Induction Heating and Healing Behaviors of Asphalt Concretes Doped with Different Conductive Additives

It is consensual that the self-healing property of asphalt concrete can repair the damage inside it during high temperature and rest period. In order to not affect the traffic, the rest period of asphalt pavement is very short and uncontrollable; so, it is necessary to obtain enough high temperature in a limited time to achieve higher healing efficiency of asphalt concrete. /e purpose of this paper is to study the induction heating efficiency and healing behaviors of asphalt concretes doped with different conductive additives. Steel fiber, steel grit, and steel slag were added to asphalt mixtures as conductive additives to prepare induction healing asphalt concretes. /e steel grit and steel slag were added to replace the aggregates of corresponding particle size by equal volume to ensure the consistency of asphalt concrete volume, which can avoid degrading the performance of asphalt concrete due to the change of porosity. /e induction heating efficiency and healing rate of asphalt concrete were quantified by infrared camera and three-point bending-healing experiment, respectively. /e results showed that the thermal properties of asphalt concrete changed with the addition of different conductive additives. /e asphalt concrete with steel fiber had the best induction heating property. While steel slag had extremely weak induction heating speed, the better thermal insulation property of the asphalt concrete with steel slag resulted in a higher induction healing rate. It was suggested to add steel slag to induction healing asphalt concrete to improve the healing rate.


Introduction
Asphalt concrete pavement has attracted more and more attention because of its excellent service performance [1][2][3][4][5]. However, due to environmental and human factors, such as temperature [6], moisture [7], ultraviolet rays [8], and heavy load [9], it is often difficult for the service life of the asphalt concrete pavement to reach the design life. Macroscopic distresses, such as potholes, cracks, and ruts, are often caused by microdamage inside asphalt concrete [10,11]. Induction heating is a new maintenance technology to repair microcracks in asphalt concrete to avoid macroscopic distresses [12][13][14]. Similar to other self-healing materials, asphalt concrete can repair the damage autonomously. Asphalt concrete has a potential to restore its stiffness and strength during rest periods. However, this internal restoration is very slow; so, this process needs to be speeded up in some way. Liu et al. first published the work on self-healing asphalt concrete with steel fiber in 2011 [15], and after that, more and more research has been carried out in this direction. Among most of those previous works, temperature is widely considered as a key factor for induction heating efficiency of asphalt concrete, and a higher induction healing rate is expected to be obtained during the road rest period. But at the moment, the most common way is to add conductive additives to asphalt binder [16,17]. A major advantage of this method is that the energy is generated from the asphalt mortar and does not directly act on the aggregate, with the coarse aggregate being not subjected to excessive temperature stress due to excessive temperature change. However, it will take longer time to transfer the temperature from the asphalt binder to the whole asphalt concrete, thus prolonging the induction healing time. Since asphalt concrete has about 95% aggregate, if conductive additives can be used to replace some aggregates without affecting the service performance of asphalt concrete, it may obtain better induction heating efficiency.
Steel slag is one of the industrial wastes, which makes up a portion of more than 10% of raw steel output [18,19], and a large amount of steel slag will be produced in the steelmaking process. e mineral composition of steel slag includes calcium silicate (C 2 S), calcium ferrite (C 4 AF, C 2 F), and calcium aluminate (C 3 A, C 12 A 7 ) [20]. Steel slag has been applied in many fields, such as the production of cement and concrete [18,20], purification engineering [21,22], and road construction [23]. However, little research has been done on the application of steel slag in induction healing asphalt concrete. On the one hand, steel slag has excellent thermal insulation; on the other hand, the energy generated by induction heating needs to be long standingly preserved in order to achieve an adequate repair of asphalt concrete. erefore, the application of steel slag in induction healing asphalt concrete can not only better induce healing performance but also save resources.
Induction healing asphalt concrete has been paid more and more attention because of its great potential, such as prolonging the pavement service life [12,13,15,16] and the application in melting snow and ice [24]. It is one of the research directions of the new generation smart pavement in the world at present. Steel slag, as a kind of solid waste, has been paid equal attention to its recycling and reuse. Its excellent performance has made it possible to use as pavement building material. At the same time, the excellent thermal properties of steel slag make it perfectly fit for induction healing asphalt concrete. e combination of them can not only improve the performance of induction healing asphalt concrete but also provide a new idea for the reuse of steel slag.
is study aims to explore the induction heating efficiency and healing rate of asphalt concrete by using conductive additives, especially steel slag and steel grit, to partially replace aggregates of certain sizes. e study investigated the induction heating and healing behaviors of asphalt concretes doped with different conductive additives by an infrared camera and universal testing machine. e thermal property of different asphalt concretes was evaluated by the thermal constants analyzer (TPS 2500S, Hot Disk, Sweden). e chemical elements and oxide of steel slag were analyzed by X-ray fluorescence. In order to avoid the change of voids, the improved method (equal volume substitution method) was used to add steel grit and steel slag into asphalt concrete.

