HMA Recycling

HMA is one of the most recycled products in the U.S. It is estimated that about 90 million tons of recycled HMA (termed "reclaimed asphalt pavement" or RAP) is used each year in the U.S. ( milled off roads during resurfacing and widening projects each year (NAPA IS 138, 2020). RAP is typically generated by rehabilitation or reconstruction projects, and nearly every HMA producer uses RAP in some fashion. The National Asphalt Pavement Association (NAPA) estimates that "...RAP use in 2019 reduced the need for 4.5 million tons (24 million barrels) of asphalt binder and more than 84 million tons of aggregate with a total estimated value of more than $3.2 billion." (NAPA IS 138, 2020, page 7). RAP can be used in a variety of ways such as:

• An addition to regular HMA.
• An aggregate in cold-mix asphalt.
• A granular base course when pulverized.
• A fill or embankment material.

HMA recycling can be divided into two basic categories based on the recycling methods used: hot recycling and cold recycling. This section presents the basic recycling process as well as typical uses and considerations for each of these recycling methods.

Hot Recycling

Hot recycling is so named because RAP is used as an aggregate in HMA (hot mix asphalt). In hot recycling, old HMA pavement is removed, broken down into aggregate-sized chunks and then incorporated into new HMA as an aggregate. There are two basic methods for accomplishing this:

• Recycled Hot Mix (RHM)
• Hot In-Place Recycling (HIR)

High-RAP Mixtures

RAP has long been used in hot mixes in limited amounts. Specifications commonly allowed RAP to replace up to 15 percent of the virgin aggregate, sometimes rising to 25 percent if the mix design was adjusted for the aging of the RAP binder. Test projects have since shown that mixes with substantially more RAP can perform well when the RAP is managed and designed for; mixes with more than 25 percent RAP are commonly called high-RAP mixes.

RAP contains both aggregate and aged asphalt binder. Its content is usually stated as a percentage of the aggregate weight or of the total mix weight, but the RAP binder also replaces part of the virgin binder, and a high-RAP mix design has to account for both. The two main concerns are materials management, from milling through stockpiling to the plant, and mix design.

Fractionation. RAP for high-RAP mixes is often fractionated, or screened into size fractions as virgin aggregate is. This gives more control over the combined gradation and can reduce variability, although unprocessed RAP milled from a single source can also be very consistent, and mixes with lower RAP contents often use unfractionated RAP. Crushing increases the proportion of fines, so over-crushing should be avoided because it can raise the dust content of the mix. Crushing and screening are best done as a separate operation before the RAP is fed to the plant, so that the selected proportions of each fraction are maintained. Variability within RAP stockpiles should be checked.

Stockpiles. RAP is stockpiled with normal practices. It does not tend to recompact, but it can form a surface crust, which a front-end loader breaks easily and which helps the pile shed water. RAP stockpiles hold water and drain less well than aggregate, so covering them and placing them on sloped, paved areas is encouraged. RAP from a single source, if available in sufficient quantity, simplifies mix design, particularly when the properties of that source are already known.

Mix design. RAP can be included in Superpave mix designs. The aggregate and binder in the RAP are both characterized so that the properties of the combined RAP and virgin materials can be determined; original construction records help when the source is known. Because the RAP binder is aged and stiffer than virgin binder, the virgin binder grade is often adjusted. For RAP contents of about 15 to 25 percent, two common approaches are to select the virgin binder from a blending chart or to use a binder one PG grade softer than normal (for example, PG 64-22 in place of PG 70-16). Field evaluations of high-RAP pavements indicate that these approaches, although approximate, are reasonable for higher RAP contents as well, and with good quality control they have been used for mixes with up to about 40 to 50 percent RAP.

Performance. The FHWA Long-Term Pavement Performance (LTPP) program included test sections containing at least 30 percent RAP paired with sections of all-virgin mix at sites across the U.S. and Canada; in most cases the high-RAP sections performed as well as or better than the virgin sections. High-RAP sections at the National Center for Asphalt Technology (NCAT) test track, under accelerated loading, gave similar results. Where differences were observed they were attributed to RAP characteristics that can be adjusted for, such as binder stiffness and dust content; some stiffer high-RAP mixes showed more cracking, probably from binder aging, but less rutting. Overall performance and ride quality were similar (FHWA, 2011¹).

Plant production of mixes with RAP is described in Recycled HMA.

Considerations When Using RAP

RAP trials began in the 1970s, when an oil crisis put pressure on the industry's traditional material sources, and RAP processing and plant equipment have improved substantially since. As of 2009, every U.S. state allowed some RAP in pavement construction.

Benefits commonly cited for RAP mixes include lower material costs, especially when asphalt prices are high; reduced environmental impact; reduced stripping, because the RAP aggregate is already coated with binder; and less rutting than in mixes made entirely of virgin material. Any plant that produces HMA can process RAP, and RAP mixes are placed and compacted with standard procedures.

Points that need attention when RAP is used:

• The mix design needs adjustment for the amount of RAP, largely because of the effect of the aged RAP binder on the binder grade.
• RAP is processed and stored much like virgin aggregate, with attention to surface crusting, moisture content, and separate stockpiles for each source.
• Because the RAP aggregate and binder have been aged by previous service, the skid resistance of surface mixes containing RAP deserves attention.

