Civil Engineering Reference
In-Depth Information
Wet concrete
Saturated concrete
Air-dried concrete
Oven-dried concrete
0.4
0.3
0.2
0.1
0.0
0
5
10
15
20
Frequency (GHz)
Figure 3.17
The variation in the reflection coefficient of concrete with concrete water
content and microwave frequency.
between the penetration depth of microwaves in concrete and microwave
frequency. This means that the higher the microwave frequency, the less the
penetration depth would be. Therefore, at higher ISM microwave frequen-
cies, the microwave energy is expected to be dissipated within the surface
layer of the concrete rather than penetrating deep into the concrete speci-
men; the higher the microwave frequencies, the thinner this affected sur-
face layer would be. In microwave-assisted selective demolition of concrete,
such relationships between the penetration depth of microwave in concrete
and the concrete's water content and microwave frequency are used to
concentrate the energy within a selected area of the concrete component,
thereby causing local surface delamination.
The delamination of the heated zone of the concrete has been shown
to be caused by a combination of two forces: differential thermal stresses
and elevated pore water pressure. The differential thermal stresses are a
direct outcome of the differential heating of the heated parts of the con-
crete caused by the relatively fast decay of microwave power within the
concrete surface layer, especially at higher ISM microwave frequencies. As
a result of the confinement of the energy within a thin surface layer of con-
crete, microwave heating at higher ISM microwave frequencies generates
high-temperature gradients inside the concrete, between the microwave-
exposed surface and the cooler interior. Figures 3.18 to 3.21 show the vari-
ation in temperature within a 10-cm (cube) concrete specimen subjected
to microwaves of three different ISM frequencies (i.e., 2.45, 10.6, and 18
GHz). The results presented in these figures have been obtained by simu-
lating the microwave-concrete behaviour using COMSOL Multiphysics
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