Cold Recycled Asphalt Mixtures Research Articles

Healing response of cold recycled asphalt mixtures with electric arc furnace slag under microwave heating and re-compaction

The objective of the present paper is the assessment of the healing efficiency of a thermomechanical treatment (microwave heating and re-compaction) on cold recycled asphalt mixtures. The asphalt mixture specimens were fabricated with different electric arc furnace slag (EAFS) contents to improve the mixtures' microwave susceptibility. The healing response was then assessed in terms of recovery of their initial indirect tensile strength. Furthermore, statistical analysis was conducted by means of a three-way analysis of variance to determine the influence of temperature, re-compaction energy, and EAFS content on the healing capability of the mixtures. Although re-compaction and heating in the ranges studied do not themselves cause healing, the combination of both produced excellent healing performance. The optimal content of EAFS was established at 5% of EAFS over the weight of the asphalt mixture.

Design, Placement, and Laboratory and Field Evaluation of Rejuvenated Cold Recycled Asphalt Mixtures

Cold recycled (CR) pavements, the combination of reclaimed asphalt pavement and a recycling agent (foamed or emulsified asphalt) at ambient temperatures, can provide a sustainable method to rehabilitate, reconstruct, and construct roadways. However, CR pavement is not a one-for-one replacement of typical asphalt base mixtures. To utilize CR pavements as an alternative to a typical asphalt base mixture or surface course, additional materials such as rejuvenating agents may be needed. This research focuses on the formulation, plant production, and placement on the National Center for Asphalt Technology pavement test track off-ramp, and laboratory evaluation of three rejuvenated CR mixtures, one foamed asphalt CR mixture, and an engineered emulsion CR mixture. These mixtures were subjected to strength and moisture susceptibility testing, along with the indirect tensile asphalt cracking test (IDEAL-CT) and high-temperature indirect tensile test (HT-IDT). The influence of laboratory production versus plant production and the impact of rejuvenators on mixture performance and density were investigated. International roughness index and rut depth data for the off-ramp sections indicate adequate performance, except for the anionic emulsion with bio-based rejuvenator section. Laboratory test results indicate that the addition of rejuvenators into two of the three CR mixtures increased density and decreased moisture susceptibility and increased the CTIndex by at least 35% compared to non-rejuvenated CR mixtures. HT-IDT strengths revealed that two of the three rejuvenated mixtures performed slightly below the non-rejuvenated mixtures.

A comparative environmental life cycle assessment of road asphalt pavement solutions made up of artificial aggregates

The construction and maintenance of road pavements entail detrimental impacts on the consumption of resources and damage to the natural environment but also make up an opportunity for the large-scale application of circular economy principles and innovative waste valorisation paths. The present study focuses on developing a comprehensive procedure to evaluate the technical and environmental sustainability of replacing high percentage of limestone aggregates with artificial aggregates from municipal solid waste incineration (MSWI) into hot or cold recycled asphalt mixtures for asphalt pavements.The technical feasibility of the designed mixtures was investigated in terms of the main physical and mechanical properties of both the raw materials and the asphalt mixtures with content of artificial aggregates or sand in the range 25–40 % by mass. The environmental feasibility of the asphalt mixtures was evaluated through the SEM-EDS technique, the analysis of the eluate of the leaching test and the ecotoxicity for living organisms.Afterwards, the life cycle assessment (LCA) was applied to detect the critical spots of the life cycle of 1 m2 of a 6 cm-thick binder layer with high percentage of artificial aggregates or sand built and maintained through 30 years analysis period according to 18 impact category indicators.The main results show that, recycling the artificial aggregates into hot asphalt mixtures has on average a negligible effect on the overall environmental performance of the life cycle, and appears to be detrimental only for the consumption of fossil resources due to the higher optimum bitumen content. Looking at the results for cold mixes, the introduction of the artificial aggregates has an effect on the predicted durability of the asphalt layers, which is maximized in the case of coarse artificial aggregates. Consequent environmental benefits regard the global warming potential, fossil resource scarcity and freshwater eutrophication indicators.

