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ACI STRUCTURAL JOURNAL TECHNICAL PAPERTitle no. 86-559

Structural Evaluation of Concrete-Backed Stone Masonry

aiDby Mohamed A. H. Abdel-Halim, Munawer R. Basoul, and Riyad A. Abdel-Karim

Experimental results of 15 test prisms, 6 beams, and 12 columns ofconcrete-backed stone masonry are reported. Six stone columns wereinfilled with plain concrete, and the other six stone columns were infilled with reinforced concrete. The prisms and the columns weretested under concentric and eccentric loads, and the beams were testedusing two equal loads at the middle third of the beam. The traditional method of building used in some of the Middle East countrieshas been adapted to simulate the actual working conditions at si te.Three kinds of stones were used: the first kind represents the veryhard stones; the second represents the hard stones; and the third represents the soft stones. All the specimens were tested under increasing load until failure.Results indicate that the adhesion between the stones and the concrete is the weakest point in the concrete-backed stone masonry. Thestresses in the concrete and stones atfailure did not compare favorably with the compressive strength of the concrete or the stone unitsdue to this weak bond between the concrete and stones. Transformedsection technique can be used to study the behavior of concretebacked stone masonry members until bond failure.Keywords: beams (supports); bond (concrete to stone); columns (supports);eomposlte eonstrue1ioD (eODereteto masoDry); eODeretes; eccentric loads; evalutiOD; failure mechanisms; infilled frames; masoDry; prisms; rocks; tests.In construction projects all over the rapidly developing countries of the Middle East, concrete-backed stonemasonry is used extensively. In most residential buildings concrete-backed stone masonry walls are used asload-bearing elements in certain types of single- anddouble-story structures. In multistory buildings, a reinforced concrete frame provides the basic skeleton, butthe external walls are built with concrete-backed stone

masonry. Stone masonry columns infilled with concrete are used sometimes as external columns, especially in balconies.The way in which the stone masonry structures arebuilt in most of the Middle Eastern countries is different from the ways used in other countries of the world.After the rubble stones are brought from quarries, theywill be regularly cut into different sizes, usually enoughto be handled by one person, using stone saws. Afterthat, stones are transported to the building site. There,experienced stonecutters cut them into different sizesand shapes according to their function in the building.Then, a professional mason lays the stones in courses608

using flush mortar joints of a nominal 7 to to-mm (0.28to OAO-in.) thickness. After building two to threecourses for a height equal to approximately 50 to 75 cm(20 to 30 in.), backing concrete is placed between thestones and the internal form work. The water-cementratio (w/c) in the backing concrete is usually on thehigh side to allow the concrete to flow and fill all thevoids behind the stones. Hand compaction is usuallyused for the backing concrete, since vibration maycause the stones to be pushed out of place. The averagethickness of the stones is not constant; it varies from 4to 8 cm (1.60 to 3.2 in.), and the thickness of the stoneat the edges is usually smaller than at the center.Stones are also built to form columns with cores inthe middle that are filled with plain or reinforced concrete. Some of these columns are built for decorationpurposes only, while others are considered as structuralsupporting members.

RESEARCH SIGNIFICANCEThere is a lack of information regarding the structural behavior of concrete-backed stone masonry mem

bers, especially at the ultimate strength level. It is important to understand the behavior of such members tobe able to make reliable estimates of permissible loading in all aspects of structural design. This paper dealswith experimental work on concrete-backed stone masonry prisms, beams, and columns. These memberswere tested under increasing load until ultimate.

