Recycled Asphalt Mixtures Research Articles

Effect of increasing preheating temperature on the activation and aging of asphalt binder in reclaimed asphalt pavement (RAP)

During preheating, the activation and aging of RAP binders occur simultaneously, and both of them are closely related to the preheating temperature. The objective of this study was to investigate the effect of increasing preheating temperature on the activation and aging of RAP binder, and to find out the appropriate preheating temperature. Three sources of RAP and nine preheating temperatures (110 °C–190 °C) were included in this study. An activation test, producing 100% RAP specimens, was conducted to evaluate the binder activation. The gel permeation chromatography (GPC) test was carried out to estimate the aging level of RAP binder. In addition, the performance of hot-recycled asphalt mixture containing 30% RAP was analyzed by low-temperature splitting test, freeze-thaw splitting test, and rutting test. Ultimately, a parameter called activation-aging index (IAA) was proposed to reflect the coupling effect of activation and aging. The results indicated that increasing preheating temperature had a significant effect on the improvement of binder activation regardless of the RAP source. In addition, there was a critical temperature, beyond which the activation was difficult to be further improved. However, the increase rate of binder aging became more significant with increasing temperature, contrary to the change law of activation. As for the coupling effect, the results of recycled asphalt mixtures showed that increasing preheating temperature improved the rutting resistance, but may damage the moisture susceptibility. The low-temperature performance, however, exhibited a non-monotonic trend and had a good correlation with IAA. It is recommended to take IAA as a reference for finding the optimal preheating temperature.

Recycled asphalt mixture's discrete element model-based composite structure and mesoscale-mechanical properties

To investigate the composite structure and mesoscale-mechanical properties of recycled asphalt mixture, this paper has established discrete element models of recycled asphalt mixture with distinct characteristics of new aggregates, recycled asphalt pavement, new asphalt, and old asphalt according to the coarse aggregate contour characteristic parameters obtained by three-dimensional scanning. The composite skeleton and micro-crack development characteristics of recycled asphalt mixture were also studied. Finally, a method for determining the recycled asphalt pavement content was proposed considering the balance between rutting and crack resistance. The results show that the recycled asphalt pavement content has a negligible impact on the number of contact points of the composite skeleton, and this component primarily impacts the interfacial mechanical properties and contact of asphalt mortar. When the recycled asphalt pavement contents were 0 %, 15 %, 30 %, 45 %, and 60 %, the bottom stresses of the rutting specimens were approximately 0.6, 0.55, 0.5, 0.48, and 0.44 times of the bearing plate stress, respectively. In the composite skeleton, there were compressive, tensile, and tensile–compressive stresses at the contact point of coarse aggregates, the proportions of which were generally 70–80 %, 15–20 %, and 5–10 %, respectively. Shear cracks mainly occurred in the recycled asphalt pavement contact interface area, while tensile cracks occurred at the aggregate–aggregate contact interface, as well as in the asphalt mortar. The number of shear cracks was 50–80 % that of the tensile cracks. The maximum micro-crack length is 8–12 mm, and its minimum length is approximately 0.3 mm with micro-crack generally in the asphalt mortar. The results can provide a reference for the recycled asphalt mixture design to improve road performance.

Developing and Testing of the Principle Prototype for Efficient Micro-Damage Fine Stripping of Asphalt on the Surface of Reclaimed Asphalt Pavement

In the current recycling process of reclaimed asphalt pavement (RAP), due to the serious damage of aggregate gradation and the large amount of aged asphalt still wrapped around the surface of the treated aggregate, the low recycling rate and poor performance of the recycled asphalt mixture are the major problems of RAP. In view of the shortcomings of RAP recycling technology, it is urgent to research new treatment methods and design specialized asphalt-stripping equipment to solve the existing problems. In this paper, based on theoretical analysis and EDEM discrete element simulation, a principle prototype for efficient micro-damage fine stripping of asphalt on the RAP surface is developed and tested. The results demonstrate that the principle prototype has a satisfactory asphalt-stripping effect and achieves fine stripping of aged asphalt on the surface of aggregate without large-scale crushing. This principle prototype has significant engineering application values, which provides design solutions and data support for further equipment development.

