Hot Recycling Research Articles

Effect of silicon carbide powder on asphalt material properties and microwave-induced self-healing

Silicon carbide (SiC) powder, an excellent microwave-absorbing material, can serve as a filler in asphalt mixtures. This study compares the particle size distribution, phase composition, pore structure, specific surface area, and electromagnetic properties of SiC powder and limestone powder. Asphalt mastic and mixtures were prepared with SiC powder replacing mineral powder at ratios ranging from 0 % to 60 %. The study examines how differences in particle size gradation, mineral composition, pore structure, and specific surface area contribute to deformation resistance and improve mechanical properties through micro-level interlocking and cementation. Electromagnetic parameter variations affect microwave heating performance, with uniform distribution of absorbing fillers ensuring heating homogenization. SiC powder also enhances the self-healing index, reaching 74.4 %, and maintaining a maximum healing index of 68.1 % after four damage-healing cycles. These findings support the practical application of SiC powder in asphalt concrete, enhancing maintenance efficiency and advancing microwave heating for self-healing and hot recycling.

Evaluation of Cracking Resistance of SMA-13 Hot Recycling Asphalt Mixtures Reinforced by Basalt Fiber.

In the context of green and low-carbon development, energy saving, and emission reduction, hot recycling technology (RT) has been researched, which is divided into hot central plant RT and hot in-place RT. However, due to the aged asphalt binders, the shortcomings of hot recycled asphalt mixtures have become apparent, as in comparison to new asphalt mixtures, their resistance to cracking was inferior and the cracking resistance deteriorated more rapidly. Therefore, it was very necessary to focus on the improvement of crack resistance of hot recycled asphalt mixtures. Basalt fiber has been proved to be able to effectively improve the comprehensive road performance of new asphalt mixtures. Therefore, this paper introduced basalt fiber to hot central plant recycled and hot in-place recycled asphalt mixtures, in order to improve the crack resistance of asphalt as a new type of fiber stabilizer. Firstly, six types of SMA-13 fiber asphalt mixtures were designed and prepared, i.e., hot mixtures with basalt fiber or lignin fiber, hot central plant recycled mixtures with basalt fiber or lignin fiber, and hot in-place recycled mixtures with basalt fiber or lignin fiber. Secondly, the trabecular bending test, low-temperature creep test, semi-circular bending test, and IDEAL-CT were used to comparatively study the changing patterns of low and intermediate temperature cracking resistance of hot recycled mixtures with conventional lignin fibers or basalt fibers. Finally, Pearson's correlation coefficient was used to analyze the correlation of the different cracking resistance indicators. The results show that the low and intermediate temperature cracking resistance of hot central plant recycled mixtures increased by 45.6% (dissipative energy ratio, Wd/Ws) and 74.8% (flexibility index, FI), respectively. And the corresponding cracking resistance of hot in-place recycled mixture increased by 105.4% (Wd/Ws) and 55.7% (FI). The trabecular bending test was more suitable for testing the low-temperature cracking resistance of hot recycled asphalt mixtures, while the IDEAL-CT was more suitable for testing the intermediate-temperature cracking resistance. The results can provide useful references for the utilization of basalt fiber in the hot recycling of SMA-13 asphalt mixtures.

Effect of RAP's preheating temperature on the secondary aging and performance of recycled asphalt mixtures containing high RAP content

Plant-mixed hot recycling technology is widely used. Due to concerns about the road performance of recycled asphalt mixtures with high RAP content, the content of RAP is strictly limited, which has negative impact on ecological and economic benefits. Low preheating temperature of RAP material is an important reason for reduced performance of recycled asphalt mixtures containing high RAP content. At the same time, excessive preheating temperature of RAP materials can cause secondary aging of asphalt in RAP. This study regarded the preheating temperature as a variable. Rheological and spectroscopic tests such as DSR and BBR, and FTIR tests were conducted to investigate the effect of RAP preheating temperature on the secondary aging of RAP. Afterwards, the influence of raising preheating temperature of RAP on the performance of recycled asphalt mixtures was explored synthetically. When the preheating temperature is below 160 ºC, there is no significant secondary aging of aged asphalt in RAP. However, when the preheating temperature increases to 180 ºC, the rheological properties of aged asphalt undergo certain changes. Increasing the preheating temperature of RAP can significantly improve the low-temperature crack resistance and fatigue performance of recycled asphalt mixtures by reason of maximally activating the aged asphalt. Increasing the preheating temperature of RAP does not weaken the high-temperature anti-rutting performance of recycled asphalt mixture. It is recommended to control the preheating temperature of RAP at 160 ºC in the design and production process of hot recycled asphalt mixtures.

