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 retainedon each sieve (g) Cumulative weightretained 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

Eq1

Eq2

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.

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