**1. Introduction**

Recycling the old asphalt pavement is a common operation used worldwide for saving natural materials, investment, and energy in highway maintenance projects [1–4]. In practice, the hot-mix recycled asphalt mixture (RHMA) has similar durability and performance to the conventional dense asphalt mixture if the RAP content is limited at 10–40%, depending on the design of mixture [5–7]. According to recent official data, China produces approximately 790 million tons of reclaimed asphalt pavement (RAP) per year through asphalt pavement rehabilitation projects [8]. This figure is expected to significantly increase in the next five years as the demand for road maintenance is increasing. Following the issuance of the most stringent environmental protection law in China's history, the exploitation of natural stone has been significantly restricted, leading to an estimated capacity reduction of over 60%. Consequently, in some parts of the country, such as the Jiangxi Province, the price of natural aggregate has nearly tripled in the past five years [9]. Thus, the Chinese transportation infrastructure sector faces years of ineffective reuse of accumulated RAP and increasing natural aggregate costs in future asphalt pavement rehabilitation projects. Therefore, increasing the RAP content in RHMA has attracted increasing attention over the past few years [10,11].

Although the aged asphalt binder in RAP is a considerable asset for construction, the incorporation of RAP could potentially reduce the durability in terms of fatigue resistance, low-temperature cracking resistance, and moisture stability due to its poor rheological

**Citation:** Zhou, J.; Li, J.; Liu, G.; Yang, T.; Zhao, Y. Recycling Aged Asphalt Using Hard Asphalt Binder for Hot-Mixing Recycled Asphalt Mixture. *Appl. Sci.* **2021**, *11*, 5698. https://doi.org/10.3390/ app11125698

Academic Editor: Amir Tabakovic

Received: 24 April 2021 Accepted: 17 June 2021 Published: 19 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

properties after volatilization and oxidation in the field [12]. To this end, the most widely and traditional solution is adding rejuvenators or using soft asphalt binder as the modifier to recover the performance of aged asphalt binder [13,14]. This solution was shown to be effective in previous publications and engineering projects. However, when RAP content reaches a high percentage (for example, over 50% by the weight of RHMA), the extra softening additives could reduce the rutting resistance of RHMA [15,16]. Thus, the use of a high percentage of RAP in RHMA has not been properly addressed in current recycling technology.

To overcome the negative effect of softening additives on RHMA, previous studies reported that the high-modulus asphalt concrete (HMAC) that originated in France in the early 1980s could be a potential solution [17]. The basic design concept of high-modulus asphalt mixtures is that through the use of hard-grade asphalt binder (with a penetration of about 10–20), fine aggregate gradation, high asphalt binder content (about 6%), and low porosity (generally 3%), the high-modulus asphalt mixture is obtained through the high modulus properties of the hard-grade asphalt binder itself [18]. For example, Ma et al. indicated that the HMAC has a higher binder/aggregate ratio, lower void content, and finer aggregate gradation compared to the conventional dense asphalt concrete [19]. The results show that the addition of RAP can be incorporated into HMAC at relatively higher contents, due to the similar stiffness between the aged asphalt binder in RAP and the high-modulus asphalt binder. To achieve good rutting resistance, hard asphalt binder is a suitable modifier of HMAC. In most cases, the hard asphalt is produced in refineries, and it also can be obtained by blending the virgin asphalt and natural asphaltite, for example, Trinidad lake or Gilsonite-like materials, whose penetration is in range of 8–28 (0.1 mm) [20]. Thus, the design concept of HMAC has the potential of incorporating more RAP contents in the asphalt mixture. However, using hard asphalt binder as the rejuvenator to recover the aged binder is rarely reported in previous studies. The traditional asphalt mixture regeneration technology is to add rejuvenator or soft asphalt to reduce the viscosity of the aged asphalt, thereby restoring its performance. Therefore, this topic may go against the concept of the traditional solutions. The objectives of this work can be summarized as follows.

