Optimizing the Performance of Asphalt Mixes with High Reclaimed Asphalt Pavement Content Using Rejuvenators

Abstract

Reusing reclaimed asphalt pavement (a.k.a., RAP) has become a routine in the production of asphalt paving materials. Theoretically, RAP is a hundred percent recyclable material since its main components are high-quality, graded aggregates coated with durable asphalt binder. Unfortunately, due to the presence of the aged asphalt binder, most agencies only approve a limited percentage of RAP, either less than 20% to 30% of RAP (by weight of the total mix) or less than 25% of RAP binder replacement (by weight of the total binder content), to be incorporated in new asphalt mixes. For decades, pavement researchers have shown great enthusiasm for promoting the RAP rate in asphalt mixes. One of the most promising approaches is to use rejuvenators to restore the aged asphalt binder to a comparable or even better performance than the virgin asphalt binder. However, there is still a lack of knowledge and practice in using rejuvenators in producing sustainable and durable recycled asphalt mixes. The gap is not only limited to the type of rejuvenators to be used, but also to the optimal dosage, compatibility with virgin and RAP binder, stability under the elevated temperature atmosphere, mix design and manufacturing process, and uncertainties about the long-term performance of the revitalized high RAP mixes. As a result, this research was initiated to investigate the use of rejuvenators for optimizing the performance of asphalt mixes with higher RAP content. This study used seven commercial rejuvenators provided by various suppliers, as well as materials collected at local asphalt plants and refineries for the experimental work, which included six bio-based oils, one petroleum-based oil, one processed RAP, graded virgin aggregates, and two virgin asphalt binders. A sequential screening methodology was applied to evaluate the rejuvenating efficacy of these rejuvenators and a soft binder using various binder and mix tests and data analysis tools. In the first part of this research, rejuvenated binder blends with different RAP binder ratios (25%, 50%, and 75%) and rejuvenator dosages (0%, 5%, and 10%) were prepared to select candidate rejuvenators with better restoration capacities in terms of reducing the viscosities at mixing and compaction temperatures as well as recovering the continuous performance grade (PG) temperatures, non-recoverable compliance, and crossover temperature measured binder testing using the Dynamic Shear Rheometer (DSR) and the Bending Beam Rheometer (BBR). Four bio-based rejuvenators were selected in the first stage and used for dosage optimization through the blending chart method and the response surface modelling (RSM) method. The blending chart method assumes a linear relationship between the rheological index and two variables, rejuvenator dosage and RAP binder ratio. Therefore, the optimal rejuvenator dosage can be determined when the rejuvenated binder blend resembles the same rheological index as a target virgin binder. Meanwhile, the response surface model was built based on a two-level factorial design, which involved the different rheological indices as the responses and the rejuvenator dosage and RAP binder ratio as the two factors. This method can verify the linear relationship and then the model can be used to calculate the optimal dosages. From the results, the blending chart method was proved to be reliable because most of the rheological indices were expected to have a linear relationship with the rejuvenator dosage and the RAP binder ratio. Furthermore, the recommended indices, the low PG temperature based on m-value (PGLm) and the crossover temperature (Tcross), exhibited the potential to ensure that the rejuvenated binder blends have adequate cacking resistance without compromising the permeant deformation resistance at high temperatures. In the mix testing phase, the Hamburg Wheel-Tracking (HWT) test results revealed concerns about increased permanent deformation and moisture susceptibility due to the additions of rejuvenators in high RAP mixes at both test temperatures, 44°C and 50°C. However, the rejuvenated high RAP mixes outperformed the control mix (with 20% RAP), particularly at the higher test temperature. The cracking test results obtained from the Indirect Tensile Asphalt Cracking Test (IDEAL-CT) indicated that the addition of rejuvenators or replacing the base binder with a softer virgin binder was beneficial in improving the cracking resistance; however, more testing results, preferably from plant trial mixes, are needed for statistical analyses. Nonetheless, the rejuvenated high RAP mixes achieved decent high-temperature stability and acceptable cracking resistance, especially after an extended oven conditioning period. The dynamic modulus test was used to investigate the effect of increased RAP content with the use of a rejuvenator (or a soft binder) on mix rheology at a variety of temperature and loading frequency combinations. The measured modulus and phase angle data were analyzed using two master curve models and several time-temperature-dependent shifting equations. Additionally, the long-term performance of the asphalt mixes was assessed by comparing test results obtained from samples before and after the long-term oven aging (LTOA) protocol. The high RAP asphalt mix with the addition of a rejuvenator showed the most stiffness increase after long-term aging; however, it retained relatively better flexibility and temperature sensitivity compared to other mixes. According to the findings of this study, using rejuvenators in asphalt mixes with a high RAP content can produce a recycled HMA with balanced rutting and cracking performance and long-term sustainability. Whereas in the high pavement in-serve temperature range, extra care should be taken for moisture susceptibility.