## 1 Introduction  
Air voids have a significant influence on the mechanical properties of asphalt mixtures. It is well established in many pieces of literature that air voids distribution can be a major factor which affects asphalt pavement performance, such as cracking, rutting, fatigue and moisture damage [1; 2; 3; 4]. Some researchers found that under the same air voids, different air voids distribution can lead to different pavement performances [2; 5]. Except for pavement performances, in open-graded pavements, air voids have a strong relationship with field permeability, and a mixture's nominal maximum aggregate size would greatly affect the permeability characteristics of the pavement [6]. Due to the lack of laboratory techniques to capture the air voids distribution in pavements, most of the research results are based on the average air voids content, no matter in pavement designing or specifying asphalt mixtures [7; 8]. The current Superpave mixture design procedure is based on developing mixtures that attain certain average percent air voids at different compaction levels. However, quantifying the air voids by an average value means assuming the material to be homogeneous, while materials with the same air voids may have different distributions of air voids, which may lead to materials showing different properties [9]. [PERSON] and [PERSON] (2019) also argued that taking the spatial variability of material properties into consideration is helpful to obtain a better insight into the actual structural reliability of the pavements [10]. Thus, the testing results of some specimens which have uneven air voids distribution would lead to certain variations in mechanical properties, affecting the testing reliability and misleading the material response [11]. To address the non-uniform distribution of air voids in materials, it is necessary to characterize and quantify the air voids inside the materials. In order to capture the internal structure of asphalt mixtures and evaluate the effects of air void distribution, X-ray Computed Tomography (CT) and digital camera processing have been widely used non-destructive imaging technologies, which could provide a 2D or 3D image of the internal structure of a solid object [12]. From the digital camera or CT image processing, the location or coordinates of aggregates, reconstructed pore geometry, void-distribution segregation, the two-or three-dimensional internal structure of hot-mix asphalt or some distresses like cracking can be obtained [13; 14; 15; 16; 17; 18]. As an increasing number of researchers have analyzed the asphalt pavement from the microstructure perspective [19; 20; 21; 22; 23], some of them have applied digital camera and CT image processing in the field of asphalt pavement, especially in terms of air voids. [PERSON] (2009) analyzed the air voids distribution along the height using the X-ray computed tomography and predicted the dynamic modulus of the asphalt mixture [24]. [PERSON] (2000) used three-dimensional X-ray computed tomography for non-destructive evaluation of air void distribution and found that the rutting performance of mixes could be qualitatively related to the aggregate structure [25]. Besides, [PERSON] et al. (2017) used virtual cut section to investigate the air voids properties and found that the air voids within the porous asphalt mixtures were homogeneously distributed with an elongated shape [26]. Nowadays, more and more researchers tend to study the effect of air voids distribution on the asphalt mixture performances, and the heterogeneity of air voids distribution has been proved to be related to material properties. Investigating the air voids distribution inside the materials could provide the outline of the structure, which would further be related to the asphalt mixture performances. Therefore, the objective of this research is to study the air voids distribution of the material, including its number and area magnitude, which could be related to mixture characteristics and pavement performances in the future.