4  Flexible Pavement Recycling

  Major Topics on this Page
  4.1 Hot Recycling
  4.2 Cold Recycling

HMA is one of the most recycled products in the U.S.  It is estimated that as much as 91 million tonnes (100 million tons) of HMA are milled off roads during resurfacing and widening projects each year (APA, 2001a).  Of this amount, 73 million tonnes (80 million tons) are recycled as "reclaimed asphalt pavement" (RAP - see Figure 2.15) (APA, 2001a).  RAP is typically generated by rehabilitation or reconstruction projects and can be used in a variety of ways such as:

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. 

 

4.1  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 (see Figure 2.16) and then incorporated into new HMA as an aggregate.  There are two basic methods for accomplishing this: conventional recycled hot mix (RHM) and hot in-place recycling. 

 

4.1.1  Recycled Hot Mix (RHM)

Recycled hot mix (RHM) is the most common way of using RAP.  Basically, new HMA is produced at a batch or drum plant to which a predetermined percentage of RAP is added.  RAP addition is typically 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, 2001c).  There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP.  Figure 2.17 shows two dense-graded HMA cores, one with RAP and one without.

Purpose: Anything for which a typical dense-graded HMA may be used
Materials: HMA and RAP
       
  Mix Design:   Superpave, Marshall or Hveem procedures.  Blending charts are typically needed when using high percentages of RAP.
       
  Other Info:   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 addition may require longer HMA plant heating times.  This can sometimes reduce plant output by as much as half.  

RAP generally contains between 3 and 7 percent asphalt by weight or about 10 to 20 percent asphalt by volume (FHWA, 2001c).  In general, RAP will be more viscous than new HMA because of asphalt binder aging.  Therefore, if enough RAP is added, a softer asphalt binder should be used.  Table 2.3 shows the AASHTO MP 2 Superpave asphalt binder selection guidelines for RAP mixtures.

In general, state DOTs allow more RAP in base and binder HMA courses than they do in surface courses.

 

After milling or crushing, RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing. 

Table 2.3: Superpave Asphalt Binder Selection Guidelines for RAP Mixtures (from AASHTO, 2001)

RAP Percentage

Recommended Virgin Asphalt Binder Grade
< 15 No change from basic Superpave PG binder requirements.
15 - 25 Select virgin binder one grade softer than normal
(e.g., select at PG 58-22 if a PG 64-22 would normally be used).
> 25 Follow recommendations from blending charts.
 

WSDOT RAP Requirements and Use
The WSDOT 2002 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 aggregate weight without specifically accounting for it in mix design (e.g., the mix design is determined using virgin aggregate only even though RAP will be included in the HMA production).  It also states that "Recycled materials shall not be used in asphalt concrete Class D."  There are no relaxations in gradation acceptance requirements for HMA with RAP.

If contractors desire to add more than 20 percent RAP, WSDOT requires a separate mix design that specifically accounts for the percentage they want to add.  This type of mix design often involves binder extraction from the RAP so that it can be graded and the virgin binder grade can be appropriately adjusted.

Figure 2.17: HMA Cores from a RAP Mix and a non-RAP Mix

 

4.1.2  Hot In-Place Recycling (HIPR)

Hot in-place recycling (HIPR) is a less common form of hot asphalt recycling.  There are three basic HIPR construction processes in use, all of which involve a specialized plant in a continuous train operation (FHWA, 2001c):

Figure 2.18: Heater Scarification Train Showing 2 Preheaters, the Heater/Scarifier, the Paver and Rollers.

Purpose: Correct shallow-depth HMA surface distress
Materials: Asphalt binder rejuvenating agent and possibly new aggregate and HMA.
       
  Mix Design:   Not well-defined, but as a minimum cores are usually taken from the existing pavement to determine the proper amount of rejuvenating agent to add.
       
