Concept: Characteristic strength of concrete (fck): The compressive strength of concrete is given in terms of the characteristic compressive strength of 150 mm size cubes tested after 28 days The characteristics strength is defined as the strength of concrete below which not more than 5% of the test results are expected to fall The test results of the sample shall be the average of the strength of three specimens. The individual variation should not be more than ±15 percent of the average. If more, the test results of the sample are invalid. Flexural strength of concrete: The theoretical maximum flexural tensile stress occurring in the extreme fibers of RC beam, which causes cracking is referred to as the modulus of rupture (fCr).