• Journal of the Korean Ceramic Society
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• v.20 no.3
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• pp.217-226
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• 1983

Adquate amount of calcia was added to the regent-grade Zirconia body. Here the amount and the form of calcia were 7-21 mol% and regent-grade calcium cabonate respectively. The specimens were fired at 175$0^{\circ}C$ for 0, 3, 5 and 7 hours respectively. The phase Strength X-ray diffraction analysis and Scaning electron microscopy. The results were as follows (1) As the additive amount of calcia was increased the firing linear shrinkage apparent density compressive strength and modulus of rupture decreased but the apparent porosity increased. (2) The specimens soaked and containing calcia displayed the grain growth. (3) Monoclinic and cubic zirconia were seen in the sepcimens containing 7 mol% calcia. When without soaking the specimens containing 7-10 mol% calcia had the volume change by monoclinic$\rightleftharpoons$tetragonal transformation. (4) The lattice parameter increased according as the calcia additive was increased. The specimens containing above 19mol% calcia had the costant lattice parameter. The value of that was from 5.1264 to 5.1396 $\AA$ in the case of 7 hours soaking.

• Journal of the Korean Ceramic Society
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• v.35 no.1
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• pp.1-4
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• 1998

SiC whisker reinforced Al2O3-SiC composite was fabricated by reaction synthesis method whish is cost ef-fective and allows good dispersion of whiskers. Fracture strength at room temperature showed the highest value with 150$0^{\circ}C$ reaction temperature because a lot of SiC whiskers was formed. Fracture strength at 135$0^{\circ}C$ did not show big differences with reaction temperature due to agglomeration of whiskers and formation of sil-icon oxynitride during hot MO(modulus of rupture) test probably promoting grain boundary sliding.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.341-344
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• 2004

In the present research, slump, modulus of rupture (MOR) and flexural toughness $(I_{30})$ of high performance hybrid fiber reinforced concrete (HPHFRC) mixed with micro-fiber (carbon fiber) and macro-fiber (steel fiber) and replaced with silica fume were assessed with the analysis of variance (ANOVA). Steel fiber was a considerable significant factor in aspect of the response values of MOR and boo Based on the significance of factors related to response values from ANOVA, following assessments were available; Slump decrease: carbon fiber >> steel fiber > silica fume; MOR: steel fiber > silica fume > carbon fiber; $I_{30}$: steel fiber > carbon fiber > silica fume. Steel fiber $1.0\%$, carbon fiber $0.25\%$ and silica fume $5.0\%$, and Steel fiber $1.0\%$, carbon fiber $0.25\%$ and silica fume $2.5\%$ were obtained as the most optimum mixture.

• Journal of the Korean Ceramic Society
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• v.29 no.8
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• pp.660-666
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• 1992

Pitchstone reacted with CaO in hot water(9$0^{\circ}C$) and increased its sedimentary volume by forming Ca(OH)2 and calcium silicate hydrates. ALC was prepared from gel at 9$0^{\circ}C$ and subsequently autoclaved at 18$0^{\circ}C$ by using the property of swelling and the physical properties of ALC was investigated with experimental conditions. When the ratio of pitchstone/CaO was 2 (CaO/SiO2 mol ratio=0.81), bulk density, modulus of rupture and thermal conductivity of the ALC were 0.75g/㎤, 73kg/$\textrm{cm}^2$ 0.150 kcal/m.hr.$^{\circ}C$, respectively. The crystalline phase of it was mainly tobermorite. Therefore ALC was turned out to be much lightweighted and good thermal insulation.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.05b
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• pp.85-88
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• 2005

The properties of pcc using BOF-slag were investigated in order to use as fine aggregate, which has been made in the steel making process and stabilized by air-jet method. The fluidity of flesh concrete has been improved with the usage of BOF-slag and it should be noted that the air content goes up. The compressive strength of hardened concrete was reduced at weight fraction over 50$\%$ and Young's modulus tends toward the trivial change. Also, the flexural and tensile strengths were increased with the usage, especially, the rupture values were increased to 1.2 times at the weight fraction of 30$\%$.

• Nuclear Engineering and Technology
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• v.53 no.9
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• pp.2953-2959
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• 2021

A new practical modeling of the Norton's power law creep is proposed and implemented to analyze the high temperature behaviors of Alloy 690 SG tube material. In the model, both the stress exponent n and the rate constant B are simply treated as the temperature dependent parameters. Based on the two-step optimization procedure, the temperature function of the rate constant B(T) was determined for the data set of each B value after fixing the stress exponent n value by using the prior optimized function at each temperature. This procedure could significantly reduce the numerical errors when using the power law creep equations. Based on the better description of the steady-state creep rates, the experimental rupture times could also be well predicted by using the Monkman-Grant relationship. Furthermore, the difference in tensile strengths at high temperatures could be very well estimated by assuming the imaginary creep stress related to the given strain rate after correcting the temperature effects on the elastic modulus.

