Detection of Flaws in Asphalt Overlaid Concrete Bridge Decks Using Ultrasonic Guided Waves

Detection of Flaws in Asphalt Overlaid Concrete Bridge Decks Using Ultrasonic Guided Waves

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Detection of Flaws in Asphalt Overlaid Concrete Bridge Decks Using Ultrasonic Guided Waves
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    This paper presents the results from laboratory experiments to determine the capabilities of a recently developed nondestructive testing (NDT) approach for detecting the flaws at the asphalt-membrane-concrete layers and rebar-concrete interfaces for asphalt-overlaid reinforced concrete bridge decks. The novel Ultrasonic Guided Wave Leakage (UGWL) method previously developed by University of Nebraska-Lincoln (UNL) and Nebraska Department of Transportation (NDOT) utilizes the steel rebar as a wave guide by placing an ultrasound transmitter at the rebar and measures the leaked energy from the top surface of concrete (or asphalt) with an array of ultrasonic receivers. This method has proved successful previously as a continuous monitoring tool in lab experiments, where early rebar-concrete delaminations as small as 0.008”, and early corrosion in specimens soaked in salt-water (as early as 9 days) have been detected in various configurations and sizes of reinforced concrete specimens. Further, two proof-of-concept studies for instantaneous field testing has been carried out. First, on an existing bridge, concrete was cored to the top rebar level and a transmitter was attached to the rebar and an array of readings were taken from the concrete surface in 6 inch increments up to 14 feet from the transmitter. While the attenuation curve from this test was as expected based on theory of ultrasonic guided waves, without a baseline data from previous years, such data is difficult to interpret for accurate flaw detection. In the second field application, a transmitter was attached on a rebar during construction and embedded in concrete. Several months of testing showed no significant change, as expected, but this experiment proved that the attachment was secure and durable in field conditions. Future readings from this field implementation can reveal change in the condition of this bridge deck since baseline data does exist in this case. None of these systems (lab or field) included asphalt overlay, therefore this latest phase of the project aimed to explore the method’s capabilities when the deck is overlaid with asphalt for wider spectrum of applications. Several asphalt-overlaid reinforced concrete laboratory specimens were tested in this study and the results showed that with careful placement of sensors and data analysis, flaws at the membrane level as well as the rebar level could be successfully detected. As stated before, the UGWL method’s strength lies in continuous monitoring: change in amplitudes from a baseline data is more meaningful than instantaneous amplitude readings at this time. This method has the potential to answer an important need in infrastructure quality control and maintenance; namely membrane inspections to detect construction flaws.
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