# Using Imaging Techniques to Analyze the Microstructure of
## 3 Scanning Electron Microscope (SEM)
### AC Microstructure Analysis Using SEM  
SEM provides high-resolution images of the internal structures of AC mixture samples, allowing mineral distribution and void characteristics to be analyzed in detail. Additionally, SEM is capable of generating 3D images of the sample's surface using focused ion beam milling and tomography techniques [35]. Moreover, energy-dispersive X-ray spectroscopy analyses can be conducted using SEM to identify the elemental composition of individual minerals in AC samples [36]. For the testing of asphalt mixtures, ESM samples are typically prepared as thin slices with a diameter or length of up to 50 mm and a depth of up to 10 mm. The ESM sample can be disc-shaped or prism-shaped, depending on the size and configuration of the original asphalt mixture specimen; see Figure 2. It is important to ensure that the prepared samples are representative of the desired areas or features of interest within the asphalt mixture for accurate SEM analysis. The samples require additional treatment to be conductive, coated with a conductive material, and ground-isolated. SEM was used to study the mechanical behaviour and microstructure features of interfacial transition zones in AC mixtures containing recycled concrete aggregate. Results showed that SEM images revealed the presence of various types of mineral phases and voids, which contributed to the mechanical behaviour and durability of the AC mixtures [37]. [PERSON] and [PERSON] [38] used SEM to estimate fine aggregate matrix (FAM) gradation. FAM refers to a combination of bituminous binder, air voids, filler, and fine aggregates. SEM was also used to estimate the amount of adsorbed bituminous binder, which is a critical parameter in AC mix design and affects the stiffness, cost, and durability of AC [39, 40]. [PERSON] et al. [41] utilized SEM analysis to examine the surface texture of natural limestone and electric arc furnace steel slag aggregates. The results showed that steel slag aggregate had higher porosity and roughness compared to limestone aggregate. Incorporating coarse steel slag aggregate into warm mix asphalt (WMA) mixtures improved properties such as Marshall stability, resilient modulus, tensile strength, moisture resistance, and resistance to permanent deformation. Based on the findings of this research, the use of coarse steel slag aggregate in WMA mixtures was recommended. SEM images provide valuable insights into the fracture mechanism of AC mixtures. These images can show the initiation and progression of micro-cracks within AC caused by adhesion failure between coarse aggregates and the fine aggregate matrix (FAM), as shown in Figure 3. Additionally, SEM can reveal cohesion failure within the asphalt binder. To characterize the impact of binder aging on AC, SEM can be employed when examining changes in microstructure, such as the formation of cracks or the degradation of binder-aggregate bonds. By comparing SEM images of aged and unaged samples, researchers can assess the effects of binder aging on the structural integrity and performance of AC. Using SEM images, [PERSON] et al. [42] found that after binder aging, the sample surface changed from a smooth surface to a tough surface with micro-cracks. Figure 3: Coarse aggregates and FAM in an AC mixture. Figure 2: The shape and size of asphalt concrete samples for SEM test. [PERSON] et al. [43] used SEM to study fracture healing mechanisms in sand asphalt mixtures. They tested three types of bituminous binders: Texaco AC-5, Texaco AC-20, and styrene-butadiene rubber latex-modified Texaco AC-5, and they could explain the incompatibility of latex with Texaco AC-5. By using an optical microscope and SEM, [PERSON] et al. [44] evaluated the self-healing efficiency and thermal stability of double-walled microcapsules mixed into small-scale AC samples. The used microcapsules did not break when subjected to high temperatures and had adequate thermal stability.