Axial compression tests on masonry walls and prisms

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Axial compression tests on masonry walls and prisms

Maurenbrecher, A. H. P.

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Axial Compression Tests on

Masonry Walls and Prisms

ANALYZED

Appeared in

Proceedings Third North American Masonry Conference

University of Texas at Arlington

June 3-5,1985

p. 19-1 - 19-14

Reprinted with permission

DBR Paper No. 1303

Price $1.50

NRCC 24710

Des essais de compression axiale ont dtg effectugs sur des

murets et des prismes constitugs de quatre types d'glgments de

ma~onnerie

:

brique d'argile

comprimge, brique d'argile

extrudge, bloc d'argile extrudg creux et bloc de bgton creux.

Les essais avaient pour but de fournir des informations sur la

rgsistance

_{B la compression et le module de compression des}

dchantillons de petite taille (prismes) et de grande taille

(murs), et de permettre d'effectuer un contr8le ponctuel des

valeurs contenues dans les tableaux et relatives

_{B la}

rgsistance

_{B la compression des glgments de ma~onnerie. Les}

essais confirment que des prismes de magonnerie peuvent ?tre

utilisgs pour dgterminer la valeur de base de r6sistance

3

la

compression si les mgthodes dlessai sont convenablement

suivies.

D'autres recherches seraient ngcessaires en ce qui

concerne lleffet de l'humidit6 sur la rgsistance des blocs de

bgton et la rgsistance des blocs alvgolaires li6s au mortier et

charggs sur la face extgrieure (charges excentrges).

AXIAL COMPRESSION TESTS ON MASONRY WALLS AND PRISMS

A.H.P.

Maurenbrecher

Division of Building Research

National Research Council of Canada

Ottawa, Canada, K1A OR6

ABSTRACT

Short walls and prisms have been tested under axial compressive load

using four types of masonry units: a pressed clay brick, an extruded

clay brick, an extruded hollow clay block, and a hollow concrete block.

The tests were designed to provide information on compressive strength

and compressive modulus of both small (prisms) and large (walls)

specimens.and to act as a spot check on tabular values relating masonry

to unit compressive strength. They confirm that masonry prisms may be

used for determining basic compressive strength if proper test

procedures are followed. Areas needing further investigation include

the effect of moisture on the strength of concrete blocks and the

strength of eccentrically loaded face-shell mortar-bedded hollow

blockwork.

AXIAL COMPRESSION TESTS ON MASONRY WALLS

AND

PRISMS

A.H.P.

Maurenbrecher

INTRODUCTION

Tests on short walls and prisms have been carried out using two types

of clay brick, a concrete block, and a clay block. The objectives were

to provide information on the compressive strength and compressive

modulus of small (prisms) and large (walls) specimens and to act as a

spot check on tabular values in the Canadian masonry design standard

relating masonry to unit compressive strength. It was hoped, in

addition, that the tests would provide a better understanding of test

procedures and workmanship.

Small, single-wythe walls in running bond with height-to-thickness

ratios of

10

and prisms with height-to-thickness ratios varying from

2.8 to 5 were built by a mason. As a check on workmanship a technician

also built prisms. The specimens were tested in axial compression to

failure, with compressive strain measured on all walls and on most

prisms. Many replicates were tested for variability to improve the

accuracy of the results.

MATERIALS

Brick and Block

A high-strength extruded clay brick, a low-strength pressed clay brick,

an autoclaved hollow concrete block, and an extruded hollow clay block

were used (Fig.

1).

Their properties are listed in Tables

1

and 2.

Mortar

Types S and N mortar were used for the clay and concrete masonry,

respectively; proportions of the ingredients by volume were

1:0.63:4.25,

1:0.5:4.5,

and 1:1:6 portland cement to hydrated lime to

sand for clay block, bricks, and concrete block, respectively. The

mortar was batched by weight and mixed in a standard mortar mixer.

Water was added to suit the mason, who sometimes added more later on

(re-tempering).

The average strength of moist-cured 50-

cubes at

7

days was 6.4 to 6.8 MPa for the

S

mortar and 4.1 MPa for the N

mortar.

l

CONSTRUCTION

The high-strength bricks were removed from the pallets, as received,

when the walls and prisms were being built. They were dipped in water

(30 s) and allowed to stand 5 to 15 min. The other units, laid dry,

were randomly redistributed to ensure a uniform distribution. The

mason was told to build according to his normal practice, with the

proviso that the specimens should be plumb. Normal practice consisted

of deeply furrowing the mortar joints for the brickwork and face-shell

mortar bedding for the blockwork. When plumbing sections of the wall,

the mason sometimes tapped a unit already in position. Walls were

built on levelled, ground steel plates covered with polyethylene. The

brickwork walls and prisms were kept level and plumb using a level.

A

course rod was provided. The blockwork walls were built to a line.

