Microwave Healing Research Articles

Microwave healing properties and moisture sensitivity of asphalt mixture containing iron powder filler

Through experimental tests, this paper investigated the effectiveness of microwave healing on asphalt mixtures containing iron powder (IP) filler to improve durability and enhance mechanical properties. The moisture sensitivity of the asphalt mixes with varying iron powder filler contents was measured using the modified Lottman test. For evaluating the asphalt mixture healing ability, two complementary methods were used: the fatigue-based method, derived from the ability of microwave healing of asphalt mixture samples damaged up to 50% level of indirect tensile fatigue (ITF), and the fracture-based method, obtained from the strength recovery rate of broken semi-circular samples by applying consecutive breaking-healing cycles. The results showed that the greater the quantity of iron powder filler in the asphalt sample, the greater the sample’s indirect tensile strength and TSR ratio, indicating that IP had a positive effect on moisture sensitivity. The findings also indicated that utilizing iron powder as a filler positively strengthens the fatigue life, increases the toughness, and increases the microwave healing indices (both fatigue and failure approaches). Finally, according to the study results, substituting 70% iron powder as the filler is the most suitable option for improving the mechanical and self-healing characteristics of asphalt mixes.

Study on self-healing mechanism of magnetic nano-powders functionalized fiber reinforced thermoplastic composites

At present, a variety of methods have been developed to realize the self-healing function of fiber-reinforced composites. However, the intrinsic mechanism of material damage recognition and self-healing initiation still lacks systematic research. In this paper, GF/(FeNip/PP) composites with multistage structures were prepared to study the self-healing properties and the mechanism of fiber-reinforced thermoplastic composites. The addition of a small amount of FeNip without affecting the melting peak temperature of the PP matrix (i.e., without reducing the use temperature of PP) can increase the electromagnetic loss (tanδ) of the composites by about 40% and the thermal conductivity by 36%. The cyclic stretching times of GF/(FeNip/PP) composites after microwave healing are doubled at 50% to 60%σmax load. It is indicated that FeNip endows the GF/PP composites with good microwave healing performance, and the purposeful repair can effectively extend the service life of the material. In addition, the self-healing mechanism of GF/(FeNip/PP) composites is discussed in detail from the aspects of microstructure change, multiphase interface effect, entropy enthalpy effect, starting and ending of self-healing procedures, etc. The magnetothermal effect, “entropy consumption” effect, and polymer chain diffusion are the fundamental reasons for the self-healing function of fiber reinforced thermoplastic composites.

Self-healing properties of steel slag asphalt mixture based on experimental characterization and 3D reconstruction

Asphalt pavement is susceptible to cracking which affects its service life. And self-healing technology helps to heal micro-cracks in mixture at an early stage, but its mechanism of action at the mesoscale is still unclear. Therefore, the composition of steel slag asphalt mixture (SSAM) was designed, microwave heating performance, healing performance and road performance of SSAM were investigated by laboratory experiments. Finally, the SSAM was scanned using X-ray CT for 3D reconstruction and quantitative analyses. The results show that steel slag has obvious magnetic loss characteristics; the surface temperature of SSAM designed by composition is higher under microwave heating to obtain a better healing rate, and the road performance of SSAM meets the specification requirements. The damage-healing process mainly affects voids in SSAM, and microwave healing can reduce the fractal dimension of voids to reduce the proportion of fissures; from the healing index based on voids, self-healing can reduce the average coordination number and throat number of connected voids, thereby improving the performance of SSAM by reducing permeability. The self-healing performance of SSAM is improved through its compositional design; the mesoscopic model based on void characteristics helps to refine the theory of self-healing of asphalt mixtures under microwave heating.

