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Re: Control joints in Elevated slabs

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Harold,

I agree that some amount of rebars should be placed at girders locations to minimise cracking, especially if concrete pour are likely to be large. Recently on a job I saw some cracking between columns but only at girder locations. There was no cracking at beam locations. Mesh was used throughout.

I think good construction practice would be to avoid pouring large areas in one pour e.g. pour floor in smaller but alternate bays, and a little extra reinforcement would minimise cracking.

The contractor probably inserted control joint due to his experience with cracking is the past. I would not recommended it.

Regards,

Nigel.



Sprague, Harold O. wrote:

Hyndman,

No. But cracking in concrete slabs on metal deck is an issue.
Number one, never use polypropylene fibers.  Use mesh or rebar.  This is
another topic.

If the slab is covered by carpet, use the minimum SDI recommended steel and
forget about it.
If the slab is going to be exposed or covered with a material that could
reflect cracks, CONTROL the cracks and consider the following.
*	Add TOP rebars about 5' long perpendicular to beams and girders.
This is not required for strength.  It is just to CONTROL the cracks that
WILL form due to negative bending along the center line of steel girders,
beams, and joists.  The only exception is if the joists or beams are spaced
at about 6 ft. or less or if you drape the mesh.  You will still need the
rebars at the girders.  The crack widths can be calculated, and when they
reach the point of ugly (architectural term), add rebar. * Use the ACI minimum reinforcing steel (as opposed to SDI).



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