• Journal of the Korea Concrete Institute
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• v.15 no.2
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• pp.163-172
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• 2003

As concrete structures are getting larger, higher, longer, and specialized, it is more required to develop steel fiber concrete and apply to the real world. In this research, it is aimed to have fatigue strength examined, varying the steel fiber content by 0%, 0.75%, 1.00%, 1.25%, by experimental study of Two-spans Beam with Steel Fibrous with repeated loads. The ultimate load and the initial load of flexural cracking were measured by static test. In addition, the load versus strain relation, load versus strain relation, load versus deflection relation, crack pattern and fracture mode by increasing weight was observed. On the other hand, the crack propagation and the modes of fracture according to cycle number and the relation of cycle loading to deflection relation and strain relation was observed by fatigue test. As the result of fatigue test, Two-spans Beam without Steel Fibrous was failed at 60~70% of the static ultimate strength and it could be concluded that fatigue strength to two million cycle was around 67.2% by S-N curve. On the other hand, that with Steel Fibrous was failed at 65~85% of the static ultimate strength and it could be concluded fatigue strength to two million cycle around 71.7%.

• Journal of the Korea institute for structural maintenance and inspection
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• v.20 no.3
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• pp.93-102
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• 2016

The purpose of this paper is to investigate the flexural behavior of high-strength steel fiber-reinforced concrete beams with compressive strength of 130 MPa. The paper presents experimental research results of steel fiber-reinforced concrete beams with steel fiber content of 1.0% by volume and steel reinforcement ratio of less than 0.02. Both of normal-strength rebar and high-strength rebar were used in the test beams. Modeling as well as compressive and tensile strength test of high-strength steel fiber-reinforced concrete was performed to predict the bending strength of concrete beams. Tension modeling was performed by using inverse analysis in which load-crack mouth opening displacement relationship was considered. The experimental results show that high-strength steel fiber-reinforced concrete beams and the addition of high-strength rebar is in favor of cracking resistance and ductile behavior of beams. For beams reinforced with normal-strength rebar, the ratio of bending strength prediction to the test result ranged from 0.81 to 1.42, whereas for beams reinforced with high-strength rebar, the ratio of bending strength prediction to the test result ranged from 0.92 to 1.07. The comparison of bending strength from numerical analysis with the test results showed a reasonable agreement.

• Journal of the Korea Concrete Institute
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• v.27 no.6
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• pp.651-659
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• 2015

Currently, social demands for long span building structures are increasing due to architectural planning purposes and economic efficiency. As a result, lighter board-type voiding materials were suggested. With the use of board-type voiding materials, a slab is able to become light weight and convenient. This process efficiently eliminates concrete where it is not required; considerably diminishing dead weight while maintaining the flexural strength of the slab. The reduction in concrete also allows for overall cost reductions and design flexibility. Also it can be ease with fixing the voided material that is composed of one body form. Although board-type voiding materials are ideal, the top and bottom concrete plates lack integrity. Because of this, test results show horizontal cracking towards the tops and bottoms of the concrete columns, or webs, connecting the slabs. The key to correcting this problem is to increase the shear strength. In order to increase the shear strength of the structure, horizontal shear area must increase. R70(100)-D-F has the largest horizontal shear area as it also shows stronger strength. As a result, shear strength ($V_{nh}$) is dependent on the horizontal shear area (N). $V_{nh}={\alpha}{\times}0.16{\sqrt{f_{ck}}}{\frac{{\pi}D^2}{4}}{\times}N({\alpha}=1.8125)$. The web columns have a shear span to depth ratio (a/d) that is less than 2; which classifies it as a deep beam. In this case, however, the shear strength of the deep beams may be as much as 2 to 3 times greater than that predicated conventional equations developed for members of normal proportions. As a result, ${\alpha}$ is suggested as an extra coefficient in the equation for shear strength ($V_{nh}$).

