• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 춘계 학술발표회 논문집(II)
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• pp.33-36
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• 2006

In manufacturing Ultra high strength fiber reinforced concrete(UHSFRC), steel fiber, super-plasticizer and silica fume are important but they are imported materials therefore very expensive. consequently it is necessary to find substitutes of them or to develop producing techniques in order to manufacture UHSFRC economically. In this study, we investigated if it was possible to substitute blast-furnace slag instead of silica fume and steel fiber of home manufacture instead of one of overseas manufacture.

• 한국주조공학회지
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• 제32권1호
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• pp.32-37
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• 2012

In this study, microstructural evaluation was carried out on secondary hardening type ultrahigh strength steel, Fe-Co-Ni composition. This paper as a first part of whole research presented the microstructural behavior by cyclic heat treatment. The cyclic heat treatment method includes normalizing, stress relieving, solution treatment and aging. Especially, solution treatments performed triple times to get maximized solution hardening. Phase transformation and microstructure were observed by using optical microscope (OM), Electron back-scattered diffraction (EBSD) and X-ray stress analyzer. During the triple solution treatment, size of grain boundary was dramatically decreased by generating a packet from the martensite transformation of residual austenite in the inner part of grain, whereas the hardness increase was not significant.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2005년도 학술발표회 논문집
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• pp.211-219
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• 2005

Nowadays, various grades of high-strength steels are available. The application of ultra-high grades of steels for building structures, however, is limited only to the elements stressed under tension. The highest grade of steels generally used has a tensile strength of around 600N/mm2. Most research is focused on lower yield ratios of high strength steel in the inelastic range to ensure the stability of structures. In this paper, however, the possibility of an effective application of high strength steel with high yield ratio to building structures is discussed. An efficient structural system and a design method based on earthquake response analysis and experimental results are proposed.

• Computers and Concrete
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• 제26권5호
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• pp.451-465
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• 2020

This paper presents experimental and numerical investigations on mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) with four types of steel fibers; micro steel (MS), crimped (C), round crimped (RC) and hooked-end (H), in two fiber contents of 1% and 2% (by volume) and two lengths of 13 and 30 mm. Compression, direct tension, and four-point bending tests were carried out on four types of specimens (prism, cube, dog-bone and cylinder), to study tensile and flexural strength, fracture energy and modulus of elasticity. Results were compared with UHPC specimens without fibers, as well as with available equations for the modulus of elasticity. Specimens with MS fibers had the best performance for all mechanical properties. Among macro fibers, RC had better overall performance than H and C fibers. Increased fibers improved all mechanical properties of UHPFRC, except for modulus of elasticity, which saw a negligible effect (mostly less than 10%). Moreover, nonlinear finite element simulations successfully captured flexural response of UHPFRC prisms. Finally, nonlinear regression models provided reasonably well predictions of flexural load-deflection behavior of tested specimens (coefficient of correlation, R² over 0.90).

• International Journal of Concrete Structures and Materials
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• 제10권2호
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• pp.177-188
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• 2016

One of the primary concerns about the design aspects is that how to deal with the shear reinforcement in the ultra-high performance fiber reinforced concrete (UHPFRC) beam. This study aims to investigate the shear behavior of UHPFRC rectangular cross sectional beams with fiber volume fraction of 1.5 % considering a spacing of shear reinforcement. Shear tests for simply supported UHPFRC beams were performed. Test results showed that the steel fibers substantially improved of the shear resistance of the UHPFRC beams. Also, shear reinforcement had a synergetic effect on enhancement of ductility. Even though the spacing of shear reinforcement exceeds the spacing limit recommended by current design codes (ACI 318-14), shear strength of UHPFRC beam was noticeably greater than current design codes. Therefore, the spacing limit of 0.75d can be allowed for UHPFRC beams.

• Journal of Welding and Joining
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• 제34권6호
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• pp.28-34
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• 2016

Galvannealed(GA) steels are now generally used in car body manufacturing for corrosion resistance. In this study, the weldability and joint mechanical behavior of a newly developed 1.2GPa grade GA ultra high strength TRIP(transformation induced plasticity) steel was investigated for three joining processes, such as adhesive bonding, resistance spot welding and weldbonding. Under both shear and peel stress conditions, the failure mode of the adhesive joints were the mixture of the adhesive cohesive failure, adhesive interface failure and coating layer failure. It means that the adhesion strength of GA coating onto the base metal was similar to that of adhesive bonding onto the GA coating. Under the shear stress condition, the weldbonding exerted to expand the optimal spot welding condition of 1.2GPa GA TRIP steel because the strength of adhesive bond overwhelmed that of the resistance spot weld. Under the peel stress condition, the weldbonding also exerted to expand the optimal spot welding condition of 1.2GPa GA TRIP steel by inducing the tear fracture mode rather than the partial plug fracture mode.

