Non-destructive evaluation (NDE) of composites: techniques for civil structures

18 Non-destructive evaluation (NDE) of composites: techniques for civil structures U. B. H A L A B E, West Virginia University, USA DOI: 10.1533/9780857093554.4.483 Abstract: This chapter presents the basic theory and applications of modern NDE techniques for testing of composite structural components such as beams, columns, and bridge decks. The use of infrared thermography and ground penetrating radar is discussed in detail using results from several laboratory and field tests. In addition, localized testing using digital tap hammer is discussed. The chapter also presents discussions on issues and challenges on the use of NDE for civil structures, in addition to future trends and sources for further information. Key Words: non-destructive evaluation (NDE), infrared thermography, ground penetrating radar (GPR), tap testing, digital tap hammer, civil infrastructure, structures, structural, fiber-reinforced polymer (FRP) composites, composites.

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Introduction

Non-destructive evaluation (NDE) plays an important role in the annual inspection and maintenance of civil structures, which is crucial for ensuring their continued structural integrity and serviceability. While the customary practice for field inspection of civil structures is to rely primarily on periodic visual inspections, there is a growing need for modern NDE techniques which can detect subsurface defects at their early stages of formation before they grow to critical sizes and show up on the surface. Early detection of subsurface defects in structural components can enable timely repair and maintenance, thus prolonging the service life of the various structural components. This aspect has become very important in the current era of aging infrastructure, growing cost of new construction, and very tight budgetary constraints. The rehabilitation of existing structures often utilizes innovative materials such as fiber-reinforced polymer (FRP) composites. For example, old concrete bridge decks can be replaced by lighter and stronger FRP composite decks, which also offer a significantly longer lifespan than their concrete counterpart (Liang and GangaRao, 2004). Deteriorating concrete and timber beams and columns in bridges, buildings, and parking garages 483 © Woodhead Publishing Limited, 2013

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can be rehabilitated by wrapping them with glass or carbon-reinforced FRP composite fabrics. These fabrics not only enhance the strength of the structural components, but also enhance their ductility (ability to deform under large wind/hurricane, earthquake, or blast loading), leading to increased safety under harsh loading conditions. Non-destructive monitoring of these rehabilitated structural components is very important for quality control during the initial construction, and periodic monitoring is needed to detect subsurface debonds that could be formed between the composite fabric and underlying material. Timely repair of the debonded locations can help to ensure the enhanced strength and ductility is continuously realized over the lifespan of the structure. Modern NDE methods for application to civil infrastructure include techniques such as infrared thermography and ground penetrating radar (GPR). These techniques utilize commercially available portable equipment and offer the capability of high-speed data acquisition in order to scan large areas in field structures. Moreover, significant reduction in equipment prices over the last decade coupled with the development of user-friendly data analysis procedures and software have resulted in increasing use of these techniques by highway agencies and field inspectors. This chapter presents the basic methodology and laboratory/field testing results for both infrared thermography and ground penetrating radar techniques. In addition, a localized but effective testing technique such as digital tap testing is also discussed. The following three NDE techniques that are widely used for condition assessment of civil infrastructure are described: • • •

infrared (IR) thermography; GPR; digital tap testing.

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Infrared thermography

18.2.1 Basics of infrared thermography testing Infrared thermography is a non-contact technique which is used to generate a surface temperature profile of the test area in the form of gray scale or color infrared images. Brighter areas in the infrared image typically signify high-temperature zones while darker areas signify lower temperatures. The first step in infrared thermography testing is to create a thermal gradient in the test object by applying either a heating or cooling source to the specimen’s surface. The rate of heat conduction through the specimen in regions consisting of subsurface defects is different from that of the surrounding defect-free area (Maldague, 1993, 2000, 2001; Maldague and Moore 2001). This produces variations in the surface temperature of the specimen. The second step is to remove the heating or cooling source and immediately

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acquire a sequence of infrared images from the specimen’s surface in order to detect the subsurface defects. This acquisition is done using infrared cameras that are sensitive to the infrared energy emitted by the test object. Modern infrared cameras can store digital infrared images using a memory card or a laptop computer which can house the image acquisition and processing software. The underlying principle behind infrared thermography can be found in several textbooks and other publications (Kaplan, 1999; Maldague, 1993, 2000, 2001; Maldague and Moore 2001). Infrared thermography measures the surface temperature of an object by detecting the amount of energy emitted by the object in the infrared band. The visible spectrum refers to the portion of the electromagnetic spectrum that is visible to the human eye and falls within wavelengths of 0.4–0.75 μm (1 μm or micrometer is one-millionth of a meter and about one-fiftieth of the diameter of a human hair). The infrared portion of the electromagnetic spectrum ranges from 0.75 to 100 μm wavelength. This infrared spectrum is divided into various sub-regions, which are: near infrared (0.75–3 μm), middle infrared (3–6 μm), far infrared (6–15 μm), and extreme infrared (15–100 μm) (Clark et al., 2003). It should be noted that most practical measurements can be made to only about 20 μm (Kaplan, 1999). As the temperature of an object reduces, the frequencies for the energy emitted by the object are lower, which means the wavelengths are higher. For example, the sun with a surface temperature of close to 6000 K, radiates most of its energy in the visible spectrum, which is what the human eye can detect. The tungsten filament glowing inside a light bulb has a temperature of around 2800–3300 K, with a significant portion of the energy in the visible spectrum. At temperatures typically found on the Earth’s surface (i.e., in the vicinity of 300 K ≅ 30 °C), most of the energy is radiated in the infrared spectrum with peak energy close to 10 μm. Energy in this range is invisible to the human eye or a photographic camera, but can be detected using an infrared camera. The lens of an infrared camera is made of special material (germanium) which is transparent in the infrared spectrum. Use of such lens material and special infrared detectors make the price of infrared cameras much higher than regular photographic cameras. The infrared detectors sense the infrared energy at various pixel locations, and the camera electronics convert the image into a visual color or gray scale image, where bright (white) colors indicate higher temperatures and dark (black) colors depict lower temperatures. These images (called thermograms or infrared images) show the surface temperature distribution of the viewed area. While old analog models allowed storage of the infrared images in a video tape, the stored image only showed the relative temperature profile and the actual temperature value was lost. The modern digital infrared cameras allow storage of

