11 th INTERNA TIONAL BRICKlBLOCK MASONRY CONFERENCE TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

DISCUSSION ON GRACK-CONTROL JOINTS

IN MASONRY BUILDINGS

Yaming Xiao

O.ABSTRACT

This paper reviews the engineering measurements of introducing crack-control joints

in masonry buildings to prevent wall from cracking suggested in Chinese building en­

gineering code and correspondent ones in a few selected foreign countries. The differ­

ences among theories underneath the codes of the various countries on the function of

crack-coutrol joints and their engineering measurements are then analysed. Proposals

on how to prevent wall from cracking are finally suggested.

1. SUB]ECT FOR DISCUSSION

WaIl cracking is a commen phenomenen related to the que.lity of masonry structures.

In order to prevent walls from cracking or to reduce the extent of cracking, various

countries have adopted their own technical measurements related to their own practi­

cal requirement, among which the introduction of crack-control joints is masonry

structures is one of the main ideas. From relevent articles of the building engineering

codes in different countries, it can be seen that clear difference exists in the rules on

the introduction of crack-control joints in masonry structures and the requirements of

the distance between neighbouring joints,and the way the joints are constructed. Why

KEY WORDS: masonry structures; wall cracks; crack-control joints

Dep. of Building Engineering , Hefei University of Technology,Hefei 230009,China.

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does the difference exist, what are the theories on which the codes are based in vari­

ou~ countries, and how do they affect the prevention of wall cracking in masonry

structures? These are unavoidable questions confronted to uso It is, therefore, natural

to discuss the subject on the intoduction of crack-control joints in masonry structures

and necessary to carry thorough study on it.

2. TYPES OF W ALL CRACKS

Observations and research has revealed that most of wall cracks in masonry structures

are caused by incompatible deformations in structural members, due to shrinkage or

thermal expansion of construction materiais . The cracks can be classified into follow­

ing cases:

(1) Wall cracks due to the temperature variation in flat roofs of steel reinforced con­

crete buildings and the shrinkage of masonry walls, such as inclined OI horizontal

cracks in walls of topmost storey. The characteristics of the cracks in walls on the

topmost storey in that it cracks most seviously at two ends, and less in the middle of

structures. The ma in reason for the cracking is that deformation differs between the

flat concrete roof and the masonry wall. This pattem of cracks is typical for cracks in­

duced by thermal expansion.

( 2) The second group of cracks are the vertical ones in places stiffness varying

sharply . The reason for this kind of cracks is that the strutures are toa long in size to

allow free shrinkage or thermal expansion of walls.

(3) For wall materiais shrinking heavily, verfical or inclined cracks usually occur on

edges of windows and doors of buildings, such as the ones below windows on the

ground floor and the vertical cracks in the middle of transverse walls. This type of

cracks usually occur in buildings made up of non- fired silicate brick or masonri

blocks. They are mainly due to the shrinkage of construction materiais. The cracks

distribute widely in buildings and the number of cracks in relatinely large, but they

are narrow in width and short in length. The pattem of this kind of cracks is typical

for cracks caused by material shrinkage.

3. RULES ON INTRODUCTION OF CRACK-CONTROL ]OINTS IN DESIGN

CODES OF SELECTED COUNTRIES

(1) China

In article 5. 3. 2 of the Chinese design code for masonry structures CGB]3-88)[I] ,it is

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stated that "Expansion joints have to be introduced in order to prevent walls fram

cracking due to thermal expansion or material shrinkage under normal workillg coudi­

tions . .•• ••. the distance between neighbouring joints can be determined according to

theoretical formula,or the values suggested in Table 5. 3. 2 (Table 1) . " The theoreti­

cal formula for calculation, however , has not been presented in the code. In note 2 on

article 5. 3. 2 it has been pointed out that " The expansion joints introduced accorcling

to values in the tist can not,in genera,prevent walls fram cracking due to thermal ex­

pansion of steel reinforced concrete raof and dried shrinkage of masonry materiais,

mentioned m article 5. 3.1 ,at same time". ~n China,double walls have been adopted

for the expansion joints, that is,a building is divided into two independent units above

the ground.

Table 1. Maximum distance between neighbouring thermal expansion joints

mansonry type roof type distance

in situ cast or prefab- with thermal insulation layer 50

ricated steel rein-

forced concrete without thermal insulation layer 40

various mansonry prefabricated steel re-

with thermal insulation layer 75 inforced concrete

with system of hori-

zontal beams without thermal insulation layer 60

clay bricK, 100 hollow brick roaf of tile or asbestos-cement sheets,

silicate block, roaf of timber or bricks 75

conerete block

(2) The United States of America

In the masonry structure design code of the U. S. A. (ACI 531-79) [2],the rules for

the distance betwee, neighbouring joints in masonry structures are as follows:

a. The distance is usually about 6-7. 5m;

b. The maximum distance is determined according tu values in Table 2;

c. For reinforced -masonry stru('tures, i. e. , the reinforcing ratio is greater than or e­

qual to O. 07% ,the distance may be chosen as whicherer is smaller "one,30m or it sat­

isfies L/H=4.

