Determining moisture content and calculating its effect on fire endurance

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Determining moisture content and calculating its effect on fire

endurance

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NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH

DETERMINING MOISTURE CONTENT AND CALCULATING I T S E F F E C T ON F I R E ENDURANCE

by

T. Z . _{H a r m a t h y}

F i r e Study No. 17

of the

DIVISION O F BUILDING RESEARCH

OTTAWA August 1967

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NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH

DETERMINING MOISTURE CONTENT AND CALCULATING ITS E F F E C T ON FIRE ENDURANCE

by

T . Z. Harmathy

F o r some t i m e t h e r e h a s been a n effort t o s e t , by the t i m e of a f i r e t e s t , the m o i s t u r e content in f i r e t e s t specimens

close t o a l e v e l corresponding t o that in equilibrium with a surrounding atmosphere of 50 p e r cent relative humidity ( a t r o o m t e m p e r a t u r e ) . F o r a g r e a t many t e s t specimens, however, i t is impossible, a t l e a s t with the p r e s e n t conditioning techniques, t o achieve t h i s m o i s t u r e condition within a r e a s o n

-

ably s h o r t t i m e ( s a y in s i x months) without inhibiting some chemical p r o c e s s e s r e l a t e d t o the maturing of the specimen, o r i n other words, without falsifying the c h a r a c t e r i s t i c s of the m a t e r i a l . T o eliminate t h i s difficulty, i t h a s been

suggested that f i r e t e s t standards be modified and allow t e s t s t o be c a r r i e d out on specimens of m o i s t u r e contents

significantly higher than that pertaining t o 50 p e r cent equili- b r i u m relative humidity. The authorization of such relaxation of the t e s t r e q u i r e m e n t s would not be possible, however,

without knowing how the m o i s t u r e content in e x c e s s of the

"standard1' level affects the r e s u l t of the f i r e t e s t . Fortunately t h e r e is now enough information t o make possible the calculation of the effect of m o i s t u r e content on the r e s u l t of f i r e t e s t s . Details of experimental observations and t h e o r e t i c a l

considerations concerning t h i s subject have been given e l s e w h e r e 1 2 ) . The application t o p r a c t i c a l p r o b l e m s of the conclusions derived in t h e s e publications will be d i s c u s s e d and i l l u s t r a t e d through examples.

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SOME BASIC CONSIDERATIONS

During the p a s t f e w y e a r s i t h a s become g e n e r a l l y a c c e p t e d p r a c t i c e to m e a s u r e and r e p o r t a t l e a s t the m a x i m u m value of t h e eqriilibrium r e l a t i v e humidity i n s i d e f i r e t e s t s p e c i m e n s p r i o r t o f i r e t e s t s .

T h e p r a c t i c e of e x p r e s s i n g t h e d e g r e e of d a m p n e s s of f i r e t e s t s p e c i m e n s i n t e r m s of e q u i l i b r i u m r e l a t i v e humidity w a s adopted on M e n z e l l s suggestion ( 3 ) . T h e a d v a n t a g e s of t h i s p r a c t i c e a r e twofold: the e q u i l i b r i u m r e l a t i v e humidity h a s a n unambiguous meaning, and i t c a n be m e a s u r e d e a s i l y and with r e l a t i v e l y high a c c u r a c y .

T h e s h o r t c o m i n g s of t h i s p r a c t i c e , h o w e v e r , soon b e c a m e obvious. A p r i o r i c o n s i d e r a t i o n s , a s well a s t h e o r e t i c a l (1) and e x p e r i m e n t a l w o r k (2), c l e a r l y i n d i c a t e d t h a t the effect of m o i s t u r e on f i r e e n d u r a n c e is not r e l a t e d d i r e c t l y t o the e q u i l i b r i u m r e l a t i v e humidity, but t o the m o i s t u r e content e x p r e s s e d i n p e r cent by

volume within t h e a p p a r e n t b o u n d a r i e s of the solid c o n s t i t u e n t s . A s t h e r e l a t i o n s h i p between the v o l u m e t r i c m o i s t u r e content and

e q u i l i b r i u m r e l a t i v e humidity, the s o - c a l l e d "sorption r e l a t i o n ,

"

d e p e n d s v e r y m a r k e d l y on the individual m a t e r i a l , and e v e n f o r a given m a t e r i a l is not unique (due to the h y s t e r e s i s in s o r p t i o n ) , the a u t h o r h a s s u g g e s t e d (4) t h a t , in addition t o the e q u i l i b r i u m r e l a t i v e humidity, t h e a b s o l u t e m o i s t u r e content of f i r e t e s t s p e c i m e n s should a l s o be d e t e r m i n e d and r e p o r t e d .

