Sampling method for measuring the moisture distribution in fire test specimens

(1)

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Building Note (National Research Council of Canada. Division of Building

Research); no. DBR-BN-42, 1963-07

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=f52482d7-708b-4a18-82dd-cefa693bd916 https://publications-cnrc.canada.ca/fra/voir/objet/?id=f52482d7-708b-4a18-82dd-cefa693bd916

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/40001559

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Sampling method for measuring the moisture distribution in fire test

specimens

(2)

CANADA Ser Trn

B92

no.

42 c. 2

SAMPLING

METHOD FOR MEASURING THE MOISTURE

DISTRIBUTION IN

FIRE

TEST SPECIMENS

T.

2. Harmathy a n d

E.

0. P o r t e o u s

NATlQNAL RESEARCH COUNCIL

7-

.

L ' , . " " -

.,

\ L A _, . .

DIVISION OF BUILDING RESEARCH NATIONAL RESEARCH COUNCIL * OTTAWA, C A N A D A

(3)

DISTRIBUTION

IN FIRE

TEST S P E C I M E N S

by

T.

2.

Harmathy a n d

E.

0. P o r t e o u s

In

1959 a

n e w r e q u i r e m e n t was i n t r o d u c e d i n the ASTM s t a n d a r d

on f i r e tests of building constructions (11, according to which a specimen cannot be subjected t o f i r e t e s t until its m a x i m u m T'dampness" becomes

*

lower than 7 5 p e r cent, in t e r m s of relative h u m i d i t y . Although i n its p r e s e n t form, this tlrnoisture r e q u i r e m e n t s f is still rather vague, the

r e a l i z a t i o n that m o i s t u r e , inevitably present in almost any build ing rnater ial, offers a serious problem t o the reproducibility of f i r e t e s t r e s u l t s a n d

that m o r e refined techniques should be u s e d for its m e a s u r e m e n t and

control, should be regarded as a v e r y important step in t h e right d i r e c t i o n . Shortly after the intr o d l n c t i ~ n of the ' h o i s t u r e r e q u i r e m e n t ,

'"

m o s t f i r e t e s t i n g l a b o r a t o r i e s adopted a technique d e s c r i b e d

b y

Menzel

(2)

far m e a s u r i n g the equilibrium relative humidity of concrete f i r e test

s p e c i m e n s . After a few y e a r s in use i t became obvious, however,

that this technique, which will be c a l l e d here "method of e m b e d d e d s e n s i n g elements, I1

did

not: suit the field of f i r e e n d u r a n c e testing.

T h e

main

difficulties

which

have been e n c o u n t e r e d in practice a r e as follows:

1 . The testing p e r s o n n e l must d e c i d e b e f a r e the construction which

points of t h e specimen a r e m a s t likely to be r e p r e s e n t a t i v e of

the "dampest s e c t i o n s t ' d ~ r i n g the drying process. After the

construction is completed, o n l y at these s e l e c t e d points can the l e v e l of e q u i l i b r i u m r e l a t i v e humidity b e m e a s u r e d , e v e n i f it becomes apparent that i n s o m e other s e c t i o n s of the specimen the d r y i n g progresses at

a

much lower r a t e ,

*

At present the f i r e test s t a n d a r d is not concerned w i t h the actual m o i s t u r e content of the s p e c i m e n , as expressed in weight o r volume

p e r c e n t a g e , but only with its equilibrium vapour pressure, e x p r e s s e d

as relative humidity. In this

paper

the latter expression w i l l b e termed 'kquiPibrium relative humidity.

"

A s t h e moisture content is generally

not a single-valued function of the relative humidity (because of

the

h y s t e r e s i s in s o r p t i o n ) , the equilibrium relative h u m i d i t y is rarely suitable for defining the m o i s t u r e content of the material.

(4)

2 . Each sensing element w i l l p r o v i d e information only on the

humidity at a certain distance from the surface of the specimen.

The

variation of the average valce

of

the equilibrium relative humidity thus cannot

be

followed, unless a l a x ge n u m b e r of

s l e e v e s (which contain the sensing elements) a r e i n s t a l l e d in t h e specimen extending t o variocs d i s t a n c e s f r o m the surface.

A s these s l e e v e s must

be

removed before the t e s t , t h e u s e of too many m a y not be d e s i r a b l e , especially i f the s l e e v e s a r e l o n g ; t h e i r r e m o v a l is a laborious operation and m a y induce cracking o r other damage t o the specimen.

