Surface index of aggregates is another way to represent the specific surface of aggregate. This method was suggested by Murdock. Murdock provided some empirical numbers to represent the specific surface of different aggregates grading (as shown in the table below).
Surface Index Values for Different Aggregate Grading
Sieve Size Within Which Particles Lie
Surface Index For Particles Within Sieve Size Indicated
80 mm to 40 mm
40 mm to 20 mm
20 mm to 10 mm
10 mm to 4.75 mm
4.75 mm to 2.36 mm
2.36 mm to 1.18 mm
1.18 mm to 600 micron
600 micron to 300 micron
300 micron to 150 micron
< 150 micron
From the above table it is clear that aggregate larger than 10 mm has less surface index value, and the value goes on increasing for aggregates within range 10 mm to 300 micron and surface index value starts to decrease with decrease in aggregate size.