An improved regression model to predict fatigue life of asphalt mixes incorporating low to moderate RAP contents

The research on the fatigue performance of recycled asphalt mixture and the establishment of its fatigue life prediction model is the key to its large-scale engineering application. The four-point bending fatigue tests of the fresh and recycled asphalt mixtures under the strain control of cyclic loading were conducted to investigate influencing factors on their fatigue performance and establish a new fatigue prediction model for RAP (recycled asphalt pavement). The hot recycled asphalt mixture samples of AC-16C were prepared by a uniform test design method, with the RAP content of 0%, 20%, 30%, and 40%. Through the fatigue test data of 20 groups of fresh asphalt mixture with 0% RAP and 40 groups of recycled asphalt mixtures with the contents of 20%, 30%, and 40% RAP, the simplified applicability of the fatigue prediction model in the current Chinese specifications for the design of highway asphalt pavement (JTG D50-2017, JTG model) and its parameters modification were studied, thereby an improved regression fatigue prediction model of recycled asphalt mixture is established on consideration of strain level, asphalt content, void ratio, and RAP mixing amount. This fatigue prediction model for recycled asphalt mixtures was proposed by modifying the parameters of the fatigue equation for new asphalt mixtures in the Chinese Highway Asphalt Pavement Design Code. (JTG D50-2017) and comparing the fatigue test data of the recycled asphalt mixture. The research results show that: The simplified JTG fatigue prediction model can effectively predict the fatigue life of fresh asphalt mixture with the content of 0% RAP, and the model parameter a is a constant of the order of 1016; The correction coefficients α and β of the initial bending stiffness modulus S0 and the Voids Filled with Asphalt (VFA) in the improved JTG model of the hot recycled asphalt mixture considering the RAP content are 0.006 and 0.0136, respectively; The model can accurately predict the fatigue life of recycled asphalt mixtures with the range of RAP content of 0–40%, with an average deviation of only 0.106, and has higher prediction accuracy for fatigue life measured close to or higher than 106 times. The average deviation of the predicted life of 20%, 30%, and 40 %RAP fatigue is only 0.057, 0.079, and 0.125. This paper’s indoor fatigue prediction model has higher accuracy for recycled asphalt mixture than the well-known traditional models.

Use of a machine learning-based framework to approximate the input features of an intrinsic cohesive zone model of recycled asphalt mixes tested at low temperatures

Although the cohesive zone model (CZM) provides numerous advancements in simulating the crack initiation and evolution in asphalt mixes, its efficiency and applicability are still challenging. This is because asphalt mixes are principally assumed to be homogeneous in CZM modeling despite intrinsic heterogeneity. Therefore, it is essential to calibrate the CZM model, by adjusting the input factors, aiming at alleviating this inconsistency between the model and reality. Accordingly, this research was aimed at presenting a model to approximate the calibrated input features as a function of a wide variety in testing conditions and mix criteria. To this end, an experimental dataset was collected by investigating the fracture mechanic responses of various recycled warm asphalt mixtures (prepared using three categories using organic and chemical materials, and using foam bitumen technology). Mixes were consisted of virgin aggregates and those containing up to 70% recycled asphalt pavement particles (i.e. 0, 30, 50, and 70%). Mixes were subjected to freeze and thaw cycles (i.e. 0, 1, and 3 cycles) and semi-circular bending test was conducted at subzero temperatures (i.e. 0, −10, and −20℃). Experimental results were analyzed using machine learning (ML) algorithms including random forest, gradient boosting regression, and multi-output regression methods. The results proved the efficiency of the random forest regression model in predicting the required input factors of CZM model. The ML model was also validated by predicting the input factors (of CZM model) for another fracture dataset, demonstrating that this ML model can be promisingly used as an approximation tool.

Evaluation of High-Temperature and Antireflection Cracking Performance of Warm Mixed Recycled Asphalt Mixture

To study the influence of temperature and reclaimed asphalt pavement (RAP) content (0%, 30%, 50%, and 70%) on the high-temperature performance and anti-reflection cracking performance of warm-mix recycled asphalt mixture, triaxial repeated creep tests and overlay tests (OT) based on digital image correlation technology (DIC) were used to analyze the high temperature and anti-reflection cracking performance of recycled asphalt mixtures. Based on the improved Burgers model, a permanent deformation (PD) model was established to evaluate the high-temperature performance of recycled asphalt mixtures. The anti-reflection cracking performance of recycled asphalt was quantitatively analyzed from macro and meso perspectives by using a macro index (load loss rate) and a meso index (vertical strain density DE). In addition, a reflection cracking damage model was developed by cumulative load loss rate relative change rate, and the fractal characteristics of reflection cracks were studied according to the fractal theory. The results show that with an increase in RAP content, the anti-deformation ability of a recycled asphalt mixture is enhanced, and the anti-reflection cracking performance becomes worse. However, with an increase in temperature, the anti-deformation ability of recycled asphalt mixture is weakened, and the anti-reflection cracking performance is enhanced. A PD mechanical model can well describe the high-temperature deformation resistance of recycled asphalt mixtures. The established reflection cracking damage model has high fitting accuracy and accurately describes the reflection cracking damage evolution process of recycled asphalt mixture. In addition, the analysis of the fractal characteristics of reflection cracks shows that the reflection cracks of a recycled asphalt mixture have fractal characteristics of statistical significance. DIC technology can be used to evaluate the anti-reflection cracking performance of recycled asphalt mixtures qualitatively and quantitatively from the mesoscopic perspective, which makes up for the current deficiency in macroscopic evaluation.

