• Proceedings of the Korea Concrete Institute Conference
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• 1994.04a
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• pp.209-214
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• 1994

The heat of hydration of cement causes the intemal temperature rise at early age, particulay in massive concrete structures. As the results of the temperature rise and restraint condition, the thermal stress amy induce cracks in concrete. The prediction of the thermal stress is very important in design and consturction slages in order to control the cracks in mass concrete. In this study, the temperature rise of high strength concrete due to the heat of hydration is investigated. Test variables are type and content of binder. As the results, the temperature rise is imcreased with increasing cement content. However, the increament is decreased in higher cement comtnet range. Fly ash is effictive in the reduction of hydration heat.

• Journal of Mechanical Science and Technology
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• v.16 no.5
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• pp.647-654
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• 2002

This study deals with the characteristic of quenching refrigerant for heat treatment deformation control of SM45C steel. Heat-treatment deformation must be controlled for the progress of production parts for landing gear. Most of deformation is occurred on inconsistent cooling. The inconsistent cooling is caused by a property of quenching refrigerant. When a heated metal is deposited in the quenching refrigerant, the cooling speed is so slow in early period of cooling because of a steam-curtain. After additional cooling, the steam-curtain is destroyed. In this progress, the cooling speed is very fast. The object of this study is to control the deformation of heat-treatment for landing gear by improving the conditions of quenching. The cooling curves and cooling rates of water, oil and polymer solution are obtained and illustrated. From the characteristics of the quenching refrigerant, the effects of heat-treatments on thermal deformation and fatigue strength are also investigated.

• Magazine of the Korean Society of Agricultural Engineers
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• v.18 no.4
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• pp.4265-4273
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• 1976

1. Fine aggregates of concrete are very important for the construction works and construction cost determination. Most of fine aggregates are from the river sand, but the amount of storage in the river side is steadily decreasing due to continuous construction works. Therefore, another source of fine aggregates is needed to meet increasied demand of sand. 2. Beach sand is a possible source of fine aggregates. But rust of steel bar is caused by CL-chemical of beach sand. Therefore, desalinization of beach sand is requested to get durable reinforced concrete. Economical methods of desalinization are as follows. (a) Flooding and drainage method. (b) Washing of beach sand with water supply and mixing. (c) Spreading of beach sand on the land and leaching by rain water for a few month. 3. Hardening of concrete with beach sand is accelerated due to salt, Thus early stage strength increase leads to make cracks. Also later stage strength decreases and durability becomes worse. By using appropriate admixture, the quality of concrete can be improved.

• Magazine of the Korean Society of Agricultural Engineers
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• v.24 no.3
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• pp.92-99
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• 1982

The objective of this study was to investigate the compressive and bending strength characteristics of epoxy resin mortar, which is still in an early stage of its use and study in Korea. The results obtained are summarized as follows; 1. The compressive strengths of epoxy resin mortar after 1 day, 2 days and 3 days were gained 87%, 91% and 95%, respectively, in view of that of mortar at the age of 7 days. This result showed that the initial compressive strength within 1 day was very high. 2. The highest compressive strength of epoxy resin mortar was 914 kg/cm2 at the point of having the mixing ratio of one to two. It reached up to 3.7 times that of the normal portland cement mortar at the age of 28 days. 3. The bending strengths of epoxy resin mortar after 1 day, 2 days and 3 days came up to 88%, 93% and 97%, respectively, in comparing that of mortar at the age of 7 days. It was expressed to be simielar to the tendency of compressive strength. 4. The highest bending strength of epoxy resin mortar was 384 kg/cm2 at mixing ratio of one to two. It came up to as much as 6.5 times in comparing with that of the normal portland cement mortar at the age of 28 days. Therefore, the epoxy resin mortar would be effective for promoting the bending strength of structural members. 5. The regression equation between compressive and bending strength was obtained as follows; oo~=0.391 oc+27.54 (r=0.99) And the estimated value of bending strength was corresponded to about 44 per cent in comparing with that of the compressive strength.

• Structural Engineering and Mechanics
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• v.49 no.6
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• pp.763-773
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• 2014

The recently developed Ultra High Performance Concrete (UHPC) displays outstanding compressive strength and ductility but is also subjected to very large autogenous shrinkage. In addition, the use of forms and reinforcement to confine this autogenous shrinkage increases the risk of shrinkage cracking. Accordingly, this study adopts a combination of shrinkage reducing admixture and expansive admixture as a solution to reduce the shrinkage of UHPC and estimates its appropriateness by evaluating the compressive and flexural strengths as well as the autogenous shrinkage according to the age. Moreover, the coefficient of thermal expansion known to experience sudden variations at early age is measured in order to evaluate exactly the autogenous shrinkage and the thermal expansion is compensated considering these measurements. The experimental results show that the compressive and flexural strengths decreased slightly at early age when mixing 7.5% of expansive admixture and 1% of shrinkage reducing admixture but that this decrease becomes insignificant after 7 days. The use of expansive admixture tended to premature the setting of UHPC and the start of sudden increase of autogenous shrinkage. Finally, the combined use of shrinkage reducing admixture and expansive admixture appeared to reduce effectively the autogenous shrinkage by about 47% at 15 days.

