A review on the heat transfer in asphalt pavements and urban heat island mitigation methods

Heat transfer in the asphalt pavement is significant because it affects asphalt pavement performance and the surrounding environment. This importance is attributed to summertime when the pavement's surface is hot. This hot surface can cause asphalt pavement distresses such as rutting and exacerbate cities' urban heat islands (UHI). Therefore, understanding the heat transfer parameters and UHI mitigation approaches is of great significance. Different thermal parameters, including thermal diffusivity, thermal emissivity, density, heat capacity, thermal conductivity, solar reflectance index, albedo, and experimental and numerical techniques for measuring and enhancing heat transfer in asphalt pavement, are explained in this study. A considerable amount of solar energy is absorbed by asphalt concrete affecting the environment and mechanical properties of asphalt pavements. Hence, there are numerous reasons for using materials altering the thermal conductivity of asphalt pavement, including transferring heat rapidly from the hot surface to the underneath structure, accelerating the snow-melting process, resistance to low-temperature cracking, cooling down the surface of the pavement to mitigate the urban heat island effect and improve rutting resistance. Indeed, developing a cool asphalt pavement is necessary to alleviate the UHI. Thus, higher thermal emissivity, albedo, and specific heat capacities are desirable for cool pavements. However, high thermal diffusivity can decrease heat capacity. Moreover, high conductive materials can reduce the surface temperature during the day, but they are hotter at night. But this has a reverse trend for low conductive materials. Hence, the combination of using high and low conductive materials in different layers can be vital. Regarding UHI mitigation methods, three ways are common, including using the cooling effects of water inside the pavement, increasing the reflectivity of pavements, and altering the thermal conductivity of pavement which are fully investigated in this study. Finally, these solutions' effects can also be evaluated by laboratory and field experiments, which are mentioned.