• Proceedings of the Korean Society of Agricultural Engineers Conference
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• 2003.10a
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• pp.375-378
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• 2003

Plastic shrinkage cracking occurs at the exposed surfaces of freshly placed concrete due to consolidation of the concrete mass and rapid evaporation of water from the surface. This so-called shrinkage cracking is a major concern for concrete, especially for flat structural such as pavement, slabs for industrial factories and walls. This study has been performed to obtain the plastic shrinkage properties of hydrophilic fiber reinforced mortar and concrete. The results of tests of the hydrophilic fibers were compared with plain and polypropylene fibers. Test results indicated that hydrophilic poly vinylalcohol fiber reinforcement showed an ability to reduce the total crack area and maximum crack width significantly (as compared to plain and polypropylene fiber reinforcement).

• Structural Engineering and Mechanics
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• v.23 no.6
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• pp.713-737
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• 2006

This paper presents the results of an experimental investigation on the behaviour of 56 reinforced concrete beams subjected to pure torsion. The reported results include the behaviour curves, the failure modes and the values of the pre-cracking torsional stiffness, the cracking and ultimate torsional moments and the corresponding twists. The influence of the volume of stirrups, the height to width ratios and the arrangement of longitudinal bars on the torsional behaviour is discussed. In order to describe the entire torsional behaviour of the tested beams, the combination of two different analytical models is used. The prediction of the elastic till the first cracking part is achieved using a smeared crack analysis for plain concrete in torsion, whereas for the description of the post-cracking response the softened truss model is used. A simple modification to the softened truss model to include the effect of confinement is also attempted. Calculated torsional behaviour of the tested beams and 21 beams available in the literature are compared with the experimental ones and a very good agreement is observed.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.11a
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• pp.323-326
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• 2005

Although many structures. with high strength concrete have been recently constructed, the flexural behavior of reinforced and prestressed concrete beams with high strength concrete is not exactly defined. This paper presents an experimental study on the flexural strength of the high strength concrete beams. Five large scale beams simply supported were tested and measured. Each beam was loaded by two symmetrical concentrated loads applied at 1.25m from the center of span. The concrete strength, the prestressed force and longitudinal tensile reinforcement ratio vary from beam to beam. From the experimental tests, the flexural strength from tests is larger than the nominal flexural strength of codes. Moreover, the initial crack-load is affected by the prestressed force and the crack width and spacing are controlled by the longitudinal tensile reinforcement ratio.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.151-154
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• 2002

The objective of this study is to find the change of signal characteristics of matrix cracks due to the different specimen shapes. As the concept of the smart structure, monitoring of acoustic emission (AE) can be applied to inspect the fracture of the structures in operating condition using built-in sensors. To understand the characteristics of matrix crack signals, we performed tensile tests by changing the thickness and width of the specimens. This paper describes the implementation of time-frequency analysis such as wavelet transform (WT) fur the quantitative evaluation of fracture signals. The experimental result shows the distinctive signal features in frequency domain due to the different specimen shapes.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.04a
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• pp.751-756
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• 2000

The structural behavior of reinforced concrete slabs strengthened by glass fiber reinforced plastic-panels experimentally investigated. The experimental variables are strengthening length, strengthening width, and pre-crack existence. The pre-cracked slabs are initially loaded to 70 percent of ultimate flexural capacity and subsequently repaired with GFRP-Panels bonded to the tension face of the slabs. Five one-way slabs were tested to failure. The main failure mode of strengthened slabs is separation failure by crack propagation from load point section to end of plate. The behavior of strengthened slabs is represented by a maximum load, load-deflection curves an load-strain curves.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.5
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• pp.411-418
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• 2008

In this work, the effect of strength mismatch on plastic limit loads is quantified for similar metal weld plates with cracks under tension load, via three-dimensional, small strain elastic-perfectly plastic finite element analyses. Relevant variables related to plate geometry and crack length are systematically varied, in addition to the weld width. An important finding is that mis-match limit loads can be uniquely quantified through strength mis-match ratio and one geometry-related parameter. Based on the proposed limit load solutions, reference stress based J-integral estimates is also investigated. When the reference stress is defined by the mis-match limit load, predicted J-integral values agree overall well with FE results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.3
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• pp.259-266
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• 2004

Detailed two-dimensional and three-dimensional finite element (FE) analyses of double-edge cracked tension (DE(T)) specimens are carried out to investigate the effect of the relative crack length and the thickness on experimental J testing schemes. Finite element analyses involve systematic variations of relevant parameters, such as the relative crack depth and plate width-to-thickness ratio. Furthermore, the strain hardening index of material is systematically varied, including perfectly plastic (non-hardening) cases. Based on FE results, a robust experimental J estimation scheme is proposed.

• Journal of Welding and Joining
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• v.19 no.3
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• pp.298-304
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• 2001

Turbine blade is subject to force of three types ; the torsional force by torsional mount, the centrifugal force by the rotation of rotor and the cyclic bending force by steam pressure. The cyclic bending force was a main factor on fatigue strength. SEM fractography in root of turbine blade showed micro-clack width was not dependent on stress intensity factor range. Especially, fatigue did not exist on SEM photograph in root of turbine blade. To clear out the fracture mechanism of turbine blade, nanofractography was needed on 3-dimensional crack initiation and crack growth with high magnification. Fatigue striation partially existed on AFM photograph in root of turbine blade. Therefore, to find a fracture mechanism of the torsion-mounted blade in nuclear power plant, the relation between stress intensity factor range and surface roughness measured by AFM was estimated, and then the load amplitude ΔP applied to turbine blade was predicted exactly by root mean square roughness.

• Proceedings of the Korea Concrete Institute Conference
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• 1992.04a
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• pp.126-130
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• 1992

Recently, fatigue problem become a critical issue in the design of prestressed concrete bridges due to the increase of traffic volumes and use of high-strength materials. Most existing studies are mainly concerned with the fatigue behavior of component materials only such as concrete, reinforcing bars, and prestressing steels and few studies exist that deals with the fatigue behavior of bridge members. An improved analytic formulation for both uncracked and cracked prestressed concrete composite section with cyclic creep effect is developed to take into account the change of neutral axis with crack propagation. The procedure also enables to investigate serviceability limit states, deflection and crack width. The present study allows more realistic analysis and design of prestressed concrete composite girder bridges under fatigue loadings.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10b
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• pp.1205-1210
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• 2000

In this research we made the mean cement particle size 4 $\mu\textrm{m}$ which can penetrate even minor cracks based on the theory of J.K. Michel who reported particles can penetrate the crack of width up to 3 times of maximum particle size. The cement slurries were produced by adding super plasticizer. The slurries were tested with slurry characterization methods and its rheological properties were characterized. The early hydrated phenomena of ultra fine cement were observed by SEM, XRD and DSC during 24 hours. Mechanical properties of hardened slurry with JIS molds were also tested in 3, 7 and 28 days. The cracked specimens which were repaired with slurries produced various conditions were tested after 3, 7 and 28 days curing in the air and adhesion properties were characterized.