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Laboratory investigation on deicing characteristics of asphalt mixtures using magnetite aggregate as microwave-absorbing materials
Construction and Building Materials 124 (2016) 589–597
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Laboratory investigation on deicing characteristics of asphalt mixtures using magnetite aggregate as microwave-absorbing materials Zhenjun Wang a,b,⇑, Hongfei Wang a, Dengdeng An a, Tao Ai a,b, Peng Zhao a,b a b
School of Materials Science and Engineering, Chang’an University, Xi’an 710061, Shaanxi, PR China Engineering Research Central of Pavement Materials, Ministry of Education of PR China, Chang’an University, Xi’an 710061, PR China
h i g h l i g h t s Asphalt mixtures containing magnetite aggregate as microwave absorber were prepared. Deicing characteristics of asphalt mixtures with magnetite aggregate were studied in laboratory. MHE test, microwave reflection and MDT test of asphalt mixtures were conducted. SEM, EDS and XRD were adopted to conduct microscopic analyses.
a r t i c l e
i n f o
Article history: Received 21 April 2016 Received in revised form 27 July 2016 Accepted 28 July 2016
Keywords: Asphalt mixtures Magnetite aggregate Microwave radiation Deicing characteristics
a b s t r a c t Natural magnetic components in magnetite are outstanding microwave absorbers, which can be used in asphalt pavement for deicing practice. Therefore, asphalt mixtures containing magnetite aggregate as microwave absorber for different replacements of conventional basalt aggregate were prepared in this work and their deicing characteristics were studied in laboratory. Properties of asphalt mixtures incorporated with magnetite and basalt aggregates were tested, respectively. Microwave heating efficiency (MHE), microwave reflection and microwave deicing time (MDT) tests of asphalt mixtures were conducted. In addition, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were adopted to conduct microscopic analyses. The results show that asphalt mixtures with magnetite aggregate present better performances at high temperature compared to those containing basalt aggregate with the same content, even though slightly lower water and temperature resistance are observed. The MHE of magnetite aggregate is 6.15 times that of basalt aggregate. The minimum reflectivity value of asphalt mixtures containing 80% magnetite content reaches 11.3 dB at 2.45 GHz frequency. Suitable magnetite content can decrease microwave reflectivity and improve microwave absorbing ability. The MHE of asphalt mixtures increases with the increase of magnetite contents when magnetite content is below 80%. Optimum magnetite aggregate content can evidently shorten the MDT. However, the MDT increases with the decrease of surrounding temperature. In a word, magnetite can be used as aggregates and microwave-absorbing materials in asphalt mixtures to achieve microwave deicing function. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nature reserves rich magnetite materials, whose main component is Fe3O4 with excellent magnetic properties [1,2]. In addition, magnetite mostly possesses excellent mechanical properties, such as higher compressive strength, lower wear value, which are required for materials used as asphalt and concrete aggregate ⇑ Corresponding author at: School of Materials Science and Engineering, Chang’an University, Xi’an 710061, Shaanxi, PR China. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.137 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
[3,4]. Therefore, magnetite materials can be used as low-cost microwave absorbers and replacement of conventional aggregates in construction and building materials [5]. Microwave absorbing material can directly transfer microwave energy into heat. Energy transfer efficiency of microwave heating method is higher than that of infrared heating, so instantaneous heating can be achieved by a microwave power system [6]. In addition, microwave heating is evidently different from conventional heating technique. For the former, heat is generated from dielectric loss and magnetic loss of materials under microwave radiation [7]; while for the latter, materials are heated from the outside to the
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597
inside through conduction, convection and radiation. Microwave heating is characterized by its uniformity and non-pollution. In a word, microwave heating technique possesses many advantages over conventional heating methods in heating efficiency and energy utilization. For the issue of road deicing in winter, the current commonly used road deicing techniques are deicer method [8,9] and mechanical method [10,11]. For the former, its operation is simple and the cost is low, but it can result in serious environmental pollution and severe corrosion for structure steels. For the latter, many vehicles are required and the pavement structure can be destroyed during deicing operation. For the rapid and effective removal of snow and ice on pavement, the novel road ice melting techniques are explored. Microwave deicing technique is one of them. However, the problems are that the traditional paving material for microwave absorption heating efficiency is low; ice melting speed is slow and the continuous and the rapid deicing performances are difficult to realize. Therefore, if the magnetite is used in asphalt mixtures, ice and snow on pavement can be removed easily under radiation of microwave pavement maintenance vehicles. There are many literatures relating to microwave technology for asphalt pavement. For example, magnetite was found enormously beneficial for microwave heating and microwave heating efficiency of asphalt pavement was studied [12,13]. Microwave deicing vehicles have been used in asphalt pavement maintenance [14] and microwave measurement could be utilized for snow and ice detection on road surface [15]. Sweeping microwave asphalt radar was developed for pavement deicing [16] and directivity of coupled antennas for microwave heating of asphalt mixture was improved [17]. Roller mountable asphalt pavement density sensor with microwave was developed [18]. Crumb rubber pretreated by microwave irradiation can improve asphalt blending efficiency [19]. In addition, improving the short-term aging resistance of asphalt by addition of crumb rubber radiated by microwave was investigated [20] and technical viability of heating asphalt mixtures with microwaves to promote self-healing was conducted [21]. In theory, heat transportation in asphalt mixtures was studied based on multiphase fluid thermal physics transfer of microwave heating [22]. On the other hand, some waste materials, such as recycled taconite [23], waste engine oil residues [24] and reclaimed asphalt pavements [25] are widely used for recycling in asphalt pavement construction. The asphalt mixtures with microwave absorbing materials can be used in asphalt pavement recycles [26]. The microwave vehicles used for deicing can also be used for asphalt reclaimed applications because the generated microwave energy is just the required heat of hot mixing asphalt pavement maintenance. Therefore, asphalt pavement with damages, such as cracks, rutting, etc., can be reheated and repaved after the microwave radiation. So asphalt pavement can be reconstructed in relatively short duration. There is no air pollution in these recycled processes [27]. Of course, it is challengeable to melt ice with large areas with the increase of today’s energy prices for microwave heating technology. It may be used in very sensitive and small applications of asphalt pavement deicing. However, the above mentioned study results show that it is performable using microwave technology in asphalt pavement. The key of promoting this technology is to improve microwave absorbing ability of materials and heating efficiency of asphalt mixtures under microwave radiation. Therefore, this study used magnetite aggregate as microwave absorbers in asphalt mixtures for pavement deicing. So, six different mixture designs were conducted to study influences of magnetite aggregate on surface temperature of asphalt mixtures. The microwave reflections under different microwave frequency were studied to explain the temperature changes. The primary aim of this work is to use
magnetite aggregate successfully as microwave-absorbing materials in asphalt mixtures and enormously improve heating efficiency of asphalt mixture under microwave radiation so as to decrease damages of chemical and physical deicing methods to asphalt pavement. 2. Experimental 2.1. Raw materials Magnetite and basalt were used as aggregates in asphalt mixtures, whose properties were shown in Table 1. The SBS (styrene-butadiene-styrene) modified asphalt was adopted as binder and its properties were given in Table 2. Hydrated lime with 2.743 g/cm3 density and 78.4% (CaO + MgO) contents was used to improve asphalt-aggregate interfacial adhesion. The mineral fillers were limestone fillers with no agglomeration, whose density is 2.895 g/cm3 and hydrophilic coefficient is 0.55. 2.2. Preparation of specimens The aggregate gradation was shown in Fig. 1. 20%, 40%, 60%, 80% and 100% basalt aggregate were replaced by magnetite aggregate in the same volume. A mixer was used for mixing at the temperature of 165 °C. The aggregate, asphalt binder, hydrated lime and Table 1 Properties of aggregates. Properties
Magnetite
Basalt
Crushing value (%) Abrasion value (%) Apparent specific density (g/cm3) Water absorption rate (%) Adhesion with asphalt ( ¥ ) <0.075 mm particle content (%)
9.5 9.9 3.774 0.23 5 0.1
15.3 13.7 2.851 0.78 5 0.1
Table 2 Properties of asphalt. Properties
Results
Specification [28]
Needle penetration (25 °C, 100 g, 5 s) (0.1 mm) Ductility (15 °C, 5 cm/min) (cm) Soften point (°C) Density (25 °C, g/cm3) Wax content (%)
54 45 85.1 1.038 1.3
40–60 P25 P70 – –
100
Passing percentage (%)
590
80
60
40
20
0 0.075 0.15
0.3
0.6
1.18
2.36
4.75
Sieve size (mm) Fig. 1. Chart of aggregate gradation.