Analysis of the grouting fullness and road performance of hot-recycled semi-flexible pavement materials

In order to enhance the utilization efficiency of reclaimed asphalt pavement (RAP) and contribute to sustainable waste management, the implementation of hot recycling technique is employed to produce semi-flexible pavement materials. This research endeavor aims to scrutinize the performance characteristics of semi-flexible pavement materials manufactured through hot recycling. To attain this objective, the grout construction issue is simulated with the aid of ABAQUS software, enabling a comprehensive investigation into the impact of various load positions, surface areas, and grout heights on the horizontal and bending features of the pavement. The findings of the study indicate that the judicious incorporation of reclaimed asphalt pavement (RAP) into the hot recycled asphalt mixture as a semi-flexible pavement material exhibits an acceptable grouting rate, outstanding resistance to high temperatures, water stability, and fatigue resistance. Such exceptional properties render these materials qualified for usage in road surfaces under heavy traffic conditions, elevated summer temperatures, and other challenging utilization scenarios [1]. Moreover, it is demonstrated that higher grouting fullness contributes to superior road performance characteristics in the context of semi-flexible pavement.

Laboratory Evaluation of the Effectiveness of Rejuvenation in Multiple Hot Recycling of Asphalt Mixtures

Abstract Because of the widespread use of reclaimed asphalt (RA) in new asphalt mixtures, multiple RA recycling is expected to become a common practice. Thus, the long-term properties of multiaged RA as well as the effectiveness of multiple-cycle rejuvenation of RA need to be properly investigated. Given this background, the present study was aimed at evaluating at laboratory scale the feasibility of multiple hot recycling of RA at both short-term aging (STA) and long-term aging (LTA) conditions. The effectiveness of rejuvenation of multiple recycled mixtures was also assessed. In particular, the effect on mixture performance of the rejuvenator addition at different recycling stages was evaluated both after mix rejuvenation and at the end of laboratory-simulated LTA. To these aims, the evolution of linear viscoelastic properties and tensile strength of a plant-produced asphalt mixture has been investigated over a series of short-term and long-term simulative aging cycles carried out with or without the addition of a rejuvenator product. The experimental findings mainly showed that recycling effect (i.e., stiffening and embrittlement) seems to stabilize over cycles with an effective rejuvenating contribution because of the recycling agent. Nevertheless, a specific design of the rejuvenator content considering the actual RA properties instead of requiring a fixed dosage based on RA binder content is only necessary because the recycling history undergone by RA (i.e., number of recycling cycles and use of rejuvenator over the different cycles) clearly affects the final properties of the recycled mixes.

Influence of the Hot-Mix Asphalt Production Temperature on the Effectiveness of the Reclaimed Asphalt Rejuvenation Process

Hot recycling of reclaimed asphalt pavement (RAP) into new hot-mix asphalt (HMA) is a complex process that must be precisely calibrated in the asphalt plants. In particular, temperature is a key parameter that, if inadequately set, can affect the final mix performance as it influences the RAP binder mobilization rate and the severity of bitumen short-term aging. The present paper aims at evaluating the effect of HMA production temperature on the behavior of mixtures including 50% of RAP and two types of rejuvenating agents. In particular, volumetric, mechanical, chemical, and rheological properties of the mixes and binder-aggregate adhesion have been investigated on the HMA produced in the laboratory at 140 °C or 170 °C. The results showed that the adoption of a lower production temperature did not significantly influence the air voids content in the mix, but determined a less stiff, brittle and cracking-prone behavior. Moreover, the decrease of the HMA production temperature was profitable for the increase of bitumen-aggregate adhesion.

