See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/283513305 Assessment of HDPE corrugated pipes used in mining applications and their compliance with specifications Conference Paper · September 2015 CITATIONS READS 0 596 3 authors, including: Mario Paredes TRI Environmental Inc. 22 PUBLICATIONS 141 CITATIONS SEE PROFILE All content following this page was uploaded by Mario Paredes on 06 November 2015. The user has requested enhancement of the downloaded file. Proceedings of Heap Leach Solutions, 2015 September 14-16, 2015, Reno, Nevada, USA Published by InfoMine, © 2015 InfoMine, ISBN: 978-0-9917905-8-6 Assessment of HDPE corrugated pipes used in mining applications and their compliance with specifications Richard W. Thomas, TRI/Environmental, Inc., USA Mario Paredes, PE TRI/Environmental Inc., USA David Cuttino, TRI/Environmental, Inc., USA Abstract Samples of HDPE Corrugated Pipe were collected from North America, South America, and the United Kingdom. Each of the dozen samples was characterized by a series of resin and pipe tests. The results were compared to the requirements found in ASTM Standard Specifications F2986, “Standard Specification for Corrugated Polyethylene Pipe and Fittings for Mine Leachate Applications,” and F2987, “Standard Specification for Corrugated Polyethylene Pipe and Fittings for Mine Heap Leach Aeration Applications.” Only two of the twelve samples met all of the requirements found in these ASTM standards. Additionally, five samples contained significant amounts of polypropylene, which indicates the presence of post-consumer recycled HDPE and two of these also had about 10% added filler. Of the twelve samples, 58% failed to achieve the minimum resistance to cracking criteria of 24 hours by the NCLS stress crack test (ASTM F2136). This paper presents all the results obtained and discusses the potential implications for the long-term performance of the different pipe products in mining applications, particularly in heap leach pads. Introduction Corrugated HDPE pipe plays an important role in heap leach pad mining applications. Since it may experience some loads or strains from the very thick ore overburden, and exposure to harsh chemicals like concentrated sulfuric acid, it should be made with consistent high quality to perform as intended. In an 1 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA effort to ensure quality pipe is used in mining applications, ASTM International published two standard specifications in 2012. The first (F2986) is for “Corrugated Polyethylene Pipe and Fittings for Mine Leachate Applications.” The second one (F2987) is for “Corrugated Polyethylene Pipe and Fittings for Mine Heap Leach Aeration Applications.” In an effort to determine if these specifications were being followed and to survey the quality of pipe used in mining, one dozen samples of corrugated pipe were obtained in the spring of 2015. Pipes were collected from manufacturers in North America, South America, and the UK. These were all new pipes headed to the field for installation. The purpose of this study was to compare the properties of these production pipes to the requirements found in the new ASTM standards. Some requirements for the two ASTM specifications are found in Table 1. Table 1: PE Compound requirements of two ASTM standard specifications Requirement Property Test method Recycled allowed ASTM F2986 ASTM F2987 No No density ASTM D1505 >0.947 g/cm3 >0.947 g/cm3 Melt index ASTM D1238 <0.4 g/10 min <0.4 g/10 min Flexural modulus ASTM D790 >758 MPa >758 MPa (110,000 psi) (110,000 psi) >21 MPa >21 MPa (3000 psi) (3000 psi) Yield stress ASTM D638 These properties are part of a Cell Classification system found in ASTM D3350. The properties have historically been measured on the actual virgin pipe resins, but more specifications are written so the properties are measured from samples from the pipe. These properties are similar to those found in highway application specifications like AASHTO M294 (AASHTO-2014). The purpose of measuring these properties is to ensure that a consistent type of polyethylene was used when manufacturing the pipes. The basic controlling properties for polyethylene are the melt index and the density. The properties in Table 1 relate to these. The pipe properties shown in Table 2 are those specifically done on finished pipe for quality control and quality assurance. 