Life-Cycle Assessment of Asphalt Pavements with Recycled Post-Consumer Polyethylene

Plastic Waste

Plastic waste typically consists of different types of plastics, with different chemical compositions and market purposes. Chin and Damen ( 12 ) differentiated between seven main categories of plastic waste and reported that polyethylene terephthalate (PET), high-density polyethylene (HDPE) and low-density polyethylene (LDPE), and polypropylene (PP) generally constitute 85% of plastic waste in Australia. Similarly, Ren et al. ( 3 ) reported a wide availability of waste polyethylene (PE), PP, polystyrene, and polyvinyl chloride (PVC) because of the relatively short lifespan of the corresponding products (two years or less). As post-consumption plastic waste is typically mixed, it is first collected, sorted, and cleaned before treatments, such as recycling ( 12 , 13 ). PET, LDPE, and HDPE are available in greater quantities and are more suitable for recycling and reuse than other plastic types, thereforer they have relatively high recycling rates ( 12 ).

Several studies have compared the environmental impacts of different end-of-life (EOL) treatments for plastic waste. Lazarevic et al. ( 14 ) summarized the results of 77 life-cycle assessment (LCA) studies that evaluated four EOL treatments: mechanical recycling, feedstock recycling, landfilling, and incineration. The results indicated that, in most cases, mechanical recycling is the preferred option from an environmental standpoint. A study by Rajendran et al. ( 13 ) showed the environmental benefits of mechanical recycling over incineration. It is noteworthy that the analyzed studies accounted for the avoided burden of virgin plastic production as a benefit of recycling ( 13 , 14 ). Khoo investigated the combinations of different EOL treatments for yearly plastic waste production in Singapore and showed how the optimum combination could be selected based on the decisionmaker’s environmental priorities ( 15 ).

Use of RP in Asphalt Pavements

While the use of polymer modification and different recycled materials (e.g., recycled asphalt pavements [RAP], recycled asphalt shingles [RAS], and ground tire rubber [GTR]) in asphalt pavements has been practiced for decades ( 11 ), RPs have not been widely used. However, the environmental initiatives and fluctuations in global markets discussed above (the example of Chinese policy changes) created the need to assess the feasibility and environmental repercussions of RP use in pavements.

The two most common ways to integrate plastics into asphalt mixtures are known as wet and dry processes. In the wet process, the asphalt binder is modified with RP, while in the dry process, RP is added directly into the mix, as a partial replacement for aggregate, mixture modifier, binder modifier, or any combination thereof ( 16 ).

The research studies focused on the wet process investigated the RP dosages ranging from 0.5% to 10% per binder weight ( 17 – 21 ), with the optimum content typically reported between 3% and 6% ( 18 , 22 , 23 ). The most commonly tested plastics type is PE ( 18 , 20 , 22 – 29 ). RP-modified binders exhibit increased viscosity, improved stiffness, raised softening point, increased penetration value, and improved high-temperature behavior ( 22 , 25 , 27 , 28 , 30 ). Asphalt mixtures prepared with an RP-modified binder present increased stiffness, improved rutting resistance, and extended fatigue life ( 21 , 23 , 31 , 32 ). RP-modified binders generally outperform conventional binders but may exhibit somewhat lower performance than virgin polymer-modified binders ( 24 ). Some studies investigated the coupled effect of binder modification with rubber and RP, and reported improved performance of binder with two modifiers ( 19 , 20 , 26 , 29 ). The feasibility of binder modifications with RP on a larger scale has yet to be evaluated. One known issue is the storage instability of modified binder and phase separation ( 33 , 34 ). Chemical compatibilizers or stabilizers can mitigate storage instability ( 33 – 35 ); however, these additional chemicals may also aggravate the environmental footprint.

