SHEAR BEHAVIOUR OF FLASHING MATERIALS IN BRICK-MORTAR JOINTS

K. S. Ibrahim I and G. T. Suter 2

1. ABSTRACT

WhiIe flashing materials typically allow some slippage to occur at the bot­tom of masonry walls and hence control their vulnerability to cracking, Iittle is known about the slip effectiveness of various flashing materiaIs. To examine the shear behavioural characteristics of flashing materials, a test apparatus was developed for the determination of shear resistance of a brick mortar joint using three-brick high prisms subjected to constant pre-compression. The shearing force was applied using a displacement controlled hydraulic jack and the rela­tive displacement between both sides of the interface was monitored using three linear variable differential transducers (LVDT's).

Three typical types of flashing materiaIs consisting of polyethylene, dual polycrepe and copperfibreen were tested at different flashing positions with re­spect to the mortar joint and brick face (centered, faced and without mortar) under three different leveIs of pre-compression. In this paper, shear load/slip relationships along with strength characteristics for different flashing materiaIs are presented. Results for mortar joints have been compared with those of the flashing materiaIs; significant reductions in the peak shear strengths of the latter have been observed.

Keywords: Flashing Materials, Mortar Joints, Shear Behaviour, Experimental, Damp-proof-course MateriaIs.

I Ph.D. Candidate, Civil Eng. Dept. , CarIeton Uni. , Ottawa, Ont., Canada, lOS 5B6

2 Professor, Civil Eng. Dept. , Carleton Uni. , Ottawa, Ont. , Canada, KIS 5B6

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2. INTRODUCTION

A number of surveys of masonry walls have concluded that ftashing materiaIs create a discontinuity which may allow some freedom for the masonry wall to slide, depending on form, loading, geometry, and the particular ftashing material being used (CIRIA, 1986).

Few publications have dealt with quantitative data such as inherent shear strength or shear load/slip relationships of ftashing materiaIs. Only, a limited number of shear strength determinations of bituminous ftashing materials were carried out by Beard et al. 1969. AIso, an experimental study on three-brick long specimens was carried out by Hodgkinson and West in 1982 to determine the shear resistance of some British ftashing materials. As part of more compre­hensive studies being carried out at Carleton University on movement problems in masonry walls, a test apparatus was developed for the determination of shear resistance of a brick mortar joint using 3-brick high prisms subjected to con­stant pre-compression. A previous paper (Suter and Ibrahim, 1992) dealt with an experimental study of the shear resistance of three typical types of ftashing materiaIs; polyethylene, dual polycrepe and copperfibreen were tested at differ­ent ftashing positions with respect to the mortar joint and brick face (centered, faced and without mortar) under three different leveIs of pre-compression.

This paper deals with the same three ftashing materiaIs but concentrates on the deformational characteristics of brick masonry bed joints containing these materiaIs. The results provide input data for nonlinear analytical studies of wall movements and the prevention of cracking.

3. EXPERIMENTAL WORK

For the present study involving ftashing materiaIs in bed joints, three-brick high prisms were utilized with bed joints oriented horizontally and the precom­pressive force applied vertically; the joint shearing force was applied horizontally.

From the many ftashing materiaIs available in North America, polyethylene, dual polycrepe and copperfibreen were chosen for testing because they represent relatively low-cost and widely used materials. The polyethylene had a thickness of 0.15mm. The dual polycrepe was composed of a 0.05mm polyethylene film asphalt bonded each side of a 81.35 g/m2 asphalt treated creped kraft paper; a 12.7 x 12. 7mm fiberglass scrim was interposed for added strength. The copper­fibreen was composed of 153 g/m2 pure copper asphalt laminated to 431 g/m2 lead with the lead si de asphalt laminated to 73.2 g/m2 creped kraft paper. A 12.7 x 12. 7mm fiberglass scrim was interposed between the lead and kraft paper for added strength.

