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Federal Highway Administration > Publications > Public Roads > Vol. 61· No. 1 > Nondestructive Evaluation for Bridge Management in the Next Century

July/August 1997
Vol. 61· No. 1

Nondestructive Evaluation for Bridge Management in the Next Century

by Steven B. Chase and Glenn Washer

The Federal Highway Administration (FHWA) is sponsoring a large program of research and development in new technologies for the nondestructive evaluation of highway bridges.

The program is currently focusing on developing technologies to address a wide range of problems. One focus area is the detection and evaluation of fatigue cracks in steel highway bridges. Another area is developing technologies for the rapid and quantitative evaluation of reinforced concrete bridge decks covered with bituminous concrete. FHWA is also developing new technologies to enable the evaluation of a bridge in a global sense. In addition, projects are underway to develop new technologies for the evaluation of prestressing steel in prestressed concrete, the evaluation of bridge substructures, and the development of special tools and methods to evaluate stay cables. And, to integrate nondestructive evaluation more fully into bridge management systems, FHWA is studying the development of new models.

This article summarizes FHWA's current research and development program for the nondestructive evaluation of bridges.

Objectives

The nondestructive evaluation (NDE) program has two main objectives:

The first objective is to develop new tools and techniques to solve specific problems. Some examples are locating, quantifying, and assessing fatigue cracks on steel bridges; quickly, efficiently, and quantitatively assessing the condition of reinforced concrete bridge decks even though they are covered with asphalt; and developing technology to evaluate the 100,000 bridges for which we do not know how deep the foundation piles extend or even, in some cases, whether or not there are piles. Any type of scour or seismic assessment is meaningless without that information.

Image of a cross section of reinforced concrete slab produced by ground-penetrating radar -imaging system, showing individual reinforcing bars and a determination.

Ground-penetrating radar image.

The second objective is to develop technologies for the quantitative assessment of the condition of bridges in support of bridge management and to investigate how best to incorporate quantitative condition information into bridge management systems. Today, the data that feeds all bridge management systems are based upon visual inspection and subjective condition assessment. We are developing technologies to quickly, efficiently, and quantitatively measure global bridge parameters, such as flexibility and load-carrying capacity.

Background

FHWA's NDE program is focused on the most important and pressing needs. The specific projects underway are intended to solve specific problems. An overview of the bridge inspection and management problem is presented to explain the priorities we have established for the NDE program.

The National Bridge Inventory contains about 570,000 highway bridges. If we exclude culverts and tunnels for the moment, the inventory still includes about 470,000 bridges. The proportions by superstructure type are shown in figure 1. Steel bridges outnumber the other types. Steel is followed by concrete, prestressed concrete, and timber. There are a few other types of bridges, such as masonry, iron, and aluminum.

Chart
Figure 1
Proportion by material type
(excludes culverts and tunnels).

Figure 2 presents data about these bridges by type and age. There have been two bridge-building booms - one in the post-depression era and the second as the interstate system was constructed. We also see that the majority of bridges built prior to 1970 were steel bridges and that the proportion of prestressed concrete bridges has been increasing steadily. There has also been a small, but steady, number of timber bridges built over the years.

Age Distribution
Figure 2
Age Distribution of Bridges
(excludes culverts and tunnels).

Figure 3 begins to show where the most critical problems are. We can see the proportion by type of all bridges compared to the proportion by type for structurally deficient bridges. A bridge is classified as structurally deficient when it has a poor or worse rating for the condition of the deck, superstructure, or substructure or when its load-carrying capacity is significantly below minimum standards. This classification includes the most serious types of deterioration.

Material Type Chart
Figure 3
Proportion by material type
(excludes culverts and tunnels).

There are about 110,000 structurally deficient bridges in the inventory. While steel bridges represent about 40 percent of the overall bridge total, they include about 60 percent of the structurally deficient bridges. Reinforced concrete bridges fare well in the comparison of proportions, and only a relatively small proportion of the prestressed concrete bridges are structurally deficient. Timber bridges, while representing only 9 percent of the entire number of bridges, represent 20 percent of the structurally deficient bridges. About half of the nation's timber bridges are classified as structurally deficient.

Taking this analysis one step further and looking at why the bridges of different types are classified as structurally deficient leads us to figure 4. The most frequent reason that steel and timber bridges are classified as structurally deficient is a low structural adequacy rating. This means the bridge has a very low load rating. It is also worth noting that more structurally deficient steel bridges have bad substructures compared to virtually equal numbers of steel bridges with bad superstructures or bad decks.

