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Re: Cracking in Concrete Slabs on Grade

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Harold,

Thanks for the response. The sawcuts were done the same day although I
believe you may have pinpointed why they cracked the way they did. I am
pretty sure that the sawcut did not match the point of the diamond. I will
be back on site tomorrow and review the situation again.
Thank you also for your proposed corrections. I will take them into account.

Domenic
----- Original Message -----
From: Sprague, Harold O. <SpragueHO(--nospam--at)bv.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, June 28, 2000 2:13 PM
Subject: RE: Cracking in Concrete Slabs on Grade


> Domenic,
>
> If the joints were cut 2 days after the concrete was placed, the joints
had
> already been made by concrete shrinkage in the form of random shrinkage
> cracks.  Control joints must be cut within about 6 hours of placing the
> concrete.  I require a Soff-Cut saw or "early entry dry cut sawing system
> with skid plate" if you want to keep it generic.  The finishers cut the
> joints on the day of placement.
>
> If you use conventional concrete sawing, you have to wait until the
concrete
> will not ravel.  If you wait that long, you've waited too long and the
> random crack beast will bite you where it hurts.
>
> The diamond isolation joint acts as a crack originator and will force the
> crack to initiate at the point.  Getting the saw cut in a timely manner is
> vital.  I stopped using the diamonds several years ago.
>
> These cracks will wear and expand as they wear with exposure to fork lift
> traffic.  Look at the shrinkage curves for concrete, and you will see that
> although the cracks appear to be slowed, they are not done by quite a
> margin.  Now that the saw cut cracks are in, you could epoxy inject the
> random cracks.  This will run about $5 to $6 per foot.
>
> Another important consideration is developing a slip plane with a well
> graded sub course and a slip sheet of polyurethane.  The joint details
> should also warrant consideration to avoid locking up longitudinal
> differential shrinkage and yet the joint should resist differential
vertical
> displacement.  You want to avoid shrinkage constraints.
>
> The saw cut joints should have a bond breaker in the base of the joint and
> be filled with a simi-rigid epoxy or polyurethane fill.
>
> These cracks will be a maintenance issue forever, and can cause wear and
> increase maintenance on his hard wheeled vehicles.
>
> I worked on a couple of projects that totaled about a half a million
square
> feet of slab on grade on expansive soils.  We had a total of about 6 or 8
> feet of random cracks, and we should not have had those.
>
> Regards,
> Harold Sprague
>
> > -----Original Message-----
> > From: Domenic DeAngelo [SMTP:domdean(--nospam--at)mediaone.net]
> > Sent: Wednesday, June 28, 2000 11:51 AM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Re: Cracking in Concrete Slabs on Grade
> >
> > I was wondering if I might get some input to a"problem" which has
occurred
> > recently. I designed a 6" concrete slab on grade with control joints at
> > column locations and mid-bay locations (the column spacing was 25' x
25').
> > Two days after the slab was poured and control joints cut, the slab
> > cracked
> > within 1/2" of the control joint. It started at the diamond shaped
> > isolation
> > joint continued for about 10 feet angled away to approximately 3" from
the
> > control joint for a distance of about 2 feet and then angled back in. On
> > the
> > other side of the column the crack followed the expansion joint. This
> > happened approximately one month ago and no further cracking in the slab
> > (other than where it was supposed to) has occurred. Needless to say, the
> > owner (it is a moving van storage facility) is not happy since the area
> > will
> > be subjected to forklift traffic. Can anyone explain why this happened
and
> > if there is another way to repair the area in question besides
non-shrink
> > elastic grout?
> >
> > Thanks in advance,
> >
> > Domenic DeAngelo P.E.
> >
>
>