Mechanical Properties of Laboratory Produced Recycled Plastic Modified (RPM) Asphalt Binders and Mixtures

In late 2016, media reports and online networks began generating an interest in the possibility of using recycled plastic waste in asphalt mixtures. The idea was marketed as an opportunity to simultaneously improve the quality of asphalt pavements and help address the issue of waste plastic in cities, towns, and waterways across the U.S. While magazine articles and videos have trumpeted positive impacts of using recycled plastic modified (RPM) asphalt, such as increased service life and reduced need for polymers to modify asphalt binders, and while preliminary research suggests some of these benefits maybe realized, a full set of research to confidently back these claims is lacking.

The current waste plastic challenge is a critical concern; however, there is equal concern about the current state of the U.S.’s aging transportation infrastructure. Investment in our system must focus on delivering long-lasting, high-performing pavements as cost-effectively as possible. This research will be used to assess the feasibility of using RPM asphalt as a sustainable solution for improving both the performance of asphalt mixtures and reducing the amount of plastic waste in the US.

The objective of this research is to evaluate the impact recycled plastics, (including but not limited to: low density polyethylene (LDPE), high density polyethylene (HDPE), and Polypropylene), have on the mechanical properties of both asphalt binders and mixtures when added to asphalt binders using a wet process or asphalt mixtures using a plant-mixed or dry process.

Currently, the use of recycled plastics being used to improve the performance of asphalt mixtures is being marketed as a solution to help stem the plastic waste issue worldwide. While marketing is being conducted, there is little to no scientific research which backs up the current claims. In the near future, state and federal agencies may be prompted to make decisions on the use of RPM asphalt binders and mixtures without adequate data to support their decisions. This research project will provide agencies with the data they can use to decide if RPM asphalt is a viable option for use on their highway infrastructure.

Standard asphalt mixture design specifications may need to be altered to include the use of recycled plastics. A new standard which include acceptable plastic dosage rates may need to be developed. Currently, the National Asphalt Pavement Association and the Asphalt Institute have a joint task force evaluating this work. This group should be included in the coordination to ensure that industry understands the research implications on operations. This research will also need to be followed with field projects and evaluations on worker health & safety, long-term field performance, environmental impacts of milling RPM asphalt mixtures, plant emissions from RPM asphalt mixtures, and recyclability of RPM asphalt mixtures.

A number of international papers have recently appeared extolling the virtues of using waste plastics in asphalt. Such “recycled” plastics can be included as a substitute for aggregates, as an aggregate coating, as an asphalt binder modifier, or some combination of the three. For example, Dalhat et al. (2019) used “Recycled Plastic Waste (RPW)” consisting of a mixture of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS) as a partial aggregate substitute in asphalt mixtures. They also modified the asphalt binder with RPW. The experiment included so many variables and modifiers that it was highly confounded, but the authors did conclude that using recycled plastics in asphalt could enhance asphalt mixture rutting characteristics.

Pre-coating aggregates with recycled plastics prior to their incorporation into asphalt mixtures has also been studied by researchers who have reported that recycled plastic materials tend to increase aggregate toughness, while decreasing water absorption. Asphalt mixtures containing plastic coated aggregates have also tended to show improved asphalt mixture moisture susceptibility properties.

Using recycled plastics as asphalt binder modifiers appears to decrease binder penetration and ductility and increase softening point and viscosity. Researchers have used such results to indicate the use of recycled plastics in asphalt binders would be good for areas that struggle with permanent deformation in asphalt pavements. These same authors are mostly silent about what such binder modification could do for pavement cracking, as stiffening binders will tend to increase cracking.

Finally, when incorporated in asphalt mixtures, researchers tend to agree that recycled plastics appear to enhance an asphalt mixture’s mechanical properties, although Dalhat et al. (2019) are one of the few groups that have used more modern mixture test methods, such as determining dynamic modulus and flow number and using a wheel tracking test. Other international researchers have used Marshall stability and flow to determine the merits of recycled plastics in asphalt, with experimental plans that are often lacking.

While there does appear to be a mounting body of literature on the use of recycled plastics in asphalt, much of the work being reported has lacked a clear experimental plan and suffers from the use of dated test methods. Additionally, from a review of the literature, it does not appear to be a cohesive, well thought out plan to answer the many questions raised about recycled plastics in asphalt.

The objective will be met by completing the following tasks:

1. Develop a work plan for completing Tasks 2 through 7.

2. Conduct a literature review of current and past research related to the use of recycled plastics in asphalt mixtures. This review should include both laboratory and field studies which provide a clearer understanding of RPM asphalt mixture performance.

3. Write an interim report including the results of Tasks 1 and 2.

4. Determine the impacts of recycled plastics on the rheological stability properties of asphalt binders. This work should evaluate the impacts of binder source, plastic dosage rate, and stabilizer, cohesive and adhesive properties, and aging on rheological and stability properties.

5. Determine the impacts of recycled plastics on the mechanical properties of asphalt mixtures using RPM asphalt binders and RPM asphalt mixtures where the plastic was introduced using a dry process. At a minimum, these mixtures should be evaluated for low temperature cracking, fatigue cracking, top-down cracking, rutting, and moisture susceptibility performance.

6. Develop a best practices manual for handling and using recycled plastics in a laboratory setting. If Tasks 2 through 5 warrant it, consideration should be also made on handling and quality control of the incoming RPM streams.

7. Produce a final report and hold a workshop with state agencies, the Federal Highway Administration, and industry to discuss the results. The final report will include changes to AASHTO asphalt standards if warranted.

Materials and Design Engineers will be responsible for using the research results. Specifications may need to be modified or changed for the use of this material. Most DOT’s have the capability within their own organizations to modify or changes specifications upon the receipt of new information or research results. AASHTO COMP is ready to review the results of this and make necessary changes in its standards if necessary.