MATERIALSStones - Three different kinds of limestone wereused in manufacturing the test samples. The stoneswere brought from three different quarries. One kindwas classified as very hard stones, the second as hardstones, and the third as soft stones. Cylindrical cores 50

ACI Structural Journal, V. 86, No. 5, September-October 1989.Received May, 10, 1988, and reviewed under Institute publication policies.Copyright 1989, American Concrete Institute. All rights reserved, includinghe making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the July-August 1990 ACIStructural Journal if received by Mar. I , 1990.ACI Structural Journal I September-October 1989


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ACI member MolItuMd A. H. Abdel-Hallm is an assistant professor of civi lengineering at Jordan University of Science cl Technology, Jordan. Dr. AbdelHalim has been involved in reseorch in polymers in concrete, segmental bridges,and finite element analysis of concrete structures. He is VicePresident of ACI'sJordan Chapter. Dr. Abdel-Halim is a graduate of Pennsylvania State University, University Park, Pa.MlllUlWerR. Bilsoul received his BS degreefrom Riyad University, Saudi Arabia, and his MS degree from Yarmouk University, Jordan. He is an independent consultant in structural design.ACI member Riytld A. Abdel-Karlm received his BS degree from the University of Jordan and his MS degree from Yarmouk University. He is at present aPhD student, Department of Civil Engineering, Pennsylvania State University.

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f- 50cm ~

....,..."";," ..:.' ..1._ ..... .... .. . . . i.~. '" :..

I;O-30em ia. The Prism Sample

EXPERIMENTAL PROGRAMThis section gives a brief description of the test spec

imens and test procedure. A more detailed descriptionis given in Ref.erences 1 and 2.

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Fig. 1 - Prisms and beam test samples

Test specimensFifteen prisms 50 cm (20 in.) wide, 100 cm (40 in.)high, and with variable thickness were tested. A sketch

for the sample is shown in Fig. l(a). Nine of the prismswere built using hard stones and six using soft stones.Twelve samples were tested under axial compressionload and three samples under eccentric load with different eccentricities. All prisms were four courses high,and the stones were laid in runing bond. The pertinentdetails of the prisms are listed in Table 1.Six beams were tested to measure the flexuralstrength (the modulus of rupture) of concrete-backedstone masonry. Three of the beams were made usinghard stones and the other three using soft stones. Thebeams were simply supported and loaded at the middlethird to have a constant moment zone, as shown in Fig.l(b). The details of tested beams are listed in Table 1.Twelve stone masonry columns were tested, six ofthem infilled with plain concrete and the other six in

filled with reinforced concrete. The column sampleshad a cross section of 18 x 18 cm (7.1 x 7.1 in.) and aclear height of 88 cm (34.6 in.). Very hard stones wereused in manufacturing the samples. The columns wereseven courses high, the course height was 12 cm (4.7

609

x 100 mm (2 x 4 in.) were drilled from each kind, andtests were conducted as prescribed in the applicableASTM specifications to determine their properties, including: ASTM C 97 for specific gravity and absorption test; ASTM C 170 for compressive strength test.The water absorption was 0.67 percent for the veryhard stones, 0.59 percent for the hard stones, and 6.36percent for the soft stones. The compressive strengthwas 80.4 MPa (11.67 kips/in.2) for the very hardstones, 40.2 MPa (5.84 kips/in. ~ for the hard stones,and 14.1 MPa (2.05 kips/in.2) for the soft stones.Concrete - The concrete was made from local materials. The coarse aggregate was crushed limestonewith a maximum size of 19 mm (.i in.). The averagecompressive strength after 28 days was 12.1 MPa (1.76kips/in.2) for the concrete used in the prism and beamsamples, and 24.4 MPa (3.540 kips/in.2) for the concrete used in the column samples. The strength of theconcrete in the prisms and beams is very low. This isbecause a high w/c is used to allow the concrete to flowand fill all the voids behind the stones and becauseconcrete cannot be vibrated as required, since this maycause the stones to be pushed out of place. In the columns, stiff concrete was placed because stones were allaround the core. There was a strength change of theconcrete in place due to the water reduction caused bystone absorption rates. The absorption rate of the softstones was higher than that of the hard stones. And,the greater the absorption of the stone units, the greaterthe consequent in-place strength of the concrete.Mortar - Mortar cubes of 50 x 50 x 50 mm (2 x 2 x2 in.) were taken during construction of samples. Theaverage compressive strength after 28 days was 32.3MPa (4.69 kips/in.2) for the mortar used in the prismsand beams and 18.6 MPa (2.70 kips/in.2) for the mortar used in the columns. Similar to the concrete, the inplace strength of the mortar was affected by the absorption rate of the stone units.Reinforcement - Some of the column samples werereinforced using four 10-mm diameter (four 0.40-in.diameter) deformed bars as the main bars and diameter6 mm (diameter 0.24 in.) as stirrups. The yield strength

of the steel was 274.4 MPa (40 kips/in.2).The samples were constructed and the materials wereselected to simulate the construction practice of concrete backed-stone masonry structures in the field.ACI Structural Journal I September-October 1989


