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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
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4The approach taken to meet the objectives mentioned in chapter 1 was to divide the project into three distinct phases. These phases were Recycled Resins, Recycled-resin Blends, and Pipe Made from Recycled-resin Blends. Phase 1—Recycled PE Resins The purpose of Phase 1 was to determine which types of recycled PE are available, where these materials can be obtained, and what their properties are. The approach was to collect samples of recycled HDPE, determine what their typical properties were, study the effects of some common contaminants, and develop test methods to characterize the materials. The results of this phase of the study allowed the assessment of the best kind of recycled PE for use in corru- gated drainage pipe. Phase 2—Recycled Resin Blends The purpose of Phase 2 was to determine how the addition of recycled HDPE would affect the properties of Canadian Petroleum Products Institute (CPPI)-certified resins and to select specific blends for trial corrugated pipe manufacturing. The approach was to prepare blends of virgin pipe resins that contained different types and amounts of recycled HDPE. Once the blends were made, key relationships were developed that allowed one to predict the properties of blends so that optimized formulations could be used in the next phase to make actual pipe. Phase 3—Pipe made from Recycled-Resin Blends The purpose of this phase of the project was to manufacture dual wall corrugated drainage pipe out of resin formulations containing recycled HDPE. Once the pipe was made, the short-term properties were measured and compared with the requirements of AASHTO M294. Additionally, some longer- term stress-crack tests were performed. And, finally, some candidate pipe samples were evaluated for their long-term creep strength, creep modulus, and stress-crack resistance. Short-Term Properties The purpose of short-term tests is to characterize a resin or pipe enough to feel confident that the properties are consistent from lot-to-lot. This is especially important for recycled resins, which are known to be variable. For example, in post-consumer recycled (PCR) HDPE the percentage of milk bottles mixed in with colored bottles will change. Since these two types of HDPE have different properties, the properties of the recycled product will change. Short-term tests can reveal these differences in properties. Another use for short-term properties is to control contam- ination. For example, the amounts of particulate matter can be measured by burning off the polymers and carbon black and measuring the ash content. The control of contaminants can also be achieved through mechanical properties. When a tensile test is performed, the test specimen will always break at a flaw. Samples with more potential flaws (particles, unblended polymers, gels) will break at lower strains. One can therefore get an idea about the level and size of contaminants by mea- suring the strain at break. And finally, the results of some short-term tests can offer a bit of information about the long-term serviceability of the material. When the flexural modulus or tensile yield stress is measured, one gets a feeling for how the material might respond to stress. Stress-crack tests, like the notched, constant tensile load (NCTL) (ASTM D5397) or notched, constant ligament-stress (NCLS) (ASTM F2136) can give an idea of the relative stress-crack resistance between materials. These are important for use with recycled materials because the particulates and other polymers may promote crack initiation or growth. And, when one measures the oxidative-induction C H A P T E R 2 Research Approach

