Civil Engineering Reference
In-Depth Information
Flexural strength is sometimes used on paving proj-
ects instead of compressive strength; however, flexural
strength is avoided due to its greater variability. For more
information on flexural strength, see
“Strength”
in Chap-
ter 1 and
“Strength Specimens”
in Chapter 16.
Aggregates
Two characteristics of aggregates have an important influ-
ence on proportioning concrete mixtures because they
affect the workability of the fresh concrete. They are:
1. Grading (particle size and distribution)
2. Nature of particles (shape, porosity, surface texture)
Grading is important for attaining an economical mix-
ture because it affects the amount of concrete that can be
made with a given amount of cementitious materials and
water. Coarse aggregates should be graded up to the
largest size practical under job conditions. The maximum
size that can be used depends on factors such as the size
and shape of the concrete member to be cast, the amount
and distribution of reinforcing steel in the member, and
the thickness of slabs. Grading also influences the work-
ability and placeability of the concrete. Sometimes mid-
sized aggregate, around the 9.5 mm (
3
⁄
8
in.) size, is lacking
in an aggregate supply; this can result in a concrete with
Water-Cementitious Material Ratio
The water-cementitious material ratio is simply the mass
of water divided by the mass of cementitious material
(portland cement, blended cement, fly ash, slag, silica
fume, and natural pozzolans). The water-cementitious
material ratio selected for mix design must be the lowest
value required to meet anticipated exposure conditions.
Tables 9-1 and 9-2 show requirements for various expo-
sure conditions.
When durability does not control, the water-cementi-
tious materials ratio should be selected on the basis of
concrete compressive strength. In such cases the water-
cementitious materials ratio and mixture proportions for the
required strength should be based on adequate field data or
trial mixtures made with actual job materials to determine
the relationship between the ratio and strength. Fig. 9-2 or
Table 9-3 can be used to select a water-cementitious mate-
rials ratio with respect to the required average strength, Â,
for trial mixtures when no other data are available.
In mix design, the water to cementitious materials
ratio,
W/CM
, is often used synonymously with water to
cement ratio
(W/C)
; however, some specifications differen-
tiate between the two ratios. Traditionally, the water to
cement ratio referred to the ratio of water to portland
cement or water to blended cement.
Table 9-3 (Metric). Relationship Between Water to
Cementitious Material Ratio and Compressive
Strength of Concrete
Water-cementitious materials ratio by mass
Compressive
strength at
Non-air-entrained
Air-entrained
28 days, MPa
concrete
concrete
45
0.38
0.30
40
0.42
0.34
35
0.47
0.39
30
0.54
0.45
25
0.61
0.52
60
20
0.69
0.60
15
0.79
0.70
8
Strength is based on cylinders moist-cured 28 days in accordance
with ASTM C 31 (AASHTO T 23). Relationship assumes nominal
maximum size aggregate of about 19 to 25 mm.
Adapted from
ACI 211.1
and
ACI 211.3
.
50
6
40
Non-air-entrained concrete
Table 9-3 (Inch-Pound Units). Relationship Between
Water to Cementitious Material Ratio and
Compressive Strength of Concrete
30
4
20
Air-entrained concrete
Water-cementitious materials ratio by mass
Compressive
2
strength at
Non-air-entrained
Air-entrained
28 days, psi
concrete
concrete
10
7000
0.33
—
6000
0.41
0.32
0
0
5000
0.48
0.40
0.3
0.4
0.5
0.6
0.7
0.8
0.9
4000
0.57
0.48
Water to cementitious materials ratio
3000
0.68
0.59
Fig. 9-2. Approximate relationship between compressive
strength and water to cementing materials ratio for concrete
using 19-mm to 25-mm (
3
⁄
4
-in. to 1-in.) nominal maximum
size coarse aggregate. Strength is based on cylinders moist
cured 28 days per ASTM C 31 (AASHTO T 23). Adapted from
Table 9-3, ACI 211.1, ACI 211.3, and
Hover 1995
.
2000
0.82
0.74
Strength is based on cylinders moist-cured 28 days in accordance
with ASTM C 31 (AASHTO T 23). Relationship assumes nominal
maximum size aggregate of about
3
⁄
4
in. to 1 in.
Adapted from
ACI 211.1
and
ACI 211.3
.
