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Reclaimed asphalt pavement (RAP) is a valuable material that can be recycled and reused in road engineering to reduce environmental impact, resource utilization, and economic costs. However, the application of RAP in road engineering presents both opportunities and challenges. This study visually analyzes the knowledge background, research status, and latest knowledge structure of literature related to RAP using scientific metric methods such as VOSviewer and Citespace. The Web of Science (WoS) core collection database identified 2963 research publications from 2000 to 2022. Collaborative networks between highly cited references, journals, authors, academic institutions, countries, and funding organizations are analyzed in this study, along with a co-occurrence analysis of keywords for the RAP research publications. Results showed that the USA has long been a leader in RAP research, China surpassed the USA in annual publication output in 2019, increasing from 2 publications in 2002 to 177 publications in 2022, and has made significant investments in technological aspects. Chang'an University ranked first in total publication output (131 publications, 4.4%). Current major research themes include road performance, recycling technology, regeneration mechanisms, and the life cycle assessment of RAP. In addition, based on cluster analysis of keywords, text content analysis, and SWOT analysis, this study also discusses RAP's challenges and future development directions in road engineering. These findings provide scholars with valuable information to gain insight into technological advances and challenges in the field of RAP. [ABSTRACT FROM AUTHOR]

This study endeavors to employ a balanced design methodology, aiming to equilibrate the resistance to rutting and cracking exhibited by hot in-place recycling asphalt mixtures containing a high dose of reclaimed asphalt pavement (RAP). The primary goal is to ascertain the optimal amount of new binder necessary for practical engineering applications, ensuring a balanced rutting and crack resistance performance of recycled asphalt mixtures. The investigation mainly employed wheel-tracking tests and semi-circular bending tests to assess the rutting and cracking performance of recycled asphalt mixtures with a different dose of RAP (in China, it is common to use RAP with 80% and 90% content as additives for preparing hot in-place recycling asphalt mixtures), and varying quantities of new binders (10%, 20%, and 30% of the binder content in the total RAP added). The results indicated that the addition of new binder reduced the resistance to rutting of the recycling asphalt mixtures but improved their resistance to cracking. Furthermore, for the recycling asphalt mixture with 80% RAP content aged for 5 days, the optimal new binder content is 1.52%, while the mixture with 90% RAP content requires 1.23% of new binder. After 10 days of aging, the optimal new binder content for the recycling asphalt mixture with 80% RAP content is 1.55%, while the mixture with 90% RAP content requires 1.28% of new binder. [ABSTRACT FROM AUTHOR]

As a pavement maintenance technology, hot in-place recycling (HIR) can realize full utilization of reclaimed asphalt pavement (RAP), significantly saving non-renewable resources and project costs. However, due to variability of material properties, recycled mixtures gradation, and temperature control in construction, HIR-maintained pavement usually suffers from segregation, affecting performance and service life. To control construction quality and prolong service life, a segregation evaluation system of HIR-maintained pavement is proposed. This evaluation system, including calculation of segregation indexes, determination of segregation zones, and discrimination of segregation types, is established based on digital image processing (DIP) and surface texture distribution characteristics. In addition, it is verified by a case study of qualified HIR projects in Jiangsu Province in China. According to the empirical acceptance requirements of maintenance projects, discriminative zones for segregation types are established at 95% confidence level. Fluctuation curves of segregation indexes and image tests of four segregation types identified by segregation zones show that segregation indexes are well correlated with texture distribution characteristics and segregation degree, confirming the effectiveness and accuracy of the segregation evaluation system. Therefore, the segregation evaluation system of HIR-maintained projects proposed in this paper can realize real-time monitoring, rapid diagnosis, evaluation, and treatment decision-making of recycled pavement segregation degree. [ABSTRACT FROM AUTHOR]

