34 PWC DECEMBER 2009
o pay my way through
college,I worked as a
man.Now,there are a
thousand things that
need to be fixed in a
hotel,and I often did-
n’t have a clue where
to start.Then I got some good advice
from old Jack,who had worked there
for 30 years.
“You’ll never know how to fix it
until you know how it’s supposed to
work and why it isn’t working,” he
told me.Keeping that in mind saved
me many hours of making futile re-
pairs that didn’t solve the problem.
It’s the same with concrete:You
need to understand the cause of a
problembefore you can decide how—
or even whether—to fix it.
To start,consider whether the con-
crete is interior or exterior,a slab or a
wall.Although obvious,these an-
swers help us begin to define the
This article will focus mainly on
damage to flatwork—floors and
slabs—although we’ll briefly mention
some wall problems.Here are some
tips to help you decide when to repair
concrete and howto make repairs that
Yes,it’s true;concrete cracks.The
reason is simple.Concrete is strong in
compression but weak in tension.A
crack in concrete means it has been
in tension.And the crack is always
perpendicular to the tension—al-
Figuring out what caused the ten-
sion is not always easy.Concrete can
crack while it is first gaining strength,
when it is new and drying out,or
after it is hard and dry.A crack after
drying usually signifies some sort of
structural problem—an overload or a
weak base beneath a slab.That sort of
crack usually calls for an engineer or
stabilization expert (see “When to
Call for Help,” page 36).
On the other hand,superficial plas-
tic shrinkage cracks or crazing form
while the concrete is setting up.Plas-
tic means the concrete was still some-
what soft.Just like mud at the bottom
on cracks, spalls
and joints —and when
to call for help.
By Bill Palmer, P.E., FACI
Ryan Klos Photography
It’s like the sponge in your sink,
which shrivels and curls when it dries.
But brittle, heavy concrete can’t slide
easily on its base, so it cracks as it
shrinks. This drying and shrinking can
go on for several months, depending on
the type of concrete, the thickness of
the slab, the relative humidity,and
other factors. But once the slab is dry
and the shrinkage complete, this type
of crack (when it’s inside a building)
seldom moves much again. Ayear-old
slab has probably shrunk as much as it
ever will, so if the owner wants this
crack repaired, filling it with a rigid
filler is often a good solution.
Start by routing (or chasing) the
crack with an angle grinder and vacu-
uming out all the debris. A clean sur-
face is critical. The purpose of the
routing is to create a clean square edge;
always avoid feathered edges with re-
I usually recommend a polyurea
filler, since it hardens quickly and
has a bit of flexibility. Epoxy and
other joint fillers are also effective,
but don’t use a flexible sealer if you
plan to cover the crack; it’s likely to
of a dried-out puddle, concrete can
crack if it dries out before it’s had the
chance to gain strength.
Plastic shrinkage cracks are usually
thin,parallel and not very deep. Craz-
ing (also called map cracks or alliga-
tor cracks) is a pattern about the size
of chicken wire that covers the con-
All of these cracks are purely sur-
face blemishes. They are dormant, not
active, and do not affect the function-
ing of the slab.Repairs are unneces-
making a big mess. The only reason-
able repair is to coat the entire surface
after prepping to remove any sealers
or curing compounds.
Shrinkage cracks are the most com-
mon type that goes through the con-
crete; they can occur on interior or
exterior slabs. When concrete is mixed,
water is added to hydrate the cement.
Extra water—what we call water of con-
venience—is added to make the con-
crete pourable and workable. When the
water of convenience eventually evapo-
rates, the concrete dries out and shrinks.
WWW.PAINTSTORE.COM PWC 35
See REPAIRS on next page
Floor joints and cracks can be repaired by filling, then applying a full self-leveling overlay.
Crazing (chicken-wire pattern) cracks do not move or affect the slab’s functionality.
tant to understand the difference.
Control joints or contraction joints
cut across the middle of a slab. Their
purpose is to “control” the shrinkage
cracks. If the control joints are cut at
the proper spacing and time, the
cracks will follow them. So a control
joint is really just a crack we have
forced to follow a clean line.
Some will argue, but my experi-
ence is that interior control joints
aren’t going to move much after the
concrete has dried for a few months.
(Exterior control joints are a differ-
ent story; they will keep moving due
to temperature changes.)
So, like shrinkage cracks, interior
contraction joints do not need to be
repaired. The only reasons to do so
are if you want to coat over them, if
they are leaking water, or if you
want to keep dirt out.
The other type of joint is an isola-
tion or expansion joint. These are
usually formed during construction,
using asphalt impregnated paper, to
isolate the slab from walls, columns
or other structural parts of the build-
ing. Isolation joints are expected to
move. If you try to bridge over them,
they’re almost sure to reflect
For experts only
Sometimes, you may want to bond
together the concrete on either side
of a crack. There are methods (such
as epoxy injection) that can do that,
but these are best left to experts.
The same goes for overload
cracks, which include slabs cracked
due to a poorly compacted sub-base.
Concrete slabs can’t support them-
selves; if not supported from below,
especially at joints, they will sink
and crack or the slab corners will
curl and break off. Repairing these
cracks requires special skills.
