• Structural Engineering and Mechanics
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• v.74 no.3
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• pp.351-363
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• 2020

Based on the dynamic tests of recycled aggregate concrete (RAC) short columns confined by the hoop reinforcement, the dynamic failure mechanism and the mechanical parameters related to the constitutive relation of confined recycled aggregate concrete (CRAC) were investigated thoroughly. The fracturing sections were relatively flat and smooth at higher strain rates rather than those at a quasi-static strain rate. With the increasing stirrup volume ratio, the crack mode is transited from splitting crack to slipping crack constrained with large transverse confinement. The compressive peak stress, peak strain, and ultimate strain increase with the increase of stirrup volume ratio, as well as the increasing strain rate. The dynamic confining increase factors of the compressive peak stress, peak strain, and ultimate strain increase by about 33%, 39%, and 103% when the volume ratio of hoop reinforcement is increased from 0 to 2%, but decrease by about 3.7%, 4.2%, and 9.1% when the stirrup spacing is increased from 20mm to 60mm, respectively. This sentence is rephrased as follows: When the stirrup volume ratios are up to 0.675%, and 2%, the contributions of the hoop confinement effect to the dynamic confining increase factors of the compressive peak strain and the compressive peak stress are greater than those of the strain rate effect, respectively. The dynamic confining increase factor (DCIF) models of the compressive peak stress, peak strain, and ultimate strain of CRAC are proposed in the paper. Through the confinement of the hoop reinforcement, the ductility of RAC, which is generally slightly lower than that of NAC, is significantly improved.

• Earthquakes and Structures
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• v.3 no.6
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• pp.889-910
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• 2012

Because of the heavy demand of confining steel to restore the column ductility in seismic regions, it is more efficient to confine these columns by hollow steel tube to form concrete-filled-steel-tube (CFST) column. Compared with transverse reinforcing steel, steel tube provides a stronger and more uniform confining pressure to the concrete core, and reduces the steel congestion problem for better concrete placing quality. However, a major shortcoming of CFST columns is the imperfect steel-concrete interface bonding occurred at the elastic stage as steel dilates more than concrete in compression. This adversely affects the confining effect and decrease the elastic modulus. To resolve the problem, it is proposed in this study to use external steel confinement in the forms of rings and ties to restrict the dilation of steel tube. For verification, a series of uni-axial compression test was performed on some CFST columns with external steel rings and ties. From the results, it was found that: (1) Both rings and ties improved the stiffness of the CFST columns and (2) the rings improve significantly the axial strength of the CFST columns while the ties did not improve the axial strength. Lastly, a theoretical model for predicting the axial strength of confined CFST columns will be developed.

• Journal of the Earthquake Engineering Society of Korea
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• v.12 no.3
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• pp.29-36
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• 2008

The TR-CFT(Transversely Reinforced Concrete Filled Steel Tube) column is proposed to control or at least delay the state of local buckling at the critical section by wrapping the CFT columns with a carbon fiber sheet. In this study, an equation to determine the flexural strength of TR-CFT is proposed. The ACI-318 code, in which the contribution of the confining effect in the concrete filled steel tube is not appropriately accounted for, may be conservative. Therefore, flexural strength design equations for CFT columns and TR-CFT columns are proposed based on the concrete strain-stress curve, which contributes to the confining effect. Finally, the predicted results for the CFT and TR-CFT columns are shown to be in good agreement with actual test results.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2005.03a
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• pp.361-371
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• 2005

The confined concrete subjected multi-axial stresses have been known as the fact it increases strength of concrete significantly compared with unconfined concrete. Many researchers have studied in confining effects of concrete, and now are studying in many fields. Although many researches about confined concrete using FRP have been studied recently, it is difficult to apply concrete confined by FRP in real structures because FRP is a brittle material. To investigate the influence of concrete strength and ductility increased by confining stiffness in steel, this study was tested and compared with 51 specimens confined by different shapes and thicknesses of steel tube. This test verified the increasement of strength and ductility in confined concrete. Also, analyzing the experimental data by regression method, this study provides stress-strain model about CSS and R4S considering effect of confinement stiffness on the stress-strain relations of concrete.