Half units, needed at the ends of every second course, were cut using a

chisel for high-strength bricks, but the other units were cut by saw.

m

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H I G H S T R E N G T H B R I C K

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L O W S T R E N G T H 8 R I C K VALUES I N BRACKETS 39 1 BOTTOM OF BLOCK 1 3 3 ,I, 140 32 (27)

7"

"'ON I 31 (27) C O N C R E T E BLOCK

PRISM Amb/Ag = 0.42 JIG-BUILT

= 0.46 M A S O N BUILT

1

i0

WALL Amb/Ag = 0.46

MORTAR BEDDED AREA

31 JIG 32 MASON 3 I I 4 I I 1 I

!

1 k I PRISM Amb/Ag = 0.42 27' 113 27' 54

'

C L A Y B L O C K 5 WALL Amb/Ag ' 0.41

Figure 1. Unit dimensions and mortar-bedded a r e a

To make t h e w a l l s more r e p r e s e n t a t i v e of a longer w a l l t h e h a l f u n i t s were placed w i t h t h e c u t end exposed, except f o r t h e high-strength b r i c k w a l l s and t h e f i r s t

s i x

low-strength b r i c k w a l l s where t h e un-cut end was exposed. This counteracted t h e mason's tendency t o provide a f u l l mortar j o i n t a t t h e ends of t h e wall.

The t e c h n i c i a n b u i l t h i s prisms i n a j i g a t t h e same t i m e ; t h e s e prisms

w i l l be designated " j i g - b u i l t . " F u l l j o i n t s were obtained by use of a metal mortar template 11 mm deep, allowing t h e mortar t o be screeded f l a t before t h e next u n i t was placed. The u n i t s were tapped down t o -

g i v e a 10- t h i c k mortar j o i n t .

Brickwork and c l a y blockwork specimens were cured under polyethylene bags f o r seven and t h r e e days, r e s p e c t i v e l y , then allowed t o s t a n d i n

t h e l a b o r a t o r y a i r u n t i l t e s t e d . Concrete masonry specimens were cured

i n

a i r

u n t i l t e s t e d . The w a l l s and s t r e t c h e r bond prisms were capped

w i t h Types S o r N mortar. The cap

was

screeded f l a t w i t h t h e h e l p of

two aluminum g u i d e s clamped t o t h e s i d e s of t h e specimen. Walls and

some prisms were whitewashed f o r e a s i e r d e t e c t i o n of cracks. TEST PROCEDURE

Prisms were t e s t e d i n a 1.8-MN R i e h l e h y d r a u l i c t e s t machine and w a l l s

were t e s t e d i n a 7-MN t e s t frame (Fig. 2, 3 ) . A l l specimens were

capped t o p and bottom w i t h ll-mm t h i c k f i b r e b o a r d over t h e whole

c r o s s - s e c t i o n f o r t h e brickwork and on t h e f a c e - s h e l l s o n l y f o r t h e

blockwork. The l o a d i n g r a t e f o r prisms where s t r a i n was n o t measured

w a s s e t s o t h a t t h e t e s t d u r a t i o n from h a l f t o maximum l o a d would t a k e

approximately 1 t o 2 min. Other specimens were t e s t e d a t a lower r a t e

t o a l l o w f o r s t r a i n measurements and c r a c k d e t e c t i o n . A t f a i l u r e t h e

l o a d c o n t r o l s were n o t a l t e r e d . S t r a i n was measured w i t h 200, 300 and

600 mm handheld, mechanical gauges (Demec).

F i g u r e 2. Walls a f t e r f a i l u r e F i g u r e 3. Concrete b l o c k p r i s m a t

( a ) c o n c r e t e block f a i l u r e

TEST RESULTS

U l t i m a t e S t r e n g t h

F a i l u r e s t r e s s e s (Tables 3-5) a r e based on g r o s s a r e a f o r brickwork and on mortar-bedded a r e a f o r blockwork. The mortar-bedded a r e a , used f o r hollow blockwork i n t h e new e d i t i o n of t h e Canadian masonry d e s i g n s t a n d a r d s 5 i s t h e h o r i z o n t a l a r e a of m o r t a r i n f u l l c o n t a c t w i t h b l o c k s above and below. For c o n c r e t e blockwork where mortar was l a i d on t h e f a c e - s h e l l s o n l y , t h e mortar-bedded a r e a f o r t h e mason-built specimens was e s t i m a t e d t o be 1.19 x a r e a , based on t h e minimum f a c e - s h e l l w i d t h F i g . 1 For j i g - b u i l t prisms t h e i n c r e a s e was 1.09 x a r e a . For c l a y blockwork w i t h s q u a r e c o r e s and uniform f a c e - s h e l l w i d t h s t h e a r e a was based on t h e a c t u a l f a c e - s h e l l w i d t h ( t h e c r o s s webs do n o t a l i g n i n t h e w a l l (Fig. 4 ) ) . The webs i n t h e prisms do o v e r l a p and a s a r e s u l t t h e r e i s a small i n c r e a s e i n mortar-bedded a r e a , b u t f o r s i m p l i c i t y i t

h a s n o t been t a k e n i n t o account. Cracking

V e r t i c a l c r a c k s were f i r s t observed on t h e s i d e s of t h e brickwork w a l l s a t 82% of t h e u l t i m a t e l o a d and a t t h e ends a t 92% ( v = 9

