• Journal of the Korea Concrete Institute
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• v.28 no.3
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• pp.317-328
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• 2016

This research intends to understand the impact of super-absorbent polymer (SAP) as an internal curing agent in Ultra-High Performance Concrete (UHPC). Two different types of SAPs of acrylic acid (SAP_AA) and acrylic acid-co-acrylamide (SAP_AM) were examined with UHPC formulation. Isothermal calorimetry and x-ray diffraction experiments revealed the impact of polymers with the different chemical bonds on cement hydration. To test its feasibility as a shrinkage reducing admixture for UHPC, a series of experiments including flowability, compressive strength, rapid chloride permeability and autogenous shrinkage profile was performed. While both SAPs showed a reduction in autogenous shrinkage, it has been concluded that the SAP size and chemical form significantly affect the performance as an internal curing agent in UHPC by controlling cement hydration and porosity modification. Between the tested SAPs, SAP_AM which absorbs more water in UHPC than SAP_AA, shows better mechanical and durability performance.

• Explosives and Blasting
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• v.36 no.4
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• pp.35-47
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• 2018

In this study, several concrete-block blast tests and AUTODYN numerical analyses were conducted to analyze the effects of different stemming and coupling materials on explosion results. Air, sand, and polymer gel were used as both the stemming and coupling materials. The stemming and coupling effects of these materials were compared with those of the full-charge condition. Soil-covered or buried concrete blocks were used for field crater tests. It was found from the concrete block tests and numerical analyses that both the crater size and the peak pressure around the blast hole were higher when the polymer gel was used than when the sand and the decoupling condition were used. The numerical analyses revealed the same trend as those of the field tests. Pressure peaks in concrete block models were calculated to be 37, 30, and 16 MPa, respectively, for the cases of the polymer gel, sand, and no stemming and decoupling condition. The pressure peak was 52 MPa in the case of full-charge condition, which was the highest pressure. But the damage area for the case was smaller than that obtained from the use of polymer gel. Full-charge was also used as a reference test.

• Journal of the Korean Geosynthetics Society
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• v.19 no.4
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• pp.41-50
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• 2020

Since cracks in concrete structures expedite corrosion of reinforced concrete over a long period of time, regular on-site inspections are essential to ensure structural usability and prevent degradation. Most of the safety inspections of facilities rely on visual inspection with naked eye, so cost and time consuming are severe, and the reliability of results differs depending on the inspector. In this study, a portable measuring device that can be used for safety diagnosis and maintenance was developed as a device that measures the width and length of concrete cracks through image analysis of cracks photographed with a camera. This device captures the cracks found within a close distance (3 m), and accurately calculates the unit pixel size by laser distance measurement, and automatically calculates the crack length and width with the image processing algorithm developed in this study. In measurement results using the crack image applied to the experiment, the measurement of the length of a 0.3 mm crack within a distance of 3 m was possible with a range of about 10% error. The crack width showed a tendency to be overestimated by detecting surrounding pixels due to vibration and blurring effect during the binarization process, but it could be effectively corrected by applying the crack width reduction function.

• Journal of the Korea institute for structural maintenance and inspection
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• v.16 no.3
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• pp.75-83
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• 2012

Porosity in concrete has close relationship with durability characteristics. Additionally mixed water can help easy mixing and workability but causes increased porosity, which yields degradation of durability performance. In this paper, cement mortar samples with 0.45 of w/c (water to cement ratio) are prepared and durability performances are evaluated with additional water from 0.45 to 0.60 of w/c. Various durability tests including strength, chloride diffusion, air permeability, saturation, and moisture diffusion are performed. Then they are analyzed with changing porosity. Changing ratios and the patterns of durability performance are quantitatively evaluated considering pore size distribution, total porosity, and additional water content.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.3
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• pp.83-90
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• 2010

Composite materials can be used economically and efficiently in civil engineering applications when standards and procedure for analysis, design, construction and quality control are to be established. Bridge systems, including the girders and cross-beams, and concrete decks behave as the specially orthotropic plates. For such systems with sections, boundary conditions other than Navier solution types, it is very difficult to obtain its analytical solution. To design the bridge made by the composite materials, cross-section was used as the form-core shape for economical reason and finite difference method was used for output of the stress value. The Tsai-Wu failure criterion for stress space is used. In this paper, the rate of tensile strength reduction due to increased size was considered. And also numerical study is made for these cases.

• Journal of the Korea Concrete Institute
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• v.26 no.4
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• pp.441-448
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• 2014

From several researches, recently, it was found that using hollowed precast concrete (HPC) column made more compact concrete casting in joint region possible than using normal solid PC (Precast concrete) column. Therefore, the rigidity of joints can be improved like those of monolithic reinforced concrete (RC). After filling the hollow with grout concrete, however, it is expected that the HPC column behaviors like composite structure since PC element and grout concrete have different materials as well as there is a contact surface between two elements. These may affect the structural behavior and strength of the composite column. A compressive strength test was performed for the HPC column with parameter of hollow ratio for the case with and without grout in the hollow and the result is presented in this paper. The hollow ratios in the test are 35, 50 and 59% of whole section of column. Concentrated axial force was applied to top of the specimens supported as pin connection for both ends. In addition, finite element (FE) analysis was performed to simulate the failure behavior of HPC column for axial compression. As a result, it was found that the hollow ratio did not affect the initial stiffness of HPC filled with grout regardless of the strength difference of HPC and grout. However the strength was increased inversely corresponding to the hollow ratio. The structural capacity of HPC without grout closely related to the hollow size. Especially, the local collapse governs the overall failure when the thickness of HPC is too thin. Based on these effect, a suitable equation was suggested for calculation of the compressive strength of HPC column with or without grout. FE analysis considering the contact surface between HPC and grout produced a good result matched to the test result.

