Some Remarks on Winter Masonry

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Some Remarks on Winter Masonry

Svendsen, S. D.; Waldum, A.

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PREFACE

Recent S c a n d i n a v i a n r e s e a r c h s t u d i e s h a v e p r o v i d e d some e x p l a n a t i o n s f o r t h e e x c e l l e n c e of masonry l a i d up i n c o l d w e a t h e r u s i n g traditional t e c h n i q u e s . T h i s h a s c u r r e n t s i g n i f i c a n c e i n North America b e c a u s e of (1) t h e i n c r e a s i n g growth of w i n t e r c o n s t r u c t i o n , and ( 2 ) b u i l d i n g code

r e q u i r e m e n t s c o n s i d e r e d r e s t r i c t i v e by t h o s e f a m i l i a r w i t h t h e t e c h n i q u e s and r e s u l t s of t h e t r a d i t i o n a l p r o c e d u r e s . T h i s t r a n s l a t i o n i s a r e p o r t of a s t u d y c a r r i e d o u t a t t h e T e c h n i c a l U n i v e r s i t y i n Trondheim. The s e n i o r a u t h o r i s a r e c o g n i z e d a u t h o r i t y on masonry i n S c a n d i n a v i a . The D i v i s i o n w i s h e s t o t h a n k M r . D.A. S i n c l a i r , T r a n s l a t i o n s S e c t i o n , N a t i o n a l R e s e a r c h C o u n c i l , f o r

t r a n s l a t i n g t h i s p a p e r and M r . J.I. Davison of t h i s D i v i s i o n who checked t h e t r a n s l a t i o n .

O t t a w a June 1 9 7 1

N.B. Hutcheon A a s i a t a n t D i r e c t o r

NATIONAL RESEARCH COUNCIL OF CANADA TECHNICAL TRANSLATION 1456 T i t l e : Some r e m a r k s on w i n t e r masonry ( L i t t om v i n t e r m u r i n g ) A u t h o r s : S . D . S v e n d s e n a n d A . Waldum R e f e r e n c e : M u r m e s t e r e n , 1-2: 3

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1 7 , 1966 T r a n s l a t o r : D . A . S i n c l a i r , T r a n s l a t i o n s S e c t i o n , N a t i o n a l S c i e n c e L i b r a r y

SOME REMARKS O N WINTER MASONRY 1. What we a l r e a d y know The s i t u a t i o n a t t h e p r e s e n t t i m e w i t h r e s p e c t t o l a b o u r i n t h e c o n s t r u c t i o n i n d u s t r y i s q u i t e c r i t i c a l . Too few y o u n g e r p e o p l e a r e e n t e r i n g i t , a n d t h e a v e r a g e age o f t h e c o n s t r u c t i o n w o r k e r i s i n c r e a s - i n g from y e a r t o y e a r . I f t h i s t r e n d c o n t i n u e s

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and i t l o o k s as t h o u g h i t w i l l

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i t can l e a d t o s e r i o u s p r o b l e m s . I n c r e a s i n g c o n v e r - s i o n t o p r e f a b r i c a t i o n , a n d a n e v e n g r e a t e r d e g r e e o f m e c h a n i z a t i o n a t t h e c o n s t r u c t i o n s i t e t h a n we now h a v e , w i l l r e l i e v e t h e s i t u a t i o n t o some e x t e n t , b u t w i l l by n o means e l i m i n a t e a l l t h e d i f f i c u l t i e s . We must a l s o e x p l o r e o t h e r p o s s i b i l i t i e s . One o f t h e r e a s o n s f o r t h e breakdown o f r e c r u i t i n g l i e s i n t h e f a c t t h a t a l a r g e p r o p o r t i o n o f b u i l d i n g a c t i v i t y i s s t i l l s e a s o n a l . The c o n d u c t i n g o f w i n t e r o p e r a t i o n s i n c o n s t r u c t i o n i s n o t a s i m p l e m a t t e r i n s u c h a s e v e r e c l i m a t e a s o u r s . On t h e o t h e r h a n d , i t i s o b v i o u s t h a t i f b u i l d i n g i s t o a t t r a c t new p e o p l e i t must b e a b l e t o a s s u r e them s t e a d y employment t h r o u g h o u t t h e y e a r . I n one way o r a n o t h e r , we must s o l v e t h e p r o b l e m by w i n t e r c o n s t r u c t i o n , a n d we must d e v e l o p t h e methods a n d t e c h n o l o g i e s s u i t a b l e f o r low t e m p e r a t u r e s . W i n t e r masonry i s one o f t h e a r e a s a b o u t w h i c h , u n f o r t u n a t e l y , we know a l l t o o l i t t l e , a n d v e r y few s t u d i e s h a v e b e e n p u b l i s h e d o n t h e s u b j e c t . T h e r e a r e some, however, a n d w e i n t e n d h e r e t o t r y t o a s s e m b l e some o f t h e a v a i l a b l e e x p e r i e n c e , a n d t o d i s c u s s t h e r e s e a r c h r e s u l t s t h a t h a v e e m e r g e d . L e t us f i r s t t a k e a c l o s e r l o o k a t what i s d e f i n i t e l y known.

We can i m m e d i a t e l y e s t a b l i s h t h a t t h e r e a c t u a l l y i s one method which s o l v e s most o f t h e p r o b l e m s o f masonry i n w i n t e r , namely, con-

s t r u c t i o n u n d e r a f u l l e n c l o s u r e . T h i s method h a s b e e n u s e d f o r some t i m e w i t h g r e a t s u c c e s s i n Canada, a n d more r e c e n t l y i t h a s a l s o b e e n t r i e d i n t h e S c a n d i n a v i a n c o u n t r i e s . Denmark h a s b e e n t h e l o n g e s t i n t h e f i e l d ( F i g u r e 1) a n d t h e r e i s a t l e a s t one f i r m t h e r e w h i c h c o n s e q u e n t l y u s e s e n c l o s u r e s i n i t s w i n t e r c o n s t r u c t i o n . The r e s u l t s h a v e b e e n v e r y good, a n d e c o n o m i c a l as w e l l , a n d i t goes w i t h o u t

s a y i n g t h a t w o r k i n g c o n d i t i o n s a r e v e r y g r e a t l y i m p r o v e d i n t h i s way ( F i g u r e 2 ) . B u i l d i n g u n d e r e n c l o s u r e s i s b e s t s u i t e d t o t h e mass p r o d u c t i o n of small b u i l d i n g s , and t h e e n c l o s u r e c a n o f t e n b e made moveable w i t h t h e a i d o f w h e e l s r u n n i n g on r a i l s a l o n g b o t h s i d e s o f a row of b u i l d i n g s . E n c l o s u r e s c a n a l s o b e u s e d i n t h e c o n s t r u c t i o n of t a l l b u i l d i n g s , h o i s t i n g them upwards as t h e b u i l d i n g g o e s h i g h e r . I n many c a s e s , e s p e c i a l l y where s k e l e t o n c o n s t r u c t i o n i s u s e d , t h e e n c l o s u r e c a n b e s i m p l i f i e d down t o a t i g h t c o v e r i n g o f t h e walls s t o r e y by s t o r e y ( F i g u r e 3 ) . If p l a s t i c i s u s e d f o r t h e e n c l o s u r e m a t e r i a l o r i t s t o p , t h e r a d i a t i o n f r o m o u t s i d e w i l l p r o v i d e a s t r o n g h e a t i n g e f f e c t , e v e n i n t h e c o l d e s t w i n t e r w e a t h e r . O b v i o u s l y t h i s g r e a t l y s i m p l i f i e s t h e h e a t i n g p r o b l e m . T h e r e a r e two a r g u m e n t s w h i c h a r e o f t e n u s e d a g a i n s t b u i l d i n g u n d e r e n c l o s u r e s . One i s t h a t i t i s d i f f i c u l t t o employ c r a n e s t o any g r e a t e x t e n t . I n t h e c o n s t r u c t i o n o f small b u i l d i n g s t h i s i s n o t a n i m p o r t a n t c o n s i d e r a t i o n , s i n c e o t h e r means o f t r a n s p o r t , e . g . , f o r k l i f t t r u c k s , c a n b e u s e d i n s t e a d . I n t h e c o n s t r u c t i o n o f t a l l b u i l d - i n g s , i t i s n e c e s s a r y t o r e s o r t t o o t h e r h o i s t i n g s y s t e m s . The o t h e r a r g u m e n t a g a i n s t f u l l e n c l o s u r e s i s t h a t t h e y a d d c o n s i d e r a b l y t o t h e c o s t o f c o n s t r u c t i o n . Danish e x p e r i e n c e , however, i n d i c a t e s t h a t t h i s c e r t a i n l y n e e d n o t a l w a y s be t h e c a s e . I n any e v e n t , t h e method h a s s u c h g r e a t a d v a n t a g e s t h a t i t s h o u l d r e c e i v e v e r y t h o r o u g h c o n s i d e r a - t i o n e a c h t i m e b e f o r e b e i n g r e j e c t e d .

However, g i v e n t h e p r e s e n t s i t u a t i o n i n Norway, we may v e r y w e l l e x p e c t t h a t a c o n s i d e r a b l e p r o p o r t i o n o f a l l w i n t e r masonry w i l l h a v e t o b e c a r r i e d o u t f o r many y e a r s t o come w i t h o u t c o m p l e t e p r o t e c t i o n .

