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United States Department of Transportation United States Department of Transportation

Public Roads - Autumn 2019

Autumn 2019
Issue No:
Vol. 83 No. 3
Publication Number:
Table of Contents

Speeding Up Bridge Deck Evaluations

by Hoda Azari and Mehdi Rashidi

FHWA researchers developed a robotic air-coupled acoustic system to reduce traffic delays and increase safety when collecting data to assess condition.

Inspecting and managing more than 600,000 bridges in the Nation's inventory with limited resources is challenging for bridge owners. The challenge is further complicated by the fact that the average age of these bridges is more than 40 years. More specifically, bridge decks require the most frequent maintenance and preservation because, on average, they deteriorate faster than all other bridge components. This is because of the routine application of deicing salts and sustained traffic load in addition to environmental effects.

FHWA developed this robotic air-coupled acoustic NDE system to speed and improve inspection of bridge decks.

Recently, bridge owners across the country worked toward implementing asset management plans and performance-based management to maintain, preserve, and improve the highway system. An essential component of this approach is to have reliable and quantitative information regarding the physical condition of structures and a greater understanding of their deterioration processes. The use of nondestructive evaluation (NDE) technologies is complementary to the current state of the practice for assessing the condition of structures, which is based on visual inspection and manual sounding techniques.

An air-coupled transducer (source) and an array of microelectromechanical system microphones (receiver) hover above a section of concrete slab.

Over the past decade, bridge owners increasingly have used NDE technologies as an assessment tool in bridge inspection because they (1) enable periodic assessment of structures without causing damage and compromising their structural integrity, and (2) provide information about defects invisible to the naked eye.

The Federal Highway Administration is researching and developing NDE technologies as a solution to myriad challenges, including a means by which to use a noncontact (or air-coupled) acoustic array to evaluate bridge decks. The contactless technology, when compared to contact sensors, significantly speeds assessment times, reducing lane closure times and increasing safety for inspectors and highway travelers.

Contact-Based Acoustic Techniques

Acoustic techniques including impact echo and ultrasonic surface waves, which are based on mechanical wave propagation, provide information about the geometry (for example, thickness) and mechanical properties (for example, elastic modulus) of a medium. Acoustic techniques are effective in detecting internal defects such as delamination, voids, and cracks within a structure. Because mechanical vibration generates different particle motions in different solid media, mechanical wave propagation velocity and amplitude are related directly to the elastic constants of the material. Internal cracks, voids, delamination, and other internal defects cause variations in the measured wave.

Bridge inspectors perform contact-based acoustic methods using a source to generate the acoustic signal and one or more receivers for capturing the response. The inspectors apply a couplant at the contact surface between acoustic probes and the structure to ensure the proper transmission of acoustic waves to the testing medium. However, this approach limits the detection of internal defects in highway structures. Depending on the size of the structure being tested and the desired spatial resolution of test results, the scan of the structure may require thousands of measurement points. Furthermore, if the concrete surface has a rough texture, it may restrict effective mechanical coupling between contact sensors and the test surface, which can make the repeatability of measurements challenging. Overall, the process of pressing the sensors against the surface at each measurement point, detaching the sensors, moving them to the next measurement point, and ensuring adequate coupling with the surface is time-consuming and costly.

Contact-based acoustic techniques increase the duration of lane closures and risk to workers and drivers. Besides safety, the longer assessment period required for contact-based acoustic techniques takes a toll on the U.S. economy and environment by contributing to traffic congestion. In 2014, U.S. highway travelers wasted 6.9 billion hours and 3.1 billion gallons (11.7 billion liters) of fuel because of traffic delays. That resulted in an overall estimated cost of $160 billion.

"Along with safety concerns, the negative impact on resources and the staggering related costs of traffic congestion are motivating factors for using technologies to speed inspections of highway structures," says Jean Nehme, Ph.D., team leader of FHWA's Long-Term Infrastructure Performance programs. "Therefore, any practice that can promote the safety of road users and bridge inspectors, while reducing traffic congestion by minimizing the duration of work zones and lane closures, would be a significant improvement to the current state of the practice."

