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H ygrot he r m a l Da m a ge of Building

M at e ria ls a nd Com pone nt s: St at e oft he

-Ar t Re por t on St udie s of H ygrot he r m a l

Da m a ge a nd Propose d Approa ch for

Da m a ge Asse ssm e nt s

N o f a l ,

M .

I R C - I R - 7 5 7

National Research Conseif natirrnal

1+

1

Hygrothermal

Damage

of Building

Materials

and

Components:

State-of-the-Alt Report

on

Studies

of Hygrothermal

Damage

and Proposed

Approach for Damage

Assessments.

(::I ST 1 / Ji3 TST NRC/i: t\!%lC: JRI: R e f S e r

H e c : ~ ? j v e d fin : 0 - : i - I?? ' 3 3

I-& t I- n.2 1. i-e L I ~ I- t, .

Mostafa Nofal

internal Report No. 757

Date of issue:

March

I n E ~ ~ C A I t - : = ! p f i r k I I - ~ < - t I t ~ t t ; - : f A N A L Y S E

J

This internal report, while not intended for general distribution, may be cited or referenced

This report is prepared for the Institute for Re-h in Construction (IRC) at the National Research Council

(NRC)

and

The

in

determining buiIdings' durability and

in

is

information a h a dmbiIity of materids and structures.

The

envelopes damage are identified as both technical and functional,

The report is split to three parts and divided into ten chapters discussing and

presenting different mdeIling and measurement aspects

of

construction materials. The first part

of

m m m i s e s the current philosophy

in

and

of construction materials. Chapter 1 introduces the problem of durability and - hygrothed damage of building materials. It states the objective of the c m n t study

and outlines the steps followed

in

theoretical basis

of

of

and

in

and

characterisation and addresses experiments aimed at predicting the bekviour of construction materials in the laboratory under changing conditions,

The second part, documented

in

4 discusses the test methodology and procedures

used in

budding

wood materials. It identifies the different darnage mechanisms sf wood due to exposure

to hvgrothermal effects. Chapter

6

in

brick

It

of materials mechanicaI and storage properties. Chapter 7 shows that concrete materSs

are vulnerable to cracking and corrosions of both comete

and

It also presents the problems in modelling concrete mic~ostructure and the chemical process lading to corrosions. Finally, Chapter 8 briefly sbws the damage of finish materials specially paint and bitumen-based materials.

The last part of the report presents detailed procedures of a novel approach for

assessing the strength of darnaged or undamaged construction materials. The proposed

approach, included

in

structures made

h m

and

on

response of structures. The p p o d method is using rigorous theories of mechanics,

inGIuding h c t m

and

approach is based on measuring stifhess degradation caused by damage accumulation as

a r e d of hygrothemd and m e c ~ c a l loads. Chapter 10 also provides conclusions and

T

my

and

Research

in

(Re),

saving on the advisory committee of

the

In

Dm

K

A. Kmgiozis, and

D.

and

This report is not regarded

in

of this

In

aad

K

TABLE OF CON'lENTS

2 MECHAMCSOF THE PROBLEM

...

...

...

...-

*...

...

=...

HH..H

...

...

...

--

...

.

...

-",*.

...

.

.

.

.

...

.

.

.

.

,

.

..

...

3.3.1 Lkfermimiorr of Storage Chmcferimmcs 18

3.3.2 Derermirmfion of Vapow Dimion C 4 c i e n t x

...

3.3.3 Determination ofCwiflmy transport Co@cients

... .

.

.

.

.

.

.

.

.. .

...

...

3.4 SOLAR RADIATION EFFECTS

...

3.5 DIKU$$IONS

.,

...,--...-..

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

...

3.6 H Y C R ~ R M A L DAMAGE

...

...1...~...

.

5.1 GENERAL

...

.

.

... ., ...--...

5.2 DAMAGE TYPES

...-...

...

...

5.2.1 SfntcfuraI Damage Model for h b e r

.

