No description available.

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. / Doctor of Philosophy / Nowadays, transportation agencies are expected to perform a large number of pavement rehabilitation projects, while facing major limitations in budgetary funds. In order to have safe, efficient, and cost-effective roadways, the economic advantage of recycling is boosting an effort to increase the amount of RAP in asphalt mixtures. In addition, over the past decades, the environmental awareness of the transportation agencies and public increased significantly, pushing even more towards the use of new green technologies. The use of RAP became noticeable in the 1970s and its popularity increased significantly since that time. However, there are still many open questions which prevent larger uses of recycled materials, mainly related to the design methodology and the field performances of recycled mixtures. Therefore, today there is a large untapped potential that would grow even more the magnitude of pavement recycling and of the associated benefits. New design procedures, based on the support of laboratory tests to characterize the mixtures, and full-scale experiments are the tools that pavement engineers can use in order to enrich the knowledge of highly recycled road materials and grow the confidence of public agencies and contractors towards these new more sustainable solutions. Throughout this dissertation it was possible to evaluate new innovative ways of incorporating more RAP in the asphalt mixtures through the analysis of current state of the art and the proposition of new procedures.

Balanced Mix Design (BMD) represents an asphalt mixture design methodology that replaces certain traditional volumetric parameters with performance-based testing to address predominant distresses such as rutting and cracking. This approach offers an avenue to properly design and produce engineered asphalt mixtures, including those with high reclaimed asphalt pavement (HRAP) contents, recycling agents (RAs), fibers, and polymer-modified binders. Laboratory performance tests are essential to the BMD process, as they ensure the production of durable, high-performance materials. Beyond laboratory performance evaluation, accelerated pavement testing (APT) plays a crucial role in bridging the gap between laboratory material characterization and field pavement performance. This dissertation aimed to assess the BMD concept for designing durable, long-lasting surface mixtures in Virginia, with particular emphasis on higher RAP content mixtures (HRAP mixtures, i.e., exceeding 30% RAP). The study involved laboratory and APT testing of six surface mixtures featuring a range of RAP contents (both conventional and high), two binder grades (PG 64-22 and PG58-28), one RA, and one warm mix additive. Findings indicated that dense-graded, unmodified surface mixtures with higher RAP contents can be successfully designed using the current Virginia Department of Transportation (VDOT) BMD special provision. These mixtures can be produced in the plant with no significant deviations in aggregate gradation and asphalt binder content from the design specifications. The combined effect of variations in different volumetric properties during production may influence the primary performance of the mixtures, potentially resulting in an imbalance. As a consequence, the produced BMD mixture may fail to meet one or more performance thresholds. Additionally, the results underscored the effectiveness of BMD concept with incorporating RAs and/or a softer binder when designing HRAP surface mixtures. Importantly, the current selected BMD tests characterized the laboratory performance of mixtures, aligning with the performance observed under APT. This research provided a steppingstone towards the examination and validation of the VDOT BMD thresholds, which ensures satisfactory field performance. The study also indicated that while current BMD thresholds provided sufficient margins for satisfactory field cracking performance, rutting resistance may become a concern for overly designed BMD HRAP mixtures. For instance, mixtures with excessively high asphalt binder content may exhibit compromised rutting resistance. Furthermore, to address the challenges uncovered during BMD test analysis—issues like the constraints of traditional pair-wise comparisons, risks of repetitive design processes, and the difficulty in pinpointing critical factors in mixture production—this dissertation proposed innovative solutions to enhance BMD application and streamline the evaluation process. First, a novel Composite Performance Index (CPI), visualized through a 3D plot, captured the "balance" status of various mixtures. Second, a machine learning-enhanced BMD framework was introduced, offering intelligent optimization throughout the design and production phases. The integration of these two tools offers significant potential for simultaneously improving multiple performance indices of asphalt mixtures. Finally, this research demonstrated that the performance of higher RAP content mixtures can exceed that of lower RAP content mixtures through the application of BMD approaches. This dissertation not only advanced the implementation of BMD for surface mixtures but also contributed to the sustainable and performance-driven evolution of asphalt mix design. The insights gained from this study provided practical guidance and strategic recommendations for enhancing asphalt mixture design, production, and performance monitoring. / Doctor of Philosophy / This dissertation evaluated a new way to design asphalt mixtures, called Balanced Mix Design (BMD). Unlike traditional methods that focus mostly on fundamental volumetric properties of materials, BMD uses testing to ensure the pavement will hold up against common issues like cracking and rutting. This approach allows for better use of materials, including high recycled asphalt pavement (HRAP), recycling agents (RAs), fibers, and modified binders. Considering economic and environmental benefits, the study evaluated the application of BMD for designing durable asphalt surfaces for pavements, particularly mixtures with over 30% recycled asphalt pavement (RAP). Tests in the laboratory and with accelerated pavement testing (APT) showed that high RAP mixtures could be designed and produced reliably using current Virginia Department of Transportation (VDOT) guidelines. The addition of recycling agents or softer binders further supported these mixtures in meeting performance standards, ensuring satisfied field performance. To address key challenges, such as the limitations of traditional pair-wise comparisons, risks of repetitive design processes, and the difficulty in finding key factors for production variability, this research introduces two innovative tools to enhance BMD. First, a Composite Performance Index (CPI) with a 3D visual tool offers a way to assess the balance status of mixtures. Second, a machine learning framework was developed to optimize BMD throughout the design and production phases. Together, these tools establish BMD as a valuable approach for designing and producing more sustainable, longer-lasting pavements.