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Re: 50' oc control joints for SOG ok?

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There are two rational approaches to jointing slabs on
grade.  

1) The oldest is to create joints at 15 to 20 feet on
center either unreinforced or with the reinforcing
stopping at the joints.  These allow the shrinkage
shortening to concentrate at the joints.  The problem
is that in many instances it is not feasible to have a
decent aspect ratio.  In addition there are instances
where the reinforcing must pass through the joints or
where the joints are not acceptable.

When the reinforcing goes through the joint the joint
cannot work as intended.  Look at the strains in the
steel at the opening and how much of the bar is
yielding.

2)  The second approach is to specify enough
reinforcing so that the cracks are tight and well
distributed and not to worry about joints at all. 
Since most of my slabs need to have continuous
reinforcing my practic has been to use 0.5%
reinforcing and not worry about crack control joints. 
This translates to #5 at 12"oc in a 5" slab.  I have
not had any complaints.

Look around you at exposed slabs in buildings and you
will typically see cracks between the crack control
joints at slabs that your initial impression was that
they had no problem.  

If you have a strong stomach look at the concrete slab
over metal deck when they remove the carpet.  You will
find significant cracks yet there likely was no
problem for the users.

Mark Gilligan


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