A Study on the Performance of Asphalt Modified by Desulfurized Waste Rubber/Ethylene Vinyl Acetate Composite with Additives

Abstract

The recycling of waste tires avoids the environmental hazards of landfills and incineration, and its application in asphalt modification achieves resource sustainability. Currently, desulfurized rubber powder (DRP) is widely used as an asphalt modifier, mainly mixed with SBS, and fewer studies have been conducted on high-dose asphalt modification with ethylene vinyl acetate (EVA). In this paper, DR/EVA-composite-modified asphalt (DR/EVACMA) was prepared using 20% DRP and 4% EVA by adding four additives: furfural extract oil (FEO), a crosslinking agent (DCP), a vulcanizing agent (sulfur), and a silane coupling agent (KH-550). The aim was to study the effects of different additives on the physical properties, storage stability, and rheological properties of asphalt. First, conventional physical property measurements were carried out, and the data were analyzed using a polar analysis to determine the degree of influence of the four additives and the optimal ratios. Then, the rheological properties and fatigue resistance of DR/EVACMA were investigated through temperature scanning experiments, linear amplitude scanning (LAS) experiments, and multi-stress creep (MSCR) experiments. Finally, the reaction mechanism and microscopic properties were analyzed through infrared spectroscopy experiments (FTIR) and fluorescence microscopy (FM). The results showed that FEO had the greatest effect on asphalt characteristics. Compared to matrix asphalt and additive-free asphalt, DR/EVACMA has higher physical properties, fatigue resistance, and high temperature rheological properties due to its internal crosslinking structure. Its storage stability is also very good, with a difference of only 0.7 °C in the softening point.

1. Introduction

Hundreds of used tires are produced every year, which are difficult to recycle and degrade, and discarding them through incineration or landfills causes great harm to the environment [1,2]. In order to reduce “black pollution”, cutting tires into rubber particles (RPs) or rubber crumb (CR) as an asphalt modifier has become one of the effective ways to solve the problem of waste tire recycling [3,4]. The preparation of waste-rubber-modified asphalt can effectively alleviate the environmental pressure and reduce landfill waste and incineration brought about by environmental pollution and costs; the secondary use of waste rubber can achieve resource sustainability. Studies have shown that waste rubber can effectively improve the deformation and aging resistance of asphalt compared to matrix asphalt, increase the durability and impact protection of asphalt [5], and improve the noise reduction and skid resistance of the pavement. A study also showed that waste tire rubber powder (WTR) exists in a granular state in asphalt, which is not conducive to the compatibility of the two, resulting in stress concentration. At present, using WTR composite with other asphalt modifiers is the mainstream way to prepare a modified asphalt with better performance, including common modified materials such as SBS, PE, EVA, and so on [6,7,8,9]. Although composite-modified asphalt prepared using WTR and SBS has excellent performance at high and low temperatures, there are some problems, the most important of which is the presence of rubber molecular chains and the difference in physicochemical properties with asphalt, resulting in high viscosity in the modified asphalt, processing difficulties, high temperature degradation, poor storage stability, hydrogen sulfide gas release processing [3,4], and mixture mixing difficulties [2,5].

Therefore, desulfurized rubber powder (DRP) has attracted much attention due to its environmental friendliness and good storage stability. After the rubber is desulfurized, the S-S and C-S bonds outside the main chain are broken, and active groups are formed on the surface. The surface becomes fluffy, and the contact area with asphalt increases, effectively increasing the solubility and dispersion of the rubber powder in the asphalt, which improves compatibility and reduces temperature sensitivity [3,10,11,12]. In addition, its stable chemical combination absorbs light components and dissolves them, reducing the production of toxic fumes and hydrogen sulfide [13,14]. Another aspect, SBS as a commonly used asphalt modifier, has excellent performance, but the price is high. This reduces its popularity among researchers [15]. Some studies have shown that SBS-modified asphalt produces large amounts of harmful fumes at high temperatures [16]. Low-cost modifiers and desulfurization powder with close performance are selected to prepare composite-modified asphalt. Not only can they improve low-end roads to meet the asphalt quality indicators, but they can also reduce production and maintenance costs.

EVA is a thermoplastic polymer. It consists of a non-polar crystalline vinyl backbone and a polar amorphous branched chain, which improves its compatibility with asphalt by increasing the polarity of the branched chain and decreasing its crystallinity. In addition, When EVA is dispersed in asphalt, the rheological properties are improved, as three-dimensional layers of crosslinked structures are formed [17]. Studies have shown that the addition of EVA can improve the overall properties of rubberized asphalt, such as temperature sensitivity, low-temperature plasticity, compatibility, and fatigue resistance [7,18]. Although there are more studies on asphalt modified with EVA and blended waste rubber, there are fewer studies on asphalt modified with high-dose DRP and blended EVA, and in-depth studies are needed to improve the storage stability of this type of asphalt. The aim of this research is to reduce the difficulty of asphalt construction and production costs and improve storage stability.