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full radiometric images in digital form in a computer’s hard drive, CD/DVD disks, or other media (such as memory card in the camera itself), and the images can be viewed later using associated software, which also enables display of temperature data at every pixel in the image. Such radiometric data can be utilized for digital image processing and for developing automated image analysis procedures and algorithms (Shepard et al., 1999). Whenever an object is thermally stimulated on its surface by heating (or cooling) the surface, conduction takes place through the thickness. The rate of thermal conduction through a defect is usually different from the conduction rate through defect-free material. If the defect is air-filled, it acts as an insulator, thus causing build-up of heat (storage of thermal energy) in the region above the defect (assuming the surface of the object is heated, not cooled). This phenomenon leads to differences in surface temperatures over defective areas compared with surrounding defect-free regions. The different parts of the surface radiate different amounts of thermal energy based on the surface temperatures in accordance with the Stefan–Boltzmann law (Kaplan, 1999). The radiated thermal energy is detected using an infrared camera, which produces thermal images showing defective areas with different surface temperatures compared with the surrounding defect-free areas. If the surface of the object is heated using a heater (an active source), and then infrared images are acquired after removal of the heat source; the images will show air-filled defects as hot spots. This phenomenon is altered if a cooling source is used instead of a heat source in order to provide thermal stimulation, or if the defect is water-filled instead of air-filled. It is important to note that consistent infrared measurements can only be made if the surface heating or cooling is uniform. In the case of bridge decks, it is often convenient to conduct infrared thermography testing under solar heating. Commercial infrared cameras are typically available in two spectral ranges (∼1–3 μm and 7–14 μm). The near infrared cameras (1 to 3 μm range) are typically used for night vision. It should be noted that the air medium between the test object and the infrared camera acts as a filter of thermal energy, often distorting the measured energy spectrum. The atmospheric transmission happens to be quite uniform in the 7 μm to 14 μm range (Kaplan, 1999); hence this range is preferred for most scientific measurements. Figure 18.1 shows a typical infrared testing set-up in the laboratory using an infrared camera that is capable of acquiring infrared images at a capture speed of up to 60 frames/s. The data acquisitions of such cameras are typically controlled by a laptop computer-based software. These cameras are expensive and the sequence of images acquired by these cameras can be used in conjunction with advanced image processing algorithms (Shepard et al., 1999). The thermal sensitivity of such cameras typically ranges from

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18.1 (a) Experimental set-up using digital infrared camera, and (b) close-up view of the camera.

0.01 to 0.06 °C at 30 °C. Figure 18.2 shows an infrared camera that produces radiometric JPEG still images, which can be stored in a SD memory card in the camera. Such cameras are typically low-cost (with thermal sensitivity of about 0.1 °C at 30 °C) and can be conveniently used for field inspections of structural components. The radiometric JPEG images from these cameras can be used to measure the temperature of any spot or average temperature over an area using computer software. Figure 18.3 shows various heating and cooling sources that can be used for thermal stimulation of a test object that is not typically heated by the sun (e.g., beams and columns under the bridges). For testing of bridge decks, it is convenient to use the sun as the heat source. For housing inspections, the thermal difference between the heated or cooled house and the ambient air is often utilized to determine areas of air and water leakage or low insulation.

18.2.2 Infrared thermography testing for glass fiber-reinforced polymer (GFRP) bridge decks This section illustrates the infrared thermography testing of glass fiberreinforced polymer (GFRP) bridge decks in the laboratory as well as field settings. The specimens used in the laboratory study consisted of GFRP

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18.2 Infrared camera with SD card that stores radiometric JPEG images.

bridge deck modules of different sizes (Halabe et al., 2007). The GFRP decks were composed of E-glass fibers and either polyester or vinyl-ester resin combination. The glass fibers were continuous strand roving and triaxial fabrics. The fiber volume fractions for the polyester and vinyl-ester decks were about 45% and 50% respectively. The top surface of some of the decks was coated with 3/8″ (9.5 mm) thick wearing surface, which consisted of an epoxy-based overlay system made of a specially selected blend of resins, hardener, powder, and aggregates. Debonds of different sizes were made by joining two polypropylene sheets with an enclosed air pocket in between them. The water-filled debonds were made by enclosing water within plastic pouches. The simulated debonds were placed on the top surface of the GFRP deck, which was followed by the application of 9.5 mm (3/8″) thick wearing surface on the entire top surface of the deck. The specimen BD1 was a bridge deck module of plan size 610 mm × 305 mm (24″ × 12″), flange thickness of 12.7 mm (0.5″), and web thickness of 8.9 mm (0.35″). This deck specimen had an overall depth of 203 mm (8″) and is shown in Fig. 18.4. The geometric crosssection, wearing surface thickness, and material composition of this specimen are the same as that used in field construction of many GFRP bridges. One side of the specimen, BD1a, consisted of two air-filled debonds of sizes 51 mm × 51 mm (2″ × 2″) and 76 mm × 76 mm (3″ × 3″) and the other side BD1b, consisted of two air-filled debonds of sizes 25 mm × 25 mm (1″ × 1″) and 13 mm × 13 mm (½″ × ½″) between the wearing surface layer and the deck. All these air-filled debonds had a thickness of 1.6 mm (1/16″). Since

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18.3 Various active heating and cooling sources: (a) 1500 W quartz heater, (b) 1500 W heating blanket, and (c) liquid carbon dioxide tank.

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18.4 Front and cross-sectional views of the GFRP bridge deck specimen BD1 with wearing surface.

this was a laboratory specimen, both sides were used for this infrared testing study, but it should be noted that both sides (with wearing surface) represented the top flange of field decks. For the infrared thermography tests conducted in this research, the top and bottom flanges essentially acted as two separate specimens. It is important to note that the polymer concrete wearing surface overlay placed on field GFRP bridge decks is an inhomogeneous mixture of coarse gravel and resin, which results in natural surface temperature variation on bridge decks. While no standard exists for thermographic testing of GFRP bridge decks, ASTM D 4788 – 03 (2007) standard for thermographic testing of steel reinforced concrete bridge decks states that the surface temperature difference must be at least 0.5 °C before an anomalous region is considered to be a debond or delamination. ASTM D 4788 – 03 (2007) also requires the use of ‘imaging infrared scanner having a minimum thermal resolution of 0.2 °C.’ The radiometric infrared camera used in this research had a sensitivity of 0.06 °C, which is more than adequate considering that only anomalies over 0.5 °C temperature difference (compared with the surrounding defect-free area) are of interest. Also, this camera was an uncooled model which is quite convenient for testing of GFRP bridge decks in the field. A 1500 W quartz heater was used as the main heating source in laboratory testing. Specimen BD1 was uniformly heated by placing the quartz

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18.5 Infrared image of specimen BD1 (a) with air-filled debonds of sizes 76 mm × 76 mm × 1.6 mm (3″ × 3″ × 1/16″) and 51 mm × 51 mm × 1.6 mm (2″ × 2″ × 1/16″), and (b) with air-filled debonds of sizes 25 mm × 25 mm × 1.6 mm (1″ × 1″ × 1/16″) and 13 mm × 13 mm × 1.6 mm (½″ × ½″ × 1/16″).

