Comparative Study on Microwave Absorbing Heating Characteristics and Microwave Deicing Performance of Airport Pavement Modied Concrete

In this paper, the airport pavement concrete has been taken as the main research object, three kinds of absorbing materials, namely silicon carbide (SiC), iron oxide (Fe 3 O 4 ) and graphite, have been respectively mixed into the concrete, and an open microwave testing system has been established. Based on this system, the basic mechanical properties, microwave heating characteristics, microwave deicing effect and its inuencing factors of modied concrete are systematically studied. In addition, a comparative analysis of the inuence mechanism of different absorbing materials on the strength and absorbing performance of pavement concrete is carried out. The results showed that the addition of SiC, Fe 3 O 4 , and graphite could effectively enhance the microwave effect of pavement concrete, and the more the addition, the more obvious the improvement. Furthermore, under the same mixing amount, the degree of improvement of microwave deicing performance of each absorbing material from large to small is graphite, Fe 3 O 4 , SiC. However, the addition of graphite will form several weak links in concrete, thereby reducing its overall mechanical properties. SiC can slightly improve the mechanical properties of pavement concrete, but it has no signicant effect on the microwave absorption properties. With the addition of Fe 3 O 4 , the strength of concrete changes little, and the effect of microwave absorbing heating and microwave deicing is remarkable. In general, the comprehensive performance of microwave deicing of Fe 3 O 4 modied concrete is optimal. This study has high scientic and practical signicance, and can be widely applied to deicing projects on airports and high-grade highways.


Introduction
The cold areas in northern China are characterized by wide distribution, large precipitation in winter and long duration of low temperature. In some areas, severe snow and ice weather occurs frequently, which causes the airport pavement to be in a state of ice for a long time. The ice cover on the pavement surface will affect the friction coe cient and roughness to a great extent and signi cantly reduce the anti-skid performance, thus seriously endanger ight safety (Yang and Jin, 2002). Therefore, rapid and effective removal of pavement ice is a hot issue that needs to be solved urgently. Traditional deicing methods mainly include clearing method, melting method and restraining method (Zhou et al., 2014). Manual clearing method has high labor intensity and low working e ciency. Mechanical removal method has low ice removal rate and great damage to road surface (Yu, 2011). Chemical melting method has great harm to the pavement performance and ecological environment (Fu, 2010). Thermal melting method has high cost and low energy utilization (Wu, 2005). Although these methods can deice the road surface, they also expose the above defects at the same time. In order to make up for the problems existing in the traditional deicing method in practical operation, it is of economic value and practical signi cance to seek a deicing method with high energy utilization rate, less damage to the road surface and environmental protection.
Microwave heating technology, as rapid development of environmental protection technology, is widely used in the deicing of road surface in winter, and has achieved certain results. In recent years, many studies have been carried out on microwave deicing. Hopstock (2003Hopstock ( , 2005 tested the improvement of microwave deicing e ciency of asphalt pavement by preparing microwave absorbing concrete with iron tunnel rock as aggregate. Wang (2010) established magnetron heating model, and calculated the electric eld distribution of heating material by HFSS simulation. Wang et al. (2016) put forward a standard evaluation method of deicing, and from the perspective of rapid deicing and optimal economy, presented the best matching scheme for the new deicing design. Gao et al. (2017) explored the feasibility of using steel slag as the aggregate of microwave deicing asphalt mixture through experiments, and determined the most effective volume and particle size to partially replace the traditional aggregate. Gallego et al. (2013) used steel ber as microwave absorbing component in asphalt concrete to increase the surface temperature of asphalt concrete, thereby improving deicing e ciency. Liu et al. (2018) improved the e ciency of concrete deicing by adding iron black into concrete. In the previous research on microwave deicing, most scholars focused on asphalt concrete. However, there are great differences in appearance and performance between cement concrete and asphalt concrete. Therefore, the existing conclusions are not suitable for microwave deicing of cement concrete pavement. In addition, most of the current scholars focus on a single microwave absorbing admixture, and there is a lack of cross-sectional comparative studies on a variety of different absorbing admixtures. Therefore, it is of great signi cance to test and study the static mechanical properties and the effect of microwave deicing of concrete mixed with different absorbing materials, so as to seek the best absorbing materials, optimize the pavement mix design, and promote the microwave deicing technology.
In this paper, silicon carbide (SiC), ferric oxide (Fe 3 O 4 ) and graphite have been respectively mixed into concrete to prepare three kinds of absorbing concrete. The microwave heating e ciency and deicing effect of concrete have been studied, and its compressive strength and exural strength have been tested. Finally, the mechanism of different absorbing materials on concrete has been compared and analyzed.
The research results provide a theoretical basis for further promotion of microwave deicing methods.

