Aging properties and aging mechanism of activated waste rubber powder modified asphalt binder based on rheological properties and micro-characterization

Abstract

The research and development of high-performance pavement materials has been intensified owing to the demand for long-life pavements. This study is performed to develop a novel pavement material using waste rubber powder, waste lubricating by-product (LBP), and asphalt. Subsequently, the aging properties and aging mechanism of activated waste rubber powder modified asphalt (ARMA) are investigated based on its rheological properties and micro-characterization. The rheological results show that, compared with waste rubber powder modified asphalt (RMA), ARMA offers a higher aging resistance and a longer fatigue life. A comparison and analysis of the rheological aging parameters of ARMA and RMA show that LBP activation diminishes the aging sensitivity of ARMA. The micro-characterization result shows that the aging of ARMA may be caused by the fact that LBP-activated waste rubber powder is more reactive and can form a dense colloidal structure with asphalt. Therefore, the evaporation loss of asphalt light components by heat and the damage to the colloidal structure by oxygen during the aging process are impeded, and the thermal-oxidative aging resistance of ARMA is improved.

Graphical abstract

Keywords

rubber powder modified asphalt / aging / mechanism / rheological / characterization

1 Introduction

Asphalt is widely used in civil engineering (such as pavement structural cementing, waterproofing, and anti-corrosion materials) owing to its excellent properties [1,2]. Although most asphalt materials offer excellent performance when used in pavements, ordinary asphalt materials do not necessarily satisfy road development requirements owing to the complexity and variability of the traffic environment, the climate environment, and other factors [3,4]. Therefore, high-performance asphalt materials have garnered the attention of researchers. Studies and practical engineering applications show that modified asphalt is an ideal alternative to ordinary asphalt [5,6], as it can not only compensate for the disadvantages of ordinary asphalt in terms of viscoelasticity and adhesion, but also effectively improve the high and low-temperature performances as well as the durability of asphalt [5,7–10]. Asphalt modified by polymers such as styrene–butadiene–styrene copolymer (SBS), ethylene vinyl acetate (EVA), and styrene–butadiene rubber (SBR) are widely used [10–13]. However, owing to the shortage of raw materials (from non-renewable fossil-based materials) for commercial polymers (such as SBS, EVA, and SBR), rubber powder from waste tire production has been used increasing [6,14,15]. This is because studies show that compared with commercial polymer modified asphalt, waste rubber powder modified asphalt (RMA) not only exhibits excellent high-temperature rutting resistance, low-temperature crack resistance, fatigue resistance, aging resistance, noise reduction, and driving comfort, but also offers more economic benefits [16–18]. In addition, the successful application of waste rubber powder in road engineering not only solves the irreparable damage to the ecology caused by the burial and incineration of waste tires, but also enables a green approach for recycling waste tires [19–21].

However, the use of RMA is challenging. First, the cross-linked structure of rubber is extremely complicated, which directly results in its inert surface and hinders the chemical reaction of rubber with asphalt [22]. Second, waste rubber is incompatible with base asphalt in terms of thermodynamics and both exhibit different densities, which ultimately causes the waste rubber powder to sink and segregate easily during storage and transportation [16,20,23]. In addition, studies have shown that waste rubber powder selectively assimilates the light components of asphalt during mixing, thus accelerating the hardening of modified asphalt and increasing its viscosity [24]. In summary, these issues have hindered the popularization and application of RMA; thus, a new technology must be developed urgently to solve these issues. Studies have shown that lubricating by-product (LBP) is an effective activated material that can undermine the inertia of rubber powder and render the rubber powder surface more active [25–27]. Nevertheless, studies regarding the aging properties and mechanism of LBP-activated waste rubber powder modified asphalt (ARMA) are few.

Therefore, in this study, the advantages of waste rubber powder and LBP are exploited to develop a new asphalt material, i.e., ARMA. Subsequently, the aging properties of ARMA are investigated using dynamic shear rheometer (DSR). Additionally, the aging mechanism of ARMA is investigated using Fourier transform infrared spectroscopy (FT-IR) and four-component analysis. This study provides support for the promotion and application of ARMA technology and a green solution for the reuse of waste tires and waste LBP.