Several research studies have demonstrated the feasibility of the inclusion of RP in asphalt mixtures through the dry process. Used contents of plastics range from 0.1% to 10% by the weight of the aggregate ( 31 , 36 – 38 ). The benefits of the inclusion of RP in asphalt mixtures through the dry process reported in the literature include increased fatigue life, rutting resistance, and Marshall stability ( 31 , 38 – 40 ). The disadvantages of the dry process include a weakened bond between aggregate and binder, as well as lower resistance to moisture damage ( 12 ).

As to the environmental impacts, the literature on RP asphalt is rather limited. White ( 41 ) reported that the addition of RP by the wet process had no detrimental effect on fume generation during mixture production or on leachate. Yu et al. ( 42 ) compared cradle-to-gate greenhouse gas emissions and energy consumption of asphalt mixtures modified with recycled PP and rubber with a mixture with a polymer-modified binder and demonstrated lower impacts of the former. The same study also reported a difference in performance between the recycled PP and rubber mixtures, which was not accounted for to contextualize the results. Santos et al. ( 43 ) investigated the effects of recycled PE in modified asphalt binder and asphalt mixtures as a partial aggregate replacement for the Australian context using LCA. Their results indicate that PR-modified binder is environmentally preferential to virgin polymer-modified binder. Asphalt mixtures with PE added through the dry process have greater increase in environmental impacts relative to the HMA baseline ( 43 ). Lastra-González et al. ( 44 ) evaluated asphalt mixtures with binder modified with 25% of PE recycled from copper cables and flexible packaging film. Their LCA evaluated end-point indicators and revealed that modified mixtures could be environmentally beneficial compared with control HMA and that service life extension increased the benefits ( 44 ). It is noteworthy, however, that Lastra-González et al. ( 44 ) accounted for the avoided burden of plastic incineration when PE is recycled and used as an asphalt modifier. The assumption of avoided burden positions PE-modified mixtures favorably and should be corroborated with the analysis of the current waste management practices for waste PE. Although several studies mention that environmental benefits are the key motivation to use RPs in asphalt pavements (e.g., 29 , 30 , 35 , 39 ), the quantitative analysis of environmental impacts is typically not conducted to support the claims of environmental benefits. Review studies recognized that the assessment of sustainability indicators (economic and environmental) is a research need ( 16 , 35 , 45 ). To address the identified research gap and inform potential future initiatives, this study focuses on quantifying the life-cycle environmental impacts of RP in asphalt.

Goal and Scope

The goal of LCA is to evaluate the environmental impacts of asphalt pavements made with the addition of RP pellets relative to pavements produced with the conventional HMA and polymer-modified asphalt from the life-cycle perspective. The evaluated RP mixtures feature recycled post-consumer PE, introduced to the mixture via a dry process. The dry process was selected over the wet process because of its relative ease of implementation, the possibility to consume larger amounts of RP, and avoiding the issue of storage instability. PE was chosen as a type of RP that is available in abundance, in comparison with PET, which has higher demand from both the bottle and textile industries ( 5 ). The polymer-modified asphalt featured the binder modified with virgin styrene butadiene styrene (SBS). Some studies in the literature focused on comparisons of recycled modifiers, such as RP, with modifiers made with virgin polymers because polymer modifiers (recycled or not) are expected to influence mixture performance in a similar manner ( 42 ). From the standpoint of mixture selection in pavement projects, depending on the agency, RP mixtures could be an alternative to conventional HMA, as well as to the polymer-modified mix. Accordingly, this study provides relevant comparisons with both mixture types organized around several scopes.

Comparisons

First, the environmental impacts of the declared unit of one short ton of different mixtures are presented in parallel. The analysis of the environmental impacts reveals the relative contributions of different processes to the overall impacts of the mixture, as well as the additional burden associated with the use of RP and SBS modifiers. The introduction of RP and SBS modification alters the properties and performance of the asphalt mixture. Therefore, the direct comparison of the impacts based on the same declared unit fails to account for different mixture performances.