Raw materials
The cement used in this paper is 42.5 ordinary portland cement, and the water reducing agent is polycarboxylic acid water reducing agent with water reducing rate of 35%. The speci c mix proportion design is shown in Table 1 below.

Specimen preparation
The size of the specimens was 150 mm × 150 mm × 150 mm, and the amount of absorbing material was 0%, 5%, 10% and 15% respectively. In order to fully ensure the dispersion and uniformity of the distribution of the absorbing admixture in the concrete matrix and promote the formation of standardized specimen properties, this paper adopted PC mixing method. The speci c steps were as follows: rst, mixed coarse aggregate and ne aggregate for 60 s, then poured in absorbing material and cement and continue to mix for 60 s, and nally, added water and water reducing agent at the same time and mixed for 120 s.
After the concrete mixing process was completed, the concrete was poured into the mold of 150 mm×150 mm×150 mm, and cured for 28 days under the standard state (T=20±2℃, relative humidity>95%). Subsequently, the concrete specimens were made.
After curing, we carried out the frozen ice layer treatment on the concrete surface. The speci c methods were as follows: Rigid PVC board was used to surround and x the specimen to ensure that the upper edge of the board was parallel to and 20 mm higher than the concrete edge, and the whole specimen was wrapped with plastic cloth. Subsequently, water was poured over the concrete surface, so that the water level just reached the upper edge of the PVC board. Finally, the specimen was placed in the lowtemperature control cabinet for freezing. After the completion of freezing, the specimen could be prepared as shown in Fig. 4 below.

Test equipment
The test equipment used for microwave deicing is an open microwave transmitting system designed independently. The equipment was composed of magnetron, water cooling device, height adjusting device, infrared temperature tester and external microwave control box, as shown in Fig. 5.
The temperature sensor was tested by WZP-128 thermal resistance with a temperature range of -30℃ 2 50℃. Its performance was stable and the temperature measurement accuracy could reach 0.1 ℃. In the test, the MEACON multi-channel paperless recorder was used with a sampling accuracy of 0.1℃ and a sampling period of 1s. It was able to collect and store the temperature data of 12 thermal resistances simultaneously.

Test method
(1) The compressive strength and exural strength of SiC, Fe 3 O 4 and graphite concrete with different content are tested by electro-hydraulic servo test system, and the effects of three kinds of absorbing materials on the mechanical properties of specimens are compared and analyzed.
(2) The microwave source port is controlled to be 20 mm above the specimen surface, and the real-time temperature of the surface center point of three modi ed concrete specimens with different dosages is monitored and recorded every 4 s.
(3) Simultaneously, the microwave source height of the device is adjusted to 20, 30, 50, 70, and 100 mm. The real-time temperature at the center of the specimen is monitored and recorded every 4 s.
(4) Subsequently, the height of the microwave source is xed as 20 mm. The ice state of SiC15, Fe15 and C15 ice covered modi ed concrete specimens in the process of deicing is investigated, and the comparative analysis of the deicing e ciency and effect is conducted.

Test Results And Analysis
Through tests, the compressive strength and exural strength of SiC, Fe 3 O 4 and graphite modi ed concrete, as well as the real-time temperature during the microwave heating process are obtained. By comparing and analyzing the data and integrating the changing trends, the following quantitative results can be obtained.

In uence of absorbing admixture on mechanical properties of concrete
In this paper, with reference to the standard method for testing mechanical properties of ordinary concrete, an electro-hydraulic servo test system was used to conduct static compressive and exural tests on ordinary concrete and wave-absorbing concrete specimens, with the loading rate of 0.5 MPa/s.
The compressive strength and exural strength of SiC, Fe 3 O 4 and graphite concrete with different dosage were obtained respectively. Based on the test data, the effects of the three kinds of wave absorbing materials on the mechanical properties of concrete were compared and plotted as shown in Fig. 6.
It can be seen from the comparative analysis in Fig. 6

