Study on the Performance Improvement of Straw Fiber Modified Asphalt by Vegetable Oil

Abstract

As a plasticizer, vegetable oil can improve the compatibility between straw fibers and an asphalt matrix and promote the uniform dispersion of fibers, thereby improving the viscoelastic properties of the composite material. This paper selected three vegetable oils: tall oil, rapeseed oil, and palm wax. Through dynamic shear rheology tests, low-temperature bending beam rheology tests, contact angle tests, and infrared spectroscopy tests, the vegetable-oil-reinforced straw fiber modification was analyzed from different points of view. The research results show that palm wax significantly improves the high-temperature rheological properties of straw-fiber-modified asphalt but has a negative impact on low-temperature properties. Tall oil can most significantly improve the low-temperature rheological properties of straw-fiber-modified asphalt. Rapeseed oil has the most obvious effect in improving the adhesion and water damage resistance of straw-fiber-modified asphalt. In addition, the research shows that all three vegetable oils exist in the modified asphalt in adsorbed form, and no new compounds are generated. These research results provide theoretical guidance value for the application of straw-fiber-modified asphalt pavement in different environments.

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.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Asphalt

The base asphalt selected in this study was Panjin 90# base asphalt. According to the requirements JTG E20-2011 [19], its performance indicators were tested. The results are shown in

Table 1. Main technical performance indicators of base asphalt.

Technical Indicators		Unit	Requirement	Results
25 °C penetration		0.1 mm	80~100	86.0
Penetration index		/	−1.5~+1.0	−0.79
Softening point		°C	≥44	44.8
10 °C ductility		cm	≥30	100
135 °C viscosity		Pa·s	≤3	0.392
Residue after RTFOT	Quality loss	%	≤±1	0.18
	Residual penetration ratio	%	≥55	68.2
	Residual ductility	cm	25	30

.

2.1.2. Straw Fiber

The straw fiber used in this paper came from a special experimental straw fiber produced in Jiangsu, China. Its technical indicators are shown in

Table 2. Technical indicators of straw fiber.

Indicators	Fiber Length	pH	Density	Water Content	Cracking Temperature
Results	1–2 mm	7.5	1.4 g/cm3	4%	>200 °C

, and the fiber sample is shown in Figure 1.

2.1.3. Vegetable Oils

The vegetable oils used in this paper came from Guangdong Fufei Chemical Technology Co., Ltd., Guangzhou, China, and the vegetable oils included tall oil, rapeseed oil, and palm wax. The technical indicators of each vegetable oil are shown in

Table 3. Technical indicators of vegetable oil.

Vegetable Oils	Indicators
Tall oil	Acid value (mg KOH/g)	Density (g/cm3)	Iodine value (g I2/100 g)	Saponification value (mg KOH/g)
	191	0.98	147	176
Rapeseed oil	Acid value (mg KOH/g)	Density (g/cm3)	Iodine value (g I2/100 g)	Saponification value (mg KOH/g)
	0.8	0.92	115	183
Palm wax	Acid value (mg KOH/g)	Density (g/cm3)	Iodine value (g I2/100 g)	Saponification value (mg KOH/g)
	5.2	0.94	17	95

, and the vegetable oil samples are shown in Figure 2.

2.2. Composite Modification Preparation

The composite-modified asphalt contained two modifiers: straw fiber and vegetable oil (tall oil, rapeseed oil, palm wax). The preparation process is shown in Figure 3.

2.3. Research on Conventional Properties of Modified Asphalt

The orthogonal experiment designed in this paper is shown in

Table 4. Orthogonal test of vegetable oil/straw fiber composite-modified asphalt.

Number	Straw Fiber (%)	Vegetable Oil (%)
		Rapeseed Oil	Tall Oil	Palm Wax
1	1	1	1	1
2	1	2	2	2
3	1	3	3	3
4	2	1	1	1
5	2	2	2	2
6	2	3	3	3
7	3	1	1	1
8	3	2	2	2
9	3	3	3	3
10	4	1	1	1
11	4	2	2	2
12	4	3	3	3
13	5	1	1	1
14	5	2	2	2
15	5	3	3	3

. Five dosage levels of straw fiber and three dosage levels of each vegetable oil were used, and the specific dosages were as follows: straw fiber at 1–5% and vegetable oil at 1–3%, increasing by 1%.

According to the orthogonal test, the penetration, ductility, softening point, and 135 °C viscosity of the composite-modified asphalts were tested. The influence of the modifier dosage on the basic indicators of asphalt was studied by the range analysis method. The range calculation formula is shown in Formula (1).

$$R_J=Max\left(K_1,K_2\dots K_t\right)-Min\left(K_1,K_2\dots K_t\right)$$

(1)

where R_J is the range corresponding to the factor and K_t is the index corresponding to any factor. The larger R_J is, the more significant of an impact the factor has on the test index.