D e t e r m i n i n g the v o l u m e t r i c m o i s t u r e content of f i r e t e s t s p e c i m e n s is, unfortunately, not always a routine p r o c e d u r e . Difficulties m a y a r i s e f o r two r e a s o n s : ( i ) in t h e c a s e of s o m e

m a t e r i a l s ( g e n e r a l l y t h o s e f o r m e d by h y d r a t i o n ) the m o i s t u r e content m a y not be uniquely definable; and ( i i ) s o m e t i m e s i t i s not p o s s i b l e t o obtain s a m p l e s of t h e t e s t s p e c i m e n i t s e l f f o r m o i s t u r e a n a l y s i s , t h e r e f o r e the a n a l y s i s m u s t be b a s e d , a t l e a s t p a r t i a l l y , on m e a s u r e - m e n t s c a r r i e d out on r e p r e s e n t a t i v e s a m p l e s .

T o o v e r c o m e t h e f i r s t difficulty, the " d r y weight" t o which the m o i s t u r e content of building m a t e r i a l s is r e f e r e n c e d m u s t be defined unambiguously. I t is s u g g e s t e d t h a t t h e weight of building m a t e r i a l s a f t e r being h e a t e d in a n oven a t 221 f 1

"F

(105 f 0. 5°C) f o r a sufficiently long p e r i o d so that no f u r t h e r weight change can be detected ahould be regarded ae t h e l r d r y weight. (Thie method of d e t e r m i n i n g the d r y weight is not a p p l i c a b l e t o g y p s u m products.

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thc p r e s e n c e of m o i s t u r e usually h a s 110 s i ~ n i f i c i i l i l cffcct o n the

f i r e test,. ) Dependent on the size of thc specimen the drying p r o c e d u r e m a y take f r o m one clay to one week.

In a l l the work done in the DDR l a b o r a t o r y in connection with studying the effect of m o i s t u r e on f i r e endurance the m o i s t u r e content w a s r e f e r e n c e d t o d r y weight m e a s u r e d i n the way just d e s c r i b e d , t h e r e f o r e s o m e c o n c l u s i o r ~ s m a y not be applicable if

the m o i s t u r e content i s d e t e r m i n e d by s o m e d i f f e r e n t d r y i n g method. In s e l e c t i n g t h i s method the DBR l a b o r a t o r y t r i e d t o comply with the m o s t commonly followed p r a c t i c e s . In the c a s e of c o n c r e t e s , which a r e p r a c t i c a l l y the only m a t e r i a l s s e n s i t i v e t o the drying p r o c e d u r e , drying a t 221 " F g i v e s a p p r o x i m a t e l y the s a m e d r y weight a s that obtained by the "dry i c e method" (5) which is t a c i t l y accepted a s the s t a n d a r d drying method in scientific l a b o r a t o r i e s .

It i s i m p o r t a n t to understand t h a t although q, s a m p l e oven- d r i e d a t 2 2 1 ° F can be r e g a r d e d a s containing no m o r e m o i s t u r e o r a s it is often t e r m e d "evaporable w a t e r , I' i t m a y s t i l l contain

a c o n s i d e r a b l e amount of "non-evaporable w a t e r ,

"

i. e.

,

w a t e r attached t o the c r y s t a l l i n e l a t t i c e by v a r i o u s c h e m i c a l bonds. T h e non-evaporable w a t e r m a y a l s o have a m a r k e d effect on the f i r e e n d u r a n c e t h i s effect, however, will not be c o n s i d e r e d i n t h i s Note.

A s a l r e a d y mentioned, obtaining s a m p l e s r e p r e s e n t a t i v e of t h e t e s t s p e c i m e n m a y p r e s e n t a n o t h e r difficulty i n d e t e r m i n i n g i t s v o l u m e t r i c m o i s t u r e content. Whether o r not t h i s difficulty a r i s e s , depends l a r g e l y on t h e type of m a t e r i a l u s e d i n the construction. B e f o r e d i s c u s s i n g s o m e methods of o v e r c o m i n g t h e s e difficulties, i t m a y be useful t o e x a m i n e briefly how i m p o r t a n t the " m o i s t u r e problem" r e a l l y is i n the f i r e t e s t i n g of v a r i o u s building m a t e r i a l s .

If spalling is not liable t o o c c u r , the p r e s e n c e of m o i s t u r e

is beneficial f r o m t h e point of view of f i r e endurance. A s a f i r s t a p p r o x i m a t i o n , one m a y expect about 5 p e r cent i n c r e a s e in f i r e e n d u r a n c e due t o e v e r y p e r cent (by volume) of m o i s t u r e . Since the reproducibility of f i r e t e s t r e s u l t s is g e n e r a l l y ' n o t m u c h b e t t e r than f 5 p e r cent (2,6), the effect of m o i s t u r e on f i r e e n d u r a n c e cannot be expected to s h o w up clearly in the c a a e of c o n s t r u c t i o n s

made f r o m m a t e r i a l s which a t the t i m e of the f i r e t e s t d o not hold m o r e than 1 p e r cent (by volume) of m o i s t u r e .

