• Journal of the Korea Concrete Institute
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• v.20 no.3
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• pp.383-391
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• 2008

The effects of concrete strength on bond-slip behavior and the failure mechanisms of glass fiber reinforced polymer (GFRP) bar embedded in concrete under direct pullout were investigated in this study. Total of twenty seven specimens were prepared by placing two different types of GFRP bars and conventional steel rebar in 25 MPa, 55 MPa, and 75 MPa concrete and tested according to CSA S806-02. The test results showed that the bond strength of the GFRP rebars as well as the steel increased with the concrete strength. However, the increase in the bond strength with respect to the concrete strength was not as significant in the GFRP series as the steel, and it was attributed to the interlaminar failure mechanism observed in the GFRP test specimens.

• Journal of the Korea Concrete Institute
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• v.28 no.5
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• pp.535-544
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• 2016

This paper aims at examining the effects of sustained load and elevated temperature on the time-dependent deformation of a carbon fiber reinforced polymer (CFRP) sheets bonded to concrete as well as the pull-off strength of single-lap shear specimens after the sustained loading period using digital images. Elevated temperature during the sustained loading period resulted in increased slip of the CFRP composites, whereas increased curing time of the polymer resin prior to the sustained loading period resulted in reduced slip. Pull-off tests conducted after sustained loading period showed that the presence of sustained load resulted in increased pull-off strength and interfacial fracture energy. This beneficial effect decreased with increased creep duration. Based on analysis of digital images, results on strain distributions and fracture surfaces indicated that stress relaxation of the epoxy occurred in the 30 mm closest to the loaded end of the CFRP composites during sustained loading, which increased the pull-off strength provided the failure locus remained mostly in the concrete. For longer sustained loading duration, the failure mode of concrete-CFRP bond region can change from a cohesive failure in the concrete to an interfacial failure along the concrete/epoxy interface, which diminished part of the strength increase due to the stress relaxation of the adhesive.

• Proceedings of the Korea Concrete Institute Conference
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• 2010.05a
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• pp.73-74
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• 2010

The objective of this research is to improve the flexural capacity of RC Beams. To delay prematured tension failure of concrete specimen and to improve flexural capacity of RC beam by increasing the contribution of FRP strengthening plates, a method for inserting a laminate to the interface between concrete and FRP materials. This method makes it possible to increase overall flexural performance of RC beam by FRP plate compared to normal RC beams and RC beam strengthened by bonded FRP plates. The new bonding technique is applicable to all types of reinforcement available FRP laminate, and in principle is also applicable to materials other than FRP.

• International Journal of Highway Engineering
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• v.6 no.1 s.19
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• pp.47-56
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• 2004

To determine the pure bond strength between substrate and its overlayed concrete material, a direct pull-off test method was introduced by using a bi-material cylinder with which a penny-shaped crack was mountained at its interface. First, to evaluate the stress magnification or concentration at the interface, the energy release rates of a penny-shaped interface crack in remote tension loading on a bi-material cylinder were determined in terms of different modulus ratios and undonded area ratios(crack ratios) using a commercial finite element program. Then the energy release rates were calibrated as non-dimensional values in consideration of structural dimensions and applied forces. And to evaluate whether this new pull-off test method gives sound test results, three different sizes of unbended area ratios were incorporated along their interface in bi-material cylinders(sulphur polymer concrete + old concrete). Test results showed that all specimens were broken off at their interfaces as intended. Also the FEM analyses and test results indicated that a bi-material specimen with unbended area ratio of 0.4$\sim$0.6 is suitable for best accurate testing.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.311-314
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• 2011

