Phase-Separation Characteristics of Bitumen and their Relation to Damage-Healing

During the service life of flexible asphalt pavements, asphalt concrete degrades due to traffic loading and environmental conditions like temperature, rain, oxidation, ultraviolet-radiation from the sun. All these environmental factors have adverse effects on the performance of bitumen, which is the binder of asphalt concrete. They are known to cause ageing and eventually lead to hardening of bitumen. As a result, ravelling (i.e. release of stones from asphalt concrete) and cracking are observed as main distress mechanisms in asphalt concrete. These distress phenomena reduce the life span of the asphalt pavement, necessitate frequent maintenance and eventually complete replacement of the asphalt. Innovative solutions with a focus on better binder properties can improve this situation. Bituminous materials with improved properties can make the rate of deterioration slower and may offer fast, efficient and cost effective repair methods. Self-healing is a desirable property in this respect, which can improve the service life as well as reduce the maintenance cost of the roads. Bitumen is self-healing by nature. Micro-cracks that occur in bitumen during service may heal at rest and the early stages of cracking are self-repairable. But the knowledge on the mechanism of damage, healing and also the fundamental properties of bitumen is inadequate. The aim of the thesis is to understand the phase-separation characteristics of bitumen at the microstructural level and their relation to damage and healing processes within the material. Atomic Force Microscopy (AFM) has been used to investigate the bitumen morphology, phase-separation and mechanical response properties at nano to micro meter length scale. From the AFM investigation, microstructure is found to be a unique fingerprint of the bitumen type. Typical bitumen microstructure possesses a two-phase morphology: the domains (i.e. bee-structures) and the matrix phase. Chemical composition of bitumen is the key parameter which influences the microstructure properties, while wax and asphaltene fractions are responsible for most of the structuring observed. The wax component has been found to induce the phase separation of bituminous materials. Temperature during construction of asphalt and its change during the service life influence the properties of bitumen to a great extent. Thus, the influence of environmental conditions like temperature, thermal history and humidity on bitumen microstructure have been investigated. From the temperature and thermal history study, hysteresis in microstructure properties of bitumens between heating and cooling cycles has been observed. The rate of cooling of the material influenced the microstructure properties. Besides, high humidity conditions can be detrimental to bitumen performance as it can introduce regions of heterogeneous properties within the material. The mechanical response properties of bitumen at the fundamental length scale have been investigated. The mechanical property maps of modulus and AFM probe-sample adhesion force of the individual phases of bitumen at the microstructural level are obtained using a special mode of AFM. The domains are found stiffer than the matrix phase, whereas the matrix phase has shown greater adhesion property. These individual phase properties have been used and a mechanics based approach has been followed to derive the composite modulus property of bitumen. With the change of temperature, changes in the mechanical properties of the individual phases and the subsequent composite response of the material are observed. Microstructural change at the onset of crack formation in bitumen has been probed during mechanical loading. After application of tensile load, micro-cracks or crazes tend to originate at the interface between the phases and localize in the domain phase- leading to a significant microstructural change. By allowing rest periods or moderate thermal changes, re-arrangement of the microstructures are observed; resulting in disappearance of cracks. The extent of blending between reclaimed binder and the fresh bitumen in the case of recycling of asphalt has been investigated. It is proposed that the degree of interaction between the binders depends on the temperature and the mixing time of the materials in the recycling process. During the process of ageing, bitumen is hardened and the adhesion property deteriorates. For service life extension of asphalt pavement, additives (i.e. rejuvenators) are used to improve the adhesion of the aged bitumen and to decrease the viscosity of the binder. This process of rejuvenation has been probed at the microstructural level. The addition of rejuvenators to the aged bitumen has shown property restoring performance from both the rheological data and microstructural properties of the binders. Understanding the micro-scale material properties can help to understand the long term macro-scale material response properties. The research presented in this thesis will guide to a better understanding of the material response in relation to both environmental and mechanical changes at microstructural level. The microscale assessment of bitumens is a step forward towards associating the observed structures with the material's mechanical response properties.Structural EngineeringCivil Engineering and Geoscience

