Monitoring Of Concrete Structures Research Articles

A novel embeddable spherical smart aggregate for structural health monitoring: part I. Fabrication and electrical characterization

Recently developed piezoceramic-based transducers, known as smart aggregates (SAs), have shown their applicability and versatility in various applications of structural health monitoring (SHM). The lead zirconate titanate (PZT) patches embedded inside SAs have different modes that are more suitable for generating or receiving different types of stress waves (e.g. P and S waves, each of which has a unique role in SHM). However, due to the geometry of the 2D PZT patch, the embedded SA can only generate or receive the stress wave in a single direction and thus greatly limits its applications. This paper is the first of a series of two companion papers that introduces the authors’ latest work in developing a novel, embeddable spherical smart aggregate (SSA) for the health monitoring of concrete structures. In addition to the 1D guided wave produced by SA, the SSA embedded in concrete structures can generate or receive omni-directional stress waves that can significantly improve the detection aperture and provide additional functionalities in SHM. In the first paper (Part I), the detailed fabrication procedures with the help of 3D printing technology and electrical characterization of the proposed SSA is presented. The natural frequencies of the SSA were experimentally obtained and further compared with the numerical results. In addition, the influence of the components’ thickness (spherical piezoceramic shell and epoxy) and outer radius (spherical piezoceramic shell and protection concrete) on the natural frequencies of the SSA were analytically studied. The results will help elucidate the key parameters that determine the natural frequencies of the SSA. The natural frequencies of the SSA can thus be designed for suitability in the damage detection of concrete structures. In the second paper (Part II), further numerical and experimental verifications on the performance of the proposed SSA in concrete structures will be discussed.

Carbon nanotube/cement composites for crack monitoring of concrete structures

The feasibility of using a carbon nanotube/cement composite as a crack sensor for concrete structures is evaluated. First, the relationship between the crack width and electrical conductivity of the composite material was investigated. Then, this analysis was repeated for the composite embedded in reinforced mortar beams, using three-point flexural loading. A conductivity model for the cracked composite was introduced and compared with experimental results. In addition, to evaluate whether monitoring was feasible under the conditions of crack closure by external loading, the cracked mortar beam was compressed along the vertical direction with respect to the crack and the conductivity of the composites embedded in the beam was measured. The conductivity of the composite decreased with increasing crack width and its reliability could be improved by increasing its original conductivity.

Highly-Sensitive Near-Infrared Spectroscopy System for Remote Monitoring of Concrete Structures

Salt- or neutralization-induced damage of reinforced concrete structures significantly affects their durability. Because most large-scale infrastructures, such as bridges, urban tunnels, and seawalls, are made of reinforced concrete, developing efficient and accurate methods or devices for early detection of concrete structure damage is important for disaster prevention. Moreover, the cost of a life cycle of a concrete structure can be much lower if surface degradation can be detected and preventive maintenance performed.In this paper, we report the development status of a remote monitoring system that implements remote imaging and diagnosis of the concrete surface degradation. The system consists of a high-sensitivity spectrometer and signal analysis software.The first prototype of the spectrometer was developed using a large-aperture and lossless optical system and a near-infrared camera. Near infrared spectra of samples that simulated salt damage were captured and diagnosed, to analyze the system’s performance. Then, specimens of pure materials constituting concrete were measured, and their absorption spectra were analyzed. Based on the absorption spectra it was concluded that the absorption characteristics of these substances had no influence on the quantification of Friedel’s salt.The prototype machine for field testing has been developed. Using this machine, a series of remote measurement of surface water with up to 10m distance was conducted. And also, measurement of surface degradation on actual concrete structures has been started, to validate the approach.The developed system is expected to rationalize inspection of concrete structures by enabling effective imaging and objective diagnosis of degradation, which will help to clarify which parts or sections of structures need intensive inspection for preventive maintenance.