Experimental application of fully recycled asphalt concretes produced with chemical additives for patch pavement rehabilitation

The reuse of recycled materials, such as Reclaimed Asphalt Pavement (RAP), is one of the most widespread and sustainable alternatives to continuous exploitation of raw materials in the road pavements sector. Over the years, numerous studies have been conducted on the use of RAP in the production of Hot Mix Asphalt (HMAs) in different percentages and with several bituminous binders and additives. At the same time, little attention was paid to the possibility of using this recycled material as HMA for small road maintenance interventions. Cold Mix Asphalt (CMA) is still the material widely adopted for potholes repairs and patch pavement rehabilitations, despite its lower performance if compared to HMA. The idea developed in the present research is the possibility of using the hot recycling technology for the production of patch asphalt for potholes and utility cuts repairs. In particular, the focus of the study was the development of an HMA produced with 100% RAP and without the addition of bituminous binders. Thus, three different chemical additives were tested for the experimental recycled HMAs. Promising results have been obtained in the laboratory characterization, showing that the recycled materials can have physical and mechanical performances comparable to HMA produced with virgin aggregates and bitumen.

Study on the blending behavior of asphalt binder in mixing process of hot recycling

The blending behavior of aged and virgin asphalt binder is one of the most critical problems in the mixing process of hot recycling. To accurately measure the participation degree of aged asphalt and optimize the mixing process of hot recycling, a series of laboratory tests were designed by using magnetite aggregate and thin-layer chromatography with flame ionization detection (TLC-FID) for the first time. In this study, 24 different lab-mixing conditions were deliberately adopted to investigate the impact on the DoB of aged and virgin asphalt binders, including mixing amount of RAP, mixing amount of rejuvenating agent, mixing time, mixing temperature, mixing orders and the thickness of asphalt film. The results indicated that the aged and virgin asphalt cannot be completely blended in the mixing process and DoB is only about 40–60 % in general. The DoB decreases with the increase of asphalt film thickness and it decreases by 30 % when asphalt film thickness is 9 µm. DOB increased with the content of RAP and rejuvenating agent up to 70 % which same as mixing sequence. Although the mixing time and temperature can promote DOB, the mixing time and temperature should be controlled within 240 s and 170 °C, otherwise the accuracy of DOB will be affected.

Quantifying the Effect of Hot Recycling on Asphalt Mixtures Based on Rod Thin-Layer Chromatography

Abstract In this study, a four-component test of aged asphalt, new asphalt, recycled asphalt on the surface of recycled old aggregate, and recycled asphalt on the surface of recycled new aggregate has been conducted based on the rod thin-layer chromatography (TLC)-hydrogen flame ion detection method to evaluate the colloidal stability index (Ic). Furthermore, the gelation index (GI) and recycling rate (P) of reclaimed asphalt pavement are defined. These parameters act as quantitative indicators of the recycling effect on the asphalt mixes and are used to characterize the degree of integration of the aged and new asphalt. The rationality of the evaluation indexes is verified by analyzing the effects of preheating, mixing temperature, and mixing time on the recycling effect. The results show that the proposed method based on rod TLC can effectively evaluate the thermal recycling efficiency of asphalt mixture, and it provides useful insights on the recycling effect of thermal recycling technology.

