• Journal of the Korea Concrete Institute
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• 제17권5호
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• pp.803-809
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• 2005

The purpose of this study is to investigate the mechanical characteristics of plain and steel fiber high strength concrete under uniaxial and biaxial loading condition. A number of plain and steel fiber high strength concrete cubes having 28 days compression strength of 82.7MPa(12,000 psi) were made and tested. Four principal compression stress ratios ($\sigma_2/\sigma_1$=0.00, 050, 0.75 and 1.00), and four fiber concentrations($V_f$ =0.0, 0.5, 1.0 and $1.5\%$) were selected as major test variables. From test results, it is shown that confinement stress in minor stress direction has pronounced effect on the strength and deformational behavior. Both of the stiffness and ultimate strength of the plain and fiber high strength concrete Increased. The maximum increase of ultimate strength occurred at biaxial stress ratio of 0.5($\sigma_2/\sigma_1=0.5$) in the plain high strength concrete and the value were recorded $30\%$ over than the strength under uniaxial condition. The failure modes of plain high strength concrete under uniaxial compression were shown as splitting type of failure but steel fiber concrete specimens under biaxial condition showed shear type failure. The values of elastic modulus were also examined higher than that from ACI and CEB expression under biaxial compression condition.

• Journal of the Korea Concrete Institute
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• 제27권6호
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• pp.669-676
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• 2015

Time dependent model for prediction of compressive strength development of concrete cured by microwave heating form was presented in this study. The presented model is similar to the equation which is given in ACI 209R-92 but the constants which is dependent on cement type and curing method in the presented model are modified by the regression analysis of the experimental data. Laboratory scale concrete specimens were cast and cured by the microwave heating form and drilled cores extracted from the specimens were fractured in compression. The measured core strengths are converted to standard core and in-situ strengths. These in-situ strengths are used for the regression.

• Journal of the Korean Geotechnical Society
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• 제24권11호
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• pp.157-166
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• 2008

To investigate the relationship between compressive strength ($q_u$) of Lightweight Air-mixed soil (LAS) and its physical deformation coefficient ($E_{50}$), a series of unconfined compressive tests have been performed on specimens of LAS according to various dredged soil types by percentage of sand, silt and clay. From the results it was found that the cement content ($C_i$) and unit weight (${\gamma}_m$) are most influence factors on strength, and percentage of sand, silt, clay by grain size analysis (KS F2302) have more effect on compressive strength than other physical properties of soil. It was also found that the rate of strength (a) increases with curing time, but it reduces with the increase of percentage of clay ($C_%$).

• Journal of Korean Tunnelling and Underground Space Association
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• 제8권2호
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• pp.183-196
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• 2006

Rock joint roughness obtained from the camera-type 3D scanner was classified into waviness and unevenness. The classification criteria were established in the previous study; digital filtering was used to distinguish one from another. The classified and original profiles were used to produce metal moulds. For accurate machining of the moulds, the WEDM(Wire-cut Electric Discharge Machining) was adopted. Specimens were cast using high strength gypsum, and joint shear tests were performed by varying normal stress from low value to high one. Roughness mobilization characteristics depending on the asperity scale and the applied normal stress were investigated. A new equation was proposed to predict shear strength of rock joint, which can consider the characteristics of roughness mobilization and roughness parameters. The roughness quantification composed of waviness and unevenness was found to be a useful method to predict the joint shear strength.

• Journal of the Korea Concrete Institute
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• 제25권4호
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• pp.411-418
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• 2013

Recently, in order to reduce a damage of chloride attack and hydration heat in marine concrete structures, blended cement in mixing the marine concrete is widely used. Long term strength development is distinct in concrete with blended cement and it also has excellent resistance to chloride attack and reduction of hydration heat. However, blended cement has a characteristic of relatively low compressive strength in early age of 28 days. On the other hand, a high level of compressive strength is required in the Standard Specification for marine concrete mix design. Such concrete mix design satisfying Standard Specification is effective to chloride attack but disadvantageous for hydration heat reduction due to large quantity of binder. In this study, the material properties of marine concrete considering water-binder ratio and binder type are experimentally investigated. Through the research results, compressive strength in blended cement at the age of 56 days is similar although it has smaller compressive strength at the age of 28 days compared with result of OPC (ordinary portland cement). Even though blended cement has a large water-binder ratio and small unit of binder content, chloride ion diffusion coefficient is still small and hydration heat is also found to be reduced. For meeting the required compressive strength in Standard Specification for marine concrete at 28 days, the increased unit content of binder is needed but the increased hydration heat is also expected.

