Reprinted from the American Concrete Pipe Association's "DISPATCH" titled "HDPE Pipe Material Requirements – Moving Forward?"
The American Concrete Pipe Association (ACPA) has worked for over 90 years to ensure that concrete pipe manufacturers supply a high quality product that will serve America's infrastructure today and for years to come. During this time we have consistently improved the quality of our product in conjunction with the AASHTO and ASTM material and testing standards that govern it. ACPA believes in, and promotes the long-standing AASHTO policy of allowing industry to be involved with the development of standards that affect their product.
From another perspective, it would appear the ACPA has worked diligently to make their product more cheaply by reducing cement content and increasing fly-ash content and by promoting non-reinforced concrete pipe for applications where reinforced pipe has been traditionally used. They have reduced the number of samples required for testing in ASTM C 655, Specification for Reinforced D-Load Culvert, Storm Drain, and Sewer Pipe . They have promoted using SIDD to give exact strength for loads, reducing the factor of safety. We are not aware of any significant research over the past 10 years to improve their products. Rather, they have chosen to attack other products and other research. History is a great teacher; this is the same approach taken by the clay pipe industry against the PVC sewer pipe industry 30 years ago and today PVC dominates that industry and application.
Recent research by the National Cooperative Highway Research Program (NCHRP) has found that not only does high density polyethylene (HDPE) pipe produced in this country lack a reliable test to assure it does not crack prematurely in the field, but that the industry has been extremely complacent in its efforts to remedy the problem. AASHTO, through its relationship with the Corrugated Polyethylene Pipe Association (CPPA), now a part of Plastic Pipe Institute (PPI) has kept the HDPE pipe industry actively involved in the process of trying to rectify this situation. However, CPPA has found it more convenient to blame AASHTO and NCHRP for HDPE pipe's failings than to improve the product.ADS Response
Members of the NCHRP panel, not CPPA or PPI , generated the negative comments regarding the failure of the data to support the recommations of the report. Only one of the nine panel members had an alliance with the CPPA. Four of the panel members recommed against the report's release in its current form.
In support of AASHTO and NCHRP, we would like to provide you with a little history concerning NCHRP 04-24 ‘HDPE Pipe Material Specification and Design Requirements' and its efforts to develop an adequate test for the determination of environmental stress crack resistance (ESCR) in HDPE pipe.
NCHRP Report Recomms Developing a Reliable Slow-Crack Growth Test for HDPE Pipe
In 1995 NCHRP 20-7, Task 68, was commissioned to review the difference in material requirements between the AASHTO design standards and material standards. In 1998 the corresponding NCHRP report recommed that the Hydrostatic Design Basis requirement be removed from Section 18 of the AASHTO Standard Specifications for Highway Bridges, but that a "reliable post-production slow-crack growth test" be initiated. The report went on to say "work on this new test should proceed rapidly." High Density Polyethylene (HDPE) Pipe producers were in full support of this action."
A more meaningful, reliable ESCR test has been a major objective of all segments of the polyethylene industry for years. An implication to the ACPA's statement is that the test described in NCHRP 429 is "the answer." In reality, this is inaccurate. Since this test's inception for geomembranes, it has gone through 9 round-robin iterations to develop the necessary precision and bias capabilities. It still does not meet ASTM standards. The researchers on NCHRP 429 did not establish the repeatability or reproducibility of the test on corrugated pipe resins either. The pipe industry conducted the only round-robin testing on this protocol. Unfortunately, it also showed variances similar to ASTM D 1693. This is the reason PPI is continuing the industry's efforts to develop a more accurate test regimen.
NCHRP Project to Develop a Quality Control Test
As a result of NCHRP Project 20-7, Task 68, and reported numerous slow-crack growth problems in installed HDPE pipe, additional public funds were appropriated for research into the slow-crack growth problem in HDPE gravity pipe. In early 1997, NCHRP Project 04-24 HDPE Pipe Material Specification and Design Requirements was initiated to develop a quality control test that could replace the Hydrostatic Design Basis (HDB) test and the environmental stress-crack resistance (ESCR) test that currently exists in the AASHTO standards.
Since its inception into ASTM D 1693 in 1959, the ESCR test has been fraught with variances up to 290%. Although the unreliability of the test was known by the HDPE pipe producers, it was not until public money was invested in NCHRP 04-24 to "develop a more practical and reliable test protocol…to qualify resins with acceptable resistance to cracking under sustained loading" that any research effort was put into effect to resolve the problem .
A round-robin test program conducted immediately after the release of the NCHRP 04-24 report found a lab-to-lab variability for the SP-NCTL test, as conducted in the NCHRP 04-24 of nearly 300%; no better than the existing ESCR test.
CPPA Contests NCHRP 04-24's Recommations
In March of 1999 the NCHRP Report 04-24 recommed to AASHTO that the Single-Point Notched Constant Tensile Load (SP-NCTL) test per ASTM D 5397 be used with an applied stress of 15% of yield stress with a failure time of 24 hours. " CPPA countered this recommation with a recommation of their own, requesting that the SP-NCTL test be used with 10% of yield stress for 10 hours .