Effect of Bitumen Production Process and Mix Heating Temperature on the Rheological Properties of Hot Recycled Mix Asphalt

Heavy traffic loads require the replacement of damaged pavements, so a huge amount of reclaimed asphalt pavement (RAP) material is now available and must be recycled in order to avoid landfill and to achieve both environmental and economic benefits. The most common and profitable solution to reuse RAP is associated with the hot recycling technique, as it allows recovering both solid and binding components of RAP. Several factors influence the performance of hot recycled mix asphalt (HRMA). Among those, this paper focuses on the role played by the origin of the virgin bitumen, i.e. the oil-distillation process, and by the mixing temperature adopted during HRMA production. The objective was to evaluate the rheological properties of mixtures produced using a high amount of RAP (50%), two different rejuvenators, two mixing temperatures (140 °C or 170 °C) and two neat bitumen types derived from different distillation processes (visbreaker and straight-run). The results showed that the addition of RAP led to an increase in the dynamic modulus and a decrease in the phase angle, while the use of rejuvenators partly tended to rebalance these characteristics. The visbreaker bitumen showed a higher sensitivity to short-term aging than the straight-run, determining higher mix stiffness and lower viscous features. The higher mixing temperature also determined an increase in the complex modulus and a reduction in the phase angle as a result of the higher mobilization of the aged bitumen from the RAP.

Features of the hot recycling method used to repair asphalt concrete pavements

Abstract The recycling methods used in the construction and repairing of asphalt concrete pavements are being constantly improved, and the improvements mostly fall under one of the following common avenues of innovation: developing new binders based on bitumen and cement; developing new varieties of asphalt concrete and other materials having an equivalent utility and function; and developing additives that can be used in the production of new types of binders that can enhance the performance properties of the pavements. This article aims to develop the composition and determine the physical-mechanical and structural-rheological properties of asphalt concrete reclaimed by the hot recycling method and reinforced by fiber of fly ash from thermal power plants (TPP). The author of this article developed a mechanism for the interaction between fiber and bitumen in asphalt binder and acquired an optimum composition of hot granular asphalt concrete. During the research, the author evaluated the utility of fiber used as an additive in reclaimed asphalt concrete, studied its effect on the properties of hot reclaimed asphalt concrete, and examined the technological and performance properties and durability of the material obtained. The fiber of fly ash used in the hot recycling method made it possible to reduce the cost and ensure the high quality and durability of the structural layer of road pavement. Our experiments with analyses of the obtained composition indicated that employing it in the construction of the structural layer of road payment would result in superior structural integrity. Hot recycling made it possible to obtain thick bound layers characterized by the homogeneity of the material. For the first time, the author studied the effect of using fiber of fly ash from Chinese TPP in hot reclaimed asphalt concrete, and the results have proved the rationality of using this composition.

Effect of Waste-Oil regenerant on diffusion and fusion behaviors of asphalt recycling using molecular dynamics simulation

In field asphalt hot recycling project, the diffusion and fusion of virgin and aged asphalt generates the weak interface under stress concentration. In this study, the representative components of waste cooking oil (WCO), waste vegetable oil (WVO) and waste engine oil (WEO) regenerant were simulated by molecular dynamics method. The influence of waste oil regenerant on the diffusion and fusion of virgin and aged bitumen was evaluated by calculating the diffusion rate, fusion amount, relative concentration in fusion area. The viscosity and solubility parameters were calculated to explore the driving mechanism of waste oil regenerant promoting fusion diffusion. The microstructure of asphalt was analyzed to determine the key components of waste oil to achieve regeneration effect. The results showed that the waste oil regenerant can promote the diffusion of virgin and aged bitumen by reducing the viscosity of aged bitumen. The promotion effect of WCO and WEO was better than WVO. When the content is higher than 6%, the promotion effect of WCO will be reduced. In terms of promoting fusion, the regeneration system containing WEO was the best. For chain molecules, the depolymerization effect of molecules (Oleic acid, Trilaurin, and Linoleic acid) with CIS structure was not as good as that of molecules (Palmitic acid, γ-Linolenic acid, N-Docosane, Eleostearic acid, and Stearic acid) with trans structure. Molecules with trans structure generally increased the binding distance between asphaltenes by 36%. The fat molecules in WCO will form new aggregates with asphaltene. Based on molecular dynamics models, the micro driving mechanism of waste-oil recycling agents on renewing the various properties of aged bitumen was demonstrated.

