STRENGTHENING AND REPAIR. OF UNREINFORCED MASONRY STRUCTURES: STATE -OF-THE-ART

Ahmad A. Hamid 1 , Abdel Dayem S. Mahmoud2 and Sherif Abo El Magd3

1. ABSTRACT

Unsatisfactory seismic perfonnance of unreinforced masonry has been observed in past earthquakes around the world. This is attributed to its lirnited tensile and shear strengths. Unreinforced masonry walls are very stiff and brittle elements with low resistance to seismic action. This paper discusses different field methods that have been used in strengthening and repair of existing unreinforced masonry buildings. Experimental in­vestigations that were carried out to study seismic upgrading of unreinforced masonry shear walls are reviewed. The paper also presents an overview of the ongoing comprehensive test program at Drexel University which aims at investigating the use of Fiber-glass Reinforced Plastic (FRP) laminates for strengthening and repair of solid unreinforced concrete masonry shear walls.

Keywords: Behavior, Seismic Upgrading, Strengthening, Repair, Unreinforced Masonry

1 Professor and Chairman, Deparonent of Civil Engineering , United Arab Emirates University, AJ-Ain, UAE, on leave from Deparonent of Civil and Architectural Engineering, Drexel University , Philadelphia, PA, USA.

2 Research Associate, Deparonent of Civil and Architectural Engineering, Drexel University , Philadelphia, PA, USA. on leave from Helwan University , Cairo, Egypt.

3 Professor, Deparonent of Civil Engineering, Helwan University, Cairo, Egypt

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2. INTRODUcnON

Although unreinforced masonry is considered one of the oldest types of construction little is known about its unique behavior. Many massive unreinforced masonry structures were built using solid units.

One of the serious problems facing the engineers today is the vulnerability of older masonry buildings in many regions around the world which become seismically acúve. These structures are usually constructed from brick or concrete blocks and in older buildings stone was used . The units are tied together by a cement mortar rnixture. Loadbearing walls are the most vulnerable elements to damage during an earthquake because they are designed primarily to carry vertical loads . However, in case of earthquake event ,they must also carry any in-plane and I or out-of-plane horizontal loads according to their relaúve rigidiúes. Generally, damage can occur in the form of cracking, spalling or possible collapse of masonry walls~

3. REASONS OF DAMAGE OF UNREINFORCED MASONRY STRUCTURES

Seismic damage to unreinforced masonry (URM) may occur due to its inability to resist seismic acúons which is attributed to :

l-deficiency in overall structural layout and distribuúon of walls in the two orthogonal direcúons of the building 2 -unsatisfactory performance of nonstructural elements 3- unsatisfactory performance of structural elements .

This paper will concentrate on the behavior of the structural elements, mainly shear walls and how to increase their ability to resist seismic lateral load . Different techniques for retrofitting of masonry structures will be reviewed and a comparison between the different methods will be presented to show the advantages and the disadvantage of each method. The quanúfiable benefits from seisrnic retrofitting can be expressed in terms of the increase in strength and decrease in damage.

4. MODES OF FAll.URE OF URM W ALLS

Unreinforced masonry walls are characterized by their brittle failure modes due to lack of ductility and energy absorpúon capacity especially for seisrnic loads. Modes of failure of unreinfoced masonry walls subjected to vertical axialload in addiúon to horizontal racking force (Fig, l.a) can be summarized as :

4.1 Shear Sliding

Shear sliding along the bed joints (Fig. l.b) will occur when the shear stress resulting from the horizontal force exceeds the shear strength of the wall plus the vertical compressive stress mulúplied by the coefficient of friction between the mortar and the units.

