# Using Imaging Techniques to Analyze the Microstructure of
## 5 Summary and Recommendations  
AC mixtures deteriorate due to traffic loading and environmental factors. The mechanical behaviour of AC mixtures is affected by many factors, such as asphalt binder type and content, mineral aggregate properties and gradation, and void content and distribution. Several studies have investigated the use of digital cameras, SEM, and X-ray CT scanning imaging techniques to analyze the performance and microstructural changes of AC mix  
Figure 7: 2D and 3D images of the microstructure of AC mixture generated via X-ray CT scan. (**a**) 3D reconstruction of AC sample (**b**). 2D image of AC sample. \begin{table}
\begin{tabular}{c c} \hline \hline
**Imaging Technique** & **X-ray CT Scan** \\ \hline Sample Size & A non-destructive technique \\  & No need to cut the AC cores \\ \hline Sample Penetration & Can penetrate the sample and reveal all details of the internal structure \\ \hline Type of Images Produced & 2D and 3D (generated by built-in image processor) \\ \hline Source & X-ray beam (photon current) \\ \hline  & X-ray CT scanning machine \\ Imaging Tools & Attached computer \\  & Image processing software \\ \hline Image Resolution & Up to 100-200 microns \\ \hline  & 3D simulation of AC cores can provide all the properties of AC mixtures \\  & Coarse and fine aggregate gradation, size and distribution \\ Produced Data & The volume of air voids \\  & Surface texture \\  & Cracks and changes due to F–T cycles \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Computed tomography (CT) scan. tures. Image quality and intensity inhomogeneity can significantly affect the efficiency of an imaging technique. Both the digital camera and SEM techniques are destructive techniques which need a sample to be cut to produce enough smooth and levelled cross-sections that represent the whole AC mixture. On the other hand, X-ray CT scanning is a non-destructive technique, and samples can be scanned in their original shape. Digital camera imaging can be used to extract gradation and size distribution of coarse aggregate as well as volumetric parameters of the mixture (VTM%, VMA%, and VFA%) using integrated models. The SEM technique can be utilized to evaluate the properties of FAM and asphalt binder content in AC mixtures. The X-ray CT scanning technique is capable of generating a 3D image of the internal structure of AC, which can be used to evaluate the geometric and volumetric properties of the mixture. Digital camera imaging cannot penetrate the sample surface, and the extracted parameters are planner projections as the images are for sample cross-sections. Correction factors and/or some assumptions are needed to estimate AC microstructure properties accurately. SEM can penetrate up to 3 \(\upmu\)m into the sample surface. However, there are some challenges in the SEM technique that make it limited, such as the need for an ultra-high vacuum, the volatility of the asphalt binder in the vacuum chamber, and the complexity of sample preparation. X-ray CT scanning uses X-rays that can penetrate through the whole sample and generate a 3D image of the internal structure. However, there are some challenges in the X-ray CT scan technique, such as dealing with noise in the images and difficulties in aggregate segmentation. Some filters and processing methods are used to improve the quality of X-ray CT scan images and reduce noise. Digital camera imaging is simple, accessible, safe, and inexpensive. The SEM technique needs operational and safety training and is only accessible in limited laboratories. X-ray CT scanning is not easily accessible, needs expensive instrumentation, and can only be conducted by a radiology technologist to ensure safety. Table 4 shows the advantages and disadvantages of each technique. In conclusion, imaging techniques, such as X-ray CT scanning, digital camera, and SEM, can be used to analyze and understand the microstructure and behaviour of AC mixtures.