-

13%). The low-strength b r i c k w a l l s o f t e n showed minor l o c a l c r a c k s a t lower l o a d s

( u s u a l l y a l o n g t h e t o p c o u r s e ) . The c o n c r e t e blockwork w a l l s developed v e r t i c a l c r a c k s a t t h e ends, down t h e c e n t r e of t h e webs, a t 58% of u l t i m a t e

(v

= 9%); t h e c l a y b l o c k w a l l s developed v e r t i c a l c r a c k s i n t h e webs a t t h e i n t e r f a c e w i t h t h e f l a n g e s a t 70% of u l t i m a t e (60-70%). The c l a y b l o c k w a l l s f i r s t cracked on t h e s i d e s a t 76% of u l t i m a t e (62-84%), b u t no c r a c k s were observed on t h e c o n c r e t e block walls b e f o r e f a i l u r e . F i n a l f a i l u r e was i n b o t h c a s e s caused by v e r t i c a l s p l i t t i n g w i t h i n t h e f a c e - s h e l l of t h e block a t t h e mortar j o i n t (Fig. 2, 3).

The brickwork p r i s m s b u i l t i n running bond cracked a t s i m i l a r l o a d l e v e l s , but w i t h g r e a t e r v a r i a t i o n . Stack-bonded prisms a r e n o t a good i n d i c a t o r of c r a c k l o a d s s i n c e t h e y cracked a t h i g h e r l o a d s ( c r a c k s were n o t monitored i n t h e blockwork prisms).

S t r e s s - S t r a i n R e l a t i o n

The s t r e s s - s t r a i n c u r v e s f o r a l l t h e specimens t e s t e d a r e shown i n F i g u r e s 5 and 6. The a v e r a g e i n i t i a l e l a s t i c moduli a r e g i v e n i n Tables 3 t o 5. Some c u r v e s f o r c o n c r e t e block prisms show a sudden change i n s l o p e , probably due t o f o r m a t i o n of a v e r t i c a l c r a c k i n t h e webs of t h e blocks. The l a r g e i n i t i a l concave p o r t i o n of t h e curve of some j i g - b u i l t , low-strength b r i c k p r i s m s i s probably due t o a n

incomplete bond between b r i c k and mortar caused, i n t u r n , by t h e h i g h s u c t i o n r a t e of some of t h e b r i c k s . J i g - b u i l t prisms t o o k l o n g e r t o b u i l d , g i v i n g more time f o r t h e mortar t o l o s e w a t e r b e f o r e t h e next b r i c k was placed. The s t r e n g t h of t h e prisms d i d n o t seem t o be

a f f e c t e d ; t h e one w i t h t h e l a r g e s t i n i t i a l concave curve f a i l e d a t t h e second h i g h e s t load. T h i s problem d i d n o t o c c u r w i t h t h e mason-built prisms. A s i m i l a r , s m a l l e r c u r v a t u r e may be s e e n w i t h t h e hollow c l a y blockwork. I n c o n t r a s t , h i g h - s t r e n g t h brickwork f o r which b r i c k s were dipped i n water beforehand shows very c o n s i s t e n t curves. The c o n c r e t e blockwork, l a i d w i t h d r y b l o c k s , a l s o shows c o n s i s t e n t c u r v e s . l---. 1.0 S T R A I N , m m l m 14 COURSES NIGH) 0

u

1.0 S T R A I N , m m l m Figure 5. S t r e s s - s t r a i n c u r v e s , brickwork; ( a ) h i g h s t r e n g t h b r i c k ; ( b ) low s t r e n g t h b r i c k

Figure 6. S t r e s s - s t r a i n c u r v e s , blockwork; ( a ) hollow c o n c r e t e b l o c k ; ( b ) hollow c l a y block

DISCUSSION Workmanship

Incomplete f i l l i n g of t h e c e n t r a l p a r t of t h e mortar j o i n t due t o furrowing of mortar (Fig. 7) reduced t h e s t r e n g t h of t h e mason-built, high-strength b r i c k prisms t o 66% of t h a t of t h e j i g - b u i l t prisms. This was not unexpected. 3 , l 1 A s a check on workmanship, a second

mason b u i l t four-course, stack-bonded prisms s i n g l y and i n rows of f i v e t o s i m u l a t e w a l l c o n s t r u c t i o n . The s i n g l e prisms were n e a r l y a s s t r o n g a s t h e j i g - b u i l t ones ( r a t i o of 0.92). Prisms b u i l t i n a , l i n e were weaker; t h e end prisms i n t h e row were 3% weaker, while t h e middle t h r e e

were

9% weaker, i n d i c a t i n g t h a t furrowing i n s i n g l e prisms i s n o t s o complete a s i n walls. Prisms should t h e r e f o r e be b u i l t i n a l i n e t o s i m u l a t e w a l l c o n s t r u c t i o n more c l o s e l y . The A u s t r a l i a n brickwork code r e q u i r e s t h i s ; t h e Canadian standard recommends it.