Effect of SiC on microwave heating e ciency of concrete
The temperature rise curve of the surface central point of concrete with different SiC content is shown in Fig. 7. It can be seen that (1) The real-time temperature curves all show a linear upward trend, and the incorporation of SiC will increase the temperature rise rate of concrete, so that the nal temperature rise range increase as well. (2) When the height of the microwave source is xed at 20 mm and the content of SiC is 5%, 10% and 15%, the temperature rise range of the central point on the corresponding concrete surface is 58.3℃, 63.5℃ and 70.8℃, which are 112%, 109% and 110% of that of ordinary concrete respectively. The greater the content of SiC, the greater the temperature rise of concrete. Each 5% increase in the amount of SiC increases the temperature rise by 6.2℃, 5.2℃, and 6.7℃, respectively. The increase in the same amount of SiC on the microwave effect of concrete is basically the same.
In order to further analyze the in uence of the addition of SiC on the microwave effect of concrete, the temperature rise amplitude of SiC5, SiC10 and SiC15 at different microwave source heights is tested, as shown in Fig. 8. It can be seen that (1) After adding SiC with different content, the change trend of temperature rise amplitude is the same as that of PC, which decreases with the increase of microwave source height. (2) The higher the content of SiC, the higher the temperature rise of concrete.

Effect of graphite on microwave heating e ciency of concrete
The temperature rise curve of the surface central point of concrete with different graphite content is shown in Fig. 11. It can be seen that (1) With the prolongation of microwave action time, the temperature rise curve of graphite concrete always shows a linear trend. (2) After graphite is added, the temperature rise of concrete in the microwave eld increases signi cantly. When the graphite content is 5%, 10% and 15%, the temperature rise is 78.1℃, 101℃ and 130.1℃, respectively, which is 149%, 193% and 249% of that of ordinary concrete.
In order to judge the improvement of the microwave effect of graphite on concrete more accurately, the temperature rise amplitude of C5, C10 and C15 at different microwave source heights is analyzed, as shown in Fig. 12. It can be seen that (1) When different amounts of graphite are added, the temperature rise amplitude of concrete decreases with the increase of the height of the microwave source, and the decrease rate is obvious. (2) When the microwave source height is 20 mm, the temperature rise amplitude increases by about 25℃ for every 5% increase of graphite content. The microwave thermal effect of concrete is greatly improved by adding graphite.

Comparative analysis of deicing effect
In order to further explore the in uence of SiC admixture on the microwave deicing e ciency of concrete, tests on the surface ice state of SiC15 specimen under different microwave action times is carried out. The results are shown in Fig. 13. It can be seen that when the microwave action time is within the range of 0~70 s, the ice layer above the SiC15 specimen always maintains the initial freezing state, and the thickness of ice does not change. When the microwave action time reaches 90 s, the melting phenomenon starts to appear below the ice layer at the central point of the concrete surface. When the microwave is applied for 120 s, the ice above the concrete central point has completely melted, and the surface melting area continues to increase. When the microwave action time reaches 160 s, the melting area of the ice layer above the SiC15 specimen expands further.
In order to explore the effect of Fe 3 O 4 on the e ciency of microwave deicing of concrete, the ice state of Fe15 specimen surface under different microwave action time is given, as shown in Fig. 14. It can be seen that when the microwave action time is in the range of 0-50s, the ice layer above the concrete is always frozen and has not changed. When the microwave is applied for 70 s, the ice layer below the Fe15 central point melts, which is shortened by 20s compared with the time when the SiC15 specimen of sic begins to melt. When the microwave action time is 110 s, the ice layer above the Fe15 central point has completely melted, and the melting area on the surface of the specimen continues to increase. When the microwave action reaches 160 s, the melting area on the concrete surface further expands, and at the same time, a considerable part of the ice layer has completely melted into water.
The ice state of C15 under different microwave time is given, as shown in Fig. 15. It can be seen that the ice layer above C15 is always frozen within 0-30 s of microwave action. When the microwave action is 50 s, the ice layer below the concrete central point has melted. Then the melting area at the center of the ice layer gradually becomes larger, while the thickness of the ice layer above the melting area becomes thinner. When the microwave is applied for 90 s, it can be seen that the central part of C15 specimen has completely melted, and the melting area of concrete surface continues to increase. When the microwave action reaches 130 s, the area where the ice layer on the concrete surface completely melts continues to increase. However, the increasing range is not obvious, and the water under ice layer has leaked out from the periphery of the concrete. To sum up, it can be analyzed that at the same 15% content, the microwave deicing e ciency of three microwave absorbing materials on concrete in order from high to low is graphite, Fe 3 O 4 , SiC.

