Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 19

conductivity of water, the conductivity of concrete increases significantly
with its water content.

1.4.3 Reflectivity and transmissivity
The mismatch between the dielectric constants at the interface between
two different media causes some of the incident waves to be reflected and
the rest to be transmitted into the next medium (Figure 1.12). The reflec-
tion coefficient R can be calculated from

ε cos θ −

+

ε cos θ

r 2 i r 1 t

R = (1.20)

ε cos θ ε cosθ t

r 2 i r 1

where ε r 1 is the dielectric constant for medium 1, ε r 2 is the dielectric con-
stant for medium 2, θ i is the angle of incidence, and θ t is the angle of trans-
mission. The square of reflection ( ∙ R ∙ 2 ), called reflectivity and denoted as r ,
is the ratio between the portion of the incident EM power that is reflected
to the first medium and the total incident EM power. Therefore, the trans-
missivity c , which is the ratio between the power transmitted to the second
medium and the total EM power, may be obtained from

c = 1 - r (1.21)

The transmissivity coefficient of concretes with various water contents
at microwave frequency ranges, calculated from the loss factor obtained
by using Equations 1.20 and 1.21, is shown in Figure 1.13. As can be seen,
the transmissivity coefficient of concrete decreases significantly with an
increase in the water content of concrete. This means that a higher propor-
tion of the incident power may be transmitted into a concrete component

Reected
Microwaves

Incident
Microwaves

θ r

θ i

Air
Concrete

θ t

Transmitted
Microwaves

Figure 1.12 Reflection and transmission of the waves at the air-concrete interface.