Deficient Bridges
Figure 4
Structurally Deficient Bridges
(comparison of ratings for key items).

Two of the highest priorities in the NDE program are developing technologies for steel bridge inspection and new technologies for bridge deck inspection.

Figure 5 shows the nation's 576,000 bridges, including large culverts and tunnels, by date of original construction. It has the same general shape as figure 2. The black line on the chart shows the percentage of each age group that is classified as structurally deficient or functionally obsolete. The number of deficient bridges steadily increases with age, with 80 percent of those bridges built between 1905 and 1910 classified as deficient. About 1 percent (5,000 bridges) becomes deficient each year.

Age Distribution of Structures.
Figure 5
Age Distribution of structures.

Today, about 187,000 bridges are classified as deficient. This figure has been reduced somewhat over the past few years, but only after federal bridge funding was increased to approximately $3 billion per year. We are currently building or rehabilitating about 10,000 bridges per year. To deal with the backlog of 187,000 deficient bridges; the 5,000 bridges that become deficient each year; and to ensure that the $3 billion is spent in an optimal manner, FHWA is mandating the implementation of bridge management systems.

The essence of a bridge management system is shown in figure 6. Data are collected from a number of sources (primarily from periodic bridge inspections) and are transferred to a large database. A sophisticated analysis of the data is performed. This generates prioritized lists of candidate projects, optimizes bridge replacement and maintenance strategies for various available funding and resource scenarios, predicts the deterioration of bridges over time, allows managers to evaluate different management options, and, in general, provides powerful decision-support tools to help formulate the best program for bridge management. The systems that have already been implemented have been a tremendous benefit to decision-makers.

Flow chart
Figure 6
Bridge Management System.

Bridge management systems, however, are still driven by the data that are collected about the condition of the bridges. No matter how sophisticated and elaborate the analysis and no matter how elegant the algorithms employed, in the final analysis, the recommended decisions cannot be any better than the data upon which they are based. Today, these data are based almost entirely upon a visual bridge inspection with condition evaluation determined by visible indications of deterioration and distress. Deterioration that does not manifest some visible symptom is not detected or quantified.

More accurate and more quantitative bridge condition data result in better decisions and more efficient and optimal allocation of bridge resources. NDE technology should be used to effectively and efficiently collect quantitative data about bridge conditions. This is especially true for certain types of hidden deterioration, such as corrosion of reinforcement in concrete or cumulative fatigue loading in steel bridges.

A portable coherent laser radar scanning system is deployed under a highway bridge. Using the computer controllable scanning system, bridge engineers are able to measure deflections of a bridge at hundreds of individual points in a few minutes.Laser Scanner

FHWA's NDE Research Program

Next is a summary of research and development projects, sponsored by FHWA's Office of Engineering Research and Development, in nondestructive evaluation for highway bridges.

Advanced Bridge Deck Inspection Technology
Development of Dual-Band Infrared Thermography Imaging System for Bridge Deck Inspection

This project adapts defense technology developed for buried land-mine detection to the quantitative inspection of bridge decks. Looking at bridge decks using two different infrared wavelengths simultaneously overcomes some of the operational problems (primarily surface emissivity variations) that have been experienced with the use of infrared thermography for the detection and quantification of delaminations on bridge decks. A first phase evaluation on test slabs demonstrated that dual-band infrared thermography could detect delaminations in both bare concrete and asphalt-covered concrete and that surface emissivity variations could be compensated for by the application of image-processing techniques. A fully operational, mobile infrared imaging system was delivered to FHWA in February 1996. It is currently undergoing field testing to more fully evaluate the benefits of dual-band infrared thermography on actual bridge decks.

This wireless global bridge monitoring system greatly facilitates the measurement of strains and deformations. Using digital radio communication technology eliminates the need for long cables.

Wireless global bridge monitoring system.

Ground-Penetrating Radar Imaging for Bridge Deck Inspection

This project will develop an engineering prototype of a new generation ground-penetrating radar for bridge deck inspection. The system will use impulse radar, synthetic aperture techniques, and sophisticated signal processing and imaging algorithms to image a 2-meter-wide portion of a bridge deck at one time. The goal is a system that will travel at traffic speeds, image a lane width of a bridge, and provide two- and three- dimensional images of the interior of the bridge deck. Using a small-scale prototype, preliminary tests have been able to provide images of the interior of a reinforced-concrete test bed; these images show test voids and reinforcement. A larger prototype system, capable of inspecting a two-meter-wide strip of bridge deck, is being assembled and is scheduled for completion in May 1997. Laboratory and field testing of this system will begin in the summer of 1997. A portable hand-held imaging system is also planned.

