• Journal of Korea Foundry Society
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• v.38 no.6
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• pp.121-131
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• 2018

The effect of additives on the castability of the AC2BS aluminum alloy, which contains 35% recycled scrap, was investigated. For the wide utilization of the recycled scrap AC2BS aluminum alloy, the research results were compared to those with the AC2B virgin alloy, which is typical Al-Si-Cu alloy system. It was confirmed that the addition of Al-5%Ti-1%B increased the ${\alpha}$-Al nucleation temperature during solidification and decreased the grain size through cooling curve and microstructural observations of the recycled alloy. It was also found that an addition of Al-10%Sr decreased the eutectic Si growth temperature during the solidification process and modified the shape of the eutectic Si of the recycled alloy. The characteristics of fluidity, shrinkage and solidification crack strength were evaluated. For the AC2BS aluminum alloy containing 35% recycled scrap, both ${\alpha}$-Al grain refinement due to Ti-B and eutectic Si modifications due to Sr contributed to the improvement of the fluidity. The macro- shrinkage ratio increased with additions of both Al-10%Sr and Al-5%Ti-1%B and the micro-shrinkage ratio increased with the addition of Al-10%Sr but decreased with the addition of Al-5%Ti-1%B. The casting characteristics of TiB and Sr-treated AC2BS aluminum alloy containing 35% recycled scrap are similar to those of AC2B virgin alloy. The improvement of the solidification crack strength of the AC2BS aluminum alloy was possible by the reduction of the grain boundary the stress concentration through the enhancement by both ${\bullet}{\cdot}$-Al refinement and eutectic Si modification. More extensive use of the AC2BS aluminum alloy containing 35% recycled scrap can be expected in the future.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.3D
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• pp.445-451
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• 2006

This study was conducted to develop the interlayer mixture that absorbs the stress between the old concrete pavement and the asphalt overlay pavement layer. The elasticity, the flexibility, the consistency and the impermeability is required for high-elastic Stress Absorbing Interlayer(HSAI) to absorb and disperse the stress that causes the flexural and horizontal movements of the joint and the crack. The HSAI developed from foreign product was satisfied with the design criteria. The specimens using the HSAI showed the significant reduction of the reflective crack compared those not using the HSAI. The significance included that the life of shear failure and horizontal displacement resistance increased 4 times. The life of the share failure increased to 5 times and the horizontal displacement increased to 9 times according to the selection of surface course material which showed the excellence of the HSAI.

• Magazine of the Korean Society of Agricultural Engineers
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• v.20 no.2
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• pp.4645-4687
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• 1978

This study was attempted to investigate the mechanical properties of concrete and crack control effects of reinforced concrete with steel and glass fiber. The experimental program includes tests on the properties of fresh concrete containing fibers, compressive strength, tensile strength, flexural strength, Young's modulus, Shrinkage and deformation of steel or glass fiber reinforced concrete. Also this study was carried out to investigate the effect of steel or glass fiber to retard the development in reinforced concrete subject to uniaxial tension and thus facilitate the use of steels of higher strength. The major conclusions that can be drawn from the studies are as follows: 1. The effect of the fibers in various mixes on fresh concrete confirmed that fibers do have a significant effect on the properties of fresh concrete, bringing much more stable and exhibiting a signiflcant reduction in surface bleeding, and that the cohesion is greatly improved and the internal resistance increases with fiber concentration. But the addition of an excess contents and length of fibers brings about the reduction of workability. 2. With the addition of steel fibers(1.5% Vol.) to concrete, the compressive strength as compared with plain concrete showed a very slight increase, but excess addition, over 1.5% Vol. of steel and glass fiber reduced its strength. 3. Splitting tensile strength of fiber reinforced concrete showed a significant increase tendency, as compared with plain concrete. In case of containing steel fiber (2.5%, 30mm), it showed that the maximum increase rate of 1.48 times as much rate, and in case of containing glass fiber (2.5%, 30mm), the increase rate of strength was 1.25 times as much rate. 4. Flexural strength of fiber reinforced concrete showed a significant tendency, as compared with plain concrete. Containing steel fiber (2.5%, 30mm) showed the maximum increase rate of 1.64 times as much rate and containing glass fiber (2.5%, 30mm) showed the increase rate of strength of 1.32 times as much rate, and in general, the 30mm length brougth the best results. 5. The strength ratio ($\sigma$b/$\sigma$c and $\sigma$t/$\sigma$c) increased, when steel fiber's average spacing was up to 3.05mm, but decreased when beyond 3.05mm, and it was confirmed that tensile or flexural strengths of steel fiber reinforced concrete are apparently governed by fiber's average spacing. 6. The compressive strain of fiber reinforced concrete showed a significant increasing tendency as the fiber was added, but Young's modulus. with the addition of steel and glass fibers, showed a slight decrease tendency. And according to the increase of flexural strength, a considerable increase was seen in toughness. 7. With the addition of fiber's the shrinkage of concrete was significantly decreased, in both case of adding steel fibers 12.5%, 30mm, and showed a significant decrease ratio, in average 30.4% and 36.7%, as compared with plain concrete. 8. With the increase of fiber volume fraction and length, the gained stress in reinforcing bar in concrete specimens increased in all crack widths, but at different rates, with the decrease of fiber diameter, the stress showed a considerable increasing tendency. And the duoform steel fibers showed the greatest improvement, as compared with the other types tested. 9. The influence of fiber dimensions in order of significanse on the machanical properties of concrete and the crack control of reinforced concrete was explained as follows: content, length, aspect ratio and dimeter.

