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[1]). It involves the same process of cold plant mix recycling except that it is done in-place by a train of equipment.
Procedure
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.
- 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.
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 the Asphalt Institute recommends and most agencies use a variation of the Marshall mix design method (FHWA, 2001b[2]).
Other Information
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
- Stripping
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[3]).
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[1]).
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.↵