Fatigue cracks in steel bridges degrade the load-carrying capacity of these structures. Fatigue damage accumulation caused by the repetitive loading of everyday truck traffic can cause small fatigue cracks initiate. Understanding the growth of these fatigue cracks is critical to the safety and reliability of our transportation infrastructure. However, modeling fatigue in bridges is difficult due to the nature of the loading and variations in connection integrity. When fatigue cracks reach critical lengths failures occur causing partial or full closures, emergency repairs, and even full structural failure. Given the aging US highway and the trend towards asset management and life extension, the need for reliable, cost effective sensors and monitoring technologies to alert bridge owners when fatigue cracks are growing is higher than ever.
In this study, an innovative Long-Term Electrochemical Fatigue Sensor (LTEFS) has been developed and introduced to meet the growing NDT marketplace demand for sensors that have the ability to continuously monitor fatigue cracks. The performance of the LTEFS has been studied in the laboratory and in the field. Data was collected using machined specimens with different lengths of naturally initiated fatigue cracks, applied stress levels, applied stress ratios, and for both sinusoidal and real-life bridge spectrum type loading. The laboratory data was evaluated and used to develop an empirically based algorithm used for crack detection. Additionally, beta-tests on a real bridge structure has been completed. These studies have conclusively demonstrated that LTEFS holds great potential for long-term monitoring of fatigue cracks in steel structures
Even in the best of economic times, funding for infrastructure maintenance, repair and rehabilitation is never adequate. As infrastructure in the United States continues to age, the funding deficit to simply maintain the existing bridges will continue to soar. Due to the inadequacy of capital allocated for infrastructure repair and rehabilitation, new, more durable construction materials with potentially longer service lives are being explored as a means of narrowing the financial deficit. One such material is fiber reinforced polymer matrix composites (FRP). By replacing conventional bridge component structural materials (i.e.; reinforced concrete and steel) with FRP, which has a higher strength to weight ratio, bridges can achieve a significant reduction in dead load weight. Bridges that have experienced substructure and superstructure deterioration can undergo a superstructure replacement with FRP rather than be subjected to the traditional load posting (vehicular load restrictions). Through reducing the bridge dead load without compromising bridge strength, original design live loads can be maintained. In order for these new bridge superstructure components to be readily accepted as viable construction materials, quick and effective means of monitoring them for degradation and overall structural health must be established and standardized. One of the most promising methods of achieving this is through the use of thermal infrared (TIR). A slight increase in temperature above ambient will allow for adequate inspection of large sections of bridge decking for detection of debonded areas between FRP components. This paper illustrates the successes and challenges of using TIR for this purpose, both in the laboratory and in field investigations. Areas for future work and improvements will be suggested.
Statistics released in the fall 1989 show that 238,357 (41%) of the nation's 577,710 bridges are either structurally deficient or functionally obsolete. New materials are being explored for use in bridge systems to solve this problem. These materials are less affected by corrosive environmental conditions than conventional civil engineering materials and thus, require less maintenance and potentially provide a longer life span. A material being considered for these applications is glass fiber reinforced vinyl ester matrix composites. Fiber reinforced plastic (FRP) composite deck systems made of this material are favorable potential replacements for deteriorating conventional bridge decks. The decreased specific weight of the FRP greatly reduces the dead load of the superstructure helping avoid load posting of bridges. However there is a lack of long-term durability data concerning this material system in typical bridge environments. Thus, an efficient and effective method must be devised to monitor the health of an FRP structure in-situ. This paper will discuss the use of Infrared Thermography as a means of detecting structural imperfections -- delaminations, disbonds, voids -- caused by conditions encountered both in fabrication and in the field. As forced convection hot air is circulated through the bridge deck, delaminations and disbonds in the top of the deck appear cold while defects in the bottom of the deck give rise to areas with higher temperatures. The discontinuities in thermal propagation patterns are detected with a thermal imaging system and indicate present and possible future structural deficiencies. Laboratory results revealing fabrication/installation problems and those from field tests will be presented.
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