• Journal of the Korea Concrete Institute
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• v.22 no.6
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• pp.777-786
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• 2010

This paper examines the flexural behavior of full-scale prestressed concrete girders that were constructed of steel fiber reinforced ultra high performance concrete (UHPC). This study is designed to provide more information about the bending characteristics of UHPC girders in order to establish a reasonable prediction model for flexural resistance and deflection for future structural design codes. Short steel fibers have been introduced into prestressed concrete T-girders in order to study their effects under flexural loads. Round straight high strength steel fibers were used at volume fraction of 2%. The girders were cast using 150~190 MPa steel fiber reinforced UHPC and were designed to assess the ability of steel fiber reinforced UHPC to carry flexural loads in prestressed girders. The experimental results show that steel fiber reinforced UHPC enhances the cracking behavior and ductility of beams. Moreover, when ultimate failure did occur, the failure of girders composed of steel fiber reinforced UHPC was observed to be precipitated by the pullout of steel fibers that were bridging tension cracks in the concrete. Flexural failure of girders occurred when the UHPC at a particular cross section began to lose tensile capacity due to steel fiber pullout. In addition, it was determined that the level of prestressing force influenced the ultimate load capacity.

• Journal of Industrial Technology
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• v.21 no.B
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• pp.331-339
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• 2001

Zero to tension fatigue tests and strain controlled fatigue tests were carried out to find how initial strain induced martensite, ${\alpha}^{\prime}$ affects low and high cycle fatigue behavior and fatigue crack growth mechanisms. Microscopic study and phase analysis were carried out with TEM, SEM, EDAX, Optical Microscope, Ferriscope, and X-ray diffractometry. The amount of Initial ${\alpha}^{\prime}$ was controlled from 0% to 33% by controlling the temperatures for cold working and heat treatment. Lower contents of initial ${\alpha}^{\prime}$ showed higher fatigue resistance in low cycle fatigue but lower fatigue resistance in high cycle fatigue because it is ascribed to the more transformation of ${\alpha}^{\prime}$ martensite during low cycle fatigue and higher ductility. In high cycle fatigue, fatigue life is attributed to the strength and phase transformation of austenite into ${\alpha}^{\prime}$ during fatigue was negligible. ${\gamma}$ boundary, ${\gamma}/twin$ boundary, and ${\gamma}/{\alpha}^{\prime}$ boundary were found to be the preferred site of fatigue crack initiation.

• International Journal of Concrete Structures and Materials
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• v.3 no.1
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• pp.71-77
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• 2009

A theoretical model of multiple cracking failure mechanism is proposed herein for fiber reinforced high performance Cementitious composites. By introducing partial debonding energy dissipation on non-first cracking plane and fiber reinforcing parameter, the failure mechanism model of multiple cracking is established based on the equilibrium assumption of total energy dissipation on the first crack plane and non-first cracking plane. Based on the assumption of the first crack to be the final failure crack, energy dissipation terms including complete debonding energy, partial debonding energy, strain energy of steel fiber, frictional energy, and matrix fracture energy have been modified and simplified. By comparing multiple cracking number and energy dissipations with experiment results of the reference's data, it indicates that this model can describe the multiple cracking behavior of fiber reinforced high performance cementitious composites and the influence of the partial debonding term on energy dissipation is significant. The model proposed may lay a foundation for the predictions of the first cracking capacity and post cracking capacity of fiber reinforced high performance cementitious composites and also can be a reference for optimal mixture for construction cost.

• Structural Engineering and Mechanics
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• v.71 no.3
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• pp.211-221
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• 2019

To study the eccentric compression behavior of ultra-high performance fiber reinforced concrete (UHPFRC) columns, six UHPFRC columns and one high-strength concrete (HSC) column were tested. Variation parameters include load eccentricity, volume of steel fibers and stirrup ratio. The crack pattern, failure mode, bearing capacity, and deformation of the specimens were studied. The results showed that the UHPFRC columns had different failure modes. The large eccentric compression failure mode was the longitudinal tensile reinforcements yielded and many horizontal cracks appeared in the tension zone. The small eccentric compression failure mode was the longitudinal compressive reinforcements yielded and vertical cracks appeared in the compressive zone. Because of the bridging effect of steel fibers, the number of cracks significantly increased, and the width of cracks decreased. The load-deflection curves of the UHPFRC columns showed gradually descending without sudden dropping, indicating that the specimens had better deformation. The finite element (FE) analysis was performed to stimulate the damage process of the specimens with monotonic loading. The concrete damaged plasticity (CDP) model was adopted to characterize the behaviour of UHPFRC. The contribution of the UHPFRC tensile strength was considered in the bearing capacity, and the theoretical calculation formulas were derived. The theoretical calculation results were consistent with the test results. This research can provide the experimental and theoretical basis for UHPFRC columns in engineering applications.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.3A
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• pp.317-328
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• 2010

The method to evaluate the flexural capacity of steel fiber-reinforced ultra high strength concrete beams was proposed in this study. An experimental program was set up and fourteen beams have been tested. Test results were compared with predictions by design code and by the proposed method, respectively. It was found that predictions by using ACI 544 Committee recommendations considerably underestimate the flexural capacity. Underestimation of flexural capacity resulted from that of tensile stress block. Three-point bending test data of notched prism specimens and their inverse analysis results were incorporated into modeling of tension stress block. The ratio of the predicted to the experimental flexural capacity was in the range of 0.98 to 1.14. The present study represents that the proposed method allows more realistic prediction of flexural capacity of steel fiber-reinforced ultra high strength concrete beams.

