Utilization of steel slag as aggregate in asphalt mixtures for microwave deicing
Introduction
In China, more than 90% expressways are constructed with hot-mix asphalt mixtures (HMA) (Chen et al., 2016, MOT, 2015). The increase of road construction imposes considerable pressure on raw building materials, especially on natural aggregates. Substitution for natural aggregates is urgently needed in order to alleviate the supply problem and help to save the dwindling natural resources.
The steel industry has been the important part of the mainstay industries of China in the past decades. As a main solid by-product of iron and steel smelting industry, steel slag is accounting for about 10%–15% of the crude steel production. Currently, China is facing a challenge on more than one billion tons of steel slag accumulations due to its less than 50% recycling rates (Guo and Shi, 2013). Consequently, a series of environmental issues, e.g. land occupation, water pollution and heavy metal pollution are caused. Obviously, recycling of steel slag in asphalt mixture is a promising way to save natural resources and reduce environmental pollution. The literature reviews demonstrate that the asphalt mixtures with steel slag possess not only higher stiffness and lower permanent deformation but also better fatigue, aging and abrasion resistance in contrast to conventional asphalt mixtures (Huang et al., 2012, Xie et al., 2013, Wen et al., 2016, Kanitpong and Bahia, 2003). However, as a kind of magnetic material, the microwave heating function of steel slag is totally neglected in pavement engineering.
Microwave heating (MH) technology has numerous applications in various industrial processes; and it has also been used in the field of highway engineering as a technique of bituminous materials hot-recycling (Benedetto and Calvi, 2013). Nowadays, the traffic safety in winter is becoming a prominent problem because of the pavement was covered with thick ice layer, which is difficult to be removed via the MH technology due to the considerably limited MH efficiency of conventional aggregate used in asphalt mixtures. Therefore, the surface temperature of asphalt mixture containing taconite under microwave radiation was carried out for the first time in 1989 (Osborne and Hutcheson, 1989). Then, the adoption of microwave absorbing materials to pavement materials has become the main methodological in this field. For example, Hopstock (2005) measured the heat capacity, thermal conductivity and microwave absorption coefficient of taconite asphalt mixtures, and established microwave heating model of pavement. Utilization of taconite as aggregate in asphalt mixtures is practical now. (Zanko et al., 2008, Zanko et al., 2009, Oreskovich et al., 2008).
In addition, Wang et al. (2014) studied the influence of carbon fiber content on the mechanical properties, microwave reflectivity and microwave deicing function of micro-surfacing asphalt mixtures. Besides, Wang et al. (2011) also conducted a series of experiments to investigate the effect of carbonyl iron powder on the microwave absorbing property of asphalt mixture. Increasing the self-healing rates of asphalt concrete is another application of MH. Gallego et al. (2013) presented the microwave heating test on steel wool asphalt mixtures and indicated that the surface temperature reached 140 °C after 120s microwave radiation. Liu et al. (2014) comparatively studied the MH self-healing rate of asphalt mixtures with different steel wool lengths and contents.
Conventional deicing methods, such as ice-crushing vehicle, snow-blower are failure to perfectly remove the thick ice layer on the pavement resulting from the presence of freezing adhesion action on the interface between ice layer and pavement surface. As is well known (Zou et al., 2011), the freezing adhesion level of material is negatively correlated with atmospheric temperature, which means that the freezing adhesion interface can be efficaciously melted by increasing its temperature.
Generally, two stages are involved in the process of MH dicing. Stage one is that the pavement surface temperature increases from negative temperature to 0 °C under the microwave irradiation, in which the freezing adhesion level is barely existed. In the following, both pavement and the water from melting ice are heated synchronously in the stage two. As a result, more ice layers are melted into water accompanied by the continued microwave irradiation. In conclusion, the best opportunity for deicing is between stage one and stage two. The reason is that the freezing adhesive interface is still having a high strength before the completion of the stage one; and the water melting from ice layer is highly intending to re-freezing under low-temperature atmosphere after the stage two. The pavement MH deicing draws support from external device, including the microwave generator and deicing vehicle. MH vehicles in China have been well developed, and their schematic diagram is shown in Fig. 1. In the process of the vehicle movement, the microwave unit plays the role of heating the pavement. After the melting of the freezing adhesion interface, ice layer is minced by the crusher and is pushed to the road side by the shovel.