Laboratory Characteristics, Performance Evaluation, and Agency Cost Analysis of Cold-Recycled Asphalt Mixtures

Laboratory Characteristics, Performance Evaluation, and Agency Cost Analysis of Cold-Recycled Asphalt Mixtures

Life Cycle Carbon Emissions and an Uncertainty Analysis of Recycled Asphalt Mixtures

The efficient recycling of road materials is an effective way to reduce carbon emissions. Approximately 400 million tons of waste asphalt pavement materials is generated annually in China, but the utilization rate is less than 50%. In this paper, a whole life cycle CO2 emission calculation model for recycled asphalt mixtures is established, and an uncertainty analysis is carried out on the basis of this model. The steps in the recycled asphalt mixture stage with the greatest impact on emissions are identified. The life cycle of asphalt mixtures is divided into raw material production, mixture preparation, material transportation and construction phases based on the carbon emission trends in the four phases. First, recycled asphalt mixtures are considered, and the carbon emissions along a two-way four-lane highway are analyzed based on the relevant carbon emission factor and other known data. The carbon dioxide emissions of each stage are also compared to identify the key links among stages. Then, the carbon emissions of hot-recycled asphalt mixtures are compared with those of ordinary asphalt mixtures and cold-recycled asphalt mixtures in each stage. Finally, an analytical uncertainty study of recycled asphalt mixtures is conducted using uncertainty modeling in different scenarios. Based on the quantitative results of the carbon emission calculations through life cycle assessment (LCA), future measures for reducing carbon emissions from recycled asphalt mixtures are proposed.

Characterization of Cold Recycled Asphalt Mixtures including Reinforcing Fibers

In recent years, cold recycling techniques have been widely used all over the world thanks to their huge environmental advantages. However, their performances are lower than the traditional hot-mix asphalt, both for the longer time to develop the final mechanical properties, which leads to delays in the reopening to road traffic, and for the lower fatigue resistance. The present paper deals with the characterization of cold recycled asphalt mixtures (CRAM), made with 100% reclaimed asphalt pavement, where synthetic fibers were included to improve the fatigue performance. The investigation involved the analysis of the curing time, volumetric properties, stiffness, strength, rheological behavior and resistance to cyclic loading. The results showed that the use of synthetic fiber, with the optimum dosage, determined a higher CRAM performance, especially in terms of fatigue resistance.

Study on the Adhesion Characteristics of Asphalt-Aggregate Interface in Cold Recycled Asphalt Mixtures

The reclaimed asphalt pavement materials have been used for sustainable cold-recycling, significantly benefiting air quality and resource conservation, while its microscale interface properties greatly affected road performance during the cold-recycling processes. In this investigation, the cold recycled asphalt mixtures were prepared with different mixing proportions, considering the effects of cement content and mixing water amount. The adhesion characteristics of the asphalt-aggregate interface in different cold recycled asphalt mixtures have been comprehensively investigated based on the atomic force microscopic (AFM). The microscale morphology and the adhesion properties, including roughness, inclination angle, adhesion force, Derjaguin–Muller–Toporov (DMT) modulus, and dissipated energy were evaluated at the interface and noninterface regions. The micromorphology results showed that the addition of cement could enhance the roughness and inclination in interface regions. In addition, the content of mixed water showed the most significant effect when the water amount was 80%. The results showed that the adhesion force, DMT modulus, and dissipated energy increased with the addition of cement, indicating that the cement could improve the microscale mechanical properties of cold recycled asphalt mixtures. With the increase of mixing water content, the adhesion characteristics showed varied results, and the optimum water content of 80% contributed to the adhesive properties. In addition, the adhesion force, DMT modulus, and dissipated energy were all higher in the interface region by comparing with the noninterface region. The correlations between the macroscale splitting tensile strength and microscale adhesion characteristics of cold recycled asphalt mixtures were established.

