TM 5-852-4/AFM 88-19, Chap. 4
e.
Heating the materials.
air content of the freshly mixed concrete meets the
requirements
of
the
applicable
specifications.
(1) It is not difficult to maintain the
Specifications are generally based on the concept of
temperature of freshly mixed concrete within a ten
having an air content in the mortar fraction of
degree range above the minimum temperature specified
approximately 9 percent; hence, the total air content of
and desired. Although it is difficult to heat aggregates
the concrete that should be obtained diminishes as the
uniformly or to a predetermined temperature, the
proportion of mortar diminishes, which it will do as the
temperature of the mixing water can be adjusted readily
quantity of aggregate increases as the maximum size of
by blending hot water or steam with cold water to
aggregate increases.
maintain the temperature of the freshly mixed concrete
c. High early strength cement. The alternative
within the desired 10 F range.
use of high early strength cement, Type III, is generally
(2) Making the basic assumption that the
permitted by the specifications. As the name implies,
specific heat of both the cement and aggregate can be
high early strength cement increases the rate at which
accurately enough represented by the factor 0.22 (0.2
concrete gains strength, thereby reducing the length of
may be used for rapid field computation) and knowing
time that the concrete surfaces must be protected.
that the specific heat of water is I, reasonably accurate
Therefore, its use is beneficial, either alone or as a
estimates of temperature of freshly mixed concrete can
supplement to heated materials during cold weather.
be made from the following equation:20
The same end result can usually be obtained by
0.22 (TaWa + TcWc + TmWm)
increasing the cement factor for Type I and II cement by
T =
0.22 (Wa + Wc) + Wf + Wm
(Equation 16)
about 30 percent. However, shrinkage characteristics
may not permit this addition.
d. Accelerators. The permissible substitution
T
= Temperature of concrete, F.
Tc
= Temperature of cement, F.
of high early strength cement, Type III (except when
Tm
= Temperature of mixing water, F.
Type V is specified), will produce the desired
= Temperature of free moisture in
Tf
acceleration of the rate at which the concrete gains
aggregates, F.
strength more positively than a chemical accelerator.
Ta = Temperature of aggregate, F.
"Calcium chloride, other salts or other chemicals in the
Wc = Weight of cement, lbs.
mix in permissible amounts will not lower the freezing
Wm = Weight of mixing water, lbs.
point of concrete to any significant degree. To avoid use
Wf = Weight of free moisture in aggregates, lbs.
of harmful materials, any such attempt to protect
Wa = Weight of aggregates, lbs.
concrete from freezing should not be permitted.'""
(The temperature of all water including the moisture in
However, calcium chloride may sometimes be needed as
the aggregates must be above 32F, as ice absorbs 144
an accelerating agent, and when its use is approved, not
Btu's of heat per pound in melting and has a specific
more than 2 percent of calcium chloride, by weight of the
heat of only 0.5.) The above equation does not take into
cement, should be allowed. The calcium chloride should
account any heat loss to the air during mixing,
be measured accurately and added to the batch in
transporting, and placing or any heat gain in hydration of
solution in a portion of the mixing water with an
the cement. Based on temperatures computed from this
acceptable commercially manufactured automatic
22
equation (using 0.2 for specific heat of cement and
dispenser .
The following precautions must be
aggregate), table 6-1 from the Bureau of Reclamation
observed in use of calcium chloride:
203
shows the effect of temperature of
Concrete Manual
1. It must not be used where sulphate
materials on temperature of various freshly mixed
resistance cement (Type V) is specified. Type III
concretes.
cement with a limitation of 5 percent tricalcium
(3) Cement as taken from usual storage is
aluminate may be used where sulphate
rarely under 32F. Heating cement should not be used
resistance is needed.
as a method of raising the concrete temperature.
2. It must not be added directly to the mixing
Cement containing frozen lumps should not be used
water in the dry state; it should be added in
since the chunks indicate the presence of moisture.
solution only.
(4) Heating the mixing water is generally
3. It must not be used in prestressed concrete.
considered the most practical and efficient means for
4. It must not be used where zinc or aluminum
obtaining the desired placing temperature of concrete
is present or when subject to sulphate conditions
dur-
contained in the aggregate or present in the
environment.
6-6
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