...

,

.

.

..

..

.

..

.

...

...

.

....,-.

...

.

...

...

...--....

... -.,.

...

...

..."...

...

. . .

...*...*

...

....,...

...

... ... ...

. .

....

. .

...,,...,,

...

..

...n...n,...w,

*.*....

...*

..,...,..

... .-.,

.... .

...

.

.

.

.

...

.

.

...

.

.

...

,

.

...-.

...

..

..,,...,,...LILILI...LILILI...

...,,, . . .

. . .

...

...

...

...

...

...*...~..

...

7.1 GENERAL

...

...-...

...*M...M....*.*..w,...

...,... *... ..- ..-".

...

7.2 C R A ~ C

...

...

...

7.3 CORRQSION AND RUSF

...,-,

... .... ...,,...

....,,...,,,

...

...,,...

.... ...

,,.,

...-...

7.3.1 Cornion of Coxmte.

...~...

TABLE

...

...

-

... ...,...,.

TABLE 3.1 TYPICAL H Y G R ~ H E R M A L MATERIAL PROPERTTES FOR C O N S T R U ~ O N MATERIALS

...

TABLE 3.2 ESSEK~AZ HYGROTHERMAL MATERIAL PROPERTIES (AFTER HENS 1996)

...

TABLE 3.3 E X F E R F ~ A L METHODS OF HYGROTHERMAL MATERIAL PROPERTIES (AFER HENS 1996)

..

TABLE 4.1 : OR DAMAGE TYPES M CONSTRUCTION MATERIALS WTH THEIR DESCRIPI~ON

...

TABLE 4.2 DAMAGE CMTERIAOF ENVELOPE SERWCEABILZTY, FUNCTIONJ~LITY AND APPEARANCE

...

TABLE 5.1 MNOR DAMAGE TYPES WCLIRF3NG IN WOOD (AFTER -EL AND MOMELL 1992)

...

T m ~ e 5.2 MINIMUM MOISTURE LEVELS FOR DECAY DEVELOPMENT

...

...

...

TABLE 5.3 O P ~ ~ M A L TEMPERATURE FOR COMMON WWWDECAY FUNGI

...

TABLE 5 -4 TYPICAL EUROPEAN COLONISIWG MOULD AND STAIN FUNGI

... .

.

.

.

.

...

TABLE 5.5 C O E F F I C I ~ TO BE USED M EQUATION [5.173

...

...

...

TABLE 5.6 OPTIMAL TEMPERATURE FOR COMMON HARMFUL WOODDECAY FUNGI IN EWRPE

...

TABLE 5.7 TYPICAL VALUES OF VARIOUS STRENGTH LOSSES I%)

...

...

TABLE 5.8 TYPICAL VALUES OF VARIOUS STRENGTH LOSSES (D/b)

...

...

TABLE 5.9 COEFFICIENTS TO BE WED M EQUATION [5

.

...

TABLE 5.1 0 OPTIMAL TEMPERATURE FOR COMMON WOOPDECAY FUNGI

...

.

.

.

.

...

TABLE 7.1 CAPILLARY P D R O S ~ ~ Y AS A FUNCTION OF THE ORIGINAL W A ~ C E M E N T RATIO

...

TABLE 7.2 DAMAGE SOURCES AND TRIGGERING LOADS IN CONCRETE MAERFALS

...

TABLE 7.3 SCALES DEFINITION A ~ R TO DTHE ~SEE REPRESENTATIVE VOLUME ELEMENT (RVE)

....

....

~ G U R E

...

..- ... ... ..-.. ....

.-I.

...

... ...

--

...

FIGURE 3.1 TYPICAL SORFTION ISOTHERM OF COMMON POROUS BUILDING MATERIALS

...

FIGURE 3.2 SCHEMATIC AND PHOTOGRAPHIC VIEWS OF LAYOUT PRESSURE VESEL

...

..,..

.