최근 콘크리트 구조물의 안전성강화를 위해 탄소섬유 강화 플라스틱(CFRP) 보강 공법이 널리 사용되고 있지만 잘 알려진 바와 같이 CFRP 보강재와 콘크리트 표면사이의 부착면 탈락은 보강재 자체의 손상보다 발생할 확률이 높고 이러한 부착면 탈락은 보강의 효과를 무의미하게 만들기 때문에 구조물 전체의 파괴로 직결될 수 있다. 이에 본 연구에서는 CFRP 부착면 탈락손상을 실시간으로 검색하기 위해 압전센서를 사용하는 구조물 건전성 평가 기술을 적용하였다. 이의 검증을 위해 CFRP로 보강된 콘크리트 보를 제작하였고 3단계로 증가하는 부착면 탈락 손상을 발생시켰다. 손상 증가 단계마다 CFRP 표면에 배열된 압전센서로부터 임피던스와 유도초음파 신호를 계측하였고 손상에 따른 신호변화를 정량화하기 위해 손상지수인 RMSD를 계산하였다. 더 구체적인 부착면 탈락 손상위치 탐색을 위해서 두 가지 계측 기법으로부터 구해진 RMSD 값를 중첩시키는 Superposed RMSD 가 제안되었다. 구해진 Superposed RMSD 값을 사용하여 커브 피팅이 수행되었고 도출된 커브의 최고값에 해당하는 위치값을 찾아 실제 손상위치와 비교함으로써 제안된 기법의 가능성을 검증해 보았다.

• 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.

• Journal of the Korea Concrete Institute
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• v.23 no.6
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• pp.781-790
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• 2011

This paper contains the experimental results on strengthening effect of RC beams strengthened with NSM CFRP reinforcement and various strengthening details. A total of 14 beams have been tested to analyze strengthening effects of NSMR with various reinforcement details. Variables were cross-sectional shape of CFRP reinforcements, strengthening areas, grooves the number and location etc. Test results revealed that failure modes of NSMR showed two types. One was bond failure at interface between concrete and filler and the other was CFRP rupture. Also, failure mode of specimens with two grooves occurred premature bond failure because of superposition of failure surfaces at concrete around grooves. failure mode of MI specimens considered the equivalent section have changed bond failure to CFRP rupture and CFRP efficiency has improved 83% to 100%.

• Journal of the Korea Concrete Institute
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• v.13 no.3
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• pp.221-227
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• 2001

Both strength and stiffness of RC structures strengthened by a steel plate greatly increase and however, their ductility might not be sufficient because premature failures usually occur at the adhesive-concrete interface. In this study, Mohr-Coulomb criterion was adopted to examine the bonding failure mechanism, and the diagonal shear bonding test, the direct shear bonding test, and the flexural test on RC beams strengthened by a steel plate were carried out to measure the bonding properties. It is found from the experimental and numerical results that the cohesive strengths of epoxy-concrete interfaces are ranging from 50 kgf/㎠ to 70 kgf/㎠ when the friction angle is 45°. Bonding failure loads can be predicted by applying the bonding properties to the structural analysis of RC beams strengthened by steel plate. By considering them in the design of strengthened beams, the premature failure would be effectively prevented.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.2
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• pp.198-207
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• 1986

This study investigates interlaminar fracture characteristics of Graphite/Epoxy composite (HFG Graphite/Epoxy) under mode I (opening mode), mode II (sliding mode) and mixed mode loading conditions. The effects on interlaminar fracture toughness due to different fiber orientations on the crack surface are also investigated. The antisymmetric test fixture proposed by M. Arcan is used for this test. Both critical stress intensity foctors and critical energy release rates were determined and several mixed mode fracture criteria were compared to the experimental data. Also fracture surfaces were investigaed to obtain informations on the fracture behaviors of Graphite/Epoxy composite by means of a scanning electron microscope(SEM).

• Journal of the Korean GEO-environmental Society
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• v.11 no.3
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• pp.23-32
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• 2010

In this study, Generally sandbag was used to reinforce slope or restore levee by using the in-situ material. To increase shear strength of sandbag, the Velcro system was effective for geosynthetic interface and make up for the weakness of shear strength between sandbag to sandbag. In this study, shear properties of geosynthetic-geosynthetic and geosynthetic-soil were evaluated from large scale direct shear tests. The cohesion and the angle of internal friction of each interface was evaluated. And laboratory model tests were performed to compare strength of reinforcement with strength of none reinforcement. As a result of this study, the cohesion and the angle of internal friction of each interface was increased, especially the cohesion was increased more than the angle of internal friction. Also according to the result of model test, the bearing capacity was increased by 20%.