Kyung-Hwan Min

Investigation of Microstructure and Mechanical Properties of KR60 Rail

The use of continuous welded rail is increasing because of its many advantages, including vibration reduction, enhanced driving stability, and maintenance cost savings. In this work, two different types of continuous welded rails were examined to determine the influence of repeated wheel-rail contact on the crystal structure, microstructure and mechanical properties of the rails. The crystal structure was determined by x-ray diffraction, and the microstructure was examined using optical microscopy and scanning electron microscopy. Tensile and microhardness tests were conducted to examine the mechanical behaviors of prepared specimens taken from different positions in the cross section of both newly manufactured rail and worn rail. Analysis revealed that both the new and worn rail had a mixed microstructure consisting of ferrite and pearlite. The specimens from the top position of each rail exhibited decreased lamella spacing of the pearlite and increased yield strength, ultimate tensile strength and hardness, as compared with those from other positions of the rail. It is thought that the enhanced mechanical property on the top position of the worn rail might be explained by a mixed effect resulting from a directional microstructure, the decreased lamella spacing of pearlite, and work hardening by the repeated wheel-rail contact stress.

A Study of Characteristics Change of Low-Shrinkage Normal Strength Concrete According to Mixing Factors and curing Temperature

This study examined the effects of the coarse aggregate maximum size and grading of fine aggregates to acquire the characteristics of very low shrinkage on normal strength concrete mixed in the field. In addition, the shrinkage characteristics of concrete under construction were evaluated in accordance with the curing temperature. The compressive strength and drying shrinkage tests were performed for nine mixing factors composed of the coarse aggregate size (13, 20, and 25 mm), types of fine aggregate (see sand, crushed sand, and blended sand), and curing temperatures (5, 20, and 35 °C). To acquire low shrinkage properties under 350 με strain on normal strength concrete, a 25 mm maximum of coarse aggregate was available, and the grading of fine aggregate affected the shrinkage of concrete. In addition, very low shrinkage properties were acquired in the curing temperature range except cold and hot weather concrete.

An Experimental Study for Longitudinal Resistance of Ballast Track on Bridge

Abstract When a ballast track of a high-speed train is constructed on a bridge, the displacement of the bridge deckscan occur because they are not fixed to the rails. Moreover, relative displacements occur between the bridge and railscaused by temperature changes and external loads. The current longitudinal resistance criteria (UIC Code 774-3, KRC-08080) on ballast tracks with continuous welded rails (CWRs) do not take into account the longitudinal movement of the bridge and the frictional force between the ballast and slabs. In addition, the magnitude of the longitudinalresistance, k, is calculated somewhat conservatively and, (therefore?) it acts as an unfavorable element in the design of long span and continuous railway bridges. Thus, in order to replicate the actual behavior more effectively, the longitudinal resistance of CWRs should take into account the additional rigidity between the slab and track. In this study, the longitudinal resistances of the ballasted track on the bridge were analyzed by carrying out an experimentalstudy with a test setup designed to simulate the deck and bed track. In the test results, the maximum longitudinalresistances of the tests were similar to the resistances of the current codes, however, the measured longitudinal stiffness designed to limit the displacement of the tests were much smaller in comparison with the longitudinal stiffness on the codes.

A Study of Shrinkage Characteristics of Low Shrinkage Normal Strength Concrete With Boundary Restraint Condition

Abstract In this study, the replacement effects of cementitious materials (fly ash, blast furnace slag, and blended mixtures) were assessed for normal strength concrete with very low shrinkage properties under 350 με strain using a powder type shrinkage reducing agent. In addition, through mock-up tests of actual size walls restrained with four sides, the shrinkage characteristics using the power type shrinkage reducing agent were measured and the crack reducing ability was assessed. The slump and air contents were measured as the properties of fresh concrete, and the length changes of the prismatic specimens, 100×100×400 mm in size, were measured for the shrinkage characteristics. To reduce the shrinkage of concrete, the maximum replacing ratio of the fly ash is effective to 20 percent; however, the use of blast furnace slag and ternary mixtures did not reduce the shrinkage.

Reduction of Drying Shrinkage of Mortar and Concrete by Expansion of Rapid Cooling Slag Fine Aggregate

It is necessary to maximize the durability of Concrete for the underground structure because its maintenance and reinforcement are difficult. For cracks due to drying shrinkage of the concrete on the characteristics of the material, there is a need for an alternative in the deterioration phenomenon that occurs. In this study, fundamental properties including drying shrinkage of mortar and concrete were investigated to replace fine aggregate from cooling slag for reducing drying shrinkage of mortar and concrete. In the case of rapid cooling slag fine aggregate, it was effective to reduce and restrain initial shrinkage of mortar and concrete, and compressive strength was increased through the all specimen in proportion to its replacement ratio.