We a r e i m m e d i a t e l y on f a r l e s s c e r t a i n g r o u n d r e g a r d i n g b o t h t h e r i s k w e t a k e a n d t h e e f f o r t t h a t c a n b e made t o i m p r o v e t h e s i t u a t i o n . We know, however, t h a t i t i s by no means i m p o s s i b l e t o c a r r y o u t b r i c k - work a t low t e m p e r a t u r e s . T h e r e a r e a number o f e x a m p l e s o f good

masonry e r e c t e d u n d e r q u i t e s e v e r e c o n d i t i o n s . The r e s u l t s show t h a t v e r y o f t e n masonry g o e s w e l l i n r e a l l y c o l d w e a t h e r . I n any c a s e , i t i s a f a c t t h a t i n n o r t h e r n Sweden and n o r t h e r n F i n l a n d , b r i c k s are l a i d a t much l o w e r t e m p e r a t u r e s t h a n i s a t t e m p t e d i n more m o d e r a t e c l i m a t i c z o n e s .

On t h e o t h e r h a n d , i t i s a l s o p o s s i b l e t o f i n d examples of damage due t o t h e f a c t t h a t t h e masonry was c a r r i e d o u t i n w i n t e r , and t h i s c a n u s u a l l y b e t r a c e d t o t h e p r o p e r t i e s o f t h e g r e e n m o r t a r . O r d i n a r y m o r t a r , e s p e c i a l l y t h e k i n d we u s e h e r e i n Norway, i s a p p l i e d w i t h a v e r y l a r g e e x c e s s o f w a t e r . If t h e t e m p e r a t u r e i s low a n d t h e w a t e r c a n n o t e v a p o r a t e v e r y r a p i d l y , i c e c r y s t a l s w i l l f o r m i n t h e l a r g e p o r e s . The c r y s t a l s o f t e n t a k e t h e f o r m o f l o n g , p a r a l l e l n e e d l e s ( F i g u r e 4 ) , a n d when t h e i c e o c c u p i e s a g r e a t e r volume t h a n t h e w a t e r , i t w i l l r e s u l t i n d e f o r m a t i o n s a n d m o d i f i c a t i o n s i n t h e s t r u c t u r e o f t h e m o r t a r . Sometimes t h i s d o e s n o harm, b u t o f t e n t h e d e f o r m a t i o n s c a u s e t h e h a r d e n e d m o r t a r t o a c q u i r e l e s s s t r e n g t h , a n d i t may c r a c k o r e v e n crumble away. I t h a s l o n g b e e n known t h a t t h i s depends e n t i r e l y on w h e t h e r t h e m o r t a r i s e n t i r e l y g r e e n o r h a s begun t h e h a r d e n i n g p r o c e s s by t h e t i m e i t f r e e z e s . A s o f t , g r e e n m o r t a r c a n w i t h s t a n d c o n s i d e r a b l e s t r a i n s w i t h o u t damage, b u t a s t i f f , p a r t l y c u r e d o r h a r d e n e d one i s v e r y s e n s i t i v e t o c r a c k i n g by i c e .

A n o t h e r i m p o r t a n t f a c t o r i s t h a t b o n d i n g a g e n t s s u c h as l i m e o r cement c u r e much more s l o w l y at low t e m p e r a t u r e s t h a n h i g h . The c a r b o n a t i o n o f l i m e b e g i n s t o s l o w down a p p r e c i a b l y a t t e m p e r a t u r e s as h i g h as 5 - 6 ' ~ , and below t h e f r e e z i n g p o i n t t h e r e a c t i o n c e a s e s a l m o s t e n t i r e l y . F o r cement a n d masonry cement i t t a k e s a l o n g e r and l o n g e r t i m e b e t w e e n m i x i n g a n d s e t t i n g as t h e t e m p e r a t u r e d r o p s .

H a r d e n i n g a l s o p r o c e e d s more and more s l o w l y , and p r e s u m a b l y c e a s e s a l t o g e t h e r a t a r o u n d - 1 0 ' ~ . T h i s means t h a t , i n masonry w h i c h f r e e z e s w h i l e i t i s s t i l l f r e s h , t h e c h e m i c a l r e a c t i o n s i n g e n e r a l d o n o t

p r o c e e d . A s l o n g as t h e m o r t a r i s f r o z e n i t n e v e r t h e l e s s p o s s e s s e s c o n s i d e r a b l e s t r e n g t h , b u t l a t e r when i t thaws i t becomes s o f t a g a i n . I n t h e meantime, t h e b u i l d i n g may h a v e a d v a n c e d c o n s i d e r a b l y , a n d t h e l o a d s may e a s i l y h a v e become s o g r e a t t h a t l a r g e d e f o r m a t i o n s a r e e x p e r i e n c e d . If t h e walls thaw f r o m one s i d e , t h i s c a n l e a d t o t h e masonry g e t t i n g o u t o f plumb. T h e r e i s a l s o a d a n g e r t h a t some of t h e m o r t a r w i l l b e p r e s s e d o u t o f t h e j o i n t s , a n d t h a t c r a c k s w i l l form i n t h i s way b e t w e e n t h e m o r t a r and t h e b r i c k s .

A l l t h e s e a r e t h i n g s we h a v e known o r b e l i e v e d , b u t i t i s o n l y i n t h e l a s t y e a r t h a t we h a v e s t a r t e d , t o some e x t e n t , t o i n v e s t i g a t e

them e x p e r i m e n t a l l y . B e f o r e w i n t e r masonry c a n b e c o m p l e t e l y m a s t e r e d , we must f i r s t s o l v e a l o n g s e r i e s o f d e t a i l e d p r o b l e m s , a n d a g r e a t many n u m e r i c a l v a l u e s a n d boundary c o n d i t i o n s must be e s t a b l i s h e d . We n e e d t o know, f o r example, how m o i s t t h e m o r t a r must b e a n d t o what e x t e n t i t must be h a r d e n e d b e f o r e i t w i l l s u f f e r s e r i o u s damage on f r e e z i n g . We must d e t e r m i n e what b r i c k s a n d m o r t a r a r e b e s t f o r w i n t e r work, a n d w h e t h e r t h e b r i c k s s h o u l d be d r y o r w e t , warm o r c o l d . We must a l s o know t h e e x t e n t o f t h e r i s k p r e s e n t i n e a c h i n d i v i d u a l c a s e o f d a n g e r o u s d e f o r m a t i o n s , a n d t h e r e a r e many o t h e r p r o b l e m s t o b e s o l v e d as w e l l . 2 . Recent l a b o r a t o r y i n v e s t i g a t i o n s

An e x t e n s i v e w i n t e r masonry i n v e s t i g a t i o n h a s now b e e n u n d e r way i n F i n l a n d f o r some t i m e , and some o f t h e r e s u l t s h a v e a l r e a d y b e e n made a v a i l a b l e . T h i s a p p l i e s f i r s t a n d f o r e m o s t t o m o r t a r t e s t s , b u t t h e F i n n i s h s c i e n t i s t s a r e a l s o i n t h e c o u r s e o f d e t e r m i n i n g a number o f masonry p r o p e r t i e s u n d e r d i f f e r e n t t e m p e r a t u r e and c u r i n g c o n d i - t i o n s .

Here i n Norway, c i v i l e n g i n e e r A l f Waldum h a s b e e n w o r k i n g on

w i n t e r masonry as t h e main r e s e a r c h p r o j e c t a t NTH, a n d h e s u b s e q u e n t l y made a number o f s u p p l e m e n t a r y t e s t s i n c o o p e r a t i o n w i t h t h e NBI

L a b o r a t o r y i n Trondheim. I n t h e p r e s e n t d i s c u s s i o n w e s h a l l pay p a r t i c u l a r a t t e n t i o n t o t h e s e Norwegians' i n v e s t i g a t i o n s , b u t a compar- i s o n w i l l a l s o b e made w i t h t h e F i n n i s h r e s u l t s where t h e y a r e a v a i l - ab l e

.

The Norwegian s e r i e s o f t e s t s f a l l i n t o t h e f o l l o w i n g c a t e g o r i e s : t e s t s w i t h masonry a n d t e s t s w i t h m o r t a r , and w i t h i n e a c h o f t h e s e c a t e g o r i e s t h e r e a r e o r i e n t a t i o n t e s t s a n d p r i n c i p a l t e s t s . F o r p r a c t i c a l r e a s o n s , masonry and m o r t a r w i l l b e d e a l t w i t h s e p a r a t e l y i n t h e p r e s e n t p a p e r .

