Thermal performance of cooling strategies for asphalt pavement: A state-of-the-art review

Asphalt pavements absorb and store more heat than natural surfaces. Thus, high temperatures are emitted from conventional asphalt pavements, subsequently releasing heat into the atmosphere and contributing to the urban heat island (UHI) phenomenon. Several cool pavement strategies, including the provision of additives and materials, surface coating and layer design, have been introduced to reduce the impact of UHI. This article provides a detailed review of the thermal properties of these mitigation measures in the context of cool asphalt pavements. The literature can be divided into three segments. The first segment discusses the impact of pavements on UHI and heat transfer mechanisms in pavements. The second segment focuses on various thermo physical properties that play an important role in mitigation measures; these properties include albedo (α), emissivity (ε), solar reflective index, thermal conductivity (k), specific heat capacity (C_p) and thermal diffusivity. The third segment discusses cool asphalt pavement strategies which specifically cover the ability of the pavement to absorb and reflect solar energy on the basis of the materials and treatments used. The literature reveals that cooling strategies that deal with the pavement surface are important due to its direct incident solar effect, which depends on surface colour, material, shape and roughness. By using high-albedo and high-emissivity surfaces, the pavement can store less heat and lower the surface temperature. These results can also be achieved by designing the materials and pavement layers with low thermal conductivity and high specific heat capacity to reduce thermal diffusivity and pavement temperature and thus combat the heat radiated by the asphalt pavement.