Research on Fatigue Performance of Fast-Melting SBS/Epoxy Resin Composite-Modified Asphalt

Abstract

A high-performance composite modifier ER-SBS-T was prepared through the mixing of epoxy resin with a fast-melting styrene–butadiene–styrene modifier (SBS-T) to enhance material properties. This study evaluated the fatigue characteristics of ER-SBS-T-modified asphalt using a linear amplitude sweep test, and 70# base asphalt, SBS-modified asphalt, and SBS-T-modified asphalt were used as control groups. The fatigue life of ER-SBS-T-modified asphalt was determined according to viscoelastic continuous damage (VECD) theory. The findings indicated that as the ER-SBS-T modifier content increased, both the maximum value and the cumulative damage limit value increased, while the rate of change of decreased. At strain levels of 2.5% and 1%, the fatigue life of the modified asphalt improved with increasing ER-SBS-T content. However, at a strain level of 5%, the modified asphalt exhibited no significant trend in fatigue life across different ER-SBS-T contents. ER-SBS-T-modified asphalt with a modifier content of >10% demonstrated favorable fatigue performance suitable for practical engineering applications.

1. Introduction

Asphalt is widely used as a pavement material owing to its excellent mechanical properties [1]. To address the increasing demand for roads driven by increasing traffic, numerous academics have studied modified asphalt. Currently, SBS and epoxy resins are commonly utilized as thermoplastic and thermosetting modifiers in asphalt modification [2].

Considering that the ordinary SBS modifier has a high melting point and is not easy to dissolve, the existing technology involves mixing, swelling, grinding, and crosslinking the SBS modifier with matrix asphalt to create SBS-modified asphalt. This modified asphalt is then transported to a mixing station for storage [3]. However, the performance of SBS-modified asphalt deteriorates during storage and transit owing to segregation and thermal breakdown [4,5]. Maintaining high temperatures for these processes also stresses ecosystems, as a significant amount of energy is consumed and large amounts of greenhouse gases are emitted [6,7,8]. To address these issues, Guolu Gaoke designed a fast-melting SBS modifier (SBS-T). This innovation improves the melting capacity of the SBS modifier and incorporates additional components to balance the melt index with the modification impact. The melt index of SBS-T is 100 times that of SBS. SBS-T melts in just one minute when added to the mixture and is ready for immediate use. The SBS-T modifier is fed into the mixing plant, where it can be directly combined with the aggregate to create a modified asphalt mixture. By integrating SBS-modified asphalt processing with the asphalt concrete mixing production process, the fast-melting SBS-modified technology offers substantial economic and social benefits. It reduces the need for repeated transportation of asphalt and enhances the durability of the asphalt surface without increasing costs [9]. Consequently, there are significant opportunities for the development of fast-melting SBS-modified asphalt technology. Its advancement provides technical support for the construction of industrial green roads [10,11,12,13].

The thermosetting polymer epoxy resin produces epoxy asphalt with exceptional strength and flexibility, resistance to aging and chemical degradation, and strong temperature resistance [14,15,16,17,18]. However, epoxy asphalt tends to be brittle at low temperatures, and temperature fluctuations can cause fractures to appear in the cured epoxy asphalt. Therefore, enhancing the low-temperature brittleness of epoxy asphalt is crucial. To address this, the fast-melting SBS/epoxy resin composite modifier (ER-SBS-T) was developed. This innovation incorporates reactive tackifying resin with a novel one-component epoxy, building upon the modification technology of fast-melting SBS (SBS-T). While maintaining the resistance to water damage and low-temperature performance of SBS-modified asphalt mixes, the ER-SBS-T-modified asphalt mixture exhibits improved resistance to rutting and load damage at high temperatures. Consequently, ER-SBS-T-modified asphalt has a longer lifespan, is less prone to damage, and is more conducive to the long-term sustainable use of roads. It is particularly suitable for intersections, long longitudinal slopes, and heavy-duty lanes.