To contextualize the results, life-cycle phases beyond material production were included in the analysis of a functional unit of one lane-mile of pavement. The functional unit in LCA should include the elements of quantity, quality, and durability ( 50 ). In this study, the quality and durability components of the functional unit (considered as performance) were analyzed further through the cradle-to-built and cradle-to-grave scope to determine contexts under which environmental benefits can be achieved.

The cradle-to-built analysis included a functional unit of one lane-mile of pavement constructed with different mixture designs. To make the comparisons based on cradle-to-built scope, pavements should be characterized by comparable performance beyond the construction phase. To account for different mixture performances, the thickness of RP pavement was varied to calculate what thickness makes it equal to other alternatives from the environmental perspective.

The cradle-to-grave comparison included all life-cycle stages for the functional unit of one lane-mile of pavement. Different mixture performance was accounted for through different intervals between the subsequent maintenance and rehabilitation (M&R) activities. Accordingly, the analysis focused on determining how much the time between the subsequent M&R interventions should be altered to achieve equal annualized environmental impacts to the alternatives.

The product system used in the study showing the pavement design alternatives and the evaluated life-cycle stages is presented in Figure 1.

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Figure 1.

Comparison of product systems: one lane-mile of pavement constructed with conventional hot-mix asphalt or polymer-modified asphalt mixture, and with recycled plastic-modified asphalt mixture (RPs). Note: The process “M&R” in the figure pertains to maintenance and rehabilitation interventions that occur during the life cycle. EOL = end of life.

Life-Cycle Inventory

The cradle-to-gate part of the analysis (A1–A3 in Figure 1) consists of material extraction, transportation, and mixing in the asphalt plant. Mixture designs used in this study originate from Transportation Research Board ( 51 ) and are summarized in Table 1. The matrix includes the conventional HMA (control) and four mixtures with addition of RP at 0.2, 0.4, 0.6, and 0.8% by the weight of the aggregate (mixtures denoted as RPs_0.2, RPs_0.4, RPs_0.6, and RPs_0.8). Control and polymer-modified mixtures had a binder content of 5.8% of the weight of the dry aggregate. To accommodate the addition of specified RP quantities into the mixture, the same weight of original materials was removed from the mixture. Specifically 50% of RP was accounted for as binder replacement and 50% as an aggregate replacement, evenly distributed across all aggregate fractions to preserve the design aggregate gradations ( 51 ). RP pellets come in one size; however, because of their pre-heating with the remainder of the aggregate, as explained in the description of the mixing process, no additional sorting and processing was needed for the incorporation.

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Table 1.

Mixture Design Matrix Analyzed in this Study

Material type	Control mixture	RPs mixtures	Polymer-modified mixture
Sieve size AASHTO (description)	% of aggregate
#7 (Crushed gravel)	15
#8 (Crushed gravel)	25
#9 (Crushed limestone)	10
#10 (Manufactured sand)	35
#10 (Natural Sand)	15
RPs content % of aggregate weight	0	0.2, 0.4, 0.6, 0.8	0

Life-cycle inventory data sources are summarized in Table 2. The chosen data sources are public, which enables the transparency and reproducibility of this study. As seen in Table 1, the asphalt binder is modeled using two inventories. The first one is based on the petroleum refining process from the National Renewable Energy Laboratory (NREL) database with the economic allocation. The inventory from the NREL database follows the process by Mukherjee and—together with data sources for aggregate, transportation, and electricity—matches the prescribed data sources from the National Asphalt Pavement Association (NAPA) EPD program for asphalt mixtures ( 52 , 53 ). This dataset is denoted in this paper as the “NAPA binder dataset.”

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Table 2.

