behaviour of interlocking mortarless block masonry

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Behaviour of interlocking mortarless block masonry

M. S. Jaafar, A. H. Alwathaf, W. A. Thanoon, J. Noorzaei and M. R. Abdulkadir

Various types of interlocking mortarless (dry-stacked)

block masonry system have been developed worldwide.

However, the characteristics of dry joints under

compressive load, and their effect on the overall behaviour

of the interlocking mortarless system, are still not well

understood. This paper presents an experimental

investigation into the dry-joint contact behaviour of

masonry and the behaviour of interlocking mortarless

hollow blocks for grouted and ungrouted prisms under

compression. Two experimental test set-ups are proposed

to evaluate the contact behaviour of dry joints, considering

the geometric imperfections in the contacting faces. The

results show that the contact behaviour of a dry joint is

highly affected by geometric imperfections in the block

bed. Different patterns of deformation are distinguished in

mortarless hollow (ungrouted) and grouted prisms. Dry

joints predominantly affected the hollow prism

deformation until the compressive load reaches 0$57 of the

maximum load. However, this behaviour is not common in

grouted prisms, because noticeable deformation

commences after 0$38 of the maximum load.

Furthermore, the variations of strength and deformation

in grouted specimens are diminished compared with those

in ungrouted specimens.

1. INTRODUCTION

Interlocking mortarless (dry-stacked) block masonry offers greatadvantages in masonry construction. The main feature of theinterlocking hollow block system is the elimination of mortarlayers: the blocks are interconnected through interlocking keys(protrusions and grooves). The goal in any interlocking systemis to ensure efficient construction formation with well-alignedmasonry structures, even without skilled masons. There havebeen several attempts to develop interlocking hollow blocks invarious parts of the world.1–5 However, these blocks vary widelyin their dimensions, shapes and interlocking mechanisms. Thereis much evidence that mortarless masonry will be as good astraditional masonry, and a competitive alternative to it, if itspeculiarities are taken into account.

Among the unconventional masonry systems, Putra Block hasbeen developed recently in Malaysia as a load-bearinginterlocking mortarless hollow block system.6 Extensive researchhas been carried out to investigate the compressive strength ofthe system under concentric and eccentric loading using

individual blocks, prisms and walls.7 The shear characteristics ofthe system have been investigated using a modified triplet testset-up under different pre-compression loads.8 The dry joint(block-to-block interface) characteristic and its influence on thedeformation and failure mechanism of the interlockingmortarless block masonry system under compression still requiremore study, not only on the Putra Block system but also on othersystems. This matter has not received sufficient attention invarious studies.5,9–11

When concrete blocks are stacked without filler or bondingmaterial, they are still in contact via virtual filler (roughness,texture, etc.) rather than via the material properties of theblock units themselves. A close-up photograph of a mortarless(dry) bed joints is shown in Fig. 1. The absence of the fillermaterial (mortar) in the block beds and head joints creates aproblem of geometric imperfection in the dry joint, which isthe main shortcoming of the system. This phenomenon needsto be investigated in terms of its effect on the joint contactbehaviour. Oh12 presented an experimental test procedurethat can estimate the dry joint contact behaviour, includingthe geometric imperfection effects. However, that test isappropriate only for simple cases and for specimens oflimited size.

In this paper, an experimental investigation is presented for thecharacteristics of dry joints and the behaviour of mortarlessmasonry under compression load. Single- and multiple-joint testset-ups have been developed to evaluate the contact behaviour ofdry joints, taking into consideration the influence and variationof the geometric imperfection of the block beds arising fromdifferent causes. The essential load–deformation curves of the dryjoints that are required for future use in numerical modelling ofthe system are also described. Furthermore, the characteristics of

Dry bed joint

Fig. 1. Mortarless (dry) masonry bed joint

Construction Materials 159 Issue CM3 Behaviour of interlocking mortarless block masonry Jaafar et al. 111

M. S. JaafarCivil Engineering Department,Faculty of Engineering,Universiti Putra Malaysia,Malaysia

Ahmed H. AlwathafCivil Engineering Department,Faculty of Engineering,Sana’a University,Yemen

Waleed A. ThanoonCivil Engineering Department,Faculty of Engineering,Petronas University ofTechnology, Malaysia

