SHEAR PERFORMANCE OF EXISTING REINFORCED CONCRETE T-BEAMS STRENGTHENED WITH FRP

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1 SHEAR PERFORMANCE OF EXISTING REINFORCED CONCRETE T-BEAMS STRENGTHENED WITH FRP Stefania IMPERATORE PhD Davide LAVORATO PhD Camillo NUTI Full Professor Silvia SANTINI Associate Professor Lorena SGUERRI Research Assistant Abstract This paper presents the experimental behaviour of two reinforced concrete (RC) beams strengthened in shear with C-FRP. The beams are extracted from an important building of the '30s in Rome. The goal of this research is to evaluate the shear strength of the retrofitted beams with the simultaneous occurrence of a negative bending moment. The tests are performed at the Experimental Laboratory of the Structural Department of the University Roma Tre. Keywords: Existing building, Shear strengthening, C-FRP, Experimental behavior. 1. Introduction A typical problem regarding the retrofitting of existing RC structures, is related to the insufficient shear reinforcement due to deficient code requirements, construction defects (irregular stirrups spacing and lack of concrete cover) and increased loads. In these cases, it has been shown that shear strengthening of a RC element can be attained applying FRP sheets. International and national guidelines and codes ([1]-[3]) give design tools for FRP reinforcement on the base of experimental researches. However, these studies are often referred to simply supported beams with FRP shear strengthening in positive moment region while the behaviour of FRP shear reinforcement in negative moment region is not clear yet. This latter is the case of continuous beams, when large shear forces are combined with large Page 1 of 8

2 negative bending moments and shear cracks start from the top of the section, with the consequent less effectiveness to enhance the shear capacity. In those cases the effectiveness of typical configurations of FRP shear reinforcement could be invalidated. Nowadays few studies try to fill this gap. Khalifa [4] tested nine continuous two-span beams with different C- FRP amounts and wrapping schemes. The tests highlight that the U-wrap FRP reinforcement is not able to control the crack, even if overall increase of the shear strength is observed. Tann et al. [5] tested four continuous beams retrofitted in shear with FRP. The comparison between the unreinforced beam and the one reinforced with U-shaped C-FRP sheets shows that the beams have almost identical load-deflection curves but the C-FRP avoids the shear strength ensuring a more ductile collapse. The experimental campaign, performed at the Experimental Laboratory of the Structural Department of the University Roma Tre, is conducted on two r.c. beams extracted from a building in Rome dated the mid-'30s. This building is subject to an intervention of consolidation (Figure 1a) in order to ensure a sufficient degree of security for vertical and horizontal loads. The extracted beams are no more necessary for bearing loads in the retrofitted structure (Figure 1b).The consolidation intervention consist in new r.c. walls, flexural reinforcement by lightweight RC slab, shear reinforcement by C-FRP diagonal strips and flexural reinforcement of the beams ends of all internal beams and column C-FRP wrapping to increase the ductility and shear strength. Figure1. a) Interventions of consolidation of the structure; b) Beam extraction. In order to evaluate the behaviour of the beams a configuration similar to continuous beams is assessed (Figure 2). The objective of this research is to evaluate the effective strength of beams reinforced for shear (by C-FRP strips) and for bending (by adding a slab to the existing one) when the imposed state of stress is similar to that of a beam placed in a structure. (a) Bending moments for a beam in a frame structure (Real configuration) (b) Bending moments for a supported with cantilever beam (Actual experimental configuration) Figure 2. Bending moments due to a distributed load: a) for a beam in a frame structure, b) for a supported with cantilever beam. Page 2 of 8

3 2. Experimental program 2.1 Beams details The two beams extracted from the structure, marked as TM1 and TM2, are characterized by a T-section, smooth rebars and stirrups with spacing from 21 to 38 cm (Table 1). The medium concrete cubic strength is of 19.1 MPa for the beam TM1 and 25.5 MPa for TM2 from destructive tests on core specimens and no-destructive SONREB tests respectively. Table 1. Geometric characteristics of the beams TM1 TM2 Beam length 461 cm 462 cm Average height 52,5 cm 52,0 cm Width web 26,0 cm 26,0 cm Width flange 52,0 cm 57,5 cm Height flange 21,0 cm 21,0 cm Upper reinforcement Lower reinforcement Support upper reinforcement Support lower reinforcement Stirrups Stirrups spacing 21/34 cm 21/38 cm To simulate a stress state similar to beam in a framed structure (negative moments at the ends and a positive moment at the center of the span, Figure 1), a cantilever 220 cm long, is added to one beam end. This cantilever is built using a concrete C28/35 and ø24 steel rebars type B450C. The mechanical properties of reinforcements for the existent beam and the cantilever are shown in Table 2. Table 2. Mechanical properties of reinforcement of the cantilever and original beam; Ø bar diameter, y yield stress, r tensile strength and A gt deformation under maximum load. Existent reinforcement Specimen label ø (mm) y (MPa) r (MPa) A gt (%) FV_ ,95 479,89 19,64 FV_ ,82 404,87 23,20 Cantilever reinforcement FP_ ,28 617,35 14, Beam strengthening The retrofit of the beams consists in the application of C-FRP strips for the strengthening in shear and the construction of a new slab for the flexural strengthening. The shear reinforcement is composed of eight C-FRP U-strips (Figure 3Errore. L'origine riferimento non è stata trovata.a) along a beam length of 140 cm from both the beam supports. Each U-strip is formed by two sheets which cross at 45 on the intrados of the beam (Figure 3b). Page 3 of 8