A High-Speed Solution

An effective solution to the time-consuming inspection process using conventional acoustic techniques is to eliminate the need for physical contact between the sensors and a structure. Researchers at FHWA's Advanced Sensing Technology (FAST) NDE Laboratory used a contactless acoustic source and receivers to develop an air-coupled (that is, no contact) acoustic array to evaluate bridge decks. The fully air-coupled system mounts on a robotic platform for high-speed evaluation of bridge decks.

In this air-coupled system, a signal generates through a waveform generator and feeds into an amplifier, which in turn sends the signal to an air-coupled transducer to excite the structure through the air medium. Microelectromechanical system (MEMS) microphones capture the response of the structure as the propagating wave leaks from the structure to the air. The resulting measurements are highly reliable and repeatable because the air-coupled system does not have the variability of the contact between the sensors and the structure, which is a major issue during conventional acoustic measurements.

This graph shows the numerical simulation of propagating leaky wave between air and concrete. The amplitude of leaky waves is significantly lower compared to other waves.

The small size of the MEMS microphones (approximately 5 millimeters), their high sensitivity, and their low cost (typically less than $1) enables a customizable array of microphones capable of high-resolution scanning of bridge decks at a low cost. To minimize the noise to improve the accuracy of the measurements, researchers at FHWA's NDE lab implemented a proper shielding of the source from the receiver. Like conventional acoustic techniques, the received signal on a delaminated section has different characteristics than that on an intact area.

By adjusting the angle of the acoustic source with respect to the surface of the structure, the air-coupled system can generate both body and surface waves. The body waves generally can detect shallow and deep flaws, but their amplitude significantly decreases because of the strong interaction with the medium. For example, body waves are highly damped in reinforced concrete because of its heterogeneity. On the other hand, surface waves typically are sensitive to shallow flaws, but their loss of amplitude is less significant than body waves, and they travel a longer distance in a medium than body waves can. Most of the surface wave energy is contained within a thickness zone of approximately one wavelength below the surface, which researchers often refer to as the wave penetration depth. The surface waves' depth of penetration in the structure can be adjusted by controlling the wave center frequency. Overall, the air-coupled system can estimate the concrete modulus through analyzing the surface waves and estimate the depth and size of flaws in bridge decks through the analysis of body waves.

Robotic Assistance Increases Effectiveness

The bridge community is increasingly using robotic platforms for the NDE of highway structures. Under the Long-Term Bridge Performance (LTBP) Program, FHWA funded a research project to develop a Robotic Assisted Bridge Inspection Tool (RABIT™). The RABIT, which is now commercialized, measures electrical, electromagnetic, and acoustic properties of reinforced concrete structures to estimate the state of damage.

The process of data collection using RABIT is faster and less labor intensive than collecting data in a manual fashion. However, RABIT uses contact sensors for measuring the acoustic properties of the medium, so the condition assessment of bridge decks is still a time-consuming task. Given the large number of measurement points required to scan a bridge deck, implementation of the air-coupled technologies on the RABIT can generate scans with a higher spatial resolution for bridge evaluation, save hours, and reduce the economical and environmental impacts of traffic congestion from lane closures.

"With promising preliminary results from the air-coupled system, we hope for fast and reliable field measurements in the near future," says Nehme. "The potential combination of the RABIT and the air-coupled acoustic system could present a huge step toward more efficient bridge deck inspections."

This circuit board contains four microelectromechanical system microphones and an amplifier. The system is not much larger than a quarter.

Hoda Azari is the manager of the NDE Research Program and Laboratory at FHWA's Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.

Mehdi Rashidi is a National Research Council research associate at FHWA's Turner-Fairbank Highway Research Center. Rashidi is conducting research in NDE of highway structures. He holds a Ph.D. in civil engineering from Georgia Tech.

For more information, contact Hoda Azari at or 202–493–3064.