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4.2 Diagonal Shear

This mode of faihrre will occur when the diagonal tension stress resulting from the compression -shear state of stress exceeds the splitting tension strength of the masonry. Failure plane normally occurs at 45 o in an zigzag shape thorough the head and the bed joints or crossing the mortar and the units l as shown in Fig. l.c

4.3 Rocking Mode of Failure

A rocking mode of failure as shown in Fig. l.d can occur when the lateral load is high compared to the vertical loads . The low tension bond between the masonry wall and the supporting foundation results in this mode of failure. For walls with high aspect ratio and low leveI ofaxial stress , the effect of lateral force will be significant due to the resulting overturning momento In this case tension crack will occur and extend towards the toe of wall causing decrease of the bearing area, concentration of stress and crushing of the toe when the compression stress exceeds the compressive strength of the masonry.

5. DIFFERENT METHODS OF STRENGTIlENING AND REPAIR

Several procedures have been found to be effective in the strengthening and repair of ma­sonry buildings . What is appropriate for one building is not necessarily appropriate for another. The methods selected must be consistent with aesthetics, building function, the original structure and its strength, ductility and stiffness. The usual methods for strengthening and repair can be summarized as :

1- Surface treatment methods (pneumatically placed concrete (gunite or shotcrete), ferrocement or reinforced plasters ) 2- Injection as repair technique 3- Jacketing or Poured in place wall and frarne elements 4-Extemal reinforcement (structural steel bracing, fiat steel bar straps or steel shear plate ) 5- Post tensioning 6 Fiber Reinforced plastic laminates.

In the next section a brief description of each method is presented. A comparison between the different methods, which points out to the advantages and the shortcomings of each, is also presented.

5.1 Surface Treatment

The surface treatment is considered one of the most common methods which has largely been developed through experience. Surface treatment is a general term which incorporates different techniques such as reinforced plaster, gunite or shotcrete and the ferrocement . All of these techniques are overlay procedures and they have been found to be very effective in regaining at least the original in-plane shear of damaged units .When used with undamaged masonry walls the shear strength is usually doubled. Prawel et al.2

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demonstrated lhat lhe out -of-plane resistance is substantially improved due to lhe composite ductility of lhe original wall and lhe surface coating .

Schneider and Dickey 3, Sheppard and Tercelj 4 and Jabarov et ai 5 conducted experimental studies on lhe in-plane strenglh of shear walls using mesh-reinforced mortar coating . They demonstrated lhat lhe strenglh of lhe coated specimens is twice lhe strenglh of lhe unreinforced walls but lhe load corresponding to lhe frrst cracking was lhe same for bolh coated or uncoated specimens. The major advantage of lhe reinforced plaster coating is lhe ease of lhe construction effort because it requires very little surface preparation and very little forrning and skilled labor.

Thin layers of ferrocement overlay have been successfully used wilh unreinforced masonry. Ferrocement, as it usually used , is an orthotropic composite material having a high strenglh cement matrix and reinforced wilh layers of fine steel wires in the form of a mesh. Reinhorn et alo 6 tested the efficiency of ferrocement coated masonry walls subjected to in-plane shear forces . The diagonal split or "blume" test was chosen for the experimental determination of in-plane shear strenglh and deformation of the ferrocement plates. The results indicated that the coated specimens developed a maximum strength, ductility and secant stiffness approximately double lhose of the bare specimens. Prawel7

and Lee 8 developed an analytical hysteric modeling to compare plain masonry walls with those coated with the ferrocement layer subjected to either in-plane or out -of-plane cyclic loads. The results indicated that lhe shear strength increased about 1.5-2 times, also the moment capacity increased about lhree times by lhe ferrocement reinforcement

Shotcrete, also called sprayed mortar, gunite or jetcrete applied concrete has been used for repair as well as for new construction for many years. Shotcrete may be applied by either a dry mix or wet mixo In the dry mix process, cement and sand are mixed dry and fed with compressed air ,a separate line delivers water and admixtures (if used ) to lhe nozzle ,which provides fmal mixing . In the wet process , ali mortar or sand are mixed prior to introduction into the delivery . A comparison between the two processes is presented in References 9 and 10.

Kahn 11&12 concluded that the application of a layer of 3 1/2-inch reinforced shotcrete to unreinforced brick masonry panels is an effective method which greatly increased the in-plane diagonal strength. lt was also demonstrated that full composite action was developed regardless of brick surface treatrnent and there is no need to add epoxy before applying of the shotcrete.