A 3D printed continuous carbon fiber reinforced composite with function of self-detecting and self-healing of internal damages

Realizing the self-detecting and self-healing of internal damage is of great significance for ensuring the safe service and extending the service life of carbon fiber reinforced (CFR) composites. Here, a CFR thermoplastic composite composed of “long CFs - short CFs” was designed and prepared through a hybrid 3D printing method. Firstly, by multi-threaded laying and hybrid printing of conductive continuous CF inside the material, it shows that there is good consistency between the mechanical properties and conductive properties induced by internal damages, providing an important non-destructive detection method for the determination and danger warning of internal damages in the material. Secondly, the printed “long CFs - short CFs” combinated structure can induce a synergistic effect in the coupling between the composite and microwave. This synergistic effect can avoid the ignition of long CFs and the local melting of short CFR polymer, thereby making it possible to realize the non-contact microwave healing of local damages. The healed material significantly improves the toughness while ensuring almost initial strength. This study is of positive significance for achieving the 3D printing of multifunctional implementation of CF composites with “nondestructive detecting - accurate positioning - self-healing” of damages.

Microwave Heating Healing of Asphalt Mixture with Coal Gangue Powder and Basalt Aggregate

Microwave heating is an effective method to achieve autonomic crack healing in asphalt mixtures, and the use of microwave-absorbing materials can largely improve this healing efficiency. As a solid waste, coal gangue contains metal oxides, which shows the possibility of microwave heating. In order to further promote the application of coal gangue in the microwave healing of asphalt mixtures, this study looks into the synergistic effect of basalt and coal gangue powder (CGP) on the microwave heating self-healing of an asphalt mixture. The mechanical performance, water stability, low-temperature crack resistance and microwave healing efficiency of the asphalt mixture were investigated using the immersion Marshall test, standard Marshall test, Cantabro test and semi-circular bending (SCB), and healing tests, respectively. The results indicated that the addition of CGP in asphalt mixture can improve the microwave heating speed, which also showed a significant advantage in water stability and fracture energy recovery. The research results will further promote the utilization rate of coal gangue.

X-ray computed tomography analysis of internal voids in steel slag asphalt mixture under freeze–thaw damage and microwave healing process

In this work, the variation characteristics of different scale voids under freeze–thaw damage and microwave healing were studied by X-ray computed tomography (CT). Firstly, the scale classification of voids was carried out to analyze the initial distribution state of voids. Then the distribution of the horizontal plane and vertical direction was quantitatively characterized, and the source of the damage-healing effect was analyzed by using iterative variation indexes. Finally, the critical void was judged by combining the total and relative iterative variation rates. The results show that the area of small voids (0.15 ∼ 2.0 mm2) accounts for 55.7 %, and the number accounts for 90.17 % of the whole, serious interference with overall development when voids are not classified. The initial distribution of voids at different scales is quite different. The void growth is relatively uniform in the horizontal plane. From the distribution of the number of voids, the inner region is more prone to coherent vertical cracks, and the outer region is more prone to discrete micro-cracks. The role of voids at different scales in the damage-healing process is different. Small voids are the critical voids in the freeze–thaw damage stage. Both small and big voids are critical voids in the healing stage. This work is helpful to better understand the damage-healing process of steel slag asphalt mixture at mesoscale and contribute to the sustainability of asphalt pavement.

Research on maintenance equipment and maintenance technology of steel fiber modified asphalt pavement with microwave heating

In this article, the 166TYHB road microwave maintenance vehicle was used as the main equipment, aiming at the investigation of the application characteristics of microwave healing technology on asphalt pavement and proving the effective of microwave heating technology in practice. The heating performance and healing behavior of steel fiber modified asphalt pavement under microwave irradiation were analyzed. The maintenance technology of asphalt pavement was designed and optimized, and the corresponding relationship between the maintenance effect of 800 W microwave oven in the laboratory test and the maintenance effect of 90 kW road maintenance vehicle in practical construction was established. The test results showed that with the curing of microwave irradiation, the steel fiber modified asphalt pavement presented an excellent healing performance, and the heating rate of the surface of asphalt pavement could reach as high as 26 °C/min. Meanwhile, as there is no need to apply another raw materials during the maintenance stage, the Pavement Condition Index(PCI) of asphalt pavement could be improved to a high degree, which is much beneficial to maintain the comfortability of pavement.

