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.
Microwave/radar sensors and techniques are widely used for detecting underground or subsurface targets in archeology, geophysics, and civil engineering. Among existing microwave/radar sensors and techniques, synthetic aperture radar (SAR) imaging enables researchers and engineers to conduct surface and subsurface detection of metallic objects with improved detectability. The noncontact, remote sensing feature of SAR imaging provides a safer approach in a dangerous mission, such as demining. The objective of this paper is to investigate the depth (d) effect of a metallic object buried in dry sand. A steel disk specimen of 15-cm diameter was buried inside a box (sandbox) filled up with dry sand at various depths (d = 10 cm, 18 cm, and 26 cm) and scanned by a 10-GHz SAR system. Three ranges (R = 15 cm, 30 cm, and 60 cm) between the SAR antenna and the sandbox were also considered in this research. It was found that the SAR amplitude and its distribution decrease with the increase of buried depth and the increase of range. Distribution of SAR amplitudes representing the buried metal disk specimen also changed with the increase of buried depth. Empirical models were also proposed for range and depth effects of subsurface metallic objects in SAR images.
In recent years, textiles are used as a structural material in externally strengthening/retrofitting deteriorated and damaged concrete structures. Formation of externally strengthened/retrofitted concrete structures creates a new type of multi-layer dielectric system for their condition assessment using non-destructive evaluation (NDE) techniques. The objective of this paper is to investigate the use of microwave/radar NDE on a one-layer textileconcrete system for condition assessment. In this paper, we use a synthetic aperture radar (SAR) imaging system at 10 GHz to study the effect of an externally attached textile layer on the SAR images of two concrete panels. One type of textile was used on a 30.48 cm by 30.48 cm by 2.54 cm concrete panels to form a one-layer textileconcrete system. Various ranges (20 cm, 30 cm, 40 cm, 50 cm and 60 cm) were considered. Our experiment results demonstrated that the SAR imaging can successfully distinguish the type of textiles. Furthermore, it was found that electromagnetic pattern of the textile layer varies with range in SAR images. Empirical models were developed to characterize the range effect on the SAR images by using textile applied on concrete panels.
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.
Detecting underground/subsurface metallic objects such as landmines and IEDs (improvised explosive devices) using efficient and effective inspection techniques is crucial in demining and mine clearance missions. The use of microwave/radar sensors in remote sensing represents a new approach to reduce demining risks and to improve efficiency. To achieve the goal, electromagnetic signatures of underground metallic objects must be thoroughly studied. The objective of this paper is to use synthetic aperture radar (SAR) imaging to investigate the size effect of an underground metallic object buried in SAR images at three ranges (15 cm, 30 cm, and 60 cm). Three different sizes (diameters = 7.6 cm, 15.2 cm, and 20.3 cm) were used and buried inside a container (sandbox) filled up with dry sand. A 10-GHz SAR imaging radar sensor was applied to generate all SAR images inside an anechoic chamber. It was found that SAR amplitude decreases with the increase of radar range. In addition, reconstructed distribution of SAR amplitudes depended on the size of underground steel disk specimens. SAR amplitudes (maximum and integrated) were related to the range and modeled by empirical equations.
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.
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.
KEYWORDS: Composites, Piezoelectric effects, Energy harvesting, Energy efficiency, Carbon nanotubes, FT-IR spectroscopy, Polarization, Crystals, Solar energy, Attenuated total reflectance
Piezoelectric materials such as polyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT) are the fundamental materials for fabricating piezoelectric based energy harvesters. However, the drawbacks for these two materials are: (i) poor mechanical properties of PZT (e.g., brittle, low fracture stress); (ii) low energy efficiency of PVDF. Extensive research work has been made on investigating the piezoelectric property of PVDF. But very few attentions have been paid on mechanical property. In this paper, we report a synthetization of PVDF using a very low multi-wall carbon nanotubes (MWCNT) fraction in PVDF to enhance both the mechanical property and piezoelectricity of PVDF/MWCNT composite. Through a series of well-controlled experiments, we found that the adding of MWCNT in PVDF affects the crystalline structure. Our experimental results also showed that the maximum tensile stress and maximum piezoelectric voltage constant both occurs at PVDF/MWCNT (0.05wt%). However, with the MWCNT fraction exceeded 0.05wt%, the maximum tensile stress and maxi- mum piezoelectric voltage constant decreased.
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