Fineness Modulus (FM)
The results of aggregate sieve analysis is expressed by a number called Fineness Modulus. Obtained by adding the sum of the cumulative percentages by mass of a sample aggregate retained on each of a specified series of sieves and dividing the sum by 100. The specified sieves are: 150 μm (No. 100), 300 μm (No. 50), 600 μm (No. 30), 1.18 mm (No. 16), 2.36 mm (No. 8), 4.75 mm (No. 4), 9.5 mm , 19.0 mm , 37.5 mm , 75 mm , and 150 mm.
Fineness modulus = 283 ÷ 100 = 2.83
• Index of fineness of an aggregate.
• The fineness modulus of the fine aggregate is required for mix design since sand gradation has the largest effect on workability. A fine sand (low FM) has much higher effect paste requirements for good workability.
• The FM of the coarse aggregate is not required for mix design purposes.
• It is computed by adding the cumulative percentages of aggregate retained on each of the specified series of sieves, and dividing the sum by 100 [smallest size sieve: No. 100 (150 μm)].
• Note: The higher the FM, the coarser the aggregate.
• It is important to note that the fineness modulus is just one number which only characterizes the average size of the aggregate, and different grading may have the same fineness modulus.
Fine Aggregate effect on concrete
• Oversanded (More than required sand)
– Over cohesive mix.
– Water reducers may be less effective.
– Air entrainment may be more effective.
• Undersanded (deficit of sand)
– Prone to bleed and segregation.
– May get high levels of water reduction.
– Air entrainers may be less effective.
• Sand grading
– gap graded or single sized may enhance bleed and segregation. Air entrainment may help fill the gaps.
• Coarse aggregate
– Poor grading may give a harsh mix at low workabilities and segregation at high workabilities.
– Effect on admixtures is small.
– Elongated or flaky aggregates may cause workability difficulties .