Microwave technology has been applied in asphalt pavement maintenance for years, but microwave heating efficiency of asphalt mixtures needs to be improved. In this paper, the components of asphalt mixtures were modified by three different means. The first was to coat a very thin layer of magnetic Fe3O4 films on the surface of coarse and fine aggregates, by using a chemical co-precipitation reaction. The second one was that steel slag particles with twenty percent iron content were modified by a coprecipitation method for improving the Fe3O4 content on its surface, and then used to completely replace fine aggregates. The third was to prepare a wave-absorbing material employing carbonyl-iron powder as magnetic loss absorbent and carbon black as dielectric loss absorbent with asphalt as matrix. On this basis, X-ray diffraction (XRD) phase analysis and microwave heating properties of prepared microwave-absorbing materials were tested compared with those of control basalt aggregrates and steel slags. XRD spectra results showed that Fe2O3 on the surface of steel slag particles was converted into magnetic Fe3O4 nanoparticles and a very thin layer of magnetic Fe3O4 films was coated on basalt fine aggregates, by using the chemical co-precipitation method. The mean grain sizes of Fe3O4 nanoparticles on the surface of steel slag particles and basalt fine aggregates were 22nm and 30nm, respectively. The heating rates of modified basalt coarse aggregate and fine aggregate were twice than those of basalt coarse aggregate and fine aggregate before modified. The temperature of fine aggregate was a little higher than that of coarse aggregate and the temperature of modified steel slag was 40?C higher than that of ordinary steel slag.
The three kinds of modified microwave-absorbing materials were added to prepare Marshell specimens and rutting slabs. Electromagnetic characteristics of the modified asphalt concretes were measured using a free space method in the range of 2-4 GHz. The FLIR thermal infrared camera and infrared thermometer were adopted to test the surface temperature of the asphalt mixture specimens. The results showed that the real part of the complex dielectric constant of asphalt mixtures prepared by adding modified basalt coarse aggregate (MCAM), modified basalt fine aggregate (MFAM), modified steel slag asphalt concrete (MSAM) and carbonyl-iron powder and carbon black (AM CIP/CB) increased by 11.3%, 11.3%, 10.6% and 1.89%, respectively, compared to their control groups (BAM). And the imaginary part of the complex dielectric constant of above asphalt mixtures increased by 30%, 28.7%, 27.3% and 5.27%, repectively. The temperature variation of asphalt concretes was linear with time and the internal temperatures of the asphalt concrete were 8-13?C higher than those of the surface temperatures. The surface temperatures rates of MCAM, MFAM, MSAM and AM CIP/CB were 0.4364?C/s, 0.4564?C/s, 0.4356?C/s and 0.4162?C/s, respectively, after microwave heating for 2min, which were 70.9%, 78.8%, 70.6% and 63% higher than those of BAM.
In order to research the microwave heating rate and temperature field of asphalt pavement, temperature sensors were used to measure the temperature distributions at 2.5cm, 5cm, 7.5cm, 10cm and 15cm depth of a 20 cm-depth asphalt concrete slab. An electromagnetic simulation software named CST was also used to measure the microwave power density and the temperature distribution. The results showed that most of the temperature variations at the same depth in different heating times were less than 8?C, indicating a good uniformity of microwave heating in the plane scope of the asphalt concretes. In the 20cm-depth asphalt concrete slab, the temperature increased firstly and then decreased in the depth direction. The temperatures at the bottom of the slab were slightly lower than those in the middle and top layers. The optimal heating depth of the asphalt concrete slab were around 10cm and the highest temperatures of the four kinds of modified asphalt mixture exceeded 120?C within 15min of microwave heating, which can basically meet the temperature requirements for asphalt pavement maintenance. The CST simulation results showed that the temperature distribution is related to the power loss density, that is, where the loss power density is higher, the temperature is higher. The maximum temperature is 11cm depth from the surface in simulation model. The simulation results also demonstrated a good accordance with the laboratory results. Although the temperature distributions were non-uniform in both horizontal and vertical direction, the temperature variations were within acceptable limits. Microwave heating has good uniformity and proper heating depth compared with conventional heating in asphalt concrete slabs.
The three point flexural failure tests and three point bending fatigue tests were conducted to evaluate the microwave healing effects of the modified asphalt mixtures. The results showed that the strength recovery of the modified asphalt mixtures was not complete and the strength recovery rate decreased with the breaking-healing cycle during microwave heating process. The strength recovery rates and microwave healing indexes of the six kinds of asphalt mixtures were listed as MCAM, MFAM, MSAM, AM CIP/CB, USAM and BAM in descending order. Microwave heating can be used to repair the cracks in asphalt mixture within certain limits. The microwave healing index mainly depends on the microwave heating time and the microwave absorbing ability of asphalt mixture. When the six kinds of specimens were heated in the same period, the microwave healing indexes of MCAM and MFAM increased by 41.4% compared to that of BAM. The microwave healing index of MSAM increaded by 36.2% compared to that of BAM and by 16.1% compared to that of USAM. The microwave healing index of AM CIP/CB increaded by 37.5% compared to that of BAM. The healing effects of six kinds of asphalt mixtures via microwave heating were three times as high as those at room temperature.
The influences of different asphalt mixtures on pavement performance were evaluated through the indexes, such as Marshall Stability, flow value, dynamic stability and so on. The results showed that MCAM, MFAM, MSAM and AM CIP/CB exhibited better dynamic stabilities and strains at low temperature than BAM, indicating that the high-temperature and low-temperature performances of the modified asphalt mixtures can be improved by the three modification methods. Water stabilities of USAM and MSAM were better than those of the other four kinds of asphalt mixtures. All the skid resistances of the six kinds of asphalt mixtures can meet the requests in the Chinese specification