• Journal of the Korea Concrete Institute
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• v.25 no.4
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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.

• Journal of the Korea Concrete Institute
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• v.23 no.2
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• pp.159-168
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• 2011

In the present study, an improved design method was developed for the punching shear strength of interior slabcolumn connections and column footings with and without shear reinforcement. In the evaluation of the punching shear strength, the possible failure mechanisms of the connections and column footings were considered. The considered failures modes were inclined tensile cracking of concrete, yielding of shear re-bars, and concrete crushing of compression zone/strut. The punching shear applied to the concrete critical section was assumed to be resisted mainly by the compression zone. The punching shear strength of the concrete compression zone was evaluated based on the material failure criteria of the concrete subjected to the compressive normal stress and shear stress. For verification of the proposed design method, its prediction was compared with the existing test results. The result showed that the proposed method predicted the strengths of the test specimens better than the current design methods of the KCI code for both the shear reinforced and unreinforced cases.

• Journal of Korean Society of Steel Construction
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• v.26 no.1
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• pp.31-42
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• 2014

In this paper, concrete-filled steel tubular columns encased with precast reinforced concrete were studied. Four eccentrically loaded columns and a concentrically loaded column were tested to investigate the axial load-carrying capacity. The test parameters were the use of fiber reinforcement for cover concrete, eccentricity, column length, and lateral reinforcement. The maximum axial loads of the specimens agreed with the nominal strengths predicted by KBC 2009. However, in some specimens, the load carrying capacity quickly decreased after the peak strength due to spalling of the cover concrete.

• Journal of the Korea Concrete Institute
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• v.21 no.6
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• pp.753-761
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• 2009

This paper presents the test results of total 24 beam-end specimens to investigate the effect of high-strength concrete and cover thickness on the development resistance capacity in tensile lap splice length regions. Based on bond characteristics that an increase in concrete strength results in higher bond stress and shortening of the transfer length, cracking behavior that thin cover thickness induced a splitting crack easily and brittle crack propagation, current design code that development length provisions as uniform bond stress assumption was investigated apply as it. The results showed that as higher strength concrete was employed, not only development resistance capacity was influenced by cover thickness, but also more sufficient safety factor reserved shorter than the lap splice length provision in current design code. From experimental research results, high-strength concrete development length was not inverse ratio of $\sqrt{f_{ck}}$ but directly inverse of $f_{ck}$, and it is also said that there is a certain limit length of the embedded steel over which the assumption of uniform bond stress distribution is valid specially for high-strength concrete not having a same embed length such as normal-strength concrete in current design criteria hypothesis.

• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.2
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• pp.168-175
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• 2022

With the recent development of nanotechnology, its application in the field of construction materials is continuously increasing. However, until now, studies on the bond characteristics of concrete and rebar for applying high-strength lightweight concrete with a compressive strength of 50 MPa and a unit weight of 16 kN/m³ to structural members are lacking. Therefore, in this paper, 81 specimens of high-strength lightweight concrete with a compressive strength of 50 MPa and a unit weight of about 16 kN/m³ were fabricated and a direct pull-out tests were performed. The design code for the bond strength of ACI-408R and the experimental results are shown to be relatively similar, and as a result of the CEB-FIP and modified CMR bond behavior models through statistical analysis, it is shown to describe well on average.

• Journal of the Korea Concrete Institute
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• v.28 no.3
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• pp.365-373
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• 2016

Ultra-High-Performance Steel Fiber-Reinforced Concrete (SUPER Concrete) exhibits improved compressive and tensile strengths far superior to those of conventional concrete. These characteristics can significantly reduce the cross sectional area of the member and the anchorage strength of a headed bar is expected to be improved. In this study, the anchorage strengths of headed bars with $4d_b$ or $6d_b$ embedment length were evaluated by simulated exterior beam-column joint tests where the headed bars were used as beam bars and the joints were cast of 120 or 180 MPa SUPER Concrete. In all specimens, the actual yield strengths of the headed bars over 600 MPa were developed. Some headed bars were fractured due to the high anchorage capacity in SUPER Concrete. Therefore, the headed bar with only $4d_b$ embedment length in 120 MPa SUPER Concrete can develop a yield strength of 600 MPa which is the highest design yield strength permitted by the KCI design code. The previous model derived from tests with normal concrete and the current design code underestimate the anchorage capacity of the headed bar anchored in SUPER Concrete. Because the previous model and the current design code do not consider the effects of the high tensile strength of SUPER Concrete. From a regression analysis assuming that the anchorage strength is proportional to $(f_{ck})^{\alpha}$, the model for predicting anchorage strength of headed bars in SUPER Concrete is developed. The average and coefficient of variation of the test-to-prediction values are 1.01 and 5%, respectively.

