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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