FIGURE 3.3 TEST SET-UP FOR DRYCUP AND W < W P MEASUREMENTS

...

...

FIGURE 3.5 ~IOTOGRAPHIC VIEW OF THE SMALL ANI3 LARGE NUCLEAR MAGNETIC RESONANCE (NhfR)

.

m)

...

FIGURE 3.7 AIR PERMEABILITY APPARATUS ( A F E R NAGATAKI AND UJIKE 1

m)

...

FIGURE 5.1 MOISTURE CONTENT FACTOR FOR DAMAGE MODEL

... ,.

..

...

FIGURE 5.2 ~ P I C A L MOISTURE CONTENTS FOR OBTIMAL FLINGPIS GROWTH

...

...

GAITER

...

...

..

FIGURE 5.6 TYPICAL GROWTH CURVE OF MESOPHILIC W00D-INWWlTING FUNGUS

...

...

...

...

....

.

.

...

...

FIGURE 5

.

...

F~GURE 5.12 FUNGAL GROWTH UNDER F L W A T I N G RELATIVE HUMIDlTY

...

...

...

...

... .

.

.

.

.

...

FIGURE 7.3 REWFORCEMENT MA- CORROSION PROCESS

...

FIGURE 7.4 TYPICAL CORROSION PROCESS OF STEm RHMFORCEhENT W CONCRETE

...

...

-...

F~GURE 9.1 PROPOSED APPROACH FOR DAMAGE FUNCTIONS OF MATERIALS 3

FIGURE 9.2 TYPICAL STRESS DISTRlBUTJOEI OF A CRACKED BAR

...

...

FIGURE 9.3 TYPICAL DEFORMATrON DUE TO CRACK 107

Moisture, temperature aud mechanical loading are the leading causes of the

continuous deterioration of

building

and

construction industry is now using new composite materials (Balaguru snd

Shah,

such as fibre or fabric-reinforced resins, to meet speci6c demands of increased load and

structure durability. However, the traditional mate&ls are stdl used

in

kcause they are thought to

be

concrete structures represent more than 65% of

all

and

are primarily

used

in

Most existing building stocks

in

emphasis on construction costs rather than the lifqcle cust of buiIdings

and

envelopes. Consequently, these m c t m ofla experience deterioration and distress

after onIy 10 years of service. The envelope may continue to

be

an

enveIopcs is the most cost-effective measure, compared to repair or replacement of the whok structure. However, in some instances, regulatory agencies are concerned with

taxes or historical signifcanee. Therefore, govermnent s and stakeholders sometimes require costly rehabilitation

and

measures usually ficus on upgrading the safety of these structures and improving their

indoor environments and exterior aesthetics,

A recent study by the Natiod Materials Advisory Board (BMAB) of the National Research Council

i

n

E

durability of structures. The study estimates that the current value of concrete infrastructure ahne in the United States

is

and

replacement costs could well reach between $1 and

$3

f

years.

The

identified

institutiod.

On the technical side, the study found "inadequate knowledge of the mechanisms of development,

and

in

Among the recommendations made by the bard to ameliorate the situation is to "develop in-depth understanding of the manner in which the compsitiorr and micrusmcfure of materials @ect

their

Among institutional issues, the report called for "the development of a proper

howledge-based expert system for providing the rules for producing durable concrete".

This conclusion is valid not ody

h r

materials. The board im this regard concIuded that "at present, only limited iqftormation is avuilable on predictirrg the long-term bekiwiour of a building material or s t m i w e in a spec$c environment, and even less is normal# made use of". The b o d recommended

the development of such a system to predict the service life of construction materials. It

wouId contain a mathematical model to document the observed degradation, to compare the data with past experience and test da@ and to pamit cdculatbn of the acceleration

The observations, concIusions and recommendations o f this distinguished board do

not apply only to the United States; but also wrldwide, including Canada. It m y

be

argued that the hdhtructure in Canada is in better shape than that

in

If this is so, then it is due more to the hstitutioml policies

and

the

structures iu Canada than to superior techGcaI know-how.

