Repair and strengthening of masonry walls with openings using FRP laminates

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1 Repair and strengthening of masonry walls with openings using FRP laminates A Moussa*, Helwan University, gypt A M Aly, Helwan University, gypt 26th Conferene on OUR WORLD COCRT & STRUCTURS: August 21, Singapore Artile Online d: The online version of this artile an be found at: This artile is brought to you with the support of Singapore Conrete nstitute All Rights reserved for C Premier PT LTD You are not Allowed to re distribute or re sale the artile in any format without written approval of C Premier PT LTD Visit Our Website for more information

2 26th Conferene on Our World in Conrete & Strutures: August 21, Singapore Repair and strengthening of masonry walls with openings using FRP laminates A Moussa*, Helwan University, gypt A M Aly, Helwan University, gypt Abstrat n the presented study, Fiberglass Reinfored Plasti laminates (FRP) have been used for strengthening and repair of masonry shear walls with and without openings. The objetive of the presented researh is to investigate (he behavior of repaired and strengthened walls under diagonal splitting tension. The experimental program onsists of three phases. The First phase presents an investigation of the geometrial, physial and mehanial properties of the used materials. The seond phase aims to investigate the behavioral harateristis of small assemblages (plain and strengthened) under axial ompression, bed joint shear and diagonal splitting tension. The third phase aims to investigate the behavior of plain, repaired and strengthened walls with and without openings under diagonal splitting tension. The methodology of fabriation and testing of assemblages and walls have been presented. For the small assemblages, tests were performed to determine ompressive strength, joint shear strength and diagonal tensile strength. The behavior of masonry walls with and without openings was studied. The main parameters were the effets of repair, strengthening, type of strengthening and size of opening on the behavior of tested masonry walls. The test results learly demonstrate the effiieny of using FRP laminates as a repair and strengthening tehnique for unreinfored load-bearing masonry walls to inrease the tension and shear apaities and the deformability for resisting lateral loading. Keywords: Masonry, Shear, Splitting tension, Opening, Repair, Strengthening, FRP 1. ntrodution One of the serious problems faing the engineers today is vulnerability of old masonry buildings in many regions around the world, whih beome seismially ative. Beause of their important role as lateral loads resisting elements, masonry shear walls have attrated the attention of many researhers in the past few years. Signifiant amount of researh was arried out to study the behavior of masonry shear walls but unfortunately fous was on solid shear walls despite of the fat that shear walls with openings are the typial walls in real buildings. Although unreinfored masonry is onsidered one of the oldest types of onstrution little is known about its unique behavior. Many massive unreinfored masonry strutures were built using solid units. Unreinfored masonry is a ommon type of onstrution in developing ountries. These strutures are usually onstruted from brik or onrete bloks and in older buildings stone was used. A ement mortar mixture ties the units together. Load-bearing walls are the most vulnerable elements to damage during an earthquake beause they are designed primarily to arry vertial loads. However, in ase of earthquake event, they must also arry any in-plane and/or out-ofplane horizontal loads aording to their relative rigidities. Generally, damage an our in the form of raking, spaling or omplete ollapse. The need of introduing a new system of repair and strengthening for many of unreinfored masonry strutures beome urgent due to the observation of damage of this type of strutures after Otober 1992 earthquake in gypt. The presented study program aims to investigate, using Fiberglass Reinfored Plasti laminates (FRP), and the behavior of strengthened and 473

