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80 Current AASHTO design methodology assumes that the long-term properties (i.e., modulus of elasticity and tensile strength) of HDPE are stress independent. Preliminary evaluations of data compiled from NCHRP Projects 4-32 and 4-39 indicate that this may not be the case. Specifically, it appears that the long-term properties are dependent on the initial stress in the pipe wall. This is very important to under- stand, as it affects the long-term structural design criteria for these pipes and, if proven true, could result in revisions to cover height or other design requirements for corrugated thermoplastic pipes manufactured with both virgin and recycled HDPE materials. To investigate this further, it is sug- gested to conduct a series of accelerated creep tests via both conventional methods and the stepped isothermal method at various initial stress conditions. The testing should be con- ducted on pipes manufactured from both virgin and recycled materials, including all the pipes investigated in this research project. The stepped isothermal method could also be used to investigate compliance with AASHTO bridge specifications for long-term strength and modulus requirements, which are rarely measured and confirmed. A second suggested area for future research is a more thor- ough investigation of the effects of various sizes of contami- nants on stress-crack initiation and propagation in thicker pipe walls. Preliminary research was conducted during this project indicating that the stress intensity factor increases in thicker walls, but more data are necessary to generate a sta- tistically valid conclusion. A thorough study to evaluate the relationship between contaminant size and quantity, speci- men thickness, and UCLS failure time would be a worthwhile pursuit. Finally, since the outcome of this project was a performance- based specification rather than a prescriptive specification, it did not address quality control criteria that should be consid- ered by pipe manufacturers. It would be helpful for both man- ufacturers and specifiers to identify some best management practices for manufacturing pipes with recycled materials. To accomplish this, it is suggested to work with pipe manu- facturers and the National Transportation Product Evaluation Program to obtain and analyze quality control and quality assurance data and to establish minimum sampling proce- dures and frequencies for testing. C H A P T E R 5 Suggestions for Future Research
