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The use of high amounts of reclaimed asphalt pavement (RAP) in asphalt pavements has many economic and environmental benefits; however, there are concerns about brittleness and potential cracking of asphalt mixtures. One of the solutions to address this concern is through the inclusion of recycling agents (rejuvenators). The purpose of this study was to explore the effect of different types of recycling agents (biooils, vegetable oils, tall oil, aromatic extract, and paraffinic oil) on the rheological, microstructural, nanomechanical, and chemical properties of asphalt binder blends with high RAP content. Rheological properties were assessed using a dynamic shear rheometer. Atomic force microscopy was used to determine the microstructural characteristics and nanomechanical properties of the asphalt binder blends. A wavelet packet transform approach was proposed to quantify surface roughness characteristics. Fourier transform infrared spectroscopy was used to evaluate the chemical properties based on carbonyl and sulfoxide indices. Results indicated a correlation between the phases observed in the microstructure and rheological performance. Biooil recycling agents were the most effective in improving the microscopic distribution and rheological properties of binder blends, followed by vegetable oils. However, chemical analysis suggested that the addition of recycling agents did not reverse oxidative aging. Finally, the study recommended a rejuvenation index (RI) that quantified the effectiveness of recycling agents in improving blending and reducing stiffness and aging susceptibility. The RI signified that tall oil was the most susceptible to aging, followed by aromatic extract and paraffinic oil, whereas biooils and vegetable oils were the least susceptible to aging. [ABSTRACT FROM AUTHOR]

Warm mix asphalt (WMA) is increasingly recognised as a green paving technology in terms of energy saving and emission reduction. The reduction in mix production temperature associated with the WMA additives is very desirable from the environmental aspect. However, researchers and practitioners desire a comparable performance with hot mix asphalt (HMA). The major issue with WMA is their moisture susceptibility especially in the pavement early life. Therefore, this study aims at investigating moisture susceptibility of WMA using both laboratory characterisation and field performance compared to HMA. Three mixtures were investigated, namely; HMA mixture, foaming and Evotherm WMA mixture. The laboratory tests included; resilient modulus (MR), Indirect Tensile test, and Hamburg wheel tracking test. Test sections on Farm to Market FM 973 Highway near Austin, Texas, were constructed using these three mixtures. The results of field performance monitoring for five years are represented in this paper. Laboratory test results revealed small differences between measured dry and wet tensile strength and MR of HMA and WMA which shows some improvements using antistripping agents. The field performance evaluation showed that the Evotherm WMA mixture had the least transverse and reflected cracks while the HMA performed better in both ravelling and longitudinal cracking. [ABSTRACT FROM AUTHOR]

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]

Determining the properties of residual binders is important to the effective use of asphalt emulsion chip seals. Yet, the effect of laboratory methods on recovered binder properties used to simulate residual binders in the field is not well understood. In this research, the residues of five asphalt emulsions were compared after recovery by three methods, a Hot Oven procedure (similar to ASTM D244-04), a Stirred-Can procedure (as reported in TXDOT 0-1710), and a Warm Oven method (ASTM D7497-09). The recovered binders were tested with size exclusion chromatography (SEC) to assess the presence of residual moisture. Properties of the original base binders and the corresponding recovered binders were compared using dynamic shear rheometry and Fourier transform infrared spectrometry. The SEC results showed no residual water in any of the recovered samples, except the samples from Warm Oven recovery, which showed a small detectable amount of residual moisture. The results from statistical analysis of binder properties using ANOVA plus Tukey's Honestly Significant Difference test suggest that recovered binders from the Warm Oven method are statistically different from their base binders. Nevertheless, considering each of the residues recovered from the three methods in paired comparisons with the other residues, none is statistically different from the others. [ABSTRACT FROM AUTHOR]

Stone-on-stone contact of the coarse-aggregate fraction is one of the main characteristics of permeable friction course (PFC) asphalt mixtures that is required to provide adequate resistance to both raveling and permanent deformation. Currently, stone-on-stone contact is determined by comparing the air voids content in the coarse aggregate (VCA), assessed in both the dry-rodded condition (VCADRC) and the compacted PFC mixture (VCAmix). The underlying assumption is that the coarse aggregate of a compacted PFC mixture with VCAmix equal to VCADRC would develop a stone-on-stone contact condition equivalent to that existing in the dry-rodded aggregate. This study focused on proposing enhancements for the quantitative determination of stone-on-stone contact of PFC mixtures. The assessment supported on both laboratory testing and application of the discrete element method and image analysis techniques, led to recommendation of a criterion to determine the breaking-sieve size. In addition, verification of stone-on-stone contact using a maximum VCA ratio of 0.9 was recommended to ensure the design and construction of PFC mixtures with fully developed stone-on-stone contact. [ABSTRACT FROM AUTHOR]