• 동력기계공학회지
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• 제14권3호
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• pp.58-63
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• 2010

A laser beam welding and an electric arc welding were combined, and the positive points of each welding method are drawn such as high speed, low thermal load, deep penetration, and high productivity. The fiber laser-MIG conjugated welding. namely the hybrid welding has been studied mainly for the automation industry of a pipeline welding. In this study, the MIG welding was combined with a fiber laser welding to make up the hybrid welding. The weld shapes, microstructures and mechanical properties for weld zones after the hybrid welding or only fiber laser welding were investigated on the 700 MPa grade Ultra Fine Grained(UFG) high strength steel. The amount of acicular ferrite in weld metals and HAZ(heat affected zone) was observed larger after hybrid welding compared with after only laser welding. The Vickers hardness of the top area of the fusion zone after fiber laser welding was higher compared with after hybrid welding.

• Earthquakes and Structures
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• 제12권6호
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• pp.603-617
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• 2017

This paper presents investigation into the behavior of beam-column joints, with the joint region concrete being replaced by steel fiber reinforced concrete (SFRC) and by ultra-high performance concrete (UHPC). A total of ten beam-column joint specimens (BCJ) were tested experimentally to failure under monotonic and cyclic loading, with the beam section being subjected to flexural loading and the column to combined flexural and axial loading. The joint region essentially transferred shear and axial stresses as received from the column. Steel fiber reinforced concrete (SFRC) and ultra-high performance concrete (UHPC) were used as an innovative construction and/or strengthening scheme for some of the BCJ specimens. The reinforced concrete specimens were reinforced with longitudinal steel rebar, 18 mm, and some specimens were reinforced with an additional two ties in the joint region. The results showed that using SFRC and UHPC as a replacement concrete for the BCJ improved the joint shear strength and the load carrying capacity of the hybrid specimens. The mode of failure was also converted from a non-desirable joint shear failure to a preferred beam flexural failure. The effect of the ties in the SFRC and UHPC joint regions could not be observed due to the beam flexural failure. Several models were used in estimating the joint shear strength for different BCJ specimens. The results showed that the existing models yielded wide-ranging values. A new concept to take into account the influence of column axial load on the shear strength of beam-column joints is also presented, which demonstrates that the recommended values for concrete tensile strength for determination of joint shear strength need to be amended for joints subject to moderate to high axial loads. Furthermore, finite element model (FEM) simulation to predict the behaviour of the hybrid BCJ specimens was also carried out in an ABAQUS environment. The result of the FEM modelling showed good agreement with experimental results.

• 대한토목학회논문집
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• 제28권5A호
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• pp.623-629
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• 2008

일반적으로 각국의 강구조물 시공현장에서 사용되고 있는 고장력볼트의 인장강도는 1,000 MPa급이 주종을 이루고 있으나, 고강도강과 극후판의 개발 및 보급과 강교량 건설기술의 발전에 따른 교량 지간의 장대화로 인하여 강도가 큰 새로운 볼트 개발이 요구되어 왔다. 따라서, 본 연구에서는 새롭게 개발된 F13T급 고장력볼트를 이용하여 마찰계수에 따른 마찰연결부 피로실험을 수행하여 피로강도를 평가하였다. 결과적으로 피로실험결과 마찰연결부의 피로강도 B등급을 만족하였다. 또한, 마찰연결부의 피로파단에 대해서 분석하였다.

• 콘크리트학회논문집
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• 제28권2호
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• pp.223-234
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• 2016

이 연구에서는 노치 도입 인장시편을 사용하여 직접인장강도 실험을 통해 UHPC의 파괴거동을 살펴보고, 강섬유 혼입률에 따른 UHPC의 초기균열강도와 인장강도를 제안하였다. 실험결과 UHPC와 초기균열강도와 인장강도, 그리고 파괴에너지 등은 강섬유 혼입률이 증가할수록 증가하는 것으로 나타났다. 균열선단에서의 응집응력은 Barenblatt의 가정을 사용하여 결정되었으며, 이를 토대로 변형경화 현상이 발생하는 강섬유 혼입률이 1% 이상인 UHPC의 최대응집응력을 예측할 수 있는 간편식을 제안하였다. 인장강도는 강섬유 혼입률과 압축강도의 함수로 제안되었으며, 파괴에너지는 인장강도의 함수로 제안되었다. 제안된 간편식들은 실험값과 비교적 잘 일치하였으며, 향후 압축강도가 140~170 MPa이고, 강섬유 혼입률이 2% 이하인 UHPC에 적용가능 할 것으로 판단된다.