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):
- Milling. A milling machine pulverizes a thin surface layer of pavement, usually from 50 to 100 mm (2 to 4 inches) deep.
- 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.
- 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.
- 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).
- 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.
- 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.
- 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.
Stabilized base course or a low volume road granular surface course.
Recycled material and a binding additive (usually asphalt emulsion, lime, portland cement or fly ash).
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:
- Rutting from excessive asphalt content or mix instability
- Wet, unstable base, subbase or subgrade materials
- Frost action
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.
- American Association of State Highway and Transportation Officials (AASHTO). (1998). Report on Cold Recycling of Asphalt Pavements. AASHTO-AGC-ARTBA Joint Committee Task Force 38 Report. American Association of State Highway and Transportation Officials. Washington, D.C.↵
- Federal Highway Administration. (2001b). Reclaimed Asphalt Pavement User Guideline: Asphalt Concrete (Cold Recycling). Web page on the Turner-Fairbanks Highway Research Center web site. http://www.tfhrc.gov/hnr20/recycle/waste/rap133.htm. Accessed 16 October 2001.↵
- Better Roads. (July 2001). Full-Depth Reclamation. Better Roads. Special insert section on full-depth reclamation in cooperation with the Asphalt Recycling and Reclaiming Association.↵