The effect of joint thickness and other factors on the compressive strength of brickwork

A mechanism for the compressive failure of brickwork is developed quantitatively, and is shown to be capable of explaining the influence that certain variables have on the compressive strength. lt is shown experimental/y and theoretically that the strength of four-brick prisms declines as the joint thickness increases and as the lateral tensile strength of the bricks diminishes in relation to their compressive strength. The effect of other well-known parameters is explained in quantitative terms.

5.-The Effect of Joint Thickness and Other Factors on the

Compressive Strength of 8rickwork

by A J FRANCIS, C B HORMAN and L E JERREMS

ABSTRACT

A mechanism for the compressive

failure of brickwork is developed

quantitatively, and is shown to be

capable of explaining the infiuence

that certain variables have on the

compressive strength lt is shown

experimental/y and theoretically that

the strength of four-brick prisms

declines as the joint thickness increases

and as the lateral tensile strength of

the bricks diminishes in relation to

their compressive strength The e./fect

of other well-known parameters is

explained in quantitative terms

b

b

d

e

m

x,y, z =

E

P

'I

p

a

NOTATION

width of brick

(as suffix) brick

length of brick

strain

(as suffix) mortar

axes of reference

modulus of elasticity

load

tb/tm

Eb/Em

a'ulr/a'r

Poisson's ratio

Gulr/a'u/t

stress

University o[ Melbourne

L'Effet de l'Epaisseur des Joints et d'Autres Facteurs sur la Résistance

à la Compression de la Maçonnerie

en Briques

Un mécanisme pour la rupture à la compression de la maçonnerie en brique est développé de façon quanti-tative, et on montre qu'il est capable d'expliquer l'inf/uence qu'exercent certaines variables sur la résistance à

la compression II est montré de façon expérimentale et théorique que

la résistance de prismes de quatre briques décroit avec I'augmentation

de I' épaisseur du joint et à mesure que

la résistance à la traction latérale des briques diminue par rapport à leur résistance à la compression

L'effet d'autres parametres bien connus est expliqué de façon quanti-tative

EinflujJ der Fugendicke und anderer Factoren auf die Druckfestigkeit von Ziegelmauerwerk

Ein Zerstorungsmechanismus von Ziegelmauerwerk unter Drucklast ist quantitativ entwickelt worden Es wird gezeigt, wie er sich zur Erkliirung des Einf/usses verschiede ner Grossen auf die Druckfestigkeit eignet ExperimenteI! und theoretisch ist bewiesen, da.fJ die Festigkeit von Prismen aus je vier Ziegeln mit grosse r werdender Fugendicke abnimmt und da.fJ die seitliche Zugfestigkeit der Ziegel im selben Verhii/tnis wie ihre Druckfestigkeit geringer wird Die Wirkung anderer gut bekannter Parameter ist quantitativ erkliirt

has been made, with partial success, to describe in quanti-tative terms the mechanism of the process of compressive failure The present paper contains an account of a simple theoretical model, and some experimental work

on the effect of bed joint thickness in four-high stack-bonded prisms which appears to support the theory put forward The model also explains a number of the features of the compressive failure of brickwork 1.2 Model of Compressive Failure of a Short Stack-bonded Prism

(1' ult compressive stress to cause failure of brick

in absence of lateral tensile stress

compressive stress to cause failure of brick

in presence of lateral tensile stress

If a short prism of bricks bonded with mortar (Figure 1 (a» is loaded in axial compression in a testing machine the mortar joints above and below a brick sufficiently remo te from the restraining influence of the platens of the testing machine tend to expand laterally more than the brick itself, since the modulus of elasticity of the mortar

is normally much lower than that of the bricks Because

of the mortar-brick bond and the frictional resistance

to slip between the bricks and mortar at the interfaces, slip will not occur at the interfaces Lateral tension is, therefore, induced in the brick, and lateral compression

in the mortar Vertical splitting, due evidently to lateral tension, is usually present in a compressive failure of brick walling

a ' r lateral tensile strength

1 A MECHANISM FOR THE COMPRESSIVE

FAILURE OF BRICKWORK

1.1 Introduction

Compressive testing of brickwork has been carried out

in various laboratories for well over half a century, and

the factors which have a bearing on the compressive

strength, and the phenomena which accompany

com-pressive failure, are now fair1y well recognized A brief

qualitative explanation was given in a recent paper,!

