Environmental Engineering Reference
In-Depth Information
the suction obtained
h
:
h
[m] ≈ 13,791 ln [RH/100]. In this way, it is possible to
reach very high suction values, as can be seen in Table 9.1.
Salts
Chemical formula
RH [%]
Suction [m]
Lithium chloride
LiCl,nH
2
O
12
-22,196
Magnesium chloride
MgCl
2
,6H
2
O
33
-15,290
Potassium carbonate
K
2
CO
3
,2H
2
O
44
-11,322
Magnesium nitrate
Mg(NO
3
)2,6H
2
O
55
-8,245
Sodium nitrite
NaNO
2
66
-5,730
Sodium chloride
NaCl
76
-3,785
Ammonium sulphate
(NH
4
)
2
SO
4
81
-2,906
Potassium nitrate
KNO
3
93
-1,001
Potassium sulphate
K
2
SO
4
97
-420
Table 9.1.
Relative air humidity obtained with saturated salt solutions
(data from NF X 15-014 norm), corresponding suctions
(
h
, θ) couples obtained with the suction plate and/or with the pressure chamber
are used to build the retention - or suction - curve; see the example in Figure 9.4.
(θ, RH) points obtained with the saline solutions define the
sorption isotherm
6
(Figure 9.5). The way this isotherm is obtained is important: if the experiment starts
with a saturated sample placed successively in chambers with decreasing RH, the
desorption isotherm is obtained. On the other hand, if it starts with a dry sample
with successively increasing RH, the sorption isotherm is obtained. Similarly,
dealing with suctions, if starting from dry state, the main
imbibition curve
is
obtained and from the saturated state, the main
drainage curve
.
Capillary hysteresis
phenomena [CRA 96] means that sorption/desorption isotherms and,
imbibition/drainage curves are not identical, as can be observed in Figure 9.5. This
hysteresis is due to several mechanisms related to the complexity of the stone's
porous structure:
- For the same water potential, the liquid phase can be arranged differently
depending on the way menisci were formed. This means that the same suction may
correspond to different water saturation rates.
- On the contrary, the same saturation rate may correspond to different suctions
depending on whether pores are being filled in or drained out. This is what happens,
for example, when water has to cross small pores to fill larger ones: a phenomenon
known as the “ink bottle neck” effect.
6 So called because it is obtained at constant temperature, generally 20°C.