A comparison of lhe behavior of various surface coating procedures based on diagonal tension tests was presented by Hutchison et al. l3. The procedures used to strengthen brick panels included prestressed walls, sprayed concrete, glass reinforced concrete coated walls, fiber reinforced concrete coated walls and ferrocement coated walls. It was concluded that the last lhree melhods are lhe most promising solutions for in-plane strenglhening of brickwork and the fiber reinforced concrete method is the best of a1l methods from economical point of view .

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5.2 Injection as a Repair Technique

Injection is the common method for repair. For damage in the fonn of relatively small cracks (0.13 nun (0.005 in.) wide). injection oflow viscosity epoxy wlúch has lúgh bond and com8ressive strength and good penetration has been found to be effective by Wamer1 .. The success of the operation of injection depends on the pro per method for perfonning the injection. Pleecnik et al 14-16 found that for cracks more than 6.4 rrun (0.25 in.) wide an epoxy mixture consisting of epoxy adhesive as a binder and various fillers such as sand or cement is most economical . Also. he found that the shear strength for the test specimens using polyester /sand was the same as the specimens with epoxy / sand. He recommended using polyester as a binding material because it is cheaper than epoxy. Sheppard and Tercelj '4 found that after repair by cement -grouting. walls built of blocks retained or slightly increased their original shear strength while in case of stone masonry walls grouting increased their strength by a factor of 2-3 .

The unique advantages of epoxy repair technique include minimum loss of utilization of the damaged structure and little or no changes in aesthetic and architectural features . The disadvantages of epoxy repair include inadequate penetration and improper curing of ep­oxy especially lúgh viscosity epoxy and presence of cavities and sensitivity of epoxy adhesives to temperatures. Plecnik et al 15 found that the shear. bond and tensile strength of epoxy repaired specimens beyond 200 °e ( 400 °F ) is nearly zero. Therefore. the nature of thennal gradients detennines the strength characteristics of epoxy repaired shear walls during fire exposure .

5.3 Jacketing

Attempts were made to tie masonry walls topether using reinforced concrete elements built witlún and adjoining the existing structure 1. In China brick walls were strengthened by casting reinforced concrete columns which are finnly connected to the walls 11 .

5.4 External Reinforcement

Strengthening masonry walls by mechanically attaching the exterior of existing masonry walls with a structural steel systern has been found greatly to improve the lateral load resistance and ductility of these walls. The relative rigidities of the original masonry structure and the new steel bracing is an important factor that should be taken into consideration .. In an earthquake. cracking in the original masonry structure is expected and after sufficient cracking has occurred • the new steel system will have comparable stiffness and be effective as outlined in Wyllie 17.

5.5 Post Tensioning

The basic concept of the post tensioning method is to improve the strength and ductility of the vertical members of the lateral load resisting frame of a structure by introducing prestressed reinforcement along the vertical member .The prestressed reinforcement is de-

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signed to take all the tension forces associated with the expected lateral loads. Kahn 11 indicated that stone masonry structures can be strengthened with vertical tendons , horizontal tendons and with both as shown in Fig 2. The lateral strength of strengthened walls with vertical steel distributed along the length of the wall and at the corners plus horizontal bars near the roof line and the base was found to resist twice the strength of the unstrengthened walls. Ganz17 gave some pracúcal examples for using the prestressing technique in Los Gatos Brick Castle in Gennany , the General Post Office in Sydney and Holy Cross Church, Santa Cruz, California Ganz conc1uded that strengthening of struc­tures with post-tensioning tendons is most effecúve since conúnuous reinforcement from anchorage to anchorage provides improved strength. In addition providing prestress improves cracking behavior of the structure.

