FINENESS MODULUS OF AGGREGATE – WHAT, WHY & HOW

What is Fineness Modulus of Aggregate?

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by 100.

Fineness Modulus of Sand
Fineness Modulus of Sand

Why to Determine Fineness Modulus?

  • Fineness modulus is generally used to get an idea of how coarse or fine the aggregate is. More fineness modulus value indicates that the aggregate is coarser and small value of fineness modulus indicates that the aggregate is finer.
  • Fineness modulus of different type of sand is as per given below.
Type of Sand Fineness Modulus Range
Fine Sand 2.2 –  2.6
Medium Sand 2.6 – 2.9
Coarse Sand 2.9 – 3.2
  • Generally sand having fineness modulus more than 3.2 is not used for making good concrete.
  • Fineness modulus can also be used to combine two aggregate to get the desirable grading.

How to Determine Fineness Modulus?

Following procedure is adopted to calculate fineness modulus of aggregate.

Procedure

  • Sieve the aggregate using the appropriate sieves (80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron & 150 micron)
  • Record the weight of aggregate retained on each sieve.
  • Calculate the cumulative weight of aggregate retained on each sieve.
  • Calculate the cumulative percentage of aggregate retained.
  • Add the cumulative weight of aggregate retained and divide the sum by 100. This value is termed as fineness modulus
  • Refer the following example calculation

Fineness Modulus of Sand – Worked Out Example

Example Calculation of Fineness Modulus of Fine Aggregate

Sieve Size Weight of sand Retained (g) Cumulative weight of sand retained (g) Cumulative percentage of sand retained (%)
80 mm
40 mm
 20 mm
10 mm 0 0 0
4.75 mm 10 10 2
2.36 mm 50 60 12
1.18 mm 50 110 22
600 micron 95 205 41
300 micron 175 380 76
150 micron 85 465 93
Pan 35 500
Total amount = 500 Total

= 246

 

So Fineness Modulus = 246/100 = 2.46

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