Construction and Building Materials 37 (2012) 812–821

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Construction and Building Materials

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Capacity of innovative interlocking blocks under monotonic loading

Majid Ali ⇑, Ronald Jansen Gultom, Nawawi ChouwDepartment of Civil and Environmental Engineering, The University of Auckland, Private Bag 9201, Auckland 1142, New Zealand

h i g h l i g h t s

" Compressive and shear capacities of novel interlocking blocks are investigated." Interlocking blocks are prepared with coconut fibre reinforced concrete (CFRC)." Different mix design ratios for CFRC are also studied." The compressive capacity of single block is more than that of the multiple blocks." The out-of-plane shear strength is more than the in-plane shear strength.

a r t i c l e i n f o

Article history:Received 4 June 2012Received in revised form 18 July 2012Accepted 4 August 2012Available online 23 September 2012

Keywords:Coconut fibresConcrete compositesMortar-freeInterlocking blockEarthquake

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.08.002

⇑ Corresponding author. Tel.: +64 9 373 7599x8452E-mail address: mali078@aucklanduni.ac.nz (M. A

a b s t r a c t

For the majority of poor people in earthquake prone regions, construction techniques for affordablelow-cost and earthquake-resistant buildings are needed. As a contribution to this requirement,innovative interlocking blocks, named as standard, bottom, top and half blocks, are invented. The blocksare fabricated with special interconnecting profiles so that they will interlock with the upper, lower andadjacent blocks to facilitate the construction of a wall. In this study, coconut fibre reinforced concrete isselected for block preparation because coconut fibre has the highest toughness amongst natural fibres.The relationship between compressive strength of individual and multiple standard blocks, and thein-plane and out-of-plane shear capacities of interlocking mechanism, are investigated. The compressivestrength and total compressive toughness of bottom block are higher than that of other blocks. It is alsofound that the compressive capacity of the multiple blocks is less than that of the individual block. Theout-of-plane shear capacity is 25% higher than that of the in-plane. However, the load required to causein-plane shear is higher than that required for the out-of-plane shear.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Many technologies are available for constructing earthquake-resistant houses. However, most of them are too expensive for themajority of people, especially those living in developing and un-der-developed countries. The available construction proceduresare also too complex. People in rural areas do not hire skilledlabour. They even build non-engineered structures by themselves,just to reduce construction cost. Often, they simply adopt oldtraditional construction technologies, which are not earthquakeresistant, resulting in collapses and ultimately causing fatalitiesand financial loss [1]. A number of major earthquakes in the past(e.g. Nepal earthquake in 2011, Haiti earthquake in 2010, Pakistanearthquake in 2005 and Sumatra earthquake in 2004) clearlyindicate the need for developing a new construction technology to

ll rights reserved.

1; fax: +64 9 373 7462.li).

be adopted in earthquake prone regions. It should be affordable tonormal people so that they are able to construct their own houseswith the available local resources and little guidance. The price ofreinforcing materials will automatically be cut if local resourcesare used. One option is to utilise natural fibres as concrete reinforce-ment. There are a wide range of natural fibres, namely sisal, bamboo,coir (coconut fibre), jute, and many others [2–4]. The researchersfocused on finding the optimum fibre length and content for thecomposites. Studies have shown that the natural fibres are goodalternatives because they are not only cost effective but also pro-mote sustainable buildings as they are renewable materials [5–7].The researchers investigated cement composites with natural fibresto produce boards for partition walls and corrugated/simple slabpanels for roofing. Natural fibres are generally abundantly availablein most developing countries where the need for proper and cheaphousing construction is in high demand [8]. Coconut fibres areselected in this study because of their highest toughness comparedto that of other natural fibres [9]. The strain of coconut fibres isreported up to 24% and 39% for coconut fibre bundles [9] and single

M. Ali et al. / Construction and Building Materials 37 (2012) 812–821 813

fibre [10], respectively, while the strains of other natural fibre spec-imens are in the range of 3–6%.

To reduce construction time and the cost of structures, theresearchers have also suggested the use of interlocking blocks toreplace the normal bricks, thus eliminating mortar from masonryconstruction [11]. Ramamurthy and Nambiar [12] compiled thehistory of the development of interlocking blocks till 2004 and ex-plained their shapes geometry, purposes and methods of construc-tion. Some other interlocking blocks are also developed from 2004to date. Most of these blocks are hollow [13–15], some are solid[16] and curved [17], and in a few cases, with provisions of holesfor the reinforcement. These blocks can be prepared mechanicallyor manually, but in some cases, these require very complicatedmoulds and manual casting. Usually, the material is concrete[13–15] but there was also stabilised soil [17,18] and fly ash[16,19] used. The thickness of these blocks also varies, makingthem suitable either for load bearing, partition or cladding walls.Interlocking mechanism is provided either by horizontal, verticalor both interconnecting keys for the in-plane and out-of-planedirections. The main purpose of these blocks is to make precisealignment and quick construction. It may be noted that the inter-locking keys of the hollow blocks alone are normally not sufficientto resist the stresses of design load for an assembled wall in astructure due to the elimination of mortar layers [20]. This mightbe because of limited key projection. To overcome this problem,normal reinforced concrete is used at regular intervals in the holesprovided in the hollow blocks. This makes the structure a little bitexpensive. In some cases, relatively less mortar (compared to thatrequired in normal brick masonry) is used with the interlockingblocks [18]. Their main objective considered so far is an easier, fastand cost-effective construction, mainly for resisting static loading.To the best of the author’s knowledge, the utilisation of mortar-freeconstruction with interlocking blocks and without steel reinforce-ment in earthquake resistant buildings has not been reported. Inthe following sections, the compressive and shear resistance ofthe CFRC interlocking block is discussed. These strengths of blocksrepresent the capacity of a structure in bearing a vertical static loadand forces at the interface of the interlocking blocks induced by thehorizontal earthquake loadings.

