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RE: Slab-on-grade control joint question

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Cliff,

I use two terms in describing joints in slab on grades, Term 1 is
Weakened Plane Joint (WPJ) and Term 2 Construction Joint (CJ).

Your description of a control joint is what I would consider a WPJ,
since it is with a soff-cut and it doesn't sound like you are going to
cut all the way through the slab. These type of joints are to control
cracking and are not deep into the slab. In a weakened plane joint, the
main requirement is to not soff-cut the slab too late. The reinforcing
is the same as everywhere else in the slab - no cutting is necessary.

Construction joints are formed joints with keys where either a greased
bar or plastic insert is required at the edge so the slabs on cast on
either side of this "joint" may shrink independently.

-gerard
Santa Clara, CA

-----Original Message-----
From: Cliff Schwinger [mailto:clifford234(--nospam--at)comcast.net] 
Sent: Sunday, August 04, 2002 10:07 AM
To: seaint(--nospam--at)seaint.org
Subject: Slab-on-grade control joint question

Is is necessary to interrupt the WWR or rebar in a slab on grade at the
control joints?

If the answer is "yes" then I have another question:

1.	If control joints will be cut into the slab-on-grade with a
Soff-Cut machine, then it seems that "rocket scientists" will be
required to the following:

2.	Mark control joint locations prior to placing WWR or rebar.

3.	Proper place reinforcing making sure to interrupt it at the
control joint locations.

4.	Place concrete.

5.	Cut the control joints over the precise locations where the
reinforcing steel was interrupted (use of "x-ray" vision goggles not
permitted).


It seems that if "breaks" in the rebar do not align with the control
joints, the slab will probably crack where the reinforcing is
interrupted (not at the control joints.

I have a feeling that most contractors do not interrupt the reinforcing
at the control joint locations (regardless of what the contract
documents require). Continuous reinforcing through control joints is
probably not a problem however, as long as the contractor installs the
reinforcing on the bottom of the slab.  As long as the WWR is just
laying on the bottom of the slab, it will not offer much restraint
against cracking and the cracks will have a better chance of occurring
at the control joints.

All of this sounds to me like a good reason to consider the use of fiber
reinforcing. (I have never specified fiber reinforcing, but I hear that
some manufacturers such as Grace have a "new generation" product that
supposedly replaces WWR in slabs on grade - trouble is, I do not want to
be the "sacrificial lamb" to try it out.)

Any thoughts?

Cliff


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