4 Figure 3a: C-FRP shear reinforcement: overview Figure 3b. Detail of the intersection of the C-FRP strips. The retrofit is designed for a beam maximum shear of 210 kn. The C-FRP has a tensile strength of 538,74 MPa, elastic modulus of 65,5 GPa; the thickness of the strip is 0.22 mm. The flexural reinforcement involved the construction of a r.c. slab with rectangular section (80 cm wide and 7 cm thick). The slab reinforcement is formed by an ø8/20 electro-welded net and two layers of ø20 longitudinal rebars. The mechanical properties of steel used in the slab are shown in Table 3. Table 3. Mechanical properties of the additional reinforcement in the slab Specimen ø r A gt label (mm) (MPa) (%) Electro-welded net EWN_ ,49 8,56 Rebars FV_ ,55 12,97 On the beam TM1 are performed two static tests: a first (TM1a) to account the influence of some construction defects and a second (TM1b) on the repaired beam. The construction defects (beam TM1a) are characterized by a slab main reinforcement of two layers 2ø20 longitudinal rebar. The connection between the beam and the new slab is realized by two sets of pegs (2ø12/20cm bars). Either the main reinforcement and the pegs are disposed only at the supports (Figure 4). The concrete used for the slab is a lightweight concrete with polypropylene fibers (40 x 12 x 0.2 mm) characterized by a mean cubic compressive strength of MPa and an elastic modulus of GPa. Figure 4. Retrofitting scheme on beam TM1a. The beams TM1b and TM2 are retrofitted using for the slab a conventional C25/30 concrete. The slab main reinforcement is formed by two layers of 3ø20 longitudinal rebars, disposed in all the slab extension. The connection between the slab and the existing beam is guaranteed by ø12/12.5 U-shape rebars disposed on all the slab length. 2.3 Test setup The tests are performed applying the loads F1 and F2 according to the scheme of figure 5: F1 was applied by a 1000 kn hydraulic jack, F2 by a 250 kn hydraulic actuator. The instrumentation (Figure 6) consists in 12 potentiometers for the measurement of the vertical displacements (located on both sides of the beam to a distance of 0.2 L L L L for the span AB and L2 - L2 for the cantilever) and 4 potentiometers for measuring the deformations of the supports (two for each support). Other 12 diagonal potentiometers measure the shear deformations (6 near the left support and 6 near the right) and 40 strain gages, with a 10 mm grid oriented at 45, are used to measure the C-FRP strains. Page 4 of 8

5 Load F 1 [kn] Load F 1 [kn] F 1 F 2 A C B D L 1 L L 2 Figure 5. Tests static scheme Figure 6. Failure test setup for the retrofitted beams. 3. Tests results 3.1 Failure test on beam TM1a In this test the specimen is characterized by some construction defects: lack of a correct connection of the new slab to the beam extrados; discontinuity of the main slab reinforcement abrupt interruption of the lower reinforcement in the beam and in the cantilever Displacement [mm] 0,2 L 0,4 L 0,6 L Figure 7. Load versus span deflections curves of the beam TM1a top middle bottom C-FRP Strain [ 0 / 00 ] Figure 8. Load versus C-FRP strain curves of the beam TM1a During the test the beam presents a minimal damage, characterized by vertical cracks. The collapse occurrs when the force F1 reaches about 240 kn and a large diagonal shear crack forms. At this step F2 is equal to 73 kn; the shear and the bending moment on the support B are 175 kn and 152 knm respectively. Figure 7 shows the load-midspan deflection curves of the tested beam; Figure 8 the load-strain curves on a C-FRP strip. The specimen TM1a didn t show a significant damage on the length AC (Figure 5) after the test. From the test results it is clear that the retrofit has not been able to work. The collapse and the damage are mainly due the construction defects and include: The detachment and the break of the slab on the support B (Figure 9) respectively due to the wrong connection between the slab and the beam and to the interruption of the slab reinforcement. The formation of a hinge in B (Figure 10) due to the interruption of the lower reinforcement in the beam and in the cantilever. Page 5 of 8

