Reinforced concrete cantilever beams were tested for flexure. Totally three strain gauges were used for measuring strains: two strain gauges on the reinforcement, one on the top of the rebar and another on the bottom of the rebar near to the end support and the third strain gauge was fixed on the surface of the concrete beam. Strains were measured by Data Acquisition (DAQ) system as well as manually. Demac buttons were fixed on the surface of the beams near the support to measure the strain manually and National Instrument (NI) DAQ card was used for assessing strain gauge data. Strains were measured for specimen with different corrosion levels. Good agreement was observed in measurement of strain gauges in mechanical and DAQ system.
This paper presents an experimental investigation for detecting defects in concrete structures using so-called "smart aggregates." The smart aggregates are small cylinders with piezoelectric patches inside that can be embedded in concrete structures and used as both actuators and sensors. Specimens with different types of defects such as notch, hole, and inclusion were used in this study. To evaluate the effectiveness of the smart aggregates for detecting real cracks in concrete structures, three-point bending tests were carried out on two reinforced concrete beams. The test results indicate that not only the passive defects (notch, hole, or inclusion) but also the real cracks in reinforced concrete structures can be detected by the smart aggregates. Sensitivities of different parameters (time-of-flight, energy content of the signals, wavelet packet decomposition-based damage index) for various defects were also investigated.
In the field of Structural Health Monitoring, there is growing interest in continuous volume measurements of material properties using embedded sensors organized in wireless sensor networks (1,2,3). In this work we propose a novel, highly reproducible piezoresistive sensor based on Carbon Nanotube (CNT) networks deposited on polymer to be used for embedded strain monitoring and crack detection in concrete. We highlight the originality of the fabrication process and describe the modalities for signal conditioning and for integration into a RFID-based wireless sensor network. We have achieved the first reported strain gauge fabricated by direct inkjet printing of CNTs on polymeric substrates, namely Ethylene tetrafluoroethylene (ETFE). The strain gauge we propose is of interest for its flexibility and sensitivity (Gauge Factor) which is higher than that of metallic sensors. For these reasons we believe that the proposed strain gauge is a promising alternative to the rigid metallic strain gauges currently on the market. The method is based on the dispersion of CNTs in a solution of 1,2-dichlorobenzene, with sodium dodecyl benzene sulphonate added as surfactant to enhance the solution’s wettability on the polymer. The solution is used as ink to print resistive layers of randomly oriented CNTs on polymer. We optimized the printing and the rinsing process for maximum control of the resistivity and uniformity of the CNT layers. The sensors show a Gauge Factor (GF) of 2.54 which is superior to commercial metallic strain gauges by 25%. Our technology is characterized by its high reproducibility, with only a 6.8% variation of GF values for different sensors fabricated on ETFE. Tests on multiple loading cycles prove the repeatability of the measurements over several loading cycles. Our results suggest that such devices could be used in real life applications. With this goal in mind, we compare several electronic circuits for signal conditioning (from Wheatstone bridge with amplification chain to Sigma-Delta conditioner) in order to select the more appropriate for embedded sensing in concrete. The application dictates two constraints on the electronics: low power consumption and small dimension. We suggest that a configuration based on RFID (Radio Frequency Identification) is the optimum solution given the imposed constraints (4,5). We propose an architecture that can be exploited in conditioning elements sensitive to other parameters of interested for SHM as humidity and pH sensors (6). Although the sensor has its own intrinsic merits, our project presents a complete system (sensor with conditioning electronics) which is innovative, original and intended for a practical application of great importance to modern societies.
In recent years, there has been an increasing interest in the adoption of emerging sensing technologies for instrumentation within a variety of structural systems. Wire- less sensors and sensor networks are emerging as sensing paradigms that the structural engineering field has begun to consider as substitutes for traditional tethered monitoring systems. A benefit of wireless structural monitoring systems is that they are inexpensive to install because extensive wir- ing is no longer required between sensors and the data acquisition system. Researchers are discovering that wire- less sensors are an exciting technology that should not be viewed as simply a substitute for traditional tethered monitor- ing systems. Rather, wireless sensors can play greater roles in the processing of structural response data; this feature can be utilized to screen data for signs of structural damage. Also, wireless sensors have limitations that require novel system architectures and modes of operation. This paper is intended to serve as a summary review of the collective experience the structural engineering community has gained from the use of wireless sensors and sensor networks for monitoring structural performance and health.
Bridge operation safety is critical to national security and people's livelihood. The structural health monitoring of bridges has emerged as an increasingly active research area. Wireless sensor networks (WSNs) technology is known to be easy to deploy and inexpensive to maintain. It is thus suitable for structural health monitoring of bridges. This paper gives a survey on structural health monitoring systems based on WSNs technology. Some basic theories and typical methods in subsystems are presented, and critical technologies are analyzed. At the end, varies issues in existing systems and directions on future work are analyzed and summarized..