Data Sources for Materials and Associated Transportation Modes and Distances

Material constituent	Data source	Transportation mode	Transportation distance [miles]
Aggregate	Marceau et al. ( 58 )	Truck	50
Asphalt binder	Mukherjee et al. ( 52 ), referred to as NAPA dataset	Diesel train	100
	Wildnauer et al. ( 54 ), referred to as Asphalt Institute (AI) dataset
Polyethylene pellets	Franklin Associates ( 55 )	Truck	50

In an LCA primer for asphalt mixtures in support of EPD programs for NAPA, chemical additives, ground tire rubber, and different polymers, including SBS, were deemed as data gaps ( 52 ). Therefore, to model polymer-modified asphalt mixture, a dataset for binder developed by the Asphalt Institute (AI) was utilized for both plain and SBS-modified binders with 3.5% of SBS ( 54 ), referred to in this paper as the “AI binder dataset.” Comparison of the two binder data sources is beyond the scope of this study. Accordingly, the comparisons were made only among the product systems with the same data source for the binder.

Modeling of mechanical recycling of waste plastics was done based on the report by Franklin Associates ( 55 ), which presents a relatively new life-cycle inventory of recycling of post-consumer PET, PE, and PP resins specific to the U.S.A. and is regarded as relevant for this study. While the original report implemented background data from the Swiss ecoinvent database, the background data in this study originated from the NREL database to preserve consistency with the remainder of the analysis. The schematic of the product system for plastics recycling based on the Franklin Associates report ( 55 ) is shown in Figure 2. In this study, plastics are the product of a polymer product system that is subsequently used in an asphalt pavement product system. Accordingly, its impacts should be allocated or partitioned among those product systems. Because post-consumer plastics are a waste product that is abundantly available and with no economic value, a cutoff allocation approach is implemented. In other words, PE pellets are introduced into the product system only with the burdens of collection, sorting, and postprocessing, as shown in Figure 2, while the burdens of virgin production are excluded. Cutoff allocation is the industry-standard allocation practice for accounting for the burden of secondary materials in LCA of asphalt mixtures ( 53 ). However, as Figure 2 indicates, PE processing is preceded by mixed waste collection, transportation, and sorting. The impacts of these three activities were partitioned based on mass in the report, which is justified because (a) it involves a physical relationship and (b) the mass of waste is a typically tracked parameter ( 55 ). Since the resins are reprocessed separately on sorting, the reprocessing inputs and outputs for the PE stream were available and modeled accordingly. It is noteworthy that the impacts of reprocessing (i.e., turning waste plastics into pellets) comprise the majority of the impacts associated with RP processing, while the impacts of collection and sorting are markedly lower ( 55 ). The same trend was reported in other studies on waste plastics collection and processing ( 43 , 56 ).

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Figure 2.

Product system for mechanical recycling of waste plastics, reproduced after Franklin Associates ( 55 ). The system boundary used in this study is limited to polyethylene (PE) processing.

Transportation distances and modes were assumed, as shown in Table 2, and transportation throughout the product system was estimated using the NREL database ( 57 ). For asphalt binder, transportation from the refinery terminal to the plant is assumed to be via diesel train ( 52 ).

The mixing process for the baseline mixture was modeled after Mukherjee ( 52 ). Based on communication with the producer, the mixing temperatures of all mixtures were set at 157°C (315°F). For RP mixtures, the duration of a dry mixing was extended from 60 s for the control to 70 s (16.7%) to allow for softening and dispersion of the RP before addition of the binder. Because dry mixing comprises approximately half of the total mixing, energy requirements for preparation of RP mixtures were assumed to be 8.35% higher compared with the control mix. With regards to polymer-modified mixture, a 17% increase in total mixing energy was assumed to account for the higher mixing temperature necessary for the modified binder with the increased viscosity, based on the literature ( 59 , 60 ).

The construction stage included the processes of asphalt hauling and placement. The processes modeled in the use stage include M&R treatments to restore pavement serviceability, while the other use phase impacts, such as excess vehicle fuel consumption, noise, and stormwater runoff, were considered equal among alternatives and therefore excluded from the analysis. M&R treatment was assumed to be milling (with the transport of the milled material) and repaving of the top 2 in. of pavement using the same mixture design as in the initial construction. Because every scenario included an equal number of interventions, corresponding emissions from traffic delay caused by maintenance works were considered equal among scenarios and therefore excluded from the analysis. EOL treatment included the removal of the full pavement thickness with a 30-mile hauling of the removed material to a landfill.