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Table 1 - Prism and beam test specimensWidth ofidth ofotalype oftone,oncrete,idth t,ccentricityo oftonesmmm,mmamples 8020000ard8070500 802000 8020000 8070500P6 807050/1O 807050/5ard807050-Soft07050-

Note: I in. = 25.4 mm.

mounted on both sides of the sample to compensate forany torsional effects. The load was transmitted fromthe loading machine to the specimen by a rigid steelbeam. Fig. 4 shows one of the column samples readyfor testing.The load was applied in increments until failure. After each increment, the readings of all the strain gagesand dial gages were recorded. The cracks were checked,traced with a black felt-tipped pen, and identified withthe load at which they formed .

FLEXURAL STRENGTH TEST RESULTSFlexural-type failure occurred in all the beams. Forthe beams with hard stones, the cracks followed the

ACI Structural Journal I September-October 1989

PRISM TEST RESULTSThe test results are summarized in Table 3. The fail

ure load for the axially loaded prisms represents theaverage load of three test samples. In all the samples,the first crack occurred at the interface between stonesand the backing concrete. The mode of failure of theaxially loaded prisms was splitting of the stones fromthe concrete along the height of the prism. For the eccentric prisms, the stones were stripped out of the concrete, but the load was still carried by the concrete. Asthe load was increased, crushing occurred for the backing concrete.The compressive stresses in the concrete and thestones were calculated when the stones were strippedout of the sample. The results obtained for the axiallyloaded prisms are shown in Table 3. It can be seen thatthese compressive stresses do not compare favorablywith the compressive strength of the concrete or thestone units. This can be attributed to the weak bondbetween the concrete and the stones in such a type ofconstruction. Also, it is clear that the higher thestrength of the concrete, the higher the stresses in theconcrete and in the stones at which failure occurs.

The behavior of the prisms was linear elastic untilbond failure occurred between the stones and concrete.This is because both materials, concrete and stones,have low stresses at failure. The conventional transformed section technique can be used to study the behavior of such members theoretically.

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in.), and provided with reinforced concrete caps asshown in Fig. 2 to apply the load eccentrically. Thecolumns were tested under eccentric loads with different eccentricities. The details of these samples are listedin Table 2, and the details of the columns infilled withreinforced concrete are shown in Fig. 2. Fig. 3 showsone of the samples being manufactured.

Fig. 2 - Column sample infilled with reinforced con-crete

Testing procedureAll the specimens were tested with static loading using a 60 ton (590 kN) capacity hydraulic testing machine. Dial gages were used to measure deflections; theywere capable of measuring to the nearest 0.02 mm.Strains were measured using 300 ohm, 120 mm (4.72in.) electrical resistance strain gages. Strain gages were610


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a-Building the Stones

Table 2 - Column test samplesWidth ofidth ofotal Eccentricitytones,ore concrete,idth I,ongitudinal,mmmmeinforcementmml 4010080- 0C2 4010080- 0.,"t3 C3 4010080 60 -._ 0 C4 4010080 -5SoS C5 4010080 90 -u C6 4010080-120C7 4010080our 100mm dia. 0C8 4010080our lO-mm dia. 000.,=:::0 C9 4010080our 100mm dia.60='0- ., CIO4010080our lO-mm dia.80...S..8 Cll 4010080our 10-mm dia.1008.... C124010080our 100mm dia.120

Note: 1 in. = 25.4 mm.

