COMPARISON OF IN-SITU AND LABORATORY TESTS OF BRICK MASONRY WALLS STRENGTHENED WITH CARBON FIBRE REINFORCED POLYMER FABRIC

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1 COMPARISON OF IN-SITU AND LABORATORY TESTS OF BRICK MASONRY WALLS STRENGTHENED WITH CARBON FIBRE REINFORCED POLYMER FABRIC Samo GOSTIČ Title University or Affiliation Address address* Mojca JARC SIMONIČ Title University or Affiliation Address address Vlatko BOSILJKOV Title University or Affiliation Address address Roko ŽARNIĆ Title University or Affiliation Address address PhD Building and Civil Engineering Institute ZRMK Dimičeva 12, Ljubljana, Slovenia B.Sc. Building and Civil Engineering Institute ZRMK Dimičeva 12, Ljubljana, Slovenia Assist. Prof. University of Ljubljana, Faculty of Civil and Geodetic Engineering Jamova 2, Ljubljana, Slovenia Prof. University of Ljubljana, Faculty of Civil and Geodetic Engineering Jamova 2, Ljubljana, Slovenia Abstract Shear failure of masonry walls is a critical failure mechanism that in many cases leads to the sudden collapse of entire buildings during earthquakes. To improve seismic resistance of brick masonry walls the reinforcement of masonry with carbon fibre reinforced polymer (CFRP) fabric is presented as innovative and efficient strengthening technique. In order to develop the most efficient way of applying CFRP fabric to brick masonry walls, in-situ and laboratory tests were carried out and their results are compared. Laboratory tests were carried out on sixteen walls with different reinforcement configuration (diagonally strengthened, horizontally and vertically strengthened, only horizontally strengthened and un-strengthened) and six walls of different configurations were tested in-situ. Specimens were tested under constant vertical load and by displacement controlled horizontal cyclic loading. Laboratory test specimens were tested as single fixed, while in-situ specimens were tested as doublefixed. CFRP strengthening favourably influenced the mechanism of wall behaviour in all cases. It significantly increased ultimate displacement, ductility and dissipated energy. The un-reinforced masonry typically failed in diagonal shear. Diagonally reinforced masonry in laboratory failed in compression due to rocking, while horizontally reinforced specimens resisted high shear and compressive stresses and failed due to masonry compressive failure within the FRP confinement. Failure mechanism of in-situ tested specimens showed compressive failure within the FRP confinement and shear cracks. In general the behaviour of horizontally reinforced masonry exhibited much higher ductility and energy dissipation in comparison to diagonally strengthened walls. Keywords: ductility, FRP, masonry, shear test, stiffness, strength, strengthening Page 1 of 8

2 1. Introduction Major part of existing stocks of public and residential buildings that were built in the last millennia represents brickwork masonry. Unfortunately unreinforced brick masonry (URM) exhibited low seismic resilience during past earthquakes. Shear failure of masonry walls is a common problem that in many cases leads to the sudden collapse of entire buildings during earthquakes. Several conventional strengthening methods to overcome these problems were developed in the past. One of them is also the new method with application of carbon fibre reinforced polymer (CFRP) fabric reinforcement. By attaching FRP stripes of fabric to the masonry the load bearing capacity and ductility of walls is greatly enhanced. At the same time the wall stiffness which could lead to unfavourable redistribution of load is not increased. One of the first studies of effect of strengthening masonry wall by fibres was done by Croci et.al. [1]. Triantafillou's research [2] includes wide spectra of composite materials for strengthening. The experimental work focused on masonry led to proposal of design equations for masonry strengthened with FRP. Valluzzi [3] performed series of diagonal tests on differently reinforced walls. Double sided, diagonal strengthening with GFRP showed to be the most efficient method. 2. Experimental program As it is not possible to reliably determine the wall characteristics needed for the efficient nonlinear seismic analysis (shear strength, displacement capacity...) only based on brick and mortar characteristics it is necessary to perform the shear and compression test on the masonry specimens. The critical elements of masonry to fail in shear during an earthquake are parts of walls between openings, so for our testing campaign dimensions of specimens are chosen to represent such elements. On the other hand the dimensions of the specimens have also to satisfy the restraints of test equipment capacity. The most realistic parameters regarding seismic resistance can be obtained by testing actual building walls in-situ with original materials, geometry and also with usual geometry imperfections. Because such in-situ tests are destructive by their nature and also complicated to perform, they are often replaced with tests on specimens built and tested in laboratory conditions. 2.1 Specimens In-situ tests In-situ tests were performed on the typical old masonry building from early 30 s. Load bearing masonry walls were made with solid clay bricks with dimensions mm and weak lime mortar (L:S=1:5 and D max = 8 mm). For each thickness of walls we prepared specimens by cutting them on 2.0 m high and 1.0 m wide pieces. As horizontal load was applied in the middle of such wall we were actually testing two walls with symmetrically fixed ends (ratio h/l=1.0). Walls were of two thicknesses: 30 cm and 45 cm and for each thickness we prepared one unreinforced specimen, one strengthened with diagonal stripes and one with horizontal and vertical stripes. As load set-up allow only one direction of applying horizontal force the diagonal stripes were glued only on the tensile diagonal of the wall (Figure 1). Surface of the wall in area designated for gluing was prepared with removing plaster and grinding the loose parts. Unevenness of the surface was corrected with epoxy based mortar in thickness up to 5 mm. After that the wet lay-up technique was used to apply CFRP to the wall. Page 2 of 8

