• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 추계 학술발표회 논문집
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• pp.673-676
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• 2006

Steel FibreReinforced Shotcrete(SFRS) is one of the main tunnel support along with the rock bolt during the excavation and after the completion of the tunnel. In the standard qualification of the SFRS defined by Korea Highway Corporation, 28 day core specimen has to meet the compressive strength of 19.6 MPa and over 90 % fibre contents. Furthermore, for the 28 days brick shaped specimen made by shooting, flexural strength should be over 4.4 MPa and flexural toughness ratio which can be calculated from flexural toughness factor has to meet more than 68% of flexural strength. In shotcrete, accelerating agent is added for the rapid strength development. Silicate and aluminate type agents are known to develop shotcrete strength rapidly, however, has such problem to degrade the middle and long term strength. Hence, using poly carboxylic super plasticizer, it was aimed to enhance the quality of the shotcrete with the lower water-cement ratio and the same level of workability. The present paper shows the part of the field test result and its analysis.

• 한국재료학회지
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• 제30권1호
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• pp.38-43
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• 2020

Sand casting 3D printing uses a binder jetting method to produce a mold having complicated shape by spraying a binder on sand coated with activator. Appropriate heat treatment process in sand mold fabrication can increase the degree of polymerization to improve flexural strength. However, long heat treatment of over 24 hours decreases flexural strength and reliability due to chemical bond decomposition through thermal degradation. The main role of the activator is to control the reaction rate between the polymer chains. As a result, when the activator composition is increased from 0.15 wt% to 0.25 wt%, the flexural strength is increased by 218 N/㎠. However, excess activator (0.40 wt%) has been shown to decrease reliability without increasing flexural strength. The main role of the binder is to control the flexural strength of the specimen. As the binder composition is increased from 2.00 wt% to 4.00 wt%, the flexural strength increases to about 255 N/㎠, indicating the maximum flexural strength increase. Finally, the reliability of the flexural strength of the fabricated specimens is evaluated by a Weibull plot. Weibull modulus calculations are used to evaluate the flexural strength reliability of the specimens, and maximum reliability value of 11.7 is obtained at 0.20 wt% activator composition. Therefore, it is confirmed that this composition has maximum flexural strength reliability.

• Structural Engineering and Mechanics
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• 제38권5호
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• pp.675-696
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• 2011

It is difficult to accurately predict the flexural strength of prestressed members with unbonded tendons, unlike that of prestressed members with bonded tendons, due to the unbonded behavior between concrete and tendon. While there have been many studies on this subject, the flexural strength of prestressed members with unbonded tendons is still not well understood, and different standards in various countries often result in different estimation results for identical members. Therefore, this paper aimed to observe existing approaches and to propose an improved model for the ultimate strength of prestressed members with unbonded tendons. Additionally, a large number of tests results on flexural strength of prestressed members with unbonded tendons were collected from previous studies, which entered into a database to verify the accuracy of the proposed model. The proposed model, compared to existing approaches, well estimated the flexural strength of prestressed members with unbonded tendons, adequately reflecting the effects of influencing factors such as the reinforced steel ratio, the loading patterns, and the concrete strength. The proposed model also provided a reasonably good estimation of the ultimate strength of over-reinforced members and high-strength concrete members.

• Advances in concrete construction
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• 제8권2호
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• pp.139-144
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• 2019

In this study, the mechanical properties of reactive powder concrete (RPC) with a constant cement to silica fume ratio of 4 were investigated. In the experimental program, reactive powder concretes with steel fiber at different ratios were produced. Five productions using quartz sand with a maximum grain size of 0.6 mm were performed. A superplasticizer with a ratio of 3% of the cement was used for all productions. $40{\times}40{\times}160mm$ prismatic specimens were prepared and tested for flexural and compression. The specimens were exposed to two different curing conditions as autoclave and standard curing condition. Autoclave exposure was performed for 3 hours under a pressure of 2 MPa. It was observed that the compressive strength of concrete, along with the flexural strength exposed to autoclave was quite high compared to the strength of concretes subjected to standard curing. The results obtained indicated that the compressive strength, along with the flexural strength of autoclaved concrete increased as the amount of cement used increases. Approximately 15% increase in flexural strength was achieved with a 4% steel fiber addition. The maximum compressive strength that has been reached is over 210 MPa for reactive powder concrete for the same steel fiber ratio and with a cement content of $960kg/m^3$. The relationship between compressive strength and flexural strength of reactive powder concrete exposed to both curing conditions was also identified.

• Advances in concrete construction
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• 제7권1호
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• pp.1-9
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• 2019

The conventional ACI rectangular stress block is developed on the basis of normal-strength concrete column tests and it is still being used for the design of high-strength concrete members. Many research papers found in the literature indicate that the nominal strength of high-strength concrete members appears to be over-predicted by the ACI rectangular stress block. This is especially true for HSC columns. The general shape of the stress-strain curve of high-strength concrete becomes more likely as a triangle. A triangular stress block is, therefore, introduced in this paper. The proposed stress block is verified using a database which consists of 52 tested singly reinforced high-strength concrete beams having concrete strength above 55 MPa (8,000 psi). In addition, the proposed model is compared with models of various design codes and proposals of researchers found in the literature. The nominal flexural strengths computed using the proposed stress block are in a good agreement with the tested data as well as with that obtained from design codes models and proposals of researchers.

