Lukas Eberhardsteiner

Layered water in crystal interfaces as source for bone viscoelasticity: arguments from a multiscale approach

Extracellular bone material can be characterised as a nanocomposite where, in a liquid environment, nanometre-sized hydroxyapatite crystals precipitate within as well as between long fibre-like collagen fibrils (with diameters in the 100 nm range), as evidenced from neutron diffraction and transmission electron microscopy. Accordingly, these crystals are referred to as ‘interfibrillar mineral’ and ‘extrafibrillar mineral’, respectively. From a topological viewpoint, it is probable that the mineralisations start on the surfaces of the collagen fibrils (‘mineral-encrusted fibrils’), from where the crystals grow both into the fibril and into the extrafibrillar space. Since the mineral concentration depends on the pore spaces within the fibrils and between the fibrils (there is more space between them), the majority of the crystals (but clearly not all of them) typically lie in the extrafibrillar space. There, larger crystal agglomerations or clusters, spanning tens to hundreds of nanometers, develop in the course of mineralisation, and the micromechanics community has identified the pivotal role, which this extrafibrillar mineral plays for tissue elasticity. In such extrafibrillar crystal agglomerates, single crystals are stuck together, their surfaces being covered with very thin water layers. Recently, the latter have caught our interest regarding strength properties (Fritsch et al. 2009 J Theor Biol. 260(2): 230–252) – we have identified these water layers as weak interfaces in the extrafibrillar mineral of bone. Rate-independent gliding effects of crystals along the aforementioned interfaces, once an elastic threshold is surpassed, can be related to overall elastoplastic material behaviour of the hierarchical material ‘bone’. Extending this idea, the present paper is devoted to viscous gliding along these interfaces, expressing itself, at the macroscale, in the well-known experimentally evidenced phenomenon of bone viscoelasticity. In this context, a multiscale homogenisation scheme is extended to viscoelasticity, mineral-cluster-specific creep parameters are identified from three-point bending tests on hydrated bone samples, and the model is validated by statistically and physically independent experiments on partially dried samples. We expect this model to be relevant when it comes to prediction of time-dependent phenomena, e.g. in the context of bone remodelling.

Towards a microstructural model of bitumen ageing behaviour

When it comes to describe the mechanical behaviour of hot mix asphalt (HMA), the change of the mechanical properties over time due to environmental impacts known as ageing has to be considered. Hardening and embrittlement of bitumen lead to a reduced resistance against cryogenic cracks and premature formation of fatigue cracks in bituminous layers. Within this work, the microstructure of bitumen is investigated by conducting static shear creep tests on artificially composed bitumen with asphaltene contents varying between 0 vol-% and 26.71 vol-% as well as on a paving grade bitumen 70/100. Both are considered in unaged and laboratory-aged (rolling thin-film oven test + pressure ageing vessel) conditions to be able to identify and describe ageing effects. While the properties of the considered material phases of bitumen (identical to saturates, aromatics, resins and asphaltenes (SARA) fractions) seem to remain unaffected, an increase of the asphaltene content due to ageing can be observed. A micromechanical model is proposed that allows a prediction of the consequences of these microstructural changes on the mechanical behaviour of bitumen. Atomic force microscopy and environmental scanning electron microscopy images confirm a composition consisting of a micelle-like structure in a contiguous matrix, a structural representative volume element concept based on SARA fractions is suggested, extending an existing multiscale model for HMA. A very good accordance between model predictions and experimental results indicates the models ability to reproduce as well as to describe the effects related to ageing.

Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen

Ageing of bitumen leads to increased stiffness and brittleness. Thus, bituminous bound pavements become more prone to failure by low-temperature and fatigue cracking. Therefore, the ageing behaviour of bitumen has a crucial impact on durability, as well as recyclability of pavements. To assess ageing of bitumen, the rolling thin film oven test and pressure ageing vessel are standardised methods for short-term and long-term ageing in the lab. For lab-ageing of hot mix asphalt (HMA), various methods have been developed in the last decades. This paper presents a study on the potential of employing a highly oxidant gas for simulating the long-term oxidative ageing of asphalt mix specimens in the lab. Based on the results, an optimised lab-ageing procedure (Viennese Ageing Procedure – VAPro) for compacted HMA specimens to assess mix performance of long-term lab-aged specimens is developed. Thus, it is possible to optimise mix design not only for short-term performance but to take into account effects of oxidative ageing during its in-service life. VAPro is based on a triaxial cell with forced flow of a gaseous oxidant agent through the specimen. The oxidant agent is enriched in ozone and nitric oxides to increase the rate of oxidation. It is shown by stiffness tests of unaged and lab-aged specimens, as well as by Dynamic Shear Rheometer tests of recovered binder from aged specimens that asphalt mixes can be long-term aged at moderate temperatures (+60°C) and within 4 days and a flow rate of 1 l/min by applying VAPro. Thus, an ageing procedure is at hand that can simulate long-term ageing at conditions that are representative of conditions that occur in the field within an efficient amount of time.

Alternative Approach Toward the Aging of Asphalt Binder

Awareness that natural, financial, and energy resources are scarce goods has increased. Thus demand is growing for infrastructure that is not only of high quality but also efficient. Efficiency, in this case, aims to optimize cost and energy consumption over the complete life cycle of a structure. The objective is to build long-lasting infrastructure with low maintenance demands and with high recycling potential after it has reached the end of its service life. For bituminous bound materials, the aging of asphalt binder has a crucial impact on durability and recyclability. Because asphalt binder is organic by nature, the thermal and oxidative aging processes affected by chemical and structural changes occur when asphalt mixes first are produced and applied and continue over the course of their service life. Increasing stiffness and brittleness of the binder make pavement more prone to thermal and fatigue cracking. The interdisciplinary research project reported here worked toward a better understanding of the physicochemical fundamentals of asphalt binder aging, as well as of the impact of binder aging on the mechanical properties of asphalt binder and asphalt mixes. Through extensive chemical and mechanical analyses, a new model was developed to explain the aging process comprehensively (i.e., on the physicochemical and mechanical levels). Aging can be determined mathematically by micromechanical modeling. With the model presented in this paper, changes in asphalt binder as a result of aging (i.e., increasing brittleness and stiffness) can be explained.

Design of bituminous pavements – a performance-related approach

Pavement design in Austria is currently based on a catalogue of standard constructions distinguishing seven load classes and eight pavement types, which can be chosen depending on the traffic-related parameters AADT (annual average of the daily traffic for each vehicle type i) or AADTT (annual average of the overall daily truck traffic). This approach does not allow one to take performance-related material characteristics or detailed traffic load information into account. To resolve these limitations, a mechanistic approach for the design of bituminous pavements in Austria was developed. Thereby, design levels for the important parameters traffic load as well as hot mix asphalt stiffness and fatigue behaviour are introduced accounting for the level of detail of these variables. This approach ensures that necessary design reserves decrease with increasing experimental effort related to the parameter identification. Hence, this method provides modern, performance-based and economic pavement design.

Using Highly Oxidant Gas for Simulating Long-Term Ageing of Asphalt Mix Specimens in the Lab

