• Advances in concrete construction
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• v.8 no.3
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• pp.207-215
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• 2019

Application of nanotechnology can be used to tailor made cementitious composites owing to small dimension and physical behaviour of resulting hydration products. Because of high aspect ratio and extremely high strength, carbon nanotubes (CNTs) are perfect reinforcing materials. Hence, there is a great prospect to use CNTs in developing new generation cementitious materials. In the present paper, a parametric study has been conducted on cementitious composites reinforced by two types of multi walled carbon nanotubes (MWCNTs) designated as Type I CNT (10-20 nm outer dia.) and Type II CNT (30-50 nm outer dia.) with various concentrations ranging from 0.1% to 0.5% by weight of cement. To evaluate important properties such as flexural strength, strain to failure, elastic modulus and modulus of toughness of the CNT admixed specimens at different curing periods, flexural bending tests were performed. Results show that composites with Type II CNTs gave more strength as compared to Type I CNTs. The highest increase in strength (flexural and compressive) is of the order of 22% and 33%, respectively, compared to control samples. Modulus of toughness at 28 days showed highest improvement of 265% for Type II 0.3% CNT composites. It is obvious that an optimum percentage of CNT could exists for composites to achieve suitable reinforcement behaviour and desired strength properties. Based on the parametric study, a tentative optimum CNT concentration (0.3% by weight of cement) has been proposed. Scanning electron microscope image shows perfect crack bridging mechanism; several of the CNTs were shown to act as crack arrestors across fine cracks along with some CNTs breakage.

• Journal of the Korea institute for structural maintenance and inspection
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• v.28 no.5
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• pp.38-46
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• 2024

In Korea, more than 60% of the population lives in apartment buildings with wall structures that exhibit brittle behavior during earthquakes. Therefore, in recent performance-based seismic design, the selection of the energy dissipation coefficient for reinforced concrete (RC) walls in nonlinear dynamic analysis is very important. Previous experimental studies have reported that the main factors affecting the energy dissipation capacity of RC walls are the axial force ratio, the spacing of transverse reinforcement of boundary element, and the aspect ratio. The Architectural Institute of Korea and the Korea Concrete Institute proposed a concentrated plastic hinge model and the energy dissipation coefficient for each RC member in the guideline 「Nonlinear Analysis Model for Performance-Based Seismic Design of Reinforced Concrete Building Structures, 2021.」 The proposed equation for the energy dissipation coefficient does not include the factors of axial force ratio and spacing of transverse reinforcement of boundary element. The aspect ratio is applied to the flexural plastic model, despite considering shear-dominated behavior. Therefore, it is necessary to examine the effect of the aspect ratio according to the analysis model. In this study, the influence of each factor on the energy dissipation coefficient was analyzed by comparing the results of existing experimental research, nonlinear analysis using the fiber element model of a nonlinear analysis program(Perform 3D), and the energy dissipation coefficient proposed in the guideline. As the axial force ratio increased, the energy dissipation coefficient decreased, and as the spacing of transverse reinforcement of boundary element decreased, the energy dissipation coefficient increased. Additionally, as the aspect ratio increased, the energy dissipation coefficient tended to increase, with the aspect ratio showing the greatest influence.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.177-180
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• 2008

Recently, the research of FRP bar as an alternative reinforcing material in reinforced concrete structures has increased to get an innovative solution to the corrosion problem. In addition to the noncorrosive nature of FRP materials, they also have a high strength-to-weight ratio. Therefore, when light weight concrete reinforced with FRP bar is used in marine environment, for instance floating structures, some advantages can be expected. But researches for the light weight concrete structure using FRP bar as a flexural reinforcement are limited to date. In this paper, the concrete shear contribution of the light weight concrete beam reinforced with GFRP bar was studied. Experiment for beams varying concrete compressive strengths and flexural reinforcement ratios was conducted and analysed. The test results showed that 75% of values obtained from proposed equation in preceding research were well agreed with the test results and were better results than the one predicted by the ACI 440.1R-06 code.

• Journal of the Korea Concrete Institute
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• v.22 no.2
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• pp.199-208
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• 2010

In order to substitute FRP bar for steel bar in new structures, it is necessary to establish a reliable design code. But relatively little research has been conducted on the material in Korea. So, a total of 22 beam specimens (18 GFRP reinforced concrete and 4 conventional steel reinforced concrete) were constructed and tested. In the first phase of the experiment, it was carried out to observe flexural behavior, and collect deflection and crack data. In order to eliminate of the uncertainty by the shear reinforcements and induce flexural failure mode, any stirrup were not used and only shear span-depth ratio were adjusted. However, almost beams were broken by shear and the ACI 440.1R, CSA S806, which were used to design test beams, showed considerable deviation between prediction and test results of shear strengths. Therefore in the second phase of the study, shear failure modes and behavior were observed. A standard specimen had dimensions of 3,300 mm long ${\times}$ 800 mm wide ${\times}$ 200 mm effective depth. Clear span and shear span were 2,800 mm, 1,200 mm respectively. Control shear span-depth ratio was 6.0. Four-point bending test over simple support was conducted. Variables of the specimens were concrete compressive strength, type and elastic modulus of reinforcement, shear span-depth ratio, effective reinforcement ratio, the effect of bundle placing method and cover thickness.