Fig. 3 - Fabrication of axial samplesTable 3 - Test results for prisms

Fig. 4 - Column sample ready for testing

Stonecompressive Concreteccentricityailureoncrete stresstone stressge,ype oftrength,trength J",,oad,at failure,t failure.aystonesMPaPam

NPaMPa 40.3.21120.0.875.132ard 40.3.53350.2.550.198 40.3.53326.7.305.748 14.16.21129.7.808.297 14.16.21188.6--7 14.16.211/1044.3--7 14.16.211/506.0--

Note: 1 ksi = 0.145 MPa; 1 in. = 25.4 mm.ACI Structural Journal I September-October 1989 611


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Fig. 5 - Failure of axially loaded columns

Fig. 6 - Close-up of splitting cracks in axially loadedcolumn612

Fig. 7 - Failure of eccentrically loaded columnsvertical and bed mortar joints at failure. However, forthe beams with soft stones, the cracks started at thevertical mortar joint in the lower course and continuedthrough the stones up to the upper course.The area of the stones was transformed into anequivalent area of concrete, and the moment of inertiafor the transformed section was calculated. Then, theflexural strengthf,. (modulus of rupture) was calculatedfrom the equation f,. = M cl IIr where M = the appliedmoment at failure; c = distance between neutral axisand bottom of the beam; and Itr = moment of inertiaof the transformed section.

The average flexural strength was 1.329 MPa (193psi) for the Bl beams with hard stones and 0.635 MPa(92 psi) for the B2 beams with soft stones. It is clearthat the flexural strength of the masonry beams increases as the strength of the stones increases.

COLUMN TEST RESULTSThe failure modes for the axially loaded columnswere tensile splitting, which initiated at the interface

between the stones and the concrete in the core, asshown in Fig. 5. A close-up for the splitting cracks isshown in Fig. 6, and failures of eccentrically loadedcolumns are shown in Fig. 7. In the eccentrically loadedsamples, cracks followed the horizontal mortar joint onthe tension side near midheight, and crushing occurredat failure for the stone units on the compression side.ACI Structural Journal I September-October 1989


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Table 4 - Test results for column samplesConcreteEccentricityailureaximum concreteaximum stonege.tone compressivetrength Ic.,oad.tress at failure.tress at failure.aystrength. MPaMPamNMPa MPa80.404.52526.735 7.8486.185C2 28 80.4024.52556.165 7.8486.1850 80.4024.525042.780 8.9278.2550 80.4024.525520.725 4.5135.7001 80.4024.525098.650 3.3352.0650 80.4024.525205.615.3509.4188 80.4024.52541.450 7.6525.7948 80.4024.52556.165 7.6525.7949 80.4024.525060.000 8.3406.5850 80.4024.525035.440 4.0204.3200 80.4024.5250013.8002.9451.0850 80.4024.525206.530.4559.710

Note: 1.0 ksi = 0.145 MPa; 1 i n. = 25.4 mm; 1 k ip = 4.448 kN.

500

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2050

--0----0- Columns Infilled WithConcrete

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The test results of the column samples are summarized in Table 4. The compressive stresses in the concrete and the stones were calculated at failure, and theresults are shown in the table. Again here, it can beseen that these compressive stresses do not compare favorably with the compressive strength of the concreteor the stone units. And this is also attributed to theweak bond between the concrete and the stones. Thebehavior of the columns was linear until ultimate because of the low stresses in the concrete, the stones, andthe steel at ultimate.

Interaction diagrams for columns infilled with plainconcrete and columns infilled with reinforced concretewere plotted in terms of axial load strengths as ordinates and moment strengths as abscissas. The diagramsare shown in Fig. 8. From these diagrams, it can beseen that the effect of the reinforcement on the strengthof the samples was small. This was due to the positionof the steel that was close to the neutral axis, whichmeans that in proportioning the sections of stone masonry columns, rectangular sections should be used foreccentric loading, and the steel bars should be placed asfar as possible from the neutral axis.