• Steel and Composite Structures
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• v.31 no.1
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• pp.69-83
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• 2019

This paper investigates the structural behavior of very high strength concrete encased steel composite columns via combined experimental and analytical study. The experimental programme examines stub composite columns under pure compression and eccentric compression. The experimental results show that the high strength encased concrete composite column exhibits brittle post peak behavior and low ductility but has acceptable compressive resistance. The high strength concrete encased composite column subjected to early spalling and initial flexural cracking due to its brittle nature that may degrade the stiffness and ultimate resistance. The analytical study compares the current code methods (ACI 318, Eurocode 4, AISC 360 and Chinese JGJ 138) in predicting the compressive resistance of the high strength concrete encased composite columns to verify the accuracy. The plastic design resistance may not be fully achieved. A database including the concrete encased composite column under concentered and eccentric compression is established to verify the predictions using the proposed elastic, elastoplastic and plastic methods. Image-oriented intelligent recognition tool-based fiber element method is programmed to predict the load resistances. It is found that the plastic method can give an accurate prediction of the load resistance for the encased composite column using normal strength concrete (20-60 MPa) while the elastoplastic method provides reasonably conservative predictions for the encased composite column using high strength concrete (60-120 MPa).

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.753-756
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• 2008

Recently, construction work period reduction is a very important topic of construction business circles. Because that is just big cost reduction. There is an important part of construction to decide the removal time of form. For prediction strength for removal form, P type schmidt hammer method and maturity method is used that. In case early strength prediction of maturity method, that is problem. Because setting duration of concrete is not proper considering. So this experimental study is a coefficient(A) of maturity method.

• Journal of the Korea Concrete Institute
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• v.25 no.4
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• pp.447-455
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• 2013

In this study, reinforced concrete slabs with steps were experimentally studied to analyze their structural performance and to suggest reinforcing details in the step. Because the stepped slabs may behave very poorly in terms of bending strength, stiffness, deflection, cracking, etc., the study is aimed to suggest proper reinforcing details such that the same bending strength is obtained as that without steps. The bending strengths of 12 test specimens with a variety of different reinforcing detail types or other parameters were compared with each other. The specimen without any additional reinforcement in the step had a very low bending strength and significant damage, and the specimens with diagonal reinforcements in the step showed substantial early cracks, experienced hinging of the step, and had a substantial loss of the bending strength. In contrast, the specimens with a combination of U-bars, reversed U-bars, L-bars, and reversed L-bars performed very well and almost reached to 100% of the slab bending strength. The U-bars and reversed U-bars were effective in controling the diagonal cracks, while the L-bars and reversed L-bars were effective in preventing from yielding of slab reinforcement near the step.

• Bulletin of the Society of Naval Architects of Korea
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• v.20 no.2
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• pp.51-59
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• 1983

The versatile practical use of electron beam welding which is very high energy density is still in early stage, but in the special welding field, the welding process is used in manufactured goods. The investigation for electron beam welding up to the present was almost achieved not for the mechanical properties of welded joint but for the process itself. On this investigation, the fatigue strength, crack propergation phenomena and hardness of weld metal and heat affected zone of partially penetrated welded joint of HT80 steel by electron beam welding was accomplished. The tensile fatigue strength in weld line direction of the joint was about $25kg/mm^2$. There still appeared spikes on the tips of penetration, and the crack initiated at the tips of spikes not from the roots. The hardness of the weld metal was higher than it of base metal because of production of martensite by rapid cooling.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.82-96
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• 2019

A semi-submersible structure has been widely used for offshore drilling and production of oil and gas. The small water plane area makes the structure very sensitive to weight increase in terms of payload and stability. Therefore, it is necessary to lighten the substructure from the early design stage. This study aims at an optimization of hull structure based on a sophisticated yield and buckling strength in accordance with classification rules. An in-house strength assessment system is developed to automate the procedure such as a generation of buckling panels, a collection of required panel information, automatic buckling and yield check and so on. The developed system enables an automatic yield and buckling strength check of all panels composing the hull structure at each iteration of the optimization. Design variables are plate thickness and stiffener section profiles. In order to overcome the difficulty of large number of design variables and the computational burden of FE analysis, various methods are proposed. The steepest descent method is selected as the optimization algorithm for an efficient search. For a reduction of the number of design variables and a direct application to practical design, the stiffener section variable is determined by selecting one from a pre-defined standard library. Plate thickness is also discretized at 0.5t interval. The number of FE analysis is reduced by using equations to analytically estimating the stress changes in gradient calculation and line search steps. As an endeavor to robust optimization, the number of design variables to be simultaneously optimized is divided by grouping the scantling variables by the plane. A sequential optimization is performed group by group. As a verification example, a central column of a semi-submersible structure is optimized and compared with a conventional optimization of all design variables at once.