time (OIT) (ASTM D-3895) or the oxidative-induction tem- perature (OITemp) (ASTM D3350), the presence of stabilizers can be detected, offering some assurance that stabilizers are present (note that the OIT or OITemp tests do not predict long-term performance). The short-term properties used for this project included: 1. Density—basic property of PE; 2. Melt Index (MI)—basic PE property relating to molecular weight; 3. High Load Melt Index (HLMI)—ratio of 2 MI tests relates to molecular weight distribution; 4. Percentage Color—organic fillers like colorants and carbon black; 5. Percentage Ash—inorganic fillers and contaminants; 6. Differential Scanning Calorimetry (DSC)—detects pres- ence of other semicrystalline polymers like PE; 7. Flexural Modulus—indicator of stiffness; 8. Tensile Yield Strength; 9. Strain at Break—sensitive to contaminants; 10. Notched, Stress-Crack Test—determines relative crack initiation and growth; 11. Un-Notched, Stress-Crack Test—sensitive to contami- nants that may initiate cracks; and 12. OIT and OITemp—indicators of stabilization. Long-Term Properties There is absolutely no doubt that the most important part of this study is to generate reliable data concerning the long- term performance of pipe made with recycled PE content. The approach for this project was to use the solid wall pipe industries’ practices as a model for the development of tests for use with corrugated pipe resins. Service Lifetime of PE The long-term service lifetime of HDPE is often presented in a graph like the one in Figure 1. An understanding of this graph is necessary to develop test methods to characterize the long-term behavior of PE. It shows three distinct stages of aging. The first stage is the likely service lifetime for materials placed under significant loads (>30% of yield). Over time, because of the time-dependent process of creep, the material will fail by yielding or stretching in a ductile manner. The second stage is at intermediate loads and involves failures by slow crack growth (stress cracking). And, finally, under low stresses the material will fail only after the additive package has been consumed and the HDPE under- goes oxidation. The first stage is somewhat dependent on the material’s yield strength, the service temperature, and the stress. Long-term tests for yield strength usually involve placing the material under a load less than its strength and waiting for failure to occur. This is called stress rupture if the sample is placed under a constant strain, or creep rupture if the sample is placed under a constant stress. Temperature is sometimes used to accelerate the process and the results are analyzed through a method called time-temperature superposition (TTS), which assumes that higher temperatures and shorter times can be related to lower temperatures and longer times. A specialized form of TTS is called the stepped isothermal method (SIM) and has been used successfully on polyester, polypropylene, and PE reinforce- ment products for civil engineering applications. The end of 5 Log Failure Time (Hrs) Lo g St re ss (p si) Stage I - Ductile Stage II - Brittle Stage III - Oxidation Equilibrium Yield Stress Figure 1. Hypothetical service lifetime for HDPE.

this stage is characterized by the equilibrium tensile strength. Any service stresses lower than the equilibrium yield stress will not cause a ductile failure during the service lifetime. The second stage involves brittle cracking through slow crack growth. During this stage, a defect in the material can initiate a craze, which can, in turn, become a running crack, eventually causing a break in the material. For a brittle crack to grow there has to be a significantly sized and shaped defect, and sufficient load. HDPE materials have an inherent stress-crack resistance that can be measured, but manufacturing defects and flaws can accelerate cracking. For PE pressure pipe, the best way to pre- dict service lifetime is the long-term hydrostatic strength test. This entire technology has been developed for pressure pipe and the results are relied upon for the design of pressure piping systems. ASTM D2837 describes a method called hydrostatic design basis for evaluating the service lifetime of pressure pipe. This involves high loads (ductile failure) at room temperature to determine the long-term hydrostatic strength (LTHS) and intermediate loads (slow crack growth) at elevated tempera- tures to validate that slow crack growth will not occur within 100,000 h. Similar tests for corrugated pipe have been presented, including the Federal Institute for Materials Research and Testing (BAM) Test (1, 2), the Florida Department of Trans- portation (FL-DOT) junction test (3), the ring stress-crack test (4), and the BAM–FL-DOT–Fathead (BFF) Test (5). It should be clearly stated that both Stage I and Stage II service-lifetime plots are necessary to determine which process will limit the service lifetime of the pipe. If the point where the two lines cross on the time scale is greater than the expected service lifetime, the most important failure mode is ductile. If the crossover point on the time scale is less than the expected service lifetime, then stress cracking is the life-limiting failure mode. In the early days of pressure pipe, most of the failures occurred by slow crack growth. But, as the stress-crack resis- tance of the resins got better and better, the primary failure mode during service became ductile. The final stage occurs at service stresses below the stress that would cause a stress crack during the service lifetime. In this case, the entire part becomes brittle through chemical oxidation and fails by many cracks starting at the same time. This region is controlled by the additive package that contains the long- term antioxidants and/or light stabilizers. Oxidation is largely a nonissue for properly stabilized resins. However, there should be some requirement for an OIT value or a specified minimum additive package placed in the final specification for resins con- taining recycled PE. An OIT of 50 minutes should suffice and there are commercial additive packages that meet the criteria. The PE pressure pipe industry has used this failure envelope for many years to ensure the quality of resins used for gas and water distribution pipelines. The main protocol for this is ASTM D2837, “Standard Test Method for Obtaining Hydro- static Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for Thermoplastic Pipe Products.” For medium- and high-density PE pipe, the method requires that at least 18 data points are generated at room tempera- ture with one point over 10,000 h (1.14 years). These points are then plotted as Log Stress vs. Log Time and the resulting line extrapolated to 100,000 h (11.4 years). The 100,000 h stress is called the LTHS. The LTHS value is then fitted within a range of values given in the standard to define the hydrostatic design basis (HDB). An example is shown in Figure 2. This shows a good example of a situation where only Stage I failures are observed during the test. The ASTM standard also accounts for Stage II or Brittle Failure through slow crack 6 y = -0.0241x + 3.2464 R2 = 0.9381 2.5 2.75 3 3.25 3.5 0 1 2 3 4 5 6 Log Time (Hrs) Lo g St re ss (p si) 100,000 Hrs LTHS = 1336 psi HDB = 1250 STAGE I Figure 2. Determination of the hydrostatic design basis (HDB).

