Material Properties of Flexible Pipe

When the AASHTO LRFD Bridge Design Specifications Section 12 were written, all buried structures were grouped together in a common document. This is convenient for the designer who is charged with evaluating and specifying buried pipe and conduit that is used for culverts, drainage, and sewers. This article focuses on flexible pipe structures.

Considering engineering mechanics of solids, there are two general classes of material used to fabricate flexible pipe. Materials that are classified as linear elastic include steel (AASHTO M36, M245) and aluminum (AASHTO M196)  ipe of various corrugation depths and steelreinforced thermoplastic pipe (AASHTO MP20-10). Materials that are classified as non-linear include corrugated, solidwall and profile-wall high-density polyethylene or HDPE (AASHTO M294, and ASTM F714-08, respectively), profilewall and solid-wall polyvinyl chloride or PVC (AASHTO M304 and AASHTO M278) and glass-reinforced polymer mortar (ASTM D3262 and D3754). Table 1 lists these flexible pipe types and compares properties.

As a linear elastic material, metals behave predictably within typical service temperatures and maintain their mechanical properties under constant load (Figures 1 and 2). Thermoplastic materials exhibit non-linear elastic behavior and are sensitive to rate of strain, temperature, and creep. Creep is a visco-elastic phenomenon where the strain in a material continues to increase with time when subjected to a constant load (Figure 3). Although glass fibers show linear elastic behavior in a small displacement field (small strain), they do, along with most polymers used to fabricate plastic pipe, behave with visco-elastic characteristics and exhibit creep under long-term loads.

 

 

 

 

 

 

 

Figure 1: Constant load applied to a material

 

 

 

 

 

 

 

 

Figure 2: Constant strain response under constant load  

 

 

 

 

 

 

 

 

 

 

Figure 3: Increasing strain response under constant load. Example shows slight recovery after load is removed.

 

The industry recognizes the difference between short-term and longterm plastic pipe properties and sets elastic modulus properties accordingly. Depending on the chemistry of the thermoplastic material, the ratio of short-term to long-term elastic modulus ranges between 3 and 5.

 

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