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101 6 Applications of Recycled Plastics in Pavements One of the greatest opportunities for using recycled plastics in infrastructure may be as a component material in pavements. Although some articles in the press and social media have claimed that âplastic roadsâ are a win-win and a solution to both the growing plastics waste problem and a means to achieve better roads and highways (Friedrich 2021; Lee 2021), the science to back those claims is not there yet. This chapter presents what is known and not known about the potential of using recycled plastics in pavements. In the United States there are three main types of pavements: flexible, rigid, and composite. The main differences among these are in their con- struction materials and design, and the way each distributes traffic loads over the subgrade. Flexible pavements are constructed using multiple lay- ers of asphalt mix. They have low stiffness and distribute traffic load over a narrow area of the subsurface. Rigid pavements are constructed with concrete and Portland cement or other pozzolanic additive cements. They have a high stiffness and distribute loads over a wide area of the subsur- face. Composite pavements are constructed with both hot-mix asphalt and Portland cement concrete and behave as rigid pavements under traffic loading (Texas DOT 2021). Figure 6-1 illustrates three general pavement structures to help explain the different types of pavements commonly used in the United States. The chapter is organized into two sections. The first focuses on the potential to use recycled plastics in asphalt pavements. The second section addresses the potential to use recycled plastics in granular base and subbase layers of any pavement structure. While concrete pavements are used in a substantial number of roads in the United States, no significant research
102 RECYCLED PLASTICS IN INFRASTRUCTURE and development on the use of recycled plastics has been undertaken for this application. Some preliminary studies have been conducted on the use of recycled plastic in Portland cement concrete for general construction applications; a summary of those studies is provided in Chapter 7 and in Appendix F. RECYCLED PLASTICS IN ASPHALT PAVEMENTS The potential to use recycled plastics in asphalt pavements seems to be a significant opportunity to impact the plastics waste management challenge simply due to the large volume of asphalt pavement materials produced and constructed each year. In the United States, approximately 94 percent of all paved roads and highways are surfaced with asphalt pavement (Buncher 2022). The other 6 percent of paved roads in the United States are con- crete pavements. In addition, asphalt pavements are also widely used in numerous other infrastructure applications such as airfields, port facilities, parking lots, as well as many recreational surfaces. To maintain the 2.5 mil- lion miles (4 million km) of roads in the United States, paving contractors produce approximately 363 million tons of asphalt paving mixtures each year (Williams et al. 2021). FIGURE 6-1 Cross-sections of flexible (asphalt) pavement, rigid (concrete) pave- ment, and composite pavement.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 103 The possible quantity of plastics that could be consumed in asphalt pavement applications will depend on the cost of the recycled plastic sup- ply relative to other additives that provide similar benefits, the dosage rate (which will depend on the method of adding the recycled plastic, described in this section), and the benefits to pavement performance, if any. An opti- mistic estimate of plastic use in asphalt pavements based on a dry process dosage rate of 0.5 percent by weight of mix used1 in 12 percent of U.S. annual asphalt mixture production would consume approximately 240,000 tons of plastic per year, which is approximately 2.4 percent of the waste polyethylene generated in the United States each year.2 This estimate pro- vides a framing context for the information that follows. Background As shown in the photograph of an asphalt pavement core in Figure 6-2, roadways are typically built using several layers of asphalt mixtures with each layer designed to provide certain characteristics at the lowest cost. Each state department of transportation (DOT) sets specifications for asphalt mixtures in their jurisdiction. Contractors design, produce, and construct the asphalt mixtures and pavements in accordance with those specifications in a competitive, lowest-bid-wins system of procurement for pavement construction and rehabilitation. In general, an asphalt mixture contains 4 to 6 percent asphalt binder and 94 to 96 percent aggregate, by mass. In some cases, other mixture ad- ditives may be required at low dosages in order to meet certain specifica- tion requirements. Most state DOT specifications allow reclaimed asphalt pavement (RAP) to be used in asphalt mixtures, although the maximum percentage allowed varies considerably from state to state and by layer and roadway classification. Nationally, the average RAP content is esti- mated to be 21 percent (Williams et al. 2021). Other recycled materials, such as asphalt shingles, ground tire rubber, and cellulose fiber from waste paper are allowedâsometimes even requiredâin asphalt mixtures by some DOT specifications. For certain pavement applications such as highways with high percentages of freight traffic, DOT specifications commonly re- quire the use of high-grade asphalt binders, commonly referred to as poly- mer-modified asphalt binders, to provide the asphalt mixture with better 1 Of the 15 documented field trials of dry process addition of recycled plastics in the United States, the highest dosage is 0.5 percent by weight of mix or aggregate (in three cases). 2 According to Asphalt Institute data, the percentage of polymer-modified asphalt sales has ranged from 11 to 14 percent of total asphalt binder sales from 2010 to 2021 (Asphalt Institute 2010-2021). Using 12 percent as a reasonable value, this estimation assumes a best- case scenario in which recycled plastics would completely displace virgin polymers in asphalt mixtures.
