Elastic modulus and strength of hollowconcrete block masonry with reference to theeffect of lateral tiesF. M. Khalaf,* BS~, MSC, PhD, A. W. Hendry,@&, POD, FICE, andD. R. Fairbairn,t BS~, PhD, MICE

NAPIER UNIVERSITY: UNIVERSITY OF EDINBURGH

This paper presents the results ofan experimental inves-tigation to derive a formula for the short-term staticmodulus of elasticity E,,, of unfilled and filled concreteblockwork masonry. The formula is based on the mod-lrlus of elasticity of each individual material used in theconstruction of blockwork masonry. The eflects of usingIateral ties as reinforcement on the behaviour, ductilityand strength of concrete blockwork masonry are alsoinvestigated.

Notation

:iEb

4

44J%

k

4

net area of concrete infill: mm;gross area of prism: mm2modulus of elasticity of brick unit: N/mm2secant modulus of elasticity of hollow block,derived from unfilled three-course high half-block prism with I-2mm dental plaster joints:N/mm2 (see Table 2)secant modulus of elasticity of concrete infill,derived from three-cube specimen: N/mm2 (seeTable 2)modulus of elasticity of aggregate: N/mm2modulus of elasticity of mortar joint: N/mm2secant modulus of elasticity of 1Omm mortarjoint, derived from unlYled two-course highfull-block prism: N/mm’ (see Table 2)modulus of elasticity of masonry: N/mm2secant modulus of elasticity of mortar, derivedfrom a three-cube specimen: N/mm2 (see Table2)modulus of elasticity of cement paste: N/mm2

‘Dqarbmt of Civil and Transportation Bngineerin& Napier Uni-vmity, hrchiston. 10 Colinton Road, Bdinburgh, EHlO ZDT,

+igLtmentofcivil~ngalldBuilding~univu‘m.~ofEdiibur&. The King’s Buildings. Bdinburgh, BH9 UL, UK.

ghb2fcxfmfm’

a

fPr

volume of aggregate per unit volume of mixbrick/block height: mmmortar joint thickness: mmcube compressive strength of block material:N/mm2cube compressive strength of concrete infIll:N/mm2characteristic compressive strength of masonry:N/llUI12cube compressive strength of mortar: N/mm2ultimate compressive strength of blockworkmasonry: N/mm2WL = 0.45 (for 1Omm mortar joint)diameter of reinforcement: mm0.25

lntrodnction

Many previous attempts have been made to find aformula for the modulus of elasticity of brickwork andblockwork masonry. Sahlin’ related the modulus ofelasticity of brickwork masonry to the moduli of boththe brick and the mortar using the following theoreti-cal equation

ai= 1(1 - d)/Ej + a/&,’ 4 G Eb (1)

hba -= h,+hj (2)

In order to determine the modulus of elasticity Ej ofthe mortar joint, Sahlin quoted the expression sug-gested by Hansen2 for two-phase material (referring toconcrete) and given by the following equation

i$= (1 - g&, + g/E, ( 3 )

185

lvhalaf et al.

Table 1. Ehtic modulus E;, for concrete masonry

Nctamacomptwsivc8trengtll of mlits:

N/mm’

241.4 - 24 13834.5 1 9 3 1 0 220692 7 . 6 1 7 9 3 1 200002 0 . 7 15 862 1 7 2 4 117.2 15 172 1655213.8 1 2 4 1 4 15 172IO.3 1 0 3 4 5 11035

Modulus of eltMticity+E,.: N/mm’

TypcNmortar

+Lii intcqmlation permitted.

Most other researchers and standards relate themodulus of elasticity of masonry to the ultimate com-pressive strength f,’ of masonry. Although relatingthe modulus of elasticity to the masonry strength isirrelevant theoretically, it does have some practicalvalue.

The British Code of Practice BS 5628 relates theshort-term modulus of elasticity for clay, calcium sili-cate and concrete masonry, including reinforced mas-onry with infill concrete, to fk as follows’

Em = fJW! (4)

The American Masonry Standard (AC1 530-88/ASCE S-88) derives the modulw of elasticity of mas-onry from a table which relates the net area compres-sive strength of units and the type of mortar.4 This isshown as Table 1.

E,,, can also be determined from the secant modulusof elasticity taken between 0.OSf,' and 0.33fd by teston prisms in accordance with the prism test method ofAC1 530.1/ASCE 6-88 and ASTM E 447-84.

