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Recycled asphalt mixtures with high amounts of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) can be excessively stiff, brittle, and prone to cracking. The use of recycling agents, or rejuvenators, can significantly reduce mixture stiffness and improve performance, specifically cracking resistance. In this study, the performance of recycled and rejuvenated asphalt mixtures from several field projects, located in different environmental zones across the United States, was evaluated considering various recycling agent dosages determined by the contractors. Field core test results and the visual distress surveys of the field projects demonstrated that using the field recycling agent dosages yielded poor mixture performance. Laboratory test results demonstrated that adding the recycling agent at the dosage to match the continuous high-temperature performance grade (PGH) of the rejuvenated binder blend (virgin/base binder, recycled binder, and recycling agent) to that of the target binder PGH specified based on climate and traffic requirements yielded improved mixture performance. The rejuvenated mixtures at this recycling agent dosage showed significant reduction in stiffness and improved cracking resistance, and facilitated the use of higher quantities of recycled materials, regardless of aging level, while maintaining rutting resistance after short-term aging. [ABSTRACT FROM AUTHOR]

Highlights • Summary of current knowledge on recycling agents (rejuvenators) and their application. • Identification of current and future challenges that prevent utilization of recycling agents. • Evaluation of recycling agents effectiveness in binder blends and asphalt mixtures. • Characterization of rejuvenated binder blends and rejuvenated asphalt mixtures. • Cost-effectiveness associated with the use of recycling agents. Abstract Recycling agents, or rejuvenators, are products with chemical and physical characteristics designed to restore the rheological properties of aged asphalt binders to improve the performance of recycled asphalt mixtures with large quantities of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) that promote economic and environmental benefits. This paper summarizes current knowledge related to the use of recycling agents in the asphalt pavement industry, including properties of recycled and rejuvenated asphalt mixtures. This information is further used to identify current and future challenges that could prevent utilization of recycling agents and production of rejuvenated asphalt mixtures with adequate performance. [ABSTRACT FROM AUTHOR]

Abstract: Durability and functionality (i.e., noise reduction effectiveness and drainability) of permeable friction course (PFC) mixtures depend on the characteristics of the air voids (AV) contained in the mixture. This study analyzes the internal structure of PFC mixtures, assessed in terms of AV characteristics; determined using X-ray Computed Tomography and image analysis techniques. Corresponding results showed: (i) heterogeneous distributions of AV in the horizontal direction of both field-compacted mixtures (road cores) and specimens compacted using the Superpave Gyratory Compactor (SGC) and (ii) limitations to compare their vertical AV distributions. Recommendations to reduce the horizontal heterogeneity included using road cores with a minimum 152.4mm diameter and coring SGC specimens to 101.6mm in diameter. Implementation of field-compaction control and future analysis of mixtures prepared accordingly was recommended to determine the pattern of vertical AV distribution that should be reproduced in SGC specimens and corresponding modifications required for fabrication of these specimens. [Copyright &y& Elsevier]

Compaction of permeable or porous friction course (PFC) mixtures is generally considered a process without major issues, and field density requirements (or corresponding total air voids [AV] content) are not currently specified for this type of hot mix asphalt. However, proper densification is one of the most important aspects to control during construction to prevent raveling, the distress most frequently reported as the cause of failure in these mixtures. This paper presents an evaluation of the effect of densification on PFC mixtures. This evaluation included both the study of the internal structure of compacted mixtures and a comparison of performance based on macroscopic response. Results from this study showed that differences encountered in the internal structure of road cores and specimens compacted using the Superpave Gyratory Compactor limit the use of these laboratory compacted specimens in durability and functionality evaluations of PFC mixtures. In addition, changes in densification, after reaching stone-on-stone contact, modified the mixture properties and performance. The magnitude of these modifications provided evidence of the ease of verifying not only stone-on-stone contact during mix design, but also of the importance of controlling the density during construction to ensure an equilibrium density that guarantees the balance between mixture durability and mixture functionality. [ABSTRACT FROM AUTHOR]

Current hot mix asphalt (HMA) mix design procedures used to determine the optimum asphalt content (OAC) for permeable or porous friction course (PFC) mixtures are based on volumetric properties, primarily total air void (AV) content. This calculated volumetric parameter depends on the bulk specific gravity (Gmb) and the theoretical maximum specific gravity (Gmm) of the mixture, which are generally difficult to measure in a laboratory due to the high asphalt contents, high total AV contents, and the use of modified asphalts for PFC mixtures. This study evaluated two methodologies for determining Gmb (vacuum and dimensional analysis) and two methodologies for determining Gmm (measured and calculated) for use in calculations of total AV content. For the mixtures assessed in this study, originally designed with a total AV content of 20 %, the alternative methodologies studied led to total AV content values outside the design range (18 to 22 %), which implies the necessity of gradation modifications or changes in the fiber content to meet AV requirements and define an OAC. Dimensional analysis and a calculation procedure, based on values of Gmm measured in the laboratory at low asphalt contents, are recommended for determining Gmb and Gmm values, respectively. In addition, dimensional analysis is preliminarily recommended to compute the water-accessible AV content of PFC mixtures based on the assessment of two methods (vacuum and a methodology proposed for dimensional analysis) to compute this parameter. Water-accessible AV content is considered as an alternative parameter for mix design and evaluation. [ABSTRACT FROM AUTHOR]

