Crumb Rubber Modified Asphalt Binder Research Articles

Effects of Aging on Rheological Properties and Microstructural Evolution of SBS Modified Asphalt and Crumb Rubber Modified Asphalt Binders

Focusing on the comparison of aging resistance between styrene-butadiene-styrene (SBS) modified asphalt (SBSMA) and crumb rubber modified asphalt binders (CRMA), the influences of aging on rheological properties and microstructural characteristics of different modified asphalts were investigated in this work. Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests were carried out, and the variations in rheological properties for different modified asphalts after the rotating thin film oven test (RTFOT) were analyzed with the rutting factor aging index (RAI) and creep rate aging index (CAI). By using Fluorescence microscopy (FM) and Fourier transform infrared (FTIR) spectroscopy, the evolutions of microstructure and chemical composition for two modified asphalts were analyzed with carbonyl index growth rate (CIR) and sulfoxide index growth rate (SIR). Then, The relationships between CIR/SIR and RAI/CAI were established to show the correlation between the deterioration of macroscopic performance and the evolution of micro-structure. The results indicated that the aging degree of asphalt increases with elevated temperatures, leading to decreasing low-temperature performance while improving high-temperature performance. Nevertheless, SBSMA exhibited strong sensitivity to aging temperature. Under thermo-oxidative aging, the RAI and CAI of SBSMA were lower than those of CRMA, whereas the regularities of CIR and SIR were opposite, indicating that CRMA was superior to SBSMA in terms of anti-aging properties due to the rupture of the cross-linked network structure of SBSMA. However, CRMA experienced aging accompanied by full swelling, and thus, relatively minor performance declined. The CIR and SIR exhibited a better correlation with the RAI and CAI, illustrating that both CIR and SIR could characterize the aging degree of modified asphalts well.

Laboratory short-term aging of crumb rubber modified asphalt: RTFOT temperature optimization and performance investigation

Improper disposal of substantial scrap tires can result in significant environmental issues, such as air pollution, water resources, and soil contamination. The scrap tires can be turned into valuable materials by preparing tire rubber powder into Crumb rubber modified asphalt (CRMA). This method can effectively reduce environmental pollution and achieve the concept of energy conservation, environmental protection, and low-carbon. However, during the production process of CRMA mixtures, the elevated construction temperature may induce more severe short-term aging of the mixtures. The standard laboratory short-term aging scheme of asphalt binders, the Rolling Thin Film Oven Test (RTFOT), cannot simulate the short-term aging of CRMA due to the increase in construction temperature. Besides, the determination methods and indices of RTFOT aging temperature are still unclear and inconsistent. In this study, three CRMA binders were treated by RTFOT with five temperatures, and the short-term oven aging (STOA) protocol was conducted on CRMA mixtures with three types of gradations. Firstly, the chemical and rheological performance of CRMA binders and mixtures were investigated by conducting Gel permeation chromatography (GPC), Fourier transformation infrared spectroscopy (FTIR), Dynamic shear rheology (DSR) tests, and Bending beam rheometer (BBR) test. Then, the equivalent aging temperatures were determined to optimize RTFOT temperatures by comparing the chemical indicators of binders and mixtures. Finally, the correlations of chemical and rheological indices were established, and the rheological properties of aged asphalt in mixtures were predicted. The analysis results indicate that suitable RTFOT temperature is not only related to the technical routes and viscosity of CRMA binders, but also relevant to the mixture gradations. Binders with higher viscosity need elevated RTFOT temperatures from 173 °C to 193 °C, especially simulating short-term aging of mixtures with higher air voids. The swelling and degradation of crumb rubber contribute to the enhancement of the anti-aging performance. The aging index (AI) can reflect the anti-aging performance of CRMA binders, and the variation of AI can validate the rationality of the optimized RTFOT temperatures.