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c o m m o n l y u s e d 1,uiltlirig m ; ~ t ~ ~ ~ . i ; l l s ir~tiic.;~l rtl ( 7 ) 111at liytl1-a1:crl portlancl cclrlent i s tlie only i~iorl?;;l~lic. l ) ~ l j l ( l i l l j : 171;1t.(?ria I of

p r a c t i c a l i m p o r t a n c e which, in e q u i l i l , r i u ~ ~ l with a 50 p e r c e n t r e l a t i v e h u m i d i t y e n v i r o n m c - n ( , can hold s i g n i f i c a n t l y m o r e than 1 p e r c c n t (by v o l u m e ) of rnoist.ure. It. i s a l s o t h e only i m p o r t a n t building m a t e r i a l , f o r which t h e " a i r d r y " condition (which

c o r r e s p o n d s r o u g h l y t o t h e 50 p e r c e n t c q u i l i b r i u r n r e l a t i v e h u m i d i t y condition) i s not r e a d i l y a t t a i n a b l e without f a l s i f y i n g i t s b a s i c p r o p e r t i e s . C o n s e q u e n t l y , d e t e r m i n i n g t h c e f f e c t of m o i s t u r e 011 t h e f i r e e ~ i d r r r a n c e is of p r a c t i c a l i n t e r e s t only in the c a s e of c o n s t r u c t i o n s c o n t a i i l i ~ l g a s i g n i f i c a n t a m o u n t ( m o r e t h a n 5 p c r c e n t by v o l u m e ) of h y d r a t e d p o r t l a n d c e m e n t . C o n s t r u c t i o n s m a d e with l e s s t h a n 5 p e r c e n t c e m e n t p a s t e , e . g . , b r i c k w a l l s m a d e with c e m e n t m o r t a r , o r f l o o r c o n s t r u c t i o n s containing l e s s t h a n 20 p e r c e n t (by v o l u m e ) c o n c r e t e , u s u a l l y c a n n o t hold m o r e t h a n 1 p e r c e n t m o i s t u r e a f t e r conditioning f o r two o r t h r e e m o n t h s , c o n s e q u e n t l y c a n be s u b j e c t e d t o f i r e t e s t s without paying a t t e n t i o n t o t h e i r a c t u a l m o i s t u r e condition. T h e above c o n s i d e r a t i o n s n a r r o w t h e r a n g e of c o n s t r u c t i o n s f o r which t h e e f f e c t of m o i s t u r e on f i r e e n d u r a n c e d e f i n i t e l y d e s e r v e s a t t e n t i o n t o t h o s e built p r e d o m i n a n t l y f r o m p o r t l a n d c e m e n t p r o d u c t s . T h u s t h e p r o b l e m of o b t a i n i n g r e p r e s e n t a t i v e s a m p l e s a l s o n a r r o w s down t o f i r e t e s t s p e c i m e n s e m p l o y i n g p o r t l a n d c e m e n t p r o d u c t s , g e n e r a l l y i n t h e f o r m of c o n c r e t e . S e v e r a l y e a r s a g o a s i m p l e t e c h n i q u e w a s d e s c r i b e d (4) f o r obtaining s a m p l e s of m a t e r i a l of t h e t e s t s p e c i m e n f o r m o i s t u r e a n a l y s i s . T h i s t e c h n i q u e h a s been s u c c e s s f u l l y u s e d i n c o n n e c t i o n w i t h s p e c i m e n s built f r o m v a r i o u s lightweight c o n c r e t e s . U n f o r t u - n a t e l y t h i s m e t h o d could n o t b e a p p l i e d e a s i l y i n t h e c a s e of t e s t s p e c i m e n s b u i l t f r o m n o r m a l ( d e n s e ) c o n c r e t e . If t h e m a t e r i a l of t h e s p e c i m e n cannot b e r e a d i l y s a m p l e d , t h e r e a r e f o u r p o s s i b l e m e t h o d s t h a t t h e t e s t i n g l a l ~ o r a t o r y m a y follow to d e t e r m i n e

the

m o i s t u r e content of f i r e t e s t s p e c i m e n s .

1. "Calculation method".

-

I t i s p o s s i b l e t o o b t a i n a f a i r e s t i m a t e of t h e d e s o r p t i o n c u r v e of c o n c r e t e by c a l c u l a t i o n s d e s c r i b e d i n d e t a i l e l s e w h e r e (7). T h e knowledge of t h i s c u r v e m a k e s i t p o s s i b l e t o c a l c u l a t e t h e m o i s t u r e c o n t e n t and i t s d i s t r i b u t i o n

i n the f i r e ten1 ~ p s c i r n o n , i f the v a l u a a of tho equilibrium

r e l a t i v e h u m i d i t y a r e known, e . g . , by m e a n s of t h e m e t h o d d e s c r i b e d by M e n z e l ( 3 ) .

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2. "Mt*tllorl of c . , ~ l l , c . t l c l ( . t l s , ~ l ~ i l ) l c . s " .