Masonry t e s t s I n t h e masonry i n v e s t i g a t i o n s h e r e i n Norway, t h e s t r e n g t h i s u s u a l l y d e t e r m i n e d on o n e - b r i c k - b y - o n e - b r i c k p i e r s , which a r e con- s t r u c t e d v e r y a c c u r a t e l y w i t h c o m p l e t e l y f i l l e d j o i n t s ( F i g u r e 5 ) . P o i n t i n g i s c a r r i e d o u t i n c o m b i n a t i o n w i t h t h e l a y i n g o f t h e b r i c k s . The b o t t o m c o u r s e i s l a i d i n m o r t a r C 100/330 on a f l a t p o l i s h e d s t e e l p l a t e , a n d b o t h b o t t o m j o i n t s a n d t o p j o i n t s a r e k e p t t o 1 5 mm. A f t e r t h e p i e r h a s b e e n c o n s t r u c t e d w i t h a t o t a l o f 1 2 c o u r s e s , t h e t o p o f i t i s s t r a i g h t e n e d s o as t o b e p l a n e p a r a l l e l w i t h t h e s t e e l p l a t e , a n d h e r e a g a i n t h e C 100/330 i s u s e d . The f u l l h e i g h t o f t h e p i e r i s a p p r o x i m a t e l y 100 cm, b u t o b v i o u s l y i t c a n v a r y somewhat w i t h t h e d e g r e e o f b a k i n g o f t h e b r i c k s and h e n c e t h e i r s i z e . A l o n g s i d e t h e s e t e s t walls i n a l l c a s e s small " p i e r s " o f t h e t y p e s e e n i n F i g u r e

6

w e r e a l s o l a i d , a n d t h e s e were u s e d t o d e t e r m i n e t h e bond b e t w e e n t h e m o r t a r and b r i c k s . F i r s t , a b r i c k i s l a i d o u t and

t h e n a s t r i p o f t h i n g a u z e i s p l a c e d on t h e t o p s u r f a c e . M o r t a r i s t h e n a p p l i e d o v e r t h e g a u z e i n t h e manner o f r e g u l a r b r i c k l a y i n g , t h e n a n o t h e r b r i c k , a n d e v e r y t h i n g i s a d j u s t e d s o as t o h a v e a j o i n t t h i c k n e s s o f 1 5 mm. These s t e p s a r e r e p e a t e d u n t i l we h a v e a s m a l l c o l u m r ~ w i t h f i v e j o i n t s , a n d a l o a d i n t h e f o r m o f t h r e e b r i c k s i s

p l a c e d on t o p o f t h i s f o r t h e f i r s t few d a y s . When t h e bond s t r e n g t h i s t o b e d e t e r m i n e d , t h e columns can b e p i c k e d up i n p i e c e s b e c a u s e t h e s t r i p s o f g a u z e p r e v e n t t h e m o r t a r f r o m a d h e r i n g t o t h e b r i c k b e n e a t h .

I n t h e w i n t e r masonry t e s t s , b r i c k l a y i n g a n d h a r d e n i n g were

c a r r i e d o u t i n t h r e e d i f f e r e n t rooms. One o f t h e s e was t h e l a b o r a t o r y i t s e l f (S) where t h e t e m p e r a t u r e d u r i n g t h e e n t i r e t e s t p e r i o d was k e p t a t a p p r o x i m a t e l y t 20°c, and t h e r e l a t i v e h u m i d i t y o f t h e atmos- p h e r e a v e r a g e d 30% ( w i t h q u i t e l a r g e f l u c t u a t i o n s ) . The o t h e r two rooms w e r e l a r g e c o l d rooms ( K 1 and K 2 ) , where t h e t e m p e r a t u r e c a n

be v a r i e d w i t h i n wide l i m i t s . I n K 1 , t h e t e m p e r a t u r e was k e p t c o n s t a n t a t - 1 5 O ~ , e x c e p t d u r i n g t h e a c t u a l masonry work, when i t was s e t a t a few d e g r e e s h i g h e r . B r i c k - l a y i n g was c a r r i e d o u t i n p e r i o d s o f a b o u t 30 m i n u t e s , and w i t h i n e a c h p e r i o d o n l y m o r t a r o f one c o m p o s i t i o n was

u s e d . The m o r t a r s w e r e k e p t a t a l l t i m e s i n t h e c o l d room, b u t s i n c e t h e m o r t a r b u c k e t was i n s u l a t e d w i t h m i n e r a l o i l a n d s t y r o f o a m ( s e e F i g u r e 5 ) , t h e t e m p e r a t u r e r e m a i n e d w e l l a b o v e t h e f r e e z i n g p o i n t . The t h i r d room, K2, was k e p t a t +6 t o 7 ' ~ d u r i n g t h e l a y i n g o f t h e b r i c k s , a f t e r w h i c h i t was l o w e r e d q u i t e s l o w l y t o - 1 5 ' ~ . F i n a l l y ,

a l l t h e t e s t p i e r s w e r e moved f r o m t h e c o l d room i n t o t h e l a b o r a t o r y , where t h e y r e m a i n e d f o r a c e r t a i n l e n g t h o f t i m e b e f o r e t e s t i n g .

F i g u r e 6 shows a l l t h e l a r g e and s m a l l p i e r s u s e d i n t h e main t e s t . A f t e r t h e c u r i n g p e r i o d , t h e l a r g e p i e r s w e r e s t r e s s e d t o b r e a k - i n g by a 500 t o n p r e s s f r o m t h e T o n i n d u s t r i e f a c t o r y w i t h a n a d j u s t - ment r a n g e o f 1 0 0 t o n s ( F i g u r e 7 ) . E a r l i e r e x p e r i e n c e h a d shown t h a t i n s u c h t e s t s t h e r e i s l i t t l e p o i n t i n m e a s u r i n g t h e d e f o r m a t i o n s , a n d t h i s was t h e r e f o r e o m i t t e d . I n s t e a d , a v e r y a c c u r a t e c h e c k was k e p t as t h e t e s t s d e v e l o p e d , a n d a l l c r a c k i n g was n o t e d w i t h t h e c o r r e s - p o n d i n g l o a d . The p i e r s w e r e p l a c e d i n t h e m a c h i n e s o n t h e b o t t o m p l a t e o n w h i c h t h e y h a d b e e n c o n s t r u c t e d , and a c o r r e s p o n d i n g p l a t e w a s l a i d b e t w e e n t h e t o p o f t h e p i e r a n d t h e h e a d o f t h e m a c h i n e . The bond s t r e n g t h c o l u m n s , as m e n t i o n e d , were d i v i d e d up i n t o i n d i v i d u a l b r i c k s , a n d e a c h o f t h e s e b r i c k s t h e n h a d one l a y e r o f m o r t a r o n t h e i r l o w e r s i d e s . The p o s i t i o n o f t h e g a u z e was c h o s e n on t h e b a s i s o f t h e common e x p e r i e n c e t h a t t h e a d h e s i o n b e t w e e n t h e b r i c k a n d t h e m o r t a r i s n o r m a l l y p o o r e s t a l o n g t h e t o p o f t h e j o i n t . I n t h e t e s t s , f i r s t a c i r c l e 8 cm i n d i a m e t e r was c u t t h r o u g h t h e l a y e r o f m o r t a r and a s h o r t d i s t a n c e i n t o t h e b r i c k ( F i g u r e 8 ) . A h a r d m e t a l d r i l l was u s e d t o make t h e c i r c l e , o p e r a t e d i n a s l o w - s p e e d d r i l l clamped f i r m l y i n a s t a n d . The work was c a r r i e d o u t a s c a r e f u l l y as

p o s s i b l e , b u t t h e r e was no way o f a v o i d i n g t h e a p p l i c a t i o n o f c o n s i d - e r a b l e s t r e s s t o t h e m o r t a r d u r i n g t h e d r i l l i n g . I n d i v i d u a l s p e c i m e n s w e r e t h e r e f o r e d e t a c h e d , a n d a l t h o u g h t h i s i s n o t e n t i r e l y c o r r e c t , t h e y w e r e g i v e n a bond s t r e n g t h o f 0 . A c i r c u l a r a l u m i n i u m d i s k w i t h e x a c t l y t h e same a r e a as t h e c i r c l e ( 5 0 c m 2 ) , w a s t h e n s e c u r e d t o t h e undamaged t e s t p i e c e w i t h epoxy g l u e . The g l u e w a s a l l o w e d t o h a r d e n f o r one day i n t h e o r d i - n a r y l a b o r a t o r y a i r , a f t e r w h i c h i t h a d a h i g h e r t e n s i l e and a d h e s i v e

s t r e n g t h t h a n a l l s t a n d a r d c o m b i n a t i o n s o f m o r t a r and s u p p o r t . I n t h e t e s t , t h e d i s k was a t t a c h e d t o a small h y d r a u l i c t r a c t i o n a p p a r a t u s a n d a c o m p l e t e l y c e n t r i c t e n s i l e s t r e s s was a p p l i e d ( s e e F i g u r e 8 ) . The t e n s i l e f o r c e s were c a r e f u l l y n o t e d a l o n g w i t h t h e a p p e a r a n c e of t h e r u p t u r e , i . e . , w h e t h e r i t was p u r e l y a f a i l u r e i n b o n d , o r w h e t h e r p a r t o r a l l o f t h e b r e a k t o o k p l a c e i n t h e m o r t o r o r i n t h e b r i c k . The p u r p o s e o f t h e o r i e n t a t i o n t e s t s was t o c h e c k t h e t e s t

methods a n d a t t h e same t i m e t o g i v e c e r t a i n i n d i c a t i o n s o f what v a r i - a b l e f a c t o r s s h o u l d b e u s e d i n t h e p r i n c i p a l t e s t . Two t y p e s o f

b r i c k s w e r e u s e d , b o t h o f which a r e m a n u f a c t u r e d by S t r i n d e n T e g l v e r k , a n d w e r e s p e c i a l l y g r a d e d s o a s t o b e e x c e p t i o n a l l y u n i f o r m . One o f t h e t y p e s was a medium-fired and r e l a t i v e l y p o r o u s f a c a d e b r i c k w i t h a n a b s o r p t i o n c a p a c i t y o f a b o u t 25g/dm2, w h i l e t h e o t h e r was a h a r d b u r n t b r i c k from t h e same s o u r c e w i t h 10g/dm2 a b s o r p t i o n c a p a c i t y p e r m i n u t e . The a b s o r p t i o n c a p a c i t y i s d e f i n e d as t h e q u a n t i t y o f w a t e r a b s o r b e d by a d r y b r i c k p e r dm2 when t h e b r i c k i s h e l d f a c e down 1 cm u n d e r w a t e r f o r one m i n u t e . The b r i c k s were l a i d d r y , i n some c a s e s warm a t t e m p e r a t u r e s o f +20°c a n d + ~ O C , s o m e t i m e s c o l d a t t e m p e r a t u r e s o f - 1 5 O ~ . Only one t y p e o f m o r t a r was u s e d , namely K C 35/65, p r o p o r - t i o n e d and mixed a c c o r d i n g t o t h e r u l e s l a i d down i n NS 422A.

