| Author: | Chan, Kin Ming |
| Title: | Investigation of the intrinsic linear viscoelastic properties and fracture behaviours of asphalt materials |
| Advisors: | Wang, Yuhong (CEE) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Asphalt materials Asphalt -- Fracture Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxii, 216 pages : color illustrations |
| Language: | English |
| Abstract: | To develop asphalt pavements with superior fracture resistance against long-term repetitive traffic loadings, a deeper understanding of the mechanical behaviours of asphalt binders and mixtures is always one of the main goals in pavement engineering. To facilitate material design, the fracture behaviours of the binders and mixtures should be studied with consideration of the linear viscoelastic properties of the materials and other factors which are controllable in the design process. However, the current understanding of the facture behaviours remains immature due to inaccurate identification of the material properties and limited considerations of factors affecting the behaviours. For example, current methods of identifying the shear viscoelastic properties of asphalt materials often show inconsistencies with regard to their theoretical relationships. Another example is that the microscopic heterogeneities of asphalt binders are seldom considered in existing fatigue life prediction models since most of them assume the binders are homogeneous. To gain a comprehensive understanding of the fracture of asphalt materials, the study presented in this thesis investigated the intrinsic linear viscoelastic properties of asphalt materials and their fracture behaviours. The first part of the study is focused on the accurate characterisation of the linear viscoelastic properties of asphalt materials. Due to the nature of viscoelasticity, the solutions of the properties given by phenomenological approaches are sensitive to experimental noises and variations. As a result, for the dynamic shear properties, existing characterisation methods often lead to inconsistent solutions deduced from the storage modulus and loss modulus; the tensile properties are commonly only prescribed empirically by the shear properties. This critically affects the results of the proposed models for studying fracture behaviours. Thus, the first part of this study provides some new methods to accurately determine the linear properties of asphalt materials. The shear properties are obtained by a joint optimization which simultaneously minimises errors between the data obtained from dynamic shear tests and the prediction models for the master curves of complex modulus and phase angle, together with the differences between relaxation spectra deduced from the storage modulus and loss modulus. The tensile properties are obtained by a tensile relaxation test developed in this dissertation. The continuous tensile relaxation spectrum is numerically derived based on the integral equation that connects the modulus to the spectrum, with L-curve regularisation to solve the overfitting issue of the ill-posed equations. The methods can then be used to accurately characterise the shear and tensile properties of asphalt materials, which are used as the inputs for the models in the later part of this study. The second part of the work establishes several novel models and simulation approaches to analyse the fracture behaviours of asphalt materials. It is well-known that fracture behaviours of aged asphalt binders play a critical role in determining the performance of asphalt mixtures, particularly for their fatigue cracking resistance. Moreover, many studies show that the fatigue lives of asphalt binders and mixtures are strongly related. It is also widely known that the performance of field-aged and laboratory-aged binders are quite different, and this is believed to be due to the difference between their microstructures formed in different ageing conditions. However, current studies of the fracture behaviours of aged binders seldom considered the microscopic details. Therefore, the second part of the study proposes a microscale two-dimensional extended finite element analysis of asphalt binders, based on the thin-film images obtained from transmission electron microscopy (TEM). Relationships between crack initiation and microstructural characteristics are evaluated. Results show that the geometry of asphaltenes in aged binders has a stress concentration effect. Based on this finding, a fatigue life prediction model is proposed for the aged binder with consideration of the asphaltene microstructures and the self-healing process in the crack initiation stage. The parameter, ηφ, the product of the viscosity of the matrix and the total volume fracture of the micro-sized colloids, can be used to effectively model the fatigue cracking resistance of aged binders, and a lower value of ηφ means better fatigue resistance. In addition, one of the well-reported facts in pavement engineering is that the fatigue lives of asphalt mixtures found in experiments are often different from the fatigue lives found in field situations. This is partially attributed to the fact that pavement strains at the cracking initiation zones induced by actual traffic loadings are both compressive and tensile while the loadings of experimental tests often only induce tensile strains at the cracking initiation zones. Inspired by the fluidity effects on the self-healing of asphalt binders, this study hypothesises that asphalt binder healing is affected by the motion of the flowable mortar, the motion is significantly influenced by the loading waveform, and the healing effect leads to the deviations of the estimations given by the tests. A discrete element method (DEM) analysis of an asphalt mixture specimen based on a realistic internal structure from the digital image is adopted to verify the above hypothesis. The proposed method shows that the compressive loading enhances the healing effect since, compared with the tension-only loading, the inward motions of the flowable mortar in the compression-tension loading will last longer and heal the cracks. The study thus suggests the influences of loading waveforms on the self-healing effects are essential in the fatigue cracking behaviours of asphalt mixtures. |
| Rights: | All rights reserved |
| Access: | open access |
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