The increasing accumulation of plastic and rubber waste in Algeria poses significant environmental challenges, highlighting the need for sustainable recycling solutions in road construction. This study investigates the influence of waste plastic (WP) and waste nitrile rubber (WNR) used as partial bitumen replacement materials on the mechanical and volumetric properties of asphalt mixtures. The bitumen binder was modified by incorporating 1%, 3%, and 5% (by weight of bitumen) of each additive. Asphalt mixtures were then prepared with these binders and evaluated through volumetric properties (bulk density, air voids (Va), Voids in Mineral Aggregate (VMA), and Voids Filled with Bitumen (VFB) and Marshall parameters, including Stability (MS), Flow (FM), and Marshall Quotient (MQ). A conventional 0/14 asphalt mixture prepared with unmodified 35/50 penetration-grade bitumen served as the control mixture. The results indicate that at an additive content of 5%, WP and WNR increased Marshall Stability by 22% and 37%, respectively, and decreased Flow by 15% and 33%, respectively, compared with the control mixture, indicating enhanced resistance to permanent deformation. However, distinct behaviors were observed. WP mainly acts as a stiffening agent, resulting in moderate mechanical improvement and stable volumetric characteristics. In contrast, WNR exhibits a swelling effect that increases the effective binder volume, leading to lower air voids, higher VFB, and improved mixture densification. Consequently, WNR-modified mixtures show higher Stability and MQ values. Despite these advantages, excessive WNR content may result in overly dense mixtures with very low air voids, potentially affecting durability. Generally, WNR provides a more effective improvement through combined densification and viscoelastic behavior, while WP primarily contributes through increased stiffness.
