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

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

Nahar, S.N.

Scarpas, A. (promotor)
Schitter, G. (promotor)

Civil Engineering and Geosciences

Structural Engineering

2016-02-08

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.

bitumen
healing
microstructure
phase-separation
Atomic Force Microscopy

https://doi.org/10.4233/uuid:670c70ff-f9f0-4cdb-aa4d-b661e7117354

978-94-6186-598-4

Institutional Repository

doctoral thesis

(c) 2016 Nahar, S.N.