• Investigative Magnetic Resonance Imaging
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• 제6권2호
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• pp.166-172
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• 2002

Purpose : To evaluate the usefulness of diffusion-weighted imaging(DWI) and quantitative apparent diffusion coefficient (ADC) maps in the patients with spinal cord infarction. Materials and methods : We studied 6 patients presented symptoms with spinal cord infarction, retrospectively (3 men and 3 women). We obtained multi-shot echo planar-based, DWI using 1.5T MR scanner at 5.4 mean days after the onset of ischemic symptoms. In six patients, signal intensity was acquired at conventional b value $1000s/\textrm{mm}^2$). The ADC value for the normal spinal cord and for infarcted lesions was measured from the trace ADC maps by using regions of interest positioned over the spinal cord. We analyzed signal intensity of lesion on MRI and DWI, and compared with ADC values in infarcted lesions and normal site. Results : T1-weighted MR image showed isosignal intensity in four of six patients and low signal intensity in two of six. T2-weighted MR image demonstrated high signal intensity in all of six. All DWI were considered to be diagnostic. All of six depicted a bright signal intensity on DWI. ADC values of infarcted lesion were measured lower than that of normal spinal cord on ADC map. The differences in ADC values between infarcted and normal spinal cord were significantly different (p＜0.05). Conclusion : It is possible to obtain DWI and ADC map of the spinal cord and DWI may be useful in the early diagnosis and localization of lesion site in patients with spinal cord infarction.

• Journal of the Korean Recycled Construction Resources Institute
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• 제10권2호
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• pp.159-167
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• 2022

The present study assessed the micro structure and durability characteristics of ternary blended cement with different types of alkali activators. Ground granulated blast furnace slag(GGBS) and ferronickel slag(FNS) was replaced until 50 % of the weight of cement. In addition, potassuim hydroxide and sodium hydroxide were used for comparing the properties of different type of alkali activator. Ternary blended cement with alkali activators showed higher peak portlandite peak than that of OPC(Ordinary Portlande Cement) and non activated ternary blended cement. Also, there was no new hydration products in ternary blended cement or/and alkali activators. Based on the mercury intrustion porosimetry(MIP) test result, ternary blended cement increased macro pore while alkali activated ternary blended cement modified pore structure and increased microp pore as compared to OPC as control. Combination with alkali activators is desirable to enhance the compressive strength and freeze thaw resistance.

• Journal of the Korea Concrete Institute
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• 제23권5호
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• pp.551-558
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• 2011

The effect of the maximum aggregate size and substitution rate of natural sand on the mechanical properties of concrete is evaluated using 15 lightweight aggregate concrete mixes. For mechanical properties of concrete, compressive strength increase with respect to age, tensile resistance, elastic modulus, rupture modulus, and stress-strain relationship were measured. The experimental data were compared with the design equations specified in ACI 318-08, EC2, and/or CEB-FIP code provisions and empirical equations proposed by Slate et al., Yang et al., and Wang et al. The test results showed that compressive strength of lightweight concrete decreased with increase in maximum aggregate size and amount of lightweight fine aggregates. The parameters to predict the compressive strength development could be empirically formulated as a function of specific gravity of coarse aggregates and substitution rate of natural sand. The measured rupture modulus and tensile strength of concrete were commonly less than the prediction values obtained from code provisions or empirical equations, which can be attributed to the tensile resistance of lightweight aggregate concrete being significantly affected by its density as well as compressive strength.

• Journal of the Korea Concrete Institute
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• 제13권5호
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• pp.438-446
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• 2001

Recently, porous concrete has been used for the purpose of decreasing the load of earth environment. It consists solely of cement, water and uniform-sized coarse aggregate. And its fundamental properties will be considerably affected by the physical properties of aggregate because the aggregate occupies for the most part in its mix proportion. For such a reason, this study was carried out to investigate the influence of the sizes and kinds of aggregate for the fundamental properties of porous concrete. It showed that the fundamental properties of porous concrete were the similar value in all sizes of aggregate except in the case of using the 2.5∼5㎜ aggregate and were varied according to the kinds of aggregate. In particular, compressive strength of porous concrete using 2.5∼5㎜ aggregate was more higher than that using other aggregate, and its void ratio and coefficient of permeability was lower. And the maintenance capacity of permeability of porous concrete was varied by the sizes and the kinds of aggregate. In particular, it was greatly decreased in case of using the 2.5∼5㎜ aggregate. And unlike dynamic modulus of elasticity of ordinary concrete, that of porous concrete was very high value in early ages and was slowly increased after that time.

• Journal of the Korea Concrete Institute
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• 제14권2호
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• pp.249-256
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• 2002

High performance concrete is prone to large autogenous shrinkage due to its low water to binder ratio (W/B). The autogenous shrinkage of concrete is caused by self-desiccation as a result of water consumption by the hydration of cement. In this study, the autogenous shrinkage of high performance concrete with and without fly ash was Investigated. The properties of fresh concrete, slump loss, air content, and flowability as well as the mechanical properties, compressive strength and modulus of elasticity, were also measured. Test results was shown that the autogenous shrinkage of concrete increased as the W/B decreased. For the same W/B, the autogenous shrinkage of high strength concrete with fly ash was considerably reduced although the development of its compressive strength was delayed at early ages. Furthermore, the autogenous shrinkage and compressive strength of high strength concrete were more rapidly developed than those of normal strength concrete. It was concluded that fly ash could improve the quality of high strength concrete with respect to the workability and autogenous shrinkage.