2 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA The dimensions, stiffness, and brittleness are specified to ensure a minimum set of properties exists when the pipe is made. The first three properties in Table 2 give some indication of how a pipe may last over many years of service. The carbon content should be controlled because research by Bell Labs showed that 2.5% black was the optimal amount to effectively block UV radiation for service lifetimes of greater than forty years (Gilroy 1985). Therefore, most engineering applications that use black polyethylene require a carbon black content of 2-3% in the compound. Table 2: Additional pipe requirements of two ASTM specifications Property Test method Carbon content ASTM D4218 Requirement ASTM F2986 ASTM F2987 2-3 % 2-3 % >24 hrs >24 hrs >20 min > 20 min -0 to +4.5% -0 to +4.5% of nominal of nominal Varies with Varies with pipe size pipe size Varies with Varies with pipe size pipe size No cracks No cracks ASTM D1603 Stress crack ASTM F2136 resistance (NCLS) (50°C, 4.14 MPa, Igepal CO-630) Oxidative induction ASTM D3895 time (OIT) Inside diameter Wall thickness Pipe stiffness Pipe brittleness ASTM D2122 ASTM D2412 ASTM D2444 (3.9°C, 2.5 kg, 1.5m) The NCLS stress crack resistance test is one of several ways to evaluate the material’s ability to withstand an environment that promotes cracking. This particular test involves a small, dumbbell shaped specimen into which a face notch of about 20% of the specimen’s thickness is cut. The specimens are then mounted in a lever arm device and placed in an aqueous solution of 10% Igepal CO-630 at 50°C. Once the samples reach the test temperature, a load equivalent to 4.14 MPa of stress is added to each. The time-to-failure 3 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA for each of five test specimens is recorded and the average time reported. The test is used to set a minimum value for the application and as a way to show that appropriate HDPE resins are being used. The oxidative induction time or OIT is a way to evaluate the additive packages found in the pipe. The OIT test has been greatly misused over the years so it’s appropriate to state what it is and what it is not: The OIT test does not, by itself, predict service lifetimes in oxidative environments. The OIT test cannot tell which stabilizers are present. A larger number in the OIT test does not necessarily mean better performance, unless the same additive package is used in both samples. The OIT test indicates the presence of added stabilizers. Loss of OIT over time can be used to evaluate the effectiveness of different stabilizers. There are a number of other properties one can measure to compare different pipe samples and to generate more information concerning the long-term performance of the corrugated pipe. The first of these is buckling strength. The buckling strength can be determined simply by extending the test used to determine the pipe stiffness. The stress on the pipe is increased at a constant rate until there is a peak in the load/deflection curve. This can be useful because a variety of things can affect this value, such as wall thickness, resin density, and the presence of fillers or other additives. Another useful test for corrugated HDPE pipe is the elongation-at-break. The tensile yield stress seems to be the only tensile property required in most pipe specifications. However, there is knowledge to be obtained from the amount a tensile bar will stretch before it breaks. During a tensile test, every specimen will fail at the location of the largest flaw. This can be a piece of dirt, a nick in the specimen, a large carbon agglomerate, or anything else that acts as a break initiator. Very clean virgin compounds exceed 800% elongation while even carbon black filled materials easily exceed 400% as long as the carbon black is of high quality. Low elongation values in tensile tests can indicate the presence of fillers, the use of poor quality carbon black, and even the presence of recycled HDPE in the sample. A proposed specification for use of recycled resins in pipe suggested a minimum elongation of 150%. (Thomas, 2011). Since low tensile failure values can indicate the presence of recycled polyethylene and/or fillers, two other tests are useful to glean more information about the pipe. The first one is the ash test. Ash is considered anything that is not organic. It could be a filler such as calcium carbonate, a processing aid such as calcium stearate, or even bits of metal or glass often found in samples of post-consumer recycled polyethylene. This test is most convenient to do along with the carbon 4 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA black test. After the polyethylene has been pyrolysedin the tube furnace or oven, all that remains is the sum of the carbon black and the ash. This result can be over 10% if there are fillers. Therefore, if one then burns off the carbon black, the percent ash will be obtained. The second test determines if post-consumer recycled HDPE is present in the pipe. This test