• Tunnel and Underground Space
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• v.23 no.5
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• pp.413-427
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• 2013

Thin Spray-on Liners(TSLs) based on polymer materials have been considered as an alternative to shotcrete and wire mesh in relatively fair rock conditions, and used in mines since 1990s. Nevertheless, Few experimental studies on their mechanical properties necessary for the evaluation of their bearing capacities as a support member have been carried out. In this study, tensile and bond strengths of two kinds of TSLs with different material compositions were measured at the age of 7 days. In addition, two kinds of bending tests proposed by EFNARC (2008) to simulate representative failure mechanisms of TSLs were carried out on the same materials and curing age as in tension and pull-out tests. From the tests, tensile strength of a TSL is shown to increase as its content of polymer is higher. In contrast, its bond strength seems to be in inverse proportion to its polymer content. Especially, the TSL material in which a cementitious component is included with relatively smaller polymer content shows a faster hardening characteristic which results in higher resistance to de-bonding between a TSL and a substrate. As a result, it is shown that the performance of TSLs might be dependent upon its corresponding polymer content.

• Journal of the Korean Ceramic Society
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• v.40 no.9
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• pp.874-880
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• 2003

The effect of h-BN content on microstructure, mechanical properties, and machinability of AlN-BN based machinable ceramics were investigated. The relative density of sintered compact decreased with increasing h-BN content. The four-point flexural strength also decreased from 238 MPa of monolith up to 182 MPa by the addition of 30 vol％ h-BN. Both low Young's modulus and residual tensile stress, formed by the thermal expansion coefficient difference between AIN and h-BN, might cause the strength drop in AlN-BN composite. The crack deflection, and pull-out phenomena increased by the plate-like h-BN. However, the fracture toughness decreased with h-BN content. The second phases, consisted of YAG and ${\gamma}$-Al$_2$O$_3$, were formed by the reaction between Al$_2$O$_3$ and Y$_2$O$_3$. During end-milling process, feed and thrust forces measured for AlN-(10～30) vol％ BN composites decreased with increasing h-BN particles, showing excellent machinability. Also, irrespective of h-BN content, relatively good surfaces with roughness less than 0.5 m (Ra) could be achieved within short lapping time.

• Journal of the Korea Concrete Institute
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• v.18 no.1 s.91
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• pp.73-82
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• 2006

Nowadays, the general stripping work of form has brought some problems; increase of total constructing cost resulted from the man-dependent form work procedure and environmental issues by wasting the debonded form. In this study, to effectively reduce unnecessary cost and resolve the environmental problems caused by these kinds of reason, a permanent form method using stainless steel fiber was introduced then its material and structural characteristics were evaluated. In the case of material characteristic, the permanent form had a good ductile behavior in the result of flexural test of the permanent form panel and pull-out test of insert bolt which is installed in the permanent form and perfect bonding capacity with a field concrete. In the case of structural characteristic, compressive and tensile behavior of the permanent form was evaluated. It also showed a good structural behavior in the view of load-deflection relationship, crack patterns and additional strengthening effect.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.11-16
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• 2003

The objective of this study is to clarify the seismic capacity and the characteristics in the hysteretic behavior of RC structures with non-seismic detailing. To do this, an exterior beam-column subassemblage was selected from a ten story RC building and six 1/3-scale specimens were manufactured with three variables; (1) with and without slab, (2) upward and downward direction of anchorage for the bottom bar in beams, and (3) with and without hoop bars in the joint region. The test results have shown that (1) the existence of slab increased the strength in positive and negative moment, 25% and 52%, respectively; (2) the Korean practice of anchorage (downward and 25 $d_{b}$ anchorage length) caused the 8% reduction of strength and the early strength degradation in comparison with the case of seismic details; and (3) the existence of hoop bars in the joint region shows significant role in preventing the pull-out.t.