9.5
13.2
16
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limestone fillers were added in order. The mixer rotated with 75 r/min speed for 3 min. The asphalt mixtures were molded within 2 min for specimens with different dimensions. Three specimens were prepared for each test. They were placed in a room with the temperature of 20 ± 2 °C for use.
MS ¼
MS1 100 MS0
591
ð3Þ
Where, MS – residual Marshall stability ratio, %; MS0 and MS1 – Marshall stability of specimens kept in water bath for 30 min and 48 h, kN.
2.3. Characterization 2.3.1. Properties of asphalt mixtures The influences of different aggregates on Marshall test, high temperature, low temperature and water stability performances of asphalt mixtures were evaluated through the indexes, such as Marshall stability, flow value, dynamic stability, residual Marshall stability and strain at low temperature in accordance with a Chinese specification (JTG E20-2011) [28]. High temperature performance of asphalt pavements was evaluated by dynamic stability. The mixtures were compacted to a slab with size of 300 mm 300 mm 50 mm employing a wheel compactor. Dynamic stability test was performed at temperature of 60 °C and a wheel-pressure of 0.7 MPa. The dynamic stability was calculated by Eq. (1).
DS ¼
42 15 d60 d45
ð1Þ
Where, DS – dynamic stability of the mixtures, cycles/mm; d60 and d45 – vertical deformation of mixtures at 60 min and 45 min, mm; 42 is the loading rate; and 15 is time lag. Strain was measured to evaluate tensile performance of asphalt mixtures at low temperature. The specimen dimensions were 250 mm 30 mm 35 mm and the bearing span was 200 mm. Loading rate was 50 mm/min and the temperature was 10 °C in an insulating chamber. The load was exerted on intermediate span until the specimen was destroyed. LVDT (Linear Variable Differential Transformer) displacement transducer was adopted to record mid span deflection. The strain was calculated by Eq. (2).
e¼
6hd L2
ð2Þ
Where, e – maximum bending strain, le; h – height of mid span section, mm; L – span of the specimen, mm; d – maximum mid span deflection, mm. Water stability performance of asphalt mixtures was evaluated by residual Marshall stability ratio. In this work, six specimens with size of 101.6 mm in diameter and 63.5 mm in height were compacted with 75 blows by Marshall compactor. All specimens were divided into two groups and were immersed in water at temperature of 60 °C for 30 min and 48 h. Then, Marshall test were conducted. Eq. (3) was adopted to calculate residual Marshall stability ratio.
2.3.2. Microwave heating efficiency measurement In order to investigate temperature changes of aggregates and asphalt mixtures after microwave radiation, microwave oven (Type MM721NG1-PS, power 800 W, frequency 2.45 GHz) was used to test their surface temperature. 500 g aggregates with the aggregate gradation shown in Fig. 1 were prepared and were put into a breaker for use. The microwave radiation time for the breaker and radiation interval were 30 s and 5 s at the surrounding temperature of 14 °C, respectively. For the mixtures, surface temperature of 180 mm 180 mm 30 mm specimen was tested by the same method with 120 s heating duration and 20 s heating interval. After microwave radiation each time, surface temperature was immediately recorded by an AR330 infrared thermometer. Nine points for each specimen were conducted and their average value was adopted as testing result. The microwave heating efficiency (MHE) was calculated by Eq. (4).
MHE ¼
T T0 t
ð4Þ
Where, MHE – microwave heating efficiency, °C/s; T – final temperature of specimen, °C; T0 – original temperature of specimen, °C; t – total heating time, s. 2.3.3. Microwave reflection test The reflection versus microwave of 180 mm 180 mm 30 mm specimen was measured by an arch reflectivity measurement at the temperature of 20 °C. The diagrammatic sketch is shown in Fig. 2. The specimen was placed in calibrated pyramidal absorbers. A network analyzer (E8362B) was connected to analyze microwave reflectivity in 2.0–4.0 GHz frequency. Test procedure can be referenced in Ref. [29]. One side of specimen was tested three times and the calculated average results were adopted. 2.3.4. Microwave deicing time test Three thermocouples were buried on the surface of specimens; then 10 mm thickness ice was frozen on specimen surface. The specimens and ice were kept in a chamber with temperatures of 10 °C, 15 °C and 20 °C to constant temperature. After they were taken out from the chamber, they were put into the microwave oven immediately and the temperature inspection instrument was used for measuring and recording the temperature of
Fig. 2. Diagram of arch reflectivity measurement.