Durability of porous or permeable friction courses (PFC) is an important aspect to address when designing this type of hot mix asphalt. At present, several agencies perform the mix design of PFC primarily by determining volumetric mixture properties. This approach ensures adequate mixture functionality, but it does not guarantee mixture durability. This paper evaluates the Cantabro loss test, the Hamburg Wheel-Tracking test (HWTT) and the Overlay test (OT) to determine the one most appropriate for mix design and laboratory performance evaluations. The Cantabro loss test, performed in both dry and wet conditions, is recommended for PFC mix design to corroborate the suitability of the optimum asphalt content defined based on volumetric determinations. The HWTT and the OT are not recommended, since the variability of the test results indicated that these tests may not be suitable for PFC mixtures. [ABSTRACT FROM AUTHOR]

Current hot mix asphalt (HMA) mix design procedures used to determine the optimum asphalt content for permeable or porous friction course (PFC) mixtures are based primarily on total air void (AV) content. Durability and functionality of PFC mixtures are also related to the total AV content. However, the connected AV content (defined as the proportion of AV that form connected pathways for air and water transport through PFC mixtures) may provide more insight into the mixture structure in terms of the AV content directly associated with functionality and durability properties and constitute an alternative parameter to conduct PFC mix design and evaluation. This study evaluated two laboratory methodologies (vacuum and dimensional analysis) for determining water-accessible AV content and two types of analysis to compute interconnected AV content based on X-ray Computed Tomography (X-ray CT) and image analysis techniques. Although both the interconnected AV content and water-accessible AV content constitute determinations of connected AV content, different nomenclature was used to differentiate the origin of the calculation. Dimensional analysis with application of vacuum and X-ray CT and image analysis with inclusion of surface AV are recommended for determining water-accessible AV content and interconnected AV content, respectively. Future work should focus on investigating the use of connected AV content as an alternative parameter to integrate in mix design and laboratory and computational evaluation of PFC mixtures. [ABSTRACT FROM AUTHOR]

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• 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]

This paper evaluates the relationship between the air void distribution in asphalt mixes and their moisture susceptibility. Asphalt mix specimens were prepared to have different air void distributions through using two different gradations, using two different compaction angles in the Superpave gyratory compactor, and trimming the specimens at different locations. An x-ray computed tomography system was used to capture the internal structure of the specimens, and image analysis techniques were used to quantify the air void distribution and connectivity. Specimens in unconditioned (dry) and moisture-conditioned (wet) states were subjected to dynamic loading, and damage was evaluated using a crack growth index based on fracture theory. The parameters required for the crack growth index were obtained from mechanical and surface energy tests. The analysis results clearly demonstrate the influence of air void distribution on moisture susceptibility. Consequently, the analysis method presented in this paper can be used to evaluate alternative mix designs and internal structures, even when the same materials are used, in order to optimize the resistance to moisture damage. [ABSTRACT FROM AUTHOR]

State highway agencies strongly desire to develop and use performance-related specifications (PRS) for highway construction. A PRS for hot mix asphalt (HMA) construction, titled HMA Spec, was recently developed as part of the WesTrack project. The beta version of HMA Spec was applied to compare pay factors and actual rutting performance to predicted performance for four Nevada Department of Transportation (NDOT) test sections on Interstate 80 (I-80) east of Reno, Nevada. Some of these sections were similar in terms of materials and construction techniques to coarse-graded sections constructed at WesTrack, so a comparison to performance at WesTrack was also made. NDOT placed the test sections on I-80 in an effort to assess the applicability of the Superpave mix design process and HMA Spec for Nevada’s conditions. Sections were designed using both Hveem and Superpave mix design methods. Several interesting observations were made while evaluating the HMA Spec output. These observations and field performance data are described in this paper. [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]

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]

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]

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]

This study aims to determine the endurance limit and macro-cracking threshold of a viscoelastic composite material using a fracture mechanics approach. Asphalt mixtures containing different recycled material contents and recycling agent doses were fabricated and long-term oven aged. Dynamic modulus tests were first conducted to obtain linear viscoelastic properties including complex modulus and phase angle, and then damage properties were determined from monotonic semi-circular bending tests. Pseudo displacement was calculated to remove the viscous effect from the total dissipated energy, and the energy balance approach and Griffith crack growth criterion were modified and used to predict the bond energy with the aid of discrete element modeling. With the bond energy computed, the endurance limit was determined to be frequency and mixture composition dependent. The macro-cracking threshold defined as the boundary between healable micro-cracks and non-healable macro-cracks was also identified to be frequency and composition dependent using a damage density concept. Both the endurance limit and the macro-cracking threshold were utilized in the asphalt mixture Black Space diagram, which is capable of illustrating the effects of aging, recycled materials content and recycling agent dose in asphalt mixtures for the prediction of fracture performance. [ABSTRACT FROM AUTHOR]