3 Figure 1. Configuration of CFRP reinforcement and test set-up for in-situ tests (of unreinforced wall; from left: strengthened diagonally, horizontally and unreinforced) Laboratory tests Sixteen walls (height/width/thickness=126/106/12 cm) have been made on reinforced concrete base footing. They were made of contemporary solid clay brick ( mm). Mortar used to build wall specimens was a mixture of cement, lime and sand (D max = 4 mm) in a volume ratio of 1:2:6. All specimens were cured for at least 1 year before test. (A) Diagonally (B) Horizontally and vertically (C) Horizontally Figure 2. Configuration of CFRP reinforcement for laboratory tests Four walls were left unreinforced. Others were strengthened with unidirectional carbon fibres in different configurations; 6 diagonally, 3 horizontally and 3 horizontally with wide strips on the sides. Strips were applied on front and back side. Wet lay-up technique was used to apply CFRP to the wall. The panels have been cleaned with abrasion before epoxy primer was applied. Epoxy adhesive, combined with filler was applied to bond the CFRP on the surface of the wall. Finally the top coat of epoxy adhesive was applied to ensure saturation of the fibres. 2.2 Test Setup Due to the capacity of testing equipment there were some differences in test setup and execution of the in-situ and laboratory tests. For in-situ tests the horizontal load of hydraulic jack (1000 kn) was applied at the middle of wall height (Figure 1) separating wall into upper and bottom specimen of the wall. The specimens were thus tested as elements with symmetrically fixed ends into the surrounding masonry (Figure 4). Walls were additionally loaded with vertical force to reach stress level at 30% of compressive masonry strength. In the laboratory the walls were tested as the shear cantilevers in the test frame as shown below (Figure 3). Free end was at the bottom of the test frame, where vertical and also Page 3 of 8

4 horizontal load is transmitted into the panel. A combination of vertical compression (400 kn which was about 25 % of compressive strength) and in-plane shear load was applied to specimen. Figure 3. Masonry wall during testing (wall is inserted up-side-down) Figure 4. In-situ configuration Principle of scales held vertical load constant during the experiment. The shear load was applied to the wall by a horizontal hydraulic jack, which was displacement controlled by computer software. 2.3 Loading sequence The horizontal load during both types of experiments was displacement controlled with some differences. Horizontal loading during laboratory testing increased in steps 0.5 mm, 1.0, 2.0, 4.0 mm, etc with each loading step repeated cyclic three times with the same amplitude and velocity (Figure 5). Shear loading was provided by two way acting servo-hydraulic actuator of 250 kn capacity. Actuator was fixed to the supporting frame, which transferred the shear load to the RC plate of the laboratory. displacement [mm] step Figure 5. Loading protocol in lab tests Figure 6. Loading protocol for in-situ tests Loading during in-situ tests was progressing with one repetition to the step (to 0.5 mm, 1.0, 1.5, 2.0, 3.0 mm etc) and release near zero (Figure 6). Loading was stopped when lateral force in the current step could not reach 80% of maximum force achieved in the test. 2.4 Instrumentation In both cases we measured horizontal displacements and deformations with linear variable differential transducers (LVDTs). Masonry deformations were also measured using LVDTs attached diagonally and vertically along the height of the wall. Load cells were used to measure vertical (pre-stress) and horizontal load. 2.5 Materials Basic materials have been tested to determine compressive strength of brick, mortar, tensile strength of FRP fabric as well as some other characteristics listed in the table below. The differences between characteristics of in-situ and laboratory masonry were dramatically apart. Contemporary materials and careful mason work (especially use of better mortar) when building laboratory specimens ended in 15-times higher compressive strength and 9-times higher elastic modulus than in case of old masonry tested in-situ. displacement [mm] step Page 4 of 8