190x75x57 mm (9% perforation) nominal size three-hole clay bricks were used in constructing the three-brick high specimens with 10 mm thick mortar bed joints. The mortar used was type S masonry mortar of 0.5:1:4.5 Portland cement:masonry cement:sand proportions by volume. The mortar's average 28-day compressive strength was determined as 11.7 MPa. The specimens were constructed by an experienced mason over a period of two days. They were then wrapped with wet rags and covered with polyethylene for 14 days; they were uncovered and exposed to laboratory conditions of about 20°C and 30 percent relative humidity until tested at an age of about three months.

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The flashing materiaIs were tested at different flashing positions with respect to the mortar joint and brick face: centered where the flashing material was placed in the middle of mortar joint; faced where the flashing material was placed in such a way that it contacted the mortar joint from one si de and the brick from the other side, and without mortar. Resultant shear resistances .were compared with the shear resistance of similar specimens with mortar joints containing no flashing materials. In a previous paper (Suter and Ibrahim, 1992), the faced and without mortar specimens illustrated the same shear behavioural characteristics. Therefore, in this paper, only the shear deformational characteristics of the centered and without mortar specimens will be presented.

Fig. 1 illustrates the apparatus used for the determination of shear resistance of flashing materials. Three different leveIs of precompression were applied: 0.3, 0.6, and 0.9 MPa; the compressive loads were kept constant during each testo A total of 90 prisms were tested; results given in this paper are the means of three determinations.

The upper and lower bricks of each specimen were restrained at one end by adjustable stops while the horizontal shear load was applied to the middle brick. The adjustable stops allowed some rotation to accommodate any slight variations in the specimens' vertical surfaces. To minimize nonuniformity of stress distribution, the two stops and the middle brick's end face were covered with a thin rubber sheet.

Fig. 1 Apparatus for Determination of Shear Resistance and Slip Behaviour of Flashing MateriaIs

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The lateral shear load was applied using a displacement controlled 90 kN MTS actuator. The rate of application of shear displacement was 1 mm/min, an arbitrary figure which was decided upon after some experimentation. The test was terminated when the overall relative displacement of the middle brick was about 10 mm. The maximum shear strength for the various fiashing materiaIs was generally reached at a displacement of less than 1 mm.

To monitor the relative displacement between the middle brick and the upper and lower stationary bricks, three LVDT's were arranged as shown in Fig. 2. The bricks' vertical faces in contact with the LVDT's were smoothed with a cementitious filler called Hydrostone. Each face was slightly sanded when the Hydrostone had hardened.

Fig. 2 Arrangement of LVDT's for Monitoring Relative Displacement

4. RESULTS AND DISCUSSION

Figs. 3 to 5 illustrate the shear load/displacement relationships for various fiashing materiaIs centered in the middle of mortar joints. The load/ displacement relationships for the case of mortar in the joints is depicted in Fig. 6 (note the difference in load scale as compared to Figs. 3 to 5). AIso, Figs. 7 to 9 illustrate the relationships for various types of fiashing materials pbced with no mortar in the joints.

Figs. 3 to 6 indicate that both the joints containing mortared-in fiashing materials and simply mortar behave in a nonlinear fashion with strain softening after reaching the peak load. On the other hand, the aforementioned strain

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softening behaviour is not noticed for the without mortar specimens. AIso, the different types of ftashing materiaIs tested with no mortar in the joint indicate very similar deformational behaviour and shear strengths as shown in Figs. 7, 8, and 9.

Fig. 10 illustrates the relationship between the maximum shear resistance and precompression for various ftashing materials for the cases without mortar in Fig. lOa and with mortar in Fig. lObo Note that for each of the two cases the various ftashing materials indicate almost the same shear resistance except for the case of copperfibreen placed in the center of the mortar joint (see Fig. 10b); that material leads to a significant increase in shear resistance.

Fig. 10 also shows a comparison between the shear resistance for various ftashing materials and for a mortar joint. The reduced shear resistance of ftashed joints compared to a standard mortar joint is deemed beneficial from a wall '5

load-slip point of view and potential cracking.

A comparison of the results shown in Figs. 10a for the ftashing materials in direct contact with the bricks indicates almost identical shear strength. This is to be expected since adhesion between ftashing materiaIs and mortar does not come into play for this case.