104 RECYCLED PLASTICS IN INFRASTRUCTURE resistance to damage caused by heavily loaded trucks and extreme weather conditions. The most common polymer used to modify asphalt binders for high-grade applications is the block copolymer styrene-butadiene-styrene (SBS), an engineered virgin plastic (specifically, a thermoplastic elastomer) designed to be compatible and storage-stable with asphalt binders and to produce specified properties in the modified binder. The cost of the raw materials and their percentages in asphalt mixtures are primary factors in a contractorâs bid for asphalt paving for new construction or rehabilitation projects. Table 6-1 provides typical costs of raw materials for asphalt mix- tures for the summer of 2022. It should be noted that asphalt binder prices vary considerably based on the crude oil market, and the costs in Table 6-1 are near the historical high points. In general, contractors will design their asphalt mixtures to meet the specifications while 1. Minimizing the asphalt content of the mixture since it is, by far, the most expensive component; 2. Minimizing aggregate costs by using local sources that will have lowest haul costs; and 3. Maximizing the RAP content because it can be produced at a lower cost than virgin materials. FIGURE 6-2 Photograph of a core from an asphalt pavement showing surface, intermediate, and base layers. SOURCE: Pennsylvania DOT.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 105 When considering the potential use of recycled plastics, or any other new material, in an asphalt pavement, one of the first questions is how it will impact pavement performance and service life. All state DOTs evaluate the condition of their highway network on an annual or semiannual basis using vans equipped with sensors and imaging systems to collect data on pavement health. Pavement performance measures include percentage of lane area cracking, rut depth, and pavement smoothness. Highway agencies establish performance criteria for pavements based on these measurements to decide when a pavement preservation treatment or rehabilitation is needed for a particular project. Figure 6-3 shows images of common types of pavement distress for asphalt pavements. TABLE 6-1 Typical Costs (2022) of Raw Materials Used in Asphalt Mixtures Raw Material Description Cost (US$/tonne) Unmodified Asphalt Binder 880 Polymer (SBS) Modified Asphalt Binder 1,060 Virgin Aggregate 22 Processed Reclaimed Asphalt Pavement (RAP) 11 SOURCE: National Center for Asphalt Technology (NCAT). FIGURE 6-3 Photographs of common asphalt pavement distresses: (a) rutting, (b) load-related cracking, (c) environmental (thermal) cracking, and (d) reflection cracking. SOURCE: NCAT. (a) (c) (b) (d)
106 RECYCLED PLASTICS IN INFRASTRUCTURE There are numerous options for the types of pavement preservation treatment or rehabilitation needed to restore a distressed pavement to good condition. By far, the most common approach to asphalt pavement rehabilitation is to cold mill the top couple of inches of the existing pave- ment (partial depth removal). Milling an existing asphalt pavement, shown in Figure 6-4, removes the significantly distressed layers, helps restore the pavementâs profile for smoothness, helps maintain curb heights and clear- ances under bridges, and generates RAP that is recycled as a component into new asphalt pavement layers. Using RAP in a new asphalt mixture reduces the virgin aggregate content and the new asphalt binder content in the mix. Historically, the primary motivation for using RAP has been economic savings. As shown in Table 6-1, the cost of RAP is typically about half the cost of virgin ag- gregate. Generally, using 20 percent RAP will reduce the new asphalt binder content of a mixture by approximately 1 percent and reduce the virgin ag- gregate content by 19 percent, saving 15 to 18 percent of the total materi- als cost for an asphalt pavement. Recently, more attention has been given to the sustainability benefits of using RAP and other recycled materials in asphalt pavements. For example, the National Asphalt Pavement Associa- tion estimates that compared to an asphalt mix with all virgin materials, FIGURE 6-4 Milling of an existing asphalt pavement, a common part of pavement rehabilitation. SOURCE: NCAT.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 107 an asphalt mix containing 20 percent RAP reduces the cradle-to-gate green- house gas (GHG) emissions by 12 percent (Shacat 2023). In 2022, the U.S. Department of Transportation (USDOT) and the asphalt paving industry established goals to achieve net-zero GHG emis- sions for pavements by 2050 as part of the global effort to halt climate change (The White House 2021; Wofford 2022). Among the strategies to achieve the net-zero goal for asphalt pavement is aggressively increasing the utilization of RAP, including reaching a national average RAP content of 50 percent by 2035. Japan and the Netherlands have already attained this target (Roos 2002; West and Copeland 2015). Lessons from Past Efforts to Utilize Waste or Byproduct Materials in Asphalt Pavements As noted previously, several recycled waste materials have been used in as- phalt pavements, some with greater success than others. Two examples are sulfur and recycled tire rubber because they illustrate some circumstances that could be repeated with the utilization of recycled plastics in asphalt pavements. Sulfur Sulfur began to be used as an asphalt binder supplement in the early 1970s when the price of crude oil and oil-derived products spiked during the oil embargo (FHWA 2012). Sulfur, primarily a byproduct of oil refining and natural gas processing, was in growing supply as a result of process controls to meet federal environmental regulations. Research studies at the time sug- gested that 20 percent of the asphalt binder in an asphalt mixture could be replaced with sulfur. This technology was commonly referred to as sulfur extended asphalt (SEA). New procedures were developed for mix design to accommodate replacement of the asphalt binder with sulfur. From 1975 to 1984, 68 SEA test sections were built in the United States; 18 sections were monitored by the Federal Highway Administration (FHWA). They concluded that the performance of SEA pavements was satisfactory, with no significant differences compared to conventional asphalt pavements. However, tests on cores from those projects, as well as other laboratory studies, indicated that mixtures containing SEA were stiffer and more sus- ceptible to moisture damage and fatigue cracking. Interest in SEA declined dramatically in the early 1980s when sulfur prices increased sharply from US$17/tonne to more than US$100/tonne (1981 dollars) (Mahoney 1982). Health and safety concerns were also significant for SEA. Gaseous emissions of hydrogen sulfide (H2S) and sulfur dioxide (SO2) from hot SEA mixtures during production and paving operations can cause irritation
108 RECYCLED PLASTICS IN INFRASTRUCTURE to eyes and throat, but are not currently regulated as an emission health hazard. Mix temperatures below 149°C (300°F) were recommended to reduce emissions during asphalt mixture production with SEA. Elemental sulfur vapor emitted from hot paving mixtures can crystalize as sulfur dust in the air and irritate the eyes of asphalt paving crews. Safety goggles were recommended for paving crews to minimize this issue. In addition to eye and skin irritation from sulfur dust, some workers also objected to odors from SEA emissions. Renewed interest in SEA and other sulfur-based technologies occurred in the early 2000s when the economics of sulfur were more favorable com- pared to asphalt binder, and additional technologies were developed to aid in handling the material and the utilization of lower mixing temperatures. Trial projects were built in Alabama, California, Missouri, Nevada, and Texas, as well as Canada, China, India, and the Middle East. Additional laboratory research demonstrated that SEA mixtures could be designed for desirable properties with greater attention given to virgin binder selection, total binder content, sulfur content, and other mix additives. Despite the reported technical success of the trial projects, there were complaints from asphalt workers about odors and eye irritation, and a distinctive sulfur odor lingered from the SEA pavements in warm weather years after construction. One important issue that has not been adequately researched is the recyclability of pavements containing SEA. Although many of the early SEA pavement trials included milling of the pavement at the conclusion of the trial, the current practice is to landfill RAP containing SEA, because at- tempting to recycle it in a new asphalt mixture would necessitate exposing the SEA RAP to temperatures well above 149°C (300°F) during the mix- ing process. The loss of circularity of used asphalt pavement materials is a strong deterrent to SEA use as motivations for greater sustainability now influence many business decisions. Recycled Tire Rubber Rubber from scrap tires can be used to produce recycled tire rubber (RTR) and used as an additive for asphalt pavements. Although RTR has been used in asphalt pavements since the 1960s, significant market adoption of RTR-modified asphalt exists in only a few states, namely, California, Arizona, Nevada, and Texas (Buttlar and Rath 2021). Common dosage rates for RTR in asphalt mixtures range from 10 to 20 percent of the as- phalt binder content, or approximately 0.5 to 1.2 percent by mass of the total asphalt mixture. In general, RTR-modified asphalt pavements have been demonstrated to have better long-term field performance compared to unmodified asphalt pavements (Buttlar and Rath 2021). Many state DOTs consider RTR modification as an alternative to commonly used
