Efflorescence on the exterior surface of masonry walls: a review of the literature

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Efflorescence on the exterior surface of masonry walls: a review of the literature

Ser

THI.

392

no. 8

c . 2 _{EFFLORESCENCE}

_ON

_{THX EXTERIOR}

_SURFACE

_OF

MASONRY IVALLS

( A review of t h e literature) by

Abstract

The t w o

general

conditions necessary

to

produce efflorescence in masonry w a l l s a r e

( a ) presence of s o l u b l e salts in the w a l l mate- rials and ( b ) moisture to c a r r y these s a l t s ts- wards a r onto &he s u r f a c e of _{t h e wall.}

No s i m p l e method, capable o f general appli-

c a t i o n ,

h a s been devised to l i m i t the soluble

salt

c o n t e n t

in

masonry materials. Hardly any masonry material is exempt f r o m t h e possibility

of

c o n t a i n i n g

sufficient

soluble s a l t s to

con-

t r i b u t e to t h e development of noticeable ef-

florescence if moisture

should

p e n e t r a t e t h e

w a l l .

Moisture p e n e t r a t i o n of t h e w a l l , and with it, efflorescence, nay be almost completely pre-

vented by p r o p e r d e s i g n , construction, and main-

tenance of the building-

7 11.' 3 . 3 ~ ; -

-

EFFLORESCZ$iCE

OM

THS EXTZRIOR SURFACE OF &tASQNRY =-- - .

.-.

WALLS

men moisture g a i n s a c c e s s to masonry walls, the water-soluble s a l t s present in the masonry m a t e r i a l s a r e taken

i n t o solution, a n d as soon as t h e weather becomes favourable

f o r evaporation of this accumulated moisture, t h e s a l t s a r e c a r r i e d in solution t o w a r d the outer f a c e of t h e wall. The concentration of the salts in solution increase w i t h c o n t i n u e d

e v a p o r a t i o n ,

until

the s a l t s f i n a l l y crystallize out, forming

a

whitish

d e p o s i t which is termed

efflorescence.

The p o i n t at which c r y s t a l l i z a t i o n o c c u r s will depend l a r g e l y on the c a p i l l a r y properties of the m a t e r i a l s ,

It

may be in a l a y e r just behind the surface, o r directly on t h e s u r - face.

In

t h e former c a s e , the crystallization may l e a d to d i s r u p t i o n and s p a l l i n s of the surface l a y e r ; in the latter case, i t p r o v i d e s disfigurement.

While

efflorescence is most common in the e a r l y s p r i n g when walls s a t u r a t e d by m e l t i n ? snow or l a t e w i n t e r r a i n s are d r y i n g out, it may occur a t any time o f t h e y e a r . It may a p p e a r q u i t e g e n e r a l l y over a wall

ar

b e c o n f i n e d to certain small areas.

The two g e n e r a l c o n d i t i o n s necessary t o produce ef- florescence, in a l l i n s t a n c e s , are:

a. Soluble s a l t s present in t h e w a l l materials. b . Moisture to c a r r y these s a l t s towards o r o n t o

the wall s u r f a c e . 2 . Nature of t h e Soluble S a l t s

Some o f t h e s a l t s frequently found in efflorescence are calcium sulphate (gypsum), magnesium sulphate (epsom s a l t s ) ,

sodium c h l o r i d e (table salt), sodium sulphate and potassium

s u l p h a t e . These a r e a l l

soluble

i n

w a t e r , though calcium sul- phate is considerably less soluble than t h e others a n d is l e s s

likely

on

this

account

to

form a

major

constituent

of e f f l o - r e s c e n c e ,

In

cases where serious

failure

o f brickwork by salG

a c t i o n

have

been r e p o r t e d at t h e

British

Building Research

S t a t i o n , it h a s n e a r l y always been found that magnesium s u l p h a t e

w a s t h e destructive agent.

The p r e s e n c e of n i t r a t e s and chlorides is usually an i n d i c a t i o n of contamination

from

some external source, such a s ground-water

or

material stored in contact with the a f f e c t e d masonry.

3 .

Origin of t h e S a l t s

-

H a r d l y a n y masonry material is exempt from t h e pos-

s i b i l i t y of c o n t a i n i r ~ g sufficient soluble s a l t s to contribute to the development of noticeable efflorescence

if

moisture

p e n e t r a t e s the wall. The

usual

sources of soluble s a l t s a r e :

( a ) F a c i n g

or

back-up m a t e r i a l , such as b u i l d i n g l i m e s t o n e s , b r i c k s , hollow tile, c o n c r e t e units,

or monolithic concrete.

