# HOW TO USE SURFACE INDEX TO FIND PROPORTION OF FINE AGGREGATE TO COARSE AGGREGATE?

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

1. Calculate the total surface index of fine aggregate (let it be x)
2. Calculate the total surface index of coarse aggregate (let it be y)
3. Let z be the total surface index of combined aggregate and a is the proportion of fine to coarse aggregate.
4. Then a = (z-y)/(x-z)

## Worked out Example

### 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 = (z-y)/(x-z)

= (0.60-0.30)/(1.065-0.60)

= 1/1.55

So fine aggregate : Coarse aggregate = 1 : 1.55

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