The Specific Pavement Studies Experiment 5 (SPS-5) in the Long-Term Pavement Performance program was designed to study the effects of overlay rehabilitation type on typical distress measures. The rehabilitation treatments compared overlay thickness, overlay type, and surface preparation before rehabilitation. The thicknesses used were 50- and 125-mm overlays. The overlay types were virgin asphalt mix and recycled asphalt that contained approximately 30% reclaimed asphalt pavement (RAP). Surface preparation consisted of either milling or not milling the existing pavement before rehabilitation. Eighteen states and provinces in North America built SPS-5 projects between 1989 and 1998. Seven distress parameters from these test pavements were analyzed, including international roughness index (IRI), rutting, fatigue cracking, longitudinal cracking, transverse cracking, block cracking, and raveling. Analyses were conducted to determine which factors affected overlay performance as measured with the above parameters. Further statistical testing compared the performance of the virgin mix sections directly with equivalent sections that contained 30% RAP. Overlays with mixes that contained 30% RAP were found to perform as well as overlays with virgin mixes in terms of IRI, rutting, block cracking, and raveling. Thicker overlays improved pavement performance, except for rutting. Milling before rehabilitation decreased IRI, fatigue cracking, and transverse cracking but increased rutting.

Several methods are available for determining the bulk specific gravity (Gsb) of reclaimed asphalt pavement (RAP). However, a clear recommendation has not been had for determining this property. In this study, asphalt mixes with known aggregate properties were produced and aged in the laboratory to simulate RAP. The aggregates were recovered, and the aggregate properties, including Gsb, were reassessed. The aged mixtures were also tested to determine maximum theoretical specific gravity (Gmm), from which estimated Gsb values could be calculated. The Gsb values from both the extraction and Gmm methods were compared with the known or true Gsb values for these aggregates. The effects of the RAP Gsb errors on voids in mineral aggregate values from the various methods were also evaluated. On the basis of the results of this study, it is recommended that the Gmm method be used to determine the RAP Gsb when a regional absorption value is known.