One of the main distresses affecting asphalt pavements is fatigue cracking, which negatively impacts their longevity and serviceability. Therefore, accurately characterizing the fatigue performance of asphalt materials is crucial for monitoring the service condition of asphalt pavements. The viscoelastic, rheological, cohesive, and adhesive properties of asphalt mixes are mainly determined by their asphalt binder, which also affects the mixtures’ fatigue life. At various material scales, asphalt binders and mixtures with superior resistance to fatigue cracking can significantly enhance the fatigue life and longevity of asphalt pavements. Many studies have noted that the fatigue parameter $(\left|G^{*}\right|\sin \delta ),$ which represents the linear viscoelastic properties of asphalt, is poorly correlated with the fatigue properties of asphalt mixtures, especially for modified asphalt and mixes. This indicates that the parameter lacks scientific rigor [19,20,21]. To measure and assess the fatigue cracking behavior of asphalt binders, Bahia et al. and Johnson [22,23] proposed the time sweep (TS) test and the linear amplitude sweep (LAS) test. In the TS test, specimens are subjected to repeated cyclic loading, while in the LAS test, samples undergo increasing oscillatory strain amplitude loading. The data from these tests can be analyzed under the theoretical framework of viscoelastic continuum damage (VECD).

In this study, the ER-SBS-T modifier was developed through the combination of SBS-T with reactive tackifying resin and one-component epoxy resin. The fatigue properties of ER-SBS-T-modified asphalt were systematically studied using the LAS test. This research provides a theoretical foundation for enhancing pavement fatigue performance through the application of ER-SBS-T composite modification technology.

2. Theoretical Background

A constitutive model that accounts for both the viscoelastic and continuous damage effects of materials is called the VECD theory. VECD theory can be used to predict the fatigue and damage behavior of engineering materials under dynamic or static loads, as well as to forecast the life and reliability of materials and structures. Schapery originally introduced continuum damage theory in 1975 and extended its application to viscoelastic materials by studying the evolution of damage in elastic materials [24,25]. Kim et al. subsequently employed this theory [26,27] to describe viscoelastic damage and mechanical response in asphalt concrete materials. In the VECD theory framework, the material is conceptualized as an elastic–viscoelastic–continuous damage continuum.

According to VECD theory, the material damage process consists of two stages: isolated damage and continuous damage. Isolated damage refers to the growth and collapse of a single microscopic crack, while continuous damage involves the interaction and growth of multiple microscopic fractures. VECD theory introduces various factors, such as damage variables, to quantify the effects of damage and characterize its behavior in materials. As material damage evolves intrinsically, a relationship between material modulus and damage evolution can be established using experimental data. Schapery explored the concept of failure and extended it to viscoelastic materials by integrating the concept of thermodynamic irreversibility. The work (W) done under a simple uniaxial stress can be expressed using the strain energy density. The strain energy density function is given by:

$$W=W({\epsilon }_{\mathrm{ij}},E)$$

(1)

The strain energy density of asphalt mixture based on VECD theory is expressed as follows:

$$W=\frac{1}{2}E\times {\epsilon }^2$$

(2)

where E represents the modulus (MPa) that denotes the uniaxial strain, and W stands for the strain energy density.

Regarding the damage rate in Schapery’s theory, the expression is as follows:

$$\frac{dS}{dt}={(-\frac{\partial W}{\partial S})}^{\alpha }$$

(3)

where S represents the internal state variables of the material, t denotes the test time, and W stands for the strain energy density. α represents the damage evolution rate of the material, which is expressed as 1 + 1/m in this paper. M refers to the index of linear viscoelastic creep compliance, determined by fitting the frequency sweep master curve.

The damage characteristics of the material are described by the modulus E, which correlates with the extent of damage. Equation (2) can be substituted into Equation (3) to compute damage accumulation. The damage parameter D can be quantified using work potential theory. Damage accumulation is determined from experimental data, as shown in Equation (4).

$$\frac{dD}{dt}={(-\frac{{\epsilon }^2}{2}\times \frac{dE}{dt})}^{\frac{\alpha }{1+\alpha }}$$

(4)

When the VECD model is applied to asphalt, the cumulative dissipated energy under strain control mode is represented by Equation (5), replacing Equation (2) in the analysis of the VECD model.

$$W=\pi \cdot I_D\cdot {{\gamma }_0}^2\cdot \left|G^{*}\right|\cdot \sin \delta$$

(5)

where I_D represents the undamaged G* value (MPa), γ₀ denotes the strain amplitude (%), G* stands for the complex shear modulus, and δ is the phase angle (°).

