Civil Engineering Reference
In-Depth Information
and shape; the middle beaker is filled with an equal
volume of small aggregate particles of uniform size and
shape; and the beaker on the right is filled with particles
of both sizes. Below each beaker is a graduate with the
amount of water required to fill the voids in that beaker.
Note that when the beakers are filled with one particle size
of equal volume, the void content is constant, regardless of
the particle size. When the two aggregate sizes are
combined, the void content is decreased. If this operation
were repeated with several additional sizes, a further
reduction in voids would occur. The cement paste require-
ment for concrete is related to the void content of the
combined aggregates.
During the early years of concrete technology it was
sometimes assumed that the smallest percentage of voids
(greatest density of aggregates) was the most suitable for
concrete. At the same time, limits were placed on the
amount and size of the smallest particles. It is now known
that, even on this restricted basis, this is not the best target
for the mix designer. However, production of satisfactory,
economical concrete requires aggregates of low void
content, but not the lowest. Voids in aggregates can be
tested according to ASTM C 29 or AASHTO T 19.
In reality, the amount of cement paste required in
concrete is greater than the volume of voids between the
aggregates. This is illustrated in Fig. 5-8. Sketch A repre-
sents large aggregates alone, with all particles in contact.
Sketch B represents the dispersal of aggregates in a matrix
of paste. The amount of paste is necessarily greater than
the void content of sketch A in order to provide workabil-
ity to the concrete; the actual amount is influenced by the
workability and cohesiveness of the paste.
Fine-Aggregate Grading
Requirements of ASTM C 33 or AASHTO M 6/M 43 per-
mit a relatively wide range in fine-aggregate gradation,
but specifications by other organizations are sometimes
more restrictive. The most desirable fine-aggregate grad-
ing depends on the type of work, the richness of the
mixture, and the maximum size of coarse aggregate. In
leaner mixtures, or when small-size coarse aggregates are
used, a grading that approaches the maximum recom-
mended percentage passing each sieve is desirable for
workability. In general, if the water-cement ratio is kept
constant and the ratio of fine-to-coarse aggregate is chosen
correctly, a wide range in grading can be used without
measurable effect on strength. However, the best economy
will sometimes be achieved by adjusting the concrete
mixture to suit the gradation of the local aggregates.
Fine-aggregate grading within the limits of ASTM C
33 (AASHTO M 6) is generally satisfactory for most
concretes. The ASTM C 33 (AASHTO M 6) limits with
respect to sieve size are shown in Table 5-3.
Table 5-3. Fine-Aggregate Grading Limits
(ASTM C 33/AASHTO M 6)
Sieve size
Percent passing by mass
9.5 mm
(
3
⁄
8
in.)
100
4.75 mm
(No. 4)
95 to 100
2.36 mm
(No. 8)
80 to 100
1.18 mm
(No. 16)
50 to 85
600
µ
m
(No. 30)
25 to 60
300
µ
m
(No. 50)
5 to 30 (AASHTO 10 to 30)
150
µ
m
(No. 100)
0 to 10 (AASHTO 2 to 10)
The AASHTO specifications permit the minimum
percentages (by mass) of material passing the 300 µm (No.
50) and 150 µm (No. 100) sieves to be reduced to 5% and
0% respectively, provided:
1. The aggregate is used in air-entrained concrete
containing more than 237 kilograms of cement per
cubic meter (400 lb of cement per cubic yard) and
having an air content of more than 3%.
2. The aggregate is used in concrete containing more
than 297 kilograms of cement per cubic meter (500 lb
of cement per cubic yard) when the concrete is not air-
entrained.
3. An approved supplementary cementitious material is
used to supply the deficiency in material passing
these two sieves.
Other requirements of ASTM C 33 (AASTHO M 6) are:
1. The fine aggregate must not have more than 45%
retained between any two consecutive standard
sieves.
2. The fineness modulus must be not less than 2.3 nor
more than 3.1, nor vary more than 0.2 from the typical
value of the aggregate source. If this value is ex-
A
B
Fig. 5-8. Illustration of the dispersion of aggregates in co-
hesive concrete mixtures.
