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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]

Although extensive work has been performed to quantify field aging of asphalt pavements and to evaluate its effect on asphalt mixture properties, little effort has been devoted towards characterizing the non-uniform field aging of asphalt pavements through laboratory testing. Due to the non-uniform air void and temperature distribution in the pavement structure, the surface of the pavement ages faster than the bottom; thus a field aging gradient exists. In this study, field cores obtained from four field projects at various in-service times were tested in the non-destructive Viscoelastic Characterization Direct Tension test to determine their mixture stiffness gradient with pavement depth. In addition, asphalt binders were extracted and recovered from the field cores at different pavement depths and then tested with the Dynamic Shear Rheometer and Fourier Transform Infrared Spectroscopy to differentiate their rheological and chemical properties. Test results obtained in this study provided a better understanding of the non-uniform field aging in asphalt pavements and its effect on asphalt binder and mixture properties. [ABSTRACT FROM AUTHOR]

Abstract: Permeable friction course (PFC) mixtures constitute one of the best options for surface paving, since they offer advantages (as compared to dense-graded hot mix asphalt) in terms of safety, economy, and environment. However, mix design and evaluation of PFC mixtures still require improvement and standardization to further promote the reliable use of these mixtures. This paper summarizes an extensive literature review focused on the basic aspects integrated in the mix design and evaluation of PFC mixtures. Although substantial advances related to these topics were reported since implementation of PFC mixtures in the 1990’s, there is still a need for integration of several of these advances in an improved mix design procedure. The paper also identifies areas that require additional research, including the optimization of the balance between mixture functionality and durability and replacement of the indirect assessment of these important aspects. [Copyright &y& Elsevier]

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]

• 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]

The economic and environmental benefits of using recycled asphalt materials continue to motivate agencies to increase their proportion in new pavement construction and maintenance and rehabilitation activities, but the resulting reduced cracking resistance remains an engineering performance challenge. The addition of recycling agents is one of the strategies to mitigate this issue caused by the increased stiffness and brittleness of the recycled mixture. This study explored the mechanism by which different types of recycling agents partially restore flexibility and ductility of asphalt binder and mixtures. A binder blend with 50% reclaimed asphalt pavement (RAP) binder was rejuvenated with seven types of recycling agents. Rheological and chemical tools were used to evaluate the binder blends and compare their performance to a control blend without a rejuvenator to study the rejuvenation mechanism. Corresponding asphalt mixtures were also tested to ensure adequate performance. Based on the results, the recycling agents were divided into three categories: (a) Softeners, (b) Replenishers, and (c) Emulsifiers. The results showed that while some agents act only on a physical level, others interact with the blend at a chemical level that affects the physical properties. A novel parameter was developed to compare the effectiveness of various recycling agents. [ABSTRACT FROM AUTHOR]

Economic and environmental demands motivate transportation agencies to increase the amount of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) used in asphalt pavements. These pavements often require a recycling agent to reduce the likelihood of premature cracking. Estimating an optimum recycling agent dosage is critical to maximize its benefit. In this study, three recycling agent dosage selection methods based on the restoration of the performance grade (PG) of the recycled blend (virgin binder, recycled binder, and recycling agent) were explored. Based on the performance of the recycled blends, the method to restore high-temperature PG is recommended to determine the optimum recycling agent dosage. [ABSTRACT FROM AUTHOR]

Abstract: The surface free energy (SFE) is a material property that can be used to identify optimum asphalt–aggregate combinations (i.e., interfaces) for improved design of hot mix asphalt (HMA) and to characterize the HMA performance through the use of micromechanical models. Based on SFE measurements, and subsequent calculation of energy indices, this paper compares asphalt–aggregate interfaces formed with asphalt rubber (AR) and polymer modified (PM) asphalt binders (or asphalts) specified for fabrication of permeable friction course (PFC) mixtures in Texas. Six PM asphalts and four AR asphalts with five different aggregates were assessed. Corresponding results suggest that, in terms of the energy indices computed, the fracture resistance, moisture damage susceptibility, and the wettability of the asphalt over the aggregate of AR asphalt–aggregate systems can be comparable to that developed by PM asphalt–aggregate systems. However, this conclusion is restricted by the variability encountered for both asphalt groups analyzed (i.e., AR and PM). Additional research is recommended to analyze the effect of lime in PM asphalts (since PFC mixtures fabricated with PM asphalts often include lime) and the effect of mineral fillers on both PM- and AR-aggregate systems. [Copyright &y& Elsevier]

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]

