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RE: SLAB - Control Joints in Concrete Slab on Grade

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I have seen similar performance.  But on occasion, I like Fibermesh in addition to rebar for control of the plastic shrinkage cracks.  But don't take away my steel.

Harold Sprague
Krawinkler, Luth & Assoc.



-----Original Message-----
From:	CLaines [SMTP:CLaines(--nospam--at)aol.com]
Sent:	Monday, April 20, 1998 10:05 AM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Re: SLAB - Control Joints in Concrete Slab on Grade

I noted in the question regarding control joints in slabs  that the specification included Fibermesh which I am assuming is in lieu of steel reinforcing. I also assume the reason control joints are of interest is to control cracks.
I am involved in a project where slab floor cracks have become an issue. The approved plans for this tract development specified welded wire mesh in the 4-inch floor/foundation slabs. The soils are not considered expansive.  The models were constructed with the welded wire fabric, but when the rest of the homes were built, the wwf was replaced with Fibermesh. That was 9-years ago. Today, the tile flooring is cracked and loose, the vinyl flooring has bumps and stains where there are cracks in the slab, and there are measured slabs cracks up to 3/16-inch wide under the carpeting, in all of the houses of this tract ...........except the models! 
The slabs of the models with the wwf have cracks, but all are hairline, with no separations and no vertical displacement. 
All concrete slabs crack, and reinforcement cannot prevent that. But steel can spread the cracks so that you have many hairline cracks, instead of a few wide cracks. The steel reinforcing also keeps the cracks tight and provides doweling between the pieces, something hair-thin Fibermesh cannot do. I personally do not specify wwf because it often ends up at the bottom of the slab due to poor installation, so I prefer #3 @ 18" for typical 4-inch slabs.  Fibermesh claims to minimize plastic shrinkage cracks, cracks that form during the first few hours after placement, but the serious deep shrinkage cracks cannot be prevented by reinforcing, only managed, and for that you need steel and properly placed joints. 
Charles Laines, S.E.
Long Beach, California


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