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RE:Slab on grade

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I find it interesting that structural engineers typically talk in terms of
crack control joints at 15-27 feet while it is not unheard of for airport
runways to be designed with much larger spacings.

The use of closely spaced joints is appropriate when there is little or no
reinforcing.  The problem is that many people add reinforcing which they
then continue through the crack control joint.  This appears to be
inconsistent at best.  Another common problem is when the concrete section
thickens yet the saw cut or troweled joint is kept at the same depth thus
becoming inefective.

In many instances the slab on grade acts as a diaphragm to help transfer
the lateral loads into the ground.  In this case it is not appropriate to
stop the reinforcing at the crack control joints.  As a result I have
fairly frequently used a higher ratio of reinforcing and not installed any
crack control joints. 

Another situation is where, because of poor subgrade the slab on grade is
designed to span between pile caps or grage beams. In this situation crack
control joints are not provided.

It is my impression that many people are stuck in designing slabs on grade
the way they were done 20 plus years ago.  We need to consider other
approaches when the joints are not desirable.

It would be interesting to hear from somebody who is knowledgable how long
runways are designed with no movement joints.  This would be especially
appropriate since the structure being discussed will house aircraft.

Mark Gilligan, SE