84 Vth International Brick Masonry Conference

11-9. Some Factors Affecting the Bond Strength of Brickwork L.R. Baker

Deakin University, Geelong, Victoria, Australia, University of Calgary, Alberta, Canada

ABSTRACT

Factors that injluence the bond strength between brick and mortar include: the age of the specimen, curing conditions, the jlow of the mortar and the composition of the mortar.

A preliminary study of these factors indicated that the bond strength did not always increase with time after the first few days and that when it did the increase was not great. DifJerent curing conditions were found to have a significant injluence although it was difficult to reach general conclusions. The jlow of the mortar had a dramatic effect on bond strength. Within the wurkable range of mortars, the greater the jlow the greater the bond strength. The magnitude of this effect was dependent upon the composition of the mortar. Of the four compositions tested the high cement content mortar gave the highest bond strength at high jlow but nearly the weakest bond strength at low jlow.

I t is clear from this study that further investigations are essential to the understanding of the factors injluencing bond strength. A disturbing feature of the tests was that ultimate bond strengths less than permissible code values were obtained for some commonly used mortars.

INTRODUCTION

The bond strength of brickwork, in a strict sense , is the "gluing" or adhesive strength of mortar to brick. This is dependent on the properties of the two materiais at the brick-mortar interface. Practical\y, however, the quantity is measured by tests in which failure may sometimes occur within the mortar joint itse\f. AIso the keying effect of mortar in the core holes, or "frog", may make a significant contribution to measured strength.

Bond strength is calculated as a direct stress in a tensile­bond test, as a shear stress in a shear-bond test, or as a modulus of rupture in a flexural-bond test. In the direct and shear tests an ultimate load is divided by an area, but in the flexural test an ultimate moment is divided by an elastic section modulus. Usual\y the effect of core holes on section properties are neglected if they constitute less than 25% of the gross area . Any shrinkage cracks that occur in the joint are neglected. Such cracks would occur at the edges of the joint and hence reduce the section modulus to a greater degree than the corresponding reduction in area.

Properties of the mortar that affect bond strength are: air content, water retention, flow, composition, sand grad­ing, shrinkage, admixtures and aging properties. Brick properties influencing bond strength are: width of brick, initial rate of absorbtion, surface texture, core pattern and surface coatings of silicone, dust etc.

Many workmanship factors affect the bond strength achieved in practice: pressure applied to joint while laying, area of contact, thickness of joint, movement of brick after initial placement, laying conditions, curing conditions and e\apsed times between mixing, spreading and laying. This paper presents some experimental data on the influence of the flow of mortar, freezing and thawing, and aging, on the flexural-bond strength of brickwork.

FLOW OF MORTAR

The effect of varying the flow of mortar on the flexural­bond strength was investigated using four different mor-

tars in Series I tests. Canadian materiais were used in mak­ing stack-bonded specimens:

PC . .. .. .. Portland Cement Type 10

MC .... .. Masonry Cement Type H

L ........ High Calcium Hydrated Lime Type S

Brick ..... 197 x 92.5 x 57 mm pressed, 3 cores 40 mm dia, with an I.R.A. of 3.2 kg/m 2/min .

Each mortar had a cementitious materiais to sand aggre­gate ratio of 1:2Y2. Cementitious materiaIs in the four mortars were PC:L, PC:MC, MC:L in equal parts by vol­ume respective\y, and MC. For each of these different composition mortars four different flows were used , rang­ing from the wettest workable mix to the driest workable mixo These extremes were judged by the bricklayer. To obtain this range of flows for each mortar the initial mix was made to the wettest workable consistency and suffi­cient mortar taken from it to make three nine-high stack­bonded piers. While these piers were being made, dry ingredients, in the correct proportion, were added to the mixer to reduce the flow a desired amount. After remov­ing sufficient mortar for the next set of specimens, the process was repeated until ali four sets of specimens had been built.

Approximately 12 minutes was required to build each set of specimens so the last sets were made 36 to 48 min­utes after initial mixing. To check that this did not have a significant effect on flexural-bond strengths, a set of specimens were built with a fresh batch of MC mortar with a flow of 115%. Comparison was made with specimens made from a similar flow mortal' obtained by adding dry ingredients and built 27 to 40 minutes after initial mixing. It was subsequently found that these strengths were prac­tically the same, thus validating the mixing procedure.

