## 1 Introduction  
Asphalt mixtures, consisting of aggregate, asphalt and air voids, can be divided into a coarse aggregate phase and an asphalt mortar phase. Coarse aggregates comprise the skeleton and occupy most of the volume, while asphalt mortar fills in the intervals of the coarse aggregates and works as a bonding material between the coarse aggregates [1, 2]. The air voids in an asphalt mixture can be distinguished as open or closed porosities, and many studies have focused on the size and distribution of the voids in a mixture and their influence on the pavement performance [3]. The air void content is a vital parameter which characterizes an asphalt mixture, and it is measured by one minus the ratio of the bulk density to the maximum theorical density [4, 5]. To obtain a stable mixture, control parameters are used, such as the passing rate, asphalt aggregate ratio and air void content. Among these constituents, the air void content has a significant effect on pavement performance [6]. The air void content is controlled by design, and the compaction has a direct impact on multiple mixture's performance, such as the fracture resistance [7; 8; 9], high-temperature creep resistance [10] and fatigue performance [11]. For example, a mixture with a larger air void content would be less stiff and have a smaller modulus compared with those with a smaller air void content, while a mixture with a certain air void percent has the best anti-rutting performance [12]. The aging rate of service pavement mixtures is also affected by the air void content [13; 14; 15]. Thus, the air void content is another vital control parameter for compaction, since dense and loose compactions in the same type of mixture have different volume properties and pavement performances [16]. The multiscale approach to asphalt mixtures has drawn increasing attention at the multiscale level, such as mortar and mastic, when comparing the asphalt materials' performance at this scale. Mortar, or FAM (fine aggregate matrix), is the transaction scale between asphalt binder and asphalt mixture, which has both the viscoelastic properties of asphalt binder and dynamic modulus and fatigue performance similar to asphalt mixture [16]. With these features, mortar research on asphalt mixtures has been vital for understanding mixture performances or distress mechanisms [17]. With the numerous studies on mixtures, increasing attention has been paid to the mortar phase in a mixture [18]. Currently, as there is no standard specification for mortar preparation, testing and evaluation, many attempts have been made to reveal the manufacturing method, various performances and relationships of binder and the mixture of a mortar [18]. For further research on mortar, when manufacturing it in laboratories, it should have similar physical properties as pavement. However, the actual void percent and distribution condition of mortar in mixtures have still not been fully revealed. Researchers have studied void size and distribution in mortar and have made much progress, but the calculating the accurate air void content of mortar is still unclear. For example, in [19], less than 25% of the air void content for a 1.18 mm mortar and approximately 50-75% of the air void content in a NMPS 2.36 mm mortar were reported [19; 20], but a more direct and accurate solution is needed. The measurement of the air void content in mixtures has been well developed. The conventional method calculates the ratio of the volume of the air void content in the mixture to that of the whole mixture containing both solids and air voids [21]. With high-tech approaches, such as computerized tomography (CT) [22; 23], air void details, such as their size and distribution in an asphalt mixture, can be characterized and analyzed. CT is a meso-structure testing approach that provides details on voids inside of samples to determine and evaluate the size and number of voids [24]. CT images with high-energy X-ray scanning of the sample can be used to illustrate an image array.