Advanced Bridge Testing and Health-Monitoring Projects
Global Bridge Measurement Using Coherent Laser Radar

This project is adapted from a system developed for the National Aeronautics and Space Administration. It is a portable laser-scanning system that quickly measures the deflected shape of a bridge with sub-millimeter accuracy. It also measures the vibration of the bridge and has the potential to facilitate the application of modal analysis (a branch of structural analysis that uses measurements of the vibrations of a bridge with respect to frequency, amplitude, shape, and decay with time to determine important structural properties of the bridge and to detect damage) as a bridge-inspection tool. The system was demonstrated in the field and was delivered to FHWA in February 1996. The system has been used to scan large test beams and substructure units, as well as to measure bridge deflection under controlled loading. It will undergo further test and evaluation through 1997.

Global Bridge Monitoring With Wireless Transponders

This project is developing a wireless bridge-monitoring system. It consists of a number of sensor-transponder modules that communicate via spread-spectrum radio to a local controller. Some modules measure strain and rotation. To minimize cost and technical risk, the system development emphasizes the use of off-the-shelf components developed for the cellular telephone and for automotive applications. The goal is to develop technology that will make it possible to instrument a bridge at a dozen locations for a cost of less than $5,000. A prototype system, consisting of a master controller and four transponders, has been delivered to FHWA. FHWA plans to purchase an additional master controller and 12 additional transponders. The system will undergo field and laboratory testing. Further development depends on gaining the sponsorship of at least 10 states in a pooled-fund study.

Bridge Deflection Measurement Using Precision Differential Global Positioning System

The TRIP steel sensor unit (in foreground) is attached to a bridge, and the maximum strain experienced by the bridge is permanently recorded by a change in the magnetic character of the steel. The sensor is read by the portable instrumentation unit shown in the background.

TRIP steel sensor

An innovative approach to monitoring and measuring large bridges is being developed under another contract, which was initiated in October 1995. This project makes the transition from a proven concept into an operational system prototype. Research sponsored by the Louisiana and Texas departments of transportation demonstrated the feasibility of a system to provide a cost-effective means of performing structural deformation surveys. The objectives are to produce a system that can be easily configured, installed, and affordably operated by state and local authorities. The resolution of the system (sub-centimeter) limits its application to large bridges. The system has been tested on the Fred Hartman Bridge over the Houston Ship Channel. (See related article in Public Roads, Spring 1997 issue, page 39.)

Bridge Overload Measurement and Monitoring Using TRIP Steel Sensors

A major contributing factor to the deterioration of the nation's bridges is overload. These overloads are caused by heavy trucks and earthquakes. To make the most efficient use of bridge inspection resources, it would be very helpful to have a passive device that could detect and measure the maximum load experienced by a bridge. A contract to develop such a system (to continue work initially sponsored by the Georgia Department of Transportation) was awarded in November 1995. The system is based upon the use of transformation-induced plasticity (TRIP) steel sensors. TRIP steel is a special steel with a special chemical formulation, and it undergoes a permanent change in crystal structure in proportion to peak strain. It changes from a non-magnetic to a magnetic steel. The change can be easily measured. This project will improve the design and development of these peak strain sensors and will test their performance on instrumented bridges. These sensors could provide a reliable, inexpensive, and easily implemented means for quantitative bridge assessment as a key element of a comprehensive bridge management system.

Advanced Fatigue-Crack Detection and Evaluation Projects
NUMAC

The New Ultrasonic and Magnetic Analyzer for Cracks (NUMAC), a new fatigue-crack detection system combining ultrasonic and magnetic inspection capabilities into a single instrument, has been successfully developed, demonstrated, and delivered to FHWA. This system consists of a backpack computer and a head-up display; it features one-hand operation, which is essential for use on a bridge. This system will greatly improve our capability to detect and quantify fatigue cracks in steel bridges even though they may be covered with paint. The prototype system, received by FHWA, has been loaned to the Colorado and Delaware departments of transportation for evaluation.

