1. Introduction
Highway and airfield pavements represent a significant element of transportation infrastructure and play essential roles in national economies, while facilitating societal progress (Lizasoain-Arteaga et al., 2019). In order to build road infrastructure and/or restore pavement smoothness and other surface characteristics, asphalt concrete has been widely used by owner-agencies such as state departments of transportation (DOTs). Asphalt mixtures are generally flexible and ductile, readily constructed, and remarkably durable. Asphalt pavements eventually begin to display deterioration modes, or distresses, during its service life resulting from traffic loads and harsh environmental conditions, such as extreme temperatures, moisture, aging, and solar radiation. In addition to permanent deformation (rutting), these factors can result in various forms of cracking in asphalt concrete, such as thermal cracking in cold regions, fatigue cracking under cycle traffic loads (Li et al., 2018a), and block cracking when severe aging occurs (Franesqui et al., 2017). Thus, cracking is a driving force behind reduction in service life and increased maintenance costs. As a result, detecting micro- and macro-cracks in asphalt concrete and attempting to heal them have gained substantial attention in pavement engineering in recent years. A viable solution to achieve this goal involves reversing damage induced in asphalt concrete via an efficient material (García et al., 2013a), heating technique (Dai et al., 2013), and healing technique (Norambuena-Contreras and Garcia, 2016).
Asphalt concrete exhibits viscoelastic, thermoplastic, and temperature dependent mechanical behavior. As the temperature increases, the stiffness of asphalt concrete decreases, and it becomes more prone to permanent (viscoplastic) deformation or rutting distress. Moreover, as the temperature decreases, thermally induced stress builds up, leading to thermally induced cracking in the asphalt layer (Jahanbakhsh et al., 2018). Regardless of these deficiencies of temperature dependency of asphalt concrete properties, these special characteristics can enhance some features of asphalt concrete. Some urgent phenomena which can be improved through increasing the temperature of asphalt concrete are induced healing by induction coil (Norambuena-Contreras and Garcia, 2016) and microwave (Wang et al., 2016, Wang et al., 2016, Wang et al., 2016), ice melting (de-icing) by induction coil (Liu et al., 2020d) and microwave (Gao et al., 2019), blending of reclaimed asphalt pavement (RAP) with virgin binder by microwave (Terrel and Al-Ohaly, 1987) and quick heating (Xu et al., 2018, Xu et al., 2018), and compaction of asphalt concrete by induction coil (Zhou et al., 2017) and microwave (Bueno et al., 2018).
Asphalt concrete is known as a self-healing material, which can repair micro-cracks when enough rest time is provided between loading cycles. When two edges of a crack in an asphalt concrete are brought in close proximity, the crack has the opportunity to heal and regain lost strength if the asphalt is able to flow and create cohesion across the surfaces. This is referred to as crack closure (Bommavaram et al., 2009). The healing process in asphalt concrete is an intricate phenomenon, which is considerably influenced by activation energy in the asphalt binder, capillary flow through the cracks, self-diffusion of molecules across the crack interface (Qiu et al., 2011), and varies depending on cracking mode, material modification, and confinement (García et al., 2012a). This viable characteristic can be further enhanced as the temperature of cracked asphalt concrete increases and its liquid binder behaves more like a Newtonian fluid. The increase in asphalt temperature under an external energy source is referred to as induced heating (García et al., 2012b) and the ability to heal cracks due to the flow of asphalt binder is called induced healing (Karimi et al., 2018). In this method, improving the electrical conductivity (García et al., 2012a) and enhancing the sensitivity of asphalt concrete to the electromagnetic field (Wang et al., 2016) have been proposed as the most efficient way to heal the cracks.
The level of temperature obtained through induced heating is the most critical factor in the induced healing process (Moreno-Navarro et al., 2017). Induced heating will be a function of heating source, material composition (i.e., aggregate, asphalt binder, and choice of conductive additive), and aggregate structure. To promote asphalt concrete induced heating, different technologies (heating sources) have been attempted. The most frequently used approaches to accelerate the induced healing are through the use of induction heating coils containing steel wool (Liu et al., 2011) and steel grit (Gómez-Meijide et al., 2018b), microwave radiation containing steel wool fiber (Norambuena-Contreras and Garcia, 2016), metallic filler (Franesqui et al., 2017), flake graphite (Wang et al., 2018), and activated carbon (Amani et al., 2020), and infrared radiation containing metal particles (Gómez-Meijide et al., 2016) and metal grit (Ajam et al., 2017). Since conventional asphalt concrete requires a great deal of time and energy to significantly increase the mixture temperature, electrically conductive additives such as fiber-based, powder-based and granular-based types have been used to facilitate the process of induced heating in asphalt mixtures (Amani et al., 2020).
The asphalt pavements experience different types of deterioration caused by traffic loading and environmental conditions (Zhu et al., 2017). Therefore, maintaining the performance and serviceability of asphalt concrete in acceptable and reliable conditions has attracted considerable attraction among pavement engineers (Lizasoain-Arteaga et al., 2019). In order to keep the serviceability of asphalt pavements satisfied enough, different treatment approaches have been proposed for prevention, maintenance, and rehabilitation (Chan et al., 2011). The mentioned methods need substantial raw materials, budget, skilled workers, times, and can cause significant environmental pollution. However, the induced heating-healing method can somewhat eliminate the mentioned shortcomings and can offer a sustainable repairing approach for asphalt pavements (Liu et al., 2020b). However, the energy consumption, environmental emission, and costs for conducting the induced healing process need to be determined and compared against the existing repairing approaches to evaluate the sustainability of this technique.
While several studies reviewed the engineering applications (Pan et al., 2015), material design (Chen et al., 2020) of conductive asphalt concrete, effective parameters on self-healing (Ayar et al., 2016), and self-healing mechanism of asphalt concrete (Sun et al., 2018, Sun et al., 2018), a comprehensive review of induced heating-healing of asphalt concrete is lacking in the literatures. Moreover, investigating the sustainability aspects (i.e., environmental impacts and cost) of the induced heating-healing technique has not received enough attention. Previous research has provided novel findings on the induced heating-healing technique as an efficient method to heal and repair cracks. Several advanced studies have focused on this method to improve induced heating efficiency and to optimize the healing capability of asphalt concrete. This paper presents a critical review on induced heating-healing of asphalt concrete. This includes differences between self-healing and induced healing, various sources of induced heating, and electrically conductive additives used in asphalt concrete. Moreover, the effects of asphalt concrete composition, specimen geometry, and type of conductive additive on induced heating-healing are thoroughly discussed. In order to investigate the sustainability and long-term efficiency of the induced heating-healing techniques, the impacts of the affecting parameters on induced heating-healing of asphalt concrete are likewise reviewed. Besides, the induced heating-healing was compared against the commonly used asphalt pavement treatment methods considering the sustainability criteria, including energy consumption, greenhouse gas emission, and costs. The last section of this review presents some considerations in the induced heating-healing process, which need further investigations in future studies. Furthermore, the necessity of new experimental setups, computational modeling, and field evaluation is discussed.
