Microwave-assisted selective demolition of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 124

Rectangular
Waveguide
Antenna
(horn)

Coaxial
Waveguide

Concentrator

(a) (b)

Figure 3.38 Schematic illustration of the applicator in a (a) microwave-assisted selective
demolition tool and a (b) microwave drill.

Apart from the microwave applicator, other main components of
microwave-assisted drills are similar to those used in microwave-assisted
selective demolition tools discussed previously and include a microwave
source and power supply, an isolator to prevent the device from poten-
tial damage caused by the reflected power, a tuner to optimise the power
absorbed by the concrete, and a microwave transmission line to transmit
the microwave power to the applicator.
As the microwave-assisted drill progresses in the concrete, the debris is
densified to the wall or converted to a glossy material [17]. The glossy
materials attached to the hole surfaces are soft and can be easily removed
mechanically. The size of the hole created by the microwave-assisted drill
can be adjusted by varying the dimensions of the concentrator and the
coaxial waveguide or through further microwave heating of the boundary
walls. The hole's depth can be increased by applying successive cycles of
microwave drilling and mechanical removal of the glossy debris from the
hole surfaces. Figure 3.39 shows a 13-mm diameter, 10-cm depth hole made
in four cycles of microwave drilling in a concrete slab using the microwave
drill tool invented by Jerby and Dikhtyar [17].
Although the microwave-assisted selective demolition tools discussed
in the previous sections rely on development of high-differential thermal
stresses and pore water pressure within the microwave-exposed region
of the concrete to cause delamination, the microwave-assisted drill relies
mainly on the thermal runway effect developed by concentrating and focus-
sing the microwave energy at a considerably smaller area on the surface of
concrete to “melt” the concrete at the heated spot.
Apart from concrete, the microwave drill is suitable for ceramics and
glass as well as the other dielectric materials with good microwave absor-
bent properties. Inventors of this technique believe that, apart from the