Search

Funding Information: The authors would like to acknowledge the contributions of the active members of the RILEM technical committee TC 264-RAP in terms of experimental results, data analysis and proofreading. The authors would also like to thank Nynas AB, Kraton Chemical BV and Technical University of Braunschweig for supporting the interlaboratory activity by providing bituminous materials, ARA and RA binder. Publisher Copyright: © 2022, The Author(s). This recommendation is based on the results of an inter-laboratory study organised by the RILEM technical committee TC 264-RAP "Asphalt Pavement Recycling"—Task Group 3 (TG3) focusing on Asphalt Binder for Recycled Asphalt Mixture. The TG3 aimed to evaluate the effect of a specific family of materials known as asphalt recycling agent (ARA) on the aged binder under different configurations. Even though ageing is an irreversible phenomenon, effective ARA must have the capability to improve the flexibility of the bituminous materials and their resistance against cracking susceptibility with no adverse effect on the rutting resistance of pavements containing reclaimed asphalt. A total of 17 participating laboratories analysed the properties of binder blends composed of aged binder from reclaimed asphalt in three different contents (60, 80, 100%), ARA and virgin binder. The physical properties of the blends were thoroughly evaluated through traditional and rheological binder testing. This recommendation proposes to restore the original material properties at low and intermediate temperatures (i.e. cracking resistance) while balancing the high-temperature characteristics (i.e. rutting susceptibility) with durable impact throughout the progression of ageing phenomena. Therefore, useing of the Dynamic Shear Rheometer is foreseen as a more suitable and sustainable means to evaluate binder blends containing an asphalt recycling agent. Compared with conventional testing, the proposed approach requires fewer materials while resulting in a faster experimental procedure with one single test.

The use of rejuvenators has seen a consistent increase over the years in the asphalt pavement industry. This is due to the need for maximizing the demand for incorporating a higher amount of Reclaimed Asphalt (RA) in the pavement asphalt mixtures. In order to tackle this challenge, the Task Group 3, focusing on asphalt binders and additives, of the RILEM TC RAP conducted an interlaboratory activity to evaluate the effect of aging on blends of virgin and rejuvenated RA binders. A set of conventional tests including penetration value at 25 °C, softening point temperature and rheological measurements by means of Dynamic Shear Rheometer (DSR) were selected to involve a large number of participants. A binder, recovered from field RA, was treated with a bio-based rejuvenator and blended with a virgin binder to simulate recycling at three different percentages 60, 80, and 100%. These blends, as well as the pure virgin binder, were next short and long term aged and tested to evaluate the changes in the corresponding properties. Relatively consistent results were obtained for the entire set of blends. The aging of combined rejuvenated RA and virgin binders was comparable to that experienced for the pure virgin binder. DSR data provided a more precise evolution of the impact of aging on the materials.

Rotational viscosities of different asphalt binders were determined at temperatures between 80°C and 185°C. Viscosity–temperature dependence of asphalt binders was described with the use of the Vogel–Tammann–Fulcher (VTF) and the William–Landel–Ferry (WLF) equations. The Vogel temperature ( Tv) and the glass transition temperature ( Tg) for different asphalt binders were determined by fitting experimental values of viscosity at different temperatures with these two equations. For asphalt binders, the difference between Tv and Tg was about 40K. Effects of asphaltenes, aging, chemical modification, and polymer content on these temperatures were evaluated. As asphaltene content increased, both temperatures, Tv and Tg, increased. Different polymers showed different effects on these temperatures. The values of Tv and Tg were correlated with the critical cracking temperature ( Tcr) determined through use of a bending beam rheometer and a direct tension tester. The results suggested that the correlations between Tv, Tg, and Tcr could be used to determine Tcr from the rotational viscosity results tested at high temperature. With simple rotational measurements, a quick estimation of Tcr of asphalt binders could be obtained. Liquid fragility theory was also used to study Tg of asphalt binders. Parameters determined with the VTF and WLF equations indicated that asphalt binders behaved as fragile liquids because of their non-Arrhenius behavior in the temperature range studied.

