CHAPTER 5

Conclusions and Future Research

This chapter provides the conclusions of the project based on the results and findings of the laboratory work plan discussed previously. The chapter also provides suggested future research on using recycled plastics in asphalt mixtures. Furthermore, a guide for using recycled plastics in asphalt mixtures via the dry process is provided as a separate deliverable of the project, which includes (1) recommendations for sourcing and certification of recycled plastics, (2) recommendations for QC testing of recycled plastics, and (3) recommendations for production, construction, and QA testing of dryprocess RPM asphalt mixtures. This guide can be found on the National Academies Press webpage for NCHRP Research Report 1143 (https://doi.org/10.17226/28867) under “Resources.”

5.1 Conclusions

5.1.1 Property Characterization of Recycled Plastics

The 12 PCR plastic samples evaluated in the project show a wide range of physical, thermal, and chemical properties, which can be attributed to the different sources, types, and recycling processes of plastics used. With the exception of one sample, the plastic samples demonstrated an initial melting temperature between 121°C and 164°C, either below or within the typical production temperature range of asphalt mixtures. All plastic samples exhibit minimal solubility in TCB at 20°C to 35°C, posing challenges for solvent extraction of RPM asphalt mixtures for rheological and chemical characterization of binders. Additionally, four plastic samples had high fume and emission potential (VOCs over 150 μg/g), and one sample had detectable benzene (at a concentration of 1.9 μg/g), which raises potential safety concerns for using PCR plastics in asphalt mixtures. These laboratory characterization results highlight the complexity of PCR plastics and the need to regulate their sources and properties for potential use in asphalt mixtures from material consistency and safety perspectives.

5.1.2 Impacts of Adding Dry-Process PCR Plastics on Properties of Asphalt Mixtures and Binders

Table 33 summarizes the performance properties of dryprocess RPM versus control mixtures evaluated in the project, including plantproduced mixtures from Experiment 2 and laboratoryprepared mixtures from Experiment 5. Adding dryprocess PCR plastics using a dropin approach, without making other mixture modifications, generally increases the stiffness, rutting resistance, and aging resistance; reduces the workability, intermediatetemperature cracking resistance, and block cracking resistance; and has negligible impacts on the lowtemperature cracking resistance, surface texture, and friction properties of asphalt mixtures. These impacts vary depending on the PCR plastics used. Specifically, the MFI of the PCR plastics has a strong

Table 33. Summary of performance properties of dry-process RPM mixtures compared to control mixtures.

positive correlation with the rutting resistance but a strong negative correlation with the workability and block cracking resistance of the resultant dryprocess RPM mixtures. There is a mixed trend in the impact of adding PCR plastics on the moisture resistance of asphalt mixtures, especially those prepared in the laboratory using different mix designs. Adding PCR plastics via the dry process does not appear to affect the properties of extracted and recovered asphalt binders. However, this conclusion is likely confounded by the lack of solubility of PCR plastics in extraction solvents, which would separate any melted and incorporated plastic from the asphalt binder. This observation warrants further investigation.

5.1.3 Laboratory Method of Adding PCR Plastics Using the Dry Process

Using different methods of adding PCR plastics in the laboratory could affect the performance properties of dryprocess RPM asphalt mixtures, as the plastics would be subject to different thermal conditions during the mixing process, impacting their interaction with other mixture components. None of the four methods evaluated in this study resulted in laboratoryprepared RPM mixtures with performance properties comparable to the corresponding plantproduced mixtures. This discrepancy between labprepared and plantproduced mixture test results may be attributed to the differences in thermal (i.e., mixture aging) and physical mixing conditions that occur during laboratory mixture preparation versus plant production. From a laboratory safety perspective, none of the four methods of adding PCR plastics released meaningful amounts of hazardous fumes or PAHs during the mixing of dryprocess RPM asphalt mixtures.