Clearly, the problem of durability is mt just due to over-stresing of stnrctuaes; a

whole range

of

and

and practices,

are

construction materials to transfer loads without experiencing distress depends on the

nature and magnitude of the applied stresses, moistwe contents, and structure

temperatures.

Moisture,

of moisture retained by the structure envelopes is also a function of the surrounding

environmental conditions. In general, environmental conditions include

all

With all these complex issues, present design

and

on

concrete and masomy, the campressive cylinder or cubic strength is often used 10 design

the whole

structure.

and

use

ns

experience to design and rehabilitate the residential dwellings.

The cummt practices uncouple the mechanical

and

is conducted

firsz

and

in

in

and

damage. Therefore, the resulting moisture and thermal loading are applied as loads to

obtain stresses and deformation of the structure. These defbrmations are predicted using elasticity or plasticity

thwries (Sih

ShYaike

and

The drawback of this solution strategy is that the elasticity

and

assume materials are homogenous

and

brittle materials. Since brittIe materials b c t u r e under loading, it was d s e d t b t the

relationship between the fracture energy wd the energy produced by the applied stresses

is the key to u n d m t m d i i the micro-mechanics of concrete (Kunin 1983; Mindless 1983;

Zaitsev 1983).

The propagation of cracks in construction materials is governed by the activation energy for kc-.

This

and to create surfaces

and

in

represents the characteristics of a brittle material versus a ductile materid. With the exception of metals, all construction materials are heterogeneous brittle solids, interspersed with

m

y

and

and

in

these defect volumes also affect their evoIution and propagation. Thus, the key to understanding the micro-mechanics

and

materials is the manner in which h&we energy

is

applied mechanical, moisture, and thermal loads.

Classicd hcture mechanics was originally developed to describe the behaviour of discrete macroscopic cracks,

but

(CIDh4).

The development

of

link

process involves initiation, evolution, and propagation of cracks in the damaged material

undw

Physical observations indicate that deterioration and undesirable deformations are in

many instances, not due only to mechanical overloading, but are also the results of excessive moistwe and temperature fluctuafion, Observed deterioration

in

In

cold climates, building envelopes experience the freeze-thaw cycIe effects. Erosion of facades, pauels and veneer wdh due to wind-driven rain is dso a common problem f i r

buildings in both climate conditions. However, special coatings rrray provide protection

against the latter effect, as welt as against ultraviolet radiation. Therefore, varying Ioads, absorbed moisture, and temperature changes causes d e t e r i ~ r a t i ~ ~ Hygrothemd and mechanical loading continue to degrade the physical, f h e d and mechanical properties of the material. Accordingfy, structure melopes subjected to any combination of these

loads d l not live their full design life. Thus, improvements in the analysis, design

and

damrage predictions of materials

and

implications.

Moisture and t h m l damage of building envelopes

is

if the

moisture carries cornding substances such as sodium cHoride or calchm chloride. The

cornding agent suspended in the moisture attacks the reinforcing, studs, and fastening metal m a t d s . The chemical reaction between the chloride ions and the metal material

causes the corrosion of these mterials and oxygen and tempmdture often accelerates the comsion processes.

The

cornsion causes mere damage a n d may even muIt

in

The severity of the comsion depends on the number

of

Customary building materials, with the exception of steel, are naturally prow. The

distribution of the pres

and

without the

In

and

mteriab becomes damaged and cracked, Adequate understanding of the durability o f

the construction materials is necessary to avoid expensive rehabilitation, or demolishing the entire

building.

Construction materials are non-homogeneous composites composed of different materials. For example, concrete materials comprise cement, water, fine and coarse

aggregates, air (voids), sometimes and other chemical admkhms. Similarly, mod, rock,

and clay-brick materials contain mtwd organic chemical, flaws, faults, and some

discontinuities. The incompatible mechanica1, hygroscopic and t h d properties of the different m a t a d s within the mimstructure are the underlying c a w s of the micro-

cracks h t exist at the microstructure level of construction materials. The presence and propagation

of

1983; Horii and Nemat-Nasser 1985; 1986).