heater at a distance of about 0.2 m from the specimen surface for a duration of 150 s. Figure 18.5 shows the infrared image of the 76 mm × 76 mm (3″ × 3″), 51 mm × 51 mm (2″ × 2″), 25 mm × 25 mm (1″ × 1″), and 13 mm × 13 mm (½″ × ½″) debonds, all with a debond thickness of 1.6 mm (1/16″). The infrared image of the specimen revealed the location of the debonds as white regions (areas with higher temperature when compared to the surrounding defect-free regions). The gray scale showing the temperatures of the deck is also available with the infrared image. The difference between the surface temperatures over the 76 mm × 76 mm (3″ × 3″) and 51 mm × 51 mm (2″ × 2″) debonded areas with respect to the surrounding defect-free area were about 11 °C and 8 °C respectively. The boundaries of the 76 mm × 76 mm (3″ × 3″), 51 mm × 51 mm (2″ × 2″), and 25 mm × 25 mm (1″ × 1″) debonds are well defined in the infrared image shown in Fig. 18.5. Though a well-defined boundary for the 13 mm × 13 mm (½″ × ½″) debond was not visible, the area shows sufficient contrast which confirmed the presence of the debond. The difference between the surface temperatures of the 25 mm × 25 mm (1″ × 1″) and 13 mm × 13 mm (½″ × ½″) debonded areas with respect to the surrounding defect-free area were about 8 °C and 4 °C, respectively. To establish the surface temperature–time curves, the bridge deck specimen BD1 was heated using the 1500 W quartz tower heater and the infrared

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images were recorded over a period of time. The infrared images from the digital infrared camera were captured at 10-second intervals. The first infrared image was captured immediately after removing the heating source from the specimen’s surface. The temperatures of the defective and defectfree areas at the 10-second time intervals were tabulated and curves were drawn for the temperatures of both the defective and defect-free areas along the Y-axis and time over the X-axis. An infrared image showing the most prominent boundary of the delamination would be obtained when the temperature difference between the defective and the defect-free area is at its maximum. The curves for all the four debonds were similar in nature, but with different values. It was observed that the maximum temperature difference between the debonded and defect-free areas for all the four debond sizes was obtained almost immediately (within 30 to 40 seconds) after the heating source was taken away. Figure 18.6 depicts the surface temperature-time curves for the 25 mm × 25 mm (1″ × 1″) and 13 mm × 13 mm (½″ × ½″) debonds. The graph also shows that the curves for the debonds and defect-free area converges. The decrease in the temperature difference is very slow for the first few minutes, after which it becomes more rapid. Images with reasonably good contrast between the defective and defect-free areas were available for about 300 s duration for the 76 mm × 76 mm (3″ × 3″) and 51 mm × 51 mm (2″ × 2″) debonds and about 200 s duration for the 25 mm × 25 mm (1″ × 1″) and 13 mm × 13 mm (½″ × ½″) debonds, after the heating source had been removed. The time available for good contrast in the infrared image reduces with the size of the defect. After this period of time the deck approaches thermal equilibrium which

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18.6 Surface temperature-time curves for air-filled debonds of sizes 25 mm × 25 mm × 1.6 mm (1″ × 1″ × 1/16″) and 13 mm × 13 mm × 1.6 mm (½″ × ½″ × 1/16″)

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is facilitated by heat conduction from the higher-temperature regions (debonded areas) to lower temperature regions (defect-free areas). Another bridge deck specimen was used to demonstrate the difference between the behavior of air-filled and water-filled debonds. This specimen, BD2 (shown in Fig. 18.7), had a plan size 610 mm × 305 mm (24″ × 12″), flange thickness of 11.4 mm (0.45″) and web thickness of 10.2 mm (0.4″). The overall depth of this deck was 102 mm (4″), which is typical of ‘lowprofile’ GFRP bridge decks used in some of the recent field construction. Two water-filled debonds of sizes 51 mm × 51 mm × 1.6 mm (2″ × 2″ × 1/16″) and 76 mm × 76 mm × 3.2 mm (3″ × 3″ × 1/8″), and an air-filled debond of size 51 mm × 51 mm × 1.6 mm (2″ × 2″ × 1/16″) were placed on the surface of the GFRP deck specimen before it was overlaid with the 9.5 mm (3/8″) wearing surface layer to create simulated air-filled and water-filled debonds. This specimen BD2 was also heated using the quartz heater. Plate VIII (between pages 296 and 297) shows the schematic view of the debond locations and the corresponding infrared image of the bridge deck. The surface temperature above the air-filled debond (white area in infrared image) is higher than the temperature above defect-free area (temperature difference: 5.7 °C). On the other hand, the temperatures above the two waterfilled debonds are lower compared to the surrounding defect-free area, with a temperature difference of −3.9 °C for the 76 mm × 76 mm (3″ × 3″) debond and −2.5 °C for the 51 mm × 51 mm (2″ × 2″) debond. Thus, the surface

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18.7 Front and cross-sectional views of the GFRP bridge deck specimens (a) without wearing surface overlay, and (b) with wearing surface overlay.

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temperature difference for water-filled debond is lower in magnitude compared with the air-filled debond. This is because the thermal diffusivity of water (0.14 × 10−6 m2 s−1) is close to that of the GFRP bridge deck whose thermal diffusivity is about 0.13 × 10−6 m2 s−1 along the flange depth (i.e., perpendicular to the fibers). On the other hand, air-filled debond has a significantly higher thermal diffusivity (33 × 10−6 m2 s−1) (Maldague, 1993) compared with the GFRP deck, which results in higher surface temperature difference. It can also be observed from Plate VIII that the top portion (6.3 mm × 76 mm or ∼0.25″ × 3″ in plan) of the two water-filled debonds showed a higher temperature as represented by small white area above the dark area. This was due to the presence of small air bubbles inside the plastic pouch that was used to make the water-filled debond. The infrared images revealed that the boundaries of the air-filled debond were more clearly defined compared with the water-filled debonds. From visual observation of the infrared images, it is more difficult to detect the presence and mark the boundaries of water-filled debonds compared to air-filled debonds. The behavior seen in the above laboratory tests have also been observed during field testing on the GFRP deck of La Chein bridge, which is a twolane bridge located in Monroe County, West Virginia, USA. The bridge deck had 9.5 mm (3/8″) thick polymer concrete overlay, which showed signs of debonding as evident from surface cracks and unevenness. The objective of infrared testing was to detect debonds present at the interface between the wearing surface and the underlying GFRP deck and to quantify the debonded area (Halabe et al., 2003b). The wearing surface overlay covered an area of 7.9 m × 12.5 m (26′ × 41′) in plan. For the purpose of infrared testing, the bridge was divided into numerous small grids and infrared images as well as digital photographs were acquired. The distance of the infrared camera from the bridge deck was about 2 m. For most part of the day, solar radiation was used as the heating source. The temperature of the deck was as high as 50 °C and the ambient air temperature was around 28–30 °C for most of the day. The test was started around noon when the sun had already heated the deck to a sufficiently high temperature to conduct the infrared testing. Typically, solar radiation is at its peak at noon while the air and deck temperatures are at their peak at about 2 p.m. The ideal time window to conduct infrared testing is from 11 a.m. to about 3 p.m. During late afternoon (around 4 p.m.), when the sun could not heat the deck efficiently, two 1500 W heating blankets of plan size 0.91 m × 0.91 m (3′ × 3′) each and shown in Fig. 18.3(b), were used as the heating source for the deck. Some representative digital photographs and infrared images identifying subsurface debonds are shown in Fig. 18.8. During field testing, it is important to use digital cameras and take photographs of the areas for which infrared images are acquired. Since the bridge was hot and dry, the debonds

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18.8 Digital photographs and corresponding infrared images of various debonded areas in the La Chein GFRP bridge deck.