According to the above orthogonal test scheme, vegetable oil/straw fiber composite-modified asphalt was prepared, and the basic indicators of the asphalt were tested. Taking rapeseed oil/straw fiber composite-modified asphalt as an example, the test results are shown in

Table 5. Basic index test results of rapeseed oil/straw fiber composite-modified asphalt.

Number	Penetration (0.1 mm)	Softening Point (°C)	5 °C Ductility (mm)	135 °C Viscosity (Pa·s)
1	112.0	40.4	256	0.372
2	122.1	40.1	273	0.369
3	152.3	39.7	309	0.361
4	103.8	44.2	124	0.388
5	114.4	43.2	218	0.372
6	140.2	40.9	263	0.364
7	96.2	46.0	122	0.422
8	112.8	44.6	182	0.391
9	134.6	41.3	249	0.379
10	92.1	47.3	108	0.496
11	108.1	45.4	124	0.422
12	127.8	42.1	170	0.396
13	70.0	48.9	85	0.523
14	99.9	46.6	102	0.486
15	107.6	44.1	130	0.411

.

Analysis of the data in Table 5 shows that the performance of the composite-modified asphalt prepared according to the orthogonal test scheme is significantly different, and different modifier dosages have different degrees of influence on the asphalt performance. Therefore, the range analysis method was used to further study the change law of the basic indicators of asphalt, as shown in

Table 6. The extreme values of basic indicators of rapeseed oil/straw fiber composite-modified asphalt.

Influencing Factors	Parameter	Penetration (0.1 mm)	Softening Point (°C)	5 °C Ductility (mm)	135 °C Viscosity (Pa·s)
Straw fiber	K1	78.8	47.1	109	0.367
	K2	75.5	48.8	92	0.375
	K3	73.5	49.0	84	0.387
	K4	69.3	49.9	74	0.408
	K5	64.5	51.5	66	0.423
	RJ	14.3	4.4	43	0.065
Rapeseed oil	K1	94.8	45.4	139	0.440
	K2	101.5	44.0	170	0.408
	K3	108.5	41.6	214	0.382
	RJ	13.7	3.8	75	0.058

.

(1)

When analyzing the variation in the penetration of composite-modified asphalt, it was found that the range corresponding to rapeseed oil was 13.7 (0.1 mm), while the effect of straw fiber on the penetration was slightly greater than that of rapeseed oil, with a range of 14.3 (0.1 mm). The penetration of asphalt decreased with the increase in fiber content, and the rate of decrease first slowed down and then accelerated, indicating that the addition of fiber had a good effect on improving the consistency of asphalt. On the contrary, with the increase in rapeseed oil content, the penetration of asphalt increased rapidly, indicating that the content of light components in rapeseed oil was high, and the addition of straw fiber to asphalt softened it and reduced its consistency.

(2)

When analyzing the change pattern of the softening point of the composite-modified asphalt, it was found that straw fiber had a particularly significant impact on the softening point, with the extreme difference reaching 4.4 °C. As the amount of straw fiber increased, the softening point of the asphalt gradually increased, and the viscosity also increased accordingly, thereby enhancing the high-temperature stability of the asphalt. In contrast, as the amount of rapeseed oil increased, the softening point of the asphalt gradually decreased, and the degree of decrease gradually increased. This is because rapeseed oil has a diluting effect on asphalt, weakening the asphalt’s ability to resist deformation in high-temperature environments.

(3)

The range analysis of the ductility of composite-modified asphalt showed that rapeseed oil had a particularly significant impact on the ductility of the asphalt, with a range of 75 mm, which is much higher than the range of straw fiber of 43 mm. As the amount of straw fiber increased, the ductility of the asphalt gradually decreased, and the rate of decrease first increased and then decreased, which indicates that the mixing of straw fiber affects the low-temperature performance of asphalt. In contrast, the ductility of the asphalt increased linearly with the increase in rapeseed oil content, thereby enhancing the ductility of asphalt in low-temperature environments.

(4)

The range analysis of the rotational viscosity of the composite-modified asphalt at 135 °C showed that the effect of straw fiber on asphalt was more obvious, with a range of 0.065 Pa·s, which is greater than that of 0.058 Pa·s of rapeseed oil. In addition, as the amount of straw fiber increased, the viscosity of the asphalt increased slowly, which is due to the resistance of straw fiber to the flow of asphalt; the increase in the amount of rapeseed oil made the viscosity of asphalt gradually decrease, indicating that the addition of rapeseed oil reduces the viscosity of asphalt.

Based on these 4 evaluation indicators, the influence of two factors on asphalt performance was noted as follows: straw fiber significantly improves the high-temperature performance of asphalt while weakening the low-temperature performance, and rapeseed oil improves the performance of asphalt in low-temperature environments. Both have little effect on its viscosity, so 4% straw fiber + 1% rapeseed oil was selected based on comprehensive considerations. The same was true for the other two vegetable oils tested.