Ageing of bitumen leads to increased stiffness and brittleness. Thus, bituminous bound pavements become more prone to failure by low-temperature and fatigue cracking. Therefore, the ageing behavior of bitumen has a crucial impact on durability, as well as recyclability of pavements. To assess ageing of bitumen, RTFOT and PAV are standardized methods for short-term and long-term ageing in the lab. For lab-ageing of hot mix asphalt (HMA), various methods have been developed in the last decades. This paper presents an optimized lab-aging procedure (Viennese Aging Procedure—VAPro) for compacted HMA specimens to assess mix performance of long-term lab-aged specimens. Thus, it is possible to optimize mix design not only for short-term performance but to take into account effects of oxidative aging during its in-service life. VAPro is based on a triaxial cell with forced flow of a gaseous oxidant agent through the specimen. The oxidant agent is enriched in ozone and nitric oxides to increase the rate of oxidation. It is shown by stiffness tests of unaged and lab-aged specimens, as well as by DSR tests of recovered binder from aged specimens that asphalt mixes can be long-term aged at moderate temperatures (+60 °C) and within 4 days and a flow rate of 1 l/min by applying VAPro. Thus, VAPro can simulate long-term ageing at conditions that are representative of conditions that occur in the field within an efficient amount of time.

Prediction of Hot Mix Asphalt Stiffness Behavior by Means of Multiscale Modeling

Cracking behavior and fatigue performance of hot mix asphalt (HMA) are the important factors, when it comes to evaluate the durability and structural lifetime of bituminous pavement constructions. Both effects are strongly affected by resulting stresses in the bituminous layers due to traffic and/or temperature change, whereat HMA stiffness has huge impact on the magnitude of these stresses and, hence, on the durability. It is obvious that a reliable characterization and prediction of HMA stiffness is crucial. A suitable way for the description of the viscoelastic response of a material is continuum micromechanics, where the mechanical, volumetric and morphologic properties of the constituents of the material are considered to predict the homogenized, “overall” material behavior. Thereby, the material is observed on different, reasonably chosen lengths scales allowing for a description of mechanical effects where they occur. These model assumptions were validated extensively showing good accordance between experimental results obtained from 4 PB-PR tests and model predictions with only the mechanical properties of the constituents (bitumen, aggregate) and the volumetric composition as model input. The results of this investigation suggest that applying the presented technique can lead to a significant reduction of experimental efforts in mix design.

Micromechanical Description of Bitumen Aging Behavior

For the design of durable pavement constructions, considering the change of material properties of hot mix asphalt over time is essential. Hardening and embrittlement of bitumen lead to a reduced resistance against cryogenic cracks and the premature formation of fatigue cracks in bituminous layers. This phenomenon called aging is induced by environmental impacts. Within this work, a micromechanical model extending an existing multiscale model for hot mix asphalt is proposed, which allows a prediction of the consequences of microstructural changes observed as a result of aging effects. On the basis of AFM and ESEM images, a microstructure of bitumen consisting of a contiguous matrix with an embedded micelle-like structure can be identified. Hence, a structural concept based on SARA fractions arranged in a representative volume element is suggested. Static shear creep tests on artificially composed bitumen with asphaltene contents varying between 0 and 30 wt% in aged and laboratory-aged (RTFOT+PAV) conditions were conducted to identify the properties of material phases as well as to validate the presented model assumptions. A very good accordance between model predictions and experimental results indicates that the model is able to reproduce as well as to describe significant microstructural effects related to aging.

Design of bituminous pavements in Austria - a mechanistic approach

The objective of pavement design is to develop pavement constructions, which are able to resist appearing traffic loads and climatic conditions during the intended life time. The concerning regulations and standards in Austria are based on semianalytical models to describe the mechanical response to these loads. As, thereby, standard constructions are derived for 7 load classes and 8 pavement types, neither actual data concerning traffic load, nor performance related material characteristics like stiffness or fatigue behavior of the hot mix asphalt (HMA) used can be considered leading to significant design reserves. Hence, a mechanistic design approach for bituminous pavements in Austria was developed, recently, and is expected to be released as a new national standard. Thereby, statistically derived model parameters or actually measured input parameters (traffic loads, HMA stiffness behavior, HMA fatigue behavior) or a combination of both are taken into account. The possible consideration of the advantages in mechanical behavior of modern bituminous mixtures not only allows more economic design results but also leads to the efficient use of bitumen, a crude oil product with decreasing availability.