• Advances in concrete construction
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• v.11 no.3
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• pp.261-270
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• 2021

In this study, a new understanding is presented on the microcracking behavior of high strength concrete (HSC) with steel fiber addition having prior compressive loading history. Microcracking behavior at critical stress (σcr) region, using seven fiber addition volume of 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, and 2.0% was evaluated, at two aspect ratios (60 and 75). The specimens were loaded up to a specified compressive stress levels (0.70fc-0.96fc), and subsequently subjected to split tensile tests. This was followed by microscopic analyses afterwards. Four compressive stress levels as percentage of fc were selected according to the linearity end point based on stress-time (σ-t) diagram under uniaxial compression. It was seen that pre-compression has an effect on the linearity end point as well as fiber addition where it lies within 85-91% of fc. Tensile strength gain was observed in some cases with respect to the 'maiden' tensile strength as oppose to tensile strength loss due to the fiber addition with teething effect. Aggregate cracking was the dominant failure mode instead of bond cracks due to improved matrix quality. The presence of the steel fiber improved the extensive failure pattern of cracks where it changes from 'macrocracks' to a branched network of microcracks especially at higher fiber dosages. The applied pre-compression resulted in hardening effect, but the cracking process is similar to that in concrete without fiber addition.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.433-438
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• 2001

In recent years, the construction methods of precast prestressed concrete beam bridges by using tendon have been increasingly proposed. The properties of flexural tensile strength between precast prestressed concrete beam and cast-in-place concrete is an important factor that affects the structural performance. This paper aims at evaluating the tensile strength between old and new concrete. Tensile strength gain, with age after placing new concrete was evaluated. Test results show that the tensile strength between old and new concrete is much smaller than that of monolithic concrete. Also, it is shown that the curing condition of concrete has the tremendous effect on tensile strength.

• Advances in concrete construction
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• v.8 no.2
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• pp.145-154
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• 2019

The present experimental study addresses the structural response of reinforced concrete (RC) beams strengthened in shear. Thirteen RC beams were divided into four different sets to investigate the effect of transverse and longitudinal steel reinforcement ratios, concrete compressive strength change and orientation for installing carbon fiber-reinforced polymer (CFRP) laminates. Then, we employed a shear strengthening solution through externally bonded reinforcement in grooves (EBRIG) and externally bonded reinforcement (EBR) techniques. In this regard, rectangular beams of $200{\times}300{\times}2000mm$ dimensions were subjected to the 4-point static loading condition and their load-displacement curves, load-carrying capacity and ductility changes were compared. The results revealed that using EBRIG method, the gain percentage augmented with the increase in the longitudinal reinforcement ratio. Also, in the RC beams with stirrups, the gain in shear strength decreased as transverse reinforcement ratio increased. The results also revealed that the shear resistance obtained by the experimental tests were in acceptable agreement with the design equations. Besides, the results of this research indicated that using the EBRIG system through vertical grooves in RC beams with and without stirrups caused the energy absorption to increase about 85% and 97%, respectively, relative to the control.

• Journal of the Korean institute of surface engineering
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• v.25 no.1
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• pp.16-23
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• 1992

Generally brittle eta phase produced during TiC deposition has a bad effect on the TRS (transverse rupture strength ; thoughness). Therefore it is necessary to reduce eta(η) phase for the improvement of tool life. At this experiment some properties (TRS, bonding strength, tool life, eta phase) have been investigated by inserting TiN or TiCN underlayer between TiC layer and substrate. The results obtained were as fellows; 1. by inserting underlayer eta phase was decreased and TRS was increased, but the bonding strength was decreased. 2. the diffusion of W, Co from the substrate was hindered by the underlayer. 3. TiC layer with TiCN underlayer had the finsest gain size. 4. by inserting underlayer (TiCN or TiN) the tool life was improved and especially notch and crater wear resistance was greatly improved.

• Korean Journal of Veterinary Research
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• v.48 no.1
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• pp.27-32
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• 2008

The present study was carried out to develop a sustained release implantable formula of bovine somatotropin (SRIF-BST) and to examine its sustained release effect. The SRIF-BST was produced by coating a solid pellet, which was comprised of BST and an excipient, made of a biodegradable polymer and poloxamer, which are capable of regulating the rate of BST release. The coated membrane of SRIFBST was observed with a field emission scanning electron microscope. The thickness of the coated membrane was approximately $1{\mu}m$, and the pore sizes of the coated membrane surface were below $10{\mu}m$. In dissolution test, the release duration of the SRIF-BST maintained for 10 days, whereas the release duration of the control BST formula maintained for 3 days. In weight gain assay and tibia test of hypophysectomized rats, the release duration of the SRIF-BST treated group was 12 days and the net weight gain was 53.16 g, also the tibia length and strength of the SRIF-BST treated group was increased 10.5% and 23.1% compared with those of the control group, respectively.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.6
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• pp.206-215
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• 2017