• The Journal of Korean Academy of Prosthodontics
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• v.37 no.4
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• pp.454-464
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• 1999

The metal ceramic crown is currently the most popular complete veneer restoration in dentistry, but in many cases, the metal cervical collar at the facial margin is unesthetic and unacceptable. Facial porcelain margin has been used in place of it. But this dose not solve the problems, such as dark gingival discoloration and cervical opaque reflection of porcelain veneer. Recently, metal copings which were designed to terminate its labio-cervical end on the axial walls coronal to the shoulder have been clinically used to solve the esthetic problem of metal ceramic crown. But in this design, porcelain veneer of labio-cervical area which is not supported by metal may not be able to resist the stress during cementation and mastication. The purpose of this study was to evaluate fracture strength and fractured appearance of crowns according to different coping designs. A resin maxillary left central incisor analogue was prepared for a metal ceramic crown, and metal dies were made with duplication mold. Metal copings were made and assigned to one of four groups based on facial framework designs: group 1, coping with 0.5mm metal collar; group 2, metal extended to the shoulder; group 3, metal extended to 1mm coronal tn the shoulder: group 4, metal extended to 2mm coronal to the shoulder. Copings and crowns were adjusted to be same size and thickness, and cemented to metal dies with zinc phosphate cement by finger pressure. Fracture strength was measured with Instron Universal Testing Machine. Metal dies were anchored in Three-way-vice at 3mm below finish line and at $130^{\circ}$ inclined to the long axis of the crown. Load was directed lingually at 2mm below midincisal edge. Load value at initial crack and at catastrophic fracture was recorded. The results obtained were as follows : 1. Fracture strength values at initial crack were higher in groups 1, 2 than in groups 3, 4 but this difference was not statistically significant(P<0.05). 2. Conventional metal collared crown had greater catastrophic fracture strength than any other collarless crowns. 3. The greater the labial metal coping reduction, the lower the catastrophic fracture strength of crowns but when more than 1mm of labial metal reduction was done, the difference in strengths was not statistically significant(p<0.05). 4. The strongest collarless coping design was group 2.

• Journal of the Korea Institute of Building Construction
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• v.17 no.3
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• pp.245-252
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• 2017

Recently, structures such as large retailers, outlets and warehouses have been increasing in accordance with changes in consumption patterns. Since these structures include ultra-flat slab members, they are thoroughly managed to control slab cracking by the plastic and drying shrinkage. In order to control the cracking of the slab member, a chemical crack reduction method is used. In particular, the use of the shrinkage reducing agent has been examined. However, domestic research results are limited. In this study, the shrinkage properties of concrete using shrinkage reducing agent and the drying shrinkage properties according to the mixing factors were investigated. The performance of domestic shrinkage reducing agent was appeared similar to that of overseas high-grade shrinkage reducing agent. As the shrinkage reducing agent usage increased, the drying shrinkage reduction effect increased. At the age of 100 days, the dry shrinkage rate of specimen with the shrinkage reducing agent of 1.5%was shown about half that of the specimen without the shrinkage reducing agent. The shrinkage reducing agent was gound to have no specific performance change for the use of the admixture.

• Journal of the Korean Recycled Construction Resources Institute
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• v.5 no.4
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• pp.403-413
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• 2017

In this paper, to increase the use of industrial byproducts for $CO_2$ reduction and to improve construction performance, it was manufactured that $CO_2$ reduction type quaternary component high fluidity concrete (QC-HFC) with Reduced cement usage by more than 80% and its quality and hydration characteristics were evaluated. QC-HFC was found to satisfy the target performance, and the flow and mechanical properties were similar to those of conventional concrete. The drying shrinkage of QC-HFC decreased about twice compared with the conventional blend, and the hydration heat decreased about 36%. As a result, it can be concluded that the amount of cracks can be reduced by reducing temperature stress due to hydration heat reduction effect and reducing deformation due to relatively small temperature difference between inside and outside. Also, As a result of the simulation of the mass structure, the temperature cracking index of QC-HFC is 1.1 or more, and the cracking probability is reduced by about 35%, so that the crack due to temperature can be reduced.

