Microwave-assisted selective demolition of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 123

Figure 3.37 Microwave drill invented by Jerby and Dikhtyar. (2000) [18]. (Courtesy of
Professor Eli Jerby, faculty of engineering, Tel Aviv University.)
elements, or installing shear connectors for better connection between the
old and new concrete. However, mechanical drilling produces considerable
noise, dust, and vibration, posing serious health hazards to drill opera-
tors and so on. A novel microwave-assisted method has been proposed
by researchers at the University of Tel Aviv in Israel as an alternative to
conventional drilling (Figure 3.37). Jerby and Dikhtyar [17,18], the inven-
tors of this technique, have reported that the microwave drill can make a
2-mm diameter, 2-cm deep hole in concrete in less than a minute, rendering
it competitive vis-à-vis conventional drilling methods in terms of perfor-
mance while eliminating most of the drawbacks of conventional methods.
In a microwave-assisted drill, the microwave energy should be concen-
trated at a very small spot, normally much smaller than the microwave
wavelength itself. This is because most holes to be drilled will range in the
order of a few millimetres to a few centimetres in diameter. In a microwave
drill, this is done using a near-ield microwave concentrator. As shown in
Figure 3.38, although the microwave-assisted selective demolition tech-
nique discussed previously makes use of an antenna to localise (spread, in
some cases) microwave power to the desired surface area of concrete, the
microwave-assisted drill is designed to localize the microwave power into
a considerably smaller area (a point). The applicator unit in microwave-
assisted drills consists of an open-ended coaxial waveguide with a movable
centre electrode that concentrates the microwave power into a specific point.
The concentrator itself is then inserted into the molten hot spot to make the
hole. Once the hole boundaries are shaped, the concentrator is extracted
from the hole and the concrete cools into a newly shaped hole. The materi-
als for the coaxial waveguide and the movable center electrode should be
selected from materials able to withstand the high temperatures generated.