마이크로폰을 이용한 콘크리트 벽체 배면의 공동 탐사

강성훈,이종섭,한우진,김상엽,유정동 한국지반공학회 2022 한국지반공학회논문집 Vol.38 No.12

Cavities behind concrete walls can adversely affect the stability of structures. Thus study aims to detect cavities behind concrete structures using a microphone in a laboratory model test. A small-scale concrete wall is constructed in a chamber, which is composed of a reinforced concrete plate and dry soil. A plastic bowl is then placed between the plate and soil to simulate a cavity behind the concrete structure. Leaky surface acoustic waves are generated by impacting the concrete plate using a hammer and are measured using a microphone. The measured signals are analyzed using natural frequencies, and cavity-free sections are evaluated. The test results show that the first natural frequency decreases at the cavity section due to the flexural vibration behavior of the plate. In addition, the amplitude corresponding to the first natural frequency decreases as the measurement location becomes farther from the cavity center and significantly decreases at the measurement locations near the rebars. This study demonstrates that a microphone may be useful to detect cavities behind concrete walls.

콘크리트 구조물 하부의 공동 탐지를 위한 충격반향(IE) 및 충격응답(IR) 기법의 모형 실험

노명근,오석훈 한국지구과학회 2011 한국지구과학회지 Vol.32 No.1

The impact echo and impulse response methods were applied to the safety inspection of concrete structure,which has the rear cavity. The concrete structure model used in this study was divided into four sections, pure concrete,concrete+cavity, reinforced concrete with iron bar, and reinforced concrete+cavity, respectively. Previous study performed by authors have showed a possibility of success to use these method for detection of the rear cavity of concrete structure. Therefore, we tried to get more enhanced result with IE and IR methods through this study. Especially, IE and IR methods are relatively accurate to map the point of measurement, which makes it possible to interpret the depth of the concrete bed and effect by rear cavity with confidence. Followings were revealed from the results; the IE method shows some small peak zones probably indicating the rear cavity in the frequency lower than the resonance frequency and the changes of mobility and dynamic stiffness in the IR method indicate the weak zones. The proposed methods can be used to delineate the weak zones of the concrete structure. 콘크리트 구조물의 안전진단을 위해 충격반향기법(Impact Echo method, IE)과 충격응답기법(Impulse Response method, IR)을 이용하여, 콘크리트 모형 하부의 공동 유무에 따른 탐사 결과를 분석하였다. 콘크리트 모형은 순수 콘크리트 부분과 철근+콘크리트 부분으로 나누고, 공동의 유무에 따라 각 기법의 적용에 의한 반응 변화를 관찰하였다. 이연구에 앞서 수행한 GPR과 IE 및 IR 기법의 복합 적용 결과, IE 및 IR 기법이 철근의 영향을 크게 받지 않고 공동존재 여부에 따른 반응이 비교적 잘 나타나는 것으로 파악되었다. 본 연구에서는 선행 연구 결과를 토대로 IE와 IR 기법의 활용도를 높여서 보다 정확한 콘크리트 구조물의 안전진단 기법을 개발하고자 하였다. GPR과 같은 비파괴 조사기법과 달리, IE 및 IR 기법은 측정이 이루어지는 위치를 정확히 알 수 있어 각 측점의 콘크리트 두께와 하부 공동의반응을 보다 정확히 파악할 수 있는 장점이 있다. 연구 결과, IE 기법에서는 공진주파수보다 낮은 저주파수에서 나타나는 작은 피크 구간이 공동에 의한 반응으로 보이며, IR 기법에서는 주파수에 따른 운동성(mobility)과 동적 강성도(dynamic stiffness)의 변화를 통해 공동의 유무를 확인할 수 있었다. 이 연구에서 제안한 방법은 콘크리트 구조물의 보수·보강에 활용될 수 있을 것으로 예상된다.

