Rethinking HDPE Pipe

As we move further into the 21st century, engineers are thinking more critically than ever about the long-term service life of products that are being used to build and repair the nation's water, wastewater and storm sewer infrastructure.

Since it is the newest pipe product to be developed during the age of computers and automation, corrugated high-density polyethylene (HDPE) pipe has been studied and analyzed from both a structural and material standpoint arguably more than any competing material. Extensive testing and analysis on raw materials, finished product and installation integrity have increased the confidence level of the product and resulted in the rapid increase in usage of corrugated HDPE pipe for storm water, culverts and other drainage applications.

Now an independent third-party study has put a life expectancy on the corrugated high-density polyethylene (HDPE) pipe that manages storm water in municipalities all across North America.

And the number will surprise you.

Drexel University researchers, led by Dr. Grace Hsuan, have developed a new test protocol for corrugated HDPE pipe, utilizing the Rate Process Method. The results were published and presented at the recent international Plastics Pipes XIII conference, held in Washington, D.C.

According to Dr. Hsuan's data presented at the Plastics Pipes XIII conference, the pipe tested far surpassed the 100-year service requirement in the harsh environmental conditions of Florida. The results were dramatic enough to remove any doubt about the 100-year issue and re-set the baseline much higher for corrugated HDPE pipe.

Highlights of the data include:

• Service life of 572 years at 7.5-percent deflection (675-psi material stress in the pipe wall)
• Service life of 949 years at 6-percent deflection (600-psi material stress in the pipe wall)
• Service life of 2,893 years at 5-percent deflection (500-psi material stress in the pipe wall)

To predict the actual service life of the corrugated HDPE drainage pipe, a test protocol utilizing the Rate Process Method (RPM) was applied to the junction where the corrugation meets the pipe liner. The RPM is a method that is well established in the pressure pipe industry to predict the long-term performance of thermoplastic pipe by testing materials at higher temperatures and stresses to accelerate the tests, then using the resulting data to extrapolate to the anticipated service conditions.

Dr. Hsuan's paper states that the 100-year stress crack resistance (SCR) of corrugated HDPE pipes was evaluated using a 600-millimeter-diameter pipe. The SCR tests were performed on the finished pipe at the liner and junction locations. The notched constant ligament stress (NCLS) test (ASTM 2163) was used for the pipe liner assessment and a similar test applied to the junction.

"As we look to rebuild our underground infrastructure, engineers are paying more attention to the service life of their construction materials and structures," said E. Lyn Heying, project manager from MECO Engineering Company, Inc. of Hannibal, Mo. "The goal is to avoid plaguing future generations with the same dire infrastructure situation that our generation is currently facing."

The service life of most drainage pipes is estimated to be between 20 and 100 years, depending on the material. But how are these numbers derived? What is the basis for service life determination? How can we be assured that the estimated service life is accurate? What defines service life? Indeed, these are some of the critical questions that must be addressed when assessing service life of drainage products.

The process for long-term service life prediction is three-fold:

• The anticipated service conditions of the drainage pipe must be assessed, including such factors as environmental conditions, soil and traffic loads, and the resulting long-term stresses and strains evident in the pipe.
• The criteria for determination of service life, including the proper identification of anticipated failure modes, must be assessed.
• The capacity or ability of the material and the manufactured pipe product and system to withstand the identified service conditions must be evaluated.

The service life of many traditional pipe products (e.g. metal, concrete) is typically predicted by determining the amount of time it takes to deteriorate the inner wall to the point that the structure is no longer capable of withstanding its service loads.

For example, the primary failure mode of corrugated metal pipe is corrosion. So for corrugated metal pipe, the predicted service life is typically based on the amount of time, taking into account the expected service conditions (including flow rate and pH of the water and abrasiveness of the flow), for the waterway to deteriorate to a predetermined level where either the structure becomes too weak to carry the loads, or otherwise fails to serve its intended drainage purpose.

Likewise, reinforced concrete pipe service life is typically estimated based on the amount of time for the pipe wall to deteriorate down to the reinforcing steel. Once the metal reinforcement is exposed to air and water, corrosion begins, swelling the metal and breaking up the concrete encasement. Once again, environmental factors such as the effluent pH, flow rate and abrasiveness play a large factor in the estimated service life. As a result, proper identification and prediction of these conditions becomes paramount.

It should be noted that neither of these service life predictors takes into account other critical pipeline considerations such as joint performance, allowed leakage rates or non-structural cracking as part of their service life criteria, although they can have a dramatic affect during actual use.

The predicted design service life of corrugated HDPE pipe isn't quite as straightforward as the more traditional materials, primarily due to the fact plastics do not corrode (they're non-metallic) and the molecular structure of HDPE makes it inherently resistant to abrasion. The material just doesn't wear like traditional materials.

Prior to Dr. Hsuan's most recent study, the Florida Department of Transportation had done the most extensive research on the subject of service life of corrugated HDPE pipe. In establishing its protocol for 100-year service life of corrugated HDPE pipe, they agreed that the limiting factor for service life would not be based on abrasion or wear of the inner wall, as it was for the competing materials.

Rather, they identified cracking via the slow crack growth mechanism as the primary mode of failure and concern for HDPE pipe. This decision was based in part due to research conducted on some older pipe that had exhibited circumferential cracking in the smooth waterway liner of the pipe wall.

While the Florida DOT acknowledged that the liner is not a structural component of the overall pipe structure (it exists for hydraulic purposes only), it was their desire to have "crack-free" pipe, and thus establish a protocol that will ensure the material will not crack over its intended service life.

Since the slow-crack growth mechanism for failure has been well researched and documented in the pressure pipe industry, and an ASTM test method was already in place to predict service life based on this failure mechanism, it was relatively easy to adapt to corrugated HDPE drainage pipes. In 2002, material changes were incorporated into AASHTO M294, the national specification for state departments of transportation for corrugated HDPE pipe, which ensured the material's long-term resistance to the slow crack growth failure mechanism.

Today, independent research like the Drexel University study and others are taking the service life issue to unprecedented levels. Some may view this amount of testing as excessive or too costly. But since corrugated HDPE pipe is relatively new in storm sewer applications, the testing provides additional confidence and security in the integrity of the product.

The HDPE pipe industry, led by the Plastics Pipe Institute (PPI), will continue to monitor, report on and deliver information and facts on relevant research that can help specifying engineers be comfortable. Our goal is to deliver a product that will still be in the ground performing its intended purpose for generations to come. According to these results, it looks like that won't be a problem.

About PPI

Michael Pluimer carries a master's degree in Mechanical Engineering from the University of Minnesota, and is the Technical and Engineering manager for The Plastics Pipe Institute (PPI). The PPI is the major trade association representing all segments of the PE piping industry. Member companies share a common interest in broadening market opportunities that make effective use of plastic piping for water and gas distribution, sewer and wastewater, oil and gas production, industrial and mining uses, power and communications, duct, and irrigation. More information is available at www.plasticpipe.org.