5.6 Fiber Reinforced Plasúc Laminates

The application of advanced composite materiais began in the 1940s and was only used in the aircraft and space industry. The most common advantages, as menúoned in References 19 and 20 are light weight, high stiffness, excellent faúgue properties and high resistance against corrosion. The main disadvantage of the fiber reinforced plasúc laminates is their cost is high compared to the stee1, but a variety of relaúvely inexpensive fibers made Df glass are now available . Fiber composite materials are made of small fibers of glass, carbon or kevlar bonded together with a resin polymer matrix. Meier et al 19 applied car­bon fiber reinforced epoxy resin plates for the post-strengthening of flexural reinforced concrete beams demonstrated improved distribuúon and control of crack width for the strengthened beams.

Saadatmanesh et al 21 attached fiber glass laminates bonded with epoxy for strengthen­ing of reinforced concrete beams at the tension face to increase the their strength. The results showed that great potential exists in the use of high strength fiber composites .

Hamid et al 22 used fiber glass reinforced laminates to strengthen small scale models Df hollow concrete prisms to study the effect of the strengthening on strength under in-plane loading. The results showed that in case of compression test and splitting tension tests the ulúmate load of the strengthened specimens was double that of the unstrengthened specimens.

6. COMPARISON BETWEEN lHE DIFFERENT METHODS OF STRENGlHENING

Adams 23 made a good comparison between different methods of strengthening in an attempt to choose the best method. The study inc1uded four methods of strengthening: pneumaúcally placed concrete (shotcrete), poured in-place concrete (jackeúng), externai reinforcement by steel bracing or steel shear plate and the surface treatment. The basis for comparison inc1udes stiffness compaúbility between the existing structure material and the material of the strengthening technique, the increase in building weight after strengthening, the good connection to the existing building, the change in the building aesthetics, the construcúon cost, and the construction period . The results indicated that from the above

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methods, the pneumatically placed concrete (shotcrete) is the preferred strengthening scheme to be used on buildings when dust from the required sand blasting is not a problem otherwise the reiIÚorced poured in-place concrete is considered to be the preferred method .

7. CURRENT TEST PROGRAM AT DREXEL UNIVERSITY

The aim of the ongoing comprehensive test program at Drexel University is to investigate the use of Fiber-Glass Reinforced Plastic (FRP) composite laminates for strengthening and repair of small scale solid unreinforced concrete masonry shear walls . The test is divided into three phases . Phase one which was completed in 1993, aimed at studying the effect of strengthening in improving the strength of small scale masonry prisms. A total of 30 prisms were constructed from two types of blocks fb 1 = 28 MPa ( 4000 psi ) and fb2 = 10.5 MPa ( 1500 psi ). There were 12 samples for compression test , 12 for in-plane diagonal tension test and 6 for shear specimen prisms. Half of these specimens were tested plain as control specimens and the second half were tested after 24 hours from applying the FRP laminates. The applying process is easy and straight forward by hand lay-up as shown in Fig. 3. Instrumentation to measure the deformation was installed on both sides of the specimens and connected to a data acquisition system . A sample of the setup and the instrumentation for the test specimens is shown in Fig. 4. The results showed an increase of the strength and deformations especially for the splitting tension and shear specimens. This indicated that the FRP is effective for strengthening of the unreinforced masonry shear walls ..

The second phase which is currently in progress, involves testing of small 1/3 scale four shear walls subjected to lateral horizontal monotonic and cyclic static load as well as vertical compressive axial load . The plain walls are [rrst tested to failure to obtain the ultimate capacity and mode of failure and then retested after applying the FRP to study the efficiency of the FRP as a repair and strengthening technique. The third phase will cover the strengthening of shear walls. Walls will be constructed and tested after applying the FRP on both sides of the walls. The test setup is shown Fig. 5 and the complete matrix of the test shear walls for phase two and phase three is shown in Table 1 .

8. CONCLUSION

Based on the above discussion it is concluded that the fiber reinforced plastic laminates is most promising method. In addition to the advantages of high strength / weight ratio and resistance to corrosion. FRP laminate is an economical method because of the ease of application by unskilled labor .These advantages can overcome it's high cost because of the fact that for strengthening and repair techniques the material cost represents only 20-25% of the total cost .

