Validation of Performance-Related Specification of High-RAP-Content Surface Mixtures Placed on a High-Volume Road

To enhance sustainability and environmental consciousness in road construction, US agencies are increasingly integrating higher reclaimed asphalt pavement (RAP) contents into the surface layers of asphalt pavements. Traditional volumetric design methods like Superpave fail to account for the impacts of elevated RAP contents on mixture performance. Thus, the implementation of performance-related specifications that evaluate mixtures both volumetrically and functionally is crucial. The Massachusetts Department of Transportation (MassDOT) currently allows up to 15% RAP in its top surface asphalt layers. In 2021, MassDOT introduced a pilot performance-related specification for mixtures with high RAP contents. This specification aims to address potential variations in RAP characteristics or asphalt binder during the production cycle, from mix design to production. This study was conducted to evaluate the pilot specification. MassDOT authorized the use of 25% to 30% RAP by weight for a high-volume field project on Interstate 495 in Massachusetts. As part of this field project, mixtures were obtained at different intervals from an asphalt plant and compared with a control mixture to establish their properties and performance. A Superpave 9.5-mm mixture containing 25% RAP was produced and placed on this highway. The study evaluated the performance of high-RAP mixtures before production (in lab conditions) and on various production dates (in plant conditions), focusing on consistency across the production cycle. Mixture performances were evaluated in terms of rutting, intermediate-temperature cracking, and low-temperature cracking. Additionally, performance grades of binders extracted from the RAP and mixtures were determined throughout the production stages. The findings indicated that the mixtures using 25% RAP, instead of the typical 15%, maintained consistent performance. Performance space diagrams were also created to illustrate potential performance outcomes for rutting and cracking based on the pilot specifications by MassDOT. These diagrams confirmed that the new specification could yield asphalt mixtures with acceptable performance balance for various types of cracking and rutting, thereby validating the effectiveness of the performance-related specifications.