Estimating the Asphalt Binder Film Thickness Using Scanning Electron Microscope and Energy Dispersive X‐Ray Spectroscopy

Abstract

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.

Assessing the Asphalt Binder Film Thickness in Recycled Asphalt Mixtures Using Micro-Level Techniques

“…According to SEM and EDS studies, the binder film thickness in recycled asphalt mixtures varies from 0.4 to 2 µm, with no correlation to recycled asphalt mixture or rejuvenator concentration, as shown in Figure 9. The results of SEM-based ABFT coincide with those of Elseifi et al [18] and Karim et al [10], but they contradict the analytical results of ABFT given in Tables 1 and 10. As indicated in Table 10, traditional ABFT values based on analytical models range from 6.4 to 13.5 µm, with film thickness being a function of recycled asphalt mixture's content, rejuvenator content, and model type.…”

“…Figure 7a-c displays images of the asphalt specimen, captured at magnification of ×5000 and a scale bar of 5 µm, which revealed asphalt mastic, comprising a mix of bitumen and mineral filler. Figure 7b shows flaky, elongated, and spherical filler particles in asphalt mastic, as well as air voids of 0.3, 0.7, and 1.4 micron sizes, which agrees with the findings of Karim et al [10] but contradicts the assertion made by Kandhal and Chakraborty [11], that voids do not occur in asphalt mastic. Figure 8c also reveals a deviating binder film, gripping the filler particles in mastic.…”

“…On the other hand, the effect of asphalt binder film thickness on asphalt pavement performance is usually neglected, especially when performance falls short of expectations. As a result, a full understanding of the effect of ABFT on the mechanical properties of compacted asphalt mixtures is required [10]. To investigate the concept of asphalt binder film thickness and its relationship to mixing temperature and binder content in conventional asphalt mixtures using SEM and EDS, F. Karim et al [10] produced Marshall Specimens at mixing temperatures of 140, 150, and 160 • C. The study revealed that the film thickness is a function of temperature, binder concentration, and model type, according to the estimated values of asphalt binder film thicknesses using analytical models.…”

“…As a result, a full understanding of the effect of ABFT on the mechanical properties of compacted asphalt mixtures is required [10]. To investigate the concept of asphalt binder film thickness and its relationship to mixing temperature and binder content in conventional asphalt mixtures using SEM and EDS, F. Karim et al [10] produced Marshall Specimens at mixing temperatures of 140, 150, and 160 • C. The study revealed that the film thickness is a function of temperature, binder concentration, and model type, according to the estimated values of asphalt binder film thicknesses using analytical models. A substantial variance in asphalt binder film thickness was reported for mix design at constant binder concentration and mixing temperature.…”

“…Table 2. Models to assess ABFT in HMA [10]. Al-Khateeb [2] Where: FT b , T F , DA, T f , F, and F be stand for ABFT, which is measured in µm (10 −6 m) or (µm).…”