2. Induced heating-healing of asphalt concrete
Self-healing is defined as the ability of materials to heal (recover/repair) damages automatically and autonomously without any external intervention or energy. The asphalt materials known as materials with the capability of self-healing (García, 2012). The healing process in asphalt concrete is performed as cohesion (re-bonding within the asphalt binder) (Xu et al., 2018) and adhesion (re-bonding between asphalt binder-aggregate) (Ayar et al., 2016). The healing in asphalt concrete is a complex phenomenon that depends on various parameters, such as the activation energy in asphalt binder, capillary flow through the cracks, and modification type of asphalt binder (Ayar et al., 2016), self-diffusion of molecules across crack edges, state of the crack, and confinement (Xu et al., 2018). Several studies revealed that temperature is the most prominent factor in the induced healing process of asphalt concrete (Liu et al., 2018). The temperature rising in asphalt concrete not only enhances the self-healing efficiency but also shortens the total time required for completion of healing. Under the induced heating process, the temperature of the asphalt binder raises, leading to melting and flowing of asphalt binder inside of micro-cracks. The melted asphalt binder can repair damages and partially recover the internal integrity. The induced heating sources, such as coil (magnetic) and microwave (electromagnetic), can be utilized to provide enough thermal energy and make the binder flow and facilitate the induced healing process.
Besides the requirement of external energy for induced healing, it has been proved that conventional asphalt concrete has low heating efficiency (González et al., 2019). In order to increase the sensitivity and induced heating-healing properties of asphalt concrete under electromagnetic energy, electrically conductive additives are added to asphalt binder or asphalt concrete to improve the potential of asphalt concrete in absorbing the external energy. The addition of electrically conductive additive in asphalt concrete may cause advantages, such as increased heating efficiency, enough sensitivity under the external sources, temperature rising, crack repairing, and enhanced the service life of asphalt pavements.
The induced healing process of the asphalt concrete is accomplished in five main steps: temperature rising, movement of melted asphalt binder inside of cracks, crack closure, crack edge wetting, and fusion of cracks (García, 2012). During the induced heating process, initial contact points are created between the asphalt binder on two sides of the crack (crack closing), providing a bridge between crack surfaces, leading to the accomplishment of the wetting throughout the crack surfaces (Bommavaram et al., 2009). To evaluate the induced healing capability of asphalt concrete, the cracking-healing process was proposed, as illustrated in Fig. 1. To this end, the healing tests were carried out at laboratory as follows: (1) specimens were conditioned as per-test standard and then tested under the cracking test to measure the maximum tolerated load (F0); (2) the broken samples were heated using a heating source to heal the crack formed under the test. After the healing step, the samples were kept at room temperature to cool down; (3) the healed samples were broken again similar to the procedure described in step (1) to measure the breaking force after healing (F1). The healing index (healing efficiency) is then calculated following Eq. (1) (Karimi et al., 2018). Fig. 2 schematically shows the induced healing process in asphalt concrete.
Fig. 1
Fig. 2Considering the fracture energy of asphalt concrete, the healing index can also be defined as the ratio of fracture energy measured in the sample after the healing process (E1) to initial fracture energy (E0) as follows (Amani et al., 2020):
In addition, the healing index in the cracking-induced healing under cyclic loading was defined using Eq. (3) (Menozzi et al., 2015).where, Np is the number of cycles until failure after healing, and N0.5 is the number of cycles with 50% probability of breaking the test samples before the healing process.
To quantify the induced healing efficiency, different fracture (crack) properties and criteria were taken into consideration. Table 1 briefly presents the criteria used to evaluate the induced healing efficacy. As it is shown in Eqs. (1)–(3), induced healing index can be reported as a function of recovery in peak load (or strength), fracture energy, and fatigue life extension. Given the induced healing index obtained through different criteria it can be inferred that the peak load criterion gives higher value of induced healing index compared to fracture energy and fatigue life extension criteria. For instance, the averaged induced healing indices obtained based on peak load, fracture energy, and fatigue life extension are 76%, 61%, and 45%, respectively. It is worth mentioning that the main goal of the induced healing process is to extend the fatigue life of asphalt concrete. While the laboratory effort needed to find the healing index based on the peak load and fracture energy is considerably more than fatigue life, the effects of the induced healing process on the long-term performance and life extension are more important.
Table 1. Comparing different calculation methods of healing index.
| Test type | Specimen shape | Loading condition | Loading rate | Testing temperature | Healing index criterion | Healing Index | Averaged healing index (%) | Reference |
|---|---|---|---|---|---|---|---|---|
| 3 PB | Semi-circular | Monotonic | 0.6 mm/min | −20 °C | Fracture energy | 0.71 | 61 | Amani et al. (2020) |
| 3 PB | Semi-circular | 0.5 mm/min | −10 °C | 0.56 | Li et al. (2020) | |||
| IDT | Cylindrical | 50.8 mm/min | 25 °C | 0.58 | Amani et al. (2020) | |||
| 3 PB | Beam | Dynamic cyclic loads |
Frequency: 4 Hz Resting period: 0.15 s |
20 °C | Fatigue life extension | 0.43 | 45 | Salih et al. (2018) |
| ITFT | Cylindrical | Frequency: 10 Hz | 10 °C | 0.31 | Menozzi et al. (2015) | |||
| 4 PB | Beam |
Frequency: 8 Hz Resting period: 18 h |
20 °C | 0.63 | Liu et al. (2012b) | |||
| 3 PB | Semi-circular | Monotonic | 0.6 mm/min | −20 °C | Peak load | 0.89 | 76 | Jahanbakhsh et al. (2018) |
| IDT | Cylindrical | 50.8 mm/min | 25 °C | 0.56 | Karimi et al. (2018) | |||
| DCT | Disk-shape | 0.01 mm/s | −20 °C | 0.89 | Wang et al. (2018) | |||
| 3 PB | Beam | 0.5 mm/min | −10 °C | 0.92 | Wang et al. (2016) | |||
| 3 PB | Semi-circular | 0.5 mm/min | −20 °C | 0.56 | González et al., 2018, González et al., 2018 |
3. Induced heating source types
As stated earlier, the temperature rising and distribution in asphalt concrete play a predominant role in the induced healing process. To this end, applying the external energy to heat the asphalt concrete and facilitate the induced healing phenomenon has increasingly gained attention. There are different technologies such as induction heating coil, microwave radiation, and infrared radiation, to accelerate the temperature rising and healing process of asphalt concrete. Among these methods, the induction heating coil and microwave heating have been more frequently applied. Notwithstanding that the infrared radiation can simulate solar radiation, it has lower efficiency in induced heating compared to coil and microwave approaches. The following sections explain the mechanism of each heating source on induced heating of asphalt concrete.
3.1. Induction heating coil
The induction heating coil is known as one of the first methods utilized to increase the temperature of asphalt concrete. To this end, electrically conductive materials were added to the asphalt concrete, and then the mixture was exposed to an electromagnetic field, generated by the coil, as shown in Fig. 3. Electrically conductive materials are needed to form closed-loop circuits and eddy current in the mixture (García et al., 2012a). Asphalt concrete is exposed to the magnetic field produced by the copper coil. According to Joule's laws, the rate of induced heating inside the asphalt concrete strongly depends on induced current, dielectric hysteresis, and electrical conductivity of asphalt concrete. According to Joule's law, the thermal energy is generated in the conductive body and then, the thermal energy is transferred to other parts of the body per conduction and convection mechanisms of heat transfer, resulting in temperature rising in asphalt binder (García et al., 2011).