growth. This is shown in Figure 3, and one can see that the failure mechanism has transitioned from Stage I to Stage II at the stresses evaluated. It’s important to understand that the first example will undergo Stage II, it’s just that it occurs at a time over 100,000 h. Figure 4 shows Stage I and Stage II for examples of both a HDB 1250 and HDB 1000 resin. These examples clearly show that the differences seen between resins is that the slope of the ductile line is shallower and, therefore, the transition from ductile to brittle (Stage I to Stage II) occurs later in time. This is why just specifying the yield stress or flexural modulus is not enough. One needs to determine the slope of the line in Figure 2 to get a good under- standing of long-term behavior under stress. Once the HDB category is found, the hydrostatic design stress (HDS) is found by applying a design factor (DF) to the HDB. The DF is similar to a factor of safety, where reductions are estimated for installation damage, lot-to-lot variability, and so forth. In the pressure pipe industry, a DF of 0.50 (or 0.62) is applied to water pipe and a DF of 0.32 is applied to gas pipe. So, a 1250 HDB resin would have a HDS of 625 psi and 400 psi in water and gas applications. 7 2.5 2.75 3 3.25 3.5 0 1 2 3 4 5 6 7 Log Time (Hrs) Lo g St re ss (p si) 100,000 Hrs LTHS = 977 psi HDB = 1000 psi STAGE I STAGE II Figure 3. Determination of the HDB when Stage II is involved. 2.5 2.7 2.9 3.1 3.3 3.5 0 1 2 3 4 5 6 7 8 Log Time (Hrs) Lo g St re ss (p si) 100,000 Hrs LTHS = 977 psi HDB = 1000 psi LTHS = 1336 psi HDB = 1250 psi Figure 4. A comparison between the examples shown in Figures 2 and 3.