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 109 polymer-modified asphalt binders that use virgin polymers to enhance the cracking- and deformation-related properties of asphalt binders. More than 260 million scrap tires are generated each year in the United States. The 2019 U.S. Scrap Tire Management Summary, produced biennially by the U.S. Tire Manufacturers Association, reported that 36.8 percent of scrap tires are used as tire-derived fuel in cement kilns, electric- ity utility boilers, and pulp and paper mills (USTMA 2019). Other uses of scrap tires include civil engineering applications other than pavements and ground rubber applications, including sports surfaces, molded and extruded products, and mulch. The 2019 summary estimated that 4.1 percent of U.S. scrap tires were processed into RTR for use in asphalt pavement mixtures and surface treatments, corresponding to approximately 163,000 tonnes of RTR. Approximately 14 percent of scrap tires are still disposed of in landfills, and 3 percent of tires are exported. Growing utilization of recycling scrap tires into other products has occurred over decades. In 1990, approximately 1 billion scrap tires were stockpiled, and many more were disposed of in unregulated manners. Tire stockpiles gained national attention as a serious environmental and health problem because of uncontrollable fires and as breeding grounds for mos- quitos and other vermin. In 1991, Congress passed the Intermodal Surface Transportation Efficiency Act, which included provisions mandating the use of RTR in asphalt pavements. The RTR-modified asphalt requirements were repealed a few years later owing to resistance from the American As- sociation of State Highway and Transportation Officials and the asphalt- pavement construction industry due largely to a lack of evidence at the time that RTR provided a benefit to pavement performance. Some challenges with using RTR in asphalt mixtures have to do with handling, storage, and testing challenges with RTR-modified binders. His- torically, the addition of RTR to asphalt mixtures used the âwet processâ in which the small ground rubber particles are blended with hot asphalt binder and reacted for at least 60 minutes using specialized blending equipment before pumping the rubber-modified binder to the asphalt plant for mixture production. Storage of rubber-modified binders can be challenging owing to the tendency of the rubber to settle in an asphalt binder storage tank. Although some technologies were developed in the mid-1990s to mitigate the settlement issue in wet-process RTR-modified binders, more recently, several companies have developed RTR products that can be added to asphalt mixtures using the so-called dry process, in which a pelletized or granulated and treated RTR product is fed directly into the mixing zone of an asphalt plant. Field trials of dry-process RTR-modified asphalt mixtures are currently being evaluated in several states. Recycling of asphalt pavements containing RTR-modified asphalt mix- tures into new asphalt paving mixtures has been commonplace in several
110 RECYCLED PLASTICS IN INFRASTRUCTURE large states for decades, which indicates that no significant issues have been identified that would challenge the circularity of RTR-modified mixtures. However, some studies (Alavi et al. 2016; Buttlar and Rath 2021) recom- mend further research to address unknowns regarding potential air quality impacts during mix production and field performance of asphalt mixtures using RAP containing RTR. Summary of Lessons Learned from Experiences with Sulfur and Recycled Tire Rubber Two principles are considered essential in evaluating the use of alterna- tive materials in asphalt paving mixtures. First, the additive must provide a pavement performance benefit to offset its cost. Second, the additive must have no significant negative environmental or health consequences. The first principle generally takes decades to convincingly demonstrate a positive impact on a pavement life cycle even if the material offers a cost advantage. Therefore, mandating a materialâs use through the legislative process or administrative policy without gathering performance data and properly conducting a life-cycle cost analysis can hinder the advancement of a technology because it can push forward a method or material that needs further research to optimize its formulation, dosage, or method of addition. The second principle to learn from these two examples is the need to evaluate environmental and health impacts of the use of a material. In the case of SEA, even relatively minor health nuisances have been a major hurdle in the path toward implementation. With regard to environmental impact, the use of a recycled material can have a positive economic benefit or initially appear to be a sustainable practice, but if it diminishes the circu- larity of the pavement as a whole, then it does not have a benefit from the environmental life-cycle assessment (eLCA) perspective. Figure 6-5 presents the life-cycle stages of pavements as presented in FHWAâs Pavement Life- Cycle Assessment Framework. Research and Background on the Use of Plastics in Asphalt Pavements The use of plastics as a modifier for asphalt pavements is not a new concept. The earliest documented use was in the 1970s in Europe, where high-den- sity polyethylene (HDPE) was used in a pourable asphaltic mixture known as Gussaphalt (Bardesi et al. 1999). In the 1990s, a proprietary plastic- modified asphalt binder, trademarked as Novophalt®, was developed in the United States and marketed with field trials in nearly 20 countries. This technology required a portable high-shear blending system (see Figure 6-6) to blend virgin low-density polyethylene (LDPE), and SBS in later formula- tions, into the asphalt binder just prior to mix production. The Novophalt®
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 111 FIGURE 6-5 Sustainability elements associated with pavement life-cycle states. SOURCE: Adapted from FHWA 2019. technology was limited by several factors, including material availability, scheduling logistics and transportation costs of the blending equipment, and higher rates of cracking for the field test sections (Williams 1993). Another proprietary polyethylene-modified asphalt binder product, Polyphalt®, followed soon thereafter, utilizing a stabilizer to mitigate sepa- ration of polyethylene-modified asphalt binders (Harbinson and Remtulla 1994). Although this product showed promising laboratory results, it was not commercially successful because of its high product costs and poor cracking performance in the early trial projects. More recently, research and development on the use of plastic-modified asphalt pavements has focused on recycled plastics, with significant efforts initially occurring in India and the United Kingdom and then spreading to other parts of the world. Greater attention was given to use of recycled plastics in roads after China and India banned imports of plastics waste. Al- though the research and development (R&D) efforts in India and the United Kingdom were both motivated by the growing waste plastics problem and the desire to improve the performance of asphalt pavements, the approaches have been quite different. In the United Kingdom, the technology developed
112 RECYCLED PLASTICS IN INFRASTRUCTURE FIGURE 6-6 Novophalt blending unit used to blend LDPE and other polymers into asphalt binder. SOURCE: Advanced Asphalt Technologies LLC. out of proprietary formulations by the startup company MacRebur Ltd. Today, MacRebur markets two recycled asphalt âextenderâ plastic prod- ucts to the asphalt pavement industry: MR6, purported to improve stiffness without a loss of cracking resistance, and MR8, purported to reduce cost and enhance environmental benefits without adversely affecting pavement performance. The composition of the MacRebur products is unknown. Projects have been built with MacRebur products in the United Kingdom, Australia, New Zealand, Turkey, Bahrain, Slovakia, South Africa, and the United States. A field test of the MacRebur MR8 product in New York City is described in Box 6-1. Most of these projects are only a few years old, so the long-term durability of the pavements has yet to be evaluated. The R&D approach to the use of recycled plastics in asphalt in India has focused on field demonstrations. India has more than 15 years of expe- rience using recycled plastic in asphalt pavements and has constructed more than 60,000 miles of such roads. Professor Rajagopalan Vasudevan of the Thiagarajar College of Engineering developed a simple process by which shredded recycled plastic is added to the aggregate conveyor of an asphalt plant before the aggregate enters the dryer. According to Vasudevan, inside the dryer, the plastic melts and coats the aggregate before the asphalt binder is mixed with the coated aggregate. In 2015, after Vasudevan gave his pat- ent for the system to the Indian government, the government mandated the use of waste plastic in asphalt pavement surface layers in large cities with populations greater than 500,000. The Indian Road Congress guide speci- fication allows only HDPE, LDPE, and polyethylene terephthalate (PET)
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 113 BOX 6-1 Field Trials of Asphalt Mixes with Recycled Plastics in New York City In August 2022, Rice Avenue and Royal Oak Road, in the New York City (NYC) Borough of Staten Island, were the locations of the first field trials of asphalt mixes containing waste plastic in NYC. This field trial included asphalt paving mixtures containing a MacRebur proprietary material derived from plastics waste reengineered into a new polymer. MacRebur estimates that the carbon footprint reduction of the trial surfaces saved more than 7,331 kgCO2e in comparison to traditional asphalt and diverted the equivalent weight of more than 214,000 single-use plastic bottles from landfill (Jones 2023). Taking place over 4 days, the NYC DOT worked with asphalt producer City Asphalt to lay four trial asphalt mixtures, using more than 2,400 tons of hot-mix asphalt. While the first mix was traditional asphalt, the remaining three each con- tained MacReburâs MR8 product at increasing dosages to replace 6 to 12 percent of the asphalt binder. The mix design used in each of the trial sections included 40 percent RAP. The NYC DOT objective of the field trial is to replace a portion of the asphalt binder with the recycled plastic material without impacting the RAP content. The trial sections will be monitored to assess field performance, and the mixtures will be tested by the Rutgers University Center for Advanced Infrastruc- ture and Transportation, with results expected before the end of 2023. Producing and laying 1.1 million tons of asphalt each year, the NYC DOT is one of the U.S. leaders in using RAP and typically uses 40 percent RAP in every mix.
