U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
The Federal Highway Administration’s (FHWA’s) Advanced Sensing Technology (FAST) NDE Laboratory houses state-of-the-art NDE equipment to support FHWA’s strategic vision and to provide training to stakeholders who maintain and manage transportation infrastructure such as bridges, pavements, tunnels, and ancillary structures. Using NDE tools and equipment, the NDE Laboratory works side by side with FHWA’s research programs, such as the Long-Term Bridge Performance Program. Some of the equipment is described below. The equipment is sorted into groups based on concrete or steel application.
One of the most effective ways to assess the susceptibility of a reinforced concrete element to corrosion is by measuring concrete surface resistivity. Low concrete resistivity is an indication of an environment supporting corrosion processes and typically leads to high corrosion rates. Measurement of resistivity is most commonly done using a four electrode probe, the Wenner probe (figure 1), which is simply pressed against a damp surface of an element and the displayed resistivity recorded. For more information about this technology available in the NDE Laboratory, visit the Electrical Resistivity page on the FHWA NDE Web Manual.
|Figure 1. Photographs. Concrete
Electrical Resistivity Meter.
The “corrosion rate” refers to the speed with which steel reinforcement corrodes. Rapid and nondestructive estimations of the corrosion rate can be performed using the GPM. The effectiveness of the method stems from its active nature, i.e., the ability to polarize rebars through the induction of small current pulses and then measure the changes of the electrochemical potential. A GPM device is available at the NDE laboratory (figure 2). For more information, visit the GPM technology page on the FHWA NDE Web Manual.
Figure 2. Photographs. Galvanostatic Pulse Measurement (GPM).
GPR is an NDE technology that can be used in the assessment of many infrastructure assets. GPR is used in the assessment of corrosion-induced deterioration; mapping of reinforcement; and thus, measurement of concrete cover; identification of layers; and measurement of their thickness in multilayer structures, such as pavements, detection of voids, etc. One of the main advantages of GPR is the speed of data collection. Detectability of features and deterioration greatly depends on the size, depth, and dielectric properties of the structure. Therefore, GPR systems with antennas of different frequencies and power (figure 3 through figure 6) need to be used in different applications. The laboratory houses multiple GPR equipment with various specifications. For more information, visit the GPR technology page on the FHWA NDE Web Manual.
Figure 3. Photograph. GPR System, Sensors, and Software, Conquest.
|Figure 4. Photographs. GSSI GPR System with
1.6 GHz (Left) and 900 MHz (Right) Antenna.
Figure 5. Photograph. Cart with a GPR antenna.
Corrosion is an electrochemical process in which a corroding rebar generates an electrical field. The HCP enables the measurement of the potential of that field, by connecting the half-cell probe (reference electrode) to the rebar through a high impedance voltmeter. The stronger negative- measured voltage will have a higher probability of active corrosion. HCP can be used on any reinforced concrete structure where a connection to the reinforcement can be made, and where there is no electrically isolating surface layer. Depending on the application, different types of probes, such as copper, copper sulphate rod, or rolling electrode, may be used. An HCP analyzing instrument with a rolling probe, which is used for the rapid scanning of large areas, is available at the NDE Laboratory (figure 7). For more information, visit the HCP page on the FHWA NDE Web Manual.
Figure 6. Photograph. Half-Cell Potential (HCP)
Corrosion Analyzing Instrument and Probe.
IR is a method that examines the dynamic response of an element or structure in order to identify the presence of anomalies. It has been used as a screening tool for the detection of voids, loss of support, or presence of large delamination or debonding. The IR measurement method involves striking the surface of an object using an instrumented hammer, and measuring the response with a nearby transducer, typically a geophone (velocity transducer), to generate mobility spectra. Multiple parameters can be extracted from mobility plots, such as dynamic stiffness, average mobility, and mobility slope, which are used in the interpretation of the measurement results (figure 8). An IR set is available at the NDE Laboratory.
Figure 7. Photograph. Impulse Response (IR) Testing System.
