# Using Imaging Techniques to Analyze the Microstructure of
## 4 X-ray Computed Tomography (CT Scan)
### Microstructure Analysis of AC Mixtures Using X-ray (CT Scan)  
The X-ray CT technique can effectively capture high-resolution images of the internal microstructure of AC mixtures. These images can be segmented using image analysis techniques, allowing various components within the AC mixture to be differentiated from one another [51]. The performance of the AC mix is affected by the distribution and orientation of aggregates and their associated air voids. [PERSON] et al. [18] used the X-ray scan technique to study the distribution of air voids in AC samples. Imaging results showed that the distribution of air voids in samples compacted by a Superpave gyratory compactor (SGC) was non-uniform and most of the voids were concentrated at the top and bottom parts of the sample. [PERSON] et al. [52] used X-ray CT scanning and stereology methods to evaluate the void systems in three WesTrack mixtures, fine (T06 and T07), fine plus (T08 and T09), and coarse (T10 and T11). The study quantified the spatial variation of void content, void size distribution, and parameters indicating damage level and interaction. The findings revealed that the coarse-graded mix had more severe inherent damage, consistent with field observations. The quantified parameters can be applied in fatigue and rutting modelling using continuum damage mechanics. Figure 5 shows some statistical results from that study. Figure 5: Some statistical results from [PERSON] et al. [52]. (**a**) The average mean solid path (mm) for all mixes. (**b**) The average percent void content of mixes. (**c**) The average void size (area, mm\({}^{2}\)). (**d**) The average specific damaged surface area of mixes. [PERSON] et al. [53] developed new methods to quantify damage parameters using stereology principles, virtual sectioning techniques, and X-ray CT scans. The study found that average spacing among damaged surfaces, comprehensive damage tensor quantity, and spacing size ratio provided effective representation of the damage state. However, the specific damaged surface area was found to be less indicative of the damaged material behaviour. The performance of AC mixtures is influenced by their permeability. [PERSON] et al. [54] used the X-ray CT scan technique to evaluate the correlation between the permeability of AC mixtures and 3D models of internal voids. A simple permeability equation was developed to calculate the permeability of AC mixtures based on volumetric indices from X-ray CT scans, which showed a good correlation with laboratory and field permeability measurements for the same mixtures  
[PERSON] et al. [55] used X-ray CT scanning to investigate the influence of aggregates on air voids in asphalt mixtures, focusing on compaction energy. X-ray CT imaging revealed that the proportion of coarse and fine aggregates played a significant role in air void morphology and distribution. Adjusting the aggregate proportion above a specific size had a noticeable impact on air voids. The complexity of the three-dimensional model and the shape of air voids were influenced by aggregate characteristics. Higher aggregate complexity resulted in larger and more complex air voids. Compaction reduced air void complexity. Overall, aggregates were identified as a crucial factor affecting air voids in asphalt mixtures, irrespective of compaction method and energy. [PERSON] et al. [56] used an X-ray CT scan, along with the fast wavelet transform (FWT) and artificial neural network (ANN), to analyze the internal structure of asphalt mixtures. The FWT effectively separates aggregates and estimates gradation, while the ANN accurately extracts aggregate properties from challenging images. The developed techniques enable comparisons of compaction methods, investigation of the effects of gradation and aggregate shape, and potential applications in micromechanical modelling. X-ray CT scanning was utilized to observe the initiation and propagation of F-T damage within AC mixtures. [PERSON] et al.