Optimizing the Use of Reclaimed Asphalt Pavement (RAP) in Hot Mix Asphalt Surface Mixes
The most common use of reclaimed asphalt pavement (RAP) is in the lower layers of a pavement structure, where it has been proven as a valid substitute for virgin materials. Instead, the use of RAP in surface mixes is more limited, with a major concern being that the high RAP mixes may not perform as well as traditional mixes. To reduce risks of compromised performance, the use of RAP has commonly been controlled by specifications that limit the allowed amount of recycled material in the mixes. However, significant cost and environmental savings can be achieved if more RAP is included in the surface layer. This dissertation develops an approach that can be followed to incorporate more RAP in the surface mix while maintaining good performance. The approach is based on the results from three studies that looked at how to optimize the design of the mix, in terms of rutting and fatigue resistance, when more RAP is used.
In the first study, a high RAP control mix and an optimized mix designed using different design compaction energy (65 and 50 gyrations respectively) were compared. The optimization process consisted in the definition of an alternative mix composition that supported the higher binder content allowed by the lower design compaction energy. Using Accelerated Pavement Testing and laboratory characterization it was possible to assess the potential of mix optimization with the objective of improving rutting resistance. The testing showed no indication that the optimized mixes would have rutting problems, supporting the implementation of the reduction of the design compaction energy level. The optimized mix exhibited a similar or superior rutting resistance in the full-scale setting, in the laboratory, and in the forensic investigation.
The second part focused on the production of highly recycled surface mixes capable of performing well. To produce the mixes, a balanced mix design (BMD) methodology was used and a comparison with traditional mixes, prepared in accordance with the requirements of the Virginia Department of Transportation (VDOT) volumetric mix design, was performed. Through the BMD procedure, which featured the indirect tensile cracking test for evaluating the cracking resistance and the Asphalt Pavement Analyzer for evaluating rutting resistance, it was possible to optimize the selection of the optimum asphalt content. Also, it was possible to obtain a highly recycled mix (45% RAP) capable of achieving better overall performances than traditional mixes while carrying a large reduction in production cost.
The final part evaluated the laboratory performance of four different highly recycled surface mixes to support their possible implementation in the state of Virginia. The mixes featured either 30% or 45% RAP, different asphalt contents, the use of a WMA additive, and a rejuvenator. To analyze the mixes' performance in great depth, a three-level (base, intermediate, and advanced) testing framework was defined. Each level was characterized by an increasing degree of complexity and included tests to characterize both the cracking resistance and the rutting resistance. The study aimed at investigating the features of the various laboratory tests. Through the review of the theoretical background, the evaluation of the test procedures, and statistical analysis of the results, it was possible to identify the strengths and weaknesses of each test and to provide guidelines to develop appropriate quality assessment criteria and mix design methodology.
In summary, throughout this research, it was possible to observe that the respect of Superpave mix design requirements alone, with particular reference to gradation limits and volumetric properties, was not guarantee of satisfactory performance in terms of both cracking and rutting resistance. To increase the confidence in the RAP properties, increase the current recycling levels, and introduce more appropriate mix design specifications, BMD could be used (even with simple laboratory tests) to check performance-based criteria.