• Journal of the Korea Concrete Institute
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• v.27 no.4
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• pp.411-418
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• 2015

This paper investigates the effects of fiber volume fraction and panel thickness on face damage characteristics of steel fiber-reinforced concrete (SFRC) under high-velocity globular projectile impact. The target specimens were prepared with $200{\times}200mm$ prismatic panels with thickness of 30 or 50 mm. All panels were subjected to the impact of a steel projectile with a diameter of 20 mm and velocity of 350 m/s. Specifically, this paper explores the correlation between mechanical properties and face damage characteristics of SFRC panels with different fiber volume fraction and panel thickness. The mechanical properties of SFRC considered in this study included compressive strength, modulus of rupture, and toughness. Test results indicated that the addition of steel fiber significantly improve the impact resistance of conventional concrete panel. The front face damage of SFRC panels decreased with increasing the compressive toughness and rear face damage decreased as the modulus of rupture and flexural toughness increased. To evaluate the damage response of SFRC panels under high-velocity impact, finite element analysis conducted using ABAQUS/Explicit commercial program. The predicted face damage of SFRC panels based on simulation shows well agreement with the experimental result in similar failure mode.

• Journal of the Korea Concrete Institute
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• v.22 no.5
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• pp.651-658
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• 2010

Sixteen concrete mixes reinforced with hybrid steel-polybinyl alcohol (PVA) fibers and a control concrete mix with no fiber were tested in order to examine the effect of the micro and macro fibers on the slump and different mechanical properties of concrete. Main variables investigated were length and volume fraction of steel and PVA fibers. The measured mechanical properties of hybrid fiber reinforced concrete were analyzed using the fiber reinforcing index and compared with those recorded from monolithic steel or PVA fiber reinforced concrete. The initial slump of hybrid fiber reinforced concrete decreased with the increase of the aspect ratio and the volume fraction of fibers. In addition, splitting tensile strength, modui of rupture and elasticity, and flexural toughness index of concrete increased with the increase of the fiber reinforcement index. Modulus of rupture and flexural toughness index of hybrid fiber reinforced concrete were higher than those of monolithic fiber reinforced concrete, though the total volume fraction of hybrid fibers was lower than that of monolithic fiber. For enhancing the flexural toughness index of hybrid fiber reinforced concrete, using the steel fiber of 60 mm length was more effective than using the steel fibers combined with 60 mm and 30 mm lengths.

• Journal of the Korea Concrete Institute
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• v.13 no.5
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• pp.457-465
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• 2001

Shrinkage strains of concrete slab in multi-story building are restrained by structural members such as columns or walls, then can induce cracks due to excessive shrinkage stress over tensile strength of member. In this study, a shrinkage stress analysis method of concrete slab in multi-story building considering not only material properties such as shrinkage, creep and reinforcement effect but also construction sequence is proposed. Tensile stresses of slab due to shrinkage are calculated by converting shrinkage strains into equivalent temperature gradients, creep that can release shrinkage stress can be considered by replacing the modulus of elasticity of concrete, Ec , to the effective secant modulus of elasticity of concrete, E$\_$eff/ Reinforcements are also considered by modeling them as equivalent beam elements in FEM program. Results of step by step analysis reflecting construction sequence summed up to calculate stresses of the whole building considering that shrinkage stresses of the building come from the difference of shrinkage between i-th floor and (i-1)-th floor, named as effecitive shrinkage, and it can be varied by construction sequence. The results of 10-story example building show that shrinkage stresses of lower floors are greater than those of upper floors, that is, stresses of lower floors(1∼2FI.) exceed modulus of rupture of concrete, but stress ratios of higher floors are in the range of 27.9∼92.8%.

• Journal of the Korean Wood Science and Technology
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• v.44 no.2
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• pp.172-183
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• 2016

Densification of the inner part of oil palm trunk (OPT) by the close system compression (CSC) method was performed in this study. The effects of the compression temperature and time on the anatomical, physical and mechanical properties of OPT were evaluated. The inner part of OPT with an initial average density of $0.3g/cm^3$ was used as samples. Oven-dried samples were immersed in water and vacuumed until fully saturated and then compressed by CSC at 120, 140, 160 or $180^{\circ}C$ for 10, 20, 30 or 40 min. The anatomical characteristics of transverse and radial sections before and after compression were compared by optical microscopy. The physical and mechanical properties, including the density, recovery of set (RS), modulus of elasticity (MOE), modulus of rupture (MOR), and compression parallel to grain were examined. It was observed that the anatomical characteristic of the inner part of OPT (i.e., vascular bundles, vessels, and parenchyma tissue) became flattened, fractured, and collapsed after compression by CSC. The RS decreased with increasing compression temperature and time. The lower RS indicated high dimensional stability. The physical and mechanical properties (i.e., density, MOR, MOE, and compressive strength) of the inner part of OPT increased with increasing compression temperature and time. Compression by the CSC method at $160^{\circ}C$ for 40 min was the optimum treatment.