2.4. Test Methods

2.4.1. Dynamic Shear Rheology Test

(1)

Temperature sweep test

The complex shear modulus (G*) and phase angle (δ) of the fiber-modified asphalt material could be obtained by temperature scanning to evaluate the viscoelastic properties of the asphalt material. The ratio of the two could be used to obtain the rutting factor (G*/sinδ), which was used to characterize the asphalt material’s resistance to permanent deformation. The larger the complex shear modulus of the asphalt material is and the smaller the phase angle is, the stronger the resistance to permanent deformation is at high temperatures. The US SHRP stipulates that the rutting factor of asphalt materials should be greater than 1.0 kPa. This study used a controlled strain of 12%, a test temperature of 36–72 °C, an increase of 6 °C, a rotor of 25 mm, and a parallel plate spacing of 1 mm.

(2)

Multi-stress creep test

The MSCR test was conducted using a dynamic shear rheometer. According to the AASHTO T350 specification, two levels of shear stress, 0.1 kPa and 3.2 kPa, were applied to the asphalt specimens to characterize the linear and nonlinear viscoelastic responses of the asphalt, respectively. In a single cycle, the creep and deformation recovery process was completed after loading for 1 s and unloading for 9 s and repeated for 10 cycles. By analyzing the deformation and stress levels at different stages, the important indicators of the MSCR test were obtained: irrecoverable creep compliance (J_nr) and creep recovery rate (R). The calculation processes of these indicators are shown in Formulas (2) and (3). The advantage of this test method is that it can more realistically reflect the viscoelastic changes of asphalt pavement during actual use by analyzing the deformation of asphalt during loading and unloading, thereby more accurately evaluating the high-temperature deformation resistance of asphalt.

$$\begin{gathered} J_{nr-\delta }(\delta ,N)={\displaystyle \frac{{\epsilon }_r-{\epsilon }_o}{\delta }} \\ {J}_{nr}={\displaystyle \frac{{\displaystyle {\sum }_{i=1}^N{J}_{nr-\delta }(\delta ,N)}}{10}} \end{gathered}$$

(2)

$$\begin{gathered} R(\delta ,N)={\displaystyle \frac{{\epsilon }_c-{\epsilon }_r}{{\epsilon }_c-{\epsilon }_o}}\times 100\% \\ R={\displaystyle \frac{{\displaystyle {\sum }_{i=1}^{N}R(\delta ,N)}}{10}} \end{gathered}$$

(3)

where ε_r is the residual deformation after 9 s recovery in a single cycle, ε_o is the initial deformation in a single cycle, ε_c is the deformation after 1 s of creep in a single cycle, δ is the shear stress (kPa), J_nr is the irreversible creep compliance (kPa⁻¹), R is the creep recovery rate (%), and N is the number of cycles.

2.4.2. Bending Beam Rheological Test

The US SHRP proposes to use a bending beam rheometer to conduct bending creep tests. Through the test, the creep modulus (S) and creep stiffness change rate (m) of asphalt materials are obtained. Among them, the bending creep modulus (S) represents the ability of asphalt materials to resist deformation when subjected to load, and the creep stiffness change rate (m) refers to the rate of change of the creep modulus of asphalt [20]. These two indicators are used to evaluate the low-temperature crack resistance of asphalt materials. According to the US SHRP performance specification requirements, the creep modulus of asphalt should not exceed 300 MPa, and the creep stiffness change rate (m) should not be less than 0.3 [21].

For this test, the TE-BBR bending beam creep rheometer produced by Cannon Company of the United States was used to test the low-temperature performance of different fiber-modified asphalts. The test temperatures were −12 °C and −18 °C. The contact load was 35 mN ± 5 mN, and the test load was 980 mN ± 50 mN. First, the prepared asphalt beam was placed on the load frame, and the load was applied to the center of the beam specimen through the load probe. The tester collected the vertical displacement of the center point and calculated the output S and m values. The calculation formulas are as follows (Formulas (4) and (5)):

$$S_{\left(t\right)}={\displaystyle \frac{P{l}^3}{4b{h}^3{v}_{\left(t\right)}}}$$

(4)

$$m={\displaystyle \frac{d\log S_{\left(t\right)}}{d{\log }_{\left(t\right)}}}$$

(5)

where P is the applied constant load (N), l is the span of the asphalt beam (mm), b is the width of the asphalt beam specimen (mm), h is the height of the asphalt beam specimen (mm), and v is the deformation of the center point of the asphalt beam specimen (mm).

2.4.3. Contact Angle Test

This study used the OCA20 optical contact angle measurement analyzer and the lying drop method to test the contact angle between the asphalt and aggregates at room temperature. The asphalt test specimen was in the form of an asphalt glass slide, and the aggregate surface needed to be polished smooth. During the test, the asphalt sample prepared in advance needs to be placed on the sample table, and the computer controls the syringe to drip different reagents. The macro camera captures the image and calculates the contact angle through the image processing software.

