Influence of Reclaimed Asphalt (RA) source on the effectiveness of rejuvenating agents by adopting a Model System

Reclaimed Asphalt (RA) represents the material obtained from the milling and crushing of old and damaged asphalt mixture employed in the construction of the flexible pavement. The recycling of RA in new asphalt mixture has become attractive thanks to its technical, economic, and environmental advantages. In fact, RA recycling leads to a reduction of costs, providing a contextual solution to disposal problems and the minimization of environmental footprint. However, when the recycling of RA exceeds a limiting amount (normally more than 50%), the final mixture will be characterized by poor performance tightly connected with the presence of the aged binder in the RA. Thus, to properly maximize recycling, the use of rejuvenating agents is required to restore the chemical and rheological properties of the aged and stiff RA bitumen. Actually, there are not any standards for the evaluation of the effectiveness of the rejuvenating agents and their principal effects related to the selection of the dosage, and its diffusion in the binder matrix. Two alternative approaches can be followed to investigate the effectiveness of such additives. The first is based on the chemical and rheological analysis of extracted binder from RA, adequately mixed with rejuvenating agents: this procedure requires the use of solvents that can damage the extracted binder. The second approach studies bituminous mixtures containing the rejuvenated RA: nevertheless, this procedure is time and money consuming. Although both methodologies have an important confirmation in the scientific literature, some concerns are still present. To overcome such disadvantages, a new approach to evaluate the effectiveness of the rejuvenating agents based on a Model System (MS) was developed in the Road Materials Laboratory of Politecnico di Torino. The Model System is constituted by a single-sized 5/8 RA material, selected to limit the presence of the finer fractions and RA cluster, ensuring the same granular composition for each MS. The MS was designed to obtain a constant value of the Voids in the Mineral Aggregate (VMA) to have a constant lithic structure and similar contact points between aggregates. Therefore, the mechanical behaviour of MS depends mainly on the binder phase, limiting the effect of aggregates interlocking. The VMA target selected is equal to 30%. The objective of the present work is to validate the previous investigations on the Model System using three different RA sources, combined with two different rejuvenating agents, in order to assess the possible dependency of the rejuvenating effect on the RA. It is important to highlight that the RA of each source has a different origin, age, gradation, bitumen type, among others. Regardless of the RA source, three different MS were prepared: (i) a black system made with 100% 5/8 RA; (ii) a rejuvenated system composed of 5/8 RA mixed with a rejuvenating agent; (iii) a white system constituted by the aggregates extracted from RA mixed with 50/70 neat binder, keeping constant the percentage of the binder characteristic of each 5/8 RA source. Once the optimal dosage was defined considering the linear viscoelastic behaviour, the cyclic direct tension test and the indirect tensile strength test were performed to study the performance at intermediate and low temperatures, respectively.