After 28 days storage in the laboratory at 20°C each joint in the prisms was tested to failure in a machine ' designed by the author and built at the University of Cal­gary. Each joint was c1amped between a fixed and sus-

Session lI, Paper 9, Some FactoH AJJecting the Bond Strength oJ Bnckwork 85

pended frame and a moment applied via the suspension shaft. Results are shown in Table I and Fig. I. lt is elear from the graphs that within the practical range of the four mortars, increased strength was obtained by increasing the flow of the mortar. This result supports some codes2. 3 that require "the maximum amount of water to produce a workable consistency." Direct tensile-bond tests by Isber­ner4 using Masonry Cement ranging in flow from 100% to 135% also indicated that bond increased with increasing mortar flow. Curves he obtained were convex-up, indicat­ing a relatively minor drop in strength as flow was reduced slightly below the maximum workable consistcncy. Ali curves in Fig. 1 (except MC:L), however, are concave-up and indicate a drastic reduction in flexural bond as flow is reduced from the maximum workable consistency. The common MC mortar used at a flow of 125%, considered by the bricklayer to be normal practice, produced a flex­ural strength only about a quarter of that obtained with mortar of maximum workable consistency.

For the materiais used the flow of the mortar was an important and very sensitive variable determining flex­ural-bond strength. One composition mortar did not con­sistenly give the highest bond strength for ali flows. The MC mortar had a much lower strength than the PC:L mortar which is often assumed to be its equivalent. A dis­turbing feature is that the measured ultimate flexural bond strength of the commonly used MC mortar was less than the permissible value in most codes of practice.

FREEZING AND THA WING

The National Building Code of Canada5 requires that "when the mean daily temperature at the job site falls below 4Y2°C (40°F), mortar, water and masonry units shall be maintained at a temperature not less than 4Y2°C during laying ... and shall be protected from freezing for 48 hours after laying." This requirement in substance is sup­ported by the Brick Institute of America6 and other codes. The British 7 and Canadian codes also prohibit the use of calcium chloride as a frost inhibitor in the mortar.

A pilot study of the effects of laying brickwork at freez­ing temperatures and of subsequent freezelthaw cyeles on the flexural-bond strength was investigated by Series 11 tests. Two mortars were used. Each contained Masonry Cement Type H and sand in the proportions 1: 3 by vol­ume. One of the mortars had Anhydrous Calcium Chlo­ride added at the rate of 1 Ib/bag of cement. Both mortars had a flow of 125%. Extruded brick 250 x 80 x 70 mm with five 25 mm dia cores were used. Seven-high stack­bonded piers were made with these materiais. Ali speci­mens were tested at an age of 45 days. One group of specimens was built inside the laboratory at 20°C and another group built outside at just below freezing. Of the former group, some were left in the laboratory until tested and hence did not go through any freezelthaw cyeles. After the first 24 hours the remaining specimens were placed outside in below freezing temperatures and some returned to the laboraory after a further 24 hours. These specimens went through one freezelthaw cyele. Those specimens that remained outside until testing day went through approximately 30 freezelthaw cyeles.

Of the group of specimens built outside, some we.·e left outside to go through the 30 freezelthaw cyeles before being tested. Others were moved into the laboratory after the first 24 hours to go through only one freezelthaw cyele. Others were moved at 24 hour intervals to go through two freezelthaw cyeles.

On the testing day, ali joints except those broken in handling, were tested in flexural-bond as described in the previous section. Results are shown in Table II and Fig. 11. lt can be seen that the mortar with calcium chloride added produced higher flexural-bond strengths for both the specimens built in the laboratory at 20°C and those built at O°C. This result confirms research by the Struc­tural Clay Products Institute8 that coneluded that this admlxture in small quantities does not significantly reduce bond strength. The amount of calcium chloride used in the present tests (I Y2%) is probably insufficient to effec­tively lower the freezing point of the mortar. Its significant effect is to accelerate initial set and liberate heat of hydra­tion at a faster rate during this initial period, resulting in increased strength.

An unexpected result was that, for both mortars, a higher flexural-bond strength was achieved by those spec­imens that initially set at O°C, than those that set at 20°C. Repeated freezelthaw cyeles led to a reduction in flexural­bond strength for specimens built with mortar containing calcium chloride. Although the detailed picture is not elear, these results are in general agreement with tests by the Structural Clay Products Institute.8 They reported that although compressive strength of brickwork is adversely affected by freezing, bond strength remains unaffected.

AGEING

lt is usually assumed that the flexural-bond strength of brickwork increases with time as the hydration of the cement becomes more complete. Some previous tests9

indicated that greatest strength occurred as early as three to seven days after building.

Two subsequent series of tests have been carried out. Series III tests used the mortars PC:L, PC:MC, MC:L as specified in Series I with flows of I 15%, 140% and 120% respectively. Pressed bricks 149 x 67 x 50 mm containing three 40 mm dia cores were used. The initial rate of absorption of the bricks averaged 3.2 kg/m 2/min .

