• 한국전산구조공학회논문집
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• 제35권5호
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• pp.287-297
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• 2022

본 논문에서는 철계형상기억합금(Fe SMA) 스트립으로 능동구속된 콘크리트 기둥의 실험적, 해석적 연구결과를 제시한다. Fe SMA과 탄소섬유보강시트(CFRP)로 각각 구속된 콘크리트 공시체의 압축실험을 통해 형상기억합금 기반 능동구속기법의 효과성을 평가하였다. 실험결과, Fe SMA 스트립으로 구속된 콘크리트 공시체가 낮은 구속력에도 불구하고 CFRP 시트로 구속된 공시체에 비해 더 우수한 변형능력을 가지는 것으로 밝혀졌다. 실험을 통해 얻은 구속된 콘크리트의 압축거동 결과를 이용해 소성힌지 영역이 각각 Fe SMA 스트립과 CFRP 시트로 보강된 콘크리트 기둥의 유한요소모델을 구축하였다. 기존 수행된 콘크리트 기둥의 수평반복가력 실험결과를 바탕으로 구축된 기둥 모델을 검증하였고, 각각의 기둥 모델에 대한 수평반복가력 해석을 수행하였다. 해석결과, Fe SMA 스트립으로 보강된 콘크리트 기둥이 CFRP 시트로 보강된 기둥모델에 비해 변형, 에너지 소산능력 향상에 효과적임을 확인하였다.

• 콘크리트학회논문집
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• 제28권5호
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• pp.535-544
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• 2016

본 연구에서는 디지털 이미지를 사용하여 DIC(Digital Image Correlation) 기법 및 부착파괴면 분석을 통해 부착파괴에너지와 부착강도의 정량적 분석뿐만 아니라 계면의 부착 면 파괴 양상의 정성적 접근을 통해 지속 하중과 온도의 복합 하중에 대한 FRP 부착 실험체의 거동을 분석하였다. 이를 위해 CFRP 쉬트를 부착한 일면전단실험체를 제작하여 사용하였다. 일면전단실험체의 지속 하중 기간의 거동은 에폭시 크리프의 영향을 상당히 받으며 지속 하중 기간 동안에 에폭시의 점탄성 특징으로 인해 응력완화가 발생하였다. 응력완화는 지속 하중 이후 실시한 계면전단실험에서 사용한 DIC 기법을 통해 관찰 하였으며 지속하중 기간 동안의 응력완화로 인해 지속하중 실험체의 최대부착파괴하중 및 계면파괴에너지가 대조실험체보다 증가하였다. 모든 실험체의 부착 파괴 면을 디지털 이미지화하여 파괴 면의 양상을 정성적/정량적으로 분석 하였다. 디지털 이미지 분석 결과 지속 하중 기간 동안 파괴 형태가 콘크리트면내파괴에서 계면부착파괴 형태로 전이가 발생하였으며 이러한 전이로 인해 지속하중 기간이 증가할수록 지속하중의 최대부착파괴하중에 대한 긍정적인 효과 감소하였다.

• 콘크리트학회논문집
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• 제16권6호
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• pp.867-873
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• 2004

It has been known that debonding failures between CFS(Carbon Fiber Sheet) and concrete in the strengthened RC beams are initiated by the peeling of the sheets in the region of combined large moment and shear forces, being accompanied by the large shear deformation after flexural cracks. These shear deformation effects are seldom occurred in small-scale model tests, but debondings due to the large shear deformation effects are often observed in a full-scale model tests. The premature debonding failure of CFS, therefore, must be avoided to confirm the design strength of full-scale RC beam in strengthening designs. The reinforcing details, so- called 'U-Shape fiber wrap at mid-span' which wrapped the RC flexural members around the webs and tension face at critical section with CFS additionally, were proposed in this study to prevent the debonding of CFS. Other reinforcing detail, so called 'U-Shape fiber wrap at beam end' were included in this tests and comparisons were made between them.

