• International Journal of Concrete Structures and Materials
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• 제8권2호
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• pp.117-128
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• 2014

Numerous investigations of RC beams strengthened in shear with externally-bonded (EB) fibre-reinforced polymer (FRP) sheets, plates and strips have been successfully conducted in recent years. These valuable studies have highlighted a number of influencing parameters that are not captured by the design guidelines. The objective of this study was: (1) to highlight experimentally and analytically the influential parameters on the shear contribution of FRP to RC beams strengthened in shear using EB FRP sheets and strips; and (2) to develop a set of transparent, coherent, and evolutionary design equations to calculate the shear resistance of RC beams strengthened in shear. In the experimental part of this study, 12 tests were performed on 4,520-mm-long T-beams. The specimens were strengthened in shear using carbon FRP (CFRP) strips and sheets. The test variables were: (1) the presence or absence of internal transverse-steel reinforcement; (2) use of FRP sheets versus FRP strips; and (3) the axial rigidity of the EB FRP reinforcement. In the analytical part of this study, new design equations were proposed to consider the effect of transverse-steel in addition to other influential parameters on the shear contribution of FRP. The accuracy of the proposed equations has been verified in this study by predicting the FRP shear contribution of experimentally tested RC beams.

• Structural Monitoring and Maintenance
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• 제7권3호
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• pp.233-259
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• 2020

Using bonded fiber-reinforced polymer laminates for strengthening wooden structural members has been shown to be an effective and economical method. In this research, properties of suitable composite materials (sika wrap), adhesives and two ways of strengthening beams exposed to bending moment are presented. Passive or slack reinforcement is one way of strengthening. The most effective way of such a strengthening was to place reinforcement laminates in the stretched part of the wooden beam (lower part in our case), in order to investigate the effectiveness of externally bonding FRP to their soffits. The model is based on equilibrium and deformations compatibility requirements in and all parts of the strengthened beam, i.e., the wooden beam, the sika wrap composite plate and the adhesive layer. The theoretical predictions are compared with other existing solutions. This research is helpful for the understanding on mechanical behaviour of the interface and design of the composite-wooden hybrid structures. The results showed that the use of the new strengthening system enhances the performance of the wooden beam when compared with the traditional strengthening system.

• Steel and Composite Structures
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• 제48권5호
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• pp.599-610
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• 2023

To avoid debonding failure, a novel type of hybrid anchorage (HA) is proposed in this study that uses a slotted plate to lock the ends of the fiber-reinforced polymer (FRP) sheet in addition to the usual bonding over the substrate of the strengthened member. An experimental investigation was performed on three groups of RC beams, which differed from one another in either concrete strength or steel reinforcement ratio. The test results indicate that the end self-locking of the CFRP sheet can improve the failure ductility, ultimate capacity of the beams and its utilization ratio. Although intermediate debonding occurred in all the strengthened beams, it was not a fatal mode of failure for the three specimens with end anchorage. Among them, FRP rupture occurred in the beam with higher concrete strength and lower steel reinforcement ratio, whereas the other two failed by concrete crushing. The beam strengthened by HA obtained a relatively high percentage of increase in ultimate capacity when the rebar ratio or concrete strength decreased. The expressions in the literature were inspected to calculate the critical loads at intermediate debonding, FRP rupturing and concrete crushing after debonding for the strengthened beam. Then, the necessity of further research is addressed.

• Structural Engineering and Mechanics
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• 제88권1호
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• pp.67-81
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• 2023

The reinforced concrete (RC) structures might need strengthening or upgradation due to adverse environmental conditions, design defects, modification requirements, and to prolong the expected lifespan. The RC beams have been efficiently strengthened using the near surface mounted (NSM) approach over the externally bonded reinforcing (EBR) system. In this study, the performance of RC beam elements strengthened with NSM-steel rebars was investigated using an experimental program and nonlinear finite element modeling (FEM). Nine medium-sized, rectangular cross-section RC beams total in number made up for the experimental evaluation. The beams strengthened with varying percentages of NSM reinforcement, and the number of grooves was assessed in four-point bending experiments up to failure. Based on the experimental evaluation, the load-displacement response, crack features, and failure modes of the strengthened beams were recorded and considered. According to the experimental findings, NSM steel greatly improved the flexural strength (up to about 84%) and stiffness of RC beams. The flexural response of the tested beams was simulated using a 3D non-linear finite element (FE) model. The findings of the experiments and the numerical analysis showed good agreement. The effect of the NSM groove and reinforcement on the structural response was then assessed parametrically.

