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The Straigt Story
on
CRACKS
By
David Thompson
Cracks
are to concrete what death and taxes are to life inevitable.
But at least cracks can be repaired. If and how a crack
gets repaired prior to a decorative concrete application
depends both on the nature of the topping to be applied and
on the nature of the crack.
Static cracks v. moving cracks
Hairline cracks generally
fall into the static crack category - they don't move and
they're not going to. They're typically surface cracks that
were caused by one-time events, like drying shrinkage. Tight,
static, surface cracks can be coated with most polymermodified
cementitious overlays and never seen again. However, if these
same hairline cracks are on a floor that's acid-stained,
they may go from practically invisible to far more noticeable
if the stain sinks in and darkens them. This could be seen
as a desired effect, but it could also be seen as a blunder.
Sealers, tinted or otherwise, can also inadvertently highlight
the hairline cracks in a floor in an unattractive way. "The
risk is, the sealer flows down into the crack and leaves
a little sink area, and when light shines across it, it accentuates
the crack," says Seth Pevamick, a technical field consultant
specializing in decorative concrete for Ardex Engineered
Cements. "In that situation, when it just a hairline,
I'd just leave it go. Yeah, you may see it when you mop
it, but when it dries you don't see it anymore."
When a crack is continuing to lengthen, or when the each
side of the cracked stab moves independently of the other,
you've got what's known as a moving crack, aka a structural
crack. Any crack that cuts clear through a slab, whether
it seems to be moving or not, ought to be considered a structural
crack. "Structural cracks are usually pretty easy to
spot," says Tim Cutright, a dealer advisor for PermaCrete. "You'll
get chipping around them, or one part of the slab will be
slightly lower than the other part, or you'll be able to
see dirt and gravel in the crack itself that's a dead giveaway."
Sometimes, though, determining whether you're dealing with
a surface crack or a structural crack is a bit of a guessing
game - though one you can get to the bottom of,
"If you had a small crack in the surface and you weren't
sure what it was, I would take a Skilsaw with a mason's blade,
and I'd cut down about one-sixteenth of an inch and see if
it bottoms out," says Outright. "If it bottoms
out, it's a surface crack."
Moving cracks demand to be repaired prior to doing a decorative
overlay. "If you don't do anything to a moving crack
and you go over it with an overlay, that crack is almost
guaranteed to transmit back up through the overlay," says
Dan Cook, president of CemTec Systems. "And once it's
transmitted up through there, if you continue to get movement,
it can pinch the overlay and cause delamination."
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