Adapted infrared technologies prove effective in detection of subsurface erosion in bridges
Every four years, the American Society of Civil Engineers takes a comprehensive look at the state of this country’s infrastructure and issues the nation a report card on its condition and needs. Grades are assigned by an ASCE Advisory Council based on the criteria of capacity, condition, funding, future need, operation and maintenance, public safety, resilience and innovation.
ASCE’s most recent assessment took place in 2013 and the Council awarded the United State’s 607,380 bridges — with an average age of 42 years — a slightly better than mediocre C-plus.
Underinvestment and delayed maintenance are cited as root problems for much of the country’s aging infrastructure, both of which are symptoms of tough economic choices in a time of shrinking federal and state revenues.
Glenn Washer, a civil engineering professor with the MU College of Engineering, is one of the country’s leading experts in nondestructive evaluation of concrete structures, particularly bridges. He has adapted existing handheld and mast-mounted infrared imaging technologies to detect sub-surface corrosion damage in concrete components as well as methodologies to increase the quality of data produced from the imaging.
“One of the biggest problems with deteriorating bridges is that concrete can fall onto cars. There were two recent incidents in Washington [state],” said Washer. “Infrared cameras have the potential to detect precursors before the concrete falls.”
Alan Jungnitsch, a civil engineering master’s student working with Washer, explained that when the steel reinforcing in concrete bridges corrodes, it expands and puts tension on the concrete, which cracks, grows and separates, leaving an air void.
“These areas of delamination heat up and cool down more quickly than the surrounding concrete, creating a thermal contrast,” Jungnitsch said.
In 2011, Washer proposed a collaboration to the Missouri Department of Transportation (MoDOT) to quantify the effectiveness and reliability of thermography for bridge assessment using broad field-testing. Findings would be compared to those obtained using standard procedures of hammer sounding and chain dragging. Both procedures produce a hollow sound when they strike or drag over delaminated bridge sections.
An advantage of the handheld infrared cameras is ability to check the structure without closing a busy bridge to traffic, as is the case when using standard manual practices. One person with a handheld device can check the bridge using infrared thermography up to 200 feet away by switching camera lenses. The technology represents a superior method for testing hard-to-reach spots on the sides and undersides of bridges.
MoDOT Research Engineer Jen Harper, a 1999 graduate of MU’s civil engineering program, served as point-of-contact for the project.
“Glenn came up with guidelines and how to implement the project,” Harper said. “Through his contacts, we determined other states that were interested and wanted to be a part of the project.”
Interested states contributed a set fee to a pooled fund, which provided them with the necessary equipment, training by Washer and his grad students and access to project results. Harper said that the funding model allowed MoDOT to undertake and participate in a large project without putting a lot of their own funding into it.
A total of 13 states have elected to participate in the project. “It’s need driven at a grassroots level,” said Washer of the states’ responsiveness.
The MU team traveled to each partner state to work with members of their departments of transportation for two days of training. The first day, they discussed and demonstrated how the handheld infrared camera worked and then practiced using it in the field on bridge decks and substructures. The following day, trainees did a blind test on their own, accompanied by Washer and his grad students.
Washer and his research team have worked to improve reliability of the process by developing complimentary software that will help users predict optimal weather conditions to collect thermal images. And they are expanding their testing with additional infrared imaging technologies that Washer has adapted for this nondestructive evaluation.
“We’ve made some real breakthroughs,” said Jungnitsch. “Handheld infrared is not quantitative. But the ultra-time domain camera we are using now takes a picture every minute or two at regular intervals, which provides more quantitative results.”
This new technology, known as Infrared Ultra-time Domain imaging (IR-UTD), was originally developed by researchers for the federal government to reveal corrosion damage under coatings such as bridge paints. Washer has worked with the developers, Fuchs Consulting Inc. of Leesburg, Va., to adapt IR-UTD to the task of imaging sub-surface damage in concrete. It differs from hand-held thermography because the cameras can monitor thermal changes over time.
Washer explained the camera can be positioned on a mount above a bridge deck to take thermal images over a 24-hour period resulting in a infrared time-lapse picture of the bridge heating and cooling, clearly revealing damaged areas.
“In post-processing the images, the rate of change of infrared radiation also gives a relative depth measurement,” Jungnitsch said of another of the procedure’s advantages over the hand held cameras.
By using standard manual methods side-by-side with thermography, the MU research team has successfully demonstrated that infrared detection is every bit as effective at revealing the scope of damage as are tried and true procedures. And in some cases it is clearly better at revealing subsurface damage.
“One of the problems we see is when they remove and replace a part of the bridge that’s bad, they may not have repaired the entire problem. [Using thermography] you can see issues outside the area repaired, so I do think it has its place,” said Harper.
Some participating states have wholeheartedly adopted hand-held thermography technologies into their existing bridge maintenance and rehabilitation programs. Both Texas and Michigan, for example, are using this technology to determine the scope of bridge rehabilitation projects. After determining repair needs, they are able to develop contracts with greater precision, reducing the number of claims and contract overruns. Texas also is using the technology for quality control of fiber-reinforced polymer (FRP) overlay applications used to strengthen bridges.
The project is set to end in December 2015, but Washer plans to continue to interest additional states in infrared bridge assessment.