Civil Engineering Reference
In-Depth Information
100
ing. It is generally expressed in megapascals (MPa) or
pounds per square inch (psi) at an age of 28 days. One
megapascal equals the force of one newton per square
millimeter (N/mm
2
) or 1,000,000 N/m
2
. Other test ages are
also used; however, it is important to realize the relation-
ship between the 28-day strength and other test ages.
Seven-day strengths are often estimated to be about 75% of
the 28-day strength and 56-day and 90-day strengths are
about 10% to 15% greater than 28-day strengths as shown
in Fig. 1-16. The specified compressive strength is desig-
nated by the symbol ˘, and ideally is exceeded by the
actual compressive strength, ¯.
The compressive strength that a concrete achieves, ¯,
results from the water-cement ratio (or water-cementitious
materials ratio), the extent to which hydration has
progressed, the curing and environmental conditions, and
the age of the concrete. The relationship between strength
and water-cement ratio has been studied since the late
1800s and early 1900s (
Feret 1897
and
Abrams 1918
). Fig.
1-17 shows compressive strengths for a wide range of
concrete mixtures and water-cement ratios at an age of 28
days. Note that strengths increase as the water-cement
ratios decrease. These factors also affect flexural and tensile
strengths and bond of concrete to steel.
The water-cement ratio compressive strength relation-
ships in Fig. 1-17 are for typical non-air-entrained con-
cretes. When more precise values for concrete are required,
graphs should be developed for the specific materials and
mix proportions to be used on the job.
For a given workability and a given amount of cement,
air-entrained concrete requires less mixing water than non-
air-entrained concrete. The lower water-cement ratio possi-
ble for air-entrained concrete tends to offset the somewhat
lower strengths of air-entrained concrete, particularly in
lean to medium cement content mixes.
Cement content: 270 kg/m
3
(454 lb/cu yd)
Normal-density concrete
w/c ratio: 0.66
75 mm (3 in.) depth
45 (1
3
/
4
)
90
80
20 (
3
/
4
)
6 (
1
/
4
)
70
60
50
800
600
400
200
Normal-density concrete
0
0.6
0.5
0.4
0.3
0.2
Normal-density concrete
0.1
0
0
75
150
225
300
375
Time of drying, days
Fig. 1-15. Relative humidity distribution, drying shrinkage,
and mass loss of 150 x 300-mm (6 x 12-in.) cylinders moist
cured for 7 days followed by drying in laboratory air at 23°C
(73°F) (
Hanson 1968
).
180
160
140
28 days
120
dry faster than voluminous concrete members with rela-
tively small surface areas (such as bridge piers).
Many other properties of hardened concrete also are
affected by its moisture content; these include elasticity,
creep, insulating value, fire resistance, abrasion resistance,
electrical conductivity, frost resistance, scaling resistance,
and resistance to alkali-aggregate reactivity.
100
80
60
40
Concrete cylinders
20
0
1
10
100
1000
10000
Strength
Age, days
Fig. 1-16. Compressive strength development of various
concretes illustrated as a percentage of the 28-day strength
(
Lange 1994
).
Compressive strength may be defined as the measured
maximum resistance of a concrete specimen to axial load-
