Michael Pluimer, Rick Thomas
Papers # 2016 Berlin
A new test method was developed to evaluate the stress crack resistance of HDPE materials containing recycled content. The method involves testing un-notched specimens in an elevated temperature bath under a specified stress. The new test method was published in March of 2016 as ASTM F3181, Standard Test Method for the Un-Notched Constant Ligament Stress Crack Test (UCLS) for HDPE Materials Containing PostConsumer Recycled HDPE. Failure data generated from this test may be bi-directionally shifted to predict the service life of pipes manufactured with recycled materials. The service life prediction model has been validated on full-scale pipes tested in both field and lab environments.
Corrugated HDPE pipes manufactured with recycled materials have been used in the agricultural and land drainage industries for decades. However, most specifying agencies in the United States do not currently allow recycled materials to be used in HDPE pipes for highway applications due to the lack of research regarding the long-term service life of these pipes. Because of the economic and environmental benefits of incorporating recycled materials into corrugated HDPE pipes, specifying agencies are expressing an interest in these pipes for their drainage infrastructure. Two National Cooperative Highway Research Program (NCHRP) projects have been recently funded to assess the long-term service life of pipes manufactured with recycled content.
One of the primary factors governing the service life of corrugated HDPE pipes is Stage II stress cracking. To address this concern, corrugated pipe specifications contain a Notched Constant Ligament Stress (NCLS) test performance requirement. An additional concern for pipes manufactured with recycled materials is the presence of contaminants that may act as stress risers that could accelerate Stage II stress cracking. To evaluate this concern, an Unnotched Constant Ligament Stress (UCLS) test was developed. Since there is no notch in the test specimen, cracking originates from the presence of contaminants (e.g. sand, non-HDPE materials, glass, paper, etc.) that may exist in recycled materials. If the test is conducted at multiple temperatures and stresses in water, bi-directional shift factors may be applied to the failure times of specimens to predict the service life in typical field service conditions.
The service life prediction model has been evaluated on several full-scale corrugated HDPE test pipes manufactured with varying levels of recycled materials in a test installation in the field and in constant deflection test in the laboratory. Based on the fullscale pipe test results, the service life prediction model via the UCLS test was validated. An ASTM standard (ASTM F3181) for the UCLS test was recently published within ASTM F17.