Equation (6) is derived by rearranging the above equation and calculating the integral:

$$D(t)\cong {\displaystyle {\sum }_{i=1}^N[\pi }{I}_D{{\gamma }_0}^2(\left|G^{*}\right|\sin {\delta }_{i-1}-\left|G^{*}\right|\sin {\delta }_i){]}^{\frac{\alpha }{1+\alpha }}{(t_i-t_{i-1})}^{\frac{1}{1+\alpha }}$$

(6)

The relationship between material properties and the degree of damage was established through model fitting. This allows for the establishment of a relationship between the fatigue life of asphalt and the amplitude of the applied load. For asphalt, $\left|G^{*}\right|\sin \delta$ serves as the material parameter. The fitted model is presented in Equation (7).

$$\left|G^{*}\right|\sin {\delta }_i=C_0-C_1{(D)}^{C_2}$$

(7)

where C₀, C₁, and C₂ represent model coefficients.

Substituting Equation (7) into Equation (5), the following can be obtained:

$$\frac{dW}{dD}=-\pi I_D{C}_1{C}_2{(D)}^{C_2-1}{({\gamma }_{\max })}^2$$

(8)

The expression for the number of asphalt failure cycles, obtained through the combination of the above formulas, is as follows:

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

(9)

$$A=\frac{f{(D_f)}^k}{k{(\pi I_D{C}_1{C}_2)}^{\alpha }}$$

(10)

where γ_max represents the estimated maximum strain value of asphalt (%), and f denotes the loading frequency (Hz).

Relevant research has shown that when $\left|G^{*}\right|\sin \delta$ is reduced by 35%, the life predicted by the LAS test has the best correlation with the fatigue life measured by traditional fatigue test [28]. Therefore, in this study, A = 35% is adopted as the fatigue failure criterion. The expression for D_f is as follows:

$$D_f=0.35{(\frac{C_0}{C_1})}^{\frac{1}{c_2}}$$

(11)

Because of its superior viscoelastic qualities, asphalt is a frequently utilized road material. However, during road use, asphalt may suffer fatigue damage after being subjected to vehicle load cycling and climate change. In real-world applications, the traffic load cycles suffered by asphalt on road surfaces generally occur at medium temperatures. Therefore, fatigue performance at medium temperatures is another important factor for road use. Studies have shown that there is a correlation between the viscoelastic properties of asphalt and the fatigue performance. If the asphalt has better viscoelastic properties, i.e., a higher modulus of elasticity and viscosity, then its deformation capacity will be relatively low when subjected to repetitive loads, thus reducing the level of fatigue damage. Therefore, this paper evaluates the fatigue performance of ER-SBS-T by performing a linear amplitude scanning test (LAS) with a dynamic shear rheometer and calculating the fatigue life of the asphalt through the VECD theory.

3. Materials and Methods

3.1. Materials

In this paper, asphalt cement with penetration grade 60/80 was used as the base asphalt binder. According to the test method in JTG E20-2011 “Technical Specification for Highway Asphalt Pavement Construction”, the basic technical properties of the base asphalt in

Table 1. Physical performance of base asphalt binder.

Technical Parameter	Values	Specification Value	Test Methods
Softening point/°C	49.5	≥46	T0606-2011
Penetration (25 °C)/0.1 mm	68.5	60–80	T0604-2011
Penetration index PI	0.4	−1.5–1	T0604-2011
Ductility (15 °C)/cm	>100	>100	T0605-2011
60 °C Dynamic viscosity/(Pa·s)	190	≥180	T0620-2011

and ER-SBS-T in

Table 2. Conventional technical indexes for modified asphalt binders.

Modifiers	Proportion	Penetration (0.1 mm)	Softening Point (°C)
SBS	6%	46.9	81.6
SBS-T	6%	58.2	89.5
ER-SBS-T	4%	49.4	58.9
ER-SBS-T	6%	46.7	60.5
ER-SBS-T	8%	44.0	62.5
ER-SBS-T	10%	42.8	73.1
ER-SBS-T	12%	42.4	88.7

were obtained. The properties of the three modifiers in

Table 3. Properties of the three types of modifiers.