but so far as the writers can discover, only one attempt2

31

The criterion of failure of a brittle material like brick under a condition of vertical compression plus biaxial lateral tension is not known, but failure will certainly occur at a lower compressive stress than would be required

in the absence of lateral tension, or if the lateral stresses were compressive

The prism shown in Figure l(a) is subjected to an axial compressive stress a y • The lateral stresses induced

32 The Effect of Joint Thickness and Other Factor s on the Compress i ve Strength of Brickwork

~~

d y

dxb

O'x d y

° x m

O

[a]

FIGURE I-Brick and mortar stresses due to applied axial com

-pressive load (ay )

in a central brick and in the mortar joint above or below

it are as shown in Figure l(b) The extensiona\ strains

in the x and z directions in the brick are therefore as

follows:

exb = ;b [(ax b + vb(a)'-a r b)]

ezb= L[(a2b + vb(ay-a ~ b)]

(1)

(2)

ex m= ~m[ -a HI1 + vm(a y- a z m)]

ez m= ~m[ -a z m + vm(a y -a Hn)]

where Eb = modulus of eJasticity of brick;

Vb = Poisson's ratio for brick;

V m = Poisson's ratio for mortar

(3) (4)

But the lateral expansion is assumed to be the same in

(5) (6) AIso, for equilibrium, the totallateraJ tensile force on the

force on the mortar joint, in both the x and z directions

In the x direction, therefore:

or

where

Similarly,

t b

rx=

-tm

t b = thickness of brick,

tm = thickness of mortar joint

G zm= CXU z b

(7)

(8) From eqns (5) and (6) we see that we can equate

eqns (1) and (3), and ais o eqns (2) and (4) Doing this,

and substituting for a xm and a z m with the aid of eqns

(7) and (8), we find that

a i f3 v m - Vb)

a x b = a zb = - "-"" -'

:.: -, 1 + rx f3 - V - rx f3vm (9)

where f3 = Eb

E m

The lateral tensile stress a b induced in the brick is

failure occurs; in the limit, ifa x b and a zb were equal to

the lateral tensile strength a ' I of the brick, a lateral tensile failure would occur even if the compressive stress a y were zero (Point A on Figure 2) At the other

"

'Cf' 1'> u 1 \

Latera l

Corrprrzs5 ; Y12 Strrzs'5

o

A La tora l Tens il e Stress

o' \

FIGUR E 2 - Theoretical envelope relating lhe tensile and

com-pressive s t r es s es i n brick at failure

allll = a' ulr the value necessary to cause failure in the absence of lateral tensile stress (Point B on Figure 2)

The way in which the va\ue of alll l varies with a xb and

a b between these extreme limits is not known for a

that, for simplicity, the linear Tresca shear criterion be adopted His relationship can be expressed by the equa-tion:

(lO)

,

where </> = a ~l l

a I

Substituting this expression for a b in eqn (9), we arrive at the following relation between aull and a' ull:

1

(11 )

The term (l - Vb) in the denominator is normally very much smaller than rx f3(1 - Vm), and p can be represented with sufficient accuracy by the following equation:

1

1 + <f>(f3 vm - v b )

rxf3(I -vm )

2 V ARIATION OF COMPRESSIVE STRENGTH

OF BRICK PRISMS WITH V ARIOUS FACTORS 2.1 Number of Bricks in Prism

In a compression test on a single brick the platens of the testing machine restrain the tendency of the brick to expand laterally, to an extent which depends on the nature of the packing material between platen and brick The brick may therefore be subjected to lateral c om

-A J Francis, C B Horman and L E Jerrems 33

pression instead of tension The failure envelope for this

can reasonably conjecture that ali I/ will exceed a' ult, its

value when a x b=O (see the broken line in Figure 2)

hand, the middle bricks may be expected to be fairly

described in Section 1

A dramatic loss of strength can be expected as the

number of bricks in the prism increases, and this is

Figure 3, obtained in tests in the Civil Engineering

r;;;-l

o

- B

><

~f

.o

L=J

OI

>

'"