2. CFRC interlocking blocks

Mortar-free construction, capable of reducing earthquake im-pact during a seismic event, is considered for seismic-resistanthousing. Coconut fibre reinforced concrete (CFRC) members withcracks have more damping than those without cracks [21]. To en-hance the damping capability of the structure, CFRC interlockingblocks are used in the mortar-free construction. Fig. 1 shows theproposed wall under gravity, and the in-plane and out-of-planeearthquake loadings. Ropes, made of coconut fibres, are utilisedas the longitudinal reinforcement of the wall to avoid its ultimatecollapse by limiting the block movement up to the key height. Thegravity load is to be taken by the compressive strength of theblocks. Because of the movability of all blocks relative to eachother, the earthquake forces induced into the structure will be re-duced. The activated lateral forces are then resisted by the inter-locking keys of the blocks in both in-plane and out-of-planedirections. This resistance will depend on the shear strength ofthe interlocking keys. Thus, the invented CFRC interlocking blockwill lead to the overall objective, i.e. easy-to-build, economicaland earthquake-resistant housing.

The shapes of the developed CFRC interlocking blocks areshown in Fig. 2. These blocks, named as standard, bottom, topand half blocks, can be utilised in constructing a wall (Fig. 1). Thebottom and top layers of the wall should be made with the bottom

and top blocks (Fig. 2b and c), respectively, so that no additionalshape is required. The block shown in Fig. 2a has overall dimen-sions of 400 mm long � 200 mm wide � 195 mm high with thein-plane and out-of-plane interlocking keys of 45 mm height. Sincethe blocks are to be used in load bearing and earthquake resistanthouses, the width is taken as a reference (equal to wall width), thelength is twice the width and the height is a little less than thewidth. This size is also selected because of (i) the provision of holesfor the rope reinforcement, (ii) relatively large interlocking keys,and (iii) the use of fibre reinforced concrete (FRC) as smaller blocksize may create problems in compaction of FRC. The weights of thestandard, bottom, top and half blocks are 24 kg, 21 kg, 20 kg and12 kg, respectively. The characteristic of the developed blocks istheir ability to move relatively to neighbouring blocks during anearthquake because of the mortar-free construction. This relativemovement then leads to energy dissipation. Because of the inclinedkeys, the blocks will come back to their original position, i.e. thewall will have self-centring ability. The maximum uplift shouldbe restricted to the key height, which is ensured by the presenceof the rope that vertically runs through the two holes providedin the block.

As part of the development process, the present study is con-ducted to determine the compressive and shear capacities of theCFRC interlocking blocks. The type (standard, bottom, top and halfblocks) and the number of blocks (single and three blocks) are con-sidered in the compressive tests. The in-plane and out-of-planeshear tests are also performed. The mix design ratio of cement,sand, aggregates and fibres is optimised for preparing the CFRC.This current work is one step towards the global goal (i.e. develop-ment of seismic-resistant houses). The consideration of mortar-free construction using the invented interlocking blocks and therope reinforcement is out of the scope of current work.

2.1. Optimisation of mix design ratio

Plain concrete (PC) properties were taken as a reference forcomparison with that of CFRC. For PC, the mix design ratio(cement:sand:aggregates) was 1:3:3 with a water cement (w/c) ra-tio of 0.48. The mix design for the respective CFRC was the same asthat of plain concrete, except that more water was added (stepwiseto avoid bleeding) because of fibre addition to make the CFRCworkable and 0.5% or 1% contents of fibres were added. Thewater–cement ratios for CFRC with 0.5% and 1% fibre content were0.54 and 0.64, respectively. It is well known that w/c ratio has aninfluence on the properties of concrete, but compaction is also animportant factor. The higher w/c ratio for the CFRC was to ensureits proper compaction with a workable mix so that a maximumstrength could be achieved. The properties of CFRC with respectivew/c ratio could be considered as the optimum one, because anyaddition of water would cause bleeding, ultimately reducing itsstrength in the hardened state. In contrast, a reduced w/c ratiocould lead to the improper compaction, again resulting in lessstrength. The considered mix design ratios of CFRC are shown inthe first column of Table 1. A total of six mix design ratios were ta-ken into account for analysing the effect of different parameters onthe properties of the resulting matrix. First, the mix design waskept constant, i.e. 1:3:3, while the fibre content was taken as 0%,0.5% and 1% by mass of concrete materials. The length of coconutfibres was 5 cm in all combinations, as this length was optimumfor CFRC [21]. The fibre content was then kept constant, i.e. 0.5%,while the mix design ratio was taken as 1:2:2, 1:3:3 and 1:4:4.In next constellation, the sand content was increased comparedto the amount of aggregates, i.e. mix design was changed from1:3:3 to 1:4:2 having 1% fibre content, so that more mortar wasavailable for grabbing the fibres. Better compaction could be donewith the mix ratio of 1:4:2, i.e. reducing air voids in fresh state, and

Horizontal earthquake loading

Foundation having groove for blocks

Possible in-plane shearing planes for interlocking keys

Top tie-beams

Horizontal earthquake loading

Foundation having groove for blocks

Possible out-of-plane shearing planes for interlocking keys

Top tie-beams

(a) (b)

mgarhpaidmorfgnidaolytivarGmgarhpaidmorfgnidaolytivarG

Fig. 1. Proposed wall assembly under gravity and earthquake loadings. (a) In-plane and (b) out-of-plane directions. (Note: Rope reinforcement is not shown for clarity.)