• Proceedings of the Korea Concrete Institute Conference
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• 2009.05a
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• pp.7-8
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• 2009

In this research, comparison and analysis were performed to understand how the cover thickness influences the equation for calculating the amount of confining reinforcement for reinforced concrete columns. And, also, an equation for calculating the amount of confining reinforcement was proposed for reasonable seismic design. In addition, appropriateness and safety of the proposed equation were examined based on the various experimental results performed at home and abroad.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.11a
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• pp.351-354
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• 2003

Triaxial behavior of concrete cylinders wrapped with FRP material has been investigated for the increase of concrete strength by lateral confinement. Using the model by Richart et al., a modified empirical equation was proposed to estimate the strength of concrete cylinders with FRP confinement based on the linear relationship between the concrete strength and lateral confining pressure. From the experimental stress-strain result of the cylinder specimens having similar confining pressure, less ductility was observed for higher strength concrete. But the compressive strength of the specimen was linearly increased by lateral confinement. The confinement effectiveness coefficient for the strength enhancement of the cylinders by FRP wrap was obtained as 2.27 from the regression analysis.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.431-436
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• 2001

Lateral confining effect due to the existence of the shear reinforcements in R.C. beam is investigate in a numerical way. For the purpose, a three dimensional constitutive model of concrete is developed based on the elasto-plasticity using non-associated plastic flow rule to control the excessive inelastic dilatancy. The plastic flow direction is determined based on the associated plastic flow direction in a way to adjust the directional angle between the two normal vector components along the hydrostatic and deviatoric axis in a meridian plane in which the loading function prescribed. The current formulation is combined with the four parameter elasto-plastic triaxial concrete model recently developed. The resulting elasto-plastic triaxial concrete model predicts the fundamental behaviors of concrete under different confining levels and the 4-points flexural test of a beam with shear reinforcements, compares with the experimental results.

• Journal of the Korean Society for Advanced Composite Structures
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• v.4 no.3
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• pp.13-21
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• 2013

In this study, uniaxial compression tests were performed to investigates the stress-strain relations of Double Skinned Composite Tubular Columns reinforced with steel tube. The confined concrete has been known as the strength of concrete increases significantly. Specimens reinforced with outer and inner steel tube were tested by uniaxial compression test. To investigate the influence of concrete strength increase by confining conditions in steel tubes, 8 specimens with different thickness of tube, hollowness ratio and concrete strength were tested and compared with other researcher's concrete material model.

• Advances in concrete construction
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• v.9 no.6
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• pp.529-535
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• 2020

The current study investigated experimentally the effectiveness of Carbon Fiber Reinforced Polymer (CFRP) in confining concrete cylinders after being subjected to high temperature. Parameters examined were: (a) the exposing temperatures (20, 100, 200, 400 600 and 700℃) and (b) the number of CFRP layers (1 and 3 layers). A uniaxial compressive testing was carried out on 36 concrete cylinders with dimensions of 150 mm×300 mm. The results obtained show that the compressive strength reduced with the increased of temperature compared to that measured at 20℃. In particular, the reduction in the compressive strength was more observed when the temperature exceeded 400℃. Further, the concrete cylinders confined with one and three layers of CFRP significantly increased the compressive strength compared to the counterpart unconfined specimen tested at the same temperature. Also, the average percentages of the increase in the compressive strength were approximately 112% and 158% when applying 1 and 3 layers of CFRP, respectively, compared to the counterpart unstrengthened specimen tested at the same temperature.

• Journal of Korean Society of Steel Construction
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• v.25 no.3
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• pp.243-254
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• 2013

Internally Confined Hollow RC(ICH RC) column consisted of concrete, transverse reinforcement, longitudinal reinforcement, and inner tube. It had good strength and ductility by core concrete was become triaxial confining state with transverse reinforcement and inner tube. There were two confining stress as external confining stress and internal confining stress in an ICH RC column. While external confining stress was researched by former researchers, internal confining stress has not researched. In this paper, confining stress of both Hollow RC column and ICH RC column was investigated using FEA program. Relation between theoretical confining stress and internal confining stress was drawn by analysis results. Modified failure condition equations of inner tube were suggested to base on failure condition equations of inner tube by former researcher. When thickness of inner tube was calculated by modified equations, it could be economic because thickness of inner tube was reduced 50% compared with former researcher equations in order to same confining stress.