Concrete, like other construction materials,
contracts
and expands with changes in moisture and temperature,
which, depending on load and support conditions, can lead to
damage and deterioration that will require attention.
Cracks can occur when provisions to accommodate these
movements are not made in design and construction. Some
common forms of cracks are:
1) Shrinkage cracks;
2) Cracks due to improper jointing;
3) Cracks due to continuous external restraint (e.g., castin
place wall restrained along bottom edge of footing);
4) Cracks due to lack of an isolation joint;
5) D-Cracks from freezing and thawing;
6) Craze Cracks; and
7) Settlement cracks.
Most random cracks that appear at an early age, although
unsightly, rarely affect the structural integrity or the service life
of concrete. Closely-spaced pattern cracks or D-cracks due to
freezing and thawing that typically appear at later ages are an
exception and may lead to ultimate deterioration.
Why do concrete surfaces crack?
According to the National Ready Mixed Concrete Association,
the majority of concrete cracks occur due to improper
design and construction practices, such as:
• Omission of isolation and
contraction joints and improper
jointing practices;
• Improper sub-grade preparation;
• The use of high-slump concrete or the excessive addition of
water on the job;
• Improper finishing;
• Inadequate or no curing.
All concrete has a tendency to crack, and it is not possible to
produce completely crack-free concrete. However, cracking can be
reduced and controlled if the following basic concreting practices
are observed:
• Sub-grade and Formwork. All topsoil and soft spots should be
removed. The soil beneath the slab should be compacted soil or
granular fill, well compacted by rolling, vibrating or tamping.
The slab, and therefore the sub-grade, should be sloped for
proper drainage. In winter, remove snow and ice prior to placing concrete and do not place concrete on a frozen sub-grade.
Smooth, level sub-grades help prevent cracking. All formwork
must be constructed and braced so that it can withstand the
pressure of the concrete without movement.
Vapor retarders directly under a concrete slab increase bleeding
and greatly increase the potential for cracking, especially
with high-slump concrete. When a vapor retarder is used, cover
it with three to four inches of a compactable granular fill, such as
a crusher-run material to reduce bleeding. Immediately prior to
concrete placement, lightly dampen the sub-grade, formwork,
and the reinforcement if severe
drying conditions exist.
• In general, use concrete with
a moderate slump (not over 5
inches). Avoid re-tempering
concrete to increase slump
prior to placement. Higher
slump (up to 6 or 7 inches)
can be used provided the mixture
is designed to produce the
required strength without excessive
bleeding and/or segregation.
This is generally
accomplished by using waterreducing
admixtures. Also
specify air-entrained concrete
for outdoor slabs subjected to
freezing weather.
• Finishing: Initial screeding must be promptly followed by bull
floating. DO NOT perform finishing operations with water
present on the surface or before the concrete has completed
bleeding. Do not overwork or over-finish the surface. For
better traction on exterior surfaces, use a broom finish. When
ambient conditions are conducive to a high evaporation rate,
use means to avoid rapid drying and associated plastic
shrinkage cracking by using wind breaks, fog sprays, and
covering the concrete with wet burlap or polyethylene sheets
between finishing operations.
• Curing is an important step to ensure durable crack-resistant
concrete. Start curing as soon as possible. Spray the surface with
liquid membrane curing compound or cover it with damp burlap
and keep it moist for at least three days. A second application of
curing compound the next day is a good quality assurance step.
• Joints: Anticipated volumetric changes due to temperature
and/or moisture should be accommodated by the construction of
contraction joints by sawing, forming, or tooling a groove about
1/4 to 1/3 the thickness of the slab, with a spacing between 24
to 36 times the 15 feet spacing for contraction joints is often
recommended. Panels between joints should be square and the
length should not exceed about 1.5 times the width. Isolation
joints should be provided whenever restriction to freedom of
either vertical or horizontal movement is anticipated—such as
where floors meet walls, columns, or footings. These are fulldepth
joints and are constructed by inserting a barrier of some
type to prevent bond between the slab and the other elements.
• Cover Over Reinforcement: Providing sufficient concrete cover
(at least 2 inches) to keep salt and moisture from contacting the
steel should prevent cracks in reinforced concrete caused by the
expansion of rust on reinforcing steel.
Follow These Rules to Minimize Cracking:
1. Design the members to handle all anticipated loads;
2. Provide proper contraction and isolation joints;
3. In slab on grade work, prepare a stable sub-grade;
4. Place and finish according to recommended and established
practices; and
5. Protect and cure the concrete properly.
Dusting or Chalking
Another sign of deterioration is the formation of loose powder resulting
from the surface disintegration of hardened concrete is called
dusting or chalking. The characteristics of such surfaces are:
1) They powder under any
kind of traffic;
2) They can be easily
scratched with a nail or
even by sweeping.