CONCLUSIONSThe experimental results obtained from this investi

gation indicate the following:1. The interface of the stones with the concrete is theweakest point in the concrete-backed stone masonrymembers. The ultimate compressive stresses in the concrete and stones do not compare favorably with theircompressive strengths; this is caused by the early bondfailure that occurs in such a type of construction. Certain reinforcement details and construction requirements must be coded to improve the composite actionbetween stones and concrete.

2. In this study, the existence of the steel bars in thecores of the stone columns infilled with concrete didnot contribute to the ultimate strength of these members. This was due to the location of the steel in thecross section that was near the neutral axis. Therefore,it is recommended to have rectangular sections for suchcolumns with steel bars placed as far as possible fromACt Structural Journal I September-October 1989

Fig. 8 - Interaction diagrams for columnsthe neutral axis. And, it is obvious also that stone columns in filled with plain concrete only should not beused as structural supporting columns in practice.3. The modulus of rupture of concrete-backed stonemasonry beams increases as the strength of the stonesincreases. After 7 days, the modulus of rupture forbeams with hard stones was equal to 16 percent of theconcrete compressive strength. For beams with softstones, it was 8 percent of the concrete compressivestrength.4. Crack patterns in concrete-backed stone masonryprisms and beams differ according to the type ofstones. For hard stones, cracks followed the bed andvertical joint, but for the soft stones, cracks started atthe vertical joints and continued through the stones.5. The method of construction followed in practicefor concrete-backed stone masonry results in very low

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strength concrete. This is because a high wlc is usuallyused to allow the concrete to flow and fill all the voidsbehind the stones and because concrete cannot be vibrated as required since this may cause the stones to bepushed away from their place.Based on the results obtained from this investigation, it is recommended to use different types of prefabricated metal connectors to serve as bonding elements between stones and concrete. Research is neededto find the allowable loads on different concrete-backed

stone masonry members provided with these metalconnectors. Otherwise, it is recommended to use concrete bearing walls and plaster on it instead, afterhardening, with thin stones for decoration purposesonly.

ACKNOWLEDGMENTThe experimental work of this investigation was carried out in the

Structural Laboratory of The Royal Scientific Society, Amman, Jordan. The financial support from the Deanship of Research, JordanUniversity of Science & Technology (JUST), is gratefully acknowledged.

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REFERENCES1. Abdel-Karim, R. A., "Structural Evaluation of Concrete Backed

Stone Masonry," Master thesis, Yarmouk University, Jordan, 1984.2. Basoul, M. R., "Structural Evaluation of Stone Masonry Col

umns Infil led with Concrete," Master thesis, Yarmouk University,Jordan, 1987.3. Hatzinikolas, M.; Longworth, J.; and Warwaruk, J., "Failure

Modes for Eccentrically Loaded Concrete Block Masonry Walls,"ACI JOURNAL,Proceedings V. 77, No. 4, July-Aug. 1980, pp. 258263.4. Khalaf, F. M.; Glanville, J. I.; and El Shahawi, M., "A Study

of Flexure in Reinforced Masonry Beams," Concrete International:Design & Construction, V. 5, No. 6, July 1983, pp. 46-53.5. Boult, B. F., "Concrete Masonry Prism Testing," ACI JOURNAL,Proceedings V. 76, No. 4, Apr. 1979, pp. 513-535.6. Drysdale, Robert G., and Hamid, Ahmad A., "Behavior of

Concrete Block Masonary Under Axial Compression," ACI JOURNAL,Proceedings V. 76, No.6, June 1979, pp. 707-721.7. Fintel, Mark, and Annamalai, G., "Philosophy of Structural

Integrity of Multistory Load-Bearing Concrete Masonry Structures,"Concrete International: Design & Construction, V. I, No. 5, May1979, pp. 27-35.8. ACI Committee 531, "Proposed ACI Standard: Building Code

Requirements for Concrete Masonry Structures," ACI JOURNAL,Proceedings V. 75, No. 8, Aug. 1978, pp. 384-403.9. ACI Committee 531, "Commentary on Building Code Require

ments for Concrete Masonry Structures," ACI JOURNAL,Proceed-ings V. 75, No. 9, Sept. 1978, pp. 460-498.

ACI Structural Journal I September-October 1989

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