• Journal of the Korean Geotechnical Society
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• v.31 no.3
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• pp.39-50
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• 2015

The installation of the expansion joints in a tunnel concrete lining and duct would minimize the cracking at the location of structural shape and stiffness change, differential settlement, big temperature change, and so on. However, it is difficult to determine the required spacing of the expansion joint in a tunnel concrete lining and duct quantitatively because the spacing is influenced by temperature change, structure construction condition, ground-structure interaction, and etc. Nevertheless, a highway specification (Korea Expressway Corporation, 2012) or a road design manual (Ministry of Land, Transport and Maritime Affairs, 2010) specifies that the expansion joint spacing in a tunnel concrete lining should be installed uniformly smaller than 25 m from the tunnel portals to 50 m inside of a tunnel and elsewhre 20-60 m in a tunnel (because there is no specifcation for a duct it is assumed that a duct follows the specfication of lining). This specification results in several construction and economic problems in relation with a tunnel construction. Accordingly, in order to minimize the problems, this study analyzed both domestic and foreign design standards and specifications. In addition, field test, theoretical and numerical analyses were carried out in relation to the expansion joint in a tunnel lining and duct. The purpose of this study is to reestabilish a criterion for installing the expansion joint in a tunnel concrete lining and duct.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.589-592
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• 2008

This study was conducted to assess the durability of Chloride-inhibiting Low-Heat Cement while being subjected to freezing-thawing during winter seasons. Although durability varies slightly depending on the conditions of the jobsite, frost damage to concrete resulting from repeated freezing and thawing over the course of seasonal changes is the leading cause behind lowered concrete durability. in addition, concrete that has been subjected to freezing and thawing during the winter season develops a significant amount of expansive force at the core and begins to exhibit signs of damage, such as cracking, peeling, and detachment from the aggregate. Therefore, this study fabricated test specimens using a Chloride-inhibiting Low-Heat Cement(CLC) and the widely used blast furnace slag cement(BFS) and Ordinary Portland Cement(OPC) with water-to-cement ratios of 35%, 40% and 45%, respectively, to assess the durability index of the CLC as per resistance to freezing-thawing. The specimens were then tested using the KS F 2456 method (Testing method for resistance of concrete to rapid freezing and thawing) to measure the dynamic modulus of elasticity. The dynamic modulus of elasticity measurements were then used to derive the durability indices. By comparing the durability indices, it was confirmed that CLC, BFS, and OPC all had superior durability.

• Journal of the Korea Concrete Institute
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• v.20 no.4
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• pp.431-437
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• 2008

The concrete pavement at Seohae Expressway in Korea has suffered from serious distress, only after four to seven years of construction. The deterioration of ASR has seldom been reported per se in Korea, because the aggregate used for the cement concrete has been considered safe against alkali-silica reaction so far. The purpose of this study is to examine the expansion behavior of aggregates of Korea due to alkali-silica reaction by ASTM C 1260 standard method of the accelerated mortar bar test (AMBT), stereo microscopic analysis, scanning electronic microscope (SEM) analysis, and electron dispersive X-ray spectrometer (EDX) analysis. The results are presented as it follows. The accelerated mortar bar test (AMBT) showed that mica granite and felsite of igneous rocks, aroke, red sandstone and shale of sedimentary rocks, slate of metamorphic rock, and dendrite and quartz of mineral rock showed more expansion than 0.1% at 14 days. But, some sedimentary rocks and metamorphic rocks expanded more than 0.1% at 28 days even though they were less than 0.1% at 14 days. The mortar bars, which showed more than occurred 0.1% expansion, resulted in cracking on surface. SEM and EDX analysis confirmed that the white gel was a typical reaction product of ASR. The ASR gel in Korea mainly consisted of Silicate (Si) and Potassium (K) from the cement. The crack in the concrete pavement was caused by ASR. It seems that Korea is no longer safe zone against alkali-silica reaction.

