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Crush Strength and Flexibility of Thermoplastic Piping

The applicability of the term "crush strength" to piping made of thermoplastic materials is a cause for concern to users, producers and installers when these products are subjected to external loads such as those encountered in underground installations. This creates a need to examine (1) the meaning of the term crush strength, (2) how it is measured, and (3) the fundamental properties of thermoplastics.

The word strength means the stress at which a test specimen ruptures or breaks or cracks. Thus, "crush strength" means the stress applied by a crush load that cracks or breaks the pipe test specimen. The term "crush strength" is also often used loosely to mean the crushing stress that will crack or break a standard test piece.

Crush strength is usually measured by placing a piece of pipe horizontally between two parallel plates or blocks, loading the assembly so that the pipe is compressed radially 180 degrees apart, and measuring the load required to crack or break the pipe. When pipes made of thermoplastic or other flexible materials are subjected to this test, fracture does not occur and therefore, there is no end point to the test. In other words, this test method is only applicable to rigid materials that do give a definite end point crack or break at some load and point of deflection, thus giving a limiting load bearing capacity.

Thermoplastic pipe behaves as a flexible conduit in the crush strength test described above. This means that the pipe can be flattened to a considerable extent without being damaged. This is a measure of flexibility or, in this case, the ability to flex under externally applied radial loads without detracting from the integrity of the pipe. For this reason, the ASTM standards for many thermoplastic pipes such as D1785 (PVC pressure pipe), D2241 (PVC pressure pipe), D3033 (PVC sewer pipe), D3034 (PVC sewer pipe), and D2751 (ABS sewer pipe) contain a flattening test that.

In summary, the crush strength methods are not applicable to thermoplastic pipes because of the high degree of flexibility of this type of material, or in the vernacular "the nature of the beasts". Therefore, the normally used term "crushing strength" is not meaningful for thermoplastic pipes. The pertinent test methods for flexible pipes are the flattening method and the pipe stiffness method. Thus, the terminology most applicable to thermoplastic pipe would be flattening, flexibility, or pipe stiffness.

Readers interested in the basic concepts of loads on rigid and flexible pipes can find more detail in chapters 24 and 25 in the book by M.G. Spangler, "Soil Engineering," 1960, International Textbook Company, Scranton, Pennsylvania.

NOTE:This PPFA User Bulletin is designed to provide guidance in achieving the efficient, effective and proper use of plastic pipe. The suggestions and advice contained in this Bulletin are offered merely to provide plastic pipe users with a general frame of reference. Because specific situations may, and often do require special treatment, the suggestions and advice are obviously not universally applicable. Therefore, the user should carefully assess the requirements of his specific situation before making practical application of anything contained in this publication.