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RE: Mat Foundation

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I would roughen the construction joints and extend the top and bottom mats of steel thru with a lap splice to the adjacent placement. (No additional dowels required.) I typically use up to a 40-ft spacing for construction joints, with at least #8(--nospam--at)12 rebar top each way to control shrinkage cracks for this thickness slab.
Although I haven't typically added extra steel at the joint as James Lutz suggested, I have often seen the phenomenon he talked about at walls placed on a foundation. The foundation slab restrains the base of the wall and causes periodic vertical shrinkage cracks for a few feet above the base of the wall. I've sometimes added additional steel near the base of wall to control such cracks, so the same concept makes some sense in a mat foundation, if you are concerned with having some minor cracking.

From: Abe Knapp [mailto:AKNAPP(--nospam--at)]
Sent: Friday, April 01, 2005 12:27 PM
To: seaint(--nospam--at)
Subject: RE: Mat Foundation

Hi Dave,
Thank you for your reply.  I am in Casper (my home town is Gillette)  Both of the replies have been very helpful.  The other item I am wondering about is: do I need to put cold joint material in a slab this thick and if so what  The joints will be doweled together (two mats of rebar top and bottom).
Thanks again,

>>> davemaynard(--nospam--at) 4/1/2005 12:22:38 PM >>>
Cold joints between pours should be dowelled together.  Construction Joints are likely spaced 20 to 30 times the slab thickness.  However, only having like 3 joints one way and two the other isn't all that great.  In a 16 inch slab, I have joints between 20 and 32 feet O.C.    That being said, with your slab, maybe put the joints at 25' O.C. along the 75' length and 21.5' O.C. along the 86' length.  Is this what you're looking for???
Dave Maynard, PE
Gillette, Wyoming