• Journal of the Korean Society for Railway
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• v.17 no.4
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• pp.270-280
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• 2014

In this study, to examine the causes of cracks in continuously reinforced concrete tracks (CRCTs) and the main factors affecting cracking, a field survey on the status of cracks and crack patterns in the Gyeong-bu high speed line was conducted, and the crack patterns of CRCT due to the temperature difference between the top of the slab (TCL) and the bottom of the subbase (HSB) and the drying shrinkage of concrete were predicted by a nonlinear finite element model considering the structure of CRCT. The results of the numerical analysis show that cracks will be developed at the interface between the sleeper and the TCL, and under the sleeper due to the temperature difference and concrete shrinkage. This corresponds well to the crack locations found in the field. Also, it is found that the most significant factors are the coefficient of thermal expansion with respect to the temperature difference, and the drying shrinkage strain with respect to shrinkage. According to the results, the reinforcement ratio should be carefully determined considering the structures of CRCT because the crack spacing is not always proportional to the reinforcement ratio due to the sleepers embedded in the TCL.

• Journal of the Korea Concrete Institute
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• v.13 no.2
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• pp.107-113
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• 2001

This study was conducted preliminarily to develop the crack source location technique for plain concrete beam using acoustic emission(AE). Before the main experiment, the test of virtual An source location was achieved in plain concrete block. The sensor layout was mutually compared between triangular layout and rectangular layout. As the results of test, AE source location by triangular layout was evaluated more effective than that by rectangular layout. The specimen to apply he source location technique was man in total nine specimens (each three in 40 %, 50%, 60% of W/C ratio) which the experiment variable was the compressive strength level(W/C ratio). The bending loading method is selected by cyclic loadings to evaluate the degree of concrete damage. It is seen that Kaiser effect and Felicity effect exists through analysis of AE parameters in coming failure experiment. As a result of analyzing the felicity ratio(FR) values, it is shown that this values can be used for evaluating the degree of concerto damage. AE activity is started highly at the 70% of failure load without the compressive strength level. Thus considered by a index in constructing the system of the failure warning at application of the field structure. And the results compared the real cracking location with the source location has perceived by AE monitoring before it is appeared the primary crack by visual observation.

• Tunnel and Underground Space
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• v.8 no.4
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• pp.307-320
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• 1998

Tunnel lining is the final support of a tunnel and reflects the results of the interaction between ground and support system. Recently it is very difficult to support and manage the tunnel because the cracks on tunnel lining cause many problems in supporting and managing tunnels. Therefore the analysis of the cracks is quite strongly required. In this study, mechanical behaviour of a tunnel lining was examined by model tests and by numerical analyses. Especially the model test was examined for double linings including shotcrete and concrete lining. The model tests were carried out under various conditions taking different loading shapes, horizontal stresses, thicknesses of linings and double lining, vault opening behind the concrete lining and rock-like medium surrounding the lining. Due to horizontal stress, compressive stress prevailed on the lining. Thus the bearing capacity of the lining increased. The existence of a vault opening behind the concrete lining reduced the bearing capacity by the similar amount of reduction of concrete lining thickness. Rock-like medium cast around the side wall of the lining restrained the deflection of the lining, and the bearing capacity for cracking and failure increased vary much. In numerical analyses a algorithm which can analysis the double lining by introduction of interface element was developed. And the results of the numerical analyses were compared with the results of the model tests.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.3
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• pp.160-170
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• 2010

Steel Fiber Reinforced Concrete (SFRC) beams has greater shear strength than typical reinforced concrete beams due to the high tensile strength of steel fibers. In this research, an experiment has been conducted to investigate the shear behavior of SFRC beams, and especially, the portion of shear resistance by uncracked compressive concrete section has been measured. Based on the test results in this study and 87 test data collected from literature, the accuracy of the existing equations for the estimation of shear strength has been evaluated. The shear strength of SFRC beams increased as more steel fibers were mixed. However, it is considered that the most efficient amount of steel fiber for enhancement of shear strength would be between 1% and 2% in that the specimen with 0.5% of steel fibers were abruptly failed after inclined cracking, and that the specimen with 2.0% of steel fibers showed a relatively low efficiency in increasing shear strength. The portion of shear resistance by the uncracked compressive concrete section was measured to be greater than 21%, and the equation proposed by Oh et al. provided the best accuracy on the estimation of shear strength of SFRC beams among the approaches evaluated in this study.