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Re: Structural importance of full-depth shrinkage cracks

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Kevin,

Do you have a theoretical estimate of what the compression would be at the
top?  After all, if there is compression there and there is not some
interlock causing the crack to resist closing (i.e. such that the
compression present is not sufficient to "break" the bind), then
compression due to flexure would more than likely close the crack.  If not
some binding due to interlock, then you might be right that there is some
other mechanism occuring such that the compression is relieved in some
manner.

In otherwords, if there is no other mechanism occuring, then there needs
to be some flexural compression to "balance" out the flexural tension.
And that compression would be either in the form of the crack closing and
full "bearing" resulting in a good compression resulant OR the compression
resultant being formed by the interlocking pieces being in compression at
the binding points (but not the whole surface and the crack would not
necessarily [completely] close).

HTH,

Scott
Adrian, MI


On Sat, 26 Feb 2005, Kevin Below wrote:

> 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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