Microscale characterization of bitumen – back-analysis of viscoelastic properties by means of nanoindentation

Abstract In order to understand the complex thermo-rheological behavior of asphalt, stemming from the viscoelastic nature of bitumen, the nanoindentation (NI) technique is employed. Hereby, the load history applied onto the indenter tip is characterized by a loading, holding, and unloading phase. As regards the identification of viscoelastic properties, a recently published back-analysis scheme, employing the holding phase of the NI test, is extended towards fractional-creep models. In fact, this type of model is found to perfectly describe the viscoelastic behavior of bitumen. Based on the identified viscoelastic model parameters, the influence of loading rate, maximum load, and temperature on these parameters is investigated. Hereby, the temperature dependence of creep parameters follows an Arrhenius-type law. In addition to the model parameters, application of the so-called grid-indentation technique within NI testing provides insight into the bitumen microstructure and the mechanical behavior of the different bitumen phases. The results obtained indicate the existence of a string-like microstructure embedded into a less viscous matrix material.