• 한국구조물진단유지관리공학회 논문집
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• 제23권3호
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• pp.17-24
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• 2019

탄소섬유보강폴리머(CFRP)는 경량이며, 성형성 및 작업성이 뛰어나 보수보강재료로서 널리 사용되고 있다. 하지만, 연성재료인 철근과는 달리 CFRP는 취성재료이므로, 철근에서 사용되는 전통적인 설계접근 방법을 적용하는 것은 부적합하다. 연성재료인 철근은 항복이후 요소사이의 응력재분배가 이뤄져 복합요소의 거동은 평균화된다. 따라서 복합요소의 응력 평균은 단위요소의 평균과 같고, 표준편차는 더 작아진다. 따라서 연성재료의 설계값은 증가시킬 수 있으나, 안전측, 실무적 접근에서 고정값을 사용한다. 반면 취성재료의 경우, 응력재분배를 기대하기 어려워 복합요소의 거동은 더 약한 요소에 의해 결정된다. 이에 복합요소의 응력의 평균값과 표준편차는 감소한다. 따라서 취성재료의 설계값은 요소수가 증가할수록 감소한다. 이 논문에서는 취성재료에서 정규분포를 가지는 단위요소가 요소 결합에 따라 와이블 분포를 가지게 됨을 증명하고, 이를 반영하여 하중이 작용하는 면적에 따른 물성치의 보정식을 제안하였다.

• Steel and Composite Structures
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• 제20권6호
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• pp.1275-1303
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• 2016

Steel plates and carbon-fiber-reinforced polymer (CFRP) laminates or plates bonded to concrete substrates have been widely used for concrete strengthening. However, this technique cause plate debonding, which makes the strengthening system inefficient. The main objective of this study is to enhance the bond strength of externally bonded steel plates and CFRP laminates to the concrete surface by proposing new embedded adhesive and steel connectors. The effects of these new embedded connectors were investigated through the tests on 36 prism specimens. Parameters such as interfacial shear stress, fracture energy and the maximum strains in plates were also determined in this study and compared with the maximum value of debonding stresses using a relevant failure criterion by means of pullout test. The study indicates that the interfacial bond strength between the externally bonded plates and concrete can be increased remarkably by using these connectors. The investigation verifies that steel connectors increase the shear bond strength by 48% compared to 38% for the adhesive connectors. Thus, steel connectors are more effective than adhesive connectors in increasing shear bond strength. Results also show that the use of double connectors significantly increases interfacial shear stress and decrease debonding failure. Finally, a new proposed formula is modified to predict the maximum bond strength of steel plates and CFRP laminates adhesively glued to concrete in the presence of the embedded connectors.

• Steel and Composite Structures
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• 제12권3호
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• pp.207-230
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• 2012

Nowadays CFRP (Carbon Fiber Reinforced Polymer) became widely used materials for the strengthening and retrofitting of structures. Many experimental and analytical studies are encountered at literature about strengthening beams by using this kind of materials against static loads and cyclic loads such as earthquake or wind loading for investigating their behavior. But authors did not found any study about strengthening of RC beams by using CFRP against low velocity impact and investigating their behavior. For these reasons an experimental study is conducted on totally ten strengthened RC beams. Impact loading is applied on to specimens by using an impact loading system that is designed by authors. Investigated parameters were concrete compression strength and drop height. Two different sets of specimens with different concrete compression strength tested under the impact loading that are applied by dropping constant weight hammer from five different heights. The acceleration arises from the impact loading is measured against time. The change of velocity, displacement and energy are calculated for all specimens. The failure modes of the specimens with normal and high concrete compression strength are observed under the loading of constant weight impact hammer that are dropped from different heights. Impact behaviors of beams are positively affected from the strengthening with CFRP. Measured accelerations, the number of drops up to failure and dissipated energy are increased. Finite element analysis that are made by using ABAQUS software is used for the simulation of experiments, and model gave compatible results with experiments.

• Steel and Composite Structures
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• 제29권4호
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• pp.465-481
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• 2018

The use of external carbon-fiber-reinforced polymer (CFRP) laminates is one of the most effective techniques existing for the confinement of circular concrete specimens. Currently, several researches have been made to develop models for predicting the ultimate conditions of this type of confinement. As most of the major existing models were developed based on limited experimental database. This paper presents the development of new confinement ultimate conditions, strength and strain models, for concrete cylinders confined with CFRP composites based on a statistical analysis of a large existing experimental database of 310 cylindrical concrete specimens wrapped with CFRP. The database is used to evaluate the performance of the proposed and major existing strength and strain models. Based on the two different statistical indices, the coefficient of determination ($R^2$) and the Root Mean Square Error (RMSE), the two proposed confinement ultimate conditions presents a good performance compared to the major existing models except the models of Lam and Teng (2003) and Youssef et al. (2007) which have relatively similar performance to the proposed models.