Modifiers	Technical Parameter	Unit	Measured Value
SBS	Block ratio	-	28/72
	Tensile strength	MPa	34.1
	Elongation	%	761
	Oil content	%	0.7
	Melt index	g/10 min	0.82
SBS-T	Individual particle Mass	g	0.21
	Ash content	%	0.48
	Melt index	g/10 min	2.39
	Dry dispersibility	-	No particle residue
ER-SBS-T	Individual particle Mass	g	<0.001
	Ash content	%	1.4
	Melt index	g/10 min	13.8
	Density	g/cm3	0.975

were obtained according to T/CHTS 20003-2018 “Technical Guideline for Construction of Direct-to-Plant SBS Modified Bituminous Pavement”. Optimization of SBS structure and pre-melting were adopted to achieve a balance between modification and fast melting. Fine pre-grinding of SBS was carried out in a dry state and vulcanization accelerators were introduced to achieve ultra-rapid cross-linking between SBS and asphalt in the mixing process. Based on this, a new generation of SBS modifier, called SBS-T, was successfully developed that can be directly put into the mixing building. The fast-melting SBS and organic polymer compounds containing a single epoxy group were reacted by cross-linking with many types of environmentally friendly curing agents under high-temperature excitation. This resulted in the formation of insoluble, non-melting polymers with a three-way network structure, called ER-SBS-T. The ER-SBS-T composite modifier, SBS-T modifier, and SBS modifier were supplied by Guolu Gaoke Engineering Technology Institute Co., Ltd. (No. 30 College Rd., Haidian District, Beijing, China). These modifiers are illustrated in Figure 1, and modified asphalt binders and the technical indexes of the three modifiers are provided in Table 2 and Table 3.

3.2. Sample Preparation

The asphalt was heated in an oven at a constant temperature of 135 °C. The performance of 6% SBS- and SBS-T-modified asphalt was good. Therefore, 6% of SBS modifier, 6% of SBS-T modifier, and varying amounts (4%–12%) of ER-SBS-T composite modifier were added to a specific amount of base asphalt. The three types of modified asphalt were then sheared using a FLUKO-FM300 asphalt shear emulsification machine (FLUKO Shanghai Equipment Co., Ltd., Shanghai, China) (manufacturer, city, country) at a rotational speed of 4500 ± 100 rpm, as shown in Figure 2. The asphalt temperature was maintained at 160 ± 10 °C during shearing, which lasted for 30 min. Finally, the prepared modified asphalt was placed in a 160 °C oven for 60 min.

3.3. Aging Methods

Asphalt pavements undergo aging owing to factors such as oxidation, ultraviolet light, and temperature changes during use, which degrade their performance and longevity. This study simulates the complex aging behavior of asphalt pavements. The asphalt samples were subjected to short-term aging and a pressurized aging vessel (PAV) process, as shown in Figure 3 and Figure 4. For short-term aging simulation, the rotating film oven test was employed to mimic asphalt aging during transportation, storage, and construction phases. Asphalt samples weighing 35 ± 0.5 g each were poured into specialized short-term aging bottles and cooled to room temperature. These samples were then placed into a rotating film oven where the test temperature was set at 163 ± 0.5 °C in short-term aging, and the aging duration was set at 85 min. The PAV was mainly used to simulate the aging of asphalt pavement under external environmental influences. Short-term aging asphalt samples weighing 50 ± 0.5 g were poured into aging trays, which were then placed inside the PAV aging chamber. The test temperature was set at 100 °C in PAV aging, and the internal pressure was maintained at 2.1 ± 0.1 MPa. The aging process lasted for 20 h.