Fig. 3It is worth mentioning that the distance between the coil and the top surface of the sample can significantly affect the distribution of electromagnetic field and heating rate within the asphalt concrete. It is found that the rate of heating considerably increases as the distance between the coil and specimen shortens (Apostolidis et al., 2017). Liu et al. (2012b) evaluated various distances (i.e., 10, 20, and 30 mm) between the coil and the surface of the specimen and proposed a gap of 10 mm as the most efficient distance for induced heating of the pavement surface in situ.
Apostolidis et al. (2017) used the finite element method and investigated the effect of operational parameters of the induction coil (i.e., supplied frequency, supplied power, distance between coil and specimen, and moving speed) on induced heating efficiency. This study evaluated different ranges of the supplied power (0.1–1 kV) and supplied frequency (10–100 kHz) of coils on heating efficiency. The results indicated that an induction coil with a greater supplied frequency provides higher electric field and heating power. Moreover, the results showed that the moving speed of the coil has considerable effects on the induced heating efficiency, such that the induced heating rate increases as the moving speed decreases.
3.2. Microwave device
Microwave radiation consists of electrical and magnetic fields emitting perpendicularly with wavelengths between 1 m and 1 mm and frequencies of 300 MHz and 300 GHz, respectively. The experimental results indicated that the microwave radiation leads to a uniform temperature rising along with the depth (Gallego et al., 2013), energy-saving (Norambuena-Contreras and Garcia, 2016), and time-saving (Wang et al., 2020b) during the heating process compared to the induction heating coil. Some previous studies conducted experimental tests (Bosisio et al., 1974) and computational analyses (Wang et al., 2020a) to investigate the capability of microwave radiation in heating the asphalt concrete. The results showed that the microwave radiation efficiently heats the deep asphalt pavement (Bosisio et al., 1974), increase the resilient modulus and resistance of asphalt concrete against moisture damage (Terrel and Al-Ohaly, 1987), increase the compaction rate and performance of longitudinal joints (Salski et al., 2017). The conventional heating method involves heating the surface and then transferring heat into materials by the mechanisms of conduction, convection, and radiation. However, microwave radiation heats the atomic level, which heats all of the asphalt mixture components (i.e., aggregate, asphalt binder, and additive) exposed to the electromagnetic field (Bosisio et al., 1974). According to Joules’ law, during microwave heating, electromagnetic energy turns into heat due to the changing of direction of bipolar molecules under high frequency within the material (Wang et al., 2020b). The produced thermal energy then moves towards the surface of the object from the core or center (Singh et al., 2015). The main advantage of microwave sources is the mechanism of transferring energy from the source to the sample compared to the induced heating coil. Moreover, the microwave radiation penetrates inside the sample and then travels to the surface (Oghbaei and Mirzaee, 2010). Unlike the conduction mechanism in transferring thermal energy that depends on the diffusion of heat from external surfaces, induced heating under microwave radiation is dependent on the electrical and magnetic fields. This provides the chance to achieve faster and more uniform heating, even in relatively thick samples (Benedetto and Calvi, 2013). Therefore, the benefits of microwave heating motivate the researchers to pay attention to this method as a beneficial induced heating source that provides selective energy absorption and temperature control (González et al., 2018), deep penetration (Sun et al., 2015), saving energy (Wang et al., 2016), and is eco-friendly (Srikant et al., 2013).
As stated earlier, the induced heating source has a decisive impact on the crack healing of asphalt concrete. Moreover, it was demonstrated that conventional asphalt concrete (neat or non-conductive) has a lower potential of induced healing efficacy under the microwave radiation (Karimi et al., 2020). This observation is because of the lower sensitivity to the electromagnetic and lower induced heating rate in the conventional asphalt concrete compared to modified asphalt concrete (Amani et al., 2020). Consequently, the conventional asphalt concrete takes higher induced heating energy and induced heating time, causing lower sustainability of induced healing technique in the conventional asphalt concrete compared to conductive asphalt concrete. As a solution, the electrically conductive additives can enhance the electromagnetic energy absorption, temperature raising, and induced healing efficiency.
3.3. Infrared radiation
As mentioned before, another technique used to increase the temperature of asphalt concrete is infrared radiation. In this method, the asphalt concrete sample is placed under infrared lamps (electromagnetic waves from 0.7 μm to 103 μm) at a specific distance, as depicted in Fig. 4. According to electromagnetic laws (Ampere, Maxwell-Faraday, and Guess), as the frequency of the electromagnetic field increases, the penetration of the electromagnetic field into the specimen decreases, and the concentration of thermal energy on the surface increases. Overall, infrared radiation possesses a lower wavelength (higher frequency) than microwave radiation, leading to lower penetration of the electromagnetic field into the solid body. It means that, under a similar power level of heating source, the penetration of thermal energy caused by infrared radiation is lower than microwave radiation. Therefore, the infrared radiation can merely heat the surface of the solid body, and then the thermal energy transfers through convection and conduction mechanisms. However, the microwave radiation penetrates the solid body and simultaneously heats the whole body, leading to a higher heating rate and uniformity. When the transmitted radiation energy of the infrared lamp hit the asphalt surfaces, a portion of this radiation is absorbed, resulting in an increase in the temperature of the asphalt concrete through conduction, whereas other portions are transmitted or reflected (Ajam et al., 2017). Researchers reported the application of infrared radiation for induced heating-healing purposes and argued that this heating method suffers from low heating rate (Ajam et al., 2017), low heating efficiency (Xu et al., 2018), higher energy dissipation (Gómez-Meijide et al., 2016), and high temperature gradient along with the depth of pavement (Salih et al., 2018).
Fig. 43.4. Comparing the induced heating rate in different heating sources
This section aims to compare the efficiency of different heating sources in the induced heating. To this end, the heating rate and distribution of temperature in the depth of asphalt concrete induced by different heating sources were examined. Byzyka et al. (2018) investigated the temperature gradient in asphalt concrete samples and their depths during infrared heat application. They concluded that the distribution of temperature under infrared heating in the depth of 45, 75, and 100 mm of pothole excavations and their host pavement were non-uniform. Salih et al. (2018) also showed that the maximum heating rate in the samples is not unique and varies almost linearly with the distance between infrared lamps and specimens.
The advantages of the induction heating coil method over the infrared are higher heating rate and absence of surface contact between material and heating source. On the other hand, the non-uniform heating of the asphalt pavement in depth and dependency on the distance between the coil and the specimen could be mentioned as the cons of the induction heating coil method (Liu et al., 2010). Li et al. (2019a) revealed that the temperature gradient grows as the heating time increases, and an increase in heating distance results in a more obvious temperature gradient. Wan et al. (2018) argued that the heating rate under the induction coil strongly depends on electrically conductive additives, such that asphalt concrete without conductive materials has insignificant heating rate and efficiency.