Materials and Experiments
2.1. Materials. Base asphalt graded 70 (penetration grade) obtained from Hubei Guochuang Hi-tech Material Co., Ltd of China was used in this paper. Steel wool fiber provided by Jiangsu Golden Torch Metal products Co., Ltd and steel grit provided by Ningbo Hongyang cleaning equipment Co., Ltd were used as the induction heating units for asphalt concrete, as shown in Figure 1. e properties of asphalt, steel wool fiber, and steel grit are shown in Table 1, while the impurity element composition of two conductive additives is shown in Table 2. During electromagnetic induction, the elements that can be heated are Fe, Co, and Ni [12]. e optimal content of fiber is 6% by the volume of asphalt according to the previous research [4]. Asphalt mixture with 6% steel wool fiber has best mechanical properties (highest strength and particle loss resistance) and quite high induction heating speed. e particle size of steel grit was 0.6-1.18 mm, which replaced the fine aggregate of corresponding size by equal volume to avoid changing the void of asphalt concrete. e basalt aggregate, steel slag aggregate, and limestone filler were used in this study. Specifically, two sizes of steel slag (4.75-9.5 mm and 9.5-13.2 mm) were used to replace basalt to study the induction heating characteristics and healing behavior of steel slag induction healing asphalt concrete. According to [25], the properties of steel slag were evaluated, as shown in Table 3. All properties meet the requirements of the code for steel slag aggregate for asphalt pavement. Although the water absorption rate was on the high side, it did not exceed the technical requirements too much.

Asphalt Concrete.
In order to study the induction heating efficiency and healing behaviors, six kinds of asphalt concretes were designed. Firstly, AC-13 basalt asphalt concrete was designed according to the Marshall design method, and the asphalt-aggregate ratio was 4.7%. e aggregate gradations and grading curves of asphalt concrete are shown in Figure 2. is kind of asphalt concrete was named BA. Secondly, the asphalt concrete was doped with 6% steel wool fiber by the volume of asphalt in BA, which was named FA. irdly, on the basis of FA, two sizes of steel slag aggregates (4.75-9.5 mm and 9.5-13.2 mm) were used to replace basalt aggregates to generate new asphalt concrete, which was named FSA. Fourthly, also on the basis of FA, steel grit (0.6-1.18 mm) was used to replace basalt aggregates to design a new asphalt concrete named FGA. Fifthly, the above steel grit and steel slag simultaneously replaced basalt aggregates to obtain GSA. Finally, FGSA was obtained by adding steel fiber into BA and replacing basalt aggregate with steel grit and steel slag. e list of different asphalt concretes ingredients is given in Table 4. It should be pointed out that steel slag and steel grit replaced basalt aggregates by the same volume, which avoids changing air void content of asphalt concrete for the future researches. In the three-point bending test and induction heating test, the specimens were rectangular beam with a notch cut by rutting board in a size of 85 mm × 50 mm × 10 mm, as shown in Figure 3.