Jamaloddin NoorzaeiCivil Engineering Department,Faculty of Engineering,Universiti Putra Malaysia,Malaysia

Mohd. Razali AbdulkadirCivil Engineering Department,Faculty of Engineering,Universiti Putra Malaysia,Malaysia

Proceedings of the Institution ofCivil EngineersConstruction Materials 159August 2006 Issue CM3Pages 111–117

Paper 14228Received 04/05/2005Accepted 24/08/2006

Keywords:brickwork & masonry/concretestructures/strength and testing ofmaterials


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an interlocking mortarless masonry system are investigated fromthe first application of the load to final failure using hollow(ungrouted) and grouted prisms. Results for deformationcharacteristics, failure mechanism, and compressive strength arepresented and discussed.

2. EXPERIMENTALTEST

PROGRAMME

The block units used in the testspecimens have been producedespecially for the current studyusing a semi-automaticblock-making machine on theuniversity campus. Theinterlocking block unit used inthe study is shown in Fig. 2.More details of the developedblock system can be foundelsewhere.6

2.1. Contact behaviour test

of bed joint

Contact behaviour is a complexproperty in the mortarless joint,andbecomesmore sophisticatedwith the unevenness of thecontacted interfaces. Thisunevenness is called thegeometric imperfection of theblock bed. Two types ofgeometric imperfection of theblock bed were observed in themortarlesswalls. Thefirst type iscaused by the variation ofregularity and roughness of theblock bed interfaces. Afterstacking blocks in a wall, animperfection in the dry bed jointcan be observed due to the

variationof the height of the adjacent blocks. This is the second typeof imperfection. Verification of the dimensions of a batch of blocksshowed that differences in the block heights were G0$25 mm. Theactual measurement of the variety of imperfections of dry joint in awall is quite difficult. Therefore a contact test is required to study thegeometric imperfection of the block bed arising from differentsources.

Two contact behaviour test set-ups were used to explore thesetwo effects: single- and multiple-joint contact tests. In thesingle-joint contact test, the closure deformation of block bedinterfaces is investigated under uniaxial compressive load asaffected by the irregularity and roughness of the contactinterfaces (Fig. 3). In the multiple-joint test (Fig. 4), the interfaceclosure is investigated as affected both by the same irregularityand roughness and by the variation of the height of the adjacentblocks (multiple effects). A face-shell bedded joint is consideredin the study because compressive load is transferred by this joint.

2.1.1. Test specimens.

(a) Single joint. In this test, 10 identical small prisms (SPR1 toSPR10) having a single joint formed between two blockquarters were used, as shown in Fig. 3(b). The top quarterwas cut from the lower block half and the bottom quarterfrom the upper block half. These parts are actually the regionlocated between two block heads, as shown in Fig. 3(a).The specimen dimension was reduced to minimise anyundesirable effects caused by different type of imperfection.

200

20

66

Face-shell

Web

10

20

150

300

Fig. 2. Details of interlocking block unit (dimensions in mm)

200

DPs

50

75 75

Joint

(a) (b) (c)

Fig. 3. Contact test of single joint (dimensions in mm)

225 300 300 75

DPs3DPs2DPs1

600

50 50 50

50 50 50

DPs4

900

Topandbottomjoints

DPs5 DPs6

(a) (b)

Fig. 4. Contact test of multiple joints (dimensions in mm)

112 Construction Materials 159 Issue CM3 Behaviour of interlocking mortarless block masonry Jaafar et al.


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(b) Multiple joints. Fig. 4(a) shows a the small test wall usedin the second contact test set-up. The blocks were stacked ina running bond utilising the interlocking system providedby the blocks. Two joints were formed between three blockcourses in the test wall. The block arrangement allows everyblock bed interface to be shared by two blocks in which theeffect of block height differences is included. Three identicaltest walls were used to verify the test results (W1, W2 andW3). Two saw-cut halves of blocks were used at the ends ofthe middle course to complete the course. The topprojections of the top course were removed to allow thetop-loaded steel block to be placed.

