Insights into the homogeneity of asphalt mixtures containing reclaimed asphalt pavement (RAP)
Liu, Quan; Oeser, Markus (Thesis advisor); Wellner, Frohmut (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021
Over the past two decades, reclaimed asphalt pavement (RAP) has received increasing attention because of climate change and resource-saving needs. The current high reuse rate of old asphalt fundamentally reduces the landfill areas of the disposal and its related environmental impacts. Hence, in addition to reducing the construction cost and the consumption of natural resources, the use of RAP can also create significant environmental benefits, such as solving the disposal problems. This thesis was carried out focusing on the homogeneity of asphalt mixture containing RAP materials from multi-scale perspectives. The homogeneity of asphalt mixtures, to a great extent, determines the performance of asphalt mixtures provided that with an appropriate material design. The distribution of aggregates, binder migration, and diffusion of binders in the mixing process of loose asphalt mixtures are influenced by the mixing apparatus, mixing temperatures, and mixing durations, etc. Under the multi-scale framework, therefore, this thesis considered the variables including mixing temperature, mixing time, thermal conductivity, aggregate type to obtain an insight into the homogeneity of asphalt mixtures containing RAP. The influence of mixing conditions on the macro-scale homogeneity was investigated. The experimental design considered a thermal-equilibrium mixing environment. Multi-direction indirect tensile modulus and morphological characterization were performed. The results indicated that a long mixing time benefited the formation of homogenous state, in particular for the breaking of clusters in RAP materials. As the mixing time increased, the macro-homogeneity of asphalt mixture experienced from ‘momentary homogeneity’ state to ‘real homogeneity’ state (Macro-scale mixing model). Whereas, a high mixing temperature played a negative role in the distribution of materials, claimed as that the increase of temperature would obstacle the breaking of clusters and prolong the state shift toward a ‘real homogeneity’ state. The influence of mixing conditions on the mesoscale homogeneity in both thermal-equilibrium and thermal-non-equilibrium environments were investigated. The dynamic shear rheometer (DSR) and Fourier Transform Infrared Spectroscopy (FTIR) are adopted as the measurement methods. The results indicated that, in terms of the migration and diffusion, the mixing temperature played a more important role than mixing time. Two-stage model (Micro- and Meso- scale mixing model) were proposed as the migration-dominated stage followed by the diffusion-dominated stage. In addition, the Asymmetric Gaussian Model (AGM) was developed to quantitatively describe the blending rate with mixing time. The diffusion between RAP binder and new bitumen was tightly associated with the meso-scale homogeneity of asphalt mixtures (migration of binders). In another word, the diffusion occurred only if the RAP binder and new bitumen interacted. The molecular dynamics simulation was performed in the investigation of aging gradient in bitumen and the diffusion between RAP binder and new bitumen. The results indicated that the aging gradient existed along with the depth. At the molecular scale, the interface between virgin bitumen and aged can be quickly achieved due to the migration of resin components. However, the diffusion process was considerably slow compared with the interface formation. Moreover, the aggregate layer, by adhering to the polar molecules in bitumen binders, can accelerate the diffusion process between new and aged bitumen. In addition to illustrating the achievements, this thesis proposed three relative research topics deserved for future work, which are (1) Quantitative characterization of sample morphology from two-dimension (2D) to three-dimension (3D); (2) The development of Multi-Scale Synchro-adjust Methodology (MSM) towards a near-perfect homogenous asphalt mixture; (3) The use of computer-aided methods in the design of asphalt mixtures containing RAP from begin to the end.