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thermocouples. When surface temperature reached 0 °C, the heating time was recorded as microwave deicing time (MDT). 2.3.5. Microscopic analyses The magnetite specimen was washed three times with ethanol and dried in an oven with temperature of 40 °C. Then, morphology of magnetite aggregate was tested by scanning electron microscope (SEM, type S4800, resolution 3.5 nm, in vacuum, test voltage 15 kV). Energy dispersive spectrometer (EDS) was used to analyze chemical element percentages in magnetite aggregate. X-ray diffraction (XRD, D/MAX 2400 diffractometer) pattern of the milled magnetite was obtained using a Cu-Ka radiation under conditions of 40 kV and 100 mA, in the scan speed of 4°/min. 3. Results and discussion 3.1. Properties of asphalt mixtures with different aggregates The properties of asphalt mixtures with total magnetite or basalt aggregates were tested and the results are shown in Table 3. It was observed that asphalt mixtures containing magnetite aggregate present higher Marshall stability and lower flow values than asphalt mixtures with basalt aggregate with the same volume percentage. However, properties of asphalt mixtures containing magnetite aggregate in water and at low temperature are slightly decreased with lower residual stability and smaller strain at low temperature. The morphology in Fig. 3 shows that the magnetite aggregate has evident angularity and possesses crystal structures, which are beneficial to form interlocking structure among aggregates and improve ability of withstanding load distortion [30]. The results indicate that asphalt mixtures with magnetite aggregate exhibit better mechanical properties and higher temperature resistance than the mixtures with conventional basalt aggregate. Water
With magnetite
Marshall stability (kN) Flow value (0.1 mm) Dynamic stability (cycle/mm) Residual stability (%) Strain at low temperature (le, 10 °C, 50 mm/min)
3.2. Microwave heating efficiency of different aggregates Surface temperature of different aggregates under microwave radiation is shown in Fig. 4. After microwave radiation 30 s, surface temperature of basalt aggregate increases by 25.1 °C and the MHE is 0.84 °C/s. However, that of magnetite aggregate increases by 155.2 °C and the MHE is 5.17 °C/s. Therefore, the MHE of magnetite aggregate is 6.15 times that of basalt aggregate. The microwave of 2.45 GHz frequency with 12.24 cm wavelength possesses strong penetration ability [32]. Asphalt mixture containing magnetite aggregate can be heated by the microwave with 2.45 GHz frequency in short duration to achieve the purpose of deicing or recycles of asphalt mixtures. Some materials cannot store but consume energy in an alternating electric field. The loss will be transferred to heating and
Limestone Magnetite
180
150
120
90
60
30
Table 3 Properties of asphalt mixtures. Properties
stability and crack resistance at low temperature of the mixtures with magnetite aggregate can meet the requests in the Chinese specification (JTG F40-2004) [31]. Therefore, magnetite aggregate can be used in asphalt mixtures for pavement engineering.
Temperature (° )
592
17.3 28.3 6874 92.4 3215
With basalt 15.6 35.2 5721 93.7 3767
Specification [31] ¥ 8.0 20–40 ¥ 2800 ¥ 85 ¥ 2800
Fig. 3. SEM picture of magnetite aggregate.
0 0
5
10
15
20
25
Microwave radiation duration (s) Fig. 4. Surface temperature of different aggregates.
Fig. 5. EDS spectrum of magnetite aggregate.
30
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597 Table 4 Elements and the percentages in magnetite aggregate. Elements
O
Si
Fe
Mg
Ca
Al
Total
Mass per./% Atom per./%
47.09 63.36
19.46 14.55
14.96 8.74
7.91 6.83
6.72 3.52
3.86 3.00
100.00 100.00
increase of temperature. The EDS spectrum of magnetite aggregate is shown in Fig. 5 and it shows the existence of Fe element. The Fe percentage in magnetite aggregate is shown in Table 4. Fe element in magnetite can generate magnetic loss and microwave absorption ability mainly depends on dielectric loss and magnetic loss [33]. In addition, the XRD pattern for characterizing mineral compositions of magnetite as shown in Fig. 6 reveals the presence of main minerals, such as magnetite, anorthite, tremolite and chlorite. The magnetic component gives better magnetic properties to the mixture, which can improve the microwave absorbing ability and decrease the microwave reflectivity.
3.3. Microwave reflectivity of asphalt mixtures Fig. 7 shows microwave reflectivity of asphalt mixtures with different magnetite aggregate contents in frequency ranges of 2.0–4.0 GHz. For asphalt mixtures without magnetite aggregate, it is shown that the reflectivity in the frequency ranges is higher than 4.0 dB, which indicates that the specimen possesses strong microwave reflecting performance. However, when the magnetite content reaches 20%, two peaks with the reflectivity of 7.2 dB and 7.4 dB occur at 2.10 GHz and 2.25 GHz. The specimen still mainly reflects microwave. When magnetite aggregate content increases to 40%, one evident absorbing peak appears. The reflectivity value is 10.3 dB at 2.70 GHz, which represents 90% microwave absorption [29]. Although there are other two peaks, their values are greater than 10.0 dB. As the magnetite content reaches 60%, there still exists one peak with lower than 10.0 dB reflectivity and it is very sharp and narrow. The reflectivity value is 10.5 dB at 2.60 GHz frequency. When the magnetite content reaches 80%, one evident absorbing peak with 11.3 dB at 2.45 GHz appears, which is consistent with the working frequency of microwave oven. The frequency width with less than 10.0 dB reflectivity is 0.16 GHz. However, when magnetite aggregate content reaches 100%, most reflectivity values are greater than 10.0 dB, but one is 10.4 dB at 2.80 GHz with the width of
593
0.04 GHz. The microwave absorbing ability of asphalt mixtures begins to decline. The minimum reflectivity of the specimens with different magnetite aggregate contents is shown in Table 5. It can also be seen that the reflectivity below 10.0 dB does not appear for the specimens with lower 40% magnetite aggregate content and the microwave reflection becomes dominant. The minimum reflectivity values tend to decrease with the increase of magnetite contents below 80%. The absorbing band width (reflectivity < 10.0 dB) for the specimen with 80% magnetite aggregate content is 0.16 GHz over the frequency range from 2.37 to 2.53 GHz. The minimum reflectivity value of asphalt mixtures reaches 11.3 dB at 2.45 GHz frequency. 3.4. Surface temperature of asphalt mixtures Fig. 8 shows the relation between surface temperature of asphalt mixtures and microwave radiation duration. In addition, Fig. 9 shows surface temperature difference of asphalt mixtures before and after 120 s microwave radiation. When magnetite aggregate content reaches 80%, surface temperature of asphalt mixtures can increase from 14 °C to 64 °C within 120 s microwave radiation duration and the MHE is 0.41 °C/s. While that of asphalt mixtures with basalt aggregate is from 14 °C to 22 °C within the same microwave radiation duration and the MHE is 0.07 °C/s. Furthermore, Fig. 8 also shows that surface temperature is decreased for asphalt mixtures with over 80% magnetite aggregate. Therefore, surface temperature and the MHE of asphalt mixtures increase with the increase of magnetite aggregate content less than 80%. However, it is not suggested to heat the asphalt pavement with too higher temperature, such as 100 °C or more. The asphalt binder can start to melt at the temperature of 100 °C and shows the soft state, which will affect the performance of asphalt pavement under the load of microwave heating equipment. In addition, higher temperature needs much more energy. The microwave propagation path in the mixtures is shown in Fig. 10. When the microwave is irradiated with asphalt mixtures, a part of the microwave is reflected on surface of asphalt mixtures. Afterwards, the microwave enters interior of asphalt mixtures and multiple reflections appear as well as absorption by magnetite aggregate. At last, rest of microwave penetrates through asphalt mixtures and the mixtures are heated by microwave radiation. The improvement of microwave heating efficiency depends on improving microwave absorption degree and reducing microwave reflection. Therefore, the decrease of microwave reflectivity is the
Fig. 6. XRD pattern of magnetite aggregate.
