• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.5
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• pp.411-422
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• 2010

Self-sensing on micro-failure, dispersion degree and relating properties, of carbon nanotube(CNT)/epoxy composites, were investigated using wettability, electro-micromechanical technique with acoustic emission(AE). Specimens were prepared from neat epoxy as well as composites with untreated and acid-treated CNT. Degree of dispersion was evaluated comparatively by measuring volumetric electrical resistivity and its standard deviation. Apparent modulus containing the stress transfer was higher for acid-treated CNT composite than for the untreated case. Applied cyclic loading responded well for a single carbon fiber/CNT-epoxy composite by the change in contact resistivity. The interfacial shear strength between a single carbon fiber and CNT-epoxy, determined in a fiber pullout test, was lower than that between a single carbon fiber and neat epoxy. Regarding on micro-damage sensing using electrical resistivity measurement with AE, the stepwise increment in electrical resistivity was observed for a single carbon fiber/CNT -epoxy composite. On the other hand, electrical resistivity increased infinitely right after the first carbon fiber breaks for a single carbon fiber/neat epoxy composite. The occurrence of AE events of added CNT composites was much higher than the neat epoxy case, due to micro failure at the interfaces by added CNTs.

• Journal of the Korean GEO-environmental Society
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• v.15 no.9
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• pp.69-75
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• 2014

Rock fall protection system installed against rock slope is one of the most conventional way to protect nearby infra structures. Despite of wide application of typical rectangular nets, virtually installed to protect rock slope face, several problems have also been pointed out up to date. Rectangular draped nets are vulnerable to a sudden external shock such as rock fall, because it doesn't have any systematical buffers or shock absorbers. Furthermore, it has been widely recognized from the some cases of rock fall accident in Korea that rock fall protection nets cause wide range of failure in the rock slope faces due to insufficient pullout bearing capacity of fixing parts. Therefore, in this study, we tried to make a consideration about the problems of existing standard rock fall protection nets in Korea, and develop a new type of hexagonal net with a shock absorber based on design rock fall energy. In this paper, laboratory and full scale test procedure is described to analysis the performance of newly developed hexagonal rock fall net, and the key results are presented and discussed.

• Journal of the Korean GEO-environmental Society
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• v.19 no.9
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• pp.5-12
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• 2018

The foundation of the multi-span greenhouse structures is designed with small shallow concrete foundation considering mainly the vertical load. However, recently, due to an abnormal climate such as strong wind, horizontal load and up-lift load over design strength are applied to the foundation, causing safety problems of the greenhouse foundation. In order to reasonably evaluate the safety of greenhouse foundations, rotational and pullout stiffness expressed by the ground-foundation interaction should be evaluated, which also affects the safety of the upper structural members. In this study, three representative basic foundation types were selected by classifying greenhouse standards in Korea according to the shape, and the horizontal loading tests and theoretical calculation were performed for each foundation type. As a result of the comparison and analysis of the test and calculation, it was found that rotational resistance of the foundation is different according to the ratio of the contact area between the foundation and ground when the conditions of the foundation - ground contact surface and the mechanical properties of the ground are the same.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.3
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• pp.76-85
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• 2017

In this study, to evaluate the mechanical properties of fiber-reinforced cement composites by strain rate, hydraulic rapid loading test system was developed. And compressive and tensile strength of the hooked steel fiber and polyamide fiber reinforced cement composite were evaluated. As a result, the compressive strength, strain capacity and elastic modulus were increased with increasing strain rate. The effect of compressive strength by type and volume fraction of fibers was not significant. The dynamic increase factor(DIF) of the compressive strength was higher than that of the CEB-FIP model code 2010 and showed a trend similar to that of ACI-349. The tensile strength and strain capacity were increased with increasing strain rate. The hooked steel fibers were drawn from the matrix. The tensile strength and strain capacity of hooked steel fiber reinforced cement composites were increased as the strain rate increased. The tensile strength and deformation capacity of the fiber reinforced cement composites were increased. And, hooked steel fibers were drawn from the matrix. On the other hand, because the bonding properties of polyamide fiber and matrix is large, polyamide fiber was cut-off with out pullout from matrix. The strain rate effect on the tensile properties of polyamide fiber reinforced cement composites was found to be strongly affected by the tensile strength of the fibers.

• Composites Research
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• v.19 no.5
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• pp.20-27
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• 2006

Self-sensing and interfacial evaluation of Ni nanowire/polymer composites were investigated using electro-macromechanical technique, which can be used fur a feasible sensing measurement on tensile and compressive loading/consequent unloading, temperature, and humidity. Mechanical properties of Ni nanowire with different aspect ratio and adding contents in either epoxy or silicone composites were measured indirectly using electro-pullout test under uniform and non-uniform cyclic loadings. Comparing apparent modulus with the conventional mechanical tensile modulus of Ni nanowire/epoxy composites, the trends were consistent with each other. Ni nanowire/epoxy composites showed the sensing response on humidity and temperature. Self-sensing on applied tensile and compressive loading/unloading was also responded for Ni nanowire/silicone composites via electrical contact resistivity showing the opposite trend between tension and compression. It can be due to the different electrically-interconnecting mechanisms of dispersed Ni nanowires embedded in silicone matrix.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.5A
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• pp.511-518
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• 2009

This study was intended to compare and evaluate the adhesion of High flowing Self-compacting Concrete (HSCC), Conventional Concrete (CC) and deformed bar based on concrete strength 3 (30, 50 and 70 MPa), among the factors affecting the bond strength between concrete and rebar, after fabricating the specimen by modifying the rebar position at Horizontal reinforcement at bottom position (HB), horizontal reinforcement at top position (HT) and vertical reinforcement type (V). As a result of measuring bond strength of HB/HT rebar to evaluate the factor of the rebar at top position, the bond strength of HB/HT rebar at 50 and 70 MPa was 1.3 or less and at 30 MPa, HSCC and CC appeared to be 1.2 and 2,1, respectively. Thus, when designing the anchorage length according to the concrete structure design standard (2007) at HSCC 30, 50 and 70 MPa, it would be desirable to reduce the correction factor of anchorage length of the horizontal reinforcement at top position, which is suggested for the reinforcement at top position, to less than 1.3 of CC.

• Journal of the Korea Concrete Institute
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• v.18 no.1 s.91
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• pp.21-28
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• 2006

This paper presents both experimental and analytical studies for the development of an ECC(Engineered Cementitious Composites) using ground granulated blast furnace slag(slag). This material has been focused on achieving moderately high composite strength while maintaining high ductility, represented by strain-hardening behavior in uniaxial tension. In the material development, micromechanics was adopted to properly select optimized range of the composition based on steady-state cracking theory and experimental studies on matrix, and interfacial properties. A single fiber pullout test and a wedge splitting test were employed to measure the bond properties of the fiber in a matrix and the fracture toughness of mortar matrix. The addition of the slag resulted in slight increases in the frictional bond strength and the fracture toughness. Subsequent direct tensile tests demonstrate that the fiber reinforced mortar exhibited high ductile uniaxial tension behavior with a maximum strain capacity of 3.6%. Both ductility and tensile strength(~5.3 MPa) of the composite produced with slag were measured to be significantly higher than those of the composite without slag. The slag particles contribute to improving matrix strength and fiber dispersion, which is incorporated with enhanced workability attributed to the oxidized grain surface. This result suggests that, within the limited slag dosage employed in the present study, the contribution of slag particles to the workability overwhelms the side-effect of decreased potential of saturated multiple cracking.