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Re: Slab on grade

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James F Fulton wrote:

> For control joints, there is a rule of thumb that says space control
> (contraction) joints in feet at between 2 to 3 times the slab thickness in
> inches, for a 6" slab this amounts to 12' to 18' joint spacing.

The rule of thumb you are proposing is also printed in ACI and PCA
publications; however, it is intended to control shrinkage cracks in
non-reinforced slabs.  We use it frequently for area paving (parking
lots) with close joint spacing and no reinforcement.

To determine joint spacing for a reinforced slab, it is more appropriate
to use the subgrade drag theory/equation.  I am surprised no one has
mentioned this.

Brian K. Smith, P.E.
Bossier City, Louisiana