BEHAVIOR OF RC CONNECTIONS STRENGTHENED WITH CARBON FIBER REINFORCED POLYMER

G. Ghodrati Amiri 1, A. Golkari 2 and Sh. Mehdizad Taleie 3 1 Professor, Center of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering,

Iran University of Science & Technology, Tehran, Iran. Email: ghodrati@iust.ac.ir 2 MSc. Student, School of Civil Engineering,

Iran University of Science & Technology, Tehran, Iran. 3 Payon Rehabilitation Group, Tehran, Iran.

ABSTRACT Devastating earthquakes in the recent years have shown that non-engineered concrete frames are particularly vulnerable to seismic action and are a major cause of loss of lives. Direct observation of damaged structures, following the Bam 2004 earthquake has shown that damage occurs usually at the beam-column joints, with failure in bending or shear. Many opportunities are becoming available for using composite materials to strengthen existing reinforced concrete (RC) structures. An experimental program about half-scale RC beam-column joints retrofitted with CFRP bonded is presented in this paper. A finite element analysis (FEA) is used to model the retrofitted specimens’ behavior. The results show the effect of CFRP composites in increasing the strength and ductility of RC joints. As confirmed by experimental results, FEA can effectively simulate the specimens strengthened by CFRP bonded fabrics. KEYWORDS FRP, RC connections, strengthening, finite element analysis, fiber model. INTRODUCTION Strengthening is often a necessary measure to overcome an unsatisfactory deficient situation or where a new code requires the structure or a member of it to be modified to achieve new requirements. These deficiencies are mainly a consequence of a lack of capacity design approach and/or poor detailing of reinforcement. Beam-column joints are one of the most important components for load transfer in RC frames, and the brittle failure of them may result in the total collapse of structures. Many studies have been done recently on the seismic assessment and retrofit of existing poorly detailed connections. In the recent years, applying Fiber Reinforced Polymers (FRP) is the most interested method for strengthening beam-column connections that has been studied. (Antonopoulos and Triantafillou 2003; Ghobarah and Said 2002; El-Amoury and Ghobarah 2002; Tsonos and Stylianidis 2002). Most of the proposed strengthening schemes have a very limited applicability, especially for interior joints, due to lack of consideration of floor members, perpendicular beams or architectural restrictions. In this fact, an investigation of four two dimensional T-shape connections will be discussed in this research. The methods used for strengthening these connections are feasible and useful for any connection in a structure even for 3D interior joints. EXPERIMENTAL STUDY

Geometry and Instrumentation The focus of the experimental program is on the construction and testing of four identical half-scale RC beam-column connections. 28-days concrete cylindrical compressive strength was selected 25 MPa for all four specimens. The yield strength of longitudinal and transverse bars is 400 MPa and 300 MPa, respectively. Elastic modulus of steel rebar (Es) is taken as 210 GPa. The CFRP used for strengthening these specimens had 0.176mm thickness with 240Gpa Modulus of elasticity. Figure 1 shows the test setup and specimen details. The test specimens were designed with lack of the transverse reinforcement to present the past constructed non-ductile joints. All connections were tested in a position rotated 90 degree from their real positions. A horizontal

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hydraulic jack was used to apply the constant axial load which was equal to 0.2Agf’c (i.e. 312.5 KN). Two horizontal hydraulic jacks were utilized to apply Reversed cyclic load at the end of the beams. Lateral loading cycles include three successive cycles at each displacement. The displacement increment was selected 1mm before yield point. After that 2mm was selected as displacement increment. Six vertical linear variable displacement transducers (LVDTs) and two horizontal LVDTs were used to measure the displacement at the top of the beam and curvature in connection area. Two more diagonal LVDTs used in connection zone to measure the shear deformation, during the test.