2.1.2. Test set-up and test procedure. Direct measurement asclose as possible to the joint is necessary to reduce theundetermined contribution of the material deformation. Pairs ofmechanical gauge Demec points (DPs) with a small gauge lengthof 50 mm were installed across the masonry joints to measure thenormal displacement (closure) of the joints, as shown in Fig. 3and 4. The DPs were installed at the mid length of the single jointnear the web, and the same location was used in the multiple jointto trace the behaviour of the joint at the same point on the blockface-shell to obtain consistent results for the two specimens.Lateral deformation was not measured because its effect on theaxial deformation can be neglected. This is because most ofthe head joint interfaces are not in contact owing to a small,visible gap arising from casting demands to facilitate productionof the blocks.

Grinding and capping were used on the top and bottom ofspecimen surfaces of the face-shell bed to obtain smooth andplane surfaces. The single-joint specimens were tested by acompression machine with a capacity of 3000 kN (Fig. 3(c)). Forthe test wall specimens, vertical compressive load was applied bymeans of a vertical hydraulic jack with a maximum loadingcapacity of 500 kN. The load was distributed uniformly using aspreader UB beam and steel block, as shown in Fig. 4(b). For bothset-ups, vertical load was applied incrementally to allowmeasuring of closure displacement of the joint during the tests.

2.2. Compression test of

prisms

This test aims to find thedeformation and failuremechanisms of interlockingmortarless hollow and groutedprisms under uniaxialcompression load until failure.The test is important toevaluate the characteristics ofa masonry system that hasvarious masonry constituents(block, grout and joints)interacting together as acomposite structure.

2.2.1. Test specimens. Twodifferent types of prism(ungrouted and grouted) werefabricated, as shown inFig. 5(a). For each type, three

identical prisms were constructed to confirm the test results. Thehollow prism specimens were designated PR1, PR2 and PR3 andthe grouted prism specimens PRG1, PRG2 and PRG3. A prismconsists of two blocks in the top and bottom and two saw-cuthalves in the middle course to reflect the actual blockarrangement in a real wall, where the webs of the hollow units donot align vertically in successive courses. This yields a realisticbehaviour. The central core of the grouted prisms was filled, andthe prisms were tested after 28 days.

2.2.2. Test set-up and test procedure. The axial deformation ofthe prisms was measured using a gauge length of 250 mm acrosstwo bed joints on both sides (L1 and L2) as well as on the frontand back faces, as shown in Fig. 5. Grinding and capping wereused on the face-shell bed at the top and bottom of the specimensurfaces to obtain smooth and plane surfaces after removing thetop projection of the top block unit. Vertical compressive loadwas applied by means of a vertical hydraulic jack with amaximum loading capacity of 500 kN.

3. TEST RESULTS AND DISCUSSION

Table 1 shows the properties of the blocks and grout used in theexperimental work.

3.1. Joint contact behaviour results

3.1.1. Single joint. Figure 6 shows typical normal displacementof the mortarless single joint under axial compressive load

300

250250600

L1 L2

DP3 DP4

DP2DP1

Front face

PRG1

(a) (b) (c)

Fig. 5. Compression test of hollow and grouted prisms (dimensions in mm)

Type ofspecimen

Compressivestrength, f 0c:N/mm2

Modulus ofelasticity, Ec:N/mm2

Splitting tensilestrength, f 0t:N/mm2

Block unit 23$4 9605 2$09COV*: %. 15$8 10$2 8$1Groutcylinders

20$4 8462$0 2$51

COV*: % 6$9 7$7 14$3

*COV is coefficient of variation.

Table 1. Block and grout properties



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measured at the DPs (see Fig. 3). This displacement represents thejoint closure at the measuring points (also shown in the figure)after removing the contribution of thematerial deformation. Non-linear gradual closure of the contact interfaces (or seating) undercompressive loading is observed in all joints. The initial increase inthe contact stiffness appears to be due to settling of the blocks andincrease of the areas that come into contact. Once the slope of thecurve approaches a high value, this means that the joint interfaceshave become almost in full contact.

Increasing the stiffness during loading is a significant differencebetween the dry joint and the conventional mortared joint. Thesolid line in Fig. 6 represents the average displacement measuredunder increasing load with the variation in the observed valuesrepresented by the upper and lower bounds. The maximumdifference in the joint displacement within the range was0$12 mm at maximum load. All the contact curves in the singlejoint lie between these limits owing to the variation of roughnessand irregularity of the block beds.

