Civil Engineering Reference
In-Depth Information
Temperature,
°
F
If stable aggregates are used and strength reduction
and the effects on other properties are accounted for in the
mix design, high quality concrete can be exposed to tem-
peratures of 90°C to 200°C (200°F to 400°F) for long
periods. Some concrete elements have been exposed to
temperatures up to 250°C (500°F) for long periods of time;
however, special steps should be taken or special materials
(such as heat-resistant calcium aluminate cement) should
be considered for exposure temperatures greater than
200°C (400°F). Before any structural concrete is exposed to
high temperatures (greater than 90°C or 200°F), laboratory
testing should be performed to determine the particular
concrete's thermal properties. This will avoid any unex-
pected distress.
0
400
800
1200
Siliceous
0.008
Carbonate
Sanded
expanded shale
0.004
0
0
200
400
600
Temperature,
°
C
Fig. 15-19. Thermal expansion of concretes containing
various types of aggregate (
Abrams 1977
).
CURLING (WARPING)
In addition to horizontal movement caused by changes in
moisture and temperature, curling of slabs on ground can
be a problem; this is caused by differences in moisture con-
tent and temperature between the top and bottom of slabs
(Fig. 15-21).
The edges of slabs at the joints tend to curl upward
when the surface of a slab is drier or cooler than the
bottom. A slab will assume a reverse curl when the surface
is wetter or warmer than the bottom. However, enclosed
slabs, such as floors on ground, curl only upward. When
the edges of an industrial floor slab are curled upward
they lose support from the subbase and become a can-
tilever. Lift-truck traffic passing over joints causes a repet-
itive vertical deflection that creates a great potential for
fatigue cracking in the slab. The amount of vertical
upward curl (curling) is small for a short, thick slab.
reduction in strength, modulus of elasticity, and thermal
conductivity. Creep increases with temperature. Above
100°C (212°F), the paste begins to dehydrate (lose chemi-
cally combined water of hydration) resulting in significant
strength losses. Strength decreases with increases in tem-
perature until the concrete loses essentially all its strength.
The effect of high-temperature exposure on compressive
strength of concretes made with various types of aggre-
gate is illustrated in Fig. 15-20. Several factors including
concrete moisture content, aggregate type and stability,
cement content, exposure time, rate of temperature rise,
age of concrete, restraint, and existing stress all influence
the behavior of concrete at high temperatures.
Temperature,
F
°
70
400
800
1200
100
Sanded expanded shale aggregate
75
Siliceous aggregate
Carbonate
aggregate
50
25
Heated unstressed, then stored
7 days at 21
F)
Avg. original strength = 27 MPa (3900 psi)
°
C (70
°
0
20
200
400
600
800
Temperature,
°
C
Fig. 15-21. Illustration of curling of a concrete slab on
ground. The edge of the slab at a joint or free end lifts off
the subbase creating a cantilevered section of concrete
that can break off under heavy wheel loading.
Fig. 15-20. Effect of high temperatures on the residual
compressive strength of concretes containing various
types of aggregate (
Abrams 1973
).
