Environmental Engineering Reference
In-Depth Information
The maximum pore radius that can be filled with water at a given RH can be
calculated with [9.13]. On the contrary, [9.14] gives the minimum RH required for
that water to condensate. Both relations clearly show the link between condensation
phenomenon and porous structure (and not only temperature) and are therefore
useful to deal with what is called
capillary condensation
. Nevertheless, results from
direct application of [9.13] and [9.14] must be interpreted cautiously, because the
true porous spectrum of the stone is always much more complex than these simple
cylindrical pores we have considered up to now. To some extent, it is possible to
generalize Laplace's law, introducing a generalized meniscus radius ℜ instead of
r
[DAI 96], but this new parameter is a function of local pore geometry, water content
and, worse, pore filling and emptying history due to wetting/drying cycles.
9.2.4.
Retention curves
9.2.4.1.
Measuring techniques
According to what has been introduced previously, it is clear that the
suction/water content relationship
h(
θ
)
is strongly linked with the stone's internal
porous structure. Actually, for a given stone sample, it can be considered to be an
intrinsic characteristic, like mineralogical composition, density, porosity, etc.
Determining this relation - called the
retention curve
- is therefore very important if
we want to understand the behavior of a particular stone towards water.
Unfortunately, a straightforward application of Laplace's law shows that in
order to explore the whole range of saturation, corresponding suctions should vary
across a huge range: typically from zero to several hundred MPa (1 MPa = 10 bar)
when talking in terms of capillary pressures. To cover this range, several
experimental devices have to be used [DAI 96, RIL 80]:
- To obtain capillary pressures
P
c
from 0 to 0.1 MPa (-10 m ≤
h
≤ 0), a
suction
plate
can be used. The desired suction is then obtained by depressing the liquid
phase, the gaseous phase being kept at atmospheric pressure.
- For
P
c
= 0.1 to 1.5 MPa, roughly (-150 m ≤ h ≤ 10 m), the suction is adjusted
by pressing on the gaseous phase. For this purpose, the stone sample is placed on a
porous plate in a
pressure chamber
. The liquid phase is kept at atmospheric pressure
(the saturated porous plate is connected to the outer atmosphere) and the air pressure
inside the chamber is adjusted to the desired value using a compressor.
- Above 1.5 MPa, directly imposing suction implies very high mechanical
stresses. For this reason, it is preferred to regulate relative air humidity RH instead.
This can be done simply using different saturated salt solutions; Table 9.1. Direct
application of Kelvin's law [9.2] gives the following equivalence between RH and