The Study on Long-term Rutting Resistance of Compound Hot In-place Recycled Asphalt Mixture Based on MLS11

In order to evaluate the long-term rutting resistance performance of recycled geothermal asphalt mixture, the accelerated loading test of long-term rutting resistance performance of recycled geothermal asphalt mixture was carried out by MLS11 (1/3 ratio accelerated loading equipment) under the environment of high temperature 60°C drying and high temperature 60°C water immersion, and was compared with the newly mixed AC-13 asphalt mixture. The test results show that, after loading 1 million times, the rutting depth of recycled asphalt mixture is 5.34mm and 4.87mm respectively, which is 0.68 times and 0.62 times of that of fresh asphalt mixture, and the rutting creep slope is 0.023mm/ 10,000 times and 0.026mm/ 10,000 times. The rutting depth of the recycled asphalt mixture was 4.22mm and 3.78mm, respectively, 0.76 times and 0.68 times of that of the fresh asphalt mixture, and the creep slope of the rutting was 0.024mm/ 10,000 times and 0.023mm/ 10,000 times. The coarse ore material gradation (small critical sieve pass rate) of the recycled asphalt mixture had the best long-term rutting resistance. In the high temperature 60°C water immersion environment, the edge deformation of the recycled asphalt mixture is 1.3 times and 1.1 times of that of the new asphalt mixture, and the long-term rutting resistance of the recycled asphalt mixture is more affected by the coupling effect of water temperature than that of the new asphalt mixture.

Effects of the addition of fatty acid from soybean oil sludge in recycled asphalt mixtures.

Recycling agents provide better additions of reclaimed asphalt pavement (RAP) in the production of new asphalt mixtures. Alternative and residual materials that have the potential as asphalt binder viscosity reducers have gained visibility in the field of paving due to the perspective of circular economy in recycled mixtures. Soybean oil sludge fatty acid is a material produced from soybean oil sludge, a waste generated in the soybean oil refining step. Thus, this paper investigated the physical, chemical, and rheological effects of the asphalt binder PG 64-XX modified by the fatty acid of soybean oil sludge in the contents of 6% and 7% by weight of the binder. The modified binder samples were submitted to penetration tests, softening point, rotational viscosity, performance grade (PG), before and after short-term aging (RTFO), and multiple stress creep and recovery (MSCR). A control asphalt mixture and recycled asphalt mixtures produced with 40% RAP and fatty acid-modified binders were subjected to tensile strength, induced moisture damage, resilient modulus, and fatigue life. A Student's t statistical test verified the significance of the data, as well as the estimation of production costs of these asphalt mixtures. The use of the fatty acid significantly reduced the stiffness and viscosity of the control asphalt binder, decreasing the mixing temperatures at 14°C and 17°C to 6% and 7%, respectively. Using higher fatty acid contents from soybean oil sludge significantly improved the performance of recycled mixtures in tensile strength, moisture damage, and fatigue life. The production cost of recycled asphalt mixtures was lower than that of the control mixture.

Laboratory and Field Testbed Evaluation of the Performance of Recycled Asphalt Mixture Using High-Penetration Asphalt

