Crumb Rubber Modified Asphalt Mixtures Research Articles

Emission Risk and Inhibition Technology of Asphalt Fume from Crumb Rubber Modified Asphalt

Crumb rubber-modified asphalt mixtures have been proven to have extensive utilization value in road engineering. However, the rubber releases more fumes during the construction period, which causes severe harm to human health and the environment. This research focused on the emission risk of asphalt fume from crumb rubber-modified asphalt, and then the inhibition technology was also optimized. Firstly, the emission behavior and the hazardous evaluation of the asphalt fume from crumb rubber-modified asphalt were investigated. Then, the characteristics of the inhibition materials were evaluated. Finally, the reduction in the emission of inhibited crumb rubber-modified asphalt fume was identified, and the optimized formula was determined based on the inhibition effect, rheological properties, and cost. The results indicate that crumb rubber-modified asphalts release more fume components with an increment in the temperature and crumb rubber content. Desulfurized rubber reduces the release of H2S and NO. Benzene compounds, including paraxylene, toluene, and benzene, are the most released pollutants that harm human health, especially DS CRA 20% and CRA 50%. Kaolin powder and expanded graphite have a sufficient pore structure and volume, the addition of which reduces the release of pollutants while possibly promoting the release of NO and H2S. Their addition also has a significant control effect on the release of particulate matter at 170 °C and 185 °C. With the consideration of emissions, rheological properties, and cost, CRA 40%-EG2%-KL2% was determined as the optimization formula. This research is helpful to the application of crumb rubber-modified asphalt in road construction and maintenance.

Effect of salt erosion on interfacial adhesion of waste tire crumb rubber-modified asphalt mixtures: A molecular dynamics study

In this study, a wetting and migration model was established using the molecular dynamics simulation method. Through in-depth analysis of parameters such as contact angle, mean square displacement, and work of adhesion, the dynamic wetting and migration characteristics of salt solution were explored, and the damage mechanism of waste tire crumb rubber-modified asphalt mixtures under salt erosion was revealed. The results showed that compared with an aqueous solution, the salt solution was more likely to be adsorbed and dispersed on the asphalt surface. This exacerbated its damage. However, the presence of salt ions reduced the wetting effect of the solution on the aggregate surface and reduced the migration rate of the solution at the asphalt-aggregate interface. Significant diffusion of Na+ and Cl- ions occurs during salt solution migration, while SO42- ions mainly accumulate near the aggregate surface. In addition, salt solution migration alters the distribution of asphalt components. This further affects the interface adhesion effect. The inclusion of crumb rubber can effectively reduce the sensitivity of asphalt to salt solutions and enhance its hydrophobicity. Simultaneously, it enhances the adhesion strength and stability of the asphalt. This effectively obstructs the intrusion of water molecules and salt ions.

Effects of freeze-thaw cycles on fatigue performance of asphalt mixture and a fatigue-freeze-thaw damage evolution model

To study the effect of freeze-thaw (FT) cycles on the fatigue performance of composite crumb rubber modified asphalt mixtures (CCRMA), a semicircular bending fatigue test based on the digital image correlation technique (DIC) was used to measure the fatigue life (Nf) of CCRMA and SBS modified asphalt mixtures (SBSMA) and the horizontal strain (Exx) of the DIC deformation field under repeated loading. The variation laws of Nf, Exx and the damage factor (D) defined based on Exx were analyzed under FT cycle conditions. In addition, considering the important influence of FT cycles on the Nf and damage evolution characteristics of asphalt mixtures, based on the classical Chaboche damage evolution model, the influence of FT cycle was considered, a fatigue-freeze-thaw damage evolution model that reflects the effects of FT cycling variables and stress levels on asphalt mixtures was constructed. The results indicate that the Exx peak of asphalt mixture shifts forward with increasing FT cycle number. After 20 FT cycles, the Nf of CCRMA decreased by 73.4 %, 62.1 %, and 75.1 %, while that of SBSMA decreased by 76.5 %, 58.6 %, and 74.6 % at stress ratios of 0.2, 0.3, and 0.4, respectively. The fatigue-freeze-thaw damage evolution model predictions were in agreement with experimental results, with the goodness of fit (R2) of all Nf prediction models being greater than 0.9. With the exception of a few outliers, the relative error between the measured and predicted Nf for the two asphalt mixtures was within 15 %. This indicates that the model can effectively predict the Nf and damage evolution laws of asphalt mixtures at different stress levels after an arbitrary number of FT cycles. The related research results provide a theoretical basis for predicting the Nf of new roads in cold regions under FT cycles.

