New technology bringing new standards to measuring bridge conditions
By Tom Kuennen, Contributing Editor
“Smart” bridges are on the way for road agencies burdened with the responsibility of monitoring bridge conditions.
Not-so-old technologies like ground penetrating radar (GPR), acoustic emissions testing and lasers for bridge condition testing are being augmented with advanced mobile radar and laser technologies and today’s nanotechnology, which puts the science of bridge condition monitoring into a whole new era with products such as bridge coatings which can sense trouble within.
In the meantime, reliable, low-cost cellular phone service has lowered the barrier to remote reporting of bridge conditions, as battery-powered systems can “call in” condition reports to an agency computer and database or alert an agency if conditions change abruptly.
Today’s benefits can include better information about bridge conditions, a better database for National Bridge Inventory (NBI) reporting, and, long term, fewer demands on personnel for inspection. But field implementation depends on the ability to make high technology marketable in the field, and on the ability of cash-strapped agencies to pay for it.
Rewards, Barriers to Implementation
Nanotechnology is the latest permutation of bridge condition monitoring, but it caps a decade-and-a-half of activity in high-tech bridge condition monitoring.
“Advanced bridge condition monitoring techniques can provide quantitative condition measurements, as opposed to the subjective assessments that a visual inspector provides,” said Dr. Steven B. Chase, research professor of civil engineering, University of Virginia-Charlotteville Center for Transportation Studies, where he works following a 30-year career with the Federal Highway Administration.
“Many of the new methods can provide indications of conditions that exist prior to a visual indication,” Chase said. “And many of the technologies can measure things that simply aren’t visual.”
Current federal regulations require that a state conduct a bridge inspection every two years. With a total of about 583,000 bridges in the national inventory, in order to inspect nearly 300,000 bridges nationally with the amount of manpower that’s available, efficient and rapid assessments and inspections are required. Today’s new technologies can provide that.
But there are built-in barriers to implementation. Past data, and systems put in place to record the results of those inspections, have been predicated on use of visual inspection, not high-tech. “It will be a long time before these new technologies supplant or add a great deal of opportunity for an agency to save money,” Chase told Better Roads. “The case will have to be made that technology can provide better inspections, with a higher probability of finding a defect that could be significant.”
And staff will have to be trained, he said. “The use of all this technology does require a higher level of capability, training and experience that the typical bridge inspectors do not have,” Chase said. “We’re trying to change that by making the technology easier to use, and produce results that are easier to interpret. We want to change bridge management and inspection practices to integrate the kind of quantitative information this technology can provide, in a way that produces better decision-making. But we have been involved in this a long time, and it will evolve over time. I don’t expect any easy breakthroughs.”
Nonetheless, Chase and his fellow researchers have been fighting to bring the technology to the field. “We are working to bring this technology to bear, to make it easier to apply and to improve the quality of information that’s available to the bridge owners from inspections,” Chase said.
Focus: Decks and Superstructures
For rating and NBI purposes, states must collect condition data on a variety of bridge structural characteristics, including, according to Chase:
The bridge deck and all wearing surfaces
The bridge superstructure, including all primary load-carrying members and connections
The bridge substructure, including the abutments and all piers
Culverts, for culvert bridges, and
Channel and channel protective systems for all structures which cross waterways.
In addition to structural condition, bridge functional adequacy also is logged. This can include load carrying capacities, whether deck geometry or lane constrictions restrict safety, the presence of low underclearances that may result in detours, and the ability of the bridge to handle water flow rates.
Bridge condition technologies are centered on decks and superstructures, and much less on substructures. “Cracks in steel and concrete, damage from corrosion, all are areas in which various technologies are available for assessment,” Chase said. “There’s a lot of work on bridge decks and superstructures. By themselves, cables have been the focus of a lot of attention.”
But except for scour, the condition technologies available for bridge substructures tend to be focused on evaluating the quality of the substructure as it’s constructed, not after it’s in-place, said Chase.
