An Experimental Investigation on Flange Bonded Retrofitting of RC Beam-Column Joints using CFRP

Transcription

1 An Experimental Investigation on Flange Bonded Retrofitting of RC Beam-Column Joints using CFRP Abolfazl Eslami 1, Hamid Reza Ronagh 2 1 PhD Candidate, School of Civil Engineering, The University of Queensland, Brisbane, Australia 2 Senior Lecturer, School of Civil Engineering, The University of Queensland, Brisbane, Australia Abstract: Retrofitting/rehabilitation of existing reinforced concrete structures using externally bonded fiber reinforced polymer (FRP) composites is increasing worldwide, due to the many advantages FRP could offer. In this paper, the results of a comprehensive experimental study performed on CFRP retrofitting/repairing of exterior RC beam-column joints are presented. Nine small-scale (1/2.85) specimens were detailed based on the recent design codes, and tested under both monotonic and cyclic loadings to investigate the efficiency of the CFRP retrofits in improving the loading capacity of the code-compliant RC joints. The unidirectional composite sheets with carbon fibers oriented parallel to the longitudinal reinforcements of beam were applied to the flange sides of the beam. This flange bonded scheme was aimed at increasing the flexural strength of the beam-column joint. A novel grooving method was used in this study to preclude debonding failure and facilitate the force transmission from beam to columns. The experimental results in terms of load-displacement curves, ultimate strength, and energy dissipation are compared before and after retrofitting. The outcomes indicate the capability of the suggested retrofitting scheme in not only enhancing the loading capacity, but also the relocation of the plastic hinges away from the column providing particular details. Keywords: beam-column joints; RC beam-column joints; CFRP; loading capacity; Hysteretic energy. 1. Introduction Compared to other techniques, rehabilitation\retrofitting of reinforced concrete (RC) beam-column joints using externally bonded fibre reinforced polymer (FRP) composites has received a lot of attention in the past decade. This is due to the advantages of FRP including light weight, high corrosion resistance, superior strength and ease of application. The efficiency of FRP composites to rehabilitate the seismic performance of deficient beam-column connections has been confirmed in many experimental studies (1-3). Le-Trung et al. (4) carried out a comprehensive experimental study on the different configurations of FRP retrofit to strengthen non-seismically designed RC joints. They concluded that the appropriately adding of composites can significantly enhance the lateral strength and ductility of test specimens. In another experimental and analytical study, Mahini and Ronagh (5, 6) upgraded the strength and ductility of exterior beam-column joints using a web-bonded retrofitting scheme. In particular, through the relocation of plastic hinges from the column interface further along the beam, they showed that this method can vary a joint core brittle failure mode to a desirable beam failure. Surprisingly, the main objective of all past studies made on the application of FRP in RC beam-column joints was to improve the seismic behaviour of RC joints suffering from a structural deficiency. However, many existing code-compliant RC buildings might be in need of retrofit due to reasons such as changes in the seismic hazard levels, applied loads, deterioration with time, and design methods. Structures located in the near-fault regions with their higher seismic demands could also be in need of retrofitting, so are those modern engineered buildings detailed based on the older seismic codes, as reported in many studies (7, 8). In addition, almost all of the retrofitting schemes available in the literature follow the application of FRP composites on web sides of the beams and joint core; a retrofitting scheme which would be impeded by the presence of cross beams and slab in real three dimensional conditions. Even in the exterior beam-column connections, the case in which both sides of the joint core were retrofitted in the earlier studies, only one side is accessible due to the existence of the transverse beam. As a result, the current experimental program was conducted on codecompliant RC joints retrofitted on top and bottom sides of the beams using CFRP sheets (herein refers to the flange-bonded scheme). Of particular interest was investigating the efficiency of a novel technique in which the CFRP sheets are inserted into the column concrete cover in order to preclude debonding failure and to facilitate force transmission from beam to columns. The main purpose of the retrofitting design was to increase the flexural strength and loading capacity of the beam-column joint sub-assemblage.