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4
-2
(d)
3
-3
2
-4
Reflectivity (dB)
Reflectivity (dB)
(a)
1 0 -1 -2
-5 -6 -7 -8 -9
-3
-10 -11
-4 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
2.00
4.00
2.25
2.50
3.00
3.25
3.50
3.75
4.00
-2
-2
(b)
(e)
-3
-3
-4
Reflectivity (dB)
Reflectivity (dB)
2.75
Frequency (GHz)
Frequency (GHz)
-4
-5
-6
-5 -6 -7 -8 -9 -10
-7 -11 -12
-8 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
2.00
4.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
Frequency (GHz)
Frequency (GHz) 1
0
(c)
-1
(f)
0 -1
-2
-2
Reflectivity (dB)
Reflectivity (dB)
-3 -4 -5 -6 -7 -8
-3 -4 -5 -6 -7 -8
-9
-9
-10
-10 -11
-11 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
Frequency (GHz)
Frequency (GHz)
Fig. 7. Reflectivity of asphalt mixtures with different magnetite content in 2.0–4.0 GHz frequency ranges: (a) 0%; (b) 20%; (c) 40%; (d) 60%; (e) 80% and (f) 100%.
key to improve surface temperature of asphalt mixtures under microwave radiation. Moreover, microwave heating power can be calculated as Eq. (5) [34], in which e and tan d are related with materials. Eq. (5) shows that the increase of microwave frequency (f) can improve microwave heating power. The heating efficiency obviously is a function of a power source for radiation. Results also show that 5.80 GHz microwave can improve microwave deicing efficiency
by 4–6 times over 2.45 GHz microwave [35]. Therefore, the enhancement of microwave frequency is an effective way to improve the MHE.
W ¼ 2pf e0 e tan d E2
ð5Þ
Where, f – microwave frequency, GHz; E – electromagnetic intensity, V/m; e0 – dielectric constant in vacuum, 8.85 1012; e – dielectric constant of materials; tan d – loss tangent.
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597
595
Table 5 The minimum reflectivity of asphalt mixtures. Magnetite content (%)
Minimum reflectivity (dB)
Frequency (GHz)
Band width below 10 dB (GHz) (frequency range)
0 20 40 60 80 100
3.4 7.4 10.3 10.5 11.3 10.4
2.10 2.25 2.70 2.60 2.45 2.80
Not exist Not exist 0.02 (2.70–2.72) 0.04 (2.59–2.63) 0.16 (2.37–2.53) 0.04 (2.76–2.80)
70 0% 20% 40% 60% 80% 100%
65 60
Temperature (° )
55 50 45 40 35 30 25 20 15 10 0
20
40
60
80
100
120
Heating duration (s) Fig. 8. Relation between surface temperature of asphalt mixtures and microwave radiation duration.
3.5. Microwave deicing time (MDT) of asphalt mixtures Freezing layer at interface between ice layer and asphalt mixtures is shown in Fig. 11(a). Due to obstacle of freezing layer, much ice shown in Fig. 11(b) is left on asphalt pavement and effect of mechanical deicing is not satisfied, which can result in serious danger to vehicles. However, the microwave can heat evenly at the interface between ice and pavement, which can eliminate influence of freezing layer and make the ice completely strip from asphalt pavement. In this work, microwave deicing time (MDT) was tested when the interface temperature reached 0 °C and the results are shown in Fig. 12. Fig. 12 shows the relation between MDT and magnetite contents with different microwave radiation time at different surrounding temperatures. When the surrounding temperature is 5 °C, the MDT is 242 s and 28 s for the asphalt mixtures with 0% and 80% magnetite contents. For 10 °C, they are 275 s and 34 s; and for 15 °C, they are 354 s and 56 s, respectively. The MDT decreases with the increase of magnetite aggregate lower 80% content. However, when magnetite aggregate content is over 80%, the MDT begins to increase because microwave absorbing ability is decreased. That is to say, suitable magnetite aggregate content can evidently shorten the MDT. One another, the MDT increases with the decrease of surrounding temperature. It indicates that the surrounding temperature can also enormously affect the microwave deicing effects. The ideal absorbing materials should absorb microwave as much as possible, which requires the materials to possess large dielectric loss or magnetic loss [36]. The magnetic loss is weak if asphalt mixtures do not contain magnetite aggregate because magnetic components of asphalt and conventional aggregate are low. Therefore, magnetite aggregate endows better magnetic
Fig. 9. Surface temperature of asphalt mixtures: (a) before microwave radiation; (b) after microwave radiation for 120 s.
Fig. 10. Propagation path of microwave though asphalt mixtures.
characteristics to asphalt mixtures, which can enhance microwave absorbing ability and improve microwave deicing performance of asphalt mixtures. On the other hand, the decrease of microwave scattering and surface reflections is also important to shorten the MDT. Nowadays, ultra thin asphalt pavement has been adopted and used in asphalt pavement. AC-10, Nominal maximum particle size 9.5 mm, and AC-5, Nominal maximum particle size 4.75 mm, have been introduced in Chinese Specification [31] and have been used
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MDT (s)
Fig. 11. Ice on asphalt mixtures: (a) before deicing; (b) after deicing with vehicles.
375 350 325 300 275 250 225 200 175 150 125 100 75 50 25 0
Surrounding temperature -5 Surrounding temperature-10 Surrounding temperature-15
c) The minimum reflectivity value of asphalt mixtures containing 80% magnetite content reaches 11.3 dB at 2.45 GHz, which is microwave oven working frequency. Suitable magnetite aggregate endows better magnetic properties to asphalt mixtures, which can improve microwave absorbing ability and decrease microwave reflectivity of asphalt mixtures. d) When magnetite aggregate content reaches 80%, surface temperature of asphalt mixtures can increase from 14 °C to 67 °C within 120 s microwave radiation duration. The microwave heating efficiency of asphalt mixtures increase with the increase of magnetite contents if the aggregate content is below 80%. e) Microwave deicing time decreases with the increase of magnetite aggregate content when magnetite aggregate content is lower than 80%. When magnetite aggregate content is over 80%, the time begins to increase because of the decreased microwave absorbing ability. Suitable magnetite aggregate content can evidently shorten microwave deicing time. However, microwave deicing time increases with the decrease of surrounding temperature.
Acknowledgements The authors acknowledge financial supports provided by the Fundamental Research Funds for the Central Universities (Nos. 310831153504 and 310831163113) and Project of Yan’an Highway Administration Bureau in PR China. References 0
20
40
60
80
100
Magnetite content (%) Fig. 12. Relation chart between MDT and magnetite contents at different surrounding temperatures.
in asphalt pavement [37]. Asphalt mixtures with less than 10 mm aggregate have been successfully designed. Therefore, it is feasible and advisable to replace conventional aggregates with magnetite aggregate with the same volume percentage to improve microwave absorption. 4. Conclusions This work presents results of a laboratory study exploring deicing function of asphalt mixtures using magnetite aggregate as substitution of basalt aggregate, which can be referenced for deicing practice through non-chemical method and promote energyefficient process in hot mixing asphalt mixtures. The following conclusions can be drawn. a) Asphalt mixtures containing magnetite aggregate present better performances at high temperature compared to the asphalt mixtures containing basalt aggregate with the same volume percentage, even though slightly lower water stability and low temperature resistance are observed. The properties of asphalt mixtures containing magnetite aggregate can reach requests of the Chinese specification. b) The microwave heating efficiency of magnetite aggregate is 6.15 times that of basalt aggregate. More heat can be generated and the temperature can be increased with the addition of magnetite aggregate in asphalt mixtures because of the existence of magnetic components.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Laboratory investigation on deicing characteristics of asphalt mixtures using magnetite aggregate as microwave-absorbing materials Zhenjun Wang a,b,⇑, Hongfei Wang a, Dengdeng An a, Tao Ai a,b, Peng Zhao a,b a b
School of Materials Science and Engineering, Chang’an University, Xi’an 710061, Shaanxi, PR China Engineering Research Central of Pavement Materials, Ministry of Education of PR China, Chang’an University, Xi’an 710061, PR China
h i g h l i g h t s Asphalt mixtures containing magnetite aggregate as microwave absorber were prepared. Deicing characteristics of asphalt mixtures with magnetite aggregate were studied in laboratory. MHE test, microwave reflection and MDT test of asphalt mixtures were conducted. SEM, EDS and XRD were adopted to conduct microscopic analyses.