We s h a l l n o t g i v e t h e r e s u l t s o f t h e t e s t s i n d e t a i l , b u t some of t h e more i m p o r t a n t c o n c l u s i o n s may b e s t a t e d . I t was f o u n d , f o r

e x a m p l e , t h a t b o t h t y p e s o f b r i c k s u b j e c t e d t h e m o r t a r t o a v e r y t h o r o u g h a b s o r p t i o n i n a v e r y s h o r t t i m e ( i n t h e c a s e o f t h e medium- f i r e d b r i c k t h e a b s o r p t i o n was s o s t r o n g t h a t i t c o n s i d e r a b l y a f f e c t e d t h e bond s t r e n g t h v a l u e s and p r e s u m a b l y a l s o i n f l u e n c e d t h e s t r e n g t h o f t h e m a s o n r y ) . The p i e r s which w e r e s t o r e d i n t h e c o l d room were k e p t below f r e e z i n g 2 1 d a y s , a f t e r which t h e y w e r e l e f t t o c u r e i n t h e main l a b o r a t o r y f o r 7 d a y s . D u r i n g t h e f i r s t few d a y s i n t h e l a b o r a - t o r y , t h e m o r t a r thawed and became v e r y s o f t a g a i n . I n s p i t e of t h e s h o r t e f f e c t i v e h a r d e n i n g t i m e ( 6 % d a y s ) t h e f r o z e n p i e r s b o t h f r o m

K 1 a n d K2 a t t a i n e d 75% o f t h e masonry s t r e n g t h o f t h e p i e r s which h a d n o t b e e n f r o z e n and which h a d b e e n k e p t 28 d a y s i n t h e l a b o r a t o r y . T h i s means t h a t f r e e z i n g i n f a c t h a d r e s u l t e d i n a c o n s i d e r a b l e

a l l , however, was t h a t i t made no d i f f e r e n c e w h a t s o e v e r t o t h e masonry s t r e n g t h o r t o t h e a d h e s i v e s t r e n g t h w h e t h e r t h e b r i c k was w a r m o r c o l d . Cold b r i c k s were a l s o j u s t as e a s y t o l a y as w a r m o n e s when t h e y were k e p t d r y i n t h e t e s t .

I n t h e main t e s t s , which embraced a t o t a l o f 38 masonry p i e r s , o n l y one t y p e o f b r i c k was u s e d

-

t h e h a r d b u r n t b r i c k s o f t h e o r i e n - t a t i o n t e s t s . The s e l e c t i o n was c a r r i e d o u t e v e n more c a r e f u l l y t h a n t h e f i r s t t i m e , a n d random s a m p l e s showed t h a t t h e r e was a l m o s t

i n c r e d i b l e agreement between i n d i v i d u a l b r i c k s b o t h i n d i m e n s i o n s , a b s o r p t i v e c a p a c i t y , b u l k d e n s i t y and s t r e n g t h . The masonry was a g a i n c o n d u c t e d i n t h e t h r e e rooms, t h e l a b o r a t o r y a t +20°c, K 1 a t - 1 5 ' ~ a n d K2 a t + ~ O C , a n d , w i t h one e x c e p t i o n , t h e b r i c k s h a d t h e same t e m p e r a -

t u r e as t h e room. E i g h t e e n o f t h e p i e r s were c o n s t r u c t e d w i t h d r y b r i c k s , and 1 8 w i t h b r i c k s which h a d f i r s t b e e n k e p t f o r a day i n w a t e r , a f t e r which t h e y w e r e a l l o w e d t o d r i p and d r y f o r a c e r t a i n t i m e . The l a s t two p i e r s were l a i d w i t h w a r m , wet b r i c k s i n c o l d room

K 1 . I n t h e s e t e s t s , moreover, t h r e e d i f f e r e n t m o r t a r s w e r e u s e d , K C

50/50, KC 35/65 a n d K C 20/80, a l l compounded and mixed a c c o r d i n g t o

NS 422A.

With d r y b r i c k s , t h e masonry work a g a i n p r o c e e d e d e a s i l y w h e t h e r t h e b r i c k s w e r e w a r m o r c o l d . With wet b r i c k s , however, t h e s i t u a t i o n was e n t i r e l y d i f f e r e n t . The s u r f a c e began q u i t e q u i c k l y t o s h e d i c e ,

t h e b r i c k s " s w a m " i n t h e m o r t a r , a n d t h e l a y i n g t o o k a l m o s t t w i c e as l o n g as w i t h t h e d r y b r i c k s . A n o t h e r d i r e c t a n d o b v i o u s r e s u l t was t h a t some o f t h e m o r t a r w a s p r e s s e d o u t o f t h e j o i n t s , a n d t h e s e "wet" p i e r s were s e v e r a l cm s h o r t e r t h a n t h e d r y o n e s . With w a r m , wet b r i c k s t h e c o n d i t i o n s were s l i g h t l y b e t t e r t h a n w i t h c o l d , wet o n e s .

D u r i n g t e s t i n g , t h e t e m p e r a t u r e i n t h e m o r t a r j o i n t s was c h e c k e d by t h e r m o c o u p l e s . I n K 1 t h e t e m p e r a t u r e a t which t h e b r i c k s were l a i d was g O c , a n d w i t h w e t , c o l d b r i c k s t h i s was r e d u c e d t o 0 i n a b o u t 2% h o u r s . The c o r r e s p o n d i n g t i m e f o r d r y , c o l d b r i c k s was 3% h o u r s , a n d

f o r w a r m , wet b r i c k s a c o m p l e t e 7 h o u r s . The d i f f e r e n c e b e t w e e n wet and d r y b r i c k s i s t h a t t h e f o r m e r h a v e a h i g h e r c o e f f i c i e n t of t h e r m a l c o n d u c t i o n . I n K2, t h e m o r t a r t e m p e r a t u r e a t t h e t i m e o f l a y i n g t h e

b r i c k s was 1 3 ' ~ . H e r e t h e room t e m p e r a t u r e was r e d u c e d q u i t e s l o w l y f r o m t6Oc t o -15Oc a n d i t t o o k

34

a n d 30 h o u r s r e s p e c t i v e l y f o r t h e m o r t a r a n d w e t a n d d r y b r i c k t o r e a c h t h e f r e e z i n g p o i n t . The w e t b r i c k t a k e s t h e l o n g e s t t i m e . The r e a s o n f o r t h i s i s t h a t i t h a s a h i g h e r t h e r m a l c a p a c i t y a n d t h e r e f o r e c o o l s l e s s r a p i d l y . A d i g e s t o f t h e t e s t r e s u l t s i s g i v e n i n T a b l e s I a n d I1 a n d i n F i g u r e s 9 a n d 1 0 . B e f o r e t h e r e s u l t s c a n b e d i s c u s s e d , h o w e v e r , i t i s n e c e s s a r y t o c o n s i d e r a l i t t l e more c l o s e l y a c o n d i t i o n w h i c h h a s a v e r y s t r o n g i n f l u e n c e on t h e w h o l e s e r i e s o f t e s t s . A s s t a t e d a b o v e , t h e w e t b r i c k was a l l o w e d t o d r y a l i t t l e b e f o r e b e i n g l a i d , a n d t h i s d r y i n g was c h e c k e d d u r i n g t h e e n t i r e t i m e a n d k e p t a t a c o n s t a n t r a t e . T h i s i s done p a r t l y by w e i g h i n g t h e b r i c k a t r e g u l a r i n t e r v a l s a n d p a r t l y by c h e c k i n g t h e a b s o r p t i o n c a p a c i t y p e r m i n u t e o f t h e b r i c k s . A t t h e t i m e o f l a y i n g , t h e b r i c k s h a d l o s t o n l y a l i t t l e o f t h e i r w a t e r c o n t e n t a n d t h e w a t e r a b s o r p t i o n p e r m i n u t e was e x t r e m e l y l o w . B e c a u s e t h e r e s u l t s w e r e somewhat u n e x p e c t e d , h o w e v e r , t h i s was s u b - s e q u e n t l y c h e c k e d i n d e t a i l . I t was f o u n d t h a t t h e b r i c k s w h i c h a p p a r e n t l y a d s o r b e d w a t e r v e r y p o o r l y p r o b a b l y h a d t h e a b i l i t y t o remove a l a r g e p a r t o f t h e e x c e s s o f w a t e r f r o m t h e m o r t a r a f t e r b e i n g l a i d . I t s h o u l d b e o b v i o u s t h a t w h a t d r i e s f i r s t i s t h e o u t e r m o s t l a y e r o f t h e b r i c k s , i . e . , t h e p a r t s w h i c h a r e d i r e c t l y i n c o n t a c t w i t h t h e m o r t a r . What was r e m a r k a b l e was t h a t t h i s d i d n o t show up d u r i n g t h e o n e m i n u t e a b s o r p t i o n , a n d we must m e r e l y c o n c l u d e t h a t t h i s method o f t e s t i n g e v i d e n t l y h a s s e r i o u s w e a k n e s s e s . T h i s c i r c u m - s t a n c e , h o w e v e r , means t h a t i n f a c t t h e m o r t a r s i n t h e " w e t " p i e r s w e r e c o n s i d e r a b l y d r i e r t h a n h a d o r i g i n a l l y b e e n e x p e c t e d . The r e s u l t s o f t h e p i e r t e s t s i n d i c a t e q u i t e d e f i n i t e l y t h a t w e t b r i c k s ( t o b e c r e d i t e d n o d o u b t w i t h a c e r t a i n s u c t i o n ) t h e r e f o r e g i v e g r e a t e r masonry s t r e n g t h t h a n d r y o n e s . T h i s h o l d s t r u e f o r a l l t y p e s o f m o r t a r , a n d i s e n t i r e l y i n d e p e n d e n t o f t h e method o f c u r i n g . Wet, w a r m b r i c k s , h o w e v e r , h a d somewhat l o w e r s t r e n g t h t h a n w e t , c o l d b r i c k s , e v e n t h o u g h t h e y w e r e e a s i e r a n d b e t t e r f o r l a y i n g p u r p o s e s . T h i s may b e d u e t o t h e f a c t t h a t i n t h i s c a s e t h e m o r t a r h a d b e g u n t o h a r d e n t o some e x t e n t b e f o r e i t f r o z e , a n d t h e r e f o r e may h a v e s u s t a i n e d l e s s damage. F o r d r y b r i c k s , masonry s t r e n g t h i s h i g h e r i n t h e c a s e of