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Table I Test Matrix for Drexel Test Program Wall Block Masonry Aspect Axial Type of Remarks

Specimen Strength Strength Ratio Stress Lateral MPa {Qsi) MPa {Qsi) MPa {Qsi 2 Load

SWA1 28 (4000) 20 (2800) 0.5 1.4 (200) M These walls SWA2 28 (4000) 20 (2800) 1 0.7 (100) M will be SWA3 10.5 (1500) 9.5(1350 ) 1 0.7 (100) M repaired SWA4 28 (4000) 20 (2800) 1 0.7 (100) C after their SWA5 28 (4000) 20 (2800) 0.5 1.4 (200) C failure

SWB1 28 (4000) 20 (2800) 0.5 1.4 (200) M These walls SWB2 28 (4000) 20 (2800) 1 0.7 (100 ) M will be SWB3 10.5 (1500) 9.5(1350 ) 1 0.7 (100) M strengthened SWB4 28 (4000) 20 (2800) 1 0.7 (100 ) C before testing SWB5 28 (4000) 20 (2800) 0.5 1.4 (200) C

M = monotonic load, C = cyclic load

9. REFERENCES:

1. Drysdale, R. , Hamid, A. and Baker, L., "Masonry Structures: Behavior and Design", Prentice-Hall Inc., Inglewood Cliffs, New Jersey , 1993.

2. Prawel, S. P. , Reinhorn, A. M. , and Kunath, S. K. , " Seismic Strengthening of Structural Masonry Walls with Externai Coatings" , Proceedings 3rd U. S. National Conference on Earthquake Engineering , Charleston, South Carolina ,1986.

3. Schneider, R. R. and Dickey, W. L. , " Reinforced Masonry Design", Prentice -Hall Inc. , Inglewood Cliffs, New Jersey , 1980 ..

4. Sheppard, P. and Tercelj, S. " The Effect of Repair and Strengthening Method for Masonry Walls" , Proceedings 7th WCEE, Vol. 6, PP. 255, Istanbul, 1980.

5. Jabarov ,M. ,Kozharimov , S.V. and Lymyov , A. A. ," Strengthening of Damaged Masonry of Reinforced Mortar Layers ", Proceedings 7th WCEE . Vol. pp. 73, Is­tanbul , 1980.

6. Reinhorn, A. M. , Prawel, S. P. , and Jia, Z. R. , " Experimentai study on External Ferrocement Coating for Masonry Walls " , Journal of Ferrocement, Vol. 15, No. 3, 1985.

7. Prawel, S. P. , Reinhorn, A. M. ,and Quzi, S. A. ," Upgrading The Seismic Resistance of Unreinforced Brick Masonry Using Ferrocement Coatings", Proceedings 8th lnternational BrickIBlock Masonry Conference , Sept. 1989.

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8. Prawel, S. P. , Lee, H. H. , " Ferrocement as a Surface Coating for The Seismic Up­grading of old Unreinforced Brick Masonry Walls Hysteric Modeling " ,Proceeding of The 9th Intemational Brick I Block Conference, Berlin, Gerrnany, 91

9. ACI Committee 311 , " ACI Manual of concrete Inspection ", Publication SP-2 of Arnerican Concrete Institute ,Detroit, Michigan , 1980.

10. Wamer, J. ," Methods of Repairing and Retrofitting ( Strengthening) Existing build­ing", Workshop on Earthquake-Resistant Reinforced Concrete Building construction (ERCBC), University of Califomia, Berkeley, July 1977.

11. Kahn, L. F. ,"Repair and Strengthening of Masonry ", Proceedings 2nd Seminar on Repair and Retrofit of Structures, pp. 247, Ann Arbor, May 1981.

12. Kahn, L. F. ," Shotcrete Retrofit for Unreinforced Brick Masonry ", Proceedings 8th WCEE , San Francisco, Vol. 1, 1984.

13. Hutchison, D. L, Yong, P. M. F. and Mckenzie, G . H. F. , " Laboratory Testing of a Variety of Strengthening Solutions for Brick wall Panels" , Proceedings WCEE, San Francisco, Vol. 1

14. Plecnik, J. M., Arnrhein , 1. E. , Jay , W . H. and Waraner , 1. ," Epoxy Repair of Structures ", Proceedings of The Intemational Symposiurn on Earthquake Structural Engineering, Missouri , 1976, pp. 1023-1036.

