MOBILE INFRARED SCANNING –
A HIGH-TECH, ACCURATE ALTERNATIVE TO TRADITIONAL BRIDGE INSPECTION METHODS
Civil engineers have a big problem on their hands: tens of thousands of bridges across the United States have been in use long past their 50-year design life. According to the Federal Highway Administration (FHWA), approximately one-quarter of the 611,845 bridges across the US are structurally defi cient or functionally obsolete. These bridges will require signifi cant maintenance, repair, or replacement, at an estimated cost of $20.5 billion annually over the next 12 years.
The traditional methods currently used to identify structural defi ciencies in concrete bridge decks can be time-consuming, inaccurate, and unsafe for both inspectors and drivers. These methods rely on an investigator’s subjective assessment. They also require bridge lane closures during the survey, blocking or severely slowing traffi c. However, a new high-tech method that combines a mobile infrared camera with analytical software offers a solution that is safer and more objective.
Identifying Bridge Delamination
Two major factors in concrete bridge damage are delamination and spalling. Delamination is the separating of concrete into layers, or the separating of the top coating from the substrate. Embedded reinforcing bar (rebar) corrodes over time, causing expansion that splits the concrete either horizontally through the layers (delamination) or into chunks that break off above the damaged area (spalling).
Finding concrete delamination typically involves a form of non-destructive testing (NDT) called acoustic chain drag inspection. The inspector drags a heavy chain across the bridge deck, listening for the distinctive hollow sound produced by delaminated areas. He can then use this data to create a delamination map of the bridge deck.
A high-tech alternative to acoustic chain drag inspections uses a truck-mounted infrared camera to pinpoint delaminated areas on concrete deck surfaces. NEXCO-West USA developed this NDT inspection technique, which incorporates images from a cooled FLIR infrared camera into maps created with NEXCO-West’s proprietary software. Company engineers are now working with the University of Central Florida to develop objective and effi cient bridge inspection procedures that could be used by state highway agencies across the US.
Visual Image
Raw IR Image
Processed Image
Visual, raw, and processed delamination images from IR survey of bridge deck Source: “Comparison of Infrared Cameras for Concrete Bridge Deck Scanning: - Vol.2 Field Test at Haymarket Bridge”, December 2014, NEXCO-West USA, Inc
A Safe, Effi cient, Mobile Method
There are disadvantages to the chain drag method. Even though this type of inspection involves shutting down the lane under survey, inspectors are frequently required to work alongside open traffi c lanes. The traffi c noise makes it diffi cult to distinguish sounds the chain makes as it crosses delaminated concrete. In addition, the chain drag method relies heavily on the knowledge and experience of the inspector, making it subjective and potentially inaccurate. In fact, an FHWA study of deck delamination surveys concluded that the chain drag method did not consistently provide accurate results.
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“NEXCO-West’s approach to testing for bridge delamination is a mobile method, using an infrared camera installed on top of the vehicle,” says company President and CEO, Masato Matsumoto. “Our approach does not require lane closures or speed limit reductions, so it keeps traffi c fl owing while simultaneously keeping inspectors safe.”
Scans are usually performed during the day or within a few hours after sunset, when large temperature shifts can be observed. For example, concrete that heated up during the afternoon will begin to cool down after sunset, creating a measurable change in temperature. Most of the deck will heat or cool evenly, but delamination interrupts the conduction path. The temperature of the damaged concrete will
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