what is fck in concrete?

what is fck in concrete?

It is comprehensive strength of concrete define as characteristics comprehensive strength of 150 mm cube size in 28 days. in designing process 5% of specimen tested are expected to fail if more than 5% of specimen is fail is said to be fail under the test result.

suppose we have 100 cube of 150 mm size of M20 grade 95 cube should be able to with stand a comprehensive load of 20 Newton per mm square if not more than 5% fail than grade of M20 of concrete design is said to be pass under the test result.so fck is characteristics of comprehensive strength of concrete like that for M20and M25 fck is 20N/mm2 and 25N/mm2 respectively.