Precast Solutions

Concrete Joins MENSA
“Smart” concrete leads to more intelligent construction.

By Greg Stutz

People generally don’t attribute human traits to basic building materials, but concrete actually can be “smart.” Other materials such as wood, steel, specialty metals and glazing are not as “smart” as concrete, because intelligence can be embedded into a concrete mix. Civil engineers, scientists, owners and regulatory agencies can then observe real-time conditions about the concrete through an array of sensor technology that is rapidly evolving every year. As a result, concrete can recite everything that is going on with its chemistry, physics and life expectancy with little effort.

You gotta love/hate concrete

The designing community will admit that the nature of economic reinforced concrete design in major structures is heavily factored because of concrete’s behavior. Until recently, failure in this material was observed through visual inspection and viewed as relatively rapid and potentially catastrophic. While concrete is recognized as the only material that grows stronger over time, reinforced concrete systems cannot successfully predict unexpected loads and forces it will experience throughout its long life, which may be centuries.

The same holds true for the other building materials. The only defense is to apply safety factors that take into account various uncertainties in load and force amplitudes and structural response. There are no second chances or makeup quizzes that will prevent a failing body. Codes require this to protect people and assets. As a result, designs tend to be more conservative and require more raw materials (aggregates, cement and water) than may be truly necessary. But now there is an increasing demand for providing online, real-time health monitoring capability to important concrete structures in order to guarantee safety and to optimize the use of financial as well as natural resources.

Architects love concrete’s fluid and expressive properties. Concrete can take the shape of almost anything imaginable. However, the same professional group will criticize concrete’s lack of efficiency in its consumption of natural resources during production and installation. Again, all building materials face similar problems. We now have technology that will potentially change how codes and specifications are written based on the knowledge coming from within the structure’s material.

Concrete is the only material that can embed this technology within itself with no effect on the operation of the sensing devices or its strength properties. The knowledge obtained by this technology is real; it will provide data for safer designs and increase the lifespan of the structure thus decreasing overall total cost of installation over time.

Concrete battles the elements

The importance of monitoring all significant structures is evident. Monitoring is the fundamental order to guarantee the safety of a structure and its users (think of a dam, a bridge or a tunnel). It also helps in the planning of maintenance intervention and to increase the knowledge of its real behavior, permitting the optimization of future similar structures and their designs.

The monitoring of a new or existing structure can be approached either from the material or from the structural point of view. In the first case, monitoring will concentrate on the local properties of the materials used (concrete, steel, timber, composite materials) and observe their behavior under load, temperature variations or aging.

Short-base length strain sensors are the ideal transmitters for this type of monitoring approach. If a great many of these sensors are installed at different points, it is possible to extrapolate information about the behavior of the whole structure from these local measurements. In the structural approach, the structure is observed from a geometric point of view. By using long gauge length deformation sensors with measurement bases much larger than the characteristic dimensions of the materials (a few yards for a concrete bridge, for example), it is possible to gain information about the deformations of the whole structure and extrapolate the global behavior of the construction materials.

The structural monitoring approach will detect material degradation such as cracking or flow only if it has an impact on the form of the structure. This approach usually requires a reduced number of sensors compared with the material monitoring approach. The availability of reliable strain sensors such as resistance strain gauges have historically concentrated most research efforts in the area of material monitoring rather than structural monitoring. This latter has usually been realized by using external measuring methods such as triangulation, dial gauges and invar wires (nickel alloys).

The steel reinforcement in concrete structures (rebar) is susceptible to corrosion when chloride ions enter into the concrete. If chlorides are present in sufficient quantity, they disrupt the passive film on the rebar resulting in corrosion. Oxygen content, moisture content and temperature also affect the corrosion rate. Corroding rebar in concrete can weaken its structural strength, creating cracking, delamination and spalling. Rebar corrosion may also affect bonding of the rebar to the surrounding concrete, caused by changes in the diameter of the rebar and from the friable iron oxide, which may accompany the corrosion.

The corrosion of steel 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 would indicate the amount of corrosion that had taken place and, therefore, reduce the quality of the rebar. However, numerous methods can be used to learn whether corrosion is currently active. If corrosion activity is monitored and found to be active for a significant period of time, then the “quality” of the rebar will become degraded, and more will be known regarding the structure’s life cycle.

How smart is smart?

Smart concrete systems are structural systems with a certain level of autonomy relying on the embedded functions of sensors, actuators and processors. These systems can automatically adjust structural characteristics in response to the change in external disturbance and environments toward structural safety and serviceability as well as the elongation of structural service life.

Today’s sensor technology is capable of recording vibrations (harmonic motion and earthquakes), wind pressure, in-service loading, component creep, snow accumulation, integrity of components, stress-strain responses of components and effects of temperature extremes. There are even situations where nuclear, biological and chemical threat detection devices can be embedded that may prove to be of great significance for security.

Concrete comes to its “sensors”

What is a sensor? A sensor is a device that converts physical, biological or chemical input into an electrical or optical signal. The signal must be measured and transformed into digital format that can be processed and analyzed efficiently by computers. The information can be used by either a person or an intelligent device monitoring the activity to make decisions that maintain or change a course of action.

What is a smart sensor? A smart sensor is simply one that acquires the physical, biological or chemical input, and converts the measured value into a digital format in the units of the measured attribute and transmits that measured information to a computer monitoring point via wireless or cabled connection.

Other sensor types are being developed through industry and academic research groups. The goal of all sensor producers is to make the devices affordable since there could potentially be several thousand sensors applied to a single structure.

Conclusion

Smart sensors work virtually calibration free and can therefore be used for long-term monitoring of concrete structures. The durability of sensors embedded in concrete can be attained by using special sensor holders protecting the sensor elements and ensuring a perfect bond between the sensor and the surrounding concrete. Because of their very small dimensions, sensors can be used as “microsensors.” Their application in concrete system research might help to contribute to the solution of related experimental problems.

Greg Stutz is NPCA’s vice president of Technical Services.

Here is a sampling of sensor suppliers. A wealth of additional information and other suppliers can be found by searching the Internet for “concrete sensors” or similar key words.

• ADC (Advanced Design Consulting USA Inc.), (607) 533-3531, www.adc9001.com
• DGSI (Durham Geo Slope Indicator), ( 425) 493-6200, www.slopeindicator.com
• Geokon, (603) 448-1562, www.geokon.com
• Physical Acoustics Corp., ( 609) 716-4000, www.pacndt.com
• Roctest Telemac, (450) 465-1113, www.roctest.com
• Sandia National Laboratories, (800) 765-1678, www.sandia.gov
• Vishay Americas (Vishay Micro-Measurements), (402) 563-6296, www.vishay.com