Field Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content (2018)

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1 S u m m a r y The purpose of this research project was to establish the field performance of corrugated high-density polyethylene (HDPE) pipe manufactured with recycled content and to propose guidelines for manufacturing these pipes to ensure they meet the service life requirements for the given application. This research project expounded on the research previously con- ducted in NCHRP Project 4-32 and reported in NCHRP Report 696 (1), which was primarily a materials and laboratory study for corrugated pipes manufactured with recycled materials. The research consisted of manufacturing several large-diameter corrugated HDPE pipes out of various blends of virgin and post-consumer recycled (PCR) materials commonly used in land drainage applications and evaluating these pipes in the field and laboratory to deter- mine their service life in typical installed conditions. PCR materials were the focus of this project as they are more readily available and typically used in the corrugated HDPE pipe industry than post-industrial recycled (PIR) materials. However, the research is applicable to both types. The recycled content in the manufactured pipes ranged from 0% to 98%. The pipes were evaluated in accordance to AASHTO M 294, “Standard Specification for Corrugated Poly- ethylene Pipe, 300- to 1500-mm Diameter” (2), the primary specification for culvert and storm drainage pipes for highway applications. Because of the successful and established history of corrugated HDPE pipes manufactured to the M 294 standard in highway appli- cations, it was decided to use this standard as a benchmark for pipes manufactured with recycled materials, even though AASHTO M 294 currently only allows the use of virgin materials. Figure 1 shows an illustration of the typical failure modes for polyethylene. At high stresses, the material fails in a ductile manner (Stage I), while at lower stresses the failure mechanism changes to brittle (Stages II and III). The primary governing failure mode for the service life of corrugated HDPE pipe is Stage II brittle failures, also known as slow crack growth (SCG). Gravity-flow corrugated HDPE pipes do not experience high enough tensile stresses to generate Stage I ductile failures, and Stage III (chemical) failures are prevented by ensur- ing sufficient antioxidants and carbon black are present in the material formulation. For these reasons, the focus of this research project from a service life standpoint was Stage II SCG failures. The SCG mechanism comprises two phases: the crack initiation phase and the crack propagation phase (3). For corrugated HDPE pipes manufactured with virgin materials, Stage II failures are prevented by utilizing a material with high stress-crack resis- tance as measured by the notched constant ligament stress (NCLS) test in accordance with ASTM F2136 (4). The NCLS test is designed to assess the crack propagation phase of the SCG mechanism, as it is conducted on specimens with an artificially manufactured notch (i.e., crack initiation site), but it does not assess the crack initiation phase. For corrugated HDPE pipes manufactured with recycled materials, the crack initiation phase is perhaps Field Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content

2the most important portion of the SCG mechanism to evaluate since the likelihood of a contaminant or void or other stress riser is greater in these materials than virgin materi- als. To assess the crack initiation phase, the BAM (German Federal Institute of Materials Researching and Testing)–Florida Department of Transportation–Fathead (BFF) test devel- oped in NCHRP 4-32 (1) was utilized. It was refined and standardized as the unnotched constant ligament stress (UCLS) test and published as an ASTM test method as part of this research project (5). When conducting the UCLS test at multiple elevated temperatures and stresses, the prin- ciples of time-temperature superposition and bi-directional shifting can be applied to shift the data and predict the long-term performance of these materials at service conditions. Using this methodology, a service life prediction model was created and evaluated for each of the test pipes. An illustration of shifted data and service life prediction for one of the test pipes is shown in Figure 2. To validate the failure model predicted by the UCLS test, full-scale pipes were evaluated in both the laboratory and in simulated field conditions. While the UCLS test is conducted at a constant stress, the validations on full-scale pipes were conducted at a constant strain Figure 1. Illustration of three failure modes for typical polyethylene materials, with failure time plotted as a function of service stress (log scale). 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Lo g St re ss (p si ) Log Time (h) UCL LCL Mastercurve at 23 deg. C y = -0.1709x + 3.7638 R2 = 1.0000 Figure 2. Elevated temperature UCLS failure data shifted to 23°C service conditions to predict service life.

3 (i.e., deflection). This is more typical of field installations. A relationship between the con- stant strain conditions typically observed in the field and the constant stress conditions of the NCLS and UCLS tests was developed. To accelerate the testing, the pipes were installed in highly deflected conditions both in the laboratory and the simulated field test. These conditions resulted in the generation of elevated tensile strains (and hence, tensile stresses) in the pipe wall. If the tensile stresses are high enough (e.g., greater than 1000 psi), one could expect failure times in a year or less, depending on the material (Figure 2). Specifically, test pipes were deflected until the vertical inside diameter was reduced by 20% in a parallel plate test in the laboratory, resulting in peak tensile strains of around 3.0% in the springline of the pipe and 3.75% in the crown. Similarly, pipes were installed with loosely compacted silt backfill materials inside fabricated precast reinforced concrete chambers and loaded to a simulated burial depth of 9.1 m (30 ft), resulting in vertical deflections of around 12% to 15%. Peak local tensile wall strains were around 3.5%, similar to that observed in the parallel plate test in the laboratory. In the parallel plate test, one of the three test pipes was predicted to develop brittle stress cracks within a year of testing based on the UCLS service life prediction model. In the simulated field test, four of the seven test pipes were predicted to stress crack. In each case, the pipes that were predicted to crack developed brittle cracks, while the ones that were not predicted to crack did not. Furthermore, it was shown that the UCLS service life prediction model based on testing coupons of pipe at elevated temperatures provided a very good esti- mate of the service life of full-scale pipes in actual field conditions. Based on these results, the service life prediction model was validated, and it was concluded that the UCLS test provides the basis for a true performance-based test that can be used to accurately predict the service life of corrugated HDPE pipes manufactured with recycled materials. To further evaluate the field performance of corrugated HDPE pipes manufactured with PCR materials, two 750 mm (30 in.) diameter pipes—one manufactured with 49% PCR materials and the other manufactured with 0% PCR materials—were installed underneath an active commuter railroad north of Philadelphia. To amplify live loads, the pipes were installed with just 2 ft of cover from the top of the pipe to the bottom of the railroad tie. Wall strains and deflections were monitored over 3 years of service, and a laboratory fatigue test was developed to evaluate the pipes’ performance relative to cyclical live loads. Test speci- mens taken directly from the walls of the pipes were tested for up to 10 million cycles with no failures. Additionally, there were no differences in performance observed between the two field pipes. Based on this, it was concluded that fatigue due to live loads in both highway and rail is not a concern for pipes manufactured with or without recycled materials. Deliverables of the project included the development of a performance-based specifica- tion based on the UCLS test method that can be used to assure the desired service life of corrugated HDPE pipes manufactured with recycled materials. Additionally, revisions were proposed to AASHTO M 294 to incorporate manufacturing and performance criteria for pipes containing recycled materials. The proposed revisions included minimum UCLS test requirements for pipes manufactured with recycled materials. A Standard Recommended Practice was developed for predicting the service life of corrugated HDPE pipes manufac- tured with recycled materials, and design guidelines were established for incorporation into the AASHTO design specifications for thermoplastic pipe. Based on this research, it was shown that corrugated HDPE pipes can be successfully manufactured with recycled materi- als to meet the service life requirements for highway and railroad applications.