1:1. i\\illi:JtIII-e tellsile :11 .. 1 It:lllel flex.llre Itl-.tlterties .tf Itl;~I~\,. ... -I(.

V A. Youl and Dr. P. K. Foster. Amalgamated Brick and Pipe Co., Ltd., Auckland, New Zealand

Introduction

Since the coming of man, his need for shelter has been a prime requirement of his environment. Research in building and construction is often neglected because of this traditional need, whilst developments such as air-<:raft, power, space traveI, and synthetics have been bom of research and are further nurtured by large research expenditures. Within the building industry, this anaIysis applies to the use of materiaIs and structural systems. Stone and brick were two of the earliest construction materiaIs known to man, these materiaIs provided many of man's early forms of shelter. Their survivaI today as building systems is due to the inherent advantages of the system and not to research and development.

New ZeaIand is in a belt of high seismic activity. This, more than any other factor, increases building costs and restricts the use of masonry structural systems.

Through research in England, Europe and the U.S.A. have developed the design of multi-story brick buildings. These buildings take into account lateraI loads occasioned by wind. Such forces seI dom equate the dynarnic conditions imposed by earthquake in relation to the period of the building. Further, multiple lateraI partitions restrict the building design and limit the structural systems.

Until earthquakes of 1929 and 1931 brick was a major structural and facing material in New ZeaIand building. The failure of many brick buildings in these disasters caused engineers and architects to distrust masonry. Such distrust will not be aIlayed until sound design criteria are established through research on, and dynamic testing of, materiaIs and practices used in building construction.

Recognizing that the most fundamental factor goveming the strength of unreinforced brickwork was the bond between brick and mortar, the N.Z. Pottery & Cerarnics Research Assn., in 1959 embarked upon an extensive research programme to evaluate the effect of the properties of materiaIs on bond

87

mechanism and bond strength. Results of early work, pub­lished by Youl1, Tytherleigh & Youl2 illustrate the mortar properties necessary to achieve maximum bond strength. Many other parameters remain to be investigated. A mathematical analysis by Kenna3 of types of brickwork showed that in a brick veneer construction some 90 percent of lateralloads are carried by the veneer and only 10 percent by the frame, which had been previously assumed to carry all the loads. This analysis has been confirmed by face loads tests on small panels reported by Kenna 13

This chapter presents results on the tensile strength of the brick-mortar interface as determined by minature tests and by the flexure of brick masonry panels. The work is part of a long-term project designed to examine factors affecting the tensile strength of masonry; it includes criteria of importance to successful design and use of masonry in addition to establishing the validity of miniature or model testing as a basis for future work.

The need for establishing the tensile behaviour of masonry is dictated largely by ignorance both of its optimum properties and of the sensitivity of these optimum properties to variations in materiaIs. The low figure of 6.7 p.s.i. for the maximum tensile load permitted by the New Zealand Model Building By-Iaw4 in the design of uninspected masonry reflects the uncertainty surrounding the properties and variability of masonry.

Confirmation of brick masonry's properties using New Zealand materiaIs would be necessary even if published oversea's work gave a sufficient understanding of masonry to permit full advantage of materiaIs to be taken in designo The latter, however, is far from being the case. Inevitably much of the work suffers from inadequate specification of materiaIs used in experiments. It is not until relationships between properties are established that the necessity for their specifica­tion in description of experiments becomes apparent. The resulting lack of detail in much of the published work makes it impossible to relate the results of different experiments. Comparisons can be made only in very broad terms, and difference between results must remain largely unexplained.

88 Designing, Engineering, and Constructing with Masonry Products

Cement-lime-sand mortars have received most attention and hlgh values of initial flow, high values of water retentivity and low values of the initial rate of absorption of brick emerge clearly as being criteria beneficiaI to the strength of brick­morta r bond. The importance and interaction of such factors as sand/cement ratio, sand grading, sand particle shape, physical and chemical properties of the cement and lime (or other plasticizer) in affecting the tensile bond strength of brick masonry, have been far from adequately studied.