5.1.4 Production and Construction of Dry-Process RPM Asphalt Mixtures

A survey of U.S. asphalt contractors for recent field projects indicated no noticeable difference in the production and constructability of dryprocess RPM asphalt mixtures compared to the same mixtures without recycled plastics. In some cases, differences were observed that were similar to those of mixtures with polymermodified asphalt binder, where RPM mixtures were found to be stiffer and more difficult for handwork than traditional mixtures with unmodified asphalt binder. The known field projects to date have used a variety of plastic types; most have used proprietary recycledplastic products. In all projects, the plastic was utilized for asphalt mixture production “as provided,” without any QC testing of the plastic material. In most cases, the

plastic was added to the asphalt plant using a fiber feeder to introduce the plastic at the point where RAP is typically added to the mixing process.

5.2 Future Research

This section briefly describes six topics suggested for further development as research needs statements to better understand and document the potential benefits or drawbacks of using recycled plastics in asphalt mixtures.

5.2.1 Long-Term Performance Monitoring and Documentation of RPM Asphalt Pavements

Since 2018, more than 20 field projects have been constructed in the United States using RPM asphalt mixtures, according to TRB Special Report 347: Recycled Plastics in Infrastructure: Current Practices, Understanding, and Opportunities (2023). Many of these projects were parking lots and privately owned roads, which provided opportunities to demonstrate the use of recycled plastics in asphalt. However, they offer limited experimental value due to the lack of control sections for comparison or absent documentation of the underlying pavement conditions and other construction aspects.

Table 34 lists field projects that may have experimental value for evaluating the longterm performance of RPM asphalt pavements. Since these projects are only a few years old, no significant distresses are yet evident to quantify the impacts of recycled plastics on pavement performance. Therefore, continued performance monitoring of these projects for 5–10 years is needed to assess the impact of using recycled plastics on the performance of asphalt pavements, especially longterm cracking performance. Furthermore, a comprehensive synthesis of the design, production, and performance data and lessons learned from these field projects is highly recommended to advance the existing body of knowledge on this topic.

5.2.2 Field Evaluation of Microplastics from RPM Asphalt Pavements

The potential release of microplastics from the weathering and milling of asphalt pavements containing recycled plastics is a major environmental concern identified in NAPAIS142. Several

Table 34. RPM field trials with experimental value to assess long-term performance.

^*VTRC = Virginia Transportation Research Council.

laboratory studies have recently investigated this topic (Enfrin et al., 2022; Boom et al., 2023; Duan et al., 2024). In these studies, RPM asphalt mixtures and conventional mixtures without recycled plastics were first subjected to surface abrasion, polishing, pore pressure cycling, or wheel loading while immersed in water to simulate the surface weathering of asphalt pavement in the field. During this process, representative samples of surface water, bulk water, and sediments were collected and analyzed to detect the presence of potential plastic residues. Although the analysis methods used by these studies varied, they typically required dissolving the asphalt binder, removing organic matter, separating microplastics from the aggregate, extracting microplastics using filters, and detecting microplastics using a fluorescence microscope. Overall, these studies concluded that using recycled plastics did not exacerbate the release of microplastics from asphalt pavements into the environment. However, it should be noted that this conclusion is based only on laboratory evaluations and should be further verified with field evaluations that consider the actual weathering and milling of asphalt pavements in the field, beyond the simulated conditions in a laboratory setting. Ongoing work with the field projects in Pennsylvania and Virginia, listed in Table 34, includes evaluating the generation of microplastics from the test sections.