6 Figure 9. Beam TM1a specimen after the collapse 3.2 Failure test on beam TM1b Figure 10. Lateral ejection of concrete due to the interruption of the lower existent reinforcement. The specimen TM1a is tested again after removing the slab and the cantilever, building a new r.c. slab for flexural reinforcement and a new cantilever added to the support A and repairing the beam on the length CB. The new slab was built using a concrete C25/30. The new specimen is called TM1b. Figure 11. Load versus span deflections curves of the beam TM1b. Figure 12. Load versus C-FRP strain curves of the beam TM1b. Figure 11 shows the force-midspan deflection curves of the tested beam until failure. The collapse of the beam occurred when the force F1 reaches the value of about 450 kn. At this step F2 is equal to 165 kn; the shear and the bending moment on the support B are 384,9 kn and 396 knm respectively. Figure 12 shows the force-frp strain curves for a strip until failure: since the strip are non anchored near the flange (where cracking starts), the lower strain is higher than the upper strain. Debonding of C-FRP strips starts near the cracks when F1 reaches 110,48 kn and it is visible when the beam reaches its shear strength (Figure 13). Figure 13. Debonding of C-FRP strips (TM1b). Figure 14. Specimen after the collapse (TM1b). Page 6 of 8

7 The first cracks develop when the beam reaches the design shear strength of 210,45 kn (corresponding to F1= 251,70 kn). The maximum average and local C-FRP deformations are between 1.2 and 1.5. The failure is characterized by concrete crushing and buckling of the original rebars (Figure 14). 3.3 Failure test on reinforced beam TM2 The beam TM2 is retrofitted without construction defects and it is tested one time until failure. The collapse of the beam occurrs when the force F1 reaches about 500 kn (Figure 15). At this step the force F2 is equal to 165 kn, while the shear and the bending moment on the support B are 416,5 kn and 396 knm respectively. The first shear diagonal crack becomes visible after a shear force of about 60 kn and assume a significant width since the beam reaches the shear strength of 210,24 kn (corresponding to F1= 250 kn). New shear cracks and beginning of debonding of C-FRP sheets near the cracks appear when the load reaches the values of 350 kn. The average maximum deformation is equal to about 2.4, while the maximum strains of the sheets are between 1.6 and 1.9. Figure 15. Load versus span deflections curves of the beam TM2 Figure 16. Load versus C-FRP strain curves of the beam TM2 4. Conclusions (a) Figure 17. Ultimate failure of specimen TM2. Two existent beams extracted from a RC structure in Rome dated the mid-'30s are retrofitted in bending and reinforced in shear with C-FRP strips. The beams are tested to investigate the shear strength of FRP reinforced concrete for a stress state similar to a beam in a framed structure and the factors which affect it. Due to this reason, the beam TM1 retrofitting has some construction defects and it has been repaired and strengthening again without (b) Page 7 of 8

8 construction defects after a first failure test, according with the strengthening intervention on the beam TM2. The experimental evidence in the current study, as well as many published reports in literature, have shown that shear strengthening of RC beams by externally bonded FRP sheets is effective in providing additional shear resistance to existing members. However when shear and negative bending develop simultaneously, shear cracks start from the beam extrados, supporting the FRP debonding if this is not adequately anchored. Finally, the occurrence of some wrong details significantly affect the final result. 5. Acknowledgements The work presented in this paper is part of the Research Project DPC - RELUIS AT 1 Line 1.1 Task Subtasks 6 and 7 founded by the Italian Department of Civil Protection. 6. References [1] CNR-DT 200/2004 (2008), Istruzioni per la Progettazione, l Esecuzione ed il Controllo di Interventi di Consolidamento Statico mediante l utilizzo di Compositi Fibrorinforzati. Materiali, strutture di c.a.. e di c.a.p., strutture murarie, CNR 13 luglio 2004, rev. 7 ottobre [2] Linee guida per la Progettazione, l Esecuzione ed il Collaudo di Interventi di Rinforzo di strutture di c.a., c.a.p. e murarie mediante FRP. Documento approvato il 24 luglio 2009 dall Assemblea Generale Consiglio Superiore LL PP. [3] FIB Task Group 9.3 FRP, Externally bonded FRP reinforcement for RC structures. Technical report on the Design and use of externally bonded fibre reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures, FIB bull. 14, July [4] KHALIFA, A. M., Shear performance of reinforced concrete beams strengthened with advanced composites, PhD Thesis, [5] TANN, D. B., DELPAK, R., ANDREOU, E., Design aspects of shear strengthening of rc beams using externally bonded FRP sheets, Structural Faults and Repair. 9th International conference, London, UK, 4th - 6th July [6] IMPERATORE, S., LAVORATO, D., NUTI, C., SANTINI, S., SGUERRI, L., Indagine sperimentale su travi in c.a. rinforzate a taglio con FRP, Giornate Aicap 11, Padova, ITA, 19th - 21th May Page 8 of 8