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.

Strength is one of the most widely used parameters for evaluating refractories. Strength can be measured at room temperature or at any temperature for which suitable test equipment exists.

Room temperature (cold) strength (CCS) measurements cannot be used directly to predict service performance, but do provide a good tool for evaluating the degree of bond formation during production. Room temperature testing also indicates the ability of brick to withstand handling and shipping without damage and to withstand abrasion and impact in relatively low-temperature applications. Strength testing at elevated temperatures is valuable in assessing the ability of a material to survive stresses caused by restrained thermal expansion, thermal shock and mechanical loading. Impact and abrasion resistance depend on material strength, as well. It is also common to use high temperature strength numbers to predict resistance to erosion and corrosion by metals and slags, although other properties, such as mineral composition and porosity, may be of equal or greater importance. Two types of strength tests are common. The modulus of rupture (MOR) test measures the flexural (transverse) breaking strength and the crushing strength test measures the compressive strength. Both tests have a version for cold (ambient) temperatures and high (service) temperatures. The cold crushing strength (CCS) and cold modulus of rupture tests are described by ASTM Method C133. In the cold crushing strength test, a sample 41/2” x 41/2” x 21/2” or 3” is loaded at a standard rate using a suitable mechanical testing machine. The load is applied vertically to the 41/2” x 41/2” face of the sample until failure. The crushing strength is calculated by dividing the maximum load supported by the sample over the surface area of the face which receives the load.

The equation for calculating is: S = W/A
where: S = cold crushing strength (CCS), psi (N/mm2)
W = maximum load, lbf (N)
A = cross-sectional area, in2 (mm2)

The test specimen, again a standard nine-inch straight, is placed on two bearing cylinders with the top and bottom (9” x 41/2”) faces oriented horizontally. The specimen is broken at mid-span in flexure at a standard loading rate.

The modulus of rupture (MOR) is calculated using the following equation:

MOR = 3PL/2bd2
where: MOR = modulus of rupture, psi (N/mm2)
P = load at rupture, lbf (N)
L = span between supports, in (mm)
b = breadth or width of sample, in (mm)
d = depth or thickness of sample, in (mm)
Strength values vary widely among classes of refractories and even among refractories of the same classification.
Typical values for a number of refractories are given in Table 3.2. Variables which significantly affect cold strength of refractories are bond chemistry, firing temperature, porosity, and the strength and sizing of the aggregate.