• 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.31 no.4A
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• pp.279-286
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• 2011

FRP (Fiber Reinforced Polymer) tendons can be used as an alternative to solve the corrosion problem of steel tendons. Material properties of FRP tendons-bond strength, transfer length, development length-must be determined in order to apply to concrete structures. First of all, in case of application for pretension concrete members with CFRP tendons, transfer length is an important characteristic. The bond of the material characteristics should be demanded clearly to apply to PSC structures prestressed with FRP tendons. This paper investigated on the bond characteristics of FRP reinforcements with various surface-type. To determine the bond characteristics of FRP materials used in place of steel reinforcement or prestressing tendon in concrete, pull-out testing suggested by CAN/CSA S806-02 was performed. A total of 40 specimens were made of concrete cube with steel strands, deformed steel bar and 6 different surface shape FRP materials like carbon or E-glass. Results of the bonding tests presented that each specimen showed various behaviors as the bond stress-slip curve and compared with the bond characteristic of CFRP tendon developed in Korea.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.45-45
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• 2009

원가 측면에서 유리한 저항점용접(Resistance Spot Welding)이 차체 용접에 80%이상으로 가장 많이 적용되고 있다. 첨단고강도강(Advanced High Strength Steel)의 저항점용접성 및 용접부 특성에 미치는 공정 변수의 영향에 대한 연구결과는 많으나, 합금원소의 영향에 대해서는 전무하다. 특히, Si는 DP(Dual Phase)강에 첨가 시 균일한 마르텐사이트의 분포를 촉진하는 원소로 저항 점용접성 및 용접부 특성에 영향을 미칠 것으로 예상되며, 이에 대한 연구는 보고된바 없다. 본 연구에서는 냉연 DP강의 저항 점용접시 중요한 인자 중 하나인 너깃경과 전단인장강도에 미치는 Si함유량의 영향을 검토하였다. 사용된 강재 및 용접기는 1.2mm 두께의 Si함유량(0, 0.5, 1.0, 1.5wt%)이 다른 인장강도 780~1000MPa급 냉연 DP강과 단상 AC용접기를 사용하였다. 용접조건은 ISO 18278-2규격에 따라 가압력 4kA, 초기가압시간 40cycle, 유지시간 17cycle로 고정하고, 용접전류만 변화하여 용접을 실시하였다. 너깃경은 용접부 단면을 컷팅 후 폴리싱 하여, 광학현미경과 Image Pro plus를 이용하여 측정했으며, 인장시편규격은 JIS Z 3137를 이용하였다. Si함유량이 증가에 따라 스패터 발생 전류는 감소했고, 너깃경은 직선적으로 증가했다. Si함유량 증가에 따른 너깃경 증가 이유는 저항(R) 측정결과, Si함유량 증가에 따라 모재의 저항이 높아져, 따라서 입열량($Q=I^2Rt$)이 많아지기 때문으로 판단되었다. 인정전단강도는 Si함유량 증가에 따라 직선적으로 증가했다. 이러한 이유는 Si함유량 증가에 따라 너깃경이 증가되기 때문으로 판단되었고, 너깃경과 인장전단강도 사이에 직선적 관계(PL(kN)=$3.2N_{dia.}$-0.81, $R^2$=0.93)를 가지고 있었다. 파단양상은 Si함유량에 상관없이 5.4kA이하에서는 계면파단이 일어났고, 6.0kA이상에서는 풀 아웃 파단이 일어났다. 계면파단주원인은 용접부 가장자리에 지름이 약 $5{\mu}m$이하의 예리한 노치가 존재하여 노치응력집중과 HAZ계면 근처에 미접합부가 존재하기 때문으로 판단되었다. 6.0kA이상에서는 예리한 노치가 없었고, HAZ부가 완전히 접합되어 있기 때문에 풀 아웃 파단이 일어난 것으로 판단되었다. 따라서, Si함유량 증가에 따라 적정용접전류 구간은 감소했고, 너깃경은 직선적으로 증가했다. 또한, Si함유량 증가에 따라 인장전간강도는 증가 했으며, 너깃경과 인장전단강도 사이에 직선적 관계를 가지고 있었다. 파단 양상은 Si함유량에 상관없이 5.2kA이하에서는 계면파단이, 6.0kA이상에서는 풀 아웃 파단이 일어났다.

