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Re: Requirement for control joints in elevated slabs.

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These documents may help to evaluate the cracks.  However, when an
elevated concrete slab is placed over a 
metal deck it appears to become an unresolved matter for many
contractors/builders. 
In the case of a composite deck that contains very little reinforcement
the concrete bonds to the metal deck , which allows tensile capacity for
positive-moment areas.   When taking into account the steel requirements
in negative-moment areas (above beams and girders) it is common to
provide steel reinforcing cross-sectional area approximately 0.0018 x Ac
, although it doe's not prevent the cracking in these areas, it does help
control the crack width. 
With regards to contraction joints  and joint spacing unfortunately there
are no published guide lines to provide answers to these questions.  Let
it crack???!!!!. maybe !!!??   Contraction joints in elevated concrete
slabs on metal decks are of little value, because the concrete deck is so
highly restrained by the metal deck and shear stud connectors, cracking
between joints becomes inevitable.  Some recommend installing reinforcing
bars in both directions,especially over beams and girders, that will help
minimize cracking and help reduce slab curling, while others prefer using
no contraction joints and as few construction joints as possible,
allowing for random cracks to form. Cracks that are wide enough to affect
serviceability could be patched.

This is my $ 0.02 worth.

djk

On Wed, 11 Dec 2002 15:42:07 EST GSKWY(--nospam--at)aol.com writes:
> In a message dated 12/11/2002 12:33:32 PM Pacific Standard Time, 
> djkiss55(--nospam--at)juno.com writes:
> 
> 
> > ACI 224   Control of Cracking in Concrete Structures
> > ACI 224.1 Causes, Evaluation and Repair of Cracks In Concrete 
> Structures.
> > ACI 201.2 Guide to Durable Concrete
> > ACI 207.2 Effect of Restrain, Volume Change and Reinforcement on 
> Cracking
> > of Mass Concrete
> > 
> > 
> 
> I'm not sure how applicable any of these documents are - a 5 in. 
> slab is 
> certainly not mass concrete.  Also,  these documents are typically 
> addressing 
> concrete slabs, not concrete on metal form deck.  While the cause of 
> cracking 
> might be the same (concrete shrinks as it hardens), the ways of 
> addressing/preventing cracking are not.
> 
> If I saw a plan that called for a control joint in a concrete slab, 
> I would 
> assume the designer didn't know what he or she was doing,  I'm not 
> so sure 
> about concrete on form deck.  If the cracking is negative moment 
> cracking 
> over the beams though I would assume it was because there was not 
> enough 
> reinforcing in the slab, that it was too widely spaced or it was at 
> the 
> bottom of the slab, doing nothing.
> 
> 
> Gail S. Kelley, P.E.


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