Micromechanical modeling has tremendous potential benefits in the field of asphalt technology for reducing or eliminating costly tests to characterize asphalt-aggregate mixtures for the design and control of flexible pavement structures and materials. In time, these models could provide a crucial missing link for the development of true performance-related specifications for hot-mix asphalt. A microfabric discrete element modeling (MDEM) approach is presented for modeling asphalt concrete microstructure. The technique is a straightforward extension of a traditional discrete element modeling (DEM) analysis, in which various material phases (e.g., aggregates, mastic) are modeled with clusters of very small, discrete elements. The MDEM approach has all the benefits of traditional DEM (e.g., the ability to handle complex, changing contact geometries and the suitability for modeling large displacements and crack propagation). These models also allow for the simulation of specimen assembly (e.g., laboratory compaction of the asphalt mixture). By modeling inclusions such as aggregates with a “mesh” of small, discrete elements, it is also possible for one to model complex aggregate shapes and the propagation of cracks around or through aggregates during a strength test. A commercially available DEM package was used to demonstrate the usefulness of the MDEM approach. A method was also presented to obtain the properties of the matrix material in an asphalt mixture, which is typically difficult to determine experimentally. This study was limited to two-dimensional analysis techniques and involved the simulation of small test specimens. Follow-up studies involving larger specimen models and three-dimensional modeling capabilities are under way.
A clustered distinct element method (DEM) approach is presented as a research tool for modeling asphalt concrete microstructure. The approach involves the processing of high-resolution optical images to create a synthetic, reconstructed mechanical model that appears to capture many important features of the complex morphology of asphalt concrete. Uniaxial compression tests in the laboratory were employed to measure the dynamic modulus of sand mastic (a very fine sand–asphalt mixture) and asphalt mixtures at three temperatures and four loading frequencies. For a coarse mixture considered in this study, it was found that a two-dimensional (2-D) clustered DEM provided good estimates of mixture dynamic modulus across a range of loading temperatures and frequencies without calibration. However, for a fine-grained mixture, the uncalibrated predictions of the 2-D model were found to reside near the lower theoretical bounds and well below experimentally determined moduli, most likely because of current limitations in scanning and modeling resolution and the nature of the 2-D microstructural description. Work is under way to extend the model to three dimensions and to consider linear viscoelastic behavior in the mastic. That notwithstanding, the current modeling approach was successfully implemented in recent follow-up studies to portray bulk material behavior in conjunction with fracture models to study crack behavior in hot-mix asphalt.
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