TM 5-852-4/AFM 88-19, Chap. 4
ing cold weather. Water is not only easy to heat, but
total mixing water on an individual batch adjustment
each pound of water has roughly 4 1/2 times as many
basis which is difficult to obtain. Heated air in pipes, or
heat units per degree Fahrenheit as are stored in a
hot water in a closed system of coils, have also been
pound of aggregate or cement. Mixing water is usually
used to keep aggregates ice-free.
heated in auxiliary tanks connected to the water
(8) Whatever method is used, the average
measuring tank. Heating can be accomplished most
temperature of an individual batch of aggregates should
115
and no aggregate should be
rapidly by injecting live steam into the water; however,
not exceed 150 F
115
as such a temperature may
heated above 212F
electric and oil or gas burning heaters are acceptable.
cause incipient cracking of the aggregate".
The mixing water should be heated under such control
f.
Mixing, transporting and placing.
The
and in such quantity and the storage tanks and lines
should be insulated to the degree that no appreciable
mixing, transportation, and placement of concrete should
fluctuation of the temperature can occur.
be accomplished with the minimum possible temperature
179
(5) Cement coming in contact with hot
loss . The batching plant, or the batching and mixing
water or hot aggregate may experience flash set. To
plant, if the concrete is centrally mixed, should be made
avoid the possibility of flash set when either aggregate or
heatable, if possible, or at least must be protected from
water is heated to a temperature in excess of 100F,
the wind. The transporting and mixing, if not centrally
mixed, and the placing must be done as quickly as
water and aggregate should come together first in the
possible so as not to allow the concrete to cool below the
mixer in such a way that the high temperature of one or
desired placing temperature. The heat losses during
the other is reduced before cement is admitted. If the
mixing, transporting, and placing are difficult to
mixer is loaded in this sequence, water temperature up
determine with any degree of accuracy; however,
to the boiling point may be used, provided aggregate is
authoritative sources indicate hourly temperature losses
cold enough to reduce temperature of the mixture of
as high as 15 percent of the difference between concrete
water and aggregate to appreciably less than 100F; in
fact, this temperature should rarely exceed 60 to
and surrounding air temperature during this period.
Consideration should be given to large transport volumes
115
80F .
and insulated and covered transport vehicles to keep this
(6) When air temperature is below
temperature loss to a minimum. Protection from the
freezing often one or more of the aggregates, as well as
wind during placement will also materially reduce the
the water, must be heated to produce the desired
heat loss through convection.
minimum temperature of the freshly mixed concrete.
g. Protection of concrete in the curing period.
The heating of all the aggregates is the more desirable
Protection required for concrete during the curing period
method since it insures that no ice, snow or frozen
will range from none in on-surface construction in
chunks are present. However, if the coarse aggregate is
summer to use of insulated forms, heated covers or
dry and free of ice, snow, or frozen chunks, the heating
complete heated enclosures, depending on the severity
of only the sand and water may be sufficient to produce
of environmental conditions and the nature of the item
the desired temperature.
involved. Problems such as work protection and heating
(7) Heating
aggregates
can
be
of materials can often be minimized through proper
accomplished most satisfactorily by the use of steam in a
schedule planning. Since most of the heat of hydration
closed system of coils in the aggregate storage bins. A
of cement is developed in the first few days after mixing,
closed system of steam coils tends to dry the aggregates
protection is intended primarily to conserve the heat
and bring about a uniform moisture content. Open
being developed and quick application of protection for
steam jets should be avoided if possible because of the
the freshly placed concrete is therefore of primary
resulting fluctuation in moisture content of the materials
importance.
so heated. If, however, emergency conditions require
(1) Insulation. Forms preferably should be
the thawing of fairly large quantities of aggregates at
well insulated before concrete is placed. Unformed
extremely low temperatures, or if railroad car or truck
surfaces, and uninsulated formed surfaces, should either
loads of aggregates must be thawed before unloading,
be covered with adequate insulating blankets or covered
then steam jets may be the only practical method. In
by vented shelters heated to maintain the desired
such instances, thawing should be accomplished as far
temperatures. The thickness of the insulating blanket to
in advance of batching as possible, the car or truck loads
be used against the forms or against the surface of the
unloaded and stockpiled, and thereafter the steam
concrete must be adequate to maintain the desired
should be cut to the minimum that will prevent freezing
temperature of the concrete under the extremes of low
and the stockpiles covered with tarpaulins to help
temperature and concurrent wind conditions to be
maintain a uniform temperature, promote a more uniform
expected during the curing period. Efforts to protect in-
moisture content, and prevent the formation of a frozen
crust on the surface of the stockpiles. Variations in the
moisture content and temperature of the aggregates
after thawing with steam jets will require control of the
6-7
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