• 한국구조물진단유지관리공학회 논문집
• /
• 제28권2호
• /
• pp.27-34
• /
• 2024

본 연구는 시멘트 복합체 기반 변형감지 센서 제조를 위한 전도성 충진재로 Fe 활성화된 목질계 바이오차를 사용하는 타당성을 조사하기 위해 진행되었다. Fe에 의해 활성화된 바이오차를 3% 혼입한 시멘트 복합체의 압축강도 및 전기적 특성을 평가하기 위해 50×50×50mm³ 크기의 입방체 시험체 3개와 40×40×80mm³ 크기의 각기둥의 시멘트 복합체 기반 센서 3개를 각각 준비하였다. 시멘트 복합체 기반 센서에는 전기저항 측정의 4탐침 방식이 사용되었다. Fe 활성화된 바이오차를 혼입한 시멘트 복합체 기반 센서의 경우 다양한 함수율 및 반복압축하에서 임피던스와 같은 전기저항과 전도성을 조사하였다. 그 결과 시멘트 복합체 기반 센서의 직류 회로에서의 전기저항률이 시간이 지남에 따라 증가하였고, 저항률의 최대 부분 변화가 900초 동안 함수율이 증가함에 따라 감소하였다. 함수율 7.5% 구간에서 전도성 충진재로 Fe에 의해 활성화된 바이오차를 3% 혼입한 시멘트 복합체의 전도성이 가장 안정적이지만, 시멘트 복합체 기반 변형감지 센서의 압축변형과 비저항의 변화비(FCR)에서 반복압축응력이 증가함에 따라 변형감지능력에 있어서 다소 떨어지는 경향을 나타냈다.

• Computers and Concrete
• /
• 제30권5호
• /
• pp.301-310
• /
• 2022

Cement-based sensors have been widely used as structural health monitoring systems, however, their long-term sensing performance have not actively investigated. In this study, a deep learning-based methodology is adopted to predict the long-term piezoresistive properties of cement-based sensors. Samples with different multi-walled carbon nanotube contents (0.1, 0.3, and 0.5 wt.%) are fabricated, and piezoresistive tests are conducted over 10,000 loading cycles to obtain the training data. Time-dependent degradation is predicted using a modified long short-term memory (LSTM) model. The effects of different model variables including the amount of training data, number of epochs, and dropout ratio on the accuracy of predictions are analyzed. Finally, the effectiveness of the proposed approach is evaluated by comparing the predictions for long-term piezoresistive sensing performance with untrained experimental data. A sensitivity of 6% is experimentally examined in the sample containing 0.1 wt.% of MWCNTs, and predictions with accuracy up to 98% are found using the proposed LSTM model. Based on the experimental results, the proposed model is expected to be applied in the structural health monitoring systems to predict their long-term piezoresistice sensing performances during their service life.

• Smart Structures and Systems
• /
• 제6권5_6호
• /
• pp.641-659
• /
• 2010

Structural Health Monitoring (SHM) gradually becomes a technique for ensuring the health and safety of civil infrastructures and is also an important approach for the research of the damage accumulation and disaster evolving characteristics of civil infrastructures. It is attracting prodigious research interests and the active development interests of scientists and engineers because a great number of civil infrastructures are planned and built every year in mainland China. In a SHM system the sheer number of accompanying wires, fiber optic cables, and other physical transmission medium is usually prohibitive, particularly for such structures as offshore platforms and long-span structures. Fortunately, with recent advances in technologies in sensing, wireless communication, and micro electro mechanical systems (MEMS), wireless sensor technique has been developing rapidly and is being used gradually in the SHM of civil engineering structures. In this paper, some recent advances in the research, development, and implementation of wireless sensors for the SHM of civil infrastructures in mainland China, especially in Dalian University of Technology (DUT) and Harbin Institute of Technology (HIT), are introduced. Firstly, a kind of wireless digital acceleration sensors for structural global monitoring is designed and validated in an offshore structure model. Secondly, wireless inclination sensor systems based on Frequency-hopping techniques are developed and applied successfully to swing monitoring of large-scale hook structures. Thirdly, wireless acquisition systems integrating with different sensing materials, such as Polyvinylidene Fluoride(PVDF), strain gauge, piezoresistive stress/strain sensors fabricated by using the nickel powder-filled cement-based composite, are proposed for structural local monitoring, and validating the characteristics of the above materials. Finally, solutions to the key problem of finite energy for wireless sensors networks are discussed, with future works also being introduced, for example, the wireless sensor networks powered by corrosion signal for corrosion monitoring and rapid diagnosis for large structures.

