Improving Bridge Inspections
Researchers at FHWA are developing innovative nondestructive evaluation technologies to assess the condition of bridges.
Shortly after the collapse of the Silver Bridge between Point Pleasant, WV, and Gallipolis, OH, in 1967, the Federal Highway Administration (FHWA) developed the National Bridge Inspection Standards to provide guidance on inspecting bridges for safety. The standards require the inspection of all bridges on public roadways in the United States on a periodic basis, normally at least once every 2 years. FHWA maintains the data from the inspections in the National Bridge Inventory, a database of information on the size, construction, and condition of bridges and culverts in the United States.
For more than 30 years, inspectors relied largely on visual inspections to evaluate the condition of bridges. Although some State departments of transportation (DOTs) have employed nondestructive evaluation (NDE) methods to complement visual inspection, widespread use of NDE technologies has been limited. New NDE technologies increasingly are sought to solve difficult inspection challenges that are beyond the capability of normal visual inspections.
The Nondestructive Evaluation Validation Center (NDEVC) at the FHWA Turner-Fairbank Highway Research Center in McLean, VA, is developing new and improved technologies to meet these needs. Recent activities at the NDEVC include research on the reliability of routine inspection methods and the development of innovative nondestructive evaluation tools. Current focus areas include laser measurement technologies, bridge deck inspections, monitoring systems, inspection of composites, automated ultrasonic testing, reactive powder concrete testing, and NDE for post-tensioned bridges.
In 1996, the U.S. Congress mandated that FHWA develop a center for evaluating NDE technologies. FHWA designed the NDEVC to provide State highway agencies with independent development, evaluation, and validation of nondestructive evaluation technologies. The center also researches new technologies to solve specific problems related to inspecting and evaluating bridges.
The validation center includes a laboratory, test bridges, and component specimens. The laboratory serves as the nucleus for preliminary testing and evaluation. Test bridges in northern Virginia and Pennsylvania provide sites for practical trials that evaluate technologies under realistic field conditions. The center also uses sections of bridges containing defects, known as component specimens, to perform capability trials in the laboratory and conduct research related to developing new technologies.
Reliability of Visual Inspection
The National Bridge Inspection Standards require that inspectors periodically inspect the Nation's bridges and report bridge conditions in a standardized format. Condition ratings range from zero to nine for each of three bridge components: the superstructure, substructure, and deck. By assigning condition ratings to each component, the standards help FHWA measure bridge performance at the national level, forecast future funding needs, determine the distribution of funds among States, and assess the maintenance needs for a particular structure. The accuracy of the ratings is important to identifying bridges in need of maintenance and repair and ensuring that FHWA programs for funding construction and renovation are equitable and meet the FHWA goal of reducing the number of deficient bridges.
In 1998, the NDEVC initiated research on the accuracy of the bridge inspection process. The study provides overall measures of the reliability and accuracy of inspections, identifies factors that may influence the results, and determines procedural differences between State inspection programs. Completed in June 2001, the research report, Reliability of Visual Inspection for Highway Bridges, Volume I: Final Report (FHWA-RD-01-020), is available online at www.fhwa.dot.gov/publications/research/nde/01020.cfm.
The study asked 49 practicing bridge inspectors from across the United States to examine the test bridges in Virginia and Pennsylvania associated with the NDEVC. Each inspector performed 10 separate tasks, including routine and indepth inspections. They used common hand tools such as a masonry hammer, plumb bob, carpenter's level, binoculars, and other nonintrusive tools. An NDEVC observer documented the performance and behavior of the inspectors during the inspection tasks.
During the routine inspections, the NDEVC staff asked the inspectors to provide a condition rating for the superstructure, substructure, and deck. The study revealed a wide distribution of condition ratings reported by inspectors evaluating the same bridge sections. On average, they assigned between four and five condition ratings for each separate component. For some components, inspectors provided as few as three different condition ratings; for others, inspectors provided as many as six. The average was between four and five.
Statistically, if the results were extrapolated to the entire population of bridge inspectors, the results indicate that only 68 percent of the reported condition ratings for these elements would vary between plus or minus (+/-) 1 from the average rating for a particular element. This data and other data from the study indicate a wide variation in the manner in which inspectors conduct routine inspections. The study concluded that the definitions of particular condition states may not be refined enough to facilitate accurate and reliable ratings.