Table 1 shows the maximum shear strengths for fiashing materials corre­sponding to the case without mortar and with mortar. The values given in Table 1 are the means of three test results. The coefficients of variation (COV) indi­cated in Table 1 illustrate reasonable leveIs of variation in most cases.

Table 1 Shear Strength of Flashing Materiais

Precompression (MPa) Polyethylene

In.itial COV %

No Mortar

0 .3 0 .116 8.5 0 .6 0 .177 7.9 0 .9 0 .274 4 .2

Centered

0.3 0.203 9.1 0 .6 0 .295 24 .5 0 .9 0.414 19.5 N.A. = not av.ulable

Mean Shear Strength (MPa)

Type of Material Polycrepe

In.itial COV %

0.121 8 .5 0 .197 10.9 0 .253 1.9

0.178 15.4 0 .283 28.9 0.402 8 .2

Copperfibreen

In.itial COV %

0 .132 12.1 0.203 12.3 0.275 9.4

0 .385 7.9 0.417 5.4 0 .603 N.A.

Statistical analyses showed that for each set of results the relationship be­tween applied precompression and shear resistance is a straight line and that the shear resistance is made up of two components, a bond component (represented by the intercept with the vertical axis) and a frictional component (represented by the slope of the line) .

AIso, an examÍnation of the specimens after testing indicated that the joints typically failed at the fiashing material contact with the mortar or brick face; joint failure generally also produced local tearing in the fiashing material . In some cases, joints may also fail within the ftashing material itself as can be seen in Fig. 11 for copperfibreen.

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..... ..... N cn

Z 6 "'O ~ -o ...J

20

15 t-:' .. I , •• _ •• _ .....

10 Il··· •• r; ......

... -.... _ .... - .... _ .... -

_ •• _ •• . 0'. ~ 0.9 MP.

0'. ~ 0.6 MP. ___ 0'. = 0.3 MPa

_ ... - ... - ...

f························· " 51f'-........ ........... .

O .~--------~------~--------~------~--------~ O 2 4 6 8 10

Displacement (mm)

Flg. 3 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Polyethylene· Centered)

Z 6 -g o

...J

20

15

10

_ •• _ ••. 0'. = 0.9MP.

/"'. • •••••.• . 0'. = 0.6 MP.

/ ". ___ 0'. a 0.3 MP. . , t "'lo, .. . " i·'" ........ " ........ ...... .............

....................... -'" .~ .... ~.- .. ~:: ::. --'-. 5

O O 2 4 6 8 10

Displacement (mm)

Flg. 5 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Copperflbreen • Centered)

Z 6 "'O ~ o

...J

20 .-----~-------r------~------r-----~

15

10

0'. = 0.9 MP.

0'. ~ 0.6 MP.

___ 0'. = 0.3 MP.

{' .. _ ............ _ ....... _ .. _ .. _-_ .. _ .. -. . [r' ........... _ ......................... .

5~

O ~.------~------~----~------~----~ O 2 4 6 8 10

Displacement (mm)

Flg. 4 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Dual Polycrepe • Centered)

Z 6 "'O ~ o

...J

40~------~----~------~------~-------.

35

30

25

,' .. -.............. . .......... -.. l .. - - .... - .. _ ... ..

_ •• _ ••. 0'. ~ 0.9 MPa

0'. ~ 0.6 MP.

- __ 0'. ~ 0.3 MP.

20 ~! ........................................... . 15r 10 W

5

O ~.--------~--------~--------~------~--------~ O 2 4 6 8 10

Displacement (mm)

Flg. 6 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Standard Mortar Jolnt wlth No Flashlng Material)

N -...J

Z 6 "O rei o

...J

20 r-----~r_----~------~------~------_, _ •• _ •• . 0'. = 0.9 MPa

••••••••. 0'. = 0.6 MP.

15 - ___ 0'. • 0.3 MP.

10

5 ~:=::~::~::~:.- .-n-n_n ._. 1° •••••••••••••••••••••••••••••

o ~r ________ ~ ______ ~ ________ ~ ______ ~ ________ ~

O 2 4 6 8 10 Displacement (mm)

Z 6 -g o

...J

20 r-----~r_----~------~------~------_,

15

10 > .