"

'-a

6

4

~ 2

U

o

o P ressed Br ie !<s Mortor I ~ 4 ~

Eoeh Fbi nt i , A v eroge

01 6 Tes\s

N umber o Cour5~s i n Pr i sm

FrGURE 3 - Effect of number of courses on compressive strenglh

of prisms

cubes These results suggest strongly that in a prism

with four or more bricks the compressive strength is

reason, four-brick prisms are adopted in the SAA

freedom from end effects, but they would be toa tall

machines designed for concrete test cylinders, and also

would take longer to make

Bricks

2.2.1 Introduction

regards the effect of the parameter ex by a series of tests

joints was varied

In order to study the effect of varying </>, two types of

brick were used: a solid pressed brick (with frog) and

(Figure 4)

2.2 2 Brick Properties

number of individual measurements, are shown in Table I

FIGURE 4-Plan view of perforated brick

Dimension

Lenglh Width Height Plan area Side area

Solid

(in.)

8 · 85 4·18 2·91 37·0 25·7

Perforated

(in.)

8·97 4·28 2·98 38·4 (gross) 26-4 (net) 26·7 (gross)

12 · 5 (net*)

*00 Sectioo X - X, Figure 4

strength a' t was determined by the 'Brazilian' tension test, in which the brick is subjected to a compressive force P applied as shown in Figure 5 This causes an

~

Br i ck

FIGURE 5 - lndirect tension (Brazilian) test method approximately uniform lateral tensile stress across the section in the plane of the force P The lateral tensile

strength is given by the expression:

, 2P

-/- TCdtb

The solid bricks were tested along a central section but the perforated bricks were tested along each outer line of holes as welI as along the central line Each value given in Table 2 for the tensile strength is for a set of

six tests

cross-sectional areas

2.2.3 Mortar Properties

The mortar mix was I: I : 6 (Portland cement: lime: sand,

34 The Eflect of Joiut Thickness aud Other Factors ou the Compressive Streugth of Brickwork

Value Coe.ff of Value Coe.ff of

Mean compressive

Minimum compressive

-Lateral tensile

strength along

Modulus of

by volume) 1·25 parts of water, also by volume, were

added, after trial mixes for workability

The compressive strength, obtained on mortar cubes

according to AS A123, was as follows:

Mean sfrengfh Coeff of

(lbf / in 2) variation

927

Other mean values used were:

(%) 8·6

Em 0·20 X 1061bf/in2

Vm 0·25*

2.2.4 Four - Brick Prisms

In each type of brick, six prisms were made with mortar

joints approximately either 0·4-in or 0·6-in thick, and

four prisms of each brick type with l-in.-thick joints,

including in each case layers of mortal' of the same

thickness at top and bottom of each prism

Four prisms of each brick type were also made with as

fhin joints as possible The prisms were cured in air in

the laboratory and tested in a Denison 200-ton

compres-sion testing machine between cardboard sheets at the

age of 14 days

On a number of the prisms, longitudinal and transverse

strains were measured with a 2-in Demec gauge The

values of Eb, V b and Em quoted above and in Table 2

were determined from these measurements, the latter

Three solid brick prisms and two perforated brick

prisms were also prepared and tested dry In these the

contacting surfaces of the bricks were polished in a

geological polishing machine until they were plane to a

high degree of accuracy Four piers with solid bricks

and three with perforated bricks, whose upper and lower

faces had been trimmed fiat with a masonry saw, were

also tested dry The frogs in the solid bricks were filled

* Value supplied by Division of Building Resean;h CSIRO ,

Melbourne

with dental plaster, sanded plane

2.2 5 Test Re s ults (See Tables 3A and 3B)