f r o z e n t h a n u n f r o z e n p i e r s , i . e . , t h e same r e s u l t a s i n t h e o r i e n t a - t i o n t e s t s . I n a d d i t i o n , i t i s w o r t h n o t i n g t h a t t h e s t r e n g t h v a l u e s o b t a i n e d f o r w e t b r i c k s and KC 2 0 / 8 0 a r e q u i t e t h e h i g h e s t s o f a r o b t a i n e d i n N B I 1 s l a b o r a t o r y .

A s f a r as bond s t r e n g t h i s c o n c e r n e d , t h e most i m p o r t a n t r e s u l t

i s t h e f a c t t h a t s u c h low v a l u e s were o b t a i n e d f o r wet b r i c k s i n t h e c o l d room K 1 . T h i s must be due t o t h e f a c t t h a t t h e b r i c k s e x p e l l e d i c e , a n d what i s s u r p r i s i n g , i n f a c t , i s t h a t t h i s d i d n o t h a v e a g r e a t e r e f f e c t on t h e masonry s t r e n g t h . The most r e m a r k a b l e f e a t u r e , o t h e r w i s e , was t h e e x t r e m e improvement o b t a i n e d w i t h wet b r i c k s i n t h e p i e r s l a i d i n t h e c o n t r o l l a b o r a t o r y . However, we s h a l l n o t e n q u i r e f u r t h e r i n t o t h e s e r e s u l t s h e r e . We s h a l l d i s c u s s them e x t e n s i v e l y i n t h e f i n a l a p p r a i s a l l a t e r i n t h e a r t i c l e . 4 . M o r t a r t e s t s I n a l l S c a n d i n a v i a n c o u n t r i e s , when i t i s a q u e s t i o n o f d e t e r m i n - i n g t h e s t r e n g t h o f a masonry m o r t a r o r p l a s t e r , t h e s o - c a l l e d S c a n d i - n a v i a n t e s t method i s a p p l i e d . The t h o u g h t b e h i n d t h i s method i s t h a t t h e m o r t a r , up t o a c e r t a i n p o i n t i n any c a s e , s h o u l d have t h e same c u r i n g c o n d i t i o n s i n t h e l a b o r a t o r y a s i t h a s i n p r a c t i c e . S i n c e most o f o u r " c o n v e n t i o n a l " m o r t a r s c o n t a i n b o t h l i m e a n d c e m e n t , t h i s i s e x t r e m e l y i m p o r t a n t . Lime h a r d e n s , i . e . c a r b o n a t e s , o n l y a f t e r t h e m o r t a r h a s begun t o become r e l a t i v e l y d r y , w h i l e cement r e q u i r e s

a c c e s s t o a b u n d a n t w a t e r i n o r d e r f o r t h e h a r d e n i n g p r o c e s s t o become c o m p l e t e . I n p r a c t i c e , b o t h w a l l j o i n t s a n d l a y e r s o f p l a s t e r h a v e a v e r y r e s t r i c t e d t h i c k n e s s . I n most c a s e s a l s o , t h e y a r e i n c o n t a c t w i t h a b r i c k o r a s u p p o r t i n g m a t e r i a l o f g r e a t e r o r l e s s e r a b s o r p t i v i t y , which t h e r e f o r e s u c k s o u t some o f t h e m o r t a r w a t e r d i r e c t l y a f t e r t h e m o r t a r h a s b e e n p l a c e d . Both o f t h e s e f a c t o r s h a v e a s t r o n g i n f l u e n c e on t h e c u r i n g p r o c e s s e s , a n d t h e t e s t method a t t e m p t s , t h e r e f o r e , t o t a k e them b o t h i n t o a c c o u n t . The t e s t s p e c i m e n s a r e made a s t h i n a s i s

p r a c t i c a l l y p o s s i b l e , a n d w h i l e t h e y a r e i n t h e mould t h e y a r e e x p o s e d t o a b s o r p t i o n i n two d i r e c t i o n s .

F i g u r e 11 shows a n example o f S c a n d i n a v i a n m o r t a r p r i s m s a n d m o u l d s . The p r i s m s a r e 25 x 25 x 1 7 0 mm, a n d w h i l e t h e y a r e s t i l l i n t h e mould, f o u r p i e c e s o f b l o t t i n g p a p e r o f a s t a n d a r d i z e d t y p e a r e p l a c e d o v e r a n d u n d e r t h e p r i s m s . T h i s r e s u l t s i n a c o m p a r a t i v e l y s t r o n g a b s o r p t i o n , and t h e p r i s m s c a n t h e r e f o r e b e s t r i p p e d a f t e r a c o m p a r a t i v e l y s h o r t t i m e , g e n e r a l l y s p e a k i n g i n two h o u r s . I n s t r i p - p i n g , a wooden e j e c t o r i s commonly u s e d ( s e e f i g u r e ) i n o r d e r t o a v o i d damaging t h e s l e n d e r s p e c i m e n s . A f t e r s t r i p p i n g , t h e p r i s m s a r e p l a c e d on a g l a s s p l a t e and k e p t f o r 7 d a y s o r 28 d a y s u n d e r s t a n d a r d - i z e d c o n d i t i o n s . G e n e r a l l y s p e a k i n g , t h i s means t h a t t h e y a r e a l l o w e d t o c u r e i n a room i n w h i c h t h e t e m p e r a t u r e i s a b o u t 2 0 " ~ and t h e r e l a t i v e h u m i d i t y o f t h e a i r i s a p p r o x i m a t e l y 7 0 % . I n c e r t a i n c a s e s , h o w e v e r , p a r a l l e l t e s t s a r e a l s o c a r r i e d o u t i n w h i c h t h e r e l a t i v e h u m i d i t y may b e e i t h e r h i g h e r o r l o w e r . F o r most b u i l d i n g m a t e r i a l s i t may b e s a i d t h a t t h e c o m p r e s s i v e s t r e n g t h i s t h e e a s i e s t s t r e n g t h p r o p e r t y t o d e t e r m i n e . A c t u a l l y , h o w e v e r , t h e t e n s i l e s t r e n g t h i s o f t e n a n e v e n more i m p o r t a n t p r o p e r t y , and t h i s i s p a r t i c u l a r l y t r u e o f m o r t a r s . It i s f a i r l y o b v i o u s t h a t t h i s i s t r u e f o r p l a s t e r where c r a c k i n g a n d f i s s u r i n g i s one o f t h e most common t y p e s o f damage. It i s a f a c t , h o w e v e r , t h a t masonry s t r e n g t h a l s o i s d e t e r m i n e d p r i m a r i l y by t h e t e n s i l e s t r e n g t h o f

m o r t a r and b r i c k s . We must a t t e m p t t o t a k e t h i s i n t o a c c o u n t by means o f m o r t a r t e s t s , b u t i n p r a c t i c e i t i s b o t h c o m p l i c a t e d and t i m e - consuming t o c a r r y o u t p u r e t e n s i l e s t r e n g t h t e s t s . We t h e r e f o r e t r y t o g e t t h e b e s t p o s s i b l e r e s u l t s i n a n o t h e r way. F i g u r e 1 2 shows s c h e m a t i c a l l y how m o r t a r s t r e n g t h s a r e d e t e r m i n e d by t h e S c a n d i n a v i a n t e s t i n g method. F i r s t , t h e b e n d i n g s t r e n g t h i s f o u n d by a p p l y i n g a s i n g l e l o a d t o t h e p r i s m a t t h e c e n t r e and i n c r e a s - i n g i t t o r u p t u r e ( a ) . T h i s t e s t t o some e x t e n t i n d i c a t e s b o t h

c o m p r e s s i v e and t e n s i l e s t r e n g t h . The two h a l v e s of t h e p r i s m r e m a i n - i n g a r e t h e n s u b j e c t e d t o a l o a d w i t h t h e a i d o f p r e s s u r e p l a t e s

25 x 25 mm i n a r e a a n d t h e c o m p r e s s i v e s t r e n g t h i s f o u n d ( b ) . F i n a l l y , t h e s o - c a l l e d c l e a v a g e t e n s i l e s t r e n g t h i s o b t a i n e d by " c u t t i n g "

p i e c e s o f t h e same p r i s m s i n two, as i n d i c a t e d a t ( c ) . The b r e a k i n g s t r e s s i n t h e l a t t e r c a s e i s c a l c u l a t e d by d i v i d i n g t h e l o a d by 2 x

t h e a r e a o f a c i r c l e i n s c r i b e d i n t h e c r o s s - s e c t i o n o f t h e p r i s m s ,

i . e .

