Civil Engineering Reference
In-Depth Information
when stress is applied, and (3) the length of time the con-
crete is stressed. It is also affected by other factors related
to the quality of the concrete and conditions of exposure,
such as: (1) type, amount, and maximum size of aggre-
gate; (2) type of cementing materials; (3) amount of
cement paste; (4) size and shape of the concrete element;
(5) volume to surface ratio of the concrete element;
(6) amount of steel reinforcement; (7) prior curing condi-
tions; and (8) the ambient temperature and humidity.
Within normal stress ranges, creep is proportional to
stress. In relatively young concrete, the change in volume
or length due to creep is largely unrecoverable; in older or
drier concrete it is largely recoverable.
The creep curves shown in Fig. 15-26 are based on
tests conducted under laboratory conditions in accor-
dance with ASTM C 512. Cylinders were loaded to almost
40% of their compressive strength. Companion cylinders
not subject to load were used to measure drying
shrinkage; this was then deducted from the total deforma-
tion of the loaded specimens to determine creep.
Cylinders were allowed to dry while under load except
for those marked “sealed.” The two 28-day curves for each
concrete strength in Fig. 15-26 show that creep of concrete
loaded under drying conditions is greater than creep of
concrete sealed against drying. Concrete specimens
loaded at a late age will creep less than those loaded at an
early age. It can be seen that as concrete strength
0.0125
0.08
Load removed
0.0100
Instantaneous recovery
Creep recovery
0.06
0.0075
Creep strain
0.04
0.0050
Irrecoverable creep
0.02
0.0025
Elastic strain
Permanent set
0
400
800
1200
1600
Time, days
Fig. 15-27. Combined curve of elastic and creep strains
showing amount of recovery. Specimens (cylinders) were
loaded at 8 days immediately after removal from fog curing
room and then stored at 21°C (70°F) and 50% RH. The
applied stress was 25% of the compressive strength at 8
days (
Hansen and Mattock 1966
).
decreases, creep increases. Fig. 15-27 illustrates recovery
from the elastic and creep strains after load removal.
A combination of strains occurring in a reinforced
column is illustrated in Fig. 15-28. The curves represent
deformations and volume changes in a 14th-story column
of a 76-story reinforced concrete building while under
construction. The 400 x 1200-mm (16 x 48-in.) column con-
tained 2.08% vertical reinforcement and was designed for
60-MPa (9000-psi) concrete.
1.5
1500
10
Concrete strength, 28 MPa (4000 psi)
Age of loading
28 days
Total
1.0
1200
90
28 (sealed)
180
5
Creep
0.5
900
0
0
Drying shrinkage
7.5
600
Concrete strength, 41 MPa (6000 psi)
1.0
Age of loading
28 days
5
0.5
300
90
28 (sealed)
360
Instantaneous deformation
0
0
250
500
750
1000
1250
0
Age, days
Fig. 15-26. Relationship of time and age of loading to creep
of two different strength concretes. Specimens were
allowed to dry during loading, except for those labeled as
sealed (
Russell and Corley 1977
).
0
500
1000
1500
Age, days
Fig. 15-28. Summation of strains in a reinforced concrete
column during construction of a tall building (
Russell and
Corley 1977
).