(b)

200 mmKey height: 45 mm400 mm

195 mm

Out-of-plane key

In-plane key

(a)

(d)

(c)

Fig. 2. CFRC interlocking blocks: (a) standard, (b) bottom, (c) top and (d) half block.

814 M. Ali et al. / Construction and Building Materials 37 (2012) 812–821

having the smooth finished surface in hardened state, particularlyfor interlocking keys of the blocks.

Ordinary Portland cement, sand, aggregates, water and im-ported brown coconut fibres were used for the preparation of CFRC.

Table 1Properties of plain concrete and coconut fibre reinforced concrete.

Matrix Cylinder testing Beam-let testing Density (kg/m3)

r (MPa) e (%) E (GPa) Tc (MPa) STS (MPa) MOR (MPa) D (mm) Pcrack (kN) Dmax (mm) TTI (–)

Plain concrete (1:3:3, 0%)a 18.8 0.18 18.2 0.19 2.33 2.89 1.05 – – 1 2318CFRC (1:2:2, 0.5%)a 39.6 0.23 37.6 0.28 4.18 4.42 1.54 3.5 9.4 5.06 2271CFRC (1:3:3, 0.5%)a 19.5 0.19 19.2 0.21 2.68 3.24 1.10 1.8 8.3 3.94 2253CFRC (1:4:4, 0.5%)a 11.6 0.17 10.4 0.15 1.77 1.98 1.01 1.4 7.8 2.63 2236CFRC (1:3:3, 1%)a 19.1 0.22 18.6 0.23 2.49 3.51 1.18 3.21 10.23 4.32 2175

CFRC (1:4:2, 1%)a 20.7 0.24 19.3 0.25 2.75 3.6 1.21 3.3 11.8 4.51 2128

Note: An average of three readings is taken. CFRC (1:4:2, 1%) is selected for preparing the interlocking blocks and their properties are shown in bold.a Mix design ratio and fibre content are shown in brackets. Fibre content is taken by mass of concrete materials. The water–cement ratio for concrete with 0%, 0.5% and 1%

fibre content is 0.48, 0.54 and 0.64, respectively.

M. Ali et al. / Construction and Building Materials 37 (2012) 812–821 815

The maximum size of aggregates was 12 mm (passing through12 mm sieve and retained at 10 mm sieve). The mean diameterof coconut fibres was 0.25 mm. The tensile strength, modulus ofelasticity and total toughness of coconut fibres were 66.3 MPa,0.74 GPa and 12.8 MPa, respectively. The preparation of CFRCwas done as per [21]. A pan type concrete mixer was used. A layerof coconut fibres (approximately one-fourth of fibre content) wasspread in the pan, followed by the spreading of aggregates, cementand sand. The first layer of fibres was hidden under the dry con-crete materials with the help of a spade. Then, another layer ofcoconut fibres followed by layers of aggregates, cement and sandwas spread. This process was repeated until the rest of the materi-als were put into the mixer pan. Water was then added, and themixer was rotated for 5 min. A slump test was always performedbefore placing it into the moulds. The slumps for CFRCs were 10–20 mm, but CFRC was workable in spite of this low slump. The rea-son for this low slump was the presence of fibres in concrete. Theworkable CFRC is one which can be placed in moulds with tampingand lifting/dropping from a height of 150–200 mm. Since compac-tion is important for CFRC, the cylinder and beam-let moulds werefilled in three layers. Each layer was compacted first by tamping 25times with 16 mm diameter rod and then the moulds were liftedup to a height of approximately 150–200 mm and then droppedto the floor for the self-compaction of the fibre concrete and to re-move air voids from the CFRC. For plain concrete, all materialswere put in the mixer pan along with the water, and the mixerwas rotated for 3 min. The slump was 50 mm.

Six cylinders (100 mm in diameter and 200 mm in height) andthree beam-lets (100 mm wide, 100 mm deep and 500 mm long)were prepared from one batch of CFRC and PC for determiningthe concrete properties. All specimens were cured for 28 days, thendried for saturated surface conditions and finally whitewashed be-fore the testing to enable a clear identification of the cracks. Specialwooden moulds were prepared in such a way that the interlockingblocks were cast inverted. Moulds were filled in four layers, i.e.approximately up to (i) 75 mm, (ii) 150 mm, (iii) 175 mm and(iv) 195 mm. The same compaction procedure was adopted foreach layer as was done for the CFRC cylinders and beam-lets. Theblocks were also cured for 28 days, then dried and finally white-washed before the testing to enable a clear identification of thecracks.

2.1.1. Cylinder and beam-let testsAll cylinders were tested in a compression testing machine to

determine the compressive strength r, corresponding strain e,modulus of elasticity E, total compressive toughness Tc and split-ting tensile strength (STS). Each cylinder was capped with plasterof Paris for uniform distribution of load. All beam-lets were testedin a universal testing machine of capacity 100 kN using 4-pointloads to obtain modulus of rupture (MOR), its corresponding mid-point deflection (D), cracking load Pcrack , the maximum midpoint

deflection (Dmax) and the flexural toughness in terms of totaltoughness index (TTI). Deflection was measured with the help ofa linear variable differential transducer (LVDT). Cracking load isthe load taken by fibres, fibre concrete bonding and part of fibre–concrete after the first visible crack is formed. An average of threereading is taken for each property.