_ •• _ ••. 0'. m 0.9MP.

. .••••••. 0'. = 0.6 MP.

____ 0'. = 0.3 MP.

5 ~./~~:.-.:~.-.:~.-.-.~: •• ~::~.-.:~ ••• ~.: •••.

, ###

.'

O ~(------~--------~------~------~------~ O 2 4 6 8 10

Displacement (mm)

Flg. 7 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Polyethylene· No Mortar)

Flg.8 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Dual Polycrepe • No Mortar)

Z 6

20r------~------~----~~----_r----__,

15

0'. m 0.9 MP.

0'. D 0.6 MP.

____ 0'. = 0.3 MP.

-g 10 o

...J .-.. - .. - .. _ .. - .... _ .... _ ... _--_ ... _ .... _--_.

5 ~ (~.~~: .••••......•..••••..••••.•••.••.....••....••....

Ir O ~r ______ ~~ ______ ~ ______ ~ ________ ~ ______ ~

O 2 4 6 8 10 Dlsplacement (mm)

Flg. 9 Shear Load Versus Shear Dlsplacement for Varlous Compresslve Loads (Copper1lbreen • No Mortar)

" c.. ; .:: Õ.

~ v; .. ~ .:: [/}

1.2

1.0

0.8

" / · .. ······················ · ······_······ · ········· ····~T .............•.............

0.6

0.4

I..........-V ~:

0.2

0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Precompression (MPa)

(a) Without Mortar

0.0 L-_ ...... __ ...... _--" __ ........ _---''"__ ........ _____ ...

0.2 0.3 0.4 0.5 0.6 0.7

Precompression (MPa)

(b) Centered

0.8 0.9 1.0

.. Polyelhylene • Polycrepe • Copperlibreen x Mortar

.. Polyelhylene • Polycrepe • Copperlibreen .. Mortar

Fig. 10 Relationships Between Shear Strength and Precompression for Various Flashing Materiais for the Cases of without Mortar

and with Mortar as Compared with a Mortar Joint

COPPERFlflREEN QUAL POL VCREPE F'.):~;h~nq Cu·nt~~ri' .• (\

Fig. 11 Joint Failure of Flashing Materiais

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5. CONCLUSIONS

• The shear loadjdisplacement relationships are found to be largely nonlinear for all the study's flashing materiaIs over the applied range of precompression (0.3 to 0.9 MPa).

• For all the flashing materiaIs placed centered in the mortar jbints, the shear behaviour of the joints experiences a strain softening phenomenon after reaching the pea.k shear load. On the other hand, for all the flashing materi­als placed in direct contact with the bricks , the shear behaviour of the joints displays no strain softening.

• The existence of mortar in the joint with flashing material controls the shear behaviour of the joint. This effect disappears when no mortar is provided in the joint; then alI the flashing materiaIs under study behave in the same manner. In other words, the adhesion at the contact surface between the mortar and each type of flashing material controls the joint shear behaviour.

6. ACKNOWLEDGEMENTS

The authors acknowledge with thanks the financiaI support provided by the Natural Sciences and Engineering Research Council of Canada. The authors also wish to thank Messrs. Mike Murray and Ken Trischuk for their assistance in conducting the experimental work.

7. REFERENCES

• CIRIA Practice Note, "Movement and Cracking in Long Masonry Walls" , London, 1986.

• Beard, R., Dinnie, A., and Richards, R., "Movement in Brickwork: (3) The Effect of Damp-Proof-Courses", Trans. Br. Ceram. Soc. 68, 1969, 87.

• Hodgkinson, H.R., and West, W .H., "The Shear Resistance of Some Damp­Proof-Course Materials", Proceedings of the British Cerarnic Society, No. 30, 1982, pp. 13-22.

• Suter, G.T. and Ibrahim, K.S., "Shear Resistance of Damp-Proof-Course MateriaIs in Brick-Mortar Joints", 6th Canadian Masonry Symposiurn, Saska­toon, Saskatchewan, June 1992.

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