• 한국농공학회논문집
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• 제50권5호
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• pp.29-37
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• 2008

Stress-strain relation and stress block parameters of polymer concrete flexural compression members were experimentally investigated. For these purposes, a series of C-shaped polymer concrete specimens subjected to axial compressive load was tested. Based on the test results, we proposed an equation by which the stress-strain relation of polymer concrete can be predicted. In this model, we took account the slope of descending branch beyond the peak stress point of single curve. The proposed equation was numerically integrated to compute the rectangular stress block parameters. Computed ${\beta}_1$ was greater than the values prescribed in ACI 318 Code for cement concrete, and $\gamma$ was about 0.85 that is similar to the value regulated in the ACI.

• Computers and Concrete
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• 제19권1호
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• pp.59-68
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• 2017

This study focused on modeling the behavior of the compressive stress using the average strain and ultrasonic test results in concrete. Feed-forward backpropagation artificial neural network (ANN) models were used to compare four types of concrete mixtures with varying water cement ratio (WC), ordinary concrete (ORC) and concrete with short steel fiber-reinforcement (FRC). Sixteen (16) $150mm{\times}150mm{\times}150mm$ concrete cubes were used; each contained eighteen (18) data sets. Ultrasonic test with pitch-catch configuration was conducted at each loading state to record linear and nonlinear test response with multiple step loads. Statistical Spearman's rank correlation was used to reduce the input parameters. Different types of concrete produced similar top five input parameters that had high correlation to compressive stress: average strain (${\varepsilon}$), fundamental harmonic amplitude (A1), $2^{nd}$ harmonic amplitude (A2), $3^{rd}$ harmonic amplitude (A3), and peak to peak amplitude (PPA). Twenty-eight ANN models were trained, validated and tested. A model was chosen for each WC with the highest Pearson correlation coefficient (R) in testing, and the soundness of the behavior for the input parameters in relation to the compressive stress. The ANN model showed increasing WC produced delayed response to stress at initial stages, abruptly responding after 40%. This was due to the presence of more voids for high water cement ratio that activated Contact Acoustic Nonlinearity (CAN) at the latter stage of the loading path. FRC showed slow response to stress than ORC, indicating the resistance of short steel fiber that delayed stress increase against the loading path.

• 드라이브 ㆍ 컨트롤
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• 제12권4호
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• pp.77-81
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• 2015

In general, the gears of mixer reducer for concrete mixer truck make use of the differential type planetary gear system to rotate mixer drum smoothly on the initial conditions. The planetary gear system is very important part of mixer reducer for concrete mixer truck because of strength problem. In the present study, calculating the gear specifications and analyzing the gear bending & compressive stresses of the differential planetary gear system for mixer reducer are necessary to analyze gear bending and compressive stresses confidently, for optimal design of the planetary gear system in respect to cost and reliability. As a result, analyzing actual gear bending and compressive stresses of the planetary gear system using Lewes & Hertz equation and verifying the calculated specifications of the planetary gear system, evaluate the results with the data of allowable bending and compressive stress from the Stress-No. of cycles curves of gears.

• 콘크리트학회논문집
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• 제23권6호
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• pp.765-772
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• 2011

이 연구에서는 알칼리활성 슬래그 콘크리트의 응력-변형률 관계를 평가하기 위한 일련의 콘크리트 실린더의 압축 실험을 요약하였다. 실험된 콘크리트의 압축강도는 8.6 MPa에서 42.2 MPa의 범위이며, 단위용적질량은 $2,168kg/m^3$ 에서 $2,343kg/m^3$의 범위이다. 34개의 콘크리트 시험체에서 얻은 결과들에 근거하여 알칼리활성 슬래그 콘크리트의 응력-변형률 모델을 수학적으로 제시하였다. 콘크리트의 탄성계수, 최대응력 시 변형률 및 곡선의 상승부와 하강부의 기울기는 압축강도와 단위용적질량의 함수로 일반화하였다. 각 시험체에서 측정한 값과 제시된 모델의 예측값 사이에서 산정된 변동계수들의 평균과 표준편차는 각각 6.9%와 2.6%이었다. 따라서 제시된 모델은 보통포틀랜드 시멘트 콘크리트에서 제시된 다른 모델들에 비해 AA 슬래그 콘크리트의 응력-변형률 특성을 보다 더 정확하고 합리적으로 나타내었다.

• 콘크리트학회논문집
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• 제28권4호
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• pp.481-488
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• 2016

콘크리트는 공용기간동안 구조물 자체의 자중 및 이동 하중에 따른 응력을 받게 되며, 이러한 응력은 구조적인 거동뿐 아니라 내구적인 거동에도 영향을 준다. 대단위 콘크리트 부재의 시공은 시공이음을 요구하는데, 면처리 불량 또는 이어치기의 지연에 의해 콜드조인트가 발생하게 된다. 이러한 콜드조인트는 전단력에 취약할 뿐 아니라 염화물 확산성에도 영향을 미친다. 본 연구에서는 응력조건과 콜드조인트가 콘크리트의 염화물 확산에 미치는 영향을 정량적으로 평가하였다. 콜드조인트를 가진 콘크리트는 인장하중 수준이 30%에서 60%로 커질 때, 확산계수는 꾸준하게 증가하여 건전부와 큰 차이를 보이지 않았다. 그러나 압축부에서는 하중재하 30% 수준부터 콜드조인트 콘크리트에서 염화물 확산계수가 크게 증가하여 확산계수가 170% 이상 증가하였다. 이러한 특성은 압축하중을 받는 건전부 콘크리트와 큰 차이가 있으므로 이음부를 가지는 콘크리트의 내구성 설계에 주의가 필요하다.

