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<p>As of 2013, the damage caused by corrosion on highway bridges has been estimated to cost approximately 14 billion dollars annually, and this cost has been increasing over the years. Corrosion is one of the natural phenomena that has been slowly deteriorating infrastructure systems across the United States. One of the most problematic types of corrosion is crevice corrosion, which is defined as the formation of rust between overlapping surfaces such as the case of a splice connection where flanges are attached by splice plates. A significant number of steel bridges in Indiana have developed crevice corrosion in splice connections. Therefore, this research focuses on the crevice corrosion, or “pack rust”, occurring in these structural elements. The application of coatings alone has not been enough to stop pack rust at these connections. In an attempt to look for approaches that can effectively mitigate this problem and maintain the designed service life of bridges, different strategies have been studied and tested. The first objective of this study is to determine the strength reduction as a function of the time of exposure to salt misting. To do this, specimens that simulate the bottom flange splice connection have been exposed to a corrosive environment for different periods of time and later tested under tension to assess the reduction in strength. The second objective is to evaluate the effectiveness of the mitigation strategies under different conditions. First, the mitigating products were initially applied before exposure to salt misting. Second, the mitigating products were applied as a repair, and in this case, the specimens corroded for a given period of time and were then repaired to evaluate any further deterioration. The assessment of the strategies’ effectiveness is based on the strength reduction and visual inspection of the specimens. The ultimate outcome of this study is a series of general guidelines to slow down crevice corrosion based on the results of the laboratory testing. </p>

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21090号 / 工博第4454号 / 新制||工||1692(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 五十嵐 晃, 教授 澤田 純男, 教授 KIM Chul-Woo / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Stålbroars approximativa livslängd bestäms av stålets utmattningshållfasthet, då utmattning är en av de främsta anledningarna till att livslängden begränsas. I Sverige existerar ett antal broar som närmar sig slutet av sin livslängd, samtidigt som behovet för kapacitet och kraven på broarna ökar. Flertalet av dessa broar är i behov av upprustning. Däremot är det inte möjligt ur vare sig ett ekonomiskt- eller miljöperspektiv att byta ut alla broar, och därför behöver de broar som är mest kritiska prioriteras. Vid utmattningsdimensionering av stålbroar beaktas hela spänningsvidden, oavsett om spänningarna är i drag eller tryck. En spricka propagerar endast vid dragspänningar, vilket innebär att tryck- spänningar egentligen inte bör vägas in i samband med dimensionering. Detta innebär att en del stålbroar skulle kunna ha en längre livslängd än vad den traditionella dimensioneringen ger. Spänningsintensitetsfaktorn 𝐾 används inom brottmekaniken för att förutspå spänningsintensiteten i närheten av sprickspetsen, och appliceras till linjärelastiska material. Det finita elementprogrammet Abaqus användes när brodetaljen modellerades och analyserades. Brodetaljen representerar en balk med en påsvetsad anslutningsplåt, som utsätts för trafiklasten på en bro och en temperaturlast för att simulera egenspänningar. Detaljen representerar problematiken med utmattning i stålkonstruktionsdelar. Motivet för denna studie är att inga sprickor har hittats under inspektioner av liknande detaljer, det till trots att en del stålbroar teoretiskt sett förbrukat sin livslängd. Studien genomfördes med en mer avancerad modell än vad som vanligtvis skapas för bedömning av utmattning, med syftet att modellera verkligheten mer korrekt. Resultaten visar hur egenspänningarna bidrar till dragspänningar, vilket leder till sprickpropagering i modellen. Vid spricklängden 9,5 mm övergår spänningarna från drag till tryck, och då upphör spricktillväxten. Resultaten visar även att utmattningssprickor kan växa i stålkonstruktionsdelar som i huvudsak utsätts för nominella tryckspänningar, ifall höga egenspänningar uppstår vid anslutningsplåten. / Steel bridges estimated service life is determined by the fatigue strength of the steel, since fatigue is one of the main reasons for limiting the service life. In Sweden there is a number of bridges that approach the end of their service life, while the need of increasing the capacity and demands on bridges grows. The majority of these bridges is in need of reparation. On the other hand, it is not possible either from a financial- or environmental perspective to replace all bridges, and therefore the bridges that are most critical needs priority. In the case of fatigue design calculation of steel bridges, the entire stress range is taken into account, regardless of whether the stresses are in tension or pressure. A crack propagates only at tensile stresses, which means that pressure should not really be considered in the design calculations. This means that some steel bridges could have a longer life span than the traditional design calculation gives. The stress intensity factor K is used within the fracture mechanism to predict the stress intensity near crack tip, and is applied to linear elastic materials. The finite element program Abaqus was used when the bridge detail was modeled and analyzed. The bridge detail represents a beam with a welded connection plate, which is exposed to traffic load at the bridge and a temperature load to simulate residual stresses. The detail represents the problem of fatigue in steel structural parts. The motive for this study is that no cracks have been found during inspections of similar details, despite the fact that some steel bridges theoretically have consumed their longevity. The study is conducted with a more advanced model than usually created for assessment of fatigue, with the purpose of modeling the reality more correctly. The results show how the residual stresses cause tensile stresses, which leads to crack propagation in the model. At a crack length of 9,5 mm, the stresses change from tension to compression, and then the crack growth ceases. The results also indicate that fatigue cracking can grow in steel structural parts that are mainly exposed to compressive nominal stresses, if tensile residual stresses appear at the connection plate.

