17. water%2c liquid

7/29/2019 17. Water%2c Liquid

1/19

Canadian mean annual

total precipitation


2/19

Rust & corrosion


3/19

Moisture content

versus

relative humidity

A materials moisture content

can increase not only bybeing immersed in water, butalso by being simply exposed

to a humid environment.

The dimensional change of a material due to a change

in its moisture content can be represented by:

Dimensional change of materials

due to moisture

L=LMC

MCw

where:

L = dimensional change (m)

= coefficient of linear expansion due to moisture (unitless)

L = length of the material (m)

MC = change in moisture content of material (%)

MCw = total moisture content range of material over which

dimensional change occurs (%)


4/19

Coefficients of expansion due to moisture for common construction materials:

Moisture expansion coefficients

material dimensionalchange (%)

moisture content range (%)

Masonrymaterials:

marblelimestoneclay & shale bricks

brick expansion after firingsandstone

less than 0.001up to 0.01

0.007

0.02 - 0.030.07

0 - 8 (saturation level)

0 - 8 (saturation level)

Cementitiousmaterials:

lime mortarPortland cementlightweight concretedense concretedense concrete blockmortar, initial shrinkage

up to 0.020.03

up to 0.020.030.040.1

0 - 20 (dry to saturated)0 - 20 (dry to saturated)0 - 20 (saturation level)0 - 20 (saturation level)0 - 20 (saturation level)

0 - 20

Roofingmaterials:

reinforced polyesterroofing felts (along length)roofing felts (across width)

less than 0.0010.21.5

0 - 300 - 30

Wood:plywood & processed woodwood (parallel to grain)wood (across grain)

0.25 - 0.500.15.0

0 - 28 (saturation level)0 - 28 (saturation level)0 - 28 (saturation level)

Wood shrinkage

The coefficients () are as follows:

parallel to the grain: 0.1% = 0.001 across the grain: 5.0% = 0.05

Example: Wood expansion due to moisture gain

Calculate the dimensional changes in a piece of wood with initial

dimensions of 0.5 m x 0.5 m x 0.5 m when its moisture contentincreases from 15% to 35%.

Wood typically expands linearly from 0% to about 28%moisture content, above which its fibres are saturatedand it shows l ittle dimensional change.

L=LMC

MCw

The equation for the dimensionalchange due to moisture is:

parallel

to grain

across grain

(radial)

across grain(tangential)

b) Similarly, for across the grain:

= (0.025 m)(0.46) = 0.0115 m

= 11.6 mm

L= (0.05)(0.5m)28%15%

28%

a) Parallel to the grain:

Dimensional change only takes place from 15% to 28% MC, and theequation becomes:

example continued

L= (0.001)(0.5m)28%15%

28%

= (0.0005 m)(0.46) = 0.00023 m

= 0.23 mm


5/19

example continued

c) Calculate the weight of water in the same piece of wood before and

after, if its dry density is 420 kg/m3.

Since moisture content is measured as a percentage of dry weight:

At 15% MC, weight of water = 15% x (420 kg/m3) (0.5 m)3 = 7.88 kg

At 35% MC, weight of water = 35% x (420 kg/m3) (0.5 m)3 = 18.38 kg

element dimension

plywood floor sheathing 19mm

38x286 floor joists 286mm

2x38x140 top plates 76mm

38 x 140 stud 2,343mm

38x140 bottom plate 38mm

2,762mm

What is the anticipated shrinkage between each

floor level of a wood frame building having the

following dimensions, if the wood is initially installed

wet with a moisture content of 30%, and after one

year its moisture content has stabilized at 12% ?

Floor-to-floor dimension is 2,762 mm, comprised of:

Example: Wood shrinkage due to moisture loss

2,7

62mm

286mm

2,3

43mm

elementparallel

to grain

across

grain

no

contribution

plywood floor sheathing 19 mm

38x286 floor joists 286 mm

2x38x140 top plates 76 mm

38 x 140 stud 2,343 mm

38x140 bottom plate 38 mm

2,343 mm 400 mm 19 mm

2,7

62mm

286mm

2,3

43mm

What is the anticipated shrinkage between each

floor level of a wood frame building having the

following dimensions, if the wood is initially installed

wet with a moisture content of 30%, and after oneyear its moisture content has stabilized at 12% ?

