1. Introduction
Gradation segregation is defined as the non-uniform distribution of coarse and fine aggregates in paved asphalt pavement [
1]. It may occur during plant manufacturing, truck transportation, and paving of asphalt mixtures. Localized areas rich in coarse and fine aggregates are termed as coarse and fine aggregate segregation, respectively. Gradation segregation is one of the main factors affecting the durability of asphalt pavement, because both coarse and fine aggregate segregation may lead to pavement failures. These failures reduce the service performance and service life of asphalt pavement [
2,
3,
4,
5]. Therefore, efforts have been devoted by various researchers for gaining improved insight into the mechanism, affecting factors, measurement and evaluation methods of aggregate gradation segregation [
6,
7,
8]. Digital image processing techniques, non-nuclear densitometers, infrared camera, and industrial x-ray computed tomography (CT) have been used to study the segregation within asphalt mixtures during transportation and paving. Bruno L et al. [
9] used an image analysis method to finalize an effective analysis of asphalt section image for automatically extracting aggregate gradation without the need of separating the bitumen from the aggregate. Wangheng J et al. [
10] investigated the spreading segregation and compaction segregation of two asphalt mixtures by sieving and non-nuclear density gauge. Based on the findings, asphalt mixtures with spreading and compaction segregation were prepared in laboratory. The corresponding moisture stability was then evaluated in laboratory. Dajin G et al. [
11] determined the feasibility of using infrared camera as the tool for analyzing the temperature segregation of asphalt mixture. Tao L et al. [
12] presented an imaging method to assess the homogeneity of asphalt concrete using X-ray computed tomography, the improved OTSU image method, and fractal theory. The pavement-quality indicators were applied to test asphalt pavement and to choose four specimens with various degrees of segregation. Finally, a segregation evaluation standard based on fractal dimensions and the imaging method was proposed.
In addition, the effect of segregation on the performance of asphalt pavement and the corresponding quality control methods were also documented in many publications [
13,
14,
15]. Based on previous studies, it was recommended that the effects of segregation and air voids should be taken into consideration when building the correlation between tensile strength and pavement performance. Besides, it is also found that the fatigue life and moisture resistance of asphalt mixture strongly related to the aggregate segregation. Mahoney JP et al. [
16] used an infrared camera to identify cooler portions of the mat.the aggregate gradation and asphalt binder content of asphalt mixture was also determined. The results showed no significant aggregate segregation within the cooler areas; however, these cooler portions of the mat consistently showed higher air voids than the surrounding pavement. The influence of aggregate segregation on rutting resistance and raveling was also reported in previous studies [
3]. Researchers also tried to determine factors affecting the gradation segregation, for instance, mixing plant, volumetric properties, aggregate shape properties, mixing temperature, paving conditions etc. [
17,
18,
19]. However, the above studies have some limitations. Both laboratory and in field tests are conducted with phenomenological methods. Changes in the micro-parameters of asphalt mixture resulting from gradation segregation are unclear. Low repeatability of test results occurs because of various experimental variables. Despite this, the correlation between gradation segregation and mechanical property of asphalt mixture still deserves more effort.
DEM has been applied to analysis road material performance because of its superiority in address non-uniformity, discontinuity, and large deformation problems [
20,
21,
22,
23]. Zelelew HM et al. [
20] using DEM to describe the microstructure of asphalt mixtures and simulate creep behavior. Coenen AR et al. [
21] developed a software for processing and analyzing 2D images of asphalt mixtures. By means of this software, radial distribution, orientation and segregation of aggregates were analyzed. Wagoner MP et al. [
22] described the development of a fracture test for determining the fracture energy of asphalt concrete. Masad E et al. [
23] investigated the stiffness anisotropy of asphalt mixtures using micromechanics-based models. Therefore, this paper presents an attempt to evaluate the effects of gradation segregation on asphalt mixture in terms of structural characteristics and mechanical properties using the DEM method. To achieve this objective, a 2D asphalt mixture model was built using PFC2D5.0 software, which is a computing software developed by the Itasca company in the USA. Based on the 2D DEM model, the aggregate skeleton in asphalt mixture and internal mechanical characteristics of asphalt mixture was studied. Furthermore, the mechanical performance such as resistance, transfer and diffusion to load of asphalt mixtures at different segregation levels was studied and compared.
2. DEM Model Development
2.1. Segregation Simulation
As the aggregate gradation in asphalt mixture changes through segregation, the differences between the control aggregate gradation and the segregated aggregate gradation can be used to determine the segregation levels. In this research, the area between the segregated gradation curve and the control gradation curve was employed to quantify the segregation of aggregate in asphalt mixture. A large area indicates a high segregation level. As shown in Equation (1),
S value can be used to reflect the area difference between the segregated and the standard gradation curves [
24].
where,
n is different sieve size level,
Pij is the passing percentage of the segregated gradation, and
Paj is the passing percentage of the standard gradation.
