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All structures and their components are subject to aging, wear and tear in the performance of their functions, and deterioration by exposure to the operating environment. Left to themselves, they will eventually become inefficient, unreliable, and fail. Maintenance problems are ubiquitous in all areas of engineering and maintenance methods are rather generally applicable. Here, the probabilistic aspects of maintenance will be emphasized and some applications to hydraulic engineering works will be discussed. Maintenance of existing structures is of continuing concern to owners and operators for economic, reliability and safety reasons. According to the definitions of the U.S. Army Corps of Engineers’ repair, evaluation, maintenance, and rehabilitation (REMR) research program (Scanlon et al. 1983), maintenance is defined as action that prevents or delays damage or deterioration, or corrects deficiencies that would otherwise lead to early repair or need for rehabilitation. Repair is restoration of damaged or deteriorated elements of a structure to continuing service, while rehabilitation is a major modification of an existing structure to bring it up to prevailing operation requirements and standards. Here the word maintenance is used sometimes in a wider sense that spans these different concepts. When a structure reaches a certain level of deterioration or obsolescence, economic or safety reasons may demand a slowdown, a halt, or reversal of the deterioration process. Maintenance usually can only reduce the deterioration rate but cannot eliminate or reverse it, as a structure can usually not maintain, let alone improve itself. Only repair or reconstruction (rehabilitation) can bring the structure or equipment back to an improved state or the as-good-as-new state. The timing and extent of this intervention must often be decided with incomplete knowledge of the actual state of the structure or equipment, its remaining strength, the loads acting on it, the true costs and benefits of rehabilitation alternatives, and other factors. One can deal with this uncertainty by allowing inputs to vary over a certain range and by considering various paths the improvement or deterioration processes can possibly take. This gives maintenance a probabilistic character, as success or failure can be predicted only with some probability. A probabilistic process can never be fully controlled, it can only be influenced by decisions to intervene in some way, e.g., through maintenance, repair, and rehabilitation.
THE MAINTENANCE PROBLEM
Maintenance will increase in importance in the years to come. As structures and equipment reach or exceed their expected life, owners and operators will have to cope with an increasing incidence of wear-out problems. There is no letup in the services demanded from the existing water resources infrastructure. Actually, these demands will further increase and diversify, making water management all the more complex (Kelley 1990). Funds, as well as opportunities for major new construction, reconstruction, or expansion, are scarce and there is public pressure to keep down rates for traditional as well as expanded services. This situation calls for cost-effective capital investments in the public and private infrastructure sector. The use of new technology and materials and of advanced analysis methods can make a significant contribution here. The magnitude of the maintenance and rehabilitation problem in the water resources sector nationwide has been analyzed in a national infrastructure study that among others stressed the need for innovative approaches (Schilling 1987). The U.S. Army Corps of Engineers estimates the number of existing dams in the United States at approximately 70,000. By the year 2000, some 20,000 of these dams will be 50-years old or older (New Perspectives 1983). Of the Corps’ ۶۰۰ major navigation, power, and multipurpose projects, about 40% will be 50-years old and older by the year 2000 (Scanlon et al. 1983). This means that a sizable portion of the existing water resources infrastructure will have reached or exceeded its expected life. In response to these needs, the Corps’ civil works appropriations began to exceed new construction appropriations for the first time by the mid-1980s (Markow et al. 1989). So far, outright failures of major dams, power plants, dikes, etc., have been rare, but the incidence of sudden failure is definitely not zero, as reported examples show (Lessons from 1988; Re-assessing the 1990; Jansen 1988). Keeping down or even reducing the relatively small probability of major failures can have an enormous effect, however, on the lives and property of many people. But even if the failure rate just stayed constant, reliability will decline, as will be shown herein. This trend must be addressed by an appropriate maintenance, repair, and rehabilitation program. Aside from keeping major failures in check, maintenance can contribute effectively to upholding efficiency in the day-to-day operation of existing projects. In the following paragraphs, some probabilistic concepts and methods will be presented that exemplify the probabilistic approach to maintenance.
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