• International Journal of Concrete Structures and Materials
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• v.18 no.2E
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• pp.79-87
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• 2006

The main objective of this research is to examine the behavior of high-strength concrete(HSC) columns. Eight test columns in one-third scale were tested under the conditions of cyclic lateral force and a constant axial load equal to 30% of the column axial load capacity. The $200{\times}200mm$ square columns were reinforced with eight DB bars constituting a longitudinal steel ratio of 2.54% of the column cross-sectional area. The main experimental parameters were volumetric ratio of transverse reinforcement(${\rho}_s$=1.58, 2.25 percent), tie configuration(Type H, Type C, Type D) and tie yield strength($f_{yh}$=548.8 and 779.1 MPa). It was found that the hysteretic behaviour and ultimate deformability of HSC columns were influenced by the amount and details of transverse reinforcement in the potential plastic hinge regions. Columns of transverse reinforcement in the amount 42 percent higher than that required by seismic provisions of ACI 318-02 showed ductile behavior. At 30% of the axial load capacity, it is recommended that the yield strength of transverse reinforcement be held equal to or below 548.8 MPa. Correlations between the calculated damage index and the damage progress are proposed.

• International Journal of Concrete Structures and Materials
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• v.18 no.1E
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• pp.49-54
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• 2006

Confinement is major contribution to bond strength between reinforcement steel bars and concrete. Cover thickness, bar spacing and transverse reinforcement are the key confinement factors of current provisions for the development and splices of reinforcement. However, current provisions are still too complicated to determine the values of the confinement, which need to be well delineated in the process of design. In this study, an experimental work using beam-end and splice specimens was performed to examine the effect of concrete cover on bond strength. The results of this experiment and previously available data are analyzed to identify the effects of confinement on bond strength. From this reevaluation, new provisions for the development and splices of reinforcement are proposed. The provisions suggest some limitations in the confinement index. The new provisions will allow the engineers to use a simple and yet satisfactory and appropriate method or a precise approach for design to determine the values of confinement on the calculation of development and splice lengths.

• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.5
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• pp.40-47
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• 2021

In this study, the structural behavior by the details of the lap region with the headed bar was estimated through finite element analysis. To solve the finite element analysis of the anchorage region with complex contact conditions and nonlinear behavior, a quasi-static analysis technique by explicit dynamic analysis was performed. The accuracy of the finite element model was verified by comparing the experimental results with the finite element analysis results. It was confirmed that the quasi-static analysis technique well reflected the behavior of enlarged headed bar connection. As a result of performing numerical analysis using 21 finite element models with various development lengths and transverse reinforcement indexes, it was confirmed that the increase of development length and transverse reinforcement index improved the maximum strength and ductility. However, to satisfy the structural performance, it should be confirmed that both design variables(development length and transverse reinforcement index) must be enough at the design criteria. In the recently revised design standard(KDS 14 20 52 :2021), a design formula of headed bar that considers both the development length and the transverse reinforcing bar index is presented. Also the results of this study confirmed that not only the development length but also transverse reinforcing bars have a very important effect.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.41-44
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• 2004

This experimental investigation was conducted to examine the behaviour of eight one-third scale columns made of high-strength concrete (HSC). The columns were subjected to a constant axial load corresponding to 30 per cent of the column axial load capacity and a cyclic horizontal load-inducing reversed bending moment. The variables studied in this research are the volumetric ratio of transverse reinforcement, tie configuration and tie yield strength. Columns with 42 per cent higher amounts of transverse reinforcement than that required by seismic provisions of ACI 318-02 showed ductile behaviour. Relationships between the calculated damage index and the observed damage such as initial crack, spalling of concrete, buckling of longitudinal bar, and crushing of concrete are propose.

• Journal of the Korea Concrete Institute
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• v.11 no.5
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• pp.61-68
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• 1999

Confinement is one of the major concepts for bond of reinforcing steel to concrete. Cover distance, and lateral reinforcement are the key factors for current provisions for development and splices of reinforcement. However, the current provisions still being complicated to calculate major variables need to be developed in the process of design. In this study, an experimental work was performed to examine the behavior of bond using beam end specimens. The test results and previous available data are analyzed to isolate the effects of confinement on bond strength. From this reevaluation, new provisions for development and splice of reinforcement are proposed. The provisions also propose some limits for confinement index. The new provisions will help engineers to decide easily the simple but conservative way for manual calculations or the exact approach for computerized design.

• Steel and Composite Structures
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• v.37 no.2
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• pp.211-227
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• 2020