• Journal of the Korea Concrete Institute
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• v.11 no.5
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• pp.79-88
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• 1999

The purpose of this paper is to develop a mathematical expression for computing crack angles based on reinforcement volumes in the longitudinal and transverse directions, member end-fixity and length-to-width aspect ratio. For this a reinforced concrete beam-column element is assumed to possess a series of potential crack planes represented by a number of differential truss elements. Depending on the boundary condition, a constant angle truss or a variable angle truss is employed to model the cracked structural concrete member. The truss models are then analyzed using the virtual work method of analysis to relate forces and deformations. Rigorous and simplified solution schemes are presented. An equation to estimate the theoretical crack angle is derived by considering the energy minimization on the virtual work done over both the shear and flexural components the energy minimization on the virtual work done over both the shear and flexural components of truss models. The crack angle in this study is defined as the steepest one among fan-shaped angles measured from the longitudinal axis of the member to the diagonal crack. The theoretical crack angle predictions are validated against experimentally observed crack angle reported by previous researchers in the literature. Good agreement between theory and experiment is obtained.

• Advances in concrete construction
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• v.8 no.4
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• pp.311-320
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• 2019

In this study, the effect of cellulose nanocrystals (CNCs) on the fire resistance properties of fiber-reinforced cement composites was investigated. The main variables were CNCs content (0.4, 0.8 and 1.2vol.% compared with cement), steel fiber ratio, and exposure temperature (100, 200, 400, 600 and 800℃). The fire resistance properties, i.e., residual compressive strength, flexural strength, and porosity, were evaluated in relation with the exposure temperature of the specimens. The CNCs suspensions were prepared to composited dispersion method of magnetic stirring and ultra-sonication. CNCs are effective for increasing the compressive strength at high temperatures but CNCs do not seem to have a significant effect on flexural reinforcement. Porosity test result showed CNCs reduce the non-hydration area inside the cement and promote hydration.

• Steel and Composite Structures
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• v.35 no.3
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• pp.371-384
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• 2020

The steel-concrete double composite girder in the negative flexural region combines an additional concrete slab to the steel bottom flange to prevent the local steel buckling, however, the additional concrete slab may lower down the neutral axis of the composite section, which is a sensitive factor to the tensile stress restraint on the concrete deck. This is actually of great importance to the structural rationality and durability, but has not been investigated in detail yet. In this case, a series of 5.5 m-long composite girder specimens were tested by negative bending, among which the bottom slab configuration and the longitudinal reinforcement ratio in the concrete deck were the parameters. Furthermore, an analytical study concerning about the influence of bottom concrete slab thickness on the cracking and sectional bending-carrying capacity were carried out. The test results showed that the additional concrete at the bottom improved the composite sectional bending stiffness and bending-carrying capacity, whereas its effect on the concrete crack distribution was not obvious. According to the analytical study, the additional concrete slab at the bottom with an equivalent thickness to the concrete deck slab may provide the best contributions to the improvements of crack initiation bending moment and the sectional bending-carrying capacity. This can be applied for the design practice.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.5
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• pp.1-9
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• 2007

This study summarizes the results of a research project aimed at investigating the inelastic rotation capacity of beam-column connections of reinforced concrete moment frames. A total of 91 test specimens for beam-column joint connections were examined in detail, and 28 specimens were classified as special moment frame connections based on the design and detailing requirements in the ACI 318-02 Provisions. Then the acceptance criteria, originally defined for steel moment frame connections in the AISC-02 Seismic Provisions, were used to evaluate the joint connections of concrete moment frames. Twenty-seven out of 28 test specimens that satisfy the design requirements for special moment frame structures provide sufficient strength and are ductile up to a plastic rotation of 0.03 rad. without any major degradation in strength. Joint shear stress, column-to-beam flexural strength ratio, and transverse reinforcement ratio in a joint all play a key role in good performance of the connections.

• Journal of the Korea institute for structural maintenance and inspection
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• v.9 no.1
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• pp.259-269
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• 2005

This paper describes the application of genetic algorithm for the discrete optimum design of reinforced concrete continuous beams. The objective is to minimize the total cost of reinforced concrete beams including the costs of concrete, form work, main reinforcement and stirrup. The flexural and shear strength, deflection, crack, spacing of reinforcement, concrete cover, upper-lower bounds on main reinforcement, beam width-depth ratio and anchorage for main reinforcement are considered as the constraints. The width and effective depth of beam and steel area are taken as design variables, and those are selected among the discrete design space which is composed with dimensions and steel area being used from in practice. Optimum result obtained from GA is compared with other literature to verify the validity of GA. To show the applicability and efficiency of GA, it is applied to three and five span reinforced concrete beams satisfying with the Korean standard specifications.

• Magazine of the Korea Concrete Institute
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• v.6 no.2
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• pp.118-128
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• 1994

There have been conducted a lot of works on shear behavior of steel fiber reinforced concrete beams. Fiber reinforced concrete beams without shear reinforcement were tested to determine their cracking shear strengths and ultimate shear capacities. Results of tests on 14 reinforced concrete beams (including 11 containing steel fibers) are reported. Two parameters were varied in the study, namely, the volume fraction of fibers and shear span-to-depth ratio.The effects of fiber incorporation on failure modes, deflections, cracking shear strength, and ul~imate shear strength have been examined. Resistance to shear stresses have been found to be improved by the inclusion of fibers, The mode of failure changed from shear to flexure when the shear span-to-depth ratio exceeds 3.4. Based on these investigations, a method of computing the shear strength of steel fiber reinforced concrete beam is suggested. The comparisons between computed values and expenmentally observed values are shown to verify the proposed theoretical treatment and steel fibers efficiency.