The application of recycled asphalt pavement (RAP) on a large scale is highly promoted to meet the current needs of carbon neutrality and sustainable development purposes. However, a majority of RAP mixture productions are currently relying on the restoring effects provided by the rejuvenators. Therefore, the study focuses on the feasibility of using high penetration asphalt binder (HPAB) in RAP mixture as a replacement for conventional rejuvenators. In this study, a recycled asphalt pavement mixture containing HPAB (RAP-H) was developed to resolve the cracking issue of RAP pavement in winter seasons owing to the rigid behavior of aged binders. To verify the applicability of the RAP-H mixture, the results of the quality standard test and mechanical performance test were compared with the reference RAP mixture having rejuvenator (RAP-R). Through the fatigue cracking test, by using Overlay Tester (OT) device, it was found that all specimens did not reach 93% load reduction after 1000 OT cycles, indicating a satisfied stress-bearing capacity. Additionally, the highest dynamic modulus of 27,275 MPa was found in the modified HPAB mixture, and this result is 4.4% higher than that of the reference mix. In the full-scale testbed, the long-term field applicability of the proposed approach was verified through field test construction. The measurement in practice reveals that the elastic modulus of RAP-H back-calculated from the FWD (Falling Weight Deflectometer) test is increased by more than 50% compared to RAP-R, which resulted in excellent performance characteristics of the HPAB pavement layer. In addition to the efficiency in the surface layer, an improved elastic modulus of the sub-base and subgrade layers in the HPAB section was found to be at 28.6% and 19.5%, respectively, compared to the RAP-R mix. In general, the performance of RAP-H satisfied all of the domestic and international quality and performance standards. The field test results confirmed the possibility of field application by showing performance higher than conventional recycled asphalt pavement.

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.

Effects of Rejuvenator Dosage, Temperature, RAP Content and Rejuvenation Process on the Road Performance of Recycled Asphalt Mixture

In this paper, the key technologies in the construction process of hot in-place recycling were investigated in order to improve the utilization rate of waste asphalt mixture; traditional lab tests including penetration, softening point and ductility tests, atomic force microscope test of recycled asphalt under different rejuvenator content, and the test of milling on grading at different temperatures were carried out. The influence of RAP content and rejuvenation processes on road performance were studied, and the low-temperature performance of mixture was analyzed by the energy analysis method, and the evaluation index was proposed. Test results indicated that the penetration and ductility increases, the softening point decrease with the rejuvenator content increasing, and the optimum rejuvenator content is 4%. The optimum mixing and compaction temperature will decrease by 2–6 °C on average for every 10% increase of RAP content by analyzing the mixture volume index. The results showed enhance rutting resistance of the mixture but lower moisture resistance and low-temperature crack resistance by adding the RAP content. The strain energy density of 10 KJ/m3 is proposed to evaluate the low-temperature performance of the mixture, and 30% RAP produces optimal mixture. The higher rutting resistance and moisture resistance can be obtained by using the construction process of RAP+ rejuvenator co-heating, and higher low-temperature crack resistance with RAP+ rejuvenator without heating.

Research on failure strength master curve and fatigue performance of asphalt mixture containing high-proportion reclaimed asphalt pavement

Reclaimed Asphalt Pavement (RAP) is a cleaner production widely used in road engineering. In order to investigate the failure strength and long-term performance of high-proportion RAP asphalt mixture, this research prepared two high-content RAP recycled asphalt mixtures (50% and 60% RAP-RAM) with the gradation of AC-20 based on Styreneic Methyl Copolymer (SMC) regenerant. Firstly, indirect tensile (IDT) strength tests were implemented on 50% and 60% RAP-RAM at four temperatures and six loading rates. Secondly, IDT fatigue tests at six stress levels were carried out. The relationships of strength and temperature, strength and loading rate were obtained based on strength test results. The master curve model of indirect tensile strength and loading rate of high-proportion RAP recycled asphalt mixture was established by linear and Sigmoidal functions. On the basis of the Sigmoidal strength master curve equation, the fatigue stress ratio was calculated. Finally, this paper established the fatigue equation of the high-content RAP recycled asphalt mixture based on the fatigue stress ratio. The analysis results show that 50% and 60% RAP-RAM based on SMC regenerant show favorable strength and fatigue properties. In addition, increasing the proportion of RAP improves the strength of the mix while reducing the loading rate sensitivity of strength. However, the fatigue performance of the mix reduces by increasing RAP content. The fatigue equation based on the fatigue stress ratio takes the factors of temperature and loading rate into account, reflecting the mix's fatigue performance more realistically. The strength and fatigue performance results verify the reliability of the high-content RAP recycled asphalt mixture based on SMC regenerant and provide a reference for the effective utilization of RAP which can generate environmental benefits of saving aggregate and asphalt and reducing energy consumption and carbon emissions.