Figure 1. Test setup and specimen details Strengthening Strategy Specimen C is defined as control one. The other three connections are strengthened with different methods, shown in Figure 2 and explained below. For retrofitting specimen R1, two layers of CFRP sheet is used extended from beam to column on both sides. On the both bottoms of the column, two layers of CFRP with 250 mm width are wrapped. Specimen R2 is similar to R1 with additional concrete ledge in the beam column interface with 70 mm width. Specimen R3 is retrofitted with concrete ledge and three layer diagonal CFRP fabrics with 50mm width installed from beam to column with two layers wrapped CFRP over them. Test Results The values of lateral load versus displacement for all four specimens were plotted as hysteretic curves. Longitudinal reinforcement of control specimen yielded in 6.5 mm of transverse deflection. The failure mode was shear-flexure at 26 mm deflection and Maximum lateral force of this connection was 64 KN, related to 17mm displacement. The maximum lateral load of connection R1 was 72 KN at displacement equal to 16 mm that is 13 percent higher than the control one. In specimen R2 the concrete ledge causes higher width of wrapped CFRP, which serves as an anchorage for longitudinal one. The ultimate load capacity of R2 is 97KN which is approximately 1.5 times more than that for specimen C. The failure seen in the specimen was shear cracks inside the beam-column connection region. Due to architectural limitations, probable beams perpendicular to the face of the connection zone and the floor members, it is not feasible to wrap the joint area. For this purpose, keeping away the forces from connection zone and transferring the moment directly from beam to the column, was performed by installing diagonal CFRP strips extended from beam to column in specimen R3. The maximum load and displacement ductility of this connection were 106 KN and 9.1, which are 65% and 185% higher than those in the control specimen, respectively. The failure mode of this specimen was the rupture of diagonal CFRP. More information in this regard is previously presented by the authors. ANALYTICAL INVESTIGATION Modelling Method Nonlinear analysis is executed by using fiber model considering flexural deformation. The longitudinal rebar slip was modeled by using nonlinear rotational spring. Also the shear deformation inside the connection panel zone was modeled by two diagonal nonlinear springs as shown in Figure 3 (Shiohara 2001). The hysteretic rule of confined concrete follows Mander, Pristley and Park’s (1988) model. However the buckling of reinforcements was neglected because of its small effect. As it is shown in Figure 3, the M-Φ curve is calculated along the member’s axis in many cross sectional positions. The bending moment based on the M-Φ curve at each fiber section must be satisfied with the bending moment at the section that obtained from the linear bending moment distribution of the member. Then the curvature distribution is obtained along the member axis, and the flexural deformation is calculated by performing the principle of virtual work.

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Figure 2. Retrofit details

Figure 3. General force and deformation in nonlinear RC member

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Total member lateral displacement measured at top of the beam which is the summation of deformations due to flexure (for beam and column), longitudinal bar slip at beam end and shear deformation of the connection panel zone. The shear deformation of beam and column was ignored. As indicated earlier, lateral displacement due to flexure can be calculated by integrating the flexural curvatures along the height of the member for a typical beam-column frame element. Elongation and slip of the tensile reinforcement at beam is not included in the flexural analysis. The slip resulting from accumulated axial strains in the bar embedded in the joint can be calculated by integrating the strains over the portion of the bar between the interface and the point with no axial strain. Using a bilinear strain distribution shown in Figure 4, the slip can be determined from the Esq. 1 and 2.

Figure 4. Bar slip deformation and forces at the beam-column interface

ysds

l ldxSlipd

εεεε ≤== ∫ 20

(1)

ysysddyy

ll

l

l lldxdxSlip

ddy

dy

dy

εεεεε

εε ≥+′

+=+= ∫∫′+

)(220

(2)