Identifying the Suitability of Warm Mix Asphalt for Reducing the Production Temperatures of Crumb Rubber–Modified Asphalt Mixtures: Economic and Environmental Perspective

Identifying the Suitability of Warm Mix Asphalt for Reducing the Production Temperatures of Crumb Rubber–Modified Asphalt Mixtures: Economic and Environmental Perspective

A comprehensive evaluation of crumb rubber modified asphalt-aggregate interfacial behavior and the effect of water: Molecular dynamics simulation and experimental investigation

The incorporation of crumb rubber (CR) makes the asphalt-aggregate interfacial properties highly complex and uncertain, while water is a key environmental factor causing interfacial failure. This paper aims to evaluate the characteristics of crumb rubber modified asphalt (CRMA)-aggregate interaction and the interfacial failure mechanism under the influence of different material properties and water at ambient temperature. Firstly, the CRMA-aggregate interfacial behavior before and after water intrusion is investigated using molecular dynamics simulations at the molecular scale. Next, the failure process and characteristics of the CRMA-aggregate interface were analyzed using the disk-shaped compact tension (DCT) test at the macro scale. Finally, the effect of water intrusion on the interface failure mode was analyzed through image processing. The results show that CR enhances binder cohesion and significantly affects its adhesion to aggregates with high CaO and MgO contents regardless of the presence of water. The asphalt concentration distribution on the CaO and MgO surfaces is more concentrated with higher peak concentration, leading to superior interfacial adhesion and water damage resistance with binder, while Al2O3 and SiO2 were inferior. The use of asphalt with higher creep stiffness, high-strength alkaline aggregates, and smaller CR particle size is beneficial for enhancing the CRMA binder-aggregate interfacial fracture performance. Water intrusion severely affected the interfacial adhesion, resulting in a significant increase in the proportion of adhesion failure, which is the main cause of the deterioration of the interface properties. The findings of this study provide insights into the factors affecting the CRMA-aggregate interfacial behavior, which facilitate the accurate design and application of high-strength durable CRMA mixtures.

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.

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.

Evaluation of Warm Mix Crumb Rubber Modified Asphalt Mixture (WCRMA) on resisting permanent deformation

Crumb rubber (CR) is generated from the recycling of used or discarded car tires and has long been utilized in the paving industry. Crumb rubber can be used in asphalt mixes in two ways: dry process and wet process. However, compared to standard combinations, crumb rubber modified asphalt (CRMA) requires much higher mixing and compaction temperatures. As a potential substitute, using warm mix asphalt (WMA) technology significantly lower hot-mix asphalt (HMA) production and placement temperatures through viscosity reduction. In accordance, the objectives of the study are to produce warm mix crumb rubberized asphalt mixtures (WCRMA) using a dry process and to evaluate WCRMA’s performance in terms of rutting resistance. WCRMA is produced by adding 1, 3, and 5% Sasobit® of the total weight of the binder. The rutting performance of WCRMA was evaluated by Dynamic Creep test. The result showed that the asphalt mixes with the largest percentages of Sasobit® (5%) showed the lowest lowest accumulated strain and permanent deformation. The modified asphalt mixture performed better and had greater rutting resistance due to the replacement of fine aggregate with crumb rubber and the addition of Sasobit® compared to control sample.