Thermographic Imaging to Detect and Quantify Fatigue Cracks in Steel Highway Bridges

This Small Business Innovative Research (SBIR) project was initiated in October 1995. It is based on the use of commercially available high-resolution thermographic imaging systems to detect surface-breaking fatigue cracks. The method, called forced diffusion thermography, uses active heating of the bridge surface with a high-wattage light to detect cracks. A special pattern of hot and cold regions is created on the steel bridge, and the thermographic imaging system presents the operator with an image of heat flow patterns. If a crack is present, a characteristic pattern is observed. A six-month, first phase study proved the concept by demonstrating the ability to detect paint-covered fatigue cracks. A second phase has been initiated to develop a fieldable and commercially viable system.

Acoustic Emission Monitor for Bridges

FHWA solicited a cooperative agreement in 1995 to work with industry to co-sponsor the development of an acoustic emission monitoring system that was specifically engineered and packaged to meet the need to monitor a fatigue crack on an in-service highway bridge. FHWA and Physical Acoustics Corp. are sharing the cost of developing this system. The new system will be small, rugged, and battery-powered, and it can be left in place for unattended monitoring for up to one week. It is important to note that this acoustic emission (AE) system could also be used to determine the effectiveness of a fatigue crack retrofit. This portable AE system will be very useful for monitoring and evaluating fatigue cracks.

Wireless Strain Measurement System

One of the impediments to the measurement of fatigue loading is the need to install a strain gauge near the fatigue crack and then to monitor the random variable amplitude strains for a period long enough to capture the loading spectrum. With traditional strain gauges, this is difficult because of the need to get to the locations where fatigue cracks typically form and the need to run long wires back to a data acquisition system. A portable, rugged, yet accurate system for measuring strains at inaccessible locations is needed.

Highway bridges present severe constraints in terms of access and power. NUMAC provides a portable, yet powerful, inspection system for the detection of fatigue cracks on steel bridges. The unit is designed to allow the user to concentrate on the inspecition task by eliminating most of the burdens associated with bringing sophisticated computer-based inspection systems to the field.

NUMAC SystemThe Small Business Innovative Research Program sponsored the development of a wireless strain measurement system. This highly innovative system consists of rugged, battery-powered (solar cells are optional) radio transponder modules. These modules are able to accept up to four standard resistive strain gauges with all power and signal conditioning provided by the transponder. The system features 16-bit analog-to-digital conversion and an effective 500-hertz (cycles per second) sampling rate. Up to 10 of these transponders can be used simultaneously. They can be configured to form local radio telemetry networks with extensive data error checking and multipath redundancy for very stable and accurate wireless data transmission. A local transponder is attached to a personal computer for data acquisition. This system should greatly facilitate the field measurement of fatigue loads.

Passive Fatigue Load Measurement Device

The wireless strain measurement system is an excellent tool, but it is somewhat expensive (about $6,000 per transponder) and has a limited battery life. A totally passive and inexpensive device for the measurement of fatigue loading is needed. Under a new contract, initiated in October 1995, a low-cost, passive device to measure fatigue load will be developed. It is based on the use of two, precracked fatigue coupons (strips of aluminum) that strain along with the bridge. The cracks in the two coupons, made of different grades of aluminum, grow at different rates. Special gauges are attached to the coupons to accurately measure the lengths of the cracks. The measurement is made by plugging a crack-length reader into the device. It is possible to measure fatigue loading by measuring how much the cracks have grown.

Fatigue Load Measurement Using Electromagnetic Acoustic Transducers

As part of a congressionally mandated study with the Constructed Facilities Center at West Virginia University, a device using an innovative technology to measure the cumulative fatigue loading of a typical highway bridge is under development. This device has electromagnetic acoustic transducers that use electromagnetic fields to generate and detect high-frequency stress waves in steel. The system can measure the strain in steel members by detecting the change in travel time of stress waves. The advantages of this system are that it attaches magnetically to the steel bridge with very little surface preparation and that dynamic stress measurements can be taken quickly. In a separate, but concurrent, developmental effort, Sonic Force Corp. produced a commercial product based on this technology, and further development of this system by FHWA is not anticipated.