15. Plecnik, J. M., Arnrhein , 1. E., Waraner, J. , Jay, W. H. and Chelapatic , C. V. ,"Repair of Earthquake Darnage Concrete Masonry Systems Subjected to Static and Dynarnic loads and Elevated Temperatures ", Proceedings 6th WCEE , New Dellú, 1977.

16. Plecnik, J. M., Corsins , T . and O' Conner , E. , " Strengthening of Unreinforced Masonry ", Joumal of structural Engineering ASCE , Vol. 112, No. 5 , May, 1986 pp. 1070-1087.

17. Wyllie, L. A. , "Strengthening Existing Concrete and masonry Buildings for Seismic Resistance ", Proceedings 2nd Seminar on Repair and Retrofit of Structures, pp. 322 ,Ann Arbor, May 1991.

18. Ganz, R. H. , " Strengthening of Masonry Structures with Post Tensioning ", Pro­ceedings of The Sixth North American Masonry Conference (6NAMC) , Drexel University ,Philadelphia ,Pa. ,June 1993.

19. Meier, U. and Kaiser , H. ," Strengthening of Structures with CFRP Larninates ", Advanced Composites Materials in Civil Engineering Structures , ASCE Proceedings of the Specially Conference , 1991, pp. 205-211.

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20. Schneider, S. , "New Structural Applicaúons of Fiber -Reinforced cussed", Swiss Materials, Feb. 1989, pp. 63-65.

plasúcs Dis-

21. Saadattnanesh, H. ,Ehsani, M. , " Applicaúon of Fiber-Composites in Civil Engineer­ing ", ASCE Conference, pp. 526-535

22. Hamid, A. A. , Larralde ,1, and Salma, A. , " Properúes of Hollow Concrete Ma­sonry Reinforced with Fiberg1ass Composite " ,Proceedings, ACI internaúonal Sym­posium on FRP Reinforcement for Concrete Structures.

23. Adams, S. , "Alternate Seismic Strengthening schemes for Masonry structures ", Proceedings of The Unreinforced Hollow Clay Tile W orkshop, San Francisco, CA., Sept. 92.

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a) URM Wall Subjected to Lateral Load and Vertical Axialload.

ç) D:tigonal Sh= Failurc

I I I I I i I I I I j I I I I I I b} Sliding Shcar Failurc

t ' I I ~ 1,1,1/,1 r I I ,

d) Rocl:ing Failurc

Fig. 1 Different Modes of Failure for URM walls subjected to

Lateral Load and Vertical Load.

Fig. 2 Externai Reinforcement and/or Prestressing Tendons Used to

Connect Transverse Wall and Walls to Floors (fram Ref. 11).

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Fig. 3 Hand-Iay-up Application of FRP to assemblages.

Fig. 4 Test Setup and Instrumentation for Diagonal

Splitting Tension Specimens.

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I. I • I ..... n-I. I. ~.,"'.'

l-Tcsuug Framc.. 2-Jad:far Anal Load. 3-Load Ccll. 4-Rollcr Allowiug Honzouw Dlsplaccmcnl af lhe Wall. 5-Load DlstribuIiDg Be:un. 6-RC:IUfura:O ConC'Ctc Top Bc:un. 7-Bra.cm for LuenJ Load. 8-L1tcraI Suppon Cor Lc:vc:r Arm. 9-l.cvr:r Arm (Maguctlc3liDIl af Urc:r:a.l Load), 10-<:outrOUed L VOT. ll-Jad: for l-uc:2J Load.. ll-Load Ccll. 13-Sc:rvo-Vaivc:. J4-Shc:u' Wall Spccimcn. lS-Rcillfora::d CODC'ClC Sas, B c::un. 16-5 tc:cJ B c::un

Fig. 5 Test Setup for Shear Walls.

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