• 한국구조물진단유지관리공학회 논문집
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• 제19권4호
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• pp.49-56
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• 2015

초고성능 콘크리트는 높은 강도와 유동성을 갖는 우수한 재료 특성을 나타내는 콘크리트이다. 그러나 고연성 시멘트 복합체에 비하여 낮은 연성을 나타낸다. 이 연구에서는 강섬유와 마이크로섬유의 조합이 초고성능 콘크리트의 인장거동에 미치는 영향을 조사하였다. 이를 위하여 강섬유와 폴리에틸렌, 폴리비닐알코올, 현무암섬유 조합에 따라 4가지 초고성능 콘크리트 배합을 결정하였고, 인장거동을 평가하기 위하여 직접인장 실험을 수행하였다. 또한 마이크로섬유가 제조과정에서 의도하지 않은 과도한 기포를 생성하는지를 확인하기 위하여 밀도실험을 수행하였다. 실험결과 인장강도가 높은 폴리에틸렌섬유는 초고성능 콘크리트의 인장거동을 향상시키는데 효과적임을 확인하였고, 현무암섬유는 초고성능 콘크리트의 균열강도 및 인장강도를 증가시키는데 효과적임을 확인하였다. 또한 마이크로섬유가 의도하지 않은 기포를 생성하지 않는다는 것도 확인하였다.

• Steel and Composite Structures
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• 제50권3호
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• pp.319-336
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• 2024

Ultra-high-performance concrete (UHPC) has attracted increasing attention in prefabricated steel-concrete composite beams as achieving the onsite construction time savings and structural performance improvement. The inferior replacement and removal efficiency of conventional prefabricated steel-UHPC composite beams (PSUCBs) has thwarted its sustainable applications because of the widely used welded-connectors. Single embedded nut bolted shear connectors (SENBs) have recently introduced as an attempt to enhance demountability of PSUCBs. An in-depth exploration of the mechanical behavior of SENBs in UHPC is necessary to evidence feasibilities of corresponding PSUCBs. However, existing research has been limited to SENB arrangement impacts and lacked considerations on SENB geometric configuration counterparts. To this end, this paper performed twenty push-out tests and theoretical analyses on the shear performance and design recommendation of SENBs. Key test parameters comprised the diameter and grade of SENBs, degree and sequence of pretension, concrete casting method and connector type. Test results indicated that both diameters and grades of bolts exerted remarkable impacts on the SENB shear performance with respect to the shear and frictional responses. Also, there was limited influence of the bolt preload degrees on the shear capacity and ductility of SENBs, but non-negligible contributions to their corresponding frictional resistance and initial shear stiffness. Moreover, inverse pretension sequences or monolithic cast slabs presented slight improvements in the ultimate shear and slip capacity. Finally, design-oriented models with higher accuracy were introduced for predictions of the ultimate shear resistance and load-slip relationship of SENBs in PSUCBs.

• Steel and Composite Structures
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• 제35권4호
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• pp.463-474
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• 2020

In this paper, the influence of adding multi-walled carbon nanotube (MWCNT) on the pull behavior of steel and GFRP bars in ultra-high-performance concrete (UHPC) was examined experimentally and numerically. For numerical analysis, 3D nonlinear finite element modeling (FEM) with the help of ABAQUS software was used. Mechanical properties of the specimens, including Young's modulus, tensile strength and compressive strength, were extracted from the experimental results of the tests performed on standard cube specimens and for different values of weight percent of MWCNTs. In order to consider more realistic assumptions, the bond between concrete and bar was simulated using adhesive surfaces and Cohesive Zone Model (CZM), whose parameters were obtained by calibrating the results of the finite element model with the experimental results of pullout tests. The accuracy of the results of the finite element model was proved with conducting the pullout experimental test which showed high accuracy of the proposed model. Then, the effect of different parameters such as the material of bar, the diameter of the bar, as well as the weight percent of MWCNT on the bond behavior of bar and UHPC were studied. The results suggest that modifying UHPC with MWCNT improves bond strength between concrete and bar. In MWCNT per 0.01 and 0.3 wt% of MWCNT, the maximum pullout strength of steel bar with a diameter of 16 mm increased by 52.5% and 58.7% compared to the control specimen (UHPC without nanoparticle). Also, this increase in GFRP bars with a diameter of 16 mm was 34.3% and 45%.