Material properties are analysed from experimental data usingthe standard procedures (NZS 3112: Part 2). Modulus of elasticity,E, is calculated as the ratio of stress change to strain change inthe elastic range. Stress–strain relationship for each cylinder isthe average of readings taken by two LVDTs attached to the spec-imens. The maximum stress and its corresponding strain value areindicated by the compressive strength r and e, respectively. Totalcompressive toughness, Tc, is calculated as the total area under thestress–strain curve [22]. Usually, other researchers have taken itas area under curve after the maximum stress up to 1% strain[23]. No spalling of the CFRC cylinder was observed in E and r test.The maximum load from load–time curve of splitting tensilestrength (STS) test is taken for the calculation of STS. CFRC cylinderswere held together by fibres after cracks. Even when the test wascontinued up to more than 800 s, the two pieces were not sepa-rated unlike the plain concrete cylinders with a brittle failure.The maximum load from the load–displacement curve of flexuretest is taken for the calculation of modulus of rupture, MOR, andthe corresponding midpoint deflection D is noted. Cracking loadPcrack and the maximum midpoint deflection Dmax are also noted.CFRC beam-lets held together even after the maximum load whilePC beam-lets broke into two pieces. After the test, CFRC beamswere intentionally broken into two halves to observe fibre failure.Two types of fibre failure were observed: (i) fibre breaking and (ii)fibre pull-out. The fibres were randomly distributed in concrete forbridging the crack. The fibres having sufficient embedment acrossthe crack on both sides were broken, and the fibres having rela-tively less embedment at one side of crack were pulled out. Flex-ural toughness is measured as the total toughness index (TTI). Itis the ratio of area under load–displacement curve up to the max-imum deflection to the area under curve up to the first-crack load.Usually, toughness index is taken as the area under the curve up to3, 5.5 or 10.5 times the first-crack deflection to the area undercurve at first-crack deflection, and they are notated as I5, I10 andI15, respectively [24]. As expected, CFRC has relatively less densityas compared to that of plain concrete because of the presence oflow density fibres. A smaller density is beneficial because less iner-tia forces will be activated in earthquakes and thus smaller struc-tural dimension is required to withstand the reduced impact of theearthquake [21]. The properties of PC and CFRC with consideredmix design ratios are presented in Table 1. For the cases of a mixdesign ratio 1:3:3 with varying fibre content of 0%, 0.5% and 1%,compressive and splitting tensile strength decrease while modulusof rupture, compressive and flexural toughness increase with anincrease in fibre content. For the cases of a constant 0.5% fibre con-

816 M. Ali et al. / Construction and Building Materials 37 (2012) 812–821

tent with varying mix design ratios of 1:2:2, 1:3:3 and 1:4:4, as ex-pected, all strength properties decrease with decreasing cementcontent. The compressive strength, E, Tc, STS, MOR and TTI of CFRCare reduced by 71%, 72%, 46%, 58%, 55% and 48% when the mix de-sign changes from 1:2:2 to 1:4:4 with the same fibre content, i.e.0.5%. One of the main purposes of the new construction technologyis to make it simple and easy, so that unskilled labour or even laypeople should be able to construct their earthquake resistanthouses by themselves. Therefore, at this stage, single storey housesare the core target, for which relatively high strength concrete(40 MPa with 1:2:2) is not required and the strength of around12 MPa with 1:4:4 may be a little low for constructing houses. Soemphasis is on the concrete strength of approximately 20 MPa,which can be achieved with a mix design of 1:3:3. The materialproperties of matrix can be improved by adding more fibres (i.e.1% fibres) or even by slight change of mix design, i.e. from 1:3:3to 1:4:2. The addition of fibres in 1:3:3 may cause some voidsand thus reduce the compressive strength, which can be avoidedin the case of 1:4:2. The compressive strength, E, Tc, STS, MORand TTI are increased by 8%, 4%, 9%, 10%, 3% and 4% when themix design is changed from 1:3:3 to 1:4:2 with the same fibre con-tent, i.e. 1% by mass of concrete materials. This shows that theproperties of CFRC are improved when the quantities of sand andaggregates are optimised from 1:3:3 to 1:4:2 for the same fibrecontent. The improvement is because of avoiding honey combingdue to better compaction and allowing more mortar for grabbingthe fibres.

3. Block capacities

For investigating the compressive and shear capacity of inter-locking blocks, a mix design of 1:4:2 with a fibre content of 1%by mass of concrete materials, a fibre length of 5 cm and awater–cement ratio of 0.64 is selected.

3.1. Compressive capacity

CFRC interlocking blocks were tested in a 2000 kN compressiontesting machine for their compressive strength, modulus of elastic-ity, total compressive toughness and Poisson’s ratio. The experi-mental setup is shown in Fig. 3. The block was placed betweentwo steel plates; one with a spherical projection so that the appliedstress was uniform during the test. Four portal strain gauges wereattached at the corners to measure the vertical deformation. Anaverage of these four readings is taken for axial strain. Portalgauges were also attached at the front and rear of the block to mea-sure the lateral strain. The same setup was used for testing the bot-tom, top, half and multiple blocks. In the case of multiple blocks,the vertical deformation was measured with the help of linear var-iable differential transducers (LVDTs) instead of portal gauges. The

Load

Steel plate

Spherical projection for uniform stress

Base Steel plate

Portal gauges(at 4 corners and

at front & rear sides)

Fig. 3. Test setup for compressive capacity.

measurements taken by the portal gauges, LVDT and applied loadwere then recorded through a computer connected to the test set-up throughout the process. Multiple blocks consisted of threeblocks [15]. This was selected to check the compressive load carry-ing capability of the invented interlocking blocks.

