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NTSB Urges More NDE in Corrosion Inspections

Monday, November 29, 2010

The National Transportation Safety Board is recommending greater use of Non-Destructive Evaluation (NDE) technologies to augment visual inspections of bridge corrosion in the wake of a fatal 2008 truck crash through a barrier on the Chesapeake Bay Bridge.

In a letter last week to the Federal Highway Administration, the Safety Board urged that state Departments of Transportation be made more aware of the risks of steel reinforcement corrosion and voids in concrete barriers and barrier attachment points, and of the NDE methods used by the Maryland Transportation Authority (MdTA) to identify internal corrosion problems in concrete bridge railings.

NDE ‘Not Widely Used’

“The NTSB concludes that the FHWA’s efforts to promote NDE technology for bridge inspectors have been limited and that NDE inspection practices are not widely used by the states because of their complexity and cost,” the letter said.

“The NTSB recommends that the FHWA (1) expand the research and development of NDE technologies to develop bridge inspection methods that augment visual inspections; offer reliable measurement techniques; and are practical, both in terms of time and cost, for field inspection work; and (2) promote the use of these technologies by bridge owners.”

The letter was addressed to FHWA Administrator Victor M. Mendez and signed by NTSB chair Deborah A.P. Hersman.

U-Bolt Anchorage

The crash occurred about 4 a.m. August 10, 2008, when a tractor-trailer traveling at 40 mph swerved to avoid a car and plunged through the Bay Bridge barrier at a 40-degree angle, dislodging a 24-foot section and knocking a 12-foot section into the bay.

The Board noted that the bridge used a U-bolt anchorage system on all parts of the barrier except the suspension span. The U-bolt system would be considered nonstandard today, but conformed to the American Association of State Highway Officials (AASHO) Standard Specifications for Highway Bridges in force when the bridge was built in 1951.

The same anchorage system was used when the bridge deck was rehabbed in 1973 and 1986—still in compliance with AASHO standards at the time, NTSB said. Removing the U-bolts in 1986 would have been too damaging to the bridge deck, Maryland officials determined. NTSB agreed.

Hidden Corrosion

After the crash, NTSB examined sections of the barrier as well as some U-bolts and found “reduced cross-sectional area due to corrosion” on the bolts, the board said. “The NTSB concludes that, based on the degree of corrosion, the attachment U-bolts for the barriers in the accident location were substantially degraded at the time of the accident.”

With those results, Maryland officials conducted NDE of the bridge barriers, using several technologies. These identified hidden corrosion that annual visual inspections could not, NTSB said.

The board noted the shortcomings of visual inspections, citing a 2001 report by FHWA that found “significant variability … in all aspects of the inspection process” and most prominently in “the assignment of Condition Ratings, where bridge owners can expect 95% of Condition Ratings to be spread over five rating points.”

ON the other hand, the board noted the availability of several NDE methods, including eddy current testing, ultrasonic testing (UT), infrared thermography, impact echo testing, and ground penetrating radar (GPR).

Noting the current popularity of performance-based bridge management, it cautioned:

“Performance-based management of bridge assets is weakened by an inspection program that is primarily conducted through visual inspection methods,” NTSB said. “The NTSB concludes that although bridge management systems may make use of sophisticated and elaborate analysis techniques, their reliance on visual inspection data is a clear limitation.”