Fluorescence Microscopy for Determining Diffusion Coefficient Between Virgin and Aged Asphalt Binders

Abstract

Diffusion between virgin and aged asphalt plays a critical role in ensuring a final uniform asphalt binder in mixtures containing recycled asphalt pavement (RAP). Understanding the diffusion phenomenon and determining the diffusion coefficient have a significant effect on asphalt mix design and asphalt mixture performance. In this study, a new method was proposed to utilize fluorescence microscopy to determine the diffusion coefficient of aged RAP binder in recycled mixtures. First, Fick’s second law was applied to quantify the diffusion process in a two-layered virgin-aged binder model and to obtain the analytical solution to the distribution of virgin and aged binders. Then, fluorescence microscopy was used in the laboratory to differentiate between virgin and aged binders and to back-calculate their concentrations from fluorescence image. The diffusion coefficient was determined by fitting the analytical solution to the laboratory concentration measurements. Comparison of the diffusion coefficient by the proposed method and that using the dynamic shear rheometer (DSR) method from the literature shows that the diffusion coefficient determined by fluorescence microscopy was of the same magnitude to, but slightly lower than, that by DSR method. The diffusion coefficient was also predicted using another theoretical method – free volume theory. Parameters describing the asphaltic model in the free volume theory were determined from analysis of the chemical structure of asphalt, viscoelastic relaxation properties, and glass transition temperature of asphalt as well as from the diffusion coefficient measured from laboratory experiments. Diffusion coefficient predicted by the free volume theory shows that diffusion coefficients of asphalt were closely dependent on temperature and asphalt type. A diffusion simulation was performed on plant mixtures using the diffusion coefficient obtained from fluorescence microscopy. The results showed that an almost complete diffusion was achieved within five minutes in a hot-mix asphalt mixture resulting in a uniform asphalt film.