Post on 26-Jul-2020
TRANSPORT OF LIQUID WATER Chapter 7
1
behaviour
• liquid water behaves in good approximation as an incompressible fluid with a density of ρl ≈ 1000 kg/m3
2
materials are exposed to moisture
• during construction – construction moisture – precipitation
• weather
– rain – melting of
snow
© Jonathan Sa’adah 3
materials are exposed to moisture
• condensation – surface condensation – interstitial condensation
• ground water infiltration • by accident
– leaking pipes – broken seals – …
© DEMA
© howtobuildahouseblog.com 4
moisture content
• the moisture content is the mass of moisture per unit volume of material – symbol: w – SI unit: kg/m3
5
for stone-like materials
moisture content at saturation
• the moisture content at saturation is the maximum mass of moisture per unit volume of material – symbol: wsat
– SI unit: kg/m3
at saturation, all open pores are filled with water
6
moisture ratio
• the moisture ratio is the mass of moisture per unit mass of material – symbol: X – SI unit: kg/kg
for wood-based materials
7
volumetric moisture ratio
• the volumetric moisture ratio is the volume of moisture per unit volume of material – symbol: ψ – SI unit: m3/m3
for highly porous materials
8
relations
• moisture ratio vs. moisture content
• moisture ratio vs. volumetric moisture ratio
• summary:
ψρρ
ρρ
mat
moist
mat
moist
mat
moist
mat
moist
VV
mmX ===
wVm
mmX
matmat
moist
matmat
moist
ρρ11
===
ψρρ moistmat wX ==
9
summary
• use the moisture content w [kg/m3] for stone-like materials
• use the moisture ratio X [kg/kg] for wood-based materials
• use the volumetric moisture ratio ψ [m3/m3] for highly porous materials
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degree of moisture saturation
• the degree of moisture saturation is the ratio between the current and the maximum amount of moisture – symbol: Sm
– SI unit: dimensionless
– formula satsatsat
m XX
wwS
ψψ
===
Sm ranges from 0 (perfectly dry) to 1 (saturated)
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saturation degree scale
Sm = 0 Sm = 1
dry material
saturated material
w = 0 w = wsat
Sm w
≤ ≤
≤ ≤
wetting process
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wetting process
dry material
saturated material
molecular adsorption
capillary condensation
wetting process
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hygroscopic curve
• equilibrium Sm in an environment at RH
00.20.40.60.8
1
0 0.2 0.4 0.6 0.8 1degr
ee o
f moi
stur
e sa
tura
tion
[-]
relative humidity [-]
MORTAR
14
00.20.40.60.8
1
0 0.2 0.4 0.6 0.8 1degr
ee o
f moi
stur
e sa
tura
tion
[-]
relative humidity [-]
hygroscopic curve
• hygroscopic curve is a material characteristic
hygroscopic materials take up a lot of water before RH=1. Non-hygroscopic materials do not
BRICK MORTAR
hygroscopic curve can be strongly non-linear! 15
capillary condensation
• consider a cylindrical pore with radius r, filled partially by gas and partially by liquid :
( )( ) ( )( )rrPP lg παγπ 2cos2 =−
gP lP2r γ α
( )αγ cos2r
PP lg =−
α = contact angle γ = surface tension Pg = gas pressure Pl = liquid pressure
Laplace’s law
force equilibrium:
capillary pressure (pc [Pa]) 16
capillary condensation
• the saturated vapor pressure above a curved meniscus differs from the one above a flat one
lP lP lP
< < ∗
satvp ,
−−=∗
TRPP
ppvl
lgsatvsatv ρ
exp,,this is described by Thompson’s law:
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capillary condensation
• at which RH does condensation start in a pore?
( )
−=
TrRvlcond ρ
αγϕ cos2exp
−−=
TRPP
vl
lgcond ρ
ϕ exp
satv
v
pp
,
=ϕsatv
satv
satv
v
pp
pp
,
,
,
∗
∗=satv
satvcond
pp
pp
satvv
,
,,
∗= = →
∗
ϕ
using Thompson’s law
using Laplace’s law
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capillary condensation
19
( )
−=
TrRvlcond ρ
αγϕ cos2exp
capillary condensation
4.0E-09 m
capillary condensation
• capillary condensation – starts in the small pores – occurs in large pores only at very high RH
• consequences – a material with many small pores will take up
moisture at low RH → hygroscopic materials – a material with only large pores will only take up
water at high RH → non-hygroscopic materials
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00.20.40.60.8
1
0 0.2 0.4 0.6 0.8 1degr
ee o
f moi
stur
e sa
tura
tion
[-]
relative humidity [-]
hygroscopic curve & pore size
• the pore-system determines the hyg. curve
mortar = hygroscopic => must have fine pores
BRICK MORTAR
brick = non-hygroscopic => must have large pores
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hygroscopic curve & pore size
0
0.5
1
1.5
2
1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03
cont
ribut
ion
to S
mpe
r po
re si
ze [l
og-1
(Pa)
]
pore radius [m]
BRICK MORTAR
00.20.40.60.8
1
0 0.2 0.4 0.6 0.8 1degr
ee o
f moi
stur
e sa
tura
tion
[-]
relative humidity [-]
BRICK MORTAR
22
relevance to practice
• hygroscopic curve describes the «moisture buffering capacity» of a material
• hygroscopic materials can contribute to stabilize RH in the indoor climate – adsorb moisture when RH in environment is high – release moisture when RH in environment is low
23
00.20.40.60.8
1
0 0.2 0.4 0.6 0.8 1degr
ee o
f moi
stur
e sa
tura
tion
[-]
relative humidity [-]
Question
• What is the impact of moisture (condensed water) on the thermal conductivity of insulation?
