odic inspection of the pipes and replacement as necessary
are recommended.
When conditions require thawing of substantial
quantities of extremely low temperature aggregates,
steam jets may be the only practicable means of providing
the necessary heat. In such a case, thawing must
be done as far in advance of batching as is possible to
achieve substantial equilibrium in both moisture content
and temperature. After thawing is completed, the
steam supply can be reduced to the minimum that will
prevent further freezing, thereby reducing to some extent
the problems arising from variable moisture content.
Nevertheless, under such conditions, mixing water
control must be largely on an individual batch adjustment
basis. Dry hot air instead of steam has been used
to keep aggregates ice free.
3.6 - Overheating of aggregates
Aggregates should be heated sufficiently to eliminate
ice, snow, and frozen lumps of aggregate. Often 3-in.
(76-mm) frozen lumps will survive mixing and remain
in the concrete after placing. Overheating should be
avoided so that spot temperatures do not exceed 212 F
(100 C) and the average temperature does not exceed
150 F (65 C) when the aggregates are added to the
batch. Either of these temperatures is considerably
higher than is necessary for obtaining desirable temperatures
of freshly mixed concrete. Materials should be
heated uniformly since considerable variation in their
temperature will significantly vary the water requirement,
air entrainment, rate of setting, and slump of the
concrete.
Extra care is required when batching the first few
loads of concrete following a prolonged period of
steaming the aggregates in storage bins. Many concrete
producers recycle the first few tons of very hot aggregates.
This material is normally discharged and recycled
by placing it on top of the aggregates in the storage
bins.
3.7 - Calculation of mixture temperature
If the weights and temperatures of all constituents
and the moisture content of the aggregates are known,
the final temperature of the concrete mixture may be
estimated from the formula
T= [0.22(TsWs+ TaWa + TcWc) + TwWw+ TsWws+ TaWw]
[0.22 (Ws + Wa + Wc) + Ww + Wwa+ Wws]
(3-1)
where
T = final temperature of concrete mixture (deg F or
C)
Tc = temperature of cement (deg F or C)
Ts = temperature of fine aggregate (deg F or C)
Ta = temperature of coarse aggregate (deg F or C)
Tw = temperature of added mixing water (deg F or C)
Wc = weight of cement (lb or kg)
Ws = saturated surface-dry weight of fine aggregate
(lb or kg)
Wa = saturated surface-dry weight of coarse aggregate
(lb or kg)
Ww = weight of mixing water (lb or kg)
Wws = weight of free water on fine aggregate (lb or kg)
Wwa = weight of free water on coarse aggregate (lb or
kg)
Eq. (3-l) is derived by considering the equilibrium heat
balance of the materials before and after mixing and by
assuming that the specific heats of the cement and aggregates
are equal to 0.22 Btu/(lb F) [0.22 kcal/(kg C)].
If the temperature of one or both of the aggregates is
below 32 F (0 C), the free water will be frozen, and Eq.
(3-1) must be modified to take into account the heat required
to raise the temperature of the ice to 32 F (0 C),
to change the ice into water, and to raise the temperature
of the free water to the final mixture temperature.
The specific heat of ice is 0.5 Btu/(lb F) [0.5 kcal/(kg
C)] and the heat of fusion of ice is 144 Btu/lb (80 kcal/
kg). Thus Eq. (3-1) is modified by substituting the following
expressions for TsWws or TaWwa, or both, depending
on whether the fine aggregate or coarse aggregate,
or both, are below 32 F (0 C).
For in.-lb units
for Ts Wws substitute Wws (0.50Ts - 128) (3-2)
for Ta Wwa substitute Wwa (0.50Ta - 128) (3-3)
For SI units
for Ts Wws substitute Wws (0.50Ts - 80) (3-4)
for Ta W wa substitute W wa(0.50Ts - 80) (3-5)
In these equations, the numbers 128 and 80 are obtained
from the heat of fusion needed to melt the ice,
the specific heat of the ice, and the melting temperature
of ice.
3.8 - Temperature loss during delivery
The Swedish Cement and Concrete Research Institute
(Petersons 1966) performed tests to determine the
expected decrease in concrete temperature during delivery
in cold weather. Their studies included revolving
drum mixers, covered-dump bodies, and open-dump
bodies. Approximate temperature drop for a delivery
time of 1 hr can be computed using Eq. (3-6)-(3-8). For
revolving drum mixers
T = 0.25 ( tr - ta)
For covered-dump body
T = 0.10 ( tr - ta)
For open-dump body
T = 0.20 ( tr - ta)
(3-6)
(3-7)
(3-8)
COLD
are recommended.
