If you’re in the lining business, “creep” is a term that is important to be familiar with. It’s the measure of a material’s deformation over time. When the material is under constant strain such as the weight of soil, water, or another material pressing against it, over time the material will change shape.

Creep applies to all materials under loading including concrete, metal, wood, and plastics, as well as hybrid materials. To illustrate creep, take a drinking straw. Thread it through your car key ring and lift each end of the straw. Depending on how heavy your key ring is, the straw will sag (deflect) or bend and fail. You’ve just observed creep in real time.

When plastics are put under constant strain, the plastic begins to deform or change shape over time. While we know all materials will change shape over time, in the case of pipe we want to predict how long it will take before we can no longer rely on it to be serviceable. We know that all materials used in a plumbing system are designed for a 50-year design life. So in the CIPP industry, the goal is to provide lining materials that will resist the failure for at least 50 years.

Since it’s not practical to build a product, put it in the ground for 50 years, then market it, we need to consider using an accelerated test that will predict failure in a shorter period of time. Depending on the materials being tested, there is a protocol developed and documented through the ASTM organization. For plastics, that standard is ASTM D2990.

To summarize the testing process for CIPP samples: Initial samples of the plastic material are prepared in prescribed sizes, put under stress and strain loads, with measurements taken of the deflection of the material. Some samples are taken all the way to failure, measuring the relationship of the deflection to the increasing pressures. Other samples are put under a constant load for up to 10,000 hours with measurements of deflection in relation to time. They measure the initial flexural modulus to the flexural modulus at the end of the test. Using the results of these testing methods, a laboratory can predict the point in time when the samples will fail in real life conditions in the field. If the samples provided have a predicted failure of less than 50 years, for our purposes, the product won’t make it to market.

This is a very brief summary of all of the factors taken into consideration, and for your CIPP customers it may still be more than they need or want to know. But you will know, and the more you know about the product you’re providing, the more value you carry as a tech. You’ll have the knowledge a customer is looking for and there will be a better chance that they stick with your services in the future.

About the author
John Heisler is the owner of Pipe Lining Supply and Quik-Lining Systems Inc. He has 20 years of experience in the CIPP lining industry and over 40 years in the underground construction industry.