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50 of mixed-color PCR. It's possible that the addition of just showed that the materials all met the requirements of Section 12 around 10% of these resins would raise the NCLS times of of the LRFD Bridge Design Specification for 50- and 75-year the recycled above the required value of 24 h. Of course, an service lifetimes. unnotched stress-crack test like the BFF test would also be The long-term stress-crack resistance was determined with a required to assess the long-term performance of the blends. new test called the BFF test. This test was found to be sensitive to contaminant particles, while other tests are not. Results were generated that suggested that recycled content-containing Phase 3--Pipe Made from pipe formulations can be developed that have estimated ser- Recycled-Resin Blends vice lifetimes greater than 1000 years. This allows one to The purpose of this phase of the project was to manufacture apply a very conservative design factor of 0.10 for a 100-year dual wall corrugated drainage pipe out of resin formulations service lifetime. containing recycled HDPE. Fifteen pipe samples were prepared The long-term oxidation resistance of pipe containing at three different manufacturing plants. Each plant made two recycled HDPE is often thought to be a significant issue. common formulations, so there was a total of 11 different However, studies over the past 20 years have consistently blends evaluated. Once the pipe was made, the short-term showed that in the absence of UV radiation, the oxidation rate properties were measured and compared with the require- for modestly stabilized HDPE materials is low. One example ments of AASHTO M294. It was determined that it is not too is the Pennsylvania Department of Transportation Deep Burial difficult to create blends within the required cell classification study in which the OIT times were measured after 20 years of required by AASHTO M294. It was more difficult to produce buried service. The results showed that very little, if any, change blends with adequate stress-crack resistance. If the 2010 had occurred to the OIT, except on the pipe ends exposed to requirement of 24 h on a plaque made from a finished pipe the sun (13). Another is a recent study that showed that an were in place, only eight of the 15 samples would meet additive package containing 0.05% Irganox 1010 and 0.10% the requirement. Again, this is a result of making blends with Irgaphos 168 had an estimated antioxidant depletion time of pipe resin with about 50 h of NCLS time. Resins tested in 130 years (18). Oxidation failures are believed to be an un- 2010 have had times greater than 150 h, which would make likely event if the formulations are made with a minimum of the resulting blends higher in stress-crack resistance. And, of 0.1% Irganox 1010 and 0.1% Irgaphos 168. course, the new bimodal HDPE resins could be used to raise The ultimate goal of this project concerning the long-term the value even more. properties is to create a master curve, similar to those used for The long-term performance of the pipe formulations was solid wall pressure pipe. An example for Sample B1 is shown also evaluated. Accelerated creep and creep rupture tests in Figure 47. were performed by the SIM on tensile dumbbells taken from This curve is an estimate of the relationship between compression molded plaques made from pipe. These results applied stress and service lifetime for a sample made from a 3.4 3.3 3.2 1188 psi Stage I from SIM Log Stress (psi) 3.1 y = -0.0469x + 3.3278 R2 = 0.9608 3 2.9 Stage II from BFF y = -0.243x + 4.3822 2.8 R2 = 0.9827 2.7 Shifted to 23C 2.6 0 1 2 3 4 5 6 7 8 Log Failure Time (hrs) Figure 47. Master curve at 23 C for Sample B1.

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51 PPI-certified, 100% Virgin HDPE. The information in this with the appropriate mass. The results of this study will be an plot includes the following: estimated service-lifetime curve. This curve will help deter- mine appropriate loads for actual field tests. 1. At applied loads less than 1,188 psi, the life-limiting failure For example, if the SIM results on pipe are similar to the mechanism is slow crack growth (Stage II). results on a dumbbell from a plaque, one could use the esti- 2. At 900 psi, the lifetime estimate is 86 years. mated lifetime curve to select loads for a field study. The SIM 3. At 500 psi, the lifetime estimate is 965 years. creep rupture curve for sample B1 on a plaque is shown in 4. The life-limiting load at 50 years is 1,026 psi Figure 48. 5. The life-limiting load at 75 years is 930 psi. Notice that one could apply loads of 40%, 38%, and 36% of 6. The life-limiting load at 100 years is 867 psi. the ultimate stress and create failure times in less than one year. The measured buckling load of Pipe B1 was 1,053 lb. So, the This plot was generated from the SIM accelerated creep corresponding loads would be 442 lb, 421 lb, and 400 lb. These rupture test and from the BFF accelerated stress-cracking test. loads should create buckling failures in less than a year. This Both of these test methods require further development, but the is the type of experiment that could validate the results of the example above should demonstrate the usefulness of the two. SIM test on formulations containing recycled HDPE. The test methods need standard methods written and round- Similar experiments on solid wall pressure pipe are ongoing robin tests conducted. There is interest in a SIM method in in these laboratories. The SIM results on a plaque are being compared to sustained burst test results at room temperature. ASTM because it is already being specified for some drainage The slopes of the log stress vs. log time curve are nearly iden- products and the only SIM standard available is for reinforcing tical, but the curves are shifted vertically. The pipe failures all geotextiles and geogrids. occur at higher stresses than the dumbbells. It's a linear offset The estimates of long-term stress and strain and of long-term and believed to be caused by sample geometry. There may also stress-crack resistance should be verified through accelerated be an offset between a pipe plaque and a pipe in compression, laboratory testing on the pipes or field studies on buried pipe. but the slopes of the lines may be the same. If the results There are some specific ways to do this. between a SIM test on a plaque and long-term compression SIM tests should be performed on pipe samples to determine tests on pipe are comparable, then a powerful new tool would the relationship between a dumbbell test on a compression exist for predicting long-term behavior. molded plaque and an actual section of pipe. This is easily In summary, the suggested follow-on research pertaining done on 12-in. diameter pipe. All that is required is an appro- to the SIM test and long-term service estimates are to do the priate environmental chamber that can hold a 12-in. 12-in. following: piece of pipe and a linear variable differential transformer (LVDT) to measure inside diameter displacement. The pipe 1. Develop a standardized method specific to performing sample would be placed between parallel plates and loaded SIM accelerated creep and creep rupture tests on HDPE. 3.4 SIM Results Shifted to 23C 3.3 B1 y = -0.0469x + 3.3278 Log Stress (psi) R2 = 0.9608 3.2 3.1 Yield Stress = 3865 psi 40% = 902 hrs 38% = 2692 hrs 3 36% = 8524 hrs 2.9 0 1 2 3 4 5 6 7 8 Log Failure Time (hrs) Figure 48. SIM creep rupture master curve for Sample B1.

