Factors affecting frost damage to clay bricks

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Factors affecting frost damage to clay bricks

I

T. R i t c h i e

Division of B u i l d i n g Research,

Ottawa, February 1968

FACTORS

AFFECTING FROST DAMAGE

T O

CLAY BRICKS

T , Ritchie

T o study the effect on clay bricks of f r e e z i n g and t h a w i n g them, a number of small samples w e r e tested under several different; f r e e z i n g conditions, Most of the samples had been used originally in other w o r k ,

such a s a study of t h e d i r e c t i o n a l effect on m o i s t u r e expansion of b r i c k s ,

and when t h e original work w a s completed, additional t e s t s w e r e m a d e

to investigate the influence of various factors on t h e damage d o n e t o the

bricks by freezing them,

The samples w e r e one i n c h square in cross section; the length

(corresponding to the l e a s t dimension of a brick) was t w o and a quarter inches. Some of the samples w e r e cut from m a n u f a c t u r e d b r i c k s made by the s t i f f -mud method, while o t h e r samples w e r e c u t from similar dry,

u n f i r e d bricks obtained from a brickworks. T h e latter w e r e cut to suitable s i z e and f i r e d in a laboratory furnace. The e n d surfaces of t h e samples w e r e g r o u n d flat, and measurements of length made w i t h a dial gauge accurate to one ten-thousandth of an inch,

The samples w e r e f r o z e n in a large c o l d chamber which operated

at a t e m p e r a t u r e varying f r o m

-15'F

to -20°F; after f r e e z i n g , they

w e r e usually thawed in air at "room1' temper atur e, approximately 7 2" F,

although in some t e s t s the samples w e r e thawed in w a t e r ,

EFFECT

OF

MOISTURE C O N T E N T O N FROST DAMAGE

Eight samples (designated

L)

w e r e cut from a brick a l o n g its length and eight samples (designated

U)

w e r e cut f r o m the same brick

at right a n g l e s to its bedding s u r f a c e . T h e d r y samples w e r e soaked in water for various p e r i o d s of time (0 min.

,

30 min. , 2 h o u r s , a n d

2 4 hours) prior to freezing, in o r d e r to study the effect of moisture content on frost damage.

The

results of

a

s i n g l e f r e e z i n g f o l l o w e d by

t h a w i n g in air a r e s h o w n in Table I ,

In these t e s t s the four samples of highest m o i s t u r e content w e r e

cracked (along t h e length, for about half the length) a s a r csult of t h e fr eezing, while the remaining samples appeared t o be undamaged.

Although d i m e n s i o n a l changes d u r i n g f r e e z i n g and t h a w i n g w e r e n o t

measured, the net dimensional change of the samples d i d n o t appear t o b e related to f r o s t damage; sample

L1,

for e x a n ~ p l e , a p p a r e n t l y undamaged, had

a

n e t expansion of 0.01 4 p e r c e n t , w h i l e that of

sample L 7 , which w a s cracked, w a s 0 , 0 0 5 per cent, Sample L8, a l s o c r a c k e d , had undergone no net dimensional change. In t h e s e

t e s t s the extent of f r o s t damage w a s apparently not affected b y t h e

d i r e c t i o n in w h i c h the samples w e r e cut f r o m t h e original brick.

ADDITIONAL TESTS

OF

THE I N F L U E N C E O F MOISTURE C O N T E N T

ON

FROST DAAdAGE

Twelve samples w e r e c u t f r o m an u n f i r e d brick, then f i r e d i n a laboratory furnace. F o u r of the samples ( d e s i g n a t e d L) w e r e cut

along the length of t h e brick, four athers

( C )

across the b r i c k , and

t h e remainder (U] a t r i g h t angles to t h e bedding surface, Prior to

t h e f r e e z i n g t e s t the s a m p l e s had been autoclaved and then soaked i n

w a t e r . T w o samples f r o m each s e t of four w e r e d r i e d for s e v e r a l

hours before f r e e z i n g , w h i l e the other t w o w e r e a l m o s t c o m p l e t e l y saturated at t h e t i m e af freezing. The results a £ a single freezing

of the samples followed by thawing in w a t e r a r e shown in T a b l e 11.