114 RECYCLED PLASTICS IN INFRASTRUCTURE to be used in asphalt mixture production at dosages of 6 to 8 percent by weight of the asphalt binder. It is unknown if the use of recycled plastics has a positive or negative impact on performance of the highway network in India. Processes Used to Add Plastics to Asphalt Mixtures Two methods have been used to incorporate recycled plastics in asphalt paving mixtures: the wet process and the dry process (see Figure 6-7). The wet process and the dry process differ in how the plastic modifier is added to the asphalt mix. In the wet process, recycled plastics are added as a modifier to the hot liquid asphalt binder at dosages ranging from 1 to 12 percent by weight of asphalt binder. These dosages correspond to about 0.5 to 6 kg (1 to 13 lb) of recycled plastics per tonne of asphalt mixture. Recycled plastics with lower melting points, such as linear low-density polyethylene (LLDPE), LDPE, and HDPE, are best suited for this process. In this process, the re- cycled plastic blends with the asphalt binder, creating a homogeneous modi- fied binder throughout (see Figure 6-8). The wet process typically requires a stabilizing additive (also referred to as a compatibilizer) or mechanical mixing to achieve and maintain a homogeneous blend of plastic-modified binder, otherwise the plastic will separate and create a highly viscous layer in the storage tank. The wet process was used for Novophalt and Polyphalt and is still used on several recent projects in the United States. In the dry process, recycled plastics are typically added directly into the asphalt plant after the aggregate is dried and heated but just prior to the point where the asphalt binder is introduced. There are some varia- tions of the dry process. In India, recycled plastic shreds are added to the aggregate conveyor belt mixture before the aggregate enters the dryer and thus exposes the plastic to gas temperatures as high as 1,400°F (760°C). This likely incinerates the plastic. In the United States, the recycled plastic is introduced further downstream in the asphalt mixture production pro- cess, where the aggregate and gas temperatures are generally below 350°F (177°C). Most studies suggest that plastics with melting points below this temperature will melt and coat the aggregate particles just prior to mixing with the asphalt binder. However, there is no scientifically established evi- dence to verify this hypothesis. For plastics with melting points above the mixing temperature in the asphalt plant, the plastic particles likely remain as discrete particles in the mixture (see Figure 6-8), which some research- ers refer to as the âaggregate replacementâ approach. For the dry process, research studies included dosage rates for recycled plastics ranging from 0.2 to 6.0 percent by weight of aggregate. These dosages correspond to about 2 to 60 kg (4.4 to 132 lb) of recycled plastics per tonne of asphalt mixture.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 115 FIGURE 6-7 Illustrations of wet and dry processes at an asphalt plant. TABLE 6-2 Summary of Advantages of Wet and Dry Methods for Adding Plastics to Asphalt Mixtures Method Advantages Disadvantages Unknowns Wet ⢠Types of plastic and dosages can be optimized to achieve enhanced asphalt binder properties. ⢠May require a compatibilizer to avoid plastic separation in the blended binder. ⢠May require a separate binder storage tank at the asphalt plant. ⢠Long-term storage stability of plastic- modified binders. ⢠Compatibility of plastic-modified binder with other liquid additives such as recycling agents, antistrip additives, and warm-mix additives. Dry ⢠Utilizes a much higher amount of plastics per tonne of asphalt mix. ⢠Preferred by most asphalt contractors. ⢠Requires additional equipment at asphalt plant to feed plastics. ⢠May require a higher mixing temperature to completely melt plastics. ⢠Laboratory mixing temperature and process to simulate plant mixing. ⢠How the plastic interacts with the binder and aggregate in the asphalt mixture.
