THE
STANDARD DEVIATION OF THE RESULTS FROM THEIR MEAN / TARGET STRENGTH IS REGARDED AS AN INDEX OF THE DISPERSION & CONSEQUENTLY
OF THE DEGREE OF SITE CONTROL.
WHEN TESTING A LARGE NUMBER OF SAMPLES OF A GIVEN GRADE OF CONCRETE UNDER
CONTROLLED CONDITIONS, THE RESULTS
WILL SHOW SOME VARIATIONS. THIS VARIATION IS ASSUMED
AS BELL SHAPE OR NORMAL DISTRIBUTION.
ACCEPTED STATISTICAL THEOREM STATES THAT " NO MATTER WHAT THE SHAPE
OF THE DISTRIBUTION, AT LEAST 75%
OF THE VALUES WILL FALL WITHIN ± TWO ( 2 ) TIMES STANDARD DEVIATION FROM THE MEAN OF THE DISTRIBUTION,
& AT LEAST 89% OF THE VALUES WILL LIE WITHIN
± THREE ( 3 ) TIMES STANDARD DEVIATION FROM THE MEAN."
ACCORDING TO THE STATISTICIANS, A SYMMETRICAL DISTRIBUTION WOULD HAVE
ABOUT 68%
OF THE VALUES WITHIN ± ONE ( 1 ) STANDARD DEVIATION FROM THE MEAN,
95% WITHIN TWO ( 2 ) AND 99% WITHIN THREE
( 3 ).
FOR QUALITY CONTROL AT SITE & FOR MIX DESIGN PURPOSE, USUALLY THE
VARIATION CURVE IS ASSUMED AS
SYMMETRICAL.
THE CHARACTERISTIC STRENGTH { FCK } MEANS THE VALUE OF THE STRENGTH OF
MATERIAL BELOW
WHICH NOT MORE THAN 5 % OF THE TEST RESULTS ARE EXPECTED TO FALL.
HENCE THE TARGET STRENGTH = FCK + K x S.
K = TOLERANCE FACTOR TAKEN AS 1.65,
AND S = STANDARD DEVIATION, TAKEN AS 4 N/MM2 FOR
GOOD QUALITY CONTROL AT SITE & 5 N/MM2 FOR AVERAGE QUALITY CONTROL.
IF THE ACTUAL STANDARD DEVIATION IS LESS THAN 5.6 N/MM2, IT WILL MEAN
A POOR SITE QUALITY CONTROL, ON THE OTHER
HAND A STANDARD DEVIATION OF AROUND 2.4 N/MM2 REFLECTS
EXCELLENT SITE QUALITY CONTROL. |