How would you determine grading of aggregate.

The grading of aggregate can be measured by following methods:

1. Specific surface and surface index
   
2. Fineness Modulus
3. Sieve Analysis

Specific surface and surface index:

The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Consideration of specific surface gives a somewhat misleading picture of the workability to be expected. To overcome this difficulty Murdock has suggested the use of ?Surface Index? which is an empirical number related to the specific surface of the particle with more weightage given to the finer fractions.The total surface index (fx) of a mixture of aggregate is calculated by multiplying the percentage of material retained on its sieve by the corresponding surface index and to their sum is added a constant of 330 and the result is divided by 1000.

Similarly, surface index can be calculated for standard grading cur....

The grading of aggregate can be measured by following methods:

1. Specific surface and surface index
   
2. Fineness Modulus
3. Sieve Analysis

Specific surface and surface index:

The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Consideration of specific surface gives a somewhat misleading picture of the workability to be expected. To overcome this difficulty Murdock has suggested the use of ?Surface Index? which is an empirical number related to the specific surface of the particle with more weightage given to the finer fractions.The total surface index (fx) of a mixture of aggregate is calculated by multiplying the percentage of material retained on its sieve by the corresponding surface index and to their sum is added a constant of 330 and the result is divided by 1000.

Similarly, surface index can be calculated for standard grading curve, and this value of surface index can be taken as the desirable surface index of the combined aggregate. This parameter of surface index can be made use of for finding out the proportion of fine aggregate to coarse aggregate available in the field to obtain specified or desirable surface index in the following way.
Let x = surface index of fine aggregate
y = surface index of coarse aggregate
z = surface index of combined aggregate
a = proportion of fine to coarse aggregate

Then ,`a =( z -y )/( x- z )`

Fineness Modulus:

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by an arbitrary number 100. The larger the figure, the coarser is the material.

The F.M of aggregates is shown below:

1. Fine aggregate:2-4
2. Intermediate Aggregate: 4-6.5
   
3. Coarse Aggregate: 6.5-8

To find fineness modulus of fine aggregate, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm.

The fineness Modulus value of an aggregate is 2.75 means that the average value of fine aggregate is in between 2nd and 3rd sieve i.e, in between 0.3mm and  0.6 mm. A sand having a fineness modulus more than 3.2 will be unsuitable for making satisfactory concrete as a fine aggregate. The various values of F.M. for different sands are listed below:

1. For finer sand, F.M= 2.2-2.6
2. For intermediate Sand, F.M= 2.6-2.9
3. For Coarse Sand, F.M=  2.9-3.2
   

To find fineness modulus of coarse, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm,10mm, 20 mm, 40 mm, 80mm.

The fineness Modulus value of an aggregate is 7.17 means that the average size of particle of given coarse aggregate sample is between 7th and 8th sieves,i.e, between 10 mm and 20 mm.
The F.M. for the following maximum size of coarse aggregate are:

1. for 20mm maximum size of coarse aggregate, f.m=6.0-6.9
2. For 40 mm maximum size of coarse aggregate, F.M.= 6.9-7.5
3. For 75 mm maximum size of coarse aggregate, F.M= 7.5-8.0
4. For 150 mm maximum size of coarse aggregate,F.M= 8.0-8.5