“Evaluating the integrity of piles, drilled shafts and other substructure elements while they’re being constructed has evolved to the point where technology such as cross hole sonic logging and other methods are routinely employed to assess whether substructure elements are constructed properly,” he said. “But once the bridge is placed in service, we tend not to be too concerned with the substructure, with the one exception of scour. That’s because we tend not to have large failures of substructure elements; most of the collapses or other failures have been things that happen in the superstructure.”
Ground Penetrating Radar
Like with pavements, ground penetrating radar (GPR) has great applicability for studying bridge deck condition. GPR systems use electromagnetic radiation at microwave frequencies, and the radiation penetrates and characterizes concrete, while reflecting off metallic material like rebar. It has the potential to replace manual chain drag testing of decks, in which experienced staff listen to the sound the chain makes; ringing indicates a sound spot, while a dull thud indicates subsurface delamination.
The FHWA pioneered GPR for bridge decks by underwriting the development of the HERMES (High-Speed Electromagnetic Roadway Measurement and Evaluation System) product at the Lawrence Livermore National Laboratories in California. HERMES was intended to provide a GPR system that can reliably detect, quantify and image delaminations in bridge decks, at normal highway speeds, and was delivered in 1998.
“HERMES was a radar system that was much, much smaller than anything at the time was commercially available,” said Chase. “It had a much higher frequency, and was based on new technology that had been developed for producing extremely short, precisely timed pulses, which was created for Livermore’s work on fusion reactors. The integration of all of that into a system that employed computer-aided tomography, with an array of antennas, was a first.”
HERMES included a computer workstation and storage device, survey wheel, and control electronics, in addition to the array of 64 antenna modules or transceivers mounted in a towable trailer. To investigate specific areas of a bridge deck that require more detailed study, a single-antenna scanning device called PERES (Precision Electromagnetic Roadway Evaluation System) was developed as an extension of HERMES.
In mid-decade they both were supplanted by HERMES II/PERES, new GPR technology using a single transmitter and receiver antenna pair configuration, developed under a participating states’ pooled fund. Work using HERMES II/PERES incorporating other technologies continued into 2009 under the auspices of the University of Vermont.
A variation of this technology is the digital synthesis arrayed radar. HERMES and most radar systems employ a very short pulse, radiated into the deck as a very broad-band signal. “The pulses are on the order in picoseconds [one trillionths of a second], and the frequency content are in gigahertz [billions of cycles per second],” Chase said. “But they are very low energy, spread out over a broad band. Short wavelengths give you more detail, but don’t propagate very far into a material. Lower frequencies give you less resolution, but go deeper.
“What the digital synthesis radar does is, rather than rely on a pulse, it synthesizes [creates] a particular frequency digitally, and radiates that specific frequency,” Chase said. “By hopping or changing frequencies rapidly you can ‘interrogate’ the bridge deck, and it makes it easier to comply with prohibitions on intentional radiation of electromagnetic energy in certain protected wavelengths.”
Acoustic Emission Technology
Acoustic emission technology is a process especially suited for steel bridges, by which engineers “listen” for characteristic signals associated with cracks forming and extending.
“It’s used primarily for detecting and locating fatigue cracks in steel highway structures, and it’s a technology used in a variety of other industries,” Chase said. “When energy is released as a crack propagates, a series of stress waves are generated that can be detected with sensitive accelerometers, combined with sophisticated signal processing that separates those signals from the rest of the noise of a highway bridge.”
Acoustic emission technology is central to a new technology from the Center for Advanced Materials and Smart Structures at North Carolina A&T State University.
There, in 2007, Dr. Mannur Sundaresan, professor of mechanical engineering, developed a single-channel continuous sensor that has the potential to detect and locate early crack growth in structures, thereby providing timely information to prevent catastrophic failures. This single channel continuous sensor can detect the leading edge of the acoustic emission event, occurring anywhere in the region covered by the sensor.
Essentially, the technology involves using commercially-available sensors deployed in a unique configuration to acoustically monitor structural integrity to remotely detect and address standard flaws via acoustic emission signals.
According to Sundaresan, the technology operates like the body’s nervous system. “If you’re hurt, the nervous system lets you know right away,” he said. “That doesn’t happen with a structure. An inspector has to go look. With small cracks, it’s like finding a needle in a haystack. Small cracks are like cancer. They’re usually not noticed until they’ve grown large enough to cause serious damage. These sensors will detect the growth of cracks in their early stages just as our nervous system alerts us of any injury immediately so that we can take action to limit the damage.”