T h e very considerable influence of m o i s t u r e content on frost r l a n ~ a g e i s c l e a r in t h e s e results. It is a l s o evident that a r e l a t i v e l y

slight c h a n g e in m o i s t u r e content d e t e r m i n e d t h e behaviour of the material. T h e samples that cracked from f r e e z i n g h a d r e l a t i v e l y

l a r g e net expansions. F o u r of the five damaged s a n ~ p l e s w e r e cracked a c r o s s the length w h i l e in the fifth the crack w a s l o n g i t u d i n a l . In

t h e s e t e s t s , as in t h e previous s e r i e s , f r o s t damage did not appear

t o b e influenced by t h e direction in which the s a m p l e s w e r e cut f r o m

the brick,

FURTHER

TESTS ON MOISTURE CONTENT

F i v e samples w e r e cut in the same d i r e c t i o n f r o m each of three

stiff-mud b r i c k s (designated A , B ,

C),

taken from t h e s a m e lot, T h e

samples w e r e soaked in water for G , 5, 30 and 6 0 m i n u t e s , a n d f o r

2 4 h o u r s , then w e r e f r o z e n and t h a w e d in air a t o t a l of eight times,

with the r e s u l t s shown in T a b l e

111.

E F F E C T O F R A T E

QF

F R E E Z I N G ON FROST DAMAGE

S i x samples w e r e cut: f r o m an u n f i r e d brick (at r i g h t angles t o

f o r 24 h o u r s , then frozen, T w o of the samples w e r e exposed d i r e c t l y

t o the c o l d air of the freezing chamber, w h i l e t h e other samples were

surrounded by plastic materials in such a w a y that t h e i r r a t e s a£

freezing w e r e l e s s than that of the uncovered samples. T h e r a t e of freezing w a s determined by e m b e d d i n g a t h e r r r ~ o c o u p l e in a sample

w h i c h w a s frozen; the time takkn for the t e m p e r a t u r e of the sample t o

fall f r o m 38°F to 2 6 - F (six degrees above and below the freezing

point of water) w a s noted and the rate of freezing w a s expressed in d e g r e e s F per minute, F o r the samples placed uncovered in the

cold chamber, t h e r a t e was about 6 d e g r e e s p e r n ~ i n u t e . A 1 samples w e r e left in the chamber until their temperatures equalled the air

temperature of the chamber, then thawed in air, T h e t e s t s , i n w h i c h

the samples w e r e f r o z e n five times, are shown in Table

IV

a n d

V.

The results of these t e s t s clearly s h o w e d that the r a t e of f r e e z i n g

of the samples had an important influence on t h e damage caused to them,

The samples frozen at t h e highest rate w e r e cracked after one and two

fr'rezings; those frozen at t h e intermediate r a t e d i d not crack u n t i l the

fifth freezing, at which point the samples f r o z e n at the l o w e s t r a t e w e r e

apparently undamaged.

ADDITIONAL TESTS ON RATE O F F R E E Z L N G

F o u r samples w e r e cut f r o m each of two e x t r u d e d bricks

( A a n d B ) f r o m the same lot. T h e samples w e r e soaked in w a t e r for 7 2 hours before freezing, w h i c h w a s carried out at various r a t e s by the method d e s c r i b e d pr cviously. The samples w e r e f r o z e n four

times, with the r e s u l t s given in T a b l e

VI.

The results of these t e s t s also clearly i n d i c a t e the important

i n f l u e n c e of rate of f r e e z i n g on the damage s u s t a i n e d by the s a m p l e s .