• Smart Structures and Systems
• /
• 제8권3호
• /
• pp.321-341
• /
• 2011

Acoustic emission (AE) monitoring was conducted for mortar specimens under three types of static loading patterns (cubic-splitting, direct-shear and pull-out). Each of the applied loading patterns was expected to produce a particular fracture process. Subsequently, the AEs generated by various fracture or damage processes carried specific information on temporal micro-crack behaviors of concrete for post analysis, which was represented in the form of detected AE signal characteristics. Among various available characteristics of acquired AE signals, frequency content was of great interest. In this study, cement-based piezoelectric sensor (as AE transducer) and home-programmed DEcLIN monitoring system were utilized for AE monitoring on mortar. The cement-based piezoelectric sensor demonstrated enhanced sensitivity and broad frequency domain response range after being embedded into mortar specimens. This broad band characteristic of cement-based piezoelectric sensor in frequency domain response benefited the analysis of frequency content of AE. Various evaluation methods were introduced and employed to clarify the variation characteristics of AE frequency content in each test. It was found that the variation behaviors of AE frequency content exhibited a close relationship with the applied loading processes during the tests.

• Structural Monitoring and Maintenance
• /
• 제9권3호
• /
• pp.259-270
• /
• 2022

This study explores the use of the recently developed "strain-sensing smart skin" (S⁴) method for noncontact strain measurements on cement-based samples. S⁴ sensors are single-wall carbon nanotubes dilutely embedded in thin polymer films. Strains transmitted to the nanotubes cause systematic shifts in their near-infrared fluorescence spectra, which are analyzed to deduce local strain values. It is found that with cement-based materials, this method is hampered by spectral interference from structured near-infrared cement luminescence. However, application of an opaque blocking layer between the specimen surface and the nanotube sensing film enables interference-free strain measurements. Tests were performed on cement, mortar, and concrete specimens with such modified S⁴ coatings. When specimens were subjected to uniaxial compressive stress, the spectral peak separations varied linearly and predictably with induced strain. These results demonstrate that S⁴ is a promising emerging technology for measuring strains down to ca. 30 𝜇𝜀 in concrete structures.

• Computers and Concrete
• /
• 제8권5호
• /
• pp.563-581
• /
• 2011

The signal-based acoustic emission (AE) characterization of concrete fracture process utilizing home-programmed AE monitoring system was performed for three kinds of static loading tests (Cubic-splitting, Direct-shear and Pull-out). Each test was carried out to induce a distinct fracture mode of concrete. Apart from monitoring and recording the corresponding fracture process of concrete, various methods were utilized to distinguish the characteristics of detected AE waveform to interpret the information of fracture behavior of AE sources (i.e. micro-cracks of concrete). Further, more signal-based characters of AE in different stages were analyzed and compared in this study. This research focused on the relationship between AE signal characteristics and fracture processes of concrete. Thereafter, the mode of concrete fracture could be represented in terms of AE signal characteristics. By using cement-based piezoelectric composite sensors, the AE signals could be detected and collected with better sensitivity and minimized waveform distortion, which made the characterization of AE during concrete fracture process feasible. The continuous wavelet analysis technique was employed to analyze the wave-front of AE and figure out the frequency region of the P-wave & S-wave. Defined RA (rising amplitude), AF (average frequency) and P-wave & S-wave importance index were also introduced to study the characters of AE from concrete fracture. It was found that the characters of AE signals detected during monitoring could be used as an indication of the cracking behavior of concrete.