2 2. Experimental program 2.1. Design of the test specimens Figure 1 shows the geometry and reinforcement details of the test specimens determined using Bukinghum theory (9) from the original structure. The scaled down specimens were extended to the column mid-height and beam mid-span corresponding to the contraflexure points of bending moment diagram under lateral loading. All joint specimens consisted of 210 mm wide and 175 mm deep beams with 210 mm square columns. A concrete clear cover of 25 mm was incorporated in all specimens. The longitudinal and transverse reinforcements were made of standard deformed N 10 bar (10 mm diameter) and deformed RW 6 wire ( 6 mm diameter), respectively. The mechanical properties of steel reinforcements were determined from direct tensile tests in a universal testing machine using a mechanical extensometer of 50 mm gauge length and are summarized in Table 1. Figure 1. The geometry and reinforcement details of the test specimens (all dimensions are in mm) Table 1. Mechanical properties of steel reinforcements Diameter Yield stress Yield strain Ultimate stress Modulus of Elasticity (Gpa) (mm) (Mpa) (%) (Mpa) Strain at ultimate load (%) Description of the Retrofitting scheme The retrofitting design was aimed to improve the flexural resistance of joints with the practical application point of view. Therefore, the flange-bonded retrofitting scheme was implemented in all retrofitted specimens. This is the optimal FRP application as it provides the maximum distance to neutral axis. A schematic illustration of the flange-bonded retrofitting scheme is shown in Figure 2. As the main test parameter, the experimental program investigated a novel technique in preventing debonding brittle failure and facilitating force transmission in beam to columns. This technique is comprised of inserting the CFRP sheets into a groove created in the column concrete cover. The depth of the groove was about 25 mm. Its width was equal to the beam width. The groove was then filled with a mixture of resin and very fine sand.

3 A L f D A : C o lu m n w ra p, T o p B : C o lu m n w ra p, B o t C : B e a m w ra p s D : F R P s h e e t, T o p E : F R P s h e e t, B o t 25 C E 35 B Figure 2. Schematic illustration of the flange-bonded retrofitting scheme adopted in this study Specimen 1 l f (mm) Table 2. Description of the test specimens CFRP sheets Position 2 ply no. t f (mm) CFRP wraps 3 Groove CS HS 175 D C - RSG 350 D C Top HSG 175 D A, C Top CS-D HSG-D 175 D A, C Top CS-C HSG-C 175 D, E A, B, C Top & Bottom RSG-C 350 D, E A, B, C Top & Bottom 1 CS: Control Specimen; HSG-D: Hinge Specimen with Groove-Damaged; RSG-C: Retrofitted Specimen with Groove- Cyclic loading. 2 See Figure 2. 3 Three layers of FRP have been applied in all cases, except for RSG where two layers are used. See also Figure 2. In total, six specimens were retrofitted to investigate the effects of different parameters on the behaviour of beam-column joints retrofitted using the flange-bonded scheme. Table 2 provides a description of all test specimens. A short description of the test specimens follows: CS and CS-C were used as control specimens under monotonic and cyclic loadings, respectively. RSG was made to evaluate the effect of column wraps. HSG and HSG-C were aimed to relocate plastic hinge in beam away from the column interface under monotonic and cyclic loadings, respectively. HS was identical to HSG, but without inserting the FRP sheets into groove. It was made to scrutinise the efficiency of the adopted grooving technique. CS-D was initially loaded to a displacement ductility level of three and unloaded. At this stage, the specimen was already cracked, but not destroyed. The surface of the pre-cracked specimen CS-D was then ground and loose spalled concrete pieces were removed using air jet pressure to assure a good bond between the CFRP sheet and concrete. Figure 3 shows the damaged specimen after surface preparation. It was also cleaned using thinner before repairing. Cracks were then filled using Concressive 2350; a low viscosity crack injection resin. After seven days of curing, the specimen was retrofitted in a way similar to the other retrofitted specimen and called HSG-D. Then, HSG-D was retested under monotonic loading up to failure. This simulates the situation in which the flange bonded CFRP is used to rehabilitate damaged joints RSG-C was identical to HSG-C, but with the length of CFRP sheets twice that of HSG-C to investigate the effect of the length of CFRP sheets.