The Canadian Standard CSA-CAN3-S304 recom-mends that the modulus of elasticity of unfilled mas-onry be expressed as follows’

Em = lOOOf,’ < 20685 N/mm2 (5)

Based on experimental data, Hatxinikolas et al.”recommended a conservative value for the modulus ofelasticity of unfilled masonry as follows

E,,, = 75Of,' 0%

Feeg et ~2.7 suggested that the modulus of elasticityof filled blockwork masonry be expressed as follows

Em, = SOOf; (7)

To study the effect of lateral ties, Feeg et al. testedreinforced blockwork masonry short columns underconcentric load. All the columns were of 400mmnominal cross-section and 144m high. Some of thecol~nswereconstructedusing4OOmm x 2OOmm x200mm blocks. The column cross-section consisted

of two blocks laid in running bond. Other columnswere built using 400 mm x 400 mm x 200mmsingle-core pilaster concrete block units. Face-shellbedding was used and a mortar joint thickness of1Omm was maintained throughout. The block unitthickness restricted the placement of the horizontalreinforcement to a spacing of 200mm; on the otherhand, the mortar joint thickness restricted the size ofthe tie reinforcement to be placed in the mortar joint.Tie diameters used in this investigation were 3.77,4.76 and 6.35 mm. The variables investigated were tic.diameter and tie location within the mortar joint,either in contact with the vertical reinforcement or inthe mortar joint between the block outer shells.

The test results showed that increasing the tie dia-meter increased the strength of the column, comparedwith columns with no ties. This increase in strengthwas also accompanied by a decrease in the amount ofvertical cracking at failure. No significant difference instrength was observed between columns having ties incontact with the vertical reinforcement and thosewhich did not, although tie strains were larger for tietlocated in the mortar joint. Rust was also noted onungalvanixed ties after failures of columns where theyhad been placed in the mortar joint.

Sahm” tested under axial load blockwork masonryprisms with helical confinement reinforcement at thecore. The test specimens were constructed from con-crete block units, 200 mm square and 200mm high,The units were horizontally laid, and the masonryprisms were built to have flush mortar joints of nomi-nal thickness 9.5 mm. After construction, helical rein-forcement, consisting of mild steel wire of diameter52mm with a core diameter of 108 mm centre-tc-centre was placed inside the prism and the core wssgrouted.

The observed mode of failure for the ungroutedprisms was tensile splitting, which was initiated in thecentral concrete block. The mode of failure for thegrouted prisms (splitting of the units and compressivefailure of the core) was similar to that of the ungrout-ed prisms, but not as explosive. Cracking was initiatedat 70-75% of the ultimate capacity. Cracking ofprisms, reinforced with confinement wire, started atthe top or bottom block at about 45-55% of ultimateload. The sudden failure was replaced by a moreductile failure. An increase in the compressive strengthof the prism of between 30 and 38%, compared withunreinforced prisms, was achieved by introducinghelical confinement reinforcement.

Sturgeon et aL9 carried out tests on concentric load-ed nine-course high blockwork masonry columns. Theshort columns were constructed using 400mm x400mm x 200mm singlecore pilaster units, andwere tested to failure. Full mortar bedding was usedand a joint thickness of 1Omm was maintained. Thehorizontal reinforcement was placed within themortar joint of the cross-section. All lateral ties were

fabricated from 6mm diameter plain steel. Theauthors concluded that the introduction of lateralreinforcement resulted in an increase in the ultimatestrength of the column of 8-28%.

This Paper presents the results of an experimentalinvestigation carried out to derive a formula for theshort-term static modulus of elasticity E,,, of concreteblockwork masonry. The results of tests on eighteenaxially loaded unfilled, filled and laterally reinforcedblockwork masonry columns are reported.

Experimental programmeThe short stack-bonded blockwork masonry col-

umns were constructed and tested under axial load tostudy the mechanism of failure, to determine thestrength, to derive a formula for the modulus of elas-ticity, and to study the effect of lateral ties. The col-umns were divided into two main series: full-block(390mm x 190mm) cross-sections; and half-block(190 mm x 190 mm) cross-sections. The coltinswere all six-course high with a slenderness ratio of6.26.

One type of mortar (1: O-25 : 3), and one type ofconcrete infill (1: 3 : 2) were used in the construction ofthe columns. The columns were constructed by anexperienced mason, thus ensuring the complete fillingof the 1Omm horizontal mortar joints between theconcrete blocks. The block at the base of each columnwas a bond beam type (Fig. l(a)) in which the endshells and mid-web had been removed to make itpossible to remove the column of mortar remainingafter construction.’ The type of block was provided bythe supplier; the rest of the column blocks were eitherfull- or half-blocks. The full-blocks for the laterallyreinforced columns were provided with 20mm wideand 20mm deep grooves, cut with a diamond saw atthe sides of the block mid-webs (Fig. l(b)), to accom-modate the lateral ties. It was not possible to obtainblocks with such grooves from the supplier, but blockswith a 20 mm or larger dip in the mid-web (Fig. l(c)),can easily be produced by using a steel mould withsuch dips, as is the case with various different shapesand types of concrete block on the market.

