Recent article from Public Roads
Developing NDE Technologies for Infrastructure Assessment
by Glenn A. Washer
This article provides an overview of the Federal Highway Administration's (FHWA's)
program for developing nondestructive evaluation (NDE) technologies for the inspection and evaluation of highway infrastructure.
This program is designed to address several key goals in FHWA's National Strategic Plan.
The article discusses: (1) how new laser technology can help attain mobility goals by reducing the number of structurally deficient
bridges, (2) new bridge deck evaluation technologies that help achieve both productivity and mobility goals by reducing traffic delays
and reducing the cost of maintenance repairs, (3) new technologies that can assist in the evaluation of bridge condition to ensure
safety and promote efficient maintenance strategies, (4) an innovative new study that will determine the reliability of existing bridge
inspection procedures, and (5) new technologies for characterizing highway building materials.
Background
The tragic collapse of the Silver Bridge on Dec. 15, 1967, resulted in the deaths of 46 people. This bridge collapse provided a catalyst
for the development of the National Bridge Inspection Standards (NBIS), adopted in April 1971. This standard outlines required qualifications
of bridge inspectors, defines the scope of bridge inspection programs, and provides for standardized methods of evaluation and
appraisal of bridge condition. Since the inception of NBIS, the periodic inspection of highway bridges has relied largely on visual
inspection to provide critical information on the condition and safety of our nation's bridges. During this period, advances in NDE technologies
have improved the tools available for inspection; however, few technologies have been widely implemented as part of routine inspection
procedures.
The emerging use by state transportation agencies of bridge management systems (BMS) to assist in the evaluation of the bridge
inventory requires more detailed and quantitative information on the condition of bridges. To make these new BMS tools most
effective, increased use of quantitative NDE technologies is required in the bridge inspection process.
The National Bridge Inventory (NBI) contains more than 25 years of information on the bridge population in the United States, and it
provides useful information on the needs of the inventory. NBI indicates that more than 104,000 bridges in the United States are rated
as structurally deficient.1 This means that the deck, substructure, or superstructure has been rated as poor or worse or that the bridge
has been determined to have a low load-carrying capacity. With such a significant number of bridges in poor condition, the development
of effective tools for inspecting and evaluating bridges is clearly urgent.
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Debris from the collapse of the
Silver Bridge on Dec. 15, 1967. |
Effective NDE technologies can impact the bridge inventory in two important
ways. By detecting deterioration in its early stages, new technologies can help ensure the safety of highway bridges. Second, these
technologies can assist in the management of the bridge system by accurately determining maintenance and repair requirements.
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A bridge inspector performs an inspection of a
steel girder bridge as part of the visual inspection study.
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The goal of FHWA's NDE program is to improve the state of the practice
for infrastructure inspection. This is accomplished by determining the reliability of existing NDE technologies and by developing
new tools to solve specific problems that are critical to maintaining safety and managing resources for the bridge inventory and
other infrastructure resources. To make this happen, FHWA established the Nondestructive Validation Center (FHWA NDE Center) at the
Turner-Fairbank Highway Research Center in McLean, Va.
FHWA NDE Center is focused on bringing innovative, state-of-the-art technologies to bear on the most critical problems affecting the nation's
infrastructure. The center works closely with the states and federal land management agencies to provide research and technology
innovations that help assess, maintain, and improve the condition of highway infrastructure.
FHWA NDE Validation Center
The objective of FHWA NDE Center is to improve the state of the practice for highway bridge inspection. The center is designed to act as a
resource for state transportation agencies, industry, and academia concerned with the development and testing of innovative NDE
technologies. FHWA NDE Center provides state highway agencies with independent evaluation and validation of NDE technologies,
develops new NDE technologies, and provides technical assistance to states exploring the use of these advanced technologies.
The center comprises three elements: the NDE laboratories; component
specimens, which are sections of bridges containing defects; and field-test bridges.
The NDE laboratories are the nucleus of FHWA NDE Center, providing a facility for the development and testing of NDE
technologies. The laboratories include a structural loading floor for constructing mock-ups of field conditions, a radiological laboratory
used for creating X-ray images of defects, a computed tomography facility for characterizing materials, and an instrumentation laboratory
used for manufacturing prototypes and developing new NDE tools.