The NCHRP 429 (04-24) report had inadequate data to support the recommations in the report. The CPPA recommations were based on the previous report funded by PPI done by Dr. Hsuan titled, "A Stress Crack Resistance Method for Evaluation of Polyethylene Materials Inted for Pipe Applications" published in 1997. The 10% of yield strength recommation proposed to ASTM & AASHTO was based on a different, more severe, sample preparation and test protocol than 04-24. It is inaccurate to compare the values. The ACPA has failed to research this adequately or chooses to ignore the facts. Profile design, as stated in NCHRP 20-7, Task 68, is more important than the resin issue. Proper wall design may make this issue moot.
At both the Bridge Engineers' and the Materials Engineers' meetings, AASHTO officials refused to accept CPPA's recommation. This was a logical choice when you consider:
NCHRP 04-24 was initiated by AASHTO, and federally funded in an effort to "develop a more practical and reliable test protocol."
CPPA's recommations are an effort to avoid any changes to the product they currently manufacture.
The recommations of NCHRP 04-24 were based on field and laboratory research of HDPE pipe failing from environmental stress-crack resistance.
The recommations of CPPA are not supported by field or laboratory research .
The current ESCR test is equivalent to the SP-NCTL test being run at 15% for 14 hours.
CPPA's recommation of 10% for 10 hours would be a reduction in the current ESCR criteria.
ADS ResponseAs mentioned previously, there is no correlation between the current 15%/24hour recommation and the earlier 10%/10hour value due to the significant difference in sample preparation.
Drexel University's Geosynthetic Research Institute was chosen to perform research for NCHRP 04-24 based on the research they had done for the Plastic Pipe Institute using ASTM D 5397 and published in "A Stress Crack Resistance Method for Evaluation of Polyethylene Materials Inted for Pipe Applications." In addition, the Executive Director of the Corrugated Polyethylene Pipe Association, as well as the former Executive Director of the Plastic Pipe Institute were appointed to the NCHRP 04-24 Panel to ensure that the industry was well represented. At the time, this was a major change in NCHRP policy, since the panels generally consist of non-biased DOT engineers and members of academia, and industry representation is not usually permitted.
Not true. Industry is represented when expertise is there. Bias of industry members is understood and not a criteria to not select a panel member.
Basis of NCHRP 04-24 Recommations
To their credit, the NCHRP researchers did a thorough job and took twice the amount of field samples as requested by the NCHRP Panel. Laboratory tests were performed on the samples to see their time to failure under the SP-NCTL test conditions.
ADS ResponseThe majority (58%) of the samples came from a pipe manufacturing technology not used for about 4 years in the United States. There were too few sites of any single profile or manufacturer to calibrate the methodology. There was no check of precision and bias.
The Single-Point Notched Constant Tensile Load Test already exists for polyethylene geomembranes and it is recommed that they be tested at 30% yield stress for 100 hours. The difficult part of the work involved in NCHRP 04-24 was establishing test criteria for high-density polyethylene pipe. All samples taken from pipe that had failed in the field were tested using the SP-NCTL test to determine their time of failure at various percentages of yield stress under test conditions.
The recommed testing requirement of 15% of yield stress for 24 hours was taken as a failure time plus two standard deviations of a specimen that had experienced slow-crack growth problems in the field. The researchers generously developed a pass/fail requirement for a laboratory test based on specimens that had failed in the field. Amazingly enough, even the development of such a low standard of quality requires the Corrugated Polyethylene Pipe Industry raise their current standards.
CPPA Hinders the Development of a QA/QC Test
NCHRP 04-24 took to task a problem that the HDPE pipe suppliers would not address on their own. NCHRP staff went out of their way to ensure that the HDPE pipe industry was involved with the research. The NCHRP researchers did their best to not unduly burden the HDPE pipe suppliers with a difficult test to pass. The result of all this? Instead of wanting to improve their standards for the good of the public, CPPA attempts to lower their standards to avoid any changes to the product they currently manufacture.
On the contrary, the CPPA has been very active in developing tighter standards and promoting QA/QC. For example,
The resin standard in M-294 is much more restrictive than the standard from just 5 years ago.CPPA proposed adding a QA/QC section to the appix of M-294 in 1998.
The PPI sponsored research into the use of the SP-NCTL test before the NCHRP 04-24projectwasinitiated.
Perhaps it is this proactive industry effort that continues to propel the corrugated polyethylene pipe industry forward. This same type of effort will undoubtedly generate further improvements in the future.
AASHTO is currently considering the NCHRP recommations and will more than likely vote on them at their Bridge and their Materials Subcommittee meetings in the year 2000. Meanwhile, CPPA has recommed TRB and AASHTO withhold any decisions until an industry-sponsored test protocol has been accepted and validated.
In 1996, NCHRP research recommed that "work on this new test (slow-crack growth test) should proceed rapidly." The Hydrostatic Design Basis test was removed prematurely in anticipation of this new test being developed in a timely fashion. For almost four years, corrugated polyethylene drainage pipe has been produced without a reliable slow-crack growth test while the NCHRP researchers painstakingly worked to develop a field verified test that was both accurate and fair. Can we really afford to allow another four years of problems to go by while the Corrugated Polyethylene Pipe Association works to disprove the facts that they have been aware of for years?"ADS Conclusion
What research has the ACPA industry funded in the past ten years? It appears that the ACPA is focussed on negative attacks without supporting research. Is this what the AASHTO Subcommittee on Bridges and Structures or the Subcommittee on Materials and Tests wants?