1. Introduction
Compared with traditional deicing methods, microwave deicing has attracted great interest from industry and academia thanks to its advantages of no damage to pavement and environmental friendliness [
1,
2]. However, the microwave heating performance of asphalt pavement is often improved by overall microwave enhancement, which limits the improvement of deicing efficiency [
3]. Meanwhile, melting ice only requires heat from the surface of the pavement, the internal heat is invalid and results in energy and material waste [
4]. Therefore, it is necessary to find a suitable pavement structure that can improve the deicing efficiency by accumulating heat on the surface.
The use of a thin-layer structure on asphalt pavement can improve the temperature rise rate of the road surface. Peng prepared a superhydrophobic coating with microwave heating function and proved that the temperature rise rate of the coated asphalt mixture was 48.37% faster than that of the uncoated asphalt mixture [
5]. Additionally, the more heat accumulates on the surface, the more efficient the ice melting will be. Wan designed an ultra-thin friction course using steel slag and studied its ice and snow melting efficiency under induction heating. The results showed that the addition of steel slag reduced the effective induction heating depth and improved the snow and ice melting efficiency of the pavement [
6]. Liu used epoxy resin as a binder to prepare a microwave-enhanced functional layer and conducted a microwave deicing test. The results indicated that the microwave-enhanced functional layer could effectively shorten the deicing time [
3]. Moreover, accumulating heat on the surface also has the effect of saving energy and reducing emissions. Liu accumulated the heat on the pavement surface by adding waste steel wool into the asphalt mixture, which saved the energy required for microwave deicing and reduced the emissions of SO
2 and NO
X [
7]. An ultra-thin wear layer is an asphalt pavement thin overlay that has emerged in recent years and has a wide range of applications [
8,
9]. It can improve the skid and wear resistance of the pavement as well as repair the original pavement microcracks [
10]. Therefore, the combination of ultra-thin wear layer and microwave deicing is beneficial for the further promotion and application of microwave deicing.
Scholars often improve the microwave deicing efficiency of pavement by adding microwave-absorbing materials [
11,
12]. According to the type of loss, the incorporated microwave-absorbing materials can be divided into magnetic loss materials, such as magnetite, and dielectric loss materials, such as activated carbon powder (ACP) [
13,
14]. The addition of 80% magnetite aggregate to the asphalt mixture increased the temperature rise rate to 0.41 °C/s and significantly reduced the deicing time [
15]. Liu partially replaced the filler in asphalt mixture with ACP and tested the ice melting efficiency, showing that the addition of ACP increased the ice melting efficiency by 2.47 times [
16]. Sun added steel slag as aggregates into the asphalt mixture to improve its ice melting performance and proved that the ice melting rate can be increased to 18.5 g/min by adding steel slag [
17]. Besides magnetite and steel slag, other microwave-absorbing materials that have been studied include steel fiber, activated carbon powder, graphite and carbon black [
18,
19,
20]. Although the addition of these microwave absorbers can improve the deicing efficiency of asphalt mixture, it also has the disadvantage of lowering the engineering performance of asphalt concrete, making construction challenging and high cost. SiC as a typical dielectric material can be added to asphalt mixture to effectively increase the temperature rise rate of the asphalt mixture under microwave heating [
21]. In addition, SiC is frequently utilized to improve the wear resistance of concrete surfaces because of its wear resistance, high hardness and chemical stability [
22]. Therefore, adding SiC into the ultra-thin wear layer can improve its microwave-absorbing performance and ensure its wear resistance.
In this work, a UML was designed to improve the deicing efficiency of asphalt pavement. The thickness range of the UML was calculated first, and then the thickness of the UML was recommended using the low-temperature splitting test. The effect of temperature uniformity and temperature rise rate on the deicing efficiency of the UML was investigated by microwave heating test and microwave deicing test. Based on the test results, the particle size and content of SiC in the UML were determined. Meanwhile, the key factors affecting deicing time were analyzed. The oil–stone ratio of the UML was designed and tested. Finally, the energy and material saving effects of the UML were evaluated.
2. Materials and Methods
2.1. Materials
The bitumen was a high-viscosity, modified bitumen produced in Hunan, China. Its properties are shown in
Table 1.
The aggregate was limestone. Its properties are shown in
Table 2; they meet the Chinese specification for the Test Methods of Aggregate for Highway Engineering (JTG E42-2005).
The SiC used in this study was produced in Henan, China. After sieving, three different particle sizes of SiC were obtained: 0~2.36 mm, 0~9.5 mm and 2.36~9.5 mm. The appearance of SiC is very similar to the aggregate used in asphalt mixture, which suggests that SiC may adhere to asphalt well and form an interlocking structure with the aggregate.