The component specimens provide a realistic test bed for the development of new NDE technologies. Component
specimens at FHWA NDE Center include sections of bridge deck containing delaminations, welded details containing cracks, cracked bridge
pins, prestressed box beams containing corroded and broken strands, cracked sign supports, and other specimens with characteristic
forms of deterioration.
Five decommissioned highway bridges are used to evaluate NDE methods under realistic environmental conditions. Two steel bridges
that are open to traffic and fully instrumented are used to test NDE methods associated with live loading, such as new instrumentation
for global bridge monitoring. These bridges are critical to evaluating the effect of restricted access, structure geometry, surface conditions,
platform stability, and human factors on the application of NDE methods during normal bridge inspections. These test
bridges provide FHWA NDE Center with a unique ability to evaluate NDE technologies under the same conditions that normal bridge inspections
are typically conducted, providing a powerful tool in the evaluation process.
Visual Inspection Study
Normal inspection practices for highway bridges rely almost entirely on visual inspection to evaluate the condition of the bridge. Since
NBIS was adopted in 1971, a comprehensive, national study to determine the reliability of the inspection process has not been conducted.
To fulfill this need and to provide a baseline for the evaluation of other NDE technologies, FHWA NDE Center initiated a study of the visual inspection
of highway bridges in 1998.
This study began with a survey of state transportation agencies concerning their practices and policies regarding bridge inspection.
Forty-two states responded to the survey, which asked questions about the training and qualifications of inspectors, management
of the bridge inspection program, and usage of NDE techniques. Detailed results of this survey will be published in 2000.
A performance evaluation was conducted to determine the reliability of bridge inspections. This part of the study had teams of bridge
inspectors from around the country perform inspections on FHWA NDE Center's test bridges. Twenty-five states sent a team of two inspectors
to participate in the study. As part of the study, the inspectors were required to inspect seven highway bridges of varying design and
condition. Ten different inspection scenarios were used to evaluate the various approaches used by state transportation agencies to
inspect bridges. Human factors, such as training, experience, and attitude, were evaluated through a series of questionnaires
administered by the FHWA NDE Center staff prior to and during the performance of the inspections.
This portion of the study was completed in October 1999, and the detailed results will be available in 2000.
New Technologies for Bridge Assessment
HERMES Ground-Penetrating Radar System
NBI indicates that there are almost 298 million square meters (more than 3.2 billion square feet) of bridge deck in the United States.
The majority of these decks consist of reinforced concrete that provides the driving surface for the bridge. The service life of a deck
can be much shorter than the substructure and superstructure of a bridge, and estimates indicate that FHWA alone is currently investing
as much as $1 billion annually for deck rehabilitation.2
Bridge decks deteriorate due to corrosion of reinforcing steel and the
resulting delaminations and spalling that can make a deck structurally deficient. The ability to detect this deterioration in its early
stages is critical in directing repairs to the most at-risk bridges and will help optimize the use of limited funds.
Currently available methods for evaluating bridge decks include inspecting the deck condition visually, sounding a bare deck
with a chain or hammer, measuring the half-cell potential of the deck, and taking cores. All these methods may require lane closure
and have limited ability to determine the internal condition of the deck over the entire deck area. In addition, these methods are not
effective in accurately determining the exact location and extent of delaminations in a bridge deck, and they are difficult to apply rapidly
to a large number of bridge decks.
Other technologies, such as the infrared thermography and existing ground-penetrating
radar (GPR) systems, are also used for the evaluation of bridge decks. These technologies have not satisfied the need for rapid,
quantitative bridge deck assessment. Infrared thermography is limited by environmental conditions and has difficulty evaluating decks
with asphalt overlays. Existing GPR systems require significant expert analysis to effectively evaluate deck condition, and they have
had difficulty providing fast and reliable results that satisfy the needs of state highway agencies.
To address these needs, FHWA has funded the development of HERMES (High-Speed
Electromagnetic Roadway Measurement and Evaluation System) by the Lawrence Livermore National Laboratories (LLNL). The goal
of the HERMES project is to develop a GPR system that can reliably detect, quantify, and image delaminations in bridge decks.
The system is designed to operate at normal highway speeds, eliminating the need for lane closure.
The HERMES system includes a computer workstation and storage device, survey wheel, control electronics, and an array of 64
antenna modules or transceivers mounted in a towable trailer.
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Scientists at FHWA NDE Center are field-testing the HERMES ground-penetrating
radar system, designed to evaluate bridge decks.
