# Using Imaging Techniques to Analyze the Microstructure of
## 2 Digital Camera Imaging
### Discussion  
The digital camera imaging technique is an effective means of analyzing AC microstructure. The geometric and volumetric characteristics of AC mixtures can be estimated via the processing of 2D images of cross-sections from compacted AC samples. The advantages of this technique are simplicity, availability, accessibility, accuracy, and low cost. However, obtaining accurate results through digital camera imaging requires careful consideration of factors such as sample preparation, lighting conditions, and image processing algorithms.  
On the other hand, this technique is a destructive testing technique as the AC sample needs to be cut into parts to obtain horizontal and/or vertical cross-sections that represent the internal structure of the AC mixture. These cross-sections must be cut slowly to achieve smooth and flat sections without any damage and to be sure that the details of the microstructure can be captured clearly.  
Digital camera images provide information about the AC microstructure at a planner level as this imaging technique cannot penetrate the sample surface. The dimensions of the objects shown in the images are projected or planner dimensions, not the real dimensions. Therefore, some assumptions and/or shape correction factors are needed in this technique.  
Digital camera imaging is an effective technique to characterize the coarse aggregate properties within a compacted AC sample. However, the fine aggregate properties cannot be accurately estimated using this technique, especially at lower image resolutions. Further research is needed to better characterize the fine aggregate properties using digital camera images. The percentage of air voids in the total mix (VTM%), the percentage of air voids in the mineral aggregate (VMA%), and the percentage of air voids filled with asphalt (VFA%) can be estimated from digital images by using the integrated models in the literature.  
However, it is challenging to distinguish the air void portions by using a 2D image for a cross-section of the AC sample. An efficient method of evaluating the morphology of air voids in AC microstructure is still needed. Moreover, this technique is limited to evaluating geometric and volumetric indices of AC mixtures with no emphasis on investigating the impact of environmental fluctuations on the behaviour of AC mixtures. Table 1 summarizes the features of using digital camera imaging techniques to analyze AC microstructure.  
\begin{table}
\begin{tabular}{l l} \hline
**Imaging Technique** & **Digital Camera** \\ \hline \multirow{3}{*}{Sample Size} & A destructive technique. \\  & AC cores are cut to produce horizontal circular sections of D of at least 50 mm or vertical rectangular \\  & sections of W of at least 50 mm, and height depends on the compaction. \\  & No. of the sections has to be representative of the sample. \\ \hline Sample Penetration & \multicolumn{1}{c}{Cannot penetrate the sample surface} \\ \hline Type of Produced images & 2D (Direct) and 3D (by image processing) \\ \hline Source & Light \\ \hline \multirow{3}{*}{Imaging Tools} & A Digital camera. \\  & A Digital system (computer). \\  & Image processing software/code. \\ \hline Image Resolution & Depending on the digital camera type can reach up to 50.6 megapixels (4 microns) \\ \hline \multirow{3}{*}{Produced Data} & Coarse aggregate gradation, size, and distribution. \\  & Volumetric properties of air voids and asphalt. \\ \cline{1-1}  & Cracks and changes due to F–T cycles. \\ \hline \end{tabular}
\end{table}
Table 1: Digital Camera Imaging.