Reconstruction of Asphalt Pavements with Crumb Rubber Modified Asphalt Mixture in Cold Region: Material Characterization, Construction, and Performance

Dry-processed rubberized asphalt mixture has recently attracted a lot of attention as an alternative to conventional asphalt mixtures. Dry-processed rubberized asphalt pavement has improved the overall performance characteristics compared to the conventional asphalt road. The objective of this research is to demonstrate the reconstruction of rubberized asphalt pavement and evaluate the pavement performance of dry-processed rubberized asphalt mixture based on laboratory and field tests. The noise mitigation effect of dry-processed rubberized asphalt pavement was evaluated at the field construction sites. A prediction of pavement distresses and long-term performance was also conducted using mechanistic-empirical pavement design. In terms of experimental evaluation, the dynamic modulus was estimated using materials test system (MTS) equipment, the low-temperature crack resistance was characterized by the fracture energy from the indirect tensile strength test (IDT), and the asphalt aging was assessed with the rolling thin-film oven (RTFO) test and the pressure aging vessel (PAV) test. The rheology properties of asphalt were estimated by a dynamic shear rheometer (DSR). Based on the test results: (1) The dry-processed rubberized asphalt mixture presented better resistance to cracking, as the fracture energy was enhanced by 29-50% compared to that of conventional hot mix asphalt (HMA); and (2) the high-temperature anti-rutting performance of the rubberized pavement increased. The dynamic modulus increased up to 19%. The findings of the noise test showed that at different vehicle speeds, the rubberized asphalt pavement greatly reduced the noise level by 2-3 dB. The pavement M-E (mechanistic-empirical) design-predicted distress illustrated that the rubberized asphalt pavement could reduce the IRI, rutting, and bottom-up fatigue-cracking distress based on a comparison of prediction results. To sum up, the dry-processed rubber-modified asphalt pavement has better pavement performance compared to the conventional asphalt pavement.

Sustainable crumb rubber modified asphalt mixtures based on low-temperature crack propagation characteristics using the response surface methodology

Fracture regime of asphalt concrete is of utmost importance in the sustainable design of optimized mixtures against low-temperature cracking. The energy dissipated in blunting the crack tip, stable crack growth, as well as post-peak resistance of the mixtures to the propagating crack comprise the overall fracture performance of the mixtures. In this research, employing the fracture resistance curve (R-curve) concept, three energy parameters: cohesive energy, fracture energy, and energy rate, are extracted to quantify the crack propagation regime of the mixtures incorporating ground recycled waste tire. A temperature range of 0 °C to −20 °C with 0 %, 10 %, and 20 % crumb rubber contents (CRC) were considered and the experiments were carried out in Single Edge notched Beam (SE(B)) fracture testing protocol. An environmental index comprising CO2 and CH4 emissions, as well as the energy consumption, was developed and the production cost of AC samples were also determined. A design of experiments based on the Central Composite Design (CCD) is applied. The Response Surface Methodology (RSM) is used to develop the best model between the fracture response, environmental factor, cost, and the input mixture properties. Multi-objective optimization scenarios were assessed by the RSM and a 4.74 % binder content with 19.5 % crumb rubber incorporation was resulted as the optimal mix design for maximum fracture response and minimized cost and environmental concerns in low temperatures. The environmental effects and costs were by average reduced by 15 % and 1 %, respectively and fracture responses have been improved compared to the reference optimum mix design (CRC = 0).

Mechanical properties of coarse asphalt Mixture-Paved trackbed under High-Speed moving train loads