When the modifier can be completely dispersed in the asphalt, the compatibility between the modifier and the asphalt is optimal, and good compatibility and network structure can reduce the phase separation of the modifier and the asphalt, thus improving the storage stability of the asphalt. The DRP and EVA dosages were determined through preliminary experiments and related studies [7,19,20]. In this study, 4% EVA and 20% desulfurized rubber powder asphalt were used as modifiers to prepare composite-modified asphalt. Considering that the molecular weight of DRP has been reduced by desulfurization, but it is still much larger than the molecular weight of the asphalt component, and the rubber powder also contains carbon black, the compatibility with asphalt still needs to be improved. To improve the compatibility and mechanical properties of the composites with asphalt, attempts were made to add furfural extract oil (FEO), a crosslinking agent (DCP), a vulcanizing agent (sulfur), and a silane coupling agent (KH-550). This was carried out to study the effect of the four additives on the performance of DR/EVACMA. FEO can supplement the asphalt with lightweight components, preventing the disruption of the internal colloidal balance of the asphalt caused by the overdosing of DRP and EVA, and ensuring that the modifiers are completely dissolved in the asphalt. Secondly, for the desulfurization of DRP after the use of sulfur and DCP crosslinking, the formation of the crosslinking network structure can significantly improve the mechanical properties of asphalt. At the same time, DCP is also commonly used as a peroxide crosslinker in EVA chemical bond crosslinking to improve the mechanical properties of EVA materials and to improve the cohesive strength and heat resistance of EVA, the crosslinking agent, and the silane coupling agent used together [21]. Silane KH-550 not only improves the bonding between carbon black filler and rubber matrix, but also prevents the uneven dispersion of carbon black in asphalt.

Firstly, nine sets of ratios were determined using orthogonal tests, the optimal ratios of additives under optimal storage stability conditions were derived from physical property measurements and polarity analyses, and in-depth analyses were carried out on the effects of each additive on the conventional physical properties of modified asphalt. On this basis, the high-temperature rutting resistance, fatigue resistance, and deformation recovery properties of the modified asphalt were investigated using dynamic shear rheometer (DSR) tests. Finally, two experiments, Fourier-Transform Infrared Spectroscopy (FTIR) and Fluorescence Microscopy (FM), were used to characterize the chemical reactions and phase distributions present in DR/EVACMA.

2. Experimentation and Methodology

2.1. Material Preparation

The asphalt raw material selected for this study was ZhenHai 90 road asphalt (referred to as ZH-90). The technical parameters are shown in

Table 1. Characterizations of the base asphalt.

Items	Test Temperature (°C)	Value	Test Method
Penetration (0.1 mm)	25	83	T 0604—2011
Softening point (°C)	-	47	T 0606—2011
Ductility (cm)	15	180	T 0605—2011
Flashpoint (°C)	-	304	T 0611—2011
Solubility (%)	-	99.8	T 0607—2011
Density(g/cm3)	15	1.030	T 0603—2011

.

2.1.1. EVA

EVA was purchased from Aladdin Co., Ltd., Beijing, China; the VA content is 28%, and its characteristic parameters are shown in

Table 2. Characterizations of EVA.

Type	VA Content (%)	Density (25 °C)/(g·cm−3)	Melting Point/°C	Melt Index (dg/min)
EVA	28	0.95	70	150

.

2.1.2. Gum Powder

Waste glue powder was purchased from Gansu Road and Bridge Shanjian Technology Co., Ltd.; it has a 40-mesh desulfurization glue powder, and its characteristic parameters are shown in

Table 3. Characterizations of DRP.

Items	Value	Requirements
Heating loss (80 °C) (%)	0.5	≤1.5
Ash (%)	7.3	≤8
Acetone extract (%)	16.7	≤20
Mooney viscosity ML 100 °C (1 + 4)	34	≤45
Carbon black content (%)	28.2	≥18
Rubber hydrocarbon content (%)	47	≥40
Iron content (%)	0.027	≤0.03
Particle size (mm)	5	≤10

.

2.1.3. Additives

Furfural extract oil was purchased from Shandong Longshengda New Material Technology Co., Jinan, China.

Sulfur was purchased from Aladdin Co.

γ-Aminopropyltriethoxysilane (KH-550) was supplied by Aladdin (Beijing, China), and it can establish a chemical bond between organic and inorganic boundaries and improve the chemical stability of the material.

Dicumyl peroxide (DCP), also supplied by Aladdin (Beijing, China), is an organic peroxide with a free-radical initiator and crosslinking properties, widely used in the rubber and plastics industries.

The physical properties of additive-free bitumen (20% DRP + 4% EVA) are shown in

Table 4. Characterizations of additive-free asphalt (0).