• Journal of the Korea Concrete Institute
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• v.22 no.5
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• pp.659-666
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• 2010

Although the compression splice needs not be longer than the tension slice due to existence of end bearing, current design codes impose a longer compression lap splice than a tension lap splice in high strength concrete. Hence, new criteria for the compression lap splice including the effects of concrete strength need to be sought for economical design involving ultra-high strength concrete. An experimental study has been conducted with column specimens in concrete strength of 80 and 100 MPa. Test results show that the splice strength can be evaluated to be proportional to square root of compressive strength of concrete. Bar stress developed by end bearing is not affected by splice length and is expressed with a function of the square root of concrete strength. Mean value of stresses developed by end bearing is 16.5 square root of $f_{ck}$. The stresses developed by bond in compression splices are nearly identical to those in tension splices and, therefore, strength increment of compression splices is attributed to end bearing only. From regression analysis of 58 tests, a design equation is proposed for compression lap splice in 40 to 100 MPa of compressive strength of concrete. By the proposed equation, the anomaly of lap lengths in tension and compression is got rid of. In addition, the equation has a reliability equivalent to those of the specified strengths of materials.

• Journal of the Korea Concrete Institute
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• v.28 no.6
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• pp.703-711
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• 2016

The present study is mainly aimed at securing adequate pores which are applicable to the aquatic environment and satisfying the required strength of porous concrete as a structure by substituting pumice for crushed stone which is usually used for the fabrication of porous concrete. Accordingly, in order to deduce the optimum mixing conditions applicable to the aquatic environment, we sought to evaluate the porosity, coefficient of permeability and compressive strength of porous concrete based on the target porosity and the mixing factors for pumice. By examining the porosity and coefficient of permeability of porous concrete and the physical properties of its compressive strength based on the target porosity and the mixing factors for pumice, it is judged that the optimum mixtures for porous concrete applicable to the aquatic environment which satisfy both the necessity of securing adequate pores and the required strength for porous concrete as a structure are PC I I-10-0, PC I I-10-5 and PC I I-10-10.

• Journal of the Korean Recycled Construction Resources Institute
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• v.5 no.1
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• pp.85-93
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• 2017

This paper presents mechanical properties and carbonation behavior of the recycled aggregate concretes(RAC) in which natural aggregate was replaced by recycled coarse aggregate and fine aggregate by specified concrete strength levels(21, 35, 50MPa). A total of 18 RAC were produced and classified into six series, each of which included three mixes designed with three specified concrete strength levels of 21MPa, 35MPa and 50MPa and three recycled aggregate replacement ratios of 0, 50 and 100%. Physical and mechanical properties of RAC were tested for slump test, compressive strength, and carbonation depth. The test results indicated that the slump of RAC could be improved or same by recycled coarse aggregate replacement ratios, when compared with natural aggregate. But slump of RAC was decreased as the recycled fine aggregate replacement ratios increase. Also, the test results showed that the compressive strength was decreased as the recycled aggregate replacement ratios increased and it had a conspicuous tendency to decrease when the content of the recycled aggregate exceeded 50%. Furthermore, the result indicated that the measured carbonation depth increases by 40% with the increase of the recycled aggregate replacement. In the case of the concrete having low level compressive strength, the increase of carbonation depth tends to be higher when using the RCA. However, the trend of carbonation resistivity in high level compressive strength concrete is similar to that obtained in natural aggregate concrete. Therefore, an advance on the admixture application and mix ratio control are required to improve the carbonation resistivity when using the recycled aggregate in large scale.

• Journal of the Korea Concrete Institute
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• v.24 no.6
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• pp.643-650
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• 2012

The limitation of the yield strength in reinforced concrete columns is given for the effective use of high-strength steel bar, because very high-strength steel bar does not yield while concrete fails in compression. In order to overcome this limitation, it is required to increase peak strain of the concrete. The objective of this study is to examine the confinement effect of plate type lateral reinforcement in reinforced concrete columns. From this experimental study, the reinforced concrete columns confined by plate type carbon fiber sheets showed higher compressive strength and peak concrete strain comparing to the unconfined columns. The confinement effect is higher when cross-sectional type is a circular one than a square one. Moreover, the confinement effect was also higher for circular type confinement. Based on this study, high-strength steel bars with strength exceeding 800 MPa can be effectively used for reinforced concrete columns confined by plate type lateral reinforcements.