3.4. Linear Amplitude Sweep (LAS) Test

The LAS test induces fatigue damage in asphalt samples by applying a linearly increasing load amplitude. The fatigue life of asphalt was predicted using VECD theory. This test is straightforward and rapid and allows precise control over testing duration, enabling measurement of fatigue damage characteristics in asphalt materials. The test process is carried out through a dynamic shear rheometer, as shown in Figure 5. The test procedure is as follows: An LAS test was conducted in controlled strain mode at a temperature of 25 °C and a frequency of 10 Hz. The strain gradient started at 0.1% and increased linearly to 30%, with increments of 1% per step. Each gradient comprised 100 loading cycles, and material parameters α were measured every 10 cycles. The results of the LAS test were calculated based on VECD theory.

4. Results and Discussion

4.1. Dynamic Modulus (G*) Master Curve

Thixotropy, strain hardening, and nonlinear viscoelastic effects can all influence the fatigue damage behavior of asphalt during fatigue testing as external loads gradually increase. Owing to the complexity of these processes, providing a suitable explanation and application within the current theoretical framework and experimental approach is challenging. The framework of linear VECD theory is employed to assess the fatigue life of asphalt binders. The fatigue life of asphalt binders and mixtures can be effectively predicted using the linear VECD theory. The complex shear modulus G* describes the resistance of modified asphalt to deformation: Higher values indicate greater stiffness and stronger resistance to deformation in the modified asphalt. ER-SBS-T-modified asphalt samples with different ER-SBS-T dosages are compared and analyzed (Figure 6). Among these, the 12% ER-SBS-T-modified asphalt exhibits the best performance; thus, it was compared with 6% SBS and 6% SBS-T, as illustrated in Figure 7. Figure 6 and Figure 7 reveal the following:

(1)

A comparison of ER-SBS-T-modified asphalts at different concentrations reveals that the complex modulus of asphalt increases with higher ER-SBS-T content, particularly in the low-frequency (high-temperature) range. This suggests that the ER-SBS-T modifier mainly enhances the deformation resistance of modified asphalt at high temperatures. The reason is that when the ER-SBS-T modifier dosage is high, the modified asphalt colloid can be fully formed within the crosslinked three-dimensional mesh structure, and the asphalt phase can be uniformly dispersed and has better compatibility with the epoxy phase.

(2)

Among the different types of modified asphalt, ER-SBS-T-modified asphalt exhibits the highest complex modulus G* (12%) at the same frequency. This demonstrates that 12% ER-SBS-T-modified asphalt exhibits excellent resistance to high-temperature rutting and low-temperature conditions. At the same frequency, the stiffness of SBS-T-modified asphalt is lower than that of SBS-modified asphalt, indicating that SBS-T-modified asphalt is more effective in resisting elastic deformation.

(3)

Aging conditions increase the complex modulus of modified asphalt. In particular, 70# matrix asphalt subjected to PAV aging exhibits a higher complex modulus than SBS- and SBS-T-modified asphalt. However, this does not necessarily indicate better performance; rather, the asphalt matrix after PAV aging exhibits higher stiffness and hardness, making it prone to brittle cracking under heavy traffic loads. Moreover, increased aging levels reduce the slope of complex modulus growth, indicating that aging reduces performance discrepancies among asphalts at various frequencies and reduces asphalt temperature sensitivity.

4.2. Fatigue Damage Curve Analysis

According to the VECD theoretical formula, the fatigue factors of 70# matrix asphalt, 6% SBS-modified asphalt, 6% SBS-T-modified asphalt, and ER-SBS-T-modified asphalt with varying contents under original, short-term aging, and PAV aging conditions are obtained. Asphalt samples with different ER-SBS-T dosages are compared and analyzed, and the results are illustrated in Figure 8. Among the asphalt samples, 12% ER-SBS-T-modified asphalt exhibits the best performance, thus it was compared with 6% SBS and 6% SBS-T, as illustrated in Figure 9.

Both the maximum value and cumulative damage limit of asphalts aged via different methods increase as aging progresses. Asphalt subjected to PAV aging exhibits a considerably greater rate of change in properties than asphalt subjected to short-term aging. This difference arises from distinct mechanisms influencing the internal state and damage evolution of modified asphalt between PAV aging and short-term aging. A comparison of ER-SBS-T-modified asphalt samples with different modifier dosages reveals that both the maximum and cumulative damage limits increase with higher modifier dosages. Furthermore, as the modifier dosage increases, the rate of change decreases.

