Hot In-Place Recycling

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HIR consists of heating and softening the existing asphalt pavement with one or more preheater units, followed by one or more heater scarification units which scarify the softened pavement with spring-loaded tines, carbide teeth, augers or hot rotary mills to the specified depths. Typical treatment depths range from 3/4 to 2 inches (19 to 50 mm); although some equipment can treat up to 3 inches (75 mm) by utilization of sequential heating, milling and windrowing the heated material.  The scarified, augered or milled pavement is then thoroughly mixed with a rejuvenating agent and the recycled mix is placed and compacted with conventional asphalt paving equipment. With HIR, 100 percent recycling of the existing asphalt pavement is completed on site. As the HIR and related equipment may be spread out over several hundred feet, the combined equipment is often referred to as an HIR “train.” 


Equipment Variations

There are subtle variations in HIR equipment trains and methodologies but they can all be categorized into one of three sub-disciplines: Surface Recycling, Remixing or Repaving.  

All HIR processes use similar equipment that may include some or all of the following equipment; preheater units, heater scarification/augering/milling units, mixing units, windrowing equipment, spreaders, paving and compaction equipment.


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Surface Recycling HIR Train


Surface Recycling is an HIR process in which softening of the asphalt pavement surface is achieved with heat from a series of preheating units. The heated and softened surface layer is then further heated and scarified to the desired treatment depth with either a series of spring activated teeth or “tines,” a small diameter rotary milling head or an auger and moldboard. As the surface is scarified, a rejuvenating agent is added and mixed, if required, and the loose recycled mixture then spreads with a paving screed. Specified treatment depths generally range from 3/4 to 2 inches (19 to 50 mm). No additives (HMA or new aggregate) are added during the Surface Recycling process so the overall pavement thickness remains essentially the same. 

Compaction of an HIR Surface Recycled mixture is performed with any combination of pneumatic-tired rollers and static and/or vibratory steel drum rollers. A surface course consisting of an asphalt overlay or surface treatment is placed in a subsequent operation for most functional classes, although the HIR Surface Recycled mixture has been left as the surface course on some low volume roads.  


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HIR Remixing Train


Remixing is the HIR sub-discipline where the existing asphalt pavement is heated, softened, scarified, augered or milled and remixed in a mixing drum or pugmill, typically with a rejuvenating agent, in a train of equipment as shown. HMA or new aggregate may be added as required for recycled mixture needs and/or grade control. The result is a thoroughly mixed, homogeneous layer. Compaction of an HIR Remixing mixture is performed with any combination of pneumatic-tired rollers, static and/or vibratory steel drum rollers. The recycled mixture has been left as the surface course but it is typically overlaid with an asphalt mix or a surface treatment such as a chip seal, slurry seal or micro surfacing depending on pavement needs.  


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The repaving method recycles and lays new hot mix in a single pass.


Repaving combines Surface Recycling with the placement of an integral asphalt overlay in a single pass. With HIR Repaving, a single heater paver unit equipped with two screeds is used. The first screed places the recycled HIR mixture while the second screed places the new asphalt overlay on top of the recycled mixture. The two layers are then compacted simultaneously with conventional compaction equipment, creating a thermal bond between the two mixtures, eliminating the need for a tack coat. 

In the Repaving process, the recycled mixture functions as a base or leveling course while the new asphalt overlay is the surface course. The new asphalt surface course can be as thin as 1/2 inch (12.5 mm) or as thick as 2.25 inches (57 mm), with the combined thickness of the of integral overlay and underlying recycled mixture limited to 3 inches (75 mm). Hence, the overall pavement thickness can be increased a significant amount in the HIR Repaving process.  


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Multi-stage HIR Train


HIR can be performed using either single stage or multi-stage heater scarification/augering/milling. In the single stage method, the existing asphalt pavement is sequentially heated and softened and scarified/augered/milled to the required depth in a single pass. Treatment depths for the single stage method are generally 1 to 2 inches (25 to 50 mm).

In the multi-stage method the existing asphalt pavement is sequentially heated, softened and scarified/augered/milled in a number of layers, usually between two to four, allowing deeper recycling depths without damaging the existing asphalt binder. Multi-stage recycling typically removes 0.5 to 0.75 inches (12.5 to 19 mm) of material with each heating scarifying/augering/milling unit. The processed material from the first layer is either picked up and carried over the heating bed of the next unit or the processed windrow is heated by tunnel heaters. The sequence is continued until the required HIR treatment depth is achieved. Treatment depths for the multi-stage method are typically 1.5 to 3 inches (38 to 75 mm). 


Material Variations

  • Rejuvenating agents typically consist of rejuvenating oil, rejuvenating emulsion or a soft binder.
  • The gradation of the existing asphalt mix can be improved with the addition of HMA or new aggregates. The Remixing HIR process is designed to allow addition of HMA or new aggregates in the mixing chamber or drum. With Surface Recycling, new aggregates have been placed in front of the HIR train. 
  • HMA and new aggregate can be used to provide additional material to improve final mixture smoothness or cross-slope correction.
  • The Repave process improves smoothness and cross-slope correction with the placement of the integral asphalt overlay as well as providing a thermally bonded longitudinal joint. 

Common Combinations

HIR + Single or Double Chip Seal: A surface course is typically placed over Surface Recycle and Remixing mixes. Where the existing pavement structure is sufficient to carry the anticipated loads chip seals are very economical.


HIR + Micro or Slurry Surfacing: Where the existing pavement structure is sufficient to carry the anticipated loads micro or slurry surfacing are additional economical surfacing options for Surface Recycle and Remxing mixtures.


HIR + HMA Overlay: To increase the structural capacity of the road, a hot mix overlay can be placed over an HIR mix. HIR Repave places the HMA overlay in a single pass during the recycling operation.


HIR + Ultra-thin Bonded Wearing Course


HIR as Wearing Surface: When HMA or new aggregate is used with HIR Remixing, and if the existing structure can carry the anticipated loads, the recycled mixture has been used as the wearing surface. For low volume road applications, a Surface Recycle mix has been used on an experimental basis as the wearing surface.