• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.1
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• pp.50-58
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• 2018

The verification of serviceability of concrete structures requires more informations on the composite behaviors between concrete and reinforcement. Among them, the investigation of crack widths and spacings is based on the tension stiffening effects. In this paper, the tension stiffening effects of high strength concrete members with compressive strength of 80 and 100MPa are investigated experimentally. It was found that the current design code which is based on the tests of normal strength concrete may not describe the tension stiffening effects in high strength concrete correctly. The coefficient that can appropriately reflect the tension stiffening effects in the high strength concrete was proposed. Also, the crack spacing was investigated through the cracking behaviors and the crack width according to the difference of the strains in steel and concrete was estimated. The results of this paper may be used to examine the tension stiffening effects of high strength concrete members.

• Journal of the Korea Institute of Building Construction
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• v.15 no.5
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• pp.465-471
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• 2015

This study proposed V-shaped tie bar method as an alternative of internal cross-tie for reinforced concrete columns in order to enhance the constructability and confinement effectiveness of the lateral tie bars. A total of 35 pull-out specimens were prepared with the parameters of concrete compressive strength and bending angle and embedment length of the V-shaped bar to examine the bond stress-slip relationship of the V-shaped tie bar. The bond strength of the V-shaped tie bars with the bending angle not exceeding $60^{\circ}$ was higher than the predictions obtained from the equations of CEB-FIP provision. Considering the constructability and bond behavior of the V-shpaed tie bar, the bending angle and embedment length of such bar can be optimally recommended as $45^{\circ}$ and 6db, respectively, where db is the diameter of the tie bar.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.7 no.1
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• pp.83-92
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• 1987

Fatigue fracture of reinforced concrete structures are characterized by considerably larger strains and microcracking as compared to fracture of R.C. structures under static loading. The strain of stirrup is increased suddenly by the occuring of inclined crack and the average strain ${\epsilon}_{\omega}$ of all stirrups in a structure at maximum load increase approximately in proportion to log N. The structures critical in longitudinal reinforcement seemed to have an endurance limit of 60~70 percent of static ultimate strengths for 1,000,000 cycles. In this test, the average fatigue strength at 1,000,000 cycles for all structures tested was approximately 65 percent of the static ultimate strength.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.3 no.4
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• pp.1-10
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• 1983

The deformation and crack width of concrete walls of slabs, plates, panels and shells reinforced by a regular rectangular net of reinforcing bars and subjected to in-plane (membrane) internal forces is analyzed on the basis of a realistic model which takes into account the frictional-dilatant behavior of rough interlocked cracks, the effect of tension stiffening, and the dowel action of bars at crack crossings. Extensive numerical computer studies are carried out, and the reinforcement designs obtained from equilibrium conditions alone on the basis of either the classical frictionless approach or the recent frictional approach are compared in terms of the resulting crack widths. It is found that the use of frictional equilibrium design based on a low friction coefficient leads to a much smaller crack width than the classical frictionless design. The influences of bar diameter and crack spacing on the crack width are also studied. The model allows more realistic deformation analysis of reinforced concrete structures.

• Science of Emotion and Sensibility
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• v.16 no.3
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• pp.319-332
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• 2013

Previous research on learning and memory has focused on how they are constructed through past experiences. Recent studies, however, have shed light on that such cognitive processes are in service of higher goals of maximizing future rewards. This review paper aims to introduce and discuss a related line of research. First, this paper introduces researches that show goal-directed model-based reinforcement learning, in which agents choose a behavior that does not necessarily bring immediate rewards but will allow future rewards, based on generalization and analogical extrapolation. It also reviews studies on neural substrates of goal-directed learning, and discusses that cognitive process implicated in striatal dopaminergic signals can also influence memory. Especially, memory is not a merely passive process of storing and retrieving past experiences homogeneously, but rather results of a decision-making process to serve higher goals. The body of research suggests that information on future rewards can have influence on current cognitive processing in a retrospective manner.

• Korean Journal of Materials Research
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• v.22 no.9
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• pp.454-458
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• 2012

Various adhesive materials are used in flip chip packaging for electrical interconnection and structural reinforcement. In cases of COF(chip on film) packages, low temperature bonding adhesive is currently needed for the utilization of low thermal resistance substrate films, such as PEN(polyethylene naphthalate) and PET(polyethylene terephthalate). In this study, the effects of anhydride and dihydrazide hardeners on the low-temperature snap cure behavior of epoxy based non-conductive pastes(NCPs) were investigated to reduce flip chip bonding temperature. Dynamic DSC(differential scanning calorimetry) and isothermal DEA(dielectric analysis) results showed that the curing rate of MHHPA(hexahydro-4-methylphthalic anhydride) at $160^{\circ}C$ was faster than that of ADH(adipic dihydrazide) when considering the onset and peak curing temperatures. In a die shear test performed after flip chip bonding, however, ADH-containing formulations indicated faster trends in reaching saturated bond strength values due to the post curing effect. More enhanced HAST(highly accelerated stress test) reliability could be achieved in an assembly having a higher initial bond strength and, thus, MHHPA is considered to be a more effective hardener than ADH for low temperature snap cure NCPs.

