Civil Engineering Reference
In-Depth Information
produce a compressive strength cylinder test result that
may not comply with specifications.
High temperatures of freshly mixed concrete increase
the rate of setting and shorten the length of time within
which the concrete can be transported, placed, and fin-
ished. Setting time can be reduced by 2 or more hours
with a 10°C (18°F) increase in concrete temperature (Fig.
13-3). Concrete should remain plastic long enough so that
each layer can be placed without development of cold
joints or discontinuities in the concrete. Retarding admix-
tures, ASTM C 494 (AASHTO M 194) Type B, and hydra-
tion control admixtures can be beneficial in offsetting the
accelerating effects of high temperature.
In hot weather, there is an increased tendency for
cracks to form both before and after hardening. Rapid
evaporation of water from freshly placed concrete can
cause plastic-shrinkage cracks before the surface has hard-
ened (discussed in more detail later in this chapter).
Cracks may also develop in the hardened concrete
because of increased drying shrinkage due to higher water
contents or thermal volume changes as the concrete cools.
Air entrainment is also affected in hot weather. At
elevated temperatures, an increase in the amount of
air-entraining admixture is required to produce a given
air content.
Fig. 13-4 shows the effect of high initial concrete tem-
peratures on compressive strength. The concrete tempera-
tures at the time of mixing, casting, and curing were 23°C
(73°F), 32°C (90°F), 41°C (105°F), and 49°C (120°F). After
28 days, the specimens were all moist-cured at 23°C (73°F)
until the 90-day and one-year test ages. The tests, using
identical concretes of the same water-cement ratio, show
that while higher concrete temperatures give higher early
strength than concrete at 23°C (73°F), at later ages concrete
strengths are lower. If the water content had been
increased to maintain the same slump (without increasing
140
Mix data:
w/c ratio: 0.45
Slump: 25 to 75 mm (1 to 3 in.)
Air content: 4.5%
Cement: Type I, Normal
120
100
80
60
Curing: specimens cast
and moist-cured at
temperature indicated
for first 28 days. All
moist-cured at 23
°
C
(73
40
20
°
F) thereafter.
0
1
3
7
28
90
365
Age of test, days
Fig. 13-4. Effect of high concrete temperatures on com-
pressive strength at various ages (
Klieger 1958
).
cement content), the reduction in strength would have
been even greater than shown.
The proper fabrication, curing, and testing of com-
pression test specimens during hot weather is critical.
Steps should be taken to make sure ASTM C 31 (AASHTO
T 23) procedures are followed regarding initial curing of
strength specimens for acceptance or quality control
testing at 16ºC to 27ºC (60ºF to 80ºF). If the initial 24 hour
curing is at 38ºC (100ºF), the 28-day compressive strength
of the test specimens may be 10% to 15% lower than if
cured at the required ASTM C 31 (AASHTO T 23) curing
temperatures (
Gaynor 1985
).
Because of the detrimental effects of high concrete tem-
peratures, all operations in hot weather should be directed
toward keeping the concrete as cool as possible.
15
Mix proportions held constant.
Initial set
Final set
Initial set
Final set
Cement A
12
COOLING CONCRETE MATERIALS
Cement B
The usual method of cooling concrete is to lower the tem-
perature of the concrete materials before mixing. One or
more of the ingredients should be cooled. In hot weather
the aggregates and mixing water should be kept as cool as
practicable; these materials have a greater influence on
concrete temperature after mixing than other ingredients.
The contribution of each ingredient in a concrete mix-
ture to the temperature of the freshly mixed concrete is
related to the temperature, specific heat, and quantity of
each material. Fig. 13-5 shows graphically the effect of
temperature of materials on the temperature of fresh con-
crete. It is evident that although concrete temperature is
9
ASTM C 403
(AASHTO T 197)
6
3
0
10
°
C (50
°
F)
23
°
C (73
°
F)
32
°
C (90
°
F)
Casting temperature
Fig. 13-3. Effect of concrete temperature on setting time
(
Burg 1996
).