The fuel usage factors from the National Cooperative Highway Research Program (NCHRP) Project 774 were used for all the modeled field activities ( 61 ), namely hauling and paving in the construction stage, milling, and paving in M&R, and pavement removal in EOL. It was assumed that the diesel fuel was used for all processes, and it was modeled using the NREL process, “Diesel, combusted in the industrial equipment” ( 57 ).

Life-Cycle Impact Assessment (LCIA)

Impact assessment method “Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts” (TRACI) 2.1 was used for LCIA as a U.S.-specific LCIA method ( 62 ). Modeling was performed in OpenLCA software ( 63 ). Five impact categories were evaluated in this study: acidification (AC), eutrophication (EU), global warming (GW), ozone depletion (OD), and smog creation (SC).

Cradle-to-Gate Analysis

Environmental impacts of a declared unit of one short ton of asphalt mixture for different mixtures normalized to the annual U.S. production from 2008 ( 64 ) are shown in Figure 3. The results indicate that the addition of RP produced an increase in environmental impacts relative to the control. Depending on the mix design, this increase ranged between 2.5% and 10.5%. Mixtures with higher RP content present a greater increase in environmental impacts relative to HMA. The comparison with the polymer-modified mixture from the AI binder data indicates that the impacts of RP mixtures are in between those of the control HMA and the polymer-modified mixture. The normalization undertaken indicates the relative importance of different impact categories compared with the U.S. industry baseline. As seen in Figure 3, the most significant impact categories are SC, AC, and GW. The magnitude of normalized OD is markedly lower than that of other impacts. OD is typically caused by chlorofluorocarbons (e.g., freons, refrigerants, air conditioners, aerosol propellants) and halons (e.g., fire extinguishers) ( 49 ). The use of such materials is not typical in the pavement industry.

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Figure 3.

Potential environmental impacts of one short ton of asphalt mixture normalized using the United States per person production from 2008 ( 64 ), comparing hot-mix asphalt (control), polymer-modified asphalt, and recycled plastics (RP) in various percentages.

Figure 4 presents the contribution of different processes to GW of different mixtures. The results indicate that RP mixtures have a somewhat lower contribution of binder impacts compared with the control because of the slightly lower binder content. However, the impact of increased energy for mixing and processing of plastic pellets yields a higher total GW for the RP mixture (5% to 8.5% in the case of the NAPA dataset and 4% to 7% in the case of the AI dataset). As seen in the AI dataset, an SBS mixture has greater impacts than the control because of the greater impacts of the modified binder, as well as higher mixing energy, amounting to a 15.6% difference. As shown in Figure 3, GW from RP mixtures is in between that of the control HMA and a polymer-modified mixture. Transportation, as shown in Figure 4, pertains to the upstream transport of all material constituents (phase A2 in Figure 1) based on the transportation distances defined in Table 1.

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Figure 4.

Contribution of different processes to global warming (GW) for one short ton of asphalt mixture.

Figure 5 shows the sensitivity of GW and SC to the transportation distance of RP pellets, as these two impact categories exhibit the highest relative increase with change in transportation distance of the RP. The analysis is performed for all RP mixtures in the study for their specific replacement rates of aggregate and binder that are elaborated in the Methodology section. The baseline scenario (0% increase in Figure 5) is based on 50-mile transportation. As seen in Figure 5, a higher rate of increase in environmental impact is seen for the mixtures with higher RP content, which is an expected trend. However, even for a twentyfold increase in transportation distance, GW and SC increase up to 1.3% and 1.8%, respectively. This trend suggests that the presence of a local RP processing facility may not be critical from an environmental standpoint. The analysis presented in Figure 5 was limited to truck transport, while the other transportation modes for longer distances were not investigated. Figure 5 pertains to the NAPA dataset, while a similar trend was identified with the AI dataset.

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Figure 5.