An experimental program was carried out to investigate the mechanical properties of high-strength concrete. High-strength concrete with compressive strengths of 80 to 120 MPa was tested. Test results are presented regarding effect of water-binder ratio on compressive strength and compressive strength gain. In addition, the effect of curing methods on compressive strength, elastic modulus, splitting tensile strength, and modulus of rupture is investigated. Test results of elastic modulus, splitting tensile strength, and modulus of rupture are compared with predictions from the current design recommendations. Predictions of elastic modulus by using KCI recommendation has good agreement with test results. However, predictions of modulus of rupture by using KCI recommendation underestimate the test results. ACI 363R recommendations predict well test results of splitting tensile strength and modulus of rupture. ACI 363R recommendations for predicting splitting tensile strength and modulus of rupture can be used for high-strength concrete with compressive strengths up to 120 MPa.

• The Korean Journal of Community Living Science
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• v.16 no.1
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• pp.59-64
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• 2005

This study is to find out effects of hot water soluble extract from green tea, one of the Korean favorites, on the calcium metabolism and bone strength in body. To do so, calcium, phosphate, creatinine concentration and ALP activity in blood and the content of calcium and ash in the organ, the length, weight, strength in bone were measured. In addition, to find the calcium metabolism, the level of calcium intake, excretion, retention were measured. Twenty male Sprague-Dawley rats were divided into two groups and isoloated soy protein was provided as the source of protein and CaCO₃ was provided as the source of calcium. 0.5% hot water soluble extract from green tea was provided to the green tea groups and for the control group deionized water was provided. The results are as follows ; 1. There is no difference between the experimental groups in diet intake, weight gain, and the feed intake. 2. Feed efficiency ratio was low in the group which hot water soluble extract from green tea was provided. 3. There is no difference between groups the level of calcium, phosphorus, creatinine and ALP activity in serum. 4. There is no difference between groups weight, contents of ash and calcium in kidney and liver. 5. There is no difference between groups in calcium intake, absorption, excretion, and retention. 6. There is no difference between groups weight, length and strength in bone. In summary, when hot water soluble extract from green tea was provided with the amount of 150-200mg, which is taken when people generally drink as favorite tea, weight gain was reduced due to the decrease of feed efficiency ratio. However, it did not affect the availability of calcium in body at all. Thus, even if a big quantity of green tea powder or solid of hot green tea extract is not provided, the quantity obtained when people drink green tea lowers the feed efficiency ratio without reducing availability of calcium in body.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.12
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• pp.1099-1104
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• 2007

In order to develop non-heated STAl(super thermal resistant Aluminum alloy) for ampacity gain conductor, the systematic research was carried out. Especailly, the effect of a very small amount of Zr, Sc element in EC grade Al ingot on mechanical and electrical properties was our priority. As a result, it was found that the strength and recrystallization temperature of designed alloy was gradually increased with Zr, Sc addition up to 0.3 wt.%. However, the electric conductivity showed no drastic change. The tensile strength and recrystalliztion temperature, $17.75{\sim}20.05\;kgf/mm^2$ and $420{\sim}520\;^{\circ}C$, was obtained at 0.3 wt.% Zr, Sc addition, respectively. Particles of the $Al_3Zr$ and $Al_3Sc$ phase affected the ambient and elevated-temperature strength of the alloys.

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2004.02a
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• pp.28-41
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• 2004

The study is aimed to assess the usefulness of the mixture design for spent resin immobilization in cement. Although a considerable amount of research has been carried out to determine the limits for the composition of an acceptable resin-cement mixture, no efficient experimental strategy exists that explores the full properties of waste form against composition relationship. In order to gain an overall view, this report introduces the method of mixture design and mixture analysis, and describes the design of experiment of the 5-component mixture with the constraint conditions. The mathematic models of 28-day compressive strength varying with the ingredients are fitted, and the main effect and interaction effect of two ingredients are identified quantitatively along with the graphical interpretation using the response trace plot and contour plots.

• International Journal of Concrete Structures and Materials
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• v.8 no.2
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• pp.173-182
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• 2014

An experimental investigation was carried out to evaluate the mechanical properties of concrete mixtures in which coarse aggregate was partially (30, 50 or 70 %) replaced with pre-washed municipal solid waste incineration (MSWI) bottom ash. Results indicated that bottom ash reduced the compressive strength, elastic modulus, and levels of heavy metals in leachate when used as a replacement for gravel, and that the maximum amount of MSWI bottom ash in concrete should not exceed 50 %. To analyze the effect mechanism of bottom ash in concrete, the degree of hydration and the following pozzolanic reaction characterized by the pozzolanic activity index, and the porosity distribution in cement mortar. The study indicates that improved properties of concrete are not solely later strength gain and reduced levels of heavy metals in leachate but also the progression of pozzolanic reactions, where a dense structure contains a higher proportion of fine pores that are related to durability.