Distributed sensors have become a great advantage for Structural Health Monitoring (SHM) as they allow for the multiple points measurement using a single sensor. Nevertheless, the installation of this technology can be time-consuming and have an impact on the overall cost of the project. For this reason, this paper explores the application of different techniques for embedding fiber optic cable into textile for Distributed Optical Sensors which could greatly reduce the installation time. This embedding also provides the ability to design sensors with different patterns that enable monitoring structures like pipelines, bridges, and others. In this paper we have identified an embedding technique that does not damage the fiber optic cable. Additionally, the sensors were tested to study their response to temperature and strain by using Brillouin Optical Time Domain Amplification (BOTDA) interrogation technique.
Differential settlement of underground pipelines is one of the major causes responsible for pipeline failures in the U.S. Due to the invisibility of underground pipeline deformation and the requirement for long-range monitoring of underground pipelines, most of the underground pipeline motions are currently undetected. In this paper, a novel long-range sensing technique using fiber optic sensors is proposed for the structural health monitoring (SHM) the deformation and motion of underground pipelines. Two laboratory 304.8x1.7cm HDPE (high-density polyethylene) pipe specimens were manufactured and tested under four-point bending for damage detection. Single mode optical fibers (10.4 ± 0.8 μm) were installed on the surface of these two HDPE pipes for distributed sensing. Four-point bending test was carried out on two HDPE pipes in the range of 445 N to 2670 N at an increment of 445 N. A BOTDR (Brillouin Optical Time Domain Reflectometer) system was applied in collecting distributed strain measurements (spatial resolution =1m, sampling interval =0.5m) from the two HDPE pipes. Fourteen conventional coil-type strain gauges (gauge factor: 2) were also instrumented on each HDPE pipe for validation purpose. From our laboratory results, it was found that the longitudinal BOTDR strain measurements near the neutral axis of the HDPE pipes can be used for detecting pipe rotation. It was also found that the longitudinal BOTDR strains at the bottom of the pipes can be used to detect pipe bending and damage detection.
Masonry structures are widely used for their low cost, durability, fire-resistance, sound isolation and other properties in civil engineering and architecture. Inspection of masonry structures is vital for maintaining their structural performance and long-term safety. Many traditional inspection technologies (e.g., acoustic/ultrasonic, thermographic, electromagnetic) have been applied for the structure health monitoring of masonry structures. The objective of this paper is to apply a synthetic aperture radar (SAR) system for characterizing the dielectric constant of masonry specimens. A 10-GHz imaging radar system was used. A masonry wall was selected, and five ranges (150 cm, 250 cm, 500 cm, 1000 cm and 1500 cm) were considered in collecting SAR images of the masonry wall. From our result, it was found that attenuation of integrated SAR amplitude exhibits a nonlinear pattern as a function of range. An algorithm was developed to estimate the dielectric constant of the masonry wall. Field collected SAR images were compared with a portable 1.6-GHz ground penetrating radar (GPR) system. It was demonstrated that SAR images can be used to estimate dielectric constant of masonry structures in the field.
Corrosion of steel rebar in reinforced concrete (RC) structures introduces internal stress at the interface between rebar and concrete, ultimately leading to the failure of structures. Detection of early-stage corrosion of steel rebar can significantly reduce maintenance cost and risks. An active photoacoustic fiber optic sensor system had been proposed for early-stage corrosion detection of steel rebars by generating and receiving surface ultrasonic waves. However, the implementation of a corrosion detection method requires knowledge of surface ultrasonic waves propagating at rebar-concrete interface. The objective of this study is to investigate the interaction of surface ultrasonic waves with local geometries (of a number four rebar) and concrete covers using the finite element method (FEM). In this study, seven three-dimensional finite element models were created to simulate surface ultrasonic waves propagating in three different cross-sections of a steel rebar with different concrete cover. Three lug locations and three types of concrete (differed by Youngs modulus) were considered. The pitch-catch mode was adopted, in which one source and one receiver were deployed at each rib of the rebar. 1 MHz sinusoidal pulse was introduced at the source while time domain radial displacements were collected at the sensor. Short-time Fourier transform was used to analyze collected time domain radial displacements. From our simulation results, it was found that high frequencies of ultrasonic waves were affected by lugs more than lower frequencies. Presence of concrete cover suppresses the amplitude of surface ultrasonic waves.
