High-thermal-conductivity steel slag asphalt pavements for road cooling

High surface temperatures in asphalt pavements, primarily caused by solar radiation absorption, accelerate material degradation and increase urban heat load. Incorporating steel slag (SS), a high-thermal-conductivity by-product of the steel industry, as a substitute aggregate offers both environmental and functional benefits. Its high conductivity can promote rapid heat transfer to the subgrade and underlying pavement layers, effectively acting as a thermal sink. This study investigates the thermal performance of SS asphalt mixtures for road cooling through laboratory testing and finite element (FE) heat transfer modelling validated under various material configurations and environmental conditions. Laboratory results show that increasing SS content improves the thermal conductivity of asphalt mixtures. In particular, mixtures containing 40% slag exhibited a 30.8% increase in conductivity and lowered daytime surface temperatures by up to 6 °C at a depth of 1.5 cm. However, higher conductivity also increased nighttime heat retention by slowing the release of stored heat. Simulations indicate that this drawback can be mitigated through several strategies, including placing the slag layer in lower pavement courses, using a lower-conductivity material at the surface, and providing a thermal buffer. Furthermore, it can also be addressed by integrating SS asphalt with other cool pavement technologies, such as heat-reflective coatings (HRCs) and permeable pavements (PPs). Reflective coatings with SS asphalt yielded daytime surface temperature reductions exceeding 20 °C compared to conventional pavements, while also limiting nighttime heat retention by enhancing radiative cooling. Permeable surfaces further moderated both daytime and nighttime temperatures by utilising moisture for evaporative cooling and buffering subsurface heat release. Overall, this study demonstrates the potential of SS asphalt mixtures to enhance road cooling, with performance strongly influenced by subsurface heat storage and surface emissivity and further improved when integrated with other cool pavement technologies.