When conditions require thawing of substantial
quantities of extremely low temperature aggregates,
steam jets may be the only practicable means of providing
the necessary heat. In such a case, thawing must
be done as far in advance of batching as is possible to
achieve substantial equilibrium in both moisture content
and temperature. After thawing is completed, the
steam supply can be reduced to the minimum that will
prevent further freezing, thereby reducing to some extent
the problems arising from variable moisture content.
Nevertheless, under such conditions, mixing water
control must be largely on an individual batch adjustment
basis. Dry hot air instead of steam has been used
to keep aggregates ice free.
3.6 - Overheating of aggregates
Aggregates should be heated sufficiently to eliminate
ice, snow, and frozen lumps of aggregate. Often 3-in.
(76-mm) frozen lumps will survive mixing and remain
in the concrete after placing. Overheating should be
avoided so that spot temperatures do not exceed 212 F
(100 C) and the average temperature does not exceed
150 F (65 C) when the aggregates are added to the
batch. Either of these temperatures is considerably
higher than is necessary for obtaining desirable temperatures
of freshly mixed concrete. Materials should be
heated uniformly since considerable variation in their
temperature will significantly vary the water requirement,
air entrainment, rate of setting, and slump of the
concrete.
Extra care is required when batching the first few
loads of concrete following a prolonged period of
steaming the aggregates in storage bins. Many concrete
producers recycle the first few tons of very hot aggregates.
This material is normally discharged and recycled
by placing it on top of the aggregates in the storage
bins.
3.7 - Calculation of mixture temperature
If the weights and temperatures of all constituents
and the moisture content of the aggregates are known,
the final temperature of the concrete mixture may be
estimated from the formula
T= [0.22(TsWs+ TaWa + TcWc) + TwWw+ TsWws+ TaWw]
[0.22 (Ws + Wa + Wc) + Ww + Wwa+ Wws]
(3-1)
where
T = final temperature of concrete mixture (deg F or
C)
Tc = temperature of cement (deg F or C)
Ts = temperature of fine aggregate (deg F or C)
Ta = temperature of coarse aggregate (deg F or C)
Tw = temperature of added mixing water (deg F or C)
Wc = weight of cement (lb or kg)
Ws = saturated surface-dry weight of fine aggregate
(lb or kg)
Wa = saturated surface-dry weight of coarse aggregate
(lb or kg)
Ww = weight of mixing water (lb or kg)
Wws = weight of free water on fine aggregate (lb or kg)
Wwa = weight of free water on coarse aggregate (lb or
kg)
Eq. (3-l) is derived by considering the equilibrium heat
balance of the materials before and after mixing and by
assuming that the specific heats of the cement and aggregates
are equal to 0.22 Btu/(lb F) [0.22 kcal/(kg C)].
If the temperature of one or both of the aggregates is
below 32 F (0 C), the free water will be frozen, and Eq.
(3-1) must be modified to take into account the heat required
to raise the temperature of the ice to 32 F (0 C),
to change the ice into water, and to raise the temperature
of the free water to the final mixture temperature.
The specific heat of ice is 0.5 Btu/(lb F) [0.5 kcal/(kg
C)] and the heat of fusion of ice is 144 Btu/lb (80 kcal/
kg). Thus Eq. (3-1) is modified by substituting the following
expressions for TsWws or TaWwa, or both, depending
on whether the fine aggregate or coarse aggregate,
or both, are below 32 F (0 C).
For in.-lb units
for Ts Wws substitute Wws (0.50Ts - 128) (3-2)
for Ta Wwa substitute Wwa (0.50Ta - 128) (3-3)
For SI units
for Ts Wws substitute Wws (0.50Ts - 80) (3-4)
for Ta W wa substitute W wa(0.50Ts - 80) (3-5)
In these equations, the numbers 128 and 80 are obtained
from the heat of fusion needed to melt the ice,
the specific heat of the ice, and the melting temperature
of ice.
3.8 - Temperature loss during delivery
The Swedish Cement and Concrete Research Institute
(Petersons 1966) performed tests to determine the
expected decrease in concrete temperature during delivery
in cold weather. Their studies included revolving
drum mixers, covered-dump bodies, and open-dump
bodies. Approximate temperature drop for a delivery
time of 1 hr can be computed using Eq. (3-6)-(3-8). For
revolving drum mixers
T = 0.25 ( tr - ta)
For covered-dump body
T = 0.10 ( tr - ta)
For open-dump body
T = 0.20 ( tr - ta)
(3-6)
(3-7)
(3-8)
COLD
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