Development of a novel infrared methodology to assess the degree of blending in cold and warm recycled asphalt mixtures incorporating a bituminous or bio-based virgin binder

2/3 of the reclaimed asphalt pavement (RAP) amount is used for recycling of European asphalt mixtures. An issue that remains unresolved is the blending between the RAP binder and the new binder. Using infrared microscopy, the binder blending is assessed in low-energy mixtures using a bituminous or bio-based binder. Infrared data were acquired and quantified using average and standard deviation indicators; their evolution is monitored over 3 months. A relationship between the average and the binder concentration is highlighted. Binders homogeneity depends on the RAP content and virgin binder. On cold mixes, infrared data evolve over time, highlighting emulsion curing.

Experimental and numerical simulation study on flow behavior of asphalt mixture using smart aggregate

A meso-scale mechanical analysis approach, which is essentially based on the use of smart aggregate sensor, was employed to inquire the dynamic responses of smart aggregate in cold recycled asphalt mixtures during gyratory compaction, including as well an investigation over the critical influence factors. The discrete element method (DEM) was further applied to study the interaction behaviors of smart aggregate with the surrounding particles. The laboratory test results showed that smart aggregate is an effective method to monitor the contact force and rotation variation between the aggregates, and the changes in the mechanical and physical properties of the mixtures can be characterized by the monitoring results of smart aggregate. In addition, the results of DEM analyses show that the mixtures with 55% coarse aggregates have higher normal force and lower kinetic energy during gyratory compaction, revealing the influence mechanism of the effect of coarse aggregate content on smart aggregate monitoring results at the meso-scale.

Effect of curing regimes on the dynamic modulus of entirely RAP cold recycled asphalt mixture

Curing condition plays a vital role in the performance of cold recycled asphalt concrete technology using cement and emulsion, that is, cement asphalt emulsion mixture (CAEM). Therefore, this article aimed to investigate the changes in the dynamic modulus evaluation (|E*|) of CAEM utilizing 100% reclaimed asphalt pavement (RAP) at different curing conditions. The changes in |E*| of 100% RAP CAEM at curing temperatures of 25°C, 40°C, and 60°C cured at 1, 3, 7, and 28 days were investigated. The results indicated that both curing temperature and curing time influence the test results of |E*|. Accordingly, the value of |E*| increases with the increase of the curing temperature and curing time; however, the rate of increase tends to slow down after seven days. In addition, |E*| values at different temperatures and frequencies showed that the CAEM exhibits the same viscoelastic properties as the traditional asphalt mixture.

Triaxial resilient modulus regression models for cold recycled asphalt mixtures

Cold recycled asphalt mixtures (CRAMs) may present a compressive resilient modulus dependent on the stress state. Currently, there is no widespread consensus on a triaxial resilient modulus (TxRM) model for CRAM analysis. A widely accepted TxRM model would favour the analysis of CRAM’s layered pavement systems. This paper assessed the adequacy of different TxRM models of CRAM’s specimens with different compositions, aggregate gradation and physical characteristics. Based on the literature, three different TxRM models were selected. The models’ non-linear estimation was performed through Levenberg–Marquardt algorithm. Model selection analysis was based on residuals square sum root (RSSR) assessment. Ni et al.’s model was found to be the most adequate for CRAM’s TxRM modelling based on RSSR analysis. The model selection was done regardless of aggregate gradation, binder type/content, air void content, RAP content, TxRM values range, active filler type and curing time of the tested specimens.

Evaluating the evolution of fiber-reinforced emulsified asphalt cold-recycled mixture damage using digital image correlation

ABSTRACT The cold recycled mixture has insufficient resistance to cracking, and the process of fracture is incompletely understood. This study used digital image correlation (DIC) technology and a semi-circular bending test to investigate the anti-crack performances and fracture propagation of basalt fiber-reinforced cold recycled mixture (BFCRM). The load-displacement, full-field strain, full-field displacement, displacement rate and crack propagation map were obtained to evaluate the BFCRM’s anti-crack performances. The control cold recycled mixture without fibers was also prepared for comparison. Meanwhile, the fracture process zone (FPZ) during loading were compared and quantitatively assessed by digital image correlation analyses, and the fiber distribution and microscopic features at fracture surface was observed by scanning electron microscopy. The results show that the formation of microcracks of BFCRM takes more time than that of the control group without fiber. Based on DIC observations, the basalt fibers maintain strong anti-crack performance in cold-recycled mixtures and reduce fracture damage. The fracture surface morphology also revealed that basalt fiber exhibited adsorption, bridging, anchoring, stability, and strengthening effects at micro/nano scale, which further improved anti-crack performance e and hindered crack development of cold recycled asphalt mixtures.

