TM 5-822-7/AFM 88-6, Chap. 8
against freezing when calcium chloride or any other
of the vibration will be limited by the time necessary
to produce a satisfactory consolidation of the
accelerator is used.
i. Placing during hot weather. During hot weather
concrete, and over-vibration will not be permitted.
Vibrators must not be permitted to touch forms,
special precautions are necessary to prevent the
dowels, tie bars, or other embedded items.
formation of plastic-shrinkage cracks which result
f. Surface vibrators. Surface vibrators will not be
from an excessive loss of moisture from the concrete
before curing is begun. The concrete needs to be placed
permitted because past experience with them has
been generally unsatisfactory. Surface vibration
at the coolest temperature practicable, and in no case
should the temperature of the concrete as placed
tends to bring excess fine material and water to the
exceed 90 degrees F. Mixing and placing of concrete
surface, which in many instances contributes to
will be controlled to keep moisture loss at a
surface deterioration and scaling. Surface-vibrator
minimum. Aggregates will be moist when added to
pans also tend to ride on high places in the concrete
the mixer, and the subgrade dampened so it will not
surface and thus cause non-uniform consolidation of
the concrete.
absorb water from the concrete. The temperature of
the concrete may be reduced by using cement with a
g. Steel reinforcement. Project drawings will show
typical details of all slab reinforcement and will
lower temperature by sprinkling the stockpiles of
indicate the pavements requiring reinforcement and
aggregate to produce cooling by evaporation, by pre-
cooking mixing water and by avoiding delays in mixing
the location and amount of steel required in the
and placing concrete. The concrete needs to be placed
slabs. This information will supplement the specifi-
and finished as rapidly as practicable, and the curing
cation requirements for reinforcement, and all
requirements will be carefully checked to insure no
started without delay. If the application of the curing
discrepancies between drawings and specification
medium should for any reason lag placement for a
requirements.
time sufficient to permit surface drying, the surface
h. Placing during cold weather. Pavement concrete
should be kept damp with a fog spray, and placement
should be discontinued until corrective action can be
should not be placed when the air temperature is
taken. The fog spray equipment will be capable of
below 40 degrees F. If unusual job conditions require
applying a very fine mist to the concrete to replace
construction at these low temperatures, special
moisture lost by evaporation. In windy areas,
provisions for placement and protection of the
concrete will be needed. For additional information
screens may be needed to protect the concrete from
on winter concreting, refer to ACI 306R. The
rapid evaporation caused by wind. Steps shall be
necessary covers and other means of protecting the
taken to avoid an evaporation rate in excess of 0.2
concrete during cold weather should be available on
pound per square foot per hour as shown in figure 1.
the job before starting concrete placement. Calcium
This is adequate for most conditions, but there are
reports of plastic-shrinkage cracking occurring
chloride as an admixture may be either required or
approved to accelerate the setting time of concrete
under adverse conditions when the indicated evapor-
placed during cold weather. No changes will be made
ation rate was as low as 0.15 pound per square foot
in the requirements for temperature of the concrete
per hour. Additional information is available in ACI
when placing or for protection of the concrete
305R.
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