One of the advantages of hydrostatic testing on plastic pipe is that one can generate both Stage I and Stage II failures with the same basic test. Additionally, a resin and carbon black formulation is certified through the room temperature testing. Then, as long as the formulation stays the same, there is no additional testing. In the present case, where we want to eval- uate pipe resins containing recycled materials, every lot may be different and accelerated tests must be used to estimate the long-term performance of the materials. Moreover, there will have to be two accelerated tests performed, one for Stage I (ductile) and another for Stage II (brittle). Along with the long-term strength (Stage I), AASHTO requires long-term creep modulus and long-term creep strain as part of the design for corrugated pipe. The approach to determine both the creep and creep rupture properties of pipe containing recycled PE is through the SIM accelerated test. The SIM for Predicting Creep and Creep Rupture (Stage I) Properties The SIM is a special form of TTS that has been used to extrapolate short-term creep results (∼24 h) into long-term estimates of creep behavior (50, 100 years). It was originally developed in these laboratories on polyester (PET) geogrids used for reinforcement applications (6, 7). The application of SIM to PET has been verified and validated by several other laboratories comparing the SIM results to conventional creep tests performed at room temperature (8). It has also been used by others on other PET fibers, Kevlar, and polyethylene naphthanate (PEN)(9). It has also been used in these laboratories to examine poly- propylene (PP) buried structures and most recently on HDPE resins used for corrugated drainage pipe. It has been validated for PP by comparing SIM results to conventional creep results. A plot of duplicate SIM results compared with two, 10,000 h conventional creep tests is shown in Figure 5. The main difference between PET and HDPE is their respec- tive temperature dependencies at temperatures from ambient to 80°C. HDPE’s properties change at a higher rate with tem- perature than PET’s properties. In fact, the low-temperature dependency of PET strength was the main reason SIM was developed in the first place. The sample-to-sample variability could be as large as the difference in creep rates at two different temperatures. A comparison for the two materials is shown in Figure 6. TTS has been used for decades and it is the basis for the validation procedures for PE pipe materials in ASTM D2837 and Plastics Pipe Institute (PPI) Technical Report TR-3 (10). TTS can be used to project the long-term hydrostatic strength of pressure pipe. Basically, increasing the temperature of a process like creep, stress relaxation, or slow crack growth is equivalent to per- forming the test at longer times. The higher the temperature, the longer the accelerated time. In the case of traditional TTS, tests are performed at various elevated temperatures on different samples and the results shifted to a lower target temperature. Because of the sample- to-sample variability, the result of TTS can be uncertain and requires tests on many specimens. Two examples of TTS are the Rate Process Method and Popelar Bi-Directional Shifting Method. SIM is a form of TTS where behavior at multiple tempera- tures is observed on a single test specimen, which reduces the uncertainty of the behavior due to sample-to-sample variability. 8 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 -4 -3 -2 -1 0 2 41 3 6 875 Log Time (hr) % S tra in Reference Temperature - 23C 10,000 hours 50 years 2 ea SIM 2 ea Conventional Creep-Modulus 1000 psi Stress Figure 5. Comparison between conventional creep and SIM for a PP storm chamber under 1,000 psi of stress.

An example SIM test for HDPE was performed under the following conditions: • Sample: Type I Dumbbell. • Strain Measurement: Extensometer. • Initial Temperature: 20°C. • Temperature Steps: 7°C (20, 27, 34, 41, 48, 55, 62, 69, 76, 83). • Stress: 500 psi. • Dwell Time: 10,000 seconds (2.77 h). The raw, unshifted data are shown in Figure 7. There are 10 temperature steps shown on the plot. Notice that the sample yielded catastrophically during the early part of the 83°C step. It’s also easy to see that at each successive temperature step, the creep rate increases. This is due to the increased temperature, but also because PE gets softer as the temperature rises. So, in reality, there is a double acceleration. The next step in the analysis is to determine what is referred to as the virtual starting time (t′) for each step above the first one. This accounts for the effects of the creep that occurred at the lower temperature. This step is necessary because the specimen “remembers” what had occurred at the previous creep step. This also allows one to rescale the individual creep curves and get them all on a common time scale. The t′ is found by plotting creep modulus vs. log time for the end of one step and the beginning of the next step. Then, one can adjust the t′ iteratively until the slopes of the two curves are parallel. A vertical shift is also added at this time 9 y = -0.0117x + 1.2639 y = -0.003x + 1.069 0 0.2 0.4 0.6 0.8 1 1.2 10 20 30 40 50 60 70 80 90 Temperature (C) R el at iv e Pr op er ty PE PET Figure 6. Temperature dependence of PET and PE. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000 Time (sec) % S tra in 20°C 83°C 76°C 69°C 62°C 55°C 48°C 41°C 34°C 27°C Figure 7. Raw SIM data.

