# Statistical Concepts of Mix Design as per IS456:2000 Reading time: 1 minute

Mix design is using specific proportions of ingredients such as aggregates, water, admixtures and cement to produce a concrete suited for a particular job or project. Here you will find the concept of mix design as per IS456:2000.

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.

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

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.

Individual test results in N/mm2 must be greater than or equal to
( fck – 3 ) N/mm2