Eddy Current Detection of Weld Cracks

The goal of this project is to develop a method to detect cracks in weld metal through bridge coatings. The eddy current method uses induced magnetic fields to inspect the surface of conductive materials, such as steel or aluminum. Traditional applications of this method are not effective in weld metal due to the wide variation in magnetic material properties. This project is developing and testing a method using a differential probe that suppresses these variations in material properties, allowing the detection of cracks in the weld crown and at the weld toe. The method has been shown to be effective through both conductive and nonconductive coatings, including zinc-based primers and lead paint. Current research is investigating the correlation between crack signals and crack depth.

Crack Detection Using ACFM

The Alternating Current Field Measurement (ACFM) method, which is related to the eddy current method, is also being evaluated. ACFM was originally developed for the offshore oil and gas industries, where crack detection methods capable of penetrating up to 5 mm of coating are required. The method uses an induced magnetic field and a unique probe detection scheme to detect and quantify longitudinal cracks at the weld toe. The method is sensitive in a variety of conductive materials, including steel and aluminum, and can penetrate typical bridge coatings. Current research is aimed at determining the accuracy of the crack depth and length measurements and exploring how the method may be used for the detection of cracks in fillet welds on light poles and sign supports.

Ultrasonic Time-of-Flight Diffraction

One of the most commonly used inspection techniques for steel structures is pulse-echo ultrasonics. A sound beam is induced in the material being inspected, and reflections of that beam are interpreted to determine the location and size of defects. FHWA is currently investigating a method know as time-of-flight diffraction, which uses a pitch-catch transducer configuration to detect cracks and determine the crack depth with a high degree of accuracy. The goal of this project is to develop a method for easily imaging crack profiles in the field, using a specially designed scanner assembly. This tool will be used to determine the severity of cracks in steel bridges.

Advanced Corrosion Detection and Evaluation Projects
Magnetic Flux Leakage Inspection System for Bridge Cables

This project is the continuation of the development of a specialized inspection system for bridge cables. A prototype system using an array of shuntable, permanent magnets has been designed and fabricated. This system detects the changes in the strong magnetic field that is set up by the magnets; these changes occur if a broken or corroded cable is present. Using similar technology, commercially available systems can inspect smaller cables, but typical bridge stay cables are too large. The system will be upgraded and developed into a dedicated cable-stay inspection system.

Impact-Echo System for Detection of Voids in Post-Tensioning Ducts

A recently completed project developed a device for the detection of voids in the grout on post-tensioned bridges. The long-term reliability and safety of these bridges depend on the integrity of the post-tensioning system. This device uses the impact-echo principle. A known energy pulse strikes a concrete surface, and the local response is measured using a piezoelectric transducer. The frequency and energy content of the response can be used to detect voids in grouted ducts. The system is smaller and better suited for use on vertical and irregular surfaces than another impact echo system designed primarily for bridge deck evaluation. The system is currently on loan to the Maine Department of Transportation.

Embedded Corrosion Microsensor

In cooperation with the Virginia Transportation Research Council, an embedded microsensor is being developed to quantitatively measure corrosion activity inside concrete. The integrated circuit in an embeddable package will provide electrochemical measurements of corrosion rate with polarization resistance and will measure chemical parameters such as acidity-alkalinity, chloride ion concentration, and temperature. The sensor will use wireless communications for power and to telemeter sensor data. The objective is to develop a small and inexpensive package that will allow hundreds or thousands of sensors to be embedded in concrete structures. It will then be possible to quickly scan a concrete structure and to quantitatively measure the rate and location of corrosion before visible deterioration has occurred. A prototype integrated circuit has been fabricated, and further development is ongoing.

A new thermographic imaging system produced this image of a 1-centimeter-long fatigue crack in a steel specimen. The remote imaging system was able to detect this crack from a distance of more that 60 centimeters. The crack was not detected by visual inspection because it was covered by several layers of paint.

Thermographic Imaging

Magnetic-Based System for NDE of Prestressing Steel in Prestressed Concrete

This project will develop a portable and versatile inspection tool for the detection and quantification of corrosion and strand breakage in prestressed concrete. It is similar to the magnetic flux leakage inspection system for bridge cables, and it also uses wireless communications. The system is built around a magnetic scanning head. The scanning head includes a strong permanent magnet; a Hall-effect sensor array, which detects changes in magnetic field strength; a position encoder; and a wireless communications unit. This portable, self-contained system will be used to scan prestressed concrete girders and beams and to telemeter the information back to a portable computer for signal processing and analysis. The results can be displayed as an image for rapid anomaly identification. It might also be useful for inspecting decks, columns, and abutments.