Thus there is an urgent need for a thorough investigation of the tensile bond strength of masonry, and the present work is one of the first steps. Because of many factors to be investigated, a miniature test method was essential to obtain information at reasonable cost and in reasonable time. Thls paper reports on the development of a miniature test method, and compares the data it gives with data from panels tested in flexure and with data from the literature. Work has been restricted to cement-lime-sand mortars at thls stage because thls gave most scope for comparison of results with published data. Future work will use the miniature test method in statistically planned experiments on the effects of variables using other plasticisers in common use in New Zealand. The intention is to confirm the more important results with full-size tests from time to time. Thls will go far towards achieving the objections of determining the optimum bond strength; the properties of the mortar and brick to give that strength, . and its sensitivity to deviations from the optimum properties.

Definitions and Standard Test Methods

AlI testing and reporting on properties of mortar and bricks is based on standard test methods wherever these apply .

Mortar Tests and Properties

The two properties measured, initial flow and flow after suction, are standard ASTM methods. In comparisons between results in this paper and other work, the only reliable comparison is when flow after suction is expressed as a percentage of initial flow.

Initial Flow. Thls property is examined in accordance with ASTM CI09-585 and is a measure of the workability or flow of mortar.

Flow after Suction. This property is examined in accordance with ASTM C91-606 . As it is a measure of mortar's resistance to loss of water due to brick suction, it is calIed water retentivity in this paper. It is also a measure of mortar's workability after placing.

Brick Tests and Properties

From early work it was well established that the rate at which brick absorbs water in the first minute of contact with the mortar bed is of prime importance in establishing a good bond performance. After checking scale effect, alI samples,

both for the miniature tests and full-size bricks, were measured by the same method.

Initial Rate of Absorption (IRA). This measurement is taken in accordance with ASTM C67-627 alI resultk of miniature and full-size brick are corrected and expressed in grams of water per 30 sq. inches of brick bedding surface.

Experimental

Miniature Tests

Mortars. Cement-lime-sand mortars, with a constant ratio of cement to sand of 1: 5 by weight , were used throughout. The cement and lime carne from New Zealand in sealed drums of the same buIk, thus preventing variation between deliveries. Natural river sand, screened to pass 3/16", was separated into size fractions and reconstituted to give the grading analysis shown in Table 13-1.

Sieve Size (NZSS 196)

7 14 25 52

100 Pan

Table 13-1

Percent Retained Each Sieve

o 14.7 29.1 38.8 12.0 5.4

AlI mortar preparation was carried out in a constan! temperature room (25°C) because previous work showed this increased reproducibility in mortar properties for a given composition. The dry hydrated lime was soaked in distilled water overnight, and mortars were then prepared following a procedure based on ASTM C305-59T8. Mortars were char· acterised by their initial flow and retentivity properties determined according to ASTM C 1 09-58 5 and C91-60r6

respectively. The required leveIs were obtained by adjusting the amounts oflime and water only.

Bricks. Special mini-bricks made commercially from a Canterbury loess in a shape approximating to a 2 in. cube, had two opposite wedge shaped faces to facilitate gripping for tensile testing. The bricks were subsequently re-fired in a laboratory kiln to give 'Initial Rate of Absorption' (IRA) values less than 25, and selected into three ranges as follows: 3-6, 8-12, and 15-20. The bonding face was ground smooth because preliminary work showed this improved the repro· ducibility of bond strength tests.

Bonding, Curing, and Testing. Five replicate couplets were prepared from each mortar. A non-absorbent plastic mo1d

produced a mortar bed 1/2 in. thick, covering the bonding face. The brick was then placed mortar-bed upwards in the bonding machine and the upper brick, positioned in the gripS

Tensile and Panel Flexure Properties of Brickwork 89

of the machine, was lowered pneumatically. Apressure of 1 ~ p.s.i . was exerted on the mortar bed. Two procedures were followed ; the second of which included relative motion 150

between the bricks to simulate the sliding imparted by the bricklayer.

Method A: Pressure was maintained for 10 seco and the grip-head was then retracted.

Method B: The upper brick was oscillated through one cycle of ± 15° about its axis perpendicular to the mortar bed while under pressure. The grip-head was then retracted.

The mini ature method (b) has advantages over the method described in ASTM EI49-59T9 . Comparison of coefficients of variation obtained in the present work and elsewhere e.g., Monk 11, show that accuracy is no less, while the reduction in size of the test pieces and therefore in time and cost of materiaIs and test equipment is significant.

