Aggregate Fineness Modulus (ASTM C136)
The single number that captures how coarse a sand is, for concrete mix proportioning: FM = the sum of the cumulative percent retained on the #4, #8, #16, #30, #50, and #100 sieves, divided by 100. Cumulative retained of 2/12/32/57/82/95 sums to 280 -> FM 2.80, within the ASTM C33 concrete-sand band of 2.3-3.1. A higher FM is coarser (needs less paste); a finer sand needs more water for the same slump, and a mix's sand should not drift more than 0.20 FM without a mix adjustment. A gradation summary, not the full sieve check -- two different gradations can share an FM. A QC / mix-proportioning aid; the sieve analysis and mix design govern.
Formula and source
FM = (cumulative % retained on #4 + #8 + #16 + #30 + #50 + #100) / 100.
Aggregate fineness modulus (ASTM C136 sieve analysis; ASTM C125 definition; ASTM C33 concrete-sand limits), by name; the sieve analysis and the mix design govern.
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