sieve is the same No 4 used as the coarsest fine-aggregate sieve. With this exception, a coarse-aggregate sieve is
designated by the size of one of its mesh openings. The size of the mesh openings in consecutive sieves is related
by a constant ratio. Size distribution graphs show the percent of material passing each sieve (see Figure 1-1).
Figure 1-1 also gives the grade limits for fine-aggregates and for one designated size of coarse aggregate. Normal
coarse aggregate consists of gravel or crushed stone, whereas normal fine aggregate is sand.
Figure 1-1. Limits specified in ASTM C33 for fine-aggregates
and for one size of coarse-aggregate.
Fineness modulus (FM). This is a number that indicates the fineness of a fine aggregate but is not the
same as its grade. Many fine-aggregate gradings can have an identical FM. To obtain the FM of a fine
aggregate (see Figure 1-2), quarter a sample of at least 500 grams of sand and sieve it through the Nos 4,
8, 16, 30, 50, and 100 sieves. Record the individual weights of the materials retained on each sieve and
the cumulative retained weights. Add the cumulative percents and divide by 100. The result is the FM of
the sample. A sand with an FM falling between 2.3 and 3.1 is suitable for concrete (see Table 1-2). In
general, fine aggregate having either a very high or a very low FM is not as good a concrete aggregate as
medium sand. Coarse sand is not as workable, and fine sands are uneconomical. Take care to obtain
representative samples. The FM of the aggregate taken from one source should not vary more than 0.20
from all test samples taken at that source.