The couplets were cured for 7 days at 25°C and 65 per cent relative humidity. Each couplet was then subjected to a tensile load perpendicular to the mortar bed in a Hounsfield tensometer. A tension spring was connected in series with the couplet to give a sufficiently slow and reproducible rate of load with the motorized drive.

Panel Tests

Mortars

The materiaIs were the same as used for the rniniature tests. Because larger quantities of mortar were needed, a different rnixer and longer mixing times were required. The latter reduced the initial flow and increased the retentivity for a given composition. The water and lime contents were adjusted accordingly to attain the same retentivity leveIs. The constant temperature room was not used. Three mixes were prepared for each panel.

Bricks

Commercial wire-cut perforated brick, 2~ X 3* X 8 in., were used. The Initial Rate of Absorption (IRA) range was 110 to 133 as received. Experiments showed that two minutes submersion in water, followed by three hours draining, reduced the IRA to between 7 and 23, with a mean of 12. Increasing the draining time by two hours increased the mean to 14. Two minutes submersion followed by three hours draining at the start of laying each panel was adopted for the work.

Laying and Testing

The procedure was generally that used by Kenna 13. Seven panels were laid in stretcher bond by a bricklayer, each 16 COurses high and three bricks wide and measuring approx­imately 48 inches by 24 inches. The panels were aged at room temperature and humidity for 28 days. Laying time for each panel was approximately 1 ~ hours.

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Figure 13-1. Effect of initial flow and retentivity on bond strength using bonding methods A and B.

Quarter-point Iive loads were applied in 100 lb. increments in a Baldwin Universal Testing Machine.

Results

Miniature Test

IRA was found to have no significant effect on bond strength, for the three ranges studied (3-6, 8-12 and 15-20).

Figure 13-1 gives the results for the bond strengths obtained with the two test methods, for mortars with different flow and retentivity properties, and bricks with IRA in the range 8-12. Each point is the mean of five couplet results.

As discussed later, the method described in Bonding, Curing, and Testing (B) was preferred, and accordingly its results were examined more closely. The Iines for three values of retentivity and the values of initial flow of 105 and 130 per cent were tested statistically for significance, using the coefficients of variation for the six points which averaged 22 percent. It was shown that the three Iines could be taken as parallel; it was confirmed statistically that their slope was highly significant. More important were that the difference between the 50 and 65 percent retentivity lines was high1y

90 Designing, Engineering, and Constructing with Masonry Products

significant, and that the difference between the 65 and 80 percent retentivity lines were significant only at the 5 percent leveI.

Panel Tests

The results of the flexural tests were expressed as the fibre stress at failure, (f) and were ca1culated from the relation

where f = M/Z M = bending moment, and Z = section modulus.

(l3-l)

They are shown in Figure 13-2 for the seven panels tested.

Discussion

In this discussion, the limitations imposed by insufficient specification of materials (mentioned in the Introduction) must be remembered. At the present stage of this work these limitations apply to a large extent, and care must be exercised in making generalisations on the detailed results reported here.

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Figure 13-2. Correlation between brick panels and minibrick couplets.

The results will first be compared with other published work. ASTM method E149-599 has been used for mini ature investigations. Ritchie and Davison 10 investigated brick IRA presenting a curve with a maximum bond strength at IRA of 15 to 20. Their data is such, however, that the bond strength could well be constant over the IRA range 5 to 20, within experimental error, and therefore not inconsistent with the present work. Ritchie and Davison 10 also published a curve of tensile bond strength using bricks of IRA 19-22 as a function of initial flow of 1: 1:6 mortars, but they do not state water retentivity. The results are approximately equal to the present results for 50 percent retentivity, using the method described in Bonding, Curing, and Testing (B). In comparing the properties of conventional cement-lime-sand mortars with latex-bearing cementitious mortars, Monk 11 compiled bond data using 16 cement-lime-sand mortars in crossed-brick couplet tests. His results are similar, but lower than those in this experiment. Fishburn 12 reported the results of many crossed-brick couplet tests and obtained bond strengths of up to 70 p.s.i. at high flows (140 to 145 percent) and high retentivities (83 to 88 percent) with brick of IRA of 4 to 9. The Figure l3-2 compares the panel results with those obtained by Kenna in the same laboratory, but using a different source of bricks. Monk 11 reported an average stress of 77 p.s.i. for 4 by 8 feet panels tested vertically in flexure using 1: 1: 6 mortar, (curing period unstated) and stresses of 42 to 61 p.s.i. obtained in diagonal tension tests on 16 in. by 16 in. wallettes using 1: 1:6 and 1: 1:4~ mortars cured for 7 days. The flow and retentivity properties of the mortars were not specified so that little comparison with the present work is possible.

