Precast Solutions - May/June 2008 - (Page 21)

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 Dennis Roach, project leader at the Sandia National Laboratories team, holds a Comparative Vacuum Monitoring (CVM) device showing galleries etched into the sensor's underside. The sensor is designed to check continuously for the formation of structural defects in bridges and highway overpasses. Photo by Randy Montoya, courtesy Sandia National Laboratories MAY/JUNE 2008 | PRECAST SOLUTIONS 21

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Precast Solutions - May/June 2008 Contents Sustainability and Practicality Security Blanket A Frank Lloyd Wright Takes Shape Concrete Joins MENSA Precast Lightens Up

Precast Solutions - May/June 2008

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