5.2.3 Identifying Suitable Chemical Solvents for Asphalt Extraction of RPM Asphalt Mixtures

This project conducted limited rheological and chemical testing of asphalt binders extracted and recovered from dryprocess RPM versus control asphalt mixtures prepared in the laboratory and at asphalt plants. The results consistently showed that the RPM binders (extracted using a blend of 85% toluene and 15% ethanol solution) had no presence of plastics and exhibited properties comparable to the control binders without recycled plastics. Although these results may indicate that adding recycled plastics via the dry process has no impact on the asphalt binder, they should be interpreted with caution because they could be confounded by the limited solubility of recycled plastics in the toluene/ethanol blend used for asphalt extraction (and in many of the other solvents allowed in AASHTO T 164). Because of this limitation, future research is needed to identify suitable solvents and solubilization conditions (i.e., mixing temperature and time) for recycled plastics that can effectively extract asphalt binders from RPM mixtures for rheological and chemical characterization.

5.2.4 Microscopy Evaluation of Dry-Process RPM Asphalt Mixtures

This project successfully determined the impacts of adding recycled plastics via the dry process by comparing the laboratory performance properties of RPM mixtures versus control mixtures without recycled plastics. However, the mechanisms of those impacts are not well understood, mainly due to the unknown fate of the recycled plastic during the asphalt mixture production process. Although several studies suggest that recycled plastics, especially those with relatively low melting points, will melt and coat the aggregate particles before mixing with the asphalt binder, that hypothesis has not been verified by scientifically established evidence. Limited work in this project found that a considerable portion of recycled plastics added via the dry process remained as discrete particles within the mixtures by visually observing the fractured or cut faces of mixture specimens. This casts doubt on the plasticcoatingaggregate hypothesis.

Figure 133 shows images from an optical microscope of a plantproduced asphalt mixture specimen cross section in normal light and under a blue light with an orange filter. From these images, it is evident that recycled plastic is not dispersed and does not coat aggregate particles, but rather it exists as deformed masses in isolated regions with the mixture. Further research is needed to better examine the fate of recycled plastics in the mixture when added via the dry process. Potential approaches include fluorescence or darkfield microscopy to detect and quantify

the presence of recycled plastics in dryprocess plantproduced RPM mixtures (Wegan and Bredahl Nielsen, 2000; Wegan, 2001).

5.2.5 Wet-Process Modification of Asphalt Binders with Recycled Plastics

The scope of this project was intentionally set to evaluate only the dryprocess modification of asphalt mixtures with recycled plastics, mainly because FHWA has funded a project through its Exploratory Advanced Research program on the compatibilization of waste plastics for asphalt binder modification via the wet process. The objective of the project, awarded to Louisiana Tech University in 2020, is to gain a systematic understanding of waste plastic–asphalt binder compatibilization, a significant challenge given the propensity of RPM binders to phase separate. According to FHWA (2021), major expected outcomes of the project include developing screening tools that can predict the compatibility of various waste polymers and asphalt binders before mixing; identifying and optimizing the most promising approaches to waste plastic–asphalt binder compatibilization; understanding the mechanism of compatibilization between waste polymers and asphalt binders; and developing a coarsegrained molecular model of waste plastic–asphalt binder blends. Future research is suggested to verify and leverage these tools to advance the use of recycled plastics in asphalt via the wet process. Future research is also needed to evaluate the compatibility of recycled plastics with other additives used in asphalt binders, such as warmmix asphalt additives, antistrip agents, and recycling agents.

5.2.6 Using High-Melting-Point Recycled Plastics as an Aggregate Replacement

The Austroads study (Giustozzi et al., 2021a; 2021b; 2022) suggests that recycled plastics with high melting points have the potential to be used in asphalt mixtures as an aggregate replacement. This suggestion is based on the hypothesis that recycled plastics with melting points that are 30°C above the asphalt mixing temperature will behave like aggregate particles without interacting with the asphalt binder; thus, these recycled plastics will not stiffen the mixture nor make it more susceptible to pavement cracking and durability issues. Another potential advantage of this approach over traditional approaches is that it allows for the use of significantly higher quantities of recycled plastics in the asphalt mixture, which could benefit regions with aggregate shortage issues. However, there is limited information on this approach in the literature; thus, future research is needed to verify its feasibility and potential applications.