  Other Info:   HIPR is only applicable to specific situations.  First, air void content of the existing asphalt binder must be high enough to accept the necessary amount of asphalt binder rejuvenator.  Second, HIPR can only adequately address shallow surface distress problems (less than 50 mm (2 inches)).  Third, pavements with delaminations (subsequent layers not binding together) in the top 50 mm (2 inches) should not be considered for HIPR projects.  Finally, pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for HIPR projects (FHWA, 2001c).

 

4.2  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 and cold in-place recycling (CIR).  

 

4.2.1  Cold Plant Mix Recycling

Cold plant mix recycling, the less common of the two cold recycling methods, 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 then typically used as a stabilized base course.  

Purpose: Stabilized base course.
Materials: RAP, asphalt emulsion or foamed asphalt, asphalt rejuvenating agent and possibly virgin aggregate.
       
  Mix Design:   No generally accepted mix design method, but the Asphalt Institute recommends and most agencies use a variation of the Marshall mix design method (FHWA, 2001b).
       
  Other Info:   Since cold in-place recycling has become more commonplace, cold plant mixing has become less popular.

 

4.2.2  Cold In-Place Recycling (CIR)

Cold in-place recycling (CIR) is the processing and treatment with bituminous and/or chemical additives of existing HMA pavements without heating to produce a restored pavement layer (AASHTO, 1998).  It involves the same process of cold plant mix recycling except that it is done in-place by a train of equipment.  The typical CIR process involves seven basic steps (AASHTO, 1998):

  1. Milling.  A milling machine pulverizes a thin surface layer of pavement, usually from 50 to 100 mm (2 to 4 inches) deep. 
  2. Gradation control.  The pulverized material is further crushed and graded to produce the desired gradation and maximum particle size.  On some jobs this step is omitted, however on others a trailer mounted screening and crushing plant is used to further crush and grade the pulverized pavement.  If needed, virgin aggregate can be added to the recycled material.
  3. Additive incorporation.  The graded pulverized material is mixed with a binding additive (usually emulsified asphalt, lime, portland cement or fly ash).  On some jobs, this is done by the milling machine, however on others a trailer mounted pugmill mixer is used.
  4. Mixture placement.  The pulverized, graded pavement and additive combination is placed back over the previously milled pavement and graded to the final elevation.  Mixture placement is most often done with a traditional asphalt paver (either through windrow pickup or by depositing the mixture directly into the paver hopper), however on some very low traffic applications the mixture can be placed by a motor grader.  Because of the larger maximum aggregate sizes of the graded mixture, the minimum lift thickness for placement is usually around 50 mm (2 inches).
  5. Compaction.  The placed mixture is compacted to the desired density.  Typical compaction efforts involve a large pneumatic tire roller and a large vibratory steel wheel roller.  If an emulsion additive is used rolling is typically delayed until the emulsion begins to break.  If a portland cement or fly ash additive is used, rolling should begin immediately after placement.
  6. Fog seal.  If the newly placed material is to operate as a high quality gravel road then a fog seal is usually applied over the top to delay surface raveling of the cold recycled mix.  A fog seal is necessary over CIR using a portland cement or fly ash additive not only to delay surface raveling but also to provide a curing membrane for the additive to properly set.
  7. Surface course construction.  On higher volume roads, the cold recycled mix is overlaid with either a BST or a thin HMA overlay.  In either case, a tack coat should be used to provide a good bond between the cold recycled mix and the surface course.

Purpose: Stabilized base course or a low volume road granular surface course.
Materials: Recycled material and a binding additive (usually asphalt emulsion, lime, portland cement or fly ash).
       
  Mix Design:   No generally accepted mix design method, but most methods are based on the Marshall or Hveem methods and equipment (AASHTO, 1996).
       
  Other Info:   CIR is best suited for cracked pavements with structurally sound, well drained bases and subgrades.  CIR is generally not appropriate for repairing pavement failures caused by:

CIR is generally suitable for lower volume roads that may only require a simple surface treatment over the resulting stabilized base course, or at most a thin HMA wearing course (Better Roads, 2001).