Even as development of GPR and acoustic systems progresses, attention is shifting to how structures themselves may be outfitted to provide condition data. These “smart” bridges comprise an exciting new opportunity for advanced bridge monitoring.
Smart systems will have several advantages over GPR-based systems. They can provide information that is continuous and that takes place between survey visits. They can measure elements that GPR cannot measure, such as stress, strain, temperature, displacement and vibration, all of which are very useful in determining what is going on in a bridge structure. And they can measure performance instead of detecting damage, providing quantitative information, such as the scenario behind the generation of fatigue cracks.
“The definition of ‘smart structures’ has changed over time,” Chase said. “The National Nanotechnology Initiative has been important in producing new technology that can be turned into a sensor at a scale much, much smaller than before. And the new wireless communication technologies and miniaturization of computers are helping to bring the application of this sensor technology to civil structures. So it’s now possible to deploy multiple sensors – that are relatively small – that do not require much power and can more easily interrogate the structure at hand.”
For example, small, wireless sensors can be can be attached to bridge superstructures to measure variables such as strain, tilt, vibrations, temperature, and seismic activity. Using this technology, it’s possible to rapidly instrument a bridge at fatigue-prone or critical details, and measure what happens under traffic and wind loading.
Another example: recently a wireless sensing system from MicroStrain Inc., Williston, Vt., was installed on the I-95 Gold Star Bridge over the Thames River at New London, Conn. The sensors are powered via 6-in. by 9-in. photovoltaic panels, linked to rechargeable batteries which power microelectronic modules that record data from inside watertight enclosures. The data are wirelessly transmitted to an agency database. Because they are solar-powered, there is no need to manually replace batteries, a benefit as the sensors may be installed in hard-to-access places.
“Just the expense of running power cables to dozens or hundreds of sensors can be more expensive than the sensors themselves,” Chase said. “If we can get away from the need to have wires, and have inexpensive sensors that are compatible with wireless communication and data acquisition systems, and are tailored to the particular job they are asked to do, then it becomes economically possible to implement a system that will actually measure what’s going in multiple locations in a bridge in response to age, deterioration or traffic.”
The most common types of sensors will be battery-powered, with an emphasis on battery life. But they don’t have to be self-powered, as unpowered receiver-transmitter transponders also will do the trick.
“There is a new focus on sensors that will ‘wake up’ when you send them radio waves, measure conditions, transmit that data, and go back to sleep,” Chase told Better Roads. “These are best used for measurements that won’t change too much. But for conditions that are constantly under traffic, or wind loads, and continuous information is required, you will have to have a continuous power supply.”
‘Smart’ can apply to bearings as well. Smart bridge bearings take advantage of the fact that the distribution of live and dead loads to the bearings through the structural systems of the bridge can be used to diagnose problems.
Non-operating bearings and the tremendous stresses that result are a common factor in bridge failures and are a common maintenance requirement.
Multi-axis fiber optic strain sensors, capable of measuring both vertical and shear strains, are integrated into a composite panel. The panel then is laminated between the neoprene bearing pads commonly used on highway bridges, and will measure the vertical and lateral forces transmitted from and to the bridge.
Nanotechnology – and its application to bridge monitoring – constitutes the next frontier in smart structures.
Nanotechnology encompasses research, development and manufacture that utilizes and manipulates the unique properties of matter existing at the “nanoscale.”
At this length scale – approximately 1 to 100 nanometers, 1 to 100 billionths of a meter – clusters of atoms and molecules exhibit properties quite different from those found at larger scales. Thus, nanoscale science and engineering provide an opportunity to gain unprecedented insight into the unique phenomena existing at the nanoscale and to use that knowledge to engineer materials and devices with new characteristics.
With nanotechnology, super-small devices can be designed and manufactured to infinitesimal degrees of tolerance. Nanotechnology involves fabrication of devices with atomic or molecular scale precision, and at such a small scale, physical forces different from those of the ordinary human dimension are at play.