Load-deformation curves were recorded during the tests, whichare then converted to average stress–strain curves for determiningblock properties. For a single standard block test, the maximumload was 370 kN and the corresponding deformation was 2.28mm. It was observed that cracks were initiated at the bottom inter-locking keys of the block, with the first crack appearing in one of thecorner interlocking keys on the bottom side of the standard blockbefore the ultimate load was reached, at approximately 90% of itspeak load, propagating upwards. The reason could be a smallerbearing area of the bottom interlocking keys in comparison to theupper keys, especially at the corners. The appearance of the firstcrack was always in one of the corner interlocking at the bottomside of the block, but not in the respective key of the other samples.Its reason could be the uneven compaction in the bottom interlock-ing keys as the blocks were prepared manually. More cracks ap-peared in the other interlocking keys on the bottom side as theload approached its maximum value. When the load decreasedafter reaching its peak value, the cracks continued to propagate up-wards and widen. The cracks were bridged by the fibres, and theparts of the block were in contact with each other at the end ofthe test.

For the bottom, top and half blocks, the maximum loads were751, 269 and 115 kN, respectively, and the corresponding displace-ments were 5.22, 7.61 and 4.15 mm, respectively. During thesetests, as expected, the cracks initiated in the bottom interlockingkeys of top and half blocks propagating upwards, and in the topinterlocking keys of bottom blocks propagating downwards.

For a multiple standard blocks test, the maximum load was344 kN and the corresponding deformation was 5.46 mm. Cracksinitiated in the bottom interlocking keys of the standard block atthe bottom, and propagated upward. The first crack appeared afterthe ultimate load had been reached. As the load continued to de-crease, the length and width of the crack increased and morecracks developed propagating upwards. Only tiny cracks were ob-served on the top and middle standard blocks, although the bottominterlocking keys of the bottom standard block had been squashedquite severely. The cracks in the top standard block were morethan those in the middle standard block. The reason for this wasthat, after the bottom keys of the lowest standard block were com-pletely crushed, the top interlocking keys of the upper most stan-dard block became the smallest contact area with the load. Thisinduced the highest stress on the top surface of the upper moststandard block, causing more cracks compared to the standardblock in the middle. At the end of the test, all standard blockshad their integrity because of the presence of fibres, with theexception of the bottom keys of the lowest most standard block.This indicates that when walls are built, the bottom and top layersof the wall should be made with the bottom and top blocks asshown in Fig. 2b and c, respectively. That is why, the tests on multi-ple blocks with top, standard and bottom blocks were also per-formed. In this test, bottom and top blocks had same contactarea with the applied load, whereas the standard block in the mid-dle had relatively larger contact interface with the upper and lowerblocks. The maximum load was 710 kN and the corresponding ver-tical displacement was 10.13 mm. Cracks initiated in the bottominterlocking keys of the top block propagating upwards. After that,cracks appeared in the bottom interlocking keys of the middlestandard block, followed by cracks in the top interlocking keys ofthe bottom block. At the end of the tests, it was observed that rel-atively fewer cracks were present compared to that in the threestandard blocks testing.

0

4

8

12

16

20

0.00 0.01 0.02 0.03 0.04 0.05

Ave

rage

d st

ress

(MP

a)

Averaged strain (-)

Fig. 5. Stress–strain curves from single and multiple blocks under compressiveloading.

M. Ali et al. / Construction and Building Materials 37 (2012) 812–821 817

Average stress–strain curves of interlocking block are analysedin a similar manner to the material properties of CFRC and PC cyl-inders. It is important to note that the block has different top andbottom contact areas, resulting in different top and bottom stres-ses. There is no guideline available for which stress is to be takento represent the compressive strength of the interlocking blocksand to calculate other engineering properties (modulus of elasticityand compressive toughness). The bottom and top contact areas ofthe tested specimens are shown in Fig. 4. These areas are used tocalculate the bottom and top stresses and then their average is ta-ken to represent the compressive strength of the block. The axialstrains at the four corners are slightly different. An average ofthe top and bottom stresses, and four vertical (or axial) strainsare taken as the average-stress and average-strain, respectively.From each standard single and multiple blocks test, the averagestress–strain curves are shown in Fig. 5. The peak stress of a singleblock is more than that of the multiple blocks, but the correspond-ing axial strain of the multiple blocks is more than that of the sin-gle block. The reason for the larger strain could be the small gapbetween the two blocks in case of multiple blocks. The reason forthe lower stress of multiple blocks is their slenderness (i.e. ratioof height to least dimension: in this case it is 2.9). The compressivetoughness and modulus of elasticity are calculated using thesecurves. For single blocks, Poisson’s ratio (i.e. the ratio of lateral toaxial strains) is calculated. The lateral (horizontal or transverse)strain is the average of strain measured at the front and rear sidesof the blocks.

The properties of single and multiple blocks are presented inTables 2 and 3, respectively. An average of three readings is taken.The compressive strength of a single standard block is 16.48 MPawith the corresponding axial and transverse strain of 1.3% and0.02%, respectively. Its modulus of elasticity, compressive tough-ness and Poisson’s ratio are 2.34 GPa, 0.56 MPa and 0.015, respec-tively. The compressive strength of the bottom block (17.02 MPa)is more than that of other tested blocks. It may be noted that thestrength of the top block (7.73 MPa) is less compared to that ofthe half block (8.66 MPa), but the average maximum load of thetop block (244 kN) is more than that of the half block(104.05 kN). Also, the contact areas of the top block are much lar-ger than those of the half block (Fig. 4). This illustrates that thepeak load is not proportional to the contact area, resulting in lessstrength for the top block and more strength for the half block.The total compressive toughness of the bottom block is also morethan that of the other blocks. The reason is that the stress dropafter the peak stress is more for other blocks compared to that of

