HOW TO BLEND AGGREGATE USING SURFACE INDEX VALUE?

Surface Index of Aggregates

Surface index of aggregates is another way to represent thespecific 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

表格1
Sieve Size Within Which Particles Lie	Surface Index For Particles Within Sieve Size Indicated
80 mm to 40 mm	-2.5
40毫米至20毫米	-2
20 mm to 10 mm	-1
10毫米至4.75毫米	1
4.75 mm to 2.36 mm	4
2.36毫米至1.18毫米	7
1.18 mm to 600 micron	9
600 micron to 300 micron	9
300微米到150微米	7
<150微米	2

从上表中可以明显看出，大于10 mm的骨料具有较小的表面索引值，并且在10 mm至300微米范围内的聚集体的增加，并且表面索引值随着聚集体大小的减小而开始降低。

上表中提到的值仅表示落入筛子大小的聚集体颗粒，如列1所示。但是在实际实践中，骨料（无论是精细的还是骨料）由不同大小的粒子组成。因此，有必要找到一种方法来计算骨料的表面索引，这些粒子粒子的颗粒组成。这是通过计算骨料的总表面指数来完成的。

How to Calculate Total Surface Index

步骤1

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.

第2步

Find out the corresponding surface index values for different aggregate grading (refer table-1)

第3步

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
		总计=	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

1. Calculate the total surface index of fine aggregate (let it bex)
2. Calculate the total surface index of coarse aggregate (let it bey)
3. 让zbe the total surface index of combined aggregate andais the proportion of fine to coarse aggregate.
4. Thena = (z-y)/(x-z)

Worked OUT示例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毫米至1.18毫米	20	7	140
1.18 mm to 600 micron	20	9	180
600 micron to 300 micron	30	9	270
300微米到150微米	15	7	105
		总计=	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毫米至4.75毫米	35	1	35
		总计=	-30
		Adding a constant of 330	300

Total surface index of coarse aggregate (i.e.y) = 300/1000 = 0.30

让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

所以比率Fineaggregate: Coarse aggregate =1：1.55