Steel rebars is a vital component in reinforced concrete (RC) and prestressed concrete structures since they provide mechanical functions to those structures. Damages occurred to steel rebars can lead to the premature failure of concrete structures. Characterization of steel rebars using nondestructive evaluation (NDE) offers engineers and decision makers important information for effective/good repair of aging concrete structures. Among existing NDE techniques, microwave/radar NDE has been proven to be a promising technique for surface and subsurface sensing of concrete structures. The objective of this paper is to use microwave/radar NDE to characterize steel rebar grids in free space, as a basis for the subsurface sensing of steel rebars inside RC structures. A portable 10-GHz radar system based on synthetic aperture radar (SAR) imaging was used in this paper. Effect of rebar grid spacing was considered and used to define subsurface steel rebar grids. Five rebar grid spacings were used; 12.7 cm (5 in.), 17.78 cm (7 in.), 22.86 cm (9 in.), 27.94 cm (11 in.), and 33.02 cm (13 in.) # 3 rebars were used in all grid specimens. All SAR images were collected inside an anechoic chamber. It was found that SAR images can successfully capture the change of rebar grid spacing and used for quantifying the spacing of rebar grids. Empirical models were proposed to estimate actual rebar spacing and contour area using SAR images.
Ultrasonic corrosion detection has been developed and widely applied in non-invasive tests in civil engineering. This paper demonstrates real time fiber optic ultrasonic corrosion detection on reinforcing rebar based on photoacoustic (PA) principle in non-invasive tests in civil engineering. The optical acoustic sensors are fabricated to monitoring the corrosion of rebar in concrete. This paper explores an approach to make an assessment for the level of rebar corrosion as well. From the experimental results, the trend of central frequency had a shift to lower based on the development of corrosion. Since the sensor can measure the rebar corrosion timely, it will have a significant step on structural health monitoring.
Structural steel members have become integral components in the construction of civil engineering infrastructures such as bridges, stadiums, and shopping centers due to versatility of steel. Owing to the uniqueness in the design and construction of steel structures, rigorous non-destructive evaluation techniques are needed during construction and operation processes to prevent the loss of human lives and properties. This research aims at investigating the application of photoacoustic fiber optic transducers (FOT) for detecting surface rust of a steel rod. Surface ultrasonic waves propagation in intact and corroded steel rods was simulated using finite element method (FEM). Radial displacements were collected and short-time Fourier transform (STFT) was applied to obtain the spectrogram. It was found that the presence of surface rust between the FOT and the receiver can be detected in both time and frequency domain. In addition, spectrogram can be used to locate and quantify surface rust. Furthermore, a surface rust detection algorithm utilizing the FOT has been proposed for detection, location and quantification of the surface rust.
In reinforced concrete (RC) structures, corrosion of steel rebar introduces internal stress at the interface between rebar and concrete, ultimately leading to debonding and separation between rebar and concrete. Effective early-stage detection of steel rebar corrosion can significantly reduce maintenance costs and enable early-stage repair. In this paper, ultrasonic detection of early-stage steel rebar corrosion inside concrete is numerically investigated using the finite element method (FEM). Commercial FEM software (ABAQUS) was used in all simulation cases. Steel rebar was simplified and modeled by a cylindrical structure. 1MHz ultrasonic elastic waves were generated at the interface between rebar and concrete. Two-dimensional plain strain element was adopted in all FE models. Formation of surface rust in rebar was modeled by changing material properties and expanding element size in order to simulate the rust interface between rebar and concrete and the presence of interfacial stress. Two types of surface rust (corroded regions) were considered. Time domain and frequency domain responses of displacement were studied. From our simulation result, two corrosion indicators, baseline (b) and center frequency (fc) were proposed for detecting and quantifying corrosion.
Steel rebar corrosion reduces the integrity and service life of reinforced concrete (RC) structures and causes their gradual and sudden failures. Early stage detection of steel rebar corrosion can improve the efficiency of routine maintenance and prevent sudden failures from happening. In this paper, detecting the presence of surface rust in steel rebars is investigated by the finite element method (FEM) using surface-generated elastic waves. Simulated wave propagation mimics the sensing scheme of a fiber optic acoustic generator mounted on the surface of steel rebars. Formation of surface rust in steel rebars is modeled by changing material's property at local elements. In this paper, various locations of a fiber optic acoustic transducer and a receiver were considered. Megahertz elastic waves were used and different sizes of surface rust were applied. Transient responses of surface displacement and pressure were studied. It is found that surface rust is most detectable when the rust location is between the transducer and the receiver. Displacement response of intact steel rebar is needed in order to obtain background-subtracted response with a better signal-to-noise ratio. When the size of surface rust increases, reduced amplitude in displacement was obtained by the receiver.