Mechanism analysis
Different wave absorbing admixtures have different effects on concrete strength. SiC is a hexagonal crystal with a speci c gravity between 3.20 and 3.25, which has the advantages of stable chemical properties, high thermal conductivity and good wear resistance (Huang and Liu, 2009;Joshi et al., 2000). After mixing with concrete, due to its high hardness, it can improve the mechanical properties of weak links such as transition zone to some extent, so the mechanical properties of concrete have been improved. However, when the specimen is cured and formed in strength, the powdered Fe 3 O 4 will not produce cohesive force, so the addition of Fe 3 O 4 will not improve the mechanical properties of concrete.
When a large amount of Fe 3 O 4 is added, the cementation of cement will be weakened due to the dilution of Fe 3 O 4 , so the compressive strength and exural strength will show a downward trend with the increase of the content. For graphite, the density is small, the pores are well developed and the surface area is large. After the specimen is formed and cured, due to the small density of graphite itself, it basically has no strength and will form many weak links in concrete. When subjected to external forces, these weak links are destroyed rapidly, which makes the concrete specimen lose strength. Therefore, the compressive and exural strength of graphite concrete show a decreasing trend, and the decreasing amplitude of the strength increases with the increase of the graphite content.
The mechanism of improving the microwave heating e ciency of concrete is also different with different absorbing admixtures. SiC material itself has low resistivity and strong electrical conductivity. Under the action of microwave, SiC will be polarized by particles inside the concrete (Shen et al., 2007). The direction of the polarization vector is shifted by an angle from the electric eld, thus producing a current in phase with the microwave eld, which generates vortices in the concrete and in turn generates heat.
After adding Fe 3 O 4 into concrete, the relative positions of Fe 3+ and Fe 2+ in Fe 3 O 4 make it highly magnetic.
The magnetic moment of Fe 3 O 4 is determined by the direction of the low-frequency alternating magnetic eld under the in uence of microwaves. Therefore, the molecules will rearrange and rotate rapidly under the in uence of the magnetic eld, thereby converting the microwave energy into heat. At the same time, due to the strong magnetism brought by Fe 3 O 4 , the magnetic induction intensity of concrete lags behind its own magnetic eld intensity in the process of repeated magnetization, which will also convert the microwave energy into heat Zhang, 2010). Graphite has excellent conductive properties and can form a whole or partial conductive network in concrete. Under the action of microwave, graphite concrete will cause heat loss because of the conduction current. Graphite particle size is small and the speci c surface area is large. When irradiated by microwave, it is easy to produce multiple scattering and re ection of microwave, which can make the microwave lose rapidly after entering concrete and emit a large amount of heat (

Conclusion
In this paper, pavement concrete with three kinds of absorbing materials doped with silicon carbide (SiC), ferric oxide (Fe 3 O 4 ) and graphite has been taken as the research object. The basic mechanical properties, microwave heating characteristics and microwave deicing effect of pavement concrete after incorporating wave absorbing materials have been tested respectively, and the action mechanism of different wave absorbing materials on concrete have been compared and analyzed. The main conclusions are as follows.
(1) The addition of SiC, Fe 3 O 4 , and graphite can effectively improve the microwave effect of concrete, and the more the amount is, the more obvious the microwave effect of concrete is. Under the same dosage, the improvement degree of the microwave deicing performance of each absorbing material on concrete is as follows: graphite, Fe 3 O 4 , SiC.
(2) After adding SiC and graphite, the temperature rise curves of concrete show a linear upward trend, and the temperature rise rate is not affected by temperature. When Fe 3 O 4 is added, the higher the temperature of concrete is, the lower the temperature rise rate is in the microwave eld. When the temperature is greater than 40℃, the temperature rise rate of concrete decreases rapidly, and the microwave effect of Fe 3 O 4 is temperature-sensitive. (4) Through a comparative analysis of concrete strength and temperature rise amplitude, among the three kinds of absorbing materials, SiC can slightly improve the mechanical properties of concrete, but the improvement of microwave effect is relatively low. Graphite can increase the e ciency of microwave heating of concrete by 9%, but the mechanical properties of concrete are greatly reduced. However, the addition of Fe 3 O 4 has a small decrease in the strength of concrete, and the microwave effect on concrete can be increased by 192%. The microwave heating effect and deicing effect are remarkable. Therefore, the Fe 3 O 4 modi ed concrete has a better comprehensive performance.
Based on the above conclusions, this paper provides a certain theoretical basis and technical support for the microwave deicing technology of airport pavement, which is of high scienti c signi cance and popularization value. In future studies, we will further study the interaction mechanism of various microwave absorbing materials and explore the optimal airport pavement modi ed concrete.  Concrete specimens of frozen ice Figure 5 Microwave deicing test equipment Real time temperature of graphite concrete Figure 12 Amplitude of rising temperature of graphite concrete Figure 13 Microwave deicing effect of SiC15 Figure 14 Microwave deicing effect of Fe15 Microwave deicing effect of C15