To explore the feasibility of eliminating the longitudinal rebars and stirrups by using ultra-high-performance fiber reinforcement concrete (UHPFRC) in concrete encased steel composite stub column, compressive behavior of UHPFRC encased steel stub column has been experimentally investigated. Effect of concrete types (normal strength concrete, high strength concrete and UHPFRC), fiber fractions, and transverse reinforcement ratio on failure mode, ductility behavior and axial compressive resistance of composite columns have been quantified through axial compression tests. The experimental results show that concrete encased composite columns with NSC and HSC exhibit concrete crushing and spalling failure, respectively, while composite columns using UHPFRC exhibit concrete spitting and no concrete spalling is observed after failure. The incorporation of steel fiber as micro reinforcement significantly improves the concrete toughness, restrains the crack propagation and thus avoids the concrete spalling. No evidence of local buckling of rebars or yielding of stirrups has been detected in composite columns using UHPFRC. Steel fibers improve the bond strength between the concrete and, rebars and core shaped steel which contribute to the improvement of confining pressure on concrete. Three prediction models in Eurocode 4, AISC 360 and JGJ 138 and a proposed toughness index (T.I.) are employed to evaluate the compressive resistance and post peak ductility of the composite columns. It is found that all these three models predict close the compressive resistance of UHPFRC encased composite columns with/without the transverse reinforcement. UHPFRC encased composite columns can achieve a comparable level of ductility with the reinforced concrete (RC) columns using normal strength concrete. In terms of compressive resistance behavior, the feasibility of UHPFRC encased steel composite stub columns with lesser longitudinal reinforcement and stirrups has been verified in this study.

• Journal of the Korea Concrete Institute
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• v.25 no.2
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• pp.217-224
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• 2013

Design provisions for the development length of headed bars in ACI 318-08 include concrete compressive strength and yield strength of headed bars as design parameters but do not consider the effects of transvers reinforcement. In addition, they have very strict limitation for clear spacing and material strengths because these provisions were developed based on limited tests. In this study, splice tests using SD600 headed bars with $2d_b$ clear spacing and transverse reinforcement were conducted. Test results show that unconfined specimens failed due to prying action and bottom cover concrete prematurely spalled. The contribution of head bearing on the anchorage strength is only 15% on average implying that unconfined specimens failed before the head bearing was not sufficiently developed. Confined specimens with stirrups placed along whole splice length have enhanced strengths in bearing as well as bond because the stirrups prevented prying action and improved bond capacity. Bond failure occurred in locally confined specimens where stirrups were placed only at the ends of splice length. The stirrups at ends of splice lengths can prevent prying action but the bond capacity did not increase. From regression analysis of test results, an equation to predict anchorage strength of headed bars was developed. The proposed equation consists of bond and bearing contributions and includes transverse reinforcement index. The average ratio of tests to predictions is 1.0 with coefficient of variation of 6%.

• Journal of the Korea Concrete Institute
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• v.13 no.6
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• pp.553-560
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• 2001

The ductility capacity should be estimated for inelastic analysis and design of reinforced concrete flexural members. Therefore, to estimate the ductility capacity, the model of moment-curvature relationship of reinforced concrete flexural member is assumed in this study. The curvature, rotation, and displacement(deflection) of reinforced concrete cantilever beams are analyzed and tested. The analytical results are compared with the test results. According to the analytical and test results, the assumed model of moment-curvature relationship in this study is adequate in flexural analysis of reinforced concrete members because the analytical results are well agreed with the test results, and it is resonable to express the ductility capacity in the rotation or displacement ductility, Because the curvature ductility is the limited index in a certain section. It is investigated that the ductility capacity is proportional to lateral reinforcement and compression reinforcement and inversely proportional to tension reinforcement.

• Journal of the Korea Concrete Institute
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• v.28 no.3
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• pp.279-288
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• 2016

The present study simplified the moment-curvature relationship to straightforwardly determine the flexural behavior of reinforced concrete (RC) columns. For the idealized column section, moments and neutral axis depths at different stages(first flexural crack, yielding of tensile reinforcing bar, maximum strength, and 80% of the maximum strength at the descending branch) were derived on the basis of the equilibrium condition of forces and compatibility condition. Concrete strains at the extreme compression fiber beyond the maximum strength were determined using the stress-strain relationship of confined concrete, proposed by Kim et al. The lateral load-displacement curves converted from the simplified moment-curvature relationship of columns are well consistent with test results obtained from column specimens under various parameters. The moments and the corresponding neutral axis depth at different stages were formulated as a function of longitudinal reinforcement and transverse reinforcement indices and/or applied axial load index. Overall, curvature ductility of columns was significantly affected by the axial load level as well as concrete compressive strength and the amount of longitudinal and transverse reinforcing bars.

• Journal of the Korea Concrete Institute
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• v.28 no.5
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• pp.611-620
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• 2016

In the construction of nuclear power plants, only 420 MPa reinforcing bars are allowed and, therefore, so many large-diameter bars are placed, which results in steel congestion. Consequently, re-bar works are difficult and the quality of RC structures may be deteriorated. To solve the steel congestion, 550 MPa bars are necessary. Among many items for verifying structural performance of reinforced concrete with 550 MPa bars, the 43 mm hooked bars are examined in this study. All specimens failed by side-face blowout and the side cover explosively spalled at maximum loads. The bar force was initially transferred to the concrete primarily by bond along a straight portion. At the one third of maximum load, the bond reached a peak capacity and began to decline, while the hook bearing component rose rapidly. At failure, most load was resisted by the hook bearing. For confined specimens with hoops, the average value of test-to-prediction ratios by KCI code is 1.45. The modification factor of confining reinforcement which was not allowed for larger than 35 mm bars can be applied to 43 mm hooked bars. For specimens with 70 MPa concrete, the average value of test-to-prediction ratios by KCI code is 1.0 which is less than the values of the other specimens. The effects of concrete compressive strength should be reduced. An equation to predict anchorage capacity of hooked bars was developed from regression analysis including the effects of compressive strength of concrete, embedment length, side cover thickness, and transverse reinforcement index.