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Re: Structural importance of full-depth shrinkage cracks

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Kevin,
 
        This is an interesting situation.  I have not experienced it before; but I do have some thoughts that may have some (if limited) value.
 
        My first thought would be to consider the crack as a deliberately cut (or cast) hole in the slab where special reinforcing at the edge of the hole has been omitted or forgotten.  This may be a bit crude; but even crude analyses can provide useful information.  At the very least one would hope to classify the problem into one of three categories: first, there is no problem, therefore, no action is required; second, there is definitely a problem and significant invasive action is required; and third, the analysis is inconclusive, i.e., it is not sufficiently detailed to determine whether or not there is a problem that requires action.
 
        I hope this is of some help.  I'll be waiting to read what the real concrete experts have to say.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Saturday, February 26, 2005 2:42 PM
Subject: Structural importance of full-depth shrinkage cracks

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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