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All Cracked Up!
BY Susan Brimo-Cox
When
it comes to concrete, are cracks inevitable? Technically,
if every aspect of the original pour was done perfectly, perhaps
not. But in the real world, it's a pretty safe bet that cracks
are going to occur. Concrete cracks are caused by improper
sub grade preparation, a bad mix design, poor thickness control,
improper curing, poorly placed control joints, temperature
changes, stress loads, ground or building movement, and a
variety of other reasons - singly or in combination. Contractors
are frequently called in to repair or camouflage concrete
cracks. A homeowner with a concrete driveway, walk or patio
may decide that the cracks are unsightly and need to be fixed.
Cracks in basement or garage floors often need attention.
Darryl Manuel, president of Vexcon Chemicals Inc. in Philadelphia,
observes a trend in turning older buildings often featuring
concrete floors with lots of cracks - into lofts and retail
spaces. And in new retail and commercial spaces, concrete
floors are often specified for their durability and longevity,
cracks not withstanding. When it comes to crack repair and
the decorative treatment of the concrete surface, a dozen
contractors have as many recommendations. Different kinds
of cracks, it seems, require different repair techniques.
"The first thing about concrete cracks is that just about
every crack is different and how you repair a crack depends
on what your coating is going to be," explains A. Ray
Anderson, owner of Only Under Pressure in Thousand Oaks, Calif.
 Being
a crack detective
Some cracking is a one-shot deal, such as when the cracks
occurred because of curing problems or an over-size truck
ran over the slab. These are static cracks, unlikely to move.
Structural and other types of moving cracks, on the other
hand, typically present ongoing problems. As Tom Ralston,
president of Tom Ralston Concrete Inc. in Santa Cruz, Calif.,
points out, "You have to honor all structural cracks.
They'll continue to move." Mike Meursing, manager at
Versatile Deck Coatings in Long Beach, Calif, says, "Once
you locate the cracks and failures, it is important to determine
the cause, otherwise your repair will be just a short term
fix." If there is still stress present, the concrete
will crack again, but there are clues to what may have caused
the cracks, he adds. Here are some examples:
- If you have a full-depth crack and
the surfaces on either side of the crack are not level it's
probably due to an unstable base.
- If the slab is on a slope and the
cracks run perpendicular to the slope, it's likely slippage
is the cause.
- If you observe random cracking and
see or know heavy machinery has crossed the concrete, excessive
load is probably the cause.
- Surface spalling or cracking where
the rebar is located indicates that corrosion of the rebar
is the culprit.
- Impact cracks are frequently indicated
by a spall.
- Freeze-thaw cycling often causes
widespread and consistent cracking across the concrete.
- And, if there isn't an apparent cause
for the cracking, but you notice curling on the surface
or sporadic cracking, it's likely due to shrinkage.
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