Moistwe in these cracks and temperature fluctuation can often increase the rate of damage

of

and

It

k

fir1

these conditions,

in

1.6 OBJECTR~ES AND SCOPE

The

also

novel approach for measuring and modelling damage in the different construction

materials

and

This report spechifically summarises the experimental and analytical work conducted

aver the

last

h m

investigations. This report also identifies areas

in

durability experiments that need kther studies

and

Existing durability testing

a d

in

current work focuses on:

(11 experimental studies of durabiIity conducted over the

lasZ

[2] the types of damage associated with various construction materials;

131 the different testing philoosophies

used

hygrothmI loading;

[4] the damage models developed h m experiments;

151 the proposal of research areas to be conducted by the National Research Council of Canada to develop leadership

in

161 the proposal of a novel approach for modelling and measuring hygrotheml damage

of

and

1.7

REPORT

Chapter 2 d s e s the theoretical h i s of the mechanics of moisture and heat lmnsfkr in construction materiaI. All theoretical models of durability

and

discusses the experimental work on the material characterisation. Chapter 3 also addrases experiments aimed at predicting the behaviow of comtmctioln materials in the

humidity and extreme temperatures are used. Thus, moisture contents far exceed actual

conditions,

and

lead

of

be

consequences

of

It is important ta rely on data collected over 20 years because of the continuous developments of new materials. Damage estimation has to

bz

tests. Howwer, the actual

field

damage process for given environmental conditions. Thus, Chapter 4 discusses the tests

on the performance and durability of actual buildings.

Chapters 5,6,7, and 8 describe current literature for estimating h y p t h t m l damage for wood, brick, concrete, md other materials

such

Chapter 9 presents the novel approach of darnage that will be used to quantify damage

both

and

on

as

mechanical loads. It also indicates the position of the Institute far Research in Construction (IRC), National Research Council of Canada, (NK) and identifies

areas

where more research is needed. Chapter 10 also d s e s the conclusions and some recommendations based on the Iinding of the cmmt study.

Finally, the report ends by providing the list of references cited in the different chapters.

M E C H A N I C S

O F

T H E

H Y G R O T H E R M A L

P R O B L E M

2.1 GENERAL

To

processes. The aim of envimnmental testing is to evaluate the physical behaviour of

construction materials under changing conditions. Experimental results also provide means of enhancing

and

The

any fluid lor solid continuum,:

I. the mass balance or continuity condition

2. the thermal energy balance

a

- I p e d v + ~ ( V - V , ) - ~ ] & -

I(g:

2

MECHANICS

THE

HYGROTAERMAL

PROBLEM

3. the momentum balance or Cauchy's fust law of mat ion

These equations are valid for any arbitrary part of the continuum of volume V

bounded by the

surface

dS

In

p is the density, v is the continuum veIocity vector, v, is the srrrface vebcity, e

is

specific internal energy, q is the heat flux, and h

is

On

the other h d , a is the stress tensor,

and

f

the

of

tensors.

When moving boundaries bound the continuum, the additional space conservation

law should

be used in

These equations are sufficient

and

continuum.

Moisture absorption in porous materials under temperature gradient occurs by

different mechanisms. Cracks and voids in the material space provide one of those

mechanisms (NoM 1997). Philip and De Vries (1957) established the. mathematical formulations of m s transfer at the macroscopic level

in

(1977) and Bear and & k t (1990) later Introduced a more fundamental solution of

the diffusion equation in porous materials. When layers of various materials used to

tom

and

among layers. The moisture

in

be

MECHANICS

THE

PROBLEM

For simultaneous heat, air and moisture actiom, 0nsager7s principle of the irreversible process of thermodynamics may be applied (Hens 1996; Duforestel

1993;

and

flux

by

represents the deriving forces of state j

.

in

,

describing

the

form of equation

12-51

flux

The change in mass can be stated as

x

am

ap,

e x - -I--- h x * - , L + q - + B r n q F ap ?Pz *3

%I at

at

P-TI

at ap,

a

w3

and

In

and

moisture. The coefficients B, I charactrise the storage capabilities of the

pornus

medium per unit change of deriving force. They are called specific capacities.

n e s e coefficients depend

on

.

