Session li, PapeT 8, Measw'ement of the Flexural Bond StTength of Masonry 79

11-8. Measurement of the Flexural Bond Strength of Masonry L.R. Baker

Deakin University, Geelong, Victoria, Australia

ABSTRACT

R esearchers have used a vaTiety of tests to measure lhe flexuml bond stTength of brickwoTk where failure occurs along a bed joint. At least thTee countries, Austmlia, America and Britain, have now adopted a standaTd test melhod, each slandard being different from lhe olher. This makes it difJicult lo relate results obtazned by the respective standards. Even within the AmeTican and British standards there are alternative optzons that znfluence the result obtained thus making comparisons even more difJicult. This paper conszders the advantages and disadvanlages of several test methods including lhe standard ones. A method of calculatmg a derzved )omt strength from each test method is proposed lhat allows direct comparison of results. Charts gzvzng the values of pararneters to be used in calculations aTe included.

INTRODUCTION

Flexural masonry specimens that span in the vertical direc­tion fail at a mortal- bed-joint. Failure initiates at the ten­sile face or the specimen, usually by a separation of the brick-mol-tar interface , but sometimes by fracture within the mortar joint. Irrespective of the mode of failure, the flexural bond strength of masonry is calculated as the extreme fibre bending stress at failure based on the e1astic section modulus of the specimen.

Many factors affect the actual flexural bond strength that develops but this paper is concerned with the differ­ing results that may be obtained if the same quality masonry is tested by different methods. Researchers have used many different beam specimens loaded in various ways.

The first standard test was that introduced in Australia in 1969 with the publication of the SAA Brickwork Code AS CA47 1969, Essentially the specified test was to be carried out on site and was consequently rather crude. The American Society for Testing and Materiais in 1974 introduced "Standard Methods of Tests for Flexural Bond Strength of Masonry" E 518-74. These are laboratory tests in which supports and loadings are well defined. The Brit­ish Standard Code of Practice for StructUl-al Use or Masonry BS 5628 : Part I : 1978, is the most recent and contains a test procedure for the "Experimental Deter­mination of Characteristic Flexural Strength of Masonry."

Within these three standards there are alternative options resulling in 13 different possible standard teslS to measure the flexural strength of masonry_ Other tests recently suggested by Hughes of the Brick Development Research Institute Australia,' Huizer and Ward,2 and the author, add another four tests to the list. This paper describes and compares these tests and provides a means of comparing the quality of brickwork tested by the dif­ferent methods,

THEORETICAL CONSIDERATIONS

Ali test specimens subjected to vertical flexure fail at a mortar joint, and, hence it is lhe strength properties of the joints that determine the load at which the test beams fai!. I f ali joints in a test beam had identical strengths then failure would occur at the joint located at the position of

maximum bending moment. The modulus of rupture cal­culated from such a test would not depend on the type of beam used.

In practice, however , ali joints do not have identical strengths and both strong and weak joints are included in a beam specimen. Where each joint in such a specimen is subjected to the same applied moment the beam will fail at the weakest joint. In general, the applied bending moment is not the same at each joinr and ali joints in the specimen do not have identical strengths so failure does not necessarily occur at the position of maximum bending moment nor at the weakest joint. Here , failure of the beam occurs at the joint where the ratio of applied bend­ing moment to joint strength is greatest.

The above considerations have been treated quantita­tively in a theory reported elsewhere." It was shown that if beam specimens are assumed to be made from a pop­ulation of joint strengths that is normall y distributed with negative values truncated, both the mean strength of the beam and its ~oefficient of variation aI-e c10sely predicted by the theory. Hence for a given mean and coeffiClent of variation of joint strengths the various strengths of the standard beam specimens can be predicted. Conversely, the mean and coefficient of variation of joint stI-engths can be derived for a particular standard beam if the mean and coefficient of variation of the beam strength is mea­sured.

Suppose the mean modulus of rupture obtained from a set of beam specimens is B and its coefficient of variation is VB . If the joints from which the specimens were made had a mean mooulus of rupture j, and coefficient of vari­ation Vi, these may be expressed as

J = m B Vi = V VB

Theoretical values of m & v have been calculated for the various test specimens as described in the next section. Hence for any or the standard beam tests J and Vi can be derived, thus allowing comparison of joint strengths that are independent of the type of bealll specilllen used.

A USTRALIAN ST ANDARD TEST

In this standard three nine-high stack-bonded prisllls are required to be lllade in a prescribed way and to be cured

80

by wrapping in a polythene sheet. At seven d ays after manufacture they a re tested as indica ted in Fig. I. Bricks are stacked on the central portion until failu re occurs. The three results are averaged.

