Keywords: multiscale, homogenization, micromechanics, creep, fatigue, asphalt, bitumen, pavement.

A reliable assessment of the performance of multi-composed materials requires suitable procedures and models for the evaluation of key material properties. In case of asphalt used for trafficked pavements, these key properties reflect its resistance to rutting caused by thermal deformation, to cracking at low temperatures, and to fatigue failure under repeated load cycles. Asphalt is composed of bitumen, aggregate, and air voids, showing a complex thermo-rheological behavior. E.g., the low viscosity of asphalt at high temperatures ( C) is a necessary prerequisite during the construction and compaction process of high-quality asphalt layers. When the surface temperature reaches C during hot summer periods, however, this viscosity should be significantly higher in order to minimize the development of permanent deformations (rutting). The desirable increase of viscosity and, hence, increase of stiffness with decreasing temperature at hot and medium temperatures ( C) are, on the other hand, disadvantageous at low temperatures ( C), causing low-temperature cracking in asphalt pavements. This optimization problem concerning the behavior of asphalt at different temperature regimes is a main objective of the Christian Doppler Laboratory "Performance-Based Optimization of Flexible Road Pavements" (TU Wien), headed by Ronald Blab. For the optimization process of a multi-composed material such as asphalt, three different modes can be distinguished:

1. variation of mixture characteristics (e.g., binder/aggregate-ratio),
2. change of constituents used (e.g. different bitumen or filler type), and
3. allowance of additives (e.g., polymers to modify the bitumen).

Within the afore mentioned Christian Doppler Laboratory, a multiscale model is currently developed. Hereby, the amount and type of bitumen, filler, and aggregate serve as input parameters, allowing to cover the wide range of asphalt mixtures resulting from the given modes of optimization. The multiscale model for asphalt is characterized by four additional observation scales below the macroscale, namely (i) the bitumen-scale (asphaltene and maltene morphology), (ii) the mastic-scale (bitumen+filler), (iii) the mortar-scale (mastic + aggregate with  mm), and (iv) the asphalt-scale (mortar + aggregate with  mm), see Figure 1. Within each observation scale, the characteristics (such as structure and material properties) of the constituents present at this scale are taken into account. Moreover, changes in scale characteristics, resulting from mechanical loading and/or environmental conditions may be considered at the respective scale of observation.

The goal of the multiscale model presented in this paper is the determination of macroscopic material parameters which serve as input for the analyses of flexible pavements. These parameters are obtained by means of upscaling procedures, bridging the scales from the bitumen-scale to the macroscale. For the assessment of the used upscaling techniques, herein formulated in the framework of continuum micromechanics, so-called verification experiments are required in addition to indentification experiments. The latter are used for identifying material characteristics at the different scales of observation. Following this hybrid character of the research work, comprising theoretical work regarding the development of appropriate upscaling techniques and experimental work for either identification or verification, upscaling of three key properties of asphalt, describing (i) low-temperature creep, (ii) thermal conductivity, and (iii) microcracking, is presented. These properties determine the risk of both cracking in consequence of thermal dilation during temperature changes and fatigue failure under repeated loading.

By establishing the upscaling scheme for key properties of asphalt, i.e., having related this key properties to the mixture characteristics of the asphalt and the behavior of the constituents, performance-based optimization of flexible pavement structures can be achieved.

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