B'

on the considered materid under testing. This issue is discussed

in

Inserting the flux of equations

12.51

p.l]

[2.2] yields:

for heat

for moisture

These equations. represent three partial differential equations defining:

(3

and

,

S,

and

.

All

ab

be

envelope.

In

three

be

are hydration

and

of

and

and

In

MECHANICS

HYGROTHERMAL

PROBLEM

thermal

and

be

Finally, it should be

of

of

building

diierence, k i t e element

and

2.3 DISCUSSION

The brief discussion of the mechanics of the problem

in

in

physicists and researchers focused on solving the tramport problem for prous materials used

in

dl

and Mwaferija (1994),

and

in

momentum requhments is that mass flow does not generate large velocities. Large velocities occur only in the

case

and

The use of computer-based models to trace

the

them1 and moisture loading has been on the rise ever the past 10 years (Hens 1996). Even the Teast sophisticated procedures o f these computer models require many material. data input parameters. The success

of

depends

boundary conditions specified for a given simulation. According to the conservation

and

flow equations discussed

in

and

needed.

The coefficients are either merial properties or m f e r capacities, depending on their

usage

in

This chapter discusses the necessary material properties, specific capacities, and

trans* tensors o f combuctien materials.

In

for each property or item will be discussed. The presentation also shows the ad-ges of some of these testing methods. Possible enhancements of these methods are aIso indicated.

3.2 MATERIAL PROPERTIES

A

full

(Hem

moisture

and

in

To

of these tests are also subject to ~ u u s ways to translate them

into

properties.

Pomns glass

0 25 50 75 100

RH (Yo)

Figure 3.1 Typical sorption isotherm of common porous building mateflak.

In

physical quantities. However, Table 3.1 shows that some of the 21 hygroscopic material

parameters are related. For example, Hokoi

and

the

m, vapour pressure pv

and

on

making

use

curves for three building materials.

These m e s explain the variation in behaviour of diEwent constmction materials,

One important ohnation is

that

and

are not the same. These curves are aka different -for different materials. Figure 3.1

also

indicates that between the dry and hIIy saturated states, the moisture contents vary

nonlinearly (Kumaran 1996).

However,

Hem

that

measured

be

Thus,

and

be

by

results (Kumararm 1996)- This approximation will affect the quality of the predicted behaviour of any computer model.

In

in

together with the main functional dependence.

Ta b k

3.2

I

I

I

T e m p m a w moisture content t h e and heat

I

3 VapwpermeabIlity Temperature, moisture content and rapour flow

-

-4 Moisture permeability Km Temperature, moishrre _{flow direction} content and moisture

I

I

I

I

Moisture diffirsivity

1

7 8

ahermal moisture &&ion ~ E c i e n t air permeability

Dr

''

temperature, moisture contmt and moisture flow direction

temperature, moisture cuntent and air flow direction

Therefibre, the number of experiments to determine these essential parameters needed to become manageable. These tests are discussed

in

It was stated earlier that some of the hygroscopic properties of construction materials

are w-dependent and interdependent. Therefore, resuJts of different experiments can

be

combined and used to estimate the rest of the material properties. The literature contains a variety of markedly different procedures to evaluate the storage characteristics, moisture

and

of

kms

of

porous material to determine:

(I) its storage characteristics;

(2) its vapour d i i i o n coefficients;

(3) Its capillary transport coefficients; and (4)

air

A brief discussion of these test procedures folIows h the subsequent sections- The

merit of each test is investigated, as well as its use

in

and

It is universally accepted that to d e t d e the storage characteristics of a budding

material, the sorption mistwe and capiIIary regions must be separated. In the sorption

moisture region, the following sirnpIe procedure is o f h used.