( $ 1

Mortars and p l a s t e r s .

In

this case, t h e s a l t s

may'be due to one

or more

of the ingredients.

Portland

cements, certain sands,-and certain limes contain

soluble salts;

and in some in-

s t a n c e s the mixing water

for

t h e mortar o r

plaster may contain

soluble

salts.

( c ) Ground-water or some other e x t e r n a l source,

S a l t s a r e introduced into t h e cement f r o m b o t h

the

r a w materials and the coal a s h . Some o f t h i s salt c o n t e n t may b e volatilized in the k i l n and c a r r i e d away in t h e dust, t h e

amount so removed i n c r e a s i n g with harder burning.

S a l t s p r e s e n t in the bricks may o r i . g i n a l l y have been present in t h e c l a y

or

tempering water. They may have been produced during the manufacturing process by t h e decomposition of compounds present

in

t h e c l a y

or

by t h e a c t i o n of s u l p h u r gases in t h e kiln. The c o n t e n t

of

soluble s a l t s in bricks v a r i e s g r e a t l y , although t h e actual q u a n t i t y of soluble s a l t s

is very m a l l , even in bricks which may produce efflorescence.

Foreover, the effect of soluble

salt

content in building mate-

rials

is _{so intimately bound up with t h e capillary properties}

and pore structures that a material should n o t be condemned on its salt c o n t e n t a l o n e , with t h e possible exception of the

Even if the f a c i n g b r i c k is carefully chosen, s a l t s c o n t a i n e d in t h e back-up units

or

in t h e mortar used f o r joint-

i n g a n d pointing may b e transferred t o facings by t h e movement

of water in t h e walls. T h i s m i g h t l e a d a c a s u a l observer to t h e e r r o n e o u s conclusion that the facing bricks were d i r e c t l y responsible, whereas they may a c t u a l l y

have

been almost f r e e

from s o l u b l e

s a l t s

o r i g i n a l l y .

In

t h i s connection, it is interesting to note t h a t t e s t s at t h e U.S. aureau of Standards on

55

types o f f a c e b r i c k

from

a s many d i f f e r e n t manufacturers in various p a r t s of t h e United S t a t e s were examined f o r s o l u b l e s a l t s , and t h e majority were found to c o n t a i n less than 0,05 p e r cent of soluble s u l - phuric a n h y d r i d e , a n amount insufficient t o cause efflorescence under c o n d i t i o n s favourable f o r such. It would seem t h e r e f o r e , that t h e face-brick manufacturers have reduced the tendency of

t h e i r p r o d u c t to e f f l o r e s c e to a c o n s i d e r a b l e e x t e n t .

It

is

claimed by t h e Structural Clay P r o d u c t s

Insti-

tute t h a t p r o b a b l y more than 90 per c e n t of t h e o u t p u t of

structural clay products in this country w i l l nut tend to ef- floresce. They p o i n t o u t , however, that second-hand brick,' because of its uncertain o r i g i n and p r e v i o u s c o n t a c t w i t h

m o r t a r and p l a s t e r , may f r e q u e n t l y be a source of efflorescence.

4

Methods of Noisture P e n e t r a t i o n

Moisture may come in c o n t a c t w i t h walls f r o m d r i v i n g r a i n s , melting snow, or ground-water, and may p e n e t r a t e the walls as a result of the following:

(a)

Insufficient a t t e n t i o n of t h e architect to p r e -

c a u t i o n a r y d e t a i l

i n

d e s i g n i n g .

( b j Poor workmanship and materials used during the

construction period.

[ c ) Improper maintenance or o p e r a t i o n of the corn- pleted b u i l d i n g .

The U . S . Bureau of S t a n d a r d s has made a d e t a i l e d s t u d y of 250 cases of efflorescence and w a l l d i s i n t e g r a t i o n , and concluded that a l l b u t ohe

or

two of the cases c o u l d have b e e n prevented by either proper-design, construction, or

maintenance.

In

Part 7 1

of

t h i s paper, some of t h e g e n e r a l l y

re-

c o g n i z e d methods o f preventing moisture penetration into t h e walls will b e d i s c u s s e d .

