• Journal of Korean Society of Steel Construction
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• v.14 no.4
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• pp.471-479
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• 2002

In this paper, 18 square CFT columns filled with high-strength concrete were tested under concentric or eccentric axial loading. Two parameters of the experimental program included the buckling length-section depth ratio ($L_K$/D) and the eccentricity of the appled compressive load (e). In additon, mechanical properties such as the compressive concrete strength and compressive and tensile steel strength were measured and incorporated into the material models for the stress-strain relationships of concrete and steel. This model was used in an elasto-plastic analysis in order to predict the behavior of the slender CFT columns. Observtions of the failure mode during the tests under axial loadig were also presented. The strengths obtained from the analysis. Recommendations for Design, and Constructions of CFT structures were presented, as verified by the experimental results.

• Advances in concrete construction
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• v.6 no.6
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• pp.645-658
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• 2018

Despite being one of the most abundantly used construction materials because of its exceptional properties, concrete is susceptible to deterioration and damage due to various factors particularly corrosion, improper loading, poor workmanship and design discrepancies, and as a result concrete structures require retrofitting and strengthening. In recent times, Fiber Reinforced Polymer (FRP) composites have substituted the conventional techniques of retrofitting and strengthening of damaged concrete. Most of the research studies related to concrete strengthening using FRP have been performed on undamaged test specimens. This contribution presents the results of an experimental study in which concrete specimens were damaged by mechanical loading and elevated temperature in laboratory prior to application of Carbon Fiber Reinforced Polymer (CFRP) sheets for strengthening. The test specimens prepared using concrete of target compressive strength of 28 MPa at 28 days were subjected to compressive and splitting tensile testing up to failure and the intact pieces of the failed specimens were collected for the purpose of repair. In order to induce damage as a result of elevated temperature, the concrete cylinders were subjected to $400^{\circ}C$ and $800^{\circ}C$ temperature for two hours duration. Concrete cylinders damaged under compressive and split tensile loads were re-cast using concrete and rich cement-sand mortar, respectively and then strengthened using CFRP wrap. Concrete cylinders damaged due to elevated temperature were also strengthened using CFRP wrap. Re-cast and strengthened concrete cylinders were tested in compression and splitting tension. The obtained results revealed that re-casting of specimens damaged by mechanical loadings using concrete & mortar, and then strengthened by single layer CFRP wrap exhibited strength even higher than their original values. In case of specimens damaged by elevated temperature, the results indicated that concrete strength is significantly dropped and strengthening using CFRP wrap made it possible to not only recover the lost strength but also resulted in concrete strength greater than the original value.

• Computers and Concrete
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• v.26 no.6
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• pp.467-482
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• 2020

This research focused on analyzing the post-fire behavior of high-performance concrete-filled steel tube (CFST) columns, with the concrete containing tire rubber and steel fibers, under axial compressive loading. The finite element (FE) modeling of such heated columns containing recycled aggregate is a branch of this field which has not received the proper attention of researchers. Better understanding the post-fire behavior of these columns by measuring their residual strength and deformation is critical for achieving the minimum repair level required for structures damaged in the fire. Therefore, to develop this model, 19 groups of confined and unconfined specimens with the variables including the volume ratio of steel fibers, tire rubber content, diameter-to-thickness (D/t) ratio of the steel tube, and exposure temperature were considered. The ABAQUS software was employed to model the tested specimens so that the accurate behavior of the FE-modeled specimens could be examined under test conditions. To achieve desirable results for the modeling of the specimens, in addition to the novel procedure described in this research, the modified versions of models presented by previous researchers were also utilized. After the completion of modeling, the load-axial strain and load-lateral strain relationships, ultimate strength, and failure mode of the modeled CFST specimens were evaluated against the test data, through which the satisfactory accuracy of this modeling procedure was established. Afterward, using a parametric study, the effect of factors such as the concrete core strength at different temperatures and the D/t ratio on the behavior of the CFST columns was explored. Finally, the compressive strength values obtained from the FE model were compared with the corresponding values predicted by various codes, the results of which indicated that most codes were conservative in terms of these predictions.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2000.04a
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• pp.202-212
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• 2000

This study investigates the ductility capacity of slender-wide reinforced concrete walls under predominant flexural moment loading. The experimental work for this study aims to provide design guidelines for bar detailing in critical regions under compressive stress in particular in case of slender-wide RC walls. According to the experimental observation the Bernoulli hypothesis of linear strain distribution is no longer valid and the ultimate compressive strain of concrete is significantly reduced, It is postulated that the nonlinear strain distribution causes the concentrated compressive stressed region and hence the premature crushing failure at the toe of walls. The reduced ultimate strain and nonlinear strain distribution need transverse reinforcement for confinement and more realistic models for the strength and displacement estimation of slender-wide RC wall.