-

A ( t l ~ c titt~c. o f j>l;~c.il~t( th(1 c o n c r c t c , c - y l i ~ l t l r i c a l h o l c s o i 1: t o L _{in. in d i , ~ n ~ c , t c r ,}_rc.;aching t o suificicrlt rlcpths, a r c lcft i n the t e s t s p e c i m e n a t a few w e l l s e l e c t e d l o c a t i o n s . Into t h e s e h o l e s , a f t e r t h e r e r n o v a l of the f o r m s , c y l i ~ l d r i c a l s a m p l e s of c o n v e n i e n t s i z e s , p r e p a r e d of t h e s a l n c c o l l c r c t c will be s e a l c d , c . g . , with s o m e putty. On t h e d a y of t h e f i r e t e s t t h e s e c y l i n d r i c a l s a m p l e s h a v e t o he r e m o v e d and a n a l y s c d both f o r e q u i l i b r i u m r e l a t i v e h u m i d i t y a n d m o i s t u r e c o n t e n t in a way d e s c r i b e d i n Ref. (4).

3. "Substitution method".

-

T h e t e s t i n g l a b o r a t o r y m a y build a I ) s a m p l e s p e c i m e n " w h i c h i s l a r g e enough t o r e p r e s e n t

a c c u r a t e l y t h e c r o s s - s e c t i o n a l d e t a i l s of t h e f i r e t e s t s p e c i m e n , but i s s c a l e d down i n one o r two d i r e c t i o n s s o t h a t i t c a n he

o v e n - d r i e d i n a l a r g e r l a b o r a t o r y oven. If t h i s s a m p l e s p e c i m e n h a s b e e n conditioned t o g e t h e r with t h e f i r e t e s t s p e c i m e n , one c a n e x p e c t t h a t t h e v a l u e of t h e a v e r a g e m o i s t u r e c o n t e n t

o b t a i n e d by p e r f o r m i n g t h e n e c e s s a r y weight m e a s u r e m e n t s on t h e s p e c i m e n w i l l a g r e e r e a s o n a b l y w e l l with what could he o b t a i n e d if t h e t e s t s p e c i m e n could b e s u b j e c t e d t o t h e s a m e m e a s u r e m e n t s .

4. "Combined weighing method".

-

T h e t e s t i n g l a b o r a t o r y m a y t a k e s a m p l e s f r o m t h e c o n c r e t e u s e d i n t h e t e s t s p e c i m e n , w h i c h a r e s i g n i f i c a n t l y s m a l l e r t h a n the one d e s c r i b e d b e f o r e , and d o not follow t h e g e o m e t r i c a l d e t a i l s of t h e t e s t s p e c i m e n . In t h i s c a s e t h e a v e r a g e m o i s t u r e c o n t e n t of t h e t e s t s p e c i m e n c a n b e d e t e r m i n e d f r o m weight m e a s u r e m e n t s p e r f o r m e d on b o t h t h e s a m p l e s a n d t h e t e s t s p e c i m e n . T h e f i r s t t w o m e t h o d s e x c e l i n t h e i r s i m p l i c i t y . T h e y h a v e t h e c o m m o n d i s a d v a n t a g e , h o w e v e r , t h a t t h e t e s t i n g l a b o r a t o r y m u s t r e l y on t h e a s s u m p t i o n t h a t t h e a v e r a g e m o i s t u r e c o n t e n t of t h e e n t i r e c o n c r e t e t e s t s p e c i m e n c a n be c a l c u l a t e d f r o m d a t a p e r t a i n i n g t o a few p a r t i c u l a r l o c a t i o n s . T h i s d i f f i c u l t y , h o w e v e r , i s o b v i o u s l y not c h a r a c t e r i s t i c of t h e s e m e t h o d s a l o n e . T h e t e s t i n g l a b o r a t o r i e s h a v e t o f a c e t h e s a m e p r o b l e m e v e n when u s i n g M e n z e l ' s m e t h o d ( 3 ) o r t h e t e c h n i q u e d e s c r i b e d in Ref. (4). With t h e "method of e m b e d d e d s a m p l e s " t h e r e is a n o t h e r d i f f i c u l t y t h a t t h e s m a l l c y l i n d r i c a l s a m p l e s u s u a l l y c o n t a i n s o m e w h a t l e s s c o a r s e a g g r e g a t e than t h e c o n c r e t e in t h e t e s t o p e e l m e n . T h i e difficulty, h o w e v e r , c a n be eliminated b y eoneiclaritrg t h a t i n d e n s e c o n c r e t e s t h e a m o u n t of m o i s t u r e h e l d by t h e a g g r e g a t e s

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i s always negligible. By del;ern~itling the density i n g r e e n condition

of both the specimen concrete a n d s a m p l e c o n c r e t e the volume

percentage of portland cement paste in both the specimen and the

s a m p l e s can be calculated [ s e e Eqs. (6) and (7)]. Then the m o i s t u r e

content of the specimen-concrete is obtained a s

With the aid of both the "substitution method1' and the

I I combined weighing method" suitable values of a v e r a g e m o i s t u r e

contents can be established. Although the f i r s t of t h e s e two s e e m s t o be s i m p l e r and m o r e convenient, under c e r t a i n c i r c u m s t a n c e s the l a t t e r can be expected t o give higher accuracy.

