Out-of-plane Behavior of Urm Walls Strengthened with Nsm Gfrp Bars

Unreinforced masonry (URM) walls are prone to failure when subjected to out-of-plane loads caused by earthquakes or high wind pressures. In this context, fiber reinforced polymer (FRP) materials may be a solution to solve or lessen the effects of this overloading. This paper describes an experimental program dealing with the flexural behavior of nine concrete URM walls strengthened with glass FRP (GFRP) twisted sand-coated bars and subjected to out-of-plane loads. The reinforcement was placed vertically in different amounts and spacing. Two different embedding materials (epoxy-based paste and latex modified cementitious paste) were used to encapsulate the bars. This research allowed to compare different behaviours and failure modes of strengthened URM walls by changing the parameters described above. Design guidelines are also provided. INTRODUCTION URM walls are one of the oldest and most widely used types of construction in the word; they are commonly used as interior partitions or exterior walls bound by steel or concrete frames forming the building envelope. Depending on design considerations, these walls can resist to lateral and gravity loads but due to weak anchorage to adjacent concrete members, or to absence of anchorage, URM walls may crack, tear and collapse under the combined effects of out-of-plane and in-plane loads generated by seismic forces. Recent failures of masonry structures around the word have identified the out-of-plane failure as the main cause of loss of lives. Thus, the development of new and effective techniques for the repair and strengthening is a need. The successful use of near surface mounted (NSM) fiber-reinforced polymer (FRP) bars for strengthening of concrete members [De Lorenzis, 2000] has been extended, in this experimental program, to URM walls. This technique consists of placing a bar in a vertical groove cut into the surface of the member being strengthened (Figure 1). The groove is first half filled with a paste, the bar is then placed into the groove and lightly pressed to force the paste to flow around the bar. The groove is also filled with more paste and the surface is levelled. Application of NSM FRP bars does not require any surface preparation work, preserves aesthetic and requires minimal installation time compared to FRP laminates [Tumialan et al., 2002]. Another advantage is the feasibility of anchoring these bars into members adjacent to the one to be strengthened (i.e. columns and beams) [Tumialan et al., 2002]. EXPERIMENTAL PROGRAM This experimental program deals with the flexural behavior of concrete URM walls strengthened with NSM GFRP bars. The parameters investigated were different dimension of bars cross-section area and amount, groove size and filling materials (latex modified cementitious-based paste and epoxy-based paste). The FRP bars were applied vertically at various spacing in order to increase the flexural loadcarrying capacity. The test setup intended to simulate walls under simply supported conditions (i.e. high slenderness ratios); thus, the influence of arching action was not present. (a) (b) (c) Figure 1. Reinforcement technique: (a) cutting of the groove, (b) filling of the groove and (c) positioning of the bar Test Matrix Nine specimens were constructed with concrete blocks. Their nominal dimensions were 600 mm (24 in.) wide by 1200 mm (48 in.) high. The nominal wall thickness was about 95 mm (3.75 in.). The masonry panels were strengthened with 6.25 mm-diameter (2/8 in.) and 9.37 mm-diameter (3/8 in.) sand-coated twisted glass FRP (GFRP) bars in different amounts (one, two or three) with a spacing of 20, 30 and 60 cm (8, 12 and 24 in.); the reinforcement was encapsulated in a square groove by using two embedding materials (epoxy-based paste and latex modified cementitious paste). In the case of 6.25 mmdiameter bars embedded in the modified cementitious paste, two depths of the groove were considered: 1.5 and 2.5 times the diameter. Table 1 reports the test matrix. Table 