until the two parallel lines line up. The matching of the slopes of the end of one step with the beginning of the next step is the critical step for accurate extrapolations. Once this is done for each step, master curves can be pre- sented as either creep modulus or strain. Master curves for this data set are shown in Figures 8 and 9. From these two curves, one can obtain both the 50-year creep modulus and 50-year creep strain. In this case, they are 23,250 psi and 2.15% respectively. These represent the behavior of the material when placed under a 500 psi load for 50 years. Notice that there are gaps between the extrapolated steps. The transition from one temperature to the next is an important variable in SIM testing. The time it takes for the specimen to equilibrate at the new temperature should be just a few minutes. Other things that occur during the transition time are thermal expansion or contraction of the specimen as well as re-equilibration of the grips and exten- someter. The researcher excludes the data from the transi- tion region, but keeps the time scale in place. The transi- tions then show up as blank spots in any plot with time as the abscissa. SIM can also be performed under higher loads to create a creep-rupture environment. Recall that the SIM test above was performed under an applied stress of 500 psi (about 12.4% of ultimate). If one does the same test at 1,000 psi, the master curve can produce the time it would take for the sample to yield under the applied load. This is shown in Figure 10. The extrapolate time to Stage I failure under these conditions is about 1,900 years. Shorter times are found for applied stresses of 1,500 and 2,000 psi (Figures 11 and 12). These three results can then be put together on a plot of Log Stress vs. Log Time, to generate a Creep Rupture Master Curve. The one for the results above is shown in Figure 13. The results from these tests show that the 50-year yield strength will be about 1,161 psi. 10 0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000 -3 -2 -1 0 1 2 3 4 5 6 7 8 LOG TIME (hr) CR EE P M O DU LU S (ps i) REFERENCE TEMPERATURE - 23C 100 Yrs50 Yrs 23,250 psi 22,531 psi Sample B1 500 psi Stress Figure 8. Creep modulus master curve under 500 psi of stress. 0 0.5 1 1.5 2 2.5 3 -4 -3 -2 -1 0 1 2 3 4 5 6 7 LOG TIME (hr) ST RA IN (% ) REFERENCE TEMPERATURE - 23C 100 Yrs 50 Yrs 2.15 % 2.22 %Sample B1 500 psi Stress Figure 9. Creep strain master curve under 500 psi of stress.

11 0 5 10 15 20 25 30 35 40 -4 -3 -2 -1 0 1 5 82 63 4 7 9 LOG TIME (hr) ST RA IN (% ) REFERENCE TEMPERATURE - 23C 1000 psi 7.22 Figure 10. Long-term yield stress by SIM at 1,000 psi. REFERENCE TEMPERATURE - 23C 2.43 1500 psi 0 5 10 15 20 25 30 35 40 -4 -3 -2 -1 0 1 52 63 4 7 LOG TIME (hr) ST RA IN (% ) Figure 11. Long-term yield stress by SIM at 1,500 psi. REFERENCE TEMPERATURE - 23C 1.11 2000 psi 0 5 10 15 20 25 30 35 40 -4 -3 -2 -1 0 1 52 63 4 7 LOG TIME (hr) ST RA IN (% ) Figure 12. Long-term yield stress by SIM at 2,000 psi.