Drying Shrinkage Behaviors of Concrete with Powder Type Shrinkage Reducing Agent and Fly Ash

Abstract In this study, series of tests were performed for drying shrinkage characteristics of concrete with powertype shrinkage reducing agent (SRA) and fly ash as a part of research to reduce drying shrinkage of concrete. Firstly,for the mechanical properties, a target strength was acquired securely. In the unrestraint shrinkage tests, the SRA decreased the drying shrinkage about 200 μe. Lastly, in the ring tests, due to the tensile creep effect, the concretewith SRA showed the cracking times as much again as the concrete with ordinary Portland cement only did. Key Words : Concrete, Drying shrinkage, Fly ash, Ring test, Shrinkage-reducing agent 본 논문은 전력산업융합원천기술개발(과제번호 : 20131010501790) 연구비 지원에 의해 수행되었음. * Corresponding Author : Dong-Gyu Lee(Chungcheong University)Tel: +82-43-230-2692 email: scarllet@ok.ac.krReceived April 7, 2015 Accepted May 7, 2015Revised May 6, 2015Published May 31, 2015 1. 서론 콘크리트 구조물은 각종 외력에 대해 강도를 확보해야 하고, 사용 연한 동안 일정한 내구성능이 확보되어야 한다. 콘크리트 구조물의 내구성능은 콘크리트 자체보다는 보강재로 사용하는 강재(steel)의 부식에 의한 영향이 훨씬 크다고 할 수 있다. 반면 최근 구조물이 대형화 되면서 내구성능의 저하에 따른 보수 보강 역시 많은 시간과 비용이 소모되므로, 설계 및 시공 시에 소요의 성능을 확보하여야 한다. 기존에는 균열의 방지를 위해 구조적인 방법론이 주로 사용되었지만, 콘크리트 혼화재료 분야의 급속한 기술발전으로 인해 재료적인 방법론이 제시되어, 보다 경제적인 설계와 시공이 가능하게 되었다[1,2,3]. 콘크리트에서 건조수축은 일정한 온도와 구속받지 않는 상태의 불포화된 공기 속의 콘크리트로부터 시간에 다른 부피의 변화로 정의할 수 있다. 콘크리트 구조물의 시공 후에 수개월간에 걸쳐 장기적으로 발생하는 건조수축은 구조물의 역학적 성능 및 내구성에 큰 영향을 미친다. 또한 이에 따라 사용 연한이 감소하고 많은 유지비용

A Study for Application of 180 MPa Ultra High Performance Concrete to Compressive Members

Abstract In the actual research fields, the studies for applications of 180 MPa ultra high performance concrete (UHFRCC) to compressive members are limited due to its very high compressive strength. In this study, in orderto find its practical use, UHPC was producted by using twin-shaft mixer batch plant. Also, to get basic research datafor the design specification of UHPC compressive members, a series of draft experiments, including short columnswith square and circular sections, were performed and its failure modes and behaviors were assessed. Keywords : Batch plant, Compressive performance, Concentrated load, Short column, Ultra high performance concrete 본 연구는 국토교통부 국토교통기술사업화지원 사업의 연구비지원(14AUDP-B069364-02)에 의해 수행되었음 * Corresponding Author : Kyung Hwan Min(Chungcheong University)Tel: +82-43-230-2678 email: alskh@ok.ac.krReceived June 2, 2015Accepted July 16, 2015Revised June 8, 2015Published July 31, 2015 1. 서론 국내에서는 건축구조물의 압축부재에 적용하기 위한 초고강도 콘크리트(ultra high-strength concrete)에 대한 연구가 1990년대 중반부터 이루어져왔다. 또한 교량 구조물의 상부에 적용하기 위한 노력의 일환으로 한국건설기술연구원을 필두로 한 180MPa급 초고성능 콘크리트(UHPC, ultra high performance concrete)의 실용화에 대한 연구가 2000년대 중후반부터 이루어지고 있다.[1] 초고성능 콘크리트(UHPC)를 압축 구조물에 적용하기 위한 연구는 국내외 모두 어느 정도 한계를 가지고 있는 실정이다. 국내외의 많은 연구에도 불구하고 특히 압축 부재로써 사용하기 위한 설계기준은 프랑스나 일본 모두 포함하지 못하고 있는 실정이다.[2] 이에 본 연구에서는 180MPa급 초고성능 콘크리트의 압축부재에의 적용성을 향상시키기 위한 연구로써, 배치 플랜트(batch plant)에서의 생산성을 검토하고, 압축부재를 제작하여 중심축하중 재하 실험을 수행하여, 파괴 형태와 거동을 평가하였다.