Probing Trace-elements in Bitumen by Neutron Activation Analysis

Trace elements and their concentrations play an important role in both chemical and physical properties of bitumen. Instrumental Neutron Activation Analysis (INAA) has been applied to determine the concentration of trace elements in bitumen. This method requires irradiation of the material with neutrons that transform the elements into radioactive isotopes. By analyzing the activity of the individual nuclides, the concentration of each detectable trace element can be determined with high precision. In this work, we perform trace elemental analyses of 13 distinct bitumens, including 2 modified and 3 bitumens from the material library of Strategic Highway Research Program (SHRP. Three elements, vanadium, nickel and cobalt are found to be present in all bitumens. Vanadium and nickel are found to be the most abundant among all the elements detected. Next to vanadium and nickel, significant concentrations of iron are found in 11 bitumens. The total number of trace elements identified varied from 17 to 28 for the bitumens studied. For modified bitumens, the concentration of trace elements is used as a parameter to measure the extent of modification. The sum of most abundant trace elements (vanadium and nickel) correlates well with the sulphur and asphaltene contents of the same bitumen. Moreover, the concentration of the latter metals are known to be an indicator for the aging characteristics of bitumen. Thus, INAA provides the content of trace elements in bitumen, where the concentrations vary (ppm to ppb) depending on the crude origin of the material. Thus, INAA can be used to trace back the crude origin of the material, which may have applications in the field of asphalt recycling (RAP and RAS).Pavement Engineerin

RILEM TC272 PIM: phase morphology of bituminous binders with liquid additives

In the past years, the use of liquid additives as bitumen modifiers has increased to tailor the rheology of bitumen for a wide range of applications. Their chemical composition and mutual interaction result in specific phase morphologies in the binders. Hence, there is a need to evaluate the phase morphology of complex binders and the impact of additives on their physical properties. The RILEM Technical Committee 272-PIM ‘Phase and Interphase behaviour of innovative bituminous Materials’, Task Group TG1 assessed the phase and interphase properties of bituminous binders. Some preliminary results are presented on blends using three liquid additives and a neat 35/50 bitumen. The goal of formulating the blends was to achieve similar consistency of a pen grade 70/100 bitumen at the original state and to evaluate the binders at both original and after aging. Physical properties were evaluated through rheological characterisation using a dynamic shear rheometer (DSR) in a wide range of conditions. The phase morphology was assessed using atomic force microscopy (AFM). Differential scanning calorimetry (DSC) was also used for the characterisation of the thermal behaviour of the binders. While conventional properties, as obtained from the routine binder testing methods, hardly distinguish between blends, the cross-over temperature, derived from DSR measurements, enabled to dictate the impact of liquid additives on the physical properties of bituminous binders at intermediate temperature. AFM confirmed a difference in phase morphology between the blends, whereas some binders displayed new phases at original and aged conditions. Glass transition, as determined by DSC, also showed a difference in the low-temperature domain that may be explained with the difference in phase morphology. Overall, an in-depth understanding of microstructure morphology and glass transition behaviour of complex binders can assist in designing future specifications to distinguish durable bituminous materials better.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Pavement Engineerin

Temperature induced healing in strained bituminous materials observed by atomic force microscopy

Bitumen is the binder in the composite material named asphalt concrete. Under cyclic mechanical loading of traffic passing over the pavement, eventually damage will initiate in the pavement, leading to eventual structural failure. This damaging process is accelerated by time dependent change of the mechanical properties of asphalt concrete due to ageing mechanisms like oxidation. Bitumen displays spatial heterogeneity at the micrometer scale, which has been observed by atomic force microscopy (AFM). The mechanical properties of the elliptical, microstructural domains of bitumen are distinct from those of the continuous phase. This introduces stiffness discontinuities in the material, which under mechanical loading will concentrate stresses at the interfaces, and thus the locations where early stages of damage will develop. This work aims at in situ probing of the crack healing of bituminous materials as a function of moderate temperature changes. The bitumen was prepared on a flexible substrate which was mechanically strained to induce damage. AFM measurements of the strained bitumen specimen provides evidence of the crack initiation at the interface and the predominant propagation of cracks through the elliptical domain phases. Healing of these cracks was observed after applying modest amounts of heat to the material. Meanwhile the process was monitored in situ with AFM. With increase of temperature one of the phases starts softening, while the material as a whole remains solid. This allows the phases to rearrange and meanwhile eliminating micro cracks at the interface

