Civil Engineering Reference
In-Depth Information
There is little advantage in using fresh concrete at a
temperature much above 21°C (70°F). Higher concrete
temperatures do not afford proportionately longer protec-
tion from freezing because the rate of heat loss is greater.
Also, high concrete temperatures are undesirable since
they increase thermal shrinkage after hardening, require
more mixing water for the same slump, and contribute to
the possibility of plastic-shrinkage cracking (caused by
rapid moisture loss through evaporation). Therefore, the
temperature of the concrete as
mixed
should not be more
than 5°C (10°F) above the minimums recommended in
Table 14-1.
material) can be assumed as 0.925 kJ (0.22 Btu) compared
to 4.187 kJ (1.0 Btu) for water.
Fig. 14-12 shows the effect of temperature of materials
on temperature of fresh concrete. The chart is based on the
equation
(
)
+
T =
0.22
TM
+
TM
T M
+
T M
a
a
c
c
w
w
wa
wa
(
)
++
022
.
MM M M
+
a
c
w
a
where
temperature in degrees Celsius (Fahrenheit) of
the f
r
esh concrete
T
=
T,T,T ,
and
T
=
temperature in degrees Celsius
(Fahrenheit) of the aggregates, cement, added mixing
water, and free moisture on aggregates, respectively;
gene
r
ally
acw
wa
Aggregate Temperature.
The temperature of aggregates
varies with weather and type of storage. Aggregates usu-
ally contain frozen lumps and ice when the temperature is
below freezing. Frozen aggregates must be thawed to
avoid pockets of aggregate in the concrete after batching,
mixing, and placing. If thawing takes place in the mixer,
excessively high water contents in conjunction with the
cooling effect due to the ice melting must be avoided.
At temperatures above freezing it is seldom necessary
to heat aggregates, the desired concrete temperature can
usually be obtained by heating only the mixing water. At
temperatures below freezing, in addition to heating the
mixing water, often only the fine aggregate needs to be
heated to produce concrete of the required temperature,
provided the coarse aggregate is free of frozen lumps.
Three of the most common methods for heating
aggregates are: (1) storing in bins or weigh hoppers heated
by steam coils or live steam; (2) storing in silos heated by
hot air or steam coils; and (3) stockpiling over heated
slabs, stem vents or pipes. Although heating aggregates
stored in bins or weigh hoppers is most commonly used,
the volume of aggregate that can be heated is often limited
and quickly consumed during production. Circulating
steam through pipes over which aggregates are stockpiled
is a recommended method for heating aggregates.
Stockpiles can be covered with tarpaulins to retain and
distribute heat and to prevent formation of ice. Live
steam, preferably at pressures of 500 to 900 kPa (75 to 125
psi), can be injected directly into the aggregate pile to heat
it, but the resultant variable moisture content in aggre-
gates might result in erratic mixing-water control.
On small jobs aggregates can be heated by stockpiling
over metal culvert pipes in which fires are maintained.
Care should be taken to prevent scorching the aggregates.
T
a
T
wa
=
mass in kilograms
(pounds) of the aggregates, cement, free moisture on
aggregates, and mixing water, respectively
If the weighted average temperature of aggregates
and cement is above 0°C (32°F), the proper mixing-water
temperature for the required concrete temperature can be
selected from Fig. 14-12. The range of concrete tempera-
tures in the chart corresponds with the recommended
values given in Lines 1, 2, and 3 of Table 14-1.
MMM
,
,
, and
M
=
a
c
w
a
Weighted average temperature of
aggregates and cement,
°
F
40
50
60
70
180
Mix data:
Aggregate = 1360 kg (3000 lb)
Moisture in aggregate = 27 kg (60 lb)
Added mixing water = 108 kg (240 lb)
Portland cement = 256 kg (564 lb)
78
160
70
Concrete temperature
62
24
C
(75
°
F)
°
140
54
20
°
C
(68
°
F)
120
46
16
°
C
(61
°
F)
12
C
(54
°
F)
°
38
100
8
C
(46
°
F)
°
30
Mixing-Water Temperature.
Of the ingredients used to
make concrete, mixing water is the easiest and most prac-
tical to heat. The mass of aggregates and cement in con-
crete is much greater than the mass of water; however,
water can store about five times as much heat as can
cement and aggregate of the same weight. For cement and
aggregates, the average specific heat (that is, heat units
required to raise the temperature 1°C (1°F) per kg (lb) of
80
0
4
8
12
16
20
24
Weighted average temperature of
aggregates and cement,
°
C
Fig. 14-12. Temperature of mixing water needed to produce
heated concrete of required temperature. Temperatures are
based on the mixture shown but are reasonably accurate
for other typical mixtures.