Other Advanced Nondestructive Evaluation Projects
Bridge Substructure Evaluation Using Forced Vibration Response

Substructure deterioration is a major reason for structural deficiency of bridges. There is also a pressing need to evaluate substructures for scour vulnerability and for post-earthquake evaluation. An innovative approach for quantitative substructure evaluation will be tested under a contract awarded in November 1995. This project will use measured structural movements caused by induced vibrations to determine the condition of the substructure. The response will enable engineers to determine the presence of piles and to establish a base line for subsequent evaluations. This technology could help evaluate the scour vulnerability of the approximately 100,000 bridges with unknown foundations.

An array of eight magnetic sensor modules is being used to inspect a 25-centimeter-diameter cable for broken wires. The magnetic flux leakage inspection system uses very strong permanent magnets to induce a magnetic field in the cable. As the array is moved along the cable, magnetic sensors detect characteristic patterns in the magnetic field if a wire is broken.

Sensor Modules

Cable-Stay Force Measurement Using Laser Vibrometers

Dynamic analysis will also be the basis for a new approach to the quantitative measurement of the forces in stay cables. This innovative approach will use non-contact laser vibrometers, which are commercially available, to provide a rapid, low-cost, yet accurate method for force measurement. The forces in the stay cables are an excellent indicator of overall structural health for these types of bridges. The concept was tested on the Stubenville Bridge in West Virginia in October 1996. The coherent laser-radar system being developed in a separate project could also be used to perform dynamic cable-stay measurements.

Integration of Quantitative Nondestructive Evaluation Methods into Bridge Management Systems

This first-of-its-kind study will investigate how to develop a unified quantitative methodology for the integration of nondestructive bridge evaluation into bridge management systems. The study will establish relevant measures of damage for bridge components; it will establish formal links between the results of NDE measurements and condition states; and it will develop a methodology for NDE-assisted bridge inspections. The methodology and procedure will be demonstrated in field inspections on at least 12 highway bridges in six states. Deliverables from this contract will include complete damage descriptions of all commonly recognized elements, a complete basis for the translation of NDE measurements to condition states, and guidance in the application of NDE-assisted inspections for highway bridges. This will be an ambitious step for the improvement of bridge management in the next century.

Improved Bridge Deck Condition-State Descriptions Using Quantitative Nondestructive Evaluation Methods

This study builds on the results of previous research (sponsored by the Strategic Highway Research Program) in measuring and predicting the condition of concrete bridge decks. This study will develop methodologies to improve bridge deck deterioration models and to incorporate preventative treatments into bridge management systems, and the study will develop criteria for using nondestructive testing for these purposes.

This integrated circuit contains all the components necessary to measure corrosion rates and replaces a bench-top full of dedicated instruments. This circuit will be integrated with wireless power and wireless communications circuits to provide a totally embeddable corrosion sensor.

Integrated Circuit

Exploratory Research Projects
Fundamental Research in Acoustic Emission

FHWA is also funding exploratory research at the National Institute of Standards and Technology in Boulder, Colo. This led to the development of an improved, wide-band AE detector, which will soon be available commercially; a unique AE laboratory test system that provides the ability to generate and detect fatigue cracks in different extension modes; and advanced finite element modeling of acoustic emission generation, propagation, and detection.

Fundamental Magnetostrictive Sensor Research

FHWA is funding fundamental research in the development of sensors based upon magnetostriction. A magnetic field produces a small change in the physical dimensions of ferromagnetic materials. By coupling a coil of wire with a bias magnet, a useful sensor can be constructed. Such senors would be low-cost, simple, and rugged. Possible applications include detection and measurement of corrosion and breakage in prestressing strands, monitoring the curing of concrete, and as embedded acoustic emission sensors.

Use of Microwaves to Detect and Quantify Fatigue Cracks

Another topic of FHWA-funded exploratory research is the development of small microwave waveguide sensors. If a microwave waveguide is placed against a steel plate, it is effectively short-circuited, and a characteristic standing wave is created. This standing wave can be detected with a very inexpensive diode. If a fatigue crack is present in the plate, the standing wave changes. A very rapid, yet low-cost, fatigue-crack detector can be produced. Current studies focus on the evaluation of lift-off (the distance from the surface being tested to the sensor) and the detection of crack edges. A paint coating acts as a spacer between the steel surface and the sensor; the thicker the paint, the greater the lift-off, and the less sensitive the sensor becomes.

