Civil Engineering Reference
In-Depth Information
is dependent upon the concrete ingredients and con-
ditions of mixing, placing, curing, and subsequent drying.
For example, air-entrained concrete is less susceptible
to damage by early freezing than non-air-entrained
concrete. See Chapter 8, “
Air-Entrained Concrete
,” for
more information.
STRENGTH GAIN OF CONCRETE
AT LOW TEMPERATURES
Temperature affects the rate at which hydration of cement
occurs—low temperatures retard hydration and con-
sequently retard the hardening and strength gain of
concrete.
If concrete is frozen and kept frozen above about
minus 10°C (14°F), it will gain strength slowly. Below that
temperature, cement hydration and concrete strength
gain cease. Fig. 14-3 illustrates the effect of cool tempera-
tures on setting time. Fig. 14-4 illustrates the effects of
casting temperature on slump. Figs. 14-5 and 14-6 show
the age-compressive strength relationship for concrete
that has been cast and cured at various temperatures.
Note in Fig. 14-6 that concrete cast and cured at 4°C (40°F)
and 13°C (55°F) had relatively low strengths for the first
week; but after 28 days—when all specimens were moist-
cured at 23°C (73°F)—strengths for the 4°C (40°F) and
13°C (55°F) concretes grew faster than the 23°C (73°F)
concrete and at one year they were slightly higher.
Fig. 14-2. Closeup view of ice impressions in paste of
frozen fresh concrete. The ice crystal formations occur as
unhardened concrete freezes. They do not occur in
adequately hardened concrete. The disruption of the paste
matrix by freezing can cause reduced strength gain and
increased porosity. (44047)
Casting temperature,
°
F
32
52
72
92
200
150
100
Mix proportions
held constant.
50
Cement A
Cement B
0
Casting temperature,
°
F
0
10
20
30
40
32
52
72
92
Casting temperature,
°
C
250
Casting temperature,
F
°
200
32
52
72
92
200
150
150
100
Mix proportions
held constant.
100
50
Cement A
Cement B
Mix proportions
held constant.
0
50
0
10
20
30
40
Casting temperature,
°
C
Cement A
Cement B
Fig. 14-4 Slump characteristics as a function of casting
temperature (
Burg 1996
).
0
0
10
20
30
40
Casting temperature,
°
C
Fig. 14-3. Initial set characteristics as a function of casting
temperature (top), and final set characteristics as a function
of casting temperature (bottom) (
Burg 1996
).