Study on Axial Compression Behavior of Cross-Shaped and L-Shaped Multi-cavity Concrete-Filled Steel Tube Special Shaped Column

Hongbo Li,Yufei Tong,Jianguang Yin,Hubiao Zhang,Changyu Yan 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.1

Multi-cavity concrete-fi lled steel tube special shaped column (CFSTSSC) combines the excellent characteristics of multicavity steel tube and core concrete. CFSTSSC has the advantages of high bearing capacity, good ductility, and strong energy dissipation capacity. At present, accurate calculation methods for these kinds of structures are limited and research into crossshaped and L-shaped multi-cavity CFSTSSCs is not available. Therefore, the axial compression behavior of cross-shaped and L-shaped multi-cavity CFSTSSCs has been investigated, though experimental research and numerical simulation, in this study. First, axial compression tests were carried out on three cross-shaped and three L-shaped multi-cavity CFSTSSCs to analyze their failure modes, axial load-strain curve, ductility index, and ultimate bearing capacity. Then, fi nite element (FE) calculation models of cross-shaped and L-shaped multi-cavity CFSTSSCs were established. The FE models are in good agreement with the experimental results, which provides a foundation for further parameter analysis and failure mechanism study of special shaped columns. Finally, combining parameter analysis and limit equilibrium theory, equations for calculating the ultimate bearing capacity of cross-shaped and L-shaped multi-cavity CFSTSSCs were proposed. The results show that the error between the simplifi ed equation and the FE result is less than 15%, indicating that the equations can provide reference for practical engineering applications.

초음파 토모그래피 다중반사 분석을 통한 콘크리트 벽체 배면 그라우트 충전 두께 평가

이성진,기성훈 한국철도학회 2023 한국철도학회논문집 Vol.26 No.12

The primary objectives of this study are to evaluate the thickness of grout behind concrete walls using ultrasonic tomography technique and evaluate certain influential factors on thickness evaluation. For these purposes, a concrete model specimen that simulates the vertical section of tunnel lining concrete is fabricated in the laboratory. The main variables in this study comprise the types of materials behind the concrete (air, concrete, shotcrete, and grout), the presence of internal waterproof layers in the concrete, grout thickness, and the presence of reinforcing steel bars in concrete. A multichannel ultrasonic measuring device is applied to the surface of the lining concrete specimen to collect ultrasonic signals, by which the thickness of grout behind concrete are visualized through ultrasonic tomography images. Based on the theory of multi-reflection of ultrasonic waves in multi-layered concrete walls, a simplified formula for estimating the thickness of each material in ultrasonic tomography images is proposed. The results of this research that demonstrate ultrasonic tomography is effective for evaluating the thickness of grout behind concrete walls.

Concrete ablation analysis for molten corium-concrete interaction mitigation strategy

Kim, Ji-Hun,Kwon, Sejin,Choi, Jinyoung,Cho, Yong Jin Pergamon Press 2019 Annals of nuclear energy Vol. No.

<P><B>Abstract</B></P> <P>The molten corium-concrete interaction (MCCI) has been receiving increased attention because it can cause the direct release of core melt into the environment via basemat melt-through (BMT). The MCCI analysis has high uncertainty and it is not fully understood according to the current level of knowledge. However, since there is a possibility that MCCI may occur, nuclear power plants (NPPs) must take action to mitigate the MCCI based on quantitative analysis. Therefore, in this study, an analysis model was proposed and MCCI analysis was conducted for NPPs using the model to provide database for establishment of MCCI mitigation strategy of real NPPs. First, the CORQUENCH code was selected as the analysis tool to conduct the MCCI analysis for NPPs, which was released in August 2018 (version 4.1b). The analysis model was proposed and validated for both small and large MCCI experiments. The validation results confirmed that the analysis model predicts the experimental results well regardless of experimental scale within a reasonable range. Second, MCCI analysis was conducted for the NPPs under a conservative assumption to consider the uncertainties in the initial conditions. The results showed that continuous ablation occurs when water is not injected into the reactor cavity and the containment liner plate is damaged by the core melt at 916 min after the melt pouring time. To verify the analysis results, sensitivity analyses were conducted for the initial corium temperature, total corium mass, melt eruption model, void fraction model, and time step. Third, possible MCCI mitigation strategies were proposed based on additional analyses. One strategy is adding new concrete at the bottom of the reactor cavity. In this case, the analysis results showed that the additional concrete should have a thickness of 40 cm or more to secure extra time for the core melt cooling. The other is injecting water into the reactor cavity. The analysis results confirmed that the core melt is effectively cooled down when the water is injected in the early phase.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A model for the molten corium-concrete interaction (MCCI) analysis is proposed. </LI> <LI> The analysis model is validated for both small and large scale experiments. </LI> <LI> The MCCI analysis for a nuclear power plant (NPP) is performed using the proposed analysis model. </LI> <LI> The strategies for MCCI mitigation for NPP are proposed based on the analysis results. </LI> </UL> </P>