5 Table 1. Mechanical characteristics of materials (laboratory tests). MATERIAL PROPERTY LAB IN-SITU METHOD Brick Compressive strength 32 MPa 20.1 MPa EN Tensile strength 5.34 MPa n/a Mortar Compressive strength 6.77 MPa ~0.5 MPa EN Flexural strength 2.08 MPa n/a CFRP Tensile strength 3400 MPa 3800 MPa ASTM D 3039/D 3039M Young's modulus 230 GPa 240 GPa Masonry Compressive strength MPa 0.83 MPa EN Young's modulus 5.74 GPa 0.64 GPa ν G c 2.29 GPa 0.21 GPa For strengthening laboratory specimens the 10 mm and 50 mm stripes were used together with appropriate epoxy resin. In case of the in-situ tests the stripes were 100 mm wide. 3. Results of masonry shear tests 3.1 Failure mechanism Most of the tested masonry walls failed by propagation of diagonal cracks. In the laboratory the failure mode started as a combination of shear and flexural mode. First cracks occurred in corners of the panel due to the rocking of the wall. Behaviour of strengthened panels was strongly related to configuration of CFRP reinforcement. The predominant mode of failure in type (B) and (C) strengthening was flexural mode, which resulted in local failure of the wall toe. Shear cracks started to develop at approx. 70 % of maximum displacement. Crushing of brick inside FRP confinement Detachment of diagonal or due to compression Figure 7. Failure modes of FRP reinforced masonry (left two in lab, right two in-situ) Cracks propagation was efficiently obstructed by the CFRP reinforcement, which resulted in appearance of many new minor cracks. Diagonally strengthened walls showed flexural and diagonal crack development. Toe crushing was the main cause of failure for this type of strengthening. For in-situ tests first diagonal cracks occurred at 70-80% of max load (or about 2-3 mm of horizontal displacement). After that cracks propagated and became wider until the end of the test. All walls failed in shear and the tests were stopped after compression failure of masonry Page 5 of 8

6 within the FRP confinement. FRP stripes in diagonal configuration first detached on uneven parts of the surface then progressed to the anchored end. The system of vertical and horizontal stripes was effective until the end of loading thou there were local detachments from the surface and even local rupture of stripes. The detachment of FRP stripes occurred mostly in the brick and not in glue or their bond. 3.2 Shear strength of masonry Typical response of combined compression-shear tests performed on section of wall is presented in the form of rotation vs. horizontal load on the diagrams bellow (Figure 8 and Figure 9). Results are shown for horizontally and vertically reinforced walls. Envelopes of such results had been collected for positive and negative direction of forced displacement in the case of laboratory tests or for lower and upper part of wall in the case of in-situ tests. An example of such envelopes is represented below (Figure 10) where crack patterns at different stages of loading are also shown. 150 Z03 H30 70 horizontal load [kn] rotation [mm/m] Figure 8. Results for lab tests horizontal load [kn] rotation [mm/m] Figure 9. Results for in-situ tests Figure 10. Envelope of rotation-force relationship for H system on 30 cm walls with corresponding development of cracks pattern. Page 6 of 8