• Journal of the Korea Concrete Institute
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• v.24 no.6
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• pp.761-768
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• 2012

The purpose of this study is to investigate bond properties of GFRP used in SFRC (Steel fiber reinforced concrete) and normal concrete. The experimental variables were rebar diameter (D13, D16), steel fiber volume fraction (0~2%) and cover thickness ($1.5d_b$, $5.4d_b$). The experimental results showed a different failure mode depending on the cover thickness. Through the tested specimens, splitting failure occurred for the specimens with small cover thickness and pull out failure occurred in the specimens with large cover thickness. Introduction of steel fiber caused the specimens to have more ductile behavior of bond stresss-lip after peak stress, but they did not increase the bond strength significantly. These failure modes were shown in both steel reinforcement and GFRP. However, from the difference of micro structure of bond failure mechanism between steel rebar and GFRP rebar, more ductile behavior was observed in GFRP-specimens after maximum bond strength was reached.

• Journal of Adhesion and Interface
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• v.19 no.1
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• pp.13-18
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• 2018

At ruthenium (Ru) catalyst was exposed from the atmosphere, the degree of catalyst activation decreased. The change of catalyst activity with the number of days of exposure to air for the Ru catalyst was confirmed using the surface tension method quantitatively. Mechanical properties and surfactant change after polymerization by DCPD using Ru catalyst for each air exposure day was evaluated. The Ru catalyst mixed with a dilution agent was exposed in the air and color was monitored for each day. Surface tension was measured using Wilhelmy and PTFE and associated with different catalyst activities. Heat was measured in real time during polymerizing DCPD with Ru catalyst. After polymerization, tensile strength was measured for p-DCPD and the change of material property was measured. Interfacial properties were also evaluated via microdroplet pull-out tests between glass fiber and p-DCPD. The surface tension was stable until the 4 days (33 dyne/cm) whereas the surface energy increased at the 10 days (34 dyne/cm), which could be correlated with oxidation of the catalyst. Tensile property and interfacial shear strength (IFSS) was also stable until the 4 days (tensile strength: 38 MPa and IFSS: 26 MPa) whereas the mechanical property decrease at 10 days (tensile strength: 15 MPa and IFSS: 3 MPa) dramatically.

• The Journal of Engineering Geology
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• v.24 no.2
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• pp.261-271
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• 2014

An almost permanent anchor (friction type) is resistant to ground deformation due to the friction between the soil and grout at a fixed length from the anchor body. The purpose of this study is to calculate the force of bearing resistance for a bearing anchor in enlarged boreholes. We conducted analytical and numerical analyses, along with laboratory testing, to find the quantities of bearing resistance prior to grouting in EBA (Enlarged Bearing Anchor) construction. The force of bearing resistance from the analytical method was defined as a function of general borehole diameter, expanded borehole diameter, and soil unconfined compressive strength. We also employed the Flac 3D finite difference numerical modeling code to analyze the bearing resistance of the soil conditions. We then created a laboratory experimental model to measure bearing resistance and carried out a pull-out test. The results of these three analyses are presented here, and a regression analysis was performed between bearing resistance and uniaxial compression strength. The laboratory results yield the strongest bearing resistance, with reinforcement 28.5 times greater than the uniaxial compression strength; the analytical and numerical analyses yielded values of 13.3 and 9.9, respectively. This results means that bearing resistance of laboratory test appears to be affected by skin friction resistance. To improve the reliability of these results, a comparison field study is needed to verify which results (analytical, numerical, or laboratory) best represent field observations.