• 한국도로학회논문집
• /
• 제8권4호
• /
• pp.87-99
• /
• 2006

본 연구는 도로포장 재료의 상대습도를 편리하고 신뢰성 있게 측정할 수 있는 방법을 파악하여 도로포장 재료의 수분변화 특성을 분석할 수 있는 가능성을 연구하기 위하여 수행되었다. 이를 위하여 먼저 대기의 습도를 여러 가지의 센서를 이용하여 측정한 후 비교 분석하여 센서의 정확성 및 측정치의 보정 방법에 대한 연구를 수행하였으며 신뢰성이 확인된 센서를 이용하여 시멘트 몰탈 시편의 초기 경화 과정 및 실외 상태에서의 습도를 측정하여 분석하였다. 또한 습도측정용 센서를 이용하여 도로포장 재료의 투수성을 분석할 수 있는 몇 가지 방법에 대한 기초 실험도 수행하여 습도 측정을 통해 재료의 투수 특성 분석 가능성도 시험하였다. 연구 결과 Hygrochron을 이용하여 도로포장 재료의 습도를 측정하는 것이 여러 측면에서 유리한 것으로 판단되었으며 이러한 센서를 장착하는 방법 및 측정값을 보정하는 방법을 파악하였다. 몰탈 시편은 시편의 습도가 대기 습도의 증가 또는 감소하는 추세에 따라 변화하기 보다는 대기 습도에 접근하는 방향으로 변화한다는 것을 알았으며, 시편의 표면과 내부에서의 습도변화에는 확연한 지연이 생기는 것도 발견하였다. 또한 습도 측정용 센서는 도로포장 재료의 투수성을 측정하기에도 매우 적합하며 습도 측정값을 이용하여 재료의 투수 속도를 예측할 수 있는 방법을 제시하였다.

• Smart Structures and Systems
• /
• 제20권2호
• /
• pp.175-180
• /
• 2017

The load-carrying capacity and structural behavior of concrete-filled steel tube (CFST) structures is highly influenced by the grouting compactness in the steel tube. Due to the invisibility of the grout in the steel tube, monitoring of the grouting progress in such a structure is still a challenge. This paper develops an active sensing approach with combined piezoceramic-based smart aggregates (SA) and piezoceramic patches to monitor the grouting compactness of CFST bridge structure. A small-scale steel specimen was designed and fabricated to simulate CFST bridge structure in this research. Before casting, four SAs and two piezoceramic patches were installed in the pre-determined locations of the specimen. In the active sensing approach, selected SAs were utilized as actuators to generate designed stress waves, which were detected by other SAs or piezoceramic patch sensors. Since concrete functions as a wave conduit, the stress wave response can be only detected when the wave path between the actuator and the sensor is filled with concrete. For the sake of monitoring the grouting progress, the steel tube specimen was grouted in four stages, and each stage held three days for cement drying. Experimental results show that the received sensor signals in time domain clearly indicate the change of the signal amplitude before and after the wave path is filled with concrete. Further, a wavelet packet-based energy index matrix (WPEIM) was developed to compute signal energy of the received signals. The computed signal energies of the sensors shown in the WPEIM demonstrate the feasibility of the proposed method in the monitoring of the grouting progress.

• Geomechanics and Engineering
• /
• 제13권5호
• /
• pp.793-807
• /
• 2017

Regularities of temperature variation were determined in points of subgrade of the highway. Measurement of temperature was performed by special sensors, based on the effect of thermal resistance. Regular measurements of temperature were performed for two sections of the highway with asphalt concrete and cement concrete pavements for continuous period from November 2010 to March 2016. Multi-year experimental data, which we obtained, allowed establishing of peculiarities for temperature variation in points of subgrade in time and temperature distribution in the depth for annual cycle. Characteristics were determined for winter period-depth, duration and freezing rate, duration and defreezing rate for pavement and subgrade of the highway.

• 한국도로학회논문집
• /
• 제7권3호
• /
• pp.79-91
• /
• 2005

포틀랜드 시멘트 콘크리트 도로 포장의 온도 패턴을 콘크리트 타설시부터 최근에 개발된 저렴하며 획기적인 센서를 이용하여 측정하였으며 그 결과를 분석하였다. 콘크리트 포장의 온도 측정은 여러 다른 지역에서 여러 다른 두께를 가진 콘크리트 포장에서 수행하였다. 콘크리트 포장의 온도 패턴은 깊이방향과 콘크리트 타설시간에 따른 종방향의 변화를 고려하여 분석하였으며, 포장 표면의 반사율, 그늘, 덮음막 등이 콘크리트 포장의 온도 패턴에 미치는 영향을 분석하였다. 본 연구 결과에서 콘크리트를 타설한 날의 콘크리트 포장의 최고 온도는 종방향을 따라 타설 시간이 다름으로 인해 생기는 차이가 두드러지는 것을 알 수 있었다. 콘크리트의 영점응력온도(zero-stress temperature)는 타설한 날의 최고 온도와 직접적인 연관이 있으므로 타설 시간에 따른 최고 온도의 차이는 콘크리트 포장의 거동 및 성능에 크게 영향을 미칠 수 있다. 또한 콘크리트 포장의 표면 상태 (표면 색상, 그늘 유무, 덮음막 유무 등)도 콘크리트 포장의 온도 패턴에 큰 영향을 미치는 인자인 것으로 분석되었다.