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In view of the cold winter in northern China and the large temperature difference between day and night, the asphalt pavement has been in low temperature service for a long time, in addition to experiencing the aging effect of water and salt erosion, jointly leading to a particularly serious pavement cracking phenomenon. Therefore, a quantity of Reclaimed Asphalt Pavement (RAP) is produced for the repair and renovation of old roads. Faced with the problems of inadequate reuse rate of RAP and high energy consumption of thermal regeneration, it is of great practical significance to explore the technology of warm mix regeneration. In order to study the adaptability of recycled SBS modified asphalt mixtures to low temperature after complex environmental erosion (long-term aging, water freeze-thaw cycle, and salt freeze-thaw cycle), two mixing methods, including hot mixing and warm mixing, were selected, and a Semi-Circular Bending Test was performed. SCB and Digital Image Correlation (DIC) techniques were used to evaluate the low temperature properties of recycled asphalt mixtures by fracture toughness, fracture energy and horizontal strain density damage. Combined with an Atomic Force Microscope (AFM) test, the microscopic analysis of recycled asphalt was carried out to verify the mechanism of low temperature cracking. The results showed that the low temperature cracking resistance of the reclaimed asphalt mixtures with warm mix was slightly worse than that with hot mix. The overall low temperature cracking resistance of the reclaimed SBS modified asphalt mixtures decreased with the increase in RAP content. The low temperature performance of the warm-mixed reclaimed asphalt mixtures deteriorated greatly when the RAP content was 80 %. The influencing factors of environmental conditions on the low temperature cracking resistance of the warm-mixed recycled SBS modified asphalt mixtures were listed as below: long-term aging > salt freeze-thaw cycle > water freeze-thaw cycle. The influence of salt on the stress field of crack tip and the microstructure of recycled asphalt after freeze-thaw was greater than that of water erosion. The changes of the microscopic adhesion were in good agreement with the macroscopic test results, which could be employed as the index evaluating the low temperature crack resistance of the warm-mixed SBS modified asphalt mixtures. The addition of warm mixing agent would play a positive role in the low temperature crack resistance of recycled asphalt mixtures with a RAP content of less than 60 % to a certain extent, so warm-mixed recycled asphalt mixtures were more suitable for the application in northern areas. • Explored the low-temperature cracking behavior and cracking mechanism of warm mix asphalt mixture. • Adverse effect of RAP content on low-temperature crack resistance of warm-mix asphalt mixture. • The micro index Fa can predict low-temperature cracking of recycled mixtures. [ABSTRACT FROM AUTHOR]

• A quantitative evaluation method for selecting recycling methods was developed. • An economic evaluation model for each recycling method was established. • Pavement recycling decision system was developed based on the. Net platform. • Case study showed that the recycling decision system was feasible and reasonable. Asphalt pavement recycling technology is widely used in China and other countries due to its economic and environmental benefits. Pavement recycling methods can generally be divided into central plant hot recycling, hot in-place recycling, central plant cold recycling, and cold in-place recycling. However, no quantitative evaluation method and calculation program for selecting an optimal recycling method are available for existing asphalt pavements. In this study, a quantitative evaluation approach for recycling method selection was proposed in consideration of pavement performance and construction cost. An asphalt pavement recycling decision system was established, and a corresponding calculation program was developed based on the. Net platform. Case study showed that the asphalt pavement recycling decision system developed based on the analytic hierarchy process in this study is feasible and reasonable. The system can help pavement managers make efficient maintenance decisions in practical projects. [ABSTRACT FROM AUTHOR]

The warm-mix technology and high reclaimed asphalt pavement (RAP) contents applied in asphalt mixture can effectively reduce the use of energy resources and waste materials disposal. To realize the performance requirement of warm-mix recycled asphalt mixture, it is significant to develop specific warm-mix rejuvenators with satisfactory performance. Diffusion and crack-resistance properties are the key factors combined with engineering characteristics for rejuvenator design. In this research, a new type of asphalt rejuvenator for warm-mix recycled asphalt mixture with high RAP contents was designed. Then, the molecular properties of the key ingredients were explored according to asphalt aging theory and molecular diffusion simulation results. The results indicate that the diffusion group should contain linear alkanes or small alkanes, aromatics with a low aromaticity, or molecules with weak or no polar groups and low cohesive energy. Rejuvenators and corresponding mixture were then prepared, followed by performance testing. The results show that the strength of the recycled mixture increased during rejuvenator diffusion, and polymer addition to certain rejuvenators effectively improved the interface performance of the asphalt fusion layer. The fine aggregate mortar beam test results showed that this approach represents a simplified test method to evaluate the low-temperature performance of rejuvenators. The overall performance of the designed rejuvenator is satisfactory and fully meets the requirement of the surface layer of provincial highways in most areas of China. The application of this technology has good environmental and economic benefits. [Display omitted] [ABSTRACT FROM AUTHOR]

Cold in-place recycling (CIR) has been increasingly applied in many areas of China, but there is no widely accepted method for the mix design of CIR. The compaction of cold recycled mixtures in the field varies significantly in different working conditions. It is therefore necessary to study the compaction characteristics of cold recycled mixtures. In this study, the primary compactibility energy index (PCEI) and secondary compactibility energy index (SCEI) of CIR were defined from the gyration compaction curve to evaluate the effect of compaction temperature, asphalt emulsion content, moisture content, aggregate gradation, and initial cure time. The control variable method was adopted to conduct the CIR compaction tests. The compaction parameters of CIR were obtained and analyzed statistically. The results from this study indicate that the SCEI of cold recycled mixtures is significantly affected by all variables and the regression of is linear relative to the compaction temperature and asphalt emulsion content. The effect of compaction temperature is much more significant than the other factors. Based on the effect of moisture content on PCEI and SCEI, a new method to determine the optimum moisture content of CIR is provided. The effect of initial cure time actually represents the effect of moisture content to some extent. [ABSTRACT FROM AUTHOR]

