Rheological, microscopic, and chemical characterization of the rejuvenating effect on asphalt binders
Introduction
As of 2011, more than 40 state agencies in the US allow to use above 30% reclaimed asphalt pavement (RAP) in mix design of Hot Mix Asphalt (HMA) [1], however, currently the average RAP use is still between 10% and 20% [2]. According to the Superpave mix design specification (AASHTO M 323), when more than 15% RAP is used in the mix design, it is required to reduce the binder performance grade (PG) by one to compensate for the aged RAP binder. Furthermore, when adding more than 25% RAP, virgin binder PG has to be determined based on the properties of extracted RAP binder. Such requirements are set to ensure that the virgin binder compensates for the deteriorated mechanical properties of the RAP binder in order to avoid cracking failures in the asphalt pavement. However, this also leads to additional expenses for contractors in terms of purchasing unconventional binder grade, installing additional hot storage tanks, maintaining a laboratory with extraction and testing equipments or outsourcing of the required additional testing. In many cases, this discourages the contractors to produce high RAP content mixtures because the savings from a relatively small increase in RAP content are outweighed by the increased expenses.
The use of rejuvenators (i.e., products that are used for restoring the mechanical properties of aged binders) is a relatively poorly understood alternative for mix design of RAP-containing HMA. This is primarily due to concerns associated with their ability to diffuse in the RAP binder and to provide the required long term performance for another service period of the pavements [3]. However, introducing rejuvenators provides major benefits as compared to merely bumping the virgin binder grade as follows:
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Unrestricted RAP content using a single rejuvenator;
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Cheap storage, since in most cases rejuvenators do not require heating;
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Simple addition to the mixture using volumetric pump or existing liquid additive dosage system;
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Ability to add the precise required dose based on the RAP binder properties.
These advantages would help increase the average amount of RAP used in HMA asphalt pavements if the rejuvenating mechanism is better understood. Previous studies suggest that rejuvenators should replenish the volatiles and light chemical fractions that have been lost during aging of pavements [4] by providing a homogeneous system where asphaltenes are well dissolved and prevented from precipitation or flocculation [5]. Other research has shown that a better rejuvenating effect can be attained with high amounts of resin or aromatic fractions [6]. However, a systematic approach of exploring how the rejuvenator modifies the chemical, microscopic and mechanical properties of the aged asphalt binders is still absent.
In this study, two virgin asphalt binders from different crude sources were aged using both Rolling Thin Film Oven (RTFOT) and Pressurized Aging Vessel (PAV) methods. In order to verify the capability of rejuvenators to restore the mechanical properties of the aged binders, two different rejuvenators were added into the aged binders. For better understanding of the rejuvenating mechanism at both macro and micro scales, rheology, surface microscopic properties, as well as chemical fractions of the virgin, aged and rejuvenated asphalt binders were characterized using dynamic shear rheometer (DSR), bending beam rheometer (BBR), atomic force microscopy (AFM), and SARA (Saturates, Aromatics, Resins, Asphaltenes) fractionation.
Section snippets
Virgin asphalt binders
Two types of asphalt binders, AAD (PG 58–28) and ABD (PG 58–10), were obtained from the Materials Reference Library of the Strategic Highway Research Program (SHRP). They were chosen because of variations in their crude source, SARA fractions, chemical elemental analysis, physical and mechanical properties, as indicated in Table 1, Table 2. Between the two virgin binders, AAD is a “softer” binder with a higher wax content and higher asphalthene content in comparison to ABD.
Rejuvenators
Two generic
Properties at intermediate and high temperatures
The rheological properties of the virgin, aged, and rejuvenated asphalt binders at intermediate and high temperatures obtained from DSR are shown in Fig. 2.
Virgin ABD was stiffer (higher complex modulus) yet more viscous (larger phase angle) than virgin AAD as shown in the master curves, which supports the results in the SHRP report (Table 2) [7]. For both binders, aging resulted in an increase of the complex modulus and a decrease of the phase angle (corresponding to a lower complex
Conclusions and future work
With the increasing interest in increasing the RAP content to produce more sustainable asphalt pavements, various rejuvenating agents have been added into the mix design so as to restore the rheological and mechanical properties of RAP materials. This is an important step for service-life extension of asphalt concrete pavements made with higher percentages of RAP. In order to better understand the rejuvenating mechanisms, two commonly used rejuvenating agents, WV oil and aromatic extract, were
Acknowledgements
The authors would like to thank R. Mallick (WPI), M. Tao (WPI), T. El-Korchi (WPI), S. Granados-Focil (Clark University), R. Frank (RAP Technologies), and D. Pelegrino (WPI) for their generous help in executing this research, and N.A. Burnham (WPI) for her detailed suggestions on AFM data analysis, as well as A. Lojoie (WPI) for her proof-reading of the manuscript.
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