4%EVA + 20% DRP
Items		Test Temperature	Value
Softening point (°C)		5 °C	62.3
Ductility (cm)		5 °C	8.1
Penetration (0.1 mm)		25 °C	48.3
Softening point difference (°C)		-	11.5
Brinell’s rotational viscosity (Pa·S)		135 °C	4.02
(RTFOT)	Softening point (°C)	5 °C	69.5
	Ductility (cm)	5°	6.0

.

2.2. Preparation Processes

2.2.1. Orthogonal Experimental Designs

Firstly, the dosages of EVA and desulfurized rubber powder were controlled at 4% and 20%. For three different dosages of four additives, including furfural extract oil, DCP, sulfur, and KH550, nine experiments were conducted using an orthogonal test to prepare different modified asphalts and performance tests. The effects of the four additives on the conventional physical and rheological properties of asphalt were analyzed. Factor levels and classification of orthogonal experimental groups are shown in

Table 5. Parameters of orthogonal test ingredients and layers.

	Ingredients
Layers	A	B	C	D
	FEO	DCP	Sulfuric	KH-550
Unit	(%)	(%)	(%)	(%)
1	2.6	0.15	0.06	0.05
2	3.0	0.25	0.16	0.15
3	3.4	0.35	0.26	0.25

and

Table 6. Orthogonal experiment groups.

Factor
Number	A	B	C	D	Program
1	1	1	1	1	A1B1C1D1
2	1	2	2	2	A1B2C2D2
3	1	3	3	3	A1B3C3D3
4	2	1	2	3	A2B1C2D3
5	2	2	3	1	A2B2C3D1
6	2	3	1	2	A2B3C1D2
7	3	1	3	2	A3B1C3D2
8	3	2	1	3	A3B2C1D3
9	3	3	2	1	A3B3C2D1

.

2.2.2. Preparation of Samples

Firstly, the matrix bitumen was heated to 160 ± 5 °C, and desulfurized rubber powder and furfural extract oil were added, during which time the mixture was stirred manually to prevent accumulation. The temperature was raised to 180–190 °C, and the bitumen was sheared with a high-speed shear at 1000 r/min for 3 min, followed by 15 min at 5000 r/min, with EVA added to keep the speed constant, and shearing was carried out for 20 min. The sheared, modified bitumen was then cooled to 150 °C and placed in a temperature-controlled electrically heated jacket. KH-550 was added at 155 °C and stirred for 15 min. Finally, the temperature was raised to 170 °C, and sulfur and diisopropyl peroxide (DCP) were added and stirred for 40 min to obtain desulfurized rubber powder/EVA-composite-modified asphalt (DR/EVACMA). The preparation process is shown in Figure 1 and Figure 2.

2.3. Performance Tests

2.3.1. General Physical Property Tests

DR/EVACMA (1–9) was prepared according to orthogonal experiments, and asphalt without additives (0) was similarly prepared as a control. Conventional properties, such as softening point, elongation, needle penetration, rotational viscosity at 135 °C, and storage stability, were tested for the above types of asphalt according to “Standard Test Methods for Asphalt and Asphalt Mixtures for Highway Construction” (JTG E20-2011).

2.3.2. Rheological Properties Testing

In this study, the phase angle (δ), complex modulus (G*), and rutting coefficient (G*/sinδ) of a total of 11 samples of matrix asphalt, additive-free asphalt, and composite-modified asphalt were measured at different temperatures using a dynamic shear rheometer (DSR) to evaluate the high temperature rheological properties of asphalt. According to the AASHTO T315 specification, the tests were performed using 25 mm parallel plates with a 1 mm gap, an oscillatory strain of 12%, an angular frequency of 10 radians/s, and a temperature scanning area from 40 °C to 82 °C at 6 °C intervals.

2.3.3. LAS Test

The LAS test is a test method that combines frequency and amplitude sweeps to predict the fatigue life of short- or long-term-aged asphalt. According to the AASHO TP 101 specification, a measurement temperature of 25 °C, an 8 mm parallel plate with a 2 mm gap between the two plates, an initial strain of 0.1%, and a frequency of 0.1–30 Hz were used to determine the α-parameter and analyze the asphalt damage.

Amplitude scanning tests were also conducted. Oscillatory shear was performed at 25 °C with a strain frequency of 10 Hz. The sinusoidal dynamic loading amplitude increased linearly in the range of 0.1% to 30% during the test, lasting 5 min. The obtained α-parameters were analyzed using the continuum damage (VECD) mechanical model (Equation (1)) to predict the fatigue life of the composite-modified asphalt at strains of 2.5% and 5%, respectively.

$$N_f={A\left({\gamma }_{max}\right)}^{-B}$$

(1)

where:

A and B: VECD model parameters;

${\gamma }_{max}$: the maximum expected binder strain for a given pavement structure (percent);

$N_f$: fatigue failure life.

