1. Introduction
Transportation is a basic, leading, and strategic industry of the national economy. The favorable economic situation has prompted the rapid development of the highway industry in China since the beginning of the new century [
1]. By the end of 2021, the total mileage of highways in China reached 5.28 million km, and the total mileage of high-grade highways exceeded 160,000 km [
2]. Asphalt pavement has been extensively employed in high-grade highways due to its advantages, including comfortable driving, skid resistance, wear resistance, and easy maintenance [
3,
4]. However, asphalt materials are susceptible to aging due to long-term exposure to coupled environments such as light, heat, oxygen, and water during service, resulting in the deterioration of pavement performance [
5,
6]. Additionally, modern traffic with large flow, heavy load, and vehicle channelization has greatly shortened the lifespan of asphalt pavement [
7,
8,
9]. To meet the increasing traffic demand, polymer-modified asphalt technology has been proposed and has become a productive method to enhance the durability of asphalt pavement [
10,
11].
Polymers commonly applied in asphalt modification include polyethylene (PE) [
12,
13], polypropylene (PP) [
14], polyethylene terephthalate (PET) [
15,
16], polyurethane (PU) [
17], styrene butadiene styrene (SBS) [
18], etc. Plastic wastes made of PP and PE are common in daily life, and their application to asphalt modification has a significant cost advantage and economic benefits [
19]. However, both PP and PE are non-polar polymers, which have poor compatibility with asphalt [
20]. PET and PU as asphalt modifiers have been hot research topics in recent years. However, the anti-cracking properties of PET or PU-modified asphalt cannot always be guaranteed effectively [
21]. Given the above issue, researchers began to explore polymer modifiers with both rigid and flexible structures in their molecular composition. The special two-phase molecular structure gives modifiers the ability to improve the high and low-temperature properties of asphalt concurrently [
22]. SBS, as a typical block copolymer, is representative of this kind of polymer and has become one of the most mature asphalt modifiers in highway engineering [
23].
However, Cao et al. found that SBS molecular structure contains unsaturated double bonds, which are easy to break and oxidize under the action of ultraviolet light and high temperature, leading to the deterioration of asphalt pavement performance [
22,
24,
25]. Therefore, some researchers use montmorillonite with SBS to compound modified asphalt, which enhances the anti-aging ability of SBS-modified asphalt to some extent [
26]. However, the results also revealed that the above inorganic material was not ideally compatible with asphalt. Meanwhile, the expensive cost makes it difficult to popularize this method on a large scale. Therefore, there is still room for asphalt modifiers to be improved. Polyolefin elastomer is a polymer material made of ethylene and octene by in situ polymerization with metallocene as the catalyst, which usually has the dual characteristics of plastic and rubber [
27]. Polyolefin elastomers are very similar to SBS in terms of molecular structure. They can also strengthen the high and low-temperature behavior of asphalt simultaneously. In addition, polyolefin does not have unsaturated bonds and few tertiary carbon atoms, which makes their heat resistant oxygen aging and ultraviolet aging ability particularly outstanding [
28].
Therefore, asphalt modifiers to cope with modern traffic still have more room for development. To face the research gap, the primary objective of the research is to further promote the development of high-performance asphalt pavement by investigating the performance and improvement mechanism of polyolefin elastomer-modified asphalt. For this goal, modified asphalt binders with different polyolefin contents were prepared by the melting blending method first. Next, the rheological properties of modified asphalt were studied, along with storage stability and workability. Additionally, the engineering performance of modified asphalt mixtures was also determined through Marshall stability, wheel-tracking, and three points bending experiments. In the end, the enhancement mechanism of polyolefin elastomers on asphalt properties was verified by infrared spectroscopy and differential scanning calorimetry. The finding of this work can provide a novel strategy for improving the durability of asphalt pavement.
2. Materials and Methods
2.1. Virgin Asphalt, Aggregate, and Polyolefin Elastomer
The virgin asphalt selected in this research is 70# petroleum asphalt. The performance parameters of virgin asphalt were measured based on JTG E20-2011 [
29], and the values are listed in
Table 1. The aggregate used for the asphalt mixture is basalt purchased from a stone factory in Jingshan City, Hubei Province, and its physical indexes are all in line with the application requirements. Polyolefin 8003 was purchased from Dow Chemical Company, Beijing, China. Its performance parameters were measured with reference to ASTM standards, and the values are displayed in
Table 2. All parameters of raw materials were tested three times in parallel.
2.2. Sample Preparation
2.2.1. Modified Asphalt Binders
The polyolefin elastomer-modified asphalt binders were prepared by the melt blending method. The specific steps follow. First, the polyolefin elastomer was heated to 120 °C for 30 min to remove water first. Second, the polyolefins of 2%, 4%, 6%, and 8% of the asphalt mass were added into the molten virgin asphalt at 170 °C. After the polyolefin elastomers were completely immersed into the asphalt, the speed of the shear machine was set at 6000 r/min for continuous shear for 1 h to obtain polyolefin modified binders. The modified asphalt samples were named P-0, P-2, P-4, P-6, and P-8 according to the content of polyolefin elastomer.