2 Experiment and measurements

2.1 Materials

LBP was purchased from the PetroChina Lanzhou Petrochemical Company, and its physical properties are listed in Tab.1. Meanwhile, 70# asphalt (penetration of 70) was purchased from Sinopec Zhenhai Refining & Chemical Company. Waste rubber powder featuring (60 mesh, 250 µm) was supplied by Jiangsu Environment Technology Co., Ltd.; its main technical specifications are listed in Tab.2. The stabilizer used was purchased from Zibo Heye Chemical Co., Ltd.

2.2 Preparation of activated waste rubber powder modified asphalt

The modified asphalt binders were prepared as follows. First, waste rubber powder and the LBP were homogeneously blended in a high-pressure reactor and then activated on a magnetic stirrer at 100 °C for 1 h to obtain activated waste rubber powder. Subsequently, 70# asphalt, which was heated to the flow state, was added to a steel container containing activated waste rubber powder and then sheared and stirred in a high-speed shear mixer for 1 h at 180 °C (The amounts of the LBP and waste rubber powder were 3% and 20% of the total weight of the modified asphalt binder, respectively.). Finally, the stabilizer was added (The amount of stabilizer was 0.5% of the total weight of the waste rubber powder.), and the mixture was stirred (3000 r/min) at 180 °C for 0.25 h to cure the ARMA. The preparation procedure is illustrated in Fig.1. The preparation process of the RMA in the reference group was similar to that of the ARMA. The technical specifications of the 70# asphalt, ARMA, and RMA are listed in Tab.3.

2.3 Aging procedures

A thin-film oven test (TFOT) was performed on the ARMA and RMA to simulate the short-term aging process, based on the specifications in ASTM D1754. The asphalt samples were placed in an oven at 163 °C for 5 h during the short-term aging tests. A pressure aging vessel (PAV) test was conducted on the ARMA and RMA to simulate the long-term aging process, based on the specifications in ASTM D6521. After the TFOT was completed, the asphalt samples were placed in a PAV at 100  °C and 2.1  MPa for 20 h during the long-term aging tests.

2.4 Measurements

2.4.1 Frequency sweep tests

The maximum temperature of an asphalt road surface can reach 60 °C in the summer. Therefore, a frequency sweep was performed at 60 °C at a frequency range of 0.2–30.0 Hz, and the strain level was set to 0.01%. The frequency sweep was performed to determine the rheological properties of the ARMA. To ensure repeatability, three replicates were performed for each modified asphalt binder.

2.4.2 Linear amplitude sweep tests

The linear amplitude sweep (LAS) test is a late-model test based on viscoelastic continuum damage theory and is typically performed to evaluate the fatigue performance of asphalt binders. For the LAS test, the following settings were specified: test temperature, 25 °C; loading frequency, 10 Hz; loading time, 600 s; loading amplitude, linear increase from 0.01% to 30.0%. To ensure repeatability, three replicates were performed for each modified asphalt binder. The damage parameter (D) was quantified using the work and potential energy theory. An integrity parameter (C) was employed to represent the integrity level of the material, which is equal to the C value when the D value reached 100 during the damage process of the asphalt binder. The formulas are shown in Eqs. (1)–(3).

(1)D(t)\cong {\sum }_{i\text{=1}}^N\text{π}{\gamma }_{\mathrm{t}}^2{(C_{i-1}-C_i)}^{\frac{\alpha }{1+\alpha }}{\left(t_i-t_{i-1}\right)}^{\frac{1}{1+\alpha }},

(2)C=\frac{\left|G^{\ast }\right|\mathrm{s}\mathrm{i}\mathrm{n}{\sigma }_t}{\left|G^{\ast }\right|\mathrm{s}\mathrm{i}\mathrm{n}{\sigma }_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}},

(3)N_{\mathrm{f}}=A({\gamma }_{\mathrm{m}\mathrm{a}\mathrm{x}}{)}^{-B},

where A = f (D_f)^k/(k(πC₁C₂)^α); D_f represents the damage accumulation at failure; f is the loading frequency, i.e., 10 Hz; k = 1 + (1 – C₂)α; B = 2α; \left|G^{\ast }\right|\mathrm{s}\mathrm{i}\mathrm{n}{\sigma }_t is the damage value at time t; \left|G^{\ast }\right|\sin {\sigma }_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}} is the initial undamaged value; α provides information regarding the properties of the undamaged material; t is the testing time; {\gamma }_{\mathrm{t}} is the applied strain (%); N_f is the number of failure cycles; γ_max is the maximum shear strain of the sample; and A and B are the VECD model coefficients determined by the characteristics of the materials [28].