7 Shape of diagrams from the laboratory testing (Figure 8) are mostly indicating rocking behaviour at first half of test followed with hysteresis opening and failure on one side of loading direction. The direction of first loading cycle was not affecting later damage development or failure mode. In-situ walls indicated constant development of cracks with highly non-elastic deformations (Figure 9). The crack patterns and damage propagation is similar for in-situ and laboratory tests. We compared cracks patterns with typical points on the envelope curve (sample is shown for test of H30 on Figure 10). To compare results obtained on walls of different dimensions (laboratory and in-situ) we calculated stress as horizontal load divided by horizontal cross section area. Below are results shown for all laboratory specimens (Figure 11, two envelopes: positive and negative for each specimen) and in-situ tests ( Figure 12, two envelopes for each specimen: upper and lower part of wall). Figure 11. Results of laboratory shear tests Figure 12. Results of in-situ shear tests on masonry Specimens prepared and tested in the laboratory yielded much higher load bearing capacity (from 0,7 to 1,1 MPa) compared to old masonry on the site (from 0,1 to 0,2 MPa). That was due to better materials used and careful mason work. From results we can also conclude that reinforcement with the CFRP glued in diagonal direction of walls is not effective solution. The best results were gained with CFRP fabric glued in vertical and horizontal direction (green curves: z08, z13, z15, H30 and H45 on diagrams above). For laboratory or in-situ tests the effect of horizontal stripes confining the wall was decisive for higher load bearing capacity and especially for higher ultimate rotation capacity. For study of FRP effectiveness the average values of three walls for each strengthening configuration (H lab, H+V lab, D lab) was compared to average values of 3 unreinforced walls in case of laboratory tests. Average of two walls of each strengthening configuration (H in-situ, D in-situ) was compared with average of 2 unreinforced walls for in-situ comparison. The biggest increase of shear strength (approx. 150%) was attained by horizontal strengthening of (weak) walls on site (Figure 13). The effect of horizontal strengthening on laboratory walls was modest 120%. Diagonal configuration did contribute just a little to shear strength (~5%) and about 110% to ultimate rotation. The highest increase (380%) of ultimate rotation was attained with applying horizontal (and vertical) CFRP str ipes of fabric on masonry on site and it was also high (around 200%) for configurations H and H+V tested in the laboratory. 4. Conclusions To improve seismic resistance of brick masonry walls the method with application of reinforcement with carbon fibre reinforced polymer (CFRP) fabric is presented. To develop the most efficient way of applying CFRP fabric to brick masonry walls, in-situ and laboratory Page 7 of 8

8 tests were carried out and their results are compared. Results show different effectiveness depending on the configuration of reinforcement. CFRP strengthening favourably influenced Figure 13. Effectiveness of CFRP strengthening variants the mechanism of wall behaviour in all cases. The biggest gain was the increase of ultimate displacement (or rotation) especially for system with vertical and horizontal stripes. Stripes in diagonal configuration did not perform so well. The primary mode of failure of diagonally strengthened wall tested in the lab was attributed to the exceedence of the compressive strength at the wall toes (due to rocking), which resulted in a localized compression failure of masonry. In-situ tests for diagonal configuration were governed by peeling of the stripes from the masonry. The difference in boundary conditions between in-situ and laboratory test set-ups proved not to be so significant. The quality of basic masonry material had the significant impact on behaviour of reinforced specimens. Better results of strengthening effectiveness were gained on initially weaker (in-situ) masonry. Best results were attained for walls strengthened with vertical and horizontal stripes. The increase of shear strength for lab specimens was 120% and 150% for in-situ compared to reference unreinforced specimens. 5. Acknowledgements The results of in-situ tests have been achieved in the project PERPETUATE ( funded by the European Commission in the Seventh Framework Programme (FP7/ ), under grant agreement n The results of laboratory tests were funded by the Ministry of higher education, research and technology of Republic of Slovenia under grant no. Z References [1] Croci, G., D'Ayala, D., D'Asdia, P., Palombini, F., 1987, "Analysis on shear walls reinforced with Fibers.", IABSE Symp. On Safety and Quality Assurance of Civ. Engrg. Struct., Int. Assoc. For Bridge and Struct., Lisbon, Portugal [2] Triantafillou, Thanasis C., 1998, "Strengthening of masonry structures using epoxybonded FRP laminates", Journal of Composites for Construction (2) May, , ASCE [3] Valluzi M.R., Tinazzi D., Modena C., 2002, "Shear behavior of masonry panels strengthened by FRP laminates", Construction and Building materials, 16, Page 8 of 8