Concrete contracts and expands with changes in moisture content and
temperature. Deflection depends on load and support conditions. Both of these
factors affect concrete performance. Cracks from drying shrinkage or
temperature changes are an inherent, unavoidable property of concrete. However,
unavoidable, they can be controlled.
Stress cracks due to applied loads are settlement cracks and can be avoided by
having a proper subbase, slab design, and proper concrete mix designed for the
load.
Common Causes
• Use of High Slump concrete (over 4
inch) which increases shrinkage.
• Low strength mix.
• Control joints too far apart.
• Isolation and control joints
omitted.
•
Insufficient depth of control joints.
•
Inadequate or improper curing
• Poor subgrade preparation
• Improper placement of reinforcement
Prevention
Minimizing cracks requires proper placement of control joints and proper curing
to hold the moisture in the concrete long enough to get adequate strength gain
before the moisture leaves and the concrete shrinks. All concrete has a
tendency to shrink and it is not possible to consistently produce completely
crack free concrete. However, cracking can be reduced and controlled if the
following basic safeguards are observed:
• The subgrade beneath the slab should
be adequately compacted by rolling, vibrating or tamping.
• The subgrade should be sloped for
proper drainage.
• If a vapor barrier is used, cover it
with a layer of 2 inches of compacted damp sand.
• Thoroughly dampen the subgrade just
prior to placing the concrete.
• Use low-slump concrete ( not over 4
inch.)
• Reduce rapid evaporation of surface
moisture, especially in hot weather. Cover the concrete with wet burlap or poly
sheeting between finishing operations in hot weather.
• Install proper control joints.
Closer spacing will be necessary for thin slabs (4 inches or less.)
• Isolation joints should be provided
whenever restriction to freedom of either vertical or horizontal movement is
anticipated.
•
Start curing as soon as possible.
• Cover cured slab for at least 5
days.
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