The specimen is placed

in

varying in stages fism less than 50% up to 90%

RH

produces

isathem The reverse of the process

yields

desorption isotherm.

Knrs

indicated that measurements above 95% RH should

not

the

is

His

on

the recommendation of DIN 5261 5. This specification states that small unavoidable

variations in the relative humidity by themselves cause very large changes

in

moisture. Small variations can be the remit

of

in

F a g e r l d (1 977) proposed

an

in

radius

produces a relatiemhip between the hezing point depression, the

radius

the Kelvin equation

(Harmathy

time is

method is reduced to 3 hours. However, the k e z h g can cause stmctud

damage

specimen. Furthermore, the measuring

and

large.

In

between the capillary pressure and water content can be determined from mercury pornshetry,

In

is obtained by incremental increases in the suction pressure. The water

is

One limitation of

this

is

specimen though higher macbnicd stresses. The method, moreover, is relatively expensive. The moisture profile has to be determined quickly and immediately after the

latter problem is avoided

by

mjar drawback of the

method

the

very

limited

I

used. The suction presswe range can

be

the

apparatus of Gardnm (1956) is wed.

h s (1996) corrected the measurements of the suction pressure using the plate apparatus. The targeted correction can be made wing three pressure vessels.

The

cylinder diameter

is

bar,

bar.

and

above

up

15

bar.

used

the test This test appasatus

is

Figure 3.2 Schematic and photographic views

of

The measurements

of

in

standard test

method E96

989)

The

known

as the 44dry-cup" and "wet-cupv' methods ( K m a n 1997; Krus 1996). The dry-cup

measurement evaluates the diffusion coefficients

in

FbI.

in

region.

The

investigation atop a vessel as its cover. The edges

of

,'--

Figure 3.3 Test set-up for dy-cup and wet-cup measurements.

The vessels sre then placed

in

and

humidity. The wata vapour diffuses through the specimen under the effect of the partial

onset of steady flow diffusion, the test vessel weight undergoes constant changes with respect to unit of time,

For thin layers of non-hygroscopic materials, the stationary diffusion flow is

established almost at once, without the start-up process. However, moisture equfi'brium

from the ambient exterior and interior conditions must be established for thicker layers of hygroscopic materials. It should be mentioned that the initial weight changes of the tes vessel, including the test specimen are attributable to changes

in

specimen, and to weight changes

in

that there are methods ~r avercoming this shortcoming by weighing the test specimen

and the sorbent separately.

Figure 3.4 Expected experimental

results for

A layer of

air

and

stagnant air exists. The air space does not affect the measurements

in

of

but it has remarkable impact h the wet-cup method. The sensitivity of the measurements in the Iatter case

is

hr

K m m (1 997) introduced an extension to this method where

he

aEso proposed

smooth the experimental results to match the typica1 graphs given in Figure 3.4 fbr non-

hypscopic and hygroscopic material~. The permeance of the test assembly according to

Kumaran (1

997)

magnitude of the derivative

pamcanoe

(RH?)=

Saturation water v a p pressure at 23 'C

Kurmaran (1997) dm w e s t e d correcting these values accord'ig to the procedures described by Lackey ef a/.

(1997)

and

account for the d c e resistance and the resistance offered by the still air inside the cup. The corrected permeance, multipfid by

the

of

W h .

elaborate procedure. The miin difference between the two approaches is only

in

2scosO

relative humidity

(RH%)

)

~ P W R D T

where the symbol o ( N / m ) stands for the

surface

(degree)

wetting angle, r ( m ) is the capiIlary radius,

RD

( kg/m3 ) is the density of the water, and

T

( K )

ullits