2.3.4. MSCR Test

The Multiple Creep Stress Recovery (MSCR) test is commonly used to evaluate the rutting resistance of asphalt. The asphalt samples used in the experiments were short-term-aged asphalt according to AASTO TP350, using 25 mm parallel plates, and the test temperature was that of the PG-grade high-temperature rating. Cyclic sequential experiments were conducted at stress levels of 0.1 kPa and 3.2 kPa, with 10 cycles run for each stress level. Each cycle consisted of two phases, the first being a 1 s shear creep phase and the second being a 9 s recovery cycle phase. The experimentally obtained unrecoverable creep elasticity (J_nr) indicates the contribution of the asphalt binder to resist permanent deformation; the smaller the value, the greater the resistance to deformation. The rate of elastic recovery (R) indicates the elastic capacity of the asphalt. The MSCR test procedure for this study was conducted at 64 °C.

2.3.5. FTIR Test

Functional groups in matrix asphalt, matrix-modified asphalt, and composite-modified asphalt were tested using an FTIR spectrometer to analyze the chemical interactions of modifiers and additives with each other and the asphalt. The wave number range was set from 400 cm⁻¹ to 4000 cm⁻¹, the number of scans was set to 32, and the frequency was set to 4 cm⁻¹ to observe specific wave peaks that could be used to identify functional groups.

2.3.6. FM Test

There is a strong correlation between the microstructure and macro performance of modified asphalt. The solubility and distribution of desulfurized rubber powder in asphalt can be clearly observed using fluorescence microscopy. The microscope has a magnification of 400×.

3. Results and Discussion

3.1. General Physical Characteristics

In this study, conventional physical property tests were conducted on the prepared additive-free asphalt and DR/EVACMA in 10 groups numbered from 0 to 9. The experimental results are shown in Figure 3.

Figure 3a–d show that the orthogonal experimental asphalt groups (1–9) and additive-free asphalt (0) have more than a 26% improvement in the softening point for the matrix asphalt, indicating that the composite-modified materials play active roles in improving the temperature sensitivity of asphalt. It is worth noting that, compared with additive-free asphalt, DR/EVACMA in the orthogonal experimental group has a lower softening point, rotational viscosity, and higher needle penetration, which are considered to be due to the softening effect brought by FEO to asphalt, and a lower viscosity reduces the difficulty of constructing asphalt. In Figure 3e, it can be seen that the modified asphalt containing additives has better low-temperature plasticity and the highest delay of 15.4 cm compared to the additive-free asphalt, showing that modifying asphalt with additives improves its low-temperature crack resistance. Asphalt is used in road construction and maintenance as a binder, and one of its most important properties is storage stability; good storage stability can give asphalt good use and reduce the performance degradation caused by segregation, as shown in Figure 3f. DR/EVACMA’s storage stability is different, but it is better than that of additive-free asphalt, with the smallest difference in the softening point being only 0.7 °C, which is in line with the technical specifications of the storage stability of road asphalt.

3.2. Range Analysis

The range analysis method (R method), also known as intuitive analysis, is commonly used in the analysis of orthogonal tests, and the content of the method is shown in Figure 4.

$K_{jm}$ represents the experimental data metrics corresponding to the level, m, of the element in column j, and $\overline{K_{jm}}$ is the mean value of $K_{jm}$. The level of excellence of factor j can be determined by the magnitude of $K_{jm}$, and the optimal combination of the experiment as a whole is determined based on the optimal level among the factors.

$R_j$ represents the extreme deviation of the element in column j. The formula is shown in (2).

$$R_{\mathrm{j}}=\left(M\mathrm{a}\mathrm{x}\right){\mathrm{K}}_{\mathrm{jm}}-\left(M\mathrm{i}\mathrm{n}\right){\mathrm{K}}_{\mathrm{jm}}$$

(2)

In this paper, the R method is used to analyze the softening point, elongation, needle penetration, 135 °C rotational viscosity, and storage stability in the conventional physical experimental data in the orthogonal experimental table to study the primary and secondary orders of the influences of four factors, namely, furfural extract oil, DCP, sulfur, and KH-550 (denoted as A, B, C, and D), on the performance of modified composite asphalt and to determine the optimal level and ratio. The range analysis is shown in

Table 7. Range analysis (softening point).

Term	Level	A (FEO)	B (DCP)	C (Sulfur)	D (KH-550)
K-value	1	179	175.7	173.9	175.6
	2	175.9	175.3	175.9	175.5
	3	172	175.9	177.1	175.8
K average	1	59.67	58.57	57.97	58.53
	2	58.63	58.43	58.63	58.5
	3	57.33	58.63	59.03	58.6
Optimum level		1	3	3	3
R		2.33	0.2	1.07	0.1

,

Table 8. Range analysis (ductility).

