IN-SITU TESTING OF BRICK MASONRY WALLS ...reinforced concrete and binding the walls with steel ties....

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

IN-SITU TESTING OF BRICK MASONRY WALLS STRENGTHENED WITH CFRP FABRIC

Gostič, Samo1; Bosiljkov, Vlatko2; Jarc Simonič, Mojca3 1 PhD, Building and Civil Engineering Institute ZRMK Ljubljana, Slovenia, [email protected] 2 PhD, Assistant Professor, University of Ljubljana, Faculty of Civ. and Geodetic Engineering, [email protected] 3 Building and Civil Engineering Institute ZRMK Ljubljana, Slovenia, [email protected]

New requirements for strengthening buildings of cultural heritage assets, apart from its efficiency demands also reversibility of proposed methods. In this regard, one of the most promising methods is application of carbon reinforced polymers (CFRP) fabric to the surface of the wall. Within the framework of European FP7 research project PERPETUATE new computation models for masonry and strengthening techniques will be developed. To support validation of models various test on masonry specimens will be performed.

In this article experimental results of in-situ shear tests of strengthened clay brick masonry walls with CFRP fabric will be presented. In load bearing walls with different thickness of 30 cm and 45 cm respectively, positioned within the building dated from around 1935 built with solid bricks in low strength lime-cement mortar, cuts were made to isolate six 100 cm wide and 200 cm high specimens. For the purpose of this study, two configurations of positions of CFRP stripes were compared with unstrengthened specimens: walls with strips of fabric placed on masonry surface in two diagonal directions and walls with strips placed in several horizontal levels providing the confinement effect to masonry brick rows. Specimens were tested by horizontal cyclic loading under the constant vertical load.

Following experimental tests, different failure mechanisms were observed and the contribution of applied CFRP strips to load bearing capacity and ductility has been found to be different depending from failure mechanisms. The advantage of the horizontally applied confinement demonstrated significant increase of both ductility and energy dissipation. The most important conclusion is that the new innovative strengthening approach favourably influence the behaviour of slender wall and that in the design of retrofitted clay masonry buildings the calculation models must check all possible failure mechanisms and not only the shear mechanism.

Keywords: masonry, FRP, strengthening, in-situ test, shear strength, ductility

Theme: Research and testing



INTRODUCTION Many of masonry buildings are classified as the most valuable architectural monuments of cultural heritage as standalone buildings or as city aggregates. Unfortunately unreinforced brick masonry (URM) showed low seismic resilience during past earthquakes. Main problems are poor connection of load bearing walls and low shear strength of masonry walls. Several conventional strengthening methods to overcome these problems were developed in the past. The most common are: changing of weak mortar in joints, jacketing of URM walls with reinforced concrete and binding the walls with steel ties. Each of these mentioned methods has its own advantages and disadvantages. They are all disruptive to residents, realization takes a lot of time, and some of the methods significantly change seismic characteristics of building. In the case of cultural heritage buildings this methods are not suitable because of the appearance alteration as well as the need to retain the original historic material. Strengthening methods using new materials (Fibre Reinforced Polymers) promise to overcome those problems. After application FRPs can be removed from the original walls if so later required by the heritage conservation. One of the first studies of effect of strengthening masonry wall by fibres was done by Croci et.al. (1987). Triantafillou's researches include wide spectra of strengthening with FRPs. The experimental work focused on masonry, led to proposal of equations for masonry strengthened with FRP. Valluzzi (2002) performed series of diagonal tests on differently reinforced walls. Double sided, diagonal strengthening with GFRP showed to be the most efficient method. In the present experimental work the in-plane shear tests were performed in-situ on masonry walls. Investigation was focused on diagonal and horizontal strengthening with CFRP fabric stripes. EXPERIMENTAL CAMPAIGN In-situ tests were performed on the (typical) old masonry building from 1930-ies. Load bearing masonry walls were made with solid clay bricks (295 × 140 × 65 mm) and weak lime mortar with coarse sand (Dmax= 8 mm). The same materials are common for cultural heritage buildings from that period. Walls were of two thicknesses: 30 cm and 45 cm and on each thickness three specimens were prepared: one unreinforced specimen, one strengthened with diagonal stripes and one with combined horizontal and vertical stripes. Specimens were prepared by cutting walls on 2.0 m high and 1.0 m wide pieces with wire saw to ensure low, undamaging vibrations and smooth sides. As load set-up enabled 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 area designated for gluing was prepared by removing plaster and grinding the loose parts. The edges were rounded on appropriate places to avoid CFRP fibbers bend cracking. Unevenness of the surface was corrected with epoxy based mortar (BE-POX CL/21) in thickness up to 5 mm. After that the wet lay-up technique was used to apply CFRP to the wall. Sheet C-240 (from S&P) cut on 10 cm wide stripes with weight of 300 g/m2 were used. They were bonded with S&P epoxy resin 55 on both sides of the masonry. The strengthening was carried out by the company GRAS from Ljubljana.



