Ronald Blab

Identification of four material phases in bitumen by atomic force microscopy

ABSTRACT The identification of material phases at the so called bitumen-scale in context of multiscale modeling of asphalt is presented. For this purpose, atomic force microscopy (AFM), providing insight into the surface topography and mechanical properties, is applied to different types of bitumen. Based on the obtained AFM results, four different material phases at the bitumen-scale are identified. These phases are related to the chemical composition of bitumen, characterized by a wide range of molecular mass. Within the anticipated multi-scale model, the properties of these phases serve as input for upscaling, providing material parameter of the bitumen phase at the next-higher scale, i.e., the mastic-scale.

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.

Enhancing triaxial cyclic compression testing of hot mix asphalt by introducing cyclic confining pressure

Permanent deformation in terms of rutting is a major deterioration mode of bituminous bound pavements. The triaxial cyclic compression test (TCCT) is a scientifically accepted and standardised test method to assess the resistance to permanent deformation. Presently, the standard TCCT according to EN 12697-25 is carried out with cyclic axial loading and constant confining pressure. In road pavements, dynamic traffic loading due to passing tyres leads to cyclic confining pressures. Thus, to bring the TCCT closer to reality, within the study presented in this paper, (a) the radial reaction and its phase lag to axial loading in standard TCCTs is measured and (b) an enhanced TCCT with cyclic confining pressure which takes into account the viscoelastic material response in terms of radial phase lag to axial loading is introduced. In a subsequent test programme, TCCTs with various confining pressure amplitudes are run on different materials and results from standard and enhanced TCCTs are analysed and compared in terms of resistance to permanent deformation. It is shown that the resistance to permanent deformation increases significantly when the viscoelastic material response is taken into account in the TCCTs with cyclic confining pressure.

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.

Experimental identification and mechanical interpretation of the interaction behaviour between concrete paving blocks

In recent years, concrete block (CB) pavements have become a favourite alternative to asphalt pavements, mainly in intra-urban regions due to their architectural design possibilities. Unfortunately, this trend is restrained by a lack of adequate design methods to assess the load capacity and durability of such pavements. Especially, the mechanical performance of the vertical joints between CBs is often not depicted realistically enough. For this reason, in this work three new experiments are proposed to determine the mechanical behaviour of the joints between the CBs, and thus the load transmission capability of different joint formations. Mechanical models and the corresponding material parameters to describe the joint behaviour are identified from the experimental results. Finally, performance optimisation of block pavements with respect to their jointing behaviour should become possible.

Assessment of Permanent Deformation Behavior of Asphalt Concrete by Improved Triaxial Cyclic Compression Testing

For the characterization of the permanent deformation behavior (rutting) of asphalt concrete (AC), the triaxial cyclic compression test (TCCT) is a standardized test method. The test simulates traffic loading by applying a sinusoidal compressive stress in vertical direction and a radial confining pressure, which simulates the confinement of the specimen within the pavement structure. As a result the permanent axial strain versus load cycles is obtained. In the standard test procedure the confining pressure is held constant throughout the test to simplify the test control. However, in reality the confining pressure oscillates in a sinusoidal way with a certain phase lag to the vertical loading due to the viscoelastic characteristics of AC. The phase lag of hot mix asphalt (HMA) depends on the temperature and load frequency. The paper presents an innovative approach to improve the TCCT by implementing sinusoidal confining pressure. Strain gauges are attached directly to the specimens surface to measure the radial deformation and to obtain the phase lag between the axial loading and the radial reaction. These experiments are carried out for various mixtures at temperatures ranging from 10°C to 50°C and frequencies from 0.1 Hz to 30 Hz. Thus, radial strain and phase lag can be analyzed as a function of temperature and loading frequency. In a second step the sinusoidal confining pressure with the obtained phase lag will be implemented into the test routine and results from standard TCCTs with constant confining pressure versus improved TCCTs with oscillating pressure can be compared and discussed.

