Civil Engineering Reference
In-Depth Information
To avoid the possibility of a quick or flash set of the
concrete when either water or aggregates are heated to
above 38°C (100°F), they should be combined in the mixer
first before the cement is added. If this mixer-loading
sequence is followed, water temperatures up to the boil-
ing point can be used, provided the aggregates are cold
enough to reduce the final temperature of the aggregates
and water mixture to appreciably less than 38°C (100°F).
Fluctuations in mixing-water temperature from batch
to batch should be avoided. The temperature of the mix-
ing water can be adjusted by blending hot and cold water.
Fig. 14-13. A bimetallic pocket thermometer with a metal
sensor suitable for checking fresh concrete temperatures.
(69881, 69882)
Temperature of Concrete as
Placed and Maintained
There will be some temperature loss after mixing while
the truck mixer is traveling to the construction site and
waiting to discharge its load. The concrete should be
placed in the forms before its temperature drops below
that given on Line 4 of Table 14-1; that concrete tempera-
ture should be maintained for the duration of the protec-
tion period given in Chapter 12 under “
Curing Period and
Temperature
.”
concrete surface is shown in Fig. 14-14. Instead of filling
the hole shown in Fig. 14-14 with a fluid, it can be fitted
with insulation except at the bulb.
Concrete test cylinders must be maintained at a
temperature between 16°C (60°F) and 27°C (80°F) at the
jobsite for up to 48 hours until they are taken to a labora-
tory for curing (ASTM C 31 or AASHTO T 23). For con-
crete mixtures with a specified strength of 40 MPa (6,000
psi) or greater, the initial curing temperature shall be be-
tween 20°C and 26°C (68°F and 78°F). During this period,
cylinders should be kept in a curing box and covered with
a nonabsorptive, nonreactive plate or impervious plastic
bag; the temperature in the box should be accurately con-
Cooling After Protection
To avoid cracking of the concrete due to sudden tempera-
ture change near the end of the curing period,
ACI
Committee 306
requires that the source of heat and cover
protection be slowly removed. The maximum allowable
temperature drop during the first 24 hours after the end of
the protection is given in Table 14-2. The temperature
drops apply to surface temperatures. Notice that the cool-
ing rates for surfaces of mass concrete (thick sections) are
lower than for thinner members.
Thermometer
Gasket stopper
Surface of concrete
CONTROL TESTS
Thermometers are needed to check the concrete tempera-
tures as delivered, as placed, and as maintained. An inex-
pensive pocket thermometer is shown in Fig. 14-13.
After the concrete has hardened, temperatures can be
checked with special surface thermometers or with an
ordinary thermometer that is kept covered with insulating
blankets. A simple way to check temperature below the
Water or
alcohol
Drilled or
preformed
hole
Table 14-2. Maximum Allowable Temperature Drop
During First 24 Hours After End of Protection Period*
Section size, minimum dimensions, mm (in.)
Less than
300 to 900
900 to 1800
Over
300 (12)
(12 to 36)
(36 to 72)
1800 (72)
28°C(50°F)
22°C(40°F)
17°C(30°F)
11°C(20°F)
Fig. 14-14. Scheme for measuring concrete temperatures
below the surface with a glass thermometer.
* Adapted from Table 5.5 of
ACI 306R-88
.