116 RECYCLED PLASTICS IN INFRASTRUCTURE Additional Background Information from a Literature Review In 2021, NCAT at Auburn University completed a comprehensive litera- ture review as part of study funded by the National Cooperative Highway Research Program (NCHRP) (NCAT et al. 2021). This review summarized more than 150 research reports and other technical documents on the growing body of knowledge related to the use of recycled plastics in as- phalt pavements. Approximately 75 percent of the literature was published within the past decade (2011-2021) and half was published since 2017. The literature comes from authors in more than 30 countries. The countries with the largest number of publications are the United States, India, Aus- tralia, China, Malaysia, and Canada. In 2022, an Austroads study (jointly FIGURE 6-8 Illustrations of the material structure of standard asphalt (no recycled plastic), and plastic-modified asphalt by the dry and wet methods. SOURCE: Adapted from Austroads (Project APT6305) and RMIT University. Pre- sentation to the study committee on July 20, 2022.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 117 funded by Australia and New Zealand) was completed under the title Use of Road-Grade Recycled Plastics for Sustainable Asphalt Pavements. The Austroads study produced four reports, but there were no associated field trials (Giustozzi and Boom 2021; Giustozzi et al. 2021, 2022a, 2022b). The following sections summarize the key findings and knowledge gaps from the literature. Types of Plastic Suitable for Use in Asphalt Paving Mixtures Polyethylene, including LLDPE, LDPE, and HDPE, have been the most studied types of recycled plastic for use in asphalt, followed by PET and polypropylene (PP). The most common properties of recycled plastics cited as important for use in asphalt include specific gravity, melting tempera- ture, melt flow index, and ash content. The Austroads study (Giustozzi and Boom 2021; Giustozzi et al. 2021, 2022a, 2022b) recommended us- ing plastics with melting points that are at least 15°C below the asphalt mixing temperature, and a melt flow index (ASTM D1238) greater than 1 g/10 min. at 190°C. However, there is very little other guidance on sourc- ing suitable plastics at this time. As mentioned in Chapter 3, variations in composition and the presence of nonplastic contaminants in post-consumer streams could be challenging for using recycled plastics as an additive in asphalt mixtures. Post-industrial recycled plastics may provide more con- sistent and less contaminated feed stock. Mixing Process In the NCAT literature review (NCAT et al. 2021), 73 percent of the studies evaluated plastics added to asphalt binders or mixtures by the wet process, 33 percent of the studies evaluated plastics added to asphalt mixtures by the dry process, and 6 percent of the studies evaluated materials using both wet and dry processes. Some studies suggest that using the wet process to modify asphalt binders with recycled plastics may be economically advan- tageous compared to traditional polymer modifiers since recycled plastics cost less than commonly used asphalt polymers. However, the handling challenges with the wet process due to the potential for separation of the plastic in the binder likely requires continuous agitation of the plastic- modified binder and/or the addition of a stabilizing agent. These additional handling and processing requirements may negate the cost advantage of the recycled material. When adding recycled plastics by the dry process, the degree to which the plastic melts and becomes part of the binder phase or coats the aggre- gate particles is unknown. Key factors affecting these phenomena are likely
118 RECYCLED PLASTICS IN INFRASTRUCTURE to include the melting point and size of the plastic particles, the temperature profile and sequence of materials addition, and compatibility characteristics of the asphalt binder and plastics. Plastics that do not melt are assumed to become an inert part of the aggregate matrix. The Austroads study referred to the âdry processâ as using high- melting-point recycled plastics (more than 30°C above the asphalt mixing temperature) as an aggregate replacement. There is very little published research on this approach. Characterization of Plastic-Modified Asphalt Binders A consistent finding from the studies reviewed by NCAT et al. (2021) that evaluated asphalt binder properties using the wet process was that the ad- dition of recycled plastics increases binder stiffness properties, which were interpreted as beneficial for rutting resistance. However, very few studies examined the effect of recycled plastics on asphalt binder properties that are related to fatigue or low-temperature cracking susceptibility. Numerous compatibilizer agents have been evaluated, and several have been found to be effective in keeping the plastic dispersed in asphalt binders. This is also a topic of ongoing research. Future research is needed to assess compatibility between recycled plastics and other additives used in asphalt binders, such as warm-mix asphalt additives, antistrip agents, and recycling agents. Characterization of Recycled PlasticâModified Asphalt Mixtures Researchers have consistently found that adding recycled plastics to an asphalt paving mixture, whether by the wet method or the dry method, increases the mixtureâs resistance to rutting. Many of the studies prior to about 2015 evaluated mixture properties that are no longer considered use- ful to assess mixture quality. In fact, even more recent studies from India and some other Asian countries still utilize outdated test methods. The Austroads study used laboratory test methods considered to be state of the art and found that the wet process for mixing plastics yielded mixtures with similar cracking resistance to that of unmodified asphalt mixtures. Field Performance of Asphalt Pavements Containing Recycled Plastics More than two-thirds of the literature examined by NCAT et al. (2021) fo- cused exclusively on laboratory testing of recycled plastics in asphalt bind- ers and mixtures, while 16 percent provided information on field projects. Unfortunately, most studies involving field projects focused on the con- struction of test sections and did not provide long-term field performance
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 119 assessments. Other references included literature reviews, cost analyses, pavement design, production information, technical guidance, environmen- tal impact assessment, or agency specifications. Environmental, Health, and Safety Concerns Three potential environmental, health, and safety concerns have been raised regarding the use of recycled plastics in asphalt pavements: (1) release of toxic components during processing of recycled plastics, (2) the generation and release of microplastics into the environment due to wear of recycled plasticâmodified asphalt pavements or future pavement recycling opera- tions, and (3) the generation of chlorine-based gases and dioxin from poly- vinyl chloride (PVC) during asphalt mixture production and construction. The few laboratory studies that evaluated the leachability of hazardous chemicals and release of toxic fumes (e.g., toluene, benzene, as well as aliphatic, cyclic, and aromatic hydrocarbons) from asphalt binders and mixtures modified with plastics found no detectable adverse effects from the recycled plastics (Giustozzi et al. 2022a, 2022b; White 2019). However, numerous reports caution against the use of chlorinated polymers such as PVC. Studies that have evaluated the potential release of microplastics are very limited at this time. No field studies have been completed on this topic. Purported Economic Benefits Some suppliers of proprietary recycled plastics for use in asphalt mixes suggest that their recycled plastic products replace a portion of the asphalt binder and, therefore, provide a savings to the overall mixture cost. This claim, which also appears in some research studies, may be caused by mis- interpreting common asphalt mixture volumetric properties that are used to determine the optimum asphalt content of traditional asphalt paving mixtures. U.S. Field Trials of Plastic-Modified Asphalt Mixtures Since 2018, more than 20 field projects have been constructed in the United States using recycled plasticâmodified asphalt mixtures. Locations of these projects are shown in Figure 6-9. Most of these projects have been for com- mercial clients that specifically requested the addition of recycled plastics without specific criteria on the plastic materials, mixing process, or mixture properties. These projects have included parking lots and privately owned roads that would not have significant experimental value since they lack control sections and underlying pavement conditions were not documented.