As the cumulative fatigue damage D(t) increases, the rate of degradation of modified asphalt under various conditions initially accelerates, then slows down, and eventually stabilizes. It is generally accepted that the higher the fatigue cumulative damage threshold value of a material, the higher the damage required to achieve damage, which means that the better the fatigue resistance is. Generally, a higher damage threshold value indicates greater durability of the material. A comparison of different types of asphalt under original and PAV aging conditions reveals that the sequence of cumulative damage limit values is as follows: 12% ER-SBS-T > SBS > SBS-T > 70#. Under short-term aging conditions, the sequence of cumulative damage limit values is as follows: 12% ER-SBS-T > SBS > 70# > SBS-T. The reason is that at a dosage of 12%, the epoxy resin material in the modifier and the asphalt fully undergoes a curing reaction, which significantly improves the hardness and resistance to deformation of the modified asphalt.

4.3. Fatigue Impedance and Load Sensitivity Analysis

The VECD model fitting parameters for each LAS test of asphalt are presented in

Table 4. Fitting parameters of modified asphalt VECD model.

Aging Method	Asphalt	C0	C1	C2	k	A35	B
Original	70#	3.90	0.02	0.60	1.91	88250014	4.56
	6%SBS	4.07	0.06	0.49	2.32	425376188	5.14
	6%SBS-T	4.81	0.06	0.52	2.22	410966399	5.07
	4%ER-SBS-T	3.86	0.03	0.59	1.95	108671236	4.62
	6%ER-SBS-T	4.87	0.13	0.42	2.43	168203866	4.93
	8%ER-SBS-T	4.27	0.03	0.56	2.10	416655582	5.04
	10%ER-SBS-T	5.35	0.03	0.58	2.11	781299616	5.26
	12%ER-SBS-T	6.44	0.02	0.60	2.12	922309552	5.54
Short-term aging	70#	4.46	0.01	0.69	1.67	365063155	4.31
	6%SBS	6.13	0.00	0.88	1.32	632592277	5.50
	6%SBS-T	6.01	0.03	0.61	1.97	553554632	4.93
	4%ER-SBS-T	5.35	0.03	0.58	2.17	299488398	5.64
	6%ER-SBS-T	6.66	0.04	0.56	2.17	257128932	5.30
	8%ER-SBS-T	7.72	0.09	0.47	2.57	616602716	5.96
	10%ER-SBS-T	8.24	0.06	0.52	2.49	969035542	6.26
	12%ER-SBS-T	9.08	0.08	0.49	2.69	1299758232	6.66
PAV aging	70#	9.76	0.07	0.53	2.45	554399740	6.17
	6%SBS	7.70	0.05	0.52	2.30	717501751	5.44
	6%SBS-T	6.80	0.03	0.61	2.06	643359500	5.49
	4%ER-SBS-T	9.39	0.09	0.51	2.40	366690829	5.73
	6%ER-SBS-T	10.37	0.07	0.52	2.53	455231460	6.34
	8%ER-SBS-T	8.53	0.06	0.52	2.51	923856674	6.34
	10%ER-SBS-T	9.22	0.04	0.56	2.47	1548689825	6.62
	12%ER-SBS-T	13.26	0.21	0.41	3.02	1806322991	6.90

. Parameter A₃₅ represents fatigue impedance, which indicates the asphalt’s ability to maintain internal structural integrity under damaging conditions. A higher A₃₅ value signifies better fatigue resistance of the material. Parameter B reflects load sensitivity, indicating how the material responds to varying load levels. A smaller absolute value of B suggests lower damage rates of asphalt under different strain levels. To more directly illustrate the fatigue resistance and load sensitivity of asphalt, A₃₅ and B are illustrated in Figure 10.