a r t i c l e
i n f o
Article history: Received 21 April 2016 Received in revised form 27 July 2016 Accepted 28 July 2016
Keywords: Asphalt mixtures Magnetite aggregate Microwave radiation Deicing characteristics
a b s t r a c t Natural magnetic components in magnetite are outstanding microwave absorbers, which can be used in asphalt pavement for deicing practice. Therefore, asphalt mixtures containing magnetite aggregate as microwave absorber for different replacements of conventional basalt aggregate were prepared in this work and their deicing characteristics were studied in laboratory. Properties of asphalt mixtures incorporated with magnetite and basalt aggregates were tested, respectively. Microwave heating efficiency (MHE), microwave reflection and microwave deicing time (MDT) tests of asphalt mixtures were conducted. In addition, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were adopted to conduct microscopic analyses. The results show that asphalt mixtures with magnetite aggregate present better performances at high temperature compared to those containing basalt aggregate with the same content, even though slightly lower water and temperature resistance are observed. The MHE of magnetite aggregate is 6.15 times that of basalt aggregate. The minimum reflectivity value of asphalt mixtures containing 80% magnetite content reaches 11.3 dB at 2.45 GHz frequency. Suitable magnetite content can decrease microwave reflectivity and improve microwave absorbing ability. The MHE of asphalt mixtures increases with the increase of magnetite contents when magnetite content is below 80%. Optimum magnetite aggregate content can evidently shorten the MDT. However, the MDT increases with the decrease of surrounding temperature. In a word, magnetite can be used as aggregates and microwave-absorbing materials in asphalt mixtures to achieve microwave deicing function. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nature reserves rich magnetite materials, whose main component is Fe3O4 with excellent magnetic properties [1,2]. In addition, magnetite mostly possesses excellent mechanical properties, such as higher compressive strength, lower wear value, which are required for materials used as asphalt and concrete aggregate ⇑ Corresponding author at: School of Materials Science and Engineering, Chang’an University, Xi’an 710061, Shaanxi, PR China. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.137 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
[3,4]. Therefore, magnetite materials can be used as low-cost microwave absorbers and replacement of conventional aggregates in construction and building materials [5]. Microwave absorbing material can directly transfer microwave energy into heat. Energy transfer efficiency of microwave heating method is higher than that of infrared heating, so instantaneous heating can be achieved by a microwave power system [6]. In addition, microwave heating is evidently different from conventional heating technique. For the former, heat is generated from dielectric loss and magnetic loss of materials under microwave radiation [7]; while for the latter, materials are heated from the outside to the
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597
inside through conduction, convection and radiation. Microwave heating is characterized by its uniformity and non-pollution. In a word, microwave heating technique possesses many advantages over conventional heating methods in heating efficiency and energy utilization. For the issue of road deicing in winter, the current commonly used road deicing techniques are deicer method [8,9] and mechanical method [10,11]. For the former, its operation is simple and the cost is low, but it can result in serious environmental pollution and severe corrosion for structure steels. For the latter, many vehicles are required and the pavement structure can be destroyed during deicing operation. For the rapid and effective removal of snow and ice on pavement, the novel road ice melting techniques are explored. Microwave deicing technique is one of them. However, the problems are that the traditional paving material for microwave absorption heating efficiency is low; ice melting speed is slow and the continuous and the rapid deicing performances are difficult to realize. Therefore, if the magnetite is used in asphalt mixtures, ice and snow on pavement can be removed easily under radiation of microwave pavement maintenance vehicles. There are many literatures relating to microwave technology for asphalt pavement. For example, magnetite was found enormously beneficial for microwave heating and microwave heating efficiency of asphalt pavement was studied [12,13]. Microwave deicing vehicles have been used in asphalt pavement maintenance [14] and microwave measurement could be utilized for snow and ice detection on road surface [15]. Sweeping microwave asphalt radar was developed for pavement deicing [16] and directivity of coupled antennas for microwave heating of asphalt mixture was improved [17]. Roller mountable asphalt pavement density sensor with microwave was developed [18]. Crumb rubber pretreated by microwave irradiation can improve asphalt blending efficiency [19]. In addition, improving the short-term aging resistance of asphalt by addition of crumb rubber radiated by microwave was investigated [20] and technical viability of heating asphalt mixtures with microwaves to promote self-healing was conducted [21]. In theory, heat transportation in asphalt mixtures was studied based on multiphase fluid thermal physics transfer of microwave heating [22]. On the other hand, some waste materials, such as recycled taconite [23], waste engine oil residues [24] and reclaimed asphalt pavements [25] are widely used for recycling in asphalt pavement construction. The asphalt mixtures with microwave absorbing materials can be used in asphalt pavement recycles [26]. The microwave vehicles used for deicing can also be used for asphalt reclaimed applications because the generated microwave energy is just the required heat of hot mixing asphalt pavement maintenance. Therefore, asphalt pavement with damages, such as cracks, rutting, etc., can be reheated and repaved after the microwave radiation. So asphalt pavement can be reconstructed in relatively short duration. There is no air pollution in these recycled processes [27]. Of course, it is challengeable to melt ice with large areas with the increase of today’s energy prices for microwave heating technology. It may be used in very sensitive and small applications of asphalt pavement deicing. However, the above mentioned study results show that it is performable using microwave technology in asphalt pavement. The key of promoting this technology is to improve microwave absorbing ability of materials and heating efficiency of asphalt mixtures under microwave radiation. Therefore, this study used magnetite aggregate as microwave absorbers in asphalt mixtures for pavement deicing. So, six different mixture designs were conducted to study influences of magnetite aggregate on surface temperature of asphalt mixtures. The microwave reflections under different microwave frequency were studied to explain the temperature changes. The primary aim of this work is to use
magnetite aggregate successfully as microwave-absorbing materials in asphalt mixtures and enormously improve heating efficiency of asphalt mixture under microwave radiation so as to decrease damages of chemical and physical deicing methods to asphalt pavement. 2. Experimental 2.1. Raw materials Magnetite and basalt were used as aggregates in asphalt mixtures, whose properties were shown in Table 1. The SBS (styrene-butadiene-styrene) modified asphalt was adopted as binder and its properties were given in Table 2. Hydrated lime with 2.743 g/cm3 density and 78.4% (CaO + MgO) contents was used to improve asphalt-aggregate interfacial adhesion. The mineral fillers were limestone fillers with no agglomeration, whose density is 2.895 g/cm3 and hydrophilic coefficient is 0.55. 2.2. Preparation of specimens The aggregate gradation was shown in Fig. 1. 20%, 40%, 60%, 80% and 100% basalt aggregate were replaced by magnetite aggregate in the same volume. A mixer was used for mixing at the temperature of 165 °C. The aggregate, asphalt binder, hydrated lime and Table 1 Properties of aggregates. Properties
Magnetite
Basalt
Crushing value (%) Abrasion value (%) Apparent specific density (g/cm3) Water absorption rate (%) Adhesion with asphalt ( ¥ ) <0.075 mm particle content (%)
9.5 9.9 3.774 0.23 5 0.1
15.3 13.7 2.851 0.78 5 0.1
Table 2 Properties of asphalt. Properties
Results
Specification [28]
Needle penetration (25 °C, 100 g, 5 s) (0.1 mm) Ductility (15 °C, 5 cm/min) (cm) Soften point (°C) Density (25 °C, g/cm3) Wax content (%)
54 45 85.1 1.038 1.3
40–60 P25 P70 – –
100
Passing percentage (%)
590
80
60
40
20
0 0.075 0.15
0.3
0.6
1.18
2.36
4.75
Sieve size (mm) Fig. 1. Chart of aggregate gradation.