,

by 2 1 / 4 IT 2 . 5 em2. The v a l u e o b t a i n e d i n t h i s way i s

p r o b a b l y n o t e x a c t l y t h e same as t h e t r u e t e n s i l e s t r e n g t h , b u t we assume t h e r e i s a c o n s t a n t r e l a t i o n s h i p b e t w e e n t h e c l e a v a g e t e n s i l e s t r e n g t h a n d t h e t e n s i l e s t r e n g t h i t s e l f . S i n c e t h r e e p r i s m s a r e u s e d f o r e a c h s e p a r a t e m o r t a r t e s t , t h r e e v a l u e s a r e t h u s o b t a i n e d f o r t h e b e n d i n g s t r e n g t h , s i x f o r t h e c o m p r e s s i v e s t r e n g t h and s i x f o r t h e c l e a v a g e t e n s i l e s t r e n g t h . G e n e r a l l y , o n l y t h e mean v a l u e s i n t h e s e t e s t s a r e r e p o r t e d . I n t h e o r i e n t a t i o n m o r t a r t e s t s , t h e S c a n d i n a v i a n t e s t i n g method was f o l l o w e d e x c e p t t h a t most o f t h e p r i s m s w e r e f r o z e n t o t h e same d e g r e e a s t h e p i e r s . The t e s t s showed, h o w e v e r , t h a t t h i s method a l o n e w a s n o t s u f f i c i e n t t o g i v e a l l t h e r e s u l t s d e s i r e d , a n d t h a t i t

was e s p e c i a l l y n e c e s s a r y t o t a k e i n t o a c c o u n t t h e amount o f w a t e r a b s o r b e d a n d t h e e x t e n t o f t h e p r e - h a r d e n i n g . T h e r e f o r e , a s e r i e s o f t e s t s was s c h e d u l e d f o r m o r t a r s o n l y , a n d t h e s e w e r e c a r r i e d o u t a f t e r t h e p i e r t e s t s h a d b e e n c o n c l u d e d .

The same t y p e s o f m o r t a r s w e r e u s e d a s i n t h e masonry i n v e s t i g a - t i o n s , i . e . , K C 50/50/610, KC 35/65/520 and K C 20/80/440. M i x i n g was c a r r i e d o u t i n a s t a n d a r d i z e d 4 l i t r e m i x e r o f t h e H o b a r t t y p e , and f o r e a c h t y p e o f m o r t a r a c o n s t a n t q u a n t i t y o f w a t e r was u s e d i n a l l b a t c h e s . T h e s e q u a n t i t i e s o f w a t e r h a d b e e n d e t e r m i n e d e a r l i e r i n t h e o r i e n t a t i o n t e s t s , a n d t h e n t h e p r o p o r t i o n s were c h o s e n s o t h a t a l l m o r t a r s h a d t h e same c o n s i s t e n c y , r e p r e s e n t e d by 20 Mo-grade ( S w e d i s h Mo-meter). C a r e f u l m e a s u r e m e n t s showed t h a t f o r a s e r i e s o f t h r e e p r i s m s t h e w a t e r c o n t e n t was 9 1 g f o r K C 5 0 / 5 0 / 6 1 0 , 89 g f o r KC 35/65/520 and 88 g f o r K C 2 0 / 8 0 / 4 4 0 . By c a s t i n g i n m o u l d s , t h e p r i s m s w e r e g i v e n f o u r d i f f e r e n t a b s o r p t i v i t y v a l u e s by t h e methods i n d i c a t e d i n F i g u r e 1 3 . Method a f o l l o w s t h e S c a n d i n a v i a n t e s t method, e x c e p t t h a t o n l y one o f t h e s t a n d a r d b l o t t i n g p a p e r s w a s u s e d on e a c h s i d e . I n l i n e w i t h t h e p r e l i m i n a r y t e s t s t h a t h a d b e e n c a r r i e d o u t on t h e l o s s o f w a t e r ( f o r t h r e e p r i s m s ) , b o t h i n t h i s method a n d i n a l l o t h e r s i t was a p p l i e d and l a y w i t h t h i n g a u z e d i r e c t l y a g a i n s t t h e m o r t a r , s o t h a t t h e bond

w a s b r o k e n . A g l a s s p l a t e was l a i d a b o v e a n d b e l o w t h e b l o t t i n g p a p e r , a n d t h e e n t i r e p r i s m f o r m was l o a d e d w i t h a b o u t 6 k g d i r e c t l y a f t e r t h e m o r t a r h a d b e e n p r e s s e d i n t o p l a c e . Method b f o l l o w e d t h e S c a n d i n a v i a n t e s t c o n d i t i o n s i n a l l d e t a i l s , w i t h f o u r b l o t t i n g p a p e r s o n e a c h s i d e , a n d h e r e a l o a d oi' 6 k g was a g a i n u s e d . The method was i n t e n d e d t o p r o d u c e a n o m i n a l water l o s s o f 25 g . I n t h e f i n a l two m e t h o d s , t h e b l o t t i n g p a p e r w a s r e p l a c e d by b r i c k s a f t e r p r e v i o u s l y c a r r y i n g o u t a l a r g e number o f c o m p r e h e n s i v e p r e l i m i n a r y t e s t s . T h e s e t e s t s i n c l u d e d some w i t h u n t r e a t e d b r i c k a n d some w i t h p o l i s h e d b r i c k ( F i g u r e 1 4 ) , a n d a l s o w i t h sawn s l i c e s o f b r i c k s . The p o l i s h e d b r i c k s g a v e by f a r t h e b e s t r e s u l t s , a n d i t was f o u n d t h a t by u s i n g v e r y c a r e f u l l y s o r t e d b r i c k s a l m o s t c o n s t a n t q u a n t i t i e s o f water c o u l d b e a b s o r b e d f r o m any o n e m o r t a r . By w a s h i n g a n d d r y i n g t h e b r i c k s d i r e c t l y a f t e r s t r i p p i n g , m o r e o v e r , t h e same b r i c k s c o u l d b e u s e d o v e r a n d o v e r a g a i n many t i m e s . I n method c _{t h e b r i c k , w i t h a l a y e r o f g a u z e , was l a i d d i r e c t l y} a g a i n s t t h e e d g e o f t h e f o r m a n d t h e t o t a l q u a n t i t y o f w a t e r a b s o r b e d s h o u l d t h e n b e 35 g . However, t h e r e was a d e v i a t i o n f r o m t h i s f i g u r e , p e r h a p s b e c a u s e t h e m o r t a r a c q u i r e d a b u b b l e i n t h e c o u r s e o f a b s o r p - t i o n a n d t h e r e b y l o s t some o f t h e c o n t a c t w i t h t h e b r i c k a b o v e .

Method d , h o w e v e r , g a v e v e r y c o n s t a n t r e s u l t s . The m o r t a r was p l a c e d i n t h e f o r m s w i t h a c o n s t a n t s l i g h t e x c e s s a b o v e t h e t o p , a n d t h e b r i c k a b o v e was t h e n p r e s s e d v e r t i c a l l y down i n t o t h e m o r t a r . T h i s method r e s u l t e d i n t h e l a r g e s t q u a n t i t y o f w a t e r b e i n g a b s o r b e d , a b o u t 45 g . A s a l r e a d y s t a t e d , t h e i n i t i a l h a r d e n i n g , a n d h e n c e a l s o t h e t i m e t h a t t h e p r i s m s r e m a i n i n t h e f o r m , p l a y s a n i m p o r t a n t r o l e i n t h e r e s u l t . I t was t h e r e f o r e n e c e s s a r y t o c a r r y o u t t h e s t r i p p i n g as q u i c k l y as p o s s i b l e , a n d by w o r k i n g v e r y c a r e f u l l y i t w a s p o s s i b l e t o c u t t h e t i m e i n t h e form t o 2 0 m i n u t e s . T h e r e w e r e o n e o r two d i s a d - v a n t a g e s t o t h i s h o w e v e r . I n t h e f i r s t p l a c e , t h e p r i s m u n a v o i d a b l y a c q u i r e d a l e s s e v e n s u r f a c e t h a n u s u a l a n d a good many i n j u r i e s o c c u r r e d . S e c o n d l y , i t was i m p o s s i b l e t o e x p o s e t h e w h o l e p r i s m i n t h i s way t o a b s o r p t i o n . T h a t i s t o s a y , s u c h p r i s m s c a n n o t b e s t r i p p e d u n t i l s e v e r a l h o u r s .

F o r e a c h t y p e o f m o r t a r and e a c h method o f a b s o r p t i o n s i x s e r i e s w e r e p r e p a r e d , and e a c h o f t h e s e s e r i e s was s u b j e c t e d t o d i f f e r e n t

c u r i n g c o n d i t i o n s . S e r i e s 1, a f t e r s t r i p p i n g , was p u t d i r e c t l y i n t h e c u r i n g room o f t h e l a b o r a t o r y a t t = 2 2 ' ~ and r e l a t i v e h u m i d i t y o f

T O % , where i t was s t o r e d 28 d a y s b e f o r e t e s t i n g . S e r i e s 2 was p u t d i r e c t l y i n - 1 5 ' ~ and r e m a i n e d 6 d a y s , a f t e r which i t was g i v e n 28 d a y s c u r i n g i n t h e c u r i n g room. S e r i e s 3 t o 6 w e r e p r e - c u r e d a t +6Oc ( i n c o l d room K2) f o r 2 , 2 4 , 48 and 120 h o u r s r e s p e c t i v e l y , a n d t h e t e m p e r a t u r e was t h e n l o w e r e d r a t h e r s l o w l y t o - 1 5 ' ~ . A f t e r 6 days s t o r a g e a t t h i s t e m p e r a t u r e t h i s s e r i e s was a l s o moved t o t h e c u r i n g room, where i t r e m a i n e d 28 d a y s . F i n a l l y , a l l t h e p r i s m s w e r e t e s t e d f o r c o m p r e s s i o n , b e n d i n g and c l e a v a g e s t r e n g t h . The r e s u l t s , a v e r a g e from t h r e e p r i s m s , a r e g i v e n i n T a b l e 111.

