CHAPTER 1

Introduction

1.1 Background

The asphalt pavement industry has a long history of using recycled materials in asphalt mixtures to achieve engineering, economic, or environmental benefits. One example is reclaimed asphalt pavement (RAP), which is one of the most recycled materials (Williams et al., 2024). Other recycled materials, such as recycled asphalt shingles, recycled tire rubber, waste engine oils, steel slag, and recycled glass, are continually used in specific markets or applications.

The use of plastics in asphalt is not a new concept. The first reported use dates to the 1970s in Europe, where high-density polyethylene (HDPE) was used in Gussasphalt for pourable asphaltic mixture applications (Bardesi et al., 1999). In the 1990s, there were considerable research efforts and field trials on a proprietary plastic-modified asphalt product marketed under the trademark Novophalt. This product requires a mobile high-shear blending unit (Figure 1) at asphalt plants to blend low-density polyethylene (LDPE), and styrene-butadiene-styrene (SBS) in later formulations, with asphalt binders just before mix production.

Although Novophalt was demonstrated on projects in nearly 20 countries, it did not gain widespread acceptance into mainstream practice due to limited material availability, transportation costs, and difficulties with on-site scheduling of blending equipment. Furthermore, early Novophalt projects had cracking performance issues, but there were no reported performance issues in later projects that used the Superpave performance grading (PG) system to guide the design of asphalt binders with LDPE. Around the same time, a product that utilizes a steric polymer stabilizer to mitigate the phase separation of polyethylene-modified asphalt binders, called Polyphalt, was also developed (Harbinson and Remtulla, 1994). Although this product showed promising laboratory results, it had economic and performance issues (i.e., cracking).

In late 2016, media reports began suggesting the use of recycled plastics in asphalt as an opportunity to improve the performance of asphalt pavements while eliminating the growing amount of waste plastics being landfilled or polluting the environment. This idea was spearheaded by the plastics industry after China and India imposed import prohibition policies on waste plastics. Since then, the plastics industry has been actively exploring new end-market opportunities for over 30 million tons of waste plastics generated every year (EPA, 2018). One potential application identified is asphalt pavements, while others include plastic composites, concrete, and wood composites (Plastics Industry Association, 2018).

The use of recycled plastics in asphalt can be challenging due to variations in composition and the presence of non-plastic contaminants. To ensure the quality of asphalt mixtures containing recycled plastics, requirements are needed to specify the key properties of these materials and how they should be used to improve pavement life-cycle benefits while protecting the environment. Furthermore, post-industrial recycled (PIR) plastics and post-consumer recycled (PCR)

Source: Image sourced with permission from Advanced Asphalt Technologies, LLC.

plastics are likely to differ in composition, consistency over time, amount of contaminates, and degradation potential.

There are two main approaches for incorporating recycled plastics into asphalt mixtures: the wet process and the dry process (Willis et al., 2020; Yin et al., 2020a). In the wet process, recycled plastics are added into the asphalt binder as a polymer modifier or asphalt replacement using mechanical mixing at high temperatures to produce a reasonably homogeneous recycled plastic–modified (RPM) binder. A primary challenge of wet process RPM binders is the tendency for the plastic to separate in tanks due to differences in specific gravity, phase, or chemical incompatibility with the asphalt binder. In the dry process, recycled plastics are added directly into the mixture as either aggregate replacement, mixture modifier, binder modifier, or a combination thereof. National Asphalt Pavement Association’s (NAPA’s) publication NAPA-IS-142 suggests that the wet process is commonly used for recycled plastics with a low melting point (in the 105°C to 150°C temperature range), such as LDPE and HDPE. Meanwhile, the dry process is applicable to virtually all types of recycled plastics, with the exception of polyvinyl chloride (PVC) due to the concern of hazardous chlorine-based dioxin emissions (Willis et al., 2020; Yin et al., 2020a). The dosage of recycled plastics varies from approximately 1.0% to 12.0% by weight of asphalt binder for the wet process, and the dosage varies from approximately 0.2% to 6.0% by weight of aggregate for the dry process.

Most existing studies on the dry process used the Marshall stability test to evaluate the impact of recycled plastics on the properties of asphalt mixtures and found that RPM mixtures had higher Marshall stability than control mixtures (Khurshid et al., 2013; Aschuri and Woodward, 2010). Because of this observation, some researchers have suggested that adding recycled plastics could extend the service life of asphalt pavements due to enhanced rutting resistance. Such suggestions lack validity since the Marshall stability has a poor correlation with field rutting performance, and it ignores the fact that the service life of asphalt pavements also depends on cracking performance. Only a few studies assessed the cracking and moisture resistance of RPM asphalt mixtures, but they did not yield consistent conclusions. A study in Europe showed that adding HDPE resulted in excessive hardening and contraction of asphalt binders during cooling (Bredael, 1993).

More needs to be learned about how recycled plastics interact with asphalt binders and aggregates in asphalt mixtures using the dry process. Questions such as “Does the plastic coat the aggregate, become part of the asphalt binder, act as an aggregate, or act as a reinforcement similar to fibers?” need to be answered. Furthermore, it remains unknown how recycled plastics affect the volumetric properties, workability, and surface characteristics of asphalt mixtures.

A comprehensive research effort is needed to understand the long-term effects of recycled plastics on various aspects of asphalt pavements, including life-cycle costs, worker safety, environmental impact, and recyclability.

1.2 Research Objectives

The overall objective of this project was to evaluate the impact of PCR plastics on the performance properties of RPM asphalt mixtures when they are added using the dry process. Specifically, this project sought to

1. Evaluate the rutting, cracking, moisture, and aging resistance of RPM asphalt mixtures using well-established mixture performance test methods.
2. Determine the impact of PCR plastics on volumetric properties and surface friction and texture characteristics of asphalt mixtures.
3. Assess the impacts of recycled plastics on mix production, process control, workability, and constructability of asphalt mixtures.
4. Develop a laboratory procedure for adding PCR plastics to simulate the production of RPM mixtures at asphalt plants.
5. Characterize the mingling of PCR plastics with asphalt binders in the dry process and their impacts on the rheological and chemical properties of asphalt binders extracted and recovered from RPM asphalt mixtures.
6. Provide guidelines on the selection of PCR plastics for use in asphalt via the dry process.

1.3 Research Approach

The research approach undertaken in this project consists of a literature review and a laboratory work plan with five experiments. The literature review was built on NAPA-IS-142, which synthesized over 110 literature documents on the use of recycled plastics in asphalt (Willis et al., 2020; Yin et al., 2020a), and included new literature published since January 2020. Experiment 1 of the laboratory work plan focused on selecting various types of PCR plastics by characterizing their key physical, thermal, and chemical properties considered important for use in asphalt mixtures via the dry process. Experiment 2 aimed to determine the performance properties and surface characteristics of plant-produced RPM versus control asphalt mixtures from two field projects as well as characterize the rheological and chemical properties of extracted asphalt binders. Experiment 3 involved surveying asphalt contractors with experience in the production of dry-process RPM asphalt mixtures and conducting exploratory quality control (QC) testing of PCR plastics using a near-infrared spectrometer. Experiment 4 aimed to develop a laboratory procedure for adding PCR plastics to simulate the production of dry-process RPM mixtures at asphalt plants. Lastly, Experiment 5 focused on evaluating the performance properties of laboratory-produced RPM asphalt binders and mixtures prepared with various sources and types of PCR plastics. Based on the findings of the literature review and laboratory work plan, conclusions and recommendations on the appropriate and effective use of recycled plastics in asphalt mixtures via the dry process are provided. Furthermore, suggested future research to address remaining knowledge gaps is provided.