Regardless of the discussed heating methods, microwave heating appears to be a promising technique for induced heating of asphalt concrete compared to other heating devices. The main advantages of microwave compared to other induced heating techniques are energy efficiency and atomic level of heating (Singh et al., 2015), heat generation throughout the material, more rapid and uniform heating (Benedetto and Calvi, 2013), heating efficiency (Salski et al., 2017), selective energy absorption (Sun, 2014), and temperature control (González et al., 2018), which makes it even more efficient than induction heating coil and infrared radiation (Norambuena-Contreras and Garcia, 2016). In this regard, Collin (2007) studied the effects of microwave radiation on asphalt binder inside the asphalt concrete and argued that the microwave radiation causes the quick change in internal friction between the molecules that leads to growing the internal temperature of the mixture. Moreover, microwave radiation transfers through all directions of the specimen and raises the temperature more homogeneously (Norambuena-Contreras and Garcia, 2016). Unlike the induction heating coil method, the application of microwave is wider, since it also can heat the aggregate phase (Amani et al., 2020). Norambuena-Contreras and Garcia (2016) compared the heating rate of asphalt concrete induced with the induction coil and microwave and revealed that the surface and asphalt binder temperatures under the microwave heating were higher compared to the induction coil, as depicted in Fig. 5.
Fig. 5Table 2 shows a summary of the advantages and disadvantages of different induced heating sources. Furthermore, a brief review of the studies on different induced heating sources, which were discussed in this section, is presented in Table 3.
Table 2. Advantages and disadvantages of different induced heating sources.
| Method | Advantages | Disadvantages | Reference |
|---|---|---|---|
| Induction heating coil |
-High dependency on distance with intended surface -Transferable -Fast, selective, and approximately uniform heating compared to infrared -Fits environmental issues |
-High manufacturing costs -Limited healing ratio compared to microwave -More complicated heating mechanism compared to conventional methods |
Agzenai et al. (2015) |
| Microwave |
-Fast, selective, and uniform heating -More reproducibility and better yields -Easy to move from one location to another -Multi-time healing -Able to penetrate asphalt pavement |
-Hard to control the depth of penetration, depending mostly on the frequency and dielectric properties of the material -Requiring more knowledge to avoid interference with other pieces of equipment -Possibility to affect the body organs, especially at high frequency -High dependency of induced heating rate on electrical conductivity of materials - Requiring specific considerations (shielding cover) to control hazardous radiations |
Agzenai et al. (2015) |
| Infrared radiation | -Reduce inflammation |
-Low efficiency -Only simulated the effect of sun radiation -Hard to penetrate deep points |
Ajam et al. (2017) |
Table 3. A review of induced heating studies of different asphalt concrete types under different heating sources.
| Material type | Conductive particle type and shape | Content of additives | Specifications of induced heating machine; Power (kW) and oscillating frequency (kHz) | Temperature at the end of induced heating machine (°C)/Exposure time under induced heating machine (s) | Optimum temperature or induced heating time | Effects of modifiers on characteristics | Test setup | Reference |
|---|---|---|---|---|---|---|---|---|
| Porous asphalt | Steel wool, fiber | 1.27% (wt of mixture) |
Coil; 50 kW; 70 kHz |
70, 85 and 100 °C | 85 °C | 63.9% extension of fatigue life | 4 PB | Liu et al. (2012b) |
| Porous asphalt | Steel wool and Aluminum, fiber | 2.5–5% (wt of mixture) | Coil | 110 °C | 110 °C | Specimen with 5% steel fibers reached the target temperature after 10 min, while the specimens with 5% aluminum fibers required 30 min to reach the target temperature | 3 PB | Pamulapati et al. (2017) |
| Porous asphalt | Steel wool, fiber | 4% (wt of binder) | Coil | 30, 50, 70, 85, 100 °C | 85 °C | Optimum heating temperature is 85 °C for porous asphalt beams to gain the most possible healing | 3 PB | Liu et al. (2012a) |
| Porous asphalt | Steel wool, fiber | 4% (v of binder) |
Coil -; 70 kHz |
70, 85, 100 and rested at 20 °C for 18 h | 85 °C | The induced heating rate of steel wool reinforced cores is 0.3442 °C/s | 4 PB | Liu et al. (2012c) |
| Dense asphalt | Activated carbon, powder | 5% and 10% (wt of binder) |
Microwave 1 kW; 2.45 × 106 kHz |
90–100 °C | 100 °C | Activated carbon significantly increased heating rate of asphalt concrete |
3 PB, IDT |
Karimi et al. (2018) |
| Dense asphalt | Steel wool, fibers | 2%, 4% and 6% (wt of binder) |
Coil 30 kW; 78 kHz |
30–130 °C | 100 °C | 60% recovery of mechanical properties | 3 PB | García et al. (2013a) |
| Dense asphalt | Steel wool, fibers | 8% (wt of binder) |
Coil; 5 kW; 120–160 kHz |
60, 80 and 100 °C | 100 °C | 73.6% recovery of original fracture strength | 3 PB | Dai et al. (2013) |
| Dense asphalt | Cast steel, particle | 6% (v of mixture) |
Coil 30 kW; 78 kHz |
80, 120, 160, 180 and 200 °C | 55 °C | Non-uniform heating can increase the risk of aging of asphalt binder | IDT | Menozzi et al. (2015) |
| Dense asphalt | Steel wool, fibers | 2%, 4%, 6% and 8% (wt of binder) |
Microwave 0.7 kW; 2.45 × 106 kHz |
10 s, 20 s, 30 s, and 40 s | 40 s | cracks were healed after 40 s induced heating | 3 PB | Norambuena-Contreras and Gonzalez-Torre (2017) |
| Dense asphalt | Steel, fiber | 3.44% (v of binder) |
Coil 5 kW;- |
60, 80 and 100 °C | 80 °C | The specimens heated until 80 °C gained the highest healing level | 3 PB | Yang et al. (2014) |
| Dense asphalt | Steel, fiber | 2%, 4%, 6% and 8% (v of binder) |
Microwave 0.7 kW; 2.45 × 106 kHz Coil 6 kW; 350 kHz |
20, 40, 60, 80, 100 and 120 s | 40 s | Microwave increased the surface temperature more than induction heating coil and this effect increased as the fiber content increased | 3 PB | Norambuena-Contreras and Garcia (2016) |
| Dense asphalt | Carbon black, powder | 10%, 20% and 30% (wt of binder) |
Microwave 1 kW; 2.45 × 106 kHz |
90–100 °C | 100 °C | Carbon black significantly improved the induced heating | 3 PB, IDT | Jahanbakhsh et al. (2018) |
| Asphalt mastic |
Steel wool, fiber Steel fiber, fiber |
Steel wool: 4%, 8%, 10% (wt of binder) Steel fiber: 10%, 20% (wt of binder) |
Coil 50 kW; 70 kHz |
85 °C | 85 °C | 83% strength recovery ratio of the specimens with 4 wt% steel fiber | 3 PB | Liu et al. (2013) |
| Asphalt mastic | Steel wool, fibers | 5.66% (v of binder) |
Coil; 5 kW; 120–160 kHz |
60, 80 and 100 °C | 60 °C | Micro-cracks can be healed at the temperature of 60 °C | 3 PB | Dai et al. (2013) |
| Asphalt mastic | Steel Slag Filler, particle | 0.2%, 0.4% and 0.6% (wt of binder) |
Microwave 0.8 kW; 2.45 × 106 kHz |
0, 20, 40, 60, 80 s | – | Asphalt mastic containing steel slag filler experienced higher heating ratio compared to limestone filler | Fatigue | Li et al. (2018a) |
| Asphalt binder | Carbon nanotube (CNT) and Graphene, powder | 10% (wt of binder) | Microwave | 10–80 s | – | The CNT is an effective modifier to improve the induced heating rate under the microwave source | BBR, DSR | Li et al. (2018b) |
3.5. Comparing the induced healing rate in the different heating sources
Referring to the impacts of heating methods on the induced healing capability of asphalt concrete, Gómez-Meijide et al. (2016) investigated the crack healing ability using infrared radiation and induction coil. The results showed that cracks in asphalt concrete could be healed through both infrared and coil. However, coil heating is more energy-efficient. Moreover, Sun et al. (2017) compared strength recovery of asphalt mixtures heated by microwave and induction coil, and concluded that the healing performance of asphalt samples heated using microwave was better than that of the induction coil. This finding was also confirmed in a study conducted by Norambuena-Contreras and Garcia (2016). Accordingly, the microwave heating is a viable and powerful method to enhance the induced heating (Phan et al., 2018) and induced healing of flexible pavements (Gulisano et al., 2020). Table 4 presents a brief review of the studies on induced healing for different types of asphalt concrete healed under triple kinds of induced heating sources.