Based on the calculated S value, the gradation segregation can be divided into four levels: no segregation, low-level segregation, medium-level segregation, and high-level segregation with the corresponding S value of less than 10%, 10–20%, 20–35%, and greater than 35%, respectively.
To evaluate the effect of segregation on the aggregate skeleton in asphalt mixture, three gradations and three segregation levels were considered in this study.
Table 1 presents the labels of the involved aggregate gradations. Control aggregate gradation (no segregation) in this study is labelled as D.
The
S value of HC, MC, LC, D, LF, MF, and HF is 37.4%, 21.9%, 11.3%, 0, 10.7%, 21.7%, and 38%, respectively and the corresponding gradations are listed in
Table 2.
2.2. Model Development
To simplify the DEM model, aggregates in asphalt mixture are modeled as spheres with different diameters. In addition, it is assumed that the density of all the aggregates is same and the surface area of aggregates passing sieve is uniformly distributed. Therefore, the mass ratio of aggregate with different diameters equals their area percentage per unit area. The number of aggregate particles in asphalt mixture can be approximately calculated through Equation (2) to Equation (3):
where,
A = total area of the designed model;
Θ = voids in the mineral aggregate;
Pi = percentage of aggregate remaining on the i-th sieve;
Ai = total area of aggregate remaining on the i-th sieve;
ni = total number of the aggregate remaining on the i-th sieve;
rmax = maximum radius of the spheres
rmin = minimum radius of the spheres.
To calculate the number of aggregates with different diameters, the air voids in mineral aggregate (VMA) should be calculated in advance. In this study, VMAs were calculated by means of trial-calculating. Specifically, let the model area equal the total area of aggregates. The amounts of aggregate with different diameters can be estimated. Then, develop the DEM model accordingly followed by calculating the VMA of aggregates within the built DEM model. For each gradation, the VMA calculation trial-calculating process was repeated 10 times to minimize errors. The trail-calculated VMAs are listed in
Table 3.
Once the VMA was obtained, the amounts of aggregate spheres can be calculated according to Equations (2) and (3). However, due to the randomness of the generated particle area, in order to ensure that the total area of each aggregate gradations is consistent with the design value, after generating the aggregate particles for each gradation according to the calculated resluts, it is necessary to count the total area of all particles. Compared with the design value, if the difference between the two is greater than , the result is modified by randomly adding or deleting one particle, and then repeat the above process until the area of each grade of aggregate particles meets the design requirement. Based on this, 2D DEM model is built.
Figure 1 shows the 2D DEM model development process. A 150 × 100 mm size model was built firstly by generating four rigid walls. Then, loose aggregates were generated uniformly in the certain space surrounded by the four walls. A certain speed was then given to the upper and lower walls for compacting the loose aggregates. Finally, to avoid the aggregates being excessively compacted, the upper wall moves up a small distance, and then the PFC “solve” command was used to delete the imbalance force in the model. Repeat this step until the stress on the upper wall is 0. The heights of the models can be found in
Table 3. As excepted the heights of the models are basically consistent and this meets the research expectation.
2.3. Contact Model and Model Parameters
In order to describe the interaction amongst the aggregates in asphalt mixture, two bonding models are available in PFC
2D, contact-bond model and parallel-bond model. The main difference between these two bond models is that the contact-bond model can only transmit force through the contact point, whereas the parallel-bond provides possibility for both transmitting force and moment. In addition, the parallel-bond model also adds the effect of binding materials [
25]. Thus, the parallel-bond model was employed in this study to simulate the mechanical response of the aggregate skeleton in asphalt mixtures. The parallel-bond model can be viewed as the combination of serval springs with normal and tangential stiffness.
The micro-parameters for developing the asphalt mixture model were collected from laboratory test, i.e. uniaxial compression test. The test was performed by the US MTS810 material test system with the test temperature and load speed of 20 °C and 1 mm/s, respectively. Vertical loadings were conducted on the DEM model and laboratory prepared asphalt samples. When the vertical deformation of asphalt mixture sample arrives at ∆
h, the corresponding vertical load in the laboratory and DEM model is recorded and labelled as
Pi and
Pi’, respectively. The difference between
Pi and
Pi’ is defined by
βi = |
Pi −
Pi’|/
Pi. Micro-parameters were adjusted for minimizing
β. The obtained micro-parameters used in the DEM model are presented in
Table 4. The mechanical response of asphalt mixture samples and the corresponding DEM models under vertical load are presented in
Table 5. As can be seen, the difference in loading between the DEM model and laboratory test is less than 8%, indicating that the microscopic parameters obtained by this method are appropriate.