The placement of lateral ties at the mortar jointswas avoided because previous studies6 had shown thatplacing lateral ties at the mortar joints produces a highconcentration of tensile splitting stresses around thetics, resulting in a reduction in the compressivestrength of the masonry assemblage. Also, placing thelateral ties at the mortar joints means that concreteblocks with thicker shells are required to comply withthe required concrete cover to the reinforcement.’

The lateral ties were placed in every course duringthe construction of the columns, including the top andbottom sides, to prevent any local failure. This gives anominal spacing of 189 mm. The lateral ties, for thefull-block columns, were placed in the 20mm wide

Fig. 1. T&es of concrete block wed in columnconstruction: (a) bond beam; (6) stan&.vd; (c) standkrdwith mid-web dip

and 20mm deep grooves during construction. In thecase of half-block columns, taking advantage of blocktapering, the dimensions of the lateral ties were madeslightly smaller than the half-block wide core. Thisenabled the ties to be held in the hollow cores, fhst byfriction and then by mortar after the construction.

Two steel brackets, 25 mm wide and 6 mm thick,were placed in prepared positions at the first and fifthmortar joints. These brackets were used later to mounttwo electrical displacement transducers (LVDTs) onboth sides of the column to measure changes in lengthwith the load increments.

After construction the six-course high columns wereleft for four days under polythene sheeting to allowthe mortar joints to gain in strength. The columnswere then filled with concrete which was batched byvolume, mixed to a high slump of 150 mm, then castin two layers. Each layer was compacted using a25mm poker vibrator until full compaction wasattained. The surface of the concrete inftll was thentrowelled level. After casting, the specimens were leftto cure under polythene sheeting for fourteen days.The polythene was then removed, and the specimensleft for a further fourteen days to cure under ambientconditions in the laboratory before testing.

Twelve of the eighteen full- and half-block columnsbuilt were unreinforced columns, tested either unfilledor filled, under axial load to determine the short-termstatic modulus of elasticity E,,, of the blockwork mas-onry. The rest were filled reinforced columns testedunder axial load to study the effect of the lateral ties.The mechanical properties for the materials used inthe construction of the blockwork masonry columnswere determined as follows (Table 2).

Concrete blockThe mechanical properties for hollow blocks were

determined by testing three blockwork masonry

Table 2. Material me&mica1 properties

Matelial

BlockSolidtPrism$Half-block

2 4 . 3 -2 1 . 9 -25.7 -

cbncrete1:5:21:3:21:1:2

8.8 8 . 3225 17.24 2 . 1 3 7 . 8

Mortar1:1:61:@5:4~51:025:3

8.1 8.016.5 1 4 . 626.6 25-0

- 2 1 1 3- 2 1 2 7- 2 1 2 7

7 . 2 1 9 7 8165 2 0 3 432.9 2 2 8 6

69 1 7 9 812.5 1 8 7 521.7 1 9 4 8

1omm nmarjoiar1:1:61:0.5:4*5l:Q25:3

---

---

- --- -

+!kant modulus of ehaticity at 3 maximum eomprrasive &engtb of spaSma.tCt~bc comprcssivc stmgtb of block materials = 24.3N/mm2.#UniYlaI thrceeoursc high half-block prism with 1-2mm dental plaster jolntn.

spcciInens orsinglc-cubc

atrcngtb:N/l2ld

llmecllbesspecimenstrength:N/Uld

prisms. Each consisted of three-course high half-blockprisms separated and capped with a I-2mm dentalplaster joint inserted between the half-blocks and bet-ween the prisms and the machine platens. This thick-ness was achieved by mixing the dental plaster withwater in a plastic bag to the desired workability. Aspirit level was used to adjust the specimen. The softdental plaster was then compressed by the machine toaccomplish the desired thickness.”

Double-axis (vertical and horizontal) elect&lstrain gauges were mounted at mid-prism height ontwo opposite sides of the prism. A computer strainlogger was used to record the strain continuouslythroughout the test until prism failure.

Unit half-blocks were tested in compression to determine their mechanical properties. Solid full-blocks,cast at the same time as the hollow blocks, were alsocompressed parallel to the bed face to compare theirmechanical properties with the hollow units. Solidblocks, sawn to the dimensions of a 190mm x190 mm x 190 mm cube, were tested in compression.The average compressive strength was then adjustedfor specimen sire” to determine the block materialcube strength fb.

