## 4 Numerical Simulations and Modeling Based on Visualization Techniques
### Numerical Methods
At present, the general and technically mature discrete element software includes PFC, a commercial granular flow software developed by Itasca; MatDEM, a matrix discrete element software independently developed by Nanjing University; Yade, a large open-source discrete element software, and SudoDEM. The investigation of morphological characteristics allows for a better understanding of the mechanical behavior of aggregate particles. The volume or mass of the stone base material is composed of mineral aggregate particles, whose morphological characteristics allow the creation of a digital sample and a discrete element modeling [138]. [PERSON] et al. [139] used an artificially generated specimen of an idealized asphalt mixture. The behavior of a highly idealized asphalt mixture was simulated by different unit models in uniaxial compression creep tests. Meanwhile, [PERSON] et al. [140] investigated discrete unit models to simulate the behavior of highly idealized asphalt mixtures under uniaxial and triaxial compression creep tests, which used single-size spherical particles to model mineral aggregates and the highly ideal model to simulate the expansion and deformation of the asphalt mixture; however, the models were clearly not accurate enough in representing  
Figure 12: Transformation of the aggregate from images into discrete element models. the shape of the aggregates. Asphalt concrete or cement concrete mixtures were simulated by scanning laboratory-prepared samples, processing the images, and reconstructing the microstructure through image pixels. [PERSON] et al. [141] developed a microstructure-based 3D discrete unit model of asphalt mixtures to study the dynamic modulus in stress-strain response under compressive loading. The 3D model predicted better than the 2D model. The effect of different void fractions and aggregate modulus on the modulus of the mix was significant. [PERSON] [142] developed a computationally efficient DEM method that directly employs spherical harmonic functions to simulate 3D irregularly shaped particles. [PERSON] et al. [143] used the 3D discrete element method to study the combined effect of aggregate particle shape and angle on the accumulation and stability of the aggregate in asphalt mixtures, and conducted triaxial tests on different types of particle specimens and obtained the results when the shape and angle of the particles had a significant effect on their properties. [PERSON] et al. [144] used overlapping spheres to establish a DEM of irregular particle shapes and discussed the effect of parameters on particle shape resolution. [PERSON] et al. [145] used DEM to study the effect of particle shape on the shear localization of granular materials. [PERSON] et al. [146] used a parallel algorithm-based DEM system to simulate aggregate and cement structures. [PERSON] et al. [147] developed an algorithm for generating 3D aggregates and used the discrete cell model generated by the algorithm to predict the mixing modulus in different loading frequency ranges. [PERSON] et al. [148] used DEM to numerically simulate asphalt concrete specimens, considering coarse recycled aggregates' distribution characteristics. The results showed that temperature and loading rate significantly affect the damage strength. Meanwhile, [PERSON] [149] investigated the differences in mesoscale characteristics between irregularly shaped and proportioned the soil-rock mixture (SRM) models using the discrete element method, which can accurately predict the mechanical properties and deformation behavior of SRM. The pros and cons of FEM and DEM are summarized in Table 7.  
Figure 13: Flowchart of DEM modeling of asphalt mixtures. [PERSON] [150] proposed a novel model to obtain quad-sphere discretization in candidate aggregate spheres, thus enabling the introduction of local discussions around selected aggregate spheres.