2.2. Low-Temperature Splitting Test
2.2.1. Sample Preparation
As an asphalt pavement thin overlay, the ultra-thin wear layer is required to be water resistant, skid resistant, wear resistant and crack resistant and to have high bonding strength. However, the thinness of the ultra-thin wear layer causes it to crack easily in service [
23]. Furthermore, the asphalt pavement layer has poorer resistance to cracking in low-temperature environments. To investigate the influence of the UML thickness on low-temperature crack resistance. The specimens were Marshall specimens with a double-layer structure to fit the situation of the UML paved on the asphalt pavement surface. The upper layer was prepared with asphalt mixture for the UML, and its grading curve is shown in
Figure 1. AC-13 asphalt mixture was used to prepare the lower layer. The UML thickness determined the upper layer height of the specimens, and the total height of the specimens was 63.5 mm. The diagram of the specimen structure is shown in
Figure 2.
The lower layer was formed according to the Chinese specification for Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). The upper layer was made on the surface of the lower layer after it had been horizontally placed for 12 h. After demolding, each group of specimens was obtained.
2.2.2. Test Process
The low-temperature splitting test was carried out on specimens with different UML thicknesses and control group specimens according to the Chinese specification for Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). Control group specimens were standard Marshall specimens formed at one time. All prepared specimens were frozen in a −10 °C low-temperature chamber for 24 h and then loaded with a multifunctional pavement material strength tester at a 1.0 mm/min loading rate. According to the maximum test load at which the specimens were damaged, the low-temperature splitting tensile strength of specimens was calculated using Equation (1).
where
,
and
h is the height of the specimen (mm).
2.3. Low-Temperature Bending Test
To ensure that the low-temperature crack resistance of UML met the specification requirements, a low-temperature bending test was conducted on specimens with different UML thicknesses according to the Chinese specification for Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). The specimens were 250 mm × 30 mm × 35 mm beams. The specimens consisted of two asphalt mixtures, the lower layer was an AC-13 asphalt mixture and the upper layer was an asphalt mixture for UML. The height of the upper layer was determined by the UML thickness and the height of the lower layer was 35 mm minus the UML thickness. At −10 °C, a pressure testing machine was used to load the specimens at a 50 mm/min loading rate. After the loading, the mid-span deflection at the time of specimen damage was recorded. Equation (2) was used to calculate the failure strain of the specimen.
where
,
h is the height of specimen at mid-span section (mm),
d is the mid-span deflection of the specimen (mm) and
L is the span of the specimen (200 mm).
2.4. Microwave Heating Test
A microwave heating test was carried out to investigate the effect of SiC particle size and content on the temperature uniformity and temperature rise rate of the UML under microwave heating. The UML was prepared by mixing SiC in different particle sizes and contents with bitumen, aggregate and mineral powder. Except for their height, the specimens for the microwave heating test were similar in structure and formation to those in the low-temperature splitting test. The upper layer of the specimens was a 10 mm UML and the lower layer was 31.8 mm AC-13 asphalt concrete. A microwave oven with 900 W output power and 2.45 GHz frequency was used for the test. The prepared specimens were put in the microwave oven at room temperature. The microwave heating time was 30 s, and the surface temperature was recorded every 10 s by an infrared thermal imager. The recorded temperature was entered into the FLIR tools for processing and analysis and provided a temperature matrix of the specimen surface. The process is shown in
Figure 3.
2.5. Microwave Deicing Test
In order to investigate the effect of SiC particle size and content on the deicing efficiency of UML, a microwave deicing test was carried out with specimens in microwave heating test. Briefly, 10 mm ice layers were made on the surface of the specimens. The thickness of the ice layers was controlled by water volume. A sealing belt was used to completely wrap the sides of the specimens to prevent any water from escaping during the making of the ice layers. The water was frozen on the surface of the specimens five times to ensure sufficient bonding between the ice layers and the specimens. The specimens with ice layers were frozen in a low-temperature chamber at −5 °C, −10 °C and −15 °C for 24 h. The low-temperature chamber temperatures were considered as initial temperatures.
The frozen specimens were taken out of the low-temperature chamber and placed vertically in a microwave oven. As soon as the microwave oven was turned on, the ice layers gradually melted as the surface temperature of the specimens rose. Gravity caused the ice layers to fall off the specimens after they had partially melted. The microwave oven was turned off when the ice layer fell off the specimens. The time taken for the microwave oven to heat the specimens was the time required for the ice layer to fall off. The time taken for the ice to fall off, the mass of melted ice and the temperature of the specimen surface were recorded. The time taken for the ice to fall off was recorded as the deicing time. The process of the microwave deicing test is shown in
Figure 4.
2.6. Oil–Stone Ratio Design Test
The oil–stone ratio significantly affects the high-temperature stability, water stability, skid resistance and other road performance of the UML, which is an important technical indicator for the UML. The Marshall test was used to determine the optimum oil–stone ratio of the UML according to the Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) in China. Four groups of oil–stone ratios of 6.5%, 7.0%, 7.5% and 8.0% were first selected. After measuring the bulk density of each group of specimens, the porosity (VV), voids in mineral aggregates (VMA), and saturation (VFA) were calculated. Following the determination of optimum oil–stone ratio, the Schellenberg binder drainage test, Cantabro test, immersion Marshall test, freeze–thaw splitting test and high-temperature rutting test were used to check the design of the asphalt mixture.