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.