The Long-Term Stress-Crack (Stage II) Resistance The long-term stress-crack resistance of pipe formulations containing recycled content will certainly be the life-limiting property for properly installed corrugated pipes. The con- taminants present can be locations for stress cracks to grow. How quickly a crack grows will depend on the size, shape, and hardness of a particle along with the inherent stress-crack resistance of the resin or resin blend. In fact, it would not be an understatement to say that the resistance to slow crack growth is the critical property for the success of this project. The success that the gas pressure pipe industry has had reducing failures can be used as a model for creating a testing protocol to evaluate recycled content–containing pipe blends. For pressure pipe, the long-term strength is determined first (Stage I) at room temperature, then a validation test is per- formed at 80°C or 90°C to ensure that brittle crack growth (Stage II) will not occur during the service lifetime of the pipe. Validation can be performed two ways; one based on the Rate Process Method (RPM) (11) and one based on Popelar bi-directional shifting (POP) (12). These two methods of TTS are described in detail in Appendix D, Section D.8.2. A test similar to the long-term hydrostatic strength test for pressure pipe is needed and will be sought as part of this study. The best features of the long-term hydrostatic strength test include the following: • The finished product can be tested. • The thickness control of the test specimen is excellent. • The applied stress is uniform throughout the specimen. • There is no notch, so failures occur at naturally occurring flaws. • The results relate to the real world. There have been three different long-term stress-cracking tests proposed for corrugated drainage pipe in the past few years (1, 3, 4). These are commonly known as the BAM Test, the FL-DOT junction test, and the pipe ring test. The FL-DOT Junction Test The FL-DOT has sponsored a research project to develop test methods to determine if corrugated pipe can have a service lifetime of 100 years (3). One of the tests involves stress-crack tests without a notch on specimens taken from the pipe. The particular location tested is the junction between the liner and the corrugation. FL-DOT Test Method FM 5-572 covers the performance of the test and FM 5-573 covers the procedures for extrapolation of the test results. The junction test specimen is an ASTM Type IV dumbbell, which has a reduced area that is 1⁄4 in. × 1.3 in. The advantages for this test include the following: • It is performed on specimens from the finished pipe. • It is related to field failures. • It is more sensitive to the manufacturing process than to the basic resin. • The well known RPM for data analysis can be applied. On the other hand, some disadvantages are • The thinnest part of the test specimen is often not the junction but the liner on either side. 12 y = -0.0459x + 3.3237 R2 = 0.9528 2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 0 1 2 3 4 5 6 7 8 Log Time (Hrs) Lo g St re ss (p si) 100,000 hrs 50 yrs 1242 psi 1161 psi 23°C Reference Temperature Figure 13. Creep rupture master curve.

• Only the thinnest part of the specimen experiences the full stress. • There is significant sample preparation stresses imparted, especially in larger diameters. • There are significant edge effects from the cutting die, especially on larger diameters. Test method FM 5-573 calls for the test to be run under three different conditions of stress and temperature in deionized (D.I.) water. These are shown in Table 1. Five replicates under each set of conditions are tested. The three sets of results are then analyzed by the RPM, which has been used for many years on pressure pipe. The test protocol also allows the tests to be terminated when they reach the following conditions: • Terminate at 110 hours for 80°C/650 psi. • Terminate at 430 hours for 80°C/450 psi. • Terminate at 500 hours for 70°C/650 psi. These times were determined with the use of an average slope of a number of stress-crack tests and POP. The BFF Test The BFF test uses a specimen, cut from a plaque, about the size of the BAM specimen (4 in. l × 0.5 in. w). The test is performed under the conditions of the FL-DOT durability protocol (80°C/650 psi, 80°C/450 psi, 70°C/650 psi) in D.I. water with a dumbbell shaped test specimen in which the ends are twice as thick as the center to reduce failures at the grips. The advantages of this test for testing pipe containing recycled HDPE are • The exposed surface area is about 5 times larger than the FL-DOT specimen. • The thickness is controlled so the stress is even throughout the specimen. • The specimen thickness (0.0040 in. to 0.0045 in.) means flaws will have a greater effect than on thicker specimens. • A wider and thinner test specimen means less edge and cutting die effects. • It is sensitive to contaminants found in PCR HDPE, like particulates. However, this test will not be able to evaluate manufacturing stresses like the FL-DOT test. A test that still holds promise for that purpose is the ring stress-crack test (7). It could not be used during this project because of the large variability in pipe-wall thicknesses measured. However, there may be a way to modify it, or test multiple sections from a full diameter that would result in a valuable test for evaluating pipes containing recycled HDPE. More time and funding would be required to develop the concept into a valuable quality control (QC) or qualification test. Similarly, if the FL-DOT junction test could be performed on wider test specimens, the results would be more meaningful. However, this would require stress-crack testing devices that were larger to accommodate a wider specimen and greater loads. The FL-DOT junction test with a Type IV (as defined by ASTM D638) specimen can be performed on most conven- tional stress-crack frames. These tests will be evaluated for use as a tool for predicting the long-term stress-crack (Stage II) resistance of pipe-resin formulations containing recycled PE. The Long-Term Oxidation (Stage III) Resistance At very low service stresses and after very long times, a PE part can become oxidized and fail by many cracks forming in a short period of time. This failure mechanism is known, especially in exposed applications where UV initiates oxidation or in potable water applications where higher concentrations of chlorine can initiate oxidation. However, for typical buried pipe applications the rate of oxidation and the extraction of long-term antioxidants are very, very slow. Over the past 20 years, accelerated aging results on PE stabilized with com- mon additive packages such as Irganox 1010* and Irgaphos 168 have suggested that the service lifetime is easily in the hundreds of years. Therefore, it is believed that as long as there is some basic stabilization, Stage III oxidation is not likely to occur in typical buried pipe applications. Specifications for pipe resins containing recycled content can specifically state that a particular stabilizer package be used. This can be verified with an OIT test. The OIT test cannot identify specific additives, but it can measure concentration if 1,010 and 168 are used. These ingredients are two of the least expensive stabilizers for PE. When used at levels of 1,000 ppm Irganox 1010 and 13 Test Temp. (°C) Applied Stress (psi) 70 650 80 650,450 Table 1. FL-DOT FM 5-573 test conditions. *The Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, and the individual states partic- ipating in the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely because they are considered essential to the clarity and completeness of the project reporting.

