Enhancement and Optimization of Workability and Physical Properties of RAP Concrete Incorporating Silica Fume and Superplasticizer for Sustainable Construction

Reclaimed asphalt pavement (RAP) is a large but underutilized resource for sustainable concrete production; however, its use in structural applications is limited by concerns regarding reduced workability and durability. This study investigates the interactions between RAP and silica fume (SF) as well as superplasticizer (SP), and identifies optimal RAP concrete mixtures through the individual incorporation of SF and SP to enhance workability, durability-related indicators, water absorption, and density. RAP replaced 0–100% of coarse aggregate, SF was added at 0–21%, and SP at 0–2.1%, with a fixed water–cement ratio of 0.48. Six mix categories were prepared: control, RAP, SF, SP, RAP–SF, and RAP–SP. SF and SP were examined separately to isolate their interactions with RAP before they were combined with other cementitious materials. RAP increased slump via a lubricating effect but increased water absorption, with the density stabilizing at 50% RAP and peaking at 75% RAP due to improved particle packing. Although SF’s influence was limited by the fixed w/c ratio, in moderate-to-high (50–100%) RAP mixes it achieved very low water absorption (≤1.1%) and increased density (up to 7.6%), confirming its pore-refinement effect. SP achieved the greatest workability gains (up to 58% slump increase) at high RAP levels but contributed less to durability, highlighting SF’s stronger pore-refinement role. Most RAP–SF and RAP–SP mixes satisfied severe-environment durability limits, confirming their potential for sustainable, high-performance RAP concrete without compromising structural reliability.