120 RECYCLED PLASTICS IN INFRASTRUCTURE Several projects that do have experimental value for assessing the effect of the added plastics include projects in Alabama, Minnesota, Missouri, New York, Pennsylvania, and Virginia (see Boxes 6-1, 6-2, and 6-3 for brief summaries of projects in New York City, Pennsylvania, and Missouri, re- spectively). Currently, these projects are less than 3 years old; no significant differences are evident in the performance of the pavement sections with and without recycled plastics. Some state and municipal highway agencies are supportive of the ongoing research projects to evaluate recycled plas- tics in asphalt pavements, but they are unwilling to accept the hypotheses that recycled plastics extend pavement life or reduce the optimum asphalt content of asphalt mixtures without proof. Ongoing Research Projects in the United States There are several ongoing national research projects exploring specific is- sues related to the use of recycled plastics in asphalt in the United States. A 3-year study funded by FHWA and being conducted at Louisiana Tech University, titled âCompatibilization of Waste Plastic to Enhance Me- chanical Properties of Asphalt Cement,â is focused on two objectives: (1) understanding the compatibility between recycled plastic and asphalt binder FIGURE 6-9 U.S. locations of pavement projects containing recycled plasticâmodi- fied asphalt mixtures. SOURCE: NCAT.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 121 BOX 6-2 Pennsylvaniaâs Experience with Post-Consumer Plastics in Asphalt The Pennsylvania Department of Transportation (PennDOT) in collaboration with Pennsylvania State University (PSU) is currently conducting a research project on recycled plastics in asphalt. The research project involves laboratory experi- mentation, analysis of the effects of recycled plastics on asphalt binder proper- ties and completed asphalt mixtures, and field performance at four pilot project locations over 3 years. PSU is evaluating laboratory data and performance of the pilot projects. A main objective of the project is to establish a framework of specification changes that could allow asphalt producers to use recycled plastic materials once field performance is shown to be acceptable and market condi- tions make these products the most economical alternative. Brief descriptions of the four pilot projects follow. Typically, the test sec- tions at each of the pilot projects are one lane of a 0.5-mile segment of road; the adjoining lane is used as the control for the experiment. ⢠Delaware County, Ridley State Park (conducted through a collaboration between PennDOTâs Strategic Recycling Partnership and Pennsylvania Department of Conservation and Natural Resources). During the mix design process, the mixtures were evaluated for rutting and cracking resistance. The test section and control were placed in 2020. The NVIAMG NewRoad® product was added to the mix at 0.5 percent of the binder weight. As part of this project, monitoring for microplastics in surface runoff is being performed by PennDOTâs environmental consultant, Rummel, Klepper & Kahl, LLP. Sampling activities will be conducted semiannually for the first 3 years, then annually for the next 2 years. Post-construction water sampling was done at 5 months and again at 11 months at two test locations and two control locations. To date, microplastics have not been detected in any of the post- construction samples. ⢠Allegheny County, SR 0051. The test section and control were placed in October 2021. During the mix design process, rutting and cracking re- sistance of the control and experimental mixture was evaluated as well as properties of the modified binder. The Green Mantra CERANOVUS⢠A115, a polyethylene wax produced from post-industrial plastics waste and post-consumer recycled plastics, was added at 1.5 percent of the binder weight. ⢠Delaware County, SR 2037. The test section and control were placed in October 2022. During the mix design process, modified binder proper- ties were determined and rutting and cracking resistance of the control and experimental mixtures were tested. The NVIAMG NewRoad® prod- uct was added to the mix at 0.5 percent of the binder weight. ⢠Lancaster County, SR 3017. The test sections will be placed in the 2024 construction season. The type of recycled plastic has not been determined for this project.
122 RECYCLED PLASTICS IN INFRASTRUCTURE and (2) developing tests to optimize the blend for plastic type, content, deg- radation status, and existing modification processes. This laboratory study is aimed exclusively at advancing the state of knowledge for the wet process approach to adding recycled plastics to asphalt binders. It is scheduled to conclude in late 2023. Another ongoing national project is funded by NCHRP but is focused on the dry process. This project, led by NCAT, is titled âMechanical Prop- erties of Laboratory Produced Recycled Plastic Modified (RPM) Asphalt Binders and Mixtures.â As implied by the title, the scope of this study is limited to laboratory experiments that will assess the impact of recycled plastics on asphalt mixture resistance to major forms of pavement distress, as well as friction properties. A limited experiment is aimed at character- izing fumes emitted during laboratory mixing of asphalt mixtures with and without recycled plastics. This study is also charged with recommending guidance on the selection of recycled plastics for use in asphalt mixtures, necessary changes to asphalt mix design procedures and production process control methods, as well as determining methods to assess the commingling of plastics with asphalt binders through the dry process. This project is also scheduled to conclude in late 2023. Four other research projects include noteworthy experimental field tri- als. Over time, these closely monitored projects will serve to provide true assessments of the performance impact of using recycled plastics in asphalt pavements as well as to provide feedback on the validity of the many dif- ferent tests that have been used in laboratory research projects. In addition, research is under way on these field projects to explore the release of micro- plastics in the environment and life-cycle assessments of GHG emissions for asphalt pavements with and without recycled plastic additives. In August 2021, the Missouri DOT partnered with the University of Missouri to design and evaluate test sections containing RTR and recycled plastics on Stadium Boulevard in Columbia, Missouri. Test sections con- taining recycled plastics include LLDPE added by the dry process at dosage rates of 0.25 and 0.5 percent by weight of the mixture. A third recycled plastic test section also used dry-added 0.5 percent LLDPE plus a compati- bilizer added to the asphalt binder. An unmodified control mix test section is also included in the field experiment for comparison. Performance of the test sections will be monitored over years to draw conclusions about the impact of the recycled plastics in asphalt pavements. A more comprehensive field project, in collaboration with the National Road Research Alliance, is planned for 2023 to include eight test sections on I-155 in Pemiscot County, Missouri. Box 6-3 provides more detail about this research program. In September 2021, two asphalt pavement test sections with recycled plastics were constructed on the NCAT Test Track as part of the Transpor- tation Pooled-Fund project TPF-5(469). The NCAT Test Track is a 1.7-mi
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 123 BOX 6-3 Missouriâs Experience with Post-Consumer Plastics in Asphalt In the summer of 2021, the Missouri Department of Transportation and the Mis- souri Center for Transportation Innovation undertook the first project in the state to use post-consumer recycled waste plastic in asphalt. The project is located in Columbia, Missouri, on a section of roadway that sees more than 8,000 vehicles per day. The purpose of the project was to examine if utilizing waste plastic and ground tire rubber would improve the performance of the overlay or at least pro- vide similar performance. The Missouri Department of Transportation strives to be environmentally responsible while providing the best value to drivers in Missouri. This project is exploring that balance. The research project Lab and Field Evaluation of Asphalt Mixtures with Post- Consumer Recycled Plastic Waste: Phase II was contracted on April 9, 2021. The research was led by Dr. Bill Buttlar at the University of MissouriâColumbia. Four test sections ranged from 0.5 miles to 1.14 miles. The recycled plastic used was in pellet form comprised mainly of LLDPE and the ground tire rubber (GTR) was an engineered crumb rubber. Both materials were added to the mix using the dry process. The asphalt overlays were 1.5 in. thick. Three sections contained LLDPE. One section used 0.25 percent LLDPE by weight of mix and the other two sections used 0.5 percent LLDPE. One of these higher-plastic-content sections used ELVALOY⢠as a compatibilizer to aid the mix in incorporation of the plastic. All mixes contained both RAP and slag, which was used to increase friction of the mix. The project used a Balanced Mix Design method to develop the four mix designs. The recycled LLDPE pellets were fed into the asphalt drum using the same equipment used to feed GTR. One of the lessons learned during the produc- tion included having to modify the feeder system, since the plastic pellets were smoother and had less friction between particles. This led to too much plastic continued