Table 4. A review of induced healing studies of different asphalt concrete types under different heating sources.
| Material Type | Conductive particle type and shape | Content of additives | Specifications of induced heating machine; Power (kW) and Oscillating frequency (kHz) | Effects on induced heating-healing | Test setup | Reference |
|---|---|---|---|---|---|---|
| Porous asphalt | Steel wool, fiber | 4% (v of binder) |
Coil -; 70 kHz |
The aging slightly decreases the fatigue life ratio by 3% | 4 PB | Liu et al. (2012c) |
| Porous asphalt | Steel wool, fiber | 10% (v of binder) |
Coil 50 kW; 70 kHz |
Porous asphalt concrete containing steel wool had a higher healing rate (fatigue resistance) than conventional asphalt concrete | ITFT | Liu et al. (2011) |
| Porous asphalt | Steel wool, fiber | 1.27% (wt of mixture) |
Coil 50 kW; 70 kHz |
The highest healing rate was 63.9% when the specimen was subjected to 70,000 cycles of loading | 4 PB | Liu et al. (2012b) |
| Porous asphalt | Steel wool, fiber | 4% (wt of binder) | Coil | Porous asphalt beams can recover 78.8% of their initial strength | 3 PB | Liu et al. (2012a) |
| Dense asphalt | Carbon, fiber |
IM8: 2% and 4% (wt of mixture) AS4: 1.5% and 3% (wt of binder) |
Microwave 1.1 kW; 2.45 × 106 kHz |
The specimens modified with 2% IM8 has the highest level of healing ratio compared to the specimens modified with carbon fiber | 3 PB | Wang et al. (2016) |
| Dense asphalt | Metal grit, particle | 11.2% (wt of mixture) |
Coil 2.8 kW; 348 kHz |
The healing rate was 90% | 3 PB | Ajam et al. (2017) |
| Infrared | ||||||
| Dense asphalt | Steel wool,fiber | 2%, 4%, 6% (wt of binder) |
Coil 30 kW; 78 kHz |
The healing rate of the modified specimens showed the recovery of up to 60% of initial strength | 3 PB | García et al. (2013a) |
| Dense asphalt | Steel, fiber | 3.44% (v of binder) |
Coil 5 kW;- |
Healing levels of samples heated at 60, 80, and 100 °C were 76.2%, 92.1%, and 84.7%, respectively | 3 PB | Yang et al. (2014) |
| Dense asphalt | Activated carbon, powder | 5% and 10% (wt of binder) |
Microwave 1 kW; 2.45 × 106 kHz |
The healing rate of specimens modified by activated carbon and made of siliceous and limestone aggregate were 56% and 64%, respectively. |
3 PB, IDT |
Karimi et al. (2018) |
| Dense asphalt | Steel, fiber | 2%, 4%, 6% and 8% (v of binder) |
Microwave 1 kW; 2.45 × 106 kHz Coil 6 kW; 350 kHz |
The healing rate of asphalt mixtures under the microwave radiation were higher than that of induction heating coil | 3 PB | Norambuena-Contreras and Garcia (2016) |
| Asphalt mastic | Steel Slag Filler, particle | 0.2%, 0.4% and 0.6% (wt of binder) |
Microwave 0.8 kW; 2.45 × 106 kHz |
The effects of induced healing on the recovery of complex modulus, fatigue life, and dissipated energy in asphalt mastic containing steel slag filler were 39.99, 10.15, and 5.82%, respectively, higher than that of asphalt mastic containing limestone filler | Fatigue | Li et al. (2018a) |
4. Electrically conductive additives used in asphalt concrete
The conventional asphalt concrete specimens contain coarse aggregates, fine aggregates, asphalt binder, and mineral fillers, which have high electrical resistivity (108 and 1012 Ωm) causing the mix to act as an electrical insulator and become insensitive to electromagnetic radiation. Therefore, the efficacy of induced heating (Gao et al., 2017) and induced healing (Wang et al., 2016) in conventional (neat and non-conductive) asphaltic materials is very low. In this regard, previous researchers have argued that asphalt concrete without conductive additives cannot properly heat and heal the existing cracks when it is exposed to external resources (Karimi et al., 2018). Electrically conductive additives can effectively increase the electrical conductivity, improve the thermal conductivity and diffusivity, and increase absorption of electromagnetic radiation leading to an improved induced heating rate in asphalt concrete (Wu et al., 2005). The electrically conductive agents used in asphalt pavements are classified as binder-based, fiber-based, granular-based, and also a combination of mentioned types. Table 5 shows the type of electrically conductive additives suggested to enhance the induced heating-healing process in different studies. Besides, in recent years, increasing the landfills of waste materials causes substantial environmental consequences. Improper disposal process of waste materials can raise the risks of environmental issues in air, groundwater, and soil. Consequently, reusing the waste materials in asphalt pavement can protect the environment by reducing the accumulation of landfills, saving raw materials, leading to an increase in sustainability. Recently, waste materials (e.g., metallic waste) are used as electrically conductive additives to improve the mechanical properties, durability and electrical conductivity. It is reported that using waste materials in the asphalt concrete causes a series positive impacts, including crack healing (Ajam et al., 2018) and deicing (Liu et al., 2020d) under the electromagnetic field. Moreover, it can be inferred that the combination of conductive additives is an alternative method for manufacturing conductive asphalt concrete to increase the induced heating efficiency and decrease the manufacturing costs. This issue needs to be determined for establishing an economically and mechanically effective condition. Moreover, the combination of different conductive materials can increase the sustainably of the induced heating-healing technique. It was shown that the combination of conductive additives (steel slag, graphite, and steel fiber) significantly increased the electrical conductivity, which can provide a more economical and efficient method to prepare conductive asphalt concrete (Wu et al., 2012).