IRT is used to detect defects, most commonly delamination, in structures by measuring the surface temperature variations as a result of uneven heating and cooling in the vicinity of defects. IRT cameras have thermal fusion functionality, which allows easier identification and interpretation of infrared images. Two models of IRT cameras are available at the NDE Laboratory (figure 8). For more details about this technology visit the IR page on the FHWA Web Manual.
Figure 8. Photograph. Infrared Thermography Camera.
The UPE method is primarily used to inspect the interior portions of the concrete structural members and tunnel linings to detect voids, delamination, and debonding; assess the grouting condition in post-tensioning ducts, and to measure thickness of members, etc. The UPE test involves emitting ultrasonic waves into the element, and recording the reflections from the objects and interfaces between the materials of different acoustic impedances. The concrete UPE application differs from traditional steel applications in two ways. The first is the use of low- frequency transducers, and the second is that the test is conducted with transducers placed in dry contact, i.e., without a couplant.
UPE equipment is available in different sizes, varying from two transducers used to conduct simple point measurements to a large number of transducers in an array arrangement used to conduct tomographic measurements. Two different models of UPE are available at the NDE laboratory (figure 9). For more information about this technology, visit the UPE page on the FHWA NDE Web Manual.
|Figure 9. Photographs. Ultrasonic Pulse Echo (UPE) system.|
The USW technique is primarily used to assess the quality of concrete through the measurement of concrete modulus. The method can also indirectly detect the presence of delamination, voids, and other major anomalies in the element. The USW test involves the measurement of the transient response of the deck to an impact by a receiver pair. The recorded response is analyzed to provide the velocity of surface waves, which can be directly correlated to the concrete modulus (figure 10). For more information about this technology, visit the USW page on the FHWA NDE Web Manual.
Figure 10. Photograph. Ultrasonic Surface Waves (USW) Testing System.
Concrete: Impact Echo (IE)The IE method is a seismic or stress wave-based method used to detect defects in concrete, primarily delamination. The objective of the IE survey is to detect and characterize wave reflectors, or resonators, in a concrete bridge deck or other structural elements. The amplitude spectrum obtained from the fast Fourier transform analysis of the time signal will show dominant peaks at certain frequencies that can be interpreted to assess the deck condition. For more information about this technology, visit the IE page on the FHWA NDE Web Manual.
Figure 11. Photograph. Impact Echo (IE) Testing System.
ECT is used to detect cracks and pitting in steel bridge members. Due to the magnetic properties of steel, only surface or near surface-breaking cracks can be detected. The technology can be effective for detecting cracks in welds; however, special probes must be used due to complex magnetic and microstructural properties associated with the weld and the surrounding heat- affected zone (HAZ). ECT works by scanning a small electromagnetic probe across the surface of a material with an objective to detect changes in eddy currents. The changes are a result of the presence of defects or changes in material properties of the tested object, electrical conductivity, and particularly magnetic permeability. Conventional handheld eddy current systems with absolute and differential point probes that have an operating frequency of 50 Hz to 12 MHz are available for use in the inspection of surface defects in metals. Two models of ECT equipment are available at the NDE Laboratory: conventional and advanced (figure 12). For more information about this technology, visit the ECT page on the FHWA NDE Web Manual.
Figure 12. Photograph. Eddy Current Testing (ECT) instrument.
ECA has several advantages over ECT, such as better detection capabilities, faster inspection, and easier data analysis and interpretation. This is a result of using multiple coils to generate eddy currents arranged in specific patterns. The advanced eddy current system is an impedance instrument with a 37-channel probe electronics unit and a high-frequency eddy current array (figure 13). The eddy current array has a drive winding with linear drive segments, and is excited with a current at a prescribed frequency. The frequency range is from less than 1kHz to 40 MHz, which provides a desired spatial distribution for the imposed magnetic field. Significant changes in the signal’s magnetic permeability readings will indicate the presence of a flaw in the material. For more information about this technology, visit the ECA page on the FHWA NDE Web Manual.
Figure 13. Photograph. Eddy Current Array Testing System.
PAUT can be applied to detect flaws, cracks, and weld flaws in steel bridge members. It can also be used for element thickness measurements. PAUT uses an array of ultrasonic transducers that are pulsed independently to create wave patterns that target specific locations. The beam from a phased array probe is moved electronically in all directions, which allows fast scanning of large volumes of a tested object (figure 14). For more information about this technology, visit the PAUT page on the FHWA NDE Web Manual.