ermal Property Test.
e thermal conductivity of steel fiber, steel grit, and steel slag is very good, which is different from that of aggregate, asphalt, and filler in asphalt concrete; so, it is necessary to study their influence on the thermal constants of asphalt concrete as a whole. e thermal constants of different asphalt concretes were measured using cylindrical samples with the size of Φ 101.6 mm × 30 mm. Different types of asphalt concrete Marshall specimens were prepared and cut to be tested at room temperature, as shown in Figure 4.
In the testing process, the particle size of the aggregate may be larger than the diameter of the Hot Disk probe, which leads to the difference of the test results. erefore, in addition to increasing the parallel test data, the test deviation was reduced by changing the test area for the same test. e test areas of thermal constants are shown in Figure 5. e straight diameter of the sample is 101.6 mm. e thermal constants were measured in 4 regions of the center, and the mean value was taken for 3 times for each part, that is, 12 times for each kind of asphalt concrete.

Induction
Heating. Induction heating test, as shown in Figure 6, was divided into two parts: induction temperature and induction healing, where the distance from the surface

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of the sample to the coil was kept constantly at 10 mm [4]. e induction temperatures of both tests were monitored by an infrared camera with a resolution of 320 × 240 pixels. e frequency of the induction heating apparatus is always 123 kHz. For the induction temperature test, the output power was 7.6 kW, while the power was 8.4 kW for the induction healing test. In order to obtain the best healing performance, the temperature of the induction healing test was set at 85°C, which was veri ed in the previous research [26]. After each induction heating test, the samples had enough time to recover to ambient temperature. In this study, for Marshall samples, the temperature was the average temperature on the upper surface, while for other samples, the temperature was the average temperature of the sample as a whole.

Fracture and Healing Test.
In this study, the three-point bending test was used to evaluate the induction healing performance of asphalt concrete. To avoid creeping, the experimental temperature was below −20°C, as shown in Figure 7 [27]. e load was applied at a rate of 5 mm/min, and the tests were stopped when the discharge curve did not change anymore. e average of ten parallel samples for each type of asphalt concrete was recorded. e healing rate of asphalt concrete was de ned as the relationship between the ultimate force (F 0 ) of the beam during a three-point bending test and the ultimate force (F t ) measured in the beam after some time healing, as shown in the following equation:

Results and Discussion
3.1. ermal Property Analysis. e difference of thermal conductivity will cause the difference of heat transfer in asphalt concrete. erefore, the thermal conductivity of asphalt concrete is the factor that needs to be considered in the process of electromagnetic induction healing. e thermal conductivity of different materials varies greatly, among which metals are prominent. e larger the thermal conductivity, the faster the temperature transfers, and the more uniform the temperature distribution of the pavement is, which is more helpful to avoid the pavement distresses caused by the temperature change. But at the same time, the lower the rate of heat exchange between asphalt concrete and external environment, the more the beneficial to the healing of internal damage of asphalt concrete, because this can make asphalt concrete in the favorable healing temperature range for a longer time. So, our goal was to get an asphalt concrete that had a faster heat transfer rate inside and a slower heat exchange with the outside, which were evaluated by thermal diffusivity and specific heat. Figure 8 shows the thermal diffusivity and specific heat of different asphalt concretes. From left to right, it was analyzed in turn how the thermal constants of asphalt concrete changed with the addition of steel fiber, steel grit, and steel slag. With the addition of steel fiber, asphalt concrete got higher thermal conductivity and smaller specific heat capacity, which was conducive to the rapid diffusion of internal temperature, but not conducive to the overall thermal insulation of asphalt concrete. Since the thermal conductivity of the metal was much greater than that of the other components of BA, and the specific heat was the opposite, it was easy to understand the change. e addition of steel grit also led to these changes, but the rate of change seemed to be greater, perhaps because steel grit