Concrete inJillTo determine the contribution of concrete infill to

the modulus of elasticity II,,, of blockwork masonry,eighteen three-course high prisms, nine full-block andnine half-block, were constructed using a 10 mm thickpolystyrene sheet inserted between the blocks to sim-ulate a zero strength mortar joint. The prisms were

Density:kg/m’

Tangent secant*modulus of modulus of

e las t i c i ty : e las t i c i ty :N/mm* N/lid

Poisson ratio

Initialstlws

Maximumstrws

19118 13014 O-20 02533 100 28977 0.15 0203 8 0 5 4 3 8 0 5 4 0 . 1 3 0 . 1 8

6 0 3 2 4 6 7 4 044 0 . 3 31 6 0 3 3 11444 0 . 1 8 042 8 3 2 0 17 180 0.14 0 . 2 2

5 6 0 31025014119

2m4 1 0 05 5 0 0

3696 0 . 2 6 0405ooo O-22 0 . 3 58 1 4 0 0.18 0 . 2 5

1 2 3 226524 0 3 7

-

-

---

then filled with three di&rent concrete infill strengths:low-strength (1: 5 : 2), medium-strength (1: 3 : 2) andhigh-strength (1: 1: 2) cement : sand : aggregate pro-portions by volume. The method used for casting andcuring the prisms was similar to that used for theblockwork masonry columns. The prisms were thentested under axial load up to failure.

To determine the mechanical properties of theprisms with different concrete infill strengths, spe-cimens consisting of three 1OOmm x 1OOmm x100 mm steel moulded cubes, separated and cappedwith a thin (I-2mm) layer of dental plaster preparedby the same method as for the hollow blocks, weretested in compression. Vertical and horizontal strainswere recorded continuously, using a data logger,on two opposite sides of the prism. Single steelmoulded cubes and cytinders were also tested incompression to compare results and determine thematerial strengths.

MortarTo determine the contribution of mortar to the

modulus of elasticity E,,, of blockwork masonry, un-filled two-course high full-block prisms were con-structed, with a 1Omm mortar joint between theblocks, to determine and compare the confined verti-cal stress versus strain curve of a 1Omm joint withvalues obtained by testing either three steel mouldedcube specimens or mortar cylinders. Three types of

mortar were used in the construction of the prisms:low-strength (1 : 1 : 6), medium-strength (1 : 05 : 45)and high-strength (1 : 0.25 : 3) cement : lime : sand pro-portions by volume. The mortar was batched by vol-ume and mixed to a suitable workability for blocklaying. After construction, the prisms were curedunder polythene sheeting for fourteen days. The pol-ythene was then removed and the specimens were leftfor a further fourteen days to cure under ambientconditions in the laboratory before testing. Electricalstrain gauges of 10 mm length were mounted on themortar joint at two opposite sides of the prism torecord the strain for the confined 10 mm mortar joint.

To determine the mechanical properties of the threedifferent types of mortar, as for the concrete infill,specimens consisting of three 1OOmm x 1OOmm x1OOmm steel moulded cubes separated and cappedwith a thin (l-2 mm) layer of dental plaster weretested in compression. Single steel moulded mortarcubes and cylinders were also tested in compression tocompare results and to determine the materialstrengths.

Before testing, all the specimens were capped with athin (I -2 mm) layer of dental plaster” prepared by themethod explained above. The rate of loading for test-ing the blockwork specimens was in accordance withBS6073 : Part 1: 198 I .” The loading pattern for all the

specimens was in accordance wtih BS 1881: Part

121: 1983 to enable themodulus of elasticity.‘3

DitxmsioIl of resalts

determination of the static

Modes of failure for jidl- and half-block columnsA common feature was observed in the modes of

failure for all the unfilled and filled full- and half-blockcolumns tested. In checking the mortar joints at dif-ferent locations after failure, it was observed that them0rt.m had been reduced to a powder at certain stagesof the loading process.

All unfilled full- and half-block columns displayeda longitudinal block shell splitting type of failure.Full-block columns showed a greater tendency to splitalong the column end shells than the side shells, withcrushing and shearing of the block mid-web. Thecolumns showed an abrupt mode of failure, with nosigns of major cracks during the loading process untilfailure.

