Markus Hospodka

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.

Fluorescence spectroscopic investigation of bitumen aged by field exposure respectively modified rolling thin film oven test

During ageing the chemical composition of bitumen changes, which is reflected by the change in the distribution of bitumen fractions. By measuring the spectrum of fractions, and by correlating fractional changes during ageing within bitumen with the corresponding spectra, a connection between bitumen’s age and the fluorescence spectrum can be drawn. While the age of bitumen increases, its fluorescence emission intensity decreases. Therefore non-aged, laboratory aged (rolling thin film oven test (RTFOT) and RTFOT + pressure ageing vessel) bitumen and bitumen extracted from asphalt slabs from a test field after different time periods were investigated. The test field confirmed the connection between intensity and ageing time, further the development of an oxidation gradient over the vertical cross section of the asphalt slabs could be observed. A modified RTFOT was conducted to study the effects of different temperatures and operating times on the fluorescence emission spectrum, as well as the influence of air (containing traces of atmospheric radicals) by conducting the test with nitrogen. Even under nitrogen atmosphere the intensity decreased, which is assigned to internal reactions.

Rolling Thin Film Oven Test and Pressure Aging Vessel Conditioning Parameters: Effect on Viscoelastic Behavior and Binder Performance Grade

This paper presents a sensitivity analysis of two parameters applied for the rolling thin film oven test (RTFOT) and pressure aging vessel (PAV) aging on the viscoelastic behavior and performance grade of asphalt binder PG 58-22. For the RTFOT, the temperature was varied from the default temperature of 163°C to 143°C and to 183°C. For the PAV, the binder film thickness was varied from the default 3.2 mm to 1.0 and 5.0 mm. Dynamic shear rheometer (DSR) tests were run with a temperature sweep from 46°C to 82°C at a frequency of 1.59 Hz on the virgin binder, as well as on the RTFOT and RTFOT+PAV-aged samples. Bending beam rheometer (BBR) tests were carried out at −12°C, −18°C and −24°C on the RTFOT+PAV-aged samples. The effect of the mentioned conditioning parameters on |G*| and δ—as well as on the upper and lower performance grade—was investigated. Results show that the effect of RTFOT temperature on the |G*| and δ is more distinct for lower DSR temperatures. In general, the effect of a change in the RTFOT temperature on the viscoelastic parameters can be considered as small. At 46°C DSR, a change of 1°C in the RTFOT temperature shifts |G*| by 2.2% and δ by 0.08°. The PAV binder film thickness has an exponential effect on |G*| and δ. A reduced RTFOT temperature leads to reduced long-term aging after PAV. Reduced short-term aging owing to reduced production temperatures results in significantly reduced long-term aging. The effect of RTFOT temperature and PAV binder film thickness on upper and lower performance grade was found to be insignificant.

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.

Impact of distillation temperature on the solvent residue and viscoelastic properties of asphalt binders

Asphalt used as a paving material is a mix of aggregates and bituminous binder. Due to reasons of acceptance tests after paving or for fault analysis, it is necessary to separate asphalt mixes into their components. Therefore, different organic solvents are commonly used. The European Standard defines a procedure for both, recovery of aggregates and the distillation of binder from solvent mixture. Within the distillation procedure, the applied maximum temperature and the time of its application are not defined clearly. This research is an approach to define the distillation temperature depending on the viscosity of the binder. Four different binder samples (unmodified, polymer-modified) were used to determine the impact of distillation temperature on solvent residue and the recovered binder and its rheological properties. The solvent residue was calculated by weight and the binder properties were obtained by a dynamic shear rheometer and rotational viscometer. These results show a dependence of the solvent residue on the viscosity at distillation temperature. Thus, it is possible to choose reasonable distillation temperatures with regard to binder viscosity instead of setting one temperature for all binders. It was also found that there is no impact of tetrachloroethene as solvent on the binder properties if its residue is low enough (<1% w/w).

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.