EVALUATION OF AGED BITUMINOUS PAVING MIXTURES REJUVENATED WITH DIFFERENT RECYCLING AGENTS

Abstract

Sustainable utilization of petroleum crude oil and its byproducts (e.g., bitumen) is the need of the hour. The recycling technology of bituminous pavements dates back to 1930. However, the bitumen industry shifted its attention to pavement recycling during the 1973’s Arab Oil Embargo. Recycling bituminous pavements require a good understanding of the material’s characteristics. However, the rheological behavior of recycled binders is still not well understood. The present study investigates the recycled binders' rheological behavior and analyzes the behavior of rejuvenated recycled mixtures. Two types of Recycling Agents (RAs), viz. soft paving grade (VG-10) and commercial rejuvenators, have been used. In the first phase of the study, the target binder grade was a hard paving grade (VG-40), which is generally used as a target binder in many states of India. Three RAP sources (RAP-D, RAP-UP, and RAP-UK) and one RA (VG-10) were used to prepare three sets of recycled binders. The RAP content in the recycled binders varied from 0, 15, 25, 40, 65, 80, and 100% by weight of the total binder. The rheological characterization includes (1) the efficacy of the mixing rules, (2) maximum and minimum RAP content from G*/sinδ contour plot for any Performance Grade (PG), and (3) conceptualization of a novel technique to plot a master curve where shift factors were calculated based on RAP binder concentration. First phase results indicated that for RA like VG-10 and target binder grade VG-40, the maximum RAP content was around 20% only. However, the practical range of RAP in a batch mix plant ranges from 30 to 50%. The national average RAP content in the USA is 20-25% only. The Indian Roads Congress (IRC) specification (IRC: 120 2015) specifies the maximum 30% RAP content in the base course of the pavement. In the study's second phase, 40% RAP-D was incorporated with three RAs (rejuvenators). Under the aegis of the Prime Minister’s Fellowship for Doctoral Research (PMRF), an industry partner, i.e., Shell India Markets Pvt. Ltd., provided three propriety commercial rejuvenators. The results of the Saturates-Aromatics-Resins-Asphaltenes (SARA) test indicated that the first rejuvenator (rejuvenator A) had more than 90% saturates. The second and third rejuvenators (rejuvenator B and rejuvenator C) was a blend of resins and aromatics. The aromatic content in Rej-C was more than in Rej-B. A thorough study was carried out to understand the behavior of rejuvenators. The Optimum Dose of Rejuvenator (ODR) was determined based on physio-rheological testing of recycled binders. The target binder grade was a 60/70 Pen binder. The ODR of Rej A, B, and C was found to be 12%, 6.5%, and 22%, respectively. It was observed that ODR not only depends on the viscosity of the rejuvenator but also on the chemical composition of the rejuvenator. To observe the effect of steric hardening on the recycled binders, the optimum dosed recycled binders were kept in an environment chamber maintained at 25°C for a period of nine months. Penetration and Softening Point (SP) tests were conducted in three months intervals. The results indicated the effect of steric hardening in recycled binders. More specifically, rejuvenator B Recycled Binders (RB- B) showed a rapid rearrangement of molecules (steric hardening) than other recycled binders and 60/70 binders. It can be attributed to the major fraction of rejuvenator B, i.e., resins, which is a complex fraction. The rejuvenated recycled mixtures were prepared with ODR and subjected to three cycles of repeated recycling. The ODR was determined in each cycle, and samples were compacted using a Marshall compactor. The experimental campaign included Marshall Stability (MS), Indirect Tensile Strength (ITS), Work of failure (Wf), and Indirect Tensile Stiffness Modulus (ITSM) tests. In each cycle, the samples were tested before and after long-term oven ageing confirming to American Association of State Transportation Highway Officials, AASTHO R 30. The results indicated that rejuvenator C Recycled Mixture (RM- C) performed better than other recycled mixtures. The higher adhesive strength between RB- C, and aggregates can be attributed to the balanced composition of rejuvenator C i.e. 23% aromatics and 76% resins. However, RM- B showed a sharp increase in stiffness after long-term oven ageing. The rapid increase in stiffness can be attributed to the major fraction of resins and low concentration of aromatics. RM- A showed the least ageing susceptibility and the least mechanical strength. The reason could be the major fraction of saturates, which is inert by nature and didn’t contribute to improve the adhesiveness of the recycled binder. In the third and final phase of the study, an empirical relationship was developed between the two rutting parameters, viz., non-recoverable creep compliance (Jnr, kPa-1) and the Zero Shear Viscosity (ZSV, Pa.s) for a wide stiffness range of asphalt binders. A new dimensionless Steady-State Parameter (SSP) was introduced to characterize the steady-state behaviors of the asphalt binders.