3 repaired URM shear walls with and without openings under diagonal splitting tension. Fiber omposite materials have been used in a variety of industries, suh as aerospae, automotive, shipbuilding, hemial proessing, et., for many years. Their appliation in ivil engineering, however, has been very limited. Their high strength-to-weight ratio and exellent resistane to eletrohemial orrosion make them attrative materials for strutural appliations The fibrous omposite is obtained when the polymer matrix (epoxy or polymer resin) impregnates the fibers (glass, arbon, graphite or aramied). The properties of the laminates depend on the amount and orientation of fiber. The permanent deformations of fibers under short-term loading are relatively negligible, but they exhibit a brittle tension failure mode. Glass fibers have relatively large reep under long-term stresses. The matrix has generally poor mehanial properties, as the behavior of polymer is dependent on time. The tensile strength of the omposite laminate is the most important when used in repair. Previous studies showed the effiieny of FRP laminates in strengthening masonry walls [1,2,3,4] 2. xperimental program Due to the high ost of full-sale testing of masonry strutures, potential problems of workmanship and apaity limitation of loading equipment a more eonomial method utilising the diret modelling tehnique is proposed as a replaement to full-sale. This tehnique has been used to model the masonry walls by 1x1 m speimens. This study program onsists of thfee phases. Phase onerned with the investigation of the geometrial, physial and mehanial properties of the omponent materials of masonry wall (perforated brik units, mortar, and FRP laminates). Phase aims to investigate the behavioral harateristis of small assemblages (plain and strengthened) under axial ompression, bed joint shear and diagonal splitting tension. These assemblages are investigated experimentally to make a hand alulation predition of the behavior of the model shear walls. Phase aims to investigate the behavior of URM walls with and without openings under diagonal splitting tension load as a first part. Three speimens 1 x1 m model shear walls were onstruted and tested for this purpose. The seond part of phase aims to investigate the effiieny of using the FRP laminates as a repair tehnique of damaged URM shear walls.the same three walls were retested after appliation of the FRP laminates whih were glued to both sides of the walls. The third part of phase aims to investigate the effiieny of using FRP laminates as strengthening tehnique of URM walls. Six speimens 1 x1 m shear walls were onstruted and tested after applying FRP laminates on both sides (three speimens strengthened with diagonal strips of FPR laminates and three speimens strengthened with FPR. laminates on whole surfae of the walls). The omponent materials an be lassified into three ategories: the brik units, the mortar and the strengthening material (FRP mat and type of resin). The strutural behavior of masonry is funtion of the properties of the omponent materials. Perforated silty sand brik unites were used. The unit dimensions were 12x25x6 m. ah unit has 1 3 m holes. The average ompressive strengh of unit brik was 122 kg/m 2, while the average splitting tensile strength was 24.1 kg/m 2. The average ompressive strength of used mortar was 234 kg/m 2, while the average splitting tensile strength was kg/m 2 3. Tests on assemblages The harateristis of the unreinfored masonry are determined by the interation of different omponents from whih the masonry is made. The basi tests of small masonry assemblages are important in studying the effet of variation in properties of omponent materials on the assemblage strength and in providing pratial methods for quality ontrol Axial ompression prisms Tests are arried out on six ompression prisms as shown in Fig.1. Three of them were plain masonry prisms and the other three were strengthened by FRP laminates. The average ompressive strength for plain prisms was 61.3 kg/m 2, while that for strengthened prisms was 69.5 kg/m 2. The failure mode was brittle due to vertial splitting raks for both ases. The test results indiate that the strengthened prisms ahieved an inrease in the strength ompared with the plain (unstrengthened) prism strength by about 13 %. The little effet of the FRP on the ompressive strength an be attributed to the debonding failure of the prisms before the full benefit of the FRP strength an be mobilised 3.2 Joint shear tests Different shapes of speimens assoiated with different test tehniques have been used by various researhers [5 & 6] to investigate the joint shear apaity of masonry assemblages. A detailed and omprehensive study' of advantage and disadvantages of the various shapes of shear speimens and testing tehniques an be found in Referene [5]. n this work, the main objetive of the shear test was to study the shear strength of brik masonry along the bed joints for the plain and the strengthened speimens. Figure 2 shows the employed test 474

4 ._""HlO=""'",, _, 11.- Plailepeeinen 11.- Plan speimen FAP Ll.minlllt'" $", FRP Lamnatlles ) '---="---} / b- strengthenedspemen Fig.1: Joint shear test speimens b- strengthened apeinen b- strengthened speimen Fig. 2: Prisms for axial ompression lesl Fig.S: Splitting diagonal tenlion speimens T FRP laminates / &H' -tit ffffff FfP FRP lanmates 13m..t t::tt if:>. K "tt-w SWPH SWst-2 SWst-3 / / g 'i ""'"l " s = ""'"l " 13m SWP2-1 SWS2-2 SWS2-3 T,, i--j P r::: ll!l / SWP3-1 SWS3-2 SWS3-3 Fig. 4: Details of tested walls 475

