Effects of a Bio-oil Additive on the Fracture Behavior of Heavy Polymer–Modified Asphalt Mixtures with High RAP Contents and Steel Slags

ABSTRACT This study explores the use of high percentages of reclaimed asphalt pavement (RAP) and steel slags in hot mix asphalt (HMA) for environmental and economic benefits. Although RAP reduces waste and offers economic advantages, technical guidelines often limit its usage to < 30 % because of performance variability with virgin materials. To address this, the study proposes techniques like recycling additives and polymer-modified binders, along with steel slags for stabilization. The goal is to design a mixture with 80 % recycled material in agreement with the European Union’s 2030 target for recycling, comprising 50 % RAP and 30 % steel slags. A bio-oil additive rejuvenates aged RAP bitumen, and a heavy polymer–modified asphalt binder ensures good rutting resistance. Three mixtures were tested: 0R (only virgin aggregates and heavy polymer–modified asphalt binder), 50R (50 % RAP, 50 % virgin aggregates, heavy polymer–modified asphalt binder, and additive), 50R30G (50 % RAP, 30 % steel slag, 20 % virgin aggregates, heavy polymer–modified asphalt binder, and additive) using Superpave indirect tensile test procedure and digital image correlation. The results showed that the presence of waste materials improved the mixture’s response in terms of strain accumulation over time. The mixtures with recycled materials exhibited lower creep compliance but maintained fracture energy thresholds well above average values for HMAs with similar aggregate gradations and medium polymer–modified asphalt binders. Strain concentration was observed in mixtures with recycled materials, whereas the mixture with virgin material displayed more evenly distributed damage. The bio-oil additive effectively reactivated aged RAP bitumen, allowing higher RAP content, and was compatible with steel slags. The heavy polymer–modified binder enhanced mixture elasticity and rutting resistance, promoting sustainable road construction practices using recycled materials.