Plastic Pipe for Highway Construction-Phase 2

Culvert pipes used for highway cross drains have historically been concrete or corrugated metal (steel and aluminum). However, over recent years, the plastic pipe industries have been pushing state transportation agencies, including the Alabama Department of Transportation (ALDOT), to give equal consideration to plastic pipes. In 2006 the political emphasis culminated in regulation that broadened the types of culvert materials that should be considered for drainage applications on federal-aided highway projects. The regulation required that "equal consideration" be given when specifying alternate pipe materials--including plastic and corrugated aluminum--that are "judged to be of satisfactory quality and equally acceptable on the basis of engineering and economic analysis." Although this requirement was relaxed through the "Moving Ahead for Progress in the 21st Century Act" (MAP-21) in 2012, the plastic pipes industries continue their push to expand their market to include widespread use of plastic pipes for cross drain applications. Although the most common types of plastic pipes, namely profiled wall high density polyethylene (HDPE) and polyvinylchloride (PVC) pipes, have been developed specifically for highway drainage applications and integrated into American Association of State Highway and Transportation Officials (AASHTO) standard specifications, there are still many concerns, and confidence in their use for cross-drain applications remains low. The most prominent of these concerns revolve around the long-term integrity of plastic pipes and their joints. Plastics such as HDPE and PVC are viscoelastic materials, and by definition, creep under loading, and their rigidity characteristics change considerably (Gabriel and Goddard 1999; Goddard 1994). Table 1 provides material stiffness and strength (modulus of elasticity and yield strength) for plastic pipes as defined by AASHTO and the Plastics Pipe Institute (AASHTO 2009, AASHTO 2010, PPI 2003), along with comparable information for concrete and metal. The drastic stiffness change over time, which is used in standard design calculations, can be noted, with the modulus of polyethylene dropping by 80% and the modulus of polyvinylchloride dropping by 65%; likewise, the yield stress used in design calculations for polyethylene and polyvinylchloride drops by 70% and 47%, respectively. As discussed thoroughly in the report "Evaluation of HDPE and PVC Pipes Used for Cross-drains in Highway Construction" from the initial plastic pipe project, these properties are merely estimates based upon accelerated test methods and modeling that were adopted from the gas pressure pipe industry (PPI 2003, McGrath et al. 2009, Stuart et al. 2011). Although material quality control has improved over recent years, it has also been demonstrated that the resins used to manufacture plastic pipes can vary significantly between pipe producers. The long-term stability concern is further complicated by the fact that, unlike concrete pipes, plastic pipes are flexible-walled conduits 10 whose strength and structural integrity relies upon the arching effect provided by the surrounding backfill. The arching effectiveness, along with tendencies for the soil and backfill to also creep through time, is highly dependent upon the backfill and compaction quality during installation.