The long-term stability of the substructure is a critical guarantee for the safety and ride comfort of high-speed railway (HSR) operations. The asphalt mixture-paved trackbed (APT) can not only play a favorable role in preventing and draining water on the top of the subgrade but also be beneficial to the gradual change of vertical stiffness between layers. This paper determines a coarse but dense-graded asphalt mixture suitable for the APT and numerically analyzes the mechanical properties of APT under high-speed moving train loads. The results indicate that the SBS-modified asphalt mixture (SBS-25) with the optimum asphalt content (OAC) of 4.2 % is more appropriate than the crumb rubber-modified asphalt mixture (CRMA-25) for the APT. The dynamic stress in the subgrade of APT is significantly determined by the conditions of the AC layer. Meanwhile, the vibration attenuation of APT is directly related to temperature change. Compared with the traditional ballastless track, the vibration acceleration of APT with trackbed thickness of 300 mm reduces by 20.1 % at 20 ℃ while only reducing by 1.7 % at 40℃. Moreover, the greater the structural stiffness of the APT, the stronger the control on dynamic deformation of each layer in the subgrade, and the influence on the formation is weaker than that on the bottom layer of the subgrade. The findings hopefully contribute to the APT substructure design; however, further verification of real engineering still needs to be conducted by field or full-scale tests.

Effect of Freeze–Thaw cycles on the pavement performance of SBS modified and composite crumb rubber modified asphalt mixtures

The effect of freeze–thaw cycles on the pavement performance of SBS modified asphalt mixture (SBSMA) and SBS/crumb rubber composite modified asphalt mixture (CCRMA) was studied using a triaxial repeated creep test, semicircular bending fatigue test based on digital image correlation (DIC), and thermal stress restrained specimen test. The mechanical model of the residual strain (RS) of the asphalt mixtures is established, and the residual viscous flow strain (εRvf,N) and residual viscoelastic strain (εRve,N) are separated. Based on the horizontal strain (Exx) of DIC, a new mesofatigue damage variable (D) is proposed. According to the characteristic curve of D, the antifatigue damage parameter (AFDP) is established and the correlation between AFDP and fatigue life (Nf) is analyzed. Simultaneously, a calculation model of the relaxation temperature stress (σRts(T)) of the asphalt mixture at different cooling rates (v) is established based on the finite element method. Results showed that the growth rates of RS, εRvf,N, and D of both asphalt mixtures accelerate, AFDP decreases, and freeze–break temperature (TD) rises with increasing freeze–thaw cycles. However, the growth rates of RS, εRvf,N, and D of CCRMA are always slower than SBSMA, the AFDP is larger than SBSMA, TD is lower than SBSMA, εRve,N is a fixed value under specific conditions, and a good correlation exists between the AFDP and Nf. The pavement performance of the asphalt mixtures worsens after the freeze–thaw cycles, and the influence of a salt freeze–thaw cycles is greater than that of a water freeze–thaw cycles. Moreover, CCRMA has better high-temperature deformation resistance, fatigue damage resistance, and low-temperature cracking resistance than SBSMA. The conversion point temperature (TZ) increases with the acceleration of v, and the σRts(T) calculation model can accurately reflect the σRts(T) of the asphalt mixture at different v.

Research on fatigue performance of composite crumb rubber modified asphalt mixture under freeze thaw cycles

To study the effect of freeze thaw cycles on the fatigue damage characteristics of composite crumb rubber modified asphalt mixture (CCRMA), the semicircle bending fatigue test and the digital speckle correlation method (DSCM) were combined and the fatigue damage characteristics of CCRMA and styrene–butadienestyrene-modified asphalt mixture (SBSMA) under freeze thaw cycles were analyzed. The plateau valve (PV) of the stiffness modulus degradation ratio curve of the asphalt mixture is proposed as the analysis index to evaluate the macroscopic fatigue damage evolution of the asphalt mixture. The horizontal strain density (DE value) obtained through analyzing the horizontal strain characteristic cloud map of the main crack zone is identified as an index to evaluate the mesoscopic fatigue damage evolution of the asphalt mixture. The results show that after a freeze thaw cycle, the PV increases, DE decreases, fatigue damage rate accelerates, and fatigue resistance decreases. CCRMA has better freeze thaw resistance and fatigue resistance than SBSMA. The fatigue damage model established has high fitting accuracy and can effectively reflect the fatigue damage evolution degree of an asphalt mixture. A correlation analysis of PV and DE values reveals that there is a good linear correlation between them, indicating that applying the DSCM method to semicircle bending fatigue tests for studying the fatigue damage characteristics of asphalt mixtures is reliable.