determines the percentage polypropylene (PP) in the sample. PP is almost always found in post-consumer recycled resins because it is largely composed of detergent bottles. Detergent bottles can contain up to about 30% PP from the bottle closures and the spouts, both made from PP. Typical lots of post-consumer HDPE can contain10-15% PP. The test involves a melting profile with a Differential Scanning Calorimeter (DSC) (Thomas – 2011). Methodology Samples obtained Twelve samples were obtained in the spring of 2015. Information about the samples is shown in Table 3. Table 3: Pipe samples Pipe number Diameter mm (in) Manufacturer Country Perforations 1 375 (15) 1 Chile No 2 250 (10) 1 Chile No 3 375 (15) 2 Chile No 4 250 (10) 2 Chile No 5 300 (12) 3 USA No 6 250 (10) 4 USA Yes 7 200 (8) 5 Chile Yes 8 125 (5) 5 Chile Yes 9 150 (6) 6 UK Yes 10 100 (4) 6 UK No 11 100 (4) 4 USA Yes 12 100 (4) 7 Mexico Yes 5 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA This group of samples represents seven manufactures and seven different sizes. Six were perforated, while the others were not. The distribution intends to provide a fair representation of drainage pipe used in mining applications. Tests performed Each of these samples was subjected to a series of more than fifteen tests. The properties evaluated included density (ASTM D1505), melt index (ASTM D1238), tensile yield stress (ASTM D638), tensile elongation-at-break (ASTM D638), flexural modulus (ASTM D790/D3350), carbon content (ASTM D4218), ash content (ASTM D5603), polypropylene content (NCHRP 696), stress crack resistance (NCLS – ASTM F2136), oxidative induction time (OIT – ASTM D3895), high pressure oxidative induction time (HPOIT – ASTM D5885), dimensions (ID, wall –ASTM D2122/F2986), stiffness and buckling (ASTM D2412), and impact resistance (ASTM D2244). Results and discussion Since there are a large number of results to be presented and discussed, they are split into three sections. These sections: Physical Properties, Mechanical Properties, and Durability Properties. Physical properties The physical properties of the pipe compounds are shown in Table 4. Table 4: Physical properties of the twelve pipe samples Sample- Density Melt index % Carbon % Ash % PP 1-375 0.962 0.25 2.68 0.1 0.1 2-250 0.961 0.35 1.61 0.1 0.2 3-375 0.962 0.28 0.71 1.1 2.3 4-250 0.961 0.41 0.60 0.9 3.0 5-300 0.954 0.20 1.37 2.9 2.0 6-250P 0.961 0.11 1.72 0.7 0.1 7-200P 0.950 0.31 2.41 0.1 0.5 8-125P 0.958 0.38 1.87 0.3 0.3 9-150P 0.960 0.37 4.01 13.0 6.2 pipe type 6 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA 10-100 1.070 0.38 4.31 9.3 5.3 11-100P 0.958 0.11 2.14 0.2 0.5 12-100P 0.963 0.45 2.04 0.2 0.7 These properties were all measured on a compression molded plaque made from the pipe. The plaques were molded according to ASTM D4703. The density and melt index are the two main control properties during the manufacturing of polyethylene resins. These are fundamental properties. The density is an indication of the crystallinity of the PE while the melt index is related to the molecular weight of the PE. Since these are independent properties, both are required to characterize PE resins. Unfortunately, when the density is done by gradient density (ASTM D1505) or specific gravity (D792), the results are influenced by other ingredients in the compound. For example, the density of carbon black is 2.27 g/cm3 and the density of calcium carbonate is 2.72 g/cm3. Therefore, anytime fillers or even are carbon black added, the density becomes more unreliable. ASTM D3350 allows for a correction for added carbon black, but experience has shown the result is not always accurate. The value of 1.070 g/cm3 for sample number 10 cannot be explained at this time. The melt index measures how much melted sample flows through a standard sized orifice at a standard temperature (190°C), and under a standard load (2.2 kg) during a period of 10 minutes. A large majority of resins used in corrugated plaque manufacturing have a melt index of less than 0.40 g/10 min. The ASTM standards for mining call for the percentage of carbon black fall between 2.0 and 3.0%. Other corrugated pipe standards have had 2.0 – 4.0% and older standards had 2.0 – 5.0%. With the requirement of 2.0 - 3.0%, only four of the twelve samples would meet the ASTM standard. This is not an overly significant issue, but it is true that as one adds more carbon black, the pipe becomes more brittle. Increased brittleness can lead to cracks during service and even reduce the buckling strength of the pipe by reducing its flexibility. The presence of ash is more alarming than the high carbon black values. First of all, when the ash test is performed on virgin polyethylene, the amount of ash is