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Moisture expansion of clay bricks and brickwork
Ritchie, T.
MOISTURE EXPAYSION OF CLAY BRICKS
AND
BRICKWORK T. RitchieOnce t h e y have been f i r e d , b r i c k s made from clay and shale undergo irreversible expansion as a result of t h e e f f e c t o f water vapour on t h e b r i c k . This increase in, volume, termed "moisture
expansionw,
continues
f o r a l o n g p e r i o d of time a f t e r t h e b r i c k leavest h e kiln. In recent years, many studies1 - have been made o f this
p r o p e r t y o f clay bricks because t h e r e s u l t i n g expansion of walls is o f t e n of sufficient magnitude to cause cracking, examples of which have been described by tIoskingB.
CAUSE
Moisture expansion o f bricks and o t h e r burned c l a y p r o d u c t s
is generally attributed to t h e h y d r a t i o n o f amorphous materials or
glasses i n t h e b r i c k that have formcd d u r i n g the burning of t h e clay.
Young ; r n c l Bro~nelll s t a t e t h a t this expansion apparently involves t h e
brick's non-crystalline, high-energy phases, which are the amorphous remnants o f clay minerals or the low-temperature glass farmed from t h e
a t k : ~ l i content of t h e c l a y . Hosking
and
h e b e r 2 also point to t h c hydration of amorphous materials and g l a s s e s in b r i c k as t h e cause of moisture expansion. They n o t e , in addition, that when b r i c k s are f i r e d a t low temperatures, The re-hydration of dehydrated clay minerals causes expansion. The studies of Vaughan and !Iinsdaleg provide evidence that the penetration o f moisture into the glassyphase of ceramic bodies causes expansion which may continue for a long p e s i a d of time.
AMOUNT AND RATE OF
EXPANSION
The amount of moisture expansion o f b r i c k s is v a r i a b l e , undoubtedly because brickmaking s h a l e s and clays are h i g h l y v a r i a b l e
in mineralogical composition. Thus, when f i r e d , they contain varying
amounts of moisture-sensitive amorphous and glassy materials. An i n d i c a t i o n of t h e w i d e r a n g e o f moisture expansion of b r i c k s i s given in Figure 1 , p l o t s o f thc cxpansion of b r i c k s manufactured at six
Can:~Jitin plants and used in cxperjrncnts hy the Division of Building 14cscarch o f t h c N a t i o n a l Ilcscarch Council. The b r i c k s were fitted
with r c f e r e n c c pins for length measurement, stored in a room maintained a t 50 pcr cent I t - [ I . , then periodically measured for l e n g t h changes.
'Thc r a t e o f moisture expansion, and p o s s i b l y the ultimate
m o u n t , depends on t h e relative humidity of the a i r in which the h r i c k is stored, a s shown in F i g u r e 2, which g i v e s the r e s u l t s of a DBR
study of t h e l e n g t h changes o f bricks obtained from one p l a n t , The b r i c k s were fitted with length-measuring p i n s and stored in rooms of different relative humidities, as well as in water. Storage at low humidity (10 p e r cent R . H . ) i n i t i a l l y resulted in shrinkage, followed
by slight expansion, such that a f t e r a year and a h a l f of storage t h e
b r i c k was 0.005 per cent l o n g e r t h a n its o r i g i n a l l e n g t h . Storage in watcr and a t 100 p e r c e n t R.H. produced rapid initial expansion, with
Storage a t 50 per c e n t
R.M.
produced expansian approximating the mean of t h e movements o f the bricks stored under t h elow
and h i g hh u m i d i t y condieions.
The moisture expansion of bricks occurs r a p i d l y after their
f i r i n g and initially i s of considerable magnitude; one of the brick
materials s t u d i e d by F r m a n and smith5, f i r e d to llOQ°C, expanded 0.042 p c r cent in t h e f i r s t 24 h o u ~ s a f t e r having been f i r e d .
The
suhscqucnt rate o f expansion, however, decreases greatly w i t h time. I.ong-term studies of b r i c k expansion by s m i t h 7 , which purposelyneglcctcd t h c f i r s t - d a y expansion, showed that expansion a-Eter 7 I J 2 years was about 66 p c r c e n t g r e a t e r than t h a t which had occurred i n
t h c f i r s t four m o n t h s of o brick's life; expansion a f t e r 60 y c a r s , hawevcr, was prcdictcd to b e about 25 p e r cent grcatcr t h a n that a t ngu 7 l / Z yc:lrs.
F I R 1 NG TEMPERATURE
The amount of moisture expansion o f bricks made of a pa~ticular Taw material depends g r e a t l y on t h e f i r i n g temperature. \%en various c l a y s were f i r e d over a temperature range from 850 to
120OQC, then steamed to produce moisture expansion, & s t 4 found t h a t the p l o t s of moisture expansion against f i r i n g temperature rose to a peak at about 9 5 0 ' ~ then decreased, with expansion of the samples fired at ~ 1 0 0 ' ~ being l e s s than tllnt of t h e samples f i r e d at 850 O C .
