 
 
Another factor is time.  The clock starts when it is placed.  You will not plateau out on the shrinkage until about 5 years time, but about 80 to 90% will happen in the first year.  The first 90 days will account for about 60% of the shrinkage. 
 
The above observations will apply to all aspects below. 
 
Sandy soils would be more prone to consolidation and could settle with vibrations. 

Regards, Harold Sprague


 
> Date: Wed, 21 Oct 2009 19:18:30 -0400
> From: akester(--nospam--at)cfl.rr.com
> To: seaint(--nospam--at)seaint.org
> Subject: SOG Cracks
>
> Harold et al,
> This is a bit of a loaded question, but I just want to test the waters on this. I have my ideas...
>
> What are your opinions as to the possiblity of settlement (slight, say, less than 1" over 10ft or so) causing hairline to 1/16" wide cracks in typical, residential, concrete slabs-on-grade (I don't like the new slab on "ground")? What if there were micro to hairline existing cracks, do you think settlement could exacerbate them?
>
> I know there is a difference in a floating slab that buts against a CMU stem wall, but for the most part I mean single story, thickened edge monolithic slab-foundations, supporting wood frame and CMU.
>
> Part 2: what about standard residential pool deck slabs and driveway slabs? Take into account that many times the pool deck slab is placed over the pool shell wall and a bond forms.
>
> Part 3: Could ground vibrations due to construction activity cause or exacerbate cracks in concrete SOG?
>
> I know, who cares, all SOG crack and are non structural. Poor workmanship, not enough CJ, better design mix and curing, proper placement of WWR, etc etc. But please humor me...
>
> Could you direct CC me your opinions also at akester(--nospam--at)cfl.rr.com ?
>
> Thanks list,
> Andrew Kester, PE
>
>
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