The preceding general comparisons with recently published work show that there are no major inconsistencies, but that significantly high stresses have been achieved, both with the miniature tests in the present work, and particular1y with panels constructed in this laboratory (Figure l3-2). Present work, Kenna 13 and Ryder l4 , achieved stresses up to 260 p.s.i. but Ryder's 14 method required fracture through brick as well as through mortar, and is therefore not a test of the bond strength alone. The differences in degree between the various sets of data confirm the need for investigating the many possible variables outlined in the Introduction.

A major object of the present study was to test the validity of rniniature test methods for investigations of the factors influencing the ultimate tensile strength of the brick-mortar junction in masonry. A broad comparison of the results in Figure l3-2 shows that this has been achieved in that, with increase in initial flow up to 130 percent, all methods give an increase in strength, and with initial flow values greater than l30 percent there is no comparable increase in performance. In making these comparisons, remember that the miniature test results in Figures l3-1 and 13-2 are each the mean of five tests, while the paneI results are from one panel at each leveI. The three results in Figure 13-2 obtained by Kenna 13 give some indication of the reproducibility to be expected under the conditions of test.

Figure l3-2 shows that the tensile strength of mini ature samples is approximately half that of the panels tested. Tlús is

Tensile and Panel Flexure Properties 01 Brickwork 91

probably largely due to a combination of the smooth, machined surface of the miniature bricks, the 7-day curing for the miniature couplets as opposed to 28 days for the panels, and imperfect simulation of the tradesman's action in the miniature tests.

The importance of curing time is shown by failures in the mortar, which occurred at high values of initial flow with the miniature test but were not reproduced by the panels. This points to the desirability of 28 day curing for the miniature test in future work.

A number of points of general importance concerning the engineering properties of brick masonry arise from the results:

1. Attention must be drawn to the inter relation between the variables initial flow, retentivity of mortar, and brick IRA. The first determines the amount of water in the mortar during initial setting if given values for the two others. The retentivity can be a measure of the ability of the mortar to retain water against the suction exerted by the brick, while the value of IRA of the brick can be a measure of its ability to reduce the water content of the mortaL

The resuIts in Figure 13-1 apply only to brick with IRA in the ranges given in the experimental section of this paper, and it is emphasized that strengths of the order given in Figure 13-1 for the same mortars would not necessarily be obtained with these mortars in contact with brick of higher IRA.

2. Retentivity is confirmed as an important property, which should be maintained at a sufficiently high value. The statistical examination of the results obtained with method B, Bonding, Curing and Testing, suggests that, with brick of IRA less than 20, little is gained by increasing the mortar retentivity beyond 65 percent. The improvement was signif­icant only at the 5 percent leveI. The results obtained with the panels confirrn this, to the extent that of five panels at 120 to 135 percent initial flow giving approximately constant strength, one was constructed. from mortar with 65 ~ercent retentivity. Results obtained by Ritchie and Davison O also indicate that the small effect of varying retentivity between 70 and 78 percent. No significance is attached to the lower strength obtained for Panel No. 7 (80 percent retentivity) compared with Panel No. 5 (65 percent retentivity); the results are probably within experimental variations and the slightly shorter curing period (24 days) of Panel No. 7 is not significant. The gain accruing from retentivities greater than 65 percent require further quantitative investigation.

3. Of all the system parameters so far studied, initial flow of mortars exerts the greatest effect on bond strength. The results in Figure 13-1 cIearly indicate the magnitude of the effect. I t also shows that for mortars with a cemen t-sand ratio of 1:5 by weight in contact with brick oflRA 5 to 20, optimum strength is obtained at an initial flow near 130 percent. Strengths consistent1y attainable with controlled materials are approximately twenty times the loads currently permitted in designo

Previous work has indicated that maximum bond strengths will be achieved at about 130 initial flow or above. The Handbook on Reinlorced Grouted Brick Masonry Con­struction 15 states "Mortar when used should have an original

flow of from 135 to 145%". Plummer and Blume 16 state, "The substantial increase in tensile bond strength with increased mortar flow indicates the importance of controlling this variable in assembling tensile bond specimens, and also of maintaining a high flow in mortar used in masonry con­struction". The "high flow" in this case refers to a range of 125-135%.

