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슬래브 측면에 선설치된 고전단 링앵커의 전단하중에 대한 콘크리트파괴강도 Concrete Breakout Strength of Cast-in High-Shear Ring Anchors in Shear Installed on Sides of Slabs

https://doi.org/10.4334/JKCI.2019.31.1.003

전상현(Sang Hyeon Jeon) ; 김문길(Mun-Gil Kim) ; 천성철(Sung-Chul Chun) ; 김재열(Jae Yeol Kim)

Shear load tests on cast-in High-Shear Ring Anchor (HRA) installed on the sides of slabs were conducted to investigate the structural behavior of concrete breakout. The test variables include slab thickness, anchor location, and placement of anchor reinforcement. All specimens showed concrete breakout failure. Test results show that inclined cracks initiated at both sides of ring at the level of anchor center in the specimens without anchor reinforcement. In the specimens with anchor reinforcement, the cracks started at the hooks of the anchor reinforcement and, therefore, the projected area of the failure surface was enlarged. Due to the large contact area between ring and concrete and the high-shear stiffness of the ring, maximum loads of all specimens developed before displacement of ring reached 2 mm. By placing anchor reinforcement, projected area of failure surface in direction perpendicular to anchor axis as well as edge distance ca1 were enlarged and the maximum loads increased by 64.3 and 127 % for h150 and h210 specimens, respectively. The average of the test-to-prediction ratios was 1.11 with a coefficient of variation of 13.2 %, when the predicted values were calculated using the existing equation with the measured concrete strength. In the test results, it was found that the concrete breakout strength of HRA can be safely predicted using the existing equation.

편심압축을 받는 철근콘크리트 벽체의 전단거동 Shear Behavior of Reinforced Concrete Structural Walls Subjected to Eccentric Axial Compression

https://doi.org/10.4334/JKCI.2019.31.1.011

엄태성(Tae-Sung Eom) ; 박은진(Park, Eun-Jin) ; 이승제(Seung-Jae Lee)

In this study, the results of an experimental investigation of the shear behavior of reinforced concrete structural walls subjected to eccentric compression loads are presented. Shear tests of six cantilever walls with a aspect ratio 2.0 were performed. The magnitude and eccentricity of the axial loads applied to the walls were considered as the test parameters. The tests showed that the failure mode and shear strength of the eccentrically loaded walls are significantly affected by the eccentricity. As the eccentricity increases, the failure mode changes from diagonal tension failure in the web to flexure-shear cracking failure, which results in a decrease in shear strength. The test strengths were compared with the shear strengths predicted in accordance with the provisions of ACI 318-14 and KCI 2012. Results indicate that the existing methods for walls and columns, based on diagonal tension and flexure-shear cracking failures, significantly underestimate the shear strengths of the eccentrically loaded walls.

콘크리트 벽돌과 모르타르 경계면에서 전단마찰응력-미끄러짐 관계 평가 Evaluation of Shear Friction Stress-Slip Relationship at Concrete Brick and Mortar Interfaces

https://doi.org/10.4334/JKCI.2019.31.1.021

이용제(Yongjei Lee) ; 양근혁(Keun-Hyeok Yang) ; 황용하(Yong-Ha Hwang)

Shear friction stress-slip relationship of masonry element interfaces of concrete brick and mortar was experimentally evaluated and generalized on the basis of the Coulomb failure criteria and test results. The main parameters investigated were the compressive strength of mortars and applied normal stresses in compression. The shear friction stress-slip curve of masonry elements was insignificantly affected by the compressive strength of mortars, whereas the shear friction strength and slip amount at the peak strength tended to increase with the increase in the applied compressive stresses. From the measured shear friction strength, the cohesion and friction angle at the interfaces of masonry elements could be fitted at 0.5 MPa and 31.4°, respectively. The proposed model exhibited a reasonable agreement with test results; as a result, it has great potential to be applied as a fundamental reference for nonlinear finite element analysis to assess the structural performance of masonry walls.