*

The low-strength brickwork was less a f f e c t e d by workmanship, probably because of i t s l a r g e c e n t r a l p e r f o r a t i o n s ( r a t i o of mason-built t o j i g - b u i l t , 0.92). The f a c e - s h e l l mortar-bedded blockwork was a l s o l e s s a f f e c t e d ; r a t i o s of mason-built t o j i g - b u i l t were 0.96 and 0.84 f o r c o n c r e t e and c l a y blockwork, r e s p e c t i v e l y ( a d i f f e r e n t mason was used f o r t h e c o n c r e t e blockwork).

Capping

Fibreboard was used a s a capping f o r t h e w a l l s and prisms because i t

was simple and quick. I n comparison t o a hard capping such a s d e n t a l p l a s t e r , f i b r e b o a r d reduces f r i c t i o n a l r e s t r a i n t , thereby lowering t h e compressive s t r e n g t h of masonry u n i t s ; t h e r a t i o of f i b r e b o a r d t o t h a t of plaster-capped u n i t s ranged from 0.82 t o 0.85, except f o r hollow c l a y block where t h e lower r a t i o of 0.73 may be due t o warped s u r f a c e s of some of t h e blocks. This r e d u c t i o n i n f r i c t i o n a l r e s t r a i n t was n o t s i g n i f i c a n t f o r masonry prisms s i n c e f a i l u r e

i s

i n i t i a t e d a t t h e mortar

joint^,^

but t h e r e can be a r e d u c t i o n i n s t r e n g t h i f t h e prism s u r f a c e i s uneven. I f s u r f a c e s a r e o u t of p l a n e (say, by more t h a n 1 mm), a hard capping should be used w i t h o r without fibreboard. The w a l l s and running bond prisms were capped w i t h mortar, while some hollow c l a y block prisms

were

capped w i t h d e n t a l p l a s t e r .

Where f a c e - s h e l l mortar bedding i s used t h e f i b r e b o a r d should be placed on t h e f a c e - s h e l l s only; otherwise, premature f a i l u r e may o c c u r O 8

As

furrowing of t h e m o r t a r could be c o n s i d e r e d e q u i v a l e n t t o f a c e - s h e l l m o r t a r bedding, t h e f u l l f i b r e b o a r d c a p s used on t h e brickwork prisms may have caused a r e d u c t i o n i n s t r e n g t h . Checks of prisms w i t h mortar s t r i p s on e i t h e r s i d e of t h e b r i c k showed a l a r g e r e d u c t i o n i n s t r e n g t h f o r low-strength brickwork (33%) b u t n o t f o r h i g h - s t r e n g t h brickwork (9%). The l a r g e r e d u c t i o n d i d n o t o c c u r f o r t h e mason-built prisms i n t h e p r e s e n t t e s t s , s o t h a t f u r t h e r s t u d y i s needed t o v e r i f y t h e e f f e c t of f u l l f i b r e b o a r d capping. Even w i t h t h e p o s s i b l e n e g a t i v e e f f e c t s of furrowed mortar and f u l l f i b r e b o a r d caps, t h e low-strength brickwork prisms gave f a i l u r e s t r e s s e s w e l l i n e x c e s s of t h e code t a b u l a r v a l u e s .

Code Tabular Values

Values f o r compressive s t r e n g t h i n t h e Canadian d e s i g n s t a n d a r d a r e based on t y p e of m o r t a r and c h a r a c t e r i s t i c compressive s t r e n g t h , f k , of t h e b r i c k o r block.5 These v a l u e s a r e compared w i t h t h o s e of t h e prisms, which may a l s o be used t o determine compressive s t r e n g t h

(Table 6 ) . Prism s t r e n g t h i s based on prisms w i t h a h e i g h t - t o - . t h i c k n e s s r a t i o of 5; lower r a t i o s a r e p e r m i t t e d , b u t a r e d u c t i o n

f a c t o r i s a p p l i e d t o t h e compressive s t r e n g t h obtained.

The t a b u l a r v a l u e s a r e l e s s t h a n t h e e x p e r i m e n t a l ones e x c e p t f o r t h e mason-built, h i g h - s t r e n g t h b r i c k prisms. Those f o r brickwork a r e t h e same a s t h o s e f o r i n s p e c t e d workmanship i n t h e B I A brickwork c o d e , l l and assume f u l l mortar j o i n t s w i t h no gaps from furrowing such a s

o c c u r r e d i n t h e p r e s e n t t e s t s . This i l l u s t r a t e s t h e importance of good j o i n t s i n load-bearing brickwork i f t a b u l a r v a l u e s a r e used. The

low-strength b r i c k prisms, on t h e o t h e r hand, gave much h i g h e r v a l u e s . Large p e r f o r a t i o n s i n t h e s e b r i c k s may make them l e s s s e n s i t i v e t o furrowing of t h e m o r t a r ; a s w e l l , t h e i r h i g h l y v a r i a b l e s t r e n g t h

( v = 24%) means t h a t c h a r a c t e r i s t i c s t r e n g t h i s low. The high

v a r i a b i l i t y w a s n o t r e f l e c t e d i n t h e w a l l s and l a r g e r p r i s m s , which had c o e f f i c i e n t s of v a r i a t i o n ranging from 9 t o 13%.