3.1.2. Multiple joints. Contact test results of multiple joints inthe test walls are shown in Fig. 7. The contact behaviour curvesmeasured at different locations (DPs are shown as dots on thespecimen in Fig. 7) lie between the upper and lower bounds, whichindicates high variations in the normal displacement (closure) inthe multiple joints. The high normal displacement value in somejoints is believed to be due to the undesirable differences of theheight of adjacent blocks due to the casting process. This type ofimperfection sometimes causes very small visible gaps between

the interfaces. Although large differences in normal displacementwere measured at different locations in the test walls (as indicatedby the upper and lower bounds in Fig. 7), the average normaldisplacement, calculated by using all the measurements at thespecified location, indicates near identical behaviour for all walls,as can be observed by the solid lines in Fig. 7. The maximumdifference in the joint displacement within the upper and lowerbounds (shown in Fig. 7) was 0$55 mm at the maximum load,which is 3$7 times greater than the measured value in the singlejoint at the same stress level.

3.2. Compression test results of prisms

3.2.1. Deformation and strength. The ungrouted prisms showedextensive axial deformation at lower load levels due to the initialseating deformation (the seating was defined in the contact testresults of the joints) at the dry bed joints, as shown in Fig. 8.Similar observations were made by Oh12 and Marzahn.13 Highvariation in the prism displacement was observed at differentlocations (the gauge lengths L1 and L2 are shown in Fig. 5, andare also shown as vertical lines on the prism sketch in Fig. 8). Themaximum difference in the prism displacement within the rangewas around 0$90 mm at the higher loads. This behaviour occurredmainly because of variation in the contact behaviour of dryjoints, which was affected by the geometric imperfection causedby block bed irregularity and variation of block height (Section3.1.2). Fig. 8 also shows the curves for load against axialdeformation for the three ungrouted prisms, calculated as theaverage of measurements at the earlier specified gauge lengthsthat fell within the average range. In general, all hollow prismsrevealed almost similar behaviour under axial compression, andthe dry joints predominantly affected the prism deformation until0$57 of the average maximum load.

A summary of the compression test results for ungrouted prisms isshown in Table 2. The average stress was calculated by dividing theload by the face-shell bedded area, and the strain was calculated bydividing the measured longitudinal deformation by the gaugelength of 250 mm. The ungrouted prisms showed an averagecompressive strength ( fm) of 11$2 N/mm2, and the average stressat which web cracks were initiated ( fwc) was 6$4 N/mm2.

The grouted prisms showed completely different behaviour fromthat of the ungrouted system, as shown in Fig. 9. The initial largedeformation at the lower loads disappears, and also the variation

0

10

20

30

40

50

60

70

80

0 0·2 0·4 0·6 0·8

Normal displacement: mm

Com

pres

sive

load

: kN

Upper bound

Lower bound

Average

0·12 mm

Fig. 6. Load–displacement curves in a single joint

Com

pres

sive

load

: kN

0

50

100

150

200

250

300

350

0 0·5 1·0 1·5 2·0

Axial deformation: mm

PR1

PR2

PR3

Upper bound

Lower bound

0·9 mm

Fig. 8. Load–axial deformation curves of ungrouted prisms

0

20

40

60

80

100

120

140

160

0 0·2 0·4 0·6 0·8Normal displacement: mm

Com

pres

sive

load

: kN

W1

W2

W3

Upper bound

Lower bound

0·55 mm

Fig. 7. Load–displacement curves of multiple joints in a test wall



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in the displacement at different locations is diminished. Thisbehaviour is attributed to the grouted core, which has nowaffected the overall deformation of the grouted prism. Unlike theblock assemblages, the concrete grout in the cores has nodiscontinuity planes. Furthermore, closure of the contactedinterfaces here depends strongly on the bond between the groutand the surrounding block shells. Curves of load against axialdeformation for all the grouted prisms are also depicted in Fig. 9,using the average of measurements at the specified gauge lengths(L1 and L2). In the grouted prisms, noticeable deformation startedafter 0$38 of the average maximum load.

In the initial stage, loads were resisted by the grout togetherwith the surrounding block shells that were bonded to thegrout. As a result, the closure deformation of bed joints wassmall compared with that for the hollow prism. This canexplain the higher slope of the load–axial displacement curvesat low loading level, as can be seen in Fig. 9. As the axialcompression load increased, bilateral deformation of the groutalso increased, as well as the longitudinal deformation. Whenthe stress in the grout reached a certain level, the bondbetween the grout and the surrounding block shell started tobreak, which in turn allowed perceptible closure in the joints.As a result, large deformation in the prism occurred at highload levels. This deformation increased according to the degreeof grout debonding.

