Compressive Strength of Concrete Cubes Lab Test Procedure Overall Strength of a structure such as flexural resistance and abrasion directly depends upon the compressive strength of concrete. According to Wikipedia, Compressive Strength of concrete is defined as the Characteristic strength of 150mm size concrete cubes tested at 28 days. crushing load is noted to calculate crushing strength of concrete according to IS 516-1959. The measuring strength of specimen is calculated by dividing the maximum load applied to the specimen during the test by the cross section area.