9.5
13.2
16
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597
limestone fillers were added in order. The mixer rotated with 75 r/min speed for 3 min. The asphalt mixtures were molded within 2 min for specimens with different dimensions. Three specimens were prepared for each test. They were placed in a room with the temperature of 20 ± 2 °C for use.
MS ¼
MS1 100 MS0
591
ð3Þ
Where, MS – residual Marshall stability ratio, %; MS0 and MS1 – Marshall stability of specimens kept in water bath for 30 min and 48 h, kN.
2.3. Characterization 2.3.1. Properties of asphalt mixtures The influences of different aggregates on Marshall test, high temperature, low temperature and water stability performances of asphalt mixtures were evaluated through the indexes, such as Marshall stability, flow value, dynamic stability, residual Marshall stability and strain at low temperature in accordance with a Chinese specification (JTG E20-2011) [28]. High temperature performance of asphalt pavements was evaluated by dynamic stability. The mixtures were compacted to a slab with size of 300 mm 300 mm 50 mm employing a wheel compactor. Dynamic stability test was performed at temperature of 60 °C and a wheel-pressure of 0.7 MPa. The dynamic stability was calculated by Eq. (1).
DS ¼
42 15 d60 d45
ð1Þ
Where, DS – dynamic stability of the mixtures, cycles/mm; d60 and d45 – vertical deformation of mixtures at 60 min and 45 min, mm; 42 is the loading rate; and 15 is time lag. Strain was measured to evaluate tensile performance of asphalt mixtures at low temperature. The specimen dimensions were 250 mm 30 mm 35 mm and the bearing span was 200 mm. Loading rate was 50 mm/min and the temperature was 10 °C in an insulating chamber. The load was exerted on intermediate span until the specimen was destroyed. LVDT (Linear Variable Differential Transformer) displacement transducer was adopted to record mid span deflection. The strain was calculated by Eq. (2).
e¼
6hd L2
ð2Þ
Where, e – maximum bending strain, le; h – height of mid span section, mm; L – span of the specimen, mm; d – maximum mid span deflection, mm. Water stability performance of asphalt mixtures was evaluated by residual Marshall stability ratio. In this work, six specimens with size of 101.6 mm in diameter and 63.5 mm in height were compacted with 75 blows by Marshall compactor. All specimens were divided into two groups and were immersed in water at temperature of 60 °C for 30 min and 48 h. Then, Marshall test were conducted. Eq. (3) was adopted to calculate residual Marshall stability ratio.
2.3.2. Microwave heating efficiency measurement In order to investigate temperature changes of aggregates and asphalt mixtures after microwave radiation, microwave oven (Type MM721NG1-PS, power 800 W, frequency 2.45 GHz) was used to test their surface temperature. 500 g aggregates with the aggregate gradation shown in Fig. 1 were prepared and were put into a breaker for use. The microwave radiation time for the breaker and radiation interval were 30 s and 5 s at the surrounding temperature of 14 °C, respectively. For the mixtures, surface temperature of 180 mm 180 mm 30 mm specimen was tested by the same method with 120 s heating duration and 20 s heating interval. After microwave radiation each time, surface temperature was immediately recorded by an AR330 infrared thermometer. Nine points for each specimen were conducted and their average value was adopted as testing result. The microwave heating efficiency (MHE) was calculated by Eq. (4).
MHE ¼
T T0 t
ð4Þ
Where, MHE – microwave heating efficiency, °C/s; T – final temperature of specimen, °C; T0 – original temperature of specimen, °C; t – total heating time, s. 2.3.3. Microwave reflection test The reflection versus microwave of 180 mm 180 mm 30 mm specimen was measured by an arch reflectivity measurement at the temperature of 20 °C. The diagrammatic sketch is shown in Fig. 2. The specimen was placed in calibrated pyramidal absorbers. A network analyzer (E8362B) was connected to analyze microwave reflectivity in 2.0–4.0 GHz frequency. Test procedure can be referenced in Ref. [29]. One side of specimen was tested three times and the calculated average results were adopted. 2.3.4. Microwave deicing time test Three thermocouples were buried on the surface of specimens; then 10 mm thickness ice was frozen on specimen surface. The specimens and ice were kept in a chamber with temperatures of 10 °C, 15 °C and 20 °C to constant temperature. After they were taken out from the chamber, they were put into the microwave oven immediately and the temperature inspection instrument was used for measuring and recording the temperature of
Fig. 2. Diagram of arch reflectivity measurement.