Floor-to-floor dimension is 2,762 mm, comprised of:

Example: Wood shrinkage due to moisture loss example continued

Therefore total shrinkage is 1.34 mm + 11.4 mm = 12.8 mm

(Note that shrinkage is concentrated near the floor assembly)

(~90% of total

shrinkage)

L= (0.001)(2,343mm)28%12%

28%

=1.34mm

a) Shrinkage parallel to the grain:

Shrinkage only takes place from 28% MC down to 12% MC,and the equation becomes:

b) Similarly, shrinkage across the grain:

L= (0.05)(400mm)28%12%

28%

=11.4mm

Values of : parallel to the grain: 0.1% = 0.001

across the grain: 5.0% = 0.05

(~10% of total

shrinkage)


6/19

Managing water

vertical surfaces

Approaches to managing water

on vertical surfaces




7/19






on vertical surfaces3) multiple element approach

(e.g. cavity walls, concealed barrier

walls, drainage cavities, rain-screens, pressure-equalized rain-screens)

Minor amounts of water whichmay penetrate the outermost

cladding layers are interceptedby a secondary line of defence.


8/19





3) multiple element approach

(e.g. cavity walls, concealed barrier

walls, drainage cavities, rain-

screens, pressure-equalized rain-screens)





3) multiple element approach

(e.g. cavity walls, concealed barrierwalls, drainage cavities, rain-

screens, pressure-equalized rain-screens)


9/19

Lets look at the impact of eachof these forces on the claddinglayer of a wall system

interiorexterior

controlled by baffling(joint covers), shingling, or

creating a dam/labyrinth

Rain falling at an angle carries

water behind the claddingdue to its momentum

most relevant foropenings > 5 mm

incorrect correct

Kinetic energy (momentum)

Kinetic energy (momentum) Gravity

controlled by avoiding inwardsloping surfaces or properly

lapping/shingling components

The force of gravity pulls any

water behind the cladding ifangles of opening are

incorrect

most relevant for

openings > 0.5 mm

incorrect correct


10/19

Gravity Surface tension

controlled by maintainingan outward slope, or addinga break in the surface such as

a drip/kerf/groove

Water adheres to the under-

side of the surface and isdrawn behind the cladding

at any opening

incorrect correct

Surface tension Capillarity (capillary attraction)

controlled by creating a gaplarger than a drop of water can

bridge (capillary gap)

Water pulled behind cladding

through narrow gap

most relevant foropenings < 0.5 mm

incorrect correct


11/19

Air pressure differences

controlled by equalizing airpressures from one side of

the cladding to the other

Differences in air pressure

push water behind claddingsystem

for openings ofbetween 0.5 mm < 5 mm

incorrect correctThe primary method for minimizing the air pressure difference across

the exterior cladding of an assembly is to ensure that there is an air-tight backup elementwhich will assume the greatest portion of thepressure difference across the total assembly.


exterior interior

pressure-equalized

cavity

3

air-tight backup

element

2air-permeable

exterior cladding

(rainscreen)

1

This concept is referred to aspressure-equalization.

Because of the openness of the cladding and the presence of adrainage cavity, the system is often referred to as a drain-screen,

open-cladding assembly, or apressure-equalized rainscreen assembly.


exterior interior

pressure-equalized

cavity

3

air-tight backup

element

2air-permeable

exterior cladding

(rainscreen)

1


12/19

The popularity ofopen joint cladding systems in recent years isbecause of its potential to operate as a pressure-equalized system.

Open joint cladding

An alternate way to think of pressure-equalization is to consider each

potential opening in a building envelope as a two-stage joint:

outer seal (baffle)1

inner seal (air-seal)2

pressure-equalized cavity3

Joint design

1-stage joint 2-stage joint

the outer seal should be loose sometimes referred to as a baffle,

the inner seal should be tight which provides the primary air seal.


13/19

Rather than rely on the sealant to determine which of

the two seals is tighter, usually the geometry of the jointcan be modified to ensure that the inner seal becomesthe air seal, as shown in the following examples:

1-stage joint 2-stage joint

Joint design Car Door Design

Car Door Design Car Door Design


14/19

Car Door Design

pressure-equalized

cavity3

2inner seal

(air-seal)

Car Door Design

Window sealing

traditional double-hung

wood window

Window sealing


15/19

Window sealing

Curtain wall

assemblies

Curtain wall mullion

components


16/19


components


components


components


components


17/19


components


components


components

Pressure-equalized rainscreens


18/19

Requirements of the moisture barrierin walls


19/19

17. water%2c liquid

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