4 Figure 3. Damage status of specimen CS-D after surface cleaning The CFRP fabrics used in the retrofitted specimens were uni-directional MBrace CF130 with tensile strength of 3900 M P a, tensile modulus of elasticity of 240 G P a, and mm in thickness. In order to improve the bond strength between the composite sheets and the concrete substrate, composite wraps with a thickness of at least 50% larger than FRP sheets were applied at both ends of the composite sheets Test set-up and loading Each beam-column specimen was tested in a beam vertical position inside a rigid frame. Figure 4 shows the experimental set-up. Both ends of the column were supported with specially designed hinge supports to release rotational deformations and fix any translation. The hinge supports at the column ends can simulate the real performance of the beam-column joint sub-assemblage at the contraflexure points under lateral loading. Column axial load was also applied in all specimens by stretching four high-strength low-elongation steel bars placed outside the column through the use of a hydraulic jack. In order to provide an even stress distribution on the column cross section, two steel plates with 20 mm thickness were also glued to both ends of the columns. During the test, the value of column axial load was controlled to remain constant at a level of 344 kn. This value represents a stress level of 0.2 f c in the column section where measured to be around 39 M P a. fc is the 28-day cylinder compressive strength and was Figure 4. Schematic illustration of test set-up The beam tip load (monotonic and cyclic) was applied at the free end of the beam using a 300 kn hydraulic actuator. The displacement of the beam was measured using a set of linear variable displacement transducers (LVDT) placed along the beam length. Also, strains of beam longitudinal reinforcements in positions prone of high inelastic deformation in the specimens under monotonic loading were obtained using strain gauges. A displacement-controlled loading was used in both monotonic and cyclic specimens. The monotonic load was simulated by displacing the beam tip in a push direction so that the beams top reinforcement

5 ( 3 N 1 0 ) undertook the tensile stresses (see Figure 1). For cyclic loading, the cyclic displacement reversals were simulated with increasing the displacement ductility factor, of beam in both push and pull directions. Two cycles were applied in each ductility level. 3. Experimental results All the tested specimens showed a ductile behaviour without any shear/shear-flexural cracking at the beam or joint core. During the monotonic and cyclic loadings, flexural cracks developed and widened in the potential plastic hinge areas of the control and retrofitted specimens, while the column was intact. This is the typical failure mode in structures designed based on the weak-beam strong-column design philosophy. In cyclic loading, failure of all specimens was attributed to the rupture of beams bottom steel bars ( 2 N 1 0 ) at the last cycle (pull direction in Figure 1), while tensile bar rupture ( was observed in some specimens under monotonic loading. In both loading type, the failure was also accompanied by crushing of the concrete in compression. No debonding or delamination was observed during the tests. However, some retrofitting configurations were not successful due to inappropriate provisions for load transmission from beam to column Monotonic loading The beam tip load-displacement curves obtained from the tests of the specimens under monotonic loads are compared in Figure 5. For the control specimen, CS, the first crack was observed at the load around 15 kn at beam-column interface and was opened up to a width of around 13 mm at the failure point. During the loading, more cracks also developed in the plastic hinge region of the beam, while concrete crushed on the compressive side. As observed in Figure 6a, failure of the control specimen was attributed to the damage in the beam near the joint core which is the typical plastic hinge location for beams. Other parts of the beam sustained only a few minor cracks. For the control specimen, CS, the maximum load and displacement were recorded at 28 kn and mm, respectively. Also, the yield displacement of the tensile reinforcements of this specimen was measured using a strain gauge as 6.3 mm. The experimental results of the specimen HS which was without a groove were almost identical to the control specimen. Figure 6b indicates the ultimate failure of the specimen. It is believed that the insertion of the CFRP sheets into the groove could eliminate this weak plane and anchor the composite sheets. This conclusion was confirmed in the experimental results of other retrofitted specimens under monotonic and cyclic loads. The specimen RSG was retrofitted without column wraps. During loading, a crack was initiated from the end of the groove at a load around 26 kn, and then developed in the concrete column on both sides. Eventually, a pullout cone failure of concrete was observed at the end of the composite sheets in the specimen RSG, as observed in Figure 6c. Other parts of the joint remained almost intact with only minor cracks in the tensile area of the beam. Comparing the results of the specimen RSG (the specimen without columns wraps) and the control specimen, a negligible increase was achieved in the ultimate load resistance and displacement of the retrofitted specimen. However, the load-displacement curve of the specimen RSG follows higher load values than the control specimen at the elastic part up to approximately the first crack point. This kind of failure mode, which has been reported for the FRP anchors too (10), is mainly attributed to insufficient embedment depth. Due to the restriction of the concrete cover thickness in the scaled joint, it was decided to wrap the column to prevent the FRP pullout failure in other specimens. In this way, the pullout forces were undertaken by the CFRP wraps. The specimen CS-D was loaded up to the tip beam displacement of,1 9.5 mm which corresponds to the ductility factor of 3, as shown in Figure 6d. The specimen was then unloaded and repaired. Similar to the control specimen CS, the first crack was observed at the beam-column interface at a load of 18 kn. At the ultimate point, the first crack width was about 2 mm. More cracks also developed in the plastic hinge region of the beam. The maximum strength of this specimen was measured to be about kn. For specimens HSG and HSG-D, the first cracks were initiated after the CFRP along the beam at the loads of about 15 kn and 18 kn, respectively. During the test, more cracks were developed near the cut-off point of the CFRP in the beam. The specimen HSG, was able to carry the maximum load of kn and tolerate the ultimate displacement of mm. These values were measured to be around N )