Based on the above discussion, utilization of steel slag as aggregate for improving microwave deicing performance of asphalt mixtures has been paid few attentions. Therefore, a novel application of recycling steel slags for microwave deicing asphalt pavement is designed in this work. Specifically, the first purpose is to confirm the validity that the steel slag as the aggregate of asphalt mixtures has the vastly MH ability. On the other hand, as a bitumen-consumption aggregate owing to its porous characteristics (Xie et al., 2012), the second purpose is to ascertain the effective volume and particle sizes for replace conventional aggregate in asphalt pavement, which is helpful to remove the economic barriers of its application. Therefore, the deicing mechanism of MH pavement was illustrated. The MH capacity of steel slags with different particle sizes was measured to determine the most effective particle sizes for replacement. Meanwhile, the surface elements distribution of steel slag was measured to reveal the active ingredient and XRD patterns for steel slag and limestone were obtained to study their mineral component. In addition, the surface temperature, thermal conductivity and heating uniformity of asphalt mixtures containing different steel slags content were tested to recommend the optimal steel slag content. Finally, the comprehensive feasibility of steel slag asphalt mixture was studied via jointly considering its sources distribution, ecological hazardous,cost performance and potential overheating problem.
Section snippets
Materials
Basic oxygen furnace slag was manufactured by crushing dense slag blocks selected from a Chinese Iron and Steel Group. Conventional aggregate was limestone aggregate. The properties of steel slag and limestone were tested in accordance with the Testing Procedures of Aggregate for Highway Engineering in China (JTG E42-2005) (Inspection and Quarantine of the People’s Republic of China. 2005), the results are summarized in Table 1.
Preparation of the specimens
Aggregate gradation is shown in Fig. 2. Due to the higher specific
Microwave heating capacity of steel slags
In order to determine whether the different sizes of steel slag have similarly MH performance, the steel slags with 10 different particle sizes (filler-13.2 mm) were placed into microwave oven together and the infrared image was captured after 15 s radiation, the infrared image of their surface temperatures are shown in Fig. 7.
In Fig. 7, the surface temperature difference between steel slags with different particle sizes can be clearly observed. It is known that steel slag is a
Conclusions
In this work, the feasibility of simultaneously using steel slags aggregate and conventional limestone aggregate in asphalt mixtures for pavement microwave deicing application was evaluated. Based on the results discussed above, the following conclusions can be drawn:
The thick ice layer removal is considered as a technological difficulty resulting from the presence of freezing adhesion action on the interface between ice layer and pavement surface. With the assistance of microwave deicing
Further study
This study has made a preliminary study of utilization of steel slag as aggregate in asphalt mixtures for microwave deicing to exploring its feasibility. The obtained conclusions are desirable, but there are several concerns should be further studied. Although we have discussed the potential overheating problem that MH pavement may facing, but there are insufficient experimental data to support our point view on this matter, thereby the long-term performance of MH pavement, for instance, the
Acknowledgements
This work was jointly supported by the National and Local Joint Engineering Materials Laboratory of Traffic Engineering and Civil Engineering, Chongqing Jiaotong University (No. LHSYS-2016-002), the Fundamental Research Funds for the Central Universities (No. 310831153504 and 310831163113), and National Science and technology support program in 12th Five-Year (2014BAG05B04). Additionally, the writers also express their thanks to Dr. Liqun Hu, Dr. Wei Jiang, Dr. Liang Zhou of Chang’an University
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