1. Introduction
The compaction of asphalt pavement is a critical step in ensuring its service life and durability [
1]. Asphalt mixtures and other civil engineering materials exhibit complex mechanical properties [
2,
3], and traditional compaction methods lack effective real-time quality assessment, which can lead to uneven compaction (either undercompaction or overcompaction) [
4]. Such unevenness can result in early pavement damage or aggregate breakage [
5,
6], thereby affecting the performance and stability of the asphalt pavement. Therefore, effective control of compaction quality is key to enhancing pavement performance.
Emerging intelligent compaction technology, equipped with integrated measurement systems, satellite-based navigation systems, thermal imaging devices, motion sensing equipment, and other tools [
7], aims to overcome the limitations of traditional asphalt pavement compaction methods to achieve superior compaction quality. This technology reflects the compaction status of the entire pavement structure rather than individual asphalt surface courses, resulting in a weak correlation between intelligent compaction measurements and asphalt surface course density [
8]. Therefore, it is necessary to delve into the compaction mechanism of asphalt pavements, particularly from the perspective of particle dynamics and rotational behavior. Particles within asphalt mixtures reduce voids by moving and spinning throughout the compaction phase, thereby enhancing the load-bearing capacity of the pavement [
9]. Hence, studying the rotation and movement of asphalt mixture particles during compaction is crucial for understanding the compaction mechanism of asphalt pavements and further optimizing intelligent compaction technology.
In recent years, SmartRock sensors have been extensively used to monitor the rotational characteristics of asphalt mixture particles during compaction processes [
10,
11]. Wang et al. [
12] studied the rotational features of aggregates during gyratory compaction using SmartRock, correlating it with the density of asphalt mixtures. Moreover, the study of particle rotational characteristics using SmartRock provided effective indicators for researching the compaction of asphalt mixtures [
9]. However, due to the limitations in the number and placement of sensors within the mixture, their data may not represent the overall compaction condition of the asphalt mixture, affecting the universality of the research findings. Currently, some researchers use Finite Element Method (FEM) and Discrete Element Method (DEM) to investigate the compaction characteristics of asphalt mixtures at both macroscopic and microscopic scales [
13]. Koneru et al. [
14] simulated the gyratory compaction of asphalt mixtures using the finite element software Abaqus. Sun et al. [
15] developed a three-dimensional finite element simulation for vibratory compaction of asphalt pavements using Abaqus, analyzing the stress distribution beneath the wheel. Masad et al. [
16] simulated the compaction process of asphalt mixtures under rolling conditions using finite element programs. Although the FEM has made some progress in simulating the compaction of asphalt mixtures, it still cannot accurately simulate the heterogeneity and particle motion characteristics of these mixtures [
17].
The FEM focuses on reducing voids through the extrusion of aggregates and colloids, while DEM emphasizes achieving denser packing through the translation and rotation of particles [
18,
19]. Several studies have utilized the DEM to simulate the compaction process of asphalt pavements. Khateeb et al. [
20] used the DEM to model the compaction of porous asphalt mixtures, aiding in understanding the mesostructural changes in the mixture during compaction. Liu et al. [
21] established a discrete element compaction model for steel bridge decks to investigate the segregation characteristics of the mixture during compaction. Man et al. [
22] proposed a dual-scale discrete element model for simulating the compaction process of a hot asphalt mixture and successfully captured the influence of particle size distribution on the compaction behavior. Chen et al. [
23] analyzed the void distribution in the asphalt mixture during compaction using the DEM, finding it effective in simulating the internal void structure of the mixture. Olsson et al. [
24] developed a novel DEM modeling approach for studying asphalt compaction, this method can reflect the force distribution network within the material. Furthermore, the angular velocity of the aggregate can reflect its rotational motion within the asphalt mixture. Wang et al. [
25] employed the DEM to simulate the precompaction of asphalt pavements, proposing a method to assess the precompaction effect of loose mixtures using the average angular velocity of aggregates. They found that the angular velocity of aggregates can assess the compaction effect of asphalt mixtures. However, angular velocity can only describe the rotation of aggregates around a specific direction and cannot measure their three-dimensional motion in space. In contrast, triaxial angular velocity (TAV) provides a comprehensive evaluation of the three-dimensional rotational behavior of mixture particles during the compaction process of asphalt pavement. This is crucial for understanding the internal compaction mechanisms of asphalt mixtures. Finally, the coordination number (CN) of the particles refers to the average number of particles in contact with a single particle. Niu et al. [
26] proposed the concept of CN as a metric to evaluate the contact characteristics of skeletal structures. They found that the CN helps assess the stability of the aggregate structure. Despite these advances, the DEM remains in the preliminary exploration stage in simulating the compaction of asphalt mixture, especially in exploring the dynamic rotational behavior of asphalt mixtures during asphalt pavement compaction and their correlation with the quality of pavement compaction.