Effect of transportation distance of recycled plastic (RP) pellets on (a) global warming (GW) and (b) smog creation (SC) for one short ton of asphalt mixture.

Based on the report that was used to model the RP mixtures ( 51 ), an increase in RP content results in decreased rutting and increased thermal cracking. As mentioned in the Methodology section, the mixtures modified with RP or SBS differ from the control HMA with regard to mixture performance, which provides different pavement performance. To contextualize the results and provide for a more relevant comparison, the scope of the analysis is broadened to pavement sections.

Cradle-to-Built Analysis

The cradle-to-built scope of the analysis includes material production, transport to the site, and construction (phases A1–A5 in Figure 1). The functional unit is one lane-mile of pavement. To compare different pavements for the cradle-to-built system boundary, the pavements’ performance beyond the construction stage should be equal. RP mixtures have some aspects of performance that are improved relative to HMA but higher cradle-to-gate environmental impacts. Conversely, a polymer-modified mix can have superior performance and higher environmental impacts relative to RP mix. In both cases, changes in pavement thickness (lower thickness of RP relative to the control or greater thickness of RP relative to polymer-modified) that do not compromise the performance beyond construction can produce two equal designs suitable for the cradle-to-built comparison. Accordingly, the focus of this section is to evaluate the change in thickness that would see the environmental impacts of RP pavement “break-even” with those of the alternatives.

In the cradle-to-built analysis, the comparisons of RP pavements were made relative to an 8 in. thick pavement constructed with HMA (Figure 6a and b ) and polymer-modified mix (Figure 6c). Results indicate that RP pavements of thickness equal to the control present higher environmental impacts. However, with thickness savings, this trend can change. As the results in Figure 6a and b show, a 1 in. reduction in thickness can be sufficient to produce environmental benefits for all RP mixtures. At equal thickness, RP pavements are environmentally preferential to polymer-modified counterparts. However, if the thickness increases over 1 in., this trend is no longer present. Figure 6 presents AC as an example, while similar trends were found for other impacts. The actual pavement thickness will be specific to the pavement design and project details. However, the use of RP mixtures should be strategic, such that their advantages (e.g., improved rutting resistance) are leveraged, and their disadvantages (e.g., aggravated thermal cracking) are minimized. In that case, use of RP can produce environmental benefits; otherwise, RP can add an environmental burden.

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Figure 6.

Acidification (AC) of one lane-mile of pavement for cradle-to-built scope, based on pavement thickness and mixture design. The baseline is 8 in. pavement made with control hot-mix asphalt (HMA) (a and b) and polymer-modified asphalt (c).

Cradle-to-Grave Analysis

Figure 7 presents a schematic of cradle-to-grave analysis, with materials extraction and construction happening at the beginning of the analysis, followed by evenly spaced M&R activities, and finalized with EOL treatment when the pavement reaches its terminal serviceability.

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Figure 7.

Schematic of cradle-to-grave analysis framework. Parameters X and X+ represent the baseline and expanded time intervals between the subsequent maintenance and rehabilitation (M&R) cycles. EOL = end of life.

The approach used in cradle-to-built analysis to develop the equivalent pavement designs for the comparisons is followed here. Analogous to pavement thickness that was varied in the cradle-to-built assessment, in cradle-to-grave analysis, the parameter of choice was the time interval between the subsequent M&R activities. Potentially improved performance of RP pavements relative to the control was accounted for through the extension of the baseline M&R interval (X and X+ in Figure 7). The comparison is made for the annualized environmental impacts after the equal number of M&R treatments when the pavements reach equal (terminal) serviceability. In comparison with the polymer-modified alternative, it was assumed that RP pavements would exhibit somewhat lower performance, represented by shortened M&R interval. In both cases, changes in the break-even intervention time periods were evaluated for RP pavements. A 30-year service life and 10-year M&R baseline intervals were assumed for the control HMA and polymer-modified alternatives, as common assumptions used in pavement life-cycle cost analysis (LCCA) by agencies in the U.S.A. ( 65 ).

