• Journal of the Korea Concrete Institute
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• v.17 no.6 s.90
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• pp.993-1000
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• 2005

Various types of FRP materials have been applied for structural strengthening of RC beams in various forms. When CFRP plates are used, a premature failure used to occur before enough strengthening effect appears. This is primarily due to the rip-off of CFRP Plate attached on RC beams. Despite of numerous studies on the rip-off failure of externally strengthened RC beams, its failure mechanism is not definitely clarified yet. Investigations from literatures have shown that the rip-off failure is dependant on the vertical and shear stresses at the level of main reinforcements in RC beams. This study suggests an analytical model to Investigate the rip-off failure load based on the stress states at the level of main reinforcements. The proposed model is relatively simple and produces very comparable results to the test data. It is believed that the proposed model can be successfully used to provide more information on the rip-off failure mechanisms and its prevention.

• Structural Engineering and Mechanics
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• v.32 no.5
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• pp.649-665
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• 2009

This paper reports part of a comprehensive research study conducted at the University of Queensland on the ability of CFRP web-bonded systems in strengthening an exterior beam-column joint subjected to monotonic loads. One 1/2.2 scaled plain and four CFRP repaired/retrofitted joints subjected to monotonic loads were analysed using the nonlinear finite-element program ANSYS and the results were calibrated against experiments. The ANSYS model was employed in order to account for tension stiffening in concrete after cracking and a modified version of the Hognestad's model was used to model the concrete compressive strength. The stress-strain properties of main steel bars were modelled using multilinear isotropic hardening model and the FRPs were modelled as anisotropic materials. A perfect bond was assumed as nodes were shared between adjacent elements irrespective of their type. Good agreement between the numerical predictions and the experimental observation of the failure mechanisms for all specimens were observed. Closeness of these results proved that the numerical analysis can be used by design engineers for the analysis of web-bonded FRP strengthened beam-column joints with confidence.

• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.645-652
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• 2002

This paper presents the flexural behavior and strengthening effect of reinforced concrete beams bonded with carbon FRP plate. Parameters involved in this experimental study were plate bond length and sheet web anchorage length. Test beams were strengthened with FRP plate on the soffit and anchored with FRP sheet on the web. In general, strengthened beams with no web anchorage were failed by concrete cover failure along the longitudinal reinforcement. On the other hand, strengthened beams with web anchorage were finally failed by delamination shear failure within concrete after breaking of CFRP sheet wrapping around web. The ultimate load and deflection of strengthened beams increased with an increased bond length of FRP plate. Also, the ultimate load and deflection increased with an increased anchorage length of FRP sheet. Particularly, the strengthened beams with web anchorage maintained high ultimate load resisting capacity until very large deflection. The shape of strain distribution of CFRP plate along beam was very similar to that of bending moment diagram. Therefore, an assumption of constant shear stress in shear span could be possible in the analysis of delamination shear stress of concrete. In the case of full bond length, the ultimate resisting shear stress provided by concrete and FRP sheet Increased with an increase of web anchorage length. In the resisting shear force, a portion of the shear force was provided by FRP anchorage sheet.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.41-44
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• 2008

Fiber Reinforced Polymers (FRPs) have recently been highly used for repairing and strengthening civil structures due to their superior material properties. By using FRPs, external bond method is the most used method in the construction industry. Although many researches on reinforced concrete externally strengthened with FRPs have been conducted, time-dependant behaviors of the beams have not been investigated yet. This paper documents a series of beam experiments that were carried out to evaluate time-dependant behavior. Three RC beams were built and the parameters considered in this experiment were types of FRPs. Through these experiments, it was proved that the beam strengthened with CFRP was more effective than that with GFRP in terms of time-dependant behavior.

• Journal of the Korean Society of Hazard Mitigation
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• v.8 no.4
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• pp.1-8
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• 2008

RC beam of existing structures often encounter shear problems for various reasons. The shear failure of RC beam is sudden and brittle. Strengthening technique jacketing with external bonding of steel plates(or CFRP and CFS) with epoxy is many use to in practice. This study presents test results on strengthening shear deficient RC beams by external bonding of slit type steel plates. Test parameters are width, interval, length, thickness and angle of slit in steel plates. The purpose was to evaluate the reinforcing effects, failure modes and shear capacities for RC beams of strengthened with various slit type steel plates. The test result confirmed that all slit steel plates improved the stiffness and strength of the specimens significantly. Failure modes of SV series and SD series showed shear fractures and flexure fractures at ultimate state respectively. SD series were ductile rather than SV series.

