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| What
is Dusting? |
Loose
powder resulting from the disintegration
of the surface of concrete.
Causes include:
- Finishing bleed water back
into the surface, thereby
producing a high
water/cement ratio and
weakening the surface.
- Placing concrete over
polyethylene vapor retarder.
- Inadequate ventilation
during cold weather
operations. Carbon
dioxide from heaters cause
carbonization.
- Allowing the concrete
surface to freeze.
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| What
is Scaling? |
The
flaking or peeling of the
finished concrete surface as a
result of freezing and
thawing. Causes include:
- Use of non-air-entrained
concrete.
Air-entrained concrete is a
must for protection against
freeze-thaw.
- Excessive amounts of
calcium chloride, used as an
accelerator.
- Any finishing operation
performed while bleed water
is on the surface.
- Overworking the surface,
reducing the air content of
the surface layer.
- Insufficient curing,
resulting in a weak surface
layer.
- Use of de-icers.
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| What
is Crazing? |
Development
of a network of fine cracks in
the surface layer of
concrete. Causes include:
- Inadequate curing or rapid
drying of the surface.
- Finishing while bleed
water exists on the surface.
- Sprinkling cement on the
surface to dry up bleed
water.
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| Why
Did My Concrete Crack? |
Expansion
and contraction of the concrete,
without means to relieve these
stresses, cause cracking.
Some common types are:
- Shrinkage cracks. (see
below)
- Cracks due to improper
jointing.
- Cracks due to external
restraint. (i.e. basement
floor)
- Crazing. (see above)
- Freezing and thawing
stresses.
- Settlement.
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| What
is Shrinkage Cracking? |
Cracks
that appear on the surface while
the concrete is still
plastic. They are usually
caused by rapid evaporation of
the surface before the concrete
has set. Can be minimized
by constructing wind breaks and
using fog sprays to keep the
humidity high, or avoid pouring
in unfavorable conditions. |
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| What
Constitutes Low Strength? |
The
strength of a set of cylinders
that falls more than 500 psi
below the specified
strength. Some common
causes are:
- Improper casting, handling
and curing procedures,
usually the most important
factor.
- High air contents.
- Addition of too much water
at the jobsite.
- Errors in production.
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| What
is Curling? |
Warping
of the slab upwards or
downwards, caused by shrinkage
or contraction in the top
relative to the bottom.
Often cause by rapid surface
drying and improper joint
spacing. |
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| What
is Delamination? |
Separation
of the top layer of the slab
from the base of the slab by a
thin layer of air or
water. Some common causes
include:
- Air-entrained concrete
used for hard-troweled
finishes.
- Overworking the surface.
- Sealing the surface, prior
to the release of bleed
water.
- Placing the concrete over
a polyethylene vapor
barrier.
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| What
Causes Discoloration? |
Some
of the primary causes of
discoloration include:
- Use of calcium chloride to
speed set times.
- Burning the concrete by
hard-troweling.
- Uneven curing.
- Batch to batch changes in
mix proportions and material
sources.
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| What
Causes Slab Moisture? |
Some
common causes include:
- High water table.
- Wicking of water through
the slab due to capillary
action.
- Water vapor, due to
temperature and humidity differences
between the slab and
underlying soils.
- Residual moisture within
the concrete.
Some ways to minimize the
effect include:
- Ensure proper drainage.
- Use crushed stone as a
base material.
- Use a vapor
retarder. Requires
proper placing procedures to
prevent cracking.
- Use a low water/cement
ratio mixture, and/or
introduce fly ash to improve
lower the permeability of
the concrete.
- Proper curing.
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