## 1 Introduction  
Pavement cracking is one of the common damages in asphalt pavements, which has an adverse effect on pavement durability. Visible cracks in the pavement structure usually initiate from local microcracks in the asphalt concrete. Since asphalt concrete is a composite heterogeneous material composed of asphalt mastic and mineral aggregates, the fracture behavior of asphalt concrete is affected by its internal composition and meso-structure [1, 2, 3]. A deep understanding of the relationship between the internal structure and fracture behavior of asphalt concrete is very important for sustainable and durable pavement design. In order to study the fracture behavior of asphalt concrete, numerous experimental studies have been carried out from a macro perspective. The three-point bending test is among the most commonly used test methods for fracture evaluation [4, 5]. Studies have shown that the mechanical behavior of asphalt concrete during pure mode I fracture and pure mode II fracture is well reflected through semi-circular bending (SCB) tests [6]. In addition, the SCB tests were also used to evaluate the fracture behavior of asphalt concrete containing recycled aggregates [7]. Although these experiments could provide the homogenized macroscopic mechanical properties of asphalt concrete, it is not easy for these experiments to reveal the internal damage mechanism of asphalt concrete from a meso or microscope. In order to further study the internal fracture behavior of asphalt concrete, experiments focusing on the local structures of asphalt concrete have been conducted. For example, pull-off tests and direct tensile tests have been used to study the fracture behavior of the interface between aggregate and asphalt mortar [8; 9; 10; 11; 12]. Table 1 offers a brief review of some of the experimental standards. These experiments provide quantitative indicators for evaluating the fracture behavior of asphalt concrete, but it is difficult for these experiments to identify crack initiation and propagation. In this case, the numerical models which involve the heterogeneous internal structure of asphalt concrete are more applicable. These numerical models are usually based on the extended finite element method (XFEM) or the cohesive zone model (CZM). Since the XFEM does not require special elements embedded at the element interface, the crack propagation is relatively flexible for XFEM, and this method has been widely used to investigate the fracture behavior of asphalt concrete [21; 22; 23; 24; 25]. Due to the heterogeneous internal structure, the damage evolution of asphalt concrete is usually complex, which includes adhesive damage at the aggregate-asphal interface and cohesive damage in asphalt mortar [26; 27; 28]. However, it is difficult for XFEM to simulate the interaction between the cracks in asphalt and in the aggregate-asphal interface [29]. Alternatively, CZM provides a convenient way for simulating damage and fracture evolution both at the interface and in asphalt mortar [30; 31; 32; 33]. The adhesion damage is usually achieved by introducing cohesive elements at the aggregate-asphal interface [34; 35; 36]. In order to avoid excessive computational cost, 2-D CZM analysis is usually used. The microstructure of asphalt concrete, such as aggregate shape and distribution, has an important influence on macroscopic properties [37; 38; 39; 40]. In order to reflect the microstructure of asphalt concrete in numerical models, numerical image identification and parametric modeling are commonly used. Numerical image identification can transform high-precision images of asphalt concrete into geometric models that characterize the microstructure of asphalt concrete [41; 42; 43]. However, this process often requires operational skills and expensive experimental equipment [30; 41].