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were air-filled and appeared as bright white spots (high-temperature areas) in the infrared image. The corresponding debonded areas were marked on the bridge deck using white spray paint as seen in the digital photographs in Fig. 18.8. The infrared testing helped to map the shape and plan sizes of the various debonds, and based on these results it was concluded that approximately two-thirds of the deck wearing surface had defective areas (Halabe et al., 2003b). Therefore, the bridge was recommended for wearing surface replacement. The debonding in this bridge was caused due to placement of the wearing surface during very hot weather (outside the temperature range specified by the resin manufacturer) which adversely affected the curing of the resin in the wearing surface layer. Some portions of the bridge deck were under shadows of the guard rail. The infrared images of these portions (Fig. 18.9) revealed areas with lower temperature (with darker shades) that were caused due to the shadow. Such areas should not be misinterpreted as debonded areas. Some other areas of the deck had dirt particles and loose gravel on the surface that had separated from the polymer concrete overlay. An infrared image of a surface with loose gravel is shown in Fig. 18.10. Proper care should be taken when analyzing the infrared images to avoid erroneous classification of the areas under the shadows of guard rail, trees, or adjacent buildings, and areas with loose gravel/dirt as debonded areas.

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18.9 (a) Photograph of the guard rail area and (b) infrared image showing the area subjected to shadow of the guard rail.

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18.10 Infrared image showing loose gravel on the surface of the GFRP bridge deck.

The above discussions have focused extensively on the detection of debonds between the wearing surface and the underlying composite bridge deck. The use of infrared thermography has also been investigated for detecting air-filled and water-filled delaminations within flanges of GFRP composite deck specimens in the laboratory as well as under solar heating, with more promising results in case of air-filled delaminations (Halabe et al., 2006, 2007; Hing and Halabe, 2010; Vasudevan, 2004). The term ‘delamination’ is used here to indicate a defect or separation within a composite layer, while ‘debond’ refers to a separation between dissimilar layers. However, some researchers have used the terms debonds and delaminations interchangeably in published literature without making a clear distinction between the two types of subsurface defects.

18.2.3 Infrared thermography testing for FRP wrapped columns This section discusses the use of infrared thermography testing of concrete or timber columns wrapped with FRP fabric. The fabrics typically consist of glass or carbon fibers and are termed GFRP or CFRP fabrics.

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Although both fabrics have been used to rehabilitate civil infrastructure, GFRP fabrics offer lower cost. However, CFRP fabrics offer a greater increase in strength and ductility properties of the structural component than GFRP fabrics (Dutta, 2010). The subsurface debonds that could form between the fabric and the underlying concrete or timber member at the time of initial wrapping or during service could lead to reduction in localized confinement, thus adversely affecting the strength and ductility of the rehabilitated member. Therefore, it is important to use techniques such as infrared thermography for condition assessment during initial rehabilitation work and for periodic monitoring of the structure in order to detect subsurface debonds and repair them by injecting resin into the debonds. If the debonds are large, then one may need to replace the fabric wrap in accordance with established standards (e.g., ACI 440.2R-08, 2008). A study was conducted by Halabe et al. (2010) to evaluate the effectiveness of infrared thermography in detecting subsurface debonds in 152.4 mm × 304.8 mm (6″ × 12″) concrete cylinders wrapped with CFRP and GFRP fabrics. The specimens were prepared with pre-inserted debonds between the fabric and the underlying concrete specimens, and were heated using 1500 W quartz heater for about one minute. The infrared images were acquired immediately upon removal of the heating source. The distance of the infrared camera to the test specimen was 0.9 m (3′). The laboratory test results (Figs 18.11–18.14) showed that infrared thermography was successful in detecting subsurface debonds in both GFRP and CFRP wrapped cylinders. It should be noted from the infrared images in Figs 18.13 and 18.14 that the water-filled debonds also showed up as hot spots similar to the air-filled debond cases in Figs 18.11 and 18.12. According to the principle of conductive heat transfer, a water-filled debond should show up as

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a cold spot or in other words, surface temperature of the region above the water-filled debond should be lower than the surface temperature of the region above the defect-free areas. This phenomenon has been observed in thicker planar composites (Plate VIII between pages 296 and 297) and is attributed to the fact that water has a higher thermal conductivity value (0.6 Wm−1 K−1) than GFRP and CFRP composites (∼0.17 Wm−1 K−1). Thus, water-filled debonds conduct heat at a faster rate through the thickness compared to the defect-free area. However, in this study, the water-filled debonds showed up as hot spots similar to the case of air-filled debonds. This is because of the small thickness of the wrapping fabric coupled with high specific heat value of water (∼4180 J/(kg K)). Thus, water stored a significant amount of heat which was then released during the cooling cycle when the infrared images were taken. This phenomenon becomes very prominent because of the small thickness of the FRP wraps. Also, it is important to note that air-filled debonds show a more clear boundary with higher surface temperature compared to the water-filled debond cases. When conducting infrared tests with a heater, it is important to note that the heater should be at least 200 mm (8″) away from the specimen surface and moved from side to side while applying the heat. This ensures uniform heating of the test surface, otherwise one may see a pattern corresponding to the two heating rods in the infrared image instead of the subsurface defects. Also, in the case of vertical members, solar heating results in higher temperature of the top area and lower temperature of the bottom area due to naturally occurring convection (hotter air moving up and colder air moving down), thus masking the effect due to the subsurface debonds (Halabe et al., 2003a). This implies that solar radiation cannot be relied upon as a heat source for infrared testing of vertical members, although it is commonly used as an effective heat source for infrared testing of horizontal members like bridge decks. The testing of vertical members can be carried out using active heating sources such as heaters, halogen lamps, or heating blankets. Plates IX and X (between pages 296 and 297) show the digital photographs and infrared images of two timber piles wrapped with GFRP fabric. The infrared images were obtained immediately after heating the test area with a 1500 W heater for about one minute. The infrared image in Plate IX(b) shows a number of debonds that were formed during wrapping because the surface of the underlying timber pile was not smooth. In contrast, the infrared image in Plate X(b) shows a very uniform surface temperature profile, which indicates a good bond between GFRP wrap and the underlying timber pile (Halabe and Periselty, 2011). Additional field testing results for GFRP wrapped timber piles and pile caps (beams) can be found in Halabe et al. (2005).