(1)

Surface free energy theory

The basic definition of surface energy is the work that must be conducted on the system to increase the unit surface area of the system under constant temperature and pressure conditions, represented by γ [22]. Young’s equation [23], γ_s = γ_lcosθ + γ_sl, is the basis for studying solid–liquid wetting. The surface energy calculation formula obtained based on the Young equation is shown in Equations (6) and (7).

$$\left(1+\cos \theta \right)=2\sqrt{{\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{d}}}{\displaystyle \frac{\sqrt{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{d}}}}{{\gamma }_{\mathrm{l}}}}+2\sqrt{{\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{p}}}{\displaystyle \frac{\sqrt{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{p}}}}{{\gamma }_{\mathrm{l}}}}$$

(6)

$${\displaystyle \frac{\left(1+\cos \theta \right){\gamma }_{\mathrm{l}}}{2}}{\displaystyle \frac{1}{\sqrt{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{d}}}}}=\sqrt{{\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{p}}}\sqrt{{\displaystyle \frac{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{p}}}{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{d}}}}}+\sqrt{{\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{d}}}$$

(7)

where γ_sl is the solid–liquid interface free energy (mJ/m²); γ_s and γ_l are the surface free energies of the solid and liquid, respectively (mJ/m₂); and γ_s^d, γ_l^d and γ_s^p, γ_l^p are the non-polar dispersion component and polar acid-base component of the solid and liquid surfaces, respectively (mJ/m²).

According to Formula (7), the γ_s^d and γ_s^p of the tested material can be obtained by linear fitting with ${\displaystyle \frac{\left(1+\cos \mathsf{\theta }\right){\mathsf{\gamma }}_{\mathrm{l}}}{2}}{\displaystyle \frac{1}{\sqrt{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{d}}}}}$ as the ordinate and $\sqrt{{\displaystyle \frac{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{p}}}{{\mathsf{\gamma }}_{\mathrm{l}}^{\mathrm{d}}}}}$ as the abscissa according to the known free energy of the liquid. The correlation coefficient R² reached above 0.9, indicating that the results are highly reliable.

(2)

Adhesion work calculation

Adhesion work is defined as the work conducted to separate two phases that are bonded together into two independent phases [24]. Therefore, the adhesion work expression between the asphalt and aggregate can be defined as Formula (8):

$${\mathrm{W}}_{\mathrm{Adhesion}}={\mathsf{\gamma }}_{\mathrm{Asphalt}}+{\mathsf{\gamma }}_{\mathrm{Aggregate}}-{\mathsf{\gamma }}_{\mathrm{Asphalt}-\mathrm{Aggregate}}$$

(8)

where γ_Adhesion is the adhesion work between the asphalt and aggregate (mJ/m²), γ_Asphalt is the surface free energy of the asphalt (mJ/m²), γ_Aggregate is the surface free energy of the aggregate (mJ/m²), and γ_{Asphalt-Aggregate} is the surface free energy of the asphalt and aggregate (mJ/m²). At this time, the calculation method of the adhesion work between the asphalt and aggregate can be rewritten as Formula (9).

$${\mathrm{W}}_{\mathrm{Adhesion}}=2\sqrt[]{{\mathsf{\gamma }}_{\mathrm{Asphalt}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{Aggregate}}^{\mathrm{d}}}+2\sqrt[]{{\mathsf{\gamma }}_{\mathrm{Asphalt}}^{\mathrm{p}}{\mathsf{\gamma }}_{\mathrm{Aggregate}}^{\mathrm{p}}}$$

(9)

2.4.4. Fourier Infrared Spectroscopy Test

This paper used an attenuated total reflection Fourier transform infrared spectrometer with 32 scans and a resolution of 4 cm⁻¹. It was necessary to collect the background first, take about 1 g of solid asphalt and cover it directly on the crystal plate, adjust the press nut to completely exhaust the air between the asphalt sample and the crystal plate, and then collect the sample data. After each sample collection, the asphalt on the stage was wiped with trichloroethylene, and the ATR crystal was then completely cleaned with acetone to ensure the reliability of the data.

3. Results and Discussion

3.1. DSR Test

3.1.1. Rheological Properties Analysis Based on Temperature Scanning Test

Temperature scanning experiments were conducted on base asphalt, straw-fiber-modified asphalt, and three types of composite-modified asphalts: tall oil/rapeseed oil/palm wax–straw fiber. The five types of asphalt were recorded as A1, A2, A3, A4, and A5. The complex shear modulus G*, phase angle δ, and rutting factor G*/sinδ measured in the experiment were used to evaluate the rheological properties of the asphalts at different temperatures. G* reflects the asphalt’s ability to resist shear deformation, and G*/sinδ reflects the asphalt’s ability to resist high-temperature deformation. The curves of the three indicators changing with temperature are shown in Figure 4.