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The most unique design feature of HERMES is the antenna array. The
arrangement of the transceivers gives samples across a two-meter width of the deck at three-centimeter intervals at a speed of
about 32 kilometers per hour. At almost 100 kilometers per hour, the intervals are six centimeters. The density of data enables
synthetic aperture radar techniques to be used in the processing of the data, and two- and three-dimensional images can be
produced. HERMES uses ultra-wide-band microwave sources developed by LLNL that produce signals with a frequency of 0.5 to
5 GHz.
The prototype system was delivered to FHWA in October 1998. A field testing of this system is currently being conducted in
cooperation with state transportation agencies. The field-testing program will fully evaluate the prototype system and identify required
improvements for a second-generation system.
As part of the HERMES project, a single-antenna scanning device capable of performing high-resolution scanning was constructed
by LLNL. Known as PERES (Precision Electromagnetic Roadway Evaluation System), this system is used for laboratory evaluation
of radar performance and for the development of image-processing algorithms. The system has been used at FHWA NDE Center for developing
algorithms and to evaluate potential uses for this innovative technology.
Laser Bridge-Deflection Measurements
Of the approximately 104,000 structurally deficient bridges, more than 21,000 bridges are classified this way due solely to a low
structural appraisal rating - that is, the bridges have a low load-carrying capacity.2
However, this load-carrying capacity is normally determined by theoretical calculation, it may not accurately reflect the true capacity
of the bridge.
This has led to the development of guidelines for load-rating bridges experimentally, a process that can be expensive and
time-consuming. Technologies that can reduce the time and cost associated with load-rating structures are critical to improving
this process. Through the effective use of accurate load-rating procedures, the actual load-carrying capacity of a bridge can be
determined, reducing the number of bridges classified as structurally deficient and increasing the accuracy of load posting.
Applications for a laser bridge-deflection system have been developed that can reduce the cost of the load-ratings. The laser
system, developed under contract for FHWA, uses a frequency-modulated laser to measure bridge deflection from a range of up to
30 meters. A computer-controlled scanning system controls the laser and allows the system to scan a large area of a structure.
Measurement resolutions of less than one millimeter are possible with the system, and no special target or surface preparation
is required.
FHWA NDE Center has developed a series of applications for this technology for making measurements of deflections for large structures.
Figure 2 demonstrates one such application. This figure shows the deflected shape of all seven beams of a bridge loaded by a
truck in the breakdown lane. The measurement of all seven girders is obtained from a single location under the bridge without the
need to mount targets on the structure. This helps reduce the cost of bridge load-rating, and it provides More Information than
is typically available from traditional instrumentation. With a range of 30 meters, the laser system is also capable of measuring
deflections of bridges crossing above open traffic lanes, eliminating the need for lane closure.
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Figure 2 - Deflection of a bridge, as measured by a
scanning laser system,
shows the load distribution for a
truck located along the shoulder of the roadway.
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Other applications of this technology that have been developed at FHWA NDE Center
include deflection measurements of large bridge piers under load and measuring out-of-plane displacements for steel plate girders.
Stress-Measurement Technologies
In the evaluation of highway bridges, it is critical to determine the distribution of loading and to evaluate the stress levels in load-carrying
members of the bridge. This is typically accomplished through the use of foil strain gauges. However, these gauges require some
surface preparation to install and are unable to measure the distribution of dead load within the structure. FHWA NDE Center is currently developing
instrumentation and applications for the ultrasonic measurement of stress in bridge members.
The first method being developed involves the use of ultrasonic birefringence to evaluate the state of stress in a steel bridge
member. A rotating transducer that launches polarized shear waves (which are confined to one plane) is used to measure the
magnitude of principal stresses. Applications of this technology for the evaluation of stress in hanger connection plates have been
previously developed.3 Current studies are focused on evaluating the stress level in the main members and lateral bracing
systems.
Also being developed are methods for evaluating the stress level in high-strength steel strand used in pre-stressed and post-tensioned
concrete applications. Strands are used in these applications to apply compressive loading in the tensile area of bridge beams,
increasing the load-carrying capacity of the member. Loss of pre-stressing force in these strands can reduce that load-carrying capacity.
The ultrasonic method being developed measures guided wave velocity to determine the force carried in the strands.
FHWA NDE Center was also involved with the assessment of the Barkhausen Noise Analysis for determining the stress carried in a
structural member, and the center assisted the state of Virginia by providing training in this and other technologies.
