The state-of-the-art review on the utilization of reclaimed asphalt pavement via hot in-place recycling technology

In recent years, the rise in raw material costs and the push towards sustainable development have made the recycling of reclaimed asphalt pavement (RAP) a prominent focus in highway engineering research. Hot in-place recycling (HIR) technology is an asphalt pavement recycling method that processes 100% of RAP through a single process involving heating, milling, adding external materials, hot mixing, paving, and rolling. This technique offers benefits such as reduced carbon emissions, environmental friendliness, stability, durability, and ease of construction. Both domestic and international researchers, as well as relevant enterprises, have extensively studied HIR technology, leading to innovations in equipment and technological advancements and accumulating considerable engineering experience. Additionally, extensive literature reviews have been conducted on rejuvenators, regeneration mechanisms, hot central plant recycling, cold recycling, and warm recycling. However, HIR technology has been only briefly discussed in existing studies. Thus, a comprehensive review of HIR technology is essential. This paper begins by outlining the HIR construction process, equipment, and common technologies, with a particular focus on the promising multi-layer synchronized recycling method. It reviews temperature control methods in HIR, evaluates the advantages and limitations of current heating modes and technologies, and suggests that coupling and multi-stage heating technology can significantly enhance heating efficiency and uniformity. Additionally, the paper introduces general design processes and mix proportion design methods, highlights strategies for optimizing mixture performance and existing shortcomings, and discusses construction quality control methods and performance variation law of HIR asphalt mixtures. Furthermore, it compares and analyzes the economic and environmental benefits. Overall, this paper provides a comprehensive review of research progress in various aspects of HIR, including design methods, construction equipment, implementation procedures, and maintenance effects. It identifies current deficiencies and outlines future research directions, aiming to foster innovation in HIR technology and contribute to achieving strategic goals in energy conservation, emission reduction, and the development of a green circular economy in highway engineering.