Because the mixtures can be used in different contexts, a variety of distresses can develop throughout the life cycle. Based on the overall pavement condition and predefined M&R trigger, agencies should decide on the time for M&R intervention to restore serviceability. The detailed analysis of distresses and pavement condition in various contexts exceeds the scope of this study. Therefore, the time to intervention is a proxy for the overall pavement section performance. Because implementation of RP mixtures is initiated because of their assumed environmental merits, this part of the analysis determines the difference in engineering performance (M&R intervals length) needed to make RP sections environmentally beneficial to the alternatives.

It is noteworthy that the impacts associated with the pavement use phase may also include the extra fuel consumption (EFC) from pavement–vehicle interaction (PVI) and different rolling resistance between the alternatives. In this study, however, M&R intervals are adjusted such that the compared sections perform similarly over the life cycle (an assumption that is typical in LCCA for pavement design decision-making). Accordingly, life-cycle EFC was considered comparable among the alternatives and was excluded from the analysis. Even though various PVI models exist, their use has been largely limited to demonstration studies by the model developers, while their independent verification has not been performed ( 66 ). Accordingly, in the light of uncertainty associated with various inputs over the life cycle and in-field use of new technology such as RP asphalt mixtures, the use of PVI models is fraught with ambiguity when used in comparative LCA or LCCA.

Calculated break-even M&R periods for various environmental impacts and RP mixture designs are presented in Figure 8. ODP is not shown because RP mixtures present slightly lower OD relative to the control because of the lower binder content. As seen in Figure 8a and b , an increase in M&R interval length by approximately 7% relative to control HMA is sufficient to justify the use of RP mixtures from the environmental perspective. The comparison with the polymer-modified alternative (Figure 8c) indicates that pavements made with higher RP contents, such as RP_0.8, should have equal performance with polymer-modified pavement to be considered equal. When RP contents are lower, the performance can be somewhat lower and translate into M&R interval reduction of approximately 3%.

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Figure 8.

Break-even maintenance and rehabilitation (M&R) intervention periods for pavements with recycled plastics based on cradle-to-grave analysis. The baseline is a 10-year intervention period for pavements made with control hot-mix asphalt (HMA) (a and b) and polymer-modified asphalt (c). Contributions of different life-cycle phases to global warming (GW) (d).

Chosen M&R interval values are specific to the analyzed mixtures and modeled scenario. As is the case with cradle-to-built analysis, the actual savings and extension of M&R cycles are context-specific and can be achieved if the mixtures are utilized where it is appropriate from the engineering performance perspective.

Consumption of Plastic Waste

Because the use of RP in asphalt mixtures is primarily motivated by the initiatives to reduce plastic waste and offload waste plastic streams, one additional parameter worth evaluating is waste consumption. Figure 9 shows the mass of RP pellets that are used in one short ton of asphalt (Figure 9a) and one lane-mile of asphalt pavement of 8 in. thickness (Figure 9b). RP contents shown in Figure 9 also represent the recycled material content in one short ton of asphalt and one lane-mile of newly constructed pavement. The numbers associated with the bars signify the number of PE milk jugs weighing approximately 60 g that can be diverted from the waste streams. Because over 75% of waste plastics in the U.S.A. end up in landfill ( 1 ), the practice of RP usage has the potential to free up the landfill space and prevent other environmental issues associated with plastics landfilling, such as leaching or creation of microplastics. Nevertheless, these potential benefits should be weighed against the impacts on asphalt mixture and pavement. As stated previously, RPs should be used strategically, based on their suitability in a specific context with regard to environmental and engineering performance.

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Figure 9.

The mass of waste plastics (RP pellets) that can be consumed through (a) one short ton of asphalt mixture, and (b) lane-mile of pavement with 8 in. thickness. Numbers associated with the bars show the equivalent number of polyethylene (PE) milk jugs that would potentially be diverted from the waste streams.