CONCEPT:\({f_m} = {f_{ck}} + 1.65\;\sigma\)where fm is target MEAN compressive strengthfck is the characteristics strength of concreteσ is the standard deviationThe standard DEVIATION for different GRADE of concrete as per Clause 9.2.4.2 IS:456-2000Grade of ConcreteAssumed Standard Deviation (N/mm2)M10, M153.5M20, M254.0M30, M35, M40, M45, M505.0Calculation:Given: fck = 20 N/mm2, fm = 27 N/mm2\(\therefore {\rm{\;\sigma }} = \frac{{{{\rm{f}}_{\rm{m}}} - {{\rm{f}}_{{\rm{ck}}}}}}{{1.65}} = \frac{{27 - 20}}{{1.65}} = 4.24\)