Tuesday, April 3, 2018

FINENESS MODULUS


DEFINITION
      An empirical figure
      Fineness modulus of is an index number which represents the mean size of the particles
      It is obtained by adding the cumulative percentage retained on sieve divided by 100

 Introduced by
      In 1925, Duff Abrams introduced the concept of fineness modulus (FM) for estimating the proportions of fine and course aggregates in concrete mixtures.

Fineness modulus range













Aggregate classification
Aggregate
Range
Cyclopean
>75mm
Coarse Aggregate
75mm-4.75mm
Fine Aggregate
4.75mm-0.075mm
Silt
0.002mm-0.06mm












Fineness Modulus and Particle size
      Fineness Modulus is roughly proportional to particle size of aggregate
      It means finer the particle (having small particle size), less is the Fineness Modulus
      Recommended Fineness Modulus is in between 2.5 to 3

Use of fineness modulus
      To combine fine aggregate and coarse aggregate
      Used in mix design
      In grading of aggregate i.e. better the grading of aggregate, better the concrete

FM OF COMBINED AGGREGATE
      Fineness modulus of combined aggregates is always between the Fineness modulus of aggregates combined
      X=(F2-F)/(F-F1) x 100
      Where, X = % of Fine Aggregate to be combined with Coarse Aggregate
      F=Fineness modulus of combined aggregate/Fineness modulus according to specified grading
      F1= Fineness modulus of Fine aggregate
      F2= Fineness modulus of Coarse aggregate

FINENESS MODULUS CALCULATION

dry weight of sample = 1000g 

sieve size
(mm)
Weight retained
 (g)
cummulative
weight retained (g)
cummulative
% weight retained (%)
4.75
0
0
0
2.36
200
200
20
1.18
300
500
50
0.6
150
650
65
0.3
100
750
75
0.15
250
1000
100

Σ=1000

Σ=310

FINENESS MODULUS MEANING
      FM OF aggregate = (Cumulative % retained)/100 = 310/100 = 3.1
      It means average value of aggregate is in between 3rd and 4th sieve.
      Now arrange the sieve sizes in ascending order and find where 3.1 lies

      Here, it lies between 0.6mm and 1.18mm sieve size














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