While this has served as an exce ll en t site control-test, it has obvious limita tions. Neither the location of the su pport reactions nor the loadi ng points are precisely known. As the load is applied by stacking bricks the recorded fai lure load may be too great by a n amount equa l to the weight of one brick . Only three results give a poor measure of variability.

Since the loading configuration on the beam is ill­defined, it is ass umed to be a third point loading case and is treated in the following section.

AMERICAN ST ANDARD TEST

Here a minimum of five stack-bonded prisms are req uired to be made in a specified manner and cured at 24 ± 8°C and at 30% to 70% relative humidity . Prisms may contain 4, 7, 10, 13 or 16 courses . At 28 days after building the specimens a re tested in the horizontal position either by the application of a uniform load or by loads applied at the third points as indicated in Figures II and II I. The average modulus of rupture and its coefficie nt of variation are recorded. When the net bedded area is less than 75% of the gross area, the net section modu lus is used in cal­culations.

A limita tion of the tes t is that some specimens, par ti c­ula rl y those sixteen courses high , are likely to fail under self-weight when lowered to the horizontal posi tion. The longer specimens will measure a smalle r modulus of rup­ture than the shorter specimens because o f the greater likelihood of weak joints being included in the former.

Uniformly distributed load method

In this case the self-weight bending moments have the same distribution as the applied bending moments. Hence the combined bending moment diagram is parabolic irre­spective of the ratio of self-weight to applied load at fail­ure . Here, failure is likely to be neither at the position of maximum bending moment nor at th e weakestjoint in the specimen, but at a j oint where there is a criticai combi­nation of applied bending moment and joint strength .

Values of mu and Vu have been calculated on this basis and are presented graphically in Fig. VIII.

For these specimens:

Third-point load method

In this test only th e central third of the beam is sub­j ected to maximum moment and it is likely that the strength of th e beam will be d ecided by th e weakest joint in this length . Less often the beam strength will be deter­mined by a weaker joint in the remainder of the beam. The standard requires that beams th at fa il in this manner must be disca rded from calculations. It is difficult to see why, for it is only fortuitous that the faili ng joint was not

Vth lnternational Brick Masonry Conference

in the central section of the beam. In fact, if it happened to occur in the central section , a lower modulus of rupture wou ld have been measured and accepted as a valid result. The standard is inconsistent in this respect beca use it accepts the uniformly loaded test as valid no matter which joint fail s. When calcu lating J & V; these tests should not be excluded. Third-point loading produces a trapezoidal bending moment diagram while that due to self-weight is parabolic. If failure results from self-weigh t on ly, the val­ues of mu and Vu apply. On the other hand , when self­weight moments are insignificant at failure, values of m . and v. applicable for the trapezoidal distribution of moments should be used. These are shown in Fig. IX. Between these extremes, intermediate values of m and v apply. For simplicity, a linear interpola tion is suggested.

In general:

J = [pmu + (l - p) m,] B

Vj = [pvu + (I - p) v,] VB

where p is the ratio of maximum extreme fibre self-weigh t stress to the average modulus of rupture , B.

BRITISH ST ANDARD TEST

Ten wall etts, two bricks wide and ten or fourteen courses high are required to be tested by this standard, twenty eigh t days after buildin g. No curing conditions are speci­fied. Four-point loading is used as indicated in Fig. IV. A characteristic strength is obtained by converting each mea­sured strength to a logarithmic value and then ca lculating a mean and standard deviation. The log-normal distribu­tion assumed by this procedure results in a higher char­acteris tic strength than would be obtained if the usual nor­mal distribution was assumed.

In the Australian and American Standard tests the beam strengths a re determined by th e strength of a single joint. The British specimens, howeve l-, a re two bricks wide and both joints in a cO Ul-se must fai l before the beam collapses. Tests4 011 specime ns made with I : 1:6 mortar have shown that such beams fail when the applied bending moment at a course is equal to or greater than the su m of the strengths of both joints in the course. Unpublished work by the author confirms that this also applies to specimens made wi th mortars rich in cement or lime. This averaging of adjacentjoint strengths tends to make the fl exural bond strength measured by the British test specimens higher than those measured by stack-bonded prisms.

