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RE: slab on grade

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I just inspected a project with the worst drying shrinkage cracks that I
have ever seen.  The cracks were 5/8" wide and there were many of them.  It
was a slab using polypropylene fibers.  I would not count on fibers except
for control of bleed water and plastic shrinkage cracks.

You have already seen posts on post tensioning which is a good solution.

Consider also shrinkage compensating concrete.  

Another thing is to eliminate as many restraints as possible.  Avoid
thickened areas.  Any penetrations by pipes, light standards, etc. should be
wrapped with a closed cell foam and sealed.  This allows the slab to shrink
without being restrained by the penetrating structures.  Add reinforcing
steel around any penetrations.  Try to create a good uniform slip plane and
eliminate restraints.

Harold Sprague
The Neenan Company
harold.sprague(--nospam--at)neenan.com


-----Original Message-----
From: jdcoombs(--nospam--at)wilsonco.com [mailto:jdcoombs(--nospam--at)wilsonco.com]
Sent: Monday, January 04, 1999 11:35 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: slab on grade


Happy New Year, y'all
I need to put in a good slab for outdoor basketball courts.  The client 
would like for them to be free of joints.  It will be an unloaded slab, no 
traffic.  The slabs will be 100' x 65'.  I can spec a proper subgrade spec. 
 How do I design for the slab thickness and reinforcing required to keep 
cracking controlled.  I have reviewed  ACI 302 and 360, and the PCA guide. 
 None really address the unloaded slab except fot the "subgrade drag 
equation", which is not really a qualitative approach.  I'm willing to add 
fibers to get a good crack control, but how does one balance between that 
and steel?  How thick a slab?  How much steel?  How much fibers?  Any 
comments are appreciated.
Jerry D. Coombs, PE
Albuquerque