Specimens for com

Top contact area (Wh

Bottom contact area (

Standard block Bottom block Top block

Fig. 4. Contact areas for calcula

the bottom block. It is because the bottom interlocking keys ofthe standard, top and half blocks are relatively weak comparedto the top ones, thus initiating cracks in bottom keys and ulti-mately providing less resistance to damage after peak stress incomparison to that in the bottom blocks with top interlocking keysonly. On the other hand, the compressive strength, modulus ofelasticity and total compressive toughness of multiple standardblocks are 15.8 MPa, 1.44 GPa and 0.56 MPa, respectively.0.7 MPa compressive strength is reduced for multiple blocks whencomparing with that of a single block. It is observed that fcm = 0.96fcs, where fcm and fcs stand for the compressive strengths of themultiple and single blocks, respectively. Jaafar et al. [15] also ob-served the similar results for their developed interlocking hollowconcrete blocks that fcw < fcp < fcb, where fcw, fcp, fcb stands for com-pressive strength of a wall panel, prism with three blocks and unitblock, respectively. Thus, the compressive strength of a wall panelmade of CFRC interlocking blocks will also be lower because of itshigher slenderness ratio, which is defined as the ratio of height tothe least horizontal dimension. In the standard multiple blockscompressive test, the smallest stress is experienced by the middleblock because of its largest contact area with the lower and upperblocks. Therefore, the crack appeared in the top block after thecracks of the bottom block. This is also observed in lateral strainsof the blocks. The average lateral strains of the top, middle and bot-tom blocks is 0.015, 0.012 and 0.019, respectively. The compressive

pressive tests

ite shaded area)

Black shaded area)

Half block

ting compressive stresses.

Table 2Compressive capacity of single block.

Block shape

Standard block Bottom block Top block Half block

Maximum load (kN) 396.03 730.98 244 104.05Compressive strength (MPa) 16.48 17.02 7.73 8.66Corresponding axial strain (%) 1.3 3.3 4.2 1.8Corresponding lateral strain (%) 0.02 0.016 0.029 0.023Modulus of elasticity (GPa) 2.34 1.05 0.21 0.53Total compressive toughness (MPa) 0.56 1.08 0.32 0.37Poisson’s ratio (–) 0.015 0.005 0.007 0.013

Note: An average of three readings is taken.

Table 3Compressive capacity of multiple blocks.

Stacked blocks

Standard blocksTop, standard and bottom blocks

Maximum load (kN) 379.13 696.79Compressive strength (MPa) 15.78 9.28Corresponding axial strain (%) 1.50 2.5Corresponding lateral strains (%)a 0.015, 0.012, 0.019 0.024, 0.014, 0.018Modulus of elasticity (GPa) 1.44 0.39Total compressive toughness (MPa) 0.56 0.48

Note: An average of three readings is taken.a Strains for the top, middle and bottom blocks.

818 M. Ali et al. / Construction and Building Materials 37 (2012) 812–821

toughnesses of the single and multiple standard blocks are thesame, i.e. 0.56 MPa. Now, in comparison to multiple blocks withtop, standard and bottom blocks, the compressive strength and to-tal toughness index of multiple standard blocks are higher. Thereason is that the former specimen has larger contact area com-pared to the latter (Fig. 4). The average peak load for multipleblocks with top, standard and bottom blocks (696.79 kN) is alsomuch more than for multiple standard blocks (379.13 kN). Again,the peak load is not proportionate to their contact areas, resultingin more strength for multiple standard blocks and less strength formultiple blocks with top, standard and bottom blocks. It is alsoimportant to mention that the single and multiple blocks havesmaller compressive strengths than those obtained from CFRC cyl-inder tests. This might be because of the block’s unique shape.

3.2. Shear capacity

The test setups for determining the in-plane and out-of-planeshear capacities of the interlocking mechanism are shown inFig. 6. Three blocks were stacked horizontally between two steelplates. These plates were connected using four steel rods and tight-ened using nuts. Four load cells were attached to the rods, one foreach rod, to measure lateral load. For the in-plane test, the setupwas put on a 2000 kN compressive testing machine so that thedirection of the load was parallel to the longitudinal bed joints(Fig. 6a). For the out-of-plane test, the setup was put on a 500 kNcompressive testing machine (because of space limitation in the2000 kN machine) so that the direction of the load was parallelto the transverse bed joints (Fig. 6b). The load was applied to themiddle block while steel blocks were placed under the left and

right blocks. The displacement of the middle block relative to theleft and right blocks was measured by means of a LVDT.

The typical load–displacement curves recorded from the in-plane and out-of-plane shear tests are shown in Fig. 7. The maxi-mum load and the corresponding deflection of in-plane shear are165.2 kN and 12.9 mm, respectively, and that of out of-plane shearare 145 kN and 7.8 mm, respectively. The shear-off areas of the in-plane and out-of-plane block testing are shown in Fig. 8a and b,respectively. The interlocking keys are numbered from 1 to 26, inorder to explain the sequence of sheared-off keys. It may be notedthat the sheared-off keys were not in a symmetrical pattern for thein-plane testing (shown in dotted rectangles in Fig. 8a). This mightbe because of the lack of confined compaction in some shearedkeys. During the in-plane testing (refer to Fig. 8a), the cracks firstappeared in the interlocking keys 14 and 15 on the bottom sideof the middle block because of keys 21 and 22 on the top side ofthe right block at a load of 58 kN, only 37% of the ultimate load.After reaching the maximum load, another crack formed in theinterlocking key 9 on the top side of the middle block due to key3 on the bottom side of the bottom block. As the load continuedto decrease, cracks also appeared in the other blocks. Rather thanspreading all over the block, the cracks in each block were confinedto its interlocking keys. The interlocking keys 5 and 6 on the bot-tom side of the left block were sheared-off by keys 11 and 12,respectively, on top side of the middle block. Similarly, the keys14 and 15 on the bottom side of the middle block were sheared-off by keys 21 and 22, respectively, on the top side of the rightblock. On the other hand, some keys on the top sides of the middleand right blocks were also sheared-off by the keys on the bottomsides of the left and middle blocks, respectively. Because of frictionbetween the two surfaces, some parts of keys 3, 16 and 17 on the

(a)

(b)

Load

Load cells(at 4 corners)

Steel rod (at 4 corners)

Steel plates(on both sides)

Steel blocks

LVDT(with stand outside rig)

Base Steel plate

Spherical projection for uniform stress

Load

Load cells(at 4 corners)

Steel rod(at 4 corners)

Steel plates(on both sides)

Steel blocks

LVDT(with stand outside rig)

Base Steel plate

Spherical projection for uniform stress

Fig. 6. Test setups for shear capacity. (a) In-plane and (b) out-of-plane.