124 RECYCLED PLASTICS IN INFRASTRUCTURE accelerated pavement testing facility located near Auburn, Alabama, that serves as a national proving ground for asphalt pavement technologies. One of the test sections containing recycled plastics used the dry process and the other test section used the wet process. The recycled plastic was an LLDPE-rich material supplied by Avangard Innovative. For the wet process, ELVALOYâ¢, a reactive elastomeric terpolymer supplied by Dow was added as a compatibilizer for the LLDPE. An unmodified mix control section was also paved as part of the experiment. The experimental test sections were designed as relatively thin pavement cross-sections for the heavy axle loads used on the test track. This results in high strain levels in the pavements that will ultimately lead to fatigue cracking, so the experi- ment will provide a clear outcome regarding the impact of recycled plastics on this common mode of pavement failure. In conjunction with this test track experiment, FHWA has funded the development of cradle-to-gate Environmental Product Declarations for the mixtures containing recycled plastics and other additives. This environmental assessment of the sum of GHG emissions associated with raw materials acquisition and asphalt mixture production, along with publicly sourced data on the construction phase and Test Track performance data, will be used to assemble LCA case studies on asphalt pavements containing recycled plastics. The NCAT Test Track experiment and the associated LCA case studies are scheduled to conclude in early 2024. In October 2021, the Virginia DOT built asphalt pavement test sec- tions containing recycled plastics on Old Stage Road in Chester, Virginia. BOX 6-3 Continued being introduced into the mix and it overloaded the drive motor. A temporary restrictor plate was used in the feeder to solve this issue. The horsepower of the feeder drive was doubled to provide a factor of safety against motor over- load. Once these minor adjustments were made, sufficient in-place density was achieved in the test sections containing recycled plastic. The pilot project has been in place for more than a year and no rutting has occurred to date. Some reflective cracking has occurred, as was expected, with the relatively thin overlay. The 0.5 percent LLDPE mix with compatibilizer has the least amount of reflective cracking. The GTR test section has the next low- est amount of reflection cracking, followed by the 0.25 and 0.5 percent LLDPE sections. A future project in southeastern Missouri is planned in 2023. Areas for further study include using additional plastic types, softer binders for highly recycled mixes, environmental impact studies of stormwater runoff, and a life- cycle assessment.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 125 The 1-mi test sections utilized two different proprietary recycled plastic products added by the dry process to the mixtures used for the 1.5-in. overlay. One of the products is a blend of polyethylenes and a propri- etary âperformance enhancerâ produced by MacRebur, LLC. This recycled plastic product, referred to as MR-6, was added at a dosage of 5 percent by weight of asphalt binder. The second product is a PET-based product trade named NEWTLAC® supplied by Kao Global Chemicals of Japan. NEWTLAC pellets were added at 3 percent by weight of asphalt binder. Virginia DOTâs research arm, the Virginia Transportation Center (VTRC), will monitor the performance of the test sections on an annual basis. VTRC is also conducting an analysis of stormwater runoff from the test sections to quantify and characterize microplastics generated from the wear of the pavement surfaces. In August 2022, the Minnesota Road Research Facility known as Mn- ROAD built a series of new asphalt pavement test sections on I-94 near Albertville, Minnesota. Two sections contained recycled plastics, one with the dry process and the other with the wet process. Like the NCAT Test Track experiment, the recycled plastic was an LLDPE-rich material supplied by Avangard Innovative. For the wet process, ELVALOY⢠was added as a compatibilizer for the LLDPE. Two control sections were paved as part of the MnROAD experiment, one control section with an unmodified asphalt binder and another with an SBS block copolymer. The MnROAD experiment is designed to focus on thermal cracking and reflection cracking performance of the asphalt mixtures. The National Road Research Alliance is supporting a comprehensive battery of laboratory characterization tests, mechanistic modeling of the pavement distresses, and a life-cycle assessment of the pavement sections with and without the recycled plastic additives. RECYCLED PLASTICS IN SUBBASE OF PAVEMENTS Background In the literature review conducted by NCAT et al. (2021), very little re- search was found about utilizing recycled plastic in the subbase of pave- ments. Some researchers have done preliminary research on the topic, but no actual pilot projects were found. Research to date included using HDPE, LDPE, PP, and PET as part of a mix for soil stabilization or sub- base aggregate. Three types of use cases were found in the literature: using recycled plastic as an aggregate type of material, using plastic as part of a soil stabilization process with cement, and using recycled plastic to make geogrid materials for pavement subbase and pavement interlayers. Among the research, there is a lack of eLCA of the implications of using recycled plastics for pavement subbase applications.
126 RECYCLED PLASTICS IN INFRASTRUCTURE State of Practice and Research Multiple research projects have conducted laboratory testing exploring the inclusion of recycled plastic for soil stabilization. No field trials were found in published documentation, although a new project led by Iowa State plans to conduct field test sections in future years. From the perspective of laboratory testing, research by Sobhan and Mashnad (2002) found that the inclusion of HDPE strips does not meaningfully increase tensile strength but can enhance overall toughness. This research looked at using HDPE fibers/ strips with Portland cement and class C fly ash. The most effective mixes utilized all three materials, although using just the HDPE fibers and fly ash had desirable strength and mechanical characteristics. Laboratory research in Australia (Yaghoubi et al. 2017) also evalu- ated using recycled HDPE and LDPE granules along with recycled con- crete aggregate for subbase materials. The research found that, as plastic content of specimens increases, the unconfined compressive strength decreased, although HDPE blends performed better than LDPE blends. Stiffness parameters were also lower. Overall, the resilient modulus3 for the plastic blends were found to be within the expected range for typical quarry materials. Additional research in Australia (Perera et al. 2019) evaluated the use of recycled shredded PET along with construc- tion and demolition wastesâin particular, recycled concrete aggregate and crushed brickâfor aggregate base. The research found that the PET could be incorporated into the base and still meet the requirements set forth by the state road authority, although decreased modulus ratios were found. Initial laboratory research results on soil stabilization using recycled plasticsâin the form of HDPE and PP pelletsâand cement combinations (Tabassum and Bheemasetti 2022) show that a combination of the plastic and cement can meet or exceed strength requirements for soil stabilization. Researchers at Iowa State are leading a project for the Iowa Depart- ment of Transportation to study the use of recycled plastics to stabilize the base of granular roadways in Iowa. The research project started in the spring of 2022 and continues through 2025.4 The project is in its early stages and, therefore, no results are yet available. One of the goals of the Iowa project is to determine the structural benefits and environmental suit- ability of using recycled plastic in a base material. As part of the research, 3 The resilient modulus is a measure of material stiffness. FHWA defines it as the ratio of the applied cyclic stress to the recoverable (elastic) strain after many cycles of repeated load- ing (FHWA 2006). 4 Information in this section relevant to Iowaâs project is based on material presented to the committee by Halil Ceylan (Iowa State University) on June 9, 2022.