2.4.3 Aging evaluation

To investigate the interaction of LBP activation on the aging behavior of modified asphalt more comprehensively, DSR parameters such as the complex modulus (G^{\ast }), phase angle (δ), rutting factor (R), fatigue factor (F), and integrity (C) were applied to evaluate the aging properties of the modified asphalt binder. Their formulas are presented in Eqs. (4)–(8).

(4)G_{\mathrm{A}\mathrm{I}}^{\ast }=(G_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}^{\ast }-G_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}^{\ast })/G_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}^{\ast }\times 100\mathrm{\%},

(5)P_{\mathrm{A}\mathrm{I}}=(P_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}-P_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}})/P_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}\times 100\mathrm{\%},

(6)R_{\mathrm{A}\mathrm{I}}=(R_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}-R_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}})/R_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}\times 100\mathrm{\%},

(7)F_{\mathrm{A}\mathrm{I}}=(F_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}-F_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}})/F_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}\times 100\mathrm{\%},

(8)C_{\mathrm{A}\mathrm{I}}=|C_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}-C_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}|/C_{\mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d}}\times 100\mathrm{\%},

where G_{\mathrm{A}\mathrm{I}}^{\ast }, P_AI, R_AI, F_AI, and C_AI are the complex modulus, phase angle, rutting factor, fatigue factor, and integrity aging indices, respectively. Subscript aged indicates short- or long-term aging, and subscript unaged indicates no aging.

2.4.4 Four-component analysis test

Although complicated physical and chemical reactions occur during the aging process of asphalt binders, their complex and undefined chemical structures render it difficult to characterize and analyze their physical and chemical reactions during the aging process. Therefore, the change in asphalt components is typically adopted to analyze the aging of asphalt. In this study, 70# asphalt, RMA, and ARMA were tested under different aging conditions based on the chemical components of asphalt (four-component method) using the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011).

2.4.5 Fourier transform infrared spectroscopy analysis

To investigate the aging mechanism of the ARMA, FT-IR was conducted to characterize the chemical structures of the ARMA and RMA before and after aging. The FT-IR analysis was performed using a VERTEX 80V (BRUKER Optics, Germany), and the wavenumber range was 4000–400 cm⁻¹. According to the Lambert–Beer principle, the absorption peak area of ​​a functional group can indicate its content. Therefore, the absorption peak areas of the carbonyl functional groups (C=O), sulfoxide functional groups (S=O), and butadiene functional groups (C=C) in the characteristic absorption peaks of asphalt were calculated, and the carbonyl aging index (C_{\mathrm{A}\mathrm{I}}^{\prime }), sulfoxide aging index (S_AI), and butadiene aging index (B_AI) were calculated using Eqs. (9)–(11), respectively.

(9)C_{\mathrm{A}\mathrm{I}}^{\prime }=A_{\mathrm{C}=\mathrm{O}}/A_{\mathrm{C}-\mathrm{H}},

(10)S_{\mathrm{A}\mathrm{I}}=A_{\mathrm{S}=\mathrm{O}}/A_{\mathrm{C}-\mathrm{H}},

(11)B_{\mathrm{A}\mathrm{I}}=A_{\mathrm{C}=\mathrm{C}}/A_{\mathrm{C}-\mathrm{H}},

where A_C=O, A_S=O, A_C=C, and A_C−H represent the absorption areas of C=O (1700 cm⁻¹), S=O (1030 cm⁻¹), C=C (1603 cm⁻¹), and C−H (2800–3000 cm⁻¹), respectively.

