| They allow for a slight amount of
crack movement, though hopefully not enough to reflect through
the overlay. Some semi-rigid crack repair systems incorporate
a fiberglass mesh layer, which is bonded to the top of the
crack. It helps to disburse any movement in the crack, to
reduce the likelihood it will telegraph through the overlay.
Old cracks, new control joints
Simply stopping or restricting
a crack's movement does nothing to address the underlying
cause of the crack and doesn't insure that the crack won't
continue cracking or reappear nearby. But short of calling
in a structural engineer and undertaking an expensive slabjacking,
slurry-pumping repair, there is a simple and crucial way
to hedge your bets: cut control joints. With a new control
joint in place a few inches away from a repaired crack, any
future cracking will be directed along the bottom of the
control joint. Hopefully.
It's standard practice to cut control joints near cracks
repaired with either semi-rigid or rigid epoxies. "If
you don't relieve the stress somewhere near that crack, you'll
develop another crack somewhere near your repair, and that
will show up through the overlay," says Ron Cottingham
of Dayton Superior.
Control joints axe especially critical near cracks repaired
with rigid materials, since the welded crack allows no movement
whatsoever. "If you had a pencil that snapped, and you
Superglued those two pieces back together and then tried
to break that pencil again, it won't break in that crack
- it will break somewhere else," Cottingham says.
Cutright recommends waiting to cut control joints until
the repair material has cured. "You never know what's
going on with the substrate, and if you start cutting control
joints prior to repairing the crack, you may possibly exacerbate
any cracking problems the concrete has," he says.
Honoring the joints
Any joint in the slab, of course, needs
to be honored through the cementitious overlay. Once that
saw cut is in place,
other purely decorative saw cuts may be used to conceal the
functioning
joints in a decorative
pattern.
The joints themselves can be made with an angled cut using
a V-shaped diamond crack-chasing blade. "More often
than not that's the tool of choice in decorative," says
Tom Roe, director of the concrete/masonry cutting division
for Multiquip, which markets crack-chasing equipment as well
as concrete saws capable of handling crack-chaser blades. "It
gives you a nice beveled look - anywhere from 30 to 45 degrees
on both sides - that's very attractive."
Typically a thinner, square-edge blade will be used to cut
the initial joints while the topping is still green.
After the concrete cures, the wider V-blade is used to finish
the joint. "Green concrete is very abrasive, and some
of the aggregate might not necessarily be set up in the cementitious
material," Roe says. "If you were go in with a
wide blade too soon, oftentimes what you're going to do is
create spatting that's very unacceptable aesthetically."
Coloring crack fillers
If a particular crack repair material
will take any stain at all, it won't stain the same way as
the surrounding concrete.
So if the idea is to make the crack disappear, an overlay
is generally used.
However, integral colors can be used with some repair materials
to match them to the surrounding concrete. "You can
use standard powder pigments, but you have to do some sort
of mock-up phase to find out what the proper pigment loading
is," says Diaz. "We have cases where we've actually
done some historical repairs and have needed to match weathered
concrete that way. It's possible to match colors pretty closely."
When a crack appears in a cementitious overlay, a patching
compound made by the overlay manufacturer is often the best
bet for a seamless repair. Patching compounds are often un-sanded
versions of an overlay material. They can be feathered onto
the topping and colormatched, integrally or with stain.
No guarantees
One thing you should never do to a crack is
warranty the repair. There's simply no way to guarantee that
a crack won’t
be back. But the return of a crack doesn't mean a repair
was all for naught. "A crack may come back, but it's
going to be less severe and hopefully less noticeable, says
Cook. "The crack that comes back to the top is usually
a hairline crack that a lot of people can live with. It's
not so unsightly."
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