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 indexof fine aggregate (let it bex)
2. Calculate the total surface indexof coarse aggregate (let it bey)
3. Letzbe the total surface index of combined aggregate andais the proportion of fine to coarse aggregate.
4. Thena = (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

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

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

= 1/1.55

Sofineaggregate : Coarse aggregate = 1 : 1.55