Donna Mae Baukat
Advanced Structural Overlay, Inc.
Advanced materials that have been proven stronger than steel in lightweight applications of aerospace and aeronautics are now found to be ideal alternatives in construction practices. Recent years have given researchers and construction experts experience in applying advanced materialsâ€”glass or carbon fiber reinforced polymer (FRP) compositesâ€”to bridge columns, joints, and other structural support beams.
The Department of Transportation and Federal Highway Administration have participated in studies with researchers using FRP composites to repair or rehabilitate concrete (reinforced and unreinforced). It might be another three years before standards are reached. Recent tests prove the ability of advanced materials to contain concrete when it fails, and may prevent the type of collapse we saw in the September 11 attacks on the World Trade Center and the Pentagon. Kevlar used in the Pentagon retrofit of the building that was hit by a passenger airplane lends further credence to the use of FRP composites in structures made of concrete and steel. Innovative technology for strengthening shear walls and structural beams is not widely accepted. Why? There is insufficient data to determine the quality and performance of FRP composites, and standards should be created to support the concept. One way to accomplish data acquisition is in the use of long-term health monitoring systems.
The concept of health monitoring systems using fiber optical strain measurements is possible on bridge structures, as well as on other concrete structures designed by civil engineers. By embedding sensors in pipelines, for instance, the location of the break will be easily obtained with continuous health monitoring. Installing sensors is possible with current structural materials technology. The challenge, however, is in weight and size. Electronic strain gauges (or gages) are commonly used to transmit strains under loads during strain measurements, as specified by highway standards; but, loads on a bridge challenge the use of heavy sensors. They are bulky and not as accurate as advanced systems using optical fibers that have multiple sensors in one cable. The use of fiber optics with sensors capable of measuring multiple points is more accurate than heavy gauges measuring a single point of a bridge, according to researchers. Optical fibers are lightweight, and they can be bonded to existing steel, wood, concrete and other alloys; preferably, embedding fiber sensors between steel and concrete could detect bends inside a support beam, before a crack appears in the concrete.
Currently, the FHWA Turner-Fairbank Research Center is studying a few bridges using fiber optical strain systems. Suppliers may be found through the American Society for Nondestructive Testing (ASNT), online at www.asnt.org, for structural materials technology in nondestructive evaluation (NDE) or nondestructive testing (NDT). Systems using fiber optical sensors (FOS) for long-term monitoring is new, and growing interest encourages researchers to improve on innovative technology using either Fabry-Perot, or Fiber Bragg Grating (FBG) sensors because of its accuracies and solution to load issues. Minute, smaller cracks are undetectable with visual inspection, as we learn in laboratory studies. With FOSS (system with sensors connected to opto-electronic unit), vital data can be collected to detect changes occurring behind a concrete wall, whether for static or dynamic loads. To clarify the types of FOS, it helps to understand that Fabry-Perot sensor is commonly used for single point sensing; FBG is more costly, due to technology measuring multiple points. Opto-electronic equipment has to be "smart," designed to receive multiple FBG sensors at the same time with as many as 80-100 per unit.
Aging structures could be repaired more economically with advanced systems that would increase their lifetime, and in historical monuments it is important to innovate with FRP composite systems. In order to develop higher performing and reliable FRP composite systems, it is critical to monitor the systems for potential delaminations that pose problems in those advanced materials. Without technical discussion, FRP composite systems have failed in performance, and researchers have pointed the finger at uncontrolled use of bonding agents and environmental exposures to moisture. That is why the use of embedded sensors in FRP composite systems is so vital to improved methodologies.
New bridge deck systems made of glass/carbon FRP have been tested recently, and the use of FOS has provided civil engineers the ability to identify points of failure under dynamic loading. In design stages, manufacturers are able to improve products before completion of process and quality higher for ensuring successful installations. During the performance stages of those improved products, critical data must be collected to expedite advancements in research during the next decade. With each year, deteriorated structures become weaker, and there will come a time when there will be far too many structures to replace or repair. In significant transportation systems that link one community to another, experts are eager to put in place long-term monitoring systems. Funding and public awareness would increase with additional data collected in the next five years using innovative monitoring systems, such as FOSS in laboratories and on existing steel or concrete structures.
In one study that we performed at a major university on a new FRP composite deck for one DOT, we used four optical fibers with FBG sensors that were UV laser embedded before it was delivered to the site, and retrieved data from the sensors with a portable opto-electronic unit during loads up to 39.2Kips. Evidence of over 2,000 micro strains at the point of the sensor during increased loads provides the customer with data that supports design and load requirements. The GFRP deck failed under the two-point test, and the FOS remain embedded for future tests. In the meantime, the portable unit is returned to safe-storage and will return to the deck to collect further strain measurements under different loads. Weigh-in motion sensor systems could be adapted to the FOS, as well as programmed to send audible or visible alerts when certain loads are reached. The alternative to reacting to hazards, or aging of structures, is to use advanced materials to strengthen them.
Why not monitor those structures throughout the year, gathering data with the seasonal conditions or other events that would damage or negatively impact the bridge load capacity with lightweight sensors? Without further research, we cannot arrive at standards in those types of testing, or decide on the least destructive method of testing existing structural components.
Whether it's a bridge, a building, a stadium, a roller coaster or auditorium, we're talking about people's lives when in use. Instead of reacting, we must seek preventive measures. The use of FRP composites is a good alternative to expensive replacements, and the life of steel and concrete could be increased with appropriately designed strengthening or repair systems. Prevention in the form of regular health monitoring is a new concept.
Current steel bridges can be adapted with FOSS (fiber optical strain systems), and bridge health monitoring can be conducted continuously throughout a season, or at intervals while optical fibers remain protectedâ€”immune to corrosion or chemicals. There are no electronic emissions, nor bulky cables as current test equipment and strain gages present noise pollution, as well as electrical hazards. Bridge inspections are dangerous, and underground inspections of steel structures not feasible, while FOSS is capable of presenting empirical data on performance during static and dynamic loads. On bridges, emergency management would be in motion with the use of FOSS health monitorsâ€”while personnel are attending other vital activities. Prevention is vital to emergency management, and we have technology already available to provide communities the key to safer public structures.
To reach Donna Mae Baukat, call (714) 279-0449 or send e-mail to email@example.com.