CONTROL OF CONCRETE CUBE STRENGTH AT SITE:
AS PER IS : 456 -2000


  PROGRAM INFO / LIMITATIONS.
( Actual program is available in Super Civil CD )

This program allows maximum of  30 records. 
Empty record between successive records are not permitted.
IS 456 - 2000, requires minimum of 30 records { average strength } for arriving at     suitable value of standard deviation at site for mix design.
Each record consists of average of 3 cube tests.
A user can also directly input average strength, if available instead of 3 individual     test results.
Some relevant useful information  are as under. 


CONCEPT OF STANDARD DEVIATION:
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.

VALUE OF TOLERANCE FACTOR { K }
ACCEPTED PROPORTION OF LOWER RESULTS K VALUE
1 IN 2 0
1 IN 5 0.84
1 IN 16 1.00
1 IN 10 1.28
1 IN 15 1.50
1 IN 20 1.65
1 IN 40 1.86
1 IN 100 2.33
1 IN 200 2.58