Recycled HMA

HMA is one of the most recycled products in the U.S.  As much as 100 million tons of HMA are milled off roads during resurfacing and widening projects each year.  Of this amount, 80 million tons (80 percent) are recycled as “reclaimed asphalt pavement” (RAP) (Bloomquist et al. 1993).  Additionally, in-place recycling techniques can also be used to resurface an existing pavement or pulverize an existing pavement for use as base material.

Reclaimed Asphalt Pavement (RAP)

RAP is essentially old pavement that is reclaimed for use.  In its most common form, it is collected in loose granular form as a byproduct of pavement rehabilitation or reconstruction (Figures 1 and 2).  RAP can be used in a variety of ways such as:

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

Figure 1: RAP Pile in Eastern Washington

Figure 2: RAP Up Close

RAP as a Constituent in HMA

The most common RAP use is as a constituent in HMA.  Basically, new HMA is produced at a batch or drum plant to which a predetermined percentage of RAP is added.  There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP.  The benefits of RAP use are two-fold:

  1. The RAP aggregate can be used in place of a portion of the virgin aggregate, which lowers cost, reduces waste and can lower the energy and greenhouse gas (GHG) footprint of the material production process.
  2. The RAP asphalt binder is reheated and used in place of a portion of the virgin asphalt binder, which lowers cost, reduces waste and can lower the energy and greenhouse gas (GHG) footprint of the material production process.

Pavement Note on RAP

In Washington State maximum limits for RAP addition are agency specific.  For instance, The WSDOT 2010 Standard Specifications for Road, Bridge and Municipal Construction (M 41-10) allows contractors the option of using RAP in the amount up to 20 percent of total HMA weight. For WSDOT, the mix design is determined using virgin material only even though RAP will be included in the HMA production.  Other agencies will allow different limits depending upon their experience and desires.

If used, RAP is most commonly added at 10 to 30 percent by weight although additions as high as 80 percent by weight have been done and additions as high as 90 to 100 percent by weight are feasible (FHWA, 2001).

Other general considerations when using RAP are:

  • When heated, RAP may give off gaseous hydrocarbons.  To minimize these emissions, HMA plants generally heat RAP indirectly (usually it is added after the aggregate is heated and thus heats up through contact with the already-hot aggregate).
  • RAP is typically added cold and thus may require longer HMA plant heating times.  This can sometimes reduce plant output by as much as half.  This can be overcome by preheating RAP, but the added energy, equipment and emissions concerns often make preheating undesirable.
  • RAP usually contains between 3 and 7 percent asphalt by weight or about 10 to 20 percent asphalt by volume (FHWA 2001).  In general, the asphalt binder in RAP will be more viscous than virgin asphalt binder due to aging effects.  Therefore, if enough RAP is added, a softer virgin asphalt binder can be used (but is not required) to counteract the more viscous RAP asphalt binder.
  • After milling or crushing, RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing.

RAP in Cold Plant Mix Recycling

Cold plant mix recycling involves mixing RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility.  A rejuvenating agent can be added to improve the recycled asphalt binder viscosity and new aggregate can also be added to improve overall performance.  The resulting cold mix is typically used as a stabilized base course.   Since cold in-place recycling has become more commonplace, cold plant mixing has become less popular.

Other Recycling Options

HMA can also be recycled in-place via hot or cold methods.

  • Hot in-place recycling (HIR).  Usually, HIR can only correct shallow surface distress problems (less than 2 inches).  HIR can be done by heater scarification, repaving or remixing.
    • Heater scarificationThe pavement surface is heated with radiant heaters, scarified using a bank of nonrotating teeth, rejuvenated using an additive to improve the recycled asphalt binder viscosity, mixed and leveled, then compacted using conventional compaction equipment.  Heater scarification is limited in its ability to repair severely rutted pavements, which are more easily rehabilitated with a conventional HMA overlay.
    • Repaving.  This method is similar to heater scarification, only the top layer is completely removed (rather than left in place) and then placed in either one or two lifts.
    • Remixing.  This method is used when additional aggregate is required to improve the strength or stability. Remixing is similar to repaving but adds new virgin aggregate or new HMA to the recycled material before it is leveled.
  • Cold in-place recycling (CIR).  CIR essentially pulverizes the existing pavement structure  to a predetermined depth (by definition, the puverization does not disturb any of the underlying unbound layers), adds a binding agent (such as an emulsion or foamed asphalt), then lays and compacts the resulting product for use as a stabilized base course.  This base course is then paved over with an HMA surface course or BST.

Full depth reclamation (FDR) involves pulverizing the existing pavement structure to include unbound aggregate base course and any other underlying layers if needed. This pulverized material is usually stabilized using a binding agent and then graded and compacted for use as a stabilized base course.