T o be able t o employ the combined weighing method the testing l a b o r a t o r y m u s t have f a c i l i t i e s t o weigh the e n t i r e f i r e t e s t

specimen. The weighing m a y be conveniently done by suspending the

specimen on a load cell. The metk~od of weighing a wall specimen

in the DBR l a b o r a t o r y is shown in F i g u r e 1.

A p r o b l e m that m a y a r i s e in connection with the combined

weighing method is that the t e s t specimen is often built in a f r a m e

o r on a s i l l the weight of which may a l s o be subject t o changes

induced by variations in the a t m o s p h e r i c conditions. The testing

l a b o r a t o r i e s have t o study how significant t h e s e weight changes may

be throughout the y e a r . If i t is found that t h e s e changes m a y

significantly falsify the r e s u l t s of m o i s t u r e m e a s u r e m e n t , vapour b a r r i e r coatings m a y be applied on t h e s e f r a m e s o r s i l l s , o r they m a y be replaced by o t h e r s built f r o m m a t e r i a l s of m o r e favourable w a t e r sorption p r o p e r t i e s . (It is often possible t o build t h e s e

components with protected o r unprotected steel. )

The Combined W e i ~ h i n e Method

The principal advantage of the combined weighing method is

t h a t the p r o c e s s of drying is d e t e r m i n e d f r o m m e a s u r e m e n t s

p e r f o r m e d on the ( c o n c r e t e ) f i r e t e s t specimen itself. The principal difficulty is that the "dry weight1' of the t e s t specimen is not

available s o that without a u x i l i a r y m e a s u r e m e n t s , weight changes cannot be e x p r e s s e d in t e r m s of m o i s t u r e content. T h i s information

haa to be d e t e r m i n e d by using a eampla of the c o n c r e t e ( c o n v e n i e n t l y

a s t a n d a r d c o m p r e s s i o n t e s t s a m p l e ) which h a s been conditioned, together with the t e s t specimen. T h i s s a m p l e , however, because of

(9)

i t s s m a l l s i z e , i s probably of slightly diffcrcnt aggregate content and will probably d r y slightly f a s t e r than the t e s t specimen. T h i s p r o b l e m can be overcome by weighing both the specimcn and s a m p l e i n the "green" condition, immediately a f t e r the removal of f o r m s ( o r m o l d s ) , and making u s e of the f a c t that a t t h i s stage the portland cement paste is s t i l l completely s a t u r a t e d with w a t e r , thus it is in a well -defined condition.

T o be able t o calculate the a v e r a g e m o i s t u r e content of the t e s t specimen, the following weights should be determined.

( a )

B y

suspending the t e s t specimen on a load cell:

W

=

weight of the g r e e n c o n c r e t e in the t e s t specimen, lb, g

W

=

weight of concrete in the t e s t specimen on the day p r i o r t o the f i r e t e s t , lb.

(b)

B y

conventional weighing methods:

W'

=

weight of g r e e n c o n c r e t e s a m p l e , lb, g

W'

=

weight of s a m p l e , a f t e r having been kept c l o s e t o the d

t e s t specimen during the conditioning period and oven-dried ( a t 221 f

1°F)

a t about the t i m e of f i r e t e s t , lb,

d

=

bulk density of g r e e n c o n c r e t e , m e a s r e d during the

_Y

placement of c o n c r e t e by using a 1 ft c o n t a i n e r , lb/ft3.

With the a i d of t h i s information, the following can be calculated. (The f o r m u l a e given below a r e based on obvious principles; the r e a d e r can e a s i l y prove t h e i r c o r r e c t n e s s . )

Volume of c o n c r e t e in the t e s t specimen:

Specific g r a v i t y of g r e e n c o n c r e t e in the t e s t specimen and i n the sample:

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Specific gravity of g r e e n portland k-ement paste:

Volume fraction of cement paste in the concrete of the t e s t specimen and in the sample concrete:

Specific gravity of sample in oven -dry condition:

4

= w;/62.4V8

Specific gravity of cement paste in oven-dry condition:

-

'a

-

(pa

-

pb)/vl

Specific gravity of concrete in the t e s t specimen:

-

'd

-

VPpd -I. ( 1 - 4 Pa

Oven-dry weight of the concrete of the t e s t specimen:

Volumetric moisture content of the concrete in the f i r e t e s t specimen a t the t i m e of f i r e test:

It may be added that, in general, the volumetric moisture content can be calculated f r o m the moisture content (by weight) as:

Obviously i t h a s not been assumed in deriving these formulae, that the sample concrete and the specimen concrete are of the same

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assuinccl, howcvcr, that t h c h y d r a t i 0 1 1 o l c c m c n t p a s t e p r o c c c d s in the s a m p l e a t the s a i n c rat:c a s it) thc t e s t specimen. A s the r a t e of hydration r e a c t i o n s dcpends on thc e q u i l i b r i u m r e l a t i v e humidity i n the c o n c r e t e (8), a n d in t u r n on the m o i s t u r e content, one can

count on the approximately equal m a t u r i t y of the cement p a s t e in the two c o n c r e t e s only i f t h e i r m o i s t u r e contents a t the t i m e of the f i r e t e s t ( a s e s t i m a t e d f r o m t h e i r p e r c e n t a g e d e c r e a s e in weight d u r i n g the conditioning p e r i o d ) a r e not too significantly different.