The mode of failure for the filled full- and half-block columns was dominated by block shell-concreteinfill separation and lateral deformation, with somesigns of block shell, mortar and concrete infill crush-ing near one of the mortar joints. The mode of failurewas leas abrupt than for the unfilled columns. Somesigns of cracking were observed at the column mid-height in the block end and side shells at 8&90% ofultimate load.

The mode of failure of columns reinforced with 6and 8 mm diameter lateral ties differed only from thatof the filled unreinforced columns. The concrete coresremained intact even after all the block shells werecrushed and had deformed outwards (Fig. 2). Thefailure was more ductile, with no complete collapse atultimate load, as was the case with unreinforced col-umns. Some signs of block shell cracking was ob-served at 80-90% of ultimate load. Full-block col-umns reinforced with 10 mm diameter lateral tiesshowed premature splitting of the block side shells andcrushing of the concrete cores. This may have beencaused by some stress concentration caused by the useof ties of larger diameter.

Short-term static mod& of ekuticity of blockworkmasonry

Any changes in length over the four courses of theunfilled and filled blockwork masonry columns weremeasured using two electrical displacement trans-ducers (LVDTs) mounted on steel brackets on oppo-site sides of the columns. The readings from thesetransducers were recorded continuously until failureusing a data logger. The average changes in lengthwere then divided by the gauge length to convert thereadings to strain over the four courses of the columnheight. Figures 3 and 4 show typical vertical stress

0 lJntllled full-bkxk columnCl Filled full-block column

E,,,=20383N/mm*(unfllled)E,,,=2316!5N/md(filled)

15.0 20-O 25-o 304Strain x lo,,’

Fig. 3. Typical stress versus strain curves for unfilled andfilled taweinforced fdl-block masonry columns

Fig. 2. Mode of faihue of j&block rmisonry cohmmreinforced with 8mm diameter lateral ties

versus strain curves for the unfilled and filled full- andhalf-block columns, respectively.

From the process of loading and unloading, theaverage short-term static modulus of elasticity forthree of each unfilled and filled, full- and half-blockmasonry columns was determined (see Figs 3 and 4and Table 3).

Based on the results of the present study and on thetheoretical expression suggested by Sahlin,’ the mod-ulus of elasticity of unfilled blockwork masonry can bedetermined as follows

Em =1

(1 - S)/aE,, + b/E,,,

In this expression the value of a is the average ofthree types of mortar (1:1:6, l:O-25:3 and1: 05 : 45) and was found to be equal to 0.45 for a1Omm mortar joint. For mortar joints of differentthicknesses, the value of a can be found by testingspecimens similar to the ones used in this investigationto determine Em,, and EB. The value of a takes intoconsideration the thickness of the mortar joint and theeffect of the concrete block confinement. It is expectedthat the value can be used in general for all unfilledblockwork masonry built with hollow concreteblocks.

The modulus of elasticity of filled blockwork mss-onry is difficult to determine owing to the presence ofthe concrete infill. Figure 5 was plotted to studyseparately the contribution of the concrete infill to thestrength and modulus of elasticity of blockwork mas-onry; this figure relates the strength of three-coursehigh full- and half-block prisms with polystyrenejoints to the concrete strength. Two relationships arcshown in Fig. 5: the first is based on the concrete infillnet area A,, equal to 19 272 mm* for full-block prismsand 9636mm* for half-block prisms; the second rela-tionship is based on the prism gross area A,, equal to74 IO0 mm* for full-block prisms and 36 100 mm* forhalf-block prisms.

Based on the prism gross area, Fig. 5 shows that fora wide range of concrete infill strengths (86-352N/mm*) the increases in strength of full- and half-blockprisms, namely 179 and 123%, respectively, are lessthan the increases in strength of the concrete infill,namely 297, and 174%, respectively.

Based on the net area of the concrete infill forhalf-block prisms, Fig. 5 shows that the strength of theconcrete infill increased by 45, 28 and 18%, corres-

Table 3. Comparison of exl:of&

Column type(tXpCiiCWl,):

N/IN’+

Full-blockUnfilledFilled

Half-blockUnfillCdFilled

2038323 165

20 952233m

Hmental and theoretical values

(thco~oai)*: ’E,,, (experimental)

N/mm2 & (thuwctical)

21535 o-9524396 o-95

21535 0.9124396 o-96

+Dcrivcd from equations (8) and (IO).

