1. Introduction
Rutting is known to represent a serious type of asphalt pavement distress that influences the safety of the road and the quality of the ride. In the SHRP-A-318 research report, pavement rutting is defined as the result of the combination of the compaction deformation and the transverse flow deformation of the asphalt mixture under numerous repeated traffic loadings [
1,
2]. B. Birgisson pointed out that the rutting was caused primarily by the shear deformation of the upper layer asphalt mixtures due to the failure to resist frequent repeated shear stress, which is now generally accepted by most road engineers [
3].
In the past several decades, the triaxial compression test and rotary shear test were employed to analyze the shear performance of asphalt mixtures [
4,
5,
6,
7]. However, it is difficult to determine the exact shear strength of asphalt mixtures by shear performance analysis. The composition of an asphalt mixture, the internal structure formed by the joint action of the aggregates, and the service conditions are the internal factors that determine its bearing capacity and service performance. Traditional tests cannot evaluate the effect of asphalt binders on the shear strength, and cannot reflect the shear mechanism of the asphalt mixture through the damage process of the test sample. In comparison, the uniaxial penetration test (UPT) method proposed by Y.F. Bi and L.J. Sun can determine the shear strength of an asphalt mixture in the pavement structure, simulate the actual shear states of the road, and better reflect the shear strength of asphalt mixtures [
8]. Chen et al. found that the uniaxial penetration test can provide repeatable results for the shear resistance of asphalt mixtures at elevated temperatures [
9]. However, it is difficult to control the variability of experimental tests due to complex artificial and environmental factors, and to build relationships between the macro-properties and micro-structure of asphalt mixtures based on experimental tests. Micromechanical modeling and virtual testing are necessary to solve the above problems and to characterize the asphalt mixture for both material evaluation and structural design purposes.
Currently, the finite element method (FEM) and the discrete element method (DEM) are the two major numerical tools used to study the mechanical behavior of asphalt mixtures. Continuum theory is the basis of FEM models with random aggregate structures for asphalt mixtures. It is difficult to characterize the contact and sliding behavior between aggregates, and the bonding behavior between aggregates and asphalt mastic. The DEM method, initially developed by P.A. Cundall and O.L. Stack, provides a promising way to model and characterize the micromechanical behavior of asphalt mixtures [
10].
DEM has been widely used to study the mechanical properties of granular materials and to solve the engineering problems of granular and discontinuous materials in recent years [
11,
12,
13,
14,
15,
16]. M. Enad used the discrete element method to analyze the impact of the aggregate gradation, shape, stiffness, and internal structure on asphalt mixture fractures [
17]. S. Hou explained how asphalt mixtures bear vehicle loadings, as well as the potential reasons the rutting forms from a micro-mechanical view, through the micro-mechanical response of asphalt mixtures based on the discrete element method [
18]. H. Feng studied the normal and shear material properties for a viscoelastic model of an asphalt mixture using DEM [
19]. T. Ma et al. studied the effect of air voids on the high-temperature creep behavior and fatigue life of an asphalt mixture based on the discrete element method [
20,
21]. J.L. Ren and L.J. Sun used the discrete element method to characterize the effect of air voids on the cracking of asphalt concrete at low temperatures [
22].
Until now, most investigations have focused on the micromechanical behavior of single-layer asphalt mixtures. In fact, an asphalt pavement surface usually consists of two or more layers. Therefore, a double-layer asphalt mixture can better reflect the deformation behavior similar to real pavement conditions. This study aims to analyze the shear deformation behavior of a double-layer asphalt mixture using the virtual uniaxial penetration test with a discrete element model. For this purpose, asphalt mixtures with two different nominal maximum aggregate sizes were designed for the preparation of the double-layer wheel tracking test specimens, which were then cored to obtain the cylindrical uniaxial penetration test specimens. The longitudinal profile images captured from cutting the cylindrical uniaxial penetration test specimens were used to reconstruct the two-dimensional virtual UPT specimens for the discrete element method analysis. In addition, the micromechanical properties and the spatial movement characteristics of the coarse aggregates in the specimens were analyzed to reveal new insights regarding the rutting failure mechanism of asphalt pavements.
5. Conclusions
In this study, a two-dimensional virtual uniaxial penetration test was built by the binarized longitudinal profile images of the UPT specimen cored from a double-layer wheel tracking test specimen to analyze the shear deformation of the asphalt mixture. The micromechanical and morphological behavior of the virtual asphalt mixture specimen during shear deformation were investigated to reveal the rutting mechanism of asphalt pavement.
Different bonding parameters were assigned to the contacts between different components within the model. The virtual test result was verified and considered to be acceptable for the virtual simulation and further analysis. Dramatic increases were observed on the tensile, compressive, shear stress, and effective bond modulus within the double-layer asphalt mixture when it was subjected to a virtual penetration load. The values of the above micromechanical parameters were greater in the upper layer, especially near the indenter, indicating that the shear deformation occurred more easily in the upper layer. The rotation and translation of the coarse aggregates within the double-layer asphalt mixture after applying the penetration load confirm that shear deformation is a process of both vertical densification deformation and lateral flow deformation. Specifically, coarse aggregates within the upper layer generate more vertical displacement, while those within the lower layer more lateral displacement.