Fiber-Optic Strain Sensor

FHWA and the Naval Research Laboratory are sponsoring development of a fiber-optic strain sensor. This project has demonstrated that it is feasible to use the Bragg grating interferometric method, which measures the changes in the frequency of reflected light, to measure strains in concrete bridge beams. A prototype system that could measure strain at up to 16 locations simultaneously is being developed.

Laboratory Support Contract to Assist in Testing, Evaluating, and Validating NDE Technologies
Laboratory Support Contract for Test and Evaluation of New Technologies

As can be seen from the wide array of new technologies being studied and under development, the testing and evaluation of these systems will be a big job. Recognizing this, FHWA awarded a contract in October 1995 to provide technical, professional, and logistical support to the Special Projects and Engineering Division for the testing and evaluation of these new systems.

Nondestructive Evaluation Validation Center

In addition to the testing and evaluation that will be conducted in accordance with the laboratory support contract, FHWA needed better facilities and capabilities to evaluate and validate the new nondestructive evaluation technologies and systems being developed by FHWA and others. Special funding was provided in fiscal year 1996 to support the design and construction of a new Nondestructive Evaluation Validation Center at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va. A contract was awarded to Wiss, Janey, Elstner Associates Inc. in September 1996 to design, construct, and operate this center. The center will renovate the existing small-structures laboratory at TFHRC to provide a modern and fully equipped NDE testing facility. In addition, three highway bridges within 250 kilometers of TFHRC will be made available for full- scale testing of NDE technologies under actual field conditions. The center will also acquire a wide variety of specimens from highway bridges that contain typical defects, which will be fully characterized. "Fully characterized" means that all aspects and properties of the bridge have been measured and quantified to the maximum extent possible. These specimens, ranging from steel elements with fatigue cracks to full- scale girders and decks, will be maintained in a library of specimens, which will be used by FHWA and other researchers and developers to test and validate existing and new NDE technologies. One of the first studies to be undertaken once the center is operational is a probability of detection study of visual inspection of highway bridges.

Conclusion

FHWA's program of research and development in nondestructive evaluation is helping to ensure the safety of the nation's highway bridges. The program is also providing more accurate and complete information about the condition of the nation's highway bridges. Many of the research and development projects produce prototypes of new inspection systems. Much of FHWA's future activities in the NDE program will involve extensive testing of these prototypes, both in the field and at the new NDE Validation Center. We plan to work with our customers, bridge owners throughout the United States, to conduct much of this testing. These tests and experiments will fully assess and validate the capabilities of new and existing inspection systems and methods, define the limits of the technologies, identify any needed iterative development, and define directions for future research. This approach ensures that follow-on implementation and demonstration projects are a logical continuation of the technology development process and involve technologies that are fully understood and are ready for widespread deployment.

For further information about FHWA's NDE research and development program, please contact one of the following: (1) NDE R&D Program Manager Dr. Steven B. Chase; phone (703) 285-2442, fax (703) 285-2766, or e-mail steve.chase@fhwa.dot.gov; (2) NDE Exploratory Research Program Manager Dr. Richard Livingston; phone (703) 285-2903, fax (703) 285-2766, or e-mail dick.livingston@fhwa.dot.gov; or (3) NDE Special Projects Manager Glenn Washer; phone (703) 285-2388 or e-mail glenn.washer@fhwa.dot.gov.

Dr. Steven B. Chase is a research structural engineer in the Structures Division of FHWA's Office of Engineering Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. He is the program manager for FHWA's Nondestructive Evaluation Research and Development Program.

He joined FHWA in 1978 as a highway engineer trainee with a bachelor's degree in civil engineering. He earned both his master's degree in civil engineering in 1984 and a doctorate in civil and environmental engineering in 1991 through the FHWA Academic Study Program. He worked as an FHWA bridge engineer in Washington, D.C.; in FHWA's regional Office of Structures in Fort Worth, Texas; and in the division offices in Massachusetts and Rhode Island. He joined the staff of the Structures Division in 1992.

Glenn Washer is a research engineer in the Special Projects and Engineering Division of FHWA's Office of Engineering R&D at TFHRC. He is the special projects manager for FHWA's NDE R&D Program. He has a bachelor's degree in civil engineering from Worcester Polytechnic Institute and a master's degree in civil engineering from the University of Maryland. He is a licensed professional engineer in Virginia.

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