There a r e no t a b u l a r v a l u e s f o r hollow c l a y block. The v a l u e s i n t h e brickwork t a b l e were s u g g e s t e d f o r p o s s i b l e u s e , l but f u r t h e r t e s t s a r e n e c e s s a r y t o check t h e behaviour of hollow b l o c k under e c c e n t r i c l o a d , where e c c e n t r i c i t y i s d i f f e r e n t t o p and bottom. Some p r e l i m i n a r y p r i s m t e s t s u s i n g c l a y b l o c k gave approximately o n e - t h i r d of t h e

expected f a i l u r e l o a d (sudden s h e a r f a i l u r e i n t h e c r o s s webs). F u r t h e r t e s t s a r e planned.

The masonry s t a n d a r d assumes t h a t a n i n c r e a s e i n h e i g h t - t o - t h i c k n e s s r a t i o from 5 t o 10 h a s no e f f e c t on t h e s t r e n g t h of a x i a l l y loaded w a l l s w i t h f l a t end c o n d i t i o n s . I n t h e p r e s e n t t e s t s , however, t h e r e

w a s a r e d u c t i o n ; t h e mean s t r e n g t h of t h e brickwork walls ( h / t = 10) was 74 and 82% of t h e s t r e n g t h of running bond prisms. S i m i l a r

r e d u c t i o n s were found i n o t h e r t e s t s . 1 2 The mean s t r e n g t h of t h e c o n c r e t e b l o c k w a l l w a s 91% of t h e p r i s m s t r e n g t h , w h i l e t h a t of t h e c l a y block w a l l was 92% (based on a d j u s t e d prism s t r e n g t h ) . The block

w a l l s had a more uniform s t r a i n on e a c h s i d e of t h e w a l l t h a n d i d t h e b r i c k w a l l s . The m o r t a r j o i n t s on one s i d e of t h e b r i c k w a l l s had a concave f i n i s h ; t h i s may e x p l a i n t h e g r e a t e r s t r a i n d i f f e r e n c e f o r t h e two s i d e s and, e f f e c t i v e l y , t h e l a r g e r e c c e n t r i c i t y of load.

Moisture Content

The compressive s t r e n g t h of c o n c r e t e block i s s e n s i t i v e t o m o i s t u r e

$ on tent.^

Blocks, b o t h a i r - d r y and soaked i n w a t e r f o r 24 h , were

t e s t e d i n September b e f o r e t h e w a l l t e s t s and a g a i n i n January a f t e r t h e t e s t s (Table 2). The r a t i o of t h e compressive s t r e n g t h of soaked t o a i r - d r y b l o c k s changed from 0.82 t o 0.69 over t h i s period. The a i r - d r y b l o c k i n c r e a s e d i n s t r e n g t h by 31% compared t o 11% f o r t h e soaked block. Humidity o v e r t h e same p e r i o d changed from 50-60% t o 20-30%. This probably meant a lower m o i s t u r e c o n t e n t i n t h e a i r - d r y block and a r e s u l t i n g i n c r e a s e i n s t r e n g t h . Blocks used a s c o n t r o l specimens s h o u l d t h e r e f o r e be t e s t e d a t t h e same t i m e a s w a l l s and prisms ( a requirement i n c o r p o r a t e d i n t h e new CSA prism s t a n d a r d 6 ) . For q u a l i t y c o n t r o l , more c o n s i s t e n t r e s u l t s w i l l be o b t a i n e d i f t h e b l o c k s a r e t e s t e d i n t h e s a t u r a t e d c o n d i t i o n ( r e q u i r e d i n t h e B r i t i s h s t a n d a r d 4 ) ; oven d r y i n g of b l o c k s i s n o t p r a c t i c a l . Moisture c o n t e n t h a s much less e f f e c t on c l a y u n i t s (Table 1 ) .

E l a s t i c Modulus

The Canadian d e s i g n s t a n d a r d g i v e s t h e e l a s t i c modulus i n terms of c h a r a c t e r i s t i c f a i l u r e s t r e s s , f k : E = 1000 f k

<

20 GPa. This a g r e e s w i t h v a l u e s f o r t h e hollow c o n c r e t e blockwork and mason-built,

h i g h - s t r e n g t h brickwork, b u t i t o v e r e s t i m a t e s t h e modulus f o r t h e o t h e r specimens where t h e r a n g e i s 570 t o 770 fk. The A u s t r a l i a n brickwork code u s e s a v a l u e of 750.