Influence of Fiber Mixing Process on the Cracking Resistance of Cold Recycled Asphalt Mixture

Fiber reinforcement is often used to improve the road performance of cold recycled asphalt mixture (CRAM). The purpose of this research is to evaluate the impact of fiber mixing process on the cracking resistance of CRAM from multiple perspectives. Four kinds of fiber mixing processes, named A, B, C, and D, were designed by changing the order of fiber addition during the mixing process. Then, semicircle bending tests and indirect tensile tests were conducted to characterize the low-temperature cracking behavior of fiber CRAM. Freeze–thaw cycle tests under both dry and water-saturated conditions were performed to investigate the freeze–thaw damage behavior of fiber CRAM. Furthermore, the fiber dispersion in CRAM was observed using scanning electron microscopy (SEM). The results show that the fiber mixing process has a significant effect on the cracking resistance of CRAM. The CRAM specimens prepared by process C have the largest fracture energy, splitting strength, and fracture work, while the specimens made by process D have the smallest value. Specially, the fracture energy of the specimens prepared by process C is 77.23% larger than that of the specimens prepared by process A, while the fracture energy of the specimens prepared by process D is 5.6% smaller than that of the specimens prepared by process A. The reason for this phenomenon is that the fiber is well dispersed in the specimens prepared by process C, which contributes to obtain a better crack resistance. For all CRAM specimens, with the increase of freeze–thaw cycles, splitting strength and fracture work of fiber CRAM decrease. However, there is an obvious difference in the reduction rate of splitting strength and fracture work, especially for the specimens under the water saturation condition. The specimens made by process C have the smallest reduction rate, which indicates a better resistance to freezing and thawing damage. According to the analysis of fiber macro-distribution state in loose CRAM, the fiber dispersion is affected by the humidity conditions in the mixing environment. The best humidity conditions are obtained for fiber dispersion in process C. Based on the SEM observation, the overlapping bridging network structure can be observed in the microstructure of the specimens prepared by process C, allowing the mixture to better transfer and disperse stress.

Research on long-term strength formation and performance evolution with curing in cold recycled asphalt mixture

This paper aims to study the long-term strength and performance evolution of cold recycled mixture with emulsified asphalt (CRME). The specimens of CRME were prepared and cured in the laboratory. The indirect tensile strength (ITS) was measured over curing time (7, 14, 40 days), considering the binder (emulsified asphalt and cement), the RAP properties and the compaction work. The asymptotic regression model was introduced to describe the strength evolution of CRME with curing time. The long-term evolution of the resistance of rutting and cracking, moisture stability and fatigue resistance were also investigated. The results indicate that the Michaelis-Menten model can effectively describes the performance evolution of CRME over time. The aged asphalt in RAP still has the bonding activity and can contribute to short-term strength formation and long-term strength growth of CRME under compaction and curing. But the aging degree of aged asphalt has a negative effect on the strength formation and growth of CRME. The emulsified asphalt provides CRME with greater strength and strength growth potential than cement. The dual effect of cement and emulsified asphalt is most pronounced in the short- and long-term strength evolution of CRME. The higher compaction power, especially in secondary compaction, is beneficial to the short- and long-term strength formation of CRME. The resistance of the mixture to rutting, cracking and fatigue also improves with increasing curing time. Nevertheless, the moisture stability is basically unchanged during the laboratory curing.

