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RE: Cracking in Concrete Slabs on Grade

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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.  

Harold Sprague

> -----Original Message-----
> From:	Domenic DeAngelo [SMTP:domdean(--nospam--at)]
> Sent:	Wednesday, June 28, 2000 11:51 AM
> To:	seaint(--nospam--at)
> 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.