# Influence of Different Fibers on Performance of Bitumen Binders and Thin-Overlay Bitumen Mixtures
## 1 Introduction  
As reported in the statistical bulletin detailing advancements in the transportation sector, China's expressway network expanded to an impressive 122,300 km by the close of 2023 [1]. This places the nation at the forefront globally in terms of highway mileage. Asthe longevity of current roadways continues to rise, road maintenance technology is also evolving. One prevalent technique in road upkeep is the application of thin overlays, which can be directly laid over the existing pavement without the need for milling, leading to considerable savings in both construction expenses and operational complexity [2; 3]. However, because of their inherent structural properties, thin overlays are susceptible to cracking under traffic stress, which ultimately shortens their lifespan [4; 5]. Consequently, improving their resistance to cracking is important to enhancing the durability and effectiveness of these thin-layer applications. Researchers have enhanced the bitumen mixture to slow the onset of cracking issues, prolong the maintenance intervals of the pavement, lower upkeep expenses, and ensure reliable, long-term performance of the bitumen surface [6; 7]. Simultaneously, investigators utilize numerical models such as the cohesive zone model and the phase field method to explore and evaluate the fracture mechanics and damage evolution in altered bitumen mixtures [8; 9]. Currently, the predominant approaches for improving crack resistance and toughness in bitumen mixtures involve both chemical and physical modifications. Chemical techniques primarily enhance bitumen by incorporating substances such as epoxy resin, activated rubber powder, polyurethane, and various other polymers. [PERSON] et al. [10] used crb to toughen epoxy bitumen mixture. CR particles enhance the toughness of epoxy bitumen mixtures, and the mixtures with less CR are susceptible to sudden fractures due to brittle coarse aggregates. [PERSON] et al. [11] examined the impact of three different polymers--high-viscosity modifier (HVM), high-modulus modifier (HMM), and anti-rutting agent (ARA)--on the toughness of mixtures. Each mixture's toughness was assessed through fracture energy (G\({}_{j}\)) and the flexibility index (_F1_). Following HMM modification, the bitumen exhibited enhanced viscosity, and the blend displayed optimal toughness. [PERSON] et al. [12] incorporated waste tire rubber (WTR), high-density polyethylene (HDPE), and styrene-butadiene-styrene block copolymer (SBS) into bitumen. Subsequent testing revealed that the inclusion of HDPE boosted both the hardness and modulus of the bitumen binder, leading to a notable enhancement in the fatigue performance of the bitumen mix. Additionally, the synergy of WTR, HDPE, and SBS improved the bond and cohesion between the binder and the aggregate particles, further improving the mixture's resistance to cracking. These polymers interact with the bitumen, resulting in the formation of a three-dimensional cross-linked network structure [13; 14; 15]. This structure restricts the mobility of unbound bitumen, increases its viscosity, and effectively bolsters the crack resistance of the bitumen mixture [16]. Nevertheless, in real-world scenarios, it has been observed that as the strength of the altered bitumen mixture rises, it becomes more susceptible to brittle failure [17]. To tackle this issue, researchers have incorporated toughening agents and various modifiers to enhance composite materials, striving to strike a balance between crack resistance and the toughness of the blend. Nevertheless, the expense associated with composite modification is considerable, the procedures tend to be quite complex, and the modified components are often susceptible to environmental influences, which can diminish their effectiveness [18; 19]. Physical modification focuses on improving the strength and modulus of the mixture by integrating nanoparticles or fiber materials, ultimately enhancing the crack resistance of bitumen pavements [20; 21; 22].