BasePDH | Course 021 | 3 Professional Development Hours

Recycled Plastics in Asphalt Pavements: Current Evidence and Practice

1. The Acceptance Context: Asphalt Economics, Circularity, and Two Precedents

Asphalt surfaces 94 percent of the paved roads and highways in the United States, with concrete making up the remaining 6 percent [1]. The network runs about 2.5 million miles (4 million km), and producers supply roughly 363 million tons of asphalt paving mixtures each year to build and maintain it [1]. Each state DOT writes its own mixture specifications, and contractors design, produce, and place mixtures against those specifications under competitive lowest-bid procurement [1]. A proposal to put recycled plastic into an asphalt pavement therefore arrives as a materials acceptance question inside an existing specification system, and the useful evaluation runs on that system's terms: what the additive is, how it enters the mixture, what it does to the properties the specification controls, and what it does to the pavement's end of life.

An asphalt mixture is 4 to 6 percent asphalt binder and 94 to 96 percent aggregate by mass [1]. Summer 2022 figures collected by the National Center for Asphalt Technology put unmodified binder at US$880 per tonne and polymer-modified binder at US$1,060, against US$22 per tonne for virgin aggregate and US$11 for processed reclaimed asphalt pavement [1]. The report carrying those figures notes that binder prices move considerably with the crude oil market and that these values sat near historical high points [1]. Contractors accordingly design to three priorities in order: minimize asphalt content, the most expensive component by far; minimize aggregate haul cost through local sources; and maximize RAP content, which is produced more cheaply than virgin materials [1]. The incumbent that a recycled plastic would displace on high-grade work is styrene-butadiene-styrene (SBS) block copolymer, an engineered virgin thermoplastic elastomer designed to be compatible and storage-stable with asphalt binders, commonly required on high-freight corridors for resistance to heavy truck loading and extreme weather [1]. Any cost argument for a recycled plastic has to be made inside this structure, where binder is a small fraction of the mixture and the modified-binder premium is the difference between two published prices.

The recycling economy already running through asphalt pavements sets the second half of the context. The most common rehabilitation approach is to cold mill the top couple of inches of a distressed pavement, which removes the distressed layers, restores the profile, maintains curb heights and bridge clearances, and generates RAP for new mixtures [1]. A mixture with 20 percent RAP reduces new binder content by about 1 percent and virgin aggregate content by 19 percent, saving 15 to 18 percent of total materials cost, and reduces cradle-to-gate greenhouse gas emissions by 12 percent according to the National Asphalt Pavement Association [1]. In 2019 the United States recycled 89.2 million tons (90.6 million metric tons) of reclaimed asphalt pavement, replacing an estimated 24 million barrels of virgin binder and 84 million tons of aggregate, together worth US$3.2 billion [1]. A 2024 industry survey found that more than 93 percent of the asphalt pavement reclaimed in 2022 was reused in new pavements [2]. The national average RAP content is about 21 percent, and industry and USDOT goals set in 2022 target a 50 percent national average by 2035 as part of reaching net-zero pavement emissions by 2050 [1]. On these figures, an additive that compromised the recyclability of the pavements it entered would put at risk a materials stream larger than any benefit the additive itself could plausibly deliver. End-of-life behavior is therefore an acceptance criterion in its own right, weighed alongside performance and cost.

State DOTs assess network pavement condition annually or semiannually, and the performance measures are percentage of lane area cracking, rut depth, and pavement smoothness [1]. Cracking is the most common mode of distress in asphalt pavements in the United States [2]. That ordering matters for everything that follows: a modifier that improves rutting resistance while degrading cracking resistance has improved the less common failure mode at the expense of the more common one.

1.1 Sulfur-extended asphalt

Two earlier additives supply most of what is known about how this class of decision goes. The first is sulfur-extended asphalt (SEA). Sulfur entered use as a binder supplement in the early 1970s, when crude oil prices spiked during the oil embargo and sulfur was in growing supply as a byproduct of refining under new federal process controls [1]. Research at the time indicated 20 percent of the binder in a mixture could be replaced with sulfur, and mix design procedures were adapted to the replacement [1]. Between 1975 and 1984, 68 SEA test sections were built in the United States, 18 of them monitored by FHWA, which concluded that SEA pavement performance was satisfactory and showed no significant differences from conventional pavements [1]. Cores taken from those same projects, along with other laboratory studies, indicated the opposite: SEA mixtures were stiffer and more susceptible to moisture damage and fatigue cracking [1]. Field observation and materials testing on the same pavements returned different answers, and the materials testing carried the warning. Interest collapsed in the early 1980s when sulfur prices rose from US$17 per tonne to more than US$100 in 1981 dollars [1].

The technology returned in the early 2000s with favorable sulfur economics, new handling methods, and lower mixing temperatures, and trial projects followed in Alabama, California, Missouri, Nevada, and Texas [1]. Hot SEA mixtures emit hydrogen sulfide and sulfur dioxide during production and paving, causing eye and throat irritation, and mix temperatures below 149°C (300°F) are recommended to limit those emissions [1]. Worker complaints about odor and eye irritation persisted despite the reported technical success of the trials, and a distinctive sulfur odor lingered from SEA pavements in warm weather years after construction [1].

The decisive failure came at end of life. Recycling RAP that contains SEA into a new mixture would require heating it well above the 149°C (300°F) limit, reintroducing the emissions that limit exists to prevent, and current practice is to landfill SEA RAP [1]. The report records that the recyclability of SEA pavements was never adequately researched, and that the loss of circularity is a strong deterrent to SEA use now that sustainability drives business decisions [1]. A pavement additive that performed adequately in service still ended as a landfill liability, because one question was left unanswered until the material was already in the ground.

1.2 Recycled tire rubber

The second precedent is recycled tire rubber (RTR), in use in asphalt pavements since the 1960s, with significant market adoption concentrated in California, Arizona, Nevada, and Texas [1]. Common dosage is 10 to 20 percent of the binder content, which corresponds to about 0.5 to 1.2 percent by mass of the total mixture [1]. RTR-modified pavements have generally demonstrated better long-term field performance than unmodified pavements, and many state DOTs treat RTR modification as an alternative to conventional polymer-modified binders [1].

The part of the RTR history most relevant here is the mandate. Around 1990, roughly 1 billion scrap tires sat stockpiled nationally, drawing attention for uncontrollable fires and as breeding grounds for mosquitoes and vermin [1]. In 1991 Congress passed the Intermodal Surface Transportation Efficiency Act with provisions mandating RTR use in asphalt pavements, and those requirements were repealed a few years later under resistance from AASHTO and the paving industry, largely because evidence that RTR benefited pavement performance did not exist at the time [1]. The evidence eventually arrived and the material earned its place in several states on performance, but the mandate preceded the data and did not survive. On circularity, RTR runs opposite to SEA: RTR-modified mixtures have been used in large states for decades with no significant identified issues challenging their recyclability, though further research is recommended on air quality during production and on the field performance of mixtures using RAP that contains RTR [1].

1.3 The two-principle screen