For projects using an asphalt emulsion additive, typical specified minimum atmospheric temperatures range from 10 to 16°C (50 to 60°F).  For projects using portland cement or fly ash as the additive, the minimum required temperature is 4°C (39°F) with no freezing temperatures expected in the next 24 hours (AASHTO, 1998).

CIR requires sunny, dry conditions in order for the additive to properly set. 

If an asphalt emulsion additive is used, it is usually added at a rate of between 0.5 to 2 percent by weight of RAP.

 

WSDOT Cold In-Place Recycling (CIR) Experience
WSDOT has been using CIR since 1982 and has place about 167 lane miles on 14 different projects.  For those projects that have been subsequently rehabilitated it appears service life was in the range of 10 years.  Some specific WSDOT guidance on CIR in addition to that listed above is:
  • Asphalt overlays place on top of CIR are typically on the order of 45 to 75 mm (0.15 to 0.25 ft.) thick and have performed reasonably well.
  • CIR relies heavily on contractor experience.
  • The necessary warm, dry climate for CIR can be found in Eastern Washington (east of the Cascade mountains) but generally not in Western Washington (west of the Cascade mountains).
  • CIR can drastically reduce aggregate or mixture haul costs because the recycled material never leaves the project site.  Therefore CIR is a more competitive rehabilitation option where limited supplies of close-by virgin aggregate would force a contractor to otherwise haul aggregate in from far away.
  • CIR has not been used in urban environments, areas of inconsistent pavement width and depth or areas of multiple pavement types within the same pavement structure.
  • Production rates can vary but are typically between 2,000 and 5,000 tons/day,
  • When used for suitable projects (e.g., no close-in supply of aggregate, low volume road, warm and dry climate, etc.) CIR costs are comparable to traditional overlay costs.

 

4.2.3  Full-Depth Reclamation (FDR)

Although referred to as "full-depth reclamation", this process is just an extension of the basic CIR principles to the entire HMA pavement depth plus a predetermined depth of the base material.  FDR can be used to depths of 300 mm (12 inches) or more but the most typical applications involve depths of between 150 and 225 mm (6 and 9 inches) (Better Roads, 2001).  The FDR process usually consists of eight steps (Better Roads, 2001):

  1. Pulverization.  A road reclaimer pulverizes existing pavement to a predetermined depth.  Road reclaimers are usually equipped to add materials such as stabilizing agents to the newly pulverized RAP.
  2. Moisture conditioning.  The road reclaimer or a separate truck adds water to the newly pulverized RAP to assist in achieving required density.
  3. Breakdown roller.  A sheepsfoot or pneumatic tire roller is typically used to compact the recently pulverized RAP to a consistent density.
  4. Shaping.  A grader is typically used to make grade and cross-slope adjustments. 
  5. Intermediate roller.  A pneumatic tire roller or a steel wheel vibratory roller is used to knead and seat any loose aggregates left from the shaping process.
  6. Finish roller.  A 12 to 14-ton static steel wheel roller is used to seat any remaining loose aggregates and create a smooth surface.
  7. Sealant.  A fog seal is typically applied to protect the finished reclaimed layer.  After the fog seal sets the reclaimed layer can generally withstand interim traffic loading.  Therefore, at this point the road is often opened to traffic until the contractor is ready to apply the surface treatment or HMA surface course.
  8. Surface treatment or surface course.  Finally, a more durable surface treatment or surface course is applied over the new stabilized base course.

Purpose: Stabilized base course.
Materials: Recycled material, asphalt emulsion or foamed asphalt, asphalt rejuvenating agent and possibly virgin aggregate.
       
  Mix Design:   No generally accepted mix design method, but the Asphalt Institute recommends and most agencies use a variation of the Marshall mix design method (FHWA, 2001b).
       
  Other Info:   FDR is generally suitable for lower volume roads that may only require a simple surface treatment over the resulting stabilized base course, or at most a thin HMA wearing course.  However, FDR has been used on major highways including interstates (Better Roads, 2001).