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597
thermocouples. When surface temperature reached 0 °C, the heating time was recorded as microwave deicing time (MDT). 2.3.5. Microscopic analyses The magnetite specimen was washed three times with ethanol and dried in an oven with temperature of 40 °C. Then, morphology of magnetite aggregate was tested by scanning electron microscope (SEM, type S4800, resolution 3.5 nm, in vacuum, test voltage 15 kV). Energy dispersive spectrometer (EDS) was used to analyze chemical element percentages in magnetite aggregate. X-ray diffraction (XRD, D/MAX 2400 diffractometer) pattern of the milled magnetite was obtained using a Cu-Ka radiation under conditions of 40 kV and 100 mA, in the scan speed of 4°/min. 3. Results and discussion 3.1. Properties of asphalt mixtures with different aggregates The properties of asphalt mixtures with total magnetite or basalt aggregates were tested and the results are shown in Table 3. It was observed that asphalt mixtures containing magnetite aggregate present higher Marshall stability and lower flow values than asphalt mixtures with basalt aggregate with the same volume percentage. However, properties of asphalt mixtures containing magnetite aggregate in water and at low temperature are slightly decreased with lower residual stability and smaller strain at low temperature. The morphology in Fig. 3 shows that the magnetite aggregate has evident angularity and possesses crystal structures, which are beneficial to form interlocking structure among aggregates and improve ability of withstanding load distortion [30]. The results indicate that asphalt mixtures with magnetite aggregate exhibit better mechanical properties and higher temperature resistance than the mixtures with conventional basalt aggregate. Water
With magnetite
Marshall stability (kN) Flow value (0.1 mm) Dynamic stability (cycle/mm) Residual stability (%) Strain at low temperature (le, 10 °C, 50 mm/min)
3.2. Microwave heating efficiency of different aggregates Surface temperature of different aggregates under microwave radiation is shown in Fig. 4. After microwave radiation 30 s, surface temperature of basalt aggregate increases by 25.1 °C and the MHE is 0.84 °C/s. However, that of magnetite aggregate increases by 155.2 °C and the MHE is 5.17 °C/s. Therefore, the MHE of magnetite aggregate is 6.15 times that of basalt aggregate. The microwave of 2.45 GHz frequency with 12.24 cm wavelength possesses strong penetration ability [32]. Asphalt mixture containing magnetite aggregate can be heated by the microwave with 2.45 GHz frequency in short duration to achieve the purpose of deicing or recycles of asphalt mixtures. Some materials cannot store but consume energy in an alternating electric field. The loss will be transferred to heating and
Limestone Magnetite
180
150
120
90
60
30
Table 3 Properties of asphalt mixtures. Properties
stability and crack resistance at low temperature of the mixtures with magnetite aggregate can meet the requests in the Chinese specification (JTG F40-2004) [31]. Therefore, magnetite aggregate can be used in asphalt mixtures for pavement engineering.
Temperature (° )
592
17.3 28.3 6874 92.4 3215
With basalt 15.6 35.2 5721 93.7 3767
Specification [31] ¥ 8.0 20–40 ¥ 2800 ¥ 85 ¥ 2800
Fig. 3. SEM picture of magnetite aggregate.
0 0
5
10
15
20
25
Microwave radiation duration (s) Fig. 4. Surface temperature of different aggregates.
Fig. 5. EDS spectrum of magnetite aggregate.
30
Z. Wang et al. / Construction and Building Materials 124 (2016) 589–597 Table 4 Elements and the percentages in magnetite aggregate. Elements
O
Si
Fe
Mg
Ca
Al
Total
Mass per./% Atom per./%
47.09 63.36
19.46 14.55
14.96 8.74
7.91 6.83
6.72 3.52
3.86 3.00
100.00 100.00
increase of temperature. The EDS spectrum of magnetite aggregate is shown in Fig. 5 and it shows the existence of Fe element. The Fe percentage in magnetite aggregate is shown in Table 4. Fe element in magnetite can generate magnetic loss and microwave absorption ability mainly depends on dielectric loss and magnetic loss [33]. In addition, the XRD pattern for characterizing mineral compositions of magnetite as shown in Fig. 6 reveals the presence of main minerals, such as magnetite, anorthite, tremolite and chlorite. The magnetic component gives better magnetic properties to the mixture, which can improve the microwave absorbing ability and decrease the microwave reflectivity.
3.3. Microwave reflectivity of asphalt mixtures Fig. 7 shows microwave reflectivity of asphalt mixtures with different magnetite aggregate contents in frequency ranges of 2.0–4.0 GHz. For asphalt mixtures without magnetite aggregate, it is shown that the reflectivity in the frequency ranges is higher than 4.0 dB, which indicates that the specimen possesses strong microwave reflecting performance. However, when the magnetite content reaches 20%, two peaks with the reflectivity of 7.2 dB and 7.4 dB occur at 2.10 GHz and 2.25 GHz. The specimen still mainly reflects microwave. When magnetite aggregate content increases to 40%, one evident absorbing peak appears. The reflectivity value is 10.3 dB at 2.70 GHz, which represents 90% microwave absorption [29]. Although there are other two peaks, their values are greater than 10.0 dB. As the magnetite content reaches 60%, there still exists one peak with lower than 10.0 dB reflectivity and it is very sharp and narrow. The reflectivity value is 10.5 dB at 2.60 GHz frequency. When the magnetite content reaches 80%, one evident absorbing peak with 11.3 dB at 2.45 GHz appears, which is consistent with the working frequency of microwave oven. The frequency width with less than 10.0 dB reflectivity is 0.16 GHz. However, when magnetite aggregate content reaches 100%, most reflectivity values are greater than 10.0 dB, but one is 10.4 dB at 2.80 GHz with the width of
593
0.04 GHz. The microwave absorbing ability of asphalt mixtures begins to decline. The minimum reflectivity of the specimens with different magnetite aggregate contents is shown in Table 5. It can also be seen that the reflectivity below 10.0 dB does not appear for the specimens with lower 40% magnetite aggregate content and the microwave reflection becomes dominant. The minimum reflectivity values tend to decrease with the increase of magnetite contents below 80%. The absorbing band width (reflectivity < 10.0 dB) for the specimen with 80% magnetite aggregate content is 0.16 GHz over the frequency range from 2.37 to 2.53 GHz. The minimum reflectivity value of asphalt mixtures reaches 11.3 dB at 2.45 GHz frequency. 3.4. Surface temperature of asphalt mixtures Fig. 8 shows the relation between surface temperature of asphalt mixtures and microwave radiation duration. In addition, Fig. 9 shows surface temperature difference of asphalt mixtures before and after 120 s microwave radiation. When magnetite aggregate content reaches 80%, surface temperature of asphalt mixtures can increase from 14 °C to 64 °C within 120 s microwave radiation duration and the MHE is 0.41 °C/s. While that of asphalt mixtures with basalt aggregate is from 14 °C to 22 °C within the same microwave radiation duration and the MHE is 0.07 °C/s. Furthermore, Fig. 8 also shows that surface temperature is decreased for asphalt mixtures with over 80% magnetite aggregate. Therefore, surface temperature and the MHE of asphalt mixtures increase with the increase of magnetite aggregate content less than 80%. However, it is not suggested to heat the asphalt pavement with too higher temperature, such as 100 °C or more. The asphalt binder can start to melt at the temperature of 100 °C and shows the soft state, which will affect the performance of asphalt pavement under the load of microwave heating equipment. In addition, higher temperature needs much more energy. The microwave propagation path in the mixtures is shown in Fig. 10. When the microwave is irradiated with asphalt mixtures, a part of the microwave is reflected on surface of asphalt mixtures. Afterwards, the microwave enters interior of asphalt mixtures and multiple reflections appear as well as absorption by magnetite aggregate. At last, rest of microwave penetrates through asphalt mixtures and the mixtures are heated by microwave radiation. The improvement of microwave heating efficiency depends on improving microwave absorption degree and reducing microwave reflection. Therefore, the decrease of microwave reflectivity is the
Fig. 6. XRD pattern of magnetite aggregate.