콘크리트매트를 이용한 개착식 지반공동 복구방법에 관한 연구

박정준,신희수,정윤석,유승경,홍기권 한국지반신소재학회 2019 한국지반신소재학회 논문집 Vol.18 No.1

This paper describes results of experimental and numerical analyses, in order to analyze a reinforcement effect of concrete mat on open cut restoration of underground cavity. The plate loading tests were conducted to evaluate a reinforcement effect of concrete mat, at reinforcement depths from the ground surface of 10 cm, 20 cm, and 30 cm. The result showed that the reduction ratio of stress (earth pressure) was about 60% at all reinforcement depth. The reinforcement effect considering ground surface settlement and reduction ratio of stress based on laboratory tests and numerical analysis was significant, at reinforcement depths from the ground surface of 10 cm∼20 cm. LFWD test results showed that subgrade modulus was the largest when concrete mat was installed 20 cm below ground surface. Therefore, it is effective to reinforce concrete mat within 20 cm from the surface, when the underground cavity due to damage of underground utilities was formed in the height direction to the bottom of the pavement layer. 본 연구에서는 최근 발생빈도가 증가하고 있는 지반함몰의 가장 큰 영향요소인 지중 공동에 대하여 콘크리트매트를 이용한개착식 복구방법의 보강효과 분석을 위한 실험적 및 수치해석적 연구를 수행하였다. 먼저, 평판재하시험을 이용하여 지표로부터 10cm, 20cm, 30cm의 보강깊이에 대하여 콘크리트매트 보강에 따른 응력감소율을 평가한 결과, 약 60%의 응력감소 효과를나타내었다. 또한 실내실험과 수치해석 결과를 이용하여 지반침하량과 응력감소율을 분석한 결과, 콘크리트매트 설치깊이가지표로부터 10cm∼20cm 깊이에서 보강효과가 우수한 것으로 확인되었다. 지반변형계수 측정을 위한 LFWD실험 결과, 콘크리트매트가 지표로부터 20cm의 깊이에 보강되었을 때의 변형계수가 가장 크게 나타났다. 따라서 지중 매설물 손상에 의한지중공동이 포장체 하부까지 높이 방향으로 발달한 경우, 지표로부터 20cm 이내에 콘크리트 매트를 보강하는 것이 효과적인것으로 분석되었다.

FWD 방향을 고려한 콘크리트 포장 하부 상태 평가

이재훈,이재훈,손덕수,유주호,정진훈 한국도로학회 2014 한국도로학회논문집 Vol.16 No.6

PURPOSES: The purpose of this paper is showing that the state of pavement sublayers can be evaluated differently according to direction of FWD. METHODS: The concrete pavement slabs above subgrade without anything, subgrade with cavity, and box culvert were modeled by finite element method(FEM). The modeled pavements were analyzed by changing the direction of falling weight deflectometer(FWD). The deflection results obtained from FEM were used to calculate radius of relative stiffness and composite modulus of subgrade reaction using AREA method. Then, the analyzed results were compared to the results of the test performed at the Korea Expressway Corporation(KEC) test road. RESULTS : The composite modulus of subgrade reaction increased with subgrade elastic modulus, while radius of relative stiffness decreased. The pavement sections of pure earth showed the consistent results regardless of FWD direction. In case there was cavity, the radius of relative stiffness was larger and composite modulus of subgrade reaction was smaller when FWD was leaving the cavity than when approaching the cavity. This pattern became clear when the cavity got larger. In case of the section with box culvert, the pattern was opposite to the case of cavity. When the soil cover depth increased, the effect of box culvert got smaller. When the load was applied far from the cavity and box culvert, the effect was also declined. The test performed at the KEC test road showed identical results to those of finite element analysis. CONCLUSIONS : The direction of FWD should be considered in evaluation of the state of pavement sublayers because it can be evaluated differently even under identical condition.