In addition, the inspectors performed two indepth inspections, defined as up-close, arms-length inspections generally conducted to identify deficiencies not normally detected during routine inspections. One indepth inspection involved evaluating a welded steel girder with fatigue-sensitive details. The inspectors were expected to search for and find seven crack indications on a specific portion of a steel bridge. Inspectors reported the crack indications at a rate of 3.9 percent. About 4 out of every 100 inspections of a particular crack indication correctly identified the indication. Test results indicated that 86 percent of the inspectors who correctly identified the indications used a flashlight for the inspection and were on average 0.2 meter (0.7 foot) from the girder during the inspection. Among inspectors who did not correctly identify indications, only 38 percent used a flashlight, and the inspectors averaged 2.7 meters (8.9 feet) from the girder during the inspection.
These results indicate that the low crack-detection rates found during the study may be related to how the inspections are performed, and additional training or retraining may increase the use of appropriate inspection practices. It was concluded that a significant portion of indepth inspections are unlikely to note defects beyond those found during a routine inspection.
Laser Measurement Technologies
Over the last 7 years, the NDEVC has developed numerous applications using laser-based distance measurements for highway infrastructure. A scanning laser system was developed as part of an FHWA research and development program. The system can measure distances with submillimeter accuracy over a range of 30 meters (98 feet). The mechanized scanning head enables the laser to scan over +/- 200 degrees on one axis and +/- 60 degrees on a second axis. Two angles and the distance measurement combine to locate a point in three-dimensional space; targets are not required.
Applications for this technology include measuring bridge deflections under calibrated load to evaluate structural behavior, calculating out-of-plane distortions in girder webs and flanges, and evaluating the as-built construction of large structures such as abutments.
This unique measurement technology recently has been applied to the fabrication of steel bridges. For many bridge construction projects, fabricators are required to set girder sections at their final elevations at the shop, prior to shipping to the job site. This ensures that bolt holes in splice plates align properly when workers construct the bridge in the field. The cost of assembling the entire bridge at the fabrication shop can be extremely high, adding between 5 and 15 percent to the project cost.
Proof-of-concept testing has been conducted using the laser system to measure the precise location of bolt holes in bridge girders following fabrication. The girders are then virtually assembled in the computer to determine the exact location of holes in splice plates that will be used to join the girders. In this manner, the bridge can be constructed virtually using laser-based measurements, eliminating the need to assemble the bridge at the fabrication shop.
Another recent application of the laser measurement technology is measuring out-of-place web distortions in a curved-girder bridge. Laser technology offers several advantages for this application. First, distortions of the web over a large field can be determined from a single measurement location. Second, no interaction with the beam under test is required because the measurements are noncontact and made at a range of up to 30 meters (98 feet), although typically at 10 to 20 meters (20 to 60 feet). The resulting images of the out-of-plane distortions of the web can be used to quantify the effects of attachments on beam performance, identify local buckling phenomena, and track beam behavior during testing.
Bridge Deck Inspections
The National Bridge Inventory indicates that there are more than 298 million square meters (3.2 billion square feet) of bridge deck in the United States. Most decks are made of reinforced concrete that provides the driving surface for the bridge. The service life of a deck can be much shorter than that of the substructure and superstructure. Decks deteriorate due to corrosion of the reinforcing steel, and the resulting delaminations and spalling can make a deck structurally deficient. Detecting deterioration in its early stages is critical to helping State DOTs repair the most at-risk bridges and optimizing the use of limited funds.
To meet that need, FHWA is developing ground-penetrating radar (GPR) systems for detecting and imaging subsurface defects in concrete bridge decks. Several prototype systems have been developed under a project known as the High-Speed Electromagnetic Roadway Measurement and Evaluation System (HERMES). The goal of the project is to develop a GPR system that can image deterioration in concrete bridge decks accurately while traveling at highway speeds. Imaging is conducted by an array of GPR antennas that operate in a synchronized manner, such that detailed images can be produced from GPR data.
Recent project efforts have focused on developing new, higher-frequency antennas to provide improved imaging resolution when integrated into the array architecture. A pooled fund study with 19 participating States has funded the development of a single antenna prototype that uses a robotic cart to scan over a bridge deck to simulate performance of the larger multichannel array. The new antenna has significantly higher bandwidth than previous antenna designs, and field testing has shown significant improvement in the system's ability to image deck deterioration. Ongoing testing is exploring the improved imaging capabilities of the antenna under a wide range of test conditions in the field and examining the ability of the system to image defects in asphalt and concrete pavements.
Bridge Monitoring Systems
The NDEVC is involved in developing various instruments to monitor the performance of civil infrastructure. Generally, these instruments are dedicated, remote data-acquisition systems that collect information on the behavior of a structure over time. They log data in a system memory that can be downloaded periodically. Designed to be inexpensive, rugged, and battery-operated, the systems operate on a flexible platform that can be customized for particular applications and installed quickly in the field.