3 Results and discussion

3.1 Analysis of rheological properties

3.1.1 G^{\ast } and δ

The rheological properties of the ARMA and RMA were determined via frequency sweep tests. Previous studies show that frequency sweep tests can be performed to characterize the linear viscoelastic behavior of asphalt binders, in which the low and high frequency information describe the viscoelastic behavior of asphalt binders under low stress (low loading or high temperature) and high stress (fast traffic or medium and low temperature) variations, respectively [29]. The frequency sweep results for the ARMA and RMA under different aging conditions are presented in Fig.2.

As shown in Fig.2, the G^{\ast } values of the ARMA and RMA under different aging conditions increased and δ decreased as the frequency increased. This indicates that both the ARMA and RMA exhibit excellent deformation resistance under different aging conditions. In addition, Fig.2 shows that the change trends of G^{\ast } and δ are similar, which implies that the linear viscoelastic behaviors of the ARMA and RMA are identical [8]. Notably, the G^{\ast } of the ARMA subjected to different aging conditions under the same loading frequency was lower than that of the RMA, whereas the δ of the ARMA was higher than that of the RMA. Although this phenomenon intuitively shows that the deformation resistance of the ARMA is weaker than that of the RMA under different aging conditions, it also indicates that the hardening rate and the increase rates of the functional group content and molecular weight of the ARMA molecules were lower than that of the RMA as aging progressed [30–32].

3.1.2 G^{\ast }/sinδ and G^{\ast }·sinδ

According to the Superpave binder specifications, the rutting factor (G^{\ast }/sinδ) can be used to characterize the high-temperature performance of asphalt. Fig.3(a) shows the variation in G^{\ast }/sinδ with the loading frequency for the ARMA and RMA under different aging conditions. The G^{\ast }/sinδ of the ARMA and RMA increased with the loading frequency regardless of the aging level, indicating the high rutting resistance of the ARMA and RMA. In addition, the G^{\ast }/sinδ ratio of the ARMA and RMA increased with the aging level at the same load frequency, which supports the conclusion that aging improves the rutting resistance of asphalt. However, as shown in Fig.3(a), the G^{\ast }/sinδ ratio of ARMA is less than that of the RMA from the unaged period to long-term aging. The shows that the high-temperature rutting resistance of ARMA is lower than that of the RMA [33], which is consistent with the results of the analysis of G^{\ast } vs. frequency. This result can be further elucidated by the fact that the activation of LBP can effectively inhibit aging damage to the asphalt components and the structure of the waste rubber powder, thus decelerating the aging of the ARMA [31,34].

The fatigue factor (G^{\ast }·sinδ) is typically used to evaluate the fatigue cracking of asphalt under medium temperature conditions. The relationship between the G^{\ast }·sinδ ratios of the ARMA/RMA and the loading frequency under different aging conditions is shown in Fig.3(b). The G^{\ast }·sinδ values of the ARMA and RMA increased as aging progressed, which implies a deterioration in their fatigue cracking resistance. However, at different aging levels, the G^{\ast }·sinδ value of the ARMA was lower than that of the RMA at the same loading frequency. This indicates that the activation of the LBP improved the fatigue resistance of the ARMA, regardless of the aging condition [30,35].

3.1.3 Linear amplitude sweep test analysis

Fig.4 shows the damage intensity and fatigue life of the ARMA and RMA at different aging degrees. According to previous studies, asphalt is not damaged when C = 1; however, when C = 0, it is severely destroyed [17]. As shown in Fig.4(a), the C of the ARMA is lower than that of the RMA under the unaged condition. This is primarily due to LBP activation, which softened the colloidal structure of the ARMA and thus deteriorated its fatigue resistance. However, as aging progressed, the damage characteristic curves of the ARMA and RMA exhibited a clear decreasing trend. This indicates that aging deteriorated the fatigue resistance of the ARMA and RMA, where the fatigue resistance of the ARMA was better than that of the RMA. Based on Fig.4(b), the fatigue lives of the ARMA and RMA shortened as aging progressed, where the fatigue life of the ARMA was longer than that of the RMA. This similarly shows that aging is conducive to prolonging the fatigue life of modified asphalt and that the ARMA exhibits excellent anti-fatigue and anti-aging properties. This may be because LBP activation decelerated both damage to the modified asphalt colloid structure and the cracking of the rubber powder [36].

