Microwave-assisted selective demolition of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 104

reviewed in the previous sections. This section discusses the working prin-
ciples of the microwave-assisted selective demolition method and explains
the phenomena leading to the development of forces big enough to break up
materials such as concrete and rocks.

3.6.1 Main causes of concrete demolition
The results of various analytical and experimental studies have confirmed
that the use of high ISM microwave frequencies can result in the selective
demolition of the microwave-exposed surface of concrete. The results of
such studies suggest that demolition of the microwave-exposed concrete
area occurs due to a combination of two phenomena: differential thermal
heating and pore pressure development (Figure 3.14). Acquiring a thor-
ough understanding of the underlying phenomena leading to the selective
removal of concrete using microwaves is essential for proper use of the
microwave-assisted concrete demolition equipment and for ine-tuning the
entire demolition process.

3.6.2 Differential thermal heating
As shown in Chapter 1, the penetration depth of microwaves in concrete
is basically a function of two parameters: the dielectric properties of con-
crete and microwave frequency. The dielectric properties of concrete are
themselves functions of various other parameters, including the concrete
composition, proportions of concrete constituents, water content of con-
crete, and microwave frequency (Figure 3.15).
It has been shown that, among these factors, the water content of concrete
plays the most important role in determining the amount of the microwave
energy absorbed by and dissipated in the concrete. The higher the water

Microwave heating at high ISM frequencies

Dierential thermal heating due to
exponential decay in the microwave power
dissipated in concrete with distance from
the microwave-exposed surface

Increased water evaporation in the heated
zone

Dierential thermal stresses within the
heated zone of concrete

Pore water pressure development in the
heated zone of concrete

Figure 3.14 The main causes of concrete demolition in microwave-assisted selective
demolition techniques.