• 한국산학기술학회논문지
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• 제22권5호
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• pp.81-88
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• 2021

최근 지진 빈도 증가로 구조물 건전도 모니터링 (SHM: Structural Health Monitoring, 이하 SHM) 시스템에 대한 관심이 증가하고 있다. Smart concrete는 전기-역학적 거동을 바탕으로 구조물 상태를 분석할 수 있는 기술이다. 하지만 콘크리트 구조물은 지진 시 정적 변형률 또는 하중 속도 보다 10배 이상 빠른 하중 속도가 작용하나 기존 연구 대부분은 정적 하중 속도에서의 감지 능력을 주로 조사하고 있다. 본 연구는 지진과 같이 높은 하중 속도에서 자가 응력감지 능력을 평가하기 위해 만능재료시험기 (UTM: Universal Testing Machine, 이하 UTM)를 사용하여 3가지 하중 재하 속도 (1, 4, 8 mm/min) 하에서 Smart Ultra High Performance Concrete (S-UHPC)의 전기-역학적 거동을 측정하였다. S-UHPC의 최대 압축 하중에서 Stress sensitive Coefficient (SC)는 1 mm/min 하중 속도 기준 -0.140%/MPa로 측정되었으나, 하중 속도가 각각 4, 8 mm/min으로 증가함에 따라 42.8 %, 72.7% 감소하였다. 전도성 재료의 변형 감소, 미세균열 증가로 인하여 S-UHPC의 감지능력이 하중속도 증가에 따라 감소하였지만, 그럼에도 불구하고 높은 하중 속도 하에서도 우수한 감지 성능을 보여 구조물 지진 하중 감지를 위한 SHM 시스템에 활용 가능함을 확인하였다.

• Computers and Concrete
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• 제18권1호
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• pp.139-154
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• 2016

Since the concrete strength around the reinforcement rebar affects the tension stiffening, the tension stiffening effect of ultra high performance concrete on the concrete members reinforced by steel rebar is examined by testing the specimens with circular cross section with the length 850 mm reinforced by a steel rebar at the center of a specimen's cross section in this research. Conducting a tensile test on the specimens, the cracking behavior is evaluated and a curve with an exponential descending branch is obtained to explain the post-cracking zone. In addition, this paper proposes an equation for this branch and parameters of equation is obtained based on the ratio of cover thickness to rebar diameter (c/d) and reinforcement percentage (${\rho}$).

• Structural Engineering and Mechanics
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• 제62권4호
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• pp.417-430
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• 2017

Ultra high performance concrete (UHPC) has aroused interest around the world owing to superior mechanical and durability properties over conventional concrete. However, the application of UHPC in practice poses difficulties due to its inherent brittleness. UHPC filled in steel tube columns (UHPC-FSTCs) are capable of restricting the brittle failure of non-reinforced UHPC columns and forming a high performance member with enhancement of strength and ductility. Currently, research on UHPC-FSTCs remains very limited and there is relatively little information about the mechanical behavior of these columns. Therefore, this study presents a review of past experimental studies to have a deeper insight into the compressive behavior of UHPC-FSTCs under axial loading on entire section and on concrete core. Based on the test results obtained from Schneider (2006) and Xiong (2012), an analysis was conducted to investigate the influence of the confinement index (${\xi}$) and diameter to steel tube thickness ratio (D/t) on the strength and the ductility in short circular UHPC-FSTCs. Furthermore, the appropriateness of current design codes including EC4, AISC, AIJ and previous analytical models for estimating the ultimate loads of composite columns was also examined by the comparison between the predictions and the test results. Finally, simplified formulae for predicting the ultimate loads in two types of loading pattern were proposed and verified.