Term	Level	A (FEO)	B (DCP)	C (Sulfur)	D (KH-550)
K-value	1	34.30	40.60	37.30	38.90
	2	42.60	38.70	40.60	38.70
	3	41.50	39.10	40.50	40.80
K average	1	11.43	13.53	12.43	12.97
	2	14.20	12.90	13.53	12.90
	3	13.83	13.03	13.50	13.60
Optimum level		2	1	2	3
R		2.77	0.63	1.10	0.7

,

Table 9. Range analysis (needle penetration).

Term	Level	A (FEO)	B (DCP)	C (Sulfur)	D (KH-550)
K-value	1	205.2	210.5	212.6	210.4
	2	209.6	210.3	209.5	210.1
	3	216	210	208.7	210.3
K average	1	68.4	70.17	70.87	70.13
	2	69.87	70.1	69.83	70.03
	3	72	70	69.57	70.1
Optimum level		3	1	1	1
R		3.6	0.17	1.3	0.1

,

Table 10. Range analysis (135 °C rotational viscosity).

Term	Level	A (FEO)	B (DCP)	C (Sulfur)	D (KH-550)
K-value	1	10.35	9.24	9.15	9.27
	2	9.49	9.37	9.35	9.33
	3	8.51	9.74	9.85	9.75
K average	1	3.45	3.08	3.05	3.09
	2	3.16	3.12	3.12	3.11
	3	2.84	3.25	3.28	3.25
Optimum level		1	3	3	3
R		0.61	0.17	0.23	0.16

and

Table 11. Range analysis (storage stability).

Term	Level	A (FEO)	B (DCP)	C (Sulfur)	D (KH-550)
K-value	1	18.8	8.1	14.5	13.2
	2	9.2	11.9	10.2	12.2
	3	7.7	15.7	11	10.3
K average	1	6.27	2.7	4.83	4.4
	2	3.07	3.97	3.4	4.07
	3	2.57	5.23	3.67	3.43
Optimum level		1	3	1	1
R		3.7	2.53	1.43	0.97

.

An analysis of the data showed that the effects of the four additives on the softening point at 135 °C, penetration, and rotational viscosity of DR/EVACMA were in the order of A > C> B > D; the effects of elongation were in the order of A > C> D > B; and the effects of storage stability were in the order of A > B > C > D. The R value of the furfural extract oil, which was affected by A, was at least twice as much as that of the other three additives. This indicates that furfural extract is the most important factor affecting the physical properties of DR/EVACMA among the four additives.

Studies have shown that when there are not enough light components in the asphalt to completely dissolve the modifier, the force between the two is weakened. FEO, as an additive to asphalt, contains a large number of aromatic hydrocarbons that are structurally similar to asphalt and are not reactive with each other. Adding modifiers to asphalt can supplement the light component, which is the colloid to restore the equilibrium state. Avoiding modifier agglomeration and asphalt hardening leads to performance degradation and a viscosity increase [22].

Studies have shown that both sulfur and peroxide (DCP) can effectively co-crosslink rubber and polymers [23,24]. Sulfur is mainly formed through crosslinking reactions between chain segments and chemical reactions with rubber macromolecules located in the dominant position of the compound to form single or multiple sulfur bonds. On the other hand, DCPs are formed mainly through reactions between free radicals and double bonds, substituting one hydrogen atom in the upper layer to form chain segment free radicals that can react with each other [25]. This crosslinking of different reaction sites results in the formation of an interlocking network structure, which improves the physical properties of modified bitumen [26]. The crosslinking process of EVA is dominated by DCP, and sulfur is not directly involved in the vulcanization of EVA [26,27]. The tertiary carbon attached to the acetate group of EVA is preferentially captured by free radicals generated by the thermal decomposition of DCP, and then macromolecular free radicals are formed, which polymerize with the polymer chains and form a crosslinked network structure between the chains [28]. As shown in Figure 3, the contents of sulfur and DCP in DR/EVACMA are positively correlated with the high-temperature performance when the FEO content is certain, which may be related to the degree of crosslinking of DR/EVACMA by both sulfur and DCP. The formation of crosslinked networks results in a more compact molecular structure, which increases the intermolecular forces and reduces the fluidity of the asphalt, which, in turn, enhances the resistance to deformation. A proper crosslinking structure also improves the storage stability of the material [29,30,31]. Studies have shown that appropriate crosslinking can improve the mechanical properties of EVA, but when the degree of crosslinking is too large, the polymer molecular chain breaks, and the strength decreases [21].

KH-550, the ethoxy functional group, reacts with the surface-active groups of carbon black in rubber powder to form chemical bonds, which effectively improves the dispersion stability of carbon black in the asphalt matrix and its adhesion with rubber. At the same time, the formation of hydrogen bonds with the hydroxyl groups in the asphalt enhances the interfacial force between the rubber and the asphalt and improves the compatibility between the two [32,33]. Some studies have shown that silane coupling agents have a certain number of polar groups in addition to -Si-O- groups, which can enhance the polarity of EVA through chemical reactions, thereby increasing the bonding force of EVA to asphalt. However, it is worth noting that Wu et al. believe that silane coupling agents, such as KH-550 and KH-560, have strong reactivity with DCP [21], and they also consider that there is a competitive relationship between the coupling reaction and crosslinking reaction of the materials in asphalt, though the strength of the reaction is not yet clear. To avoid the above problems, this study adopts the method of adding KH-550 and DCP separately.