Effect of Temperature Segregation on Crumb Rubber-Modified Asphalt Mixture of Pavement Compaction Degree

Effect of Temperature Segregation on Crumb Rubber-Modified Asphalt Mixture of Pavement Compaction Degree

R-Curve Characterization of Crumb Rubber Modified Asphalt Mixtures Incorporating Warm Mix Additive at Low Temperatures

In a previous research by authors, a methodology was developed to derive J-R curves for Hot Mix Asphalt (HMA) mixtures using an elastic-plastic approach where a comprehensive understanding of crack propagation regime could be achieved. In this research, the effect of crumb rubber modification of HMA binder is studied in terms of R-curves and crack propagation at low temperatures. Mode I Single edge notched beam (SE(B)) fracture tests were conducted in temperature levels of 0 °C, -10 °C, and -20 °C. PG58-22 and PG64-22 binders were used in the fabrication of HMA samples. Modified specimens consist of 20% crumb rubber along with the incorporation of 3% warm mix admixture. Crack growth resistance curves were obtained in SE(B) tests by means of image processing and recording of the progressive crack length. Elastic-plastic J-R curves revealed that crumb rubber modified mixtures exhibit higher crack growth resistance for each bitumen performance grade. As well, increased ductility and cohesive energy can be observed according to the R-curves as the mixtures are modified by crumb rubber.

Predict the Phase Angle Master Curve and Study the Viscoelastic Properties of Warm Mix Crumb Rubber-Modified Asphalt Mixture.

To identify the most accurate approach for constructing of the dynamic modulus master curves for warm mix crumb rubber modified asphalt mixtures and assess the feasibility of predicting the phase angle master curves from the dynamic modulus ones. The SM (Sigmoidal model) and GSM (generalized sigmoidal model) were utilized to construct the dynamic modulus master curve, respectively. Subsequently, the master curve of phase angle could be predicted from the master curve of dynamic modulus in term of the K-K (Kramers–Kronig) relations. The results show that both SM and GSM can predict the dynamic modulus very well, except that the GSM shows a slightly higher correlation coefficient than SM. Therefore, it is recommended to construct the dynamic modulus master curve using GSM and obtain the corresponding phase angle master curve in term of the K-K relations. The Black space diagram and Wicket diagram were utilized to verify the predictions were consistent with the LVE (linear viscoelastic) theory. Then the master curve of storage modulus and loss modulus were also obtained. Finally, the creep compliance and relaxation modulus can be used to represent the creep and relaxation properties of warm-mix crumb rubber-modified asphalt mixtures.

Workability, compactibility and engineering properties of rubber-modified asphalt mixtures prepared via wet process

The purpose of this study is to compare the performance of rubber-modified asphalt mixtures prepared with incorporation of latex and crumb rubber. The service characteristics of the rubber-modified asphalt mixtures were evaluated using workability index and compaction energy index to determine the ease of placing, handling, and compacting of the asphalt mixture. The engineering properties of the asphalt mixtures were also evaluated in terms of indirect tensile strength (ITS), resilient modulus, permanent deformation, moisture susceptibility, and the Leutner shear test. Crumb rubber and latex were separately used as modifiers in this study. The percentage of crumb rubber and latex used were 5% and 10%, while organo silane additive was added at a rate of 0.1%, all by the weight of asphalt binder. Through the evaluation, the modified asphalt mixture required comparable energy for compaction through the assessment of compaction energy index (CEI) as compared to the control asphalt mixture. Based on the results of performance test, it can be concluded that the crumb rubber-modified asphalt mixture showed a better performance than the latex-modified asphalt mixture in terms of fracture resistance, permanent deformation, resilient modulus, shear resistance, and moisture resistance. Overall, the rubberised asphalt mixture has better engineering performance properties that can prolong the lifespan of the flexible pavement compared to the conventional asphalt mixture.

