Civil Engineering Reference
In-Depth Information
Aggregate temperature,
°
F
kept moist by sprinkling. Do not spray salt water on ag-
gregate stockpiles. Since evaporation is a cooling process,
sprinkling provides effective cooling, especially when the
relative humidity is low.
Sprinkling of coarse aggregates should be adjusted to
avoid producing excessive variations in the surface mois-
ture content and thereby causing a loss of slump unifor-
mity. Refrigeration is another method of cooling materials.
Aggregates can be immersed in cold-water tanks, or
cooled air can be circulated through storage bins. Vacuum
cooling can reduce aggregate temperatures to as low as
1°C (34°F).
Cement temperature has only a minor effect on the
temperature of the freshly mixed concrete because of
cement's low specific heat and the relatively small
amount of cement in a concrete mixture. A cement tem-
perature change of 5°C (9°F) generally will change the
concrete temperature by only 0.5°C (1°F). Because cement
loses heat slowly during storage, it may still be warm
when delivered. (This heat is produced in grinding the
cement clinker during manufacture.) Since the tempera-
ture of cement does affect the temperature of the fresh
concrete to some extent, some specifications place a limit
on its temperature at the time of use. This limit varies
from 66°C to 82°C (150°F to 180°F) (
ACI Committee 305
).
However, it is preferable to specify a maximum tempera-
ture for freshly mixed concrete rather than place a tem-
perature limit on individual ingredients (
Lerch 1955
).
70
80
90
100
110
90
Concrete temperature:
30
80
70
20
60
50
10
40
0
32
20
30
40
Aggregate temperature,
°
C
Fig. 13-5. Temperature of freshly mixed concrete as
affected by temperature of its ingredients. Although the
chart is based on the following mixture, it is reasonably
accurate for other typical mixtures:
Aggregate
1360 kg (3000 lb)
SUPPLEMENTARY CEMENTITIOUS
MATERIALS
Moisture in aggregate
27 kg (60 lb)
Added mixing water
109 kg (240 lb)
Cement at 66°C (150°F)
256 kg (564 lb)
Many concrete producers consider the use of supplemen-
tary cementitious materials to be essential in hot weather
conditions. The materials of choice are fly ash and other
pozzolans (ASTM C 618 or AASHTO M 295) and ground
granulated blast-furnace slag (ASTM C 989 or AASHTO
M 302). These materials generally slow both the rate of set-
ting as well as the rate of slump loss. However, some cau-
tion regarding finishing is needed; because the rate of
bleeding can be slower than the rate of evaporation,
plastic shrinkage cracking or crazing may result. This is
discussed in greater detail under “Plastic Shrinkage
Cracking” below.
PREPARATION BEFORE CONCRETING
Before concrete is placed, certain precautions should be
taken during hot weather to maintain or reduce concrete
temperature. Mixers, chutes, conveyor belts, hoppers,
pump lines, and other equipment for handling concrete
should be shaded, painted white, or covered with wet
burlap to reduce solar heat.
Forms, reinforcing steel, and subgrade should be
fogged or sprinkled with cool water just before the con-
Fig. 13-6. Substituting ice for part of the mixing water will
substantially lower concrete temperature. A crusher
delivers finely crushed ice to a truck mixer reliably and
quickly. (44236)