1,000 ppm Irgaphos 168, the OIT value will be around 70 min- utes and the pipe will be well stabilized. Specifications Ensuring consistent properties for blends containing recycled resins will be challenging. There are a number of different kinds of companies that will be involved once AASHTO allows recycled use. At the beginning of the process are the recycled-resin suppliers. These companies specialize in locating, cleaning, reprocessing, and selling recycled materials. Some are small family-owned businesses that simply grind and clean recycled materials while others are fairly sophisticated with auto- mated cleaning and optical color sorting. And there are many in-between. Some will be happy just selling reprocessed re- cycled HDPE and others will want to prepare and sell fully formulated blends directly to the corrugated pipe manufac- turer. Next will be the compounders. This group may include the recycled-resin supplier, but other companies with estab- lished sources of sub-prime virgin resins will use their blending and formulating technology to make fully formulated recycled blends for direct sales to the corrugators. And, finally, some pipe manufacturers themselves may want to prepare their own blends and only buy high quality reprocessed recycled material from the recycled suppliers. Because of this complicated way in which recycled resin may make its way into AASHTO approved pipe, there could be up to five different specifications. First, one may need a specification for post-consumer recy- cled, mixed-color reprocessed (PCR-MCR) HDPE in pellet form. This will be used by compounders and corrugators to ensure that they are receiving a consistent stream of material from which to make recycled blends. Its focus will be mostly on how well the material is melt filtered and how much polypropylene is present, among other things. These are believed to be important properties for good recycled material. Then there may be two specifications for fully formulated recycled content–containing blends. One will be for use in pipe for AASHTO M252 and drainage applications and the other for M294 applications. Apparently, these two applications are different enough that a different quality and type of resin is used for each one. These are necessary for the corrugators to know what is being purchased to be converted to pipe. Finally, there will also be two specifications—M252-Recycled and M294-Recycled—for pipe made from recycled content– containing resins. These specifications will include both resin properties and the properties of the final pipe; all testing will be conducted on the pipe or on samples taken from the pipe. 14

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Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 696: Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content provides potential specifications for corrugated drainage pipe manufactured with recycled high-density polyethylene (HDPE). The report includes proposed draft specifications for recycled HDPE, formulations of virgin and recycled HDPE, and drainage pipe containing recycled HDPE.

The following three appendixes of NCHRP 696 are available in electronic format only.

Appendix B: Recycled Polyethylene Resins

Appendix C: Recycled-Resin Blends

Appendix D: Pipe Containing Recycled HDPE

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