Series IV tests used three mortars composed of Ordi­nary Portland Cement, Hydrated Lime and Sand. The proportion of cementitious material to sand aggregate was 1: 3 for ali mortars. Cementitious materiais for each of the three mortars were PC:L in equal volumes, PC and L. Flows were 90%, 95% and 90% respectively. Extruded bricks 230 x 110 x 75 mm with 5 cores 64 x 20 mm were used, having an initial rate of absorption of AO kg/m2/min. In Series IH tests, two seven-high stack-bonded piers were used and in Series IV, ten single joint specimens were used to establish flexural-bond strengths at each age. Ali specimens in both series were cured in the laboratory in air at 20°C.

Results are shown in Table lU and Fig. lIl. No elear relationship exists between flexural-bond strength and age. In the Fig. IIIb the 28 day strength has been stan-

86

dardized and the ratio of the flexura l strength at age t, FI> to the strength at 28 days, F2s , has been plotted. Here, there is no fixed value for this ratio in the early stages of curing. For example, the ratio of seven day strength to 28 day strength, F7/F2s , varies from 0.43 to 1.25. Also it can be seen that reductions in flexural strength sometimes occur as the age of the specimens increase.

Apart from the variability of the material, there are sev­eral possible reasons for this erratic behaviour. Firstly, shrinkage cracking of the outer portions of the joints may reduce the effective section modulus. If shrinkage cracks extend 5 mm in from the surface of the joints in a normal sized stack-bonded pier, an apparent reduction in strength of about 28% would be measured.

Secondly, as pointed out by Isberner,4 hydration of the cement continues only while the relative humidity of the mortar exceeds about 85%. He found that the relative humidity at a depth of 'li! the thickness of the brickwork reduced to 80% after three days of drying at 28°C and 50% relative humidity. Hence hydration in the outer por­tion of the joint ceased after three days. Other measure­ments showed that hydration at the quarter-points and centre of the brickwork underwent 12 and 15 days hydra­tion, respectively. As flexural strength is largely deter­mined by the strength of the material in the outer fibres, the drying rate of the outer portion of the mortar joints is most important. In previous tests9 specimens cured in air for seven days showed a marked increase in strength when water cured. Another consideration is that hydrated­lime does not harden in water but requires carbon dioxide in the air for carbonation to occur. This was indicated in Series IV tests where some specimens made with lime mortar were placed under water after initially setting in air for 24 hours. When removed from the water after seven days, they had practically zero strength but then gained strength at the same rate as the specimens that had been air cured from the time of building. This is shown in Fig. IlIa. Shrinkage cracking, hydration of cement and carbonation of lime, appear to have a complicated inter­action in the development of flexural-bond strength in brickwork that is not well understood.

CONCLUSIONS

The flow of mortar is a sensitive and important para meter influencing the flexural-bond strength of brickwork. Max­imum strengths are obtained with mortars of wettest work­able consistency.

The flexural-bond strength of brickwork made with masonry cement morta r was not greatly influenced by con­struction at freezing temperatures, freezelthaw cyeles, or small additions of calcium chloride to the mixo

Vth lnternational Brick Masonry Conference

The apparent erratic behaviour of the flexural-bond strength of brickwork as it ages points to a complicated interaction of shrinkage cracking, hydration of cement, and carbonation of lime that needs more detailed inves­tigation.

The varying values of flexural-bond strengths measured in the experimental work described here indicates that present knowledge of this property is insufficient to pre­dict it from a few simple parameters. Direct measurement is desirable for ali structural brickwork, particularly in view of the low values obtained with commonly used materiais.

ACKNOWLEDGMENT

The work reported in this paper as Series I, 11 and 111 tests was performed in the masonry research laboratory of the Department of Civil Engineering, University of Cal­gary, Alberta, Canada, while the author was there on study leave. Work was financed jointly by I. X.L. Industries Lim­ited, Medicine Hat Alberta and the Department of Civil Engineering. Special acknowledgement is accorded the Technical Staff of the Department, particularly H. john­son and R. Rodney.

The remaining work was performed at Deakin Univer­sity, Victoria, Australia, financially supported by the Brick Deveiopment Research Institute, Melbourne, Victoria. Grateful acknowledgment is made to R. Randall and R. johnson for their valued experimental work.

REFERENCES

I. Baker, L.R., "Measurement of the Flexural Bond Strength of Masonry" paper presented at this conference. 2. Brick Institute of America Standard Specification for Portland Cement-Lime Mortar for Brick Masonry BIA-MI-72. 3. American Society for Testing and MateriaIs "Tentative Speci­fications for Mortars for Unit Masonry" Part 12, 1967 ASTM C270-G4T. 4. Isberner, A.W., "Properties of Masonry Cement Mortars" Designing Engineering and Constructing with Masonry Products. Gulf Publishing Company, Houston, Texas, 1969. 5. National Building Code of Canada, 1965, Part 4. 6. Brick InsLitute of America "Recommended Practice for Engi­neering Brick Masonry" Virginia, 1975. 7. British Standards Institute "Code of PracLice for Structural Use of Masonry" BS 5628 : Part I : 1978. 8. "Cold Weather Masonry ConstrucLion", Technical Notes I, IA, Structural Clay Products Institute, December 1967, January 1968. 9. Baker, L.R. and Franken, G.L. "Variability Aspects of the Flex­ural Strength of Brickwork", Proceedings Fourth International Brick Mãsonry Conference, Brugges, April 1976.

