Hot In-Place Recycling

Hot in-place recycling (HIR) is a less common form of hot asphalt recycling. There are three basic HIR construction processes in use, all of which involve a specialized plant in a continuous train operation (FHWA, 2001c[1]):

  • Heater scarification (Figure 1). This method uses a plant that heats the pavement surface (typically using propane radiant heaters), scarifies the pavement surface using a bank of nonrotating teeth, adds a rejuvenating agent to improve the recycled asphalt binder viscosity, then mixes and levels the recycled mix using a standard auger system. The recycled asphalt pavement is 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.

Figure 1. Heater Scarification Train Showing 2 Preheaters, the Heater/Scarifier, the Paver and Rollers.

  • Repaving. This method removes (by heating and scarification and/or grinding) the top 25 to 50 mm (1 to 2 inches) of the existing HMA pavement, adds a rejuvenating agent to improve the recycled asphalt binder viscosity, places the recycled material as a leveling course using a primary screed, and simultaneously places a thin (usually less than 25 mm (1 inch)) HMA overlay. Conventional equipment and procedures are used immediately behind the train to compact both layers of material (Rathburn, 1990[2] as cited in FHWA, 2001c[1]).
  • 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.


Correct shallow-depth HMA surface distress


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

HIR 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, HIR 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 HIR projects. Finally, pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for HIR projects (FHWA, 2001c[1]).

Footnotes    (↵ returns to text)
  1. Federal Highway Administration.  (2001c).  Reclaimed Asphalt Pavement User Guideline: Asphalt Concrete (Hot Recycling).  Web page on the Turner-Fairbanks Highway Research Center web site.  Accessed 16 October 2001.
  2. Rathburn, J.R.  (March 1990).  One Step Repaving Speeds County Work.  Roads and Bridges.