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Crack Control Joints - Unreinforced Slab

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ACI 302 Committee Report (ACI Materials Journal, May-June, 1989) states,

"[Contraction] [j]oint spacing in feet for nonstructural slabs should be 2 to
3 times the slab thickness in inches.  Aggregate size, unit-water content,
and local experience with the concrete ingredients will assist the designer
in selecting the proper spacing.  Shorter intervals should be used whenever
there is reason to expect shrinkage to be high or the thermal range to be
great.  Temperature steel may be used to restrict crack width."  (8- to
12-feet for a 4" slab.)

Exposed slabs that I have seen with polypropylene fibers look like they were
given a light broom finish with a broom that had the fibers falling out.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Bill Polhemus wrote:

. > I've got to design an unreinforced slab on grade for a nightclub. The
. > "unreinforced" part is the owner's requirement; the mall in which the
. > nightclub is being built uses unreinforced concrete throughout. The slab
. > I'm in charge of is about 160' X 120', with 40' square construction pours.

. > I'm debating what sorts of measures to take to ensure the slab doesn't
. > crack (it will be exposed over about half the surface area, with some
. > sort of clear seal applied). I was thinking of using synthetic fibers but
. > am not sure if that would create a problem with application of the clear
. > seal.

. > Also, I'm contemplating control joints at 20' (quartering the pour
. > panels), but see very little in the way of guidelines for control joint
. > spacings in the literature.

. > Does anyone have any enlightenment to share?

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