Steel slag asphalt mixture for cool pavements: evaluation of mechanical and thermal performance

Asphalt pavements absorb solar radiation during the day and store it, raising surface temperatures and contributing to the urban heat island effect. High-conductivity aggregates such as steel slag draw heat from the pavement surface into deeper layers, lowering daytime surface temperatures. However, the same conductivity returns that stored heat to the surface at night. This study evaluates a layered configuration—arranging steel slag by depth—to separate daytime cooling from night-time heat release, and characterises the mechanical and thermal performance of steel slag as a fine-aggregate substitute. Asphalt concrete mixtures incorporating 0%, 20%, and 40% steel slag were prepared and tested. Mechanical performance was characterised through wheel-tracking (rutting resistance), British Pendulum (skid resistance), and indirect tensile strength and retained Marshall stability tests (moisture susceptibility). Thermal behaviour was assessed through thermal conductivity measurements and 24-hour heating–cooling simulations under various layered pavement configurations to examine their influence on surface and near-surface air temperatures. The results showed that, mechanically, 40% of steel slag improved rutting resistance (rut depth −15.8%; dynamic stability +26.8%) and skid resistance (British Pendulum Number +24.2%), with moderately lower moisture resistance (TSR from 86.94% to 83.64%; RMS from 95.07% to 90.19%) that nevertheless remained within specification limits, and raised thermal conductivity by 30.9% relative to the control. In the laboratory 24-hour heating–cooling simulation, a full-depth configuration of 40% of steel slag reduced daytime surface temperatures by up to 5.21°C but raised nocturnal near-surface air temperatures by up to 2.40°C, indicating a day–night trade-off in full-depth steel-slag substitution. Layering mitigated this trade-off under the tested laboratory conditions: the SS-40–40–0 configuration (40% steel slag in the upper two layers over a 0% base) produced the largest daytime near-surface air-temperature reduction (−3.46°C) while limiting the night-time air-temperature rise to + 1.40°C, far below the full-depth case. Under the conditions tested, layer arrangement influenced the day–night temperature response more than slag content alone, indicating that steel-slag content and its placement through the pavement depth should be specified together in cool-pavement design.