• Computers and Concrete
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• 제24권1호
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• pp.85-94
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• 2019

The ACI building code is allowing for higher strength reinforcement and concrete compressive strengths. The nominal strength of high-strength concrete columns is over predicted by the current ACI 318 rectangular stress block and is increasingly unconservative as higher strength materials are used. Calibration of a rectangular stress block to address this condition leads to increased computational complexity. A triangular stress block, derived from the general shape of the stress-strain curve for high-strength concrete, provides a superior solution. The nominal flexural and axial strengths of 150 high-strength concrete columns tests are calculated using the proposed stress distribution and compared with the predicted strength using various design codes and proposals of other researchers. The proposed triangular stress model provides similar level of accuracy and conservativeness and is easily incorporated into current codes.

• Computers and Concrete
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• 제6권3호
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• pp.235-252
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• 2009

Due to the reduction of bond strength resulting from the high corrosion level of reinforcing bars, influence of this reduction on flexural capacity of reinforced concrete (RC) beam should be considered. An extreme case is considered, where bond strength is complete lost and/or the tensile steel are exposed due to heavy corrosion over a fraction of the beam length. A compatibility condition of deformations of the RC beam with partially unbonded length is proposed. Flexural capacity of this kind of RC beam is predicted by combining the proposed compatibility condition of deformations with equilibrium condition of forces. Comparison between the model's predictions with the experimental results published in the literature shows the practicability of the proposed model. Finally, influence of some parameters on the flexural capacity of RC beam with partially unbonded length is discussed. It is concluded that the flexural capacity of the beam may not be influenced by the completely loss of bond of the whole beam span as long as the tensile steel can yield; whether or not the reduction of the flexural capacity of the beam resulting from the loss of bond over certain length may occur depends on the detailed parameters of the given beam.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2011년도 춘계 학술논문 발표대회 1부
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• pp.69-72
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• 2011

The purpose of this study is to identify basic property of porous concrete using cooper slag as fine aggregate. The specimens were made with cooper slag with various mixing ratio(10, 20, 30, 50%), porous concrete and porous concrete containing river fine aggregate and crushed fine aggregate, which W/B ratio fixed 0.25. Compressive strength, Flexural strength, coefficient of permeability. From the test results, various fine aggregate mixing ratio improves compressive strength and flexural strength, but cooper slag fine aggregate mixing ratio over 20%, concrete indicates trend to decrease performance of permeability. Concrete containing fine aggregate is improved the performance of permeability and strength compared to other specimen, when age 28days, and cooper slag mixing ratio less than 20% concrete indicates better performance than cooper slag mixing ratio 20% over.

• 한국건설순환자원학회논문집
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• 제7권2호
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• pp.158-165
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• 2019

이 연구의 목적은 알칼리활성 슬래그 기반 섬유보강 복합재료를 보 부재의 재료로 활용하였을 때 휨거동을 해석적으로 분석하는 것이다. 7% 이상의 초고연성이 나타날 수 있는 알칼리활성 슬래그 기반 섬유보강 복합재료를 제조하기 위하여 재료 및 배합을 선정하였고, 재료의 압축강도와 인장성능을 평가하였다. 복합재료는 압축강도 32.7MPa, 인장강도 8.43MPa, 인장변형성능 7.52%를 나타내었다. 초고연성 복합재료로 구성된 보의 휨거동을 분석하기 위하여 4가지 단면에 대하여 비선형 단면 층상화 방법을 사용하여 해석을 수행하였다. 해석결과 초고연성 복합재료로 부분적으로 보강된 경우 8.0%, 콘크리트가 복합재료로 전부 치환되어 전체 보강된 경우 24.7%의 휨강도 증진효과가 있는 것으로 나타났다. 휨강도 증진 효과가 크지 않은 이유는 인장 연단의 변형률이 최대 1.38%로 초고연성 복합재료의 인장변형성능의 18.4%밖에 되지 않기 때문인 것으로 나타났다.

• The Journal of Advanced Prosthodontics
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• 제11권1호
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• pp.1-6
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• 2019

PURPOSE. The purpose of this in vitro study was to evaluate the effect of surface grinding and polishing procedures using high speed zirconia diamond burs with different grit sizes on the phase transformation and flexural strength of zirconia. MATERIALS AND METHODS. Forty disc shape specimens ($15{\times}1.25mm$) with a cylindrical projection in the center of each disc ($1{\times}3mm$) were fabricated with 3Y-TZP (Prettau, Zirkonzahn, Italy). The specimens were divided into 4 groups (n=10) according to the grinding and polishing procedures: Control group - grinding (coarse-grit diamond bur), Group 1 - grinding (coarse-grit diamond bur) + polishing, Group 2 - grinding (fine-grit diamond bur) + polishing, and Group 3 - grinding (fine grit diamond bur). Each specimen was analyzed by 3D-OM, XRD analysis, and biaxial flexural strength test. RESULTS. Based on the surface morphology by 3D-OM images, polished specimens showed smoother surface and lower roughness value (Ra). In the result of XRD analysis, partial phase transformation from tetragonal to monoclinic zirconia occurred in all groups. Control group, ground with a coarse grit diamond bur, showed more $t{\rightarrow}m$ phase transformation and lower flexural strength than Groups 1 and 2 significantly. CONCLUSION. The flexural strength in all specimens after grinding and polishing showed over 500 MPa, and those were clinically acceptable. However, grinding with a coarse grit diamond bur without polishing induced the phase transformation and low strength. Therefore, surface polishing is required for the occlusal adjustment using a high speed zirconia diamond bur to reduce the phase transformation and to prevent the decrease of flexural strength of zirconia.