CONCLUSIONS

The s t r e n g t h of c o n c r e t e b l o c k s can vary w i t h age and m o i s t u r e c o n t e n t . It i s t h e r e f o r e i m p o r t a n t t h a t t h e y s h o u l d be t e s t e d a t t h e same t i m e a s w a l l s o r prisms t o e n s u r e good c o r r e l a t i o n .

Fibreboard capping makes t e s t i n g e a s i e r , b u t i t i s i m p o r t a n t f o r t h e cap t o have a c o n f i g u r a t i o n s i m i l a r t o t h a t of t h e mortar j o i n t . For example, i f f a c e - s h e l l bedding i s u s e d , f i b r e b o a r d s h o u l d be p l a c e d only on t h e f a c e - s h e l l a r e a . Stack-bonded prisms should be b u i l t i n a l i n e t o s i m u l a t e w a l l c o n s t r u c t i o n . This b e t t e r r e f l e c t s f a c t o r s s u c h a s furrowing of t h e mortar j o i n t and l e n g t h of time t h e mortar i s l e f t exposed b e f o r e t h e n e x t u n i t i s placed. Although t h e i r e f f e c t on

s t r e n g t h h a s n o t been shown t o be s i g n i f i c a n t , c u t ends of h a l f u n i t s s h o u l d f a c e outward a t t h e ends of t e s t specimens s o t h a t t h e w a l l o r p r i s m w i l l be more r e p r e s e n t a t i v e of a l a r g e r specimen.

The p r e s e n t t e s t s c o n f i r m p r e v i o u s work showing t h a t masonry prisms p r o v i d e a good b a s i s f o r determining compressive s t r e n g t h i f p r o p e r t e s t procedures a r e followed. T e s t s a r e normally c a r r i e d o u t 03

a x i a l l y loaded prisms, b u t t h e y a r e n o t always a good i n d i c a t o r of how masonry behaves under e c c e n t r i c load. A f u r t h e r t e s t on a n

e c c e n t r i c a l l y loaded prism should be i n c o r p o r a t e d i n any prism s t a n d a r d t o be used f o r new t y p e s and s h a p e s of u n i t . The l o a d s h o u l d be

e c c e n t r i c a t t h e t o p ( s a y , t / 6 ) and a x i a l a t t h e bottom.

For load-bearing masonry b u i l d i n g s , e s p e c i a l l y t h o s e u s i n g s o l i d

masonry u n i t s , it i s important t h a t t h e r e be i n s p e c t i o n t o e n s u r e f u l l m o r t a r j o i n t s i f s t r e s s e s a r e based on t a b u l a r v a l u e s o r on prisms w i t h f u l l mortar j o i n t s .

F a c t o r s t h a t need f u r t h e r i n v e s t i g a t i o n i n c l u d e :

1) The e f f e c t of humidity on t h e s t r e n g t h of c o n c r e t e b l o c k s : i n c r e a s e d humidity can reduce s t r e n g t h .

2) The s t r e n g t h of e c c e n t r i c a l l y - l o a d e d , hollow blockwork w i t h m o r t a r on t h e f a c e s h e l l s only: l o a d s may be much lower t h a n expected i f t h e e c c e n t r i c i t y i s d i f f e r e n t t o p and bottom.

3 ) The e x p e r i m e n t a l e l a s t i c modulus f o r masonry u s i n g c l a y u n i t s i s lower t h a n t h e recommended v a l u e i n t h e Canadian s t a n d a r d : a v a l u e s u c h a s 700 f k may be more a p p r o p r i a t e .

4 ) The s l e n d e r n e s s r e d u c t i o n f a c t o r s f o r low s l e n d e r n e s s r a t i o s may need t o be changed ( w a l l s l e s s t h a n s t o r e y h e i g h t ) .

ACKNOWLEDGEMENT

T h i s paper i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, National Research Council of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e Division.

APPENDIX 1

-

REFERENCES

1. ASTM C109-77, Compressive s t r e n g t h of h y d r a u l i c cement m o r t a r s , 1977.

2.

ASTM

C140-75, Sampling and t e s t i n g c o n c r e t e masonry u n i t s , 1975.

3. Anderson, G.W., Small specimens of brickwork a s d e s i g n and c o n s t r u c t i o n c r i t e r i a , Department of Works, Commonwealth

Experimental B u i l d i n g S t a t i o n , A u s t r a l i a , S p e c i a l Report No. 24, 1970.

B r i t i s h S t a n d a r d s I n s t i t u t i o n , BS6073: P a r t 1: 1981, S p e c i f i c a t i o n f o r p r e c a s t c o n c r e t e masonry u n i t s , 1981.

Canadian S t a n d a r d s A s s o c i a t i o n , CAN3-S304-M84, Masonry d e s i g n f o r b u i l d i n g s , 1984.

Canadian S t a n d a r d s A s s o c i a t i o n , CAN3-A369.1-M84, Method of t e s t f o r compressive s t r e n g t h of masonry p r i s m s , 1984.

Canadian S t a n d a r d s A s s o c i a t i o n , CAN3-A82.2-M78, Method of sampling and t e s t i n g b r i c k , 1978.