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Table 2. 'Maximam distance between expansion joints

vertical distance between horizontal mansúnry structure

reinforcement (mm) building unit without reinforce-

ment 600 400 200

Length -height ratio 2 2. 5 ·3 4

(L/h)

maximum length 12 14 i5 18

L(m)

reduced nitio of

reinforcement (%) 0.0 0.01 0.015 0.03

It can be seen that the distance between expanison joints are related to the vertical

stiffness (L/H) of buildings and their ratio of wall reinforcement. With a little rein­

forcement in masonry walls, the distance can be enlarged. The masonry structures

with reinforcing ratios in Table 2 are considered as non-reinforced masonry structures

(u<O. 07%) according to the design codes of the U. S. A. and the Great Britain. In

the notes on Table 2 it has been pointed .out that the distance values in the Table are

restricted to the masonry structures y.rith moisture-controJed and the distance should

be halfed for the structures without control on moisture. Expansion joints should be

put where cracks may occur, such as edges of windows anc doors, areas where wall

height and thickness varies sharply. This type of crack-control joints are only intro­

duced locally in walls and there is no need to seperate the whole structure.

(3) The Great Britain

In the design code for masonry structures of the Great Britain (BS 5628-80 )[3J , the

rules for the distance between crack-control joints in masonry structures (without re­

inforcement) are, the distance is about lO-15m for day brick; it should be, in gener­

ai, reduced to below 6m. For the local joints in walls, not only should they pass

through the right structure members, but a1so the wall surface layers, sue h as plaste­

ing layt.[ and the width of vertical joints should be about 12 mm.

(4) Germany

In Germany design code for morta r masonry structures (DIN1053-74 )[4J, the rules

for the distance between joints in externai walls of double wall system are presented

in Table 3,and it is further pointed out that they are only for walls without windows,

and the joints have to be constructed on edges of windows and doors if there ·are win­

dows or doors in the wall. It is written in the German code that in order to let the

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whole building free to shrink or expand so that potential vertical cracking in the mid­

dle of building can be prevented the maximum distance between neighbouring joints is

. suggested to be about 25-30m for buildings of mortar-brick and without thermal in­

sulation layer.

Table 3. Suggestiongs on distance between externai joints of double wall systems

wall type distance between joints

externaI brick walls with doubIe Iayers of mortar and one Iayer of air 6-8

on ·surface

externaI brick walls with doubIe Iayers of morta r ,one Iayer of air and

thermaI insuIation 6-8

' . externaI brick walls with double Iayers of mortar but no air Iayer 8-12

In addition, it hasbeen written in the code of U. S. A, Britain and Germany that slid­

ing layers (sliding joints) have to be introduced in walls where steel reinforced con­

crete raof slabs, flaor slabs ar beams are placed to seperate the two building compo­

nents for the prevention of load bearing walls from cracking due to incompatible de­

formations in structural members induced by dried shrinkage or thermal expansion.

4. MAIN FUNCTIONS OF CRACK-CONTROL JOINTS IN SELECTED COUN­

TRIES. AND THEIR ANAL YSIR

(1) lt can be seen from the Chinese code that the function of expansion joints in Chi­

na is to avoid buildrigs being toa long so that through vertical cracks due to tLmpera­

turedifferentiation and dried shrinkage can be . well coutroled. Engineering practice in

China has shown that cracking occurs little if expansion joints are coustructed accord­

ing to,thecode and rules in the code can be ajusted with practical experience lt should

,' ,be known that the joints described in the Chinese code can prevent from neither wall

cracking on topmost storey in steel reinforced concrete flat-raof buildings nor shrink­

age induced wall cracking in buildings of heavy shrinkage materiais.

(2) ,In the codes of U. S. A. ,Britain. and Germany, various joints constructed In

buildings are ali known as crack - control joints and they are funher classified into

expansion joints,shrinkage joints and sliding joints. a. Expansion joints,the neigh-

. bouring distance 1S about 30m, similar to that of the Chinese code, its function is to

prevent them' from vertical cracking in the middle of buildings; b. Shrinkage joints,

its neighbouring 'distance is, in general , relatively smaller and its standard value is

about 6'......:7:-5tn· and its main funct;ion is to prevent walls from dry shrinkage induced

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cracking in builings of heavy shrinkage materiais. c. ,. Slidi,I;lg joints,its ,lllain ,func~ion

is to prevent load bearing walls fram cracking induced 9Y the inc~mpati,l:)le deforma­

tion between the wall and steel reinforced concrete roof slabs, floor slabs or beams . • : • I I