Characterization of asphalt mixture using X-ray computed tomography scan technique after freeze-thaw cycle and microwave heating

• Recommend the optimum healing moment of different asphalt mixtures suffering from moisture damage. • HEL and SRL are introduced to compare the self-healing properties of different asphalt mixtures. • There are three moisture damage and microwave healing modes of asphalt mixtures on pores. • Valid pores and invalid pores are proposed to characterize the internal pores of asphalt mixtures. • The self-healing properties and characterization of different asphalt mixtures are compared. Microwave heating technology could heal micro-cracks caused by freeze-thaw (F-T) cyclic damage. The main objectives were to recommend the optimum heating moment and evaluate the difference between two typical mixtures during F-T cycle and microwave heating based on microstructure. To this end, cyclic F-T split tests and microwave heating tests were conducted to determine the best heating moment. In addition, the internal structure was scanned using X-ray computed tomography and evaluated using digital image processing. The results indicated that the optimum timing to heal two different asphalt mixtures (AC-13 and OGFC-13) occurred when the degrees of moisture damage were 39.8% and 42.4%, respectively. AC-13 exhibited better heating uniformity, and OGFC-13 exhibited better heating transferring. Moreover, F-T damage and microwave heating could change the internal structure: three types of moisture damage were seen in the voids: pore generation, connections, and expansion; pore number, porosity, pore area, and equivalent diameter first increased after the F-T cycles and then decreased after microwave heating, but the effect for OGFC-13 was more obvious; The F-T cycles mainly increased the number of valid pores, and microwave healing mainly increased the number of invalid pores; The damage and healing process of AC-13 and OGFC-13 mainly occurred on the 0.15–1.18 mm and 0.15–4.75 mm pores. This research could contribute to maintenance cost savings and revelation of F-T damage and microwave healing mechanism.

Self-Healing Potential of Porous Asphalt Concrete Containing Different Aggregates and Metal Wastes Through Microwave Heating

Asphalt is known as a self-healing material due to its temperature-related flow behavior. When damage occurs in asphalt concrete, bitumen can flow into cracks with heat and provide recovering by filling the cracks. Much of the current literature on self-healing pays particular attention to artificially heat asphalt concrete by several methods, including microwave heating. Although there is a growing body of literature that recognizes using steel fibers in microwave healing asphalt, there are no data on the use of aluminum fibers to improve microwave healing. This paper evaluates the microwave healing potential of asphalt concrete that contains metal wastes such as aluminum and steel shavings. Besides, the use of basalt and limestone aggregate (BA and LA) in asphalt concrete were evaluated. To achieve this, firstly, microwave absorption capabilities of BA and LA were determined. In the second step of the study, porous asphalt concrete specimens were prepared, and they were damaged by the indirect tensile (IDT) strength test at low temperatures. After that, damaged specimens were healed via microwave heating, and they were damaged again by the IDT strength test. In the end, the healing index of the specimens was determined as the proportion of healed specimen’s strength to the original. It has been found that BA is almost three times more microwave absorber material than LA, and asphalt specimens containing BA showed better healing performance. It has also been demonstrated for the first time that aluminum shavings might be a better option than steel shavings to accelerate microwave healing.

Laboratory Investigation of Microwave Healing Characteristic of Hot Mix Asphalt Incorporating Steel Slag and Fly Ash

Abstract: This research aims to analyze the usage of fly ash (FA) as filler and Steel slag as coarse aggregate in hot mix asphalt concrete to monitor its healing characteristic under microwave heat. Both fly ash and steel slag are the industrial by-products. Using Marshall, Hot mix asphalt (HMA) mixtures were prepared for control and three different modified specimens having 10%, 20% and 30% steel slag as coarse aggregate, and 75% optimum fly ash content (OFC) as filler. Then thermal distribution and optimum healing time of test specimens were recorded using infrared thermometer. A three-point bending (TPB) tests were applied to the samples before and after the microwave healing procedures. Test results demonstrated that adding steel slag and fly ash in asphalt concrete mixtures increases the heating rates and improve the healing performance of hot mix asphalt using microwave heat. The replacement of natural coarse aggregate by 30% steel aggregate is extremely promising since it has not only improved healing results but also enhance the load-displacement relationship of the HMA mixtures with more ductile behavior. Overall, the use of steel slag and fly ash in hot mix asphalt is a significant option that helps to the sustainable pavement construction as it improves healing and cracking resistance. Keywords: Fly ash, Steel slag, Hot mix asphalt, Microwave heating, Three-point Bending