Fiber optic acoustic generators have generated a lot of interest due to its great potential in many applications including nondestructive tests. This paper reports four acoustic generation configurations. All the configurations are based on gold nanoparticles/polydimethylsiloxane (PDMS) composites. Since gold nanoparticles have high absorption efficiency to optical energy and PDMS has a high coefficient of thermal expansion, the composites can transfer optical energy to ultrasonic waves with high conversion efficiency. The strength and bandwidth of ultrasonic waves generated by the composites can be changed by different designs and structures of the composites. This paper explores the relation between the structure of fiber optic acoustic generators and the profile of generated ultrasonic waves. Experimental results also demonstrated that four ultrasonic generation configurations have similar features of ultrasonic transmission on a steel plate, which is important for future choices of ultrasonic receivers.
Corrosion of steel reinforcing bars (rebars) is the primary cause for the deterioration of reinforced concrete structures. Traditional corrosion monitoring methods such as half-cell potential and linear polarization resistance can only detect the presence of corrosion but cannot quantify it. This study presents an experimental investigation of quantifying degree of corrosion of steel rebar inside cement mortar specimens using ultrasonic testing (UT). A UT device with two 54 kHz transducers was used to measure ultrasonic pulse velocity (UPV) of cement mortar, uncorroded and corroded reinforced cement mortar specimens, utilizing the direct transmission method. The results obtained from the study show that UPV decreases linearly with increase in degree of corrosion and corrosion-induced cracks (surface cracks). With respect to quantifying the degree of corrosion, a model was developed by simultaneously fitting UPV and surface crack width measurements to a two-parameter linear model. The proposed model can be used for predicting the degree of corrosion of steel rebar embedded in cement mortar under similar conditions used in this study up to 3.03%. Furthermore, the modeling approach can be applied to corroded reinforced concrete specimens with additional modification. The findings from this study show that UT has the potential of quantifying the degree of corrosion inside reinforced cement mortar specimens.
The use of microwave and radar sensors in the nondestructive evaluation (NDE) of damaged materials and
structures has been proven to be a promising approach. In this paper, a portable imaging radar sensor utilizing
10 GHz central frequency and stripmap synthetic aperture radar (SAR) imaging was applied to steel and wood
specimens for size and range determination. Relationships between range and properties of SAR images (e.g.
maximum amplitude and total SAR amplitude) were developed and reported for various specimens including a
steel bar (2.5 cm by 2.5 cm by 28.5 cm), a wood bar (2.5 cm by 2.5 cm by 28.5 cm), a steel plate (39.7 cm by
57.9 cm by 1.75 cm), and a wood board (30.5 cm by 30.5 cm by 1.8 cm). Various ranges from 30 cm to 100 cm
were used on these specimens. In our experiment, attenuation of radar signals collected by the imaging radar
system on different material specimens was measured and modeled. Change in the attenuation of maximum SAR
amplitude was observed in different materials. It is found that SAR images can be used to distinguish materials
of different compositions and sizes.
Failures of aging light poles can jeopardize the safety of residents and damage adjacent structures. The need for reliable and efficient damage detection methods is raised. Any change in structural properties (e.g., mass, stiffness and damping) can lead to differences in the dynamic response of structures (i.e., modal frequencies). As a result, changes in dynamic responses can be used as indicators for damage detection. In this study, relationships between artificial damages and modal frequencies are determined by investigating the modal frequencies of intact and damaged light pole models using the finite element method (FEM). Finite element (FE) models were built with 5,529 C3D8R elements in ABAQUSR. New parameters (sensitive and insensitive modes) were defined and used to evaluate the sensitivity of the first ten modes of FE models. It is found that combinations of sensitive and insensitive modes are unique for each damage location and can be used to locate artificial damages in light pole models. Empirical equations are proposed to quantify damage level and damage size.
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