ASTM C133 determines the cold crushing strength and the modulus of rupture (MOR) of dried or fired refractory shapes of all types. This test method determines room temperature flexural strength in 3-point bending for cold modulus of rupture and compressive strength for cold crushing strength. This method is useful in research, design, manufacturing, and developing purchasing specifications

The relative ratio of the largest grain size to the smallest specimen dimension may significantly affect the numerical results. For example, smaller cut specimens containing large grains may present different results than the bricks from which they were cut. Under no circumstances should 6 by 1 by 1-in. (152 by 25 by 25-mm) specimens be prepared and tested for materials containing grains with a maximum grain dimension exceeding 0.25 in. (6.4 mm).