FM in Sieve Analysis FM = Fineness Modulus

 FM in Sieve Analysis

FM = Fineness Modulus

It is an empirical index number that represents the average particle size of aggregates (fine or coarse).

It is obtained by adding the cumulative percentage retained on a specified series of standard sieves and dividing the sum by 100.

According to IS 2386 (Part I):1963, for fine aggregates, standard sieves used are: 150 µm, 300 µm, 600 µm, 1.18 mm, 2.36 mm, 4.75 mm, 10 mm.

Interpretation

Higher FM → Coarser aggregate

Reduces water demand

Improves pumpability and reduces shrinkage

But may lower workability if too coarse

Lower FM → Finer aggregate

Increases water demand for same workability

Improves cohesiveness

But may cause more shrinkage and higher cement consumption

Effect of FM on Concrete

Workability:

Coarse sand (high FM) gives harsh mixes → less workable.

Fine sand (low FM) gives higher cohesion but requires more water.

Strength:

Optimum FM (balanced grading) → denser packing of particles → higher compressive strength.

Durability:

Proper FM ensures reduced voids and permeability.

Too fine → weak and shrink-prone.

Too coarse → honeycombing risk.

Economy:

Balanced FM reduces cement consumption (since voids are minimized).

Ideal FM Values

Fine Aggregate (Sand): 2.3 – 3.1 (as per IS recommendations)

Coarse Aggregate: 5.5 – 8.0 (depending on maximum size used)

📌 For concrete mix design, sand with FM around 2.6 – 2.9 is usually preferred (neither too fine, nor too coarse).

As per Indian Standards (IS), the relevant codes for the Fineness Modulus (FM) of aggregates are mainly:

-IS 383:1970 (Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete)

This code specifies that fine aggregate (sand) shall have an FM typically in the range of 2.2 to 3.2. It classifies sand into zones by grading, with limits on FM:

Fine Sand: 2.2 to 2.6

Medium Sand: 2.6 to 2.9

Coarse Sand: 2.9 to 3.2

Sand with FM less than 2.2 is considered very fine, and above 3.2 generally unsuitable for good concrete .

IS 2386 (Part 1):1963 (Methods of Test for Aggregates for Concrete - Part 1: Particle Size and Shape)

This code covers the procedure for sieve analysis that is used to determine the FM by adding cumulative % retained on specified sieves and dividing by 100. It is the standard reference for the test method .

IS codes mention that the FM is an index number used to determine the average particle size of aggregate, which influences mix proportioning, workability, strength, and durability of concrete.

Summarized IS Code Guidelines:

For Fine Aggregate (Sand), FM usually should be between 2.2 and 3.2.

Sand outside this range should be avoided or handled with adjusted concrete mix designs.

Cumulative percentages retained on standard IS sieves (from 150 µm to 10 mm for fine aggregate) are used for calculation as per IS 2386 (Part 1) procedure.

If desired, detailed sieve sizes and percentage retained for FM calculation are found in IS 2386 (Part 1) .