Radiography
FHWA NDE Center has a nuclear instrumentation laboratory,
which is used to develop and evaluate nuclear NDE techniques for highway applications.
The nucleus of the laboratory is a state-of-the-art X-ray computed tomography (CT) system. The CT imaging system consists of
dual-focus 420-kilovolt and microfocus 160-kilovolt continuous X-ray sources. The system can benefit many industrial and scientific
applications, including materials research, nondestructive testing, core-sample characterization, weld inspection, and failure analysis.
Projects carried out at the laboratory include the evaluation of available radiographic systems for the detection of broken wires
in cable-stayed bridges, imaging of post-tensioning strands in concrete beams, and the detection of voids in the grouted post-tensioning
ducts.
Projects were conducted to evaluate the use of tomographic imaging for the evaluation of air entrainment and air-void distribution
in concrete cores taken from highway bridges. A three-dimensional rendering of a concrete core is shown in figure 3. Imaging of crack
propagation caused by chemical attack in concrete cores was also demonstrated.
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Figure 3 - A three dimensional rendering of a concrete core,
showing aggregate, cement paste, and air voids.
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Another project conducted in the laboratory was the design, fabrication, and evaluation of a portable system for nondestructive determination
of chloride concentration in reinforced concrete structures.4 This system identifies areas susceptible to accelerated
corrosion.
Epithermal neutron detectors for nondestructive measurement of concrete hydration are also being developed to monitor concrete-curing
processes.5
Fatigue-Crack Detection
Since the time of the Silver Bridge collapse, FHWA has had an ongoing interest in the development of more effective tools for the detection
of fatigue cracks in steel bridges. Recent work at FHWA NDE Center has focused on developing methods for the application of electromagnetic
crack-detection systems. This has included studies of the eddy current method and alternating current field measurement for detecting
cracking in the area of welded connections. The primary advantage of these technologies is the ability to "see" through paint
with minimal surface preparation required.
These innovative crack-detection technologies have been fieldtested in locations around the country to provide technical assistance
to state highway agencies exploring new methods for detecting cracks. Field tests were conducted for agencies in Alaska, the District
of Columbia, Georgia, and Virginia and for the Eastern Federal Lands Highway Office.
FHWA also funded the development of a coating-tolerant, forced-diffusion thermography system for the detection of cracks in steel structures.
The goal of the system being developed is to provide a bridge inspector with a full field representation of critical details with characteristic
patterns indicating the presence of a crack. The project, funded under the Small Business Innovation Research (SBIR) Program, is scheduled
to be completed later this year. Field testing at FHWA NDE Center will be used to determine the capability of the system.
Partnerships
FHWA NDE Center has developed partnerships with state highway agencies, other government agencies, and other teams at the Turner-Fairbank Highway
Research Center (TFHRC) to develop NDE technologies and improve the state of the practice.
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At FHWA NDE Center, engineers from state departments of
transportation are trained on the use of advanced NDE technologies.
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HERMES II Project
The HERMES system described earlier in this article has garnered attention from many sources. Caltrans (California Department of Transportation)
is convinced that this new technology has the potential to provide a cost-savings tool that can change the way bridge decks are evaluated.
However, further development of this prototype system is required, and the participation of state highway agencies is essential. Caltrans has
taken the lead in sponsoring a pooled-fund study to develop a comprehensive workplan for the development of a second-generation prototype.
As part of that effort, FHWA NDE Center has moved much of its experimental test program to states across the country. Field tests conducted
with the cooperation of state highway agencies in Colorado, Minnesota, Missouri, New Jersey, Pennsylvania, Tennessee, and Virginia
improved the evaluation process and allowed more than 200 state and local transportation officials to witness the system in action.
At a meeting of the participating states in January 2000, the groundwork for the development of the next generation of HERMES technology will be laid out.
Box-Beam Testing
FHWA NDE Center has been leading a cooperative study with the state of New York to perform full-scale structural testing of five pre-stressed box beams
that were removed from a bridge in upstate New York. The testing is determining the remaining strength of the girders and evaluating the
potential of composite wrapping systems to strengthen the girders back to their original load-carrying capacity.