5 model. Mode of failure for the plain speimens was generally a sudden shear slip along the bed joint. The failure initiated by debonding at brik and mortar interfae as shown in Fig. 5. The mode of failure of strengthened speimens was started by debonding of the FRP laminates around the gap joint followed by bukling of the FRP at the area of the gap joint whih resulted in the joint bond failure. Test results learly show that strengthening of joint shear speimens by FRP laminates is very effetive. The average failure load for plain prisms was 1.4 tons, while that of strengthened prisms was 3.2. t is also found that the strain at maximum stress for the strengthened speimens are 3 times more than that of the plain speimens. These results learly demonstrate that the use of FRP laminates does not only inrease the joint shear strength but also inreases the deformation apaity and dutility of masonry assemblages. Beause this is one of the most ritial modes of failure of solid masonry walls, it is lear that the use of FRP laminates an be very effetive in inreasing the shear strength of plain (unreinfored ) shear walls. 3.3 n-plane splitting tests One of the important parameters, whih affet the behavior of masonry strutures, espeially for unreinfored masonry, is the tensile strength. Shear walls are subjeted normally to in-plane horizontal fores from wind or seismi fores whih produe tensile stresses. When the tensile stresses exeed the ultimate tensile strength of masonry, a diagonal tension rak starts and propagates. The main objetive of this phase of the test program is to determine the diagonal tensile apaity for plain as well as for strengthened speimens. A total number of 6 model plain and strengthened speimens were onstruted with a hexagonal shape in a running bond. All the speimens were tested under a diagonal load (45 with respet to the bed joint). The details of th test speimen are shown in Fig 3. Proedures and onstrution tehniques were similar to that used for the ompression and joint shear test speimens. All model speimens were air ured in the laboratory under ontrolled temperature and humidity. At the age of two weeks, the loading surfaes were apped using hydrostone gypsum ement. For the strengthened speimens, the FRP laminates were prepared and applied by the hand-lay-up. After at least 24 hours from applying the laminates the speimens were tested under splitting line loads using a universal mahine. The entroid of the speimen-bearing surfae was arefujly aligned vertially with the enter of thrust of the spherial seat of the testing mahine. The load was applied through roller bearing at the top and the bottom of the model panel. A omputerised system was used to measure the deformation from whih the splitting tension stress-strain urves are obtained. The speimens were inrementally loaded at a slow rate to permit reording of deformations. The modes of failure for most of the test speimens were splitting a long the loaded plane aused by the transverse tensile stresses. The shape of failure plane was different aording to the type of test speimens (plain or strengthened). For plain speimens the general failure plane was a frature rak extending through the briks and mortar joints starting at the points of load appliation and then followed the mortar-brik interfaes in a zigzag plane of failure as shown in Fig. 6. The mode of failure in this ase Garl be desribed as a mixed shear (slip at the brik-mortar interfae) and tension (splitting of brik). The mode of failure of the plain speimens was stepped wise plane through the mortar/unites interfae. The mode of failure of strengthened speimens was a plane rossing the briks and the mortar and in some ases diverged from the plane onneting the two applied loads. This inqiates the signifiant influene of FRP laminates on modifying the behavior and strength of solid unreinfored masonry. The spitting tensile strength and the shear deformation of the speimens inreased signifiantly by the FRP strengthening. The average failure load for plain prisms was 3.73 tons, while that of strengthened prisms was This means that the strength of plain speimens inreased by 83 %. Stress-strain urves shown in Fig. 8 indiate that the strengthened speimens exhibited a muh more dutile behavior than that of plain speimens. 4. Tests on masonry walls n this phase nine URM walls were onstruted and tested under inplane monotonially inreasing load. The sope of the proposed researh is foused on unreinfored brik masonry walls with and without openings and the effet of repair and strengthening on its behavior. All walls have the same overall dimensions (13x96.5x12 m) as shown in Fig. 4. Failure modes, rak patterns, load-defletion relationships, dutility and ultimate apaity of masonry walls are investigated. The main parameters studied in this investigation are: 1- ffet of presene and size of openings. 2- ffetiveness of repair and strengthening using FRP laminates. 3- ffetiveness of alternative types of strengthening (FRP strips or whole surfae). 476