Compressive strength evaluation by non-destructive techniques: An automated approach in construction industry

Non-destructive techniques (NDTs) for the evaluation of concrete structure are widely used for the assessment and monitoring of concrete structures. Previous researches showed a large variation in results due to highly dependent on the number of cores and sensitivity of the NDTs. In this experimental work, concrete mixtures were cast in a wide range of compressive strength varying from 20 to 50MPa. Ultrasonic pulse velocity test, rebound hammer test and the crushing compressive strength test were conducted at the age of 7, 28 and 56 days. Relationships between ultrasonic pulse velocity-compressive strength and rebound number-compressive strength were established. At the end, a combination of both NDTs was used to predict the compressive strength. The predicted values were verified by comparison with experimental results and previous studies. A close correspondence was found between experimental and predicted values, as the data was scattered within 15% of line of equality. ISO-strength curves were also drawn, which may be used to predict the compressive strength from 20 to 50MPa accurately.

RFID sensor systems embedded in concrete – requirements for long–term operation

Structural Health Monitoring (SHM) is an important topic for Industry 4.0. More and more systems are embedded in different materials and are connected to each other. To embed sensors for a long time in concrete, an exact preparation is required. In this paper, we identify Radio-Frequency Identification (RFID) as promising technology for monitoring of concrete structures. This paper present the concept for long term monitoring, defines the requirements and shows first development steps, for example, the system design, the possibilities of sensor connection, and an encapsulation for embedding in concrete.

Analysis and Design of a Concrete Embedded Antenna for Wireless Monitoring Applications [Antenna Applications Corner

In this article, we describe an embedded circularly polarized patch antenna for the batteryless sensor monitoring of concrete structures. In particular, we focus our effort on designing a patch antenna that is able to work in concrete and has different dielectric properties without changing the input impedance to achieve an optimal power transfer. The antenna is designed and optimized by exploiting an analytical model that is suitably customized for the case at hand and a full-wave numerical analysis. The achieved results give physical insights into the antenna performance and explain its robustness against variations in the concrete. Experimental measurements fully confirm the model predictions. Although our work is focused on the particular application concerned with concrete health monitoring, the developed analysis can be extended in a conceptually easy fashion to other fields such as antenna design for biomedical and subsurface microwave imaging.

An Innovative Active Sensing Platform for Wireless Damage Monitoring of Concrete Structures

An Innovative Active Sensing Platform for Wireless Damage Monitoring of Concrete Structures

Acoustic emission monitoring of reinforced concrete beams subjected to four-point-bending

The aim of this study is to apply acoustic emission (AE) technique and study the damage mechanism of the reinforced concrete (RC) beams under four-point bending. Laboratory experiments are performed on three types of RC beams of grade M30 with 42, 64 and 93% of longitudinal steel against balanced section. The damage in the beams are classified into four zones symbolizing formation of micro cracks, visible cracks, steel yielding and concrete crushing. The AE parameters such as amplitude, rise time, count, duration and average frequency are quantified in each damage levels and a parametric analysis is performed between average frequency and RA value. The results showed that as the level of damage increased, the values of AE parameters such as count, hits, rise time, acoustic emission energy and duration increased except for average frequency. This results coincided with the visual observation results according to crack modes. The adequacy of the crack classification is also evaluated by Gaussian mixture modelling (GMM), a probabilistic based approach. GMM is used as a parametric model to overcome the randomness found in the data set generated by AE testing. The results of the present investigation can be utilized in health monitoring of concrete structures subjected to flexural load.

Feasibility study of using smart aggregates as embedded acoustic emission sensors for health monitoring of concrete structures

Acoustic emission (AE) is a nondestructive evaluation technique that is capable of monitoring the damage evolution of concrete structures in real time. Conventionally, AE sensors are surface mounted on the host structures, however, the AE signals attenuate quickly due to the high attenuation properties of concrete structures. This study conducts a feasibility study of using smart aggregates (SAs), which are a type of embedded piezoceramic transducers, as embedded AE sensors for the health monitoring of concrete structures. A plain concrete beam with two surface mounted AE sensors and two embedded SAs was fabricated in laboratory and loaded under a designed three-point-bending test. The performance of embedded SAs were compared with the traditional surface mounted AE sensors in their ability to detect and evaluate the damage to the concrete structure. The results verified the feasibility of using smart aggregates as embedded AE sensors for monitoring structural damage in concrete. Potentially, the low cost smart aggregates could function as embedded AE sensors, providing great sensitivity and high reliability in applications for the structural health monitoring of concrete structures.