The calculation p r o c e d u r e m a y be b e t t e r understood by studying the following s a m p l e calculation.

E x a m p l e 1.

-

F o r a wall f i r e t e s t a s s e m b l y built f r o m d e n s e c o n c r e t e , t h e following weights (of c o n c r e t e only) have been m e a s u r e d :

A s t a n d a r d c o m p r e s s i o n t e s t s p e c i m e n h a s been u s e d as a s a m p l e , f o r which A w a t e r - c e m e n t r a t i o of w d c

=

0.572 w a s m e a s u r e d . It is known, f u r t h e r m o r e , t h a t t h e v a l u e s PC

=

3.15 and p a

=

2.65 a r e applicable t o m o s t commonly m e t c a s e s . CALCULATIONS f r o m

Eq. ( 2 )

f r o m Eq. (3) f r o m Eq. (4) f r o m Eq. (5)

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f r o m Eq.

(6)

f r o m Eq. (7)

~h

=

27.862/62.4 x 0.19635

=

2.2741 f r o m Eq. (8) P ~ d

=

2.65

-

(2.65

-

2.2741)/0.2739

=

1.2776 f r o m Eq. (9) Pd

=

0.2474 x 1.2776

+

(100.2474) x 2.65 2 2.3105 f r o m Eq. (10) W

=

62.4 x 44.5 x 2.3105 = 6423 1b d f r o m Eq. (11) f r o m Eq. (12)

Some additional calculations indicated t h a t the end r e s u l t of t h e s e calculations depends only slightly on the s e l e c t i o n of the value of pa. Selecting pa

=

2.65, which s e e m s t o be a good a v e r a g e f o r a v a r i e t y of a g g r e g a t e s , will t h u s probably suffice i n m o s t p r a c t i c a l c a s e s . It w a s a l s o found, however, t h a t the end r e s u l t depends v e r y significantly on the a c c u r a c y of the load c e l l m e a s u r e m e n t s . With t h e load c e l l u s e d i n the DBR l a b o r a t o r y (20,000-lb capacity) a n a c c u r a c y of f 10 lb can be expected.

Calculation of the Effect of M o i s t u r e

T h e gain in f i r e e n d u r a n c e due t o m o i s t u r e ( a s r e f e r r e d t o m o i s t u r e l e s s , i. e., o v e n - d r y condition) c a n be d e t e r m i n e d with the aid of the n o m o g r a m in F i g u r e 2. T h i s s a m e n o m o g r a m can a l s o be u s e d t o c o r r e c t the r e s u l t of f i r e endurance t e s t s , when the t e s t w a s c a r r i e d out a t a n o n - s t a n d a r d m o i s t u r e level,

or

_{t a d e t e r m i n e the} value

of

f i r e endurance f o r any o t h e r m o i s t u r e l e v e l of i n t e r e s t .

When the m o i s t u r e is supposedly s y m m e t r i c a l l y d i s t r i b u t e d with r e g a r d to the c e n t r a l plane of the f i r e t e s t s p e c i m e n , the

p a t t e r n of the a c t u a l m o i s t u r e d i s t r i b u t i o n is i m m a t e r i a l , and should be i n t e r p r e t e d a s the a v e r a g e v o l u m e t r i c m o i s t u r e content of the t e s t specimen.

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m o i s t u r e content, 'pa, the f i r e cndlirancc of a t e s t s p e c i m e n w a s found t o be T ~ , what would b e i t s f i r e e n d u r a n c e a t an a v e r a g e m o i s t u r e content m ? T o calculate T the f i r e endurance of the

P

B '

s p e c i m e n in m o i s t u r e l e s s (oven-dry) condition, T h a s f i r s t t o be d e t e r m i n e d . d

'

1 T h e calculation p r o c e d u r e is a s follows. F i r s t c a l c u l a t e w h e r e b

=

5.5 f o r d e n s e and gun-applied c o n c r e t e s , 8.0 f o r lightweight c o n c r e t e s , 10.0 f o r c e l l u l a r c o n c r e t e s .

