Fineness Modulus:

The fineness modulus is a numerical index, which gives general idea about the coarseness (mean size) of aggregates but does not provide any information regarding particle size distribution in the sample of aggregate. It is determined by the following formula:

Fineness Modulus = ∑(Cumulative percentage retained on each sieve)/100

In the above example (Table 3.2), the fineness modulus = (277)/100 = 2.77. It can be viewed as weighted average size of a sieve on which particles are retained. The sieve being counted from the finest sieve as one. So, fineness modulus of 2.77 indicates that average size of particles is among 300-600 μm sieves and more precisely, close to 600 μm.

The value of fineness modulus is higher for coarser aggregates and lower for finer aggregates. For the aggregates generally used, the range of fineness modulus is as below:

Fine Aggregate         2.0 to 3.5

Coarse Aggregate    5.5 to 8.0

All-in-aggregate       3.5 to 6.5

Further, the range of fineness modulus for various categories of sand is as below:

Fine Sand                   2.0 to 2.6

Medium Sand           2.6 to 2.9

Coarse Sand              2.9 to 3.5

Generally, the sand with fineness modulus ranging between 2.2 to 3.2 is considered suitable for making concrete.

Comments

The fineness modulus is useful in grading the given aggregate for designing the mix economically for the given strength and workability with the minimum amount of cement paste. Aggregates with higher fineness modulus produce harsh concrete and with the lower fineness modulus will give uneconomical mix. The fineness modulus is also used to give a rough check on the batch-to-batch variation within the grading of aggregates acquired from the same source.