Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 32

detailed understanding of the mass transfer phenomenon in concrete. The
mass conservation equation for concrete may be expressed as

∂
∂ +⋅ =

w
t

JHD () (1.46)

Here, specific water content, time, gradient operator, water flux vector, and
change in free-water content because of hydration and dehydration. The
free-water content in the liquid phase within the concrete can be deter-
mined by means of the so-called Equation of state of pore pressure [22].
For temperatures above the critical point of water (374.15°C), all free water
is assumed to have been vapourised; thus, there is no liquid phase. For tem-
peratures below the critical point of water, the free-water content depends
on the temperature and the ratio of water vapour pressure to saturation
vapour pressure [23]. In the following, the semiempirical expressions from
Reference 22 are used. For nonsaturated concrete, the following formula
has been proposed:

1
()

w
C

W
C

mT

s 1

= × h h ≤ 0.96 (1.47)

Here, is the water content of concrete, W s 1 is the saturation water content
at 25°C, C is the mass of (anhydrous) cement per cubic metre of concrete,

P
PT
s

h = ()

where P s ( T ) = saturation pore pressure at temperature T , and m ( T ) is an
experimentally determined empirical expression as follows [22]:

( )

2

10
22 32510

T +

() =

mT 104 . − (1.48)

( ) ++ ( )

2 2

. + T 10

For saturated concrete, the ratio of free water to cement is determined by [22]

w
C

W
C

( )

s 1

= 1012 + . h − 104 . h ≥ 1.04 (1.49)

The transition between h = 0.96 and h = 1.04 is assumed to be linear
between the values of w 0.96 and w 1.04 ; thus,