1. Introduction
As global resources become increasingly scarce and environmental awareness continues to increase, the search for renewable and environmentally friendly materials to replace traditional petroleum-based products has become one of the hot topics in materials science research [
1,
2]. In road construction, asphalt is an important paving material, and research on its modification has always been an important way to improve the performance of roads and extend their service life [
3]. Traditional asphalt modifiers such as polymers, rubber, and petroleum resins have significant effects in improving the mechanical properties, durability, and aging resistance of asphalt, but the sources of these modifiers rely on non-renewable petroleum resources, and their production and use processes may have negative impacts on the environment [
4,
5]. Therefore, the development of environmentally friendly and renewable asphalt modifiers has become a new research direction.
Vegetable oils and natural fibers have gradually become popular materials in the field of asphalt modification due to their renewability, low cost, and environmental friendliness [
6]. As agricultural waste, straw fiber is not only large in quantity and low in price but also has good mechanical properties and durability and can effectively improve the high-temperature performance and low-temperature crack resistance of asphalt [
7]. Vegetable oils such as tall oil, rapeseed oil, and palm wax have excellent toughening and adhesion properties and play an important role in improving the low-temperature crack resistance and water damage resistance of asphalt [
8]. However, the current research on the composite modification of straw fiber and vegetable oil asphalt is not systematic; in particular, research on its composite modification mechanism and rheological properties is relatively scarce. Therefore, systematic research on the composite mechanism and performance of straw fiber and vegetable oil in modified asphalt is of great significance for the development of high-performance and environmentally friendly road materials.
Most traditional rheological tests (basic asphalt index tests) are based on single performance predictions of indicators, which may not be able to fully predict the performance of asphalt in actual use. The interpretation and application of many traditional indicators are based on empirical rules, which may not be comprehensive enough for the evaluation of new modified asphalt. Modern rheological tests can simulate conditions that are closer to the actual use environment so as to obtain more accurate performance data, which can better reflect the performance of modified asphalt in actual use [
9,
10]. Therefore, this paper uses a variety of testing methods such as the dynamic shear rheology test (DSR), low-temperature bending beam rheology test (BBR), contact angle test, and infrared spectroscopy test (FTIR) to analyze the rheological properties, low-temperature performance, and microstructure of modified asphalt from multiple points of view.
A dynamic shear rheometer is an instrument widely used to evaluate the high-temperature performance of asphalt materials. By testing the shear modulus and phase angle of modified asphalt at different temperatures and frequencies, its complex modulus and viscoelastic properties can be obtained, thereby evaluating its high-temperature anti-rutting performance and creep recovery properties [
11,
12]. A low-temperature bending beam rheometer is mainly used to evaluate the crack resistance of asphalt under low-temperature conditions. By testing the creep stiffness modulus and creep rate of modified asphalt at low temperatures, the improvement effect of different modifiers on the low-temperature performance of asphalt can be analyzed [
13,
14]. As an important means to study the adhesion characteristics of asphalt and aggregates, the contact angle test has the advantages of simple operation and intuitive results. Many scholars have widely used it and achieved fruitful results [
15,
16]. In addition, an infrared spectrometer, as an important tool for analyzing the molecular structure and chemical composition of materials, can analyze the changes in chemical groups and intermolecular forces in modified asphalt by measuring the infrared absorption spectrum of the material [
17,
18].
In summary, this paper aims to systematically reveal the rheological properties and micro-structural characteristics of vegetable-oil-reinforced and straw-fiber-modified asphalt in order to provide a scientific basis and theoretical support for the development of high-performance and environmentally friendly asphalt modifiers. The high-temperature performance of the modified asphalt was tested by DSR test, and the influence of the modifiers on the viscoelastic properties and anti-rutting performance of asphalt was analyzed. The low-temperature performance of different modified asphalts was tested by BBR test, and the improvement effect of composite modification on the anti-cracking performance of asphalt was evaluated. The surface free energy of different modified asphalts and aggregates was tested by contact angle test, so as to characterize the adhesion characteristics of the asphalt and aggregates. The molecular structure changes of asphalts modified by different modifiers were analyzed by FTIR, and the modification mechanisms and their relationships with macroscopic performance were discussed. A variety of experimental methods were used to comprehensively analyze its performance changes. The research results not only help to deeply understand the microscopic mechanism and rheological properties of composite modified asphalt but also provide theoretical support and practical guidance for the application of environmentally friendly asphalt materials in actual engineering.
5. Conclusions
This paper systematically studied the rheological properties and microstructure of vegetable-oil-enhanced, straw-fiber-modified asphalt by selecting three vegetable oil modifiers: tall oil, rapeseed oil, and palm wax. The study makes the following contributions:
- (1)
The effect of vegetable oils on the high-temperature rheological properties of straw-fiber-modified asphalt was revealed, showing that palm wax significantly improved the complex modulus and rutting resistance of straw-fiber-modified asphalt and exhibited excellent creep under stress recovery performance for high-temperature or heavy traffic conditions. However, the impact of tall oil and rapeseed oil on high-temperature performance was not as good as that of palm wax, suggesting the compatibility of vegetable oil selection with the road application environment.
- (2)
An in-depth analysis of the contribution of vegetable oils to low-temperature rheological properties showed that tall oil and rapeseed oil significantly improved the low-temperature crack resistance of straw-fiber-modified asphalt, reduced the creep stiffness modulus, and improved the creep resistance of straw-fiber-modified asphalt. rate, indicating suitability for cold climate conditions. On the contrary, palm wax is not conducive to the maintenance of asphalt properties at low temperatures, indicating that different vegetable oil types have different effects on asphalt properties at different temperatures.
- (3)
The mechanism of action of vegetable oils on asphalt adhesion and water damage resistance was explored. Through contact angle tests and FTIR analysis, it was found that the three vegetable oils improved the adhesion of modified asphalt to aggregates through physical adsorption, and rapeseed oil was particularly outstanding in enhancing water damage resistance. This result provides a new direction for improving the durability of modified asphalt in humid environments.
Overall, this study systematically analyzed the action mechanism of vegetable oil modifiers in straw-fiber-modified asphalt, revealed the influence of vegetable oils on asphalt’s rheological properties and microstructure from multiple perspectives, and provides a basis for the development of high-performance, environmentally friendly asphalt modifications. It provides a scientific basis and theoretical guidance for their application under different environmental conditions.