• Advances in Computational Design
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• 제7권4호
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• pp.281-295
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• 2022

Tensile property is one of the excellent properties of ultra-high performance concrete (UHPC), and uniaxial tensile test is an important and challenging mechanical performance test of UHPC. Traditional uniaxial tensile tests of concrete materials have inherent defects such as initial eccentricity, which often lead to cracks and failure in non-test zone, and affect the testing accuracy of tensile properties of materials. In this paper, an original integrated design scheme of mould and end fixture is proposed, which achieves seamless matching between the tension end of specimen and the test fixture, and minimizes the cumulative eccentricity caused by the difference in the matching between the tension end of specimen and the local stress concentration at the end. The stress analysis and optimization design are carried out by finite element method. The curve transition in the end of specimen is preferred compared to straight line transition. The rationality of the new integrated design is verified by uniaxial tensile test of strain hardening UHPC, in which the whole stress-strain curve was measured, including the elastic behavior before cracking,strain hardening behavior after cracking and strain softening behavior.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 추계 학술발표회 논문집
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• pp.493-496
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• 2006

Ultra high performance concrete(UHPC) has an excellent strength, toughness, and durability. It seems that it is very efficiently applicable for various structures such as bridge, building. When it is used to bridge girder, It is possible to reduce the amount of concrete and steel, to cut down costs for construction. This paper estimated whether it was applicable and how it was efficient. It was confirmed that the height of girder could be reduced by 40% or more in using UHPC. We can also think that the stirrups can be removed considering the ductile tensile behavior of UHPC and that its very high compressive strength make the anchor plate smaller from this study.

• Advances in concrete construction
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• 제15권6호
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• pp.405-417
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• 2023

While ultra-high performance concrete (UHPC) is commonly reinforced with micro straight steel fibers in existing applications, studies have indicated that the use of deformed steel macro-fibers leads to enhanced ductility and post-peak responses for UHPC structural elements, which is of particular importance for earthquake-resistant structures. However, there are potential concerns regarding the use of UHPC reinforced with macro-fibers due to the issues of workability and fiber distribution. The objective of this study was to address these issues by extensively investigating the restricted and non-restricted deformability, filling ability, horizontal and vertical velocities, and passing ability of UHPC containing macro hooked-end steel fibers. A new approach is suggested to examine the homogeneity of fiber distribution in UHPC. The influences of ultra-fine fillers and steel macro-fibers on the workability of fresh UHPC and the mechanics of hardened UHPC were examined. It was found that although increasing the ratio of quartz powder to cement led to an improvement in the workability and tensile strain hardening behavior of UHPC, it reduced the fiber distribution homogeneity. The addition of 1% volume fraction of macro-fibers in UHPC improved workability, but reduced its compressive strength, which is contrary to the effect of micro-fiber inclusion in UHPC.

• Steel and Composite Structures
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• 제23권6호
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• pp.669-682
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• 2017

The use of ultra high performance concrete (UHPC) in composite columns offers numerous structural benefits, and has received recent research attention. However, the information regarding the behavior of steel tube confined concrete (STCC) columns employing UHPC has been extremely limited. Thus, this paper presents an overview of previous experimental studies on circular STCC columns with taking into account various concrete strengths to point out their distinctive features. The effect of the confinement factor and the diameter to thickness ratio on both strength and ductility in circular STCC columns employing UHPC was investigated. The applicability of current design codes such as EC4, AISC, AIJ and some available analytical models for concrete confined by steel tube was also validated by the comparison of ultimate loads between the prediction and the test results of Schneider (2006) and Xiong (2012). To predict the stress-strain curves for confined UHPC in circular STCC stub columns, a simplified model was proposed and verified by the comparison with experimental stress-strain curves.