Microstructural Changes in Bitumen at the onset of Damage-healing

Self-healing of bitumen is a property that positively contributes to the sustainability, maintenance requirements and cost effectiveness of asphalt pavements. Ideally one would like to design an asphalt mix with a well-defined healing potential. Although substantial research efforts have been dedicated to the healing mechanism in bitumen, complete understanding of the fundamental mechanisms that govern the property of healing is still lacking. Here we investigate the manifestation of damage and healing of bitumen at the microstructural level. Three distinct bitumen grades are subjected to mechanical loading conditions, and the damage is investigated at the microstructural level by atomic force microscopy combined with finite element simulations. One of the bituminous phases appears to display visible signs of cracks, which are found to (partly) disappear at moderate temperature changes. Simulations of mechanical loading of experimentally derived finite element meshes are corresponding well with these experimental observations. Moreover, the simulations provide a measure of mechanical response, i.e. stiffness, of the material as a function of strain level. From this it is found that the microstructural cracks lead to diminished structural response properties, whereas after healing these properties are partly recovered. The experimental observations, together with the simulations, support earlier ideas that relate the phenomenon of self-healing in bitumen to their rheological property of thixotropy. Moreover, the work presented hints that the property of self-healing is governed by processes at the microstructural length scale.Pavement Engineerin

Vergelijkend AFM Onderzoek: Microstructuur van bitumen in relatie tot healing

In this report we present the background, the scientific and experimental approach and the results of AFM experiments performed on two different batches of bitumen. The specific bitumen researched in this project has also been studied in the context of the InfraQuest project ‘Pragmatisch Healing Onderzoek’. It has been known for quite some time already that bitumen posessess a microstructure at the typical length scale of micrometers. This can be shown experimentally by imaging the bitumen surface with Atomic Force Microscopy (AFM). As is the case for many other engineering materials (e.g. steel), the microstructure will manifest itself by the macroscopic mechanical response of the material; thus on the typical length scales where it performs its load bearing function in pavement structures. Together with the hitherto not precisely specified properties of the many available bitumen grades, this justifies further research into the origin and properties of this microstructure. Here we also anticipate that a better understanding about the origin and properties of the bitumen microstructure will lead to improved bitumen grades (material appraisal) and possibly to better criteria for selecting a bitumen for a specific application. In the context of this research first the objectivity of the AFM imaging technique has to be established. Therefore two independent laboratories (TNO and CiTG, TU Delft) have prepared and conditioned bitumen samples for the AFM. All samples have been prepared from the same batch of bitumen. Then each laboratory has imaged its ‘home made ‘samples’ as well as the samples prepared at the other lab. The results appear to be qualitatively identical. Thus one may conclude that the microstructure of bitumen is a reproducible quantity. It was also found that the (thermal) conditioning of the bitumen (prior to imaging) has a significant impact on the microstructure observed. One may conclude from this that the sample conditioning procedure is a very important aspect in the AFM imaging process. In other words: an AFM image of bitumen is meaningless, unless the conditioning procedure of the samples is reported extensively. The next step was to find the influence of temperature on the observed bitumen microstructure. Identical samples have been prepared by TU Delft and both laboratories have imaged the microstructure as a function of temperature. A similar observation as stated before has been made: the microstructures observed by both laboratories were very similar. Moreover, it has been observed that the microstructure gradually disappears when the temperature is raised. However, even at the highest (experimental) temperatures (70 °C) traces of the microstructure remain visible. Apparently the ordering process that governs the bitumen microstructure has an associated interaction energy in the order of 400 kB, i.e. 30-40 meV (kB, Boltzmann constant). It was also found that (chemically) reclaimed bitumen (from an asphalt test beam) does show a microstructure as well. Surprisingly however, it was found that the microstructure of harder bitumen grades disappears at lower temperatures compared to softer bitumen grades. This is against the intuition that in harder bitumen molecules are more tightly bound together than in softer grades, and that for harder bitumen the microstructure would ‘melt’ (disappear) at higher temperatures. The molecular mobility appears to be higher in harder bitumen grade, hence they are anticipated to be better ‘healers’. Macroscopic fatigue test have shown similar trends.Structural EngineeringCivil Engineering and Geoscience