• Journal of the Korean Ceramic Society
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• v.48 no.4
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• pp.323-327
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• 2011

The effects of $Dy_2O_3$ and $Er_2O_3$ co-doping on electrical properties and temperature stability of barium titanate ($BaTiO_3$) ceramics were investigated in terms of microstructure and structural analysis. The dielectric constant and the insulation resistance (IR) of 0.7 mol% $Dy_2O_3$ and 0.3 mol% $Er_2O_3$ co-doped dielectrics had about 60% and 20% higher than the values of undoped one, respectively, and the temperature coefficient of capacitance (TCC) met the X7R specification. The addition of $Dy_2O_3$ contributed to electrical properties caused by increase of tetragonality; however, preferential diffusion of $Dy^{3+}$ ions toward A site in $BaTiO_3$ grain exhibited an adverse effect on temperature stability by grain growth. On the other hand, The $Er_2O_3$ addition in $BaTiO_3$ could affect the TCC behavior and the IR with suppression of grain growth caused by reinforcement of grain boundary and electrical compensation. Therefore, the enhanced electrical properties and temperature stability through the co-doping could be deduced from the increase of tetragonality and the suppression of grain growth.

• Journal of the Korea Concrete Institute
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• v.13 no.5
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• pp.499-506
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• 2001

Eight direct tension tests were conducted to study the bond characteristics and crack behavior in high-strength concrete axial members. The main variable was the concrete strength up to 61-63 MPa. The specimens consisted of two different types of the short specimens modeled the part between transverse cracks and the long specimens having numerous transverse cracks. The results obtained show that the bond strength increases in proportion to compressive strength. Thereby, in high-strength concrete the length of stress-disturbed region is shortened and the space of adjacent transverse cracks become smaller. Although the concrete strength varies from 25 MPa to 61 MPa, the split cracking loads remain constant, while transverse cracking loads vary as variation of concrete tensile strength. Accordingly, the current code provisions for development length may need reconsideration in high-strength concrete members, and it is recommended that either thicker cover or transverse reinforcement should be additionally provided for high-strength concrete members.

• Explosives and Blasting
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• v.25 no.1
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• pp.53-65
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• 2007

When doing a blasting demolition on RC structures made of scale models, scale model members considering both a proper scale factor and mechanical characteristics of materials have to be similar to prototype RC members to analyze the collapse behavior of RC structures. In this study. a similitude law considering the density of prototype materials is calculated. Both mix of concrete and arrangement of reinforcement have been described referring to Concrete Standard Specification as well as Design Standard of Concrete Structure. The scale factor on scaled concrete models considering maximum size of coarse aggregate is about one-fifth of a cross section of prototype concrete members. A scale factor on staled steel bar models is about one-fifth of a nominal diameter of prototype steel bar. According to the mechanical test results of scale models, it can be concluded that the modified similitude law may be similar to compressive strength of prototype concrete and yield strength of prototype steel bar.

• Structural Engineering and Mechanics
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• v.72 no.5
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• pp.631-642
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• 2019

The objective of the present study is to examine the flexural behavior of two-span post-tensioned lightweight aggregate concrete (LWAC) beams using unbonded tendons and the reliability of the design provisions of ACI 318-14 for such beams. The parameters investigated were the effective prestress and loading type, including the symmetrical top one-point, two third-point, and analogous uniform loading systems. The unbonded prestressing three-wire strands were arranged with a harped profile of variable eccentricity. The total length of the beam, measured between both strand anchorages, was 11000 mm. The test results were compared with those compiled from simply supported LWAC one-way members, wherever possible. The ultimate load capacity of the present beam specimens was evaluated by the collapse mechanism of the plasticity theorem and the nominal section moment strength calculated following the provision of the ACI 318-14. The test results showed that the two-span post-tensioned LWAC beams had lower stress increase (Δf_ps) in the unbonded tendons than the simply supported LWAC beams with a similar reinforcement index. The effect of the loading type on Δf_ps and displacement ductility was less significant for two-span beams than for the comparable simply supported beams. The design equations for Δf_ps and Δf_ps proposed by ACI 318-14 and Harajli are conservative for the present two-span post-tensioned LWAC beams, although the safety decreases for the two-span beam, compared to the ratios between experiments and predictions obtained from simply supported beams.