Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 4

are the most commonly used frequencies for microwave heating applica-
tions. At these frequencies, the wavelength of radiation in the transmitting
medium (dielectric material) ranges from 1 to 10 cm. The most commonly
used source of microwave energy, primarily for reasons of efficiency, is
the magnetron. Because of mass production, magnetrons at 2.45 GHz are
particularly cheap; however, magnetrons are available for other frequency
ranges as well. Other sources available include traveling wave tube (TWT),
klystron, gyrotron, and solid-state devices. Each has characteristics that
can be exploited to suit the needs of the user.
At present, almost all common applications of microwave heating rely
on the ability of lower-frequency (916 MHz and 2.45 GHz) microwaves to
heat the material volumetrically and uniformly. This is because, at this fre-
quency range, the penetration depth of microwaves usually exceeds the typ-
ical thickness of the materials being processed by most current microwave
heating user industries, resulting in a uniformly heated material. Higher
ISM microwave frequencies, such as 10.6 and 18 GHz, usually dissipate
much faster in the dielectric materials and thus may result in localized heat-
ing, which is usually undesired for cooking and drying purposes. However,
although not suitable for cooking and drying purposes, higher microwave
frequencies have been shown to be highly effective in applications for which
localised and selective heating of materials is crucial. These include the
applications in cutting, drilling, and demolition of hard dielectric materials
such as hard rock and concrete, for which localised microwave heating is
used to generate high-differential thermal stresses. The use of microwave
heating in such an application can significantly improve the efficiency and
result in considerable economic and environmental benefits. With recent
technological developments in microwave technology, especially in produc-
ing cheaper high-frequency cum high-power microwave generators, the
use of microwave heating technology in such innovative applications in
place of the less-eficient traditional heating technologies makes more sense
than ever before.
1.2 APPLICATIONS OF MICROWAVE HEATING
IN CONCRETE TECHNOLOGY
The unique capabilities of microwave heating have also attracted the atten-
tion of civil engineers, especially concrete specialists. Since the 1980s,
microwave heating has found various applications in concrete production,
demolition, and recycling. These applications take advantage of the capa-
bilities of a range of microwave frequencies to heat concrete to different
extents and degrees of uniformity. Replacement of the often-ineficient con-
ventional techniques with microwave-assisted techniques may significantly
decrease the acknowledged negative environmental impacts associated with