Mechanism of physical hardening on the fracture characteristics of polymer-modified asphalt binder

This article aims to reveal the physical hardening mechanism of polymer-modified asphalt binder (PMA) and accurately assess its impact on the low-temperature fracture properties of asphalt binders. Base asphalt binder (BA), styrene–butadiene–styrene modified asphalt binder (SBSMA), and crumb rubber modified asphalt binder (CRMA) were used as the research objects. Molecular dynamics (MD) simulation and differential scanning calorimetry (DSC) test were used to investigate the physical hardening mechanism of PMA at the microscopic level. The fracture characteristics of PMA under physical hardening were analyzed based on the single-edge notched beam (SENB) test. The results show that under prolonged cold service conditions, the asphalt molecular mobility is weakened, the non-bond energy and the free volume are reduced, and the molecules undergo aggregation state transformation, leading to the physical hardening of asphalt materials. In the process of physical hardening of asphalt molecules in the micro-Brownian motion of the main active force from the aromatic and saturate fractions. Considering the effect of physical hardening, it is recommended that the method used to test the low temperature properties of asphalt binders should ensure a constant temperature for at least 24 h. The crack tolerance (Ct) is recommended as the evaluation index of low-temperature cracking resistance of PMA in cold regions. The Ct of BA, SBSMA, and CRMA are reduced by 52.88 %, 12.04 %, and 35.24 %, respectively, after 24 h of constant low temperature. This indicating that the polymer modifiers can improve the fluidity of asphalt molecules, inhibit molecular aggregation, and reduce the effect of physical hardening on asphalt binders. Thermo-oxidative aging can aggravate the physical hardening phenomenon of asphalt binder, and the higher the degree of thermo-oxidative aging, the more severe the physical hardening of asphalt binder. The related research results provide the theoretical basis and technical support for the accurate design and development of the low-temperature performance of asphalt binders for roads in cold regions.

Presentation of machine learning methods and multi-objective optimization of fracture indices for asphalt rubber mixtures containing wax-based warm mix additives modified by nano calcium carbonate

Warm mix asphalt (WMA) additives have been proposed to overcome the high viscosity problem of crumb rubber modified asphalt (CRMA) binders. Various additives have been used to improve the low-temperature cracking performance of asphalt mixtures, but no study has been conducted to present the optimum additive content in CRMA binders in different loading modes. Therefore, this research aims to present the optimum content of nano calcium carbonate (NCC) to improve the fracture toughness and energy of asphalt rubber mixtures containing WMA additives (slack wax (SW) and polypropylene wax (PPW)). The semi-circular bend (SCB) fracture test was applied under pure mode I, mixed mode I-II, mixed mode II-I and pure mode II loadings at subzero temperature. Machine learning methods, including multivariate regression (MVR) and artificial neural network (ANN) models of group method of data handling (GMDH) and multilayer perceptron (MLP), were used to provide the prediction models of effective stress intensity factor (Keff) and fracture energy (Gf). Finally, the multi-objective optimization of Keff and Gf was performed to obtain optimum NCC content. The results indicated that in MVR model, the outputs had a small correlation with laboratory values, so that R value of MVR was 0.8406 and 0.8011 for Keff and Gf, respectively. Also, it was revealed in MVR that NCC had the highest impact on Keff and Gf significantly. GMDH model results showed that the relationships between predicted and laboratory values of Keff and Gf are appropriately described with R value of 0.9546 and 0.9229 for Keff and Gf models, respectively. In MLP model, different layer structures of the feed-forward neural network were developed to obtain the most accurate structure. It was indicated that MLP with 4–22–1 and 3–19–1 structures had a higher accuracy for Keff and Gf prediction models with R value of 0.9951 and 0.9978, respectively. Finally, by the use of the best model relationships, the results of the multi-objective optimization indicated that 4.91% and 6.37% NCC were the design optimum contents resulting in a maximum of Keff and Gf simultaneously for SW and PPW-modified CRMA mixtures. Moreover, the optimum NCC contents in loading modes of mode mixity Me of 1, 0.8, 0.4 and 0 were 3.62%, 5.12%, 4.08% and 3.42% for SW-modified CRMA mixtures and 4.35%, 6.51%, 7.19% and 2.84% for PPW-modified CRMA mixtures, respectively.

Effects of aging behavior on macro and micro characterizations of surfactant–crumb rubber asphalt binders

Asphalt aging process degenerates the performance and shortens the lifetime of the pavement, while using warm mix additive and crumb rubber modifier can improve the main negative distress. In this study, aged crumb rubber modified asphalt binder (CRMAB) with and without surfactant (0.6% SK) additive were designed through rolling thin film oven and pressure aging vessel test. The dynamic shear rheometer, bending-beam rheometer, fourier transform infrared spectroscopy, and atomic force microscopy tests were conducted to identify the effects of aging behavior on the macro-rheological properties, chemical constituents, and morphological characteristics of CRMAB and surfactant–crumb rubber modified asphalt binder (S-CRMAB). The findings from micro and macro analysis showed that aging increases carbonyl, sulfoxide, and aliphatic components, decreases aliphatic branched components, causing higher high-temperature rheology, lower low-temperaturen and fatigue performance. However, SK improves its aging resistance and relaxation ability. Moreover, the appearance of uniform hills–valleys and “round and bright lamellar” structures on the surface of 3D and 2D topographic images of S-CRMAB, respectively, indicating the surface roughness decreases, and microstructural stability gets enhanced.