Based on the scope analysis, the roles of the four additives were synthesized. It is considered that the silane coupling agent plays more of an auxiliary role. FEO dissolves the modifier completely in asphalt, which is conducive to improving the compatibility; KH-550 binds the surface of the modifier with asphalt, and secondly, crosslinking occurs in the modifier that is uniformly dispersed in the asphalt by sulfur and DCP to form a perfect three-dimensional network structure. Combined with the physical property graph and polarity analysis, the optimal ratio of DR/EVACMA additives in this study is the fourth orthogonal test: 3% FEO, 0.15% DCP, 0.16% sulfur, and 0.25% KH-550.

3.3. Temperature Sweep

The asphalt rutting coefficient (G*/sinδ) is used in the SHRP specification to describe the rutting resistance of asphalt binders. In Figure 5a, the G* of the modified asphalt is much larger than that of the base asphalt, which indicates that the composite-modified asphalt with the addition of gum powder and EVA has an improved rigidity, resulting in better rutting resistance [7]. Meanwhile, the G* gradually decreases with an increasing temperature, indicating that the bearing capacity of asphalt is negatively correlated with temperature. However, it is worth noting that there are differences in the size of the G* of asphalt, which, intuitively, may be directly related to the content of FEO, with a higher G* for asphalt Samples 1, 2, and 3, which have the smallest amounts of FEO. One explanation is that FEO maintains the colloidal equilibrium of the asphalt, allowing the modifier to dissolve effectively and completely in the asphalt, but the significantly lower G* values for asphalt Samples 7, 8, and 9 are thought to be due to the excessive softening of the FEO, which leads to the softening of the asphalt and a large reduction in the resistance to load deformation.

The phase angle (δ) is the coefficient of proportionality between the viscous and elastic components of an asphalt material and decreases with an increasing temperature. The matrix asphalt itself is a viscoelastic material, and in Figure 5b, with the addition of DRP and EVA, the phase angle of asphalt decreases dramatically, which originates from the elastic effect brought by the modifiers to the asphalt. After the addition of additives, the phase angle has a small tendency to increase, considering that FEO is a viscous component, but FEO also promotes the modifier to dissolve in the asphalt, so that DCP and sulfur can effectively crosslink, forming a crosslinked network structure, and at the same time, KH-550 also ensures that there is effective dispersion of the modifier and the bonding between the modifier and the asphalt. However, when the temperature increases, the phase angle of DR/EVACMA with 3.4% FEO has the largest slope change, and it cannot maintain good elastic recovery ability, which may be due to the addition of too much FEO, leading to the over-saturation of colloidal equilibrium within the asphalt, and the viscous component is too much.

According to the SHRP specification, the G*/sinδ is derived from the G*, which represents the rutting resistance coefficient of asphalt binder. In Figure 6, the magnitude of the G*/sinδ for different binders is in the following order: matrix-modified asphalt (0) > orthogonal test group (1–9) > matrix asphalt. When the amount of composite modifier incorporated is too large, the light component content of asphalt is not enough to dissolve completely, the asphalt macromolecule content increases, and the viscosity increases. The addition of FEO will bring its colloid into equilibrium and produce a softening effect, which will reduce the rutting resistance of unfilled asphalt. However, it is worth noting that sulfur and diisopropylbenzene peroxide improve the rutting resistance of asphalt by inducing crosslinking reactions in the asphalt. Therefore, the combined effect of these two actions needs to be analyzed experimentally. It is worth noting that the higher the FEO dosage, the smaller the G*/sinδ, and this type of binder is more prone to rutting at high temperatures. At the same FEO level, the sulfur content is proportional to the G*/sinδ, which indicates that sulfur has a positive effect on the high temperature performance of rubber asphalt. With the increase in FEO, the difference in the G*/sinδ of the binder in the same group becomes larger, which may be due to the fact that the softening effect of FEO on the binder is larger than the crosslinking effect. Therefore, it is very important to choose the appropriate filler ratio to obtain asphalt with excellent rutting resistance.

