HOW TO BLEND AGGREGATE USING SURFACE INDEX VALUE?
Surface Index of Aggregates
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 Single Graded Aggregate
TABLE1 

Sieve Size Within Which Particles Lie 
Surface Index For Particles Within Sieve Size Indicated 
80 mm to 40 mm  2.5 
40 mm to 20 mm  2 
20 mm to 10 mm  1 
10 mm to 4.75 mm  1 
4.75 mm to 2.36 mm  4 
2.36 mm to 1.18 mm  7 
1.18 mm to 600 micron  9 
600 micron to 300 micron  9 
300 micron to 150 micron  7 
< 150 micron  2 
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.
The values mentioned in the above table only represent the aggregate particles which fall in the sieve size as shown on column1. But in actual practice the aggregate (whether fine or aggregate) consist of particles of different sizes. Therefore it is necessary to find a way to calculate the surface index of aggregate consisting particles of different sizes. And this is done by calculating total surface index of aggregate.
How to Calculate Total Surface Index
Step1
Sieve the combined aggregates as per the sieves mentioned below and record the amount of material retained by corresponding sieves.
Sieves required for sieving = 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron & 150 micron.
Step2
Find out the corresponding surface index values for different aggregate grading (refer table1)
Step3
Multiply the percentage of material retained on its sieve by the corresponding surface index. Then add all the multiplied values and to their sum add a constant 330 and divide the result by 1000. And the answer is the total surface index of the aggregate.
Example Calculation for Total Surface Index
Sieve size within which particle lie  % of particles within sieve sizes  Surface index for particles within sieve size  Calculating Total Surface Index 
20 mm – 10 mm  55  1  55 
10 mm – 4.75 mm  15  1  15 
4.75 mm – 2.36 mm  7  4  28 
2.36 mm – 1.18 mm  7  7  49 
1.18 mm – 600 micron  7  9  63 
600 micron – 300 micron  7  9  63 
Total =  164  
Adding constant 330  494 
So Total Surface Index = 494/1000 = 0.494
Blending of Aggregate Using Total Surface Index Value
Our aim is to mix the fine aggregate & coarse aggregate in such a ratio so that the final combined aggregate gives the specified or desirable surface index. Follow the below mentioned steps to find the proportion of fine to coarse aggregate.
Procedure
 Calculate the total surface index of fine aggregate (let it be x)
 Calculate the total surface index of coarse aggregate (let it be y)
 Let z be the total surface index of combined aggregate and a is the proportion of fine to coarse aggregate.
 Then a = (zy)/(xz)
Worked Out Example for Aggregate Blending
Calculating total surface index of fine aggregate
Sieve sizes within which particles lie  Percentage of particles within sieve size  Surface index for particles for within sieve size  Calculation of total surface index 
4.75 mm to 2.36 mm  10  4  40 
2.36 mm to 1.18 mm  20  7  140 
1.18 mm to 600 micron  20  9  180 
600 micron to 300 micron  30  9  270 
300 micron to 150 micron  15  7  105 
Total =  735  
Adding constant of 330  1065 
Total surface index of coarse aggregate (i.e. x) = 1065/1000 = 1.065
Calculating total surface index of coarse aggregate
Sieve sizes within which particles lie  Percentage of particles within sieve size  Surface index for particles for within sieve size  Calculation of total surface index 
20 mm to 10 mm  65  1  65 
10 mm to 4.75 mm  35  1  35 
Total =  30  
Adding a constant of 330  300 
Total surface index of coarse aggregate (i.e. y) = 300/1000 = 0.30
Let the total surface index of combined aggregate (i.e. z) = 0.60
So a = (zy)/(xz)
= (0.600.30)/(1.0650.60)
= 1/1.55
So the ratio of Fine aggregate: Coarse aggregate = 1:1.55