Physical properties of crumb rubber modified asphalt binders and environmental impact consideration

The use of Crumb Rubber Modifier (CRM) with asphalt binders improves their properties by reducing their temperature susceptibility; depending on the way CRM and binder react with each other. The CRM particles tend to absorb some oils from asphalt binder and swell, resulting in a higher viscosity and thus stiffer binder. This paper discusses the effect of adding CRM to asphalt binder produced in Jordan by applying three different percentages of CRM on two asphalt binders, namely, Grade 60-70 and Grade 85-100. A total of 120 specimens were prepared to test their basic physical properties such as penetration, softening point, ductility, flash and fire points, and rotational viscosity. It was found that modified asphalt binders with higher crumb rubber content have lower ductility and penetration number, but higher softening point and viscosity values. Hence, this could be sign of rutting resistance and extended service life. Furthermore, different analytical methods were utilized to validate the experimental results. It was reported that only penetration displayed change in behavior for grade 60-70 and 85-100 asphalt binders. CRM content was found to be influential on all performance-related physical tests. Finally, the combined effect of asphalt-types and CRM content is statistically significant on softening point, ductility and rotational viscosity test results. On the other hand, the energy saving due to CRM incorporation coupled with other benefits including less cracking and maintenance makes rubberized asphalt very attractive and beneficial in highway construction. Rubberized asphalt technique reduced the environmental impact of highway construction by 5% to 10% compared to traditional practice.

A Laboratory Investigation Regarding Storage Stability of the CRM-Modified Bitumen—CRM Processing Method (Untreated vs. Treated)

The aim of this study is to analyze the phase separation that occurs between treated and untreated rubber crumb particles produced by wet processes in the laboratory. The percentage of replacement used for both the treated and untreated crumb rubber-modified asphalt (CRMA) was 5%, 10%, 15%, and 20%. Tests to evaluate binder properties were performed using a rotational viscometer and a DSR, and the following properties were determined—viscosity, G*/sinδ, % recovery, and Jnr. The phase separation study was analyzed using the viscosity and G*/sinδ results. In general, the results of the study show the following. (1) The treated CRMA binders had higher viscosity values than untreated CRMA binders, although some values could not be measured due to the high viscosity values. (2) The G*/sinδ, % recovery and Jnr results also had a similar trend with viscosity results. (3) The viscosity and G*/sinδ phase separation values demonstrate that treated CRMA binders perform better than untreated CRMA binders. (4) Different experimental methods have shown variations in the calculated SI value; hence, a more improved approach should be explored to accurately assess the storage stability of asphalt binders containing various additives.

Volumetric Dilatometer Study of Thermoreversible Aging Properties in Crumb Rubber–Modified Asphalt Binders

Abstract The primary objective of this paper is to investigate the thermoreversible aging mechanism of crumb rubber–modified asphalt binders (CRMAs). In order to achieve this goal, an extended bending beam rheometer (Ex-BBR) test and volumetric dilatometer test were conducted. Additionally, isothermal crystallization kinetics (Avrami theory) and thermal stress calculation using the Hopkins and Hamming algorithm were employed to process the test data. The low-temperature properties of the asphalt specimens were characterized through calculations of creep stiffness (S), creep rate (m-value), grade loss (GL), thermal stress, and Avrami index (n). The experimental results indicate that thermoreversible aging is present in all asphalt samples during the long-term isothermal hardening process. This aging is characterized by continuous macroscopic volume shrinkage, accumulation of thermal stress, a crystallization process, and a process akin to crystallization (i.e., crumb rubber adsorbed oil at low temperatures). It was observed that CRMAs exhibit more severe thermoreversible aging and faster physical hardening rates due to their unique properties at a constant low temperature when compared with base asphalts. However, it was also observed that CRMAs can reduce the thermal stress by reducing the volumetric shrinkage coefficients and S. It is important to note that the volumetric shrinkage coefficients are not constant, which are related to the base asphalts, crumb rubber, and conditioning time. Therefore, it is not appropriate to use a single empirical constant to calculate thermal stress. Instead, multiple coefficients may need to be incorporated to account for the variability in shrinkage behavior.