T h e n in the n o m o g r a m in F i g u r e 2 d r a w a line f r o m point A to the a p p r o p r i a t e value of b r b on the right-hand side s c a l e . D r a w a n o t h e r line, p a r a l l e l t o t h i s , f r o m the point indicating the e x p e r i m e n t a l l y obtained f i r e e n d u r a n c e value, T ~ on the left-hand side s c a l e . , A

h o r i z o n t a l line d r a w n f r o m the point w h e r e the l a t t e r line i n t e r s e c t e d the c u r v e of the n o m o g r a m , will yield the value of ~d (the f i r e

e n d u r a n c e i n m o i s t u r e l e ss condition) on the left -hand side s c a l e . T o find the f i r e endurance of the s a m e t e s t s p e c i m e n , T 8 8 a t

s o m e o t h e r m o i s t u r e content rDB, calculate bcp and connect t h i s value

P

on the right-hand side s c a l e with point A. D r a w a line p a r a l l e l t o t h i s f r o m the point obtained previously on the c u r v e of the nomogram. T h e i n t e r s e c t i o n of t h i s line with the left-hand s i d e s c a l e will yield T

B

'

If the m o i s t u r e distribution i n the t e s t s p e c i m e n is m a r k e d l y a s y m m e t r i c a l (this is g e n e r a l l y due t o the g e o m e t r y of the construction,

not

ta the: conditiarring procadiire), calculate firet the "equivalent

e y m m e t r i c a l " m o i s t u r e content a s w h e r e M, t h e " m o i s t u r e m o m e n t , " is r e f e r r e d t o t h e exposed s i d e ( w h e r e x

=

o ) and is defined as

a

Thie may a l s o be w r i t t e n as n

(14)

In t h e s c calculations a continuous a i r lnycr ( of any t h i c k n e s s ) should be r e g a r d e d a s a 1-in. -thick< m o i s t u r e l e s s layer.

The m o i s t u r e level of p a r t i c u l a r i n t e r e s t i s that pertaining t o 50 p e r cent equilibrium relative humidity. The m o i s t u r e content of a reasonably m a t u r e portland cement paste in equilibrium with an a t m o s p h e r e of 50 p e r cent relative, humidity a t r o o m t e m p e r a t u r e is g e n e r a l l y between 10 and 20 p e r cent by volume. Since, a s mentioned e a r l i e r , in a concrete the moisture-holding capacity of the a g g r e g a t e s is usually v e r y s m a l l , one can w r i t e that the volumetric m o i s t u r e content of c o n c r e t e s in "standard" ( a i r -dry) condition is

The wideness of t h i s range is due p r i m a r i l y t o h y s t e r e s i s effects, and only i n a l e s s e r d e g r e e t o the c h a r a c t e r i s t i c s of the anhydrous cement used. If during the conditioning period the f i r e t e s t specimen was always subjected t o desorption, in other w o r d s , w a s

'

continuously losing weight, the upper half of the above range should be used.

The p r o c e d u r e of "correcting" f i r e endurance t e s t r e s u l t s f o r

I I _{standard" m o i s t u r e level} _is _{i l l u s t r a t e d by the following two examples.}

Example 2.

-

The wall f i r e t e s t a s s e m b l y d e s c r i b e d in Example 1 yielded ra

=

3.33 h r f i r e endurance. What would be i t s f i r e endurance a t "standard" m o i s t u r e level?

The m o i s t u r e distribution in wall t e s t s p e c i m e n s is always reasonably closely s y m m e t r i c a l . F o r heavy c o n c r e t e b

=

5.5, thus,

bqa = 48.1. With t h i s and

--ra

=

_{3.33 h r one obtains ( s e e F i g u r e}₂₎_that

the f i r e endurance i n m o i s t u r e l e s s condition is T

=

2.58 h r . d

F o r t h i s c o n c r e t e v

=

0.2474 (volume p e r cent of cement p a s t e ) w a s previously calculated. Assuming (by m e a n s of Eq. (17)) t h a t in the a i r - d r y condition the volumetric

moisture

content is 15v, one g e t s CQB 1 5 ~ 0 . 2 4 7 4

=

3.71% by vol.

(A

c l o s e r e s t i m a t e could be obtained by calculating ( s e e Ref. (7)) o r experimentally determining the desorption c u r v e of the m a t e r i a l . ) With t h i s

l

m

₌

20.4, and

again using F i g u r e 2 one obtains T

_a

=

2.90 h r , whi@h is t h e r e f o r e t h e

expected p e r f o r m a n c e of the wall in the "standard condition. I'

Example 3.

-

A steel-jointed floor construction, sketched in F i g u r e 3a, yielded T~

=

2.6 h r ( t h e r m a l ) f i r e endurance. With the aid of "embedded samples" a m o i s t u r e distribution shown i n F i g u r e 3b

(15)

h a s b e e n found in the c o n c r e t e l a y e r p r i o r to the f i r e test. What would be the f i r e endurance of this construction i f tested in a i r - d r y condition?

The model of the construction, a s simplified f o r t h e s e kinds of calculations, is shown in F i g u r e 3b. In t h i s sketch the thickness of the a i r l a y e r h a s been reduced to 1 in. in accordance with what was said in connection with Eq. (1 6). The m o i s t u r e in the gypsum p l a s t e r h a s been neglected.

