RCA–RAP Hot-Mix Asphalt Mixtures: Mass–Volume Non-Equivalence and Recycled Binder Availability in Mix Design and Performance Interpretation

Combining recycled concrete aggregate (RCA) and reclaimed asphalt pavement (RAP) in hot-mix asphalt (HMA) creates a coupled aggregate–binder design problem. This review critically synthesizes RCA–RAP studies, supplemented by relevant single-material evidence, to clarify how material-specific mechanisms govern mix design and mixture performance. Differences in aggregate bulk-specific gravity (Gsb) make mass- and volume-based replacement non-equivalent, while RCA breakage and RAP agglomerate dissociation modify gradation through distinct mechanisms. RAP binder content does not directly represent recycled binder availability (RBA), and binder absorption by RCA further alters effective binder content. Consequently, optimum asphalt content (OAC) cannot be compared meaningfully across studies without considering the replacement basis, RAP-binder accounting, and RCA absorption. Performance evaluated at the respective OACs likewise reflects concurrent changes in material composition and binder content rather than isolated RCA or RAP effects. Rutting resistance generally improves, whereas fatigue, low-temperature cracking, and moisture resistance show less consistent trends; at high replacement levels, improved rutting resistance may coexist with voids in mineral aggregate (VMA) or voids filled with asphalt (VFA) below design limits. Current evidence remains insufficient to isolate the effects of gradation adjustment and measured RBA or to establish statistically supported RCA × RAP interactions. Future studies should employ controlled mix-design comparisons and replicated factorial designs in which RCA and RAP contents are varied independently to quantify their main and interaction effects.