Micromechanics modeling of the electrical conductivity of carbon nanotube cement-matrix composites

Abstract The incorporation of Carbon Nanotubes (CNTs) as nanoinclusions for the development of electrically conductive cement-based composites opens a vast range of possibilities for monitoring of concrete structures. A key issue for the design and optimization of these composites is the development of theoretical models capable of providing a quantitative prediction of their overall electrical conductivity. Experimental results have evidenced the strong influence of the waviness and dispersion of the nanotubes on the overall conductivity of these materials, what makes the consideration of these two phenomena essential for the development of realistic theoretical models. Nevertheless, both waviness and agglomeration have been often neglected in the literature or, when considered, have been reproduced with very simple modeling approaches not suitable to catch the complexity of the problem at hand. This paper presents an improved micromechanics model of the effective electrical conductivity of CNT cement-based nanocomposites based on enhanced approaches for reproducing waviness and non-uniform spatial distributions of the nanoinclusions. The two mechanisms that govern the electrical conductivity of these composites, electron hopping and conductive networks, are incorporated in the mixed micromechanics model. On the basis of scanning electron microscopy inspections, a helical waviness model and a two-parameter agglomeration approach are proposed. In order to assess the accuracy of the proposed analytical model, cement-based specimens have been manufactured and tested for providing data to use as the basis of comparison. In particular, specimens of cement pastes, mortars and concretes with different concentrations of Multi-Walled Carbon Nanotubes (MWCNTs) have been prepared. It is shown that the consideration of straight uniformly distributed nanotubes, as typically done in the literature, leads to an overestimation of the overall conductivity. On the contrary, it is highlighted that the wavy state of the fibers as well as their agglomeration in bundles play a crucial role in the conductivity of cement-based nanocomposites, which is demonstrated by achieving a good fit to the experimental data when using the proposed models for waviness and agglomeration. Overall, the paper highlights the physical mechanisms governing the overall electrical conductivity of cement-based composites with MWCNTs and provides a powerful analytical tool for their design.

Influence of axial loads on the health monitoring of concrete structures using embedded piezoelectric transducers

Piezoceramic-based smart aggregate has been widely used to evaluate early-age concrete strength and to detect damage in concrete structures. In these structural health monitoring systems, they are generally verified and calibrated through experiments under load-free condition. However, the stress levels of actual concrete members are different. The microstructures of concrete will change with the variation of external load, and the high-frequency waves used in the monitoring system may be highly sensitive to these changes. In this study, the effects of axial compressive loading on the monitoring results are investigated. Specifically, three loading cases, that is, single cycle load, cyclic load, and step-by-step load, are employed to stress the concrete specimens embedded with smart aggregates. The amplitude and velocity of monitoring signals were measured before, during, and after each loading case. The test results show that the axial load lower than 30% of failure load still have a significant impact on the received signals. The amplitude attenuation is dependent on both frequency and load history, while the velocity is highly stress-dependent. The results indicate that the baselines of monitoring signals obtained from the same concrete structure in its healthy state can vary under different stress levels. The axial load variation should be carefully considered during the monitoring process. This study also provides a potential method to assess stress state in concrete structures using smart aggregates.

Distributed Long-Gauge Optical Fiber Sensors Based Self-Sensing FRP Bar for Concrete Structure

Brillouin scattering-based distributed optical fiber (OF) sensing technique presents advantages for concrete structure monitoring. However, the existence of spatial resolution greatly decreases strain measurement accuracy especially around cracks. Meanwhile, the brittle feature of OF also hinders its further application. In this paper, the distributed OF sensor was firstly proposed as long-gauge sensor to improve strain measurement accuracy. Then, a new type of self-sensing fiber reinforced polymer (FRP) bar was developed by embedding the packaged long-gauge OF sensors into FRP bar, followed by experimental studies on strain sensing, temperature sensing and basic mechanical properties. The results confirmed the superior strain sensing properties, namely satisfied accuracy, repeatability and linearity, as well as excellent mechanical performance. At the same time, the temperature sensing property was not influenced by the long-gauge package, making temperature compensation easy. Furthermore, the bonding performance between self-sensing FRP bar and concrete was investigated to study its influence on the sensing. Lastly, the sensing performance was further verified with static experiments of concrete beam reinforced with the proposed self-sensing FRP bar. Therefore, the self-sensing FRP bar has potential applications for long-term structural health monitoring (SHM) as embedded sensors as well as reinforcing materials for concrete structures.

