5.12:
Fineness Modulus
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The fineness modulus (FM) of aggregate is a numerical index that measures the coarseness or fineness of the particles. It is calculated by adding the cumulative percentages of aggregate retained on each of a specified series of sieves and dividing the sum by 100.
Consider performing sieve analysis on sand through a set of ASTM sieves. The weight of aggregate retained in each sieve and pan placed at the bottom is recorded, as given in Column B of Table 1.
To determine the fineness modulus of sand, calculate the cumulative weight of aggregate retained on each sieve (Column C of Table 1) and express it as a percentage of the total weight (Column D of Table 1). Finally, the sum of the cumulative percentages of aggregate retained on each sieve is determined and multiplied by 100 to obtain the fineness modulus of the sand.
Table 1: Sieve analysis on sand
| Column A | Column B | Column C | Column D |
| ASTM Sieve | Weight retained on each sieve (g) | Cumulative weight retained on each sieve (g) | Cumulative percentage retained on each sieve (%) |
|
⅜ inch |
0 | 0 | 0 |
| No. 4 | 5 | 0+5 = 5 | 5/500 = 1 |
| No. 8 | 55 | 5+55 = 60 | 60/500 = 12 |
| No. 16 | 65 | 60+65 = 125 | 125/500 = 25 |
| No. 30 | 90 | 125+90 = 215 | 215/500 = 43 |
| No. 50 | 160 | 215+160 = 375 | 375/500 = 75 |
| No. 100 | 90 | 375+90 = 465 | 465/500 = 93 |
| Bottom pan | 35 | Total weight = 465+35 = 500 |
In general, sand has a fineness modulus of around 2.3 to 3.1. Lower values indicate finer sand, while higher values mean the sand is more coarse.