University of California Pavement Research Center

The construction of three partial-depth recycling (PDR) pilot projects was monitored in late 2021. These studies focused on the benefits of adding supplemental aggregates to PDR materials, comparison of emulsified asphalt (EA) and foamed asphalt (FA) recycling agents in PDR applications, comparison of the gradations produced by single- and multi-unit recycling trains, and the effect of recycling train forward speed on gradation. Initial findings from the study can be summarized as follows:• Statistical analyses of quality control results on in-place recycling projects are challenging given the variability in materials and pavement structure along the length of the project. The problem is intensified on pilot projects with multiple experimental sections on which performance is being compared.• Supplemental aggregates can be used to reliably increase the density and strength of PDR layers without increasing the recycling agent or active filler contents and by not requiring pre-milling of the road to accommodate the materials without changing grade height.• There was no discernable difference in the density and strengths of PDR layers produced with the single- and multi-unit trains. The main benefit of the multi-unit train is better control of maximum aggregate size by the on-board screens and crushing unit. However, the crushing unit does not appear to change or improve the finer portion of the gradation (i.e., material passing the #4 [4.75 mm] sieve), which will have a larger influence on compaction density, air-void content, strength, and moisture resistance.• On coarse gradations, higher foamed asphalt contents were required to achieve the minimum indirect tensile strength requirement compared to emulsified asphalt. This is attributed in part to the coating action provided by emulsion treatments being more effective than the “spot welding” action provided by foam treatments on coarse, high air-void content gradations.• Marshall compaction overestimated the in-place density of PDR layers to a greater extent than gyratory compaction.• Rerolling can result in a small increase in density on PDR-EA layers. The timing of rerolling will influence the extent of this increase.• The densities recorded on specimens produced for strength and stability tests were not always consistent with the density results measured on the layer. This difference was attributed in part to inherent variability in the materials and pavement structure, sampling and handling procedures, and different specimen preparation procedures used by the contractors.• Relationships between gradations of field samples and field densities were inconsistent, which was also attributed to inherent variability in the materials that may not be captured in the small samples taken to represent a relatively large area of the layer.The pilot projects should be monitored to evaluate long-term performance. Monitoring should include annual visual surveys, annual or biannual falling weight deflectometer testing, and biannual coring and dynamic cone penetrometer testing. This study has highlighted a number of issues and suggested changes within the PDR mix design and quality control procedures followed in these projects (CT 315), which have been discussed with the method owner.