• Proceedings of the Korea Concrete Institute Conference
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• 1994.10a
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• pp.87-92
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• 1994

The main objective of this study is to determine the critical chloride ion concentrations in the pore solutions causing depassivation of steel reinforcement in concrete made with cements of different $C_3A$ contents. Cement pastes with water-ratio of 0.5 were prepared using four cements with $C_3A$ contents of 0.46, 5.97, 9.14, and 9.65 percent. The pastes were allowed to hydrate in sealed containers for 28days and then objected to pore solution expression. The expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. It was found that the free cholride concentration in the pore solution decreases significantly with an increase in the $C_3A$ content of the cement. With increasing level of chloride addition, although the alsolute amount of bound chloride increase, the ratio of bound to total chlorides decreases.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.05a
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• pp.1023-1030
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• 2002

From the early 1980s, the New Austrian Tunnelling Method (NATM) has been developed as a one of the standard tunneling method in Korea. Approximately 10 years ago, wet-mix shotcrete with sodium silicate accelerator (waterglass) was introduced and widely used to tunnel lining and underground support. However, this accelerator had some disadvantages due to the decrease of long-term strength compared to plain concrete (without accelerator) and low quality of the hardened shotcrete. In order to compensate for these disadvantages, recently developed alkali-free accelerator has been successfully demonstrated in numerous projects and applications as a new material to make tunnels more durable and safer. An experimental investigation was carried out in order to verify the strength behavior of wet-mix Steel Fiber Reinforced Shotcrete (SFRS) with alkali-free accelerator. Compressive strength, flexural strength and equivalent flexural strength were measured by testing specimens extracted from the shotcrete panels. From the results, wet-mix SFRS with alkali-free accelerator exhibited excellent strength improvement compared to the conventional shotcrete accelerator.

• Journal of the Korean Society for Precision Engineering
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• v.20 no.5
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• pp.148-155
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• 2003

Most manufacturing processes such as welding, cutting and molding generate residual stresses on the surface of manufactured parts. Between compressive and tensile residual stresses, the tensile residual stress is harmful to the surface integrity, which results in reduced fatigue life and causes other structural failures when the service stresses are superimposed on the residual stresses. In the research, the residual stresses in the hardened tool steel (SKD11) were measured using hole-drilling method. The specimens were prepared through hard turning. Most of residual stresses in the machined surface were compressive.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10b
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• pp.36-39
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• 2003

Due to the strength and biocompatibility requirement, the stainless steel SUS 316L is widely used for trauma internal fixation device. SUS 316L can be hardened and strengthened only by cold work. In this work, the material compression test is performed both in laboratory and computer simulation by a FEM analysis software DEFORM to correlate the hardness to strain. This data is then used for preform design and predict the hardness of the finish bone plate forging. Finally, we compared the hardness between the actual forging and computer analysis results. Although the predicted hardness from computer simulation. is 55HV higher than the final forging sample, we can get good compatibility on the hardening tendency of cold forging.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.437-442
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• 1999

Corrosion of steel reinforcment is the most significant factor of deterioration in reinforced concrete structures. Chloride ion is considered one of the most common culprits on the corrosion of steels in concrete. This study is effect of cement hydraulic degree on the chloride binding in hardened cement pastes. With increasing the cement hydration, cement products such as CSH and Ca(OH)2 were increased, and the part of added chloride were binded with them. With respect to A type, in first, the additional contents of chloride of 27.08mM/L in pore solution were reduced as 4.3mM/L at 3 days, 4.0mM/L at 7 days, 3.6mM/L at 28 days.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.05a
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• pp.653-657
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• 2000

Machining performance in hard turning of hardened AISI M2 steel has been studied. Ceramic tools were used in the cutting tests without coolants and workpiece was prepared by heat treatment to increase its hardness up to Rc 60. Cutting state parameters such as cutting forces, temperature, and tool wear were measured in the experiments and effects of tool wear on cutting states were investigated.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1999.10a
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• pp.307-314
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• 1999

By using AIP(Arc Ion Plating) of a physical vapor deposition for the first time in Korea a ceramic tool whose surface is coated single layeredly with TiN is developed. In addition, cutting resistance appearing in the process of finishing cut of hardened carbon tool steel, STC3 is studied. The principal and radial components of cutting resistance in those cutting conditions appear to be the same or similar, and the feed component is relatively small. The feed component is found to be in proportion to cutting width, and the radial component in proportion to cutting thickness. Owing to coating the cutting resistance of a TiN coated ceramic tool increases compared with that of a general ceramic tool.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2000.04a
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• pp.34-38
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• 2000

The trend of cutting process today goes toward higher precision and higher efficiency. Many thermal/frictional troubles occur in high-speed machining of die and mold steels.In this paper, the thermal characteristics are evaluated in high sped ball end-milling of hardened steel(HRc42). Experimental work is performed on the effect of cutting environments on tool life and cutting temperature. Cutting environments involve dry, wet(20bar), compressed chilly air at -9$^{\circ}C$, compressed chilly air at -35$^{\circ}C$. The measuring technique of cutting temperature using implanted thermocouple is used. The cutting temperature is about 79$0^{\circ}C$, 35$0^{\circ}C$ and 54$0^{\circ}C$ in dry, wet and compressed chilly air at +9$^{\circ}C$, respectively. The tool life for compressed chilly air at -9$^{\circ}C$ is longer than all other cutting environments in experiment.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1994.10a
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• pp.73-78
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• 1994

Wear and fracture mode of ceramic tool for hardened SKD11 steel was investigated by face milling in this study. The cutting force and Acoustic Emission(AE) signal were utilized to detect the wear and fracture of ceramic tool. The following conclusions were obtained : (1) The wear and fracture modes of ceramic tool are characterized by three types: \circled1wear which has normal wear and notch wear, \circled2 wear caused by scooping on the rake face, \circled3 large fracture caused by thermal crack in the rake face. (2) The wear behaviour of ceramic tool can be detected by the increase of mean cutting force and the variation of the AE RMS voltage. (3) The catastrophic fracture of ceramic tool can be detected by the cutting force(Fz-component). (4) As the hardness of work material increased, Acoustic Emission RMS value and mean cutting force(Fz) increased linearly, but the tool life decreased.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.565-566
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• 2006

High speed machining experiment on the inclined surfaces of hardened mold steel(STAVAX at hardness HRC 53) is carried out using the long-neck type ball endmill. Surface texture and roughness are compared fur various cutting conditions. Tool overhang length greatly affects the roughness of machined surface. It is found that, fur this type of long-neck endmill, the chip load should be carefully selected by reducing either the axial depth of cut or feedrate to avoid tool vibration. Feedrate adjustment is more appropriate method in terms of tool wear.