• ETRI Journal
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• v.41 no.6
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• pp.739-749
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• 2019

Security and accuracy are two issues in the localization of wireless sensor networks (WSNs) that are difficult to balance in hostile indoor environments. Massive numbers of malicious positioning requests may cause the functional failure of an entire WSN. To eliminate the misjudgments caused by malicious nodes, we propose a compressive-sensing-based multiregional secure localization (CSMR_SL) algorithm to reduce the impact of malicious users on secure positioning by considering the resource-constrained nature of WSNs. In CSMR_SL, a multiregion offline mechanism is introduced to identify malicious nodes and a preprocessing procedure is adopted to weight and balance the contributions of anchor nodes. Simulation results show that CSMR_SL may significantly improve robustness against attacks and reduce the influence of indoor environments while maintaining sufficient accuracy levels.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.11a
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• pp.229-232
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• 2006

The HPFRCCs show that the multiple crack propagation, high tensile strength and ductility due to the interfacial bonding of the fibers to the cement matrix. Moreover, performance of cement composites varies according to type and weight contents of reinforcing fiber. and HPFRCCs with hybrid fiber have better performance than HPFRCCs with single fiber in damage tolerance. Total four cylindrical specimens were tested, and the main variables were the type and weight contents of fiber, which was polyvinylalchol (PVA), polyethylene (PE). In order to clarify effect of hybrid types on the characteristics of fracture and damage process in cement composites, AE method was performed to detect micro-cracking in HPFRCCs under cyclic compression. Loading conditions of the uniaxial compression test were monotonic and cyclic loading. And from AE parameter value, it is found that the second and third compressive load cycles resulted in successive decrease of the amplitude as compared with the first compressive load cvcle.

• Journal of Ocean Engineering and Technology
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• v.18 no.5
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• pp.43-49
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• 2004

The lightness of components required in the automobile and machine industry necessitates the use of high strength components. In particular, the fatigue failure phenomena, which occurs when using metal, increases the danger to human life and property. Therefore, antifatigue failure technology is an integral part of current industries. Currently, the shot peening is used for removing the defect from the surface of steel, while improving the fatigue strength on surface. Therefore, in this paper, the effect of compressive residual stress of spring steel(JISG SUP-9) by shot-peening on fatigue crack growth characteristics in a stress ratio(R=0.1, R=0.3, R=0.6) was investigated, giving consideration to fracture mechanics. By using the methods mentioned above, following conclusions are drawn: (1) The fatigue crack growth rate(da/dN) of the shot-peening material was lower than that of the un-peening material and in stage I, ΔKth, the threshold stress intensity factor of the shot-peen processed material is high in critical parts, unlike the un-peening material. Also m, fatigue crack growth exponent and number of cycle of the shot-peening material, was higher than that of the un-peening material, as concluded from effect of da/dN. (2) Fatigue life shows more improvement in the shot-peening material than in the un-peening material, and the compressive residual stress of surface on the shot-peen processed operate resistance of fatigue crack propagation.

• Journal of the Korean Geotechnical Society
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• v.37 no.11
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• pp.23-36
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• 2021

In this paper, the effect of shear rate on internal friction angle and unconfined compressive strength of non-cemented and cemented sand was investigated. A dry Jumunjin sand was prepared at loose, medium, and dense conditions with a relative density of 40, 60 and 80%. Then, series of direct shear tests were conducted at shear rates of 0.32, 0.64, and 2.54 mm/min. In addition, a cemented sand with cement ratio of 8% and 12% was compacted into a cylindrical specimen with 50 mm in diameter and 100 mm in height. Unconfined compression tests on the cemented sand were performed with various shear rates such as 0.1, 0.5, 1, 5 and 10%/min. Regardless of a degree of cementation, the unconfined compressive strength of the cemented sand and the angle of internal friction of the non-cemented sand tended to increase as the shear rate increased. For the non-cemented sand, the angle of internal friction increased by 4° at maximum as the shear rate increased. The unconfined compressive strength of the cemented sand also increased as the shear rate increased. However, its increasing pattern declined after the standard shear rate (1 mm/min). A discrete element method was also used to analyze the crack initiation and its development for the cemented sand with shear rate. Numerical results of unconfined compressive strength and failure pattern were similar to the experimental results.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.33-38
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• 2002

Concrete placed under cold weather has some defects such as the decrease of initial strength by hydration delay, strength unrecovery at unhardened concrete freezing, and structural failure and crack by expansion pressure. So, in this study, we tried to evaluate the JIS mortar which was made under cold weather using de-icing agency. In mortar test, the do-icing agency increased compressive strength under standard curing, and the de-icing agency made by NaNO$_2$ gave the highest strength. However, as pre-curing time under 21$^{\circ}C$ was short, the de-icing agency made by NaNO$_2$ and Ca[NO$_3$]$_2$ had the highest strength.

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.57-66
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• 2000

The ductility is an important consideration in the design of reinforced concrete structures. In the seismic design of reinforced concrete columns, it is necessary to allow for relatively large ductilities that the seismic energy be absorbed without shear failure of significant strength degradation after the reinforcement yielding in columns. Therefore, prediction of the ductility should be as accurate as possible. This research investigate the ductile behavior of rectangular reinforced high-strength concrete columns like as bridge piers with confinement steel. The effects on the ductility of axial load, lateral reinforcement ratio, longitudinal reinforcement ratio, shear span ratio, and compressive strength of concrete were investigated analytically using layered section analysis. as the results, it was proposed the proper relationship between ductility and variables and formulated into equations.