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 quanti­tative, 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 quanti­tative.

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 verschiedener Grossen auf die Druckfestigkeit eignet. ExperimenteI! und theoretisch ist bewiesen, da.fJ die Festigkeit von Prismen aus je vier Ziegeln mit grosser 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 quanti­tative 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 stack­bonded 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 com­pressive 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 Factors on the Compressive Strength of Brickwork

~~

d y

~ c1Zb

d xb

O'x d y

° xm O

zm 0y

[a]

FIGURE I-Brick and mortar stresses due to applied axial com­pressive 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 [(axb+ vb(a)'-arb)]

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

(1)

(2)

SimiJarly, in the mortar joint, the extensionaJ strains are:

exm= ~m[ -a.HI1 + vm(ay- azm)]

ezm= ~m[ -azm+ vm(ay -a.Hn)]

where Eb = modulus of eJasticity of brick; Em = modulus of elasticity of mortar;

Vb = Poisson's ratio for brick; Vm = Poisson' s ratio for mortar.

(3)

(4)

But the lateral expansion is assumed to be the same in brick and mortar. Thus,

(5)

(6)

AIso, for equilibrium, the totallateraJ tensile force on the brick must be balanced by the total lateral compressive force on the mortar joint, in both the x and z directions. In the x direction, therefore :

or

where

Similarly,

tb rx = ­tm

tb = thickness of brick,

tm = thickness of mortar joint.

Gzm = CXUz 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 zm with the aid of eqns. (7) and (8), we find that

ai f3 vm- Vb) a xb = a zb = - --"-""-----':.:..-,--

1 + rxf3 - Vb - rxf3vm (9)

where f3 = Eb . Em

The lateral tensile stress a xb induced in the brick is bound to reduce aull, the value of ay at which compressive failure occurs ; in the limit, ifaxb and azb 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 Tensil e Stress

o' \

FIGURE 2- Theoretical envelope relating lhe tensile and com­pressive stresses in brick at failure.

extreme the limit would be the compressive stress allll = a' ulr. the value necessary to ca use failure in the absence of lateral tensile stress. (Point B on Figure 2).

The way in which the va\ue of allll varies with a xb and a zb between these extreme limits is not known for a brittle material like brick, but HILSDORF2 has suggested that, for simplicity, the linear Tresca shear criterion be adopted. His relationship can be expressed by the equa­tion:

(lO)

, where </> = a ~ll

a I

Substituting this expression for a xb 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 rxf3(1 - Vm), and p can be represented with sufficient accuracy by the following equation:

1 p= (12)

1 + <f>(f3vm - vb) 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 com-

A. J. Francis, C. B. Horman and L. E. Jerrems 33 pression instead of tension. The failure envelope for this condition is again not known for brick material, but we can reasonably conjecture that ali I/ will exceed a' ult, its value when axb=O (see the broken line in Figure 2).

In a prism with three or more bricks, on the other hand, the middle bricks may be expected to be fairly free from the influence of the platens, and to be subjected to some tensile stress according to the mechanism described in Section 1.

A dramatic loss of strength can be expected as the number of bricks in the prism increases, and this is confirmed by experiment, as shown by the results in 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 Extruded Br icks

o Pressed Br ie!<s

Mortor I ~ 4 ~

Eoeh Fbi nt i , Averoge 01 6 Tes\s

Number o f Cour5~s i n Prism

FrGURE 3- Effect of number of courses on compressive strenglh of prisms.

Department, University of Melbourne. A similar result was obtained by WEST and others3 using 9-in. brickwork cubes. These results suggest strongly that in a prism with four or more bricks the compressive strength is not appreciably influenced by the end effects. For this reason, four-brick prisms are adopted in the SAA Brickwork Code, AS CA47-1969, as a standard test specimen for the measurement ofthe compressive strength of brickwork. Probably five or even six-brick prisms, as in current USA practice, would have given even greater freedom from end effects, but they would be toa tall to be accommodated in many compression testing machines designed for concrete test cylinders, and also would take longer to make.

2.2 Thickness of Joint and Lateral Tensile Strength of Bricks

2.2.1 Introduction The simple theory outlined in Section 1 was verified as regards the effect of the parameter ex by a series of tests on four-brick prisms in which the thickness of the mortar joints was varied.

In order to study the effect of varying </>, two types of brick were used: a solid pressed brick (with frog) and an extruded brick with seventeen circular perforations (Figure 4).

2.2.2 Brick Properties

The mean dimensions of the bricks, taken from a large number of individual measurements, are shown in Table I.

The compressive strength of each type was measured on twelve single bricks according to the procedure set

FIGURE 4-Plan view of perforated brick.

TABLE l - BRICK DIMENSIONS

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.

out in AS CA47-1969, Rule 6.4. The lateral tensile 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

P P

~

Br ick

i t P P

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 a - -­/- 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.

AlI values for perforated bricks are based on gross 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 TABLE 2-STRENGTH ANO PROPERTIES OF BRICKS

Solid bricks I Perforated brick s Property

Value Coe.ff of Value Coe.ff of vaI'. varo ( %) ( %)

Mean compressive strength, (x) 9530 8·5 8070 6,5

Minimum compressive strength (C) 8592 - 7470 -

Lateral tensile { 310 16·8 strength (a ' ,) along 364 8·1 section X-X (Fig. 4) 298 10·7

Lateral tensi le - - f 308 9·8 strength along section Y -Y (Fig. 4) - - l 277 16·5

Modulus of elasticity Eb (lbf;in2) 3·8 x IG6 2·95 X 106

Poisson's ratio Vb 0·25 0·25

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/in2) variation

927

Other mean values used were:

( %) 8·6

Em 0·20 X 106 1bf/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 compres­sion 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, Vb and Em quoted above and in Table 2 were determined from these measurements, the latter from measurements across the joints.