Laboratory investigation on blending process of reclaimed asphalt mixture

With the extensive application of hot recycling technology in maintenance of asphalt pavement, there is a concern on the blending between reclaimed asphalt pavement (RAP) binder and virgin binder. Two main stages, viz. the mobilization stage of RAP binder and the diffusion stage of virgin binder, in the blending process between RAP and virgin binder are investigated in this study. In order to quantify the mobilization rate of RAP binder, a blending chart relating RAP binder content to carboxyl index (CI) of blended binder was established by using Fourier transform infrared spectrometer (FTIR). The effects of RAP particle size, RAP temperature and mixing sequence were taken into account in laboratory experiment. The diffusion of virgin binder was studied by using the tracing method. Cobaltous (Co) naphthenate solution as a tracer was blended with the virgin binder. Based on the distribution of Co element in RAP binder detected by scanning electron microscope equipped with energy dispersive spectrometer (SEM/EDS), an index was proposed to quantify the diffusion degree of virgin binder. The obtained results showed that the proposed indicators could well reflect the variation of mobilization rate and diffusion degree under different test conditions and can be used to comprehensively evaluate the blending efficiency between RAP binder and virgin binder.

Towards very high RAP content asphalt mixes: A comprehensive performance-based study of rejuvenated binders

Hot recycling of reclaimed asphalt pavement (RAP) from replacement of deteriorated asphalt pavements is a widespread technique, and able to ensure clear economic and environmental benefits. For this reason, paving industries, agencies and researchers are spending significant efforts in order to maximize such advantages by increasing the amount of RAP within new asphalt mixtures. Nevertheless, adequate performance of recycled mixtures must be guaranteed despite the re-use of high amounts of RAP which involves the presence of aged, oxidized and stiffened binder. In this perspective, the present study aimed at evaluating the rheological properties at mid-service and high-service temperatures of bituminous blends composed by 40% of virgin binder and 60% of aged rejuvenated bitumen (simulating the aged hard bitumen coming from the RAP). Such proportion was selected to represent a high recycling rate of RAP (about 60% of the total weight of aggregates, RAP included). A chemical commercial additive at the liquid state was selected as rejuvenator; it was dosed at the optimum content based on empirical and rheological preliminary characterization. The performance of the recycled blend was then compared to that of a virgin reference bitumen. Dynamic shear rheometer tests were carried out to determine the master curves and the failure characteristics (at mid and high service temperatures) of the selected binders. The effect of aging (related to the durability over the material lifetime) was also studied analyzing binders’ responses in unaged, short-term and long-term aged conditions. Overall, experimental findings demonstrated the efficacy of the rejuvenation of recycled binders containing very high amount of aged bitumen. • Rejuvenation of aged RAP binder was able to restore original rheological properties. • Dosage of rejuvenator was crucial to restore adequately the binder properties. • Rejuvenated blend showed comparable or enhanced performance than the virgin binder.

ДОСЛІДЖЕННЯ СПОСОБІВ ПОВТОРНОГО ВИКОРИСТАННЯ ДОРОЖНЬО-БУДІВЕЛЬНИХ МАТЕРІАЛІВ ПРИ РЕМОНТІ ДОРОЖНІХ ОДЯГІВ КАПІТАЛЬНОГО ТИПУ

Introduction. The paper considers the issues of increasing of road construction materials reuse when repairing of major type pavements, by simultaneous use of hot and cold recycling technologies (in the plant and on the road). Problem statement. At many road construction objects, especially when repairing roads of state importance, designing engineers meet the problem of the need in a significant amount of bituminous materials. There is a question of how to provide such a quantity of these materials and do it economically efficient. Research has shown that it is necessary to use hot and cold recycling technologies economically efficiently. Purpose. Searching for the possibility of arranging of foundation layers, by simultaneously applying of hot and cold recycling technologies. Research methods. Analytical and experimental with the use of computer simulation of a multilayer structure. Results. It has been determined that the upper layers of the foundation can be strengthened by asphalt concrete manufacturing by hot recycling technology (in the plant), and the lower layers by cold recycling mixtures using different types of binders and manufacturing methods (in the plant and on the road). Conclusions. The conducted research allowed to confirm the thesis about the effectiveness of the solution to restore the strength of the capital-type road pavement. The obtained solution will reduce the cost of pavement repair and solve the issue of bituminous materials utilization, which will positively affect the environmental safety of the region.