In the perforated brick prism tests with mortal' joints, slight cracks began to appear at low compressive stresses, sometimes at 6001bf/in2 or less, and there was some

i

I

2050t 0·65

I

2450

• Polished faces

t Masonry saw-cut faces

I

8760*

0·35 I

I

I

* Polished faces

t Masonr y saw-cut faces

A J Francis, C B Horman and L E Jerrems 35 spalling of brick faces The solid bricks were obviously

less brittle

One of the dry piers with polished contacting faces

(perforated brick) developed the remarkably high strength

of 9760Ibf/in2 failing with explosive force The dry

piers with saw-cut contacting surfaces, however, were

much weaker, and it was noticed that slight cracking

and spalling began under very low stresses (of the order

of 300 Ibf/in2 in some cases) This was probably caused

by uneven bearing between the dry surfaces, which

indicates the important part played by the mortar

joints in reducing or eliminating these stress

concentra-tions, as had been demonstrated by WEST and others.3

The compressive strengths of the prisms are plotted

against the mean joint thickness in Figures 6 and 7

o 0 2

So l id Br ic" Pr i sms Four Br i cks H i gh

04

o Dr y Pr i sms , Pol i shQd FOCQS

o Dry Prismsí Mosonry

joint thickness - solid bricks

lt was hoped to obtain a 'ull from the tests on the

prisms with dry joints, but it was obvious that the

strengths of these prisms depended very much on the

degree of planeness of the contacting surfaces;

further-more, the values bore no consistent relation to the trend

of the results for various joint thicknesses A mean line

was drawn, therefore, through the results for the prisms

for each brick type with mortar joints and the values at

zero joint thickness taken as a'u/t: these were 4150 and

5600 Ibf/in2 for solid and perforated bricks respectively

With the values of E, 'I, a't, etc given earlier, eqn (12)

can be plotted for the two types of brick The values

of the relevant parameters are given in Table 4

The theoretical and experimental values of p( = ao/t / a 'u/t)

are shown in Figure 8

The most striking aspect of these results is the effect

of joint thickness on the strength of the prisms This

phenomenon has been observed before in the Iiterature.4

Obviously, the thinner the joints the stronger the

brickwork, and this is the reason for the requirement in

the SAA Brickwork Code that in structural brickwork

(i.e brickwork requiring engineering design to the

Four Br i c" s H I 9h

t760 I bf/ i n'

60

" " ":' O~ I I' 5 00 Jbf /i n'

o Dr y Pr i sms ; Po li shQd FOCQS

"

b 4

'"

>

:::

::'

a

E 20

8

O

o

00

o

o

Av.roge Jo i nl Th i ckn.ss , Im [in]

joint thickness - solid bricks

Paramefer Solid bricks Per/orafed

bricks

'" 2·91 2·98

fi 3-80= 19.0 2'95=14'75

0·20 0 · 20

'" 4150364 = 11040 5600298 =18'80

requirements of the Code) the joints must not be more than 1 in thick, and shall preferably not exceed i in

in thickness (The ancient Egyptian, Greek and Roman

engineers, in whose stone temples, aqueducts, and other major structures the joints are usually extremely thin, often with no morta r at all, evidently had an intuitive sense of good practice in masonry construction.) Another interesting aspect of the results is the greater loss of strength with increase in joint thickness for the perforated bricks compared with the solid bricks It

seems certain that a major cause of this is the lower ratio of </>, i.e the greater weakness in lateral tensile strength in the perforated bricks caused by the presence

of the holes The theory indicates a less pronounced difference between the performance of the two types of brick than was found experimentally

The reason for this discrepancy probably lies in the method of estimating the lateral tensile strength, which

is admittedly crude and probably only gives a rough

approximation to the true value of this parameter

36 The Effect of Joint Thickness and Other Factors on the Compressive Strength of Brickwork

12

•

Q

0"4

0 2

o

00

02

o PQrfor " o\Qd Bricks

[12J So li d Bricks

0 "4

••

•

AV Qrog Q Joi n Th i ck n Qss , tm [i n]