Today, nano- and micro-electrical mechanical systems (MEMS) sensors have been developed and used in construction to monitor or control the environmental condition, and the materials and structural performance. One advantage of these sensors is their small dimension; such sensors could be embedded into the structure during the construction process.
Larger than MEMS, “smart aggregate” has been used to monitor early age concrete properties such as moisture, temperature, relative humidity and early age strength development. The sensors can also be used to monitor concrete corrosion and cracking.
In a structural concrete matrix, smart aggregates can monitor internal stresses, cracks and other physical forces, and can be capable of providing an early indication of the health of the structure before failure can occur.
For example, researchers at Johns Hopkins University’s Applied Physics Laboratory developed a robust wireless embedded sensor suitable for long-term field monitoring of corrosion in rebar, particularly in bridge decks. These smart aggregate sensors can be embedded throughout a structure during construction, added to the mix right before placement. The smart aggregates are interrogated by a data reader that can be mounted on a car or truck; the transmitted energy from the reader excites the aggregates as it passes over them and collects their radiated sensor data onto a PC.
Each Johns Hopkins smart aggregate contains a wireless power receiver and data transmission coils, and incorporates ceramic hybrid integrated circuit technology to withstand mechanical stresses and concrete’s high pH environment. The aggregates are built to have a lifetime of 50-plus years.
“Nanotechnology will impact smart structures because the ability to manufacture sensors integrating nanotechnology gives us the potential to sense things that we could not in the past,” Chase said.
“For example,” he said, “there is work going on to develop chemical sensors that will serve as ‘artificial noses’, that can provide a very broad band of response to a variety of atmospheric gases. You can create a sensor that will be sensitive specific to a particular chemical, small enough that they will fit into a particular capsule, and mount that sensor on a structure. It then can tell you when the chloride ion concentration in the concrete has increased to the point where it might cause corrosion, but nondestructively, at a stage before there was any visible indication that damage had been done, with no inspector required to visit the bridge.”
Smart Bridges in Michigan
In early 2009, a new $19-million project on smart bridges was launched by the University of Michigan-Ann Arbor, with cooperation of the Michigan DOT.
The five-year project aims to create the ultimate infrastructure monitoring system and install it on several test bridges whose precise locations are not yet determined.
The monitoring system is envisioned to include several different types of surface and penetrating sensors to detect cracks, corrosion and other signs of weakness. The system would also measure the effects of heavy trucks on bridges, which is extremely difficult. And through enhanced antennas and the Internet, the system would wirelessly relay the information it gathers to an inspector on site or in an office miles away.
Funded in large part by nearly $9 million from the National Institute of Standards and Technology’s Technology Innovation Program, the project involves 14 UM researchers with the College of Engineering, and the UM Transportation Research Institute (UMTRI). In addition, engineers at five private firms in New York, California and Michigan are key team members.
The remaining funding comes from cost-sharing among the entities involved and the Michigan DOT. MDOT has offered unfettered access to state bridges to serve as high-visibility test-beds showcasing the project technology.
“This project will accelerate the field of structural health monitoring and ultimately improve the safety of the nation’s aging bridges and other infrastructures,” said Jerome Lynch, principal investigator on the project, and assistant professor in the UM Department of Civil and Environmental Engineering. “We want to develop new technologies to create a two-way conduit of information between the bridge official and the bridge.”
Four types of sensors will contribute to gathering data. Victor Li, professor of civil and environmental engineering, has developed a high-performance, fiber-reinforced, bendable concrete that’s more durable than traditional concrete and also conducts electricity. Researchers would measure changes in conductivity, which would signal weaknesses in the bridge. On test bridges, the deck would be replaced with this concrete.
A carbon nanotube-based “sensing skin” that Lynch and a colleague in chemical engineering are developing would be glued or painted on to “hot spots” to detect cracks and corrosion invisible to the human eye. The skin’s perimeter is lined with electrodes that run a current over the skin to read what’s happening underneath based on changes in the electrical resistance.
The sensing skin that Lynch and his colleagues created is an opaque, black material made of layers of polymers. Networks of carbon nanotubes run through the polymers. Carbon nanotubes are a fundamental building block of the nanotechnology revolution.