As shown in Figure 4a, with the increase in temperature, the complex shear modulus of the five asphalts tended to decrease. The size of the complex shear modulus (G*) can reflect the ability of asphalt to resist deformation under external force. The larger the G* value is, the stronger the shear resistance of the asphalt is and the smaller the possibility of deformation is, thereby improving the high-temperature rutting resistance of the asphalt pavement. As the temperature increased, the G* value gradually decreased, indicating that the asphalt softened and the anti-deformation ability of the five asphalts weakened. In the temperature range of 36–60 °C, the G* value decreased sharply, while in the 60–72 °C range, the G* value gradually became stable; during the decline process, the gap between the four modified asphalts and the base asphalt gradually shrank. It can be seen from the curve chart that the five curves basically coincide at this time. In the early stage of heating, the five asphalts had different decreases. Among them, the complex shear modulus values of the four modified asphalts were all higher than that of the base asphalt, which shows that the incorporation of straw fiber and vegetable oil enhances the shear resistance of asphalt. Among the four types of modified asphalt, the modification effect of palm wax led to the most significant improvement in high-temperature rutting resistance.

As shown in Figure 4b, the phase angles of the five asphalts increased with increasing temperature. The size of the phase angle (δ) can reflect the proportion of viscous components in asphalt. The larger the δ value is, the higher the proportion of viscoelastic components is and the closer the state of the asphalt is to viscosity. As the temperature increased, the phase angles of the five asphalts increased, indicating that their elasticity decreased and their anti-deformation ability was impaired. During the entire heating process, the increase rate of the initial δ value was higher than that of the later stage of heating. It is worth noting that the δ values of the four modified asphalts were all lower than that of the base asphalt, which shows that the addition of straw fiber and vegetable oil enhanced the recoverable deformation ability of asphalt. This is because the presence of straw fiber hinders the flow of asphalt, increases the proportion of elastic components, and enables asphalt to maintain good elastic properties under high-temperature conditions. The curves of the tall oil and rapeseed oil/straw fiber composite-modified asphalt are similar. Compared with straw-fiber-modified asphalt, the addition of palm wax further reduced the phase angle, and the phase angle value of the palm wax/straw fiber composite-modified asphalt increased the most. This shows that palm wax has a significant effect in improving the recoverable deformation ability of asphalt.

As shown in Figure 4c, the change patterns of the rutting factor of the five asphalts with temperature are consistent with those of the complex shear modulus, but contrary to the change patterns of the phase angle, all of which show a trend of gradually decreasing with increasing temperature. The size of the rutting factor (G*/sinδ) can reflect the ability of asphalt to resist permanent deformation. The larger the rutting factor is, the stronger the rutting resistance of the asphalt is under vehicle load at high temperature, and the better the high-temperature stability is. As the temperature increased, the G*/sinδ values of the five asphalts decreased, indicating that their high-temperature deformation resistance weakened. In the early stage, the rutting factor decreases rapidly with the increase in temperature; then, the curve tends to be flat, and finally, the curves basically overlap. Compared with the base asphalt, the rutting factors of the four modified asphalts all increased to varying degrees, indicating that the addition of straw fiber and vegetable oil improved the high-temperature deformation resistance of asphalt. Among them, the G*/sinδ curve of the palm wax/straw fiber composite-modified asphalt is at the top, indicating that its rutting resistance is the best.

3.1.2. Rheological Properties Analysis Based on Multiple Stress Creep Recovery Test (MSCR)

The MSCR test results of the five types of asphalt are shown in Figure 5.

Figure 5 shows the creep recovery curves of different asphalts over 10 cycles at stress levels of 0.1 kPa and 3.2 kPa. It can be seen that under the two stress levels, the cumulative shear strain of the five asphalts at the high stress level is significantly greater than that at the low stress level. This shows that during the actual service of the road surface, long-term heavy traffic loads will significantly increase the risk of asphalt rutting and other defects, seriously affecting road performance and shortening the service life of the road surface. Under the two stress levels, the difference in the curves of the asphalt creep stage in the first second is small, but with the passage of time and the gradual accumulation of shear strain, the five curves gradually disperse. The curve position of the matrix asphalt is the highest, which shows that the incorporation of modifiers effectively improves the rutting resistance of asphalt. Among them, the curve of the palm wax/straw fiber composite-modified asphalt is at the bottom, and its strain value is significantly lower than those of the other asphalts, reflecting a stronger anti-deformation ability and indicating the effect of palm wax in improving the rutting resistance of straw-fiber-modified asphalt most notably. The curves of the tall oil and rapeseed oil/straw fiber composite-modified asphalts basically overlap, indicating that tall oil and rapeseed oil have similar effects on improving the high-temperature deformation resistance of straw-fiber-modified asphalt.

Figure 6 shows the calculation results of the creep recovery rate of the five types of asphalt.

