Theory regarding standard deviation calculation
Firstly we have to know the limitation of assumed standard deviation
as per coral provision IS10262
SL.Nos
|
GRADE OF CONCRETE
|
Assumed standard deviation (N/mm2) or MPA
|
1
|
M10
|
3.5 N/mm2
|
2
|
M-15
|
|
3
|
M-20
|
4.0 N/mm2
|
4
|
M-25
|
|
5
|
M-30
|
5.0N/mm2
|
6
|
M-35
|
|
7
|
M-40
|
|
8
|
M-45
|
|
9
|
M-50
|
|
10
|
M-55
|
Please be noted as per Is code we considered tolerance for standard
deviation (+/- 1) N/mm2 ,with
reference to the quality of building materials received at site.
While designing the Grade of
concrete we consider Target mean strength in which we conform to our
client that while doing
testing for compressive strength of cube not more than 5% will fail
or fall
Target means strength=
fck ( i.e grade of
concrete)+1.65 *
Standard deviation
Formula
to find standard deviation-√
(∑(x-x¯)2/n-1)
x
=Compressive Strength of cube (average of 3 nos of cube)
x¯ =Mean of x (means total sum of strength divided by total nos of cube
N = Total number of counts
Steps
to find the standard deviation :-
-
Find all the required parameter from the format of cube test report maintained on the daily basis.
-
After that put all values of parameter on the given formula for finding the standard deviation
-
Be noted take square root after calculation of inside variance value.
Definition
:
If we are getting less value of standard deviation as per the
required value of deviation then it means we have a good and better
quality control of materials used for production of concrete