Maurenbrecher, A.H.P., E f f e c t of t e s t procedures on compressive s t r e n g t h of masonry p r i s m s , Proceedings, Second Canadian Masonry Symposium, Ottawa, 1980, pp. 119-132.

S e l f , M.W., S t r u c t u r a l p r o p e r t i e s of load-bearing c o n c r e t e masonry, S M a s o n r y : P a s t and P r e s e n t , ASTM, STP 589, 1975, pp. 233-254.

Standards A s s o c i a t i o n of A u s t r a l i a , AS1640-1974, SAA Brickwork Code, 1974.

S t r u c t u r a l Clay Products I n s t i t u t e (now Brick I n s t i t u t e of America), Recommended p r a c t i c e f o r e n g i n e e r e d b r i c k masonry, McLean, VA, 1969.

S t r u c t u r a l Clay P r o d u c t s Research Foundation (now BIA),

Compressive, t r a n s v e r s e and r a c k i n g s t r e n g t h t e s t s of four-inch b r i c k w a l l s , SCPRF, Geneva, Ill., Research Report No. 9, 1965. S t r u c t u r a l Clay Products Research Foundation, Compressive, t r a n s v e r s e and s h e a r s t r e n g t h t e s t s of s i x and e i g h t - i n c h

s i n g l e - w y t h e w a l l s b u i l t w i t h s o l i d and heavy-duty hollow c l a y masonry u n i t s , SCPRF, Geneva, Ill., Research Report No. 16, 1969.

APPENDIX 2

-

NOTATION Afs = f a c e - s h e l l a r e a Ag = g r o s s a r e a Amb = mortar-bedded a r e a = n e t a r e a Em = mean i n i t i a l e l a s t i c modulus f m = mean compressive s t r e n g t h f k = c h a r a c t e r i s t i c compressive s t r e n g t h = f m ( l

-

1.5 v ) h = h e i g h t n = number of r e p l i c a t e s t = t h i c k n e s s v = c o e f f i c i e n t of v a r i a t i o n APPENDIX 3

-

TABLES TABLE 1

-

Brick High S t r e n g t h Low S t r e n g t h

n

= 30 n = 48 P r o p e r t y Mass

,

kg Density

,

kg /m3 24-h w a t e r a b s o r p t i o n , %, by w t by v o l IRA, kg/m2 /min Dimensions, mm, l e n g t h h e i g h t t h i c k n e s s P e r c e n t s o l i d Compressive s t r e n g t h , MPa Dry P l a s t e r cap F i b r e board cap Fibreboard cap

Wet Fibreboard cap

( 1 ) n = 42; ( 2 ) n = 82; ( 3 ) n = 30

A l l t e s t s on a i r - d r y b r i c k s s t o r e d i n t h e l a b o r a t o r y o r on w e t b r i c k s t o r e d i n w a t e r f o r 24 h.

IRA based on n e t a r e a . Compressive s t r e n g t h based on g r o s s a r e a of h a l f b r i c k s e 7

TABLE 2

-

Block P r o p e r t y Concrete Clay n = 20 n = 12 Mean v ( % > Mean v ( % ) Mass, kg Density, kg/m3 24-h w a t e r a b s o r p t i o n , %, by w t by v o l IRA, kg/m2 /min Dimensions, mm, l e n g t h h e i g h t t h i c k n e s s P e r c e n t s o l i d Compressive s t r e n g t h , MPa Dry P l a s t e r cap F i b r e b o a r d cap F i b r e b o a r d cap F i b r e b o a r d c a p F i b r e b o a r d cap

Wet Fibreboard cap

( 1 ) A l l t e s t s on a i r - d r y b l o c k s s t o r e d i n t h e l a b o r a t o r y o r on wet b l o c k s s t o r e d i n w a t e r f o r 24 h. Compressive s t r e n g t h of whole c o n c r e t e b l o c k s and h a l f c l a y b l o c k s based on n e t a r e a ( p e r c e n t s o l i d ) , 2 , Where two v a l u e s a r e shown t h e y a r e based on a n

a v e r a g e of t e n b l o c k s t e s t e d a month b e f o r e t h e s t a r t of t h e w a l l

t e s t s and two months a f t e r completion. ( 2 ) End b l o c k s , An/A = 0.62.

( 3 ) Face-shell a r e a f32-mm wide f i b r e b o a r d s t r i p s ) . Afs/Ag = 0.43 (average c o n t a c t a r e a t o p and bottom).

