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.
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