Engineering Inc. - May/June 2007 - (Page 38)

GuEst ColuMN tEChNoloGy “Smart” ConCrete LeadS to more InteLLIgent ConStruCtIon By Greg Stutz Concrete is considered less glamorous than other basic building materials, so it may come as a surprise that, in fact, it can be much “smarter” than other components. Rapidly evolving smart sensor technology brings a new dimension to precasting, with computer chips embedded in the concrete when it is poured. The sensors then monitor everything that is going on with the chemistry, physics and life expectancy of the product. Smart sensors that monitor the physical, biological or chemical input of the concrete transmit data to a computer via wireless or cable connection. Until recently, failure in concrete could be observed only through visual inspection. And while concrete is known as the only building material that grows stronger with time, manufacturers of reinforced concrete systems cannot successfully predict unexpected loads and forces that a product will experience throughout its long life—which could be centuries. But there is an increasing demand for structures that can provide real-time monitoring to guarantee safety and to optimize the use of financial and natural resources. Precast concrete is ideally suited for this type of smart manufacturing. Concrete is the only material that can embed smart sensor technology within itself with no effect on the operation of the sensing devices or on the structure’s strength properties. The Greg Stutz knowledge obtained with this technology can increase the lifespan of the structure—thus decreasing the total cost of installation—as well as provide information for safer designs. Smart sensor technology also may eventually change how codes and specifications are written, as now they can be based on data coming from within the structure. While smart sensors can be embedded in the concrete mix, they also can be used externally. The monitoring of a new or existing structure can be approached either from the material or from the structural perspective. In the first case, monitoring will concentrate on the local properties of the materials used (concrete, steel, timber, composite materials) and record their behavior under load, temperature variations or aging. Short base-length strain sensors are the ideal transmitters for this type of monitoring approach. If many of these sensors are installed at different points, it is possible to extrapolate information about the behavior of the whole structure. In the structural approach, the structure is observed from a 38 ENGINEERING INC. MAy / JuNE 2007 geometric point of view. By using long-gauge length deformation sensors with measurement bases much larger than the characteristic dimensions of the materials (for example, a few yards for a concrete bridge), it is possible to obtain information about the deformations of the whole structure and extrapolate to the global behavior of the construction materials. This approach, which usually requires fewer sensors than the material approach, will detect material degradation—such as cracking or flow—only if it has an impact on the form of the structure. The availability of reliable strain sensors, such as resistance strain gauges, has concentrated most research efforts on material monitoring rather than structural monitoring. The latter typically has been realized using external measuring methods like triangulation, dial gauges and invar wires (nickel alloys). Steel reinforcement (rebar) corrosion in concrete is an electrochemical process that produces an electric current similar to that of a battery. This electric current spreads out from the rebar into the surrounding concrete, and the resulting voltage is measurable at the surface of the concrete. The amount of current flow is directly proportional to the rate of loss of the steel mass. There are no geophysical techniques that indicate the amount of corrosion that takes place, reducing the quality of the rebar. However, there are numerous methods used to find if corrosion currently is active. If the quality of the rebar becomes degraded while corrosion activity is monitored by a smart sensor, which finds the corrosion to be active for a significant period of time, more will be known regarding the structure’s life cycle. Today’s smart sensor technology also is capable of recording vibrations (such as harmonic motion or earthquakes), wind pressure, in-service loading, component creep, snow accumulation, integrity of components, stress-strain responses of components and effects of temperature extremes. There even are situations in which nuclear, biological and chemical threat detection devices can be embedded. Smart sensors work virtually calibration-free and therefore can be used for the long-term monitoring of concrete structures—for an expected 50 years. The data collected from these sensors could lead to technological advances that would improve the already impressive characteristics of precast concrete structures. n Greg Stutz, P.E., is vice president of technical services at the National Precast Concrete Association.

Table of Contents for the Digital Edition of Engineering Inc. - May/June 2007

Table of Contents
From ACEC to You
News & Notes
Market Watch
Legislative Ac tion
Interview with Congressman James Oberstar
Port Security
Water: Managing our Precious Commodity
Engineering Excellence Awards
“Smart” Concrete
Business Insights
Members in the News
One on One

Engineering Inc. - May/June 2007