almost immeasurable, and will be below 0.1%. We only report the value to 0.1% because the results are not as reliable as the value gets very small. Ash content above a couple of tenths of a percent is often a sign of the pipe having recycled PE content. Recycled materials are not necessarily bad. Research for the National Academies through the Transportation Research Board (TRB) and within the National Cooperative Highway Research Program (NCHRP) demonstrated that pipe containing 100% recycled PE can be made to exceed the requirements for highway applications and have estimated service lifetimes of more than 100 years (Thomas - 2011). However, recycled resins should only be used in low-risk applications or by those that truly understand the limits involved with using recycled PE. In a situation where there are very high sustained loads, harsh 7 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA chemicals, and high temperatures, one would want to minimize the risk by using only virgin materials. The two samples with nearly 10% ash should be a concern because large amounts of fillers negatively impact the mechanical properties of HDPE. Once more, the long-term cracking resistance can be heavily affected by added inorganic fillers. When one looks at the percent black, the percent ash, and the percent polypropylene, a clearer picture concerning recycled PE emerges. First of all, polypropylene can come from two different sources. The first is the carrier resin in the carbon black concentrate. Color concentrates are most often sold as 50:50 blends of carbon black and PE. The PE used in color concentrates is most often post-consumer recycled (PCR) resin, unless virgin resin is specified. If one starts with a 50% blend and wants to make 2.5% black in the final compound, the concentrate must be diluted 20:1. So, assuming there is 10-15% polypropylene in the PCR carrier resin, the final PP content will be between 0.5 and 0.75%. Notice that PP was detected in all twelve samples. Three samples had just a trace (<0.2%), four samples had between 0.3% and 0.7%, and the remaining five had from 2.0 to 6.2%. The last group shows that PCR-HDPE was added to the final pipe formulation. Mechanical properties The next set of test results demonstrates the differences between the pipe samples when they are subjected to mechanical stress. The results of different mechanical tests are shown in Table 5. Table 5: Mechanical properties of the twelve pipe samples Sample- Yield Break Flexural 5% pipe stress (MPa) strain (%) modulus Stiffness1 (MPa) (kg/cm/cm) type 1-375 24.58 139 967 2.952 2-250 27.32 120 1027 3.806 3-375 24.98 350 1008 3.057 4-250 26.85 131 899 4.073 5-300 25.96 81 1012 3.884 6-250P 25.19 251 1053 3.885 8 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA 7-200P 23.42 91 925 4.017 8-125P 26.62 48 983 3.252 9-150P 23.88 12 1118 3.915 10-100 22.79 8 1109 3.979 11-100P 25.70 239 1000 6.595 12-100P 27.63 80 1167 3.460 1: Force per displacement per unit length 2: Force per unit length The minimum required tensile yield stress is 20.60 MPa (3000 psi) in both ASTM pipe standards. All of these pipes are well above the requirements. However, both standards also require a minimum base resin density of 0.948 g/cm3. Based upon the relationship between yield stress and density found in the Plastic Pipe Institute’s TR-43 (PPI – 2003), one can see that in order to meet the density requirement, the tensile yield stress needs to be above 24.13 MPa (3,500 psi). Because the density measurements of those compounds containing fillers and recycled materials are unreliable, the yield stress-density relationship can be used to evaluate the densities of the base resins. The pipes with yield stresses below 24.13 MPa include samples 7, 9 and 10. There is nothing out of the ordinary with sample 7 but samples 9 and 10 have high concentrations of carbon, fillers, and polypropylene. The strain (elongation) at break values are related to the “flaws” in the pipe. Six of the twelve pipe samples had break strains of less than 100%, and only three had break strains greater than 150 %. Samples 9 and 10 are particularly worrisome because they broke at a strain near or at the yield point. This should not be acceptable for any pipe product, especially for one used in designed structures like mine heap leach pads. The presence of flaws can also affect the stress crack resistance of the pipes. A sufficiently large flaw (critical defect) can initiate a stress crack. All the flexural modulus results indicate that each of the pipe samples met the minimum required value of 759 MPa. And, just like tensile strength, there is a relationship between density and flexural modulus (PPI 2003). Therefore, with the density