I

•

1 0

FIGURE 8 - Variation of p with joint thickness: theoretical and

experimental resu l ts compared

Further, the failure criterion adopted in the theory

is only conjecturaI The value used in eqn (12) for

Poisson's ratio of the mortar also has an important

bearing on the result The figure ofO·25 was not measured

but was suggested by the Division of Building Research

CSIRO, Melbourne

3 FURTHER DISCUSSION OF THE MECHANISM

OF COMPRESSIVE FAILURE

On the basis of the model proposed above, other observed

phenomena associated with the compressive strength

of brickwork can be explained

3.1 Strength of Short 4t-in WalIs

The presence of vertical joints, which have a much lower

lateral tensile strength than even the bricks, may be

expected to reduce further the compressive strength

under axial load, and the greater their frequency in the

brickwork, the lower should be the compressive strength

Thus we would expect the four-high prisms used in the

present investigation, and standard brickwork

com-pressive strength specimens in the SAA Code, to be

stronger than 4t-in walling of small slenderness ratio

This is in agreement with experimental results, and Rule

6.7.1.5 in the Code requires that the basic strength of

brickwork (F'm) for the purposes of establishing design

stresses is to be taken as 0·75 of the minimum prism

strength

If an average bond strength of 60 Ibf/in2 is assumed at

the vertical joints, then since a tensile crack will pass

through joint and brick in alternate courses the mean

value of a ' l for the solid bricks in this study would be

(364+60)/2 = 212Ibf/in2 Eqn (J 2) then yields a value for

the strength of 4t-in walling which is in reasonable

agreement with the test results given by FRANCIS.4

3.2 Strength of Short 9-in WalIs

Continuing the above reasoning, we should expect 9 in

or thicker waIling to have a lower compressive strength than 4t-in walling, because of the presence of vertical joints in both directions This is again confirmed by experiment: Swiss results quoted by MONK5 are given below

Wal/ constru c tion

Single brick width Single brick width Single brick width Multiple brick width

Wall Relative thickne ss s trength

(in.)

5 1·00

6 0 · 89

7 - 10 0 " 80

10 - 15 0·68

Account is taken ofthis in the Swiss regulations govern-ing structural masonry.6

3.3 Brick and Mortar Properties The theory indicates that the strength of brickwork should increase with the compressive strength of the bricks, and with increase in the compressive strength (and therefore in the modulus of elasticity) of the mortar These two effects are well known and allowed for in all modem codes dealing with the form of con -struction Poor lateral tensile strength in the bricks compared with their compressive strength is known to have a deleterious effect on the strength of brickwork, not only from the work reported in this paper but also from the extensive test programme recently conducted

on storey-height walls by the British Ceramic Research Association,7 and from tests on storey-height walls and on wallettes for the Brick Development Research Institute.8 Provision for this effect has not been made

in codes in the past, but it may be desirable to place some limit on the reduction in cross-sectional area of perforated bricks to be used in highly stressed construction

3.4 Bond Strength Since the lateral strength or vertical joints depends mainly on the bond strength between bricks and mortar,

it is obvious that the axial compressive strength of brickwork must be improved if good bond strength

is achieved Bond strength is of COLme, of paramount importance wherever bending or eccentricity of load causes tensile stresses

3.5 Joint Reinforcement Steel reinforcement placed in the bed joints, even if only light-gauge, will substantially increase the compres-sive strength of the mortar and particularly the effective

value of E m and the Poisson's ratio of the jointing Tests by HENDRy9 showed that the compressive strength

of 4t-in storey-height walls was increased by over 60 %

when every course was reinforced with a patent woven mesh, but that if only every fifth COLme was reinforced there was no strengthening effect

4 CONCLUSIONS The mechanism of compressive failure developed in this paper certainly does not take account of all the relevant factors of importance, but it is a start in the right direc-tion

It appears to be capable of explaining a number of well-known but apparently unconnected phenomena associated with the behaviour of brickwork in com-pression, in particular the effect of the thickness of joints and of the lateral tensile strength of the bricks

A J Francis, C B Horman and L E Jerrems 37

ACKNOWLEDGEMENTS Thanks are due to Mr 1 C McDowall, formerly Director,

help with the project, and to Mr C Tonta for assistance

REFERENCES

Pittsburgh (May, 1965)

Strength of Brick Walls and Wal1ettes Special Rpt No I,

Ceram Soe (4) , 67, 1965