• Journal of the Korea Institute of Building Construction
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• v.19 no.3
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• pp.229-237
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• 2019

This study examines the estimation of strength through a ultra-high strength concrete mock-up specimen using the rock compressive strength test hammer. According to the test result, the commonly used strength estimation formulae showed differences among them when the data of this test were applied. In additional, it show that these formulae underestimated the actual measurements further when the compressive strength was 30MPa or greater and deviated the distribution range of actual measurements in all strength ranges. The rock test hammer showed a higher correlation than type N Schmidt hammer regardless of the direction of hit for each type of W/B and the inclusion of coarse aggregate, and mortar showed a little higher correlation than concrete. As a result, it can be suggested that the coefficient of variation and the standard deviation of the mortar(2.26%/1.36) are lower than those of the concrete(4.06%/2.5), and the smaller the size of the coarse aggregate, the smaller the coefficient of variation and the more accurate the value.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.11a
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• pp.611-614
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• 2005

This study concerns the strength of concrete cores drilled from existing structures. The test factors are core size, drilled position of core, concrete age and concrete strength. The test results are as follows; (1) Under the filled condition of curing, concrete strength for three years are larger than that of 28 days by $15\~20\%$ (2) According to the core size effect from diameter of 75mm to 150mm , the variation of core strength are by $8\~18\%$ (3) According to the wall height of 1m, the strength of lower point of wall is than larger that of the upper point by $5\~20\%$. (4) In Accessing the core strength of concrete as a basis, the effect of core size and drilling position should be considered.

• Proceedings of the Korea Concrete Institute Conference
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• 1995.10a
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• pp.235-240
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• 1995

A 36 stories high multi-use building was designed with the specified concrete strength range of 300~400kg/$\textrm{cm}^2$ On the ground of the concept of compressive strength, adequate mix designing for the concrete, which has the target strength range of 390-520kg/$\textrm{cm}^2$, was carried out to provide enough strength margin. And with due regard to the workability and transportation time, the slump and slump flow ranged 16~21cm and 30~45cm respectively, maintaining these properties up to 2-hours from the beginning of the mix. The high-range water reducer is incorporated into the mix as a admixture. The building construction is controlled satisfactorily, so far. The actual average 28-day compressive strength is 370kg/$\textrm{cm}^2$, the standard deviation is 28kg/$\textrm{cm}^2$ and the coefficient of variation is 7.6% for concrete of 300kg/$\textrm{cm}^2$ specified strength.

• Computers and Concrete
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• v.5 no.1
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• pp.21-36
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• 2008

Steel fiber reinforced concrete is increasingly used day by day in various structural applications. An extensive experimentation was carried out with w/cm ratio ranging from 0.25 to 0.40, and fiber content ranging from zero to1.5 percent by volume with an aspect ratio of 80 and silica fume replacement at 5%, 10% and 15%. The influence of steel fiber content in terms of fiber reinforcing index on the compressive strength of high-performance fiber reinforced concrete (HPFRC) with strength ranging from 45 85 MPa is presented. Based on the test results, equations are proposed using statistical methods to predict 28-day strength of HPFRC effecting the fiber addition in terms of fiber reinforcing index. A strength model proposed by modifying the mix design procedure, can utilize the optimum water content and efficiency factor of pozzolan. To examine the validity of the proposed strength model, the experimental results were compared with the values predicted by the model and the absolute variation obtained was within 5 percent.

• Proceedings of the KIEE Conference
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• 2006.07c
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• pp.1349-1350
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• 2006

FRP has been used much for core materials of insulator. FRP consists of fiber and plastics(resin and binder). The fiber contributes strength to FRP. The fiber orientation in FRP has a great effect on the strength of FRP because the strength of FRP mainly depends on the strength of fiber. The direction of applied stress of FRP is different from the kinds of insulators. In this study, inner part of FRP rod was made unidirectionally by pultrusion method and outer part of FRP rod was made by filament winding method. Compressive strength and stress of FRP rods were simulated according to the winding orientation of glass fiber. Simulated value and real evaluated compressive strength were compared each other.