24
liquid transport
• the pore-system determines the permeability
1E-20
1E-18
1E-16
1E-14
1E-12
1E-10
1E-08
1.E-10 1.E-08 1.E-06 1.E-04
liqui
d pe
rmea
bilit
y [lo
g(s)
]
pore radius [m]
25
liquid transport
• there are 3 possible transport mechanisms – due to capillarity
– due to gravity
– due to (external) pressure
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transport due to capillarity
• imagine a tube with radius r, in contact with a water surface
2r
α gP
( ) ( )( )rghrl παγπρ 2cos2 =
h γ
gravity surface tension
( )gr
hlρ
αγ cos2=
27
transport due to capillarity
• EXAMPLE: how much is the capillary rise of water in a tube with radius 0.001 m? (α=0°, ρl=1000 kg/m3, g=9.81 m/s2, γ=0.0725 J/m2)
( )gr
hlρ
αγ cos2=
( ) m 0148.0001.081.91000
0cos0725.02=
⋅⋅⋅⋅
=
© www.funsci.com 28
transport due to capillarity
• EXAMPLE: how much is the capillary rise of water in a tube with radius 0.00001 m? (α=0°, ρl=1000 kg/m3, g=9.81 m/s2, γ=0.0725 J/m2)
( )gr
hlρ
αγ cos2=
© www.funsci.com 29
transport due to capillarity
• capillary rise is most pronounced in materials with fine pores, such as many cementitious materials
© Ilan Kelman 2000
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transport due to capillarity
• possible problems – «bubbles» in paint – detachment of
plaster/render – decrease of material
strength – …
© Ilan Kelman 2000
31
transport due to capillarity
• solution: install/foresee a moisture barrier (aka DPC – damp proofing course)
inside outside
ground basement/ground
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transport due to capillarity
• solution: install/foresee a moisture barrier
33 www.wbdg.org
transport due to capillarity
new constructions
existing buildings
© abacus
quality control is an issue! © DELTA®
34
transport due to capillarity
• capillarity works in all directions! – in vertical direction, it is countbalanced by gravity – in horizontal direction, … not
-20
-15
-10
-5
brick
mortar
Sm = 1 Sm = 0
log(
perm
eabi
lity)
[log
(s)]
for short-term exposure, mortar can act as barrier 35
transport due to capillarity
• nowadays facades are designed as rain barrier
(1) run-off
(2) absorption in brick
(1) (2)
(3) drainage in cavity
(3) 36
transport due to capillarity
• nowadays facades are designed as rain barrier
37 www.wbdg.org
liquid transport
• there are 3 possible transport mechanisms – due to capillarity
– due to gravity
– due to (external) pressure
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transport due to gravity
• in wide pores, gravitational forces dominate over capillary forces and cause flow
capillary forces
keep water island
in place
water island moves due to gravity
39
transport due to gravity
• possible problems – «bubbles» in paint – detachment of
plaster/render – decrease of material
strength – soiling – …
40
transport due to gravity
• solution
41 www.wbdg.org
transport due to gravity
• solution: – foresee a moisture barrier on horizontal surfaces – design architectural details carefully
outside
inside
42
transport due to gravity
without drip-moulding
with drip-moulding
± 1 year after construction ± 15 years after construction 43
transport due to gravity
44 www.wbdg.org
transport due to gravity
• windowsills: foresee sufficient overhang…
45
transport due to gravity
• windowsills: foresee a tiny «side-wing»…
46
transport due to gravity
• Other details
47 www.wbdg.org
transport due to gravity
• Other details
48 www.wbdg.org
transport due to gravity
• Other details
49 www.wbdg.org
liquid transport
• there are 3 possible transport mechanisms – due to capillarity
– due to gravity
– due to (external) pressure
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transport due to pressure
• the pressure at a point depends on the height of the water column above
• therefore, the water level in two connected water columns is always equal
ghPP lgl ρ+=
h
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transport due to pressure
• ground water
water table
water pressure
© basement systems
52
transport due to pressure
• solution: a water-thight barrier – at the outside: the only good solution – at the inside: only a cosmetic solution – most
likely the problems will get worse
© brehmann.de
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transport due to pressure
• ground water
pressure
© www.mainline waterproofing.com
54
• solution: install a drainage pipe
transport due to pressure ©
ww
w.e
pa.g
ov
55
transport due to pressure
• rain penetration through facade cladding
- one layer is simultaneously wind and rain barrier
- if the joint sealant degrades the pressure difference across the wall will cause leakage
−aP+
aP
56
transport due to pressure
• solution: employing a two-step system
(1) water thightness (rain)
(2) pressure equalization
(3) air thightness (wind)
(1) (2) (3)
+aP −
aP+aP
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transport due to pressure
58 www.wbdg.org
summary transport
• capillary suction is connected to the pore width and is mainly active in small pores
• gravity is important in pores that show limited capillary suction, i.e. large pores
• air as well as water induced pressure differences are mainly important for sealants, material interfaces and fractures
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