The newly developed method to estimate asphalt viscosity and aging trends using Low Field Nuclear Magnetic Resonance (LF-NMR) was applied to characterize the effect of incorporating recycled materials and recycling agents in asphalt mix samples. The measurements were conducted on asphalt mix samples prepared with different binders: (1) a virgin binder, (2) a recycled binder from Reclaimed Asphalt Pavement (RAP), (3) a blend of virgin and RAP binder (recycled blend) and (4) a rejuvenated blend that consists of the recycled blend with the addition of a recycling agent (tall oil). The LF-NMR measurements were performed at room (25 °C) and magnet (36.4 °C) temperatures. The Relative Hydrogen Index (RHI) obtained from LF-NMR captured the expected binder viscosity trends for the asphalt mix samples prepared with each of the different binders. The viscosity of the binders and blends was estimated from Dynamic Shear Rheometer (DSR) measurements. The relationship between dynamic viscosity and RHI agreed with previous findings. The samples prepared with the rejuvenated blend had lower viscosity and higher RHI than the recycled blend, indicating that the tall oil was effective in lowering its viscosity. Small discrepancies in the trends of the rejuvenated blend as compared to the virgin binder were attributed to their similar detectable hydrogen content, obtained in the rejuvenated blend with the addition of tall oil, but different oxidation state, molecular distribution and steric hindrance. This indicates that the LF-NMR method is more influenced by the chemistry of the asphalt binders rather than their viscosity. The asphalt mix samples with virgin and rejuvenated binders were also subjected to an aging treatment at 70 °C for a month. A significant decrease of the RHI was detected for the samples prepared with rejuvenated binder whereas no significant change was detected for those prepared with virgin binders, which indicates that rejuvenated binders were more susceptible to additional aging that resulted in increased viscosity. [ABSTRACT FROM AUTHOR]

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]

In this study, the effect of recycling agents on recycled blends obtained from base and recycled asphalt binders and the performance of the rejuvenated binders with aging was investigated by means of Atomic Force Microscopy (AFM) and Dynamic Shear Rheometer (DSR). This study arises as the amount of recycled materials in asphalt pavements continues to increase. Recycled binders are significantly stiffer than base binders; however, the performance grade can be restored by inclusion of recycling agents. Nevertheless, intermediate temperature cracking performance evaluated by the Glover-Rowe (G-R) parameter indicated that the effect of the recycling agents is not sustained with subsequent aging, which could jeopardize its effectiveness in service. The AFM images provided microstructural information, which offer indications about changes in molecular mobility and degree of association with aging, presence of agglomerations, and the dispersive effect of recycling agents. [ABSTRACT FROM AUTHOR]

Permeable Friction Courses (PFC) are asphalt mixtures characterized by high air void contents that are used as surface layers in conventional pavements, principally to achieve safer roads and control noise pollution. This paper summarizes existing information related to the performance of PFC mixtures in the United States (US), including properties of the mixture that determine the durability and functionality of the pavement and mix design methodologies that are currently being used in the US. The objective of this work is not only to analyze this information but also to identify current and future challenges to enhance the performance of PFC mixtures and promote its responsible use. [ABSTRACT FROM AUTHOR]

• Laboratory and field results from selected chip seal highway sections were reviewed based on 6 °C increment SPG specification and climate selection map in implementation. • A causal effect analysis was used to evaluate the effect of SPG specification to chip seal field performance. • A nearest-neighbor matching algorithm was applied to minimize selection bias in the causal model. • The SPG specification was proven to have an improvement to chip seal field performance through a statistical hypothesis test with 95% confidence level. The Texas Department of Transportation (TxDOT) has implemented a Surface Performance-Graded (SPG) specification for chip seal binders as a special provision available for use in their annual statewide chip seal program. The SPG specification was established to improve chip seal field performance by limiting aggregate loss and bleeding during the critical first year after construction. The objective of this study is to evaluate the effect of the SPG specification on chip seal field performance by reviewing selected highway sections (HSs) from previous TxDOT annual statewide chip seal program built from 2011 to 2014. The review of chip seal binder laboratory characterization and field monitoring results is based on: (a) the finalized SPG specification with validated high and low temperature limits, (b) an updated SPG climate-based map with 6 °C increments for practical consideration, and (c) an improved Surface Condition Index calculation method to eliminate subjectivity from field inspection personnel. The review results are used to compare the SPG criteria with field performance. A causal effect analysis is performed to assess the average treatment effect on HSs from implementing SPG specification for chip seal binders. HSs are categorized into treatment and control groups based on SPG results, and 19 well-matched pairs were selected by utilizing a nearest-neighbor matching algorithm to minimize selection bias. Given the random selection of chip seal HSs, an improvement of field performance on HSs that satisfied the SPG specification is validated by a statistical hypothesis test at 95% confidence level. [ABSTRACT FROM AUTHOR]

• The effect of FDR mixture sample size and testing temperature on IDT strength was evaluated. • Revised TxDOT IDT acceptance criteria were proposed for FDR 6-inch diameter samples. • TxDOT IDT acceptance criteria were proposed for FDR vacuum saturated samples. • IDT strengths of dry and soaked FDR samples were highly dependent on testing temperature. Full depth reclamation (FDR) is a cost-effective recycling strategy that reuses both asphalt bound materials and unbound granular materials to refurbish existing pavement subsurface layers. In this study, the effect of sample size, conditioning procedure, and testing temperature on the indirect tensile (IDT) strength of FDR mixtures was evaluated. Test results indicated that all 6-inch diameter samples had lower IDT strength as compared to the 4-inch diameter samples, regardless of the material source, binder type, binder content, or conditioning procedure. Revised IDT acceptance criteria were proposed for the Texas Department of Transportation (TxDOT), and recommendations on testing temperature were also provided. [ABSTRACT FROM AUTHOR]