6 4.1 Tested walls The tested walls are lassified into three groups: Group (1): without opening. Group (2): with a small opening whih has side length/ wall length of.25. Group (3): with a large opening whih has side length / wall length of.39. ah group ontains one plain wall and two strengthened walls. Strengthening is either by strips of FRP laminates or with overing the whole surfae as mat of FRP. The tested plain walls were repaired and retested. Table 1 Tested walls Group Wall Type Opening Opening Strengthening dimension side ratio type Repaired _._-,--- _._... _-- SWP1-1 Plain SWR1-1 1 SWS1-2 Strengthened Strips SWS1-3 Strengthened Mat SWP2-1 Plain 27x23.5 m SWR2-1 2 SWS2-2 Strengthened 27x23.5 m.25 StrjQs SWS2-3 Strengthened 27x23.5 m.25 Mat SWP3-1 Plain 39.5x38.5m SWR SWS3-2 Strengthened 39.5x38.5m.39 Strips SWS3-3 Strengthened 39.5x38.5m.39 Mat Fabriation of walls All walls were onstruted in running bond with a half brik overlap. The mortar used with a joint thikness 1 mm. All the joints in the masonry walls were tooled on both sides to be fl,jrther ompated. The exess mortar was arefully leaned using a brush before setting of the mortar. Walls were ured by spraying with water (two times per day) for 15 days. Mortar ontrol speimens were made during the onstrution of the walls and ured in the laboratory under the same onditions as the orresponding walls. Repair and strengthening of walls were done simply as follows. Two piees of FRP mat were ut with a horizontal dimension equal to the length of the wall and a vertial dimension smaller than the height of the wall to prevent diret touhing between load and FRP whih an ause early debonding of FRP laminates. The two piees were attahed to the wall one from eah side and lamped at the top. The polyester resin was then plaed in plasti bowl and mixed with obalt and peroxide as a atalyst using a ratio of 1:2: 1. The three omponents were mixed very well. The first oat of resin was applied on the wall surfae at top part over the level of the two lamps by using a small hand brush to insure uniform distribution of the resin over the whole surfae of the wall followed by the attahment of the FRP mat. The seond oat of the resin was then applied on the top of the mat within half an hour before the first oat started to set. A small brush was used to insure omplete wetting of the mat and to intermix the resin from both oatings. The proess was repeated for the lower part. Great are was paid to prevent trapping air gaps under the mat whih an affet on the interfae botld onditio.! between ihe laminate and the wall. The resulting thikness of the FRP laminate was approximately 1.75mm. This gives a ratio of FRP laminate ross-setional area to the area of the wall setion approximately equal to.3. Beause the walls were unreinfored they were very sensitive to any movement or any impat load during handing. Great are and ertain steps were followed for transportation of wall from the loation where it was built to testing frame Test setup and instrumentation The wall panels were tested as a speimen subjeted to in-plane vertial line load inlined 45 with the bed joint. The vertial load was applied through a roller made with 1. in. diameter steel rod and bearing steel plate at the middle of the wall. The vertial load was applied through a hydrauli jak of maximum apaity 25 tons supported at the top by the testing frame. This onnetion has limited rotation ability. The hydrauli jak was attahed to 'a alibrated load ell (37 ton apaity and 1 kg auray) to monitor the load during the test. The load ell was plaed in vertial position between the hydrauli jak and the top of the wall panel. The wall was guided and prevented from out-of-plane displaement by a system of non fritional supports (four vertial steel angle and piees of rubber. Mehanial dial gages had been used to monitor the wall response with loading. Two rows of demi point gages were arranged vertially and horizontally with a spaing of ten m to monitor the shear deformation of the wall during the test

7 a. Plain speimen b. Strengthened speimen Fig. 5: Mode of failure for joint shear test a. Plain speimen o. Strengthened speimen Fig. 6: Mode of failure for inp!ane splitting test Wall SWP1-1 Wall SWR3-1 Wall SWS2-3 Flg, 7: Failure modes for some ofthe tested walls 478

