Like many other state highway agencies, the Virginia Department of Transportation (VDOT) is extensively working to determine how to best incorporate recycled materials such as recycled asphalt pavement (RAP), recycled crumb rubber, and crushed concrete into their roads. With the change in recycling streams over the past few years, the interest in diverting plastics from the waste stream for reuse in asphalt has been a growing topic for highway agencies. In 2017, plastics accounted for 35.4 million tons of waste in the United States with only 3 million tons (8% of the plastic waste) being recycled, 5.6 million tons (16% of the plastic waste) undergoing combustion, and 26.8 million tons (76% of the plastic waste) being stacked and landfilled. Hence, the current plastic waste challenge became a critical concern.
Researchers and engineers started looking at asphalt mixtures as a potentially viable “home” to incorporate some of the recycled plastic waste commodity. There is a growing body of literature on the use of recycled plastic waste in asphalt, but much of the documented efforts lack a clear experimental plan and suffer from the use of dated test methods. Moreover, it is still unclear if producing and paving recycled plastic modified (RPM) asphalt mixtures would require any changes to typical paving practices, especially to those in Virginia. Finally, future research was recommended with a particular focus on sourcing and methods of incorporating recycled plastics, material characterization of laboratory and field produced asphalt mixtures modified with recycled plastic waste, plant operations, health and safety, and mainly construction of field demonstration projects and associated short- and long-term performance.
What’s in It for Virginia?
The initiative of incorporating recycled plastic waste into asphalt mixtures has been on the agency’s radar for years. This initiative promotes defining a sustainable solution for both improving the performance of asphalt pavements in Virginia and diverting waste plastic from being placed in a landfill or incinerated into a secondary use as a commodity replacement for other raw materials. Seven types of plastics are currently identified with various chemical and physical properties (such as melting points and others). With the existence of multiple plastic types and the major uncertainties due to the variabilities in their properties, such an initiative will help the agency gain a gradual knowledge about the types of recycled plastic waste that may be compatible with the paving material typically used in Virginia (e.g., asphalt binders, aggregates and others) and that are expected to result in longer-lasting pavements. As much as we care about diverting plastics and any other material coming from various waste streams (tire rubber, RAP, glass, etc.), we also care about including these materials in a very responsible manner that would guarantee good performance.
Summer 2021 Field Trials
One major benchmarking experiment/project was planned and developed for the 2021 paving season. The experiment included two RPM 12.5 mm dense-graded surface mixtures (SM-12.5 P1 and SM-12.5 P2) alongside two typical VDOT control dense-graded mixtures (SM-12.5 D and SM-12.5 E). The first RPM mixture featured the use of a complex arrangement of polyethylene-based polymers designed for the extension and enhancement of asphalt binders used in road surfaces at a rate of 5% by total weight of the employed asphalt binder (herein referred to as “P1”). The second plastic trial featured the use of polyethylene terephthalate-based plastomeric amorphous polymers designed to improve the overall performance at a rate of 3% by total weight of the employed asphalt binder (herein referred to as “P2”). The RPM mixtures were designed to meet the agency’s specifications in terms of gradations and volumetric properties. Both RPM mixtures included 15% RAP and were produced using a PG64S-22 asphalt binder.
The mixtures were produced following typical contractor practices. The only difference is that the plastics were added to the mixtures following a “dry process.” The plastics were introduced at the RAP collar behind the flame at the target rates using carefully calibrated fiber feeders to ensure a consistent flow of plastic. They were then mixed with the hot aggregates prior to introducing the asphalt binder as shown in Figure 1. The RPM mixtures were produced at a temperature range of 320 to 330°F. About 700 tons of RPM asphalt mixtures per each recycled plastic product were produced and were placed in the field in a 1.5-inch single lift over the course of 1-mile long adjacent stretches on Old Stage Road in Chester, Richmond, Virginia.
The first few tons of RPM mixtures with plastic P1 were produced at a relatively lower temperature (~290 to 300°F) which resulted in the initial roller pattern and control strip not meeting the density specifications. This could be due to the slight increase in stiffness of produced asphalt mixtures when recycled plastic particles were included. To account for that, the production temperature of asphalt mixtures at the plant was increased to 320 to 330°F. As the night progressed, all core densities passed the requirements. The first night of trials generated several lessons learned. No issues were encountered when producing and paving the second RPM mix (with plastic P2). Figure 2 shows some of the paving operations. No changes from routine established practices in relation to surface preparation or paving operations were reported; standard construction practices and equipment were used. Finally, personnel involved with this effort had not encountered any safety-, health- or environmental-related considerations specific to RPM mixtures that do not apply to standard conventional asphalt mixtures.