• Survey conducted on the state of practice to account for recycled binder activity. • Two recycled binder activity quantification methods were compared. • Recycled binder activity depended on RAP source, conditioning, and mixing temperatures. • Using 100% RAP sources to estimate recycled binder activity is a promising approach. Many environmental and economic benefits can be achieved by employing reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures. However, due to the high stiffness and brittleness of recycled materials, cracking issues may arise when utilizing them at high contents. Recycled binder activity is one of the primary factors that influences the performance of recycled asphalt pavements. Recycled binder activity represents the amount of RAP or RAS binder that becomes active at production temperatures for mixing with the virgin binder. Previous studies have indicated that recycled binders are only partially active; however, the majority of state department of transportation's assume full activity. Designing recycled asphalt mixtures under the assumption of full recycled binder activity may result in mixtures with insufficient binder content and thus increasing the susceptibility for cracking and moisture damage. This paper compares two recycled binder activity quantification methods; the first method quantifies activity using recycled asphalt mixtures of virgin and RAP materials, while the second method uses 100 percent recycled materials. The two methods showed to produce relatively similar recycled binder activity values; however, the use of 100 percent RAP materials showed to be a more promising approach due to its practicality. [ABSTRACT FROM AUTHOR]

Abstract: Permeable friction course mixtures (PFC) are special hot mix asphalt (HMA) mixtures that are designed to improve motorist safety and reduce traffic noise (i.e., tire–pavement noise). In terms of pavement structural design and construction, PFC mixtures are typically used as the surfacing course layer where in addition to improving the skid resistance, also serve as the surface drainage layer. This paper compares PFC mixtures designed and fabricated using performance grade (PG) and asphalt rubber (AR) asphalt-binders (or asphalts). The experimental design included assessment of total air voids (AV) content, durability, drainability, stone-on-stone contact, and internal structure of the PFC mixtures fabricated using both AR and PG asphalts; denoted herein as AR–PFC and PG–PFC mixtures, respectively. The analysis conducted provided evidence of differences between AR–PFC and PG–PFC mixtures that suggest the need for differentiation and refinement of the existing specifications to consider these mixtures as independent materials. Future research should, therefore, focus on defining particular specifications for mix design and control for AR–PFC and PG-PFC mixtures. [Copyright &y& Elsevier]

This study introduced a cracking parameter, called the Cracking Resistance Index (CRI), derived from the load-displacement response obtained during a semi-circular bending test, and compared it to the recently developed Flexibility Index (FI). Laboratory test results indicated that both FI and CRI are sensitive to binder Performance Grade, recycled materials content, and recycling agent dosage. The results indicated the dependency of the two indices on specimen thickness and air voids content. A reasonable correlation between FI and CRI was verified. As compared to the FI, the proposed CRI provided better distinction between different asphalt mixtures, less variability, and easier calculation. [ABSTRACT FROM AUTHOR]

Understanding the performance of rejuvenated asphalt mixtures is critical for cleaner pavement design and construction, as the use of rejuvenators lead to environmental and economic benefits with the ability to increase recycling but promote rutting and moisture susceptibility at a high recycled binder ratio. Existing models have not systematically evaluated rejuvenated asphalt mixtures in terms of rutting performance from a mechanistic approach, and there is a lack of research related to the third stage of rutting induced by high-temperature moisture damage. To overcome these shortcomings, two-stage and three-stage mechanistic-empirical models based on viscoelastic stress-strain response were proposed in this study to predict and evaluate rutting and moisture susceptibility of rejuvenated asphalt mixtures. Wheel tracking tests in wet and dry conditions were performed on a virgin asphalt mixture (without recycled materials) and on rejuvenated asphalt mixtures (with recycled materials and rejuvenator), and the cumulative rutting curves were investigated. In addition, linear viscoelastic properties at a reference temperature were used as input parameters in a finite element model (FEM) to calculate the viscoelastic response of virtual specimens under repeated loading. The FEM results were then substituted into the proposed models to determine the material parameters for wet and dry conditions separately. Two prediction results were combined to determine the proportion of rutting caused by stripping in rejuvenated asphalt mixtures. Four indices were used in the evaluation of rutting in wet conditions. Based on the wheel tracking test results, the rejuvenated asphalt mixtures with the highest recycled material content performed worst in wet conditions and best in dry conditions, which indicates that anti-stripping agent might be necessary for rejuvenated asphalt mixtures. [ABSTRACT FROM AUTHOR]