-------- ----------- - ---

20-o -

%i.--_l-_l-_l--- - -w---e-- --

E 15-O-ii

g,o.o-//

0 lJnfllkd hnlf-bbdr column0 Flnedhd-bbkwlumn

&,,=209!2N/m~(unRlled)23 320 N I mm* (filled)

Sfain x lo-’

Fig. 1. Typical stre.w ver.rc.strain curves for un$&d andj&d unreinforced half-block masonry columns

ponding to concrete cube strengths of 12*9,20*2 and3S*2N/mm2, respectively. This is due to confinementof the concrete by the block units. The same trend wasalso observed in the full-block prisms. The differences

in the percentage increases in concrete stmngth sug-gest that the concrete block provides more confine-ment to the softer concrete infill than to the stifferconcrete infill. Also, soft concrete deforms more later-ally than stiff concrete, owing its high Poisson ratio.

The formula, derived from a line passing throughthe results of both the full- and half-block prisms withpolystyrene joints (Fig. S), and based on the prismgross area, is as follows

fA = @25f, + l-3 (91

The gradient of this line is 0.25, which represeuts thecontribution of the concrete infill to the strength of thethree-course high prism with polystyrene joints. Theconstant value of 1.3 N/mm2, which is the intersectionof the broken line with the Y-axis, indicates that theunfilled blockwork masonry prism has some strengtheven when the mortar joint strength approaches zero.

Since the contribution of the concrete infill to thestrength of filled blockwork prisms is only 25%, itmay be assumed that the concrete inflll will contributethe same percentage to the modulus of elasticity. Onthis basis, the short-term static modulus of elasticity of

0 !so 100 t&ilOIE;gd ao-0 .95-o 400

8Yg. 5. E#kct of concrete injill strength on #l&d thm-corowse h&h prims strengthwith polystyrene joints

TWe 4. Strain measurements

ColUmntrpe

Lateral ties+nominal dia.:

mm

Yield A=mF Averagestrain tran8ducon horizontal tierx lo-’ strain x lo-’ strain x IO-’

FW-blockUntilledFilledFilledFilledFilled

--

68

10

- - 23.3- - 13.1

441.5 2 1 16.4527.9 26 15.6519.1 26 IO.9

--

532

- - -6 2 1 38 26 2

1 0 26 4

*6 mm dia. lateral ties, hot rollcd plain low-yield steel bars; 8 mm dia. lateral tics, hot rollcd dcformcd high-yield steel bars; 10 mm dia. lateraltics, hot rolled &formal high-yield &ccl bars.

Half-blockUllflUcdFilledFilledFilledFilled

- --

441.5527.9519.1

- 21-912.317.31 6 ‘ 618.9

-

filled bloclcwork masonry can be expressed as follows

Em =1

(1 - @/a&,, + S/Eb + rE- (lo)

The above two formulae for E,,, were found to giveexcellent results when compared with the average ex-perimental values obtained for the modulus of elastic-ity of unfilled and filled blockwork masonry (Table 3).

Strain meawementsTable 4 gives the results of the average strain mea-

surements, for the concrete over four courses of thecolumn, and also for the lateral ties, obtained duringtests on both the unreinforced and reinforced col-umns.

All values of average strain reported herein repre-sent measurements taken either at ultimate load or atthe limit of the ascending load versus strain curve.Although the strain was monitored continuouslyduring the loading process, it was difficult to monitorthe strain over the descending part of the load versusstrain curve owing to the sudden failure of specimenscaused by the sudden release of energy stored in themachine at failure.

The strain values recorded over four courses for theunreinforced full- and half-block columns indicatethat the strain in a filled column is 44% lower thanthat for an unfilled column, suggesting that theamount of shortening over four-courses of a filledcolumn is lower than that for an unfilled column. Thiscan be attributed to an increase in stiffness resultingfrom the presence of the concrete infill.

Full-block columns, reinforced with 6 and 8mmdiameter lateral ties, show an increase in the recordedstrains over four courses of 25 and 19%, respectively,compared with filled unreinforced columns. This

means that the laterally reinforced columns are mereductile than the filled unreinforced columns. On theother hand, columns reinforced with 10 mm diameterlateral ties show a decrease of 17%, compared withfilled unreinforced columns. The decrease in strainmay be due to failure of the column caused by the highconcentration of tensile splitting stresses around thelateral ties near the block mid-webs: this resulted inpremature splitting of the column side shells. Therehigh splitting stresses are related to the size of thelateral ties used.

Half-block columns, reinforced with 6.8 and 1Ommdiameter lateral ties, show increases in the recordedstrains over four courses of 41, 35 and 54%, resprc-tively, compared with filled unreinforced columns.The increase in strain for the half-block column, rein.forced with 1Omm diameter lateral ties, supports theexplanation given previously for the decrease in strainof the full-block column reinforced with IOmm dia-meter lateral ties, namely premature splitting of theblock side shells.