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4
-2
(d)
3
-3
2
-4
Reflectivity (dB)
Reflectivity (dB)
(a)
1 0 -1 -2
-5 -6 -7 -8 -9
-3
-10 -11
-4 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
2.00
4.00
2.25
2.50
3.00
3.25
3.50
3.75
4.00
-2
-2
(b)
(e)
-3
-3
-4
Reflectivity (dB)
Reflectivity (dB)
2.75
Frequency (GHz)
Frequency (GHz)
-4
-5
-6
-5 -6 -7 -8 -9 -10
-7 -11 -12
-8 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
2.00
4.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
Frequency (GHz)
Frequency (GHz) 1
0
(c)
-1
(f)
0 -1
-2
-2
Reflectivity (dB)
Reflectivity (dB)
-3 -4 -5 -6 -7 -8
-3 -4 -5 -6 -7 -8
-9
-9
-10
-10 -11
-11 2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
Frequency (GHz)
Frequency (GHz)
Fig. 7. Reflectivity of asphalt mixtures with different magnetite content in 2.0–4.0 GHz frequency ranges: (a) 0%; (b) 20%; (c) 40%; (d) 60%; (e) 80% and (f) 100%.
key to improve surface temperature of asphalt mixtures under microwave radiation. Moreover, microwave heating power can be calculated as Eq. (5) [34], in which e and tan d are related with materials. Eq. (5) shows that the increase of microwave frequency (f) can improve microwave heating power. The heating efficiency obviously is a function of a power source for radiation. Results also show that 5.80 GHz microwave can improve microwave deicing efficiency
by 4–6 times over 2.45 GHz microwave [35]. Therefore, the enhancement of microwave frequency is an effective way to improve the MHE.
W ¼ 2pf e0 e tan d E2
ð5Þ
Where, f – microwave frequency, GHz; E – electromagnetic intensity, V/m; e0 – dielectric constant in vacuum, 8.85 1012; e – dielectric constant of materials; tan d – loss tangent.
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Table 5 The minimum reflectivity of asphalt mixtures. Magnetite content (%)
Minimum reflectivity (dB)
Frequency (GHz)
Band width below 10 dB (GHz) (frequency range)
0 20 40 60 80 100
3.4 7.4 10.3 10.5 11.3 10.4
2.10 2.25 2.70 2.60 2.45 2.80
Not exist Not exist 0.02 (2.70–2.72) 0.04 (2.59–2.63) 0.16 (2.37–2.53) 0.04 (2.76–2.80)
70 0% 20% 40% 60% 80% 100%
65 60
Temperature (° )
55 50 45 40 35 30 25 20 15 10 0
20
40
60
80
100
120
Heating duration (s) Fig. 8. Relation between surface temperature of asphalt mixtures and microwave radiation duration.
3.5. Microwave deicing time (MDT) of asphalt mixtures Freezing layer at interface between ice layer and asphalt mixtures is shown in Fig. 11(a). Due to obstacle of freezing layer, much ice shown in Fig. 11(b) is left on asphalt pavement and effect of mechanical deicing is not satisfied, which can result in serious danger to vehicles. However, the microwave can heat evenly at the interface between ice and pavement, which can eliminate influence of freezing layer and make the ice completely strip from asphalt pavement. In this work, microwave deicing time (MDT) was tested when the interface temperature reached 0 °C and the results are shown in Fig. 12. Fig. 12 shows the relation between MDT and magnetite contents with different microwave radiation time at different surrounding temperatures. When the surrounding temperature is 5 °C, the MDT is 242 s and 28 s for the asphalt mixtures with 0% and 80% magnetite contents. For 10 °C, they are 275 s and 34 s; and for 15 °C, they are 354 s and 56 s, respectively. The MDT decreases with the increase of magnetite aggregate lower 80% content. However, when magnetite aggregate content is over 80%, the MDT begins to increase because microwave absorbing ability is decreased. That is to say, suitable magnetite aggregate content can evidently shorten the MDT. One another, the MDT increases with the decrease of surrounding temperature. It indicates that the surrounding temperature can also enormously affect the microwave deicing effects. The ideal absorbing materials should absorb microwave as much as possible, which requires the materials to possess large dielectric loss or magnetic loss [36]. The magnetic loss is weak if asphalt mixtures do not contain magnetite aggregate because magnetic components of asphalt and conventional aggregate are low. Therefore, magnetite aggregate endows better magnetic
Fig. 9. Surface temperature of asphalt mixtures: (a) before microwave radiation; (b) after microwave radiation for 120 s.
Fig. 10. Propagation path of microwave though asphalt mixtures.
characteristics to asphalt mixtures, which can enhance microwave absorbing ability and improve microwave deicing performance of asphalt mixtures. On the other hand, the decrease of microwave scattering and surface reflections is also important to shorten the MDT. Nowadays, ultra thin asphalt pavement has been adopted and used in asphalt pavement. AC-10, Nominal maximum particle size 9.5 mm, and AC-5, Nominal maximum particle size 4.75 mm, have been introduced in Chinese Specification [31] and have been used
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MDT (s)
Fig. 11. Ice on asphalt mixtures: (a) before deicing; (b) after deicing with vehicles.
375 350 325 300 275 250 225 200 175 150 125 100 75 50 25 0
Surrounding temperature -5 Surrounding temperature-10 Surrounding temperature-15
c) The minimum reflectivity value of asphalt mixtures containing 80% magnetite content reaches 11.3 dB at 2.45 GHz, which is microwave oven working frequency. Suitable magnetite aggregate endows better magnetic properties to asphalt mixtures, which can improve microwave absorbing ability and decrease microwave reflectivity of asphalt mixtures. d) When magnetite aggregate content reaches 80%, surface temperature of asphalt mixtures can increase from 14 °C to 67 °C within 120 s microwave radiation duration. The microwave heating efficiency of asphalt mixtures increase with the increase of magnetite contents if the aggregate content is below 80%. e) Microwave deicing time decreases with the increase of magnetite aggregate content when magnetite aggregate content is lower than 80%. When magnetite aggregate content is over 80%, the time begins to increase because of the decreased microwave absorbing ability. Suitable magnetite aggregate content can evidently shorten microwave deicing time. However, microwave deicing time increases with the decrease of surrounding temperature.
Acknowledgements The authors acknowledge financial supports provided by the Fundamental Research Funds for the Central Universities (Nos. 310831153504 and 310831163113) and Project of Yan’an Highway Administration Bureau in PR China. References 0
20
40
60
80
100
Magnetite content (%) Fig. 12. Relation chart between MDT and magnetite contents at different surrounding temperatures.
in asphalt pavement [37]. Asphalt mixtures with less than 10 mm aggregate have been successfully designed. Therefore, it is feasible and advisable to replace conventional aggregates with magnetite aggregate with the same volume percentage to improve microwave absorption. 4. Conclusions This work presents results of a laboratory study exploring deicing function of asphalt mixtures using magnetite aggregate as substitution of basalt aggregate, which can be referenced for deicing practice through non-chemical method and promote energyefficient process in hot mixing asphalt mixtures. The following conclusions can be drawn. a) Asphalt mixtures containing magnetite aggregate present better performances at high temperature compared to the asphalt mixtures containing basalt aggregate with the same volume percentage, even though slightly lower water stability and low temperature resistance are observed. The properties of asphalt mixtures containing magnetite aggregate can reach requests of the Chinese specification. b) The microwave heating efficiency of magnetite aggregate is 6.15 times that of basalt aggregate. More heat can be generated and the temperature can be increased with the addition of magnetite aggregate in asphalt mixtures because of the existence of magnetic components.
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