Effect of specific heat capacity of aggregates and nano-graphite on self-healing of hot mix asphalt under microwave radiation

This paper aimed to investigate the effect of nano-graphite (NG) as a conductive additive and specific heat capacities of aggregates on the self-healing performance of the asphalt mixtures. In this research, three types of aggregates, including Limestone, Granite, and Dolomitic Limestone, with different contents of NG (5%,10%, 20%, and 30% by the weight of bitumen) were used. The indirect tensile tests (IDT) were applied to evaluate the microwave healing capability of broken samples. The healing index (HI) analysis showed that the optimum amount of NG was 10% by weight of bitumen, which restored tensile strength in healed samples above half of their original strength. Also, it was found that among the mixtures containing optimum NG, the sample with Granite aggregate consumes 16% and 30% less heating energy than samples with Dolomite Limestone and Limestone aggregates, respectively. In addition, results indicated that the HI of broken samples in the 24-hour rest period at ambient temperature has increased by 15 to 30 % compared to the 12 h.

Effect of palm oil capsules on the self-healing properties of aged and unaged asphalt mixtures gained by resting period and microwave heating

Palm oil was used as a new rejuvenating agent for manufacturing calcium alginate capsules to study the self-healing properties of aged and unaged asphalt mixtures. To optimize the palm oil capsule for use in the asphalt mixture, nine types of calcium alginate capsules with various oil-to-water (O/W = 5, 10, and 15 %w) and calcium chloride-to-water percentages (CaCl2/W = 2, 4, and 6 %w) were prepared, and compressive strength, thermal sensitivity, size measurement, density measurement, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) tests were performed on them. Asphalt mixtures with various percentages of optimum capsules (0, 0.35, and 0.7% by total weight of the mixture) were prepared in unaged and long-term-aged conditions, and the effect of capsules on the stiffness modulus of aged and unaged asphalt mixtures was investigated by the indirect tensile stiffness modulus (ITSM) test. Via the semicircular bending (SCB) test, self-healing with a resting period and microwave healing were used to investigate the self-healing properties of aged and unaged asphalt mixtures with and without capsules. To reduce heating time and energy consumption, the effect of incorporating steel wool fibers (SWF) on the microwave heating of palm oil capsule-containing asphalt mixtures was investigated. The results showed that the use of palm oil capsules significantly increased the self-healing capability of the aged and unaged asphalt mixtures. The microwave healing method promoted the healing speed and healing level of the capsule-containing aged and unaged asphalt mixtures. Moreover, with the use of SWF in microwave healing, a healing level similar to that of SWF-less asphalt specimens was obtained in a shorter heating time.

Characterization and microwave healing properties of different asphalt mixtures suffered freeze-thaw damage

Asphalt materials display the inherent ability to heal micro-cracks and the microwave heating technique can help accelerate this process thus promoting the creation of a durable asphalt pavement. The objective of this study is to investigate the difference in healing potential between two representative types of asphalt mixtures, namely dense asphalt concrete (AC-13) and open-graded friction course concrete (OGFC-13), suffered freeze-thaw (F-T) damage. In particular, OGFC concrete is especially susceptible to water damage due to its high percentage of air voids. The effect of gradation and F-T damage on the healing performance of asphalt mixtures is appraised by means of three-point bending test, freeze-thaw splitting test and X-ray computed tomography test. Results demonstrate that the gradation exerts a clear influence on the microwave healing behavior and microwave absorber content. In addition, the mechanical strength of mixtures characterized by F-T damage can be partially restored with microwave heating treatment: the healing efficiency of OGFC asphalt specimen after three cycle F-T cycles is still satisfactory. As probed with X-ray computed tomography, the proportion of micropores increases, while the amount of mesopores and macropores decrease after the microwave treatment. However, thermal heating technics are not suitable for further healing when the structural deformation of the OGFC sample occurs due to the F-T damaging process. The research outcomes can contribute to understand the microwave healing behavior for different asphalt mixture types and characterize the healing mechanism of mixtures suffered F-T damage.