Laboratory and field performance investigation of pre-swollen crumb rubber modified asphalt mixtures

The pre-swollen crumb rubber (PSCR) is a pelletized rubber produced by the reaction of scrap tire rubber particles with bitumen-compatible oil at elevated temperatures. It does not require a prior blending with bitumen, and it can be added directly at an asphalt plant. A laboratory investigation was carried out to evaluate the performance of plant-produced PSCR modified mixtures for surface and intermediate courses and analyze their suitability for low-traffic volume road in cold environment applications. Linear viscoelastic properties, rutting and thermal cracking performance were investigated in the laboratory. The presence of pre-swollen rubber particles and, in particular, the bitumen-compatible oil softened the mixtures as indicated by the dynamic modulus master curves. The different mechanical responses led to an enhancement of thermal cracking resistance but increased the rutting susceptibility of the PSCR mixtures. Despite the laboratory findings, no rutting was observed after five years of service life based on visual inspection of the field test sections constructed in Michigan with PSCR-modified Hot-Mix Asphalt (HMA). A few transverse cracks were generated in the lanes where the control mix with neat bitumen was used. Those cracks did not propagate into the PSCR modified material, in agreement with the laboratory findings.

The Discrete and Continuous Retardation and Relaxation Spectrum Method for Viscoelastic Characterization of Warm Mix Crumb Rubber-Modified Asphalt Mixtures.

To study the linear viscoelastic (LVE) of crumb rubber-modified asphalt mixtures before and after the warm mix additive was added methods of obtaining the discrete and continuous spectrum are presented. Besides, the relaxation modulus and creep compliance are constructed from the discrete and continuous spectrum, respectively. The discrete spectrum of asphalt mixtures can be obtained from dynamic modulus test results according to the generalized Maxwell model (GMM) and the generalized Kelvin model (GKM). Similarly, the continuous spectrum of asphalt mixtures can be obtained from the dynamic modulus test data via the inverse integral transformation. In this paper, the test procedure for all specimens was ensured to be completed in the LVE range. The results show that the discrete spectrum and the continuous spectrum have similar shapes, but the magnitude and position of the spectrum peaks is different. The continuous spectrum can be considered as the limiting case of the discrete spectrum. The relaxation modulus and creep compliance constructed by the discrete and continuous spectrum are almost indistinguishable in the reduced time range of 10−5 s–103 s. However, there are more significant errors outside the time range, and the maximum error is up to 55%.

Decision support for selecting optimal method of recycling waste tire rubber into wax-based warm mix asphalt based on fuzzy comprehensive evaluation

Recycling crumb rubber (CR) derived from waste vehicle tire as asphalt modifier is an effective waste management approach with both engineering and environmental merits. However, the high viscous asphalt rubber (AR) brings higher energy consumption and hazardous construction emission. Wax-based warm mix asphalt (WMA) additive has proven to be able to alleviate these concerns. Application of WMA additives enriches the methods of recycling CR into asphalt. Preliminary investigations documented the effects of CR recycling methods on the service performance of warm crumb rubber modified asphalt mixture (WCRMA). Therefore, making a decision on the optimal recycling method considering the overall service performance is necessary. Nevertheless, there are limited studies focusing on the recycling method assessment and decision-making approach. To fill this gap, WCRMAs with different crumb rubber recycling methods were prepared and tested in laboratory. The effect of the recycling method on the overall service performance of WCRMAs was characterized. A fuzzy comprehensive evaluation (FCE) method was employed to quantify the performance grade of asphalt mixtures according to fuzzy logic. The weight matrix of all the service properties was developed based on the Analytic Hierarchy Process (AHP) under the considering of the circumstance of southern China. The experimental work revealed the significant influence of the recycling method on the service performance of WCRMAs. By means of the FCE method, this study suggested that for southern China with hot and humid climate, the optimal method of recycling CR into wax-based WCRMA is mixing AR and Sasobit first followed by incorporating them into aggregate. In rainy regions, asphalt mixture produced by directly mixing AR, Sasobit, and aggregate at 160 °C may perform better because it showed the best moisture damage resistance and good overall performance.