Civil Engineering Reference
In-Depth Information
high shrinkage properties, high water demand, and poor
workability and placeability. Durability may also be
affected. Various options are available for obtaining
optimal grading of aggregate (
Shilstone 1990
).
The maximum size of coarse aggregate should not
exceed one-fifth the narrowest dimension between sides of
forms nor three-fourths the clear space between individual
reinforcing bars or wire, bundles of bars, or prestressing
tendons or ducts. It is also good practice to limit aggregate
size to not more than three-fourths the clear space between
reinforcement and the forms. For unreinforced slabs on
ground, the maximum size should not exceed one third the
slab thickness. Smaller sizes can be used when availability
or economic consideration require them.
The amount of mixing water required to produce a
unit volume of concrete of a given slump is dependent on
the shape and the maximum size and amount of coarse
aggregate. Larger sizes minimize the water requirement
and thus allow the cement content to be reduced. Also,
rounded aggregate requires less mixing water than a
crushed aggregate in concretes of equal slump (see
“Water Content”).
The maximum size of coarse aggregate that will pro-
duce concrete of maximum strength for a given cement
content depends upon the aggregate source as well as its
shape and grading. For high compressive-strength con-
crete (greater than 70 MPa or 10,000 psi), the maximum
size is about 19 mm (
3
⁄
4
in.). Higher strengths can also
sometimes be achieved through the use of crushed stone
aggregate rather than rounded-gravel aggregate.
The most desirable fine-aggregate grading will
depend upon the type of work, the paste content of the
mixture, and the size of the coarse aggregate. For leaner
mixtures, a fine grading (lower fineness modulus) is desir-
able for workability. For richer mixtures, a coarse grading
(higher fineness modulus) is used for greater economy.
In some areas, the chemically bound chloride in
aggregate may make it difficult for concrete to pass chlo-
ride limits set by
ACI 318
or other specifications.
However, some or all of the chloride in the aggregate
may not be available for participation in corrosion of
reinforcing steel, thus that chloride may be ignored.
ASTM PS 118 (to be redesignated ASTM C 1500), Soxhlet
extracted chloride test, can be used to evaluate the
amount of chloride available from aggregate. ACI 222.1
also provides guidance.
The bulk volume of coarse aggregate can be deter-
mined from Fig. 9-3 or Table 9-4. These bulk volumes are
based on aggregates in a dry-rodded condition as des-
cribed in ASTM C 29 (AASHTO T 19); they are selected
from empirical relationships to produce concrete with a
degree of workability suitable for general reinforced con-
crete construction. For less workable concrete, such as
required for concrete pavement construction, they may be
increased about 10%. For more workable concrete, such as
may be required when placement is by pump, they may
be reduced up to 10%.
Nominal maximum aggregate size, in.
Air Content
0
0.5
1
1.5
2
2.5
3
0.9
Entrained air must be used in all concrete that will be ex-
posed to freezing and thawing and deicing chemicals and
can be used to improve workability even where not required.
0.8
0.7
Table 9-4. Bulk Volume of Coarse Aggregate Per Unit
Volume of Concrete
0.6
Nominal
Bulk volume of dry-rodded coarse
maximum
Fineness modulus = 2.4
Fineness modulus = 2.6
Fineness modulus = 2.8
Fineness modulus = 3.0
aggregate per unit volume of concrete for
size of
different fineness moduli of fine aggregate*
0.5
aggregate,
mm (in.)
2.40
2.60
2.80
3.00
9.5 (
3
⁄
8
)
0.50
0.48
0.46
0.44
0.4
12.5 (
1
⁄
2
)
0.59
0.57
0.55
0.53
0
25
50
75
19 (
3
⁄
4
)
0.66
0.64
0.62
0.60
Nominal maximum aggregate size, mm
25 (1)
0.71
0.69
0.67
0.65
37.5 (1
1
⁄
2
)
0.75
0.73
0.71
0.69
Fig. 9-3. Bulk volume of coarse aggregate per unit volume
of concrete. Bulk volumes are based on aggregates in a
dry-rodded condition as described in ASTM C 29 (AASHTO
T 19). For more workable concrete, such as may be re-
quired when placement is by pump, they may be reduced
up to 10%. Adapted from Table 9-4,
ACI 211.1
and
Hover
(
1995
and
1998
).
50 (2)
0.78
0.76
0.74
0.72
75 (3)
0.82
0.80
0.78
0.76
150 (6)
0.87
0.85
0.83
0.81
*Bulk volumes are based on aggregates in a dry-rodded condition as
described in ASTM C 29 (AASHTO T 19). Adapted from
ACI 211.1
.
