Fineness Modulus (FM) is an empirical calculation used to support the optimisation of a concrete mix. Fine aggregate or sand grading affects consistence and finishability.

FM 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 retained (%) rather than the amount passing on each sieve as follows:

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

Example of a 0-4mm MP G_f85 Concrete Sand:

FM = (4% + 8% + 15% + 45% + 90% + 98%) ÷ 100 = 2.6

Points to note:

• A 0-4mm with high amount passing 4mm & 2mm can skew a high value.
• Analysis of a number of 0-4mm Concrete Sand sources showed at the 500µm sieve, passing values ranged between 27% to 62%, and gave a similar FM. Therefore, there does not appear to be a correlation between FM and aggregate grading.