Do RPM Mixtures Perform Well?
An extensive suite of laboratory tests are being performed on asphalt binders and specimens fabricated from the RPM and corresponding control produced mixtures to assess the compatibility of using recycled plastic waste with the raw materials locally available in Virginia. The asphalt specimens include plant mixed plant compacted specimens (no-reheat), plant mixed laboratory compacted specimens (reheat) and plant mixed field compacted specimens (cores) right after paving. The selected tests belong to three levels of testing complexity: “basic” tests characterized by a short time for specimen preparation and testing without requiring any specific cutting, coring and gluing; “intermediate” tests requiring a longer time for specimen preparation and testing; and “advanced” tests requiring expensive machinery and multiple days to complete and analyze the test results. Figure 3 presents some of the results of the Virginia Balanced Mix Design (BMD) tests when performed on RPM and control specimens fabricated during design and production (i.e., reheats and non-reheats). Overall, the RPM mixture exhibited Cantabro mass loss values relatively greater than the ones observed for SM-12.5 E mix but lower than the ones observed for SM-12.5 D mix; thus, indicating promising durability. As expected, no issues related to rutting were observed when evaluating all mixes using the asphalt pavement analyzer (APA) rut test at 147°F (64°C). The indirect tensile cracking test (IDT-CT) at 77°F (25°C) was performed on all mixtures to evaluate the resistance to cracking. In general, RPM mixes showed cracking tolerance index (CT index) values similar or greater to those of SM-12.5 D and SM-12.5 E mixes. Although, current efforts are comprehensively assessing the feasibility of using these tests to evaluate the “true” performance of RPM mixtures, promising preliminary performance of RPM mixtures were observed, making them qualify to act as “D premium mixtures.”
Environmental Aspect of the Initiative
In addition to evaluating the performance properties of the produced RPM paving materials, there is a major interest in better understanding the potential environmental impacts associated with the use of recycled plastics in asphalt mixtures. For example, per night, this trial helped save the equivalent weight of approximately 635,000 single-use plastic bags going to landfills. This was accompanied by an offset of approximately 10,770 kg of CO2. Although these numbers might seem low because only one trial was done, an increase in using such alternatives can help reduce fossil fuel usage, leading to a reduction in carbon footprint and helping to foster a circular economy.
On the other side, along with an increased interest in the reuse of waste plastics, a growing concern has been raised regarding the topic of microplastics in the environment. Lewis N. Lloyd, research scientist at VTRC with expertise in waste management and beneficial reuse of waste material, is handling the environmental aspect of this initiative. His efforts will attempt to detect and quantify the presence of microplastics in material generated from pavement wear that could potentially be introduced into the environment through stormwater runoff. This portion of the research is still in its early stage, and no firm or clear conclusions have been derived yet.
Ongoing and Future Work
We learned many lessons from this trial about practices covering the mix design and production stages, but there is still more to figure out. Multiple field trials are being planned for the upcoming 2022 paving seasons. These trials are expected to feature similar and new types of plastic waste. Moreover, the research team will continue evaluating the RPM and control mixtures in the laboratory with a major focus on critical long-term aging and in the field through non-destructive testing.
One major item on the team’s radar in 2022 is about evaluating the feasibility and the process of recycling asphalt mixtures already containing recycled plastic. This will highlight the impact of such processes on the properties and production of such mixtures, and the potential generation of hazardous emissions on the environment.
This important and extensive effort could not be accomplished without the contribution and hard work of several parties. Appreciation is extended to Thomas Schinkel, the material engineer for VDOT’s Richmond District, for allowing this trial to take place in his district. Support from personnel at VDOT’s Richmond district and VDOT’s Materials Division is greatly appreciated. Sincere appreciation is expressed to Travis Cable, Vice president of quality control, and the team at Colony Construction Inc. In addition, the assistance and support provided by the plastic suppliers MacRebur and Kao Chemicals and the supplier of fiber feeders/machines, Hi-Tech Asphalt Solutions, are greatly appreciated. Finally, appreciation is extended to VTRC leadership and staff for their continuous support of innovative efforts.