3.2 Aging sensitivity analysis

The rheological index has been reported to be a novel index for evaluating the aging sensitivity of asphalt binders. Fig.5 shows the changes in the rheological aging index before and after ARMA and RMA aging. Based on Fig.5, the values of the rheological aging indices (G_{\mathrm{A}\mathrm{I}}^{\ast }, P_AI, R_AI, F_AI, and C_AI) of the ARMA and RMA increased as aging progressed, where those of RMA were higher than those of the ARMA from short to long-term aging. This implies that as the aging environment deteriorated, the aging sensitivity of the RMA became higher than that of the ARMA [34]. This may be because the hardening rate (or the destruction of the colloidal structure) of the ARMA was lower than that of the RMA under different aging conditions. The C_AI of the ARMA was lower than that of the RMA, based on aging via the TFOT and PAV (see Fig.5(e)), which implies that the aging sensitivity of the ARMA is lower than that of the RMA. Hence, one can conclude that LBP activation can effectively decrease the aging sensitivity of ARMA.

3.3 Four-component analysis

The chemical composition test results of the 70# asphalt, RMA, and ARMA at different aging degrees are shown in Fig.6. The asphaltene content of the three asphalts increased as aging progressed, whereas those of the saturates, aromatics, and resins decreased. This indicates that the aging of the asphalt resulted in an intrinsic shift, i.e., aromatics → resins → asphaltenes [37,38]. Notably, the increase rate of the asphaltenes in the short term aging stage was significantly higher than that in the long term aging stage, which is primarily attributed to the fact that the asphaltenes underwent ring-opening and chain-breaking reactions at high temperatures, where some asphaltenes transformed into saturates and aromatics; hence, the amount of asphaltenes increased gradually in the long term aging stage [39–41]. A comparison of the changes in the four components of the 70# asphalt, RMA, and ARMA during the aging process shows that the changes in the components of the RMA and ARMA were similar to those of the 70# asphalt; nevertheless, the change rates of the asphaltenes, saturates, aromatics, and resins in the ARMA were lower than those of the RMA and 70# asphalt. Hence, one can conclude that the saturated chain scission reaction, oxidation reaction of aromatic condensation dehydrogenation, and ring-opening and chain scission reactions of asphaltenes in the ARMA were slower than those of the 70# asphalt and RMA. This implies that the transformation rate of the ARMA is lower than those of the other components during the aging process, which indirectly indicates that the aging resistance of the ARMA is better than that of the 70# asphalt and RMA [42].

3.4 Fourier transform infrared spectroscopy analysis

FT-IR can reveal the chemical bonds or functional groups in materials; additionally, it is used for the qualitative or quantitative analysis of material properties. The FT-IR spectra of the ARMA and RMA at different aging degrees are shown in Fig.7. The functional groups determined from the FT-IR spectra of the ARMA and RMA are listed in Tab.4. The conclusions inferred from Fig.7 and Tab.4 are that the absorption peaks (functional groups) of the ARMA and RMA did not change significantly from the unaged period to long-term aging. This indicates that the aging mechanisms of the ARMA and RMA under different conditions are similar [34]. However, the intensities of these absorption peaks were different, indicating that the ARMA and RMA have different anti-aging properties. Additionally, a new absorption peak appeared at 1030 cm⁻¹ for the ARMA and RMA after short- and long-term aging (see Fig.7). This absorption peak was confirmed to be that of sulfoxide (S=O), which was due to oxygen absorption by sulfur during aging. Additionally, as aging progressed, the intensity of the carbonyl absorption peak at 1700 cm⁻¹ increased significantly, which is attributed to the absorption of oxygen by the unsaturated carbon chain. Therefore, one can conclude that the ARMA and RMA primarily underwent oxidation reactions during the aging process [25].