The use of crumb rubber modifiers (CRMs) to change asphalt binders was evaluated. Blending two base asphalt binders of performance grade 70-22 and 64-22 with three sizes of a CRM at two rubber contents resulted in varying grades of asphalt binders. In addition to these binders, two binders modified with reacted crumb rubber using a patented process were included in the study. Testing was conducted at different temperatures, testing frequencies, and strain conditions, using methods recently developed as part of the NCHRP 9-10 project (Superpave Protocols for Modified Asphalt Binders) for complex binders. The new testing includes the particulate additives test (PAT), which determines the volume of particulate material greater than 75 μm in asphalt, and the laboratory asphalt stability test (LAST), which measures the potential for separation and degradation of additives in asphalt. Strain sweeps, fatigue testing, and low-temperature failure and creep properties were also measured. The results indicate that the concentration and size of crumb rubber affect the viscosity of the resulting modified binders significantly and influence the volume of residue collected in PAT. It was observed that concentration and size of rubber are, however, not important compared with other factors in LAST and that keeping the binders agitated during storage can reduce separation significantly. On the basis of the rheological testing, it was found that both fatigue and strain dependencies are highly sensitive to asphalt binder type, rubber size, and rubber content. The failure properties of CRMs measured using the direct tension test indicate that failure stress and strain are reduced by increasing rubber size and concentration.

In the current Superpave® binder tests, the rheological properties of asphalt binders are determined by the dynamic shear rheometer. The asphalt binders are tested using a gap size of either 1 or 2 mm. In the asphalt mixture, the thickness of the asphalt film between aggregates can vary between a few microns and a few millimeters. Because of the interaction between the asphalt and the aggregate, rheological behavior in thin films can differ from the behavior determined at large gap sizes. In this study the rheological properties of asphalt binders were measured at different gap sizes that simulate the range in film thickness of a typical mixture. Significant changes in rheological properties were observed as the gap size changed. The results indicate that different modified binders show different trends as the gap size changes. A review of the literature was conducted for an explanation of this behavior. Four mechanisms were offered to explain similar gap effects observed for other materials. Using the data collected in this study, the disentanglement mechanism caused by adhesion to testing plates appears to fit the observations for asphalt binders. The results suggest that the current gap settings in the binder tests might not be sufficient to determine the performance of the binder in the mixture. The findings also suggest that more work is required to understand the effect on the rheological behavior of asphalt binders of physical and chemical interactions with aggregates.

The Laboratory Asphalt Stability Test (LAST) was introduced as part of the NCHRP 9-10 project to evaluate storage stability of asphalt binders. The unique design allows varying the temperature, time of storage, and level of agitation during storage and allows sampling of the material continuously throughout the test. The test design is based on scaling down one of the commonly used storage tanks used for modified asphalts in the field. The test results can be used to guide the production of the modified binders and determine the performance of these binders during the construction process. This paper discusses the design and introduces the details of the testing protocol proposed to evaluate thermal stability of binders. The protocol follows a systematic procedure for detailed evaluation of effects of phase separation and possible degradation during storage. A set of parameters is proposed for use as indices to reflect the potential of binders for phase separation and/or thermal degradation. A large number of binders have been tested with the LAST and the results are summarized and discussed in this paper. The results indicate that binders can vary significantly in their thermal stability characteristics and thus their stability characteristics need to be measured and considered in selection of binders for different applications. The LAST results indicate that some binders are very stable for extended times, others can be kept homogeneous only with minor agitation while others require more rigorous agitation. Time for separation is an important factor that should be considered. The results also indicate that agitation can result in degradation of modifier and/or significant additive hardening. The rheological measurements conducted indicate that phase separation and/or degradation appear to have influences on rheological type of binders, such that frequency dependency of temperature dependency changes with storage. (A)

In a recent survey of users and producers of modified asphalts, stability and short-term aging were two of the main concerns regarding the use of modified asphalts. In an NCHRP project, a concentrated effort was put into development of revised or new testing procedures to better characterize the nature of modified asphalt and to solve some of the difficulties with using existing aging procedures developed for neat asphalts and modified asphalts. Three main procedures have been proposed to complement the existing Superpave procedures: the laboratory asphalt stability test, for measuring the storage stability of asphalt binders; the particulate additive test, for separation of particulate additives; and the modified rolling thin film oven test for short-term aging. The background behind the development of these test methods is explained, and typical data collected for a number of modified asphalts are presented. The results indicate that these tests show high promise in covering characteristics not covered by the current Superpave binder specifications and in solving some of the problems with the existing aging methods. The results also indicate that the behavior of modified binder can be very complex and that the method of data interpretation is very critical.