Study on performance deterioration regularity of hot regenerated asphalt mixture under multiple aging factors

Several elements, including ultraviolet light, humidity, temperature, vehicle load, etc., have an impact on the asphalt pavement concurrently during the road service process, gradually diminishing the road’s service level. The self-created Life Cycle Analysis System (LCAS) was employed to simulate the cumulative effects of multiple aging components during the service process. The freeze–thaw splitting test, dynamic creep test, and notched semi-circular bending test were conducted after the hot recycled and the fresh asphalt mixtures were subjected to varying durations of simulated aging. The results indicate that initially, the road performance of the hot recycled asphalt mixture is comparable to the fresh asphalt mixture. However, the fresh asphalt mixture outperforms the hot recycled mixture in all but water stability over time. The attenuation degree of the flow number (FN) differs by 22.5% between the two kinds of mixtures, while the fracture energy differs by 53.3%. The performance of the hot recycled asphalt mixture degrades much more rapidly than that of the fresh asphalt mixture. The LCAS method can well simulate the actual service environment of asphalt pavement constructed with RAP materials and provide help for the performance evaluation of RAP pavement in road engineering and subsequent related research.

Study on performance of recycled asphalt mixture based on blending state analysis of virgin and aged asphalt

ABSTRACT The blending state of virgin and aged asphalt is one of the key factors that influence the cracking resistance and fatigue performance of recycled asphalt mixtures. In this paper, the complex modulus of the virgin and aged double-layered asphalt samples was measured by DSR experiments, and the degree of blending (DOB) was calculated based on the complex modulus. The cracking resistance and fatigue performance of the mixture prepared under different blending conditions were tested and related to the DOB to obtain a prediction model. Results showed that the DOB and the complex modulus G* increased with blending time and temperature. The blending effect of modified asphalt was not as good as 70-pen asphalt. The contribution of blending time and temperature to DOB was 40% and 50%, respectively. The influence of DOB on cracking resistance and fatigue performance of recycled asphalt mixtures was consistent with DSR test. Good correlations between DOB, cracking resistance and fatigue performance were observed, which were 79.14% and 82.82%, respectively. Based on the DOB model, optimal blending temperature and blending time could be found according to the softening point difference between virgin and aged asphalt.

Study of Fatigue-Damage Characteristics of Recycled Asphalt Mixture

To investigate the influence of methyl styrene block copolymers (SMC) modifier on the fatigue performance and damage evolution of recycled asphalt mixture (RAM), RAM with reclaimed asphalt pavement (RAP) ratios of 30%, 50%, and 70% and 90# asphalt mixture were used for four-point bending fatigue test. The fatigue performance and damage evolution process of RAM were analyzed using the stiffness attenuation rate and the relative change rate of dissipated energy (RDEC). Linear amplitude sweep (LAS) tests and atomic force microscopy (AFM) tests were carried out for mixed asphalt in the mixture. The results show that under the same test conditions, with the increase of RAP content, the damage development of RAM is faster, and the fatigue damage resistance is worse. Compared with 90# asphalt mixture, RAM has better fatigue resistance. The fatigue property of the mixed asphalt is better than that of the 90# asphalt. With different RAP contents, the modulus changes of RAM and mixed asphalt are almost the same on macro and micro scales, which proves that the difference of fatigue performance between 90# asphalt mixture and RAM probably is caused by asphalt. A fatigue failure criterion based on the peak dissipation energy is proposed for the LAS test, and the applicability and accuracy of the criterion were proved by grey correlation analysis. Under this criterion, the power function model is introduced to describe the development process of fatigue damage in LAS test, which has high fitting reliability.

Viscoelasticity of Recycled Asphalt Mixtures with High Content Reclaimed SBS Modified Asphalt Pavement

For the concerns of investigating the viscoelastic properties of recycled asphalt mixtures incorporating high content reclaimed styrene-butadiene-styrene (SBS) modified asphalt pavement (RAP-SBS), asphalt mixture performance tester (AMPT) was applied to analyze the dynamic modulus and phase angle of recycled mixtures by the influence of RAP-SBS content, temperature, loading frequency, long-term aging (LOTA), and the incorporation of a rejuvenating agent. Master curves of recycled asphalt mixture regarding dynamic modulus and phase angle are developed, and the viscoelastic properties of recycled mixtures within a wide frequency range are characterized with the Christensen–Anderson–Marastean (CAM) model. Eventually, the one-way analysis of variance (ANOVA) was applied to investigate the role of factors on the viscoelasticity of recycled mixtures. The research indicates that (1) the elastic component of recycled mixtures elevates with the increasing of RAP-SBS content and loading frequency; as a result, the high-temperature stability of it enhances, while it is prone to cracking at low temperatures; (2) RAP-SBS content should be selected according to specific characteristics of pavement. For most cases, a content of 50% is recommended; (3) the recycled mixtures incorporating high-content RAP-SBS mixed with a rejuvenating agent has outstanding aging resistance performance; (4) RAP-SBS content is observed to have a significant influence on the viscoelasticity of recycled 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.

