Factors affecting stress crack resistance of corrugated high-density polyethylene pipe

In the AASHTO M294 specification for high-density polyethylene corrugated pipe, the stress crack resistance (SCR), property is determined on the basis of the performance of virgin pipe resins. The specification requires pipe resins to have a failure time greater than 24 h by using the notched constant ligament stress test. However, SCR of the finished pipe can be significantly different than that of the corresponding resin because of the effects of additives such as carbon black, regrind, and manufacturing processing. As part of the NCHRP 4-26 project, the effects of carbon black, percentage of regrind material, and pipe processing on SCR were evaluated. Twenty-four pipes and 13 resins from five manufacturers were tested. For the majority of resins, carbon black decreased SCR of the resin. The effect of regrind on SCR varied substantially from pipe to pipe; however, increasing the regrind from 10% to 20% did not show significant changes. The manufacturing process was found to have the greatest effect on the reduction of SCR. The influencing factors and 95% confidence interval were established between pipe resin and pipe plaque and between pipe liner and pipe plaque. The proposed minimum failure times for SCR of virgin resin, pipe plaque, and pipe liner are 33, 24, and 18 h, respectively. The data indicate that pipes with qualified resin may not necessarily pass the pipe liner proposed value. Thus, an SCR specification on pipe liner is more critical than a virgin resin.