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replaced aggregates, rather than being wrapped in asphalt binders like steel ber. Although steel slag could not be heated quickly by induction, which would be analyzed in the next section, the addition of steel slag seemed to increase the speci c heat of asphalt concrete. For example, FSA reached to 3.165 MJ/m 3 K, which was more conducive to the insulation of asphalt concrete for better induction healing rate. is could be explained by the physicochemical characteristics of steel slag con rmed in the research by Chen et al. [1]. Although the limited metallic components and porous structure led slower induction heating rate of steel slag, its thermal insulation e ect was better, and the purpose of using energy more reasonably was achieved. To sum up, although steel slag could not be e ectively heated by induction, the asphalt concrete replaced traditional aggregates by steel slag can improve the e ciency of induction energy utilization, which helps to increase the induction healing rate. Figure 9 shows induction heating ability of steel ber, steel grit, and steel slag after 5 s when the output power of the induction heating instrument is 8.3 kW. It can be seen that the induction heating abilities of the three materials were in the order of steel ber > steel grit > steel slag, where steel slag had extremely weak ability for induction heating. rough Wan et al. research [28], only iron and Fe 3 O 4 can be heated by induction. e content of chemical elements and main oxides of steel slag were measured by XRF (PANalytical Axios, RIGAKU ZSX Priums), as shown in Table 5. From the table, it was found that all iron elements exist in the form of Fe 2 O 3 , which did not help much with induction heating. Figure 10 shows average surface temperature of di erent kinds of asphalt concretes after induction heating for 60 s, 120 s, and 180 s, and the 120 s infrared image is shown in Figure 11. In the light of the above study, the di erence between the induction heating properties of the three materials was very clear. Because the induction heating ability of the steel ber was better than that of the steel grit, it was observed that the induction heating ability of FSA was better than that of GSA. erefore, the induction heating ability of conductive additives is paramount for induction heating asphalt concrete. FGA had the highest induction temperature, reaching 122.5°C after 180 s. Comparing SA with FSA, also FGA with FGSA, it was found that, even though the internal inductive ability of asphalt concretes was the same, the temperature of asphalt concretes added with steel slag was lower than that of basalt after same induction time. is phenomenon veri ed the conclusion of the previous section. Although the energy generated by induction was the same, the speci c heat capacity of asphalt concrete increased after adding steel slag, which resulted in slower temperature rising speed. It was also found that, because of the di erent induction heating e ciencies of above three materials, the temperature di erence of di erent asphalt concretes became more obvious with time increasing. Higher speci c heat can result in slower energy exchange and lower temperature loss, which improve the induction healing rate of asphalt concrete. erefore, it is harmful to pursue the exorbitant induction heating e ciency, and the heat storage performance of asphalt concrete should also be considered, because too low thermal storage performance and too high induction heating e ciency will cause asphalt concrete to experience more serious thermal expansion. It is more scienti c to study the induction heating ability of asphalt concrete on the premise of considering its temperature storage ability. Figure 12 shows the variation tendency of F 0 of di erent asphalt concretes. From BA to FA and GSA to FGSA, F 0 increased obviously, resulting from toughening e ect of steel ber. e strength of asphalt concrete with steel ber is composed of the cohesion of asphalt, the intercalation force between aggregate and aggregate, the tensile stress of ber, and the adsorption force between ber and asphalt. e overall strength of asphalt concrete depends on the performance of asphalt, properties of aggregates, and steel ber. As shown in Figure 13, when ordinary asphalt concrete is subjected to tensile stress, the strength is mainly borne by the cohesion between asphalt. Because asphalt has poor e ect of resisting external force, the tensile strength of ordinary asphalt concrete is low. A sudden brittle failure occurs when the ultimate tensile stress is reached. After the addition of steel ber, the tensile stress is supported by the cohesion of asphalt and the force between asphalt and steel ber. When asphalt concrete is subjected to force, the cohesion of asphalt rst disappears and then relies on the adhesion between asphalt and steel ber. e tensile stress is transferred to the contact surface of the asphalt and steel bers, and the tensile strength of the asphalt concrete increases with the addition of ber because of the enhanced bond between the asphalt and the ber. From FA to FSA and other asphalt concretes with steel slag, F 0 dropped dramatically. is is contrary to our previous study [4]. Steel slag shows weak alkalinity due to the presence of metal oxides, which is more conducive to the adhesion to asphalt.

