# 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)
[57] utilized X-ray CT scanning to investigate physical impacts on EB14 HMA asphalt, an asphalt mixture with a nominal aggregate size of 0/14 mm (EB14, Quebec standard), under different saturation conditions. X-ray imaging allowed freezing points to be determined and deformations during temperature cycles to be examined. The results showed that water-saturated samples exhibited more significant deformations compared to brine-saturated samples. Additionally, X-ray analysis facilitated the measurement of the linear coefficient of thermal expansion (LCTE), which indicated different expansion and contraction behaviours in the samples. The findings highlight the importance of X-ray imaging in studying the effects of saturation and temperature variations on asphalt properties. Using a CT scan, [PERSON] et al. [22] analyzed the internal structure of asphalt mixtures under freeze-thaw cycles. The tested mixtures were a conventional dense-graded asphalt mixture with 80/100 penetration-grade binder (CM), a stone mastic asphalt mixture with 80/100 penetration grade binder (SMA), and an open-graded asphalt mixture with rubberized asphalt binder of 80 grade (OGFC). Changes in void contents, number, and shape were quantified to understand deformation and micro-crack formation. The study revealed that the internal structure evolves through void expansion, coalescing, and new void formation. The asphalt mixture exhibited different internal structure changes due to aggregate size distribution. The study emphasized the significance of design air void content and saturation, as higher levels negatively impact internal structure stability and promote microcrack propagation. Figure 6 shows the change in air void content for each mixture with the increasing of F-T cycles. [PERSON] and [PERSON] [58] developed an intelligent collection system for coarse aggregate attitude using 3D printing and wireless sensing. In this study, X-ray CT scanning was used to monitor the movements of the coarse aggregate particles at different compaction stages and to validate the proposed system. The use of X-ray CT scanning and image processing techniques in AC microstructure analysis has shown possibilities for numerical simulations that can accurately predict the mechanical performance of AC. [PERSON] et al. [59] used X-ray CT scanning and digital image processing to analyze void characteristics in an epoxy-porous asphalt mixture. The results showed a strong correlation between the mesoscale CT scanning data and macroscale measurements, indicating the effectiveness of this approach. The connected void fraction was found to be the most influential parameter in predicting the sound absorption coefficient. This technology has potential for studying drainage characteristics and road surface skid resistance in pavement research. [PERSON] et al. [60] employed X-ray CT scan analysis to assess the performance of asphalt mixtures containing steel slag as a substitute for limestone aggregates. The study determined that the optimal steel slag content was 75%, resulting in enhanced high-temperature stability, water stability, shear resistance, dynamic modulus, freeze-thaw resistance, and uniaxial penetration strength. However, there was a slight reduction in low-temperature cracking resistance and a minor increase in volume expansion.