Civil Engineering Reference
In-Depth Information
100
80
60
40
20
Optional,
see text
Fig. 5-4. Range of particle sizes found in aggregate for use
in concrete. (8985)
0
Sieve sizes
(AASHTO M 6/ M 80) plus an additional six more coarse
aggregate size numbers. Fine aggregate or sand has only
one range of particle sizes for general construction and
highway work.
The grading and grading limits are usually expressed
as the percentage of material passing each sieve. Fig. 5-6
shows these limits for fine aggregate and for one size of
coarse aggregate.
There are several reasons for specifying grading limits
and nominal maximum aggregate size; they affect relative
Fig. 5-6. Curves indicate the limits specified in ASTM C 33
for fine aggregate and for one commonly used size number
(grading size) of coarse aggregate.
aggregate proportions as well as cement and water
requirements, workability, pumpability, economy, poros-
ity, shrinkage, and durability of concrete. Variations in
grading can seriously affect the uniformity of concrete
from batch to batch. Very fine sands are often uneconomi-
cal; very coarse sands and coarse aggregate can produce
harsh, unworkable mixtures. In general, aggregates that
do not have a large deficiency or excess of any size and
give a smooth grading curve will produce the most satis-
factory results.
The effect of a collection of various sizes in reducing
the total volume of voids between aggregates is illustrated
by the simple method shown in Fig. 5-7. The beaker on the
left is filled with large aggregate particles of uniform size
Fig. 5-7. The level of liquid in the graduates, representing
voids, is constant for equal absolute volumes of aggre-
gates of uniform but different size. When different sizes are
combined, the void-content decreases. The illustration is
not to scale.
Fig. 5-5. Making a sieve analysis test of coarse aggregate
in a laboratory. (30175-A)