( 4 ) 40 b l o c k s t e s t e d on t h e same d a t e s a s t h e prisms and w a l l s . ( 5 ) Face-shell a r e a (291nm wide f i b r e b o a r d s t r i p s on a f u l l ,

plaster-capped b l o c k ) ,

AFS /A

= 0.41. g

TABLE 3

-

Clay Brickwork

Wall Prisms Brick

15x3( 1 ) . . 7x1.5 7x1 4x1 4x1 7x1 1x0.5 (7d) ( j i g ) ( p l a s t e r cap) Low-strength b r i c k f MPa 12.5 - 15.3 v?;) 13 9 n 12 12 h / t 11.2 5.2 High-strength b r i c k f MPa 20.3 27.1 v?; ) 8.2 17 n 12 12 h / t 10.8 5.1 28 d ( + I d ) t e s t u n l e s s o t h e r w i s e n o t e d ; s t r e s s based on g r o s s a r e a . ( 1 ) Height x l e n g t h i n terms of c o u r s e s and b r i c k l e n g t h s .

( 2 ) L i n e a r s l o p e of t h e s t r e s s - s t r a i n c u r v e from t h e o r i g i n t o approximately 20% of t h e f a i l u r e s t r e s s .

( 3 ) n = 11.

( 4 ) F a u l t i n s t r e s s - s t r a i n c u r v e s (Fig. 5).

TABLE 4

-

Concrete Blockwork

Wall 7x3(2) Prisms 2 x 1 Block

28 d 28 d 7 d 1 x 1

(mas on) (mason) ( j i g ) (mason) ( j i g ) ( p l a s t e r c a p )

f m , MPa ( 1 ) 13.9 v

( % I

4.9

n 10

h / t 10.1

( 1 ) Net a r e a f o r b l o c k and mortar-bedded a r e a f o r w a l l s and prisms.

h b / A = 0.42 ( j i g - b u i l t ) and 0.46 (mason-built).

( 2 ) I-Ieighf x l e n g t h i n terms of c o u r s e s and b l o c k l e n g t h .

( 3 ) Assumed f o r p l a s t e r capping = 1.18 x 17.4 ( f i b r e b o a r d capping). ( 4 ) L i n e a r f i t of t h e s t r e s s - s t r a i n c u r v e from t h e o r i g i n t o

TABLE 5

-

Clay Blockwork

Wall Prisms Block

15x30

aso on

J i g 1x0.5 5x 1 5x 1 4 ~ 1 ( 6 ) ( P l a s t e r cap) f MPa (1) 27.7(7) 32.7 38.9 44.9 92.4 v s )

-

13 7.8 13 9.3 n 3 12 12 6 12 h / t 10.8 3.5 3.5 2.8 0.64 E GPa (5) 15.7 19.1(3) 19.9(4) 20.7

-

v?;)

-

11 4.8 3.9 Em/f k

-

7 25 580 570

-

(1) Net a r e a f o r b l o c k and mortar-bedded a r e a f o r prisms and walls.

Amb/Ag = 0.41. Wall t e s t s (2 a t 28 d; 1 a t 56 d ) ; 5 x 1 prism t e s t s ( 8 a t 28 d; 4 a t 56 d ) ; 4 x 1 prism t e s t s (28 d ) .

(2) Height x t h i c k n e s s i n terms of c o u r s e s and b l o c k l e n g t h s . ( 3 ) n = 10; two v a l u e s d e l e t e d because of i n i t i a l concave curve. (4) n = 9; t h r e e v a l u e s d e l e t e d because of i n i t i a l concave curve. (5) L i n e a r f i t o f t h e s t r e k s - s t r a i n curve from t h e o r i g i n t o

approximately 20% of t h e f a i l u r e s t r e s s . ( 6 ) B u i l t a f t e r t h e w a l l t e s t s .

(7) Maximum 30.3, minimum 24.8

TABLE 6

-

Tabular v s Experimental Values

S t r e n g t h , ( f k ) MPa R a t i o

U n i t Table Prism Prism/Tabular

Mason J i g Mason J i g

-

B r i c k High-strength 101 25(1) 20.2(2) 40.0 0.81 1.60 Low-strength 21.4 8.1 13.2 15.9 1.63 1.96 Block Concrete 17.1 10.1 13.8 13.1 1.37 1.30 Clay 79.5 22.8 24.3(3) 31.8(3) 1.07 1.40 (1) For b r i c k s t r e n g t h

>

90 MPa. (2) Running bond prisms.

T h i s , p a p e r , w h i l e being d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be reproduced i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o ,

KlA

0R6.

C e document est d i s t r i b u g s o u s forme de tirC-3-part par l a D i v i s i o n d e s r e c h e r c h e s e n b l t i m e n t . L e s d r o i t s d e r e p r o d u c t i o n s o n t t o u t e f o i s l a p r o p r i C t Q de 1 ' C d i t e u r o r i g i n a l . C e document n e p e u t S t r e r e p r o d u i t en t o t a l i t 6 ou en p a r t i e s a n s l e consentement de l ' d d i t e u r . Une l i s t e des p u b l i c a t i o n s de l a D i v i s i o n p e u t Ctre obtenue en Q c r i v a n t

a

l a S e c t i o n d e s p u b l i c a t i o n s , D i v i s i o n d e s r e c h e r c h e s en b l t i m e n t , C o n s e i l n a t i o n a l de r e c h e r c h e s Canada, Ottawa, O n t a r i o ,

K1A

OR6.