The Role of Flexural Strength in CIPP Liner Design and the Correct Means to Estimate its Long-Term Values

Abstract

Understanding time-dependent material properties such as flexural modulus and flexural strength is essential for design and to ensure the longevity of CIPP liners. Flexural strength adjustments over time are commonly estimated by many designers as being proportional to creep retention of the flexural modulus in a constant load creep retention test such as that detailed in ASTM D2990. These tests are constant load 3-point bend tests that determine apparent flexural modulus of elasticity versus time in the circumferential (hoop) direction at representative working stress levels causing secondary creep deformation, but do not take the samples to failure. To use these tests to estimate flexural strength reduction is and has long been understood to be technically incorrect, as it was not a concern in most circular liners with limited imperfections. In non-circular host pipes, however, and in circular pipe with large imperfections, long-term flexural strength can govern many designs, and this strength reduction must be estimated by constant load 3-point bend tests over a range of higher stress levels producing flexural creep rupture with time (i.e. the samples must be taken to failure). This paper presents an overview of the appropriate testing methods used on a number of polyester resins common in use in North American market and its impact on design from a number of actual projects, largely ASCE MOP 145 design but also for severely deteriorated circular designs at the limits of ASTM F1216 applicable design methods.