Investigating the effects of antioxidants on the aging characteristics of crumb rubber modified asphalt

Crumb rubber modified asphalt (CRMA) has gained significant interest due to its eco-friendly properties. However, there is still a limited understanding of its anti-aging properties, particularly with regards to the impact of antioxidants on its physical and chemical structure. This study aims to investigate the effect of three antioxidants (Irgafos 168, Irganox 1098, and Irganox 1076) on the chemical and physical properties of CRMA binders during short-term and long-term aging. The physical, rheological, and chemical properties of different samples were analyzed before and after aging using conventional physical and rheological tests, as well as Fourier transform infrared spectroscopy (FTIR). The results showed that the plastic deformation behavior of CRMA at low temperature was enhanced by modification with antioxidants, with Irgafos 168 exhibiting the most pronounced effect. After aging, CMRA showed a decline in both physical and rheological properties. In contrast to the control sample (CMRA), the three types of antioxidants displayed superior ability to alleviate the decrease in asphalt properties during short-term aging. Nevertheless, their effectiveness decreased significantly during long-term aging, possibly due to antioxidant degradation. The FTIR results revealed that the inclusion of antioxidants can effectively retard the increase of carbonyl index, with Irganox 1098 displaying the most notable impact. Nonetheless, the presence of sulfides in rubber powder limits the utility of sulfoxide index as an indicator for the degree of CRMA aging.

Stage-aging characteristics and stages division of crumb rubber modified asphalt binder

Currently, the aged crumb rubber-modified asphalt binder (CRMA) pavements have recycling demands; however, the understanding of the aging behavior of CRMA remains limited. Furthermore, few studies have been conducted to divide the aging stage of CRMA, which is an essential basis for the study of recycling. To solve the above problems, this study aims to analyze the stage-aging characteristics and divide the aging stages of the CRMA to understand better the aging and recycling mechanism of CRMA. The evolution characteristics of the rheological performance of aged CRMA were assessed by high-temperature performance indicators. In addition, thermal gravimetric analysis (TGA) is employed to investigate the degradation degree of crumb rubber modifier (CRM). Finally, the rheological-performance results are combined with the TGA results to divide the aging stages. The results demonstrate that the CRM in CRMA degrades in aging, which shows an initial rapid degradation, then slows down. The process of CRM degradation is accompanied by changes in CRM composition; the natural rubber content of CRM decreases the most significantly, followed by synthetic rubber and filler. The evolution of the properties of aged CRMA is characterized by stages due to CRM degradation, and the CRMA is divided into three aging stages. In addition, the gray correlation entropy analysis demonstrates that the MSCR test is more sensitive to the degradation of CRM in CRMA; it can be used to characterize the stage-aging process of CRMA. In summary, this study's results might fill a significant gap in understanding the aging mechanism of CRMA and provide the basis for recycling the aged CRMA.

Effect of modifier on Low-Temperature reversible aging behavior of asphalt binder and its morphology analysis

• Rubber powder can alleviate the effect of reversible hardening on asphalt binders. • The addition of modifiers can reduce the reversible hardening degree of the asphalt binder. • Cone penetration test can be used to characterize the reversible hardening degree of the asphalt binder. • With the extension of conditioning time, the average area of asphalt binder “bee” structure increases gradually. Reversible aging of asphalt binders is one of the reasons for the premature failure of asphalt pavements. To better understand the effect of modifier types on the reversible aging in asphalt binder, styrene–butadienestyrene (SBS), styrene-butadiene rubber (SBR), crumb rubber (CR), and Nano-CaCO 3 were selected to prepare modified asphalt binder. Firstly, an extended bending beam rheometer (Ex-BBR) test and a cone penetration test (CPT) were carried out on base asphalt (BA) and modified asphalt binders under different conditions. Secondly, the reversible aging behavior of asphalt binders was characterized using a logistic regression model. Furthermore, the mechanisms of asphalt reversible aging were analyzed by atomic force microscopy (AFM) and molecular dynamics (MD) simulation. The results show that the addition of modifiers affects the reversible aging behavior of asphalt binders. Compared with SBS, SBR, and nano-CaCO 3 , CR can improve the mobility of asphalt molecules and mitigate the aggregation of asphalt molecules, thereby alleviating the impact of reversible aging on asphalt binder performance. After 72 h low-temperature conditioning, CR modified asphalt binder has the smallest grade loss of 1.7 °C. The low-temperature conditioning time can greatly affect the microstructure of the asphalt binder. With the increase of low-temperature conditioning time, the average area of the asphalt binder’s “bee” structures increases linearly. Furthermore, the reversible aging behavior of asphalt binders can be better understood using MD simulation.