2.2.2. Modified Asphalt Mixtures
The AC-13 polyolefin modified asphalt mixtures were prepared regarding the Marshall design method in this work. The gradation composition of AC-13 is listed in
Table 3. The ratio of asphalt to aggregate was set at 4.5% according to the preliminary experiments. The modified asphalt mixtures are named AM-0, AM-2, AM-4, AM-6, and AM-8 according to the content of polyolefin.
2.3. Measurement and Characterization
The glass transition temperature of polyolefin elastomer was determined by differential scanning calorimetry (DSC). The heating rates and cooling rates were set to 10 °C/min. Nitrogen as protective gas was continuously swept at a rate of 50 mL/min. Two cycles were performed for each experiment: the first cycle was used to eliminate the thermal history of the sample; the result of the second cycle was used for analysis.
The storage stability of polymer modified asphalt should receive attention since it will directly affect the durability of asphalt pavement. Softening point difference (SPD) tests were performed in this work to evaluate the storage stability of polyolefin modified asphalt. Three repeated tests were carried out on each sample to ensure the reliability of the results.
The workability of polyolefin-modified asphalt with different dosages was characterized through rotary viscosity tests, which were carried out at five different temperatures including 120 °C, 135 °C, 150 °C, 165 °C and 180 °C. Three parallel experiments were conducted for each sample at each test temperature.
A dynamic shear rheometer was used to measure the rheological properties of the modified binders at high temperatures. The experimental temperature was set in the range of 46~70 °C with 6 °C intervals. The complex modulus and phase angle was determined through high-temperature scanning experiments using a plate with a diameter of 25 mm. In addition, the temperature of the fatigue performance test was set in the range of 16~34 °C with 3 °C as the interval. The diameter of the plate used was 8 mm. The angular frequency and strain values were set at 10 rad/s and 12%, respectively. Each sample was tested once because of the high reproducibility of DSR on rheological properties of asphalt.
The low-temperature behavior of modified binders with different polyolefin contents was determined by the bending beam rheometer (BBR) experiment. The test was carried out at −12 °C. In addition, three parallel experiments were conducted on each specimen to ensure experimental accuracy.
Residual Marshall stability (RMS) was employed to estimate the moisture stability of polyolefin-modified asphalt mixture; the wheel-tracking experiment was employed to evaluate the high-temperature stability of the asphalt mixture; and the three points bending experiment was performed to determine the anti-cracking of the asphalt mixture under cold conditions. The above experiments were performed according to JTG E20 (2011). Three replicate experiments were conducted for each sample to ensure the reliability of the results.
The functional group information of modified asphalt was tested by infrared spectroscopy to confirm whether chemical reactions occurred between polyolefin elastomer and virgin asphalt. The samples were prepared by the potassium bromide tableting method. The wave number scan range was set as 4000~400 cm−1. The baseline correction and peak identification were achieved through the OMMIC software. In addition, repeat experiments were performed on all samples.
2.4. Research Plan
The research plan of the work is summarized in
Figure 1.
4. Conclusions and Future Work
In this study, the feasibility of using polyolefin elastomers with a two-phase molecular structure to simultaneously improve the high and low-temperature performance of asphalt was investigated. Through the preceding analysis and discussion, the following findings can be obtained:
The polyolefin with a unique two-phase structure was detected by DSC to have a glass transition point (−43.7 °C and 43.1 °C) at high and low temperatures, respectively, which laid the foundation for improving the high and low-temperature properties of asphalt at the same time. In other words, it is reliable to select high-quality asphalt modifiers from the perspective of molecular structure to improve the durability of asphalt pavement.
The results of rheological properties demonstrate that polyolefin elastomer can significantly enhance the anti-deformation and anti-cracking abilities of asphalt binder, while the fatigue resistance of asphalt is weakened slightly by the hardening of polyolefin. In addition, no concerns are found about the storage stability and workability of polyolefin-modified asphalt. It is suggested that the exact dosage of polyolefin should be based on the premise that asphalt has sufficient anti-fatigue properties.
The engineering performance of the asphalt mixture indicates that the introduction of polyolefin can improve the moisture damage resistance, rutting deformation resistance, and cracking resistance of the asphalt mixture. The residual Marshall stability, dynamic stability, and ultimate tensile strain of the modified asphalt mixture are 1.05 times, 1.31 times, and 1.17 times of those of the contrast sample, respectively.
The infrared spectrum analysis of modified asphalt shows that asphalt modified by polyolefin is mainly mechanical blending. The improvement of polyolefin on asphalt performance can be explained by the existence of both “rigid” and “flexible” structures in polyolefin. It can be said that polymers with similar structures have the potential to simultaneously improve the high and low temperature performance of asphalt.
Overall, this work confirms that polyolefin elastomers have great potential as asphalt modifiers. However, there is a need for more extensive studies on field applications. Additionally, a deeper understanding of the aging resistance of polyolefin-modified asphalt and its adhesion to aggregates remains an outstanding research need in the future.