Technical Blog

Welcome to John’s Blog. Answers to frequently asked questions are periodically posted here. The objective is to share information about PVC pipe with readers as well as with utilities, design engineers and pipe installers. The blog provides the latest information on PVC pipe design, installation, and application for water and wastewater infrastructure projects.

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John Houle: Senior Technical Consultant, PVC Pipe Industry

John Houle holds a Master’s Degree in Civil Engineering from the University of Missouri and an MBA from the University of Oregon. He has more than 25 years of experience in the plastic pipe industry in applications engineering, market development, forensic analysis, technical writing, and standards development.

PVC Pipe: “Loss of Strength” with Time? – No!

Posted By John Houle on Oct 22, 2015

Plastics have a material property that sets them apart from the traditional materials that most engineers studied in school. For traditional materials, there is no distinction between short-term loading and long-term loading – the material responds the same in either case. For plastics, however, there is a significant difference: plastics can handle much higher short-term loads than long-term.

A quick example: AWWA C900 DR18 pipe has a long-term rating of 235 psi, but its short-term rating jumps up to 376 psi. This is equivalent to hoop stresses of 2000 psi and 3200 psi, respectively. For this example, the short-term rating is 60% higher than the long-term rating.

When a log-log plot is made of PVC pipe stress at failure vs. time of load application, the failure points will lie along a line that slopes downward as time increases. This is logical, given the discussion above.

Proponents of non-plastic materials have chosen to use this downward-sloping line as evidence that PVC pipe “loses strength with time” or “degrades over time.” However, this is not the case – the line’s slope merely proves that PVC pipe can withstand higher short-term stresses than long-term stresses.

To read more, click here for the Tech Brief on this subject.