Cold-in-place recycling, according to FP2, is an onsite recycling process in which an asphalt pavement with structural failures is reconstructed using 100 percent reclaimed asphalt pavement (RAP) to a typical treatment depth of 2 to 6 inches, using a mobile train of equipment that may include tanker trucks, a milling machine, crushing and screening equipment, a mixer, binder rejuvenating additives and a paver.

With CIR recycling, existing pavement materials are removed, crushed, sized (if needed) and rejuvenated with a recycling agent. Its benefits include major pavement structural improvement via overhaul of the existing pavement materials, and CIR can cure most types of pavement distress. As with HIR, material hauling costs are minimized, while the blue smoke associated with HIR is eliminated.

Laydown may be by asphalt paver, or in some instances the material can be placed with a motor grader. Compaction to obtain the required density can be achieved by either a heavy pneumatic, vibratory or static steel roller, or any combination of rollers. Typically, the pavement can be opened to traffic at the end of the work day.

AASHTO’s Center for Environmental Excellence defines CIR recycling as a process involving specialized plants or processing trains, whereby the existing pavement surface is milled to a depth of up to 6 inches, processed, mixed with asphalt emulsion (or foamed asphalt), and placed and compacted in a single pass.

CIR is suitable for roadways with moderate to severe distresses where reflection cracking is a concern, AASHTO says. CIR involves milling the existing pavement, screening for oversize, addition of asphalt emulsion, and mixing; then, this cold renewed material is spread, reprofiled and compacted on the roadway in one continuous operation. There is no processing required prior to the actual recycling operation.

An alternate to cold-in-place recycling, in-plant cold mix recycling, mixes RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility. A rejuvenating agent can be added to improve the recycled asphalt binder viscosity, and new aggregate can also be added to improve overall performance. The resulting cold mix is typically used as a stabilized base course. RAP is trucked to the plant and recycled mix is returned by truck.

Hot-in-place recycling heats a pavement and removes it to a certain depth, or surface-scarifies it, prior to application of a new or recycled surface, FP2 says. It may be performed either as a single-pass, one-phase operation that monolithically combines the recycled pavement with virgin material, or as a two-pass procedure, in which the restored, recycled material is recompacted prior to placement of a new wearing surface.

The AASHTO Center for Environmental Excellence defines HIR as a process of repaving that is performed as either a single- or multiple-pass operation using specialized heating, scarifying, rejuvenating, laydown and compaction equipment.

HIR recycling can treat surface defects like corrugation, rutting and cracks, but like the CIR method, needs room for the equipment train. Heating the asphalt surface may produce blue smoke. The process can result in a pavement being opened to traffic just minutes after passage of a recycling train.

With heater scarification, the pavement’s surface only is heated with radiant heaters, then scarified or disturbed using a bank of nonrotating teeth, rejuvenated using an additive to improve the recycled asphalt binder viscosity, mixed and leveled, then compacted using conventional compaction equipment.

Repaving is a process that heats, scarifies and applies a recycling agent to an existing, aged driving course, and then replaces that material as a new leveling course. A screed on the same machine immediately tops that recycled leveling course with a virgin hot-mix asphalt overlay fed into the repaver from the front, which bonds thermally with the recycled leveling course to form a durable, monolithic pavement.

The repaving process also eliminates the longitudinal joint between two lanes of pavement, because repaving also reheats the edge of adjacent paved lanes, resulting in a more durable, higher-density seam between the driving lanes. In essence, the longitudinal joint is eliminated.