The T a b l e shows q u i t e c l e a r a n d l o g i c a l r e s u l t s a s f a r a s compres- s i v e s t r e n g t h s a r e c o n c e r n e d . Both t h e b e n d i n g s t r e n g t h v a l u e s a n d t h e c l e a v a g e t e n s i l e s t r e n g t h v a l u e s h a v e s t r i c t l y t h e same t e n d e n c i e s , b u t t h e r e a r e q u i t e a few l a r g e d e v i a t i o n s . The r e a s o n f o r t h i s i s u n d o u b t e d l y t o be f o u n d i n t h e i n j u r i e s a n d i r r e g u l a r i t i e s c a u s e d by t h e e a r l y s t r i p p i n g . T h e s e small s u r f a c e i n j u r i e s p l a y v e r y l i t t l e p a r t i n t h e c o m p r e s s i o n t e s t , b u t t h e y c a n c a u s e a c o n s i d e r a b l e d i f f e r e n c e i n t h e t e n s i l e s t r e n g t h o f t h e p r i s m . F u r t h e r t r e a t m e n t o f t h e n u m e r i c a l v a l u e s i s t h e r e f o r e c o n c e n t r a t e d on t h e c o m p r e s s i v e s t r e n g t h . F i g u r e 1 5 shows i n d i a g r a m m a t i c f o r m how t h e c o m p r e s s i v e s t r e n g t h v a r i e s w i t h t h e c o m p o s i t i o n , a b s o r p t i v i t y , p r e - h a r d e n i n g a n d t h e c o l d t r e a t m e n t o f t h e m o r t a r . The d i a g r a m r e v e a l s many i n t e r e s t i n g d e t a i l s . F o r t h e m o r t a r s w i t h t h e l a r g e s t l i m e c o n t e n t t h e c o l d t r e a t m e n t i n a l m o s t a l l c a s e s r e s u l t e d i n a n i n c r e a s e of t h e s t r e n g t h . T h i s h o l d s r e g a r d l e s s of t h e k i n d o f p r e - h a r d e n i n g o r t h e d e g r e e of a b s o r p t i o n , b u t i s most p r o n o u n c e d f o r t h e d r i e s t m o r t a r s which were p u t d i r e c t l y

i n t h e c o l d room o r which h a d r e c e i v e d e s p e c i a l l y l o n g p r e - h a r d e n i n g a t + ~ O C . T h e s e m o r t a r s a c q u i r e d maximum v a l u e s a l m o s t t w i c e a s g r e a t

a s t h o s e o b t a i n e d w i t h " n o r m a l " , f r o s t - f r e e c u r i n g . The l o w e s t v a l u e s w e r e o b t a i n e d f o r s m a l l a b s o r p t i o n v a l u e s and s h o r t o r i n t e r m e d i a t e

I n t h e m o r t a r s w i t h a h i g h e r p r o p o r t i o n o f cement t h e r e was a c e r t a i n c h a n g e . Here i t i s f i r s t a n d f o r e m o s t t h e l o n g p r e - h a r d e n i n g t i m e w h i c h l e a d s t o a p o s i t i v e o u t c o m e , a n d i t i s c l e a r t h a t t h e m o r t a r s t h e n a c q u i r e s u c h a h i g h i n i t i a l s t r e n g t h t h a t t h e y c a n n o l o n g e r b e i n j u r e d by s e v e r e f r e e z i n g . I n t h e c a s e o f wet m o r t a r a n d a s h o r t p r e - h a r d e n i n g t i m e , on t h e o t h e r h a n d , e x p o s u r e t o f r o s t h a s a d i s t i n c t i v e l y n e g a t i v e e f f e c t , a n d some r e s u l t s g o as low as h a l f " n o r m a l " s t r e n g t h . With i m m e d i a t e f r e e z i n g i t i s p a r t i c u l a r l y t h e d r y m o r t a r s w h i c h a c q u i r e a n i n c r e a s e i n s t r e n g t h . The w e t t e s t m o r t a r s i n t h e m i x t u r e p r o p o r t i o n K C 20/80 o n t h e o t h e r h a n d r e m a i n e d w e a k e r . F i g u r e 1 6 , p e r h a p s , g i v e s a n e v e n b e t t e r o v e r a l l i m p r e s s i o n o f t h e t e s t r e s u l t s . H e r e t h e t r u e w a t e r c o n t e n t h a s b e e n c a l c u l a t e d a f t e r a b s o r p t i o n i n e a c h s e p a r a t e s e t o f p r i s m s , a n d a l s o t h e w a t e r - cement r a t i o . I n t h i s c o n n e c t i o n , t h e r e f o r e , t h e l i m e c o n t e n t i s d i s - r e g a r d e d . The c o m p r e s s i v e s t r e n g t h s i n kp/cm2 a r e p l o t t e d f o r t h e i n d i v i d u a l s e r i e s a g a i n s t t h e w/c r a t i o . The c u r v e s i n d i c a t e t h a t t h e m o r t a r s w h i c h w e r e " c u r e d n o r m a l l y " a r e i n f a c t i n d e p e n d e n t of t n e w a t e r c o n t e n t a t t h e b e g i n n i n g o f c u r i n g , w h i l e a l l o t h e r s e r i e s show a c l e a r i n c r e a s e f o r d e c r e a s i n g w/c. T h i s shows p e r h a p s more t h a n a n y t h i n g e l s e how i m p o r t a n t i t i s f o r m o r t a r s u s e d i n w i n t e r masonry t o b e combined w i t h a n a b s o r p t i v e b r i c k . The g r e a t e r t h e p r o p o r t i o n o f cement t h e s t e e p e r t h e c u r v e b e c o m e s , and e s p e c i a l l y f o r K C 2 0 / 8 0 , s t r o n g v a l u e s a r e l a c k i n g f o r a m o r t a r w h i c h h a s n o t r e c e i v e d any a b s o r p t i o n a t a l l . Such a m o r t a r w i l l h a v e h a d a w/c r a t i o o f a b o u t 1 . 0 , a n d t h e r e i s l i t t l e d o u b t t h a t i t w i l l h a v e a c q u i r e d a v e r y low s t r e n g t h

.

When we compare F i n n i s h a n d Norwegian m o r t a r t e s t s we f i n d a d e f i n i t e a g r e e m e n t f o r m o r t a r s p l a c e d d i r e c t l y i n t h e f r e e z i n g room. F o r p r e - h a r d e n e d m o r t a r s , h o w e v e r , t h e F i n n s f o u n d n o i n c r e a s e i n s t r e n g t h , b u t i n d e e d , i n g e n e r a l , a d e c r e a s e . T h e r e may b e s e v e r a l r e a s o n s f o r t h i s . F i r s t , i n F i n l a n d o n l y s h o r t p r e - h a r d e n i n g t i m e s , up t o 24 h o u r s , w e r e u s e d , i . e . , p e r i o d s w h i c h g a v e c o m p a r a t i v e l j r p o o r r e s u l t s i n t h e Norwegian t e s t s as w e l l . S e c o n d l y , d u r i n g p r e - h a r d e n i n g a n d f r e e z i n g t h e F i n n i s h p r i s m s w e r e w r a p p e d i n p l a s t i c , s o t h a t g e n e r a l l y s p e a k i n g t h e r e was n o l o s s o f m o i s t u r e d u r i n g t h i s t i m e .

The Norwegian p r i s m s l a y i n c o m p a r a t i v e l y l a r g e b o x e s f i t t e d w i t h c o v e r s , a n d t h e measurements showed t h a t t h e r e l a t i v e h u m i d i t y a r o u n d them w a s a b o u t 7 0 % . T h e s e s p e c i m e n s t h u s h a d a c e r t a i n o p p o r t u n i t y t o d r y up somewhat d u r i n g t h e p r e - h a r d e n i n g , a n d o b v i o u s l y t h i s c a n h a v e a f a v o u r a b l e e f f e c t on t h e r e s u l t s . I t i s d i f f i c u l t t o s a y which o f t h e s e methods i s c l o s e s t t o c o n d i t i o n s i n p r a c t i c e

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n o d o u b t i t w i l l depend l a r g e l y on t h e p o s s i b i l i t y of t h e masonry d r y i n g d u r i n g t h e f i r s t p e r i o d a f t e r c o n s t r u c t i o n . I n F i n l a n d , a n a t t e m p t was made t o f i n d a n e x p l a n a t i o n o f t h e i n c r e a s e t h a t c a n t a k e p l a c e i n t h e c o m p r e s s i v e s t r e n g t h on f r e e z i n g . I t h a s b e e n p r o v e d t h a t when m o r t a r s f r e e z e t h e y u n d e r g o a change w i t h r e s p e c t t o t h e s i z e a n d d i s t r i b u t i o n o f t h e p o r e s . T h i s i s p a r t i c u - l a r l y p r o n o u n c e d i n many f r e s h m o r t a r s , a n d no d o u b t c a n p l a y a l a r g e p a r t i n d e t e r m i n i n g t h e f i n a l s t r e n g t h . 5 . D i s c u s s i o n o f r e s u l t s I t i s t e m p t i n g t o a s k o n e s e l f w h e t h e r t h e s e i n v e s t i g a t i o n s i n

F i n l a n d a n d Norway r e a l l y l e a d t o some c l a r i f i c a t i o n o r some new p o i n t s o f view r e g a r d i n g t h e p r o b l e m o f w i n t e r masonry w i t h o u t p r o t e c t i o n . To t h i s q u e s t i o n one must r e p l y b o t h y e s a n d n o . On t h e one h a n d ,

t h e r e r e m a i n a number o f q u e s t i o n s which h a v e n o t b e e n a n s w e r e d a t a l l . T h i s a p p l i e s f i r s t a n d f o r e m o s t t o t h e c o n d i t i o n s o f l o a d i n g o f t h e masonry, i

.

e .