In-situ non-destructive testing of durability in cementitious materials is essential to the early prediction and prevention of structural failures. Whereas degradations in cementitious materials, and henceforth durability loss, are brought about and controlled by the characteristics and evolutions of microporosity, there isn't to our knowledge any method for the in-situ non-destructive testing of microporosity itself. To evaluate in-situ the durability of cementitious materials, we put forward an innovative concept based on in-situ instrumentation of their microstructure. Individual micropores are to be probed by high-frequency ultrasonic waves generated and detected by capacitive ultrasonic microtransducers (μ-cMUT) embedded in large number within the material. The vibrating plate of the μ-cMUT devices is to be made of a thin layer of densely aligned single-walled carbon nanotubes, in order for the devices to satisfy the applicative and technological requirements. Relevance of this instrumentation method has been studied : we have used an elasto-acoustical model to describe the interaction between the vibrating plate of a μ-cMUT device and the fluid (water or air) filling a pore of micrometric size. The specificity of this model lies in the integration of fluid viscosity. It has required us to develop ad-hoc solving techniques. We have found out numerically that in this problem dissipation leads to a decrease in resonance frequency compared to non-visquous acoustics. The boundary layer is large compared to the domain size. The vibration amplitudes of the plate are very sensitive to pore content and to water-filled pore geometry. We have deduced from these results that the μ-cMUT devices we envision may be relevant to study hydration and to monitor degradations in cementitious materials. Feasibility of a high-frequency, nanotubes-based μ-cMUT device operating in water or air has also be evaluated : using first a dielectrophoretic deposition technique, we have made thin, dense membranes of well-aligned nanotubes. One of our deposition reaches mono-layer thickness, which is remarkable for dielectrophoretic depositions. We have suspended the nanotubes, thus obtaining long and rigid membranes. They are very thin and have a high form factor compared to state-of-the-art cMUT devices. Finally, we have used laser vibrometry to observe membrane vibrations. Membrane vibration amplitudes reach 5 nm at low frequency. As far as we know, it is the first time vibrations of carbon nanotubes have been successfully observed with laser vibrometry. These results prove that we have overcome one of the most significant technological bottle-neck of the whole feasibility study. Moreover, they indicate short-term feasibility of air microdetectors that could be valuably employed to monitor gaseous microporosity in cementitious materials. By associating a numerical study on relevance and a technological study on feasibility, this work contributes significantly to the development of a new durability monitoring method for cementitious materials. Central to this method is the use of a large number of embedded microsensors integrating nanotechnologies
The application of the electromechanical impedance (EMI) method to monitor the condition of structures is an actively researched area. This article extends the method to allow it to be incorporated into a wireless sensing device, which is embedded into freshly poured concrete to monitor initial curing and subsequent structural health. The results show that the hydrating concrete has an effect on the sensing system and that it is sensitive enough to monitor the strength development of concrete. Initial results also show that the embedded EMI method is sensitive to the removal of formwork. The response of the system to compressive testing is also investigated, and the initial results show a good correlation with previously published reports on compressive testing of concrete. Finally, the ability of the system to be incorporated into a previously developed wireless-sensing platform is investigated. The AD5933 impedance chip offers this possibility, and its response is investigated and compared with the response of the HP4192A. The results show that it is feasible to design a completely wireless-sensing device for the monitoring of the strength gain of concrete and its deterioration.
Health Monitoring of Bridges prepares the bridge engineering community for the exciting new technological developments happening in the industry, offering the benefit of much research carried out in the aerospace and other industrial sectors and discussing the latest methodologies available for the management of bridge stock.
Health Monitoring of Bridges:
• Includes chapters on the hardware used in health monitoring, methodologies, applications of these methodologies (materials, methods, systems and function), decision support systems, damage detection systems and the rating of bridges and methods of risk assessment.
• Covers both passive and active monitoring approaches.
• Offers directly applicable methods as well as prolific examples, applications and references.
• Is authored by a world leader in the development of health monitoring systems
• Includes free software that can be downloaded from the accompanying website at www.wiley.co/go/wenzel, and provides the raw data of benchmark projects and the key results achieved.
This book provides a comprehensive guide to all aspects of the structural health monitoring of bridges for engineers involved in all stages from concept design to maintenance. It will also appeal to researchers and academics within the civil engineering and structural health monitoring communities.
In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.
Strain gauges are widely used in structural health monitoring (SHM) systems because they are inexpensive, easy to install, and sensitive enough to detect the potential danger of collapse of a building or structure. However, off-the-shelf equipment for the strain gauges is usually either wired to the sensors or wireless with very limited range, which significantly increases the total cost as well as restricting the performance of the deployment. In this paper, we propose a structural health monitoring system based on wireless sensor nodes equipped with inexpensive strain gauges. The performance of the system is not limited, because multi-hop deployment is possible. Our system consists of a sensor board, which properly amplifies and converts the signal from the strain gauges, and communication software, written in TinyOS, that is in charge of reliable transport of the strain data. We evaluated our system in comparison with the commercial equipment. The experiment results show that the proposed system is reliable and effective for use in structural health monitoring.