• Structural Engineering and Mechanics
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• v.17 no.3_4
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• pp.291-302
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• 2004

In this paper, load monitoring technologies using U-shape Magnetoelastic (EM or ME) sensors have been exploited systemically for the first time. The steel rod to be tested is the Japan 7 mm piano steel rod. The load dependence of the magnetic properties of the piano steel rod was manifested. Two experimental designs of U-shape magnetoelastic sensors were introduced, one with double pick-up concentric coils wound on the rod to be tested, the other with pick-up coil on one yoke foot. The former design is used to derive the correlation of the relative permeability with elastic tension, while the latter is aimed to reflect the stress induced magnetic flux variation along the magnetic circuit. Magnetostatic simulations provide interpretations for the yoke foot sensing technology. Tests with double pick-up coils indicate that under proper working points (primary voltages), the relative permeability varies linearly with the axial load for the Japan 7 mm piano steel rod. Tests with pick-up coil on the yoke foot show that the integrated sensing voltage changes quadratically with the load, and error is more acceptable when the working point is high enough.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.8
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• pp.2055-2061
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• 2009

There can be some variation in the load carrying capacity of high-tension bolt friction joints when oversize bolt holes are made on the base plate and the cover plate. This study performed a static tensile test in order to examine the variation of slip load and slip coefficient according to standard bolt hole and oversize bolt hole in high-tension bolt friction joints. According to the results of the static tensile test, the slip coefficient changed to some degree according to oversize bolt holes on the base plate and the cover plate, but it was somewhat unreasonable to find a pattern in the change. Sliding strength showed a difference of up to 26% between the use of standard bolt holes and the use of oversize bolt holes. Because this exceeds the design sliding strength, however, its effect on the serviceability of joints under service load is insignificant. Thus, if the regulation on oversize bolt holes, which may be inevitable in making steel members, is applied flexibly, we may improve efficiency and economy in the design and construction of structures.

• Journal of Korean Association for Spatial Structures
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• v.5 no.2 s.16
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• pp.89-97
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• 2005

As building structures are becoming high-rise, large-scale, and specialized, the use of high-strength materials increase. Therefore, an analytical model is necessary to appropriately predict the shear strength of reinforced concrete (RC) beams with high-strength materials. This study presents a truss model which is able to reasonably predict the shear strength of the RC beams having high-strength steel bars. Test results of 107 RC beams reported in the technical literatures were collected to check the validity of proposed model, TATM, for the shear strength of the RC beams with high-strength reinforcing bars. They were compared to theoretical results obtained from proposed model, TATM, and existing truss models. The experimental results were better predicted by TATM rather than other truss models, and the ratios of experimental results to theoretical results obtained from TATM were almost constant regardless of the yield strengths of tension and shear reinforcements.

• Computers and Concrete
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• v.22 no.3
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• pp.337-353
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• 2018

The present study focuses on examining the structural behaviour of steel-fibre-reinforced concrete (SFRC) beams under high rates of loading largely associated with impact problems. Fibres are added to the concrete mix to enhance ductility and energy absorption, which is important for impact-resistant design. A simple, yet practical non-linear finite-element analysis (NLFEA) model was used in the present study. Experimental static and impact tests were also carried out on beams spanning 1.3 meter with weights dropped from heights of 1.5 m and 2.5 m, respectively. The numerical model realistically describes the fully-brittle tensile behaviour of plain concrete as well as the contribution of steel fibres to the post-cracking response (the latter was allowed for by conveniently adjusting the constitutive relations for plain concrete, mainly in uniaxial tension). Suitable material relations (describing compression, tension and shear) were selected for SFRC and incorporated into ABAQUS software Brittle Cracking concrete model. A more complex model (i.e., the Damaged Plasticity concrete model in ABAQUS) was also considered and it was found that the seemingly simple (but fundamental) Brittle Cracking model yielded reliable results. Published data obtained from drop-weight experimental tests on RC and SFRC beams indicates that there is an increase in the maximum load recorded (compared to the corresponding static one) and a reduction in the portion of the beam span reacting to the impact load. However, there is considerable scatter and the specimens were often tested to complete destruction and thus yielding post-failure characteristics of little design value and making it difficult to pinpoint the actual load-carrying capacity and identify the associated true ultimate limit state (ULS). To address this, dynamic NLFEA was employed and the impact load applied was reduced gradually and applied in pulses to pinpoint the actual failure point. Different case studies were considered covering impact loading responses at both the material and structural levels as well as comparisons between RC and SFRC specimens. Steel fibres were found to increase the load-carrying capacity and deformability by offering better control over the cracking process concrete undergoes and allowing the impact energy to be absorbed more effectively compared to conventional RC members. This is useful for impact-resistant design of SFRC beams.

• Journal of the Society of Naval Architects of Korea
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• v.29 no.4
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• pp.214-226
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• 1992

In this paper, the characteristics of fatigue crack growth in the spot welded joint of the same kinds of specimens($HS{\times}HS,\;GA{\times}GAB$) and different kinds of specimens($HS{\times}GA,\;HS{\times}GAB$) which consist of dual phase high strength steel(HS) and monogalvanized steel(GA) were examined with static tension tests and axial tension fatigue tests. Some of the important results are as follows : 1. The divergence of tensile strengths among the same and different kinds of spot welds under the same conditions is comparatively low regardless of the difference of stiffness. 2. At the low load bevel and long life legion, the fatigue crack is initiated near the nugget. However, in the high load level and short life region, it occurs a tittle far from the nugget. 3. It has shown a linear relation between maximum stress Intensity factor, Kmax and fatigue life, $N_f$ among each of the spot welds and has gathered in a narrow band on the log-log graph paper. $Kmax=H{\cdot}{N_f}^{P}$ where H and P are a material constant.