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Nanometers? Tiny sensors embedded in concrete will collect physical and chemical data
By Deborah Snoonian, P.E.

These mini measuring devices need no external power source—they’re powered by radio waves. Photography: Courtesy Advanced Design Consulting

The National Science Foundation has awarded a grant to Advanced Design Consulting (ADC) of Lansing, New York, to develop tiny passive sensors that can be mixed into concrete to collect data on the physical and chemical characteristics that affect its strength and performance.

The company will build the sensors using a silicon-based nanotechnology known as a microelectromechanical system (MEMS). Each device will consist of four or five MEMS sensors that measure information such as pH, moisture, temperature, and concentrations of chloride, sodium, and potassium ions within the concrete. It will be paired with a radio frequency identification device (RFID) for ID and measurement purposes. The devices would then be encapsulated within a durable, noncorrosive material, similar to a pill, that would allow them to be mixed into concrete bridge decks, walls, or forms. ADC will manufacture the MEMS devices at nearby Cornell University’s NanoScale Science and Technology Facility.

By detecting physical and chemical changes in concrete as they occur, engineers can head off rebar corrosion and cracking before they become a critical problem. Steel rebar in concrete is protected against corrosion by the high pH of Portland cement. But deterioration is kick-started when deicing salts penetrate concrete, or when carbon dioxide permeates through concrete’s pores and combines with the lime in cement to create calcium carbonate. Both of these processes upset the pH balance of the mix, which makes the steel vulnerable to decay and compromises concrete’s long-term compressive strength. “These devices will provide critical data for evaluating concrete performance from its freshly mixed stage to its casting, through the concrete’s service life to its period of deterioration and repair,” says Eric Johnson, vice president of research at ADC.

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The company also aims to displace shopworn practices in construction. “We want to get rid of the slump test and the cylinder test,” says Johnson, referring to two long-used, low-tech physical methods to predict performance before—or while—concrete is poured, not during its service life.

Researchers have previously developed moisture sensors for concrete, but they have limitations when embedded in the material, says Alex Deyhim, president of ADC. Most sensors need external power, for instance, which increases their size and shortens their life span.

Within the first six months of the grant period, ADC will demonstrate that the sensors can be manufactured and data collected from them without an external power source. The sensors will be in “off” mode when buried in the concrete, until a handheld monitor emitting radio waves passes over them—then they will “kick on,” take measurements, and reradiate the information back to the monitor. “The low cost and ability to send real-time data will make this system particularly useful,” says Deyhim. The devices could be mixed randomly within a road deck or form during a pour, says Johnson, but for easier data collection, they would likely be placed along a linear or known distribution pattern (parallel to rebar, for instance).

ADC’s work adds to a growing body of nondestructive inspection and measurement technologies in construction, which assess the condition of in-place materials without the need for collecting cores or samples that break the material apart. Reinforced-concrete bridge decks will serve as the first case studies for evaluating the viability of the MEMS sensors; in the future, ADC expects tiny sensors of all stripes to be viable for buildings, roads, and other types of infrastructure. Good things, it seems, will come in small packages.