Suggested Citation:
Louw, Stephanus, David Jones, Joseph Hammack, Heather Tom (2025)

Cold Central Plant Recycling Study: Preliminary Determination of Maximum Time That Processed Material Can Be Stockpiled and Compacted

. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-23-91

Abstract:

The objective of this preliminary study was to investigate the effect of delayed compaction and/or stockpiling of cold central plant recycled materials with foamed asphalt recycling agents and cement active fillers. The results are also considered applicable for emulsified asphalt recycling agents and in-place cold recycling. Given that hydrated lime is currently not permitted as an active filler for cold recycled materials in Caltrans specifications, no tests were conducted with lime, and no recommendations are made regarding stockpile time if lime is considered in the future. A separate study will need to be completed for lime if justified.

The following conclusions were drawn from the test results:

• Delayed compaction or stockpiling results in reduced material workability characterized by lower densities, strengths, and stiffnesses—which in turn will result in shorter pavement life. 

• Delayed compaction or stockpiling can further increase variability in terms of density, strength, and stiffness of cold recycled materials, given that the material could be at various levels of curing/cementation throughout the stockpile window. 

• The densities obtained using Marshall compaction do not represent the densities achieved in the field. This can result inoverestimation of the strength for quality control.

The following recommendations are made based on the results of this preliminary study:

• The current specification requirement that layers of cold recycled material be placed and compacted within two hours of mixingtime (i.e., from when mixing moisture is added to the cement) should be maintained. If delayed compaction or material stockpiling is permitted for any reason, quality control testing should be performed on specimens compacted to the breakover density achieved at the sampling location in the area of compaction delay.

• On cold recycling projects where the processed material is discharged from the lane-width track-driven recycler in a windrow, consideration should be given to limiting the distance between the recycler and paver to ensure that the two-hour limit for completion of compaction is met.

• On cold recycling projects where wheel-driven recyclers (i.e., 8 ft. [2.4 m] recycle width) are used, consideration should be given to requiring that two recyclers be used in tandem, each with a primary roller, to eliminate the need for reversing the recycling train back to the start of a section to do the second pass, along with the associated delays in starting blading to final level and final compaction.

• The importance of meeting the two-hour compaction completion specification requirement should be highlighted in all just-in-time training sessions for all cold recycling projects.

These recommendations may change if lime is permitted as an active filler in the future, in which case additional testing will be required to determine whether the two-hour limit can be extended.