It

should be n o t e d a t

this

p o f n t , however,

that

In

t h e ease of

a

new buPlding, a uniform e o a t f ng of _{deposf t e d s a l t s} may appear,

even

when

all

reasanable pracautfons have been

taken. Thf s c o n d i t f

on

is the r e s u l t of the v e r y l a r g e quantf ti e s

of _{water used}

_in

_{t h e}

_construction

_procezs,

_After

_{t h e walls}

_have

had an opportunfty to dry thoroughly, a f h a l cleaning or some- times n f e w raFns w P l l wash away the efflorescence a n d it

se Edom appears agaf

n,

The

.vfew fs held by some that new brickwork is partl- c u l a r l y liable to damp p e n e t r a t i o n troubles, and t h a t the n a m ~ l progress of _{weathering dfmfnfshes t h e permeability of the w a l L} by a process of s5ltfng up p o r e s by d P r t and v e g e t a t f o n ,

Why Eff

loreacence

Should be Prevented

Efflorescence should be prevented because of I t s un- sfghtliness and because crystallization

i n

a layer gust behfnd

t h e wall s u r f a c e may cause the smface Eager of t h e masomy

to

spall,

Another, and even more aarfous a s p e c t of _{t h e sftuatlon,}

is

t h a t efflorescence, e x c e p t

Pn

t h e case of

a

new bufldlng ad

noted above, 1s a symptom of m o i s t w e p e n e t r a t i o n into ths WELS,

Wet walls r e s u l t Xn decreased fnsulatfon value of the wall and

In damp f n t e r i o r s , and may e ~ e n t a a l l y lead t o partfal

or

eom-

pEete dfsintegratfon o f the wall,

It

i s essential t h a t e x c e s - sf

vs mot

s t w e penetrat9on be p ~ e v e n t e d , and if thiw f s acrlfeved,

then at t h e same t h e the passibll9ty of ,efflorescence o c c w r l n g

will have been almost completely elfmLnatedo

BART

11

-

PREVENTTON OF EFFLORESCENCE 1 Contrpl- by- Manufacturers of Masonry Material#

The manufacturenaa af masonry materials have not y e t

suoceedsd f n devjlsfng any simple method, capable of general, ap- p l i c a t l o n , to llrnf t; the _{soluble s a E t s}

In

_{t h e f r products,}

'Numerous attempts have been made to develop admixtures whfch w u l d combine wfth t h e solwble s a l t s p r e s e n t t o form InsoIubB

compounds, BarSwn carbonate f s one such admfxture Ghf ch has been used and is

stflE

used; however, fnvestfgations to date fndfcata t h a t admfxt;ures a r e n o t completely effectfve f n r e -

By

exercising c l o s e control

of

t h e d r y i n g and

burning

operations, t h e manufacturers of face-brick have done

much

to

r e d u c e

the

possibility

of efflorescence

i n

t h e i r products. F o r

example, c e r t a i n s a l t s t e n d to be decomposed a n d e x p e l l e d from

bricks by hard f i r i n g , and

in

a kiln where c l o s e temperature

control is n o t m a i n t a i n e d , t h e r e mag often be c o n s i d e r a b l e v a r i a t i o n s in t e m p e r a t u r e , r e s u l t i n g in a h i g h e r concentration

of s a l t s in those b r i c k s coming from t h e c o o l e r p a r t

sf

t h e kiln. The r e l a t i v e l y c l o s e c o n t r o l s maintained by t h e face- b r i c k industry in p a r t j u s t i f y

the

c o s t d i f f e r e n c e between

face-brick and the cheaper clay products, s u c h a s hollow tile

a n d common b r i c k , w h i c h a r e generally u s e d as b a c k i n g materials,

From a n economic s t a n d p o i n t , it is n o t always p r a c t i c a b l e f o r t h e manufacturers of t h e s e c h e a p e r p r o d u c t s to adept the pro- cedures used by t h e face-brick industry.

A t p r e s e n t , the surest preventative of efflorescence

is to keep water o u t o f the masonry. T h i s f a c t o r m u s t be con-

trolled by the architect in d e s i g n i n g t h e building, by t h e can-

t r a c t o r in e r e c t i n g t h e building, a n d by t h e owner in maintaining

and o p e r a t i n 5 t h e b u i l d i n g .

Keeping IYalls Dpy by Proper De-sJgn-

-

and-

- ,Const.ru,ctiqn.