Effects of recycling agents and methods on the fracture and moisture resistance of asphalt mixtures with high RAP contents

The design guidelines for recycled asphalt mixtures recommend rejuvenating the oxidized binder by adding recycling agents, which are viscosity and, consequently, temperature-reducing additives. As it is a component of recent use in pavement and its variety, there has yet to be a consensus on the determination of the recycling agent content or the ideal moment of its addition in the recycled asphalt mixture production process. This study aimed to analyze the fracture and moisture performance of asphalt mixtures with the addition of high contents of RAP (25%, 50%, 75%, and 100%) and two recycling agents (ADCAP WM and residual engine oil) in stages distinct from the production of recycled asphalt mixture (directly to the RAP and directly to the virgin asphalt binder). The recycling agents were added at 12%, 15%, and 18% residual engine oil by mass of RAP and 2%, 2.5%, and 3% of ADCAP WM by the total mass of asphalt binder. The asphalt mixtures were submitted to mechanical tests of tensile strength, resilient modulus, ice and thaw cycles, bending fracture, and fatigue life. The results showed that adding residual engine oil in the asphalt binder enables using up to 75% of RAP. In the case of ADCAP WM additions, identical additions of RAP can be performed. Mixtures with ADCAP WM showed higher consumption of fracture energy, while blends with residual engine oil showed a lower slope of the post-peak curve. Conditioning by the freeze–thaw (F-T) cycle method increased the susceptibility of mixtures to the deleterious effect of water. Statistical analyzes showed that the parameters RAP content, recycling agent, and time of addition of the recycling agent presented adequate significance for the analyzed tests. The maximum RAP content to be applied should be 75%, and adding ADCAP WM directly to the binder showed slightly better results than the other combination.

Exploration of the design theory of crack-resistant rejuvenator for warm-mix recycled asphalt mixtures with high RAP contents

The warm-mix technology and high reclaimed asphalt pavement (RAP) contents applied in asphalt mixture can effectively reduce the use of energy resources and waste materials disposal. To realize the performance requirement of warm-mix recycled asphalt mixture, it is significant to develop specific warm-mix rejuvenators with satisfactory performance. Diffusion and crack-resistance properties are the key factors combined with engineering characteristics for rejuvenator design. In this research, a new type of asphalt rejuvenator for warm-mix recycled asphalt mixture with high RAP contents was designed. Then, the molecular properties of the key ingredients were explored according to asphalt aging theory and molecular diffusion simulation results. The results indicate that the diffusion group should contain linear alkanes or small alkanes, aromatics with a low aromaticity, or molecules with weak or no polar groups and low cohesive energy. Rejuvenators and corresponding mixture were then prepared, followed by performance testing. The results show that the strength of the recycled mixture increased during rejuvenator diffusion, and polymer addition to certain rejuvenators effectively improved the interface performance of the asphalt fusion layer. The fine aggregate mortar beam test results showed that this approach represents a simplified test method to evaluate the low-temperature performance of rejuvenators. The overall performance of the designed rejuvenator is satisfactory and fully meets the requirement of the surface layer of provincial highways in most areas of China. The application of this technology has good environmental and economic benefits.

Improving the Mechanical Properties of Recycled Asphalt Pavement Mixtures Using Steel Slag and Silica Fume as a Filler

Due to its environmental and economic advantages, the use of recycled materials in asphalt mixes is witnessing increased interest, where the properties of those mixes are significantly affected by the properties of the recycled materials in them. This paper discusses the results of an experimental study conducted to evaluate the performance of recycled asphalt mixtures made with reclaimed asphalt pavement aggregate (RAP). These mixtures were also prepared with two filler additives, namely steel slag (SS) and silica fume (SF), at four different percentages by weight of the aggregate. A total number of 234 mixtures were tested. The laboratory results indicated the effectiveness of using such additives as a filler material. The Marshall stability showed improvement for mixes prepared with steel slag ranging from 11.73 to 32.73 kN as the RAP level increased; the highest stability load was recorded for the 75% RAP with a 50% steel slag mix. On the other hand, the silica fume depicted variance in its strength, yet the maximum load value of 31.02 kN was for the 75% RAP with 100% silica fume. The use of steel slag in the presence of water decreased the stability results, while satisfying the ASTM standards.