I N F L U E N C E O F DIRECTION

O F

FREEZING

ON

FROST DA,MAGE T e n samples w e r e cut from an extruded clay brick, soaked in water for 3 6 hours then p l a c e d in tho freezing chamber, some of them

a r r a n g e d as usual so that t h e c o l d air s u r r o u n d e d t h e m on all s i d e s , w h i l e the others w e r e arranged t o have only one end e x p o s e d t o t h e

c o l d a i r , a l l other s u r f a c e s being c o v e r e d by an i n s u l a t i n g m a t e r i a l , The samples w e r e f r o z e n once, w i t h t h e results shown in Table VIT.

Although the results clearly indicate a d i r e c t i o n a l i n f l u e n c e of

d i f f e r e n c e in r a t e of freezing, which w a s not measured or controlled in t h e s e tests, also had an influence. A difference in the rates of

freezing would have r e s u l t e d from the different conditions of exposure, An interesting anornoly in the behaviaur of the four s u r v i v i n g

sarnpkes of the above test w a s noted in a second freezing test, in

w h i c h t w o of t h e samples w e r e frozen omnidirectienally and the other t w o w e r e frozen unidirectionally,

The

former samples w e r e unaffected by this second freezing, while the latter cracked.

EFFECT

OF

FREEZING

CRACKED

SAMPLES

Some samples which h a d cracked in freezing w e r e thawed then f r o z e n again, sometimes several times, with measurements made

of length changes, Usually cracks appeared in t h e samples without their complete breakage and t h e sample could be h a n d l e d and measured.

The crack sometimes extended partly across the sample near the mid- point or ~ a r t l y along its length f r o m one end, but in many cases

cracking o c c u r r e d in both directions near the centre of t h e sample,

giving it the appearance of having burst from within. T w o d i f f e r e n t

behaviour s of c r a ~ k e d samples subsequently r e -frozen a r e shown in

Table VIII,

T h e first sample cracked d u r i n g its second freezing with a net

expansion equal t o that of the other sample, which cracked during its fir st f r eeaing. The behaviour on subsequent fr e e z i n g s d i f f e r e d ; the

fir st sample expanded greatly on the t h i r d freezing and b r o k e after

the fourth, whereas subsequent freezings of the other sample produced

no additional net expansion,

SUMMARY

Several important influences on the damage t o clay bricks from

f r e e z i n g have been demonstrated, in confirmation of similar r e s u l t s d e s c r i b e d in papers f r o m other laboratories which have investigated t h e effects of freezing on masonry materials.

The m o i s t u r e content of a clay brick at the time of f r e e z i n g w a s clearly a major factor in its resistance to damage. A s l i g h t change of moisture content w a s frequently critical to t h e behaviour of the brick. T h e r a t e of t e m p e r a t u r e change of the b r i c k d u r i n g its

f r e e z i n g was also shown to be an important factor, and one in w h i c h

The d i r e c t i o n of f r e e z i n g of a brick, whether cooling w a s from all surfaces or from a single surface, w a s indicated to be important t o t h e p e r f o r m a n c e of the brick.

The net dimensional change of a brick from f r e e z i n g a n d

thawing w a s usually small even i f the brick cracked. Freezing

of a c r a c k e d brick in some c a s e s resulted in no additional expansion

of the material; in others there w a s gross expansion and breakage.

The direction of cutting samples from a brick did not appear to

T A B L E I

EFFECT

O F MOISTURE CONTENT

ON

FROST DAMAGE

M o i s t u r e Content Condition Length C h a n g e ,

Sample ( P e r cent D r y Weight) After F r e e z i n g Per cent*

unchanged unchanged unchanged unchanged unchanged unchanged cracked cracked unchanged unchanged unchanged unchanged unchanged unchanged cracked cracked $0.01 4 0 -0. 005 - 0 , 0 0 5 -0.019 -0. 0 0 5 SO, 0 0 5 0

*

Expansion t; shrinkage

-.

