Building Materials, Third Edition

IVT f2w—
—

sand in the concrete will be
2

1

21.82

1.1

, instead of 2 per unit volume of cement. The mix

proportion then would be 1:1.82:4 rather than 1: 2: 4. Which indicates lesser production of
concrete. Also, there will be chances of segregation, honeycombing and reduced yield of
concrete.

Bulking of sand can be determined, in field, by filling a container of known volume (A) with
damp sand in the manner in which the mixer hopper will be filled. The height of sand in the
container is measured. The sand is then taken out of container carefully, ensuring no sand is
lost during this transaction. The sand is then either dried and filled back into the gauge box, or
the container is filled with water and the damp sand is poured in to displace the water.
Whichever method is adopted, the new depth of aggregate in the container gives the unbulked
volume (B).

Then percentage bulking expressed as a percentage of the dry volume =

AB

100

B



xX„2'2—2 '2——2@ A2—2™™'2— 2—2& 

p2TP i    ™22w    2g   22f %22ƒ—

p


2wX It is a numerical index of fineness, giving some idea about the mean size
of the particles in the aggregates. The fineness modulus (F.M.) varies between 2.0 and 3.5 for
fine aggregate, between 5.5 and 8.0 for coarse aggregate, and from 3.5 to 6.5 for all-in aggregate.
Aggregate, whose F.M. is required, is placed on a standard set of sieves (80, 63, 40, 20, 12.5, 10,
4.75, 2.36, 1.18 mm and 600, 300, 150
m) and the set vibrated. The material retained on each
sieve after sieving represent the fraction of aggregate coarser than the sieve in question but
finer than the sieve above. The sum of the cumulative percentages retained on the sieves
divided by 100 gives the F.M. A fineness modulus of 3.0 can be interpreted to mean that the
third sieve i.e., 600
m is the average size. The test procedure is given IS: 2386 (Part I).
The object of finding F.M. is to grade the given aggregate for the required strength and
workability of concrete mix with minimum cement. Higher F.M. aggregate result in harsh
concrete mixes and lower F.M. result in uneconomical concrete mixes.