3. Although knowledge of the equilibrium relative humidity of the

fire t e s t _specimen_is_{v e r y}_{valuable because it}_{enables the}

m o i s t u r e condition of the construction t o be r e l a t e d t o the conditions existing in s e r v i c e , it must

b e

r e m e m b e r e d that it is the a c t u a l moisture content, a s e x p r e s s e d in weight or volume percentage, that a f f e c t s the p e r f o r m a n c e of t h e c o n s t r u c t i o n in f i r e . W i t h t h e u s e of the method of embedded s e n s i n g elements t h e latter information is n o t available; the e f f e c t of the dampness

of the specimen on the result, of f i r e t e s t , t h e r e f o r e , cannot

b e

assessed.

4. Because the equilibrium relative h u m i d i t y a n d the r e s p o n s e of the

sensing elements a r e temperature -dependent, reliable relative h u m i d i t y measurements can only b e taken

at

the t e m p e r a t u r e for which the s e n s i n g e l e m e n t s have been calibrated. Consequently,

if the test specimen is s t o r e d in a ccoditioning chamber at an

elevated temperature, it s h o u l d be r ernoved from the chamber a n d kept at s o m e c o n t r o l l e d temperature l e v e l for s e v e r a l days

before valid measurements can 5 e m a d e , !This difficulty l e d in

1 9 6 1

to a hasty modification of the ' F m c i s t u r e r e q ~ f r e m e n t " of

ASTM E l 19 ( 3 ) which abolished the lower limit of t h e allovable equilibrium relative h u m i d i t y of f i r e t e s t s p e c i m e n s .

1

Even if these p r a c t i c a l difficulties d i d n o t e x i s t , there would s t i l l

be objections to the method of e m b e d d e d sensing e l e m e n t s . B e e s u s e of t h e

disturbance

in the drying p r o c e s s caused by the p r e s e n c e of the s l e e v e s , measurements taken t h r o u g h the sleeves a r e p r o b a b l y s o m e w h a t e r r o n e o u s .

Two y e a r s ago t h e Fire Research S e c t i o ~ of the Division of Building

R e s e a r c h i n t r o d u c e d a new technique for obtaining information on the

'Uarnpnessll of fire t e s t s p e c i m e n s . T h e essence of this technique, which w i l l

(5)

ature r o o m .

The

equipment u s e d to obtain these samples consists

of

a

*-in.

electric d r i l l , t o which a diamond c o r e d r i l l of 1 in. i n s i d e diameter

i s attached ( F i g u r e 1). C o m p r e s s e d air can be supplied through the nozzle attached to the

neck

of the cox e d r i l l in ~r d e r to prevent the specimen and

the cutting edge of the c o r e d r i l l f r o m being o v e r h e a t e d . It has been found,

however, that e v e n without introducing cooling a i r , the temperature rise can be kept within a few degrees i f , by switching the motor on a n d off, the

ovex-all

time of cutting i s extended to 3 min or m o r e , and between the p e r i o d s of cutting sufficient time is left far

the

heat t o be conducted a w a y

along the metallic s t e m

of

the tool.

W h e n the

r

squir

e d

depth ( g e n e r a l l y the centre of the s p e c i m e n )

is r e a c h e d , the c y l i n d r i c a l sample f o r m e d inside t h e core d r i l l is s e p a r a t e d from the r e s t of the t e s t specimen, and the hole r err.aining in the specimen is f i l l e d with f a s t - s e t t i n g

high

t e m p e r a t u r e m o r t a r .

T h e

d i a m e t e r of the

sample is about 7/8 in. Immediately after separating from the specimen i t

is divided into a number of smaller c y l i n d e r s

( g e n e r a l l y

three) of 5 1 8 to 1 1 /4

in.

in length (depending on t h e total length of the sample), and t h e s e smaller c y l i n d e r s are crushed s e p a r a t e l y i n t o p i e c e s n o t l a r g e r than 1/4 in.

Because

t h e

s o f t e r components oi the m a t e r i a l ( t h e cementitious components, as a r u l e ) break m o r e easily than the harder ones, it is good practice t o

s o r t out the h a r d e r particles after an initial crushing, and continue the operation in s e v e r a l s t e p s . When combining the p a r t i c l e s {corniflg f r o m the

s a m e s m a l l e r c y l i n d e r ) , c a r e should b e taken not t o lose even that portion of the sample which accidentally b e c a m e completely

powdered.

Leaving the powder behind in the c r u s h i n g bawl m a y b e equivalent t o falsifying the

carnpos ition of the material. It is advisable to crush the samples in a r o o m of about 50 p e r cent r e l a t i v e humidity. T h e d u r a t i o n of the c r u s h i n g must nut exceed 15 rnin.