• Computers and Concrete
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• 제20권2호
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• pp.215-227
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• 2017

A finite element model (FEM) for predicting early-age behavior of reinforced concrete (RC) bridge deck slabs with fiber-reinforced polymer (FRP) bars is presented. In this model, the shrinkage profile of concrete accounted for the effect of surrounding conditions including air flow. The results of the model were verified against the experimental test results, published by the authors. The model was verified for cracking pattern, crack width and spacing, and reinforcement strains in the vicinity of the crack using different types and ratios of longitudinal reinforcement. The FEM was able to predict the experimental results within 6 to 10% error. The verified model was utilized to conduct a parametric study investigating the effect of four key parameters including reinforcement spacing, concrete cover, FRP bar type, and concrete compressive strength on the behavior of FRP-RC bridge deck slabs subjected to restrained shrinkage at early-age. It is concluded that a reinforcement ratio of 0.45% carbon FRP (CFRP) can control the early-age crack width and reinforcement strain in CFRP-RC members subjected to restrained shrinkage. Also, the results indicate that changing the bond-slippage characteristics (sand-coated and ribbed bars) or concrete cover had an insignificant effect on the early-age crack behavior of FRP-RC bridge deck slabs subjected to shrinkage. However, reducing bar spacing and concrete strength resulted in a decrease in crack width and reinforcement strain.

• Structural Engineering and Mechanics
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• 제54권3호
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• pp.521-543
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• 2015

Depending on the damage type as well as the level of damage observed after the earthquake, certain measures should be taken for the damaged buildings. In this study, structural repairing of two different types of damaged RC beam-column assembly by carbon fiber-reinforced polymer sheets is investigated in detail as a member repairing technique. Two types of 1:1 scale test specimens, which represent the exterior RC beam-column connection taken from inflection points of the frame, are utilized. The first specimen is designed according to the current Turkish Earthquake Code, whereas the second one represents a deficient RC beam-column assembly. Both of the specimens were subjected to cyclic quasistatic loading in the laboratory and different levels of structural damage were observed. The first specimen displayed a ductile response with the damage concentrated in the beam. However, in the second specimen, the beam-column joint was severely damaged while the rest of the members did not attain their capacities. Depending on the damage type of the specimens, the damaged members were repaired by CFRP wrapping with different configurations. After testing the repaired specimens, it is found that former capacities of the damaged members were mostly recovered by the application of CFRPs on the damaged members.

• Structural Engineering and Mechanics
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• 제56권6호
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• pp.899-916
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• 2015

Upgrading or strengthening of existing reinforced concrete (RC) infrastructure is an emerging demand nowadays. Near Surface Mounted (NSM) technique is very promising approach for flexural strengthening of RC members. However, premature failure such as concrete cover separation failure have been a main concern in utilizing this technique. In this study, U-wrap end anchorage with carbon fiber reinforced polymer (CFRP) fabrics is proposed to eliminate the concrete cover separation failure. Experimental programs were conducted to the consequence of U-wrap end anchorage on the flexurally strengthened RC beams with NSM-steel. A total of eight RC rectangular beam specimens were tested. One specimen was kept unstrengthened as a reference; three specimens were strengthened with NSM-steel bars and the remaining four specimens were strengthened with NSM-steel bars and U-wrap end anchorage using CFRP fabrics. A 3D non-linear finite element model (FEM) was developed to simulate the flexural response of the tested specimens. It is revealed that NSM-steel (with and without end-anchors) significantly improved the flexural strength; moreover decreased deflection and strains compared with reference specimen. Furthermore, NSM-steel with end anchorage strengthened specimens revealed the greater flexural strength and improve failure modes (premature to flexure) compared with the NSM-steel without end anchorage specimens. The results also ensured that the U-wrap end anchorage completely eliminate the concrete cover separation failure.