Multiscale study of the bitumen-aggregate interfacial behavior based on molecular dynamics simulation and micromechanics

Fan, Zepeng; Oeser, Markus (Thesis advisor); Wang, Linbing (Thesis advisor); Wang, Dawei (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

The bitumen-aggregate interfacial phenomena are an area where chemistry, physics, and engineering intersect. While continuous research efforts have been devoted to this issue in the past decades, much less is known about the fundamental mechanisms controlling the origins and the evolution of interfacial failure. The complexity lies in the multifactorial and multiscale nature of bitumen-aggregate interfacial behavior. The interaction between bitumen and aggregate relies directly on an intricate interplay of bitumen chemistry, aggregate mineralogy, and surface topography; and the interfacial performances in service are also closely related to the random heterogeneous microstructure of asphalt mixture, the periodical climate conditions, and the repeated vehicle loads. Moreover, the heterogeneity of the interacting components ranges across nine orders of magnitude in length scales. The current thesis is dedicated to developing a "bottom-up" approach which handles the enormous number of factors across the micro-to-macro length and time scales. A mechanistic study using molecular dynamics simulation was carried out to uncover the adsorption configuration of bitumen-aggregate interface at the molecular scale and how the aggregate mineralogy affects it. The microstructural of the adsorbed bitumen layer was found to be a superposition of two configurations: the layered configuration in the near-surface region arising from aggregation and parallel orientation of the bitumen molecules, and the gradient descent configuration in the region further away from the surface. The degree of concentration and radius of influence are significantly impacted by the mineral surface. The hypothesis of selective adsorption was tested by probing the distribution characteristics of different fractions in bitumen, and the results suggest a rejection of the hypothesis.For purpose of investigating the water-induced damage between bitumen and aggregate, the rolling bottle tests were conducted for six kinds of aggregates, and the ternary bitumen-water-aggregate interface models were established to perform molecular dynamics simulations. The results indicate the existence of competitive adsorption between bitumen and water molecules at the mineral surface, and the penetration capacity of bitumen molecule is greatly affected by the mineral property. Aggregates with higher content of nepheline, chlorite, pyroxene and olivine minerals are more likely to exhibit better moisture damage resistance while aggregates with higher content of quartz, plagioclase, and calcite minerals do the opposite. The influence of surface topography on the adhesion and water-induced debonding behaviors of bitumen on aggregate surface was studied through wetting theory. The contact angle tests were performed to measure the surface energies of bitumen and aggregate surfaces varying in both mineralogy and roughness, based on which the interaction energies between bitumen and aggregate in both air and water environments were quantified, respectively. The negative interfacial adhesive energy for the air/bitumen/aggregate interface and interfacial debonding energy for the water/bitumen/aggregate interface imply that both bitumen wetting and water-induced bitumen dewetting on flat surface are thermodynamically favorable. The Wenzel model was found to describe the rough interface systems well. The interfacial adhesive energy and interfacial debonding energy are enhanced geometrically by the roughness factor r, which indicates that the textured aggregate surface is in favor of force-induced interfacial cracking resistance but leading to an adverse effect on moisture damage resistance. The interfacial cracking behavior of asphalt mixture was exploited at the mesoscale through micromechanics method. A micromechanical damage model was established by incorporating the bilinear Cohesive Zone Model (CZM) into the Mori-Tanaka model. It is found that the interfacial debonding between bitumen and asphalt mortar exhibits a significant dependency on the aggregate size. A critical aggregate size has been identified, above which the damage behavior of asphalt mixtures changes from hardening to softening. The critical aggregate size increases as the mortar modulus increases but decreases with the increase of the interfacial stiffness, Poisson’s ratio of mortar, and aggregate volume fraction. The interface strength and fracture energy also show significant influences on the fracture behavior of asphalt mixtures. The strength of asphalt mixtures increases as the interface strength increases, but it is independent of the fracture energy. Increased fracture energy can improve the fracture resistance of the asphalt mixture, while increased interface strength has the opposite effect.In general, this thesis has exploited a mechanistic investigation on the interfacial interaction between bitumen and aggregate. The fundamental knowledge regarding the influence factors as well as the way how they works were created at multiple length/time scales. The findings from this thesis open up an avenue for predicting the bitumen-aggregate interfacial behavior based on the material genomes.