Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 31

where C w is the specific heat of water, and J is the water flux. The water flux
J can be expressed in terms of the gradient of pore pressure:

a
g

J =− P (1.43)

where a is the water permeability of concrete, and g is Earth's gravity. Hence,

C
g

w

qCTJ = = − a TP (1.44)

cv w

However, because the water permeability of concrete is about three orders
of magnitude smaller than heat conductivity, this term can be neglected;
thus, Equation 1.39 may be simplified as

∂
∂

T
t

( ) + ()

ρ C =− ⋅− kT Px (1.45)

It is noteworthy that Equation 1.43 is the classical Darcy's law; hence,
its validity is known to be limited to saturated porous materials. However,
previous studies have shown that Darcy's law is applicable to a heated
unsaturated material provided that P is interpreted as the pressure of water
vapour in the pores rather than the pressure of liquid capillary water [21].
Once the heat source P ( x ) is known, Equation 1.45 may be used to cal-
culate the temperature distribution inside the microwave-heated concrete.

1.10 MASS TRANSFER PHENOMENON AND THE PORE
PRESSURE DEVELOPMENT IN MICROWAVE-
HEATED CONCRETE

Another phenomenon that has to be fully comprehended for a better under-
standing of the applications of microwave heating in concrete technology
is the way microwave heating affects the movement of water and water
vapour in the concrete pores. As a result of microwave heating of con-
crete, part of the pore water may turn into vapour. If the vapour generation
rate exceeds the rate of the vapour migration from the surface of concrete,
substantial pore water pressure may be developed within the concrete,
which may affect its structural integrity. Understanding this phenomenon
and predicting the behaviour of water and vapour in concrete requires a