Cold Recycling

Cold recycling is so named because RAP is used as an aggregate in cold mix asphalt. In cold recycling, old HMA pavement is removed, broken down into aggregate-sized chunks and then combined with an emulsified or foamed asphalt. This mix is then typically used as a stabilized base course for reconstructed pavements. There are two basic cold recycling methods:

• Cold Plant Mix Recycling
• Cold In-Place Recycling (CIR)

CIR can also be extended to the full pavement as Full Depth Reclamation (FDR):

• Full-Depth Reclamation (FDR)

In-Place Recycling

In-place recycling reuses the existing pavement on the project site instead of removing it and hauling it to a plant as RAP. Each method adds specialized recycling equipment to an otherwise conventional paving train. The three main methods differ chiefly in how deep they work, and so in which distresses they can correct:

Hot In-Place Recycling

HIR uses a mobile heating and mixing plant working in a continuous train with a paver and rollers. It is done in three ways:

• Surface recycling. The surface is heated with propane or diesel heaters and scarified with a bank of spring-loaded tines. A rejuvenating agent is added to the softened material, which is then blended, shaped and recompacted.
• Remixing. The heated surface layer is removed by milling or grinding and mixed with a rejuvenating agent and with virgin aggregate or new plant mix (up to about 30 percent), so that the result is partly recycled and partly new material. A separate wearing course is often not needed.
• Repaving. The heated, scarified and rejuvenated material is placed as a leveling course by a primary screed while a second screed places a thin overlay of new mix on top. The two are compacted together with conventional rollers.

Remixing and repaving give better grade control and better mixing of additives than surface recycling. HIR comes closest of the in-place methods to producing and placing conventional HMA, so it may not need an overlay, but because it works only near the surface it corrects only surface distress, and the properties of the existing mix can limit where it can be used.

Cold In-Place Recycling

In CIR the pavement is milled without heating. The millings may pass through screening and crushing units to reach the desired gradation, and virgin aggregate can be added to improve the mix; for example, angular aggregate can be added to improve rut resistance where the existing mix contains a large fraction of rounded gravel. The material is mixed in a pugmill with a binding agent, typically emulsified asphalt (often polymer modified) or foamed asphalt, and any stabilizing additives, then placed with a paver and compacted much like HMA. With an emulsion, compaction is usually delayed until the emulsion begins to break.

CIR produces a granular material that behaves like a high-quality gravel. On a low-volume road it may serve as the surface if a treatment such as a fog seal is applied; on higher-volume roads it is treated as a stabilized base under a thin asphalt overlay. Most applications need a wearing course of some type. Because the additives must cure, CIR may be limited to sunny, dry weather in some regions.

Full-Depth Reclamation

In FDR a road reclaimer pulverizes the full asphalt layer and the top of the underlying base. Stabilizing agents and other additives are mixed in during pulverization, along with water for moisture conditioning and compaction; they may be supplied by a separate truck or added by the reclaimer itself. The material is shaped with a motor grader, which can also correct the profile and cross slope, and compacted, commonly with a padfoot roller followed by conventional rollers that seat loose aggregate and leave a smooth surface.

FDR produces a thick stabilized base that needs a surface treatment or overlay. Where deficiencies extend into the base or subgrade below the reclaimed depth, additional work is needed; the same equipment can also be used for soil stabilization.

Benefits

In-place recycling reduces the need for virgin aggregate and asphalt binder and the hauling of material to and from the site, which can lower construction costs. Used appropriately, it may also reduce construction delays to road users.

Recycled Asphalt Shingles

Asphalt roofing shingles can also be recycled into HMA as recycled asphalt shingles (RAS). Shingles contain about 20 to 36 percent asphalt, which binds fine aggregate, granular aggregate, and a fiberglass or organic felt mat, much as asphalt binds aggregate in a pavement.

Sources

There are two shingle waste streams:

• Manufacturer waste. Scrap from shingle manufacturing. Its composition is well known, which makes it the preferred source, but it is available only near shingle plants (in 2011, about 30 states had them).
• Tear-off shingles. Shingles removed during re-roofing or roof removal. This is the much larger and more evenly distributed source: in 2011, about 7 to 10 million tons a year in the U.S., against roughly a tenth of that in manufacturer waste. Tear-offs can contain deleterious materials such as wood and nails and, in older shingles, asbestos, so they must be sorted from other construction waste and regularly tested for asbestos, and their use is more tightly regulated. Asbestos was dropped from shingle production in the late 1970s and early 1980s, and with a shingle life of about 12 to 25 years, asbestos-containing shingles are becoming rare in the waste stream.

Processing and Use

Shingles are screened to remove deleterious materials and then ground, which makes the asphalt in them more available to the rest of the mix. The specified maximum particle size is usually about 6 to 13 mm (0.25 to 0.5 inch), and finer grinding generally performs better. In 2011, state DOTs that allowed RAS generally limited it to about 5 percent of the mix, and many allowed only manufacturer waste. RAS replaces part of the virgin asphalt binder and some of the virgin fine aggregate. Experimental studies have reported replacing up to 50 percent of the binder, but 20 to 30 percent is more realistic in most cases.

Benefits and Concerns

Using RAS diverts shingles from landfills, reduces the need for virgin binder and aggregate, and avoids landfill tipping fees. Mixes with RAS are stiffer and have been reported to resist rutting better and reach higher density with less compactive effort.

The added stiffness can raise the risk of thermal cracking in cold climates. Mix working time may be reduced and moisture retention in the pavement may increase. Processing requires facilities for screening, grinding and storage, a capital investment that depends on how much RAS will be used over the long term, and shingles wear both processing and paving equipment.

@misc{pavint-hma-recycling,
  author = {{Pavement Interactive}},
  title = {HMA Recycling},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/pavement-types-and-history/pavement-types/hma-recycling/}},
  note = {Accessed [date]}
}