Civil Engineering Reference
In-Depth Information
ceeded, the fine aggregate should be rejected unless
suitable adjustments are made in proportions of fine
and coarse aggregate.
The amounts of fine aggregate passing the 300 µm
(No. 50) and 150, µm (No. 100) sieves affect workability,
surface texture, air content, and bleeding of concrete. Most
specifications allow 5% to 30% to pass the 300 µm (No. 50)
sieve. The lower limit may be sufficient for easy placing
conditions or where concrete is mechanically finished,
such as in pavements. However, for hand-finished
concrete floors, or where a smooth surface texture is de-
sired, fine aggregate with at least 15% passing the 300 µm
(No. 50) sieve and 3% or more passing the 150 µm (No.
100) sieve should be used.
Fineness Modulus.
The fineness modulus (FM) of either
fine or coarse aggregate according to ASTM C 125 is calcu-
lated by adding the cumulative percentages by mass
retained on each of a specified series of sieves and divid-
ing the sum by 100. The specified sieves for determining
FM are: 150 µm (No. 100), 300 µm (No. 50), 600 µm (No.
30), 1.18 mm (No. 16), 2.36 mm (No. 8), 4.75 mm (No. 4),
9.5 mm (
3
⁄
8
in.), 19.0 mm (
3
⁄
4
in.), 37.5 mm (1
1
⁄
2
in.), 75 mm
(3 in.) and, 150 mm (6 in.).
FM is an index of the fineness of an aggregate—the
higher the FM, the coarser the aggregate. Different aggre-
gate grading may have the same FM. FM of fine aggregate
is useful in estimating proportions of fine and coarse
aggregates in concrete mixtures. An example of how the
FM of a fine aggregate is determined (with an assumed
sieve analysis) is shown in Table 5-4.
Degradation of fine aggregate due to friction and
abrasion will decrease the FM and increase the amount of
materials finer than the 75 µm (No. 200) sieve.
Table 5-4. Determination of Fineness Modulus of
Fine Aggregates
Percentage
of individual
Cumulative
fraction
Percentage
percentage
retained,
passing,
retained,
Sieve size
by mass
by mass
by mass
9.5 mm
(
3
⁄
8
in.)
0
100
0
4.75 mm
(No. 4)
2
98
2
2.36 mm
(No. 8)
13
85
15
1.18 mm
(No. 16)
20
65
35
600
µ
m
(No. 30)
20
45
55
300
µ
m
(No. 50)
24
21
79
150
µ
m
(No. 100)
18
3
97
Pan
3
0
—
Total
100
283
Fineness modulus
= 283 ÷ 100 = 2.83
given maximum-size coarse aggregate can be varied over
a moderate range without appreciable effect on cement
and water requirement of a mixture if the proportion of
fine aggregate to total aggregate produces concrete of
good workability. Mixture proportions should be changed
to produce workable concrete if wide variations occur in
the coarse-aggregate grading. Since variations are difficult
to anticipate, it is often more economical to maintain
uniformity in manufacturing and handling coarse aggre-
gate than to reduce variations in gradation.
The maximum size of coarse aggregate used in con-
crete has a bearing on the economy of concrete. Usually
more water and cement is required for small-size aggre-
gates than for large sizes, due to an increase in total aggre-
gate surface area. The water and cement required for a
slump of approximately 75 mm (3 in.) is shown in Fig. 5-9
for a wide range of coarse-aggregate sizes. Fig. 5-9 shows
that, for a given water-cement ratio, the amount of cement
required decreases as the maximum size of coarse aggre-
gate increases. The increased cost of obtaining and/or
Coarse-Aggregate Grading
The coarse aggregate grading requirements of ASTM C 33
(AASHTO M 80) permit a wide range in grading and a
variety of grading sizes (see Table 5-5). The grading for a
Table 5-5. Grading Requirements for Coarse Aggregates (ASTM C 33 and AASHTO M 80)
Amounts finer than each laboratory sieve, mass percent passing
Size
Nominal size, sieves
100 mm
90 mm
75 mm
63 mm
50 mm
number
with square openings
(4 in.)
(3
1
⁄
2
in.)
(3 in.)
(2
1
⁄
2
in.)
(2 in.)
1
90 to 37.5 mm
(3
1
⁄
2
to 1
1
⁄
2
in.)
100
90 to 100
—
25 to 60
—
2
63 to 37.5 mm
(2
1
⁄
2
to 1
1
⁄
2
in.)
—
—
100
90 to 100
35 to 70
3
50 to 25.0 mm
(2 to 1 in.)
—
—
—
100
90 to 100
357
50 to 4.75 mm
(2 in. to No. 4)
—
—
—
100
95 to 100
4
37.5 to 19.0 mm
(1
1
⁄
2
to
3
⁄
4
in.)
—
—
—
—
100
467
37.5 to 4.75 mm
(1
1
⁄
2
in. to No. 4)
—
—
—
—
100
5
25.0 to 12.5 mm
(1 to
1
⁄
2
in.)
—
—
—
—
—
56
25.0 to 9.5 mm
(1 to
3
⁄
8
in.)
—
—
—
—
—
57
25.0 to 4.75 mm
(1 in. to No. 4)
—
—
—
—
—
6
19.0 to 9.5 mm
(
3
⁄
4
to
3
⁄
8
in.)
—
—
—
—
—
67
19.0 to 4.75 mm
(
3
⁄
4
in. to No. 4)
—
—
—
—
—
7
12.5 to 4.75 mm
(
1
⁄
2
in. to No. 4)
—
—
—
—
—
8
9.5 to 2.36 mm
(
3
⁄
8
in. to No. 8)
—
—
—
—
—