Figure 1: Three configurations (D, un-strengthened and H) were prepared for testing

Basic materials have been tested to determine compressive strength of brick, mortar, tensile strength of FRP fabric as well as some other characteristics. Samples of bricks were randomly taken from different locations and tested. Compression strength was 20,1 MPa with quite high standard deviation (11,72 MPa). CFRP has E modulus 240 GPA with tensile strength 3800 MPa. COMPRESSION TESTS To determine the compressive strength and modulus of elasticity tests on two brick wall samples (T30 and T45) were performed. Dimensions of the first were (width/height/thickness) 100 x 100 x 30 cm and the other 100 x 85 x 45 cm. The result of the investigations gave the compressive strength of the wall which had to be determined also to set the vertical pre-stress of shear tested walls. In-situ compression tests were carried out with a hydraulic jack of 1300 kN capacity in a range up to 335 kN (Figure 2). Preparations of the samples were similar to those for shear tests - samples were cut -isolated from existing walls with wire saw, cleared of a plaster and evened on the top surface with cement mortar. With the system of steel ties and displacement controlled hydraulic jack the samples were tested in-place. Vertical and horizontal deformations were measured with LVDTs mounted on both sides of the wall (Figure 3). Sample T30 was tested with monotonically increasing load up to failure. Sample T45 was released at 45% of the maximum load and then loaded again up to failure. Mechanical characteristics gained in the compression tests are presented in Table 1.



Table 1 Results of compression tests T30 T45 average

fmc MPa 0,92 0,73 0,83 Ew MPa 519,92 754,54 637,23

w 0,65 0,50 0,58

Gc MPa 157,74 250,93 204,34 Gc/Ew 0,30 0,33 0,32

Figure 2: Compression test set up Figure 3: Diagrams of vertical

deformations for T30 and T45 SHEAR TESTS Six 2 m high and 1 m wide specimens with thicknesses 30 cm and 45 cm were cut out from load bearing walls for shear tests. Un-strengthened specimens for comparison were labelled O30 and O45. Diagonally strengthened specimens were D30 and D45 while H30 and H45 had vertical and horizontal reinforcement. At the middle of wall height (Figure 4) the horizontal load of hydraulic jack was applied separating wall into upper and bottom ‘specimen’ of the wall each with ratio h/l = 1.0. The specimens were thus tested as elements with symmetrically fixed ends into the surrounding masonry. Walls were additionally loaded with vertical force (with second hydraulic jack) to reach stress level at 30% of compressive masonry strength to simulate load of two more stories above tested walls.

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Figure 4: Shear test set-up

Figure 5: Shear test

instrumentation set-up Displacements and deformations were measured with linear variable differential transducers (LVDTs, Figure 5). Vertical pre-stress load (V) and horizontal load (H) were measured with load cells. Loading during tests was displacement controlled and it was progressing in steps 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 previously achieved.