Characterization and Multiscale Modeling of Asphalt - Recent Developments in Upscaling of Viscous and Strength Properties

The assessment and prediction of the performance of multi-composed materials, such as e.g. asphalt, requires suitable procedures for identification of their mechanical properties. In case of asphalt used for trafficked pavements, these properties vary with the underlying mix design (volume fractions and used constituents) and additives (e.g., polymers). In the past, the mix design and the allowance of additives were optimized, aiming at (a) a low viscosity at high temperatures (T > 135 °C) for the construction and compaction process of high-quality asphalt layers, (b) a significantly higher viscosity at medium temperature in order to minimize the development of permanent deformations (rutting), and (c) sufficient relaxation behavior at sub-zero temperatures, avoiding low-temperature cracking (see failure modes in Figure (left)). Motivated by the large variety of asphalt mixtures resulting from this optimization process and the necessity of predicting the future performance of pavements, a multiscale model for asphalt is currently developed at TU Wien. It relates macroscopic properties to finer-scale information (such as volume fractions, morphology, and the behavior of material phases) by introducing, in addition to the so-called macroscale (i.e., the scale at which prediction analyses are performed), four finer scales of observation, ranging down to the so-called bitumen-scale (see Figure (right)). Open image in new window (left) failure modes in asphalt pavements and (right) multiscale model for asphalt

Low-temperature performance prediction of asphalt mixtures used for LLP—new approach based on fundamental test methods and numerical modeling

A major mode of deterioration in fully-flexible long-life pavements (LLP) is surface-initiated top–down cracking. If surface maintenance is suspended, surface-initiated cracks may conflict the long-life concept, as they will propagate into the structural layers and structural deterioration will start. The risk of surface-initiated top–down cracking is influenced by a number of interacting factors. Critical stresses may result, e.g. from the 3D loading situation at or near the pavement surface due to contact stresses between the tire and the pavement surface, from a sudden change in the asphalt layer temperature gradient, from traffic loading at low-temperatures and from material related factors like asphalt ageing and particle segregation due to stripping of the binder. Cracks may also be initiated by early construction micro-cracks induced by the asphalt roller. Depending on the respective mechanism of initiation, surface cracks are observed to be orientated in both directions, transversally to the road axis and longitudinally and longitudinal cracks are found inside as well as outside the wheel-path. In this study, the combined effect of tensile stresses resulting from both traffic and thermal loading in a typical fully-flexible LLP structure is investigated. Supposed that the risk of surface-initiated cracking can be minimized by using an appropriate asphalt material, the main objective of this paper is to find important missing links between the characterization of fundamental low-temperature properties of an asphalt mixture, on the one hand, and its in-service behaviour with special regard to the risk of surface-initiated top–down cracking at low-temperatures, on the other hand. Based on fundamental test methods, material modeling and numerical simulation, an overall concept is presented that will enable a better prediction of the low-temperature performance and a better risk evaluation of surface-initiated cracking at low-temperatures. First promising results are given for a typical fully-flexible LLP structure that is exposed to prescribed temperature and loading scenarios. Realistic critical stress distributions at the road surface in consequence of combined thermal and traffic loading are found that allow assessment of the risk of longitudinal surface-initiated top–down cracking at low-temperatures.

Influence of compaction direction on performance characteristics of roller-compacted HMA specimens

Hot mix asphalt (HMA) slabs produced by roller compaction can be used to core and cut specimens for further testing. The relation between the direction of compaction and testing in the laboratory is not always the same relation as it is between the direction of compaction and actual loading in the field. This paper presents outcomes of a study analysing the influence of the compaction direction on performance characteristics of roller-compacted HMA specimens. Performance parameters of a base layer mix are obtained from performance-based test methods, including high-temperature, stiffness, fatigue and low-temperature tests. The relation between direction of compaction and specimen testing is varied in all three dimensions to find relevant influences. From the results, it can be concluded that all obtained performance parameters are sensitive to the anisotropy of the material due to compaction, especially for stiffness and fatigue performance. For the high-temperature performance, specimens from path- and force-controlled compaction were compared. The applied compaction work rather than the compaction method is linked to the difference in the corresponding results. The uniformity of the compaction in terms of the variation of bulk density of the specimens reflects on the scattering of test results.