Civil Engineering Reference
In-Depth Information
Table 15-1. Effect of Aggregate Type on Thermal
Coefficient of Expansion of Concrete
Low Temperatures
Concrete continues to contract as the temperature is
reduced below freezing. The amount of volume change at
subfreezing temperatures is greatly influenced by the
moisture content, behavior of the water (physical state—
ice or liquid), and type of aggregate in the concrete. In one
study, the coefficient of thermal expansion for a tempera-
ture range of 24°C to -157°C (75°F to -250°F) varied from
6 x 10
-6
per °C (3.3 x 10
-6
per °F) for a low density (light-
weight) aggregate concrete to 8.2 x 10
-6
per °C (4.5 x 10
-6
per °F) for a sand and gravel mixture. Subfreezing tem-
peratures can significantly increase the compressive and
tensile strength and modulus of elasticity of moist con-
crete. Dry concrete properties are not as affected by low
temperatures. In the same study, moist concrete with an
original compressive strength of 35 MPa at 24°C (5000 psi
at 75°F) achieved over 117 MPa (17,000 psi) at -100°C
(-150°F). The same concrete tested ovendry or at a 50%
internal relative humidity had strength increases of only
about 20%. The modulus of elasticity for sand and gravel
concrete with 50% relative humidity was only 8% higher
at -157°C than at 24°C (-250°F than at 75°F), whereas the
moist concrete had a 50% increase in modulus of elasticity.
Going from 24°C to -157°C (75°F to -250°F), the thermal
conductivity of normal-weight concrete also increased,
especially for moist concrete. The thermal conductivity of
lightweight aggregate concrete is little affected (
Monfore
and Lentz 1962
and
Lentz and Monfore 1966
).
Aggregate
Coefficient
Coefficient
type (from
of expansion,
of expansion,
one source)
millionths per °C
millionths per °F
Quartz
11.9
6.6
Sandstone
11.7
6.5
Gravel
10.8
6.0
Granite
9.5
5.3
Basalt
8.6
4.8
Limestone
6.8
3.8
Coefficients of concretes made with aggregates from different
sources may vary widely from these values, especially those for
gravels, granites, and limestones (
Davis 1930
).
perature drop will crack concrete regardless of its age or
strength, provided the coefficient of expansion does not
vary with temperature and the concrete is fully restrained
(
FHWA and Transtec 2001
and
PCA 1982
).
Precast wall panels and slabs and pavements on
ground are susceptible to bending and curling caused by
temperature gradients that develop when concrete is cool
on one side and warm on the other. The calculated amount
of curling in a wall panel is illustrated in Fig. 15-18.
For the effect of temperature changes in mass concrete
due to heat of hydration, see
Chapter 18
.
High Temperatures
Temperatures greater than 95°C (200°F) that are sustained
for several months or even several hours can have signifi-
cant effects on concrete. The total amount of volume
change of concrete is the sum of volume changes of the
cement paste and aggregate. At high temperatures, the
paste shrinks due to dehydration while the aggregate
expands. For normal-aggregate concrete, the expansion of
the aggregate exceeds the paste shrinkage resulting in an
overall expansion of the concrete. Some aggregates such as
expanded shale, andesite, or pumice with low coefficients
of expansion can produce a very volume-stable concrete in
high-temperature environments (Fig. 15-19). On the other
hand, some aggregates undergo extensive and abrupt
volume changes at a particular temperature, causing dis-
ruption in the concrete. For example, in one study a
dolomitic limestone aggregate contained an iron sulfide
impurity caused severe expansion, cracking, and disinte-
gration in concrete exposed to a temperature of 150°C
(302°F) for four months; at temperatures above and below
150°C (302°F) there was no detrimental expansion (
Carette,
Painter, and Malhotra 1982
). The coefficient of thermal
expansion tends to increase with temperature rise.
Besides volume change, sustained high temperatures
can also have other, usually irreversible, effects such as a
Warm side
T
1
= 20
°
C
Cold side
T
2
= -6
°
C
(T
1
- T
2
)
L
2
8t
=
= Coefficient of expansion per
°
C
t = Panel thickness
When = 0.00001 per
°
C
= 2 mm
(20 + 6) x 0.00001 x 3000
2
8 x 150
=
= 2 mm
Concrete thickness
t = 150 mm
Fig. 15-18. Curling of a plain concrete wall panel due to
temperature that varies uniformly from inside to outside.