• Journal of the Korea Concrete Institute
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• v.28 no.2
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• pp.197-204
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• 2016

Concrete walls of neutron generating facilities such as fusion reactors and fission reactors become radioactive by neutron irradiation. Both low-activation and neutron shielding are a critical concern at the dismantling stage after the shutdown of facilities with a requirement of radioactive waste management. To tackle this, two types of additives were investigated in fabricating mortar specimens: synthetic high polymers and boron carbide. It is well known that a hydrogen atom is effective in neutron shielding by an elastic scattering because its mass is almost the same as that of the neutron. And boron is an effective neutron absorber with a big neutron absorption cross section. In this study, the effect of the type, shape, and size of polymers were investigated as well as that of boron carbide. Total 16 mix designs were prepared to reveal the effect of polymers on mechanical properties and neutron shielding performance. The neutron does equivalent of polymers-based mortar for fast neutrons decreased by 36 %, and the count rate of boron carbide-based mortar with regard to thermal neutrons decreased by 90 % compared to conventional mortar. These results showed that a combination of polymers and boron carbide compounds has potential to reduce the thickness of neutron shields as well as radioactive waste from reactors.

• Journal of the Korea Concrete Institute
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• v.21 no.2
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• pp.169-178
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• 2009

An equation for calculating confining reinforcement amount of RC bridge columns, specified in the current bridge design codes, has been made to provide additional load-carrying strength for concentrically loaded columns. The additional load-carrying strength will be equal to or slightly greater than the resistant strength of a column against axial load, which is lost because the cover concrete spalls off. The equation considers concrete compressive strength, yield strength of transverse reinforcement, and the section area ratio as major variables. Among those variables, the section area ratio between the gross section and the core section, varying by cover thickness, is a variable which considers the strength in the compression-controlled region. Therefore, the cross section ratio does not have a large effect in the aspect of ductile behavior of the tension-controlled region, which is governed by bending moment rather than axial force. However, the equation of the design codes for calculating confining reinforcement amount does not directly consider ductile behavior, which is an important factor for the seismic behavior of bridge columns. Consequently, if the size of section is relatively small or if the section area ratio becomes excessively large due to the cover thickness increased for durability, too large an amount of confining reinforcement will be required possibly deteriorating the constructability and economy. Against this backdrop, in this study, comparison and analysis were performed to understand how the cover thickness influences the equation for calculating the amount of confining reinforcement. An equation for calculating the amount of confining reinforcement was also modified for reasonable seismic design and the safety. In addition, appropriateness of the modified equation was examined based on the results of various test results performed at home and abroad.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.3A
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• pp.215-225
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• 2011

In general, the precast columns can obtain its homogeneous quality as they are produced in a factory with a hollow concrete block type by using high strength concrete, so that they can generate the reduction of dead load. Such a method of precast hollow concrete columns is already implemented in USA and Japan and used for connecting between blocks which use PC tendons. However, it is inevitable to have uneconomical construction with excessive cost in early stage when PC tendons are used. This study aims to develop an economical precast column with high quality and constructability which consists of only splice sleeve and general reinforcing bar without using PC tendons in order to reduce the construction period and cost. To achieve this goal, this study tested the performance of total 5 minimized models in the experiment with the variables such as hollowness, diameter of main reinforcement bar and cross-sectional size for the cross section of precast column by using grouting type splice sleeve which is a new type joint rebar. And it also verified the performance of column in the experiment for a large-sized model in order to overview its applicability by excluding large scale effect.

• Journal of the Korean Recycled Construction Resources Institute
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• v.3 no.3
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• pp.220-227
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• 2015

Ultra high performance concrete (UHPC) uses large quantities of steel fiber, silica fume, filler and superplasticizer for a low water-to-binder ratio (W/B). Despite of exceptional mechanical performances, UHPC exhibits increased viscosity due to the adoption of silica fume and its fabrication cost is costlier than ordinary concrete because of the use of large quantities of expensive materials. Following, this study evaluates the mechanical properties of 180MPa-UHPC using zirconium silica fume (Zr) instead of silica fume with respect to the quantity and type of superplasticizer (SP) and the size of filler. The results reveal that the Zr-UHPC using W/B of 20%, 100% of Zr, amount of SP-L of 2 to 3% and $4{\mu}m$-filler with steel fiber in 1.5 vol.% can develop better fluidity than the traditional mix composition using silica fume and secure a compressive strength higher than 180 MPa. In addition, the proposed mix composition is shown to enable a reduction of the fabrication cost by 33% compared to traditional UHPC.