Characterization of Residual Moisture Distribution in Cold-Recycled Mixtures with Foamed Asphalt Using Hyperspectral Technology

Cold-recycling technology with foamed asphalt is used widely because it has the incomparable advantages of environmental protection and resource conservation. In this study, cold-recycled mixtures with foamed asphalt were prepared with different gradation types, mixing water contents, and curing times to study the distribution characteristics of residual moisture. An optimum foaming water content of 2.5% and foaming temperature of 165°C were determined for producing the foamed asphalt that was used for the cold-recycled asphalt mixtures. Hyperspectral imaging tests and spectral processing technology were used to obtain the image information and spectral information of residual moisture from 44 specimens; the moisture content of all specimens ranged from 0.25% to 4.70%. Pseudocolor-coding technology was utilized to realize visualization of moisture distribution. In addition, the residual moisture content prediction model was established based on the partial least-squares regression (PLSR) model. Correlations of the residual moisture distribution characteristics and moisture damage resistance evaluation indicators were developed. The residual moisture distribution characteristics and moisture damage resistance are affected significantly by aggregate gradations, mixed water content, and curing time. After 3 days of curing, the strength of the recycled mixture tends to be stable, and the moisture damage resistance can reach the specification requirements, and is suitable for open traffic. It was confirmed that there is a certain correlation between the internal residual moisture content value distribution range and the moisture damage resistance of the cold-recycled mixtures with foamed asphalt. This study could contribute to a better application of cold-recycled asphalt pavement for environmental protection and carbon stabilization.

Void distribution, interfacial adhesion and anti-cracking mechanisms of cold recycled asphalt mixture based on AFM and X-ray CT

To reveal the anti-cracking mechanisms of cold recycled mixture (CRM) prepared by waterborne epoxy resin/styrene butadiene rubber latex (WER/SBR) modified emulsified asphalt (EA), the void distributions in CRM were analyzed by X-ray computed tomography. Then a method for calculating the adhesion work of asphalt-aggregate interface was proposed based on atomic force microscope tests. Finally, the anti-cracking properties were discussed by low-temperature bending and semicircular bending fatigue tests. Results indicated that the surface void contents of CRM specimen were all smaller in the middle and larger at both ends along the height direction. The WER/SBR modifier exerted a significant filling effect on the voids, reducing the void content and size. Moreover, WER/SBR modified EA (WSEA) showed stronger adhesion property and deformation resistance. WSEA-fresh aggregate interface had the largest adhesion work of 267.25 mJ/m2. Further, the interfacial adhesion failure between EA and fresh aggregate, as well as the cohesive failure of the syncretic interface between EA and aged asphalt were the main cracking forms of CRM specimen. A three-dimensional network in the WSEA-aggregate interface was formed by the WER/SBR modifier, improving the interfacial adhesion property. The void distributions and interfacial adhesion property in CRM specimen significantly affected the cracking resistance.

Permanent Deformation Assessment of Cold Recycled Asphalt Mixtures

The objective of this study was to evaluate the behavior in terms of the permanent deformation of four cold recycled asphalt mixtures stabilized with asphalt emulsion and foamed asphalt. The analysis was conducted considering three flow number test procedures adapted for cold recycled mixtures: (i) ABNT NBR 16505 (method A); (ii) adaptation of the FN test for CRAM according to Kim and Lee’s method (method B); (iii) 3D-Move analysis software simulation of pavement structures to address the stress magnitude for the FN test (method C). The influence of the curing process on the performance of the materials was also verified. The study used an extensive database of computed pavement responses from the four different asphalt pavement structures of the experimental section subjected to axial loading of a tandem axle using the 3D-Move pavement response analysis program. The FN results using the 3D-Move data (method C) demonstrated the good mechanical behaviour with respect to permanent deformation of the cold recycled mixtures subject to axial stress and temperature. The method proposed by Kim and Lee (method B) showed little efficacy for the analysis of the CRAMs tested in this work. The ABNT NBR 16505 method (method A), conventionally used for HMA, highlighted the influence of temperature (60°C) on the CRAM mixtures. For all FN test conditions, the curing was a preponderant factor for the performance of cold recycled asphalt mixtures and should be carefully evaluated to simulate the early stage of the mixtures in the field. It is also important to verify the influence of humidity and temperature on the permanent deformation mechanism of cold recycled asphalt mixtures.

