• Steel and Composite Structures
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• v.48 no.2
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• pp.131-143
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• 2023

The CFRP bar was used to achieve more ductile and durable headed-stud shear connectors in composite components. Three series of push-out tests were firstly conducted, including specimens reinforced with pure steel fibers, steel and CFRP bars. The distributed stress was measured by the commercial PPP-BOTDA (Pre-Pump-Pulse Brillouin optical time domain analysis) optical fiber sensor with high spatial resolution. A series of numerical analyses using non-linear FE models were also made to study the shear force transfer mechanism and crack response based on the test results. Test results show that the CFRP bar increases the shear strength and stiffness of the large diameter headed-stud shear connection, and it has equivalent reinforcing effects on the stud shear capacity as the commonly used steel bar. The embedded CFRP bar can also largely improve the shear force transfer mechanism and decrease the tensile stress in the transverse direction. The parametric study shows that low content steel fibers could delay the crack initiation of slab around the large diameter stud, and the CFRP bar with normal elastic modulus and the standard reinforcement ratio has good resistance to splitting crack growth in headed stud shear connectors.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.2
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• pp.38-46
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• 2014

Effects of various parameters on bond property between reinforcing bar and concrete are investigated in many researchers, and various study is on going to improve the bond strength. Properties of interface between reinforcement and concrete is important role in bond property. This study analyzed the interfacial bond mechanism between deformed bar and concrete by finite element analysis (FEA) to evaluate the effect of rib shape. The FEA model in this study is simplified 2D plane stress model. The variables of analysis are selected by rib angle, rib height, rib spacing and relative rib area. From the results of analysis, reinforcing bars with rib angle $30{\sim}60^{\circ}$ showed better bond strength than the others. Bond strength ratio following to the rib height is proportionally increased up to the $0.12d_b$, but rib spacing has little effect on bond strength. The results also indicated that relative rib area can be efficiently represented the properties of deformed shape in reinforcing bars, and zigzagged rib height shape showed excellent bond strength increase.

• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.6
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• pp.1-9
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• 2015

Transverse steel bars are used in the plastic hinge zone of columns to insure adequate confinement, prevention of longitudinal bar buckling and ductile behavior. Fabrication and placement of rectangular hoops and cross-ties in columns are difficult to construct. Details of reinforcement for rectangular section require a lot of rectangular hoops and cross-ties. In this paper, to solve these problems, the new lateral confinement method using oblong hoop is proposed for the transverse confinement of the flared column. It can be the alternative for oblong cross-section and flared column with improved workability and cost-efficiency. The final objectives of this study are to suggest appropriate oblong hoop details and to provide quantitative reference data and tendency for seismic performance or damage assessment based on the drift levels such as residual deformation, elastic strain energy. This paper describes factors of seismic performance such as ultimate displacement/drift ratio, displacement ductility, response modification factor, equivalent viscous damping ratio and effective stiffness.

• Journal of the Korea Concrete Institute
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• v.27 no.6
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• pp.615-623
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• 2015

The previous strut-and-tie models (STMs) to evaluate the shear strength of squat shear walls with aspect ratio less than 2.0 do not consider the axial load transfer of concrete strut and individual shear transfer contribution of horizontal and vertical shear reinforcing bars in the web. To overcome the limitation of the existing models, a simple STM was established based on the crack band theory of concrete fracture mechanics. The equivalent effective width of concrete strut having a stress relief strip was determined from the neutral axis depth and effective factor of concrete strength. The shear transfer mechanism of shear reinforcement at the extended crack band zone was calculated from an internally statically indeterminate truss system. The shear transfer capacity of concrete strut and shear reinforcement was then driven using the energy equilibrium in the stress relief strip and crack band zone. The shear strength predictions of squat shear walls evaluated from the current models are in better agreement with 150 test results than those determined from STMs proposed by Siao and Hwang et al. Furthermore, the proposed STM gives consistent agreement with the observed trend of the shear strength of shear walls against different parameters.

• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.6
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• pp.67-76
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• 2020

This paper reports an experimental study to evaluate the flexural behavior of concrete beams reinforced using Fe based shape memory alloy (Fe-SMA) bars. For the experiment, a concrete beam of 200mm×300mm×2,200mm was produced, and a 4% pre-strained Fe-SMA bar was used as a tensile reinforcement. As experimental variables, type of tensile reinforcement (SD400, Fe-SMA), reinforcement ratio (0.2, 0.39, 0.59, 0.78), activation of Fe-SMA (activation, non-activation), and joint method of Fe-SMA bar (Continuous, welding, coupler) were considered. The electric resistance heating method was used to activate the Fe-SMA bar, and a current of 5A/㎟ was supplied until the specimen reached 160℃. After the upward displacement of the specimen due to the camber effect was stabilized, a three-point flexural loading experiment was performed using an actuator of 2,000 kN capacity. As a result of the experiment, it was found that the upward displacement occurred due to the camber effect as the Fe-SMA bar was activated. The specimen that activated the Fe-SMA bar had an initial crack at a higher load than the specimen that did not activate it. However, as with general prestressed concrete, the effect of the prestress by Fe-SMA activation on the ultimate state of the beam was insignificant.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.6
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• pp.81-88
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• 2018

