Microwave-assisted selective demolition of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 112

3

2

z

1

x

y

1

3 2

E
H

Figure 3.23 Field lines for the TE10 mode in a rectangular waveguide.

assumed to excite the TE 10 mode. As seen in Figure 3.22, the sinusoidal
shape of the TE 10 mode results in a sine 2 shape temperature distribution
across the microwave incident surface.
As illustrated through the example (Figures 3.18 to 3.22), the exponential
decay in the microwave power within concrete, especially at high concrete
water contents or high microwave frequencies, may result in consider-
ably high temperature gradients within the concrete's surface layer. Such
nonuniform heating within a very short time leads to a high differential
temperature gradient and thus high thermal stresses. Figures 3.24 to 3.26
show the thermal stresses developed in the concrete specimens subjected
to microwaves at different frequencies and at a constant microwave power
of 1.1 MW/cm 2 . Furthermore, the stress developed across the microwave
incident surface of the concrete is plotted in Figure 3.27. Such plots may be
of use when choosing the appropriate microwave specifications for a given
nominal strength and depth of spalling. Similarly, for a specific incident
microwave power, microwave frequency, and concrete water content, the
approximate spalling depth can be easily estimated. As seen in Figures 3.24
to 3.26, considerably high differential thermal stresses may be developed
within a concrete specimen after just a few seconds of microwave heating,
especially at high frequencies and water contents, at a typical microwave
power used in industrial heating. Such stresses are commonly in the form
of radial compressive stresses and may result in delamination of the surface
layer of the concrete when the compressive strength of concrete is exceeded
under confined conditions.