3.4. Results of LAS Experimental Tests

Based on the continuous viscoelastic damage model (VECD), Figure 7 shows the fatigue life of different bitumen at strain levels of 2.5% and 5%. In general, the fatigue life decreases with increasing strain levels. It is noteworthy that the additive-free asphalt (0) has the smallest fatigue life, which may be attributed to the fact that EVA and DRP do not form a proper crosslinked network structure in the bitumen, resulting in insufficient stiffness and ductility and weak resistance to fatigue damage [17]. By adding FEO to asphalt and changing the ratio of the four components, the rubber powder and EVA in the asphalt completely absorbed, dissolved, and dispersed uniformly, avoiding the phenomenon of lumps and agglomerates. This effect also promotes DCP and sulfur in the asphalt in line with the modifier to ensure a good crosslinking effect, involving the formation of a crosslinked mesh structure, thus improving the fatigue resistance. A silane coupling agent, KH-550, increases the degree of adhesion between the modifier and asphalt through coupling and also improves the fatigue resistance of asphalt to a certain extent. Interestingly, the fatigue performance of FEO-enhanced binders is not absolute, as can be clearly seen in Figure 7, where the fatigue life of asphalt modified with 3.4% FEO is less than about half of the fatigue life of asphalt modified with 2.6% FEO. In conclusion, FEO is mainly an aromatic component in MA, and its excessive content will cause the excessive softening of asphalt, reducing the deformation resistance and negatively affecting the fatigue performance.

3.5. Results of Multi-Stress Creep Recovery Tests

In order to better evaluate the high-temperature performance of DR/EVACMA under traffic loading conditions, the unrecoverable creep elasticity (J_nr) and creep recovery (R) at creep stress levels of 0.1 kPa and 3.2 kPa were measured using MSCR simulation experiments as an evaluation index.

Figure 8a,b show that the J_nr0.1 value of the modified bitumen is about 40/1 times that of the matrix bitumen, and the J_nr3.2 value is about 10 times that of the matrix bitumen compared to the matrix bitumen. In a further step, the J_nr0.1, J_nr3.2, R_0.1, and R_3.2 values of the orthogonal test group were also reduced and increased to different degrees compared to the additive-free asphalt (0). It is shown that the conformed modified asphalt after filler has more significant high-temperature characteristics and elastic behavior.

Research has shown that using carbon black in rubber has a positive effect on the aging performance of asphalt under thermal and photo-oxidizing conditions, which improves asphalt’s high-temperature deformation resistance and elastic recovery [34]. When rubber is incorporated into asphalt, carbon black is released and free from asphalt, and KH-550 connects it to asphalt in the form of a “bridge” to increase adhesion and dispersion and prevent buildup. As the stress level increases, the R of the modified asphalt decreases and the J_nr increases, indicating that the internal structure of the asphalt is damaged to a certain extent under high stress. Unlike the no-filler group (0), J_nr and R are better in the orthotropic group. However, there is a difference in the variation of Jnr and R of asphalt. A certain pattern can also be seen in Figure 8a,b, where the difference in the variation of J_nr and R of asphalt is the largest when the DCP content is 0.35% (the value of Sample 3 does not fluctuate significantly). Combined with the storage stability charts of conventional physical properties, it can be seen that the most severe segregation occurs in Samples 3, 6, and 9. When the contents of sulfur and the lightweight component reach certain amounts, the high DCP content leads to the crosslinking of the EVA network, the structure is more compact, the original crosslinking moderate and flexible polymer chain are elongated, the EVA molecular structure is destroyed, and the molecular chain has different degrees of fracture and cannot effectively provide better elasticity and recovery performance; at the same time, due to the DCP crosslinking of the C-C bond, the sulfur crosslinking of the flexible system produced by the C-S and S-S bond is shorter and more rigid. Therefore, proper DCP and sulfur crosslinking can provide good deformation resistance and elastic recovery for modified bitumen [26].

It is worth noting that the J_nr and R of the composite-modified asphalt with different additive ratios may also be related to the amount of FEO added, and in the figure, there is a tendency for J_nr to increase and for R to decrease as the amount of added FEO increases. This is because the higher the content of lightweight components in asphalt, the more obvious the lubrication between rubber molecules. Under the action of external forces, it may cause greater plastic deformation, reduce the deformation resistance, and reduce the elastic recovery.

3.6. Results of FTIR Spectroscopy Tests

Figure 9 shows the infrared spectra of three different asphalts (matrix asphalt, additive-free asphalt (0), and DR/EVACMA (4)) in the wavelength range of 400 cm⁻¹ to 4000 cm⁻¹.

