Characteristic strength
This means that value of the strength of concrete below which, not more than 5 percent of the test results are expected to fall.
Target mean strength
ft = fck + K.s
where
ft = target mean strength
fck = characteristic strength
K = a statistical constant, depending on the definition of fck and is derived from the mathematics of Normal Distribution
s = standard deviation
The value of K is equal to 1.65 where not more than 5% of test results are expected to fall below the characteristic strength
=>ft = fck + 1.65 s
Acceptance Criteria (IS 456 : 2000)
Compressive strength
Both the following conditions must be met :
a) The mean strength of any four consecutive test results compiles with the appropriate limits in Table 3.
For M15 grade
Mean of 4 consecutive results in N/mm2 must be greater than or equal to
fck + 0.825 x SD or
fck + 3 N/mm2 , whichever is greater
For M20 or above grade
Mean of the group of 4 consecutive results in N/mm2 must be greater than or equal to
fck + 0.825 x SD or
fck + 4 N/mm2 , whichever is greater
Acceptance Criteria
b) Any individual test result complies with the appropriate limits in Table 3.
For M15 grade
Individual test results in N/mm2 must be greater than or equal to
( fck – 3 ) N/mm2
For M20 or above grade
Individual test results in N/mm2 must be greater than or equal to
( fck – 4 ) N/mm2
Acceptance Criteria
Flexural strength
Both the following conditions must be met :
a) The mean strength of any four consecutive test results exceeds the specified characteristic compressive strength by at least 0.3 N/mm2
b) The strength determined from any test result is not less than the specified characteristic strength less 0.3 N/mm2