FWD 방향을 고려한 콘크리트 포장 하부 상태 평가

이재훈,이재훈,손덕수,유주호,정진훈 한국도로학회 2014 한국도로학회논문집 Vol.16 No.6

PURPOSES: The purpose of this paper is showing that the state of pavement sublayers can be evaluated differently according to direction ofFWD. METHODS: The concrete pavement slabs above subgrade without anything, subgrade with cavity, and box culvert were modeled by finiteelement method(FEM). The modeled pavements were analyzed by changing the direction of falling weight deflectometer(FWD). Thedeflection results obtained from FEM were used to calculate radius of relative stiffness and composite modulus of subgrade reaction usingAREA method. Then, the analyzed results were compared to the results of the test performed at the Korea Expressway Corporation(KEC) testroad. RESULTS : The composite modulus of subgrade reaction increased with subgrade elastic modulus, while radius of relative stiffnessdecreased. The pavement sections of pure earth showed the consistent results regardless of FWD direction. In case there was cavity, the radiusof relative stiffness was larger and composite modulus of subgrade reaction was smaller when FWD was leaving the cavity than whenapproaching the cavity. This pattern became clear when the cavity got larger. In case of the section with box culvert, the pattern was opposite tothe case of cavity. When the soil cover depth increased, the effect of box culvert got smaller. When the load was applied far from the cavity andbox culvert, the effect was also declined. The test performed at the KEC test road showed identical results to those of finite element analysis. CONCLUSIONS : The direction of FWD should be considered in evaluation of the state of pavement sublayers because it can be evaluateddifferently even under identical condition.

초음파 토모그래피를 활용한 터널 라이닝 콘크리트 배면공동의 그라우팅 상태 평가

이성진,기성훈 한국철도학회 2023 한국철도학회논문집 Vol.26 No.7

The main purpose of this study was to investigate the effectiveness of ultrasonic tomography in evaluating the condition of a lining cavity filled with grout material. For this purpose, a concrete lining model specimen was fabricated to simulate the tunnel lining at the top end, considering various construction conditions (presence of reinforcing steel, spacing of reinforcing steel, and cavity thickness). Air cavities behind concrete lining were filled with a grout material. A multichannel ultrasonic measuring device was used to collect ultrasonic signals on the surface of concrete. Ultrasonic tomography images reconstructed by the synthetic aperture focusing technique (SAFT) were used to visualize the conditions of aircavities filled with grout material. The estimated conditions were compared with the as-built conditions. The accuracy of the grout depth estimated by tomography images was about 90% in the un-reinforced section and about 85% in the reinforced sections. As a result, it was demonstrated that ultrasonic tomography is effective at evaluating the condition of aircavities behind lining concrete before and after that lining is filled with grout material.

지표투과레이더를 활용한 터널 라이닝 콘크리트 배면 공동 및 충전 그라우트 상태 평가 시 보강철근의 영향

이성진(Sung Jin Lee),레지데스티오코 하르세노(Regidestyoko Wasistha Harseno),기성훈(Seong-Hoon Kee) 한국철도학회 2024 한국철도학회논문집 Vol.27 No.8

The primary objective of this research is to investigate the influence of reinforcing steel (rebar) in tunnel lining concrete during the evaluation of tunnel lining concrete structures using Ground-Penetrating Radar (GPR) and propose an optimal GPR frequency. For these purposes, a concrete lining model specimen was fabricated to simulate the lining of the tunnel crown. Main variables of study include the number of rebar layers (bare concrete, single, or doble layered rebar) and the thickness of air-void and grout. GPR signals from the concrete surface were collected using three types of ground-coupled GPR antennas at 400 MHz, 900 MHz, and 1,600 MHz. The condition of the tunnel concrete backside was visualized with GPR B-scan images. Overall, the use of a 1600 MHz GPR showed the most improvement in accuracy of assessing the depth and thickness of cavities. In un-grouted section without reinforcement, accuracy values of cavity depth and thickness estimates were approximately 95% and 90%, respectively. However, in reinforced sections (single and double layered), the accuracy values of cavity depth and thickness decreased to approximately 85% and 90%. After grouting, the increased signal attenuation made it difficult to assess the quality of grout application using GPR, and it was challenging to discern the effects of the reinforcement.