As can be seen from Figure 6, the R values of the four modified asphalts are all high, indicating that these modified asphalts can recover to their original state more quickly after being loaded and have better deformation recovery ability and elasticity. At the stress level of 0.1 kPa, the creep recovery rate of the palm wax/straw fiber composite-modified asphalt was the highest, reaching 2.18%. However, at the stress level of 3.2 kPa, the R values of the five asphalts were all reduced, reflecting the weakening of the deformation recovery ability of asphalt under high stress, which is consistent with the law of the time–strain response curve. Asphalt is a viscoelastic material, and its recovery ability will be weakened at high stress levels. Among the asphalts, the creep recovery rate of the straw-fiber-modified asphalt at the stress level of 3.2 kPa was the highest, reaching −2.78%. This is because the addition of straw fiber increases the viscosity of asphalt, thereby reducing irreversible creep and enhancing the asphalt’s resistance to high-temperature deformation.

Figure 7 shows the calculation results of the irreversible creep compliance of the five types of asphalt.

As can be seen from Figure 7, the J_nr value at the stress level of 0.1 kPa is lower than 3.2 kPa, indicating that at low stress levels, asphalt has stronger resistance to deformation, which is consistent with the trend of the creep recovery rate curve, further verifying that in the actual use of the pavement, high load levels are more likely to cause rutting and other defects. Under the same stress level, the irrecoverable creep compliance value of straw-fiber-modified asphalt is the smallest, showing the strongest resistance to permanent deformation, followed by palm wax/straw fiber composite-modified asphalt, while the J_nr values of tall oil and rapeseed oil/straw fiber composite-modified asphalt are similar and lower than that of the base asphalt. Combined with the analysis of the creep recovery rate curve, it can be seen that compared with pure straw-fiber-modified asphalt, the addition of vegetable oil destroys the internal structure of asphalt to a certain extent. This is because the light components in the vegetable oil will surround the asphalt flocs and reduce resistance to molecular movement, thereby weakening the ability of asphalt to resist permanent deformation. According to the comparison of the J_nr values of the three vegetable oil composite-modified asphalts, the palm wax/straw fiber composite-modified asphalt showed the strongest anti-rutting ability, while the effects of tall oil and rapeseed oil were similar. This is consistent with the results of the temperature scanning experiment, further proving the superiority of palm wax in enhancing the high-temperature performance of asphalt.

3.2. BBR Test Results

The creep modulus (S) indicates the stiffness of asphalt under a constant load at a certain temperature. The larger the S value is, the greater the elasticity of the asphalt is, the lower the viscosity is, and the more likely it is to produce low-temperature cracking when subjected to loads [25]. The creep rate represents the stress relaxation ability of asphalt. A decrease in the creep rate indicates that the material is less likely to recover after deformation, and the stress relaxation ability decreases, which makes it more likely to cause cracks. The BBR test results of the modified asphalts are shown in Figure 8.

It can be seen from Figure 8 that as the temperature decreases, the creep stiffness of the various asphalt materials increases significantly, making the asphalt materials more susceptible to cracking at low temperatures. Nonetheless, the S values of all asphalt samples were less than 300 kPa, meeting the requirements of the US SHRP performance specifications. As the temperature decreases, the creep rate of asphalt gradually decreases, which shows that the lower the temperature is, the lower the rate of change of the stiffness curve is with time, and the material is less likely to recover when deformed, resulting in a decrease in stress relaxation capacity and thereby increasing the crack formation possibility. Temperature factors have a significant impact on the creep rate of asphalt materials. After incorporating fibers, the stiffness modulus of the fiber-modified asphalt was greater than that of the base asphalt. This is because the addition of fiber reduces the amount of free asphalt, reduces the viscosity of the asphalt, and simultaneously enhances the elasticity, resulting in an increase in the creep stiffness modulus of the fiber-modified asphalt, which in turn has an adverse effect on low-temperature performance.

In addition, the incorporation of fibers also reduces the creep rate of asphalt materials, indicating that the addition of fibers weakens the stress sensitivity of asphalt. In terms of the influence of the modifiers, tall oil and rapeseed oil significantly improved the low-temperature properties of straw-fiber-modified asphalt. The fatty acids and rosin acid components in vegetable oil interact with the aromatic hydrocarbons and saturated hydrocarbons in asphalt, thereby enhancing the toughness and ductility of the asphalt, helping to prevent asphalt from becoming brittle, and improving its low-temperature crack resistance under low-temperature conditions. In contrast, palm wax has a detrimental effect on the low-temperature properties of straw-fiber-modified asphalt. Palm wax contains a higher proportion of stearic acid and wax ester components. These components increase the rigidity of asphalt, making it more susceptible to brittle cracking at low temperatures, resulting in a decrease in low-temperature crack resistance.

3.3. Research on Asphalt Adhesion Performance Based on Surface Free Energy Theory

3.3.1. Surface Energy Results of Asphalt and Aggregate Based on Contact Angle Test

In order to make the calculation results of the surface free energy of the asphalts and aggregates accurate, the test used three standard solutions with good stability and incompatible with asphalt, namely distilled water, formamide, and propylene glycol. The surface energy parameters of the three standard solutions are shown in

Table 7. Surface energy parameters of standard solutions.

