SUMMARY

Evaluation of Post-Consumer Recycled Plastics in Asphalt Mixtures via the Dry Process

NCHRP Project 09-66, “Performance Properties of Laboratory Produced Recycled Plastic Modified (RPM) Asphalt Binders and Mixtures,” evaluated the impact of post-consumer recycled (PCR) plastics on the performance properties of asphalt mixtures when added using the dry process. To that end, a literature review was conducted to synthesize existing studies and identify knowledge gaps on topics relevant to using recycled plastics in asphalt, including the availability and sourcing of recycled plastics, methods of incorporating recycled plastics, laboratory characterization of recycled plastic–modified (RPM) asphalt binders and mixtures, plant operations, construction, health and safety, environmental impacts, field project performance, and other potential civil engineering applications.

A laboratory work plan was then conducted to address several knowledge gaps in the literature review. The work plan consisted of five experiments designed to (1) characterize the critical properties of recycled plastics for use in asphalt mixtures; (2) evaluate the performance properties and surface characteristics of plant-produced RPM mixtures versus control mixtures without recycled plastics from two field projects; (3) explore the use of a near-infrared spectrometer for quality control testing of PCR plastics; (4) evaluate different laboratory procedures for adding PCR plastics to simulate the production of dry-process RPM mixtures at asphalt plants; and (5) characterize the performance properties of laboratory-prepared RPM mixtures with different sources and types of PCR plastics.

Based on the experiment results and findings, the following conclusions can be made:

• The source, type, and recycling process of PCR plastics will impact their physical, thermal, and chemical properties. Most PCR plastics composed primarily of polyethylene or polypropylene will have melting temperatures below or within the typical production temperature range of asphalt mixtures. Some PCR plastics could have high levels of volatile organic compounds and contain detectable benzene, which raises safety concerns for potential use in asphalt mixtures.
• Adding dry-process PCR plastics using a drop-in approach, without making other mixture modifications, generally increases the stiffness and rutting resistance; reduces the workability, intermediate-temperature cracking resistance, and block cracking resistance; and has negligible impacts on the low-temperature cracking resistance, surface texture, and friction properties of asphalt mixtures. The effect on moisture susceptibility appears to be mix design–specific and warrants further investigation. Since cracking is the most common mode of distress in asphalt pavements in the United States, using recycled plastics via the dry process is not expected to provide performance benefits.
• Adding PCR plastics via the dry process does not significantly affect the rheological and chemical properties of extracted asphalt binders. However, this finding should be interpreted cautiously because it could be confounded by the lack of solubility of PCR plastics in asphalt extraction solvents.

• Different laboratory procedures for adding PCR plastics via the dry process affected the performance properties of the resultant RPM mixtures since the plastics are exposed to different thermal conditions during the mixing process. The four procedures evaluated in the project did not release meaningful amounts of hazardous fumes or polycyclic aromatic hydrocarbons, but none of the procedures yielded laboratory-prepared RPM mixtures that matched the properties of corresponding plant-produced mixtures.

This report includes five chapters. Chapter 1 discusses the background, research objectives, and research approach of the project. Chapter 2 presents literature review findings on the use of recycled plastics in asphalt mixtures. Chapter 3 presents the experimental design of the laboratory work plan, which consists of five experiments. Chapter 4 presents the test results and findings from each of the laboratory experiments. Finally, Chapter 5 summarizes the conclusions of the project and suggested future research needs based on the findings of the study.

In addition, a guide document was developed to provide state highway agencies and the asphalt pavement industry with suggestions for sourcing PCR plastics and ensuring their safe handling and use in mix design, production, construction, and quality assurance testing of dry-process RPM asphalt mixtures. The guide is not intended as an endorsement for using recycled plastics in asphalt mixtures but rather as guidance for future technological advancement and evaluation of recycled plastics that could contribute to the performance and sustainability of asphalt pavements. Workshop materials and the contractor questionnaire, an annotated bibliography, and a mixture results database are also provided. These materials are available on the National Academies Press webpage for NCHRP Research Report 1143: Evaluation of Post-Consumer Recycled Plastics in Asphalt Mixtures via the Dry Process (https://doi.org/10.17226/28867) under “Resources.”