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عنوان فارسی مقاله:
خصوصیات ترانسفورمر بتن در دمای بالا
عنوان انگلیسی مقاله:
Dielectric characterization of concrete at high temperatures
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
After water, concrete is the second most consumed product on Earth [1]. Because of its broad usage in industry works and civil engineering its processing and utilization are urgent issues. Traditionally, for demolition and dismantling of concrete structures, mechanical electric rotary or pneumatic tools are used. Besides pneumatic breakers, also hydraulic and ultrasonic breakers are sometimes used. All the above methods, however, have significant health hazards because of generation of high level of noise, dust and/or vibrations. In recent decades, microwave technologies are progressively replacing traditional heating and processing techniques. Similarly, in the concrete industry, the application of microwave has shown a high potential for applications, such as a selective demolition and drilling of concrete [2]. Such advantages as contactless and low noise processing, time and energy saving, sufficient reduction of dust and debris, and much less cross-contamination (when used for surface deactivation of radioactive concrete) make it attractive for industry. The first patents for breaking of rocks [3] and demolition of concrete with microwaves [4,5] in the end of 1960s were followed by research activities in Japan (JAERI), UK (Harwell Laboratory) and USA (ORNL) [6,7] on the methods and necessary equipment for microwave demolition of concrete. Later, the development of sophisticated, non-destructive methods for investigation of micro-structure of concrete (see e.g. Ref. [8]) as well as the progress in theory and numerical simulations of microwave heating of concrete (see Refs. [9e12]) have advanced the understanding of the mechanisms of the breakage and durability of concrete. For calculation of thermal stresses and strains which lead to breakage of concrete, the absorbed microwave power Pabs has to be known. It is directly proportional to the relative dielectric loss factor εr 00 of the material and reads as follows: Pabs ¼ 2pf ε0ε 00 rE2 rms. Here f, ε0 and Erms are the frequency, dielectric constant of vacuum and the root mean square amplitude of the electric field in the material, respectively. The latter one also depends on the reflection from a concrete surface that is influenced by the real part of the relative permittivity of concrete εr 0 . Therefore, for process modelling and reliable design of a microwave applicator the complex permittivity (εr ¼ εr 0 -j·εr 00) has to be known precisely and within a full range of the working temperatures. The necessity of temperature dependent permittivity data for correct modelling of microwave demolition of concrete was recognized in the work of W. Li et al. where the power dissipation and temperature distribution profiles were modelled for four frequencies: 0.896, 2.45, 10.6 and 18.0 GHz [10]. Also in their work, W. Li and coauthors have performed the temperature dependent measurements of concrete dielectric properties within 20 … 250 C with a step of 50 C and with the use of the coaxial probe method.
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