Table 5. The different conductive additives used in asphalt concrete.
| Type | Material composition |
|---|---|
| Fiber-based | Carbon fiber (Wang et al., 2016), Metallic fiber (Norambuena-Contreras and Gonzalez-Torre, 2017), Aluminum fiber (Pamulapati et al., 2017), Steel Wool Fiber (SWF) (Xu et al., 2020, Xu et al., 2020), and Steel fiber (Apostolidis et al., 2020) |
| Binder-based | Coke breeze (Fromm, 1976), Activated carbon (Karimi et al., 2018), Carbon black (Jahanbakhsh et al., 2018), Graphite (Wang et al., 2018), and Carbon Nano Tube (CNT) (Li et al., 2018b) |
| Granular-based | Graphite (García et al., 2009), Magnetite aggregate (Wang et al., 2016), Steel filing (Franesqui et al., 2017), Ferrite filler (NiZN ferrite) (Zhu et al., 2017), Iron particle (Jeoffroy et al., 2018), Metal particle (Obaidi et al., 2018), Steel slag (Apostolidis et al., 2018), Steel grit (Gómez-Meijide et al., 2018b), Metal grit (Gómez-Meijide et al., 2018a), Metal waste (González et al., 2018), Silicon Carbide (SiC) (González et al., 2019), Copper slag (Fakhri et al., 2020), and Metal shaving (Liu et al., 2020a), |
| Combination of conductive additives |
Graphite + Carbon fiber (Wu et al., 2005); Carbon black + Carbon fiber (Wu et al., 2005); Graphite + Steel fiber (García et al., 2009); Steel slag + Graphite (Wu et al., 2012); Steel slag + Carbon fiber (Wu et al., 2012); Steel slag + Graphite + Carbon fiber (Wu et al., 2012); Steel slag + Graphite + Steel fiber (Wu et al., 2012); Iron powder + Steel fiber (Apostolidis et al., 2016); Steel fiber + Steel slag (Sun et al., 2017); Steel wool + Steel filing + Metallic powder (Franesqui et al., 2017); CNT + Graphite (Li et al., 2018b); Powders of Fe3O4, Iron, Traditional mineral, and Steel slag (Wan et al., 2018); Steel wool + Graphite (Wang et al., 2020b); Electric Arc Furnace (EAF) slag + Graphene Nanoplatelets (GNPs) (Gulisano et al., 2020) |
4.1. Fiber-based electrically conductive additives
Fiber-based conductive additives with a fiber-like shape were often added to asphalt concrete. In some research studies, the fiber-based conductive materials, such as steel wool fibers (SWFs) (Liu et al., 2011), carbon fiber (Wang et al., 2016), aluminum fibers (Pamulapati et al., 2017), and metallic fibers (Norambuena-Contreras and Gonzalez-Torre, 2017), were utilized to boost the electromagnetic sensitivity of asphalt concrete. Metallic fibers (e.g., SWF), as the most frequently used conductive additives, can be used to increase the induced heating rate of asphalt concrete. Additionally, the steel fiber obtained from the scrap tires (waste tires) is another kind of conductive material used in asphalt concrete (Ajam et al., 2018). It has been reported that, from 2015 to 2018, 67 million tons of scrap tires were added to landfills (Ajam et al., 2018). The use of waste steel fibers obtained from scrap tires can decrease the vulnerability of the environment and increase sustainability. The conductive fibers not only improve the electrical conductivity and sensitivity of asphalt concrete to the electromagnetic field, but they can also increase the thermal conductivity and heat transfer characteristics more than other components of the asphalt concrete (i.e., aggregates and asphalt binder) (Liu et al., 2010). The addition of steel wool fiber can remarkably improve the induced heating (Liu et al., 2010) and induced healing (García et al., 2013a) efficiency. Despite higher heating rate of this additive, they may cause disadvantages, such as clustering, non-uniform temperature distribution (Norambuena-Contreras and Garcia, 2016), increased air void content, decreased integral integrity, and lower mechanical performance (Karimi et al., 2019). Moreover, based on the non-uniform distribution of SWF within the mixture, the electrical conductivity was localized, leading to non-uniform electrical conductivity and localization of absorbed electromagnetic radiation (Li et al., 2019a, Li et al., 2019b). Consequently, the temperature at the surrounding of SWF clusters is considerably higher than the average temperature in the specimen, which can cause aging of the asphalt binder in the vicinity of conductive materials (Menozzi et al., 2015). Also, it was concluded that the diameter and length of SWF can affect the induced heating rate (García et al., 2014). It was demonstrated that as the diameter of SWF increases, the heating rate of asphalt concrete samples increases, and the formation of clusters in asphalt concrete decreases (García et al., 2013c). The SWFs with large diameters are stiffer and offer a lower probability of cluster formation. As the diameter decreases, the fibers are more easily clustered during the mixing and compaction process and decrease the internal integrity and mechanical characteristics of asphalt concrete. Fig. 6 schematically shows the effects of SWF on induced heating-healing properties of asphalt concrete.
Fig. 6Carbon fibers have the general characteristic of carbon materials such as high-temperature resistance, resistance to friction, thermal conductivity, and corrosion resistance. Besides, they have substantial effects on the mechanical performance (Liu et al., 2011), thermal conductivity, and electrical conductivity of asphalt concrete (Wang et al., 2016). Based on previous studies, Table 6 shows the effect of electrically conductive fiber types on the volumetric (i.e., density, air voids, and homogeneity of mix), mechanical, and induced healing features of asphalt concrete.
Table 6. Effect of different fibers types on properties of asphalt concrete.