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 Results (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

T ABLE 3A- PRISM COMPRESSIVE STRENGTHS (allll ):

SOLID BRICKS

i

Average Average joint alll! joint aUll

thickness (lbflin 2) thickness (lbflin 2)

(in.) (in.) I

4070* 0·38 3030

4720* OAO 3080

zero (dry) 5660* 0·67 2300

22401' 0·63 2660

2050t 0·65

I

2450

1840t 0·64 3120

O 4220 0·67 2980

0·02 3950 0·62 3180

0·05 4600 0·68 2570

0·02 4020 1 ·00 2130

OA2 2920 1·00 2290

0·41 3150 1·00 2190

OAO 2890 1·00 2020

0,41 2810

• Polished faces . t Masonry saw-cut faces.

TABLE 3B- PRISM CO:\1PRESSIVE STRENGTHS (aul,): PERFORATEO

BRICKS

Average Average joint aull

I

joint aul, thickness (lbfl in 2) Ihickness (lbf/in 2)

(in .) (in.)

5430* I 0·31 2460

8760* I

0·33 2440

zero (dry) 3810t 0·31 2810

3940t 0·54 2305

3930t 0·57 2213

3990t 0'56 1780

O 5450 0·57 2530

O 5770 0·53 2305

0·02 5475 0·55 2420

0·04 4450 1·00 1128

0·35 2970 1·00 1255

0·32 I 2625 J ·00 1255

0·35 I

2750 \·00 123\ I

I * Polished faces . t Masonry 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 concentra­tions, 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 isms Four Bricks High

04

o Dr y Pr isms , Pol ishQd FOCQS

o Dry Prismsí Mosonry So",­Cul FOCQS

• V orious Jo inl Thi cknesses

--- Averog. Curve

0 8 10

AvorogQ Joinl Thi cknQss , trJl Qn]

FIGURE 6-Variation of prism compressive strength with mortar 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; further­more, 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/in 2 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

PQrforotQd Br ick Prisms

Four Br ic" s HI9h

t760 I bf/ i n'

60

"" ":'--O~ I I' 5600 Jbf/in' Q o

o Dr y Pr i sms ; Po li shQd FOCQS

" b 4

'" >

::: ::' a. E 20 8

O

o 00

o

• Dry Pr i 5ms , Mosonry Sow­Cul Foces

o Vori ous Jo inl Th ick ness.s

- AverogQ CurvQ

o

Av.roge Jo inl Th ickn.ss , Im [in]

FIGURE 7- Variation of prism compressive strength with morta r joint thickness- solid bricks.

TABLE 4-PARAMETER VALUES

Paramefer Solid bricks Per/orafed bricks

'" 2·91 2·98 ---- ----Im 1m

fi 3-80 = 19.0 2'95 = 14 '75 0·20 0 ·20

'" 4150 = 11040 5600= 18'80 364 298

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

• 501 i d Bricks

o PQrfor"o\Qd Bricks

[12J Solid Bricks

0 "4

•• •

06 0 8

AVQrog Q Joint Th i cknQss , tm [i n]

I •

10

FIGURE 8- Variation of p with joint thickness: theoretical and experimental resu lts compared.

Further, the fail ure 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 com­pressive 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/in 2. 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/ construction

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

Wall Relative thickness strength

(in.) 5 1·00 6 0 ·89

7- 10 0"80 10- 15 0·68

Account is taken ofthis in the Swiss regulations govern­ing 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 con­struction. 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 compres­sive strength of the mortar and particularly the effective value of Em 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 direc­tion.

It appears to be capable of explaining a number of well-known but apparently unconnected phenomena associated with the behaviour of brickwork in com­pression, 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, Brick Development Research Institute, Melbourne for help with the project, and to Mr C. Tonta for assistance in the testing.

REFERENCES 1. BRADSHAW, R. E., Structural Ceramics. Conference on Industrial

Building and the Structural Engineer. Inst. Str. Eng., 1966. 2. HILSDORF, H. K., Investigation into the Failure Mechanism of

Brick Masonry Loaded in Axial Compression. ' Designing, Engineering and Construction with Masonry Products.' Edited by F . B. Johnson. Houston, Texas, Gulf Publishing, 1969. pp. 34-41.

3. WEST, H. W. H. , EVERILL, J . B. and BEECH, D. G ., The Testing of Bricks and B10cks for Load-bearing Brickwork. Proc. X Int. Ceram. Congr., Stockholm, 1966. pp. 559- 565.

4. FRANCIS, A. J., The S.A.A. Brickwork Code: The Research Background. Inst. Engrs. Aust. , Civil Eng. Trans., October 1969. pp. 165-176.

5. MONK, C. B. Jr., Old and New Research on Clay Masonry Bearing Walls. 1st Nat. Brick and Tile Bearing Walls Conf., Pittsburgh (May, 1965).

6. SCHWEIZERISCHER INGENIEUR UND ARCHITEKTEN, Technical Note No. 113. Verein, SIA, Oct. 1963.

7. WEST, H. W. H ., HODGKINSON, H. R. and DAVENPORT, S.T.E., The Performance of Walls Built of Wire-cut Bricks with and Without Perforations. Trans. Brit. Ceram. SOCo 67, (10) , 434, 1968.

8. McDoWALL, L c., McNEILLY, T. H. and RYAN, W. G., The Strength of Brick Walls and Wal1ettes. Special Rpt. No. I, Brick Development Research Ins!., Melbourne, Nov. 1966.

9. PRASAN, S., HENDRY, A. W. and BRADSHAW, R. E. Crushing Tests on Storey-height Wal1s 4·Hn. Thick. Proc. Brit. Ceram. Soe. (4) , 67, 1965.