Thermochromic functionalization of asphalt mixtures as an urban heat island mitigation strategy: assessing spray-coating performance and microclimatic impacts in a tropical environment

Abstract

Urban heat has become one of the most critical environmental challenges in contemporary cities. The extensive presence of impervious surfaces, particularly asphalt pavements, intensifies solar absorption and heat storage, significantly contributing to the formation and persistence of urban heat islands (UHIs). Because pavements occupy large portions of urban areas, they constitute a strategic surface for implementing sustainable cooling solutions. In this context, the thermochromic functionalization of asphalt pavements emerges as a promising cool pavement strategy, as it enhances the surface’s capacity to modulate temperature. Thermochromic materials reversibly change color according to temperature, modifying surface albedo and increasing solar reflectance under high thermal conditions. The main objective of this research is to evaluate how the thermochromic capacity of asphalt pavement surfaces, functionalized through a surface spraying technique, influences the formation and mitigation of UHI effects. This dissertation is organized into four main parts. First, a bibliometric analysis followed by a systematic literature review investigates the current scientific panorama of thermochromic asphalt pavements’ functionalization, identifying research trends and knowledge gaps. Second, experimental studies evaluate the effects of thermochromic spray coatings applied to recycled asphalt mixtures on surface texture and cracking resistance. Laboratory analyses include colorimetry, texture characterization, pull-off adhesion tests, accelerated aging, and semi-circular bending (SCB) tests under long-term aging conditions. Third, outdoor experiments assess optical-thermal balance, cooling performance, and durability under environmentalexposure. Finally, a case study simulates the microclimatic impacts of the thermochromic strategy, including pedestrian thermal comfort, in different urban morphologies of a tropical city using computational fluid dynamics (CFD)-based modeling. Results indicate that thermochromic coatings reduce surface temperatures by 5–9 °C during peak solar radiation. The functionalization shows good adhesion, minimal changes in surface texture, and improved cracking resistance in aged specimens with 6% thermochromic content. However, accelerated weathering reveals reductions in solar reflectance over time, highlighting durability challenges. At the urban scale, simulations demonstrate surface temperature reductions of up to 9.6 °C and air temperature reductions of 1.7 °C. Nevertheless, thermal comfort analysis using the universal thermal climate index (UTCI) reveals that lower thermophysical variables do not necessarily result in improved daytime comfort, due to increased mean radiant temperature and the influence of urban canyon geometry. The findings confirm the potential of thermochromic pavements for UHI mitigation, while emphasizing the need for context-sensitive and integrated urban climate strategies.