Civil Engineering ETDs

Publication Date

Fall 8-22-2017

Abstract

Healing is defined as the restoration of the material property during the rest period after damage. The healing property of the asphalt binder is opposite to fatigue damage, which occurs due to repeated traffic loading. On the other hand, healing occurs in the rest period between loading. Most of the existing methods for characterizing the fatigue damage and healing of asphalt are empirical in nature and do not consider the mechanism of healing. Although the asphalt concrete is a mixture of asphalt binder, fines and aggregates, most of the healing occurs in the asphalt binder. Therefore, it is important to understand the healing mechanism of the asphalt binder. In this study, the healing mechanism and the effects of several factors and asphalt chemistry are discussed. Then a constitutive fatigue damage model is developed with the incorporation of healing. The overall healing has two parts: instant healing and long-term healing. Instant healing is defined as the instantaneous recovery of damage at the beginning of the rest period due to the wetting of the micro-crack surfaces. Wetting is the process of the crack surfaces coming in contact with each other. It depends on the cohesive strength of the asphalt binder. In this study, energy of separation from tack test is used as a measure of the cohesive strength of the asphalt binder. It is observed that the instant healing is directly proportional to the energy of separation, which is correlated to the asphaltene and resin contents of the binder. Asphaltenes and resins represents larger molecules in the asphalt binder. An increase in the larger molecules in the binder reduces their mobility, which results in a decrease in wettability and instant healing. On the other hand, long-term healing is the time dependent recovery of damage over the rest period due to the inter-molecular diffusion at the micro-crack surfaces. The molecular diffusion rate depends on molecular chain length. In this study, molecular chain length is measured as the methylene to methyl ratio using Fourier Transform Infrared (FTIR). This study shows that the long-term healing rate of the asphalt binder is proportional to the molecular chain length. The effects of aging and moisture conditioning on the healing of asphalt binder is also studied. Results showed that aging and moisture conditioning decrease both instant and long-term healing, resulting in a reduction in overall healing. The grade of the performance grade (PG) binders has significant effect on aging and moisture conditioning. The effects of different filler contents are studied for two different PG binders. Results showed that mastic healing is mostly controlled by the asphalt binder type. The instant and long-term healing of the mastics decrease with the addition of filler. This is expected since the addition of fillers decreases the molecular mobility by interrupting the continuous binder medium, resulting in a decrease in wettability and diffusion rate in the mastic. In this study, a constitutive damage model is utilized to incorporate the healing and fatigue damage of the asphalt binder under cyclic shear loading with and without rest period. Healing is incorporated as a variable in this model. Shear loading is considered since previous studies showed that the force-interaction between the wheel and the pavement generates shear stress, which affects the fatigue damage the most. Motivated by experimental observations, the proposed model divides overall damage evolution into two parts: recoverable and permanent damage. The micro-damage healing is captured by the decrease in recoverable damage during rest periods. At the same time, this damage and healing model is coupled with the viscoelastic and viscoplastic model to simulate the non-linear behavior of the asphalt binder. For continuous loading, initially the permanent damage is almost close to zero. It accumulates with continuous loading which eventually results in the failure of the material. For loading with rest period, the recoverable damage heals during the rest period, resulting in a reduction in overall damage. However, after repetitive loading-rest period sequences the healing decreases and permanent damage becomes larger. The model prediction agrees with the experimental measurements under cyclic loading with and without rest periods. Thus, the model developed in this study can capture the overall response of the asphalt binder under cyclic or repetitive loading with and without rest period. The incorporation of healing in the damage model will provide an accurate measure of fatigue damage in the asphalt binder.

Keywords

Asphalt, Damage, Healing, Fatigue, FTIR, DSR

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Committee Member (Chair)

Rafiqul A. Tarefder

Second Committee Member

Tang-Tat Ng

Third Committee Member

Mahmoud R. Taha

Fourth Committee Member

Yu-Lin Shen

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