Experimental investigation of the feasibility of using recycled asphalt pavement (rap) and chromite waste containing magnetite in hot mix asphalt wear courses

Increasing traffic loads, dwindling natural aggregate resources, rising costs, and environmental concerns have made the use of sustainable materials mandatory in the hot mix asphalt (HMA) wearing courses commonly used in Türkiye; as a result, recycling practices that reduce the consumption of unprocessed aggregate and asphalt have become widespread. In this study, the feasibility of using recycled asphalt pavement (RAP) aggregates obtained by milling existing pavements and magnetite-containing chromite waste (MCT) generated during the chromite enrichment process as alternatives to natural aggregates in road wearing courses was investigated. The mixtures were designed based on the Type-1 wear course criteria of the Highway Technical Specifications, including a control mixture, mixtures with 10%, 20%, and 30% RAP content, mixtures with 50% and 100% MCT content, and hybrid mixtures containing both RAP and MCT. Density and void ratio analyses, Marshall stability and flow tests, indirect tensile strength (ITS) tests, as well as SEM and EDS analyses were performed on the samples. The results showed that as the RAP content increased, Marshall stability and flow decreased, while the Marshall quotient (MQ) increased and ITS values decreased. This finding indicates that the aged binder layer observed by SEM and the high carbon (C) content detected by EDS support the hard structure of the RAP. In mixtures containing MCT, the 100% MCT mixture exhibited higher stability and lower flow compared to the 50% MCT mixture, and showed lower ITS values compared to the control mixture. The results indicate that MCT’s fine crystalline structure and mineral composition, which is rich in Mg-Fe-Cr, contribute positively to resistance to deformation but do not yield a corresponding improvement in tensile strength. Upon evaluation of the findings, it was determined that the hybrid mixture containing 30% RAP-100% MCT meets the specification requirements and yields more balanced results in terms of performance parameters. This mixture has also yielded the best results in terms of Marshall performance, and it has been concluded that it may be possible to use it at even higher concentrations.