Investigating mechanical performance and interface characteristics of cold recycled mixture: Promoting sustainable utilization of reclaimed asphalt pavement

The recycling and sustainable utilization of reclaimed asphalt pavement (RAP) are the key method to improve resource conservation and reduce energy consumption in roadway construction. The cold-recycled technology has been regarded as a promising recycling one because it can achieve mixing at room temperature and high utilization of RAP. Investigating the mechanical mechanism of cold-recycled asphalt mixture (CRAM) is the basis for large-scale application. Thus the splitting tests were carried out to study the influencing law of the material components on the strength performance of CRAM by varying RAP, cement and asphalt content. Then the effects of material component on the strength mechanism were further revealed by the interface characteristics analysis, including void and filler parameters, binder and interface transition zone (ITZ) thickness. The results show that the CRAM with more RAP has a higher splitting strength and the aged asphalt contributes to the reduction of void number around the aggregate interface, indicating a stronger bonding effect with binder. The asphalt mixture has lower void size, void number and porosity than cement mixture, and shows more uniform distribution of binder thickness and lower void distribution difference between the ITZ and the mortar phase, which is conducive to force uniformly and prevent stress concentration within mixture. The above outcomes confirm the effects of strength improvement from RAP and the void increment from cement, and the contents could be reasonably increased for RAP and strictly controlled for cement to ensure strength performance of CRAM while promoting saving energy and environmental protection.

Study on Cold Recycled Asphalt Mixtures with Emulsified/Foamed Asphalt in the Laboratory and On-Site

Millions of tons of reclaimed asphalt pavement (RAP) and reclaimed aggregate or reclaimed inorganic binder stabilized aggregate (RAI) is produced every year in China. The cold recycled mixture (CRM) technology reduces fuel consumption, emissions, and cost and utilizes the high content of RAP. In this paper, six types of CRM with varying RAP/RAI composition and asphalt binders were investigated. The laboratory tests included strength indicators, high temperature stability, low temperature crack resistance, water stability, and dynamic modulus. A full-scale trial section was constructed after the laboratory tests. Except for low temperature failure strain without secondary compaction in the mixture design, test results illustrated that the performances of different CRMs met the specifications. The cement addition limited the thermo-viscoelastic behavior of the CRM. The RAI contents had reduced the water sensitivity of CRM, and the emulsified asphalt CRM had better performance than the foamed asphalt CRM. The performances of samples cored from the test section in the field met the specifications and were lower than that in the laboratory. The curing conditions in the field were not as effective as in the laboratory. The curing conditions and compaction method should simulate the conditions in the field to guide the CRM selection and mixture design.

Fatigue Cracking Evolution and Model of Cold Recycled Asphalt Mixtures during Different Curing Times.

This paper aims to investigate the fatigue cracking evolution of cold recycled asphalt mixtures with asphalt emulsion (CRME) under different curing times. The fatigue cracking model of CRME based on damage mechanics and fracture mechanics was analyzed according to the fatigue loading curve. Firstly, the fatigue cracking evolution of CRME was studied through an SCB strength test and SCB fatigue test. Then, the fatigue damage mechanics were used to establish a nonlinear fatigue cracking model, and the damage degree of CRME at the initial cracking point was determined. The Paris formula was used to characterize the law of fatigue crack propagation. Finally, the microstructure of CRME was observed by scanning electron microscopy (SEM) with the backscattering method. The results indicate that the initial cracking point appears at around 60% of the fatigue life according to the SCB fatigue test by means of image analysis. The damage variable was obtained through the cracking model, and the value of the damage variable was determined as 0.06–0.17 at the initial cracking point. In addition, the Paris formula showed that the crack growth of CRME can be reflected by the stress intensity factor and correlative parameters. Moreover, cement hydration products were mixed with the asphalt membrane to form a denser spatial structure during the curing process, which may provide higher fatigue performance of CRME. This research may provide a theoretical reference for studying the fatigue cracking behavior of CRME.