• Journal of the Korea Concrete Institute
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• v.18 no.6 s.96
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• pp.811-817
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• 2006

The primary purpose of this study is to estimate the guaranteed strength and construction quality of the combined high flowing concrete which is used in the slurry wall of underground LNG storage tank. The required compressive strength of this type of concrete become generally known as a non economical value because it is applied the high addition factor for variation coefficients and low reduction factor under water concrete. Therefore, after estimation of the construction quality and guaranteed strength in actual site work, this study is to propose a suitable equation to calculate the required compressive strength in order to improve its difference. Application results in actual site work are shown as followings. The optimum nix design proportion is selected that has water-cement ratio 51%, sand-aggregate ratio 48.8%, and replacement ratio 42.6% of lime stone powder by cement weight. Test results of slump flow as construction quality give average 616~634mm. 500mm flowing time and air content are satisfied with specifications in the rage of 6.3 seconds and 4.0% respectively. Results of strength test by standard curing mold show that average compressive strength is 49.9MPa, standard deviation and variation coefficients are low as 1.66MPa and 3.36%. Also test results by cored cylinder show that average compressive strength is 66.4MPa, standard deviation and variation coefficients are low as 3.64MPa and 5.48%. The guaranteed strength ratio between standard curing mold and cored cylinder show 1.23 and 1.32 in the flanks. It is shown that applied addition factor for variation coefficients and reduction factor under water concrete to calculate the required compressive strength is proved very conservative. Therefore, based on these results, it is proposed new equation having variation coefficients 7%, addition factor 1.13 and reduction factor 0.98 under water connote.

• Korean Journal of Optics and Photonics
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• v.17 no.6
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• pp.548-555
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• 2006

We investigate the effect of the reflectivity of both facets and the phase of a phase tuning section on the self-pulsation (SP) characteristics of multisection index-coupled (IC) DFB lasers composed of two index-coupled DFB sections and a phase tuning section between them in terms of yield. In the case of weak coupling strength, as the reflectivity of both facets increases, the effect of reflected fields from both facets and the other DFB section on the mode characteristics of one DFB section increases. Thus the number of mode hoping increases and yield decreases for the variation of phases of both facets. In the case of strong coupling strength, as the reflectivity of both facets increases, the spatial hole burning effect increases, so that the yield decreases. The maximum yield and the range of the phase of a phase tuning section with yield more than 40% decrease as the facet reflectivity increases irrespective of coupling strength. As the coupling strength increases, the variation of yield for the variation of the phase of a phase tuning section increases and the variation of the phase of a phase tuning section with the maximum yield for the variation of the reflectivity of both facets decreases. The yield characteristics of the cases with the coupling strengths of 2 and 3 are better than those with the coupling strengths of 1.2 and 4.

• Journal of the Korean institute of surface engineering
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• v.29 no.4
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• pp.238-252
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• 1996

In order to study the effect of interface oxides on the adhesion strength of the copper/epoxy system, copper foils were immersed in black oxide or brown oxide forming solutions before lamination with epoxy prepregs, and variation of peel strength with the treatment time were investigated. Results showed that peel strength decreased rapidly up to 1 minute of treatment lime and remained constant in the case of the black oxide treated specimens, which was accompanied by the thickening of $Cu_2O$ at the Copper/Epoxy interface during the period. In contrast, peel strength increased rapidly up to 1 minute of treatment time and remained constant in the case of the brown oxide treated specimens, which could be ascribed to the thickening of CuO. Subsequent heat treatments of the Copper/Epoxy laminations at $120^{\circ}C$ in air showed that peel strength remained constant in the case of the black oxide treated specimens but decreased gradually in the case of the brown oxide treated specimens. Following XPS analyses revealed that the latter was possibly caused by the coalescence of CuO at the Copper/Epoxy interface into $Cu_2O$.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.947-952
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• 2001

The prediction model is proposed to estimate the variation of compressive strength of fly ash concrete with aging. After analyzing the experimental result with the model, the regression results are presented according to fly ash replacement content and water/cement ratio. Based on the regression results, the influence of fly ash replacement content and water/cement ratio on apparent activation energy was investigated. According to the analysis, the model provides a good estimate of compressive strength development of fly ash concrete with aging. As the fly ash replacement content increases, the limiting relative compressive strength and initial apparent activation energy become greater. The concrete with water/cement ratio smaller than 0.40 shows that the limiting relative compressive strength and apparent activation energy are nearly constant according to water/cement ratio. But, the concrete with water/cement ratio greater than 0.40 has the increasing limiting relative compressive strength and apparent activation energy with increasing water/cement ratio.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.741-744
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• 2003

In this study, the influence of chemical admixture on early strength development of concrete is discussed. According to the results, fluidity with variation of kinds of chemical admixture is lower in the case of acceleration type than in the case of normal type. Setting time of naphthalene acceleration type is shortened by I hour, and that of melamine is nearly same, but that of polycarbonic acid is somewhat retarded in comparison with that of naphthalene normal type. Early compressive strength gains 5MPa in about 18hours regardless of the kinds of chemical admixture. But as time elapses, compressive strength is higher in order of polycarbonic acid, naphthalene and melamine type. The relativity between compressive strength and the rebound value of P-type schmidt hammer is also favorable at early age, and compressive strength of 5MPa is estimated at the rebound value of 22.