Several systems have been assembled for various applications. Transducers, for example, can monitor the displacement of a wing-wall relative to the abutment, using an eddy-current sensor. Designed at the NDEVC, the sensor measures the relative movement and tilt of the wing-wall, providing a stable measurement of displacement over long periods of time with minimal power consumption. As of November 2003, the system had been in place for 38 months on a bridge in Washington, DC.
Inspection of Composite Structures
The growing use of composites in civil infrastructure presents many challenges in terms of post-construction inspection for quality control and monitoring of the long-term performance of materials. To address these challenges, the NDEVC is developing thermographic methods for evaluating composite bridges and composite bridge repairs.
Thermographic systems operated under ambient weather conditions are used to detect anomalies in heat transfer that occur due to delaminated or debonded material. Applications include detecting delaminations in a carbon-fiber laminate used to strengthen concrete bridges and debonding of epoxy overlays in composite bridges. The data are collected under ambient environmental conditions without the use of external heat sources. Diurnal (daily) temperature variations and the significantly different thermal conduction properties of the overlay and the substrate material provide the thermal gradients necessary to create thermal contrasts at defects. The technique, known as passive infrared thermography, enables fast scanning of structures and reduces the need to access structures at close distances to apply active heating.
Automated Ultrasonic Testing
Automated ultrasonic testing (AUT) combines traditional ultrasonic testing methods with computerized data acquisition and processing. AUT technologies have been available for several years, and inspectors increasingly use these methods during routine inspections of pipelines and in aeronautical applications.
AUT offers several advantages over traditional ultrasonic testing techniques:
Images created by an AUT system can be easier to interpret, especially in areas with complex geometries.
AUT systems preserve a record that inspectors can use to confirm the completeness of inspections and archive for future use.
AUT systems can be combined with robotic scanning systems to provide efficient and repeatable inspections of standard weld geometries.
Both ultrasonic and radiographic testing are used to inspect steel bridges during fabrication to ensure weld quality. Radiographic testing is more common for steel bridges in the United States than ultrasonic testing, although requirements vary by State and bridge member design. Because radiographic testing is a well-proven method that provides a more complete record than manual ultrasonic testing, bridges owners frequently prefer it. The health issues related to radiographic testing, however, introduce logistical difficulties in the fabrication process that result in increased costs. AUT, on the other hand, provides a more complete record than manual UT and may represent a safer alternative to radiographic testing.
In 2002, the NDEVC began developing and evaluating AUT systems to inspect steel bridges during fabrication. The goal was to determine if AUT technology can provide an alternative to radiographic testing as a quality-control tool. The study examined the use of AUT technology for inspecting butt-welds and compared the results with those using radiographic testing.
To date, researchers have conducted more than 150 hours of in-line testing at fabrication shops, in parallel with industrial radiographic services. Results indicate that AUT can be an effective inspection tool that could be used in place of radiography under certain conditions.
A final report detailing the testing conducted during this study should be available in 2004.
Reactive Powder Concrete Testing
A new class of concrete known as reactive powder concrete (RPC) is becoming available in the United States for fabricating bridge members. Classified as ultrahigh performance concrete, the material consists of sand as its largest aggregate and fine steel fibers distributed within the concrete itself. RPC achieves compressive strengths of up to 200 megapascals, MPa (29 kips per square inch, ksi), compared with maximum compressive strengths of 50 to 100 MPa (7 to 15 ksi) for high-performance concretes. Young's modulus values greater than 50 gigapascals (7,000 ksi) are common for RPC. Obviously civil engineers would be very interested in an easily formable material with high compressive strength and stiffness, but introducing this new material brings new challenges for nondestructive evaluation. Applying RPC in the field will result in lighter sections, longer spans, and innovative new section geometries. Effective tools to assess bridge condition will play an important role in integrating this new material into the civil engineering community.
Because of the homogeneous, highly packed nature of the RPC microstructure, it is possible to use ultrasonic testing methods for inspection and materials characterization in ways not possible with traditional concrete. Transducer frequencies of 10 to 20 times those used in normal concrete can be used to launch and receive ultrasonic waves over distances on the order of several hundred millimeters. Initial research indicates that ultrasonic wave velocities can help determine the elastic properties of the material, and traditional pulse-echo ultrasonic testing can be used to detect cracks in the cement matrix. Ongoing research is exploring how ultrasonic velocity measurements can be used as a quality-control tool during construction and how ultrasonic testing may be used for in-service inspection of bridges constructed of RPC.
