• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.6
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• pp.409-416
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• 2019

An improved numerical model for non-linear analysis of reinforced concrete (RC) slabs subjected to blast loading is proposed. This approach considers a strain rate dependent orthotropic constitutive model that directly determines the stress state using the stress-strain relation acquired from the data obtained using the biaxial strength envelope. Moreover, the bond-slip between concrete and reinforcing steel is gradually enlarged after the occurrence of cracks and is concentrated in the plastic hinge region. The bond-slip model is introduced to consider the crack direction of the concrete under a biaxial stress state. Correlation studies between the numerical analysis and the experimental results were performed to evaluate the analytical model. The results show that the proposed model can effectively be used in dynamic analyses of reinforced concrete slab members subjected to explosive loading. Moreover, it was determined that it is important to consider biaxial behavior in the material model and the bond-slip effect.

• Structural Engineering and Mechanics
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• v.82 no.1
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• pp.121-131
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• 2022

The main goal of this study is to prepare a program for analyzing High Strength Steel Fibrous Reinforced Concrete (HSSFRC) slabs and predict the response and strength of the slab instead of preparing a prototype and testing it in the laboratory. For this purpose, new equations are proposed to represent the material properties of High Strength Steel Fibrous Reinforced Concrete. The proposed equations obtained from performing regression analysis on many experimental results using statistical programs. The finite element method is adopted for non-linear analysis of the slabs. The eight-node "Serendipity element" (3 DoF) is chosen to represent the concrete. The layered approach is adopted for concrete elements and the steel reinforcement is represented by a smeared layer. The compression properties of the concrete are modeled by a work hardening plasticity approach and the yield condition is determined depending on the first two stress invariants. A tensile strength criterion is adopted in order to estimate the cracks propagation. many experimental results for testing slabs are compared with the numerical results of the present study and a good agreement is achieved regarding load-deflection curves and crack pattern. The response of the load deflection curve is slightly stiff at the beginning because the creep effect is not considered in this study and for assuming perfect bond between the steel reinforcement and the concrete, however, a great agreement is achieved between the ultimate load from the present study and experimental results. For the models of the tension stiffening and cracked shear modulus, the value of Bg and Bt (Where Bg and Bt are the curvature factor for the cracked shear modulus and tension stiffening models respectively) equal to 0.005 give good results compared with experimental result.

• Journal of the Korea Concrete Institute
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• v.18 no.1 s.91
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• pp.91-100
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• 2006

This paper presents an analytical model for evaluation of crack widths in structural concrete members. The model is mathematically derived from the actual bond stress-slip relationships between the reinforcement and the surrounding concrete, and the relationships summarized in CEB-FIP Model Code 1990 are employed in this study together with the assumption of a linear slip distribution along the interface at the stabilized cracking stage. With these, the actual strains of the steel and the concrete are integrated respectively along the embedment length between the adjacent cracks so as to obtain the difference in the axial elongation. The model is applied to the test specimens available in literatures, and the predicted values are shown to be in good agreement with the experimentally measured data.

• Journal of the Korean Society of Safety
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• v.27 no.6
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• pp.99-108
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• 2012

Ultra High Performance Concrete(UHPC) has compressive strength higher than 180 MPa. The use of steel fibers in the dense UHPC matrix increases tensile strength, ductility and bond strength between UHPC and rebars. However, to apply the advance material behavior of UHPC to the design of a structure, we need design formulas. The crack formula is one of them. This paper investigated experimentally the bond behavior of a rebar and K-UHPC, the UHPC developed by Korea Institute of Construction Technology, and, modified CEB-FIP crack formula based on the test. In addition, this paper tested the crack behavior of K-UHPC reinforced with rebars to verify the modified crack formula. The result showed that the modified formula is reasonable to predict the width of cracks in the reinforced K-UHPC structures.

• Journal of the Korean Society for Railway
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• v.12 no.6
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• pp.944-952
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• 2009

This paper presents an analytical model for calculation of crack widths in reinforced concrete structures. The model is mathematically derived from the actual bond stress-slip relationships between the reinforcement and the surrounding concrete, and the relationships summarized in CEB-FIP Model Code 1990 and Eurocode 2 are employed in this study together with the numerical analysis result of a linear slip distribution along the interface at the stabilized cracking stage. With these, the actual strains of the steel and the concrete are integrated respectively along the embedment length between the adjacent cracks so as to obtain the difference in the axial elongation. The model is applied to the test results available in literatures, and the predicted values are shown to be in good agreement with the experimentally measured data.

• Computers and Concrete
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• v.19 no.5
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• pp.443-449
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• 2017

Taking high geothermal tunnels as background, the deformation of concrete for shotcrete use was studied by simulating hot-humid and hot-dry environments in a laboratory. The research is made up by two parts, one is the influence of two kinds of high geothermal environments on the deformation of shotcrete, and the other is the shrinkage inhibited effect of fiber materials (steel fibers, polypropylene fibers, and the mixture of both) on the concrete in hot-dry environments. The research results show that: (1) in hot and humid environments, wet expansion and thermal expansion happened on concrete, but the deformation is smooth throughout the whole curing age. (2) In hot and dry environments, the concrete suffers from shrinkage. The deformation obeys linear relationship with the natural logarithm of curing age in the first 28 days, and it becomes stable after the $28^{th}$ day. (3) The shrinkage of concrete in a hot and dry environment can be inhibited by adding fiber materials especially steel fibers, and it also obeys linear relationship with the natural logarithm of curing age before it becomes stable. However, compared with no-fiber condition, it takes 14 days, half of 28 days, to make the shrinkage become stable, and the shrinkage ratio of concrete at 180-day age decreases by 63.2% as well. (4) According to submicroscopic and microscopic analysis, there is great bond strength at the interface between steel fiber and concrete. The fiber meshes are formed in concrete by disorderly distributed fibers, which not only can effectively restrain the shrinkage, but also prevent the micro and macro cracks from extending.