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Ground penetrating radar (GPR)

18.3.1 Basics of GPR testing GPR equipment typically consists of a mainframe system whose settings are controlled by a data acquisition laptop computer. The mainframe is connected to devices known as antennas to propagate short pulse (1 to 2 nanoseconds) electromagnetic waves into a medium and receive echo signals back from subsurface layer interfaces formed by debonds, delaminations, and inclusions such as steel reinforcing bars in case of reinforced concrete structural components. The wave velocity, signal amplitude, and wave attenuation characteristics of electromagnetic waves depend on the complex dielectric permittivity of the propagation medium (Halabe et al., 1993). For a medium with low electrical conductivity, the wave velocity (v) in the medium is given by the following equation: v=

c εr

[18.1]

Where c is the velocity of electromagnetic waves in vacuum (= 3 × 108 m/s) and εr is the relative dielectric constant of the propagation medium. The depth of a subsurface interface or inclusion (d) can be obtained using the following equation: d=

vt 2

[18.2]

where t is the total round-trip time taken by the electromagnetic pulse to travel from the antenna to the subsurface feature and back to the antenna. Equation (18.2) assumes that the radar antenna is placed on the surface of the test specimen and the same antenna transmits and receives the echo signal (pulse-echo mode). Since the waves attenuate as they propagate through a medium, it is customary to use linear or exponential gain (increasing with travel distance) during radar data acquisition. The wave attenuation coefficient increases with antenna frequency and moisture content of the propagation medium (Halabe et al., 1993). While low-frequency antennas (15–900 MHz) are typically used for subsurface investigation in pavements and soils, higher-frequency antennas (1–2.5 GHz) are typically used in cases of concrete and composite structural components where the required penetration depth is much lower (few cm to 1 m). The higher frequencies offer shorter wavelengths and better resolution, so smaller defects can be detected. Some researchers have successfully used very highfrequency systems known as microwave or millimeter wave systems for composite applications (e.g., Akuthota et al., 2004; Fallahpour et al., 2011; Kharkovsky et al., 2008).

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2.0GHz horn antenna Computer

Mainframe processor

18.15 Components of a GPR system along with an air-launched radar antenna.

GPR antennas are typically of two types, air-launched and groundcoupled. Air-launched antennas (1–2 GHz) can be attached to the back of a vehicle and kept 0.15–0.3 m above the road or bridge surface. This allows for faster scanning of a pavement or bridge, with vehicle speeds ranging from ∼30–65 kph (20 to 40 mph). An air-launched antenna mounted on a push cart housing a GPR system is shown in Fig. 18.15. The ground-coupled antennas (15 MHz to 2.5 GHz) are dragged over the test surface (using attachment handles or belts) and transmit higher amount of energy into the ground, which is required for deeper penetration (>1 m). Also, highfrequency (1–2.5 GHz) ground-coupled antennas (Fig. 18.16) typically have a smaller footprint (as small as 0.1 × 0.2 m), which results in a higherresolution scan where small subsurface debonds and rebar layout are clearly visible. In GPR applications, it should be noted that defects which have a higher dielectric contrast than the surrounding medium are easier to detect. This means water-filled defects and metallic inclusions (e.g., steel rebars) are much easier to detect than air-filled defects (Halabe et al., 1995). Also, the electromagnetic pulses transmitted by commercial GPR antennas are linearly polarized. This means a conductor such as a rebar produces much higher signal amplitude if it is oriented along the polarization direction of the antenna. While the electromagnetic waves propagate easily through GFRP composites, they cannot penetrate conducting media such as CFRP composites owing to the high electrical conductivity of the carbon fibers. Some researchers have shown that electromagnetic waves can be propagated into CFRP composites if the polarization direction of the radar antenna is oriented perpendicular to the carbon fibers (Akuthota et al.,

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(a)

(b)

18.16 High-frequency ground-coupled radar antennas: (a) 1.6 GHz ground-coupled antenna mounted on a hand-held cart and (b) 0.9 GHz ground-coupled antenna.

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2004; Kharkovsky et al., 2008). This approach can be effectively used for testing unidirectional CFRP composites. However, the CFRP fabrics used in civil structures often consist of multidirectional fabrics with fiber orientation in 0° and 90°, ±45°, or a combination of all four directions. Such multidirectional CFRP fabrics offer better ductility and earthquake resistance, but they can be tested only using other NDE techniques such as infrared thermography, but not GPR (Halabe et al., 2010).

18.3.2 GPR testing for GFRP bridge decks Figure 18.17 shows a low-profile (102 mm thick) GFRP composite deck, similar to the one shown in Figure 18.7. The deck size was 1830 mm × 915 mm in plan. Several simulated air-filled and water-filled debonds were inserted between a 9.5 mm polymer concrete wearing surface and the underlying deck (Hing and Halabe, 2010). Figure 18.18 shows the schematic layout of the subsurface debonds. A radar scan was acquired by moving a 1.5 GHz ground-coupled antenna along the line of the subsurface debonds. This longitudinal GPR scan (vertical cross-section), also known as B-scan, is shown in Plate XI (between pages 296 and 297). The water-filled debonds

18.17 GFRP bridge deck with simulated debonds inserted between the wearing surface and the underlying GFRP composite deck.

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127.0 mm

457.2 mm

177.8 mm

101.6 mm × 101.6 mm × 1.5 mm water-filled debond

177.8 mm

50.8 mm × 50.8 mm × 1.5 mm air-filled debond

127.0 mm

101.6 mm × 101.6 mm × 1.5 mm air-filled debond

127.0 mm

457.2 mm

177.8 mm

101.6 mm × 101.6 mm × 2.25 mm water-filled debond

177.8 mm

50.8 mm × 50.8 mm × 2.25 mm air-filled debond

127.0 mm

101.6 mm × 101.6 mm × 2.25 mm air-filled debond

127.0 mm

457.2 mm

177.8 mm

101.6 mm × 101.6 mm × 0.75 mm water-filled debond

177.8 mm

50.8 mm × 50.8 mm × 1.5 mm water-filled debond

127.0 mm

505

101.6 mm × 101.6 mm × 0.75 mm air-filled debond

18.18 Layout and dimensions of simulated air-filled and water-filled debonds.

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are clearly visible in this image but none of the air-filled debonds was detected. The features between the debonds are echoes produced by the flange–web junction in the underlying GFRP deck. The deck was also scanned along multiple longitudinal path and these scans were combined to form a three-dimensional image (C-scan) which is shown in Plate XII (between pages 296 and 297). This image shows the location of the waterfilled debonds very clearly. While GPR antennas with 1–2 GHz frequency range has difficulty in detecting air-filled debonds, researchers have shown that microwaves at 10–24 GHz frequency range can detect air-filled debonds as well (e.g., Akuthota et al., 2004). It should be noted that GPR offers superior penetrating capabilities compared to near-surface testing techniques such as infrared thermography. The GPR technique has shown potential in detecting defects in the web and bottom flange of the composite deck with scan taken from the top side which is more accessible (Hing and Halabe, 2010).

18.3.3 GPR testing for FRP wrapped columns A study conducted by Halabe et al. (2010) showed that a GPR system with 1.5 GHz ground-coupled antenna was successful in detecting subsurface water-filled debonds in 152.4 mm × 304.8 mm (6″ × 12″) concrete cylinders wrapped with GFRP fabrics. Figure 18.19 shows the laboratory set-up for GPR testing of the FRP wrapped cylinders. Figure 18.20 shows the GPR scans from specimens with subsurface water-filled debonds of various sizes placed between the GFRP fabric and the underlying concrete cylinder. For cylinders wrapped with CFRP fabric, the debonds did not show up very well in the GPR scans, and infrared thermography was shown to be a more effective NDE technique (Halabe et al., 2010). Also, infrared thermography

Data acquisition system

Rotating table

Antenna

Antenna deployment frame

18.19 Laboratory set-up for GPR testing of FRP wrapped cylinders.

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Debond

(a)

507

Debond

(b)

(c)

18.20 GPR image of GFRP wrapped cylinders with water-filled debonds of sizes (a) 35.6 mm × 35.6 mm, (b) 50.8 mm × 50.8 mm and (c) 76.2 mm × 76.2 mm.

was able to detect both air-filled and water-filled debonds as explained earlier in this chapter.