Microwave heating properties of steel slag asphalt mixture using a coupled electromagnetic and heat transfer model

Microwave heating technique is a promising asphalt pavement maintenance method with minimal external intervention. In order to employ this technology in real engineering applications, massive research efforts have dealt with avoiding overheating and non-uniform heating issues. Microwave heating frequency and power are two relevant inputs which deserve a thorough investigation. Coupled electro-magneto-thermal modeling of microwave heating is an innovative tool to understand the system performance. In this study, numerical simulations are performed to investigate the effect of the mentioned parameters on the electromagnetic and thermal fields developed inside steel slag asphalt specimens. Results illustrate that microwave heating is highly sensitive to frequency. 2.45 GHz is the optimal frequency ensuring the most uniformly distributed electromagnetic and thermal fields, and can achieve the maximum energy efficiency; furthermore, a larger input power contributes to higher temperature and better microwave healing performance. In addition, numerical simulations are compared with experimental findings for the frequency equal to 2.45 GHz and for these power values: 120 W, 280 W, 460 W and 700 W. The research characterizes the mechanism of microwave heating process and documents a good agreement between numerical and experimental findings. Furthermore, the investigation identifies the electromagnetic and thermal fields induced in asphalt mixtures and thereby contributes to optimize the microwave applicators.

The Prospect of Microwave Heating: Towards a Faster and Deeper Crack Healing in Asphalt Pavement

Microwave heating has been shown to be an effective method of heating asphalt concrete and in turn healing the damage. As such, microwave heating holds great potential in rapid (1–3 min) and effective damage healing, resulting in improvement in the service life, safety, and sustainability of asphalt pavement. This study focused on the microwave healing effect on porous asphalt concrete. Steel wool fibres were incorporated into porous asphalt to improve the microwave heating efficiency, and the optimum microwave heating time was determined. Afterwards, the microwave healing efficiency was evaluated using a semi–circular bending and healing programme. The results show that the microwave healing effect is largely determined by the steel fibre content and the mix design of the porous asphalt concrete.. Besides, the uneven heating effect of microwave contributes to an unstable damage recovery in the asphalt mixture, which makes it less efficient than induction heating. However, microwaves exhibited the ability to penetrate further into the depth of the test specimen and heat beneath the surface, indicating deeper damage recovery prospects.

Improving microwave healing characteristic of asphalt concrete by using fly ash as a filler

The paper aims to evaluate the usability of fly ash (FA), which is an environmental pollutant, as a filler in asphalt concrete in order to accelerate its microwave heating rate to promote self-healing. In the study, microwave heating rates of asphalt concrete specimens with and without FA were determined via thermographic imaging. Then the three-point bending test was applied to specimens before and after the microwave healing procedure. The crack size of the damaged specimens was also measured via a crack microscope in order to determine the effect of damage levels on microwave healing performance. Test results demonstrated that crack sizes both before and after healing could be interpreted as an indicator of the self-healing performance of the specimen. Microwave heating temperature is the key factor of self-healing, and FA has improved microwave heating rate of asphalt concrete. FA has also improved the healing capability of specimens at some damage levels. It can be concluded that fly ash has great potential as a filler in self-healing asphalt concrete because it can improve self-healing and mechanical characteristics of asphalt concrete, and FA could be reclaimed by using this way.

