Reflectance Spectra of Thermochromic Asphalt Binder: Characterization and Optical Mixing Model

Abstract

Asphalt pavement constructed with traditional asphalt binder, which features high solar absorbance, is characterized by high surface temperatures during summer. This causes undesirable performance degradation (i.e., due to accelerated rutting, bleeding, etc.) and environmental consequences (i.e., urban heat island effects and volatile gas emission). Thermochromic asphalt binder, which is asphalt binder mixed with thermochromic materials, has been developed by the authors to dynamically control the solar reflectance and modulate the surface temperature of asphalt pavement. The goal is to improve its long-term durability and environmental friendliness. Thermochromic materials are substances that can reversibly change their colors in response to temperature. The binary thermochromic asphalt binder mixture is produced by incorporating thermochromic powders into a conventional asphalt binder. Measurements of spectral reflectance of thermochromic powders indicate that thermochromic materials feature higher solar reflectance at higher temperatures. In this paper, the spectral reflectance of thermochromic powders as functions of wavelength and temperature are fitted with common types of optical mixing models based on experimental data. Optical measurements conducted on the thermochromic asphalt binder mixture show that they feature more solar reflectance than conventional asphalt binders; furthermore, reflectance increases with temperature. The spectral reflectance of thermochromic asphalt binder also increases as the powder content increases. The common types of optical mixing formulas, including those based on Maxwell, Mori-Tanaka, and effective medium theory (EMT), etc., are used to establish the relationship between spectral reflectance of thermochromic asphalt binder and powder content. These optical mixing formulas provide a convenient way to predict the reflectance spectra of thermochromic asphalt binder and will assist its multifunctional design and optimization.