| Properties of asphalt concrete | Type of fiber | |||||
|---|---|---|---|---|---|---|
| Wool (Ajam et al., 2018) | Grit (Ajam et al., 2018) | Shaving (Ajam et al., 2018) | Tire (Ajam et al., 2018) | Carbon | ||
| Volumetric properties | Density | Slight increase | Slight increase | Slight increase | Slight increase | Decrease (Arabzadeh et al., 2019) |
| Air voids | Slight increase | Slight increase | Slight increase | Slight increase | Increase (Arabzadeh et al., 2019) | |
| Homogeneity in mix | Moderate decrease | Slight decrease | Slight increase | Slight decrease | Decrease (Arabzadeh et al., 2019) | |
| Mechanical properties | Indirect tensile strength | Slight increase | Moderate increase | Slight increase | Slight increase | Increase (Mawat and Ismael. 2020) |
| Resistance to water damage | slight decrease | Moderate decrease | No significant effect | No significant effect | Increase (Mawat and Ismael. 2020) | |
| Stiffness modulus | Slight increase | Slight increase | Slight increase | Slight increase | N.A | |
| Particle loss resistance | No significant effect | Slight decrease | No significant effect | Slight decrease | N.A | |
| Skid resistance | Slight increase | Moderate increase | Moderate increase | Sharply increase | N.A | |
| Induced heating-healing properties | Induced heating capability | Moderate increase | Moderate increase | Slight increase | Sharply increase | Increase (Arabzadeh et al., 2019) |
| Healing properties | Sharply increase | Moderate increase | Slight increase | Moderate increase | Increase (Arabzadeh et al., 2019) | |
4.2. Binder (powder)-based electrically conductive additives
The binder-based conductive additives were widely used as an asphalt binder modifier (Karimi et al., 2018). Different types of binder-based materials, including graphite (size ranges: 75% of 0.15 mm and 25% of 0.075 mm) (Wang et al., 2017c), activated carbon (size ranges: 45–75 μm) (Amani et al., 2020), carbon black (size ranges: 0.03 μm) (Rew et al., 2017), and coke breeze (Fromm, 1976), were utilized to boost the conductivity and electromagnetic sensitivity of asphalt concrete. Graphite is a commonly used binder-based conductive additive that improves the high-temperature resistance (Wen et al., 2019), electrical conductivity (Park et al., 2014), thermal conductivity (Wang et al., 2018), and aging resistance (Huang et al., 2009) of asphalt concrete. Concerning the effect of graphite on the cracking resistance of conductive asphalt concrete, some studies demonstrated that graphite enhanced the mechanical strength of asphalt concrete (Wu et al., 2010), which this observation relates to the hydrophobic nature of graphite and the strong adhesion between hydrophobic asphalt binder and graphite (Rew et al., 2017). However, some studies argued that the usage of graphite causes degradation of mechanical performance of conductive asphalt concrete (Liu and Wu, 2011) due to the stiffening effect of graphite caused by the absorption of the lightweight fraction of asphalt binder (Wang et al., 2016). Recently, some studies have shown that activated carbon can effectively improve the ability of asphalt concrete and increase the absorbance of electromagnetic field (Wang et al., 2019, Wang et al., 2019). Furthermore, it is reported that modification of asphalt concrete by activated carbon results in substantial improvement in mechanical properties such as aging resistance (Amani et al., 2020), moisture and freeze-thaw damages (Kavussi et al., 2020), rutting resistance (Wang et al., 2019), fatigue and thermal cracking resistance (Karimi et al., 2018).
Another binder-based material, which is used as a conductive agent to boost the electromagnetic sensitivity and accelerate the induced heating-healing rate, is carbon black. Jahanbakhsh et al. (2018) studied the microwave heating and induced healing of asphalt modified with carbon black as the conductive component. The results indicated that carbon black improves the induced heating-healing and substantially increases the mechanical strength at low and intermediate temperatures, acid corrosion, and oxidative resistance of asphalt concrete.
4.3. Granular-based electrically conductive additives
The granular-based conductive additives were mostly used as filler or coarse aggregate replacement in asphalt concrete. Different types of granular-based materials, containing steel slag (size ranges: 4.75–12.5 mm) (Lou et al., 2021), steel particles (size ranges: 2.36–4.75 mm, and less than 2.36 mm) (Cox et al., 2019), iron particles (size ranges: 0.05–0.18, 0.13–0.43, 0.30–1.00, 0.71–1.40, 1.20–1.70, 3.00, 4.00, and 6.00 mm) (Jeoffroy et al., 2018), metal waste particles (replaced as fine aggregate (<0.063 mm)) (Franesqui et al., 2017), and magnetite aggregate (size ranges: < 0.063, 0.063–0.25, 0.25–1.00, 1.0–2.8, and 2.8–4.0 mm) (Wang et al., 2016), were utilized to increase the electromagnetic absorption of asphalt concrete. It is worth mentioning that the magnetite additives were used with granular shape as the replacement of coarse aggregate (Wang et al., 2016) and with powder shape as the filler replacement in asphalt concrete (Guan et al., 2019) to enhance the electrical conductivity and sensitivity of asphalt concrete to the electromagnetic field. Amongst different granular-based materials, steel slag, as the most frequently used additive containing iron and Fe3O4, was applied in asphalt concrete to improve the induced heating characteristics (Wan et al., 2018). As reported by Phan et al. (2018), the steel production industry produces hundreds of million tons of steel slag annually. The considerable natural mineral resources consumed to construct asphalt pavements can cause substantial costs and environmental consequences. Given this issue, replacing aggregates with industrial solid waste, such as steel slag, leads to reduced costs and environmental impacts, and increased sustainability.
Liu et al. (2017) reported that replacing the steel slag with coarse aggregate partially enhanced the particle loss resistance, water stability, and fracture energy. Another study showed that the substitution of 30% normal coarse aggregate with steel slag is a viable solution to provide more efficient induced healing results (Phan et al., 2018). Regarding the size effect on the induced heating, Wan et al. (2018) indicated that steel slag shows a decreased induced heating rate as the range of particle size decreases.
Since metal shaving is an electrically conductive material, it can increase the microwave heating (Ajam et al., 2018) and crack healing properties of asphalt concrete (González et al., 2018). Gonzalez et al. (2018) showed that uniformly distributed metal shavings in the mixture can increase the crack healing properties of asphalt concrete. The utilization of metal shavings, as a waste material, in asphalt concrete not only enhances the induced heating-healing process but also decreases the environmental consequences caused by leaving this in landfills.
Obaidi et al. (2017) investigated the induced heating of asphalt concrete modified by steel girt as another granular-based additive. The results revealed that the maximum average surface temperature reached by asphalt concrete constantly increased as the content of the steel girt increased.
4.4. Effects of conductive additives on asphalt concrete features
This section investigated the effects of type, shape, and content of conductive additives on different features of asphalt concrete. Table 7 presents a brief review of commonly used types of conductive additives utilized in asphalt concrete to improve the conductivity and efficiency of the induced heating-healing process. This table also provides details regarding the shape of additives, conventional range, optimum contents, and effects of each additive on mechanical properties, electrical conductivity, thermal conductivity, induced heating, and induced healing of asphalt concrete. According to Joule's laws, the rate of induced heating inside the asphalt concrete strongly depends on induced current and electrical conductivity (Karimi et al., 2018). The electrical conductivity firmly depends on the conductive paths formed by dispersed conductive materials in the asphalt concrete (Pan et al., 2015). The electrically conductive additives make the mixture able to absorb the electromagnetic field imposed on the pavement and transfer it to the lower points of pavement layers.
