Synthetic aperture radar (SAR) imaging is one of the radar NDE techniques used for remote sensing of civil infrastructures. SAR imaging has been used for remote detection of moisture content, chloride content, and steel rebar corrosion in concrete specimens. Parameters extracted from SAR images are used for condition assessment, including integrated SAR amplitude (Iint), average maximum SAR amplitude (Iavg), and critical contour area (Ac). The objective of this paper is to investigate the effects of moisture and chloride content on the interdependency of critical contour area with integrated SAR amplitude and average maximum SAR amplitude. Laboratory portland cement concrete panel specimens of different water-to-cement (w/c) ratios (0.4, 0.5, and 0.55) were manufactured and scanned inside an electromagnetic anechoic chamber during the drying process of the specimens. From our result, it was found that the interdependency of critical contour area with integrated SAR amplitude and average maximum SAR amplitude during moisture variation is different from the one during chloride variation for different w/c ratios.
Presence of chloride ions is the main cause responsible for the corrosion of steel rebars and other components in concrete structures. Detecting the presence and amount of chloride ions in concrete, however, is a challenging task. In this paper, we present our development on the use of synthetic aperture radar (SAR) imaging for the remote detection of chloride ion content in concrete. SAR imaging is a remote sensing technique capable of performing noncontact subsurface inspection of dielectric materials like Portland cement concrete. Such techniques can be applied to field inspection of concrete structures and laboratory material characterization. The objective of this paper is to demonstrate the application of SAR imaging on characterizing chloride content inside oven-dried concrete specimens, using a 10-GHz central frequency SAR imaging sensor. Twelve concrete specimens (0.3x0.3x0.05 m3 ) with a 0.45 water-to-cement ratio were manufactured in six groups of different chloride contents (0%, 2%, 4%, 6%, 8%, and 10% of cement weight). Four SAR image-based parameters were developed from each SAR image of concrete specimens for chloride, including integrated amplitude Iint, average maximum amplitude Imax, critical contour area Ac, and average Gaussian curvature of critical contour Kavg. A parametric analysis on the combined use of different numbers of SAR image parameters was carried out to determine the optimal application of SAR image parameters on the chloride detection problem in this paper. From our result, it is found that, for the purpose of chloride detection inside concrete specimens using single SAR parameter, integrated amplitude Iint. When using two SAR image parameters, combination of integrated amplitude Iint and critical contour area Ac shows the best performance among other two-parameter combinations. When using three SAR image parameters, combination of integrated amplitude, critical contour area, and average maximum amplitude shows the best performance among other three-parameter combinations. Furthermore, it is interesting to report that combination of three SAR image parametersprovides the overall best performance among all other combinations for the chloride detection problem in oven-dried concrete specimens.
Synthetic aperture radar (SAR) imaging has become an emerging technique in the remote subsurface sensing of construction materials (dielectrics) such as concrete, FRP-concrete, timber, and masonry. Increased center frequency, bandwidth, and synthetic aperture can improve the resolution of SAR images with better contrast and signal-to-noise ratio for condition assessment. However, since most reported SAR applications are for surface sensing, little is known about how to interpret SAR images for subsurface sensing of construction materials. In this paper, extraction, interpretation, and application of parameters from SAR images are presented for condition assessment, using two sides of a CFRP-concrete specimen as an example. A 10-GHz center frequency laboratory SAR imaging system was used for data collection. Seven incident angles were considered (0-deg, 15-deg, 20-deg, 30-deg, 35-deg, 45-deg, and 60-deg.) Multi-dimensional SAR image parameters were defined and applied to the fourteen SAR images of the CFRP-concrete specimen. We have found that performance of SAR image parameters depends on the type of condition assessment problems at hand. Combined use of SAR image parameters of different dimensionalities is encouraged, but a systematic understanding on the physical meaning of each SAR image parameter is necessary. Maximum SAR amplitude is easily affected by background noise. Integrated SAR amplitude in theory performs better than the maximum SAR amplitude. Range curves of SAR images are similar to the A-scan curves in GPR, but they contain more coupling effects from geometry and material's. Cross-range curves of SAR images can also used for surface profiling of concrete specimens. Critical contours of SAR images show the spatial distribution of the backscattering response of concrete. K-R-I curves of SAR images extract features of the images and facilitate quantitative comparison between two SAR images collected at different orientations and image resolutions.