• Journal of the Korea Concrete Institute
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• v.20 no.5
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• pp.601-609
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• 2008

The purpose of this study is to investigate the flexural strengthening effectiveness for the beams combined reinforced with NSM CFRP strips and EBR CFRP strips. To accomplish this objective, a total of nine concrete T beams were tested. From this study, it is found that the flexural stiffness and strength of the beams combined reinforced with NSM and EBR strips were significantly improved compared to the beams strengthened only with NSM CFRP strip. The maximum increase of flexural strength was 347% compared to the beam without CFRP strip. Failure of the beam combined reinforced with NSM and EBR strips (T shape) is initiated by debonding of EBR strips attached on the bottom face, and it was succeeded a part of separatio-n of NSM strips along the longitudinal direction and secondly failure of NSM strips was occurred, eventually sudden explosive failure with the separation of concrete cover in the shear region. This result shows that the NSM and EBR strips have good combination to resist applied load and the combined reinforcement with NSM and EBR strips can redistribute appropriately the total stress subjected concrete beam to the EBR and NSM strips.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.11a
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• pp.145-148
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• 2006

Near surface mounted (NSM) is a recent strengthening technique based on bonding fiber reinforced polymer (CFRP) bars (rods and laminate strips), the use of NSM FRP bars is Emerging as a promising technology for increasing flexural strength of deficient concrete. In order for this technique to perform effectively, the structural behaviour of RC element strengthened with NSM FRP bars to be fully characterized. Totally, 10 beams were tested using symmetrical two-point loads test. The parameters examined under the beam test were a diffrent type of strengthening length.

• Structural Engineering and Mechanics
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• v.70 no.3
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• pp.303-310
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• 2019

Any revision of seismic codes usually demands a higher capacity from structural members, making existing structures unsafe particularly from strength considerations. Retrofitting of capacity deficient members is very suitable for tackling such situations. This paper presents an experimental study on different retrofitting measures adopted for strengthening a series of reinforced concrete (RC) beams. Four identical RC beam specimens were casted, out of which three specimens were strengthened by different schemes (viz., bolted hot rolled flat, bolted cold-formed steel channel, and carbon fibre reinforced polymer (CFRP) laminate, respectively) on their tension face and tested under four-point monotonic loading. This study focuses on the investigation of the flexural behaviour of these retrofitted beams, observed in terms of strength and stiffness. It was concluded that all retrofitting measures improved the structural performance of these beams. However, the cost involved with each strengthening mode was proportional to the improvement in the performance achieved.

• Earthquakes and Structures
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• v.15 no.5
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• pp.499-511
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• 2018

This paper aims to investigate the seismic performance of the prestressed steel strips retrofitted RC beam-column joints. Two series of joint specimens were conducted under compression load and reversed cyclic loading through quasi-static tests. Based on the test results, the seismic behavior of the strengthened joints specimens in terms of the failure modes, hysteresis response, bearing capacity, ductility, stiffness degradation, energy dissipation performance and damage level were focused. Moreover, the effects of the amount of the prestressed steel strips and the axial compression ratio on seismic performance of retrofitted specimens were analyzed. It was shown that the prestressed steel strips retrofitting method could significantly improve the seismic behavior of the RC joint because of the large confinement provided by prestressed steel strips in beam-column joints. The decrease of the spacing and the increase of the layer number of the prestressed steel strips could result in a better seismic performance of the retrofitted joint specimens. Moreover, increasing the axial compression ration could enhance the peak load, stiffness and the energy performance of the joint specimens. Furthermore, by comparison with the specimens reinforced with CFRP sheets, the specimens reinforced with prestressed steel strips was slightly better in seismic performance and cost-saving in material and labor. Therefore, this prestressed steel strips retrofitting method is quite helpful to enhance the seismic behavior of the RC beam-column joints with reducing the cost and engineering time.

• Journal of the Korea institute for structural maintenance and inspection
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• v.12 no.6
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• pp.89-96
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• 2008

The purpose of this study is to investigate the flexural strengthening effectiveness for the beams reinforced with NSM CFRP strips. To accomplish this objective, concrete T beams were made and tested. From this study, it is found that the flexural stiffness and strength of the beams reinforced with NSM strips were significantly improved compared to the beams without CFRP strip. The maximum increase of flexural strength was 247%. Failure of the beam reinforced with NSM was initiated by a part of separation of NSM strips along the longitudinal direction, and the second failure of strips was investigated. After the first rupture of the NSM strips, the load dropped suddenly and the second rupture was succeeded. This result shows that a perfect composite reaction with NSM strips and concrete is possible in the beam reinforced with NSM CFRP strips the NSM strips and Near surface mounted(NSM) is one of the most recent and promising strengthening techniques for reinforced concrete structures.