The present study evaluated the effective arrangement approach of transverse reinforcement in the jacket section for seismic strengthening of reinforced concrete columns. To simulate the full-scale columns, the section dimensions were determined as $450{\times}450mm$ for non-seismic existing columns and $750{\times}750mm$ for section enlargement strengthening columns. Over-lapped channel-shape bars and prefabricated bar units were proposed for closed-hoops in the jacket section, and conventional cross-ties anchored into existing columns and V-ties were considered for the supplementary ties. Test results showed that the axial capacity of the existing column and section enlargement columns with over-lapped channel-shape hoops was similar to the nominal strength calculated using ACI 318-14 procedure whereas the section enlargement column with prefabricated bar units possessed 1.25 times higher axial capacity than the nominal prediction. Furthermore, the axial ductility ratio of the section enlargement column with prefabricated bar unit was 139% higher than that of the existing column despite the potential size effect on ductility of concrete. Thus, it can be concluded that the developed prefabricated bar unit technique is practically useful for preventing the premature buckling of longitudinal reinforcement and confining core concrete in the section enlargement strengthening columns.

• Journal of the Korea Concrete Institute
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• v.28 no.4
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• pp.473-480
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• 2016

The purpose of this study is to provide analytical method to reasonably evaluate the complicated failure behaviors of shear friction of reinforced concrete shear wall specimens using grade 500 MPa high-strength bars. A total of 16 test specimens with a variety of variables such as aspect ratio, friction coefficient of interface in construction joint, reinforcement details, reinforcement ratio in each direction, material properties were selected and the analysis was performed by using a non-linear finite element analysis program (RCAHEST) applying the modified shear friction constitutive equation in interface based on the concrete design code (KCI, 2012) and CEB-FIP Model code 2010. The mean and coefficient of variation for maximum load from the experiment and analysis results was predicted 1.04 and 17% respectively and properly evaluated failure mode and overall behavior characteristic until failure occur. Based on the results, the analysis program that was applied modified shear friction constitutive equation is judged as having a relatively high reliability for the analysis results.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.12
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• pp.772-781
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• 2020

A direct shear member resists external forces through the shear transfer of reinforcing bars placed at the concrete interface. The current concrete structural design code uses empirical formulas based on the shear friction analogy, which is applied to the horizontal shear of concrete composite beams. However, in the case of a member with a large amount of reinforcing bars, the shear strength obtained through the empirical formula is lower than the measured value. In this paper, the limit state of newly constructed composite beams on an existing concrete girder is defined using stress field theory, and material constitutive laws are applied to gain horizontal shear strength while considering the tension-stiffening and softening effects of concrete struts. A simplified method of calculating the shear strength is proposed, which was validated by comparing it with the related design code provisions. As a result, it was confirmed that the method generally shows a similar tendency to the experimental results when the shear reinforcing bar yields, unlike the regulations of the design code, where differences in the predicted value of shear strength occur according to the shear reinforcement ratio.

• Smart Structures and Systems
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• v.17 no.4
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• pp.593-610
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• 2016

In this study, an innovative and smart glass fiber-reinforced polymer (GFRP) hybrid bar was developed for stronger durability of concrete structures. As comparing with the conventional GFRP bar, the smart GFRP Hybrid bar can promise to enhance the modulus of elasticity so that it makes the cracking reduced than the case when the conventional GFRP bar is used. Besides, the GFRP Hybrid bar can effectively resist the corrosion of conventional steel bar by the GFRP outer surface on the steel bar. In order to verify the bond performance of the GFRP hybrid bar for structural reinforcement, uniaxial pull-out test was conducted. The variables were the bar diameter and the number of strands and pitch of the fiber ribs. Tensile tests showed a excellent increase in the modulus of elasticity, 152.1 GPa, as compared to that of the pure GFRP bar (50 GPa). The stress-strain curve was bi-linear, so that the ductile performance could be obtained. For the bond test, the entire GFRP hybrid bar test specimens failed in concrete splitting due to higher shear strength resulting in concrete crushing as a function of bar deformation. Investigation revealed that an increase in the number of strands of fiber ribs enhanced the bond strength, and the pitch guaranteed the bond strength of 19.1 mm diameter hybrid bar with 15.9 mm diameter of core section of deformed steel the ACI 440 1R-15 equation is regarded as more suitable for predicting the bond strength of GFRP hybrid bars, whereas the CSA S806-12 prediction is considered too conservative and is largely influenced by the bar diameter. For further study, various geometrical and material properties such as concrete cover, cross-sectional ratio, and surface treatment should be considered.

• Journal of the Korea Concrete Institute
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• v.26 no.3
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• pp.299-306
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• 2014

Coating is one of the methods used to solve the problem of corrosion of reinforcement in concrete structures. There are few research reported in the literature regarding the effect of zinc-coating on flexural behavior compared to epoxy coating. The objective of this study was to determine whether zinc-coated rebar adversely affects flexural behavior. Concrete beams reinforced with black or zinc-coated steel were tested in flexure. The test variables included the presence of rebar surface coating with zinc, steel ratio used and cover depth. The study concentrated on comparing crack pattern, crack width, deflection and strain. The ultimate flexural capacity of beams reinforced with zinc-coated bars was not different from that of black steel reinforced beams. The results from deflection and crack width measurements were indicative of no significant variation for the different rebar surface conditions. In addition, it was found that load-strain curve of beam reinforced with zinc-coated steel was similar to that of beam reinforced with zinc-coated steel. Therefore, the test results indicated that the use of zinc-coated rebar had no adverse effect on flexural behavior compared to the use of black rebar.