## 1 Introduction  
There is overwhelming evidence regarding the significance of the fabric of asphalt mixtures, which is referred to as the internal structure of asphalt mixtures, on the compaction of asphalt pavements. With respect to material properties, the internal structure of asphalt mixtures is influenced by asphalt binder type, aggregate gradation, and aggregate shape/texture [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. In compaction circumstances, the behavior of asphalt mixtures depends on compaction methods, tools, and temperatures [6; 13; 14; 15; 16; 17; 18]. The importance of internal structure in asphalt mixtures has been recognized by experimental researchers in that the internal structure of an asphalt mixture affects not only the compaction performance but also the pavement performance and distresses such as rutting, fatigue, and thermal cracking [18; 19; 20; 21; 22; 23; 24; 25]. As a part of the Strategic Highway Research Program (SHRP) study for a new asphalt mixture design, the adoption of the Superpave Gyratory Compaction (SGC) method, which is a convenient way of assessing the compaction process of asphalt mixtures, shows pavement technologists to volumetrically characterize asphalt mixtures and its constituents on a more rational basis [26; 27; 28]. Asphalt mixtures consist mainly of an asphalt binder, aggregate, and air voids. However, intensive efforts have been devoted to improve the properties of asphalt binders [28; 29; 30; 31; 32]. Since the aggregates in asphalt mixtures are over 90% by mass, the aggregates play an important role in the compaction of asphalt mixtures, as well as in determining the mechanical behavior of the asphalt pavement. The response of aggregates during compaction depends on the compaction temperature and applied pressure by rollers [17; 33; 34]. Accordingly, the internal structure of an asphalt mixture has been indirectly determined by investigating the preferred aggregate orientation, interface contact between aggregate and binder, and adhesive characteristics between aggregates [4; 5; 15; 35; 36; 37; 38; 39; 40; 41; 42; 43]. In order to explain the compaction mechanism in an asphalt mixture, it is necessary to investigate the behavior of aggregates in the mixtures. However, it is difficult to define a simple compaction concept of asphalt mixture related to the behavior of aggregates. Although previous research focused on linking the internal structure of asphalt mixture to the engineering properties of each constituent of mixture, changing not only the material types as well as the compaction methods, limited information is currently available on the internal structure of asphalt mixtures. Nevertheless, [PERSON] et al. [38] firstly introduced asphalt-aggregate responses into a well-established structure of asphalt mixture along with compaction conditions. For characterizing the macrostructure of asphalt mixtures, in terms of the orientation and distribution of the aggregate as well as the distribution of air voids, many of the internal structure studies employed the image processing technique and X-ray tomography as tools to measure and quantify the distribution or orientation of aggregate and air voids from an analysis of photographed images of asphalt mixtures [1; 2; 8; 15; 22; 35; 38; 40; 41; 42; 43; 44; 45]. According to [PERSON] et al. [28], there is the preferred tendency for aggregate orientation and distribution up to a certain compaction effort (loading pressure), and the optimum asphalt binder viscosity makes it possible for aggregates to be uniformly distributed and preferably oriented. In addition, the brittleness of the binder leading to the fatigue can be decreased by the compensation of a preferred orientation in aggregates. They found that aggregates in the mixture also can be rearranged according to the magnitude and duration of loading. [PERSON] et al.