It is believed that strengths obtained with initial flows greater than 130 are irrelevant to brick masonry, when used in conjunction with brick of mean IRA in the range 8-12. Under laboratory conditions, they were not considered workable by the bricklayer, and certainly resulted in excessively dirty and therefore unacceptable brickwork. To this extent, the lower strengths obtained for the panels at high initial flows are unlikely to be met in practice, and differences between the results of the two miniature test methods need not be resolved. Figure 13-1 shows that loss of strength is large with values of initial flow lower than 100 percent. Accordingly, it is conc1uded that for brick of IRA range less than 20, the range of initial flow of interest is 100 to 130 percent.

Summary

A good correlation exists between strengths obtained from panel tests and from a miniature test method. The latter is suitable for further investigation of the effects of variables on brick mortar bond strengths.

At present, considerable care must be exercised in gen­eralising from particular experimenlal results.

For the cement-lime-sand mortars used, properties have been found in broad agreement with published work as follows:

1. Bond strength was found to increase with initial flow up to a value of about 130, and remained approximately constant up to 155, for brick IRA of 8-12.

2. Bond strength was found to increase strongly with retentivity up to a value of 65% and only slightly from 65% up to 80%.

3. Bond strength was insensitive to IRA of brick for the range 3 to 20. For IRA less than 20, the range of initial flow of practical interest is given an upper limit of 130 by workability, and a lower limit of 100 by loss of bond strength.

Consistent flexural strengths of brick panels 20 times greater than the figure perrnitted by the N. Z. Standard Model Building By-law4 can be obtained.

References

1. Youl, V. A., "Design of Buildings," Pape r 2 c.B.I., Part I, Australian Building Research Congresso 1961.

2. Tytherleigh E. St. J. and Youl, V. A., "Design of Buildings," Paper 2 C.B.I., Part 11, Australian Building Research Congresso

3. Kenna, L. F., "Some structural features of brick veneer construction," Technical Report No. 12, New Zealand Pottery & Ceramics Research Assn., 1963.

4. "Design and Construction : Masonry," Chapter 9.2 New Zealand Standard Model Building By-Law, NZSS 1900, New Zealand Standards Inst., 1965.

92 Designing, Engineering, and Constrncting with Masonry Products

5. "Standard method of test for compressive strength of hydraulic cement," ASTM CIOS-58, American Society for Testing and Materials, 1958.

6. "Standard specification for Masonry cement," ASTM C91-60T, American Society for Testing and MateriaIs , 1960.

7. "Standard method of sampling and testing brick," ASTM C67-62, American Society for Testing and MateriaIs, 1962.

8. "Tentative method for mechanical mixing of hydraulic cement pastes and mortars of plastic consistency," ASTM C305-59T, AmeI. Soco for Testing and MateriaIs, 1959.

9. "Tentative method of test for bond strength of mortar to masonry units," ASTM E 149-59T, American Society for Testing and Materials , 1959.

10. Ritchie, T. and Davison, J. 1., "Factors affecting bond strength and resistance to moisture penetration of brick masonry," ASTM Special Tech. Publ. 320: 16-30 , 1963.

11. Monk, C. B., "Testing high-bond c1ay masonry assem_ blages," ASTM Special Technical Publication 320:31-66 1963. '

12. Fishbum, C. C., "Effect of mortar properties on strength of masonry ," Monograph 36, US National Bureau Standards 1961,p.45. '

13. Kenna, L. F., "Brickwork and earthquakes in New Zealand," Proc. 3rd World Conference Earthquake En­gineering, 3, Session IV: 292-311 , 1965 .

14. Ryder, J. F., "The use of small brickwork panels for testing mortars," 62(8):615-29 Trans. Brit. Ceramic Society, 1963.

15. Handbook on Reinforced Grouted Brick Masonry Cano strnction, 2nd ed, Brick Institute of California, Los Angeles, 1962.

16. Plummer, H. C. and Blume, J. A., "Reinforced brick masonry and lateral force design," Structural Clay Products Inst., Washington, D. C., 1953.