Environmental Engineering Reference
In-Depth Information
10.2.3.2.2. Hydric perturbations
If the isotherm of the system is ensured, [10.3] is reduced to:
θ
∂
K
∂
θ
l
=∆+
D
[10.8]
θ
∂
t
∂
z
Here, Aouaïssia-Abdallah [AOU 97, AOU 98] can be quoted. By putting a disk
of material near saturation into a centrifuge, he models the hydric transfer equation
like this:
2
2
K
ω
∂
θ
1
∂
∂
θ
l
=
(
r
)
−
[10.9]
∂
t
rr r
∂
∂
g
An evolution of the radial distribution of water content (measurable, for instance,
by gammametry) results, from which we can deduce the hydric diffusivity and
permeability (here again, by comparison of theoretical forecasting obtained by
resolution of [10.9] and the experimental results).
In the case of a rock in hygroscopic conditions, [10.8] can be written:
∂
φ
=∆
D
φ
[10.10]
θ
∂
t
with
Ø
the relative humidity of air in equilibrium with the material [BAS 84].
If we have a time scale and model of conditions at the limits, analysis of the
response to a hygric perturbation in relative humidity allows us to determine the
mass diffusivity
D
θ
(also written as
α
m
) and dimensions that have been introduced by
analogy with the thermic:
λ
m
(close to the water vapor permeability in a permanent
regime),
ρ
0
c
m
capacity of humidity absorption and effusivity
b
m
[BAS 97]. Here we
blow air over the material inside the channel (drilled into the material) that is drier -
or more humid - than that initially in contact with the material. The drying or
wetting process is kept isothermal by means of external ventilation.
The vapor permeability,
π
or
δ
in the II.2 RILEM test, obtained by static methods
(implementation of a permanent vapor flux), may in principle be linked to the
diffusivity
D
θ
, resulting itself from a dynamic determination. It can be shown that
formally we have:
PT
()
∂
φ
vs
D
θ
=
δ
[10.11]
ρ
∂
θ
l