Study on the Properties of Waste Oil-Activated Crumb Rubber-Modified Asphalt Based on Molecular Dynamics Simulation and Rheology

To investigate the effect of activated crumb rubber content on the molecular interactions and the properties of crumb rubber-modified asphalt (CRMA) with waste oil, six models of asphalt with various rubber dosages were developed using Materials Studio software, and the molecular dynamics performance of the system was further examined. Then, the fatigue and high- and low-temperature performances of the CRMA binders were characterized by dynamic mechanical experiments in the laboratory. The mean square displacement and diffusion coefficient were used to quantify the migration of molecules. The aggregation state of the components was evaluated using a radial distribution function. The bulk modulus of the CRMA models was calculated to study the mechanical properties. Dynamic shear and bending beam rheometer tests were implemented to evaluate the road performances of the CRMA binders. The results show that increasing the amount of powder could improve the mechanical properties of the asphalt, that is, the modulus of 70% of the asphalt was improved by 57.5%. The rubber and waste oil were evenly dispersed in the system, and the distribution of asphalt components was in accordance with the colloid theory. The temperature-sensitive properties of the rubber led to the improvement of road properties of the CRMA binders with the increase of the admixture. Combined with the distribution of molecules in the asphalt model, the results of rheological indexes show that the waste oil could improve the rheology and stability of binders. This will provide theoretical support for upgrading the content of crumb rubber in CRMA binders.

Statistical modeling of low-temperature properties and FTIR spectra of crumb rubber modified asphalts considering SARA fractions

In this study, the correlation between low-temperature performance of the crumb rubber modified asphalt (CRMA) binders and their chemical fractions was investigated. The low-temperature properties of CRMA binders were firstly characterized by creep stiffness and creep rate using the bending beam rheometer (BBR) test. Secondly, the SARA fractions, namely saturates, aromatics, resins, and asphaltenes, were separated from CRMA binders using an improved column chromatography method. Meanwhile, the functional groups and the derived indexes of CRMA binders and SARA fractions were characterized using Fourier transform infrared (FTIR) spectra, respectively. Finally, statistical modeling was conducted between low-temperature properties and functional groups of CRMA binders and their SARA fractions. The results showed SARA fractions from CRMA binders contained substances released from the partially degraded crumb rubber modifiers, indicated by the emergence of peaks at 1100 cm−1, 967 cm−1, and other wavenumbers in FTIR spectra. The substance exchanges played a positive role in improving the low-temperature performance of CRMA binders. Meanwhile, the addition of crumb rubber could effectively inhibit the increase of polar functional groups in the aging process, such as carbonyl of aromatics, resins, and asphaltenes fractions. This thus significantly resisted the aging and improved the low-temperature performance of CRMA binders. Finally, the statistical modeling showed the stiffness modulus and creep rate of CRMA binders were highly correlated with functional groups of aromatics, resins and asphaltenes. This indicated that SARA fractions significantly affected the low-temperature properties of CRMA binders.

Ageing of Crumb Rubber Modified Bituminous Binders under Real Service Conditions

Due to their environmental advantages, crumb rubber modified asphalt binders constitute an interesting alternative to conventional binders for road surfaces of a more durable and sustainable nature. However, in practice, they remain less commonly used than conventional polymer modified binders. This research aims to study the real ageing of crumb rubber modified asphalt binders during their service lives when exposed to various factors, including temperature gradients, the presence of water and oxidation. To this end, research was conducted on a selection of highways built with these binders and located in regions with severe climatic and traffic conditions. The binders from cores of highway surface layers were recovered and tested using the DSR (Dynamic Shear Rheometer) to determine the evolution of the rheological parameters. Crumb rubber modified asphalt binders were studied in comparison with traditional polymer modified bitumen. The analysis of the complex modulus and phase angle was conducted based on frequency and temperature sweep tests, while the evolution of the elastic recovery, Jnr, L-Index and T-Index were assessed from the multiple stress creep and recovery test. The results obtained indicate that crumb rubber modified binders show similar ageing and rheological parameters to those of conventional polymer modified bitumen, even under severe traffic and climate conditions. Furthermore, it was observed that, at high temperatures, the effect caused by real service life ageing was different to that obtained in the laboratory through the RTFO and PAV tests.