0

40

80

120

160

0 25 50 75 100

Loa

d (k

N)

Displacement (mm)

In-plane testing

Out-of-plane testing

Fig. 7. Load–displacement curves from selected shear tests.

(a)

(b)

Bottom side

Rightblock

Middle blockLeft

block

Bottom side edispoTedispoT

Interface Interface

1 2

3 4

5 6

78 9

10

11 1213

14 15

16 17

18 19

20

21 22

23

24 2526

Bottom side

Rightblock

Middle blockLeft

block

Bottom side edispoTedispoT

Interface Interface

1 2

3 4

5 6

7

8 9

10

11 12

13

14 15

16 17

18 19

2021 22

23

24 2526

Fig. 8. Sheared interlocking keys highlighted as shaded black from selected tests;(a) in-plane and (b) out-of-plane.

M. Ali et al. / Construction and Building Materials 37 (2012) 812–821 819

bottom sides of the left and middle blocks were torn off, but thekeys were not sheared. This might be due to a lack of proper com-paction during manual casting in the interlocking keys on the bot-tom sides of the block. However, the key 4 on the bottom side ofthe left block was sheared-off by key 8 on the top side of the mid-dle block. And this was not observed for the interface of the bottomside of the middle block with the top side of the right block. Com-paction during casting could be the reason for this non-uniformbehaviour. This is claimed because, among many factors, compac-

tion is the one which also affects the strength of the concrete. Sinceother factors were same for the considered case and compactionwas done manually, this might have varied a little for interlockingkeys of different blocks by the labour even it was instructed toadopt the same method of compaction for all blocks. This mightbe taken as human carelessness. Also, compaction of the fibre rein-forced concrete demanded much more care compared to that ofplain concrete. So it was very likely that this would have beenthe reason for the non-uniform behaviour. The key 7 on the topside of the middle block was sheared-off by the combined effectof friction with central bottom sides of the left block and thesheared key 9 on the top side of the middle block. Similarly, thekey 26 on the top side of the right block was sheared-off by thecombined effect of friction with central bottom sides of the middleblock and the sheared keys 24 and 25 on the top side of the rightblock. It can be said, in general, that the corner keys 1, 2, 5 and 6(or 14, 15, 18 and 19) on the bottom side were weak in comparisonto keys 8, 9, 11 and 12 (or 21, 22, 24 and 25) on the top side.Whereas, if proper compaction was done especially for interlockingkeys, middle keys 3 and 4 (or 16 and 17) on the bottom side wouldbe strong in comparison to keys 8, 9, 11 and 12 (or 21, 22, 24 and25) on the top side for the in-plane direction. In the out-of-planetests (refer to Fig. 8b), alike patterns were observed for the inter-face between the bottom side of the one block and the top sideof the other block. Again, cracks were confined to the interlockingkeys. The edged keys 2, 4 and 6 on the bottom side of the left blockwere sheared-off by the out-of-plane keys 7, 10 and 13, respec-tively, on the top side of the middle block. Similarly, the edged keys14, 16 and 18 on the bottom side of the middle block weresheared-off by the out-of-plane keys 20, 23 and 26, respectively,on the top side of the right block.

The maximum load from the load–displacement curves is di-vided by the total sheared-off area to get the shear capacity. The

Table 4Shear capacity of interlocking keys.

Shear

In-plane Out-of-plane

Maximum load (kN) 165.2 148.5Shear strength (MPa) 2.65 3.30Total energy required to cause shear (kN m) 3.61 3.53Total shear toughness (–) 7.59 7.11

Note: An average of three readings is taken.

(a)

(b)

0

40

80

120

160

Ap

plie

d ve

rtic

al lo

ad (k

N)

Lateral reaction force (kN)

Top-rightTop-leftBottom-rightBottom-left

Projectedcurve path

Load cellslocations

0

40

80

120

160

0 2 4 6 8 10 12

0 2 4 6

Ap

plie

d ve

rtic

al lo

ad (k

N)

Lateral reaction force (kN)

Top-right

Top-left

Bottom-right

Bottom-left

Load cells locations

Fig. 9. Vertical load vs. lateral reaction force curves from (a) in-plane and (b) out-of-plane tests.

820 M. Ali et al. / Construction and Building Materials 37 (2012) 812–821

area under the load–displacement curve is taken as the energy re-quired to cause shear failure of the interlocking keys. The authorshave defined a parameter shear index describing the shear tough-ness as the ratio of the total area under the load–displacementcurve to the area under the load–displacement curve up to themaximum load. It is a measure of how much energy is requiredto cause further displacement after its strength decreases whensubjected to shear loading. The shear capacity, total energy re-quired for shear, and shear toughness of the in-plane and out-of-plane testing are presented in Table 4. An average of three readingsis taken. It can be observed that the out-of plane shear strength(3.3 MPa) is more than the in-plane shear strength (2.65 MPa),but slightly more energy (3.61 kN m) is required to cause in-planeshear then out-of-plane shear (3.53 kN m). Actually, the sheared-off area of in-plane direction is much larger than that of out-of-plane (Fig. 8), but the load required to cause in-plane shear(165.2 kN) is slightly more than that of out-of-plane (148.5 kN).This means that the load does not proportionally increase thesheared-off area (indicated in black in Fig. 8) for in-plane testingwhen compared to out-of-plane testing. Therefore, the slightincrease in the numerator and the considerable increase in

denominator results in a lesser ratio, representing relatively lowshear strength, i.e. 2.65 MPa.

