Microstructural association model for upscaling prediction of asphalt concrete dynamic modulus

Multiscale modeling is becoming an increasingly useful method of evaluating the behaviors of asphalt concrete. Reasons for this increased interest include the fact that many of the critical behaviors of this material are affected by localized behaviors that cannot be completely captured using more traditional continuum approaches. Computational methods are popular for this type of evaluation because in principle they can directly account for many of the localized mechanisms. However, computational expense can be excessive, particularly if all of these localized mechanisms are accounted for rigorously. An alternative method of multiscale modeling relying on analytical models is developed and presented in this paper. The model is referred to as the microstructure association model because it accounts for the ways that the multiple scales within asphalt concrete associate together to yield the gross behaviors of the finished composite, asphalt concrete. This model is formed from the hypothesis that asphaltic composites at different length scales evolve along a continuum, which is related to the degree of internal structuralization that exists within the composite. In this way asphalt mixture may behave in a quantitatively similar way as asphalt mastic if the concentration of particles within the mastic produces the same degree of internal structuralization as that which exists in the asphalt mixture. Through this modeling effort two separate, but possibly related, mechanisms are identified: (1) interaction of particles separated by some distance, but still influenced by an adsorbed and nonadsorbed asphalt binding medium; and (2) close-proximity contacts of aggregate particles that lead to a significant increase in stiffness with relatively small changes in volumetric concentration. Each of these mechanisms is modeled separately within the microstructural association model and combined using a rational mechanical analog approach. The model is characterized using only experiments on asphalt mastic at different concentrations, but is verified with its ability to upscale to the modulus of asphalt concrete. In the final section of this paper the ability of the model to perform useful engineering tasks is shown when it is used to assess the impact of asphalt content and air void content changes on the modulus of two different materials.