T h e length change is the the difference b e t w e e n the l e n g t h after t h a w i n g and the length before freezing, e x p r e s s e d as a percentage of the l e n g t h b e f o r e f r e e z i n g ; t e m p e r a t u r e and moisture cantent a s s u m e d to be constant.

TABLE I1

EFFECT O F SEVERE MCXSTURE CONDITIONS ON FROST DAMAGE

M o i s t u r e Content

Percentage of Saturation Condition Length Change

Sample Content After Freezing Per c e n t

cracked* cracked* unchanged unchanged cracked* cracked** unchanged unchanged unchanged cracked* unchanged unchanged

*

C r a c k e d across t h e sample.

**

Cracked along t h e length, about half the length.

+

cxpansiun

TABLE 111

EFFECT

O F MOISTURE

CONTENT ON

FROST DAMAGE AFTER

REPEATED FREEZE -THAW CONDITIONS

M o i s t u r e Content Condition After

Sample (Per cent D r y Weight) Eight

F r

eezings

AZ

0 unchanged 131 0 unchanged C1 0 unchanged unchanged unchanged unchanged unchanged unchanged unchanged cracked ( 8 ) " cracked ( 8 ) c r a c k e d ( 6 ) cracked ( I ) c r a c k e d ( 1 ) c r a c k e d ( 1 )

*

N u m b e r s in parentheses a r e the number of freezings

TABLE

IV

E F F E C T

OF

RATE

OF

FREEZING

ON

FROST

DAMAGE

Rate of Condition of Sample After

F r

e e z i n g Number F r e e z i n g

Sample ( " ~ / r n i n , ] 1 2 3 4 5

1

6

unchanged cracked

-

-

Y

2

6

cracked

-

-

-

-

5 4 unchanged unchanged unchanged unchanged c r a c k e d

6 4 unchanged unchanged unchanged unchanged cracked

3 2 unchanged unchanged unchanged unchanged unchanged

8 2 unchanged unchanged unchanged unchanged unchanged

TABLE

V

E F F E C T

OF

RATE

OF

F R E E Z I N G ON LENGTH CHANGE O F SAMPLE

R a t e of Length Change of Sample (per cent) After F r e e z i n g Number

Freezing

Sample a ~ / m i n . ) 1 2 3 4 5

+

expansion

T A B L E VI

ADDITIONAL TESTS

ON

THE EFFECT

OF

RATE

O F F R E E Z I N G

O N

FROST DAMAGE

R a t e of Condition of Sample After Freezing Number

F r e e z i n g Sample ( * F/rnin. ) 1 2 3 4 A l 6 cracked I A2

6

cracked

-

8 1 6 cracked

-

-

I BZ

6

cracked I

-

I A 3

2.4

unchanged cracked c.

-

B 4 2. 4 unchanged cracked

-

3

A5 0. 7 unchanged unchanged unchanged unchanged

B5 0.7 unchanged unchanged unchanged unchanged

TABLE VIH

I N F L U E N C E

OF

D R E C T L O N

OF

FREEZING OX FROST DAMAGE

Sample F r e e z i n g Condition Condition of Sample

omnidir ectional omnidir ectional omnidir ectional omnidirectional omnidirectional omnidir ectional u n i d i r e c t i a n a l u n i d i r e c t i o n a l unidirectional unidirectional cracked cracked c r a c k e d cracked cracked cracked unchanged u n c h a n g e d unchanged u n c h a n g e d

TABLE VIII

EFFECT O F ADDITIONAL FREE ZhVG

ON

CRACKED SAMP LES

Expansion After Each Freezing [Percentage of Original Length)

Sample Freezing 1 F r e e z i n g 2 Freezing 3 Freezing 4

A 4 0 , 0 0 5 0 , 0 1 9 0.137

(not cracked) (cracked) {cracked) ( d i s c a r d e d )

L5 0 . 0 1 9 0,014 0.014 0.014