• Structural Engineering and Mechanics
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• 제83권4호
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• pp.415-433
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• 2022

Eighteen (18) (120×300×2200 mm) beams were prepared and tested to evaluate the shear strength of Glass Fiber Reinforced Concrete (GFRC) beams with no shear reinforcement, and evaluate the effectiveness of various innovative strengthening systems to increase the shear capacity of the GFRC beams. The test variables are the amount of discrete glass fiber (0.0, 0.6, and 1.2% by volume of concrete) and the type of longitudinal reinforcement bars (steel or GFRP), the strengthening systems (externally bonded (EB) sheet, side near-surface mounted (SNSM) bars, or the two together), strengthening material (GFRP or steel) links, different configurations of NSM GFRP bars (side bonded links, full wrapped stirrups, side C-shaped stirrups, and side bent bars), link spacing, link inclination angle, and the number of bent bars. The experimental results showed that adding the discrete glass fiber to the concrete by 0.6%, and 1.2% enhanced the shear strength by 18.5% and 28%, respectively in addition to enhancing the ductility. The results testified the efficiency of different strengthening systems, where it is enhanced the shear capacity by a ratio of 28.4% to 120%, and that is a significant improvement. Providing SNSM bent bars with strips as a new strengthening technique exhibited better shear performance in terms of crack propagation, and improved shear capacity and ductility compared to other strengthening techniques. Based on the experimental shear behavior, an analytical study, which allows the estimation of the shear capacity of the strengthened beams, was proposed, the results of the experimental and analytical study were comparable by a ratio of 0.91 to 1.15.

• 한국농공학회논문집
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• 제48권1호
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• pp.61-68
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• 2006

The p-version nonlinear finite element model has been developed to analyze the nonlinear behavior of simply supported RC slabs strengthened with carbon fiber reinforced plastic sheets. The shape function is adopted with integral of Legendre polynomials. The compression model of concrete is based on the Kupfer's yield criterion, hardening rule, and crushing condition. The cracking behavior is modeled by a smeared crack model. In this study, the fixed crack approach is adopted as being geometrically fixed in direction once generated. Each steel layer has a uniaxial behavior resisting only the axial force in the bar direction. Identical behavior is assumed fur tension and compression of steel according to the elastic modulus. The carbon fiber reinforced plastic sheets are considered as reinforced layers of equivalent thickness with uniaxial strength and rigidity properties in the present model. It is shown that the proposed model is able to adequately predicte the displacement and ultimate load of nonlinear simply supported RC slabs by a patch with respect to reinforcement ratio, thickness and angles of CFRP sheets.

• Advances in concrete construction
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• 제13권 2호
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• pp.191-198
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• 2022

FIB is introduced as the sole guideline for the design purpose that results in a practical relationship for the torsional capacity of concrete beams strengthened with carbon fiber-reinforced polymer (CFRP). This study applies first-order reliability method to assess the reliability evaluation of the torsional capacity of CFRP-strengthened beams on the basis of FIB guidelines. In terms of steel reinforcement losses, this study applies a corrosion model to investigate the ceaseless deterioration of the existing structure. Hence, the average of reliability indices varies between 2.68 and 2.80, indicating the reliability viewpoint of the design methodologies. The average values are somehow low compared to the target values of reliability (3.0 or 3.5) applied in the calibration stage of the FIB guideline. In this way, the partial safety factors may change in the forthcoming guideline revisions. For this aim, the reliability of strengthening ratio was applied to assess the variation in the average value of the reliability index with different partial safety factors. The performance of parametric study for the factor proved that minimum values of 1.60 and 2.32 are required for target values of reliability (3.0 and 3.5), respectively.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 추계 학술발표회 제20권2호
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• pp.253-256
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• 2008

복합섬유(Fiber Reinforced Polymers)가 외부 부착된 철근콘크리트 보에 관한 연구는 국내 및 세계 여러나라에서 활발히 진행되고 있으나 장기 거동에 관한 연구는 많이 미흡한 실정이다. 본 연구에서는 현재 국내에서 많이 사용되고 있는 탄소섬유(Carbon Fiber Reinforced Polymers)와 유리섬유(Glass Fiber Reinforced Polymers)를 철근 콘크리트 보에 외부 부착시킨 후 300일간 지속하중하에서 철근의 변형률, FRP의 변형률을 측정함으로써 FRP 보강 실험체의 시간의존적 거동을 파악하였다. 또한, Adjusted Effective Modulus Method, (AEMM)과 Ghali and Farve의 방법을 사용하여 시간경과에 따른 크리프와 건조수축에 의한 응력과 변형률의 변화를 예측하였다. 실험결과, RC보의 휨 보강 측면으로 보았을 때, CFRP가 GFRP보다 장기거동에 있어 우수한 성능을 보이는 것으로 나타났으며, 이론 식으로 산정된 값은 무보강 실험체의 철근 변형률의 경우 비교적 유사하게 예측하고 있으나 FRP로 보강된 실험체들의 인장철근은 다소 과대평가하는 결과를, 압축철근은 과소평가하는 결과를 나타냈다.