18.4

Digital tap testing

Tap testing or sounding technique is commonly used to detect subsurface debonds between FRP laminates or fabrics and the underlying structural component. Areas with air-filled subsurface debonds often produce a hollow sound when tapped with objects such as wooden stick, metallic rod, or FRP rebar. However, this kind of testing is very subjective and depends on the hearing ability of the testing personnel. A significant improvement in the area of tap testing has been achieved through the use of a digital tap hammer (Fig. 18.21). Such a device uses an instrumented hammer to record the echo signal received after tapping. This signal can be converted into a reading using parameters such as peak amplitude, area under the Fourier transform or power spectral density of the received echo signal. In the case of debonded areas, the echo from the debond results in a larger reading while defect-free areas produce smaller readings since the elastic waves propagate right through the test object and a small echo from the back wall is received. In other words, the more rigid or stiff the area, the smaller is the reading. Advanced tap testing devices often record the contact surface echo as well as the slightly delayed echo coming from the subsurface debonds or voids. This enables the normalization of the debond echo by dividing it with the corresponding parameter for the surface echo. Such normalization makes the displayed tap test reading relatively immune to the tapping force. In other words, different users would achieve more or less similar normalized reading from a given area and the readings are repeatable within a narrow range. However, it is important to first establish a baseline reading

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18.21 Digital tap hammer.

from a defect-free area using average of several tap tests before testing other areas to detect possible subsurface debonds or voids. Any area which results in a reading that is at least 10% higher than the baseline would have a good probability of a subsurface debond or void. Also, the higher the reading, the higher is the probability that a subsurface debond or void exists. Table 18.1 illustrates the readings from defective and defect-free regions in case of a GFRP bridge deck specimen and concrete cylinders wrapped with GFRP fabrics. It can be seen from Table 18.1 that the digital tap testing results for any given case are fairly consistent and repeatable. Also, the data from the third specimen shows that the air-filled subsurface debond leads to much higher readings compared to the water-filled subsurface debond. The above results show that tap testing is a very easy-to-use and effective NDE technique for detecting subsurface debonds. However, it is a slow technique and cannot cover large areas quickly. If coverage of large areas is needed, then infrared thermography and GPR offer more efficient alternatives. Infrared thermography is very effective for detecting near-surface defects while GPR can provide significantly higher penetration depths.

18.5

Issues and challenges in using non-destructive evaluation (NDE) techniques

Unlike the aerospace industry, the civil infrastructure community invests only a small percentage of their budget on NDE related work, and most of the budget is designated for new construction or repair work. Agencies such

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Table 18.1 Tap testing results obtained from laboratory specimens

Specimen type

Type of test area

Deck specimen BD1a with subsurface debonds between wearing surface and GFRP deck

Defect-free

Concrete cylinder wrapped with one layer of GFRP fabric

Above 76 mm × 76 mm air-filled debond Above 51 mm × 51 mm air-filled debond Defect-free

Above 51 mm × 51 mm air-filled debond

Concrete cylinder wrapped with two layers of GFRP fabric

Defect-free

Above 25 mm × 25 mm waterfilled debond

Above 12.7 mm × 12.7 mm air-filled debond

Tap test readings from successive tests

Average tap test reading

Percentage change compared to defectfree area (%)

1134, 1124, 1160, 1178, 1107, 1142, 1177, 1162, 1149, 1141, 1144, 1150 1513, 1539, 1491, 1436, 1589 1441, 1584, 1455, 1546, 1518

1147

–

1514

31.9

1509

31.5

1089, 1090, 1104, 1088, 1101, 1105, 1098, 1109, 1120, 1121, 1123, 1101 1796, 1821, 1803, 1805, 1813, 1778, 1792, 1839, 1774, 1781, 1850, 1797

1104

–

1804

63.4

1225, 1209, 1154, 1255, 1187, 1129, 1225, 1223, 1228, 1132, 1161, 1206 1447, 1390, 1420, 1390, 1361, 1415, 1413, 1433, 1405, 1457, 1396, 1407 2152, 2180, 2079, 2190, 2456, 2760, 2587, 2328, 2917, 3024, 2339, 2451

1195

–

1411

18.1

2455

105.5

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as the Federal Highway Administration and State Departments of Transportation are continually looking for highly cost-effective NDE solutions. This requires the use of low-cost NDE equipment which is fairly easy for a field inspector or highway personnel to use. For in-service testing, portability of the NDE equipment is very crucial. Also, development of userfriendly data analysis software that can be used in the field to quickly identify problem areas during field inspection is important. Future development of automated data processing algorithms for various NDE techniques can greatly help to enhance the data analysis procedure and possibly minimize human errors in data interpretation. If such algorithms are incorporated into user-friendly software, it can help in increasing the use of NDE by field engineers and testing personnel. Also, it is important to understand the challenges faced during field testing. For example, structural components under the bridge are often difficult to access and require lane closures for NDE testing. Development of NDE systems with accessories adapted for rapid testing of structural members under the bridges can greatly facilitate the increased use of NDE. At the same time, such equipment and associated data processing software need to be available at a reasonable cost for increased use of NDE for condition assessment of civil infrastructure.

18.6

Future trends

Significant research and development work in the area of NDE of composites for civil infrastructure has taken place in the last ten years. This includes development of more robust and sensitive NDE systems, as well as easyto-use field testing equipment. A significant number of researchers are devoting their time towards development of quantitative data analysis algorithms. For example, in the infrared thermography area, algorithms have been developed to predict width and depth of subsurface debonds by analyzing a sequence of infrared images acquired during the cooling cycle, that is, immediately after removal of heat. These algorithms make use of parameters such as break time, contrast analysis, derivative images, and determination of inflection points in the surface temperature profile over the debonds (Halabe and Dutta, 2010; Maldague, 2000, 2001; Maldague and Moore 2001b; Plotnikov, 1999; Ringermacher et al., 2007; Shepard et al., 1999, 2003, 2007; Starnes et al., 2003; Shepard, 2006, 2007; Vavilov, 2007). Computer-controlled short-duration flash lamps are often used in conjunction with more robust cameras (with capture rates of up to 500 frames per second) and advanced data acquisition and processing methodologies such as pulsed thermography, lock-in thermography, and vibrothermography (Spring et al., 2011). However, owing to the very high cost and bulky nature of these systems, they have been predominantly used only in the laboratory