Figure 14. Photograph. Portable Phased Array Ultrasonic System.
UT technology can be applied to truss members, steel girders, or other steel bridge components with a plate-like geometry (i.e., parallel surfaces). UT is used to detect cracks and weld flaws, and to determine the thickness or length of a tested member. UT uses a transducer placed on the surface to emit high-frequency acoustic waves into the structure. The waves are reflected from discontinuities in the material, which are then detected by the transducer. The technology is typically implemented using longitudinal (straight beam) or shear wave (angled beam) methods. Longitudinal wave methods are used to detect cracks in bridge pins, trunnion shafts, or eyebars, or measure the thickness of a steel plate in order to detect section loss as a result of corrosion (figure 15). Shear wave methods (angled beam) are commonly used to inspect welds for flaws (figure 16). For more information about this technology, visit the UT page on the FHWA NDE Web Manual.
Figure 15. Photograph. Ultrasonic Testing (UT) for thickness measurement.
Figure 16. Photograph. Ultrasonic Testing (UT) for flaw detection.
The laboratory is equipped with a 22-kip load frame with an environmental testing chamber. The environmental chamber enables the mechanical testing of materials and components across a broad range of temperature, humidity, and caustic conditions. The equipment is ideal for conducting tension, compression, bending, and cyclic fatigue testing of metals, composites, and ceramics.
An X-ray CT and digital radiography imaging system was acquired to accommodate the research needs of the laboratory. The system can assist many applications including materials research, nondestructive testing, core sample characterization, weld inspection, failure analysis, and reverse engineering. The X-ray CT enables the visualization of the interior of scanned objects by directing X-rays at an object from different directions and examining the attenuation or strength of reflections along a series of linear paths. Unlike traditional X-ray imaging, the digital radiography utilizes X-ray sensors to transfer and enhance images digitally. The system has been used by researchers at the NDE Laboratory for various projects, such as the determination of air-void parameters, crack propagation, and internal structure characterization of portland cement concrete and asphalt concrete structures.
Two specimen scanning systems were developed to accommodate high resolution experimental testing performed in the NDE Laboratory.
The xy scanner platform covers an 8-foot by 8-foot scanning area. The scanning head is fitted with a manipulator attachment designed to hold various NDE probes. The scanning head manipulator also provides means for probe orientation, including probe downward/upward positioning, surface contact, predetermined contact pressure, and duration. A scan, using various NDE sensor technologies, can be conducted at the nodal points of a virtual grid with as small as elements as desired. Computer control and data acquisition are available to integrate and automate new sensor technologies into the scanning system. The system is designed to be adaptable to vertical surfaces, such as tunnel walls, as well as horizontal slabs.
Figure 17. Photograph. XY scanner in foreground and robot
scanner in background for high volume lab specimen testing.
The robot scanner includes a KUKA KR R110 robot mounted on an external stepper motor controlled x-axis to extend the range of the robot. The robot system includes the KRC4 compact controller with EtherCAT communication implemented to control the external axis. The robot enables high precision, high volume measurement scans across complex surfaces. The robot scanner system is able to automate measurements with legacy via pneumatic actuation.
Figure 18. Photograph. Robot scanner for high volume lab specimen testing.
A portable acoustic array for multiple simultaneous impact echo (IE) and ultrasonic surface wave testing (USW) was developed. The system covers a 4-feet-wide testing strip. The array has five sources and eight receivers. The spacing between the impact sources is 12 inches. The sensors and sources are coupled to the surface pneumatically, which enables the array to compensate for uneven surfaces and ensure that all the sources and sensors are coupled to the surface with equal force.
Figure 19. Image. A schematic of the pneumatic acoustic array.
The five source and eight sensor arrangement enables eight IE tests and eight USW tests. The sources are solenoid impactors, while the sensors are accelerometers. The accelerometers cover a frequency range of 100 Hz to 25 kHz. The system is portable for field use.
Figure 20. Photograph. Portable pneumatic acoustic array.
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