Solution Type	Surface Energy γ (mJ/m2)	Dispersion Component γd (mJ/m2)	Acid-Base Content γp (mJ/m2)
Distilled water	72.8	21.8	51
Formamide	58	39	19
Glycerol	64	34	30

.

At the same time, in order to reduce the influence of operating errors, each sample was tested three times in parallel—that is, three different positions were found on the same slide—and the average of the three measurement results was taken as the final result. The same standard solution was measured three times. The test results of the contact angles between different asphalts and aggregates are shown in

Table 8. Test results of contact angle between asphalts and aggregates.

Sample Type	Distilled Water	Formamide	Glycerol
	Contact Angle (°)	Contact Angle (°)	Contact Angle (°)
A1	108.4	94.2	100.8
A2	107.2	93.2	99.7
A3	105.7	91.9	98.3
A4	101.9	88.6	94.8
A5	104.2	90.7	97.1
Limestone	53.4	37.9	46.1

.

In order to further verify the validity of the test results, for any asphalt sample, the surface energy γ of different test liquids was linearly correlated with cosθ, so the γ and cosθ of different asphalts and aggregates were linearly fitted, and the fitting results of the material surface free energy parameters calculated according to Formula (7) are shown in

Table 9. Fitting results of contact angle between asphalts and aggregates.

Sample Type	γ and cosθ Fitting Curve R2	$\frac{\left(1+\mathbf{cos}\mathsf{\theta }\right){\mathsf{\gamma }}_{\mathbf{l}-\mathbf{g}}}{2}\frac{1}{\sqrt{{\mathsf{\gamma }}_{\mathbf{l}-\mathbf{g}}^{\mathbf{d}}}}$$\mathbf{and}\sqrt{\frac{{\mathsf{\gamma }}_{\mathbf{l}-\mathbf{g}}^{\mathbf{p}}}{{\mathsf{\gamma }}_{\mathbf{l}-\mathbf{g}}^{\mathbf{d}}}}$ Fitting Curve	R2
A1	0.99828	Y = 1.28615x + 3.34181	0.95811
A2	0.99987	Y = 1.37547x + 3.36086	0.96385
A3	0.99990	Y = 1.48399x + 3.39078	0.96926
A4	0.99988	Y = 1.76693x + 3.45958	0.97745
A5	0.99945	Y = 1.60812x + 3.39586	0.96906
Limestone	0.91285	Y = 5.0425x + 4.69223	0.99375

, with the fitting curve shown in Figure 9.

According to Table 9, R² is in the range of 0.95~1.0, so the test results are highly reliable. According to the contact angle test results, the contact angles of the different samples with formamide in the three standard solutions are all minimal. Among the five asphalt samples, no matter which standard solution was used, the θ value of the rapeseed oil/straw fiber composite-modified asphalt was the smallest. The smaller the contact angle of the liquid is, the greater the surface energy is and the easier it is to wet.

The slope $\sqrt{{\gamma }_s^p}$ and intercept $\sqrt{{\gamma }_s^p}$ of the fitting curve in Figure 9 were converted into the surface free energy parameters of the material: the acid-base component and the dispersion component. The sum of the two is the surface free energy of the material; that is, $\gamma ={\gamma }^p+{\gamma }^d$. The surface free energy parameters of the asphalts and aggregates are shown in

Table 10. Calculation results of surface free energy of different types of asphalt and aggregate.

Sample Type	Acid-Base Content γp (mJ/m2)	Dispersion Component γd (mJ/m2)	Surface Energy γ (mJ/m2)
A1	1.65	11.17	12.82
A2	1.89	11.30	13.19
A3	2.20	11.50	13.7
A4	3.12	11.97	15.09
A5	2.59	11.53	14.12
Limestone	25.43	22.02	47.45

.

It can be seen from the results in Table 10 that the surface energy dispersion components of the five asphalts account for more than 79% of the entire surface free energy. The addition of modifiers increased the surface free energy of the asphalt. Compared with the base asphalt, the surface free energy of the rapeseed oil/straw fiber composite-modified asphalt was 15.09 mJ/m², which had the largest increase. The surface free energy of straw-modified asphalt was 13.19 mJ/m², which had the smallest increase. This is consistent with the test results in Table 8. According to the surface free energy theory, the asphaltene content has a significant impact on the surface free energy of asphalt, and the content is inversely proportional to the surface free energy, indicating that the incorporation of straw fiber and vegetable oil reduces the asphaltene content in asphalt, leading to increases in the surface freedom of modified asphalt. Compared with single straw-fiber-modified asphalt, among the three vegetable oil modifiers, rapeseed oil had the most obvious effect in increasing the asphalt surface free energy. For limestone, the polar acid-base component of the surface free energy was slightly higher than the non-polar dispersion component, accounting for 53% of the surface free energy.

3.3.2. Calculation of Adhesion Work between Asphalt and Aggregate

The adhesion work between the five asphalts and limestone was calculated according to Formula (9), and the calculation results are shown in Figure 10.

