Materials to Mitigate the Urban Heat Island Effect for Cool Pavement: A Brief Review

Abstract

The urban heat island (UHI) effect has a significantly negative impact on the living environment in urban areas. Asphalt pavement is one of the most widely used infrastructures that absorbs solar energy, which leads to the UHI effect and premature failure. As a result, cool pavement technology has been rapidly developed in recent years to mitigate the UHI effect originating from asphalt pavement. Although several outstanding review articles have analyzed previous studies on cool pavement technologies, very few review articles have focused on how to design and expand cool pavement technology from a materials perspective. In this mini-review article, the theoretical and practical factors of the solar reflective coatings and phase-change materials, which are significantly dependent on the design of new materials, have been summarized. The main challenges and potential problem-solving ideas have been presented. In a cool pavement, the solar reflective coatings are composed of epoxy resin or acrylic polymer matrix filled with solar reflective nanoparticles, such as TiO2, SiO2, ZnO, Al2O3, or Fe2O3. The main challenges of the solar reflective coatings are the spalling of the coating polymers from the asphalt pavement surface and the dispersion of the solar reflective nanoparticle in the polymer matrix. Most importantly, it is critical to harmonize the balance between the bonding strength, aging rate, solar reflectance, curing requirements, mechanical properties, and durability of the solar reflective coating. For the nanofillers, the cost of the filler materials, the balance between UV, visible light, and near-infrared reflectance and the dispersion status of the nanofillers in the polymer matrix are the primary factors that must be concerned. For the phase-change materials (PCMs), the interaction between the asphalt and the PCMs, the decomposition of the PCMs, the toxicity of the PCMs, the distribution status of the PCMs in the asphalt matrix, and the cost are the main factors that have to be considered in constructions. This review article can not only provide basic knowledge for the development of new solar reflective pavement materials but also serve as a guide for practical applications of cool pavement in the field.