"Properties of Reinforced Concrete, Compressive Strength. The values obtained for the compressive strength of concretes, f 'c, is determined by testing to failure 28-day-old cubes, 150/ 100 mm on each side or by 150x300/ 100x200 mm cylinder. The testing of 150x300 mm cylinders provides compressive strengths equal to about 80% of the values determined with the cubes, i.e., the measured values of compressive strength using cubes are equal to 1.2 x the measured values of compressive strength determined with cylinders. The stress–strain curves of Fig. 1 represent the results obtained from compression tests of sets of 28-day-old standard cylinders of varying strengths.