Understanding the particle effects and interaction effects of crumb rubber modified asphalt regarding bonding properties

• Base asphalt, conventional CRMA and drained CRMA were evaluated. • BBS test, TSR and HWT test were performed for performance evaluation. • Rubber particles selectively absorb the light components of asphalt. • BBS test can only highlight the interfacial bonding strength of asphalt and aggregate. • Improving the elasticity of the mixture is helpful to improve the water resistance of the mixture. The bonding performance and moisture resistance of crumb rubber modified asphalt (CRMA) has been a hot debate for many years. On the other hand, the influence of crumb rubber particles can be divided into two parts: the filler nature of crumb rubber particle, namely the particle effect (PE); and the chemical interaction between crumb rubber particle/base asphalt, namely the interaction effect (IE). This research aims to understand and quantify the PE and IE in terms of CRMA binder bonding performance. To do so, base asphalt, conventional CRMA and drained CRMA were evaluated. Drained CRMA were prepared by removing rubber particles from conventional CRMA via sieving. Asphalt binder bonding strength (BBS) test, tensile strength ratio (TSR) and Hamburg Wheel Tracking (HWT) tests were employed for performance evaluation. Furthermore, rheology master curves, Fourier Transform Infrared Spectroscopy (FTIR) and fluorescence microscopy were conducted to help reveal the influence of PE and IE. The results show that in terms of BBS testing, the PE and IE may vary with different CRMA formulas, but overall, a large negative PE with a small positive IE is seen, leading to inferior bonding strength of all CRMA samples. On the other hand, CRMA shows the best mixture performance (TSR, HWT) with both positive PE and IE. It is believed that even though the rubber particle is not favorable in terms of binder bonding performance, it is beneficial to the mixture water-resistance by improving the mixture elasticity.

Assessment of the Aging Process of Finished Product–Modified Asphalt Binder and Its Aging Mechanism

Styrene-butadiene-styrene-modified asphalt binder (SBSMA), crumb rubber–modified asphalt binder (CRMA), and high-viscosity-modified asphalt binder (HVMA) have been widely used in porous asphalt concrete. However, the aging of the modified asphalt binder significantly affects the performance of the porous asphalt concrete. The main aim of this research is to quantitatively assess the aging degree of finished product–modified asphalt binders of SBSMA, CRMA, and HVMA at high, medium, and low temperatures, and to explore the aging mechanism at the microscopic level. This study used rolling thin-film oven and pressurized aging vessel to simulate the aging of asphalt binders. Dynamic shear rheometer, bending beam rheometer, Fourier transform infrared spectrometry, thin-layer chromatography with flame ionization detection, and scanning electron microscopy were employed to investigate how aging affects the rheological and microscopic properties of finished product–modified asphalt binder. The results showed that at high temperatures, short-term aging significantly impacted HVMA [change rate of the rutting factor (CRRF)=18.9%], while long-term aging had an enormous influence on CRMA (CRRF=179.5%). Short-term aging had the greatest effect on the fatigue property at medium temperature of CRMA [change rate of Nf(CRNf)=59.9%], followed by SBSMA (CRNf=46.1%) and HVMA (CRNf=26.2%). However, the effect of long-term aging on the fatigue properties of HVMA (CRNf=99.2%) is the largest, followed by that of CRMA (CRNf=95.8%) and SBSMA (CRNf=56.6%). At low temperature, compared with CRMA and HVMA, aging had the greatest influence on the low-temperature rheological properties of SBSMA. In addition, the three finished product–modified asphalt binders exhibited similar changes in microscopic level after aging.

Cracking Resistance of Recycled Rubber Asphalt Binder Composed of Warm-Mix Additives.