[pt] Pontes rodoviárias são estruturas sujeitas à ação conjunta de fadiga e corrosão. Nesse sentido, este trabalho propõe um protótipo de sistema para auxiliar na tomada de decisão sobre a manutenção de pontes de viga de aço, no qual apenas pontes simplesmente apoiadas são consideradas. O sistema considera os efeitos da corrosão-fadiga e estima o tempo de vida útil dessas estruturas a partir de uma análise de confiabilidade estrutural. Tal análise é baseada nas curvas S-N para fadiga da norma americana AASHTO e na regra do acúmulo de dano de Miner. Para considerar a perda de material e a perda de resistência causadas pela corrosão, são realizados ajustes nos parâmetros de fadiga do modelo de confiabilidade utilizado. Além disso, algumas alternativas de manutenção que reduzem a taxa de degradação são consideradas no sistema com a correção dos parâmetros de corrosão do material. O sistema utiliza uma rede neural artificial para estimar as tensões nas vigas de aço com a passagem de um veículo de fadiga da AASHTO a partir de dados geométricos e de material da ponte. A base de dados utilizada para o desenvolvimento da rede neural foi criada a partir de resultados de simulações por elementos finitos. O ciclo de vida de uma ponte de aço projetada conforme a norma americana é simulado usando o sistema proposto. Através dessa simulação, conclui-se que o tempo de vida útil da mesma está relacionado com a combinação dos parâmetros de tráfego e da agressividade do ambiente. Conclui-se também que o aumento no volume de tráfego médio diário de caminhões (ADTT) pode causar uma redução de 48 por cento a 76 por cento na vida útil e dependendo do aumento na sua taxa de crescimento anual a redução pode ser de até 80 por cento. As alternativas de lavagem da superestrutura proporcionam ganho de vida útil para todos os ambientes, sobretudo no ambiente marinho (podendo chegar até 30 por cento). No entanto, em alguns dos cenários de tráfego testados nas simulações esse ganho não é suficiente para garantir o tempo de vida útil mínimo recomendado pela AASHTO. / [en] Highway bridges are structures subjected to the both action of fatigue and corrosion. In this sense, this research proposes the prototype of a system to assist in the maintenance decision making process of steel beam bridges, in which only simply supported steel girder bridges are considered. The system contemplates the effects of corrosion-fatigue and estimates the lifetime of these structures based on a structural reliability analysis. Such analysis is based on AASHTO S-N fatigue curves and Miner s damage accumulation rule. In order to account for material and resistance loss caused by corrosion, adjustments are made to the fatigue parameters of the reliability model used. In addition, some maintenance strategies that reduce the rate of corrosion are considered in the system by correcting the corrosion parameters of the material. The system uses an artificial neural network model to estimate the stresses in steel beams under the passage of an AASHTO fatigue vehicle using geometric and material data from the bridge. The database used for the development of the neural network was created from finite element simulations results. The life cycle of a steel bridge, designed according to the AASHTO American standard, is simulated using the proposed system. Through this simulation, it is concluded that the structure s lifetime is related to the combination of traffic parameters and the environment corrosivity. It is also observed that the increase in the average daily truck traffic (ADTT) can cause a reduction from 48 percent to 76 percent in the bridge s lifetime and depending on its annual growth increase rate the reduction can go up to 80 percent. Maintenance alternatives for the superstructure such as washing provide a lifetime gain for all environments, especially in the marine environment (up to 30 percent). However, in some of the tested traffic scenarios this gain is not sufficient to guarantee the minimum service life recommended by AASHTO.

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