The strain measurements for the ties in the laterallyreinforced full- and half-block columns show thatthese values are all less than the yield strain of the steel(Table 4). A maximum strain value of 5 x lo-’ wsarecorded on the 6mm diameter ties.

Cohmn strengthTable 5 gives values of the compressive strength of

all the unreinforced and laterally reinforced full- andhalf-block columns. The table shows that the com-pressive strength of the filled unreinforced full- andhalf-block columns (based on gross area) decreases by13 and 33%, respectively, compared with unfilled ccl-umns (based on net area). This reduction is mainly dueto the presence of the concrete infill, which applica

T&le 5. Compressive strength of unreiqforced ond laterally retiforcedjdl-block ond hoof-black columns

colmnn Lateral tial ultimatetype nominal dia.: load:

mm kN

tYndnforced hl-block$Ud%d -Filled -

17.5-

---

23.4-

72a.91128.5

Laterally reirsorad full-block$Filled 6 1328.7Filled 8 1327.2Filled 10 10*5

Unreinforced half-blockgUnfilled - 465.1Filled

I- 561.4

Laterally reiniorced half-bloc&jFilled 6 570.8Fil led 8 586.0Filled 1 0 676.6

*Avungc and S.D. for unreinforced colmnna are calculated for three specimens.fCubc compressive strength of block material fb = 24.3 N/mm’.$Not arca = 41700mm2. gross ama = 390 x 190 = 74 100mm2.metarea = 19900mm2. gross area = 190 x 190 = 36100mm2.

---

Avoragecom~w SD.:strength? N/mm2 N/llU212

NOtUXl Groasarea Mortar

f,Ill5Uf.

9.8 Os/o*31 27.1 -15.2 o-92 271 21.6

17.9 - 28.6 18.317.9 - 28.6 18.314.0 - 28.6 18.3

12.9 0.78/@4315.6 o-96

15.8 -16.2 -18.7 -

-19.7

17917-917.9

Material cubeOOlllpressive

atmgtbt: N/mm2

tensile stresses as a result of the high value of thePoisson ratio of the concrete.‘t20 These large tensilestresses result in the failure of the column beforeattaining the apparent material strength of the block&.

The strength of the unfilled full-block columns, onthe other hand, is 25% less than that of the half-blockcolumns. This is caused by differences in the aspectratio l/t (column length-to-thickness) and mortar bed-ded area between the full-block columns (l/t = 2.05)sad half-block columns (l/t = 1.0). A small differenceia compressive strength was observed between theBled full columns and half-block columns.

Table 5 also shows that the compressive strength ofthe full-block columns, reinforced with 6 and 8mmdiameter lateral ties, is 18% more than the strength ofthe filled unreinforced columns. On the other hand,columns reinforced with 1Omm diameter lateral tiesfail at a compressive strength which is 8% less thanthat for filled unreinforced columns. This increase incolumn strength can be attributed to the confinementof the concrete infill by the lateral ties. These confine-ment stresses cause a reduction in the harmful tensilestresses exerted on the block shells by the concrete andan: not a result of any increase in the concrete8Wxqth. The decrease in strength of the column rein-forced with 10 mm diameter lateral ties may be due topremature failure of the column as a result of the highconcentration of splitting stresses around the lateralties, as explained above.

20%, with provision of lateral ties of diameter, 6, 8and lOmm, respectively.

As with the case of filled unreinforced columns, nosignificant difference in strength was observed bet-ween the laterally reinforced full- and half-block col-umns. The only anomaly is the full-block columnreinforced with 1Omm diameter lateral ties, whichshows a lower strength than the corresponding half-block column, for the reason mentioned above.

conchsio~

A semicmpirical formula (equations (8) and (10))has been proposed to determine the short-term staticmodulus of elasticity of unfilled and filled blockworkmasonry.

The presence of lateral ties changes the mode offailure of blockwork masonry cohmms from a suddenexplosive failure to a more ductile failure. Columnsreinforced with 1Omm diameter lateral ties show pre-mature splitting of the block side shells and crushingof the concrete cores. This may be caused by a highconcentration of tensile splitting stresses around thelarge diameter lateral ties near the block mid-webs.

More consistent results for values of strain andultimate load were obtained for columns reinforcedwith hot rolled deformed high yield 8mm diameterlateral ties than for columns reinforced with 6 and1Omm diameter lateral ties.

On the other hand, the half-block columns show a The strengths of filled full- and half-block columnsclear tendency to increase in strength, by 1, 4 and (based on gross area) were 13 and 33% less, resp

tively, than the strengths of un8lled columns (based onnet area). This reduction was attributed to the highvalue of the Poisson ratio of the concrete infill com-pared with that of the block material.