RETRACTED: Research on Fatigue Prediction Model of Asphalt Mixture with High RAP Content

Asphalt pavement hot recycling technology can improve resource utilization efficiency, reduce energy waste and CO2 emissions, and bring huge economic and social benefits, further promoting the realization of “carbon peak and carbon neutrality”. This paper uses single logarithmic regression fitting on the fatigue failure life of recycled asphalt mixture with high RAP content to characterize the influence of RAP content (R), stress ratio (C), loading frequency (H) and test temperature (T) on the fatigue life of recycled asphalt mixture. Subsequently, the relationship between fatigue life and stress ratio of recycled asphalt mixture, under the influence of different RAP content, loading frequency and test temperature was analyzed. The results showed that the fatigue life of recycled asphalt mixture decreased with an increase in RAP content, stress ratio and test temperature, and decreased as loading frequency increased. Fatigue life and stress ratio were found to be approximately logarithmic, and based on the relationship between fatigue equation parameters a, b and R, T, H, the fatigue prediction model for recycled asphalt mixtures with independent variables R, C, H, and T was established.

Fatigue characteristics and prediction of cement-stabilized cold recycled mixture with road-milling materials considering recycled aggregate composition

During road maintenance, a large volume of milling materials is produced, including asphalt pavement milling mixtures (AMM) and cement-stabilized base-course milling mixtures (CMM). AMM has been widely used in the hot recycling and cold recycling for asphalt pavements. CMM is mainly used in the cold recycling with asphalt and cement for the base-course, while it is not always appropriate for all grades of highways because of high cost of asphalt materials. The cold recycling using CMM with pure cement, namely the cement-stabilized cold recycling mixtures (CSCRM), is alternative considering a high utilization of CMM and relatively low construction costs. However, there are few studies on this recycling technique, especially on fatigue behaviour and the effects of recycled aggregate (RA) composition. Therefore, the effects of RA content, RA composition, and stress level on the fatigue life of the CSCRM using CMM with pure cement were revealed via 40 sets of the splitting fatigue tests in this study. Finally, a fatigue equation considering RA content, RA composition, and stress level was established to predict the fatigue life of the CSCRM under different cement contents.

Research on the Maximum Amount of Reclaimed Asphalt Pavement Based on Balanced Design of Performance and Economy

By controlling the variability of Reclaimed Asphalt Pavement (RAP) and achieving the goal of high content, it is proposed that the milling speed of RAP should be controlled at 5 ∼ 7m / min, the moisture content should not be greater than 2%, and there should be at least two levels of pretreatment technology. The maximum RAP content of AC-25 hot recycling is 50% according to the grading calculation curve under the condition that the current standard grading range and powder binder ratio are not greater than 1.3. At the same time, according to the grading calculation of the project, the empirical formula of coarse and fine aggregate mix to realize the balanced design of utilization and economy is put forward.

Study on the Performance of the Physical Foaming Warm-mix Recycled Asphalt Mixture

In order to evaluate the performance of the physical foaming warm-mix recycled asphalt mixture (WRAM), the hot mix plant recycling technology was adopted to develop AC-20 hot-mix recycled asphalt mixture (HRAM) and the WRAM of different reclaimed asphalt pavement (RAP) contents. The high temperature stability, water stability and low temperature crack resistance performance of the reclaimed asphalt pavement were evaluated. The results indicate that the physical foaming WRAM feature better high temperature stability, water stability and low temperature crack resistance compared to those of HRAM. With the increase in RAP contents, the high temperature stability of the physical foaming WRAM and HRAM rises notably but the water stability and low temperature crack resistance performance continues to drop.

Study on the Blending Behavior of Aged and Virgin Asphalt in Process of Hot Recycling

Study on the Blending Behavior of Aged and Virgin Asphalt in Process of Hot Recycling