Crack propagation and microwave healing of Ferrite-filled asphalt mixes based on image correlation observations

One of the most important features of asphalt mixes is their ability to undergo self-healing after certain time intervals. Increasing the temperature can accelerate the healing process. In this study, the heating process will be triggered by using microwave technology when ferrite powders is added in asphalt mixes. Fracture tests were performed on open-graded friction course (OGFC) asphalt and ferrite-filled asphalt mixes under three-point bending with different heating and resting time periods. To examine the improvements in the microwave healing and fracture response of ferrite-filled asphalt mixes, the fracture process zone (FPZ) was characterized and quantified through digital image correlation. The FPZ lengths of the specimen with and without ferrite powder were compared. The asphalt specimens including ferrite powders are proved to have a relative longer FPZ length providing an improved cohesive response and a higher cracking resistance.

Effect of moisture and freeze-thaw damage on microwave healing of asphalt mixes

Microwave heating/healing is a kind of maintenance technique to repair micro-cracks in asphalt mixes. This method was conducted mostly on unaged materials, and the effects of moisture and freeze-thaw damages have not completely been studied. The fluidity of the asphalt binder, accomplished by heating, increases the ability of binder to fill micro-cracks and heal the asphalt mixes. Moisture and freeze-thaw damage increase the modulus of asphalt binder resulting decrease of fluidity of material. In contrast, moisture and freeze-thaw damages could reduce the ability of mixes to heal. In this research, mixes were prepared using neat and activated carbon modified asphalt binders. In order to assess the microwave heating properties, mixes were subjected to different levels of moisture and freeze-thaw conditions. The microwave heating performance was investigated under electromagnetic radiation. It was found that activated carbon is a proper binder-based additive that increases electromagnetic absorption of asphalt mixes. Indirect Tensile (IDT) and Semi-Circular Bending (SCB) specimens were fabricated, and different levels of moisture and freeze-thaw cycles were applied. For quantification of microwave healing capability of damaged asphalt concrete mixes, the cyclic fracture-healing tests were conducted on IDT and SCB specimens. Individual and combined effects of mechanical damage, fracture temperature, notch length, moisture, and freeze-thaw cycles on microwave healing performance of asphalt concrete mixes were investigated. Results showed that moisture and freeze-thaw damages have adverse effects on microwave healing performance of asphalt concrete mixes. Increased critical factors caused further detrimental effects on microwave healing performance of mixes.

Microwave Healing Performance of Asphalt Mixture Containing Electric Arc Furnace (EAF) Slag and Graphene Nanoplatelets (GNPs)

Pavement preventive maintenance is an important tool for extending the service life of the road pavements. Microwave heating seems to be a promising technology for this application, as bituminous materials have the potential to self-repair above a certain temperature. As ordinary asphalt mixture has low microwave absorbing properties, some additives should be used to improve the heating efficiency. In this paper, the effect of adding Electric Arc Furnace (EAF) slag and Graphene Nanoplatelets (GNPs) on the microwave heating and healing efficiency of asphalt mixtures was evaluated. Microwave heating efficiency was assessed by heating the specimens using several heating times. In addition, the electrical resistivity of the mixtures was measured to understand its possible relationship with the microwave heating process. Furthermore, the healing rates of the asphalt mixtures were assessed by repeated Indirect Tensile Strength (ITS) tests. The results obtained indicate that the additions of graphene and EAF slag can allow important savings, up to 50%, on the energy required to perform a good healing process.

Microwave loss percolation effect and microwave self-healing function of FeNip/PP nanocomposites

Based on the study of microwave loss percolation effect of FeNip/PP nanocomposites, the microwave healing technology of FeNip/PP nanocomposites was proposed and its mechanism was studied. There is a unique microwave loss percolation characteristic between the microwave loss factor (tanδ) of FeNip/PP nanocomposites and the content of FeNi powders (0–20 wt%), and the percolation area is the range of 2–10 wt%. The microwave healing technology can realize the self-healing property of the PP-based composites. When the composites received the microwave signal, the magnetic nanopowders generate heat effect to heat the resin matrix, which forms a mobile phase. The mobile phase, melted PP matrix with some magnetic nanopowders, moved toward the microcracks, thereby healing or reducing microcracks inside the composite. The optimum time for microwave treatment was 15 min. After microwave healing, the microcracks of the composites were closed, and the tensile strength of FeNip/PP nanocomposites can be restored to 90% of that before damage.