6 Applied load, kn kn and mm for specimen HSG-D. At failure, the maximum cracks opened to a width of 14 mm and 18 mm, in specimen HSG and HSG-D, respectively. Damage to these specimens showed that the retrofitting design used for these specimens was capable of relocating the plastic hinge away from the column faces. The location of plastic hinges in specimens HSG-D and HSG can be observed in Figures 6e and 6f, respectively. This type of failure, with plastic hinges away from the column interface, is desirable in seismic retrofitting because it prevents joint brittle failure and respect the weak-beam strong-column failure mode (2, 6, 11). In addition, the experimental results of the specimens HSG and HSG-D proved the efficiency of the CFRP flange-bonded scheme in increasing the load carrying capacity of the code-compliant RC joints (see Figure 5) CS HS RSG HSG CS-D HSG-D Beam tip displacement, mm Figure 5. Distribution of the beam tip load versus displacement for the specimens under monotonic loading (a) CS (b) HS (c) RSG (d) CS-D (e) HSG-D (f)hsg Figure 6. Failure modes of the specimens under monotonic loading

7 Applied Load, kn 3.2. Cyclic loading The hysteretic curves obtained from the beam tip loads versus corresponding displacements of all specimens under cyclic loading are drawn in Figure 7. Also, the corresponding envelope curves are compared in Figure 8. Failure of the control specimen occurred at a load of 5 kn and the displacement of mm in the pull direction. Figure 9a shows the ultimate failure of the control specimen under cyclic loading history. Other parts of the beam sustained only minor damages whereas the joint core and column remained completely intact. During testing of the specimen HSG-C, the retrofitted part of the beam and the whole parts of the column and joint area were undamaged. Rupture of the beam tensile bars in the pull direction at cycle 16, followed by crushing of concrete resulted in the failure of the specimen, as observed in Figure 9b. Compared to the control specimen, CS-C, the experimental results of the specimen HSG-C confirmed the effectiveness of the adopted flange-bonded scheme in not only increasing the load carrying capacity in both directions, but also relocating the plastic hinge away from the column face and further into the beam. (a) (b) (c) Figure 7. Hysteretic responses of the specimens (a) CS-C, (b) HS-C, and (c) RS-C CS-C HSG-C RSG-C Beam tip displacement, mm Figure 8. Envelopes of the hysteretic curves of the cyclic specimens As observed in Figures 9a, and 9b, the failure of the specimens CS-C and HSG-C were accompanied by the formation of a small length plastic hinge at the beam-column interface, and after CFRP sheets along the beam, respectively. The length of the plastic hinge was estimated to be around half the beam height. This was also suggested by other researchers for conventional RC structural beams and columns (11). For the specimen RSG-C, the first loading cycle resulted in beam cracking after the CFRP retrofits. However, at the third cycle, a flexural crack was initiated after the grooves in the beam-column interface on both sides and penetrated to the beam and opened up to 20 mm during the test. As shown in Figure 9c, failure of the specimen was attained at the 15 th cycle, by a rupture of both beam tensile bars in pull direction and column FRP wraps in shear. This implies the inadequacy of applied composite sheets in relocating the plastic hinge in this specimen due to the greater CFRP length

