Creep! No Not The Perv On The Park Bench

Creep for our discussion here is measuring a materials deformation over time and not the creep on the end of the park bench. When the material is under constant strain such as the weight of soil, water, or other 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 slide it through the key ring between both ends. Now lift each end of the straw and depending on how heavy your key ring is, it will sag (deflect) or bend and fail. You’ve just observed creep in real time.

When plastics are put under constant strain, the plastic pipe begins to deform or change shape over time and while we know all materials will change shape over time, we want to predict how long it will take before we can no longer rely on the pipe to be serviceable or as in the case of the straw, how long it will take to fail. We are talking about drain, waste, vent and sewer piping and we know that all materials used in a plumbing system are designed for a 50 year design life. So for our lining materials, our goal is to provide a material 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. Unless you’re an engineering whizz kid who wants the formulas and details, I’ll summarize in laymen’s terms here what is involved in testing cured in place pipe samples. Initial samples of the plastic material is prepared in prescribed sizes, put under stress and strain loads, with measurements taken of the deflection of the material. Some samples are taken 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.

My description is a very brief summary of all of the factors taken into consideration, but for our purposes and for your explanation to your customers installing a cured in place pipe product, this explanation may be more than they need or want to know, but you will know. The more you know about the product your providing your customer, the more value you carry as the tech who has the knowledge a customer is looking for and the more likely they will stick with you in the future.