Abstract: Permafrost temperatures have been monitored through six boreholes drilled up to 15m in depth under asphalt pavement along the centerline of an embankment along the Qinghai–Tibetan Highway. Temperatures have been measured twice monthly from 1996 through 2007. Soil temperatures at 1.0m depth under asphalt pavement in the northern areas of the Tanggula Mountains have shown a significant increasing trend, but a significantly decreasing trend in the southern Tanggula Mountains during the period of record. Mean annual permafrost temperatures at 10m depth have increased from 0.26°C to 0.64°C, with an average increase of about 0.39°C from six monitoring sites during the period of record. The artificial permafrost table depth has increased significantly over the period of record for all six monitoring sites, varying from 0.16 to 2.6m, with an average of 1.67m. The heat accumulation produced by heat absorption of the asphalt pavement dominates the long-term thermal effect of asphalt pavement on permafrost. With permafrost thawing and permafrost temperature rise, thaw settlement increased significantly. The amount of thaw settlement is positively correlated with the permafrost thawing rate. A noteworthy feature is that lower temperature permafrost''s response to climate change is more sensitive than to engineering; however, the response of high temperature permafrost to engineering is more sensitive than to climate change. [Copyright &y& Elsevier]

• Climate simulation test system was used for fine aggregate matrix (FAM). • Natural environment to couple aging conditions was considered. • The torsion bar fixture of DSR was used for fatigue performance of FAM. • The effects of RAP content and aging time on fatigue performance were different. Recycled asphalt mixture has been widely used in all grades of asphalt pavement in the world, but its unstable fatigue performance limits the addition of reclaimed asphalt pavement (RAP) materials. According to the characteristics of the two hot recycled methods commonly used in China, nine kinds of fine aggregate matrix (FAM) were produced in the laboratory. Based on the torsion bar fixture of dynamic shear rheometer (DSR), linear viscoelastic (LVE) test, strain sweep, frequency sweep and time sweep test were carried out on FAM. The results showed that aging time and the increase of RAP content would have a negative effect on the fatigue performance of FAM, and the effect of aging time was more significant. Both time sweep test and strain sweep test could predict the fatigue properties of FAM, but strain sweep test could save more time. Therefore, it was suggested that strain sweep test could be used as a prediction method of fatigue properties of FAM. [ABSTRACT FROM AUTHOR]

Hot in-place recycling (HIR), 100% recycling reclaimed asphalt pavement (RAP) directly on a construction site, has been widely used as a sustainable maintenance treatment. Since the RAP provides aged asphalt and dense skeleton structure, the HIR is beneficial to restoring pavement rutting resistance. With ever-increasing maintenance requirements whereas limited maintenance budgets, a rutting-based optimum maintenance decision strategy of hot in-place recycling in semi-rigid base asphalt pavement is proposed. It contains the selection of maintenance sections using the peak cutting method in a multi-objective optimization model and the determination of maintenance timing by the arching depth rather than long-term pavement performance. To verify, a case study based on a HIR project of an existing expressway in Jiangsu Province in China is conducted. Results confirms the economy and effectiveness of the peak cutting method in selecting maintenance sections by comparison with the traditional empirical method. With the finite element simulation, the arching depth is proved as the dominant factor of the temperature of asphalt mixtures in milling process. Based on the confidence level of milling temperature, maintenance timing related threshold of arching depth are recommended. Consequently, the proposed maintenance decision strategy could provide the highway management department with guidance in selecting maintenance sections and determining maintenance timing at a critical pre-requisite of saving maintenance funds and restoring pavement serviceability. [ABSTRACT FROM AUTHOR]

• A pavement maintenance project at Southeast China is studied. • The variability of RAP obtained form 16 sources are analyzed. • RAP from 16 sources are grouped by cluster analysis. • The maximum addition of the grouped RAP sources is determined. The variability of reclaimed asphalt pavement (RAP) materials obtained from different sources is a major concern for the industry when recycling RAP for recycled asphalt mixtures. Many efforts have been made to minimize RAP variability by pre-mixing, crushing, and sieving of the RAP stockpile. In this study, a perspective for reducing RAP variability is proposed by grouping RAP sources with similar material characteristics, which is expected to minimize such variability from the origin. To is end, an asphalt pavement maintenance project located in South China with 16 RAP sources was selected for the study. Furthermore, material characteristics including aggregate gradation, binder softening point, penetration, and dynamic viscosity of recovered asphalt, for RAP obtained from various sources are analyzed by testing on core samples. After the aggregate gradation is presented via Fréchet similarity, a K-means cluster analysis is conducted with the involvement of aggregate gradation, asphalt content, and softening point of recovered asphalt. Then, the RAP sources are grouped into three groups with similar material characteristics. The results show that there is a significant difference in RAP material characteristics even for the same pavement, and the procedure developed in this study helps to reduce the RAP variability while increasing the RAP maximum addition amount for producing recycled asphalt mixture. The results may be of interest to construction contractors and road management departments because this study was intentionally conducted for use in practical applications. [ABSTRACT FROM AUTHOR]