초고성능 섬유보강 콘크리트로 보강된 이방향 슬래브-기둥 접합부의 뚫림 전단 강도 평가 Evaluation of Punching Shear Strength of Two-Way Slab-Column Connections Strengthened with Ultra-High Performance Fiber Reinforced Concrete

https://doi.org/10.4334/JKCI.2019.31.1.029

염현수(Hyun-Soo Youm) ; 홍성걸(Sung-Gul Hong) ; 임우영(Woo-Young Lim)

This paper presents the test results on 5 UHPFRC-RC two-way slab-column connections to investigate retrofitting effect of section enlargement using ultra-high performance fiber reinforced concrete (UHPFRC) on punching shear capacity. The test parameters were the section enlargement thickness and the amount of steel re-bar in it. Sand blasting method was conducted to roughen the interface. The test results showed that by adding a thin layer of UHPFRC, considerable increases in global punching shear resistance and structural rigidity were achieved. Furthermore, although it is noticeable that two governing cracking patterns (diagonal tensile cracking in RC section, near interface cracking) were observed, all the composite slabs appeared to experience punching shear mode due to propagation of diagonal tensile cracking. The test results were compared with predicted strength by refined code provisions using equivalent design parameters. The comparison results showed that the refined code provisions addressed comparatively good prediction of the test results with reasonable safety margin.

초고성능 콘크리트 (UHPC) 부재에서 긴장재의 전달길이에 관한 실험적 연구 Experimental Study on Transfer Length of Prestressing Strands in UHPC Members

https://doi.org/10.4334/JKCI.2019.31.1.041

한선진(Sun-Jin Han) ; 박민국(Min-Kook Park) ; 정재훈(Jae Hoon Chung) ; 김강수(Kang Su Kim) ; 서수연(Soo-Yeon Seo)

In pretensioned concrete members, a certain distance is required to transfer the prestressing force in strands to the concrete; this is the so-called transfer length, and it is dominated by bond performance between prestressing strands and surrounding concrete. Compared to the normal strength concrete, however, newly developed ultra-high performance concrete (UHPC) shows different bond characteristics of prestressing strands because it has a very dense matrix. Therefore, this experimental study was conducted to investigate the transfer length of prestressing strands in UHPC members, in which the compressive strength of UHPC and volume fraction of steel fiber were considered as the main variables. In addition, by comparing the test results with current code provisions and other models proposed by previous researchers, the applicability for estimation of the transfer length of UHPC members was examined.

3차원 Four-pile Cap 스트럿-타이 모델의 스트럿 유효강도 Effective Strength of Concrete Struts for a Three-dimensional Four-pile Cap Strut-and-Tie Model

https://doi.org/10.4334/JKCI.2019.31.1.049

윤영묵(Young Mook Yun) ; 이영재(Young-Jae Lee) ; 박성식(Sung-Sik Park) ; 이성철(Seong-Cheol Lee) ; 임성근(Sung Gun Lim)

For the rational analysis and safe design of reinforced concrete pile caps by using the three-dimensional (3D) strut-and-tie model method, the effective strength of concrete struts must be determined accurately. In this study, an equation of the effective strength of concrete strut for a typical and most appropriate 3D strut-and-tie model for four-pile caps is proposed. In the development of the equation, the effects of 3D stresses associated with tensile strains in reinforcing bars crossing a strut, deviation angle between strut orientation and compressive principal stress flow, the degree of confinement provided by the flexural reinforcement, and shape ratios defining the geometry of a four-pile cap, are considered. 3D strut-and-tie model analyses of 115 reinforced concrete four-pile caps were carried out to evaluate the proposed equation and to compare the proposed equation with those equations in several design specifications. The ultimate strengths predicted by using the proposed strut strength equation were most consistent and agreed reasonably well with experimental results.

콘크리트 표면 보호용 생태학적 코팅 모르타르의 내황산성 평가 Evaluation of Sulfuric Acid Resistance of Biomimetic Coating Mortars for Concrete Surface Protection

https://doi.org/10.4334/JKCI.2019.31.1.061

윤현섭(Hyun-sub Yoon) ; 양근혁(Keun-hyeok Yang) ; 이상섭(Sang-seob Lee)

The objective of this study is to evaluate the sulfuric acid resistance of biomimetic coating mortars based on glycocalyx substance generated around the bacteria cell to protect the substrate of concrete. Rhodobacter capsulatus was isolated as a bacteria strain, and expanded vermiculate or super absorbent polymer, employed for immobilizing the bacteria, were used at 30 % volume for replacement of fine aggregates in coating mortars. The coating mortars using expanded vermiculate containing bacterial agents exhibited better performance than the other mortars with respect to the following: the 28-day compressive strength and population of viable bacteria were 40.7 MPa and 4.6×106 cell/mL, respectively; the variations of compressive strength and mass measured in specimens immersed in 5 % sulfuric acid solution were less than 2 %; and the gypsum production due to the hydrate chemical action against sulfuric acid was 17 % lower than that of the mortar without bacteria. As a result, the proposed biomimetic coating mortars have great potential as protective materials for blocking deterioration of concrete.