,

t h e d e f o r m a t i o n s t h a t can o c c u r u n d e r d i f f e r e n t c o n d i - t i o n s . On t h e o t h e r h a n d , a good d e a l of what w a s a l r e a d y known o r b e l i e v e d h a s b e e n c o n f i r m e d , a n d i n a d d i t i o n c e r t a i n i n d i v i d u a l s o l u - t i o n s h a v e b e e n p u t f o r w a r d which a r e more o r l e s s c o n t r a r y t o t h e g e n e r a l o p i n i o n . F i n a l l y , i n t h i s a r t i c l e we s h a l l a t t e m p t t o sum up t h e f a i r l y p r a c t i c a l c o n c l u s i o n s which c a n b e drawn w i t h some c e r t a i n t y from t h e i n v e s t i g a t i o n s .

F i r s t o f a l l , i t a p p e a r s t h a t w i n t e r masonry, q u i t e g e n e r a l l y , i s

c o n s i d e r a b l y l e s s r i s k y t h a n most p e o p l e b e l i e v e . T h i s i s p a r t i c u l a r - l y t r u e where b r i c k s h a v i n g a c e r t a i n minimum a b s o r p t i v i t y a r e u s e d . We do n o t know e x a c t l y what t h i s a b s o r p t i v i t y must b e , b u t e v e r y t h i n g i n d i c a t e s t h a t i t can b e c o m p a r a t i v e l y m o d e r a t e , and t h a t i t i s a l s o s u f f i c i e n t f o r t h e o u t e r f a c i n g o f b r i c k s t o b e a b s o r p t i v e . However,

a t t h e same t i m e we must c l e a r l y r e c o g n i z e t h a t t h e b r i c k s which w e r e u s e d i n t h e i n v e s t i g a t i o n s were o f a t y p e h a v i n g a c o m p a r a t i v e l y s m a l l number o f a b s o r b i n g p o r e s . A b r i c k w i t h a g r e a t e r a b s o r p t i o n c a p a c i t y can r e d u c e t h e w a t e r c o n t e n t o f t h e m o r t a r , when l a i d i n e i t h e r w i n t e r o r i n summer, t o s u c h a n e x t e n t t h a t i t w i l l r e d u c e t h e bond s t r e n g t h a n d t h e s t r e n g t h o f t h e masonry. I f t h e b r i c k h a s a d e q u a t e a b s o r p t i v i t y , i t c a n b e l a i d r e g a r d l e s s o f t h e t e m p e r a t u r e , and i n f a c t a l s o r e g a r d l e s s o f v a r i a t i o n s i n t e n - p e r a t u r e . I t i s m e r e l y n e c e s s a r y f o r t h e m o r t a r t o be warm enough a n d t o r e m a i n w e l l above t h e f r e e z i n g p o i n t d u r i n g t h e e n t i r e l a y i n g o p e r a t i o n . I n t h i s c a s e i t d o e s n o t a p p e a r t o m a t t e r e i t h e r w h e t t i e r t h e b r i c k s l a i d a r e c o l d o r w a r m , a n d i t c a n e v e n h a p p e n t h a t warm b r i c k s may p r e v e n t p r e - h a r d e n i n g a n d i n c r e a s e t h e i n i t i a l s t r e n g ~ h . T h i s a p p l i e s p a r t i c u l a r l y t o p e r i o d s d u r i n g which t h e t e m p e r a t u r e g o e s a b o v e z e r o i n t h e d a y t i m e a n d t h e r e i s s e v e r e n i g h t t i m e f r o s t . I f b r i c k s a r e l a i d u n d e r s u c h c o n d i t i o n s , e v e n w i t h a n a b s o r p t i v e b r i c k , t h e masonry s t r e n g t h v a l u e can b e e x p e c t e d t o b e a t l e a s t a s good a s f o r summer b r i c k l a y i n g , a n d p o s s i b l y a good d e a l h i g h e r . T h i s i s

p r o b a b l y a s s o c i a t e d b o t h w i t h c h a n g e s i n t h e p o r o u s s t r u c t u r e a n d w i t h a s l o w e r d r y i n g and more c o m p l e t e h a r d e n i n g o f t h e masonry m o r t a r .

N e v e r t h e l e s s , one p r o b l e m r e m a i n s i n c o n n e c t i o n w i t h t h e l a y i n g o f a b s o r p t i v e b r i c k s , namely, d e f o r m a t i o n s . We know, h o w e v e r , t h a t m o r t a r s which h a v e l o s t somewhere n e a r h a l f of t h e i r o r i g i n a l w a t e r c o n t e n t w i l l h a v e a c e r t a i n s t r e n g t h e v e n i f no h a r d e n i n g h a s t a k e n p l a c e . I t s h o u l d b e s u f f i c i e n t f o r small l o a d s , b u t c e r t a i n l y n o t f o r a v e r y h e a v i l y l o a d e d p i e c e o f masonry. I t w i l l b e a n a d v a n t a g e t o u s e a m o r t a r w i t h a h i g h cement c o n t e n t , p r e f e r a b l y K C 20/80 o r some c o r r e s p o n d i n g masonry cement m o r t a r . M o r t a r s o f t h i s t y p e g a i n s t r e n g t h more r a p i d l y a n d h a v e b e t t e r h a r h d e n i n g p o s s i b i l i t i e s a t low t e m p e r a t u r e s t h a n m o r t a r s w i t h a h i g h e r p r o p o r t i o n o f l i m e . The l o w e r t h e a b s o r p t i v i t y o f t h e b r i c k t h e g r e a t e r t h e r i s k o f f r o s t damage. I f one c a n b e e n t i r e l y s u r e t h a t t h e masonry w i l l h a v e s e v e r a l d a y s o f p r e - h a r d e n i n g a t t e m p e r a t u r e s w e l l above O'C b e f o r e f r e e z i n g , t h e n i t i s p r e s u m a b l y s a f e i n t h i s c a s e a s w e l l . We can

c o u n t on a h i g h masonry s t r e n g t h and good a d h e s i o n b e t w e e n t h e b r i c k a n d m o r t a r , and i n t h i s c a s e t h e r e w i l l b e no s p e c i a l d a n g e r o f damage d u e t o d e f o r m a t i o n s .

I f t h e b r i c k s a r e l a i d i n r e a l l y c o l d w e a t h e r and h a v e p o o r a b s o r p t i v i t y , o r n o n e a t a l l , t h e masonry i t s e l f w i l l p r o c e e d s l o w l y a n d b e much more d i f f i c u l t . The d a n g e r o f u n s i g h t l y d e f o r m a t i o n s a p p e a r i n g when t h e masonry t h a w s i s c o n s i d e r a b l e , b u t on t h e o t h e r h a n d one can c o u n t o n a h i g h s t r e n g t h i n t h i s c a s e a s w e l l when t h e m o r t a r e v e n t u a l l y h a r d e n s . T h i s i s p r o b a b l y one o f t h e r e a s o n s t h a t good r e s u l t s a r e s o o f t e n o b t a i n e d u n d e r r e a l l y c o l d , s t e a d y w i n t e r c o n d i t i o n s , a n d h e r e c h a n g e s i n t h e p o r e s t r u c t u r e o f t h e m o r t a r b e g i n t o p l a y a n i m p o r t a n t p a r t . The w o r s t s i t u a t i o n , p r e s u m a b l y , i s when t h e t e m p e r a t u r e v a r i a - t i o n s a r e g r e a t a n d t a k e p l a c e r a p i d l y . If b r i c k s w i t h p o o r a b s o r p t i - v i t y a r e u s e d i n s u c h c a s e s , f r o s t damage c a n e a s i l y o c c u r i n t h e m o r t a r . W a r m b r i c k s i n t h i s c o n n e c t i o n may i n c r e a s e t h e d a n g e r , b e c a u s e t h e y p r o l o n g t h e p r e - h a r d e n i n g p r o c e s s . I a m t e m p t e d t o c o n c l u d e t h i s summary w i t h a r e s u l t w h i c h h a s n o t h i n g t o do w i t h w i n t e r m a s o n r y : t h e masonry t e s t s r e f e r r e d t o e a r l i e r i n d i c a t e d v e r y c l e a r l y t h a t u n d e r n o r m a l c o n d i t i o n s , i . e .

,

d u r i n g summer, w e t b r i c k s c a n p r o d u c e masonry w i t h s u p e r i o r s t r e n g t h a n d t i g h t n e s s . I t i s p e r h a p s a method w h i c h i s t o o c o m p l i c a t e d t o b e c a r r i e d o u t i n e v e r y d a y p r a c t i c e , b u t i t may b e u s e f u l t o k e e p i t

c l e a r l y i n mind. The e x t r a s t r e n g t h and t i g h t n e s s may b e d e s i r a b l e i n s p e c i a l c a s e s w h e r e t h e demands o n t h e masonry a r e e x t r a o r d i n a r i l y h i g h .