## 5 Applications and Outlook
### Compaction Method of Aggregate Specimens  
The application of different compaction techniques and compaction environments to aggregate specimens may vary considerably. The performance of asphalt mixes is greatly influenced by the quality of the compaction [48]. Excessive or premature permanent deformation and deterioration damage are often the results of inadequate compaction. Commonly used laboratory compaction methods include SGC, vibratory compaction, and plane compaction [92]. [PERSON] [161] et al. used SGC compaction, vibratory compaction, and field compaction to investigate the mechanical properties of asphalt mixtures from stiffness, modulus, permanent deformation, and fatigue resistance after compaction. The results showed that the mechanical properties were not the same after the same degree of compaction.  
The Marshall impact compaction method [162], as the first and most widely used method, takes into account volumetric parameters as well as Marshall stability and flow rate, etc. The SGC method is usually [163] used for testing purposes such as analyzing volumetric properties of asphalt mixtures, evaluating mixture denseness, estimating sensitivity to aggregate shape, and field quality control. Specimens using the SGC method can well simulate the density, aggregate orientation, and structural characteristics of asphalt mixtures on real pavements [109]. Both methods have been used extensively by domestic and international researchers for evaluating and analyzing the performance of asphalt mixtures, and the two methods have been compared to complement each other [164; 165].  
The purpose of compaction is to reduce a void fraction, optimize aggregate contact behavior and increase bulk-specific gravity [166]. The compositional properties of the asphalt mix, compaction environment, compaction method and field conditions may lead to changes in compaction results. [PERSON] et al. [167] investigated the development of software that can be used for 2D image analysis and aggregate structure characterization of asphalt mixtures, which has the advantage that the images can be processed prior to the analysis so as not to affect the accuracy of the results. By using the SGC method, the aggregate structure characteristics of the specimens were analyzed under different compaction conditions, including the orientation, spatial distribution, and a number of contact points of the aggregates in the images, and key features describing the influence of the aggregate structure and compaction conditions were proposed. [PERSON] [168] developed a new method to generate 3D models of real aggregate shapes to characterize aggregate segregation during the Superpave gyratory compactor (SGC). In addition, the orientation and distribution of aggregates in asphalt mixtures have an important influence on the performance of asphalt mixtures. [PERSON] et al. [169] used different compaction methods ( gyratory compaction, vibratory compaction and slab compaction) to compact gap-graded asphalt mixture specimens and analyzed the image characteristics using the X-ray CT technique to characterize the orientation and distribution of the ground in the compacted specimens. The results showed that the aggregates near the edge of the samples tended to be arranged along the circumferential direction, while those near the center of the samples were randomly oriented. Figure 14 presents the random asphalt mixture and virtual compaction model.  
In summary, scholars have explored the changes in asphalt mixtures under different compaction parameters by varying their relevant properties. Moreover, the existing studies mainly stay in the compaction process without changing the loading parameters. Nevertheless, the loads in the real working environment are adjusted, and how to change the simulated compaction loads at different stages is a problem to be solved in subsequent studies.