A Highly Circular Asphalt Surface Mixture with Steel Slag Aggregates and Reclaimed Asphalt Pavement: Laboratory-to-Field Validation and Life Cycle Assessment

The increasing demand for sustainable road infrastructure has encouraged the development of asphalt mixtures incorporating recycled materials and industrial by-products. This study developed and validated a highly circular AC14 asphalt surface mixture incorporating steel slag aggregates (SSA) and reclaimed asphalt pavement (RAP). The laboratory-designed mixture contained 63.8% SSA and 17.2% RAP, corresponding to 81.0% secondary materials, or 83.0% when recovered filler is included. Its volumetric and mechanical performance was compared with that of a conventional AC14 surface mixture with natural aggregates. The highly circular formulation was then produced in an asphalt plant and applied in a full-scale field trial. A life cycle assessment (LCA), following EN 15804:2012+A2:2019, and a production-stage cost analysis were conducted using plant-specific data. The highly circular mixture showed improved rutting resistance, higher stiffness modulus, very high resistance to water damage, and better fatigue indicators than the conventional reference mixture. The field trial supported its feasibility under real production and construction conditions. The LCA showed reductions in 12 of the 13 product-stage environmental impact indicators, including reductions of 18.1% in total global warming potential, 26.6% in abiotic depletion potential for fossil resources, 77.6% in abiotic depletion potential for minerals and metals, and 81.5% in water deprivation potential. The estimated production-stage unit price was 36.4% lower than that of the conventional mixture and 45.4% lower than the Portuguese market benchmark. These results demonstrate the technical, environmental, and economic potential of highly circular asphalt surface mixtures incorporating SSA and RAP.