Present knowledge of b u i l d i n g construction h a s n o t

r e a c h e d t h e stage xihere t h e problem o f k e e p i n g the w a l l s d r y can be s a i d to be on a satisfactory d e s i g n b a s i s . The problem is complicated by the fact t h a t t h e a p p r o a c h must b e d i f f e r e n t

f o r d i f f e r e n t farms of construction. F a r example, solid walls of dense, n o n - a b s o r p t i v e m a t e r i a l , such a s concrete w a l l i n g , r e q u i r e a different t e c h n i q u e t h a n walls of porous, absorptive m a t e r i a l s , such as b r i c k .

Some of the d e t a i l s which a r e i m p o r t a n t in t h e design and c o n s t r u c t i o n o f masonry walls in general if w a l l s a r e to be k e p t d r y a r e listed below.

F o u n d a t i o n s

( a ) I n s t a l l a d e q u a t e d r a i n s a l o n g f o o t i n g s ,

( b ) Provide membrane waterproofing on t h e outside of foundation walls.

( c ) I n s t a l l damp-proofing in a masonry course im- m e d i a t e l y above g r a d e .

Walls

Above Foundation?

( a )

Start

the

ftrst

c o u r s e of

masonry

units

well

away

from contact with t h e e a r t h and extend r a i n water

l e a d e r s t o the ground to a v o i d s p l a s h i n g the brickwork,

( b ) Protect w a l l s under construction to p r e v e n t rain or melting snow from entering, When masons a r e not w o r k i n g , cover the t o p of the w a l l s with

weighted tarpaulins, overhanging one or t w o feet on each side.

Bricks

stored on t h e j o b should

a l s o

be k e p t d r y .

Ec)

A m o r t a r p o s s e s s i n g a high degree of w o r k a b i l i t y

and water retentiveness should be u s e d , S l a k e d quicklime putty a n d a number sf hydrated l i m e s

have

such characteristics and

will

assure a s a t i s -

factory mortar if used in quantities up t o 2 p a r t s lime s o l i d s to

I

of cement. Workability will be improved by the use of well-graded, clean sand w i t h a f i n e n e s s modulus

sf

about 2.2. The m o r t a r should be a c c u r a t e l y p r o p o r t i o n e d and be mixed f o r at least 2 minutes in a mechanical b a t c h mixer. Pjiortar t h a t is between

5

and 2

hours o l d w i l l have the most p l a s t i c c h a r a c t e r i s - t i c s .

A l l j o i n t s in masonry w a l l s s h o u l d be f i l - l e d . Joints should

be

concave t o o l e d , first while q u i t e s o f t and a second t i n e when t h e m o r t a r

starts to stiff en.

( d )

If

possible, a b r i c k w i t h a 24-hour cold a b s o r p - tion of between

4

and

LO

per

c e n t

should be

s e l e c t e d . Although moisture-proof w a l l s can b e built w i t h b r i c k s h a v i n g

an

a b s o r p t i o n lower t h a n

4

p e r c e n t or h i g h e r than 10 p e r c e n t ,

t h e h a z a r d s are i n c r e a s e d , and extra precautions must be

taken,

( e )

It

i s p r e f e r a b l e t h a t bricks be kept and laid dry,

but if b r i c k s are used with

an

a b s o r p t i o n h i g h e r than 1 0 p e r c e n t , they s h o u l d be wetted. T h i s moisture may c a r r y s o l u b l e salts t o the surface

of

t h e masonry and t h e r e b y contribute to efflo- rescence; but w e t t i n g the bricks will prevent them

from absorbing excessive moisture from t h e mor- t a r , which would have t h e more

serious

consequence

of reducing the ultimate w a t e r - t i g h t n e s s of t h e w a l l .

If

it i s necessary t o wet t h e bricks, t h e y should

be w e t t e d as uniformly as possible 2 4 to

4€!

hours

in advance of the time needed f o r use, and t h e pile s h o u l d be k e p t covered with a t a r p a u l i n to

prevent l o s s of moisture.

B r i c k s should never be w e t t e d in f r e e z i n g weather.

[f]

On some building projects it may be c o n s i d e r e d

d e s i r a b l e to make a chemical analysis of t h e b r i c k ,

L . A . P a l m e r s t a t e s in Technological P a p e r No. 370

of the

U.

