Table of Contents
Surface Index of Aggregates
The Surface Index of an aggregate is another way of representing the specific surface of an aggregate. This method was suggested by Murdock. Murdock provided some empirical numbers to represent the typical surface grading of various aggregates (as shown in the table below).
Surface Index Values for Single Graded Aggregate
Table 1 

sieve size 
The surface index for particles within the sieve size is indicated 
80mm to 40mm  2.5 
40mm to 20mm  2 
20mm to 10mm  1 
10mm to 4.75mm  1 
4.75mm to 2.36mm  4 
2.36mm to 1.18mm  7 
1.18 mm to 600 µm  9 
600 micron to 300 micron  9 
300 micron to 150 micron  7 
<150 µm  2 
It is clear from the above table that aggregates larger than 10 mm have a lower surface index value, and for aggregates within the range of 10 mm to 300 µm, the value increases and the surface index value decreases with the decrease of aggregate size. .
The values mentioned in the above table represent only the total particles which fall in the sieve size as shown in column1. But in actual practice the aggregate (whether fine or aggregate) consists of particles of different sizes. It is therefore necessary to find a way to calculate the surface index of an aggregate consisting of particles of different sizes. And this is done by calculating the total surface index of the aggregate.
How to Calculate Total Surface Index
step 1
Filter the combined aggregate according to the sieve mentioned below and record the amount of material held by the respective sieve.
Sieve required for sieve = 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 μm, 300 μm and 150 μm.
step 2
Find the corresponding surface index value for different composite grading (see Table1)
step 3
Multiply the percentage of material placed on its sieve by the corresponding surface index. Then add up all the multiplied values and add a constant 330 to their sum and divide the result by 1000. And the answer is the total surface index of the total.
Example calculation for total surface index
the size of the sieve within which the particle is located  % of particles within sieve size  Surface index for particles within the sieve size  Calculation of Total Surface Index 
20mm – 10mm  55  1  55 
10mm – 4.75mm  15  1  15 
4.75mm – 2.36mm  7  4  28 
2.36mm – 1.18mm  7  7  49 
1.18 mm – 600 µm  7  9  63 
600 micron – 300 micron  7  9  63 
total =  164  
continuously 330. Add  494 
So total surface index = 494/1000 = 0.494
Composite Aggregate Using Total Surface Index Value
We aim to mix fine aggregates and coarse aggregates in such a ratio that the final composite aggregate gives the specified or desirable surface index. Follow the steps below to find the ratio of fine to coarse aggregate.
Process
 Calculate the total surface index of the fine aggregate (let it be x,
 Calculate the total surface index of the coarse aggregate (let it be You,
 Let it happen jade be the total surface index of the composite set and a The ratio of fine to coarse aggregate is
 Then a = (z)/(xz)
O . Work doneUT example for total blending
Calculating the total surface index of fine aggregates
Filter out shapes that contain particles  Percentage of particles within the size of the sieve  Surface index for particles within the sieve size  Calculation of Total Surface Index 
4.75mm to 2.36mm  10  4  40 
2.36mm to 1.18mm  20  7  140 
1.18 mm to 600 µm  20  9  180 
600 micron to 300 micron  30  9  270 
300 micron to 150 micron  15  7  105 
total =  735  
330. adding a constant of  1065 
Total surface index of coarse aggregate (ie.) x) = 1065/1000 = 1.065
Calculation of Total Surface Index of Coarse Aggregates
Filter out shapes that contain particles  Percentage of particles within the size of the sieve  Surface index for particles within the sieve size  Calculation of Total Surface Index 
20mm to 10mm  65  1  65 
10mm to 4.75mm  35  1  35 
total =  30  
330. adding a constant of  300 
Total surface index of coarse aggregate (ie.) You) = 300/1000 = 0.30
Let the total surface index (ie.) of the composite set be jade) = 0.60
therefore a = (z)/(xz)
, (0.600.30)/(1.0650.60)
= 1/1.55
so the ratio Foffline Aggregates: = Coarse Aggregates 1:1.55
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