Method of Combining Aggregates:

It is desirable to proportion the available materials in such a way that the grading of the combined aggregate corresponds to one of the four standard grading curves. It could be achieved as suggested in Road Note No. 4 method.

Analytical Method

The grading of fine aggregate and coarse aggregate available at works site are shown in Table. The fine aggregate and coarse aggregate of 20 mm maximum size have to be combined in order to approximate to the coarsest grading of the standard grading. This technique is explained by a numerical example. In standard grading, 25% total aggregate passes through 4.7 mm IS sieve.

Table: Grading of Fine and Coarse Aggregate Available at Site

Let x and y be the proportions of fine aggregate and coarse aggregate in combined state. Thus, to satisfy the percentage passing 4.75 mm IS sieve, we have

 80 x/100 + 6 y/100 = 25 (x + y)/100

0.8 x + 0.06 y = 0.25 (x + y)

0.8 x - 0 .25 x = 0.25 y - 0.06 y

0.55 x = 0.19 y

x: y = 1 : 3 (Approx.)

It means in which fine aggregate and coarse aggregate should be combined within ratio 1: 3. The grading of combined aggregate is determined by multiplying,

(a) By 1.0, and

(b) By 3.0 and then dividing the sum of these products by 4.0.

The resulting merged grading could be acquired as shown in last column.