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RE: dead end slab control joint

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The design of the slab and its connection to the perimeter walls play a huge
part in shrinkage cracking.  I MEAN HUGE......I preach pour strips like
crazy and nearly always have to suggest overruling detailing to the engineer
about location of joints, but I rarely if ever get well thought out jointing
on structural plans on the first go except in tilt-up engineering. Why you
folks think tilt-up needs it while masonry wall building engineering doesn't
truly baffles me.   You almost cannot pour a slab on grade for even a 20x20
garage and connect it to a perimeter footing or wall without losing control
of the cracking.  Too much can go wrong and it is, indeed, smarter than us.

IMO, engineers all need to design complete independence of a slab on grade
from anything that will stop it from moving until it cures. Keep it simple
for the slab.  That means no connection to footings or walls AT ALL, no
re-entrant corners, nothing that will hinder movement.  I'd advise you all
to JUST SAY NO if anyone tells you otherwise.  OR, perhaps I'm nuts and
someone here can tell me what I can do to prevent shrinkage cracks and keep
such connections. I just don't think it's physically possible.  

-DB

PS  T intersections greencuts are an AUTOMATIC crack unless none of the cuts
crack at all.

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
Sent: Sunday, June 17, 2007 11:58 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: dead end slab control joint

In general, yes, it is wrong.  I would only terminate a control joint at an 
expansion joint.  Keep in mind that concrete will try to shrink to the 
center of the section of slab.  If constrained, it can not shrink to the 
center, and it will shrink and form its own joints.

Dead ending a slab control joint will result in a stress riser at the 
termination.  This will become a point of generation of shrinkage cracks.

Regards,
Harold Sprague

 


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