Towards sustainable roads: A State-of-the-art review on the use of recycling agents in recycled asphalt mixtures

The use of asphalt mixtures containing previously paved asphalt mixture from milled pavements, so-called reclaimed asphalt pavements or RAP, has been around for more than half a century. Historically, relatively low RAP contents were used, around 15% of the total mix. Recently, the use of high RAP contents has increased and now it is common to design mixtures with 30% or higher RAP contents, especially in the US. At these higher recycled contents, the characteristics of the recycled material begin to significantly affect the behaviors of the asphalt mixtures and alter the pavement performance. The use of recycled materials in asphalt mixtures at these contents may result in diminishing returns in terms of prospective performance at or beyond a certain incor-poration dosage level. To overcome this barrier, recycling agents (RA) have been proposed to extend the break-even point and/or provide savings, both in terms of cost-effectiveness as well as environmental impacts. This paper is an examination of the use of RAs in recycled asphalt mixtures. There have been many reviews regarding RAs in the recent past, notably the review compiled as part of the United States National Cooperative Highway Research Program (NCHRP) Project 09-58. However, this field is evolving very quickly and there have been notable advances that require a renewed examination of the state of the art with respect to RA usage. In these intervening years, some specific advances have been made to propose novel methods for understanding the chemical nature of RAs, how RAP interacts with the virgin binder and RA, and how acceptable performance of recycled asphalt binder blends and mixtures can be achieved through dosage selection methods. However, the scientific knowledge on the selection and use of RAs in asphalt pavements is limited, and there is a need to develop robust methodologies that establish threshold criteria and performance metrics to facilitate their use on a regular basis. While there do exist some general trends with respect to RA effectiveness, the specific benefits and limitations found in individual studies vary greatly. Thus, given the current state of knowledge, RAs should be evaluated on a material-by-material basis using asphalt mixtures designed and delivered using local practices.