Application of ultrasonic surface wave techniques for concrete bridge deck condition assessment

Ultrasonic surface wave (USW) is a well-established technique for the performance monitoring of concrete structures. In order to investigate the capability and reliability of this technique for concrete bridge deck condition assessment, a portable seismic property analyzer (PSPA) with USW capabilities was used to assess the condition of a reinforced concrete bridge deck exhibiting visible evidence of significant deterioration. After the investigation was completed, variable thicknesses of concrete were removed from upper surface of the concrete deck by milling and hydrodemolition, with greater thickness being removed where the concrete was more deteriorated. The thickness of removed concrete during the hydrodemolition process was mapped by Light Detection and Ranging (LiDAR). A comparison of the thickness of concrete removed and the USW data indicates that there is a qualitative correlation between the USW results at each test location and the thickness of concrete removed at those same test locations. Results suggest that the PSPA, and comparable USW techniques, could be potentially effective for estimating the thicknesses of concrete that would be removed during milling and hydrodemolition, although more work is needed to study the relationship between USW and removal thickness data in order to be used for quantity estimations.

Smart Aggregate-Piezoceramic Patch Combination for Health Monitoring of Concrete Structures

A new method combining an embedded smart aggregate and surface mounted piezoceramic patches is introduced for health monitoring of concrete structures. The smart aggregate is embedded in a concrete beam as an actuator (or transmitter), and piezoceramic patches are attached on the surface of the concrete beam as sensors. Two tests using the smart aggregate and the piezoceramic patches are conducted. The first test investigates the sensitivity of the recorded signal amplitude-frequency relation on the piezoceramic patches. To explain the significant amplitude fluctuations in the results, the possibility of resonance occurring in the piezoceramic patches in a certain frequency range is verified through finite element modeling. In the second test, a damage index is proposed to evaluate the health of concrete structures and a three-point bending test is conducted to induce damage in the concrete beam. It is observed that, with increasing severity of damage in the concrete beam, the recorded signal amplitude at the patches decreases gradually while the value of the damage index increases significantly. The experimental results show that the proposed method is an effective tool for health monitoring of concrete structures.

Investigations on scalable fabrication procedures for self-sensing carbon nanotube cement-matrix composites for SHM applications

Electrically conductive cement-based composites doped with carbon nanotubes possess the functional properties of being strain- and damage-sensitive, thus providing a cost-effective solution for monitoring of concrete structures. The weak point of the technology is the dispersion of the nanotubes, typically based on special procedures, such as sonication, that are not suitable for large scale applications. This paper presents a systematic investigation on various procedures for fabricating carbon-nanotube-cement pastes, mortars and concretes. Dispersion of nanotubes in water is achieved using chemical dispersants and different mixing strategies, while quality of nanotubes' dispersion is assessed by measuring the rate of nanotube separation and by SEM inspections. Electrical percolation and strain-sensitivity are investigated in order to assess the quality of the fabricated composites. The results of this research allow to identify a processing procedure that, without sonication, might be potentially effective for fabricating self-sensing cement-based composites in a way that is compatible with large scale deployment. In particular, similarly to what observed in the case of sonicated specimens, the percolation thresholds of mortars and concretes fabricated with the “scalable” procedure are seen to be around 1% w. of MWCNTs content with respect of the weight of cement, while the same percolation threshold is less evident for pastes. Electrical conductivity and gauge factor of the same samples are also similar to those of sonicated composites, whereby conductivity of composite pastes, mortars and concretes assumes values of 5.2E-4, 1.8E-4 and 1.0E-4 (Ωcm)−1, respectively, while gauge factors are equal to 130, 68 and 23, respectively.

Amplitude and phase measurements of continuous diffuse fields for structural health monitoring of concrete structures

Measuring amplitude and phase of continuous ultrasonic waves with a lock-in amplifier is shown to give similarly sensitive indicators of concrete damage as pulsed coda wave analysis, but maintains its sensitivity at considerably much lower signal levels. Continuous and pulsed measurements were performed on a concrete slab subjected to cyclically increased damage level. In the unloaded phase each measurement type was performed at varying transmit signal levels. The result indicates the possibility of using a larger distance between transducers in high frequency health monitoring systems of concrete structures, where attenuation of propagating waves is strong.