For f i r i n g temperatures in the range generally employed in commercial p r o d u c t i o n , t h e amount af moisture expansion u s u a l l y
dccreascs w i t h jncrensing f i r i n g temperature. In the study by Laird
:~ncl W ~ C k c n s G of the moisture expansion of commercially manufactured
hri cks, r l ~ c minimum moistlrre e x p a n s i a n was generally t h a t of t h e F i ; ~ r t l - l ~ t ~ r n ~ . t l I ~ r i cks
.
Onc of t h e b r i c k s they studj cd, for example, b i 1 1 1 - l'rcslt w h c r l the i n i ti ;I 1 measurement was made and subsequentlys t o r e d n u t s i c l c 1 ~ 1 an cxpnscd p o s i t i o n for one year, expanded 0.093, 0 . (144 and 0.038 per ccnr of the o r i g i n a l length, respectively, far
s i m p 1 cs r a p r c s u n t i n g l i g h t - , medium- and h a r d - l ~ l l r n c d b r i c k s . Of t h e
tcrl raw nlatcria 1 s , studicd by smithJ, which were f i r e d a t various
tcrnpcratures and measured fox moisture expansion after 7 1 / 2 y e a r s sf
storage, seven showed m i n i m u m moisture expansion
for
the h i g h e s t burning temperature employed; for two of the remaining materials t h emoisture expansion was relatively low regardless of the f i r i n g
temperature. RESTRAIN'I'
Bccnusc bricks in a wall arc bonded to the surrounding
mortar t f ~ c y may bc expected to h e restrained by it to some e x t e n t .
I n a d d i t i o n , loads and restraints such as those imposed by walls, floors :lnd R ~tructura1 frame may a l s o a f f e c t t h e amount of moisture cxpzrnsior~ o f t h e b r i c k s in service. A DBR study in w f l i c h
b r i c k s were r e f i r e d a t a h i g h temperature (980°C) showed t h e e f f e c t of r e s t r a i n t on t h e i r moisture expansion. Expansion was induced by
a u t o c l a v i n g t h e samples. Some of them were r e s t r a i n a d by collars over their ends, connected by s p r i n g s which applied a stress of
50 p s i d u r i n g autaclaving. The results of t h e s e t e s t s showed t h a t t h e amount of moisture expansion of each of t h e seven different
I ~ r i c k s s t u d i e d was less f o r the restrained b a ~ than f o r t l ~ c r t ~ r r c l ; p o n d i ~ i g
unrestrained bar. Thc r e d u c t i o n
in
expansion, however, varied~ r c a t l y i n amount, from 4 to 24 pcr cent of t h e expansion o f the f r c c
1 1 ; ~ ~ .
WALL
FXI'ANSlONS t u d j s s rcportcd t o d a t e of t h e cxpansion of individual hsicks i n relation to t h a t of walls made of c o r r e s p o n d i n g bricks h a v e
invalved the u s e of free-standing, non-load-bearing walls, Because
such walls a r e not part o f a building and b e a r only t h e weight o f t h e i r b r i c k s and m o r t a r , they are not necessarily subjected to a l l o f the restraining factors that may operate on the walls of b u i l d i n g s .
In a s t u d y by klosking8 o f t h e expansion of individual bricks in relation t o t h a t of walls made o f corresponding bricks it was found
t h a t t h e wall expansion exceeded t h a t o f t h e b r i c k s , t h u s indicating ,
t h a t some o t h e r f a c t o r , a d d i t i o n a l to moisture expansion, c o n t r i b u t e d
t o t h c wall cxpansion. Comparisons of t h e expansions of bricks and
wal Is mndc hy s m i t h l o i n d i c a t e d that, in general, wall expansion was
lcss than t h a t nf t h c individual h r i c k s , the r a t i o of wall to brick
e x p a n s i o n t ~ c i n g about 0 . 6 . In some cascs, however, t h e r a t i o
cxcccdcd 1.0, which w a s a t t r i b u t e d to wetness of the walls resulting
i r l sulphatc cxpansion of t h e mortar. Comparisons by Bcard and h i s
associates'
l of t h e measured expansion of walls, and t h e i r expansioncomputcd on t h e h a s i s of t h e dimensional changes of samples of the bricks and mortars, i n d i c a t e d that t h e actual movement was generally
less t h a n t h a t computed.
The maximum expansion o f t h e walls s t u d i e d by kIosking8 was of t h e order of 0.2 per c e n t a f t e r seven years. A similar amount of
expansion was measured b y Beard and associatesll in a wall s i x years
o l d ; t h e maximum wall expansion measured by smithlo, for walls four
ycars o l d , was 0.075 per cent.