1. Test matrix Diameter (d) Space between two Bars Depth of the Groove Specimen Code Embedding Material mm [in] Amount of Bars cm [in] Times of d E1-9-S 9.525 [3/8] 1 60 [24] 1.5 E2-9-S 9.525 [3/8] 2 30 [12] 1.5 E3-9-S 9.525 [3/8] 3 20 [8] 1.5 E2-6-S 6.35 [2/8] 2 30 [12] 1.5 E3-6-S EpoxyBased Paste 6.35 [2/8] 3 20 [8] 1.5 C1-9-S 9.525 [3/8] 1 60 [24] 1.5 C2-9-S 9.525 [3/8] 2 30 [12] 1.5 C2-6-B 6.35 [2/8] 2 30 [12] 2.5 C3-6-B Latex Modified Cementitious Paste 6.35 [2/8] 3 20 [8] 2.5 The specimen designation refers to the following parameters: type of embedding paste – number of bars used for reinforcing – diameter of the bar– dimension of the groove. As an example, E2-6-S refers to a wall having epoxy paste as embedding material (“E”) and reinforced with two GFRP No. 6 bars (“2-6”) placed in a small groove (“S”). Reinforcement terminated before the reaction point so that it would not touch the roller supports used for testing. Materials Tests were performed to characterize the mechanical properties of the materials used in this investigation. The average compressive strengths of concrete blocks (dimension 102 x 203 x 305 mm / 6 x 8 x 16 in.) obtained from the testing of prisms (ASTM C1314) were 10.5 MPa (1520 psi). Type N mortar was used; standard mortar specimens were tested according to ASTM C109. An average value of 7.6 MPa (1100 psi) at an age of 28 days was found. Splitting tensile test (ASTM C496) for both the embedding materials were performed since the most important mechanical properties that are used in design of NSM reinforcement are the tensile properties. The splitting tensile strength was found to be 3.58 MPa (0.518 ksi) after 7 days and 5.59 MPa (0.81 ksi) after 28 days in the case of latex modified cementitious paste, and 16.31 MPa (2.36 ksi) after 7 days and 18.54 MPa (2.7 ksi) after 28 days in the case of epoxy-based paste. Tensile tests were performed on GFRP bars to determine their mechanical properties, which are related to fiber content and not to composite area. No. 6 GFRP bars presented a tensile strength of 825 MPa (120 ksi) and a modulus of elasticity of 40.8 GPa (52900 ksi), while No. 9 GFRP bars presented a tensile strength of 760 MPa (110 ksi) and a modulus of elasticity of 40.8 GPa (52900 ksi). Test Procedure The masonry panels were tested under simply supported conditions; the influence of arching action was also not present. Figure 2 shows the test setup. Due to the brittle nature of URM and to the weight of the test equipment (i.e. steel beam, hydraulic jack, etc.) no control specimen was tested. The load was generated with a 12-ton capacity hydraulic jack and was transferred to the specimen by means of a steel beam supported by two rollers, which applied a load along two lines spaced at 20 cm (8 in.). The line loads rested along the full width of the walls. The load was applied in cycles of loading and unloading. Deflection at midspan was measured using two LVDTs (linear variable differential transducers) and strain gauges were placed in the GFRP bars to record strains at different levels of load an in different. Figure 2. Test setup Test Results The walls exhibited the following modes of failure: (1) debonding of the FRP reinforcement and (2) shear failure in the masonry near the support. Table 2 reports the test results. Table 2. Test results Reinforcing Area Ultimate Load Maximum Bending Moment Displacement at Midspan Specimen Code mm [in.] kN [kips] kN-m [kip-in.] mm [in.] Mode of Failure E1-9-S 84.32 [0.131] 6.85 [1.54] 1.56 [13.85] 17.23 [0.68] D E2-9-S 168.64 [0.262] 17.21 [3.87] 3.93 [34.82] 16.93 [0.66] S E3-9-S 253 [0.393] 24.36 [5.48] 5.57 [49.3] 18.69 [0.74] S E2-6-S 66.46 [0.103] 7.38 [1.66] 1.68 [14.94] 27.1 [1.06] D E3-6-S 99.69 [0.154] 9.91 [2.23] 2.27 [20.43] 26.38 [1.04] S C1-9-S 84.32 [0.131] 4.35 [0.98] 0.94 [8.82] 9.7 [0.38] D C2-9-S 168.64 [0.262] 7.16 [1.61] 1.64 [14.58] 10 [0.4] D C2-6-B 66.46 [0.103] 9.03 [2.03] 2.06 [18.27] 9.03 [0.7] D C3-6-B 99.69 [0.154] 12.81 [2.88] 2.93 [25.9] 22.67 [0.9] D Legend: D = debonding failure, S = shear failure