U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Vol. 62 No. 4 Date: Jan/Feb 1999|
Publication Number: Vol. 62 No. 4
Date: Jan/Feb 1999
The National Bridge Inspection Program (NBIP), established by the Federal Highway Act of 1968, requires the states to periodically inventory and inspect all highway structures on the federal aid system. Congress later expanded this mandatory inspection program to include all structures on public roads, including those not on the federal system. This congressional mandate is still one of the driving forces behind the massive inspection, rating, and inventory programs of state departments of transportation throughout the United States.
Regular inspection of the nation's bridges is a massive undertaking. There are about 583,000 bridges in the inventory, and in a report to Congress in May 1997, the Federal Highway Administration (FHWA) reported that more than 182,000 of these bridges - more than 31 percent of the bridges in the inventory - are either structurally deficient or functionally obsolete.
The principal method for inspecting bridges has been visual inspection. However, deterioration and distress that do not manifest some visible symptom are not detected or quantified.
"Unfortunately, many of the things an inspector needs to see are hidden under or within the structure, and we have no measure of the validity or reliability of those inspections," said Robert J. Betsold, former FHWA associate administrator for research and development.
Since the initiation of NBIP, FHWA and other organizations have invested much time, effort, and money toward the development and advancement of nondestructive evaluation (NDE) techniques and technologies. A number of NDE technologies - such as infrared thermographic imaging, ground-penetrating radar imaging, laser-radar scanning, acoustic emission monitors, electromagnetic acoustic transducers, embedded corrosion microsensors, and laser vibrometers - are being tested and used to effectively and efficiently collect quantitative data about bridge conditions.
However, many bridge inspectors do not have a clear understanding of the factors that affect the reliability and performance of these methods. It is clear that before NDE systems can be used with confidence, all factors affecting the reliability must be identified, and the systems must be validated considering these factors.
To accomplish this, FHWA established an NDE Validation Center (NDEVC). This center, which was initiated by congressional funding in 1996, is the only center in the world dedicated entirely to the evaluation and validation of NDE technologies for highway infrastructure.
"With new technology, some invented [at the NDE Validation Center] and some adapted from the [Department of] Defense programs and other sources, we can now do a better inspection job," Betsold said. "This [center] will allow us to: (1) determine the reliability of visual inspections, (2) verify the usefulness and reliability of new inspection methods and devices, and (3) teach others the value of NDE technologies.
"More than $4 billion in highway funds are spent each year on bridge construction and maintenance. Decisions on which bridges to renovate or reconstruct are too often based on incomplete information that comes from the visual inspections. The use of NDE technologies will improve the quality of the decision-making process and allow the highway community to make better investments of their highway pavement and bridge funds."
The grand opening ceremony for the center was held on Oct. 15, 1998, at the NDEVC Laboratory at the Turner-Fairbank Highway Research Center in McLean, Va. The laboratory and offices provide a modern and fully equipped NDE testing facility and serve as the headquarters of the center. The center will acquire a wide variety of specimens from highway bridges with typical defects. In addition, test bridges in Virginia and Pennsylvania have been made available for full-scale testing of NDE technologies under actual field conditions.
FHWA awarded a three-year contract to Wiss, Janney, Elstner Associates Inc. (WJE) to design, build, and operate the NDE Validation Center.
This article describes NDEVC's resources and goals in greater detail.
The NDEVC Laboratory contains 439 square meters (4,725 square feet) of newly renovated laboratory and office space for "probability of detection" investigations, capability trials, and other testing. In addition to the general testing and setup areas, the laboratory has various specialized testing areas. One principal testing area includes an 11.8-meter by 3.6-meter strong floor used to conduct tests on mock-ups and manufacture component specimens.
Essential to assessing the reliability of any NDE method is a performance evaluation under actual field conditions. Bridge inspection conditions are neither predictable nor easy to accurately reproduce in the laboratory. Additionally, any evaluation of an NDE system without including environmental factors will likely overestimate the reliability.
The field bridges provide a test bed for investigating the effect of the environment on NDE reliability and system performance. In essence, the field bridges are an extension of the NDEVC Laboratory, where additional variables affecting reliability can be effectively investigated. The test bridges provide a mechanism to assess the physical field suitability of NDE systems.
