Improving bridge inspections: researchers at FHWA are developing innovative nondestructive evaluation technologies to assess the condition of bridges

Public Roads, Nov-Dec, 2003 by Glenn A. Washer

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.

Validation Center

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.tfhrc.gov/hnr20/nde/01020.htm.> 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.