With the notation u s e d in F i g u r e 3b the m o i s t u r e moment can be calculated a s follows:

M = x c p &

+

x c p m

+

x r p A x 3 + x c p A x 1 1 1 2 2 2 3 3 4 4 4

+

x5CD5m5 where x

=

0.3125 in. 1

wl

=

0

axl

= 0.625 in. x

=

0.8125 in. 2

m2

=

1.0 in. x

=

2.125 in. CP3 = 5.80 3 Ax3

=

l o o in* x

=

3.125 in. 4 cp4

=

4.65 Ax4

=

1.0 in. x

=

4.125 in. 5 cp5

=

3.80 Ax5

=

1.0 in. T h e r e f o r e M

=

2 . 1 2 5 ~ 5 . 8 0 x 1 , 0 + 3 . 1 2 5 ~ 4 . 6 5 ~ 1 . 0 + 4 . 1 2 5 ~ 3 . 8 0 x 1.0

=

42.531 in. vol

%

and the equivalent s y m m e t r i c a l m o i s t u r e content

-.

is

Since b

=

5.5 a g a i n , b@a

=

21.9. By making u s e of F i g u r e 2 one

obtaine

r

=

2.3 hr.

d

It h a s been established by e x p e r i m e n t s that the m o i s t u r e content of the c o n c r e t e u s e d i n the construction in a i r - d r y condition is 3.0 p e r cent by volume. Thus the m o i s t u r e moment pertaining t o a i r - d r y conditions is ( s e e F i g u r e 3b):

and the equivalent s y m m e t r i c a l m o i s t u r e content in a i r - d r y condition is:

@

=

2 _r_{2 8 . 1 ~ 5 / 4 . 6 2 5 ~}

₌

_{2.63% by vol.}

(16)

With t h i s b13

=

14. 5. Again using F i g u r e 2 one obtains, f o r the a i r - d r y conftruction, r = 2. 5 hr.

8

NOTATION

I

Symbols denoted by the s u p e r s c r i p t r e f e r t o a sample.

m o i s t u r e content,

7'0

by weight e m p i r i c a l value

bulk density, lb/ft 3

o v e r - a l l thickness of f i r e t e s t specimen ( c o r r e c t e d when continuous a i r l a y e r i s p r e s e n t ) , in.

m o i s t u r e moment, in. 2 vol.

%

number of s t r i p s ( s e e F i g u r e 3b)

volume f r a c t i o n of portland cement paste volume of c o n c r e t e , f t 3

water -cement r a t i o , lb/lb

W

=

weight of concrete; without subscript:

weight of c o n c r e t e on day of f i r e t e s t , lb. x

=

d i s t a n c e f r o m s u r f a c e exposed t o f i r e , in. Ax

=

width of s t r i p ( s e e F i g u r e 3b), in. G r e e k L e t t e r s p

=

specific g r a v i t y r

=

f i r e endurance, h r cp

-

= m o i s t u r e content,

%

by volume

cp

=

"equivalent s y m m e t r i c a l " m o i s t u r e content,

%

by volume

S u b s c r i p t s

a

=

of aggregate

c = of anhydrous portland cement

d

=

of d r y c o n c r e t e , in m o i s t u r e l e s s (oven-dry) condition

g

=

of g r e e n c o n c r e t e i

=

l , 2 , 3 , . .

.

pg

=

of g r e e n portland cement p a s t e pd

=

of d r y portland cement p a s t e

a

=

i n s o m e

a

condition (generally i n the condition a r i s i n g during f i r e t e s t )

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REFERENCES

1.

Harmathy, T. Z. Ef::ect of m o i s t u r e on the f i r e endurance of building elements. ASTM Spec. Tech. Pul)l. No. 385, 1965, p. 74.

2. Harmathy, T. Z. F i r e Technology, E x p e r i m e n t a l study on m o i s t u r e and f i r e endurance.

-

2, 52 (1966).

3. Menzel, C. A. A method f o r determining the m o i s t u r e condition of hardened c o n c r e t e in t e r m s of r e l a t i v e humidity. p. 1085-

1109, P r o c . ASTM, -9 55 1 (1955).

4, Harmathy,

T.

Z. and E. 0. P o r t e o u s . Sampling method f o r m e a s u r i n g the m o i s t u r e distribution in f i r e t e s t specimens. Natl. R e s , Council, Pivision of B ~ i l d i n g R e s e a r c h , Building R e s e a r c h Note 4 2 , July 1963.

5. Copeland, L. E. and 3. C. Hayes. Determination of non- evaporable w a t e r in hardened portland-cement paste. ASTM Bulletin, No. 194, Dec. 1953, p. 70-74.

6. S h o r t e r , G. W. and J, A. C. Blanchard. Some f a c t o r s influencing the p e r f o r m a n c e of c o n c r e t e m a s o n r y unit walls i n ASTM E l 19 f i r e t e s t s (in preparation).

7, Harmathy, T. 2. Moisture sorption of building m a t e r i a l s (paper in preparation).