*Article* **Mechanism and Performance of Bituminous Mixture Using 100% Content RAP with Bio-Rejuvenated Additive (BRA)**

**Anqi Chen 1,2, Yazhen Qiu 1, Xiangyu Wang 1, Yuanyuan Li 1,\*, Shaopeng Wu 3, Quantao Liu 3, Fan Wu 1, Jianlin Feng <sup>1</sup> and Zhuowei Lin <sup>4</sup>**


**Abstract:** The low RAP content, hot mixing conditions, and the addition of a high ratio of new bitumen and aggregates result in low economic and environmental benefits for current regeneration technologies. A bio-rejuvenated additive (BRA) that can fully (100%) regenerate the RAP without heating is proposed in this paper. To reveal the mechanisms of BRA-rejuvenated RAP, the effects of BRA on the chemical structure and molecular weight of the RAP were investigated using Fouriertransform infrared spectroscopy and gel permeation chromatography. The mechanical performance and water damage resistance of BRA-rejuvenated RAP were studied. Low contents of new bitumen or epoxy resin were suggested to increase the mechanical performance of 100% RAP. The results show that the 1.5% BRA-rejuvenated RAP had the best mechanical performance. The blending of BRA with recycled RAP is a completely physical process, without any chemical reactions. The molecular weight of BRA is lower than that of bitumen; it can substantially increase the content of light components in aged bitumen, and play the role of adjusting and restoring the balance of the components of aged bitumen. The mechanical performance of BRA-rejuvenated RAP is enhanced significantly by adding low dosages of new bitumen or epoxy resin.

**Keywords:** bio-rejuvenated additive; 100% rejuvenation; reclaimed asphalt pavement; regeneration mechanism; pavement performance

**1. Introduction**

Due to the good viscoelastic properties of bitumen, asphalt pavements are used as the main form of pavement structure worldwide [1]. Bituminous mixtures consist of aggregates, fillers, and organic bituminous binder. The bulk bituminous mixture can build strength after being paved and compacted on the road [2]. During service, the organic bituminous binder is aged by various effects, including environmental factors and vehicle loadings [3], leading to a gradual degradation of pavement performance and the occurrence of stresses in roads, such as cracks, spalling, ravelling, and potholes [4]. Typically, when the pavement condition index (PCI) of a bituminous pavement is lower than 75, the pavement must be repaired (or partially repaired) or reconstructed, resulting in a significant waste of bituminous mixtures [5]. Almost 35% of asphalt pavements in China needed to be repaired from 2016 to 2020, which produced more than 3.4 billion tons of reclaimed bituminous mixture (RAP), causing significant environmental harm and resource waste without being effectively regenerated. The construction of bituminous pavement requires large amounts

**Citation:** Chen, A.; Qiu, Y.; Wang, X.; Li, Y.; Wu, S.; Liu, Q.; Wu, F.; Feng, J.; Lin, Z. Mechanism and Performance of Bituminous Mixture Using 100% Content RAP with Bio-Rejuvenated Additive (BRA). *Materials* **2022**, *15*, 723. https://doi.org/10.3390/ ma15030723

Academic Editor: Giovanni Polacco

Received: 1 December 2021 Accepted: 5 January 2022 Published: 18 January 2022

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of petroleum bitumen and aggregate. In 2020, China's petroleum bitumen production was 62,796,000 tons, with approximately 85% applied to roads and airport runways, and the bitumen and aggregate consumption was estimated to be 53,377,000 tons and 1.07 billion tons, respectively [6,7]. Therefore, the recycling of RAP is of great significance, as it can not only reduce the consumption of petroleum resources, but also reduce the cost of bituminous concrete.

In recent years, the research on RAP has gained more and more attention. Yu [8,9] investigated the fracture properties of the mixtures containing up to 40% RAP with polymermodified bituminous (PMA) binder, and the results indicated that RAP mixtures with PMA exhibited lower damage accumulation rates than common RAP mixtures. The study of Juliana [8,9] to determine the variability of RAP properties obtained from different projects or sources indicated that there is significant variability in the performance of the RAP. Han [10] studied the impact of RAP on the pavement performance of bituminous mixtures. The findings indicated that the low-temperature performance and fatigue resistance decrease with the increase in RAP content; meanwhile, the water damage resistance of bituminous mixture first increases (with 0–40% RAP), and then decreases (>40% RAP). Wu [11] indicated that the bituminous mixture containing 40% RAP had the greatest shear strength. The RAP increases the potential for low-temperature cracking in bituminous mixtures [12]. Therefore, the RAP and its dosage have a significant influence on the pavement performance [10,13–15] and workability [16,17] of bituminous mixtures. To ensure that the bituminous mixtures containing RAP have good pavement performance [18], the "Technical Specifications for Highway Asphalt Pavement Recycling" in China indicate that the optimal RAP content should be between 15% and 30%, and most uses in actual engineering also do not exceed 30% [19,20]. Currently, both hot and cold recycled RAP technologies require the addition of new materials, such as new bitumen and aggregates [21,22], and the hot mixing process requires heating of both the new materials and the RAP (old and new aggregates are heated separately, with the new aggregates being heated to a higher temperature and acting as a secondary heat source to heat the RAP). These requirements result in high energy consumption [23,24] and low-content RAP recycling [25]. To increase the used content of RAP in bituminous mixtures, it is essential to stimulate and restore the technical properties of the aged bitumen covering the surface of the RAP [26]. The cold mixing technology can reduce the energy consumption and the elimination of lightweight components during heating, which can prevent acceleration of the aging of bitumen, and also reduce the volatility of volatile organic compounds (VOCs).

A bio-rejuvenated additive (BRA) for regenerating 100% RAP with cold mixing technology is proposed in this research. After mixing the BRA with RAP, the BRA can stimulate the aged bitumen, which is coated in a thin layer on the surface of the RAP aggregate, and restores its components and technical properties. To enhance the strength and toughness of BRA-rejuvenated RAP [27], low doses of new bitumen or epoxy resin were added to the 100% cold mixing of RAP with BRA to create a spatial three-dimensional network. Fourier-transform infrared spectrometer (FTIR) and gel permeation chromatography (GPC) were used to determine the chemical structure and molecular weight distribution of BRArejuvenated bitumen; the results can reveal the regeneration mechanism of BRA-rejuvenated RAP. In addition, the high-temperature performance, low-temperature performance, water damage resistance, and fatigue resistance of BRA-rejuvenated RAP were investigated extensively to evaluate the pavement performance of BRA-rejuvenated RAP.

#### **2. Materials and Experimental Methods**
