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Grain Size Analysis of Aggregates

Fine aggregate is the sand used in mortars. Coarse aggregate is the broken stone used in concrete .The coarse aggregate unless mixed with fine aggregate serves no purpose in cement works. The size of fine aggregate is limited to a maximum of 4.75 mm gauge beyond which it is known as coarse aggregate.

Fineness Modulus of Aggregates

Fineness modulus is only a numerical index of fineness, giving some idea of the mean size of the particle s in the entire body of the aggregate. To a certain extent it is a method of standardization of the grading of the aggregate. It is obtained by adding the percentage weight of material retained in each of the standard sieves and dividing it by 100. The objective of finding the fineness modulus is to grade a given aggregate for the most economical mix and workability with minimum quantity of cement Certain limits of fineness modulus for fine coarse aggregates are given in the table below and a sample under test should satisfy these results so that the aggregate may give good workability under economic conditions.

Limits of Fineness Modulus of Sand

If the test aggregate gives higher fineness modulus the mix will be harsh and if on the other hand gives a lower fineness modulus it gives uneconomical mix .

Effective Size of Aggregates

Effective size (in microns) is the maximum particle size of the smallest 10% of the aggregate or it is the sieve opening corresponding to 10% finer and is designated by the symbol D10.

Uniformity Coefficient of Aggregates

This is the ratio of the maximum size of the smallest 60% to the effective size. Uniformity coefficient = D60/D10

Apparatus for Grain Size Analysis

Indian standard test sieves, weighing balance, sieve shaker etc . Size of sieves to be used:

1. For fine aggregate- 4.75mm, 2.36mm, 1.18mm, 600 microns, 300 microns, 150 microns.
2. For coarse aggregate-25mm,20mm 12.5mm, 10mm, 4.75mm.

Procedure of Grain Size Analysis of Aggregates

For Fine Aggregates

1. Take one kg of sand from the laboratory sample.
2. Arrange the sieves in order of IS sieves no’s 480, 240, 120, 60, 30 and 15, Keeping sieve no.480 at the top and 15 at the bottom and cover the top.
3. Keep the sample in the top sieve no.480.
4. Carry out the sieving in the set of sieves for not less than 10 minutes.
5. Find the weight of sample retained in each sieve.
6. Tabulate the values in given tabular column .

For Coarse Aggregates

1. Take one kg of coarse aggregate
2. Arrange the sieves one over the other in relation to their size of opening.(25mm, 20mm,12.5mm, 10mm, 4.75mm)
3. Carry out the sieving for the specified time
4. Find the weight of aggregate retained on each sieve taken in order and tabulated in table.

Plot the Graph Draw a graph with sieve opening to log scale on the X-axis and % finer on Y-axis .The curve iscalled a grading curve.

Calculations in Grain Size Analysis

For Fine Aggregates

1. Effective size in microns(Di0 sieve opening corresponding to 10%finer in the graph ) =
2. Uniformity coefficient (D60/ D10),D to be obtained from the graph ) =
3. Fineness modulus (Sum of cumulative % wt retained /100) =

For Coarse Aggregates

1. Effective size in microns (D10,sieve opening corresponding to 10% finer in the graph)
2. Uniformity coefficient (D60/ D10),D to be obtained from the graph ) =
3. Fineness modulus (Sum of cumulative % wt retained /100) =

Results of Grain Size Analysis

For Fine Aggregates

1. Effective size =……………..micron
2. Uniformity coefficient =
3. Fineness modulus =

For Coarse Aggregates

1. Effective size =……………..micron
2. Uniformity coefficient =
3. Fineness modulus =