Civil Engineering Reference
In-Depth Information
When using crushed ice, care must be taken to store it at a
temperature that will prevent the formation of lumps.
When ice is added as part of the mixing water, the
effect of the heat of fusion of the ice must be considered;
so the equation for temperature of fresh concrete is modi-
fied as follows:
the ice. The volume of ice should not replace more than
approximately 75% of the total batch water. The max-
imum temperature reduction from the use of ice is limited
to about 11°C (20°F).
If a greater temperature reduction is required, the in-
jection of liquid nitrogen into the mixer may be the best
alternative method.
The liquid nitrogen can be added directly into a cen-
tral mixer drum or the drum of a truck mixer to lower con-
crete temperature. Fig. 13-1 shows liquid nitrogen added
directly into a truck mixer near a ready mix plant. Care
should be taken to prevent the liquid nitrogen from con-
tacting the metal drum; the super cold liquid nitrogen
may crack the drum. The addition of liquid nitrogen does
not in itself influence the amount of mix water required
except that lowering the concrete temperature can reduce
water demand.
Aggregates have a pronounced effect on the fresh
concrete temperature because they represent 70% to 85%
of the total mass of concrete. To lower the temperature of
concrete 0.5°C (1°F) requires only a 0.8°C to 1.1°C (1.5°F to
2°F) reduction in the temperature of the coarse aggregate.
There are several simple methods of keeping aggre-
gates cool. Stockpiles should be shaded from the sun and
0.22 (
T
a
M
a
+
T
c
M
c
) +
T
w
M
w
+
T
wa
M
wa
-80
M
i
T (C°)
=
0.22 (
M
a
+
M
c
) +
M
w
+
M
wa
+
M
i
0.22 (
T
a
M
a
+
T
c
M
c
) +
T
w
M
w
+
T
wa
M
wa
-112
M
i
T (F°)
=
0.22 (
M
a
+
M
c
) +
M
w
+
M
wa
+
M
i
where
M
i
is the mass in kg (lb) of ice (
NRMCA 1962
and
Mindess and Young 1981
).
The heat of fusion of ice in metric units is 335 kJ per
kg (in British thermal units, 144 Btu per pound). Calcu-
lations in Table 13-1B show the effect of 44 kg (75 lb) of ice
in reducing the temperature of concrete. Crushed or
flaked ice is more effective than chilled water in reducing
concrete temperature. The amount of water and ice must
not exceed the total mixing-water requirements.
Fig. 13-6 shows crushed ice being charged into a
truck mixer prior to the addition of other materials.
Mixing time should be long enough to completely melt
Table 13-1B (Metric). Effect of Ice (44 kg) on Temperature of Concrete
Specific heat
Joules to vary
Initial temperature
Total joules
Material
Mass,
M
, kg
kJ/kg • K
temperature, 1°C
of material,
T
, °C
in material
(1)
(2)
(3)
(4)
(5)
Col.1 x Col. 2
Col. 3 x Col. 4
Cement
335 (
M
c
)
0.92
308
66 (
T
c
)
20,328
Water
123 (
M
w
)
4.184
515
27 (
T
w
)
13,905
Total aggregate
1839 (
M
a
)
0.92
1692
27 (
T
a
)
45,684
Ice
44 (
M
i
)
4.184
184
0 (
T
i
)
0
2699
minus
44 (
M
i
) x heat of fusion, (335 kJ/kg) =
-14,740
65,177
65,177
2699
= 24.1°C
Concrete temperature =
Table 13-1B (Inch-Pound Units). Effect of Ice (75 lb) on Temperature of Concrete
Btu to vary
Initial temperature
Total Btu's
Material
Mass,
M,
lb
Specific heat
temperature, 1°F
of material,
T
, °F
in material
(1)
(2)
(3)
(4)
(5)
Col.1 x Col. 2
Col. 3 x Col. 4
Cement
564 (
M
c
)
0.22
124
150 (
T
c
)
18,600
Water
207 (
M
w
)
1.00
207
80 (
T
w
)
16,560
Total aggregate
3100 (
M
a
)
0.22
682
80 (
T
a
)
54,560
Ice*
75 (
M
i
)
1.00
75
32 (
T
i
)
2,400
1088
minus
75 (
M
i
) x heat of fusion, (144 Btu/lb) =
-10,800
81,320
81,320
1088
= 74.7°F
Concrete temperature =
*32
M
i
- 144
M
i
= -112
M
i
