Civil Engineering Reference
In-Depth Information
0.06
Steam curing will also reduce drying shrinkage.
Computer software is available to predict the effect of
curing and environmental conditions on shrinkage and
cracking (
FHWA and Transtec 2001
).
Hedenblad (1997)
provides tools to predict the drying of concrete as effected
by different curing methods and type of construction.
0.05
0.04
C
N
M
X
0.03
TEMPERATURE CHANGES
OF HARDENED CONCRETE
0.02
Cement content
= 323 kg/m
3
(545 lb/yd
3
)
Concrete expands slightly as temperature rises and con-
tracts as temperature falls, although it can expand slightly
as free water in the concrete freezes. Temperature changes
may be caused by environmental conditions or by cement
hydration. An average value for the coefficient of thermal
expansion of concrete is about 10 millionths per degree
Celsius (5.5 millionths per degree Fahrenheit), although
values ranging from 6 to 13 millionths per degree Celsius
(3.2 to 7.0 millionths per degree Fahrenheit) have been
observed. This amounts to a length change of 5 mm for 10
m of concrete (
2
⁄
3
in. for 100 ft of concrete) subjected to a
rise or fall of 50°C (100°F). The coefficient of thermal
expansion for structural low-density (lightweight) con-
crete varies from 7 to 11 millionths per degree Celsius (3.6
to 6.1 millionths per degree Fahrenheit). The coefficient of
thermal expansion of concrete can be determined by
AASHTO TP 60.
Thermal expansion and contraction of concrete varies
with factors such as aggregate type, cement content,
water-cement ratio, temperature range, concrete age, and
relative humidity. Of these, aggregate type has the greatest
influence.
Table 15-1 shows some experimental values of the
thermal coefficient of expansion of concretes made with
aggregates of various types. These data were obtained
from tests on small concrete specimens in which all factors
were the same except aggregate type. In each case, the fine
aggregate was of the same material as the coarse aggregate.
The thermal coefficient of expansion for steel is about
12 millionths per degree Celsius (6.5 millionths per degree
Fahrenheit), which is comparable to that for concrete. The
coefficient for reinforced concrete can be assumed as 11
millionths per degree Celsius (6 millionths per degree
Fahrenheit), the average for concrete and steel.
Temperature changes that result in shortening can
crack concrete members that are highly restrained by
another part of the structure or by ground friction. Consider
a long restrained concrete member cast without joints that,
after moist curing, is allowed to drop in temperature. As the
temperature drops, the concrete wants to shorten, but
cannot because it is restrained longitudinally. The resulting
tensile stresses cause the concrete to crack. Tensile strength
and modulus of elasticity of concrete both may be assumed
proportional to the square root of concrete compressive
strength. And calculations show that a large enough tem-
0.01
ASTM C 157
0
0
8
16
24
32
Age, weeks
Fig. 15-16. Drying shrinkage of concretes made with
selected high-range water reducers (N,M, and X) compared
to a control mixture (C) (
Whiting and Dziedzic 1992
).
Effect of Curing on Drying Shrinkage
The amount and type of curing can effect the rate and ulti-
mate amount of drying shrinkage. Curing compounds,
sealers, and coatings can trap free moisture in the concrete
for long periods of time, resulting in delayed shrinkage.
Wet curing methods, such as fogging or wet burlap, hold
off shrinkage until curing is terminated, after which the
concrete dries and shrinks at a normal rate. Cooler initial
curing temperatures can reduce shrinkage (Fig. 15-17).
0.1
0.08
0.06
0.04
Cement content
= 307 kg/m
3
(517 lb/yd
3
)
0.02
ASTM C 157
0
0
8
16
24
32
40
48
56
64
Age, weeks
Fig. 15-17. Effect of initial curing on drying shrinkage of
portland cement concrete prisms. Concrete with an initial 7-
day moist cure at 4°C (40°F) had less shrinkage than
concrete with an initial 7-day moist cure at 23°C (73°F).
Similar results were found with concretes containing 25%
fly ash as part of the cementing material (
Gebler and
Klieger 1986
).