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 127 test sections will be constructed to assess the performance. At the end of the project, a practitionerâs guide will be developed to document best prac- tices and accelerate implementation. Preliminary studies will examine three experimental treatment groups: pure soil, soil treated with 4 percent PET shredded by soil weight, and soil treated with 4 percent HDPE pellets by soil weight. Structural benefits to be explored include durability and freeze- thaw damage. The research will also look at the environmental aspects such as possible microplastics being released from the roadway. Once the test sections are constructed, they will be monitored for typical granular road failure types such as rutting, washboarding, potholes, and loose aggregate. Leachate potential will be measured by taking water samples from nearby groundwater, lysimeters under the roadway, and soil samples collected adjacent to the roadway. Impacts and Concerns While researchers have explored the use of recycled plastics for subbase ma- terials or for soil stabilization of subbases, no field work was found in the literature. Laboratory work shows promise, but the long-term performance and environmental effects are unknown. Field testing needs to be conducted to make sure that the appropriate strengths are maintained throughout the life of the project. There are also concerns with microplastics being introduced into the environment as the subbase is exposed to moisture, which could lead to rapid degradation of the recycled plastic with related generation of microplastic particles and their release to water and soil. This concern is higher for those mixes that do not utilize cement, but there still needs to be testing of both methods to ensure there are no negative envi- ronmental consequences. An additional issue that has not been adequately researched is the recyclability of pavements that incorporate recycled plas- tics for subbase materials or for soil stabilization of subbases. A common rehabilitation technique is full-depth reclamation which may be hindered depending on how the recycled plastics are incorporated. Gaps in Knowledge or Processes Only preliminary research has been done to determine the overall durability and performance of using recycled plastic in subbase or granular material. While testing procedures are the same as for typical unbound materials without plastic, specifications will still need to be developed for maximum dosages. Until pilot tests have been completed and environmental concerns are addressed, this technology is not ready for deployment.
128 RECYCLED PLASTICS IN INFRASTRUCTURE FINDINGS ⢠Numerous marketing claims and media reports suggest that using recycled plastics in asphalt pavements is a win-win, solving both an environmental problem of plastics waste while lowering the cost of and improving the life of asphalt pavements. However, there is essentially no evidence at this time to support the claims that the addition of recycled plastics will extend the life of asphalt pave- ments, and the purported cost savings have not been adequately documented. Field research projects under way to assess the impact of plastics on asphalt pavement performance will take several years to complete. ⢠Based on a literature review, the types of plastics that appear to have the highest potential for use in asphalt pavements as an as- phalt binder modifier are LDPE, LLDPE, HDPE, and PP. Further research is needed to understand the degree of purity needed for establishing âroad-gradeâ recycled plastics specifications. The po- tential of using plastics with melting points above typical asphalt mixture production temperatures as an aggregate replacement still demands further evaluation in field trials. ⢠The possible quantity of plastics that could be consumed in asphalt pavement applications will depend on the cost of the recycled plas- tic supply relative to other additives that provide similar benefits, the dosage rate, and the benefits to pavement performance, if any. An optimistic estimate of plastic use in asphalt pavements based on a dry process dosage rate of 0.5 percent by weight of mix used in 12 percent of U.S. annual asphalt mixture production would consume approximately 240,000 tons of plastic per year, which is approximately 2.4 percent of the waste polyethylene generated in the United States each year. ⢠The most critical gap in knowledge yet to be answered is the impact of specific plastic types on the life of the asphalt pavements. If the answer to that question is positive, additional topics that must also be answered include (1) what criteria are needed for specifying the quality and consistency of the recycled plastic supply; (2) how will the supply of the desired types of plastics meet the demand created by the opportunity to use recycled plastics in asphalt pavement; (3) how will cost of recycled plastics compare with other additives, such as virgin polymers or recycled tire rubber, that may provide similar performance benefits; and (4) how will the eLCA of recycled plasticâmodified asphalt pavements compare to pavements with similar performance lives using existing additives. Currently, there is very limited research under way to assess whether microplastics
APPLICATIONS OF RECYCLED PLASTICS IN PAVEMENTS 129 generated from the wear of pavement surfaces containing recycled plastics will have negative consequences on air quality or stormwa- ter runoff from real projects. There is also very limited information on the recyclability of asphalt pavements containing plastics waste. Disrupting the current circularity of asphalt pavements would be a game-ending consequence. ⢠Limited research is available related to the use of recycled plastic in the subbase of pavements. The evaluations to date have involved preliminary laboratory studies. While these initial evaluations sug- gest that using recycled plastic can achieve the strength require- ments for soil stabilization, much remains to be studied from the perspective of engineering and environmental performance prior to evaluating this application in pilot projects. REFERENCES Alavi, Z., Hung, S., Jones, D., and Harvey, J. 2016. Preliminary Investigation into the Use of Reclaimed Asphalt Pavement in Gap-Graded Asphalt Rubber Mixes, and Use of Re- claimed Asphalt Rubber Pavement in Conventional Asphalt Concrete Mixes. Report for the California Department of Resources, Recycling and Recovery. https://escholarship. org/uc/item/1n7574h6 Asphalt Institute. 2010-2021. Annual Asphalt Usage Surveys for United States and Canada. http://www.asphaltinstitute.org Bardesi, A., Brule, B., Corte, J. F., Diani, E., Gerritsen, A., Lefevre, G., and Watkins, S. 1999. Use of modified bituminous binders, special bitumens and bitumens with additives in pavement applications. Technical Committee Report: On Flexible Roads. (C8) World Road Association, PIARC. Buncher, M. 2022. What percentage of our roads are asphalt? Asphalt, The Magazine of the Asphalt Institute. http://asphaltmagazine.com/94percent Buttlar, W. G., and Rath, P. 2021. State of Knowledge Report on Rubber Modified Asphalt. U.S. Tire Manufacturers Association and The Ray. Federal Highway Administration (FHWA). 2006. Geotechnical Aspects of Pavement Ref- erence Manual. FHWA NHI-05-037. https://www.fhwa.dot.gov/engineering/geotech/ pubs/05037/index.cfm âââ. 2012. An Alternative Asphalt Binder, Sufur-Extended Asphalt (SEA). Technical Brief FHWA-HIF-12-037. Office of Pavement Technology. âââ. 2019. Tech Brief: Building Blocks of Life-Cycle Thinking. FHWA-HIF-19-027-d. https:// www.fhwa.dot.gov/pavement/sustainability/pubs/hif19027.pdf Friedrich, M. 2021. Innovative Plastic Asphalt Shows Promise for Paving Roads. https://www. good.is/plastic-asphalt-could-pave-the-roads-of-the-future Giustozzi, F., and Boom, Y. J. 2021. Use of Road-Grade Recycled Plastics for Sustainable As- phalt Pavements: Overview of the Recycled Plastic Industry and Recycled Plastic Types. Austroads Publication No. AP-R648-21, Austroads, Ltd. Giustozzi, F., M. Enfrin, M., Xuan, D. L., Boom, Y. J., Masood, H., Audy, R., and Swaney, M. 2021. Use of Road-Grade Recycled Plastics for Sustainable Asphalt Pavements: To- wards the Selection of Road-grade PlasticsâAn Evaluation Framework and Preliminary Experimental Results. Austroads Publication No. AP-R663-21, Austroads, Ltd.
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