As illustrated in Figure 10, the parameters A₃₅ and B of 4% ER-SBS-T-modified asphalt are roughly the same as 70# asphalt, indicating that a low-content ER-SBS-T modifier does not markedly enhance the fatigue performance of asphalt. However, as the content of the ER-SBS-T modifier increases, the parameters A₃₅ and B also increase, demonstrating that the ER-SBS-T modifier can improve both the fatigue impedance and load sensitivity of asphalt. Short-term aging increases parameters A₃₅ and B for each type of asphalt. Compared with short-term aging, PAV aging can further improve the parameters A₃₅ and B of asphalt. The increase in the A₃₅ and B parameter values of the modified binder after the aging process indicates that short-term aging and PAV can not only effectively increase the fatigue impedance of asphalt but also increase the load sensitivity of asphalt. This may be because as asphalt ages, the structure of the asphalt material changes. The large molecular chains in the asphalt begin to crack and the short-chain molecules gradually increase, causing the asphalt to become more flexible at low temperatures. These short-chain molecules also fill the cracks and pores within the asphalt, improving parameters A₃₅ and B. The load sensitivity of 70# asphalt significantly increases after PAV aging, indicating that the damage rate of matrix asphalt under different strain levels is notably high following pressure aging.

4.4. Fatigue Life Analysis

In addition to parameters A35 and B, which compare the fatigue resistance of SBS-modified asphalt and SBS-T-modified asphalt, the number of fatigue damage cycles (Nf) of asphalt at different strains can also be used to evaluate the fatigue performance of asphalt. Reference studies have found that the predicted maximum strain γ_max is 2.5% and 5%, corresponding to the actual strain levels in thick and thin layers of asphalt pavements, respectively [13]. Therefore, fatigue life is computed at maximum strains of 1%, 2.5%, and 5%, respectively. A γ_max value of 2.5% simulates the actual strain levels experienced by asphalt pavement. When the maximum strain γ_max is 5%, the N_f value of asphalt shows a strong correlation with the fatigue life of the mixture in this study. The fatigue lives of asphalt samples under these three strain levels are calculated separately. ER-SBS-T-modified asphalt samples with different modifier dosages are compared, and the results are illustrated in Figure 11. Among the samples, 12% ER-SBS-T-modified asphalt exhibits the best performance; thus, it is compared with 6% SBS and 6% SBS-T (Figure 12).

Under low-strain conditions with a peak strain of 1% and the same aging degree, 70# asphalt exhibits the lowest fatigue life. This indicates that the addition of modifiers can enhance the fatigue resistance of asphalt at low strain levels. The fatigue life of SBS-T-modified asphalt is slightly lower than that of SBS-modified asphalt. As the ER-SBS-T modifier content increases, the fatigue resistance of ER-SBS-T-modified asphalt improves and the fatigue life of 6% ER-SBS-T-modified asphalt approaches that of SBS-T-modified asphalt. At ER-SBS-T dosages exceeding 8%, the fatigue resistance of ER-SBS-T-modified asphalt shows significant improvement. This enhancement is due to the activation of the modification mechanism by the epoxy resin component in ER-SBS-T. As the aging degree deepens, the fatigue life of all asphalt types gradually increases. The reason why aging contributes to the fatigue life of asphalt is due to the fact that aging volatilizes or converts the lighter components of the asphalt to the heavier components, resulting in an increase in the heavier components. This increases the modulus of the asphalt and improves its fatigue impedance. Thus, the asphalt after the aging process can withstand multiple repetitive loads at small strains. When the strain increases, the damage rate of asphalt is accelerated, and the effect of aging on the fatigue life of asphalt is more complex and needs to be further studied.

At a peak strain of 2.5%, the fatigue life of the original asphalt samples follows the order 70# < SBS < SBS-T < 12% ER-SBS-T. Short-term-aged samples exhibit the following order: 70# < 12% ER-SBS-T < SBS < SBS-T. PAV-aged samples exhibit the following order: 70# < 12% ER-SBS-T < SBS-T < SBS. The fatigue life of different types of asphalt varies with increasing aging degree. The fatigue life of 70# and SBS-modified asphalt continues to increase with deeper aging. Short-term-aged SBS-T-modified asphalt exhibits a significantly higher fatigue life than the original asphalt and a notably higher fatigue life than short-term-aged SBS-modified asphalt. This suggests that SBS-T-modified asphalt exhibits strong anti-fatigue properties when used in asphalt pavement. However, after PAV aging, the fatigue life of SBS-T-modified asphalt decreases noticeably, indicating that the anti-fatigue performance of the asphalt deteriorates under prolonged road load conditions. Although 12% ER-SBS-T-modified asphalt demonstrates good fatigue life initially, the fatigue life decreases significantly with increasing aging degree. The reason is that the aging destroys the structure of ER-SBS-T-modified asphalt, which reduces its cohesion under large strain and easily produces cracks. A comparison of ER-SBS-T-modified asphalt samples with different modifier dosages reveals that the fatigue life consistently increases with higher dosages under the same aging condition. However, the growth trend in fatigue life slows down at ER-SBS-T modifier dosages exceeding 10%.