Where, slip = amount of reinforcing bar slip at beam-column interface, ld = elastic development length, l’d = development length over the inelastic portion of the bar, ldy = development length corresponding to reinforcing bar yielding at interface, εs = strain in reinforcing bar, and εy = yield strain. The development lengths over the elastic and inelastic portions of the bar can be calculated based on the equilibrium of forces in the bar at the interface, and the assumption of bi-uniform bond stress distribution, ub: ld = fsdb/(4ub), l’d = (fs-fy) db/(4u’b), where ub = elastic uniform bond stress, u’b = inelastic uniform bond stress, fs = stress in reinforcing bar, fy = steel yield stress, and db = bar diameter. Results and Comparison between Experimental and Analytical Outputs

Figure 5 illustrates the envelop curves of hysteretic loops for control and retrofitted specimens. The envelop curves are the averages envelop of hysteretic loops in both directions. The dashed lines indicate the curves for analytical results and the continuous ones indicate the experimental. The initial stiffness in analytical analysis is predicted with reasonable accuracy. As shown in these figures the analytical model shows good agreement with the experimental results except for specimen R1. The additional concrete ledge in the beam column interface was not modeled in analytical analysis and the end of CFRP elements were assumed fixed. According to experimental tests, the effect of longitudinal CFRP in specimen R1 was less because the lack of complete anchorage. The additional concrete ledge in specimen R2 causes the complete and uniform anchorage which is compatible with modeling system, so the analytical and experimental envelop curves of specimen R2 are nearly the same. Diagonal spring of specimen R2 exceeded the maximum capacity at the end of analytical results which leads to shear failure in the connection panel zone. This phenomenon is compatible with the failure mode observed at the end of experimental test. As mentioned earlier the connection R3 was strengthened with concrete

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ledges and diagonal CFRP strips, extended from beam to column. Diagonal CFRP were modeled by additional linear spring in the connection panel zone. This idea could successfully change the load transferring route from beam to column. Decreasing the shear deformation, measured by diagonal LVDTs in the connection zone of this specimen in comparison with previous ones proves this. Also, the analytical results show decreasing of diagonal spring which is obvious because of additional diagonal spring. The failure mode of specimen R3 in analytical analysis was rupture of diagonal CFRP which is exactly the same as happened for this specimen in experimental tests.

Figure 5. Analytical and Experimental envelop curves of hysteretic loops Although there are good agreement between the analytical and experimental results for specimens C, R2 and R3 but the slight differences are visible. In such analytical investigations, we couldn’t model local damages which may occur during the tests. Furthermore, the Poisson’s ratio of concrete is taken constant along loading of the specimen, since it varies during the test. At last, the shear deformation of beam and column was not considered in analytical modeling. CONCLUSIONS In this paper, an attempt was made to introduce a rational and comprehensive procedure for modeling FRP strengthened RC connections for non-linear FE analysis. Appropriate elements from the software were chosen to account for the realistic behavior of each component in the connection, and the modeling and the analysis procedure were verified using some existing experimental data. The following conclusions can be drawn from this investigation:

• The ultimate moment and ductility of beam-column connections retrofitted with CFRP fabrics are higher compared with those of control specimens.

• Retrofitting method used for specimen R1 seems to be less effective due to the inability of wrapped FRP to anchor the longitudinal FRP efficiently.

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• Concrete ledge at the interface of beam and column is beneficial to anchor the longitudinal CFRP and also to increase the beam and column section depth at the ends, leading to enhance their moment capacity.

• By utilizing diagonal CFRP, excessive loads could transfer directly from beam to column and vice versa. Test results show that this is a beneficial method to strengthen the connections against flexure and shear for even 3d interior connections.

• Analytical analysis including fiber model and slip model for longitudinal bars, showed good agreement with experimental results.

• The behavior of diagonal nonlinear spring could almost present the shear deformation of the connection panel zone.

ACKNOWLEDGMENTS The authors wish to acknowledge financial support provided by the “Payon Rehabilitation Group”. The authors also extend their gratitude to Mr. M. Nikbakht for his valuable helps during analytical analysis. REFERENCES Antonpoulos, C.P. and Triantfillou, T.C. (2002), “Analysis of FRP-strengthened beam-column joints”, Journal

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