• 콘크리트학회논문집
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• 제15권6호
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• pp.769-777
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• 2003

최근 철근콘크리트(RC) 구조물의 보강기법으로 큰 인장강도와 탄성계수를 갖는 탄소섬유계열 보강재를 사용한 보강공법 개발과 이와 관련된 많은 실험 및 이론적 연구가 진행되고 있다. CFS 및 강판 보강재에 의한 보강시 야기되는 문제점을 해결하기 위하여 대체 재료로 국내에 도입된 탄소섬유메쉬(CFM)를 실 구조물의 보강공사에 적용하기 위하여 우선적으로 CFM을 이용한 보강기법 및 보강된 부재의 구조성능 규명에 대한 연구가 요구되고 있다. 본 연구에서는 탄소섬유메쉬와 콘크리트의 부착특성 규명을 위한 실험적 연구를 수행하였다. 일반적으로 탄소섬유 부착 보강공법은 보강재와 기존 부재와의 부착성능에 의해 보강효과가 지배받게 된다. 즉 부착강도가 충분한 보강효과를 기대할 수 없을 경우의 부착파괴의 가장 큰 원인으로는 계면에서의 전단강도에 기인한다고 할 수 있다. 따라서 본 연구에서는 CFM을 콘크리트에 부착하는데 있어서 정착철물(Clip)의 설치 유무, 정착철물의 정착위치, 정착철물의 설치 열 수, 부착 모르타르의 바름 유무, 부착 모르타르의 바름 두께 등의 실험 변수를 설정하고 인장전단 실험을 수행하였다. 실험결과 적절한 정착철물의 부착위치 및 정착철물 및 부착 모르타르의 부착특성을 규명할 수 있었다. 또한 본 연구에서는 범용 비선형 유한요소 해석 프로그램인 ABAQUS를 이용하여 CFM의 부착특성을 규명하기 위한 유한요소 모델 및 해석기법을 개발하였고 이를 실험결과와 비교하여 이에 대한 검증을 하였다.

• 콘크리트학회논문집
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• 제21권4호
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• pp.513-521
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• 2009

최근 들어 외부 강선을 이용한 프리스트레스트 콘크리트구조물의 건설이 증가하고 있다. 그러나 극한거동 해석시 단면 적합조건을 이용하는 내부 부착 강선과는 다르게 외적 비부착 강선은 부재의 전체거동에 의해서 응력 증 가량이 결정된다. 또한 편향부에서의 미끌림 효과와 강선의 편심 변화 효과 등이 발생하게 된다. 따라서 본 연구는 외 부 강선을 가지는 프리스트레스트 콘크리트(PSC) 보의 거동 특성을 평가하기 위하여 강선량, 긴장력 등을 변수로 하여 정적 휨실험을 수행하여 외부 강선 부재의 휨거동 특성을 얻었다. 실험 결과에 의하면 균열발생 이전의 외부 강선 PSC 부재는 부착 강선 PSC부재와 거동 차이가 크지 않음을 알 수 있었다. 그러나 균열이 발생한 이후의 거동에서는 철근의 항복하중, 극한하중, 강선 응력 등이 외부 강선 부재에서의 값이 부착 강선 부재에 비해서 작게 나타나고 있다. 하중-강 선 변형률 관계에서 보면, 외부 강선 부재들의 경우, 하중 증가에 따른 외부 강선 변형률의 증가량은 초기 긴장력의 크 기 순서와 거의 반대의 순서로 외부 강선의 변형률이 증가한 것으로 나타났으나, 다만 초기 긴장력이 크다할지라도 강 선의 유효응력이 작은 경우의 강선 변형률은 강선의 유효응력이 큰 부재들보다는 다소 작게 증가하고 있는 것으로 나 타났다. 외부 강선 부재의 콘크리트에 발생된 압축 변형률의 크기는 외부 강선의 유효응력 크기 순서와 일치하는 것으 로 나타나, 콘크리트의 압축변형률은 외부 강선의 유효응력에 비례함을 알 수 있다. 실험 결과와 기존의 설계식과 비교 해본 결과, ACI-318에 의한 결과는 긴장력 혹은 유효응력이 차이를 전혀 반영하지 못하고 있고 특히, 그 결과가 실험 결과보다 상당히 작게 나타나, 지나치게 보수적인 것으로 판단된다. 한편 AASHTO 1994는 ACI-318과는 다르게 강선량, 초 기 힘 및 유효응력 등의 변화에 적절하게 영향을 받고 있는 것으로 나타났지만 내부 비부착 강선의 실험 결과를 이용 하여 작성된 이유로 외부 강선 실험 결과보다 지나치게 큰 결과를 유발하고 있는 것으로 평가된다. 이러한 이유로 외 부 강선의 극한응력을 정확하게 예측할 수 있는 새로운 규정이 필요하다.