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Structural importance of full-depth shrinkage cracks

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I sent this a couple of weeks ago, but I don't think it went through.  So once again : 
I have two current projects in reinforced concrete with full-depth cracks at mid-span of a slab.  One is 20 years old.  The other 3 months.
 
I am wondering what happens when a shrinkage crack gets so big that it opens up on the compression face of a suspended reinforced concrete slab.
What happens to the compression zone ?  If there is no contact between the two sides of the crack, and there is no compression steel, then there is no compression.   (Well, maybe a little, if there is any interlocking going on that I can't see.)  So the only link across the crack is the tension steel.
That means that the suspended span becomes two cantilevers meeting at the crack, and connected together at the ends of the cantilevers by the tension steel across the crack.
They are not really hinged together, since the tension steel is (probably) ensuring some degree of moment continuity across the crack.
 
Does that sound reasonable ? 
 
If that is so, then I guess it is important to inject the crack to restore the compression zone.  If so, what should be injected ?  Epoxy ?  Any comments from the concrete specialists ?   (Gil and Gail ??)  It would be much appreciated.
 
I read all the discussion from the 2003 archive about cracking in a PT slab.  It didn't really answer my question. 
 
 

Kevin Below, ing., Ph.D.

GÉNÉCOR CIVIL INC.

290, rue Seigneuriale

Beauport, (Québec) G1C 3P8

Tél. : (418) 660-6969 poste 272

Fax : (418) 660-6463

 

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