https://doi.org/10.1155/2021/8894970
“Asphalt binder film thickness has relation to mixing temperature and binder content in hot mix asphalt, which influences mixture’s performance. A significant variation in assessing the asphalt binder film thickness has been observed in the literature. Development of state of the art technology and Superpave specification requires the study of actual asphalt binder film thickness at micro-level. This study estimates asphalt binder film thickness at micro-level and compares results with those obtained through analytical models from the previous studies. The study utilizes different asphalt mixtures at various mixing temperatures and binder contents. The asphalt binder film thickness around the finest particles of 500 nm (0.5 micron) size in asphalt mastic was detected and measured by image analysis (using scanning electron microscope) and elemental analysis (using energy dispersive X-ray spectroscopy) at magnifying power of ×30,000. The analytical estimation revealed that the asphalt binder film thickness for the aforementioned conditions varies from 9 μm to 13 μm, with a fair relationship to binder content and mixing temperature. However, results obtained from image analysis revealed that the asphalt binder film thickness varies from 0.5 μm to 2.4 μm, with no relation to binder content and mixing temperature. The image analysis showed that the asphalt mixtures mostly contain asphalt mortar and asphalt mastic, occurring in irregular shape. It was also found that the asphalt binder film does exist as a separate entity inside the asphalt mastic in the form of a band around the filler particles as non-absorbed binder, which fills the approximate distance of 0.5 to 2.5 microns among filler particles.”
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4.3. Image Analysis Using SEM and EDS
SEM-JEOL JSM IT 100 available with secondary electron detector (SED), backscattered electron detector (BSED), and energy dispersive X-ray spectroscopy (EDS) features was used in this study at magnifications between ×200 and ×30,000, as shown in Figure 6(a). Due to high experimental cost, the current study is limited to SEM to capture high resolution images in order to detect asphalt binder film and EDS is only used for elemental composition of asphalt binder film already detected by SEM in compacted asphalt mixtures. The sample preparation started by cutting Marshall specimens with a diamond saw to reveal the internal structure of the material. The sample was further cut and obtained 8 mm × 8 mm×6 mm and 12 mm × 8 mm×6 mm specimens, as shown in Figures 7(b) and 7(c). A spot of interest was then marked in the specified specimen, and the sample was then coated with gold (4 nm thick) thin film by Gold Sputter Coater in order to make the surface of the sample conductive as shown in Figures 6(b) and 6(c). This metal-coated layer is so thin that all the microtexture at the surface of HMA is preserved and observed during the SEM process, as shown in Figure 7(c). In order to detect and measure thickness of asphalt binder film by SEM and EDS, three asphalt samples were prepared and analyzed at each optimized binder content of 4.36%, 4.23%, and 4.12% by weight of Marshall sample at each mixing temperature of 140°C, 150°C, and 160°C. The samples were thoroughly examined through image analysis using SEM. The asphalt mastic and asphalt binder film thickness were measured at various spots of the asphalt samples.
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5.2. Analysis of Asphalt Samples Based on SEM and EDS
SEM images were taken from low to high magnification in order to investigate different features on the surface of the asphalt specimens, extracted from asphalt samples. These asphalt samples were prepared at optimized binder content of 4.36%, 4.23%, and 4.12% and mixing temperature of 140°C, 150°C, and 160°C, respectively. The asphalt samples at the aforementioned conditions were analyzed experimentally by SEM and EDS, and entities as small as 500 nm (0.5 μm) were detected in the images provided in Figures 8–10. The results from the images lead to the following observations:
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