8 8 7 6 i 5 "' 1ii strain Fig. 8: ffet of strengthening on (j-g urve for axial ompression prisms o o strain Fig. 9: ffet of strengthening on (j-g urve for joint shear prisms C> 4 -f) 3 f)... -en Strain Fig. 1: ffet of strengthening on (j-g urve for splitting tension prisms 25 2 ""2 g 15 "C 'iii u Opening size Fig. 11: ffet of opening size on load apaity of walls 479

9 .2.15 Large opening Small opening. :1 " Without opening -.1. " -.15 :> -.2 arge opening Load (ton) Small opening Fig. 12: ffet of opening on wall deformations.2 " :r: s Plain. " -.3 Strip strength. :> -.4 Load (ton) Fig. 13: ffet of repair and strengthening on p-8 Curve foe walls with small openings "C OJ "C.8 :>.4 Plain -.1 Repaired tho Whole strength. Strip stren tho 6 8 Whole strength Load (ton) Fig. 14: ffet of repair and strengthel lillg 11 p-6 Curve for solid walls s.2.q 1;; -.4 J2 " " :> -O.S.8 Load (ton) Fig. 15: ffet of repair and strengthening on p-o Curve for walls with large openings 48

10 Table 2: Test results of masonry wall speimens Group Type of Model prisms Opening side Ratio Maximum Vertial Horizontal load Pmax Deformation at Deformation at (ton).5 Pmax (mm).5 Pmax (mm) 1 Plain SWP1-1 Strengthened SWS1-2 Strengthened SWS not reahed - - Repaired SWR Plain SWP2-1 Strengthened SWS2-2 Strengthened SWS Repaired SWR " Plain SWP3-1 Strengthened SWS3-2 Strengthened SWS Repaired SWR Analysis of test results 5.1 ffet of openings The effet of openings an be determined from omparing the results of walls SWP1-1, SWP2-1 and SWP 3-1. The load apaity of wall dereases as the opening size inreases as shown in Table 2 and Fig. 11. The load apaity of wall with small opening was 1% lower than that of plain wall without opening. Large opening wall had a load apaity 8% lower than plain wall without opening. The load deformation urves demonstrate that the stiffness of wall with openings is less than that of wall without opening. Mode of failure depends on the presene and size of opening. Wails without openings showed shear sliding failure mode, while walls with openings had splitting along the vertial axis and the right part, and sliding in the left part (along the mortar joint) as shown in Fig. 7. t should be noted that the unsymmetrial failure mode is dependent on the orientation of brik units. Figure 12 shows that horizontal and vertial deformation (relative to' plain solid wall) inrease when opening size inreases and rate of this inrease was high in the walls with large openings ffet of repair For walls without openings, the load apaity of repaired wall was about 95 % of that plain wall. At low load levels, stiffness of the repaired walls were less than those of piain walls due to previous damage. At higher loads, repaired walls keep its stiffness, espeially in tension zones; This leads that the stiffness of repaired walls exeeds that of plain walls and the deformation derea$ed. Failure started by suessive debonding of FRP laminates. For repaired walls with small and large openings, the exhibited strengths were.94 and 2.8 of that of plain wall respetivly. This means that repah- is more effetive in ase of large openings. Also, dutility and the energy absorption are improved by repair with FRP laminates. 5.3 ffet of strengthening The load arrying apaity of walls has been greatly inreased by strengthening using FRP laminates. This inrease was 15% for solid walls, 94% for walls with smaii opening and 28% for walls with large opening. The average load apaity of wall strengthened by FRP strips was 2% less than that of walls strengthened on the whole surfae. This means that strengthening with strips gives good results and may be adequate for most ases. n general in all groups, there was not great differene in the initial stiffness between the strengthened and plain walls as shown in Figs 12 to 15. This an be attributed to at early stages, the load is mainly transferred by ompression and the unraked briks still an sustain tension. Sine FRP is not a stiff material, it inreases the deformation and tension apaities without inreasing the rigidity of the wall. n addition, stiffness degradation of the strengthened wall had not been observed until failure. The strengthened wall exhibited higher stiffness ompared to the plain and the repaired walls. 481

