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Re: Concrete slab on grade - natural cracking

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Ken and Gary,
The reason for the sawcuts is to create a regular pattern of limited size
cracks that are fine enough to permit the transfer of shear by the irregular
patterns of coarse aggregate.  The sawcut is only about a quarter of the
depth of the slab but it provides a slightly weakened location for the first
cracks to appear if it is done soon enough (less than 12 hours and
preferably closer to 6 hours depending on atmospheric and exposure
conditions) after the pour.
Without the joints the first crack will tend to widen to a degree that will
not permit the shear lock that would otherwise result.
Curling occurs because large cracks have been produced and the top of the
slab dries out much faster than the underside.  This is generally due to
improper base preparation, too much water in the mix, improper curing of the
concrete surface or a combination of these factors.  It is less likely to
occur if the pour is properly done and the sawcuts are made.  Contractors
who argue otherwise do not know how to do the work correctly and want to
keep the extra money.  This is especially true if the slab is to be covered
and not noticed until years later  when the failure becomes more pronounced.

Richard Hess
----- Original Message ----- 
From: "Gary Hodgson & Associates" <ghodgson(--nospam--at)bellnet.ca>
To: <seaint(--nospam--at)seaint.org>
Sent: Saturday, June 04, 2005 4:39 AM
Subject: Re: Concrete slab on grade - natural cracking


> Ken,
> I did an A&P store a few years ago and the A&P
> consultants also didn't want saw cuts-let it crack
> where it will.
> Gary
>
> On 2 Jun 2005 at 16:54, Ken Peoples wrote:
>
> > We have a project with a wood gymnasium floor on sleepers.  We have
> > specified control joints in the slab (both sawcut and formed keyed
> > joints).  The contractor is telling us that he would prefer to not put
> > the sawcut control joints in and just leave the slab crack wherever it
> > needs to crack.  His reasoning (besides saving the cost of sawcutting)
> > is that he has had bad experiences with curling at the sawcuts that
> > have needed to be ground down.  Apparently when the slab just cracks
> > wherever it wants to, the aggregate keeps it from curling.  I have not
> > heard this argument before, but I can see the logic in it.  I know
> > that this would be a problem where reflective cracking might be a
> > concern (such as a tile floor), but since there are wood sleepers and
> > then a wood floor above that, I wonder if he is correct.  This may be
> > a case where less work is better.  He claims that many large slabs
> > (like K-Mart or Wal-Mart type buildings) are being done this way
> > today.
> >
> > I look forward to reading the collective wisdom on this topic.
> >
> > Best regards,
> >
> >
> > Ken
> >
> > Kenneth S. Peoples, P. E.
> > LVTA
> > Lehigh Valley Technical Associates, Inc.
> > 1584 Weaversville Road
> > Northampton, PA 18067
> >
> > Phone: 610-262-6345
> > Fax:610-262-8188
> > Email: kpeoples(--nospam--at)lvta.net
> >
>
>
>
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