Firstly, the two typical peaks at 2850 cm⁻¹ and 2920 cm⁻¹ are caused by saturated hydrocarbon (C-H) and its derivative vibrations of bitumen. In contrast to the matrix asphalt, the infrared wave peaks at 1250 cm⁻¹ and 1730 cm⁻¹ for both modified asphalts, caused by C-H and C=O vibrations, respectively, proved that EVA chemically reacted when blended with asphalt [7]. Meanwhile, the IR peaks at 1591 cm⁻¹ and 1450 cm⁻¹ were similar for the three binders, indicating that the desulfurized rubber powder was mainly physically dissolved with asphalt. However, some studies have shown some chemical reactions between the rubber powder and asphalt at 1030 cm⁻¹, as the strength of the sulfoxide group (S=O) increases with the increase in rubber powder, which is manifested as a decrease in transmittance, which is reflected in the figure [35]. Most notably, there is a difference in the wave peaks at 720 cm⁻¹ between the three binders, which is due to C-S bond oscillations. It can be concluded from the study that the addition of modifiers increases the relative concentration of C-S in the binder, and with the addition of sulfur, monosulfur or polysulfur bonds are formed, which confirms the participation of sulfur in the chemical reaction of asphalt [36]. It is interesting to note that the IR light absorption of the hydroxyl group (-OH) at 3350 cm⁻¹ is significantly lower than that of the unfilled binder, which may be attributed to the chemical reaction between the added silane coupling agent KH-550 and the hydroxyl group (-OH) in DR/EVACMA [37]. Meanwhile, FEO does not react chemically with bitumen because of its similar composition. The positions of the major groups in the FTIR spectrum are shown in

Table 12. Locations of major groups in FTIR spectra.

Wavenumber (cm−1)	Functional Group (Chemistry)	Vibration Pattern
3350 cm−1	-OH	stretching
2850 cm−1, 2920 cm−1	C-H	stretching
1730 cm−1	C=O	stretching
1450 cm−1	C-H of-(CH2)n-	bending
1030 cm−1	S=O	stretching
720 cm−1	C-S or C-H	stretching

.

3.7. Results of FM

Fluorescence microscopy allows for the observation of the phase distribution between the modifier and the bitumen. Since the phase distribution of the matrix asphalt could not be seen under the fluorescence microscope, Figure 10a,b only show the microstructure of Sample 0 (20% DRP + 4% EVA) and Sample 4 (20% DRP + 4% EVA + 3% FEO + 0.15% DCP + 0.16% sulfur + 0.25% KH-550). As shown in the figure, the shape and distribution of the composite modifier as a dispersed phase in the additive-free asphalt was disordered under fluorescence microscopy. On the contrary, in Sample 4, the modifier formed a homogeneous continuous phase with the bitumen and a crosslinked network structure. This reveals the reason for the excellent storage stability of DR/EVACMA and confirms the ability of the addition of furfural extract oil, DCP, sulfur, and KH-550 to improve the storage stability of the modified bitumen.

4. Conclusions

• In this paper, the effects of the addition of different ratios of FEO, DCP, sulfur, and KH-550 on the conventional physical properties, medium-high rheological properties, storage stability, and fatigue resistance of DR/EVACMA were investigated. A microanalysis was also carried out through infrared spectroscopy experiments and fluorescence tests, and the following conclusions were drawn:
• The conventional physical properties of the prepared DR/EVACMA were analyzed using an orthogonal test and polar analysis of variance, and it was determined that DR/EVACMA containing 3% FEO, 0.15% DCP, 0.16% sulfur, and 0.25% KH-550 had excellent storage stability, and the difference in the softening point was only 0.7 ℃. Compared to additive-free asphalt (0), the softening point difference decreased by 10.6 °C, the viscosity decreased by 0.97 Pa-S, and the needle penetration increased by 2.14 mm, while the low-temperature plasticity was improved, and the elongation increased by 7.3 cm. kh-550 improved the adhesion of the modifier to the asphalt. The crosslinking of DCP and sulfur improved the temperature sensitivity and mechanical properties. The addition of FEO improved the compatibility of the modifier with the asphalt and the low-temperature plasticity and slightly lowered the softening point. The performance of DR/EVACMA depended on the combined effect of the four additives.
• The TS experiment showed that with the increase in FEO and the temperature, the G*/sinδ gradually decreased, and the rutting resistance was weakened. In the MSCR experiment, the composite-modified asphalt had a lower Jnr value and a higher R value, showing good high-temperature rutting resistance and elastic recovery. However, the excessive crosslinking of EVA by DCP and the softening effect of FEO weakened this phenomenon. In the LAS experiment, the fatigue resistance of the asphalt binder was proportional to the degree of crosslinking of the modifier, and the fatigue life correlated well with the elasticity. In the FTIR experiment, the interaction between rubber powder and asphalt was mainly physical solubilization, and KH-550 could connect the modifier and asphalt, play the role of molecular bridge, and increase the dispersion of the modifier in asphalt to improve the adhesion between the two.
• Fluorescence experiments showed that under the premise that FEO provides lightweight components to ensure the complete dissolution of the modifier, the dispersing and connecting effect of KH-550 makes the modifier uniformly dispersed in asphalt, and the crosslinking of sulfur and DCP makes the rubber powder and EVA form a crosslinking network structure so as to improve the mechanical properties and storage stability of asphalt.
• Rubber/polymer-composite-modified asphalt can be used to obtain higher performance and storage stability by crosslinking the two together to form an interactive network structure.