Other teams at TFHRC are playing key roles in the project. The Geotechnical Team constructed full-scale abutments using
innovative construction techniques, and the High-Performance Materials Team is applying their expertise in full-scale structural testing to assist in the study. At
the conclusion of the study, a test bridge constructed from three of the girders will be left in place for the future development of NDE
systems that evaluate the condition of pre-stressed concrete beams.
Teaming
FHWA NDE Center is providing assistance and technical expertise to enhance the efforts of other programs at TFHRC. In conjunction with the Asphalt
Team, for example, new instrumentation is being developed to calibrate gyratory compactors that are used to characterize asphalt pavements.
The previously mentioned laser system is being used to measure the deflections of a full-scale curved-girder bridge being tested in the Structures
Laboratory at TFHRC. The unique capabilities of the laser system, coupled with processing algorithms developed at FHWA NDE Center, are being used
to measure the out-of-plane distortion of the curved girders during testing. The laser system presents a full-field view of the deflections during the testing, allowing engineers to detect
and quantify buckling at the early stages of failure.
FHWA NDE Center has also actively assisted the Federal Lands Highway Program by providing technical expertise to bridge inspectors who ensure the safety
of bridges in their jurisdictions. The assistance has included the use of thermography systems to evaluate the quality of new bridge decks,
demonstrating crack-detection methods for steel bridges and training inspectors on the use of instrumentation for load-rating bridges.
Teaming with LLNL, FHWA NDE Center demonstrated the use of a variety of NDE techniques for evaluating bridge pins. This study included the evaluation of cracks
in pins by ultrasonic and radiographic methods. LLNL imaged the cracked pins using computed tomography; FHWA NDE Center imaged the cracked pins using
ultrasonic and radiographic methods.
FHWA NDE Center provides technical assistance to state transportation agencies through its active field-testing program, which brings
innovative technologies and prototype systems to the field to be evaluated. The center has also been active in providing training and
technical expertise to the states, using a unique team of technical experts to provide a powerful resource. The staff of the center includes experts in the area of ultrasonics,
radiography, optics and lasers, instrumentation, crack detection, wireless data acquisition, bridge inspection methods, structural engineering,
and ground-penetrating radar.
Conclusion
The development of new NDE technologies for the inspection of highways and highway bridges will assist FHWA in attaining its stategic goals
by helping to effectively manage the highway system in the 21st century. While many NDE tools are available or are being developed for highway
application, few have been widely accepted for use in normal bridge inspections. The FHWA NDE Validation Center, through the development
and evaluation of NDE technologies, will hopefully broaden the use of NDE and improve the state of the practice for highway bridge inspection.
The work described in this article is being conducted by the staff of the FHWA NDE Validation
Center: Dr. Fassil Beshah, Dr. Paul Fuchs, Ben Graybeal, Dr. Brent Phares, Dr. Ali Rezaizadeh, Dennis Rolander, Dr. Mike Scott, and Dr. Habib Saleh.
References:
- E. Small and J. Cooper.
"Condition of the Nation's Highway Bridges: A Look at the Past,
Present and Future," TR News, Transportation Research Board,
Washington, D.C., January-February 1998, pp. 3-8.
- S. Chase. "Dynamics
and Field Testing of Bridges in the New Millennium: A Look Forward,"
white paper prepared for Transportation Research Board Committee A2C05,
Washington, D.C., January 1999.
- A.V. Clark, C.S. Hehman,
D. Gallagher, M. Lozev, and P. Fuchs. "Ultrasonic Measurement
of Stress in Pin and Hanger Connections," Structural Materials
Technology III, SPIE, Vol. 3400, eds. D. Medlock and D. Laffrey, San
Antonio, Texas, 1998, pp 176-187.
- R.A. Livingston and H.
Saleh. "Experimental Evaluation of a Portable Neutron-Based Gamma
Spectroscopy System for Elemental Analysis of Reinforced Concrete,"
Journal of Radioanalytical and Nuclear Chemistry, 1999 (in press).
- R.A. Livingston and H.
Saleh. "Development of an Epithermal Neutron Detector for Nondestructive
Measurement of Concrete Hydration," Nondestructive Characterization
of Materials VIII, pp. 535-540.
Glenn A. Washer is the
program manager of the Federal Highway Administration's Nondestructive Evaluation Validation Center at the Turner-Fairbank Highway Research Center
in McLean, Va. 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 currently a doctoral candidate at The Johns Hopkins University's Center for Nondestructive Evaluation. Washer
is a licensed professional engineer in Virginia.