탄소나노튜브(CNT)를 혼입한 초고성능 콘크리트(UHPC) 복합재료의 전자파 차폐효과 향상을 위한 연구 Study on Improvement in Electromagnetic Interference Shielding Effectiveness of Ultra-High Performance Concrete (UHPC) / Carbon Nanotube (CNT) Composites

https://doi.org/10.4334/JKCI.2019.31.1.069

정명준(Myungjun Jung) ; 이영순(Young-soon Lee) ; 홍성걸(Sung-Gul Hong)

In this study, to evaluate the electromagnetic shielding effect of UHPC/CNT composites by the content of CNTs, electromagnetic shielding effectiveness (SE) test was performed based on MIL-STD-188-125-1 using specimens similar to actual size of exterior material. Then, experimental results were verified by applying Antenna theory of Schelkunoff. As the CNT contents increased, the SE at the same frequencies increased. However, contrary to the theory, the SE in the magnetic field was decreased with increase in frequency regardless of content of CNTs. The reason was due to the electromagnetic coupling effect and proved by using formula. Percolation threshold of CNT used in this experiment was 0.8∼1 wt.% of cement. In this case, the SE was maximized while maintaining the compressive strength and workability at levels similar to reference proportion. The SE when considering the electromagnetic coupling effect in the magnetic field was 18 dB at 10 kHz. And the SE in the plane wave (1 GHz) was about 22 dB. In conclusion, this composite could be used as a shielding material.

반복가력실험 및 비선형해석에 의한 내부접합형 철골프레임 내진보강공법의 내진성능 Seismic Performance Evaluation of Internal Steel Frame Connection Method for Seismic Strengthening by Cycling Load Test and Nonlinear Analysis

https://doi.org/10.4334/JKCI.2019.31.1.079

정희창(Hee-Chang Jung) ; 정주성(Ju-Seong Jung) ; 이강석(Kang Seok Lee)

In this study, a seismic strengthening method by internally attaching steel frame was proposed for strengthening medium-to-low-rise reinforced concrete (RC) buildings. The main technique proposed in this study was a welding method for connecting between existing R/C frame and internal steel frame strengthening element. The connection technique was developed considering the structural integral behavior between existing frame and strengthening member. The method is one of the strength design approach by retrofit which can easily increase the ultimate lateral-load capacity of R/C buildings controlled by shear. The cycling load test, designed using an existing school building in Korea, was carried out in order to verify the seismic retrofitting effects of the proposed method in terms of the maximum load carrying capacity. The nonlinear static analysis was also conducted to establish a hysteresis model of the proposed strengthening method, and to investigate the consistency between experiment and analysis results. The results revealed that the proposed internal strengthening method installed in R/C frame enhanced conspicuously the strength capacities, and the test results can reasonably compared to those of the non-linear static analysis.

유사동적실험에 의한 HSWF 외부접합공법으로 내진보강 된 실물 2층 철근콘크리트 골조의 내진성능 평가 Seismic Capacity Evaluation of Full-size Two-story R/C Frame Strengthened with HSWF External Connection Method by Pseudo-dynamic Test

https://doi.org/10.4334/JKCI.2019.31.1.089

이복기(Bok-Gi Lee) ; 황대성(Dae-Sung Hwang) ; 최윤철(Yun-Chul Choi) ; 김유석(You-Sok Kimi) ; 이강석(Kang-Seok Lee)

A new HSWF (H-section Steel Window-typed Frame) external connection method was proposed for seismic strengthening of existing medium-and low-rise reinforced concrete buildings in this study. The external connection technique was developed considering the structural integral behavior between existing frame and strengthening member. The HSWF method, proposed in this study, is capable of carrying out the seismic rehabilitation construction while residents can live inside structures. The method is one of the strength design approach by retrofit which can easily increase the ultimate lateral load capacity of concrete buildings controlled by shear. Two full-size two-story reinforced concrete frames based on the existing school building without seismic details in Korea were designed and fabricated for the structural test. The pseudo-dynamic test was carried out in order to verify the seismic strengthening effects of the proposed method in terms of the maximum load carrying capacity and ductility. Test results revealed that the proposed HSWF strengthening method installed in reinforced concrete frame enhanced conspicuously the strength and displacement capacities, and the method can resist markedly under the large scaled earthquake intensity level.