Automated Surface Wave Measurements for Evaluating the Depth of Surface-Breaking Cracks in Concrete

The primary objective of this study is to investigate the feasibility of an innovative surface-mount sensor, made of a piezoelectric disc (PZT sensor), as a consistent source for surface wave velocity and transmission measurements in concrete structures. To this end, one concrete slab with lateral dimensions of 1500 by 1500 mm and a thickness of 200 mm was prepared in the laboratory. The concrete slab had a notch-type, surface-breaking crack at its center, with depths increasing from 0 to 100 mm at stepwise intervals of 10 mm. A PZT sensor was attached to the concrete surface and used to generate incident surface waves for surface wave measurements. Two accelerometers were used to measure the surface waves. Signals generated by the PZT sensors show a broad bandwidth with a center frequency around 40 kHz, and very good signal consistency in the frequency range from 0 to 100 kHz. Furthermore, repeatability of the surface wave velocity and transmission measurements is significantly improved compared to that obtained using manual impact sources. In addition, the PZT sensors are demonstrated to be effective for monitoring an actual surface-breaking crack in a concrete beam specimen subjected to various external loadings (compressive and flexural loading with stepwise increases). The findings in this study demonstrate that the surface mount sensor has great potential as a consistent source for surface wave velocity and transmission measurements for automated health monitoring of concrete structures.

Hybrid optical-fibre/geopolymer sensors for structural health monitoring of concrete structures

In this work, we demonstrate hybrid optical-fibre/geopolymer sensors for monitoring temperature, uniaxial strain and biaxial strain in concrete structures. The hybrid sensors detect these measurands via changes in geopolymer electrical impedance, and via optical wavelength measurements of embedded fibre Bragg gratings. Electrical and optical measurements were both facilitated by metal-coated optical fibres, which provided the hybrid sensors with a single, shared physical path for both voltage and wavelength signals. The embedded fibre sensors revealed that geopolymer specimens undergo 2.7 mϵ of shrinkage after one week of curing at 42 °C. After curing, an axial 2 mϵ compression of the uniaxial hybrid sensor led to impedance and wavelength shifts of 7 × 10−2 and −2 × 10−4 respectively. The typical strain resolution in the uniaxial sensor was 100 . The biaxial sensor was applied to the side of a concrete cylinder, which was then placed under 0.6 mϵ of axial, compressive strain. Fractional shifts in impedance and wavelength, used to monitor axial and circumferential strain, were 3 × 10−2 and 4 × 10−5 respectively. The biaxial sensor’s strain resolution was approximately 10 in both directions. Due to several design flaws, the uniaxial hybrid sensor was unable to accurately measure ambient temperature changes. The biaxial sensor, however, successfully monitored local temperature changes with 0.5 °C resolution.

An experimental investigation on the health monitoring of concrete structures using piezoelectric transducers at various environmental temperatures

The application of piezoelectric transducers in in situ health monitoring of concrete structures has been widely investigated. However, previous experimental studies were normally performed in an isothermal room environment, which offers insufficient considerations for temperature variations that real engineering structures experience in practical monitoring cases. In this article, the temperature effects on smart aggregate–based monitoring results are treated by performing active structural health monitoring on two plain concrete specimens at various temperatures. Experimental results show that the amplitude of the monitoring signal increases with temperature, with low-frequency signals being more sensitive to temperature variation while high-frequency signals less temperature dependent. This research demonstrates the necessity of temperature compensation in smart aggregate–based monitoring techniques.

OS6-10 Amplitude and Phase Measurements of Continuous Diffuse Fields for Structural Health Monitoring of Concrete Structures(Ultrasonic NDT of Cracks and Damages (2),OS6 Ultrasonic non-destructive testing and evaluation,MEASUREMENT METHODS)

OS6-10 Amplitude and Phase Measurements of Continuous Diffuse Fields for Structural Health Monitoring of Concrete Structures(Ultrasonic NDT of Cracks and Damages (2),OS6 Ultrasonic non-destructive testing and evaluation,MEASUREMENT METHODS)