As can be seen from Figure 10, the adhesion work of the matrix asphalt to the aggregate is the smallest, and it is more likely to be damaged when it adheres to the aggregate. It is more likely to have defects such as peeling during the actual pavement service process. The four modified asphalts show increases to varying degrees in comparison. Among them, the adhesion work of the rapeseed oil/straw fiber composite-modified asphalt is the largest, which increased by 13% compared with the matrix asphalt and increased by 11% compared with the single straw-fiber-modified asphalt, indicating that it has the best adhesion to the aggregate, which is consistent with the calculation results of the surface free energy above. However, the surface roughness of the aggregate determines the surface free energy of the aggregate to a large extent. Therefore, in the process of polishing the aggregate surface, the roughness of all aggregate surfaces should be made as close as possible to reduce the difference in surface free energy caused by the different surface roughness of the aggregate.

3.4. Infrared Spectrum Test Analysis

In this paper, ATR-FTIR technology was used to conduct infrared spectroscopy tests on the five types of asphalt. The original spectrum results are shown in Figure 11.

As can be seen from Figure 11, there is not much difference in the characteristic peaks of the infrared spectra of the base asphalt and the different modified asphalts. The main common characteristic peaks are as follows: stretching vibration of the C-H bond at 2920 cm⁻¹ and 2850 cm⁻¹; stretching vibration of the aromatic ring C=C at 1600 cm⁻¹; bending vibration of C-H at 1460 cm⁻¹ and 1375 cm⁻¹. Therefore, according to the results of the original spectrum, the positions of the main absorption peaks before and after modification are basically the same, indicating that the chemical groups of the base asphalt and the modified ones have not changed significantly, and there is only a certain change in the peak intensity, indicating that no new chemical bonds were generated or broken during the modification process due to physical modification—that is, the modifiers physically mixed and dispersed in the base asphalt rather than chemically reacting to achieve the modification effect.

Some studies have pointed out that using the second-order derivative of the infrared spectrum to evaluate changes in characteristic peaks can significantly improve the resolution and sensitivity of the spectrum; amplify the detailed features of the peaks, especially weak absorption peaks and overlapping peaks; and help more accurately identify and distinguish samples. The existence and changes of different chemical groups are shown in [26]. This method can effectively remove the influence of baseline drift, provide clearer and more accurate spectral information, and facilitate in-depth analysis of the molecular structure and chemical composition of the sample. Therefore, this article continues to use the second-order derivative for analysis, and the results are shown in Figure 12.

As can be seen from Figure 12, in addition to the absorption peaks in Figure 11, it can also be observed that at 1730–1750 cm⁻¹, C=O groups appear in the three vegetable oil/straw fiber composite-modified asphalts. The enhancement of the C=O group indicates that the asphalt has undergone an oxidation reaction during use. This is because after the addition of vegetable oil, the asphalt continues to be stirred and undergoes secondary aging, so the C=O group appears, which may lead to changes in the molecular structure and performance degradation. However, the characteristic peak vibration of the palm wax/straw fiber composite-modified asphalt is the smallest, indicating that it has the best anti-aging performance. The stretching vibration of free hydroxyl O-H appears at 3650 cm⁻¹. Free hydroxyl is more active in asphalt and can form hydrogen bonds with water molecules. This makes asphalt more likely to absorb moisture in a humid environment, resulting in a decrease in adhesion and an increase in the risk of peeling. The characteristic peak intensity of straw-fiber-modified asphalt at 3650 cm⁻¹ is higher, indicating that the free hydroxyl content is the highest and that its water damage resistance is the worst. As the affinity of free hydroxyl with water makes asphalt more susceptible to moisture erosion, it weakens the adhesion between asphalt and aggregates. However, the vibration of the free hydroxyl groups of the modified asphalts weakened or disappeared after adding vegetable oil, indicating that their water damage resistance was significantly enhanced, among which the rapeseed oil/straw fiber composite-modified asphalt had the strongest water damage resistance.

4. The Practical Engineering Significance Corresponding to the Research Results

The practical engineering significance of this study is given below.

By applying vegetable oil as a plasticizer to straw-fiber-modified asphalt, the compatibility between the fibers and the asphalt matrix can be effectively improved, and the uniform dispersion of the fibers can be improved, thereby significantly improving the viscoelastic properties of the modified asphalt. This provides a scientific basis for the practical application of roads under different environmental conditions. Specifically, the research results show the following:

(1)

The application of palm wax helps to significantly improve the rheological properties of asphalt under high-temperature conditions, making it suitable for high-temperature areas or high traffic load sections, but its adverse impact on low-temperature performance needs to be considered.

(2)

Tall oil has an outstanding performance in improving the low-temperature properties of modified asphalt. It can provide optimized solutions for road construction in cold climate areas and reduce the risk of cracking of asphalt pavements at low temperatures.

(3)

Rapeseed oil has a significant effect on improving the adhesion and water damage resistance of asphalt. It is especially suitable for environments with frequent rain or high humidity to extend the service life of roads.

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.