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setting for aerospace and automotive applications, especially for thin composites. It is conceivable that in the near future these advanced algorithms and robust NDE hardware will be incorporated into small low-cost handheld units so that they can be conveniently used for inspection of civil structures in the field environment. In addition, it is expected that better heating systems will become available for application of uniform heating to field structural components. In the area of GPR applications, algorithms are being developed to combine two-dimensional GPR cross-sectional scans into threedimensional images for better rendering of the structural components’ subsurface features. The three-dimensional GPR imaging techniques have been extended from flat objects such as composite bridge decks to cylindrical objects such as columns. Further development in this area includes the ability to create circular cross-sectional scans and the capability to slice a three-dimensional cylindrical image into vertical and horizontal slices at various distances and depths for easy viewing of the subsurface defects or position of inclusions such as steel rebars (Halabe and Pyakurel, 2010; Pyakurel, 2009; http://www.gpr-survey.com/). Currently, these algorithms require significant amount of intermediate steps with human intervention to produce three-dimensional images. In the future, it is expected that these algorithms will be packaged into more user-friendly software with automated data processing features and incorporated into portable field testing equipment so that their use will become widespread within the civil structures community. Other NDE techniques such as fiber optics-based strain measurement, ultrasonics, acoustography, laser shearography, Raman spectroscopy, electromagnetic and eddy current, microwaves, and millimeter wave techniques are being developed and utilized for aerospace composite applications (e.g., Akuthota et al., 2004; Fallahpour et al., 2011; Kharkovsky et al., 2008; Smith, 2007; Wang et al., 2011). It is conceivable that these NDE techniques, which are also being tested in the laboratory or manufacturing setting for composite structural components (e.g., Akuthota et al., 2004; Kharkovsky et al., 2008; Washer and Blum, 2011; Wang et al., 2011), will be adapted for field application to composite components in civil structures in the near future. The data acquisition hardware for some of these NDE techniques could also be made more robust and adapted for permanent installation on field structures and the acquired data could be transmitted wirelessly to a central data processing location. This approach would become feasible for large field structures with the development of automated data processing algorithms that can integrate the results from different NDE techniques to reliably predict the condition of the structure as a whole and help in making timely repair and maintenance decisions so that a long service life for the structure can be achieved.

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18.7

References

ACI 440.2R-08 (2008), Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, Report by Committee 440, American Concrete Institute, Farmington Hills, MI 48331. Akuthota, B., Hughes, D., Zoughi, R., Myers, J., and Nanni, A. (2004), ‘NearField Microwave Detection of Disbond in Carbon Fiber Reinforced Polymer Composites Used for Strengthening Cement-Based Structures and Disbond Repair Verification,’ Journal of Materials in Civil Engineering, ASCE, 16(6), 540–546. ASTM D 4788 – 03 (2007). ‘Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography,’ Annual Book of ASTM Standards, Vol. 04.03, American Society of Testing and Materials (ASTM), West Conshohocken, Pennsylvania, USA. Clark, M. R., McCann, D. M., and Forde, M. C. (2003), ‘Application of Infrared Thermography to the Non-Destructive Testing of Concrete and Masonry Bridges,’ NDT&E International, 36, 265–275. Dutta, S. S. (2010), ‘Nondestructive and Destructive Evaluation of FRP Composite Wrapped Concrete Cylinders with Embedded Debonds,’ Ph.D. Dissertation, West Virginia University, Department of Civil and Environmental Engineering, Morgantown, WV. Fallahpour, M., Ghasr, M. T., Case, J. T., and Zoughi, R. (2011), ‘A Wiener Filter-based Synthetic Aperture Radar Algorithm for Microwave Imaging of Targets in Layered Media,’ Materials Evaluation, 69(10), 1227–1237. Halabe, U. B., and Dutta, S. S. (2010), ‘Quantitative Characterization of Debond Size in FRP Wrapped Concrete Cylindrical Columns Using Infrared Thermography,’ Proceedings of the Fourth Japan-US Symposium on Emerging NDE Capabilities for a Safer World, conference jointly conducted by the Japanese Society for NonDestructive Inspection (JSNDI) and the American Society for Nondestructive Testing (ASNT), Maui Island, Hawaii, U.S.A., June, 64–68. Halabe, U. B., and Perisetty, N. K. (2011), ‘Composite Materials Use for Railroad Infrastructure – Rehabilitation of Timber Railroad Bridges Using Glass Fiber Reinforced Polymer Composite: Nondestructive Testing and Evaluation (Task 3),’ USDOT-FRA Agreement No. FR-RRD-0010-10-01-00, Final Report, submitted to US Department of Transportation – Federal Railroad Administration (USDOTFRA), Washington, DC, pp. 1–26. Halabe, U. B., and Pyakurel, S. (2010), ‘2D and 3D GPR Imaging of Cylindrical Wooden Logs and Concrete Columns,’ Proceedings of the Fourth Japan-US Symposium on Emerging NDE Capabilities for a Safer World, conference jointly conducted by the Japanese Society for Non-Destructive Inspection (JSNDI) and the American Society for Nondestructive Testing (ASNT), Maui Island, Hawaii, U.S.A., June, 81–87. Halabe, U. B., Sotoodehnia, A., Maser, K. R., and Kausel, E. A. (1993), ‘Modeling the Electromagnetic Properties of Concrete,’ American Concrete Institute (ACI) Materials Journal, 90(6), 552–563. Halabe, U. B., Maser, K. R., and Kausel, E. A. (1995), ‘Condition Assessment of Reinforced Concrete Structures Using Electromagnetic Waves,’ American Concrete Institute (ACI) Materials Journal, 92(5), 511–523.