Is Atomic Force Microscopy suited as Tool for fast Screening of Bituminous Materials? An Inter-laboratory Comparison Study

Bituminous binders are known to have microstructures at typical length scales of micrometers. This microstructure can be probed with Atomic Force Microscopy (AFM). Now that worldwide several research groups are reporting AFM results on bitumen, it is becoming important to improve the understanding of the reproducibility and objectivity of the technique for studying bituminous samples. When reproducibility and stability are proven, AFM can be a tool for asphalt professionals to rapidly screen bituminous binders. In this context two independent laboratories have developed a standard method for preparing and conditioning bitumen for AFM imaging. By means of an inter-laboratory comparison of independently imaged specimen, the reproducibility of microstructure measurements was investigated. A quantitative comparison on different microstructures was developed, and the consistency of independently obtained results was confirmed. The results from both labs were comparable: the microstructural properties were found to be randomly distributed within a 5% interval. Also the influence of temperature on the microstructure was demonstrated to be reproducible and consistent. With the increase of temperature, the microstructure gradually disappeared, however traces of the microstructure remained visible up to the highest measurement temperature of 60°C. It is concluded that given well defined sample preparation and measurement procedures, the microstructure of bitumen can be reproducibly imaged by AFM from room temperature up to temperatures where bitumen has become soft and too sticky to be probed by the same setup as used for lower temperatures.Structural EngineeringCivil Engineering and Geoscience

Turning Back Time: Rheological and Microstructural Assessment of Rejuvenated Bitumen

Countermeasures to the ageing of bituminous asphalt binders is a highly important topic, both for service-life extension of asphalt ‘in the field’ and for recycling old pavements (RAP) into new structures with similar functional requirements as the original structure. Usually this is achieved by applying additives that restore the adhesive and mechanical properties of the original bituminous binder. These additives are commonly termed (asphalt) rejuvenators. Here we examine the performance of two very distinct rejuvenating agents. Usually the effectiveness of rejuvenators is measured by comparing the penetration and softening point of the rejuvenator-aged bitumen blend to reference values of the virgin binder. First, the rejuvenating capabilities of the two additives are evaluated in terms of rheology using a dynamic shear rheometer. Then the microstructures of the virgin binder and the rejuvenated blends are obtained by means of atomic force microscopy. Subsequently the rheological results are related to the microstructure morphologies. One finds from rheology that both rejuvenators exhibit the desired softening and property restoring performance. Though, one rejuvenator does so at much lower dose rates. By correlating rheology to the microstructural observations one finds that the effect of both rejuvenators is very distinct at microscopic length scales: rejuvenation is achieved by distinct chemo-physical mechanisms. One of the rejuvenators restores the virgin microstructure, whereas the other rejuvenator generates a new morphology. Thus, it is demonstrated that by combining rheological and microstructural techniques, the mechanism and performance of rejuvenation can be understood. This may guide future designs and optimization of asphalt rejuvenating agents.Structural EngineeringCivil Engineering and Geoscience

Impact of maltene and asphaltene fraction on mechanical behavior and microstructure of bitumen

As a widely accepted concept, bitumen consists of four fractions that can be distinguished by their polarity. Highly polar asphaltene micelles are dispersed in a viscous phase of saturates, aromatics and resins (maltene phase). Different concentrations of asphaltenes in the bitumen result in a range of mechanical response properties. In an interdisciplinary study the impact of the maltene phase and asphaltenes on the linear viscoelastic behavior and the microstructure of bitumen were analyzed by creep recovery testing in a DSR and by atomic force microscopy (AFM). Therefore, bitumen was separated into the maltene and asphaltene fractions and artificial bitumen samples with different, pre-defined asphaltene concentrations were produced and investigated. It was found that the artificially produced, precipitated bitumen samples can be regarded as a representative, bitumen-like material in terms of mechanical behavior and microstructure. Asphaltenes play an important role in the typical viscoelastic behavior of bitumen being mainly responsible for stiffness and elasticity. Also, their concentration appears to be correlated to the occurrence and shape of the bee-like inclusions which can be typically observed by AFM.Structural EngineeringCivil Engineering and Geoscience