To summarize, traditional compaction methods lack real-time quality assessment, which leads to uneven compaction and consequently affects the compaction of asphalt pavement. Although intelligent compaction technology allows for real-time monitoring, it still does not adequately reflect the compaction quality of individual asphalt surface courses. Moreover, SmartRock sensors and numerical simulation methods provide new pathways for a deeper understanding of the compaction mechanisms of asphalt mixtures. These technologies have revealed the characteristics of particle rotation and their relationship with compaction quality, yet a full understanding of these relationships requires further exploration. This paper aims to utilize the DEM software EDEM (Enhanced Discrete Element Method) V2020.1, developed by Altair Engineering, Inc. (Troy, MI, USA), to establish a model for asphalt pavement compaction, to investigate the compaction mechanism of asphalt mixtures, and to further investigate the dynamic response of particle rotation during the vibratory compaction process and its correlation with pavement compaction quality. This can help reflect the compaction quality of individual asphalt pavement courses through particle rotation characteristics. Then, this study has significant implications for improving the control of asphalt pavement’s compaction quality and advancing the development of intelligent compaction technology.
5. Conclusions
This study utilized the DEM to construct a compaction model for the middle surface course of the asphalt pavement and verified its effectiveness. Initially, by monitoring the changes in the particle angular velocity and TAV of the mortar, aggregates, and mixture during compaction, the compaction mechanism of the asphalt pavement was preliminarily explained. Subsequently, the variations in the angular velocity amplitude of the mixture and the TAV amplitude of the mortar, aggregates, and mixture under different vibratory compaction passes were analyzed. Finally, the correlation between the TAV amplitude, the CN amplitude, and the compaction degree of the mixture was studied, respectively. The main conclusions are as follows:
Both static and vibratory compaction cause asymmetric wave deformation in asphalt pavement, but vibratory compaction significantly reduces this deformation, enhancing pavement smoothness. This helps optimize compaction methods to enhance pavement smoothness, thereby improving driving comfort and durability.
The particle angular velocity reflects the dynamic rotation of asphalt pavement particles during compaction, with mixtures primarily rotating within vertical planes during the first six passes and horizontal planes during the seventh pass. This helps to understand the rotation patterns of asphalt pavement particles during compaction.
The TAV reflects the evolution of asphalt mixtures from a loose to a dense and stable state during compaction and reveals the three-dimensional dynamic rotational characteristics of particles. This provides crucial indicators for understanding the compaction mechanism of asphalt pavements.
The TAV amplitude of the asphalt mixture, aggregates, and mortar all show a clear linear correlation with the mixture’s compaction degree, and their fitting formulas offer an effective quantitative method for controlling the compaction quality of asphalt pavements. This helps optimize the compaction process, ensuring the pavement achieves the desired density.
The change in the particle CN and the close correlation between its amplitude and the compaction degree of the mixture provide an effective quantitative index for an in-depth understanding of the density of asphalt mixtures and the structural stability of aggregates. This helps monitor the stability of the aggregate structure during the compaction process of asphalt pavement courses.
This research explores the particle rotational motion characteristics of the middle surface course of asphalt pavement and their correlation with the compaction degree of the mixture. However, the relationship between different surface courses (top surface course, middle surface course, and bottom surface course) and particle motion (speed, angular velocity, and TAV) still requires further investigation. Therefore, it is hoped to promote further research to predict the compaction quality of asphalt pavements, thereby providing important theoretical support for achieving true “intelligent compaction“.