The Examination of Asphalt Binder Microrheology Using Atomic Force Microscopy and the Finite Element Method
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In order to obtain a better understanding the complex mechanical behavior of asphalt binder, its microstructure must be examined. In addition, the interrelationship between asphalt binder microstructure, damage, and mechanical properties must also be determined. In this work the unique relationship between asphalt binder microstructure and damaging mechanisms is established. Here, the AFM is used to image the microstructure of two different types of asphalt binders. A micrometer driven mechanical device named a "Micro-Loading Frame" is used to obtain AFM images of bitumen surface after subjecting it to high levels of tensile strain. Using image analysis techniques the microstructural changes due to the applied tensile strain is established. Furthermore, using a specific AFM creep indentation protocol the viscoelastic properties of asphalt binder are extracted for each microstructural phase observed. In addition, this concept was applied to rolling thin film oven test (RTFOT) aged and pressure aging vessel (PAV) aged binders. The geometry obtained from AFM imaging combined with the microrheology obtained from AFM creep indentation experiments were used to construct finite element simulations examining local stress distributions. The numerical solutions were compared to experimental observations. Using AFM imaging technique three unique phases were found within the asphalt binder at the micron length scale i.e. bee, bee casing, and interstitial. All three phases were found to have uniquely different viscoelastic properties. Finite element analysis results showed that heterogeneity within the asphalt binder led to localized stress amplification. Application of high levels of tensile loads resulted in phase separation and cracking referred to as load induced phase separation (LIPS) zones. The occurrence of the LIPS zones were primarily found to be within interstitial regions. Application of tensile strains also resulted in the reduction of the number of bee structures, and also the area occupied by the bee structures. The location of LIPS zones coincided with the location high stress zones determined using the finite element simulations suggesting the possibility of phase separation and cracking occurring due to localized high stresses. Application of tensile strains to RTFOT, and RTFOT+PAV aged samples also resulted in the formation of LIPS zones within the interstitial regions. However, an increase in the level of aging the led to a reduction of LIPS zone formations for the same level of applied tensile displacements. The effect of geometry of asphalt binder microstructure on the mechanical response was determined. An increase in the area fraction of the bee/bee casing phase, a reduction in the distance between adjacent bee/bee casing features led to an increase in the maximum stress magnitudes. An increase in the number of bee/bee casing features led to a reduction in the maximum stress magnitude when the area fraction was kept constant.
Jahangir, Rezwan (2015). The Examination of Asphalt Binder Microrheology Using Atomic Force Microscopy and the Finite Element Method. Doctoral dissertation, Texas A&M University. Available electronically from