Table 7. The effects of type and content of conductive additives on characteristics of the asphalt concrete.
| Shape of conductive additives | Conductive agent type | Conventional range of additives | Optimum content of additive | Effects on the performance of asphalt concrete |
|---|---|---|---|---|
| Fiber-based | Steel wool fiber | 2, 4, 6, and 8% (volume of binder) | 4% (volume of binder) |
Mechanical: The SWF improved the particle loss resistance of asphalt concrete. However, the increase of the SWF content caused the air void to increase and the stiffness modulus of asphalt concrete to reduce (Norambuena-Contreras and Garcia, 2016). Electrical resistivity: The electrical resistivity reduced by approximately 52% (Norambuena-Contreras et al., 2018). Induced heating: The heating rate increased by approximately 43% (Norambuena-Contreras and Garcia, 2016). Induced healing: The healing index of asphalt concrete comprised of 4% of SWF (volume of binder) was approximately 93% (Norambuena-Contreras and Garcia, 2016). |
| Carbon fiber | 2 and 4% (wt of mixture) | 2% (wt of mixture) |
Electrical resistivity: The electrical resistivity was reduced by approximately 60% (Wang et al., 2016b). Thermal conductivity: The thermal conductivity was increased by approximately 20% (Wang et al., 2016b). Induced healing: The carbon fiber recovered damages by approximately 92% (Wang et al., 2016b). |
|
| Binder-based | Activated carbon |
5 and 10% (wt of binder) |
5% (wt of binder) |
Mechanical: The activated carbon enhanced fatigue, low temperature (Karimi et al., 2018), aging resistances (Amani et al., 2020), moisture and freeze-thaw resistance (Kavussi et al., 2020), and reduced rutting potential (Karimi et al., 2018). Electrical resistivity: The electrical resistivity was reduced by approximately 67% (Karimi et al., 2018). Induced heating: The heating rate was increased by approximately 44% (Karimi et al., 2018). Induced healing: The activated carbon recovered intermediate temperature and low temperature cracking by approximately 50% and 70% (Karimi et al., 2018). |
| Carbon black | 10, 20, and 30% (wt of binder) | 10% (wt of binder) |
Mechanical: The carbon black enhanced fatigue, low temperature, and rutting resistances (Jahanbakhsh et al., 2018). Electrical resistivity: The electrical resistivity was reduced by approximately 71% (Jahanbakhsh et al., 2020b). Induced heating: The heating rate was increased by approximately 33% (Jahanbakhsh et al., 2018). Induced healing: The carbon black recovered cracks occurred at intermediate and low temperatures by approximately 60% and 70% (Jahanbakhsh et al., 2018). |
|
| Graphite | 5 and 7% (wt of binder) | 7% (wt of binder) |
Mechanical: The graphite can enhance the fracture properties (Wang et al., 2017c), dynamic modulus (Wang et al., 2017a), and rutting resistance (Wang et al., 2017a) of asphalt concrete. Electrical resistivity: The electrical resistivity decreases as the graphite content increases (Wang et al., 2017a). Thermal conductivity: The thermal conductivity increases as the graphite content increases (Wang et al., 2017a). Induced heating: The heating rate increased by approximately 18% (Wang et al., 2018). Induced healing: The graphite recovered damages by approximately 97% (Wang et al., 2018). |
|
| Granular-based | Steel slag | 30% of coarse aggregate to 100% of total aggregate | N.A. |
Mechanical: The steel slag improved the Marshall Stability, residual Marshall Stability ratio, and fracture energy (Liu et al., 2017). Induced heating: The heating rate of asphalt concrete containing steel slag was 1.5 times greater than that of the control mixture (Phan et al., 2018). Induced healing: The replacement of coarse aggregate by steel slag recovered damages by approximately 80% (Phan et al., 2018). |
| Steel shaving | 2, 4, 6, and 8% (volume of binder) | 2% (volume of binder) |
Mechanical: The steel shaving can enhance the indirect tensile strength and stiffness modulus of asphalt concrete (Ajam et al., 2018). Thermal conductivity: The thermal conductivity of asphalt concrete containing steel shaving was lower than that of the control mixture (Norambuena-Contreras et al., 2018). Induced heating: The heating rate increases as the steel shaving content increases (Norambuena-Contreras et al., 2018). Induced healing: The healing index of asphalt concrete containing steel shaving was approximately 71% (Norambuena-Contreras et al., 2018). |
Magnetic permeability is another parameter affecting the induced heating process in asphalt concrete (Liu et al., 2019). A few studies have investigated the magnetic permeability of asphalt concrete and its influences on the induced heating process (Apostolidis et al., 2016). The magnetic permeability represents the response and distribution of the magnetic field in the asphalt concrete exposed to an electromagnetic field (Wang et al., 2019). In other words, this parameter can control the penetration and distribution of the magnetic field in the depth of asphalt pavement (Liu et al., 2020d). It was concluded that the magnetic field penetration in the asphalt concrete and efficacy of induced heating increase as the magnetic permeability increases (Wang et al., 2019). Moreover, the results showed that the magnetic permeability depends on the temperature, exciting frequency of the induced heating source, and density of asphalt concrete (Jaselskis et al., 2003). As the temperature and density of asphalt concrete increase and the frequency of the magnetic source decreases the magnetic permeability increases (Jaselskis et al., 2003). This observation indicates that the higher frequency of induced heating source leads to lower penetration of magnetic field in depth (Liu et al., 2020d). However, according to Joule's law, the heating rate increases as the frequency increases (Liu et al., 2019). Therefore, an optimum frequency level needs to be determined to acquire the most efficient heating rate and distribution in the asphalt pavement. In other words, while the increase of frequency can increase the heating rate, the thermal energy concentrates on the pavement surface, and the deep points of asphalt pavement cannot receive enough magnetic field to be heated.
Previous studies (García et al., 2009) regarding the electrically conductive asphalt concrete indicated that the electrical resistivity directly depends on the volume content of conductive additive in the mixture, as schematically shown in Fig. 7. In the physical and chemical sciences, the percolation theory describes the conduction mechanism in composite materials. Percolation in conductive asphalt concrete denotes the connection of conductive additives distributed in non-conductive asphalt material to form conductive paths (Rew et al., 2017). The percolation threshold is defined as critical conductive additives content near which the resistivity substantially decreases, and above the percolation threshold, the contact resistance of conductive additives determines the conductivity of asphalt concrete. However, the optimum content is defined as the content from which the further increase in the content of conductive additives has no impact on the resistivity of asphalt concrete. As shown in Fig. 7, the change in the electrical resistivity versus the content of conductive additives comprises four phases: insulated phase (before percolation threshold), insulated-conductive transition phase (near percolation threshold), conductive phase (beyond percolation threshold), and excess of conductive additive phase. The change of conductivity in the asphalt concrete alters as the mechanism of conductivity changes the phase. Therefore, it can be indicated that, there is an optimum content of conductive particles, which results in the most possible electrical conductivity (minimum electrical resistivity). For the conductive contents below the optimum point, the electrical resistivity of the mixture drops to that of a non-conductive material, in which the induced heating rate is not significant. In this case, a low thermal conductivity makes a non-uniform temperature distribution inside of the mixture. However, the contents of conductive additives beyond the optimum point have negligible effects on the electrical conductivity and may weaken the mechanical properties of mixtures (Wu et al., 2005). Adding excessive contents of conductive additives can deteriorate the internal integrity of asphalt concrete, leading to overheating and excessively age the binder (Karimi et al., 2019). Garcia et al. (2014) indicated that the increase of the air void content caused by the high content of SWF leads to a minor change in the thermal conductivity and weakens the mechanical