As the shear load (i.e. applied vertical load in Fig. 6) increasesduring the in-plane and out-of-plane shear tests, the variation oflateral reaction forces is shown in Fig. 9. Parts of the curves fromthe in-plane shear tests are projected because the maximum dis-placement of the middle block could not be measured due to spacelimitations in the 2000 kN compression machine. It can be ob-served that the lateral reaction force at the lower part (bottom-leftand right) is larger than that of the upper part (top-left and right)in both in-plane and out-of-plane cases. When considering eitherthe lower or upper part, it can also be seen that there is a little dif-ference between the left and right load cells readings. This mightbe caused by the initial minute gaps between the block and steelplate at some locations and/or the development of very small gapsbetween sheared-off keys and the central block at a few locationswithin the interface. Since the blocks were cast inverted, it was dif-ficult to have a very smooth bottom surface of the block with fibrereinforced concrete. When the block was placed between the steelplates, very small (minute) gaps at some locations between theblock bottom surface and the steel plate could be seen with thenaked eye. These gaps are named as ‘‘initial minute gaps’’. Theblocks between the steel plates were tight enough to avoid move-ment/slippage of the blocks without any load. At this stage, theload measured was around 5 kN. An effort was made to have min-imum gaps. These were not measured, but were kept in mind if theoutput detects these in terms of minor differences in readings ofthe left and right load cells. At the sheared portions, the small gapswere also created for a very short duration due to falling particlesof cement and sand, causing the slightly different readings in theleft and right load cells. For one of the in-plane tests, the corre-sponding lateral reaction forces at the maximum vertical load fortop-left, top-right, bottom-left and bottom-right locations are3.18, 3.41, 5.76 and 6.0 kN, respectively. In one of the out-of-planecases, the maximum lateral forces at top-left, top-right, bottom-leftand bottom-right locations are 4.25, 4.39, 5.30 and 5.34 kN, respec-tively. The lateral forces at the maximum vertical load at the corre-sponding load cells are 3.48, 3.42, 5.05, and 4.61 kN, respectively. Itmay also be noted that the corresponding lateral forces at maxi-mum vertical load for the upper portion (top-left and right) are lessfor the in-plane tests compared to the out-of-plane tests. The samefor lower portion (bottom-left and right) are more for the in-planetests compared to that for the out-of-plane tests.

4. Conclusions

Novel interlocking blocks are presented. Coconut fibre rein-forced concrete (CFRC) is used in manufacturing of blocks whichis also new. The 20 MPa strength of CFRC may be sufficient forthe single-storey earthquake-resistant houses. The compressivecapacity of single (standard, bottom, top and half blocks) and threeblocks (standard as well as the combination of top, standard andbottom blocks) were determined experimentally. The in-planeand out-of-plane shear capacities of the blocks were also investi-gated. An average of three readings is taken to represent a partic-ular property. The shear toughness of the blocks is measured bythe shear index, defined as the ratio of the total area under theload–displacement curve to the area under the curve up to themaximum load. The following conclusions are drawn:

� CFRC with a mix design of 1:4:2 (cement:sand:aggregates), a w/c ratio of 0.64, a fibre length of 5 cm and a fibre content of 1% bymass of concrete materials is recommended for manufacturingblocks, because of better properties compared to those of CFRCwith 1:3:3 having 0%, 0.5% or 1% fibre content.

M. Ali et al. / Construction and Building Materials 37 (2012) 812–821 821

� The compressive strength and total compressive toughness ofbottom block are higher than that of other blocks (standard,top and half blocks).� The compressive strength of multiple standard blocks is only

slightly less than that of the single standard block because ofthe slenderness effect, i.e. for standard blocks, fcm = 0.96 fcs,where fcm and fcs stand for compressive strength of multipleand single blocks, respectively.� Compressive toughness is the same for the single and multiple

standard blocks.� The average of compressive peak load for multiple blocks with

top, standard and bottom blocks is 84% larger than that formultiple standard blocks, but the compressive strength of latteris 70% higher.� The out-of-plane shear strength is 25% larger than that of the in-

plane.� More energy is required to generate the in-plane shear failure

than the out-of-plane shear.� The in-plane shear index is 7% higher than the out-of-plane

shear index.� During the manual casting, the compaction of CFRC in interlock-

ing keys of the developed block should be done with great careto ensure uniform compaction.

This pilot study is the first step towards exploring the behaviourof the invented block for the deemed technology. Mechanical cast-ing can help in proper compaction, and production in large num-bers will reduce the relative manufacturing costs. The blocksunder cyclic loading should also be investigated for measuring itsfatigue. The interlocking blocks may provide a practicable solutionfor the low-cost seismic-resistant housing.

Acknowledgements

The authors would like to thank all persons who helped themthroughout the research, particularly Golden Bay Cement and Win-stone Aggregates for support of this research. The careful reviewand constructive suggestions by the anonymous reviewers aregratefully acknowledged. The authors also wish to thank PakistanHigher Education Commission for supporting the Ph.D. study ofthe first author at the University of Auckland.

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