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Halabe, U. B., Steele, W. E., GangaRao, H. V. S., and Klinkhachorn, P. (2003a), ‘NDE of FRP Wrapped Timber Bridge Components Using Infrared Thermography,’ Review of Progress in Quantitative Nondestructive Evaluation, Vol. 22 (AIP CP657), ed. D. O. Thompson and D. E. Chimenti, American Institute of Physics, Melville, NY, 1172–1177. Halabe, U. B., Vasudevan, A., and GangaRao, H. V. S. (2003b), ‘Field Testing of the La Chein Composite Bridge Deck Using Infrared Thermography,’ Final Report No. CFC-03-112, submitted by Constructed Facilities Center, West Virginia University, to West Virginia Division of Highways (WVDOH), District 9, Lewisburg, WV, October 2003, pp. 1–17. Halabe, U. B., Vasudevan, A., and GangaRao, H. V. S. (2005), ‘Field Testing and Evaluation of FRP Wrapped Timber Railroad Bridge Components Using Infrared Thermography,’ CD-ROM Proceedings of the Third US-Japan Symposium on Advancing Applications and Capabilities in NDE, American Society for Nondestructive Testing and the Japanese Society for Nondestructive Inspection, Maui, Hawaii, June, 342–348. Halabe, U. B., Roy, M., Klinkhachorn, P., and GangaRao, H. V. S. (2006), ‘Detection of Air and Water-Filled Subsurface Defects in GFRP Composite Bridge Decks Using Infrared Thermography,’ Review of Progress in Quantitative Nondestructive Evaluation, Vol. 25 (AIP CP820), ed. D. O. Thompson and D. E. Chimenti, American Institute of Physics, Melville, NY, 1632–1639. Halabe, U. B., Vasudevan, A., Klinkhachorn, P., and GangaRao, H. V. S. (2007), ‘Detection of Subsurface Defects in Fiber Reinforced Polymer Composite Bridge Decks Using Digital Infrared Thermography,’ Nondestructive Testing and Evaluation, 22(2–3), 155–175. Halabe, U. B., Dutta, S. S., and GangaRao, H. V. S. (2010), ‘Infrared Thermographic and Radar Testing of Polymer-Wrapped Composites,’ Materials Evaluation, 68(4), 447–451. Hing, C. L. C., and Halabe, U. B. (2010), ‘Nondestructive Testing of GFRP Bridge Decks using Ground Penetrating Radar and Infrared Thermography,’ Journal of Bridge Engineering, ASCE, 15(4), 391–398. Kaplan, H. (1999). Practical Applications of Infrared Thermal Sensing and Imaging Equipment, SPIE Press, Bellingham, Washington. Kharkovsky, S., Ryley, A. C., Stephen, V., and Zoughi, R. (2008), ‘Dual-Polarized Near-Field Microwave Reflectometer for Noninvasive Inspection of Carbon Fiber Reinforced Polymer-strengthened Structures,’ IEEE Transactions on Instrumentation and Measurement, 57(1), 168–175. Liang, R., and GangaRao, H. V. (2004), ‘Applications of Fiber Reinforced Polymer Composites,’ Proceedings of Polymer Composites III Conference on Transportation Infrastructure, Defense and Novel Applications of Composites, Robert C. Creese and Hota Gangarao, eds., Destech Publications Inc., Lancaster, PA, 173–187. Maldague, X. P. V. (1993), Nondestructive Evaluation of Materials by Infrared Thermography, Springer-Verlag, London. Maldague, X. (2000), ‘Applications of Infrared Thermography in Nondestructive Evaluation,’ Trends in Optical Nondestructive Testing (invited chapter), ed. P. Rastogi, 591–609 (paper also available at http://www.gel.ulaval.ca/∼maldagx/r_1123. pdf).

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Maldague, X. P. V. (2001), Theory and Practice of Infrared Technology for Nondestructive Testing, John Wiley & and Sons, New York. Maldague, X. P. V. and Moore P. O. (2001), Nondestructive Testing Handbook – Infrared and Thermal Testing, American Society for Nondestructive Testing, Columbus, OH. Plotnikov, Y. A. (1999), ‘Modeling of the Multiparameter Inverse Task of Transient Thermography,’ Review of Progress in Quantitative Nondestructive Evaluation, Vol. 18, ed. D. O. Thompson and D. E. Chimenti, Kluwer Academic/Plenum Publishers, New York, 873–880. Pyakurel, S. (2009), ‘2D and 3D GPR Imaging of Wood and Fiber Reinforced Polymer Composites,’ Ph.D. Dissertation, Department of Civil and Environmental Engineering, West Virginia University, Morgantown, WV. Ringermacher, H. I., Knight, B., Li, J., Plotnikov, Y. A., Aksel, G., Howard, D. R., and Thompson, J. L. (2007), ‘Quantitative Evaluation of Discrete Failure Events in Composites Using Infrared Imaging and Acoustic Emission,’ Nondestructive Testing and Evaluation, 22(2–3), 93–99. Shepard, S. M. (2006), ‘Understanding Flash Thermography,’ Materials Evaluation, 64(5), 460–464. Shepard, S. M. (2007), ‘Thermography of Composites,’ Materials Evaluation, 65(7), 690–696. Shepard, S. M., Rubadeux, B. A., and T. Ahmed (1999), ‘Automated Thermographic Defect Recognition and Measurement,’ Nondestructive Characterization of Materials IX, AIP Conference Proceedings 497, ed. R. E. Green, Jr., American Institute of Physics, Melville, New York, 373–378. Shepard, S. M., Lhota, J. R., Rubadeux, B. A., Wang, D., and Ahmed, T. (2003), ‘Reconstruction and Enhancement of Active Thermographic Image Sequences,’ Optical Engineering, 42, 1337–1342. Shepard, S. M., Lhota, J. R., and Ahmed, T. (2007), ‘Flash Thermography Contrast Model Based on IR Camera Noise Characteristics,’ Nondestructive Testing and Evaluation, 22(2–3), 113–126. Smith, R. A. (2007), ‘Advanced NDT of Composites in the United Kingdom,’ Materials Evaluation, 65(7), 697–710. Spring, R., Huff, R., and Schwoegler, M. (2011), ‘Infrared Thermography: A Versatile Nondestructive Testing Technique,’ Materials Evaluation, 69(8), 934–942. Starnes, M. A., Carino, N. J., and Kausel, E. A. (2003), ‘Preliminary Thermography Studies for Quality Control of Concrete Structures Strengthened with Fiberreinforced Polymer Composites,’ Journal of Materials in Civil Engineering, ASCE, 15(3), 266–273. Vasudevan, A. (2004), ‘Application of Digital Infrared Thermography for NDE of Composite Bridge Components,’ M.S. Thesis, Department of Civil and Environmental Engineering, West Virginia University, Morgantown, WV. Vavilov, V. P. (2007), ‘Pulsed Thermal NDT of Materials: Back to the Basics,’ Nondestructive Testing and Evaluation, 22(2–3), 177–197. Wang, C., Faidi, W., Tralshawala, N., Vadde, A., and May, A. (2011), ‘Online Monitoring of Composite Joining Process,’ Materials Evaluation, 69(12), 1393–1398. Washer, G., and Blum, F. (2011), ‘Developing Raman Spectroscopy for the Nondestructive Testing of Carbon Fiber Composites,’ Materials Evaluation, 69(10), 1219–1226.

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76 mm 51 mm Air-filled (51 mm × 51 mm)

102 mm 51 mm

Water-filled (51 mm × 51 mm)

127 mm

76 mm

Water-filled (76 mm × 76 mm)

76 mm

127 mm

152 mm

152 mm (a)

(b)

Plate VIII (Chapter 18) (a) Schematic view, and (b) infrared image of bridge deck specimen BD2 showing the air-filled and water-filled debonds.

49.9 °C

45.8 °C

35.9 °C

12.7

(a)

(b)

Plate IX (Chapter 18) (a) Photograph of a pile, and (b) infrared image showing debonds.

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39.1 °C

31.6 °C

17.2

(a)

(b)

Plate X (Chapter 18) (a) Photograph of a pile, and (b) infrared image showing good bond.

Water-filled debonds

Plate XI (Chapter 18) Results of GPR test showing water-filled debonds.

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Water-filled debonds

Plate XII (Chapter 18) Three-dimensional GPR image showing location of water-filled debonds.

× 10–6 0

w 65 nm

r

∂w/∂x

z

Undetected defect zone

–1 –2 –3 –4 0

0 nm

(a)

0.005 0.01 0.015 0.02 0.025 0.03 r (m) (b)

Plate XIII (Chapter 20) Results of the finite element simulations for a defect diameter equal to 40 mm and an applied depressure ΔP = 100 Pa.

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