In a cooperative effort with the Virginia Department of Transportation, two in-service bridges near Washington, D.C., were selected to be part of NDEVC. These bridges have steel superstructures, multiple spans, less than 15 degrees skew, and were considered ideal locations to evaluate many NDE techniques. One bridge is heavily traveled with a large percentage of truck traffic. This structure is an ideal place to investigate NDE systems intended to locate and size fatigue cracking and other high load-cycle-initiated defects. The other bridge has an extremely low average daily traffic count (less than five vehicles per day) and will allow access to the structure for extended periods without causing major traffic problems.
In 1968, the Pennsylvania Turnpike Commission (PTC) abandoned an 18-kilometer section of the Pennsylvania Turnpike. This section has been preserved by PTC for highway-related research and is operated by PTC as the Safety Testing and Research (STAR) facility. STAR includes two tunnels, five bridges, and 18 kilometers of four-lane roadway.
Under a cooperative agreement with PTC, NDEVC is using STAR as part of its extended laboratory for evaluating highway-related NDE technologies. STAR bridges provide an excellent resource for evaluating the effectiveness of NDE techniques under field conditions. STAR bridges have many desirable qualities: (1) These bridge have conditions typical of the aging infrastructure. (2) The fact that they are decommissioned allows for structural modification of the bridges. (3) They allow complete access.
Component specimens are, in essence, small sections of typical bridge details. Component specimens allow NDE technologies to be tested in the laboratory to determine their capabilities. Testing of NDE techniques on component specimens is the first step in evaluating their reliability.
Among the component specimens currently collected by NDEVC are more than a dozen reinforced concrete deck sections, each approximately 1.8 meters by 3.8 meters. These panels have been removed from bridges undergoing rehabilitation and contain defects and deterioration typical of structurally deficient bridge decks.
In addition, NDEVC has an extensive collection of fatigue-crack specimens. These specimens contain a variety of defect types and sizes and can be used to investigate a wide range of NDE techniques. It should be pointed out that all specimens maintained by NDEVC are available for on-site use or for temporary loan to interested organizations.
FHWA's NDEVC is a national resource for the evaluation of existing and emerging NDE techniques. The resources of NDEVC are available to federal and state agencies, the academic community, and industry. NDEVC will perform critical evaluations of NDE technologies and will be a source of information and expertise.
The first task given to the NDEVC staff is to assess the effectiveness of visual inspection. To accomplish this, the factors that may affect the reliability of visual inspection will be identified and will then be evaluated through a series of tests by experienced bridge inspectors. The validation of the visual inspection method will provide an important baseline against which other NDE techniques can be compared.
To find more information about the NDE Validation Center, visit the NDEVC Web site or the NDEVC quarterly newsletter. The Web site URL is http://www.fhwa.dot.gov/publications/ndec/index.cfm. If you would like to receive the newsletter, e-mail Glenn Washer (Glenn.Washer@fhwa.dot.gov).
Dr. Brent M. Phares is a research engineer for Wiss, Janney, Elstner Associates Inc., a consultant to FHWA's Special Projects and Engineering Division. He works in the NDE Validation Center on the systematic and quantitative validation of NDE technologies related to bridges and other civil structures. Currently, Dr. Phares is working on the validation of visual inspection, a remote acoustic emission system, and a ground-penetrating radar system. He received his doctorate in structural engineering from Iowa State University.
Glenn Washer is a research structural engineer in the Special Projects and Engineering Division of FHWA's Office of Engineering Research and Development (R&D) at the Turner-Fairbank Highway Research Center in McLean, Va. He is the special projects manager for FHWA's Nondestructive Evaluation R&D Program. He has a master's degree from the University of Maryland, and he is a licensed professional engineer.
Mark Moore is a senior consultant for Wiss, Janney, Elstner Associates Inc. and serves as the program manager for the NDE Validation Center project for FHWA. He has more than 16 years of engineering practice in the area of structural evaluations and investigations. Much of this practice has been related to evaluation and repair of highway bridges. From 1983 to 1987, Moore was the co-principal investigator on another FHWA-sponsored research program involving the design and testing of a two-span, three-girder, 0.4-model, steel-girder bridge designed using Alternative Load Factor Design, or Autostress. To date, the Autostress model bridge study is the largest structural test program to be completed at the Turner-Fairbank Highway Research Center. He has a bachelor's degree from Purdue University and a master's degree in engineering from the University of Texas at Austin. He is a registered