requirement of 0.948 g/cm3, all the flexural modulus values should be greater than 863 MPa, which they are. These first three properties were measure on test specimens taken from compression molded plaques. The next three properties were measured directly on samples of pipe. The 5% stiffness results represents the load required to reduce the inside diameter of the pipe 5%. The units of kg/cm/cm are unusual. The value reported is the force (kg) required to compress the pipe, 9 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA normalized to the distance traveled (cm) and the length of the pipe sample (cm). Three of the pipe samples failed to meet the minimum requirement for stiffness. Durability properties In an application such as this, there are a number of factors that will affect the long-term properties of the HDPE pipe. These include the time-dependent response to stress and strain, the resistance to oxidizing chemicals such as sulfuric acid, and the accelerating effects of elevated temperatures. Unfortunately, there are only a couple of tests that address these important design considerations. Three tests were performed that can be related to the materials long-term performance. They are shown in Table 6. All three of these tests were performed on compression molded plaques made from the pipe. The first test is a measure of the stress crack resistance of samples. The test is called the Notched Constant Ligament Stress (NCLS) test and is a measurement of the time it takes for a face notch (cut about 20% through the thickness of the specimen) to grow when the specimen is subjected to 4.137 MPa (600 psi) of stress in a surfactant solution at 50°C. The required minimum value for this test in the two ASTM pipe specifications is 24 hrs. Seven of the twelve samples did not meet this requirement. In fact, the average values ranged from 6 to 548 hrs. This shows a very large difference in the pipes ability to resist cracking. Table 6: Durability properties Stress crack OIT HPOIT pipe type Sample resistance (hrs) (min) (min) 1-375 11 40 246 2-250 10 18 244 3-375 16 13 88 4-250 7 6 62 5-300 71 74 326 6-250P 40 21 173 7-200P 548 60 335 8-125P 38 9 95 9-150P 13 14 78 10-100 14 6 66 11-100P 48 28 273 10 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA 12-100P 6 13 139 The next test is the Oxidative Induction Time (OIT) test. This measures how long it will take a sample to be consumed by oxygen at a test temperature of 200°C. The onset of oxidation is delayed by added process stabilizers and long-term antioxidants. The minimum required value is 20 minutes in the specifications. These samples showed a large range of values which went from a low of 6 minutes to a high of 74 minutes. It seems reasonable to assume anti-oxidants would be consumed more rapidly in low pH environments but this was shown to not be true for HDPE geomembranes (Abdelaal-2012). The test can also be performed at a lower temperature (150°C) under an applied oxygen pressure of 3.448 MPa (500 psi). This High Pressure OIT (HPOIT) test is sensitive to a class of additives known as Hindered Amine Light Stabilizers (HALS) (Thomas – 1993). These additives dramatically improve the resistance of HDPE to UV exposure as well as acting as long-term antioxidants. A pipe containing HALS and having a high number in this test should be more resistance to the effects of exposure to sulfuric acid. This test is not required by the ASTM specifications, but one can see that the results ranged from 62 to 335 minutes. Summary and conclusions The purpose of the study was to measure various properties on a selected number of pipes made around the world for mining applications. The focus has mostly been on heap leach pads because this is where the environment is the most severe. The overburdens can be very high, the temperatures can be significantly above ambient, and the pipes can be exposed to concentrated sulfuric acid. The results obtained on a dozen different samples has demonstrated that the pipe sold into this market is variable in terms of its composition and its mechanical properties. Some of the properties that are believed to affect the pipes performance over its service lifetime are discussed below. Recycled polyethylene Five of the twelve pipes (42%) contained significant amounts of recycled PE based on the percentage polypropylene in the samples. The presence of PP is specific for post-consumer recycled HDPE. Results of an NCHRP study (Thomas – 2011) demonstrated that contamination is a main concern with the use of PCR-HDPE and this must be controlled to ensure consistent performance in corrugated drainage pipe. Contaminants influence the strain-at-break and the stress crack resistance, and probably play a role in the buckling strength and long-term resistance to stress. Contamination can be controlled by specifying a minimum strain-at-break and a maximum ash and polypropylene content. 