Steel tendons that provide compressive forces on post-tensioned concrete bridges are critical structural elements. The forces provided by these tendons counteract tensile forces that result from the substantial dead weight of the structure and traffic loading. The construction elements that use post-tensioned tendons include segmental and cast-in-place concrete bridges, integral pier caps, substructures, and piers.
Steel tendons typically are located inside metal ducts within the concrete member. Workers fill the ducts with a cementitious grout that protects the corrosion-sensitive, highly stressed tendons. The grout is intended to fill the duct completely so water cannot collect there, and the highly alkaline environment created by the grout around the tendon inhibits corrosion.
Substantial evidence in Europe and the United States indicates that tendons may be susceptible to failure due to corrosion at locations where the duct has not been grouted properly. Improper grouting may result in a void or pocket around the tendon where water can collect, creating a corrosive environment. Since the tendons are located within the concrete, they represent a significant challenge for inspectors. The most widely used approach to finding and evaluating voids is to excavate the concrete to expose the tendon, open the duct, and examine the strands. This process, however, is destructive and provides only intermittent results since it is impractical to expose the entire duct.
As an alternative to excavation, FHWA is examining the effectiveness of using high-powered linear accelerators to radiograph the internal features at duct locations. Portable linear accelerators that have energies of 6 megaelectron volts are available from service providers. NDEVC staff conducted laboratory experiments using this technology, and the Central Artery Tunnel Project in Boston, MA, and the Florida Department of Transportation have used the technology in field experiments. Although studies indicate that radiographic testing can detect broken strands and voids in the grout under field conditions, the technology remains cumbersome to apply and expensive. Future efforts at FHWA may include developing a system engineered for application to civil infrastructure.
Ultrasonic methods to monitor tendon condition are another approach to examining post-tensioning systems. FHWA developed electromagnetic acoustic transducers (EMATs) that encircle individual strands. EMATs launch and receive acoustic waves traveling within the strand and may be capable of serving dual roles. First, acoustoelastic methods could monitor the level of force carried in the strand. Second, waves launched from the EMATs could be used to detect broken wires within the strand. EMAT sensors embedded in a structure during the construction process could monitor the condition of the system over the life of the bridge. Proof-of-concept testing is ongoing at FHWA.
Safety Is the Goal
The goal of the Nondestructive Evaluation Validation Center at FHWA is to improve the state of the practice for highway bridge inspection. Staffed with a multidisciplinary team, the facility will continue to evaluate the reliability and accuracy of existing NDE technologies and work to develop new ones. By improving tools for inspecting and evaluating bridges, FHWA and the NDEVC staff are helping inspectors ensure the safety of the Nation's bridge infrastructure.
Moore, M., Phares, B., Rolander, D., Graybeal, B., Washer, G., Reliability of Visual Inspection for Highway Bridges, Volume I: Final Report, USDOT (FHWA-RD-01-020), Washington, DC, 2001.
Mindness, S. and J.F. Young, Concrete, Prentice Hall, Inc., New Jersey, 1981. Test and Assessment of NDT Methods of NDT Methods for Post-Tensioning Systems in Segmental Balanced Cantilever Concrete Bridges, DMJM Harris, Florida Department of Transportation Central Structures Office, February 2002.
Glenn A. Washer, Ph.D., P.E., is the director of the NDEVC. Dr. Washer received his Ph.D. in materials science and engineering from the Johns Hopkins University in 2001. He received a master's in structural engineering from the University of Maryland in 1996 and his bachelor's in civil engineering from Worcester Polytechnic Institute in 1990. He has been with FHWA for more than 13 years, during which time he has been involved with the development and testing of many NDE technologies for highway bridges and has published more than 40 related conference and journal papers. In 2001, he received the Arthur S. Flemming Award in Applied Science from The George Washington University for his role in developing the NDEVC. Dr. Washer is a registered professional engineer in Virginia.
The following staff from the NDEVC contributed to this article:
Mark Moore, M.S., P.E., Program Manager, Wiss, Janney, Elstner Associates, Inc. (WJE)
Paul Fuchs, Ph.D., Fuchs Consulting, Inc.
Michael Scott, Ph.D., WJE
Ali Rezaizadeh, Ph.D., WJE
Habeeb Saleh, Ph.D., WJE
For more information on projects at the NDEVC, visit www.fhwa.dot.gov/publications/ndec/index.cfm. For information on the use of high-energy x-ray on the Central Artery Tunnel Project, contact Structural Engineer Daniel Wood with the FHWA Division Office in Massachusetts at 617-494-2462 or email@example.com.