As soon as t h e c r u s h i n g is completed, the particles obtaified f r o m t h e d i f f e r e n t s m a l l c y l i n d e r s are p l a c e d i n different glass bottles of about

1 0 0 cu crn volume, A humidity sensi.ng element is then i n s e r t e d into e a c h bottle.

As

F i g u r e

2

shows, t h e s e n s i n g elements a r e equipped with rubber

stopper c o l l a r s which seal the opening of the bottles air -tight. Finally, identifyfng l a b e l s are attached t o the bottles (indicating which portion of the

test specimen the sample came f r o m ) a n d the whole a s s e m b l y i s moved into a r o o m of c o n t r o l l e d t e m p e r a t u r e (74'E in this laboratory).

(6)

The

equilibrium between the condensed and g a s e o u s phases in

*

the bottles develops in a period

generally

not longer than 2 4

hr.

After this period

final

equilibrium relative humidity r e a d i n g s can b e taken.

The instrument u s e d in this laboratory for r e l a t i v e humidity

rneasur ement s is a Model 10 3

''El

-T

rtsnic s" portable h y g r orneter (4).

Although m o r e accurate instruments a r e available, this hygrometer has been chosen because a s i n g l e s e n s i n g e l e m e n t can

be

used f o r the whole

.,- &

-4- q.

range of the instrument ( 1 5 to 1 0 0 p e r cent ~ e l a t i v e humidity),

After obtaining the final equilibrium relative humidity readings,

the samples are transferred in small m e t a l containers ( m a r k e d to identify the samples), weighed, and put in a vacuum oven, T h e furnace ternper

-

ature is set at 1 0 5 ° C for samples of concrete or clay products, and at 4 0 ° C f o r m a t e r i a l s containing gypsum, After 2 4 to

48 hr

the samples a r e

removed from the furnace. T h e r n o i s t u r e content of the samples (prior to

the d r y i n g p r o c e s s ) is calculated f r o m the loss of weight. Both the

equilibrium relative humidity a n d the moisture content ( r e f e r r e d to oven

-

d r y w e i g h t ) m e a s u r e m e n t s will be included i n the f i r e t e s t report.

It is obvious that with the u s e of t h i s "sampling technique1'

all

the difficulties associated with the ?'method of embedded s e n s i n g elements"

a r e eliminated, In a d d i t i o n ,

this

technique gives an accurate picture of the d i s t r i b u t i o n of both t h e e q u i l i b ~ i u m relative humidity and t h e actual

m o i s t u r e content across the f i r e t e s t specimen.

It should be observed, however, that the apnlication of

this

method may become tedious when the d r y i n g p r o g r e s s of a f i r e test specimen i s t o

be

followed.

In o r d e r

to avcid taking too m a n y successive samples f r o m the specimen, the following p r o c e d u r e is r e c o m m e n d e d :

W h e n the m o i s t u r e c o n d i t i o n of the s p e c i m e n is supposedly in t h e c o r r e c t range, take one sample and a n a l y z e it for equilibrium relative

B e c a u s e

2,1

mg

water is capable of completely s a t u r a t i n g 100 cu cm

air at

74"F,

the

moisture lost o r gained b y the sample d u r i n g t h e development

of equilibrium is obviously negligible.

$*

_{T h e}_sensing_{elements must not be exposed to I00}_{p e r}_{c e n t}_relative

humidity f o r l o n g e r periods. T o e n s u r e an acceptable accuracy it is r e c o m m e n d e d that t h e e l e m e n t s be r e c a l i b r a t e d in t w o 3 -month intervals.

(7)

i t

by

m e a n s of "Quickmount" s e l f - s e t t i n g plastic (5). After about 30 min,

a sensing element can be inserted in the sleeve which, even if n o t calibrated f o r the temperature of the c o n d i t i o n i n g chamber, w i l l indicate the t r e n d of variation of the equilibrium relative humidity in the specimen d u r i n g the following d a y s or w e e k s . When the readings s h o w that the m o i s t u r e

condition of the specimen m a y be at the correct level, take another sample from a similar location and if the analysis confirms that the specimen

i s ready for the test, include the result of the analysis in the t e s t r e p o r t .

REFERENCES

1. Standard methods of f i r e t e s t s of building c o n s t r u c t i o n a n d materials

(El

19 -58). Tentative revision submitted J a n u a r y 15, 1459.

1959

Supplement t o Book of

ASTM

Standards, Part 5, p . 2 0 1-202.

2 . M e n z e l ,

C.

A. A method f o r determining the moisture condition of

h a r d e n e d c o n c r e t e in t e r m s of relative humidity. P r o c . ASTM, Vol, 55, 1955, p. 1 - 2 6 ,