(3)From the result of in situ investigation in C.hina[5],~racks . mainly occur in.walls 00

topmost storey for buildings of fired elay bricks. For buildi~gs.oÍ pon - fired silicate

bricks or masonry blocks, wall cracks often occur and sometimes very seviously, The

main reason is that the dry shrinkage ratio of non - fired silicate bricks or masonry

blocks is as large as twice or more of the fired elay bricks and their resistance to .shear

and stretch is more weak than that of the fired elay brick at !,ame t,ime. According to

the Intevnational Standand ISO 9652/1 «non-reinforced masonry»[6],the shrinkage

ratios for various masonry materiaIs are, O. 1mm/m for fired elay bricks; O . . 2mm/lTI

for silicate bricks and dense concrete; O. 3mm/m for light concrete. The shrinkage can

reach 1. 2~1. 5mm if the neighbouring distance of standard shrinkage joints is about

6 ~ 7. 5m and the shrinkage ratio O. 02 %. If shrinkage joints were not constructed,

the calculated shrinkage stress in walls would exceed the tensile strength oí masonry

blocks and the shrinkage induced cracks would occur. This kinds of cracks dis.tribute . .. ,.";

widely in buildings,but their width is relatively narrow. The distance of neighboUl;ing

joints set in the Chinese code (See Table 1 )already exceeds greatly that in other coun­

tries and it is obvious lhat the shrinkage induced cracks can not be prevented. It is for

the prevention of shrinkage induced cracks that local shrinkage joints have been intro­

duced in U. S. A. ,Britain. etc.

5. CONCLUSIONS AND SU,GGESTIONS

(1 ) U pon previous analysis, the crack - control joints (expansion joints, shrinkage

joints and sliding joints)introduced in the design codes for masonry buildings in U. S.

A. ,Britain, Germany etc. is a key engineering me~susement in prevention of wall

cracks. The construction of crack - control ,joints ,is, in other words; to ajust th,e pc­

tential incompatible deformation in buildings, and the deformation will be controled

under certain limit. If cracking is unaviodable, let it occur where the joints have been

designed in advance and the messy cracks in walJs ·can be avoided. TherefQ,E!>1:the~e

should be clear requirement on the position of and the distance between the j0ints and

the way how they should be constructed. Not only should walls be free to deform, but

also supply necessary lateral stability , weathertightness and sound insulation. Wall

cracks are, in general, under well control through the construction of crack - control

joints, local reinforcement and other addifional measurements.

(2)Fram the rules in the Chinese design code,the function of the thermal expansion

joints in Chi'1a matches only that of the expansion joints in other countries and they

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do not function as shrinkage joints or sliding joints do. There is no clear classificafion

on the shrinkage properties in the Chinese code and it is obvious that shrinkage cracks

are, in practice, difficult to control as rules are only relevent to the thermal expansion

joints . Ccnsequently, buildings 01 new type of matevials, such as masonry blocks,

~ortar bricks and other silicate materiais, crack relative seriously[7], and the waIl

.cracking is one of current existing problems.

(3) It is concludeed that the Chinese design code for masonry buildings faIls behind

tr.at of U. S. A. and Britain in rules' on how to construct crack - control joints and

prevent waIls from ~racking. :.1ore att;:ntion should be paid to the waIl cracks in

buildings of heavy shrinkage materiais. On the basis of engineering practice in our

own country and successful experience and methods abroad, there should be more

clear description on moistures ratio and shrinkage ratio of masonry materiais and

more rules on the contruction of local shrinkage joints and sliding joints. Vulnerable

parts of buildings should be locaIly reinforced. The properties of masonry bricks and

mOl tar materiais have to be improved. The construction quality should be further im­

proved as weIl. The waIl crack problem can only be solved completely if comprehen­

sive technical measurements are adapted te: the reality in China and put into engineer­

ing practice.

6. References:

1. Design Code of Masonry Structures (GBJ3-88) ,Beijing, 1988.

2. RanJal,F. A. , Concrete Masomry Handbook,USA,1982.

3. Curtin, W. G. , Stuctural Masonry Desigbners Manual,Britain, 1982.

4. Schneider,K. H. , Ks-Mauerwerk Konstruktion and Statik,Frankfut,1980'.

5. Xiao, Y. M. , A Summary of the Research on the Problem of Masonry Structural

Cracks and Control,Supplement to Engineering Mechanics,1994,pp. 1447-1452.

6. Yen.Z. F. , Discussion on how Local Reinforcement in Masonry Structures Affects

Crack Control and the Distance Between Neighbouring Expansion Joints, Engineer­

ing Construction Standardiza tion , 1996 , pp 12 - 15.

7. Xiao, Y. M. , An EXpE'rimental Research on SmaIl - Size BIock Houses under the

Action of Temperature,Journal of Southeast University,oct.1995,pp640-645.

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