Under high strain with a peak strain of 5%, the fatigue life order of the original asphalt is 70# < SBS < SBS-T < 12% ER-SBS-T. The order for short-term-aged samples is 70# < 12% ER-SBS-T < SBS < SBS-T, while the order for PAV-aged samples is 70# < 12% ER-SBS-T < SBS-T < SBS. The fatigue life of different asphalt types does not consistently increase with deeper aging. The fatigue life of 70# asphalt and ER-SBS-T-modified asphalt continues to decrease with increased aging degree. The results indicate that the fatigue life of both asphalt mixtures decreases as the aging degree increases. The fatigue life of SBS-modified asphalt is comparable to that of SBS-T-modified asphalt. Both show increased fatigue life after short-term aging. However, after PAV aging, the fatigue life of SBS and SBS-T decreases, with SBS-T showing a greater decline. This suggests that SBS-modified asphalt exhibits better resistance to pressure aging. In contrast, the fatigue life of ER-SBS-T-modified asphalt decreases rapidly with increasing aging depth, indicating poor anti-aging properties for the ER-SBS-T-modified asphalt mixture. Compared with ER-SBS-T-modified asphalt with different content, the fatigue life of the original asphalt generally increases when the content is 4%~10%. However, short-term- and PAV-aged ER-SBS-T-modified asphalt samples exhibit no clear trend in fatigue life, indicating that the dosage has little effect on the fatigue properties of ER-SBS-T-modified asphalt mixtures after aging.

5. Conclusions

According to VECD theory, this study conducts dynamic shear rheological tests on original, short-term-aged, and PAV-aged 70# matrix asphalt; 6% SBS-modified asphalt; 6% SBS-T-modified asphalt; and ER-SBS-T-modified asphalts with different modifier dosages. Fatigue properties of the asphalt samples are evaluated through LAS tests, and the following conclusions are obtained:

(1)

The cumulative damage curve results indicate that under original and PAV aging conditions, the cumulative damage limit values of the materials rank as follows: 12% ER-SBS-T > SBS > SBS-T > 70#. Under short-term aging conditions, the order is 12% ER-SBS-T > SBS > 70# > SBS-T. When the dosage of ER-SBS-T reaches 12%, the epoxy resin material in the modifier and the asphalt experience a full curing reaction, which significantly improves the hardness and deformation resistance of the modified asphalt. Therefore, the 12% dosage of ER-SBS-T has the highest cumulative damage threshold value.

(2)

Analysis of the fatigue life of modified asphalt at different strain levels reveals that at a low strain level of 1%, the fatigue life of ER-SBS-T-modified asphalt increases with increasing modifier content. The fatigue performance considerably improves when the modifier content exceeds 8%. Fatigue life of ER-SBS-T-modified asphalt increases after aging. At a medium strain level of 2.5%, the fatigue life of ER-SBS-T-modified asphalt increases with increasing modifier content. The fatigue life of ER-SBS-T-modified asphalt decreases after the aging process. However, the fatigue life decreases with aging. At a high strain level of 5%, the fatigue life of ER-SBS-T-modified asphalt no longer shows a clear increasing trend with increasing modifier dosage. The fatigue life of ER-SBS-T-modified asphalt decreases very rapidly after the aging process.

(3)

For ER-SBS-T-modified asphalt, the fatigue performance of dosage from 8 to 12% is better than that from 4 to 8%. Considering the comprehensive fatigue life and economy of ER-SBS-T-modified asphalt, the use of ER-SBS-T-modified asphalt with a modifier content of over 10% is recommended for practical engineering applications.