Influence of compaction methods and active fillers on physical-mechanical behavior and performance of cold recycled mixture

Cold recycling technologies are emerging as important solutions for asphalt pavement rehabilitation, offering environmental sustainability and enabling the construction of new pavement layers with reduced greenhouse gas emissions and minimal material transport. The most common products of cold recycling technologies are cold recycled mixtures (CRMs). CRMs can be produced in mobile plants or directly in place, using up to 100% of milled material from a damaged existing asphalt pavement (reclaimed asphalt pavement or RAP) without the need to heat aggregates during the mixing stage. They are considered partially bonded materials with intermediate mechanical properties between hot mix asphalt (HMA) mixtures and unbound materials, exhibiting a temperature- and stress-dependent mechanical response, particularly influenced by confining pressure. The main objective of this research study was to improve the understanding of the mechanical behavior of CRMs, focusing on key factors affecting their properties, and therefore performance: laboratory compaction method and active filler type and content. The influence of compaction methods (modified Proctor, gyratory compactor, and vibratory hammer) on the physical-mechanical properties of CRMs was evaluated. Tests included density and moisture content measurements, volumetric analysis, indirect tensile resilient modulus (MR), indirect tensile strength (ITS), and shear properties obtained through the monotonic triaxial shear strength (TSS) test. In addition, results were analyzed in the context of long-term pavement performance. The findings indicated that not only the amount of compaction energy but also how it is applied significantly affects the physical and mechanical properties of the mixtures. In particular, the compaction method affects the moisture-density relationship, with moisture content playing a critical role in the mechanical response. Gyratory compaction yielded the highest overall physical-mechanical properties, whereas the vibratory hammer, though a viable alternative, tended to underestimate these properties, confirming that the two methods are not equivalent for CRMs evaluation. Additionally, mixtures compacted at the optimum moisture content (OMC) obtained from the Proctor method exhibited lower mechanical properties compared to those compacted at moisture contents derived from gyratory or vibratory hammer compaction. The effect of active filler type and content was also examined. In particular, cement and hydrated lime were incorporated at varying dosages in the production of CRMs with either foamed bitumen or bitumen emulsion. Mechanical behavior was evaluated through indirect tensile strength (ITS) and monotonic triaxial shear strength (TSS) tests, supported by a shear stress to strength ratio analysis in order to predict pavement performance in terms of rutting potential. In addition to analyzing the overall stress-strain behavior, key mechanical parameters such as peak stress, strain at peak, and initial stiffness modulus were examined. Results showed that the mechanical response of CRMs is strongly influenced by both the type and amount of active filler, as well as its interaction with the bituminous agent. Cement contents above 1.5% enhanced stiffness and cohesion, shifting the material from a granular to a more cohesive behavior, although with increased risk of brittleness. Hydrated lime, on the other hand, seemed to be effective only at contents between 2% and 3%, where it provided a favorable balance of strength, stiffness, and ductility. A 1% hydrated lime dosage appeared insufficient, likely due to poor particle bonding, while contents above 3% led to excessive stiffness and reduced overall performance. Overall, this research provided valuable insights into optimizing CRMs mix design and construction practices, highlighting the critical role of both binder-active filler combinations and compaction techniques in achieving durable, high-performing recycled pavements.