3 . Bureau o f Standards that a

brick

c o n t a i n i n g

Q 0 0 5

per c e n t or less of water-soluble sulphuric anhydride in its o u t e r

l j 8

i n c h of ex-

posed surface, is not a p t to contribute to efflo-

rescence.

A t e s t which is mare r e a d i l y performed t h a n chemical analysis is t h e wick t e s t , o r i g i n a l l y d e s c r i b e d in Research Paper iqo, 1015

of

the

U.

3 . K a t i o n a l Bureau of S t a n d a r d s by Messrs. McBurney and P a r s o n s . A summary of this test is included %n Appendix 11 of the Tentative Nethod o f Test

f o r S u c t i o n and Efflorescence of

Brick,

A . S , T . 2 < , Designation

C67-42T.

( g )

Avoid

forming a water t a b l e e f projecting bricks,

To

c o r b e l out w i t h one o r more c o u r s e s of b r i c k

or

any o t h e r

masonry

material i s always a danger-

ous procedure unless the projecting eouYses are so situated as to' be protected f r o m exposure to moisture p e n e t r a t i o n .

(h) Provide adequate drips on cornices, chinney c a p s , sills, and other masonry projections. G ~ o o v e s s h o u l d b e at l e a s t

r/$

inches w i d e and

3/$

inches deep, a n d projections s h o u l d be

at

l e a s t 2 inches to keep the d r i p away from t h e w a l l .

(i)

Non-corrosive metal o r bituminous f l a s h i n g s h o u l d b e placed under cornices, chimney c a p s , s i l l s and

( j ) Caulk around v5ndows and door frames.

(k)

Use l u g s i l l s at windows a n d wherever p r a c t i c a l a v o i d joints in s i l l s .

(1) Flash the washes o f pediments and dormers.

( m )

When h i g h humidities may be present w i t h i n masonry buildings careful consideration should b e given to the c o n t r o l of water vapour flow in o r d e r that

c o n d e n s a t i o n may b e prevented

i n

c o l d weather,

C o n d e n s a t i o n may be p r e v e n t e d by t h e c a r e f u l

s e l e c t i o n and a p p l i c a t i o n of vapour b a r r i e r s , a n d where p o s s i b l e by the provision sf v e n t i l a t i o n

to remove vapour-laden air, Parapets

( a ) The u s e of p a r a p e t walls s h o u l d b e a v o i d e d when- ever possible,

If

they must be used, t h e y s h o u l d be c o n s t r u c t e d 1 6 t h g r e a t care, u s i n g goad q u a l i t y

materials, F l a s h i n g s h o u l d cover t h e back side of p a r a p e t

walls

but it should be free-standing,

in o r d e r that there may b e circulat5on of air in t h e space between f l a s h i n g and

wall,

(b) A c o p i n g of low permeability

and

good frost r c s i s - t a n c e s h o u l d be used, and this coping should have

a

t h r o u g h f l a s h i n g under it extending at l e a s t

E

inch

past the f a c e of t h e w a l l on efther s i d e .

3 .

Keeping Walls .

-

D r y -. by-_Careful Mainteng>,"c-ce.

Walls

my be p r o p e r l y built, b u t t h e y must also be

properly maintained t a s t a y dry.

It

is especially i m p o r t a n t that

f l a s h i n g s , g u t t e r s , dovimspouts and copings be k e p t in good s t a t e of repair and t h a t c r a c k e d o r p o o r l y f i l l e d joints be promptly raked out and r e p o i n t e d .

When efflorescence a p p e a r s on t h e walls af

a

b u i l d i n g that has been in service f o r

several

y e a r s , it is _{evidence t h a t}

m o i s t u r e is p e n e t r a t i n g the

walls.

The

source

o f p e n e t r a t i o n

'?The l o c a t i o n o f efflorescence does n o t necessarily

mean

t h a t w a t e r i s e n t e r i n g t h e w a l l at t h a t p o i n t , but f r e - q u e n t l y provides a c l u e to t h e

source of

t h e

trouble,

F o r

example, s t r e a k i n g from t h e top of a

wall

down

o r ,

a t

times,

p a t c h e s some d i s t a n c e from t h e t o p , would p o i n t

t o

d e f e c t i v e c o p i n g s

or

g u t t e r s .