• 콘크리트학회지
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• 제8권6호
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• pp.141-149
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• 1996

동일한 인장철근배근상태($\rho$/$\rho_b$=constant)에서 휨을 받는 고강도 철근 콘크리트 보는 보통강도의 철근 콘크리트보에 비해 더욱 취성적인 거동을 하게 된다. 본 실험결과 연성적인 파괴를 유도하기 위하여 콘크리트의 강도가 830kg/$cm^2$이상일 경우 철근비를 $0.6\rho_b$이하로 배근하여야 할 것으로 나타났다. 또한 콘크리트 강도가 830kg/$cm^2$ 이상일 경우 사각형응력 블록을 사용한 ACI 휨강도식의 안전율이 감소하였으며, 삼각형에 가까운 압축응력 분포를 나타내었다. 이는 콘크리트 강도가 증가할수록 응력-변형률 곡선이 거의 선형적으로 증가하는 재료적 성질에 기인하는 것으로 사료된다.

• Structural Engineering and Mechanics
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• 제64권6권
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• pp.793-802
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• 2017

The damage of concrete due to the expansion of alkali-aggregate reaction (AAR) and thermal-chemical reactions affecting the strength of concrete is studied. The empirical equations for the variations of expansion of AAR, compressive strength and degradation of the modulus of elasticity with time, and compressive strength with degradation of the modulus of elasticity are proposed by analysing numerous experimental data. It is revealed that the expansion of AAR and compressive strength increase with time. The proposed combination of the time variations of chemical and mechanical parameters provides a satisfactory prediction of the concrete strength. Seismic analysis of the aged Koyna dam is conceded for two different long-term experimental data of concrete incorporating the proposed AAR based properties. The responses of aged Koyna dam reveal that the crest displacement of the Koyna dam significantly increases with time while the contour plots show that major principal stress at neck level reduces with time. As the modulus of elasticity decreases with ages the stress generated in the concrete structure get reduces. On the other hand with lesser value of modulus of elasticity the structure becomes more flexible and the crest displacement becomes very high that cause the seismic safety of the dam reduce.

• Computers and Concrete
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• 제22권6호
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• pp.563-572
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• 2018

Uniaxial compressive strength test and uniaxial compression creep one were produced on four groups of twelve concrete specimens with different hole number by RLW-2000 rock triaxial rheology test system. The relationships between horizontal holes and instantaneous failure stress, the strain, and creep failure stress, the strain, and the relationships between stress level and instantaneous strain, creep strain were studied, and the relationship between horizontal holes and failure mode was determined. The results showed that: with horizontal hole number increasing, compressive strength of the specimens decreased whereas its peak strain increased, while both creep failure strength and its peak strain decreased. The relationships between horizontal holes and compressive strength of the specimens, the peak strain, were represented in quadratic polynomial, the relationships between horizontal holes and creep failure strength, the peak strain were represented in both linear and quadratic polynomial, respectively. Instantaneous strain decreased with stress level increasing, and the more holes in the blocks the less the damping of instantaneous strain were recorded. In the failure stress level, instantaneous strain reversally increased, creep strain showed three stages: decreasing, increasing, and sharp increasing; in same stress level, the less holes the less creep strain rate was recorded. The compressive-shear failure was produced along specimen diagonal line where the master surface of creep failure occurred, the more holes in a block, the higher chances of specimen failure and the more obvious master surface were.

• Computers and Concrete
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• 제33권3호
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• pp.309-324
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• 2024

Internal curing, a widely used method for mitigating early-age shrinkage in concrete, also offers notable advantages for concrete durability. This paper explores the potential of internal curing by partial replacement of sand with fine lightweight aggregate for enhancing the behavior of high-performance concrete at elevated temperatures. Such a technique may prove economical and safe for the construction of skyscrapers, where explosive spalling of high-performance concrete in fire is a potential hazard. To reach this aim, the physico-mechanical features of internally cured high-strength concrete specimens, including mass loss, compressive strength, strain at peak stress, modulus of elasticity, stress-strain curve, toughness, and flexural strength, were investigated under different temperature exposures; and to predict some of these mechanical properties, a number of equations were proposed. Based on the experimental results, an advanced stress-strain model was proposed for internally cured high-performance concrete at different temperature levels, the results of which agreed well with the test data. It was observed that the replacement of 10% of sand with pre-wetted fine lightweight expanded clay aggregate (LECA) not only did not reduce the compressive strength at ambient temperature, but also prevented explosive spalling and could retain 20% of its ambient compressive strength after heating up to 800℃. It was then concluded that internal curing is an excellent method to enhance the performance of high-strength concrete at elevated temperatures.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1998년도 가을 학술발표논문집(II)
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• pp.501-506
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• 1998

This paper is an experimental study on the flexural strength and ductility capacity of reinforced high performance concrete beams with the concrete which has compressive strength of 600~700kg/$\textrm{cm}^2$, slump value of 20~25cm and slump-flow value of 60~70cm. Total 8 beams with different tensile reinforcement ratio and pattern of loading were tested. Form the results of reinforced high performance concrete beams, the equivalent stress block parameters proposed by MacGregor et al. or New Zealand code are recommended to use. Also, an extreme fiber concrete compressive strain of reinforced high performance concrete beams are distributed 0.0033~0.0048. In reinforced high performance concrete beams, reinforcement ratio in order to insure curvature ductility index 2 and 4 propose by ACI code should be less than those of reinforced normal strength concrete beams.