Moisture content inside concrete affects the properties of Portland cement concrete structures. It also indicates the likelihood of structural damages (e.g., freeze-thaw, steel corrosion) in reinforced and prestressed concrete structures. While several laboratory techniques (intrusive/destructive) are available for moisture determination, it is a challenging task to quantify the moisture content of concrete in real structures without using intrusive methods/techniques. In this paper, an SAR (synthetic aperture radar) image analysis approach using critical contour area for subsurface moisture sensing inside concrete specimens is proposed. Concrete specimens cast and conditioned in laboratory environments were air-dried for three months and continuously monitored for their moisture variation by SAR imaging. Dimensions of concrete specimens were 0.3 m-by-0.3 m-by-0.05 m. A 10.5-GHz center frequency radar system with 1.5 GHz bandwidth was used to generate SAR images of the concretes at various moisture contents from 0% to 3.85% (by mass). All SAR images of concrete specimens in this paper were collected inside an anechoic chamber. To determine the critical contour area (Ac) of an SAR image for subsurface moisture sensing inside concrete, a selection criterion is proposed and applied to SAR images of concrete specimens at different moisture contents (ψ). A critical SAR amplitude is chosen at the highest coefficient of determination (R-square, R2) for all values of moisture content. From our SAR imaging result, it was found that critical contour area increases nonlinearly with a linear increase of moisture content of concrete specimens.
Moisture content in Portland cement concrete structures is commonly associated with many durability problems (e.g., steel corrosion) and can be used in structural health monitoring and nondestructive evaluation/testing (NDE/T) of concrete structures. While several intrusive techniques are available for quantifying moisture content inside concrete structures, it is a challenging task to estimate the moisture content of concrete in the field without using embedded moisture sensors. In this paper, synthetic aperture radar (SAR) imaging as a nonintrusive technique and the K-R-I (curvature-area-amplitude) transform are applied to concrete panel specimens made of two water-to-cement (w/c) ratios (w/c = 0.4 and 0.5) for moisture determination. Concrete panel specimens with dimensions of 0.3 m-by-0.3 m-by-0.05 m were manufactured and conditioned in a laboratory environment by air-drying for three months. Its time-dependent moisture variation was simultaneously monitored by a 10.5- GHz center frequency SAR imaging sensor inside an anechoic chamber. Quantitative analysis of SAR images was carried out by using the K-R-I transform to understand the simultaneous change of SAR amplitude and distribution (contour shape) at different moisture contents. It was found that SAR amplitude and its distribution increase with the increase of moisture content inside concrete panel specimens. Spatial distribution of SAR amplitudes can be used to indicate subsurface moisture distribution inside concrete. The area-amplitude (R-I) curve of SAR images can be used to quantify the relationship between moisture content and its distribution.
In civil engineering, determining the subsurface moisture content and the water-to-cement (w / c) ratio of concrete structures in the field is a challenging task. Moisture content of concrete structures regards the cement hydration level and short-term strength development of concrete, as well as the phase-distribution (solid, gaseous, and liquid) inside concrete and its long-term durability. We describe a microwave nondestructive evaluation technique using noncontact/nonintrusive synthetic aperture radar (SAR) imaging to estimate both the w / c ratio and subsurface moisture content of concrete panels. Concrete panels made of three w / c ratios (0.4, 0.5, and 0.55) were air dried for ∼3 months and scanned by a laboratory 10-GHz SAR imaging system. From this research, we found that SAR imaging can capture the moisture content variation and its spatial distribution. A parameter of SAR images, the critical contour area, is identified and proposed for estimating subsurface moisture content and the w / c ratio of concrete panels. At a given w / c ratio, the critical contour area increases with the increase of moisture content. At a given moisture content, the critical contour area decreases with the increase of the w / c ratio.
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.
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.
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.
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.
Nondestructive evaluation (NDE) is an indispensable approach for the sustainability of critical civil infrastructure systems such as bridges and buildings. Recently, microwave/radar sensors are widely used for assessing the condition of concrete structures. Among existing imaging techniques in microwave/radar sensors, synthetic aperture radar (SAR) imaging enables researchers to conduct surface and subsurface inspection of concrete structures in the range-cross-range representation of SAR images. The objective of this paper is to investigate the range effect of concrete specimens in SAR images at various ranges (15 cm, 50 cm, 75 cm, 100 cm, and 200 cm). One concrete panel specimen (water-to-cement ratio = 0.45) of 30-cm-by-30-cm-by-5-cm was manufactured and scanned by a 10 GHz SAR imaging radar sensor inside an anechoic chamber. Scatterers in SAR images representing two corners of the concrete panel were used to estimate the width of the panel. It was found that the range-dependent pattern of corner scatters can be used to predict the width of concrete panels. Also, the maximum SAR amplitude decreases when the range increases. An empirical model was also proposed for width estimation of concrete panels.
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