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**Fineness Modulus Method of Concrete Mix Design**

The index number, which describes the relative sizes of coarse and fine aggregates, is called as fineness modulus (FM). Fineness modulus is determined separately by sieving coarse and fine aggregates through the following set of sieve.
IS sieves for coarse aggregates are 80 mm, 40 mm, 20 mm, 10 mm and 4.75 mm.
IS sieves for fine aggregates are 1 mm, 600 , 300 , 212 , 150 , and 75 .
Where, X= Fineness modulus of coarse aggregate,
Y= Fineness modulus of fine aggregates, and
Z= Economical value of fineness modulus for combined aggregate.
The above formula will fix the proportion of fine aggregate to coarse aggregate. The percentage of the coarse aggregate is equal to 100 minus the percentage of fine aggregate.
Table below gives the economical value of F.M. for combined aggregate.
**Table: Economical Value of F.M. for Combined Aggregate**

S. No. |
Particle size (mm) |
Fineness modulus |
Average F.M. | |

Minimum |
Maximum | |||

1 | 20 | 4.7 | 5.1 | 4.90 |

2 | 25.4 | 5.0 | 5.5 | 5.25 |

3 | 31.5 | 5.2 | 5.7 | 5.45 |

4 | 38.1 | 5.4 | 5.9 | 5.65 |

5 | 40 | 5.7 | 6.3 | 6.00 |

6 | 80 | 6.5 | 7.0 | 6.75 |