As shown in Table 2, the grouted prisms showed an averagecompressive strength ( fm) of 11$5 N/mm2, and the load and

average stress at which cracks in the web were initiated ( fwc) was7$9 N/mm2. The concrete grout core area was added to theface-shell’s bedded area to find the total loaded area in stresscalculation of the grouted prisms. Because the effect of the dryjoint is reduced in the grouted prisms, the variation in theirstrength becomes less than in the ungrouted prisms, as can beseen in the coefficient of variation (COV) in Table 2.

3.2.2. Web splitting and mode of failure. In the ungroutedprisms, before failure, cracks were observed at webs in the

0

50

100

150

200

250

300

350

400

450

0 0·5 1·0 1·5 2·0

Axial deformation: mm

Com

pres

sive

load

: kN

PRG1

PRG2

PRG3

Upper bound

Lower bound

Fig. 9. Load–axial deformation curves of grouted prisms

Type of prism Specimen Maximum load: kNCompressivestrength, fm: N/mm2

Web splitting load:kN

Web splitting stress,fwc: N/mm2

Ungrouted PR1 216$1 9$0 122$8 5$1PR2 299$5 12$5 171$9 7$2PR3 289$6 12$1 164$1 6$8Ave. 268$4 11$2 152$9 6$4COV* % 13$9 14$1

Grouted PRG1 402$7 11$8 270$1 7$9PRG2 383$0 11$2 270$1 7$9PRG3 392$7 11$5 272$5 8$0Ave. 392$8 11$5 270$9 7$9COV* % 2$0 0$4

*COV is coefficient of variation

Table 2. Test results of compressive strength and web splitting loads of prisms

3500

3500

Fig. 10. Web splitting of an ungrouted prism; the numbersrepresent the level of applied load in lb/in2 (1 lb/in2Z6$89 kN/m2)at which the crack(s) appeared

2700

Front face

DP1 DP2

Fig. 11. Face-shell cracking of an ungrouted prism; the numbersrepresent the level of applied load in lb/in2 (1 lb/in2Z6$89 kN/m2)at which the crack(s) appeared



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plane of the prism, as shown in Fig. 10. At the first appearanceof web cracks, the bed joints at the contacted interfaces tendedto resettle for a new contacted state. Web splitting in the planeof the wall has also been observed in conventional mortaredmasonry, which occurred via a mechanism similar to deepbeam bending.14–17 Although the webs of the block cracked,failure occurred after face-shell cracking at one or more blocks,as shown in Fig. 11. The failure was sudden and explosive forall test prisms.

Premature failure occurred for specimen PR2 (PmaxZ216$1 kN: seeTable 2) because of the bed unevenness of the two halves in the

middle course, which caused a small visible gap at one half joint.This induced an additional flexural stress as a cantilever action inthe block that was not fully in contact. Fig. 12 shows the concretecrushing at the bottom of the top block. The crack did not extend tothe top because of the end plate restraint.

In the grouted prisms, cracking of the web started at higher loadsthan in the hollow prisms (see Table 2) and occurred near thejunction of webs and face-shells, as shown in Fig. 13. In adifferent mechanism from that for the hollow prisms, websplitting occurred because of lateral deformation of the groutrather than a flexural effect. Final failure occurred afterface-shell cracking at one or more blocks, as shown in Fig. 14. Asshown in Fig. 15, some face-shell parts were debonded from thegrout. This observation occurred mostly in specimen PRG2,which had the lowest strength.

4. CONCLUSIONS

The proposed contact tests of dry joints were used successfully toassess the contact behaviour of mortarless joints and todetermined the ranges of contact variation due to different typesof geometric imperfection of block beds for single and multiplejoints. Non-linear gradual closure of the contact interfaces(or seating) under compressive loading is observed in dry joints.The results show that the contact behaviour of dry joints isstrongly affected by the geometric imperfection of block beds,and especially by the imperfection caused by differences in theheights of adjacent blocks in a wall.

