Author: Tan, Zhifei
Title: Multiscale characterization and modeling of the viscoelastic tension-compression asymmetry of asphalt concrete
Advisors: Leng, Zhen (CEE)
Degree: Ph.D.
Year: 2023
Subject: Asphalt concrete
Pavements, Asphalt concrete
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xi, 125 pages : color illustrations
Language: English
Abstract: The mechanical behavior of asphalt concrete (AC) plays a dominant role in the service life of asphalt pavements, and a better understanding of AC can contribute to enhancing the durability of asphalt pavements. Conventionally, AC has been regarded as an isotropic linear viscoelastic (LVE) material, and its measured moduli in compression are used for pavement design. However, AC displays significant tension-compression (TC) asymmetry even at small strains by showing much lower stiffness in tension than in compression. Overestimating AC's tensile performance may lead to tensile failures in asphalt pavements. Nevertheless, the understanding of such asymmetric behavior is limited and quantitative analyses of its effects on AC and asphalt pavement are still lacking.
To fill the knowledge gap, this study aims to explore the underlying mechanisms of AC's TC asymmetry and quantify its effects on the response of AC and asphalt pavements. Due to the high volumetric proportion of AC, such asymmetric behavior was postulated to be induced by the microscale aggregate contacts, namely, the interaction characteristics of the neighboring aggregates in AC. Considering the huge scale gap between microscale aggregate contacts and macroscale asphalt pavement, multiscale experimental characterizations and numerical modeling were performed to hierarchically study AC's TC asymmetry at multiple scales. At the microscale, the aggregate contact characteristics in the contact region (CR), i.e., the narrow contact area of the contacting aggregates in AC, were identified through the nanoindentation tests. It was found that CR has a sandwich-like structure composed of two interfacial transition zone (ITZ) layers with the gap filled with asphalt mastic and large filler particles. Accordingly, the micromechanical model of CR was developed to predict its viscoelastic properties in tension and compression (T&C). The modeling results showed that the large rigid filler particles play the role of contact points in compression, resulting in the higher compressive moduli of CR, but there is no such role in tension. At the mesoscale, the mesostructural AC model with CR was developed through digital image processing (DIP) techniques. The aggregate contact effect was considered in AC modeling through CR. The micromechanical modeling demonstrated that AC inherits the properties of CR and thus also presents significant TC asymmetry. Despite a tiny volumetric proportion in AC, CR can help develop the stress chains to improve AC's compressive moduli significantly. At the macroscale, a dual viscoelastic constitutive model was developed to consider the TC asymmetry of AC. This model was further applied to the numerical model of asphalt pavement for predicting the effects of AC's TC asymmetry on pavement response. The modeling results showed that AC's lower tensile moduli significantly increase the vertical deformation and strains in asphalt pavements. These increases are more significant at a higher temperature or lower vehicular speed.
Overall, this study comprehensively explored the TC asymmetry of AC and its effects on asphalt pavements through multiscale characterization and modeling. The outcomes of this study have led to the following major conclusions: 1) The microscale contact characteristics induce AC's TC asymmetry; aggregate contacts can significantly improve stress transmission among aggregates in compression but cannot play their role in tension, which leads to remarkable TC asymmetry of AC; 2) the TC asymmetry of AC is time- and temperature-dependent; the stiffening matrix (FAM) can help reduce the TC asymmetry of AC at higher frequencies or lower temperatures; and 3) The lower tensile modulus of AC can increase the strain in asphalt pavements; ignoring AC's TC asymmetry may overestimate AC's performance and lead to various pavement distresses, such as fatigue cracking and rutting. The outcomes of this study are expected to help improve the design and maintenance of asphalt pavement toward enhanced durability.
Rights: All rights reserved
Access: open access

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