Laboratory Evaluation of Environmental Performance of Photocatalytic Titanium Dioxide Warm-Mix Asphalt Pavements

The use of titanium dioxide (${\mathrm{TiO}}_2$) coating for pavements has received considerable attention in recent years to improve air quality near large metropolitan areas. However, the proper method of applying TiO2 to asphalt pavements is still unclear. This study evaluated the benefits of incorporating ${\mathrm{TiO}}_2$ in the preparation of warm-mix asphalt (WMA). Two application methods to integrate ${\mathrm{TiO}}_2$ were evaluated, a water-based ${\mathrm{TiO}}_2$ solution applied as a thin coating and using ${\mathrm{TiO}}_2$ as a modifier to asphalt binder in the preparation of WMA. On the basis of the results of the experimental program, it was determined that the photocatalytic compound was not effective in degrading ${\mathrm{NO}}_{\mathrm{x}}$ in the air stream when used as a modifier to the binder in the preparation of WMA. This could be attributed to the fact that only a small amount of ${\mathrm{TiO}}_2$ is present at the surface. When used as part of a surface spray coating, ${\mathrm{TiO}}_2$ was effective in removing nitrogen oxide (${\mathrm{NO}}_{\mathrm{x}-}$) pollutants from the air stream with an efficiency ranging from 31 to 55%. The maximum ${\mathrm{NO}}_{\mathrm{x}}$ removal efficiency was achieved at a coverage rate of $0.05\mathrm{L}/{\mathrm{m}}^2$. However, durability of the surface spray coating requires further evaluation. In addition, the increase in flow rate and relative humidity negatively impacted the effectiveness of ${\mathrm{NO}}_{\mathrm{x}}$ reduction efficiency. In contrast, the increase in ultraviolet (UV) light intensity improved the ${\mathrm{NO}}_{\mathrm{x}}$ removal efficiency of the surface coating.