Efflorescence

u n d e r windows would p u t t h e s i l l s a n d caulking around the frame u n d e r s u s p i c i o n . E f f l o -

r e s c e n c e on the courses of a f o u n d a t i o n , c l o s e t o t h e ground, e s p e c i a l l y w i t h a r a t h e r p o r o u s brick, would i n d i c a t e ground

water drawn up by c a p i l l a r y suction. A s i n g l e p a t c h o f ef- florescence on a w a l l , w i t h no o b s e r v a b l e r e l a t i o n to masonry

openings, may sometimes b e e x p l a i n e d on the grounds of a b a d l y

defective m o r t a r j o i n t

or

b y a projecting brick forming

a

w a t e r

t a b l e S t .R

If

a s e r i o u s defect

has

been built into t h e

wall,

it w i l l o f t e n be d i f f i c u l t to remedy t h e condition permanently by a s u r f a c e r e p a i r . An error o f omission

can

be e q u a l l y serious and d i f f i c u l t to remedy. k damp-proof membrane, f o r example,

is p r a c t i c a l l y impossible t o i n s e r t after the wall h a s been

b u i l t .

PART 111- RBIQVAL

OF

EFFLORZSCEIiCE

When efflorescence has resulted from excessive s o a k i n g during csnstructfon, a n d t h e w a l l has f i n a l l y dried out and r e p e l s m o i s t u r e successfully, then t h e efflorescence will g r a d u a l l y b e washed away by t h e r a i n s a n d disappear wi-t;h t i m e . T h i s process may be h a s t e n e d by washing away w i t h water applied w i t h stiff

scrubbing b r u s h e s ; o r , if t h i s is n o t e f f e c t i v e , wet t h e wall with

water,

t h e n scrub w i t h water c o n t a i n i n q not more t h a n

10

p e r cent

of

muriatic (hgdrochlaric)

a c i d and, immediately t h e r e a f t e r , r i n s e thoroughly w i t h p l a i n water.

If

muriatic a c i d is used, it

i s

particularly i m p o r t a n t that t h e w a l l be wetted w i t h water first,

and it is i m p o r t a n t also that t h e solution s h o u l d not c o n t a i n

more

than

10 p e r c e n t rnuriat5c a c i d .

Obviously,

if

moisture is p e n e t r a t i n g a wall due to f a u l t y d e s i g n , c o n s t r m c t i o n , or maintenance, then t h e measures

described above

hi11

b e useless, inasmuch a s t h e efflorescence w i l l reappear. The underlying d e f e c t must first be remedied in order to f i n a l l y b e r i d of t h e efflorescence,

- , .--.

-

.-.. .

.

A - 2 . * L . . L. -+-&--

- - - -

-

-

A -- + - -

R "Brick E n g i n e e r i n g , Handbook o f DesignTt,

H.

C . Plwnmer and L , J ,

Reardon;

Structural

Clay P r o d u c t s Institute, h'ashington, D , C ,

1943.

P.

171.

REFERENCES

(1)

"Principles

o f Modern

Buildingw,

Volume

1,

R . F i t z r n a u r i c e ;

H A ,

Stationery Office,

London,

1939.

₇

(2)

17Wet

Walls

and Efflorescence", A

.I

. A .

File No.

5m.

-

( d -

+-4

L . A , Palmer; American Pace Brick A s s o c i a t i o n , ,

Chicago, 1928,

-

. . , 1 LL r z ~ ~ n ~ r .

-"sL1 L ~ L -

LC-4

e.d' 8 ~ i * .

( 3

) "Brick E n g i n e e r i n g , Randbook o f Designt',

A . C . B l m e r and

L. J.

R e a r d o n ; Structural Clay Products Pnstitute, Washington,

D.

C., 1943.

( 4 )

"The Chemistry o f Cement and Concrete",

I?.

E,

Lea and C. H. Oesch; Edward

Arnold and

Co.,

London, 193

5

.

(

5

) "Semi-Fireproof C o n s t r u c t i o n t 1

H.

R-. S t a l e y ;

D. Van

Aostrand Company, I n c . , Mew York,

1 9 4 g .

( 6 )

t l F a c t o r s in

t h e

Resistance of B r i c k i4asonry l i a l l s to Tkyoisture PenetrationK, C. C.

Connor;

A,S.T.fi, P r o c e e d i n g s , V a l .

4G,

1948.

4

{a

b tYk&&h f++

1

C F j - k d i g - L k

f

h c ( a . tx

unk-I-.