Problems with slabs aren’t limited
to cracking. Others include dusting,
popouts, and scaling or spalling.
Dusting occurs with a thin soft sur-
face layer that actually rubs off onto
your finger or clothes. If the dusting
is not too bad or deep, it can some-
times be repaired with a surface
hardener, such as a silicate or sili-
conate, which actually grow new
crystals within the concrete.Ex-
36 PWC DECEMBER 2009
Some points about joints
Joints are the straight lines tooled
into soft concrete during finishing or
cut in with a saw after hardening.
These are functional, not decorative,
joints that allow the slab to crack or
move. Jagged cracks in a slab typi-
cally mean that the joints were in-
stalled at the wrong time or in the
Joints are of two types; it’s impor-
REPAIRS from last page
When to Call for Help
Cracks in concrete are not all created equal. You should not try to repair
cracks that have a structural cause unless you have experience with this type
For one thing, your repair is likely to fail. And, just as important to your
business, attempting to repair a structural crack can impart some level of li-
ability. Even if you just recognize that a crack might indicate some danger,
you probably have some liability. So immediately inform the owner in writing.
In this litigious world of ours, it’s best to cover yourself.
Cracks that should raise a red flag include:
• Circular cracks in a slab (often around the intersection of two contraction
joints). This can imply a subbase failure or overloaded condition. The only so-
lution is slab jacking or ripping out and replacing the damaged section.
• Basement wall cracks that seem to bulge. Put an eight-foot 2x4 across
the crack. If there’s a bulge,that could mean a failing wall, due to saturated
soil outside the wall or heavy loads near the foundation. Basement walls may
also have innocuous, thin, vertical
shrinkage cracks that can be repaired
just like shrinkage cracks in a slab.
But wide or horizontal cracks in walls
are cause for concern.
• Basement floor cracks a few feet
away from,and parallel to,the
walls—especially if the floor at the
wall is higher or lower. This can be
caused by the lack of an isolation joint
between the floor and foundation
wall,by subbase failure,or by heaving
from expansive soils below the slab.
• Cracks in structural reinforced concrete, such as columns or beams,
especially diagonal cracks on beams near the supports. These are shear
cracks and imply an overloaded member.
• Cracks that have rust stains imply that there is embedded reinforcing
steel that is corroding. This can be repaired,but it means removing concrete
down to the steel, which can affect the structure’s stability.
Radial cracks indicate either that the slab
was overloaded or the subbase failed.
These typically require an expert repair.
See REPAIRS on page 38
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tremely soft surfaces may require me-
chanical removal of the top layer but,
depending on the size involved, try
the chemical means first. Do a couple
applications of a silicate hardener,
followed by a sealer. If that doesn’t
work, you’ll have to grind off the sur-
face to get rid of the soft
material. The slab below
is usually good and
Popouts and scaling
are most common on ex-
terior flatwork that gets
wet and freezes. Pop-
outs are small round
holes in the surface with
a piece of aggregate in
the bottom. The aggre-
grate is the culprit here. You can drill
it out and fill the hole with a repair
material (something with shrinkage
compensation and polymers to in-
crease adhesion and strength).
Surface scaling is all too common
on exterior concrete slabs—and com-
pletely unnecessary. Exterior concrete
must have adequate air entrainment
to withstand freeze-thaw cycles. Un-
38 PWC DECEMBER 2009
REPAIRS from page 36
fortunately, air can be a little unsta-
ble in concrete—and deicing salts,
especially on new concrete, exacer-
bate the freeze-thaw damage. Re-
pairs are relatively simple, though:
Remove all unsound concrete (sand-
blasting is best), apply a bonding
material of a neat portland cement
slurry, and coat with a repair mortar.
I recommend one with some poly-
mers to increase adhesion and pro-
vide an impermeable coating.
Toppings are available in a wide va-
riety of formulations, for nearly any
desired characteristic. Meticulously
follow the manufacturer’s instruc-
tions,and you’ll get good bond and
One final note: If you are consider-
ing an overlay or coating, you’ll need
to think about moisture emissions
from the slab. Water vapor moves
through concrete fairly easily. If the
concrete has not dried out or there is
no vapor barrier beneath the slab,
water vapor is coming out of the sur-
If you apply an impermeable sealer,
overlay or flooring, the vapor will get
trapped—but not for long. Sooner or
later, the pressure will pop off most
sealers and overlays. The solution is to
test the moisture vapor emission rate
or internal relative humidity—sub-
jects for a different article, I’m afraid.
Concrete repair isn’t difficult, but
failed repair jobs are all too common.
Start by understanding the cause of
the defect,then make sure you’ve pre-
pared the surface and used the proper
Bill Palmer, P.E.,FACI is a con-
sultant to the concrete industry and
the former editor in chief of Concrete
Construction magazine. Contact him
Left to right: This deep shrinkage crack was repaired with a rapid-setting polymer material, then ground off to match the existing concrete.
You need to understand
the cause of the problem
before you can decide how
(or even if) to fix it.
38 PWC DECEMBER 2009
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