Tn the threc preceding studies8* l o * 1 1 , the use of kiln- . Trcsli I ~ r i c k s t o Fluild walls resulted i n g r e a t e r wall expansion than d i d
t l r c ! l r s r ol' similar hricks employed a f t e r a p e r i o d o f storage because tl~r stor;tgc of thc l ~ r i c k s reduced t h c amount of ex ansion t h a t t h e y
could undergo in t h c wall. Two of t h e ~ t u d i c s ' ~ ~ F1, a l s o showed t l w t t h c c x p ; ~ n s i o n rlcnr t h e t o p of a wall d i f f e r e d from that a t t h e
I ~ c b t t o n r . W a I 1 s oxposcd to the weather expanded more a t the bottom tfl:~n a t tllc t o p and w i l l 1s built indoors and in sheltered locations
c x l ~ n l ~ d c r l rnorc a t t h c t o p t h a n at the bottom. lrlall rnovcments measured
in tlic horizontal and vertical d i r e c t i o n s ncse similar f o r walls
MOVIJMENT JOINTS
lhc expansion of b r i c k walls may be considerable, depending on thc moisture expansion characteristics o f t h e bricks and how soon
a f t e r manufacture they are used in c o n s t r u c t i o n . Tn the worst case i t could b e
of
the o r d e ~ of 0.2 per cent in six or seven years a f t e r construction, but withoutmuch
further expansion taking place t h e r e -a f t e r . It would b e necessary to have one expansion j o i n t 3 / 8 - i n .
t h i c k i n every 1 6 - f t length of wall t o accommodate this amount of movement (0.2 per c e n t ) . As a result of h i s studies, IIosking8
s u g g e s t e d t h a t f o u r 1 / 2 - i n . or t h r e e 3/4-in. w i d e movement j o i n t s
p r o v i d e d in cash 100-ft length of wall would b e adequate to accommodate expansion, with t h e first j o i n t s located w i t h i n 10 ft of t h e corners because cracking usually takes place near the corners of a building.
The moisture expansion of b r i c k s i s much greater t h a n t h e i r
thermal expansion, for which a value of 0.07 per cent l e n g t h change
113s I x c n .qivcni2, The jointing provided f o r moisture expansion i s
thcrcforc cnnsidered adequate f o r bath moisture expansion and thermal niovcrncnts
.
Ikicks madc from clay a d s h a l e expand a f t e r firing due to t h c c f f c c - c s o f nroisturc. The expansion c o n t i n u e s f o r many y e a r s ,
although t h e rate decreases greatly w i t h time. The amount af moisture
cxpansjon is n o t ttniform, v a r y i n g over a range from about 0.03 p e r cent
to about 0 - 2 0 per cent, depending on t h e raw materials used in brick manufacture and on t h e temperature at which the bricks are fired. The moisture expansion of b r i c k s results in t h e expansion of walls t h a t may l e a d to t h e i r c r a c k i n g
if
movement j o i n t s are not provided toaccommodate the expansion.
1 Young, .J.B:. :rnd W.E. Brownell. Moisture expansion of clay ~ ~ s o d t ~ c t s . .Journ;il of t h e American Ceramic Socicey, Val. 4 2 , Nn. 1 2 , 1959.
7 I l n s k i r ~ g , . I . S . and I1.V. Hcubcr. Moisture expansion of c l a y
pr.oducts wi t 1 l s ~ ~ c c j a 1 ref ercncc to b r i c k s . Transactions of t h e V 1 1 Intcsnational Ceramic Congress, London, 1960.
3 I l o s k i n g , J . S . and H.V. Heuber. Dimensional changes due to
moisture in b r i c k s and brickwork. American S o c i e t y f s ~ T e s t i n g
West,
H.W.H.
Moisture movement of b r i c k s and brickwork.Transactions of t h e British Ceramic Society, Vol. 66, No.
4,
1967.Freeman,
I.L.
andR.G.
Smith. Moistme expansionsf
structuralceramics; unrestrained expansion. Transactions
of
the BritishCeramic Society, Vol. 6 6 , 1967.
Laird, R.T. and A.A. Wickens. The moisture expansion
of
f u l l - size b r i c k s . Transactions, British Ceramic Society, Vol. 6 7 ,No. 12, 1968.
Smith, R.G.
Moisture expansion of structural ceramics; longt e r m
unrestrained expansion of test bricks. T r a n s a c t i o n s , British Ceramic S o c i e t y , Vol. 72,No.
1 , 1973.Hosking, J . S . Prevention of damage due to moisture expansion
in
ceramic structures. C l R Bulletin, No. 1, 1964.Vaughan, F. and A. Dinsdale. Moisture expansion. Transactions, British Ceramic Society, Val. 61,
No. 1,
1962.Smith, R . G . Moisture expansion sf structural ceramics;
expansion of u n r e s t r a i n e d Fletton brickwork. Transactions,
British
Ceramic Society, Vol. 73,No.
6 , 1974.Beard,
R.,
A. D i n n i e and R.Richards.
Movement of brickwork. Transactions, B r i t i s h Ceramic Society, V o P . 68, No, 2 , 1969.Baker, M . C . Thermal and moisture deformations
in
buildingmaterials. Canadian Building Digest, No. 56, Division of
Building Research, National Research Council of Canada, Ottawa, 1964.
0 1 0 0 2 0 0 3 0 0 400 5 00 6 0 0 D A Y S
F I G U R E 1
<
I
z
-
-
EACH POINT REPRESENTS THE AVERRGE VALUE OF
MEASUREMENTS O F TWO STORAGE = 509b R . H .