The strength of full-block columns reinforced with6 and 8 mm diameter lateral ties was 18% higher thanthe strength of filled unreinforced columns. On theother hand, columns reinforced with 10 mm diameterlateral ties failed at loads which were 8% less thanthose for filled unreinforced columns. This increase incolumn strength results from confinement of the con-crete infill by the lateral ties. These confinement stres-ses cause a reduction in the harmful tensile stressesexerted on the block shells by the concrete and are nota result of an increase in the concrete strength. Thedecrease in the strength of columns reinforced with1Omm diameter lateral ties may be caused by prema-ture failure of the column due to the high concentra-tion of tensile splitting stresses around the large dia-meter lateral ties. On the other hand, the results of thehalf-block columns show increases in strength of 1,4and 20%, with provision of lateral ties of diameter, 6,8 and 10 mm, respectively. This tendency to increase instrength is similar to that in reinforced concrete col-umns.

Acknowledgements

The Authors wish to express their appreciation andthanks to Forticrete Concrete Masonry Ltd, Bootle,Merseyside, for supplying the materials for the exper-imental investigation. This work was carried out at theUniversity of Edinburgh, Scotland.

References1. sAIiuN s. structulnl matowy. PmnbHall he., Eugl@

cliffs, 1971.2. HANSEN T. C. Creep of concrete. !hwdish Cement and Cm-

emto Reaedl IMtitution, Bulletin 33, 1985,24-33.3. Burn&~ STANDARDS IIWITWTION. Use of mawnry. BSI, Len-

don. BS 5628. Part 1: 1978; Parta 2 and 3 : 1985.4 . A M E R I C A N (Immtm I - . nuilm code leqdment

for mawmry .vtructnree. AC1 530/ASCE 5, 1988.

5 .

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

CANADIAN STANDARIH ASSOCIATION. hfasonry &a&n on&mstruction fw bdiingx. CSA-CAN3-S304, 1978.HA-o~ M. et al. Concrete mawnry walk StmctdEngineering Report No. 70, Civil Engineering Dcparhncnt,University of Alberta, 1978.FEW C. et al. Effects of rei+formnent tktatlittg for muetematmy columnt. Structural Engineering Report No. 76,Civil Engineering Department, University of Alberta, 1979.SALIM A. H. The effect of confinement reinforcement on loadcarrying capacity of mawnfy prisms. Proc. 2nd C~&Jmasonty eympoaium. Ottawa, 1980,65-72.STUROBON 0. R. et al. An investigation of reiqforced concretehfoc& mmonry cohmou. Structural Engineering Report No,91, University of Alberta, 1980.KHALAP F. M. and HENDRY A. W. Effect of bed-face ptqn~.ration in compressive testing of masonry units. Proc. 2nd int,masonry con$ British Masonry Society, London, 1990, No. 4,1 2 9 4 3 0 .NEVILLEI A. M. Properties of concrete. Pitman, London, 1981,3rd edn.B R I T I S H STANDAROS I - . Precatt concrete matanyunits. BSI, London, 198 I. BS 6073 : Parts I and 2.BRITISH .!+TANDARDfi INSnTUnON. Testing concrete: methodfor &tem&ation of static modah of ehticity in comp~~~ien.BSI. London, 1983. BS 1881 :Part 121.HAMID A. A. and DRYWALE R. G. Suggested failure a-it&for grouted masonry under axial compression. AC1 J., 1979,76, No. 10, lO47-1061.DWSDALW R. G. and Hlua~ A. A. Behaviour of concreteblock masonry under axial compression. AC1 J., 1979,76,No. 6.707-721.CHREMA T. S. and KLINE R. E. Comprwsive strength dconcrete masonry prisms. ACI J., 1986,8, No. 2.88-97.HAMID A. A. and C~JKWIJNENW A. 0. Compression b&n-viour of concrete masonry priama. J. Struct. Div. Am. Sec.Civ. Engrs, 1986, 112, No. 3. 605613.KHALAP F. M. The performance of concrete blocka loadedparallel to the bed face. M-y Intwnational, 1988.2, No.1.2~2f4.NATIONAL CONCRKIII MMONRY Aiwcwm~. Research imw-tigation of theproperties of mawmr ygroutincomcretemamnry,NCMA Engineering Concrete Masonry Design Committee,NCMA Grout Research Report, USA, 1988.&HARI F. A. and KALDJIAN M. J. Finite element aaaly& olconcrete masonry prisms. ACI J., 1989,86. No. 5,525-530.