The results indicate that the overall behaviour of the mortarlesssystem is strongly affected by dry joint behaviour. Two differentpatterns of deformation were observed in ungrouted and groutedmasonry. In ungrouted masonry prisms, high initial axialdeformation (seating deformation) takes place until thecompressive load reaches 0$57 of the maximum load. In groutedmasonry this behaviour is not common, because high axialdeformation at the higher load levels depends on the degree ofgrout debonding, and noticeable deformation occurred after 0$38of the maximum load. In grouted prisms, the structuralperformance is enhanced because the effect of dry joints isreduced. The undesirable high initial deformation disappears in

5500

4700

Fig. 13. Web splitting of grouted prism; the numbers representthe level of applied load in lb/in2 (1 lb/in2Z6$89 kN/m2) at whichthe crack(s) appeared

DP1 DP2

4800

Fig. 14. Face-shell cracking of grouted prism; the numbersrepresent the level of applied load in lb/in2 (1 lb/in2Z6$89 kN/m2)at which the crack(s) appeared

Fig. 15. Block shells debonding in grouted prism

Concretecrushing

Back face

DP1 DP2

25002700

Fig. 12. Mode of failure of ungrouted prism PR1; the numbersrepresent the level of applied load in lb/in2 (1 lb/in2Z6$89 kN/m2)at which the crack(s) appeared



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the grouted prisms, and the variation in their strength becomesless than in the ungrouted specimens.

Similar to conventional mortared masonry, web splitting takesplace in the mortarless masonry. Web splitting occurs at higherstress in grouted prisms than in ungrouted prisms. In ungroutedprisms web splitting occurs via a mechanism similar to deepbeam bending, whereas in grouted specimens it occurs as a resultof lateral expansion of the grout.

REFERENCES

1. THALLON R. Dry-tack block. Fine Homebuilding Magazine,1983, August, 50–57.

2. HAENER. Stacking mortarless block system. In EngineeringDesign Manual, Atkinson Engineering, Inc., Hamilton,Ontario, 1984.

3. GALLEGOS H. Mortarless masonry: the Mecano system.International Journal of Housing Science and itsApplications, 1988, 12, No. 2, 145–157.

4. HARRIS H. G., OH K. and HAMID A. A. Development of newinterlocking and mortarless block masonry units for efficientbuilding systems. Proceedings of the 6th Canadian MasonrySymposium, Saskatoon, 1992, pp. 15–17.

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6. THANOON W. A., JAAFAR M. S., ABDULKADIR M. R., ALI A. A., TRIKHAD. N. and NAJM A. M. Development of an innovativeinterlocking load bearing hollow block system in Malaysia.Construction and BuildingMaterials, 2004, 18, No. 6, 445–454.

7. JAAFAR M. S., THANOON W. A., NAJM A. M., ABDULKADIR M. R. andALI A. A. Strength correlation between individual block,prism and basic wall panel for load bearing interlockingmortarless hollow block masonry. Construction and BuildingMaterials, 2006, 20, No. 7, 492–498.

8. ALWATHAF A. H., THANOON W. A., JAAFAR M. S., NOORZAEI J. andABDULKADIR M. R. Shear characteristic of interlockingmortarless block masonry joint. Masonry International,2005, 18, No. 3, 139–146.

9. DRYSDALE R. G. and GAZZOLA E. A. Strength and deformationproperties of a grouted, dry-stacked, interlocked concreteblock system. Proceedings of the 9th InternationalBrick/Block Masonry Conference, Berlin, 1991, pp. 164–171.

10. OH K., HARRIS H. G. and HAMID A. A. New interlocking andmortarless block masonry units for earthquake-resistantstructures. Proceedings of the 6th North American MasonryConference, Philadelphia, 1993, pp. 821–836.

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12. OH K. Development and Investigation of Failure Mechanism ofInterlocking Mortarless Block Masonry System. PhD thesis,Department of Civil and Architectural Engineering, DrexelUniversity, Philadelphia, 1994.

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15. BECCIA I. J. and HARRIS H. G. Behaviour of hollow concretemasonry prisms under axial load and bending. The MasonrySociety Journal, 1983, 2, No. 2, T1–T26.

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17. XIE H., PAGE A. W. and KLEEMAN P. W. An investigation of thecompressive failure mechanism for face-shell bedded hollowmasonry. Proceedings of the 6th Canadian MasonrySymposium, Saskatoon, 1992, pp. 97–108.

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