Fracture Resistances.
is should lead to higher fracture strength. In the previous study, after the steel slag replacing the traditional aggregate, the bitumen-aggregate ratio increased, because the porous structure of the steel slag would need more asphalt binders. erefore, su cient asphalt can be combined with steel slag  to provide adhesion force. Unfortunately, in this study, in order to control the in uencing factors, the bitumenaggregate ratio was the same for each kind of asphalt concrete, so the asphalt binder of asphalt concrete with steel slag was insu cient, and the fracture strength was a ected. Nevertheless, after a reasonable design of the bitumenaggregate ratio of steel slag asphalt concrete, the steel slag still has a good application prospect in induction healing asphalt concrete because of promoting e ect on improving healing rate. Overall, the addition of steel grit had a negative e ect on the strength of asphalt concretes. is may be due to the shape and surface properties of the steel grit, which was not conducive to become stronger adhesion with asphalt, and the smooth surface also had a negative e ect compared to steel ber. As shown in Figure 14, the surface of the steel ber was not a completely smooth structure after magnifying the steel ber 50 times taken by optical microscope (BXF-150). It could be seen that there were more textures on the surface of steel ber, and the existence of the texture enhanced the adhesion between the asphalt and the steel ber. us, the shear stress was increased at the interface. When the stress acting on the interface was greater than the shear strength between the two interfaces, the ber was gradually pulled out and destroyed, which did not happen on steel grit.

Induction
Healing Rate. Figure 15 shows the healing rates of asphalt concretes doped with di erent conductive additives. Firstly, although the fracture resistances of the samples with steel slag were relatively low, when they were heated to 85°C, the healing rates were high, of which FSA was the highest. e defective bitumen-aggregate ratio led to the decrease of fracture resistance, but the better heat preservation property of steel slag led to a better healing rate of asphalt concretes. In this study, the excellent performance of steel slag was rstly combined with electromagnetic induction heating, which may provide a new direction for the utilization of steel slag. In addition, the steel ber not only gave the asphalt concrete better healing rate but also improved its fracture resistance. However, the performance of steel grit was unsatisfactory.

Conclusions
In this research, the induction heating and healing behaviors of asphalt concretes doped with di erent conductive additives were investigated. Based on the results discussed above, the following conclusions could be drawn: (1) e addition of steel ber and steel grit made asphalt concretes obtain better thermal conductivity, and the addition of steel slag improved the heat storage performance of asphalt concretes, which was benecial to induction healing. (2) Steel ber had the best induction heating properties, and its induction heating temperature was 93.8°C   after 120 s, the idea of replacing aggregate with steel grit in order to achieve better induction heating e ciency was not very successful, and steel slag had extremely weak induction heating ability due to the lack of elements or oxides that can be induction heating.    Figure 13: Failure mechanism of asphalt concrete: without steel ber (a) and with steel ber (b).

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(3) Although the insu cient bitumen content lowered the mechanical strength of asphalt concretes with steel slag, its better thermal insulation property resulted in higher induction healing rate. (4) It was suggested to adding steel slag to induction healing asphalt concrete.

Future Researches
e pavement performance of asphalt concretes needs to be evaluated and tested, such as high temperature rutting performance test, low temperature cracking performance test, fatigue performance test, water damage performance test, and freeze-thaw cycle test. More types of steel slag need to be evaluated to determine a new index on the application of steel slag in induction healing asphalt concrete.
Data Availability e data used to support the ndings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no con icts of interest. Advances in Materials Science and Engineering 9