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| What
is Hot Weather Concrete? |
Concrete
placed when high temperatures
require special procedures for
handling, placing, finishing and
curing concrete. These
procedures include, but are not
limited to:
- Unloading concrete within
90 minutes of batching.
- Adding a chemical retarder
to delay the set.
- Adding ice or chilled
water to lower temperature
of the plastic mix.
- Minimizing the amount of
water added on the job.
- Begin barrier or water
curing as soon as finishing
is complete.
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| What
is Cold Weather Concrete? |
Concrete
placed when temperature falls
below 40 F for more than three
successive days. Special
procedures for cold weather
concrete include, but are not
limited to:
- Using heated water and
aggregates.
- Avoid placing concrete on
frozen subgrade.
- Using accelerators to
speed the set time.
- Protect the finished
concrete with blankets to
prevent freezing.
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| Should
I Add Water at the Jobsite? |
Water
can be added at the jobsite,
provided that the target
water/cement ratio of the
mixture has not been
exceeded. Excess water
should be avoided for several
reasons, most important of which
is the direct reduction in
strength of the hardened
concrete. A loss of
approximately 150 psi per gallon
of excess water added can be
assumed. To avoid this
effect, if high slumps are
desired, the inclusion of
high-range water reducers should
be considered. |
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| What
is Segregation? |
Segregation
is when the aggregates separate
from the rest of the concrete.
This causes weakening and
excessive curling and
shrinkage. Some of the
ways to avoid segregation
include:
- Placing the concrete as
close as possible to its
final position.
- Do not drop from higher
that 2-3 feet.
- Avoid high slumps.
- Do not move the concrete
with a vibrator.
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| What
is a Vapor Retarder? |
A
vapor retarder is a material
used to minimize the
transmission of water from the
soil through a concrete
slab. They are usefull
when the concrete will be
covered with tile or floor
coverings, and can help prevent
radon penetration. Proper
concrete placement is required
to prevent problems with
cracking, curling and
delamination due to confinement
of bleed water. |
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| How
Important is the Subgrade? |
The
subgrade, or the underlying
support materials, are vital to
the performance of a slab.
They should be graded to provide
a uniform depth of concrete to
avoid cracking. Soft areas
should also be removed to
prevent unequal
settlement. Drainage
should also be a
consideration. As stated
in the cold weather section, the
subgrade must be free of frost
before concrete placement. |
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| How
Important is Reinforcement? |
For
concrete slabs, the primary
purpose of reinforcement is to
keep cracks closed
tightly. The product
normally used in this
application is wire fabric,
although success has been
achieved using synthetic fibers,
commonly known by the brand name
of FiberMesh. |
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