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52 3.12 BFF Results Shifted to 23C 3.1 3.08 B1 3.06 y = -0.243x + 4.3822 Log Stress (psi) R2 = 0.9827 3.04 3.02 Yield Stress = 3865 psi 3 28% = 40 yrs 2.98 23% = 90 yrs 18% = 248 yrs 2.96 2.94 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 Log Failure Time (hrs) Figure 49. BFF stress-cracking master curve at 23C for Sample B1. 2. Perform SIM on 12-in. diameter pipe samples to deter- The stress-cracking master curve for sample B1, generated mine the relationship between a dumbbell specimen and by the BFF test, is shown in Figure 49. an actual pipe. Notice that the failure times for the brittle (Stage II) por- 3. Validate the SIM results by laboratory tests on pipe under tion of the predicted curve are very long. Therefore, it is not compressive loading at room temperature. practical to try to validate these results at room temperature. The same BFF test results shifted to 50C (122F) are shown The BFF test also needs to be validated in a similar manner in Figure 50. to the SIM test. Multiple tests should be performed on the In this case, applying loads of 295 lb, 242 lb, and 190 lb to same samples across time to determine how reproducible the a pipe sample at 50C (122F) should create stress-crack failures test is in a single laboratory. Then, tests should be performed by in about a year. This experiment could be done in a room different laboratories to determine the lab-to-lab variability. controlled to 50C. The results from this study will show if Only then would the true value of this test be known. the master curves generated by the BFF test actually relate to 3.2 BFF data shifted to 50C 3.1 y = -0.2346x + 3.9008 R2 = 0.9787 3 Log Stress (psi) 2.9 Yield Stress = 3865 psi 28% = 1,299 hrs 2.8 23% = 3,005 hrs 18% = 8,543 hrs 2.7 2.6 2 2.5 3 3.5 4 4.5 5 Log Failure Time (hrs) Figure 50. BFF stress-cracking master curve at 50C for Sample B1.

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53 a corrugated pipe under a direct compressive load. This test density or low elongation at break or have poor stress-crack is especially important for pipe formulations containing post- resistance. They vary in their stress-crack resistance in an consumer mixed-color recycled HDPE because they are the unnotched test. The poorest had a failure time about 25% of ones that are more likely to stress crack over their service the best's time. There are some that have peak loads less than lifetimes. 20% deflection and others that are over 40% in pipe deflection Validation of the BFF test results for predicting long-term tests. And, there are some that won't meet the required 50-year stress-crack failures is very important because it is crystal creep modulus of 22,000 psi. These pipe samples are well suited clear that this is the main concern for corrugated pipe con- to evaluate some of the assumptions and long-term projections taining recycled HDPE. Of equal importance is validating made for corrugated drainage pipes. the proposed BFF test under a single set of temperatures Unfortunately, the times to grow stress cracks are quite and stresses as a quality control test. Such a test is necessary long in a field study. The research suggests that the maximum to ensure that pipe formulations containing recycled HDPE stress that will cause a stress crack instead of local buckling is have adequate stress-crack resistance over their intended 1,000 to 1,200 psi, depending upon the yield stress. So, in a service lifetime. field environment at 23C, it would take over 25 years to grow Finally, there are 80 feet of pipe left for each of the 15 samples. a crack in a sample with 30% recycled content. Acceleration The pipes have a variety of properties. Some do not meet the through temperature seems to be the only way to verify the cell classification for AASHTO M294 pipe. Some display low results of the lifetime estimates based on the BFF test.