Concrete is giving up its secrets. Prodded by engineers and contractors frustrated with having to imagine, rather than measure, the changing forces inside sullen masses of hardening mix, scientists and technologists are working to perfect sensors that finally allow a look inside.

Sensor technology is riding high on a decade of heady advances in electronics, computers and communications. As sure as concrete, sensors embedded into highways, bridges and buildings are rising stars of construction.

It’s no longer just researchers using sensors to study concrete and measure changes in integrated structures that can affect the material. Contractors and engineers are also applying sensors to verify designs and build and maintain facilities more efficiently. They are using sensors to speed construction by quickly and accurately monitoring concrete cure. Sensors are tracking the evolution of load and strain on structures during assembly, which helps engineers fine tune shoring and sequencing schemes on the fly.

Currently, at the visitors center being built at the U.S. Capitol in Washington, D.C., engineers are using wireless sensors to read transient strains that may affect concrete within several levels of composite decks as supporting steel beams, temporarily exposed to winter weather, react to temperature change (ENR 6/17/02 p. 30). Findings may change shoring plans.

Wireless sensors, particularly ones married to self-networking "motes" that automatically extend a communications quilt by linking with each other to relay data, are also in development and testing. They may be particularly suited to temporary monitoring systems, or placement in areas that are difficult to access.

Research at universities all over the country is forging on with trials and studies to refine practices for designing and installing sensor arrays.

At West Virginia University in Wheeling, Samir N. Shoukry, who holds a double chair in mechanical and aerospace as well as in civil and environmental engineering, has worked for decades with sensors that test both concrete design and techniques for installing sensors in road and bridge construction. "The sensors and instrumentation technology is available, but it has to be very carefully used," he says.

It’s in the Details

Links. Wireless mote sensors automatically network. Shoukry (right) holds array. Photo left by Greg Ellis, West Virginia University; right by Tom Sawyer for ENR)

Shoukry says high-quality, shielded cables are a must to avoid collecting noise instead of data. Data must be processed religiously, or else it becomes like an old, unbalanced checkbook: good for nothing. Sensors must be properly placed and bonded to avoid failure or interference with concrete performance. "Once the sensor is placed and the concrete is poured you can’t do anything about it," he says. "Mistakes cannot be corrected." He also notes that installing sensors takes time. Contractors need to plan for it.

Shoukry is a strong believer in not only the promise, but the need for instrumentation in infrastructure construction. His latest project is a 1,100-ft-long concrete bridge near his institution. It carries 850 sensors bonded to rebar and supporting steel. They monitor many phenomenon, including strain on reinforcing rods and crack development, and sheer and buckling and bending moment in girders. They also track weight-in-motion of rolling loads. Readings are correlated to weather data collected at the site and are accessible over the Internet. It is a heavily instrumented bridge by any measure, and Shoukry says more such projects are needed.

"Compare the cost of infrastructure in the U.S. to anything….Boeing, General Motors, add them together even, and anything would look small compared to the cost of our infrastructure," notes Shoukry. "To put an investment into instrument sensors should be a national initiative. It’s the basic health insurance of our infrastructure."

Other researchers continue to build data from long-running embedded sensor studies that are testing concrete durability and patch techniques. Daniel Cusson, a researcher at Canada’s Institute for Research in Construction, at the National Research Council in Ottawa, has ac-cumulated data from within a concrete bridge over several years. His prime goal is evaluating materials and techniques for repairs, but there is the overriding goal of adding to the body of knowledge of how sensor data can be used.

"We are interested in the use of sensors for the information they provide on the actual performance of the structures under monitoring [but] we hope that the use of embedded sensors in the construction of structures for real-time condition/safety assessment will increase," Cusson says.

He hopes owners, "after several successful demonstration projects," will see the benefits of using sensors in the construction of important structures. As confidence increases and costs come down, the technology would be applied to construction of common structures. Cousins says wide use of sensors for long-term "health"...