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RE: Structural importance of full-depth shrinkage cracks

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Scoot, the spans in the new structure are 28 feet.  These cracks formed
very soon after pouring, and were evident on the top surface even before
form stripping, under no-load conditions.  After stripping, they are
still open.  Maybe as the loads increase with occupation of the
building, they will close up.  Unless there is so much dirt in the
cracks that they can no longer do so.

I don't really see how interlock could be contributing to the
compression, since the crack would simply close up if it came under net
compression.

I guess that the answer lies in the magnitude of loading.  Under zero
load after the pour, shrinkage creates uniform tension in the section,
and it cracks full-depth.  That relieves the tension elsewhere.
After stripping, self-weight creates a little compression, but maybe not
enough to create net compression anywhere.  The crack would still be
open at the top.
With more load, more compression, and at some stage, enough compression
will be created to close the crack.  

I will follow this on the site.  We will soon be at clean-up stage, and
doing final slab inspections before floor coverings are installed.  But
I guess I won't see it under full load because by then the floors will
be covered.

On the 20-year-old project, the floors have been covered by travertine
tiles, and the cracks through the tiles are open.  We will be removing
them and cleaning the surface for treatment.  I will be able to observe
the cracks more closely at that stage.

I think I will need to restore the compression capability across the
crack by filling it with epoxy, probably by gravity, not
pressure-injection, after sealing the under-surface.  The local Sika rep
says he has treated full-depth cracks this way before just prior to
applying epoxy floor finish, and never had the cracks reappear through
the new floor finish.

It still seems to be a complex mechanism at work.  

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



-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: 27-Feb-05 02:46
To: seaint(--nospam--at)seaint.org
Subject: Re: Structural importance of full-depth shrinkage cracks


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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