11 6. Conlusions 1 n general, the test results show that modes of failure for unreinfored masonry shear walls are signifiantly affeted by the type of strengthening and the size of the opening. For both repaired walls and strengthened walls, it has been found that FRP laminates inrease the load and deformation apaities. 2 Results of tests on plain walls show that the behavior and the load arrying apaity of the masonry walls are highly sensitive to presene of opening. Large size of opening dramatially derease the strength and stiffness of the wall. This derease is nonlinearly proportioned to the inrease in size of opening. Failure of wall without opening started by splitting and loal rushing at loading zones followed by sliding over the whole length of the wall. For walls with opening failure started by splitting along the vertial axis from the opening orner towards the loading points. Splitting in right part and sliding on the left part (// briks) followed. 3 t has been found that FRP laminates is an effiient repair tehnique for damaged unreinfored masonry walls. For ases of small opening and solid walls, the load arrying apaity of the repaired walls ahieved about 95% of that of original plain walls. n ase of large opening, the load arrying apaity was muh higher than that of original plain walls (185%). This means that FRP repair tehnique is more effiient in ase of large openings. FRP laminates is very effetive in eliminating debonding of the mortar joint-brik interfae. Walls repaired with FRP laminates showed higher deformability than original walls. 4 Strengthening of brik walls with FRP laminates inreases the load arrying apaity without signifiant inrease in the wall stiffness. This leads to great improvement of deformability, dutility and seismi behavior of the strengthened walls. The strengthened walls reserved its stiffness without signifiant degradation up to failure espeially for walls with openings. The load arrying apaities of strengthened walls are about four times higher than that of plain walls for ases of large opening, and nearly twie for small opening and solid walls. 5 Walls strengthened with FRP strips ahieved about 8-9% of the failure load of the walls strengthened on whole surfae. This means that laminate strips are also effetive and an be used in retrofitting of walls espeially in ase of openings. 6 Results of tests on small masonry assemblages showed reasonable onurrene with that of walls. Strengthened speimens showed a muh more dutile behavior than plain speimens. Strengthening with FRP laminates is very effiient in inreasing the assemblage strength espeially the joint shear strength. The joint shear strength for the strengthened speimens were 3 times that of the plain speimens. The ompressive strength of strengthened prisms inreased by 15% than that of the plain prisms. Strengthening of diagonal tension speimens results in 8% inrease in the splitting tensile strength ompared with plain speimens. Referenes [1] Ghanem, G.M., Abo Zied, M., and Salama, A.., "Repair and Strengthening of Masonry Assemblages Using Fiber Glass", Pro. of 1 th B2 Ma, University Calgary, Calgary, Alberta, Canada July, [2] hsani-mr, Saadatmanesh-H, A.-Saidy-A, "Shear Behavior of URM Retrofitted with FRP Overlays ", Journal of CompOSites for Constrution Feb [3] Saadatmanesh,"Fiber Composites for ew and xisting Strutures" AC Strutural Journal May-June [4] HamidA-, Mahmoud,A.S., Abou l Magd, S., "Strengthening and Repair of Unreinfored Masonry Strutures: State-of-the Art", Pro. of 1th HB2 Ma, University of Calgary, Alberta, Canada, July, [5] Ghanem, G.M., "Behavioral Charateristis of Partially Reinfored Load-bearing Masonry Wall Strutures, PhD. Thesis Helwan University, [6] Hamid, A.A. and Abdoud, B.., "Diret Modelling of Conrete Blok Masonry under Shear and in-plane Tension ", Journal of Testing valuation, JTVA, Vol. 14. o.2 Marh [7) Mahmoud, A.S., " Strengthening of Load-bearing Unreinfored Masonry Strutures", Ph.D. TheSiS, Helwan University, Cairo gypt, [8] Amerian Soiety for Testing and Material ASTM C-15, "Standard Speifiation for Portland Cements" Annual Book of Standard, Vol. 4.2, Philadelphia, PA,