• Earthquakes and Structures
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• v.7 no.5
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• pp.691-704
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• 2014

Reinforced concrete (RC) shear walls are commonly used for building structures to resist seismic loading. While the RC shear walls can have a high load-carrying capacity, they tend to fail in a brittle mode under shear, accompanied by forming large diagonal cracks and bond splitting between concrete and steel reinforcement. Improving seismic performance of shear walls has remained a challenge for researchers all over the world. Engineered Cementitious Composite (ECC), featuring incredible ductility under tension, can be a promising material to replace concrete in shear walls with improved performance. Currently, the application of ECC to large structures is limited due to the lack of the proper constitutive models especially under shear. In this paper, a new Cyclic Softening Membrane Model for reinforced ECC is proposed. The model was built upon the Cyclic Softening Membrane Model for reinforced concrete by (Hsu and Mo 2010). The model was then implemented in the OpenSees program to perform analysis on several cases of shear walls under seismic loading. The seismic response of reinforced ECC compared with RC shear walls under monotonic and cyclic loading, their difference in pinching effect and energy dissipation capacity were studied. The modeling results revealed that reinforced ECC shear walls can have superior seismic performance to traditional RC shear walls.

• Journal of the Korea Concrete Institute
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• v.17 no.3 s.87
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• pp.375-384
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• 2005

The effects of bar deformation properties on bond of steel reinforcing bars to concrete are experimentally studied to predict the bond strength. Based on the previous research about high relative rib area, bond strength between reinforcing bars and concrete can be improved by the control of rib height and spacing. But, the equations in Korean code provisions to estimate development and splice length do not include these specifications of reinforcing bars. So the purpose of this paper is to determine the effect of relative rib area to the bond strength. This paper describes 2 kinds of experimental researches. Thirty beam-end specimens were tested to investigate the effects of bar size and relative rib areas ranging from 0.112 to 0.162. And, twelve lap-splice beam specimens were tested to the same variables. Each test results are normalized and compared with the proposed equations of ACI 408 committee. The results show that bond strength is increased as bar size and the relative rib area(Rr) increase. The distribution of flexural cracks and failure aspect do not appear to be affected by $R_r$.

• Journal of the Korea Concrete Institute
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• v.23 no.2
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• pp.135-144
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• 2011

Recently, researches about fiber reinforced polymer (FRP) which has excellent durability, corrosion resistance, and tensile strength as a substitution material to steel tendon have been actively pursued. This study is performed to examine FRP tendon used prestressed beam's safety under service load. The specimen was a prestressed concrete beam with internal bonded FRP tendon. In order to compare the member fatigue capacity, a control specimen of a prestressed concrete beam with ordinary steel tendon was tested. A fatigue load was applied at a load range of 60%, 70%, and 80% of the 40% ultimate load, which was obtained though a static test. The fatigue load was applied as a 1~3 Hz sine wave with 4 point loading setup. Fatigue load with maximum 1 million cycles was applied. The specimen applied with a load ranging between 40~60% did not show a fatigue failure until 1 million cycles. However, it was found that horizontal cracks in the direction of tendons were found and bond force between the tendon and concrete was degraded as the load cycles increased. This fatigue study showed that the prestressed concrete beam using FRP tendon was safe under a fatigue load within a service load range. Fatigue strength of the specimen with FRP and steel tendon after 1 million cycles was 69.2% and 59.8% of the prestressed concrete beam's static strength, respectively.

• Structural Engineering and Mechanics
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• v.65 no.3
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• pp.291-302
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• 2018

Rigid body spring method (RBSM) is an effective tool to simulate the cracking process of structures, and has been successfully applied to investigate the behavior of reinforced concrete (RC) members. However, the theoretical researches and engineering applications of this method mainly focus on two-dimensional problems as yet, which greatly limits its applications in actual engineering projects. In this study, a three-dimensional (3-D) RBSM for RC structures is proposed. In the proposed model, concrete, reinforcing steels, and their interfaces are represented as discrete entities. Concrete is partitioned into a collection of rigid blocks and a uniform distribution of normal and tangential springs is defined along their boundaries to reflect its material properties. Reinforcement is modeled as a series of bar elements which can be freely positioned in the structural domain and irrespective of the mesh geometry of concrete. The bond-slip characteristics between reinforcing steel and concrete are also considered by introducing special linkage elements. The applicability and effectiveness of the proposed method is firstly confirmed by an elastic T-shape beam, and then it is applied to analyze the failure processes of a Z-type component under direct shear loading and a RC beam under two-point loading.