8 compared to the specimen HSG-C. Similar to HSG-C, the application of CFRP retrofits in the specimen RSG-C also resulted in increasing the ultimate load carrying capacity. (a) (b) (c) 4. Comparison of results 4.1. Strength and ductility Figure 9. Failure modes of the specimens under cyclic loading For monotonic loading, the retrofitting configurations implemented in the specimens HSG and HSG-D led to a significant improvement in the behaviour of the joints. The maximum load capacity of the specimen HSG-D was recorded at 30.4% and 27.1% higher than those measured for the specimens CS-D and CS, respectively. This indicated the efficiency of the CFRP flange-bonded scheme, not only in the strengthening of existing beam-column joints, but also in the rehabilitation of damaged joints. Also, compared to the specimen CS-C, the maximum loading capacity of the HSG-C and RSG-C were increased by 31.4% and 45.4% in the push direction. These increments were about 19.4% and 37.6% in the pull directions. Table 3 provides a summary of the results for all specimens. Specimens Table 3. Summary of the results for all test specimens Maximum load Ultimate Dis. f # Increase (%) Ductility factor Decrease* (%) (kn) (mm) c Push Pull Push Pull Push Pull Push Pull Push Pull CS HS RSG HSG CS-D HSG-D CS-C HSG-C RSG-C #Cylinder compressive strength of concrete measured on the testing day. The 28 day cylinder compressive strength was about M P a. *Ductility decrease of specimen HSG-D was calculated related to CS In addition to strength, ductility is also an important parameter in seismic assessment of RC beamcolumn joints. The displacement ductility factor,, defined as the ratio of ultimate displacement,, to the yield displacement, y. In this study, the yield displacement was assumed to be the displacement at first yielding of the tensile reinforcements and this was considered to be around 6.5 mm and 7 mm for control and retrofitted specimens, respectively. The yield displacements were also controlled using nonlinear finite element analyses performed in ANSYS (12). The ultimate displacement was assumed as that displacement after which a significant drop was observed in the load. The ultimate displacement values and the ductility factors of all test specimens are given in Table 3. The experimental results indicated a reduction of the displacement ductility factor of the retrofitted specimens, although this decrease was not considerable under monotonic loading. For cyclic specimens, the ductility factors of the specimen CS-C were calculated to be around 9.1 and 8.7 u

9 Cumulative dissipated energy, kn.m in push and pull directions. These values reduced to 7.4 in both loading directions for the specimen HSG-C, while they were calculated as 7.5 and 5.6 for the specimen RSG-C, in push and pull, respectively. The lower amount of ductility in specimen RSG-C was particularly due to the separation of the grooves at the end of the retrofitted part of the beam which resulted in a drop in strength. Providing a deeper groove and/or FRP anchors with the grooving method may improve the loaddisplacement behaviour of the retrofitted joint in this case. Technically speaking, the ductility capacity depends to a great extent on the retrofitting scheme. A retrofitting architecture might aim to improve strength, ductility, or both, in an RC beam-column joint sub-assemblage. Ductility enhancement could be obtained through the retrofitting techniques aimed to provide confinement. However, an improvement in the displacement capacity could not be expected with the suggested flange-bonded scheme mainly aimed at increasing the strength. This conclusion has also been addressed by other researchers for RC beams (13) and beam-column joints (3, 14). In addition, Ronagh and Eslami (15) recently showed that decreasing the ductility of the nonlinear plastic hinges of structural members retrofitted using flange-bonded CFRP has a negligible effect on the overall displacement capacity of the structure. Their pushover results confirmed a significant increase (almost 80%) in the lateral resistance of an 8-story building retrofitted using flange-bonded CFRP at plastic hinge regions of beams and columns. More investigations however, need to be conducted to improve the flangebonded technique with the suggested grooving method Energy dissipation The energy dissipation during a particular loading cycle is assumed to be the area enclosed by the corresponding load-displacement hysteretic loop. The cumulative energy dissipated was calculated by accumulating the energy dissipated in consecutive loops throughout the cyclic load reversals. The cumulative energy dissipated versus the loading cycles is plotted for the cyclic specimens in Figure 10. While the cumulative energy dissipated by all three specimens is approximately equal in the first cycles, it stands higher for the retrofitted specimens after the eighth cycle. The greater amount of energy dissipation in the retrofitted joints, particularly for the specimen HSG-C, can provide the RC structure with the ability to survive the lateral forces at a lower level of damage. This is aligned with retrofitting purposes to ascertain the structural performance at an acceptable level during strong ground motions. This is in addition to the higher strength provided by the CFRP retrofit. Of particular interest is an almost identical amount of energy dissipation at the ultimate failure point of the specimens HSG-C and control specimen, CS-C, despite the lower loading cycles sustained by the former. It should be mentioned that failure of specimen RSG-C was attained in smaller cycles due to the inadequacy of the anchoring method. Therefore, more energy dissipation might also be achieved in this specimen, providing a deeper groove or FRP anchors CS-C HSG-C RSG-C Cycle no. 5. Conclusion Figure 10. Cumulative energy dissipation for the cyclic specimens The results of an experimental investigation on CFRP retrofitting of code-compliant RC connections have been discussed in this study. Various strategies, including the use of composite wraps and grooving technique were implemented to ascertain the accomplishment of retrofitting purposes and to preclude the debonding brittle failure under both monotonic and cyclic loadings. The main experimental findings can be summarised as follows:

