Hot In-Place Recycling: How HIR Heats, Remixes and Relays Old Asphalt

Hot in-place recycling (HIR) is a road rehabilitation method that heats the top inch or two of an existing asphalt pavement, loosens it with scarifiers or rotary mills, mixes in a rejuvenating agent (and sometimes new hot mix), then spreads and compacts the recycled material right where it came from, all in a slow-moving train of equipment. It reuses the old surface without hauling it away, fixes surface cracking, raveling and minor ruts, and can be completed in a single pass. It isn't a cure for structural failures, weak base or thin pavement, which need deeper methods like cold in-place recycling or full depth reclamation.

Full depth reclamation →

How the HIR Process Works

1. Preheating. One or more heater units with infrared or hot-air heaters warm and soften the existing surface.
2. Loosening. Scarifier teeth or heated milling heads break up the softened layer to a set depth, commonly up to about 2 inches.
3. Rejuvenating. A rejuvenating agent is added to restore some of the flexibility lost as the old binder aged.
4. Remixing. The loosened material is mixed, sometimes with new hot mix asphalt added for gradation or volume.
5. Placing and compacting. A screed spreads the recycled mix and rollers compact it.

The Three HIR Methods

Method	What happens	Typical use
Surface recycling (heater-scarification)	Heat, scarify, rejuvenate and relevel the surface	Minor cracking and raveling; often followed by an overlay or surface treatment
Repaving	Surface recycling plus a new thin overlay placed at the same time	When a fresh wearing surface is needed
Remixing	Recycled material blended with new mix to adjust gradation or binder	Surfaces with mix problems like rutting or poor gradation

Benefits

• Reuses the existing asphalt in place, reducing the amount of new material needed.
• Less trucking, since the old surface isn't hauled to a plant and new mix doesn't all come from a plant.
• Single-pass operation that can reopen lanes relatively quickly.
• Treats surface distress such as cracking, raveling, oxidation and minor rutting.
• Maintains road height, which helps with curbs, bridges and overhead clearance.

Limitations

• Surface only. HIR treats the top inch or two. Cracks and failures that come from the base or subgrade will return.
• Needs enough asphalt thickness to heat and process without disturbing the base.
• Not every pavement suits it. Surfaces with lots of patches of different materials, heavy crack-sealant buildup, or multiple chip seals and slurry layers can be difficult to heat and remix uniformly.
• Smoke and emissions from heating must be managed, and work near buildings and trees needs care.
• Specialized equipment and crews, so it's mostly used by agencies on roads and large lots.

HIR vs Other Recycling Methods

Method	Depth	Heat	Best for
Hot in-place recycling	About 1 to 2 inches	Yes	Surface distress
Cold in-place recycling	Several inches of asphalt	No	Deeper cracking in the asphalt layers
Full depth reclamation	Asphalt plus part of the base	No	Structural and base failures
Mill and overlay	Removed and replaced with new mix	New mix is hot	General resurfacing

See our guides to full depth reclamation and mill and overlay for the other methods.

HIR vs Infrared Patching

The same basic idea, heating old asphalt so it can be reworked, is used at small scale for patching. Infrared patch machines heat a single damaged area, a crew rakes in rejuvenator and a little new mix, and the patch is compacted seamlessly into the surrounding pavement. Our infrared asphalt repair guide covers that method, which is more common on driveways and parking lots.

Rejuvenators

As asphalt ages, the binder loses lighter components and becomes stiff and brittle. Rejuvenating agents are formulated to restore some of those components so the recycled mix is less prone to cracking. The type and rate are chosen by testing the existing pavement. Products sold for homeowners to spray on driveways are a different category and don't substitute for a road recycling process.

Checking Mix Temperature

Quality control on HIR focuses on the temperature of the heated material, the depth of treatment, the rejuvenator rate and final density. Crews use an infrared thermometer for spot checks, along with the equipment's built-in controls. Overheating can damage the binder and create smoke; underheating leaves material that won't loosen or compact properly.

Is HIR an Option for My Driveway or Lot?

Full HIR trains are sized for roads, so they're rarely used on residential driveways. Larger parking lots and private roads may be candidates if a contractor in the area offers it. For smaller areas, infrared patching, crack sealing, or a mill and overlay are the usual options. If you're comparing an overlay, estimate the new asphalt with the asphalt tons calculator so you can weigh quotes.

Safety Around HIR Operations

The heater units produce intense heat and some smoke. Stay well back from the equipment, never walk between machines, and wear a high-visibility vest in the work zone. Burners and blowers on the heater units are loud, so hearing protection is a good idea for anyone working near them for long. Residents along the route may notice an asphalt odor while the train passes; it usually fades once the section is compacted and cooled. Freshly placed recycled material is hot and soft, just like new hot mix.

Bottom Line on Hot In-Place Recycling

Hot in-place recycling heats and reworks the top inch or two of an aging asphalt road, rejuvenates it and relays it in one pass, saving material and trucking. It's effective for surface cracking, raveling and minor rutting on pavements with a sound structure underneath. For deep or structural problems, cold in-place recycling, full depth reclamation or reconstruction is the better fit.