A concrete floor dusts under
traffic because the wearing
surface is weak. This
weakness can be caused by:
• Any finishing operation performed
while bleed water is
on the surface or before the
concrete has finished bleeding.
Working this bleed water back
into the top 1/4 inch of the slab
produces a very high water-cement
ratio and, therefore a
low-strength surface layer.
• Placement over a non-absorptive sub-grade or polyethylene
vapor retarder. This reduces normal absorption by the subgrade,
increases bleeding and, as a result, the risk of surface
dusting.
• Floating and/or troweling operations following the condensation
of moisture from warm humid air on cold concrete. In
cold weather, concrete sets slowly— in particular, cold concrete
in basement floors. If the humidity is relatively high, water
will condense on the freshly placed concrete, which, if
troweled into the surface, will cause dusting.
• Inadequate ventilation in enclosed spaces. Carbon dioxide from
open salamanders, gasoline engines or generators, power buggies
or mixer engines may cause a chemical reaction known as
carbonation, which greatly reduces the strength and hardness of
the concrete surface.
• Insufficient curing. This omission often results in a soft surface
skin, which will easily dust under foot traffic.
• Inadequate protection of freshly placed concrete from rain,
snow or drying winds. Allowing the concrete surface to freeze
will weaken the surface and result in dusting.
Preventing Dusting
Concrete with the lowest water content with an adequate slump
for placing and finishing will result in a strong, durable and wear resistant
surface. In general, use concrete with a moderate slump
not exceeding 5 inches.
Concrete with a higher slump may be used provided the mixture
is designed to produce the required strength without excessive
bleeding and/or segregation. Water-reducing admixtures
are typically used to increase slump while maintaining a low
water content in the mixture. This is particularly important in
cold weather when delayed set results in prolonged bleeding.
NEVER sprinkle or trowel dry cement into the surface of plastic concrete to absorb bleed water. Remove bleed water
by dragging a garden hose across the surface. Excessive bleeding
of concrete can be reduced by using air-entrained concrete,
by modifying mix proportions, or by accelerating the setting
time.
DO NOT perform any finishing operations with water present
on the surface or while the concrete continues to bleed.
Initial screeding must be promptly followed by bull floating.
Delaying bull floating operations can cause bleed water to be
worked into surface layer. Do not use a jitterbug, as it tends to
bring excess mortar to the surface. DO NOT add water to the
surface to facilitate finishing operations.
Do not place concrete directly on polyethylene vapor retarders
or non-absorptive sub-grades as this can contribute to
problems such as dusting, scaling, and cracking. Place 3 to 4
inches of a trim-able, compactable fill, such as a crusher-run
material, over vapor retarders or non-absorptive sub-grade prior
to concrete placement. When high evaporation rates exist,
lightly dampen absorptive sub-grades just prior to concrete
placement, ensuring that water does not pond or collect on the
sub-grade surface.
Provide proper curing by using liquid membrane curing compound
or by covering the surface with water, wet burlap, or other
curing materials as soon as possible after finishing to retain
moisture in the slab. It is important to protect concrete from the
environment at early ages.
Placing concrete in cold weather requires concrete temperatures
exceeding 50°F, as well as an accelerating admixture.
To prevent dusting, consider the following:
• Use moderate-slump concrete not exceeding 5 inches;
• Do not start finishing operation while the concrete is bleeding;
• Do not broadcast cement or sprinkle water on concrete prior to
or during finishing operations;
• Ensure that there is adequate venting of exhaust gases from
gas-fired heaters in enclosed spaces; and
• Use adequate curing measures to retain moisture in concrete for
the first three to seven days.
Repairing Dusting
1) Sandblast, shot-blast, or use a high-pressure washer to
remove the weak surface layer.
2) To minimize or eliminate dusting, apply a commercially
available chemical floor hardener, such as sodium silicate
(water glass) or metallic zinc or magnesium fluosilicate, in
compliance with manufacturer’s directions, on thoroughly
dried concrete. If dusting persists, use a coating, such as latex
formulations, epoxy sealers or cement paint.
3) In severe cases, a serviceable floor can be obtained by wetgrinding
the surface to durable substrate concrete. This may
be followed by properly bonded placement of a topping
course. If this is not practical, installation of a floor covering,
such as carpeting or vinyl tile covering, is the least expensive
solution to severe dusting.
This option will require some prior preparation since adhesives
for floor covering materials will not bond to floors with a
dusting problem and dusting can permeate through carpeting. ❑
Source: National Ready Mixed Concrete Association.
