Aggregate comprises
about 55 per cent of the volume of mortar and about 85 per cent

volume of mass concrete.
Mortar contains aggregate of size of 4.75 mm and concrete contains aggregate
upto a maximum size of 150 mm.

One
of the practical methods of arriving at the practical grading by trial and
error method is to mix aggregates of different size fractions in different percentages
and to choose the one sample which gives maximum weight or minimum voids per
unit volume, out of all the alternative samples. Fractions which are actually
available in the field, or which could be made available in the field including
that of the fine aggregate will be used in making samples.

**Sieve Analysis**

This is the name given
to the operation of dividing a sample of aggregate into various

fractions each
consisting of particles of the same size. The sieve analysis is conducted to determine
the particle size distribution in a sample

of aggregate, which we
call gradation.

A convenient system of expressing
the

gradation of aggregate
is one which the

consecutive sieve
openings are constantly doubled,

such as 10 mm, 20 mm, 40
mm etc. Under such a

system, employing a
logarithmic scale, lines can be

spaced at equal
intervals to represent the successive

sizes.

normally of the maximum
size 80 mm, 40 mm, 20

mm, 10 mm, 4.75 mm, 2.36
mm, 600 micron, 300

micron and 150 micron.
The aggregate fraction

from 80 mm to 4.75 mm
are termed as coarse

aggregate and those fraction
from 4.75 mm to 150

micron are termed as
fine aggregate. The size 4.75

mm is a common fraction
appearing both in coarse

aggregate and fine aggregate (C.A. and
F.A.).

Mechanical devices are
actually designed to

give motion in all
possible direction, and as such, it is more systematic and efficient than
handsieving.

For assessing the
gradation by sieve analysis, the quantity of materials to be taken on

the sieve is given Table 3.8.

Many a time, fine
aggregates are designated as coarse sand, medium sand and fine
sand. These
classifications do not give any precise meaning. What the supplier terms as
fine

sand may be really
medium or even coarse sand. To avoid this ambiguity fineness modulus could be
used as a yard stick to indicate the fineness of sand.

The following limits may
be taken as guidance:

Fine sand : Fineness
Modulus : 2.2 - 2.6

Medium sand : F.M. : 2.6
- 2.9

Coarse sand : F.M. : 2.9
- 3.2

A sand having a fineness
modulus more than 3.2 will be unsuitable for making

satisfactory concrete.

**Specific Surface and Surface Index**

The importance of a good grading of the coarse and fine aggregate has already been

discussed. The quantity of water required to produce a given workability depends to a large extent on the surface area of the aggregate.

The total surface index
(fx) of a mixture of aggregate is calculated by multiplying the

percentage of material
retained on its sieve by the corresponding surface index and to their sum is
added a constant of 330 and the result is divided by 1000. The method of computing
the total surface for any given grading is shown in Table 3.13

Surface Index (fx) =

507/1000

= 0.507

Similarly,
surface index can be calculated for standard grading curve, and this value of surface index can be taken as
the desirable surface index of the combined aggregate. This parameter of surface index
can be made use of for finding out the proportion of fine aggregate to coarse aggregate
available in the field to obtain specified or desirable surface index in the following way.

Let x =
surface index of fine aggregate

y
= surface index of coarse aggregate

z
= surface index of combined aggregate

a
= proportion of fine to coarse aggregate

Then
a = (z-y )/(x-z)

The
following example will show how to combine the available fine aggregate with available coarse aggregate
whose grading patterns are known to get the desirable surface index of the combined
aggregate. The desirable surface index of the combined aggregate could be calculated from the
grading pattern of the standard grading curve.

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