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.
Highlights
E asphalt binder film thickness is often mentioned in the literature as mix design and quality assurance criteria to withstand traffic and environment
Estimation of asphalt binder film thickness in hot mix asphalt (HMA) is based on various analytical models, as presented in Table 3. ese models are based on surface area factors of aggregates where the aggregates gradation is linked to its total surface area based on certain assumptions. ese assumptions are the spherical shape of aggregates, uniform binder film thickness on them, and no information on porosity, and degree of compaction of the mixture
According to the Marshall procedure described in ASTM D1559 [42]. e Marshall specimens were prepared by filling Marshall mold, having a diameter of 4 inches and thickness of 2.5 inches, and compacted with 75 blows of the standard Marshall hammer for heavy traffic on each side of the sample
Summary
E asphalt binder film thickness is often mentioned in the literature as mix design and quality assurance criteria to withstand traffic and environment. Erefore, some researchers recommend the average asphalt binder film thickness as design criteria in asphalt mixtures rather than minimum VMA in Superpave volumetric mix design [2, 3]. It was reported that the asphalt binder film on the aggregate surface in compacted asphalt mixture is the source of cohesion and adhesion, thereby affecting bonding performance in terms of durability of asphalt mixtures [2]. E coarser asphalt mixtures have thick asphalt film due to lower surface area but commonly fail to meet the minimum VMA criteria. Fine asphalt mixtures have lower asphalt film due to higher surface area and commonly meet the minimum VMA criteria. Estimation of asphalt binder film thickness in HMA is based on various analytical models, as presented in Table 3. ese models are based on surface area factors of aggregates where the aggregates gradation is linked to its total surface area based on certain assumptions. ese assumptions are the spherical shape of aggregates, uniform binder film thickness on them, and no information on porosity, and degree of compaction of the mixture
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