• Journal of the Korean Recycled Construction Resources Institute
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• v.8 no.1
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• pp.88-96
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• 2020

In this study, the effect of cyclic wetting-drying on the self-healing of cementitious materials containing superabsorbent polymers (SAPs) were experimentally evaluated. In each cycle, cracked cement paste specimens containing various SAP dosages were exposed to wet conditions for 1 h, during which the capillary water absorption tests and water flow tests were conducted, and then exposed to dry conditions for 47 h. The capillary water absorption test results showed that the sorptivity values of the specimen without SAPs, SAP 0.5%, SAP 1.0%, and SAP 1.5% specimens were decreased by approximately 22.9%, 36.8%, 42.8%, and 46.3%, respectively, after 8 cycles. In addition, the water flow test results showed that the amount of water runoff through the cracks of all cracked specimens gradually decreased over wet/dry cycles, especially the reduction ratio of the amount of water runoff increased with increasing SAP dosage. Furthermore, the swelling behavior of SAPs in cracks by in gress water was con firmed via X-ray computed tomography (CT) analysis. These results indicate that the effective crack width can be reduced as SAPs absorb water and swell, while the water absorbed in SAPs can be released to crack surfaces under dry conditions, further promoting healing product formation. This study demon strates that the in corporation of SAPs can in crease the water tightness of cracks, thereby improving the self-healing efficiency of cementitious materials.

• Korean Journal of Materials Research
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• v.30 no.6
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• pp.292-300
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• 2020

In this study, the effect of tempering on the stretch-flangeability is investigated in 980 MPa grade dual-phase steel consisting of ferrite and martensite phases. During tempering at 300 ℃, the strength of ferrite increases due to the pinning of dislocations by carbon atoms released from martensite, while martensite is softened as a consequence of a reduction in its carbon super-saturation. This strength variation results in a considerable increase in yield strength of the steel, without loss of tensile strength. The hole expansion test shows that steel tempered for 20 min (T20 steel) exhibits a higher hole expansion ratio than that of steel without tempering (T0 steel). In T0 steel, severe plastic localization in ferrite causes easy pore formation at the ferrite-martensite interface and subsequent brittle crack propagation through the highly deformed ferrite area during hole expansion testing; this propagation is mainly attributed to the large difference in hardness between ferrite and martensite. When the difference in hardness is not so large (T20 steel), on the other hand, tempered martensite can be considerably deformed together with ferrite, thereby delaying pore formation and hindering crack propagation by crack blunting. Eventually, these different deformation and fracture behaviors contribute to the superior stretch-flangeability of T20 steel.

• Journal of the Korea Concrete Institute
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• v.20 no.5
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• pp.661-668
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• 2008

It is necessary to develop a new technology for effectively reducing hydration heat and controlling thermal cracking caused increasing construction of large size massive concrete structures such as mat foundation of high-rise building, grandiose bridge, and LNG tank. Therefor, to develop a new technology for reducing hydration heat of large size massive concrete in this study, after developing the latent heat binder for controling hydration heat of concrete by application of latent heat material, it was investigated basic properties and durability such as slump, air content and compressive strength, shrinkage properties, permeability, freezing and thawing resistance, corrosion, and hydration heat generation properties of concrete using latent heat binder. As a test result, it was confirmed that latent heat binder was not affected adversely the basic property and durability of concrete, and was advanced on the reduction of hydration heat and control of thermal crack. It is expected to be applied as the excellent technology on the management of hydration heat and thermal crack in large size mass concrete structures.

• Journal of the Korea Concrete Institute
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• v.23 no.3
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• pp.393-400
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• 2011

The needs for high strength structural materials have recently increased, because construction and cost efficiencies are demanded by the costumers. But, the use of high strength reinforcing bars requires longer development and splice lengths compared to normal strength bars. This restriction may cause reduction in construction efficiency and require more complicated details. The purpose of this paper is to evaluate the bond strength on the rib shape of reinforcing bars to decrease development and splice lengths of high strength reinforcements. Total of 5 simple beam specimens were tested, and the main test variable was a rib shape of reinforcing bars. Test data was analyzed in the viewpoint of bond strength, load-deflection relationship, and crack pattern. Test results indicated that the bond strength of high relative rib area reinforcing bars increased up to 11% compared to normal strength reinforcements. And the improved rib shape reinforcements, which were formed with high and low height rib, increased the bond strength up to 23% even though the relative rib area was same as the high relative rib area reinforcements. Serviceability performances such as deflection number of cracking, and maximum crack width were similar in all specimens, so it is safe to conclude that the improved rib shape reinforcements can be applied to the structural members.