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Fineness Modulus of Fine Aggregate

Fine aggregate (sand) grading affects consistence and finishability. Fineness Modulus (FM) is an empirical calculation used to support the optimisation of a concrete mix design. It is used to determine ‘degree of uniformity of the grading’. This can create a relationship between grading and consistence and/or the amount of cement or water to fill the voids. An example would be that sand with a high FM will have lower voids between the particles.

In BS EN 12620 Aggregates for Concrete, FM is calculated by amount of material passing (%) each sieve as follows:

FM = (4.0mm + 2.0mm + 1.0mm + 500µm + 250µm + 125µm) ÷ 100

Example of a 0-4mm MP Gf85 Concrete Sand:

FM = (96% + 91% + 83% + 48% + 9% + 1%) ÷ 100 = 2.6

Points to note:

  • A 0-4mm with high amount passing 4mm & 2mm can skew a high value.
  • Analysis of number of 0-4mm Concrete Sand sources gave a similar FM even though the fraction passing the 500µm sieve varied between 27 to 62%. There does not appear to be a correlation between FM and aggregate grading.

Acknowledgement: Concrete Society