As this standard requires the beam specimens to be tested in th e vertical attitude there is ap parently no self­weight moment. When the specimen bends however, it tends to be supported on the bottom edge at the tension face. This produces a self-weight moment that res ists the applied lateral load and, unless a llowed for, resu lts in toa high a calculated modulus of rupture. To remove uncer­tainty it wo uld be preferable to definitely locate the reac­tion from th e floor at the centre of th e specimen by plac­ing rollers at this location. In this case no self-weight bend­ing moment would result. The effect of compressive stress on mb and Vb is negligible and both self-weight compres-

Session 11, P.aper 8, Measuremenl of lhe Flexural Bond Strenglh of Masomy 81

sive stress and moment were neglected in calculating the values in Fig. X.

For this case:

OTHER TESTS

Three other tests are considered here. In each , end moments are applied to a stack bonded specimen so that each joint is subjected to essentially the same momento Failure in these cases is at the weakest joint. An advantage of these methods is that they are capable of testing single joints so that J and Vi can be obtained by direct measure­ment. Devices made by the Brick Development Research Institute fit on the end bricks of a prism and provide roller bearings through which forces from the upper and lower plattens of a standard testing machine apply end moments. The arrangement is shown in Fig. VI.

Metal beam extensions used by Huizer and Ward , clamp onto the end bricks of a prism to effectively increase its length. A four point loading system is used in a standard testing machine to apply a uniform moment over the brickwork section of the beam. As originally used the total weight of the specimen was carried by the outer supports, leading to high self-weight mornents. The author used the device as shown in Fig. V. Here the self-weight moments are greatly reduced and each joint is subjected to approx­imately the same moment.

In a special testing machine designed by the author, an end brick is clamped to a solid frame and the other end brick is clamped to a frame suspended from a central shaft. Turning the shaft applies end moments to the spec­imen. Two universal joints in the shaft make the system self-aligning and do not confine the specim.en .to bend about a predetermined neutral axis. The machll1e IS shown in Fig. VII. Since the machine is specially designed to measure joint strengths, it can do so quite rapidly. A test­ing rate of 30 joints per hour is comfortably achieved.

In ali these devices, specimens containing one to four joints may be tested. Equal moments are applied to each of these joints and this condition has been used to calculate me and Ve in Fig. XI:

DISCUSSION

A method of comparing the results of various flexural bond tests by calculating a derived joint strength has been presented. From the mean and coefficient of variation of the modulus of rupture measured from beam tests , the mean and coefficient of variation of the composite joint strengths can be derived. The derived values are largely dependent on the measured coefficient of variation of the beam tests. Hence the accuracy of the derived values is largely dependent on the number of specimens tested. The Australian standard with only three tests and the American with five tests, do not give good estimates of this quality. Even the ten tests specified in the British code do not give a high levei of confidence. Despite this diffi­culty if one is to make a valid comparison 01' results from different beams, such a procedure must be followed. A gl-éater number of tests would be desirable to more accu­rately define the properties. If say 30 beam tests were required to be performed, costs cou ld beco me prohlbltlve.

On the other hand, if ali joints in five seven-high stack­bonded piers were tested , thirty results would be obtained and accurate direct measurement of joint strengths would be obtained economically. Tests on single joints can be carried out in standard machines using the adaptions described earlier. Where numberous specimens are to be tested the special machine mentioned would become eco­nomical.

It is suggested that for research purposes the direct measurement of the flexural bond strength of single joints is desirable. Whenever standard beams are used to mea­sure flexural bond, it is suggested that a derived joint strength be also reported, or at least sufficient infor~ation given to allow others to derive it from the charts glven 111

this paper.

REFERENCES

I. Hughes , D.M., Brick Development Research Institute Aus­tralia. Technical Note. 2. Huizet·, A. and Ward , M.A. , "A Simplified Flexural Bond Test for Clay Brick Masonry" Pl'oceedings North Amel'ican Masonry Conference, Boulder, August 1978. 3 . Baker, L.R. and Franken , G.L. , "Variability Aspects of the Flexural Strength of Bl'ickwork" Proceedings Fourth Interna­tional Brick-Masonry Conference, Brugge, April 1976. 4. Bakel', L. R. , "The Failul'e Criterion of Bl'ickwol'k in Vertical Flexure" Proceedings Sixth International Conference on Load Bearing Bl'ickwol'k , No. 27 London , December 1978.

82 Vth International Brick Masonry Conference

or 14 Courses

FiguTe I . Australian Standard Figure IV. British Standard

4,7,10,13 or 16 Courses

Figure 11. American Standard Figure V. Huizer and Ward

4,7,10,13 or 16 Courses

Figure IIl . American Standard Figure VI. Hughes

Session 11, Paper 8, MeasuremenL of Lhe Flexural Bond SLrengLh of Masonry 83

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