Warm-mix asphalt technology has been applied to recycled rubber asphalt binder (RAB), which forms warm-mixed crumb rubber-modified asphalt binder (W-RAB) as a “green” material for environmental conservation and to enhance road performance. Furthermore, low-temperature cracking is one of the major distresses for asphalt pavement, which drastically restricts ride quality and service level. Therefore, the main objective of this study is to comparatively analyze the low-temperature properties of W-RABs based on thermal stress and the simple fractional model. W-RABs were obtained by mixing 60 mesh recycled rubber (CR) and two different types of warm-mix additives, namely viscosity reducer (1, 2, and 3%) and surfactant (0.4, 0.6, and 0.8%). First, Hopkins and Hamming’s numerical algorithm and the Boltzmann superposition principle were used for obtaining thermal stress . Subsequently, critical cracking temperature was derived using the single asymptote procedure (SAP) theory. Second, the simple fractional viscoelasticity model was used to calculate the creep compliance, damping ratio, and dissipation energy ratio, and the results were compared with the Superpave protocol results obtained with bending beam rheometer (BBR) tests. The results showed that a combination of CR and warm-mix additives could slightly improve the thermal crack resistance of the asphalt binder. The addition of 0.6% surfactant yielded the optimum performance, while only a high dosage (3%) of viscosity reducer provided a marked improvement in efficiency, which decreased with a decrease in temperature. This study recommends the use of RAB composited with 0.6% surfactant for areas with extremely low temperature.

The Effects of Activation Treatments for Crumb Rubber on the Compatibility and Mechanical Performance of Modified Asphalt Binder and Mixture by the Dry Method

Compatibility and mechanical performance of crumb rubber (CR)-modified asphalt and mixtures prepared by the dry method are poor at present, which limits the application of dry-mixed asphalt rubber in paving engineering. To solve this problem, various activated treatments for CR were performed, and the modified asphalt binder was prepared by the dry process in this study, aiming to activate the CR as the modifier and improve the compatibility of CR-asphalt blends. The effects of pre-treating methods of high temperature, pre-swelling, and microwave treatment on rutting resistance and compatibility of the asphalt binder and asphalt mixture were evaluated. Furthermore, the dynamic shear rheometer (DSR) of temperature sweep, Brookfield rotary viscosity, physical properties, rutting resistance, and microstructure of modified asphalt were measured. The results indicated that the three treatments have a pronounced effect on the high temperature performance of the crumb rubber-modified asphalt (CRMA) binder in the dry process and modified asphalt storage stability improvement. The high-temperature and microwave pre-treatment for CR lead to the largest complex modulus, viscosity, and rutting factor. Moreover, high-temperature, pre-swelling, and microwave treatment for CR significantly increases the dynamic stability of dry-mixed asphalt mixtures, which indicates the enhanced resistance to permanent deformation. In microscopy, homogeneous and compatible states were obtained for asphalt modified by high temperature-treated CR and pre-swelled CR, in which the strong interaction occurs among rubber molecules and the asphalt component. The microstructure of dry CR-asphalt blends sheds light on the mechanical performance. Consequently, activated CR modification for the asphalt binder by the dry process in this study is a promising technology, and other properties need to be verified in the future.

Quantitative determination for effective rubber content in aged modified asphalt binder

During the aging process, the desulfurization and degradation process of crumb rubber modified asphalt binders (CRMA) consistently occurs. In this paper, a toluene insoluble test, an electrochemical titration method, viscosity, FTIR and GPC were developed to determine the effective crumb rubber content of CRMA after exposure to various aging conditions. Centering on efficiency and accuracy, these five methods were compared with the Pearson coefficient method. The results show that when the degree of aging was severe, the desulfurization and degradation of rubber were aggravated, causing the CC bond in the rubber to break and an increase in the large molecular weight of the asphalt binder. The effective rubber content in the aged CRMA was 7.7–14.2% less than the unaged CRMA. At the same time, the correlation coefficients among the five methods were all above 0.90, but the electrochemical titration and FTIR had priority over the other methods.

Aging characteristics of crumb rubber modified asphalt binder and mixture with regenerating agent

At present, the research hotspots of recycled asphalt mainly focus on the recovery of performance, but less attention has been given to the secondary aging of recycled asphalt. Therefore, this article mainly studied the effects on the physical properties of initial aging, regeneration and secondary aging of crumb rubber modified asphalt (CRMA) and mixtures. The results show that a rubber regenerating agent (RA) can restore the performance of aged CRMA to its original level, and the optimal RA content was 8%. During the secondary aging, RA-CRMA also experience performance degradation similar to virgin CRMA due to the loss of light components and the degradation of rubber. However, on the whole, the aging index of RA-CRMA and mixture was better than that of virgin CRMA and mixture, indicating that RA improved the aging resistance of CRMA. Further, the Pearson correlations between the various aging indexes of binder and mixture were significant, and the state of secondary aging can be quickly determined through a simple binder experiment.