Do all rejuvenators improve asphalt performance?

Abstract

Reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) contain significant amounts of bitumen and their application in new constructions have been promoted as a sustainable practice in the pavement industry and to promote circular economy. However, this effort has faced challenges such as the aged bitumen’s inferior properties compared to the virgin counterpart and its unknown contribution to new pavements. To address the latter issue, liquid additives were used under the general title of rejuvenators. That poses an additional challenge associated with the lack of clear metrics to differentiate between softeners and rejuvenators. While revitalising aged asphalt binder to regain its desirable properties would be an ideal solution, progress in this area has been limited by a lack of understanding of rejuvenation mechanisms at the molecular level and the absence of representative test parameter(s) to evaluate both efficacy and durability of rejuvenators. In this study, both laboratory experiments and computer modelling were combined to not only compare efficacy of several rejuvenators, but also explain why some rejuvenators are effective while others are not. It was found that all studied rejuvenators have a softening effect on aged asphalt, but only a few of them revitalised aged asphalts’ physicochemical and rheological properties at the binder level. At the mixture level, a lower number of gyratory compaction and reduced rate of crack propagation were observed after the addition of high performing rejuvenators to the asphalt mixtures containing RAP. The reduction in crack propagation rate could be attributed to both enhanced blending between an aged and virgin bitumen and the revitalisation of aged bitumen properties. The latter phenomenon was explained at the molecular level based on the rejuvenator’s ability to interact with aged asphaltene molecules intercalating into and separating asphaltene nanoaggregates. Accordingly, this molecular-level phenomenon is reflected in two ways: an increase in crossover modulus and crossover frequency at the binder level, and a reduced crack propagation rate at the mixture level.