The calculated values for C_{\mathrm{A}\mathrm{I}}^{\prime }, S_AI, and B_AI before and after aging are shown in Fig.8. C_{\mathrm{A}\mathrm{I}}^{\prime } and S_AI increased significantly as the ARMA and RMA progressed gradually from short to long-term aging, whereas B_AI decreased significantly. This indicates that the amount of oxidized components (carbonyl and sulfoxide groups) in the ARMA and RMA increased during aging, whereas the polymer dispersed in asphalt degraded significantly, and the amount of the dienyl group decreased [43]. More importantly, the differences in the C_{\mathrm{A}\mathrm{I}}^{\prime }, S_{\mathrm{A}\mathrm{I}}, and B_{\mathrm{A}\mathrm{I}} of the ARMA were lower than those of the RMA. This suggests that the aging resistance of the ARMA is higher than that of the RMA [23], which is attributable to the fact that the aromatic components (an important component of asphalt) of asphalt in the ARMA were consumed less compared with those in the RMA [44]. Therefore, the LBP activation during the aging process decelerated the transformation of the ARMA from a sol structure to a gel structure, thereby decelerating aging and improving the anti-aging performance.

3.5 Aging mechanism analysis

Based on the FT-IR analysis above and the four-component measurement results, a possible anti-aging mechanism for the ARMA is suggested, as shown in Fig.9. Because the ARMA is composed of waste rubber powder and 70# asphalt, the thermal-oxidative aging of the ARMA is twofold. One is the aging of 70# asphalt under thermal-oxidative conditions, which involves many complicated physicochemical reactions. Such physical reactions are primarily associated with the heat volatilization of small molecules with low boiling points and some light components generated by the high-temperature chain scission of saturates. Chemical reactions are represented by the oxidation and condensation of some polar groups (aromatics) in asphalt to form oxidized functional groups (such as carboxyl and sulfoxide groups) or resins. In addition, some polar groups (such as aromatics and resins) polymerize into large asphaltenes, whereas some resins or asphaltenes may decompose into small molecules [43,45]. Therefore, the complexity of the physicochemical reactions induces significant changes in the components and colloidal structure of asphalt.

Meanwhile, the aging of the rubber powder in the ARMA was primarily due to a significant increase in the specific surface area of the rubber powder compared with that of a tire; therefore, some of the rubber molecular chains disintegrated in the hot oxygen environment, which resulted in the generation of small molecules in the asphalt phase. Another portion of the rubber powder was oxidized and fractured, which generated a small area in the network structure and a few chains [46]. As the reaction continued, the active ingredients in the rubber powder (such as sulfur, carbon black, iron oxide, and silicon oxide) entered the asphalt phase and reacted with the polar components, which resulted in changes to the components and colloidal structure of the modified asphalt. Eventually, the modified asphalt became hard and brittle, the composite shear modulus increased, and the phase angle decreased [24]. However, the exhaustive chemical reaction of the 70# asphalt and the comprehensive chemical reaction of the waste rubber powder in the asphalt should be investigated further. The activation of the LBP adequately inflated the cross-linked network structure of the rubber powder (Fig.9(b)), and the colloidal structure formed with the asphalt effectively hindered the intrusion of oxygen, thereby alleviating the oxidative condensation of aromatics, polymerization of resins, decomposition of asphaltenes, and oxidative cracking of rubber powder. This extended the destruction time of the original equilibrium colloidal structure and improved the anti-aging performance of the ARMA.

4 Conclusions

In this study, LBP was used to activate waste rubber powder, which was then applied to modify asphalt. The aging performances of ARMA and RMA were compared and analyzed by determining the changes in their rheological indices and chemical components under different aging degrees. Some conclusions inferred based on comparison and analysis are as follows.

Compared with the RMA, although the activation of LBP weakened the high-temperature rutting resistance of the ARMA to a certain extent, the ARMA exhibited a higher aging damage resistance and a longer fatigue life.

The values of the rheological aging indices of the ARMA and RMA increased as aging progressed. However, the aging sensitivity of the ARMA was lower than that of the RMA, which indicates that LBP activation decreased the aging sensitivity of the ARMA.

Based on an analysis of the functional groups of the ARMA, the ARMA did not react chemically with asphalt. However, the change in the aging index of particular functional groups (C=O, S=O, and C=C) shows that LBP activation can prevent damage to the colloidal structure of the ARMA via aging, which is conducive to improving the anti-aging performance of the ARMA.

Based on the FT-IR spectra and four-component results, the potential aging mechanism of the ARMA was suggested. Waste rubber powder activated by LBP can form a dense and stable colloidal structure with asphalt, thereby improving the aging resistance of the ARMA by hindering oxygen intrusion and impeding the loss of asphalt components.

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