10 The adopted retrofitting scheme can improve the lateral strength of existing RC joints providing the groove with adequate depth and wrapping the column near the joint core. In addition, under particular circumstances, the flange-bonded CFRP retrofits can also relocate the plastic hinge away from the column face, preventing the joint brittle failure. The experimental results of the specimens CS-D and HSG-D, confirmed the effectiveness of the flange-bonded retrofitting scheme in rehabilitation of damaged RC connections. The ultimate load carrying capacity of the specimen HSG-D was increased by 30.4% using CRFP retrofits compared to pre-cracked control specimen, CS-D. This is of particular of interest to repair of RC structures which suffer severe damage under seismic motions. As far as the ductility capacity is concerned, the experimental outcomes showed a reduction in the displacement ductility factor in the CFRP flange-bonded RC specimens. This decrease is anticipated due to the higher amount of reinforcement provided by CFRP composites. It should be mentioned that the lower the amount of tensile reinforcement in the RC structural member the higher the ductility capacity. The cumulative energy dissipated to any particular cycle showed that the CFRP retrofitted joints dissipated a higher amount of energy compared to the control specimen. The increase in cumulative energy dissipation is more remarkable for specimen HSG-C where the total energy dissipation of the specimen at the failure point is approximately equal to the control specimen. 6. References 1. Ghobarah, A., El-Amoury, T., Seismic rehabilitation of deficient exterior concrete frame joints, Journal of Composites for Construction, 9(5), 2005, pp Karayannis, C.G., Sirkelis, G.M., Strengthening and rehabilitation of RC beam-column joints using carbon-frp jacketing and epoxy resin injection, Earthquake Engineering and Structural Dynamics, 37(5), 2008, pp Attari, N., Amziane, S., et al., Efficiency of Beam-Column Joint Strengthened by FRP Laminates, Advanced Composite Materials, 19(2),. 2010, pp Le-Trung, K., Lee, K., et al., Experimental study of RC beam-column joints strengthened using CFRP composites. Composites Part B: Engineering, 41(1), 2010, pp Mahini, S.S., Ronagh, H.R., Strength and ductility of FRP web-bonded RC beams for the assessment of retrofitted beam-column joints. Composite Structures, 92(6), 2010, pp Mahini, S.S., Ronagh, H.R., Web-bonded FRPs for relocation of plastic hinges away from the column face in exterior RC joints. Composite Structures, 93(10), 2011, pp Liao, W.I., Loh, C.H., et al., Earthquake responses of RC moment frames subjected to nearfault ground motions. Structural Design of Tall Buildings, 10, 2001, pp Alavi, B., Krawinkler, H., Behavior of moment-resisting frame structures subjected to nearfault ground motions. Earthquake Engineering & Structural Dynamics, 33(6), 2004, pp Noor, F.A., Boswell, L.F., (Eds). Small scale modelling of concrete structures. London: Elsevier Applied Science, Ozdemir, G., Mechanical properties of CFRP anchorages. MSc thesis. Department of Civil Engineering: Middle East Technical University, Turkey, Paulay, T., Priestley, M.J.N., Seismic design of reinforced concrete and masonry buildings. New York: Wiley, ANSYS Manual. ANSYS, Inc ed: Canonsburg, PA 15317, USA; El-Mihilmy, M.T., Tedesco, J.W., Deflection of reinforced concrete beams strengthened with fiber-reinforced polymer (FRP) plates. ACI Structural Journal,,97(5), 2000, 14. Dalalbashi, A., Eslami, A., et al., Plastic hinge relocation in RC joints as an alternative method of retrofitting using FRP. Composite Structures, 94(8), 2012, pp Ronagh, H.R., Eslami, A., Flexural retrofitting of RC buildings using GFRP/CFRP A comparative study Composites Part B: Engineering, 46(0), 2013, pp