Figure 6: Loading protocol

EXPERIMENTAL RESULTS Un-reinforced specimens O30 and O45 started to show first cracks at 70..80% of max load (or about 2..3 mm of horizontal displacement). They failed by propagation of diagonal cracks to width of 12 mm (Figure 7) after reaching maximum load (34 kN for O30 and 61 kN for O45) and continuing till ultimate displacement of 10 mm. On the Figure 8 the rotation versus horizontal load with its envelope is shown together with the pattern of cracks at the end of test. Cracks propagation was efficiently obstructed by the CFRP strips, which resulted in formation of many minor cracks for specimens D. First diagonal cracks occurred at 60% of max load (or about 5..8 mm of horizontal displacement). FRP stripes in diagonal configuration detached on uneven parts of surface where the weak part for detachment was in the brick and not in the glue (Figure 9). Load reached during D30 test was even lower (30 kN) than for un-reinforced while D45 reached 73 kN (Figure 10). The system of vertical and horizontal stripes was the most effective. Failure mechanism showed compressive failure within the FRP confinement and shear cracks after reaching load 59 kN and 84 kN (H30 and H45) with ultimate displacements of 34 mm and 45 mm

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respectively. There were local detachments from the surface and even local rupture of stripes but the specimens finally fail by masonry crushing in compression zones (Figure 11).

Figure 7: Diagonal cracks on

unreinforced specimen Figure 8: Envelope of rotation vs. load

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Figure 9: Failure of FRP stripes (detachment)

Figure 10: Envelope of rotation vs. load and the pattern of the cracks

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Figure 11: Local detachment of FRP

stripes from the surface Figure 12: Envelope of rotation vs. load

and the pattern of the cracks To compare results obtained on walls of different dimensions we calculated stress as horizontal load divided by horizontal cross section area (Figure 13, for upper-z and lower-s part of wall separately). The best results both in terms of strength and ductility were gained with H configuration of vertical and horizontal stripes (green lines).

Figure 13: Hysteresis envelopes for all shear tested walls

For study of effectiveness the average values of strengthening configuration was compared to average values of URM walls. The biggest increase of shear strength and ultimate displacement was achieved by configuration H (Figure 14).

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Figure 14: Effectiveness of CFRP strengthening configurations D and H

CONCLUSION The application of CFRP stripes for in-plane strengthening of masonry have been tested in-situ. Tests were performed on walls of two thicknesses under constant vertical load and with half-cyclic displacement controlled horizontal load. With the method we obtained good results for strength and ductility and due to reversibility of application it is suitable for strengthening the cultural heritage buildings. Two different configurations of CFRP stripes have been tested and compared with un-reinforced specimen. Best results were attained for walls strengthened with vertical and horizontal stripes (config.H). The average increase of shear strength over un-reinforced specimen was 150% with 380% increase of ultimate displacement. Stripes in diagonal configuration did not perform so well because the failure mechanism was governed by detachment of FRP from the masonry surface. The un-reinforced masonry failed in diagonal shear while the horizontally and vertically reinforced specimens failed by masonry compressive failure within the FRP confinement combined with diagonal shear cracks. ACKNOWLEDGEMENTS The results have been achieved in the project PERPETUATE (www.perpetuate.eu ), funded by the European Commission in the Seventh Framework Programme (FP7/2007-2013), under grant agreement n° 244229. REFERENCES 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 Triantafillou, Thanasis C., 1998, "Strengthening of masonry structures using epoxy-bonded FRP laminates", Journal of Composites for Construction 2 (2) May, 96-104, ASCE Valluzi M.R., Tinazzi D., Modena C., 2002, "Shear behavior of masonry panels strengthened by FRP laminates" Construction and Building materials, 16, 409-416

IN-SITU TESTING OF BRICK MASONRY WALLS ...reinforced concrete and binding the walls with steel ties....

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