## 3 Digital Image Acquisition, Processing and Modelling Reconstruction
### Digital Modelling Reconstruction Technology  
The microscopic scale properties of aggregated particles have an influential effect on their mechanical properties. In order to further investigate the microscopic scale properties and their macroscopic counterparts, the connection between the above two aspects can be facilitated by 3D digital model reconstruction techniques. In contrast to the established 2D model, the 3D morphological characteristics of the aggregates are more abundant. In combination with characteristic parameters such as shape and angularity, the 3D aggregate has indicators such as surface texture and roughness, which makes the mechanical behavior of the contact interface more complex. In addition, the extension of 3D spatial coordinates makes the anisotropy of the aggregate accumulation behavior more significant, and the distribution characteristics, such as opening, closing, and connecting of voids, are difficult to predict. X-ray CT, a completely nondestructive technique, allows visualization of the features inside opaque solid objects to obtain digital information on 3D geometric shapes and properties. [PERSON] et al. [97] investigated the damaging effect of asphalt mixtures on microstructure and aggregate contact based on X-ray CT imaging and DIP methods and analyzed the relationship between the damage factor and contact properties. The results showed that there is a strong linear relationship between the distance of the main skeleton aggregate and the breakage factor [98]. [PERSON] et al. [69] developed algorithms to calculate the aggregates' size, location, contact points and orientation, which is an excellent solution for better acquisition of microscopic images of aggregates based on the X-ray CT technique. The main representations are the segmentation of close aggregates and processing noisy or poorly contrasted images. The proposed image processing algorithm can guarantee the accuracy of acquisition and recognition by over 95%. Moreover, their findings also indicate that the number of contact points increases significantly with increasing compaction level and direction. [PERSON] et al. [99] proposed a framework that can be used to simulate the microscopic response of asphalt mixtures under various loading conditions. Identifying the microstructural composition of asphalt mixtures by X-ray CT imaging and other image processing techniques, the 3D microstructure of asphalt mixtures was obtained and converted into a finite element model to describe its mechanical behavior. The models in their findings reflect the effects of temperature, loading rate, repeated loading and mix ratio design on the microstructure response. Thus, macroscopic design specifications for aggregates can be achieved and guided by X-ray CT-based microscopic modeling methods. [PERSON] [100] used an X-ray CT system and image analysis techniques to analyze the pore distribution in asphalt concrete specimens. During Superpave gyratory compaction, when the number of gyrations increased from 50 to 174, the void fraction distributed in the top and bottom regions also increased from 65% to 80%. The distribution of void fractions may be due to the presence of some factor that weakens the compaction effect in the top and bottom regions. The gyratory compactor's top and bottom regions can restrict the aggregate's flow. Alterations in temperature can also lead to changes in the compaction characteristics of the specimens, which can affect the roadworthiness of the aggregates. [PERSON] et al. [101] tested the compaction levels of asphalt mixture specimens (AC13, AC16 AC20, and SMA13) under different compaction parameters at different temperatures and obtained high-quality images based on X-ray CT imaging for the microstructural characterization of asphalt mixtures. Variations in the size of the aggregates at the same temperature condition affect the compaction properties. The smaller the particle size, the higher the level of compaction.