Each layer of the sensing skin can measure something different. One tests the pH level of the structure, which changes when corrosion is happening. Another layer registers cracks by actually cracking under the same conditions that the structure would.
The perimeter of the carbon nanotube skin is lined with electrodes that are connected to a microprocessor. To read what’s going on underneath the skin, scientists (or inspectors) send an electric current through the embedded carbon nanotubes. Corrosion and cracking cause changes in the electrical resistance in the nanotube skin. The microprocessor then creates a two-dimensional visual map of that resistance. The map shows inspectors any corrosion or fracturing too small for human eyes to detect.
Lynch says the skin could be a permanent veneer over strain- and corrosion-prone hot spots including joints on bridges, buildings, airplanes and even spacecraft. When it’s time to examine the health of the structure or aircraft, an inspector could push a button and in minutes, the skin would generate an electrical resistance map and wirelessly send it to the inspector.
Also in the Michigan tests, low-power, low-cost wireless nodes could look for classical damage responses like strain and changes in vibration. These nodes would harvest energy from vibrations on the bridge or even radio waves in the air. They are being developed by Dennis Sylvester, an associate professor in the Department of Electrical Engineering and Computer Science, and Khalil Najafi, chair of the Electrical and Computer Engineering division.
The fourth type of sensor would be housed in the vehicles that travel on the bridge. UMTRI researchers will outfit a test vehicle to measure the bridge’s reaction to the strain the vehicle imposes. This information generally is not available today. But how vehicles, especially trucks, affect bridges is a critical piece of information that could help predict the structure’s lifetime.
Leading this effort is research professor Tim Gordon, head of UMTRI’s Engineering Research Division. “Our work will add to what is currently done, not replace it,” Gordon said. “The infrastructure problem and the feasibility of new monitoring strategies are emerging at the same time. We believe we have ways of testing the performance of bridges as integrated structures, not just inspecting their components.”
“The technologies from this project could prove very beneficial to the citizens of Michigan in the longer lasting, smarter, safer and ultimately more sustainable roadways,” said state transportation director Kirk T. Steudle, P.E.
Carbon Nanotubes at Work
Carbon nanotubes also will figure into two research projects announced in early April by FHWA. The project, under way at Florida State University, seeks to develop technologies to inhibit corrosion for new in-situ materials, and methods to repair or retrofit structures located both above and underwater. The research will utilize carbon nanotubes to develop an on-site spray-based method to develop both a structural capacity enhancement, and a barrier layer for corrosion resistance.
FHWA also issued a cooperative agreement for the University of Minnesota-Duluth to develop new, intelligent, self-sensing concrete pavement that can monitor its own structural health by continuously detecting internal stress level changes of the pavement. In the proposed pavement structure, the concrete will be mixed with carbon nanotubes, the piezoressitive property of which will enable the concrete to detect the changes in the mechanical stress.
Phase I of the proposed work will develop and test a prototype of self-sensing CNT concrete in a lab environment, and Phase II, which will be conducted in partnership of the Minnesota DOT, will fabricate and test the self-sensing concrete in a real but controlled road environment at the Minnesota Road Research Facility just north of Minneapolis.
In the meantime, tried-and-true technologies like GPR are continuing to make their way into the field, but it’s only possible through improved technology.
“There are companies that will conduct a commercial GPR survey, and provide you with information that is based on more than just looking at echoes,” Chase said. “The work on phased array, on synthetic aperture, computer-aided tomography, and storing all this data on a computer and processing it to get information about what’s going on under the surface, all has evolved significantly in the last 15 years. A number of commercially-available systems have the capability of doing this type of signal processing, where it was not the case 15 years ago.”
And much more has transpired over the last decade-and-a-half. “The great thing is that simultaneous technological advances are making things possible today that weren’t possible just 15 years ago,” Chase said. “There have been tremendous advances in battery technology driven by the cell phone and wireless community that are now making it possible to have battery lives that are much longer than before. There also has been a focus on the development of low-power components, again, driven by the wireless technology industry, and they all have benefits for the bridge monitoring community.” v