Why do we do long-term creep testing?

Why do we do long-term creep testing?

The reason cured-in-place-pipe (CIPP) is used as a rehabilitation material is because the user is looking to extend the life of their pipe infrastructure. When plastic materials are stressed they behave differently than metals do. Under typical conditions, when a metal is stressed at a stress less than its yield strength, it will be able to withstand this force indefinitely.

Plastics don’t respond in the same manner. Even when stressed below their yield stress, plastic materials will creep and given sufficient time under load plastics can fail. It’s critical to understand this creep characteristic when designing a CIPP liner installation because we’re looking to extend the life of the deteriorated pipe by at least 50 years.

The flexural creep (ASTM D2990) test setup is very similar to the short-term flexural properties test ASTM D790. A rectangular specimen of the CIPP is placed in simple beam bending but instead of a continuously increasing load as is the case in ASTM D790, a static load is applied instead. Over time, the deflection of this statically loaded beam will increase. The rate of increase will depend on the level of load applied to the specimen.

Multiple specimens are tested simultaneously at a range of loads for up to 10,000 hours. The zero hour loading condition is effectively the same as the short-term flexural modulus. Creep testing is typically carried out at ambient conditions but can also be modified to include the impact of different environmental conditions.

This 10,000 hour deflection/time data is plotted as a log/log relationship and the data is then extrapolated to 50 years. This flexural modulus which is predicted at 50 years then typically forms the basis for the liner design.

So you can see the long term performance is based initially upon the short term modulus. This is why it is critical to establish the short term modulus for each installation since the extent of life extension is directly related to the initial modulus.

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Will you get the 50 year plus life you expect from your CIPP installation? A video inspection and thickness measurement of your completed installation will not tell you this. Each CIPP installation encounters variables that affect its initial cured-in-place mechanical properties. The present state-of-theart in mechanical inspection techniques uses a destructive flexural test to determine the initial flexural strength and modulus of each installation. This data, along with the final thickness is then used to confirm that the installation complies with the liner’s design, such as by F1216 Appendix X1 calculations. Normally variation can be expected between the results obtained for specimens from lab prepared samples and results obtained for specimens from field prepared samples. It is also not unusual for results obtained from different labs to present additional scatter. The sources of this latter variation have not been well-documented, and consequently, they create unnecessary doubt about the method used to determine these properties. This paper describes the ASTM D790 test method that is used to determine flexural strength and modulus; and examines the sources of variation that exist from specimen preparation techniques as opposed to CIPP sampling techniques. The D790 method provides guidance on test specimen selection based upon the material to be tested. CIPP is not a specific material configuration in the D790 method. As a result, it is left to the user (or test lab) to interpret which D790 material configuration most closely matches the CIPP material. This ambiguity in D790 can lead the tester to choose from a wide selection of different specimen types. This paper documents the results of experiments designed to quantify the effect of 1) specimen types and 2) testing directions on the flexural modulus and strength of CIPP field samples.