Session lI, Paper 9, Some Factors Affecting the Bond Strength of BTickwork 87

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90% 100% 11 0% 120% 130% 140% Fl ow

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~ . 1 I n i t I a 1 set at 20°C ::> x LU ..J ....

o L-~ __ ~ ____________________________ ~

No . o f Freeze!Thaw Cyc 1 es

Figure 2. Effect of Freezing and Thawing

Figure 1.

Effect of Flow

30

T ABLE I -Effect of Mortar Flow-Series I Tests

I Portland I Portland I Masonry I Masonry Cement ) Cement ) Cement ) Cement )

I Lime ) PC:L I Hasonry PC:MC I Lime ) MC:L 2V2 Sand ) MC 5 Sand ) Cement ) 5 Sand )

Mortar 5 Sand )

Flow% 145 125 115 100 150 130 11 5 95 140 125 100 90 140 125 11 5 95

N 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24

J .70 .51 .37 .36 .6 1 .24 .11 .04 .32 .31 .24 .2 1 .26 .07 (.049 O (.052*

V .09 .20 .23 .29 . 15 046 .55 .81 .19 .20 .29 .24 .29 .74 .67 -

N = No. of Tests . J = Mean Joint Strength (MPa) . V = Coefficient of Variation . 'Value oblained using fresh ly mixed morlar

TABLE 2-Effect ofFreeze/Thaw Cycles-Series 11 Tests

Specimens Built in Specimens Buil t outside

No. of Freezerrhaw Cycles Laboratory at 20°C at O°C

O I 30 I 2 30

I Masonry Cement ) N 7 8 12 10 12 11 ) Flow 125% J .15 . 18 .19 .22 .21 .20

'- 3 Sand ) V .34 AO .20 .43 .31 .35 ~ '-o

:2: I Masonry Cement ) N 10 12 12 12 12 12 )

Flow 125% J .26 .26 .2 1 .37 26 .25 3 Sand ) V .55 .32 .2 1 .2 1 .16 .17 Plus Calcium Chloride

88 Vth lnternational Brick Masonry Conference

TABLE 3-Effect of Ageing

Age in Days

Mortar 1 2 3 7 14 28 35

1 Portland Cement ) PC:L

N 4 9 9 9 1 Lime )

Flow 115% J .11 .23 .27 .227 5 Sand ) V .18 .32 .23 .49

-- 1 Portland Cement 9 9 9 - ) N '" PC:MC QJ 1 Masonry Cement ) J .48 .47 .43 'C Flow 140% QJ 5 Sand ) V .18 .15 .28 VJ

I Masonry Cement ) MC:L

N 9 9 9 I Lime )

Flow 120% J .16 .17 .27 5 Sand ) V .08 .25 .33

I Portland Cement ) PC:L

N lO 10 10 lO \O 10 I Lime )

Flow 90% J .32 .22 .33 .27 .25 .61 6 Sand ) V .16 .31 .31 .27 .42 .38

I Portland Cement ) PC

N 10 10 lO 10 lO 3 Sand )

Flow 95% J .32 .61 1.55 .98 1.34 > V .29 .28 .38 .26 .40 -'" QJ

'C I Lime ) N 10 10 10 lO QJ L VJ

3 Sand ) J .02 .03 .11 .23 Flow 90%

V .25 .1 8 .12 .14

I Lime ) L

N lO 3 Sand )

Flow 90% J Cured in Water for 7 Days .12 V .39

I Portland Cement ) PC:L

N .30 30 20 20 1 Lime )

Flow 95% J .60 .77 .70 .56 Q') 6 Sand ) V .19 .17 .25 .24 ....;

QJ

~ I Portland Cement ) N 14 16 15 8 16 PC:L

1 Lime ) Flow 95% J .36 .3 1 .37 .41 .41

6 Sand ) V .33 .37 .19 .39 .39

N = No. ofTests,j = MeanJoint Strength (MPa), V = Coefficient of Variation

Session lI , Paper 9, Some Factors AJJecting the Bond Strength of Brickwork

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1.4

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1.2

1.1

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O.~~----~~~--+---------------~------~ 14 28 35 1 2 3 7

Time days

F t

F28

Time days

Figure 3. Effect of Ageing

89