11 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA Stress crack resistance In other buried pipe applications, the resistance to cracking under a sustained load or strain has been identified as a service life-limiting property. Therefore, minimum stress crack resistance requirements are placed on corrugated HDPE pipe, even if it contains recycled and is used for land drainage, where the overburden can be just 0.3 m. Our results show that the pipes used for mining varied from 6 to 550 hours of average failure time in the NCLS stress crack test. To put this in perspective, the worst HDPE for stress crack resistance, 100% post-consumer recycled HDPE and HDPE homopolymer have stress crack failure times around 6-8 hrs. HDPE homopolymer is used to make milk jugs and is not good enough for detergent bottles because it cracks when it is used for detergent bottles. Additionally, PCR-HDPE is primarily made from detergent bottles and milk jugs. A reasonable NCLS average failure time is 24 hours, which was the value established for corrugated pipe under highways about fifteen years ago. Seven of the twelve pipes tested (58%) failed to meet an average failure time of 24 hrs. Oxidation resistance Pipes used in copper mining may be exposed to hot concentrated solutions of sulfuric acid. Since this acid initiates oxidation, the service lifetime could be partly controlled by the long-term antioxidants present in the pipe compound. Tests that have been used to evaluate additive packages are the OIT test and the HPOIT test. The two test methods for pipe used in mine applications have a modest OIT test requirement of 20 minutes. This is less than the typical value of 100 minutes for HDPE geomembranes, but the pipe has higher natural density (0.948 vs. 0.938 g/cm3), which gives it higher natural resistance to oxidation. There really is not enough knowledge about how this pipe reacts to concentrated sulfuric acid to establish a minimum value. However, seven of the twelve pipes (58%) failed to meet a modest requirement of 20 minutes. Conclusions The corrugated HDPE piping being used in mining operations today varies greatly in composition and properties. The differences surely contribute to variable performance in the field. It would be beneficial to the mining industry to understand how these products perform in service and how their long-term survivability can be improved to increase the output and profitability of the mine. However, until product specifications with defined minimum properties are adopted, the industry will continue to deal with variability in product performance. ASTM International has published two specifications that if followed, could bring more continuity to the corrugated pipes used in mining applications. 12 HEAP LEACH SOLUTIONS, 2015 ● RENO, NEVADA, USA References Abdelaal, F.B., Rowe, R.K., Smith, M., Brachman, R.W. and Thiel, R. 2012. Antioxidant depletion from HDPE and LLDPE geomembranes without HALS in an extremely low pH solution, Proceedings of GeoAmericas 2012, Lima, Peru. AASHTO. 2014. Standard Specification for Corrugated Polyethylene Pipe, 300- to 1500-mm (12- to 60-in) Diameter, American Association of State Highway and Transportation Officials, Washington, D.C. ASTM International F2986-12 Standard Specification for Corrugated Polyethylene Pipe and Fittings for Mine Leachate Applications, ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania, 2012. ASTM International F2987-12 Standard Specification for Corrugated Polyethylene Pipe and Fittings for Mine Heap Leach Aeration Applications, ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania, 2012. Goddard, James B. Profile wall polyethylene pipe specification development, Beijing International Plastic Pipe Exchange Conference, Beijing, China, November, 2011. Gilroy, H.M. 1985. Polyolefin longevity for telephone service, SPE/ANTEC 31. PPI. 2003. Design service life of corrugated high density polyethylene (HDPE) pipe, Plastic Pipe Institute Technical Report TR43, Washington D.C. Smith, M.E. 2004. Drainage pipe deflection for high heaps, The Mining Record, 4th Quarter. Thomas, R.W. and Ancelet, C.R. 1993. The effect of temperature, pressure, and oven aging on the high pressure oxidative induction time of different types of stabilizers, Proceedings of the Geosynthetics ’93 Conference, Vancouver, British Columbia. Thomas, R.W. and Cuttino, D.M. 2011. Performance of corrugated pipe made with recycled polyethylene content, NCHRP Report 696, Transportation Research Board, Washington, DC. 13 View publication stats