Bernhard Hofko

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.

Effect of short-term ageing temperature on bitumen properties

Properties of asphalt mixtures after ageing are fundamental parameters in determining long-term performance (e.g. durability) of these materials. With increasing popularity of reduced temperature mixtures, such as warm-mix asphalt, WMA, the question remains how a reduction in short-term ageing affects the properties after long-term ageing of bituminous materials. This paper aims to improve our understanding of the effect of asphalt manufacturing temperature on ageing and the resulting mechanical properties of bituminous binder by studying the effect of short- and long-term ageing of different bitumen samples as a function of short-term ageing temperatures. For this purpose, round robin experiments were conducted within the RILEM technical committee (TC) 252 chemo-mechanical characterisation of bituminous materials by 10 laboratories from 5 countries using four binders of the same grade (70/100 pen) from different crude sources. The short-term ageing was carried out using the standard procedure for rolling thin film oven test (RTFOT), but varying the temperatures. Long-term ageing was carried out using the standard procedure for pressure aging vessel (PAV) in addition to RTFOT. For the mechanical characterisation, rheological data were determined by using the dynamic shear rheometer (DSR) and conventional tests, with needle penetration and softening point using the ring and ball method. The results show that although different short-term ageing temperatures showed a significant difference in the mechanical properties of the binders, these differences vanished after long-term ageing with PAV.

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.

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.

Technological and environmental performance of temperature-reduced mastic asphalt mixtures

Temperature reduction of mastic asphalt (MA) mixtures can decrease costs and energy demand, as well as health-relevant emissions of particulate matter throughout the life cycle. A state-of-the-art method for temperature reduction is wax modification of the bituminous binder to reduce its viscosity. In this paper, the results of an extensive study. On mechanical performance, particulate matter emission and life-cycle analysis of temperature-reduced MA are presented. Therefore, a reference MA is compared to three temperature-reduced MAs: a state-of-the-art reduction by modification with amide wax (AW) and an alternative method of substituting crushed aggregates by rounded ones. For both methods, a temperature reduction of 30°C can be realised. In addition, a combination of both methods, wax modification and use of rounded aggregates, is investigated. For this combination a reduction of 50°C is possible. The results show that the resistance to permanent deformation is not decreased by using rounded aggregates and that it can be doubled by employing AW regardless of the aggregate shape. Resistance to low-temperature cracking is not affected by any of the studied methods for temperature reduction. Emission analysis reveals that more than 80% of the emitted particulates are below 2.5 µm a.d. (aerodynamic diameter). From a life-cycle perspective, a main benefit of temperature-reduced MAs is the significant decrease in particulate emissions by up to 80% in case of 50°C temperature reduction. Also, up to 20% of production process energy can be saved when the mixing temperature is reduced by 50°C. Application of a wax additive reduces process energy costs, but increases the total life-cycle costs. Based on the considered scenarios, the application of additives is controversial and the substitution of crushed aggregates by rounded aggregates seems to be beneficial.

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.

A laboratory procedure for predicting skid and polishing resistance of road surfaces

Abstract Skid resistance of road surfaces is an important safety parameter. Decreasing skid resistance correlates with increasing number of accidents. The presented study aims for a correlation between lab-based polishing simulation and skid resistance measurement and evolution of the skid resistance under traffic in the field. A prediction model for skid and polishing resistance based on the Wehner/Schulze device was developed. The lab procedure as a basis for the model works on the hypothesis that a maximum level of skid resistance exists that can be regenerated even from a highly polished state by sandblasting of the surface. This hypothesis was verified for an asphalt and concrete surface material. The prediction model was set up by correlating field data on skid resistance and traffic volume from 14 test sections to cores taken from these test sections and tested in the lab. A linear regression links the cumulated traffic volume in the field with the number of polishing passes in the Wehner/Schulze device. Thus, it is possible to simulate millions of wheel passes within a couple of hours in the lab to generate equal skid resistance level and predict the skid resistance level of a road surface after years under traffic.