Use of Complementary Imaging Techniques in Concrete Deterioration Studies
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Use of Complementary Imaging Techniques in Concrete Deterioration Studies K.E. Wagner, E.A. Draper, and J. Skalny Introduction The infrastructures of the United States and other industrialized nations are, to a large degree, based upon concrete. With such a pervasive application, the importance of accurately analyzing concrete structure failures and addressing durability problems is obvious, as is the timely recognition of premature structure deterioration, so that an appropriate recommendation for remediation may be made. Although concrete is one of the most important engineering materials in use today, it is perhaps also the least understood, primarily because it is chemically reacting and changing throughout its service life. The chemical and physical changes that occur in this material can have degenerative effects on its mechanical properties and, ultimately, on its durability. Understanding how these chemical reactions occur within the concrete and affect its physical characteristics is often not straightforward. For example, the reactions can be affected by internal components (cement, fine and coarse aggregate), mixing procedures, curing cycle parameters, and/or the choice of additives. External influences on these reactions include weather exposure, chemically aggressive environments, and climate-mitigating surface treatments. Furthermore, there are some deleterious internal reactions that tend to occur sequentially and others that may occur concurrently. The overlap of simultaneous reactions can make it difficult to
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isolate the initial occurrence and to identify the deleterious component of deterioration. To resolve such questions, it is necessary to understand the interrelationship among the concrete components and their processing, and to examine the material with a variety of complementary techniques and sample configurations.
Adequate sample preparation is necessary for obtaining accurate information. The hard, abrasion-resistant aggregate in the relatively soft cement paste matrix presents a challenging problem with respect to preparing samples of concrete for microscopic analysis. Some of the more common sample configurations generally used for analysis are polished sections, petrographic thin sections, fracture surfaces, fluorescent dye-impregnated sections, and Transmission Electron Microscope (TEM) thin foil samples (see Table I). Each of these sample configurations has its merits and limitations. For instance, a polished section is well-suited for Scanning Electron Microscopy (SEM) and reflected light microscopy, but standard petrography cannot be performed on it. A fracture surface is ideal for evaluating the local fracture process and studying the spacial relationships among paste, aggregate, and reaction products; it is not, however, appropriate for microstructural analysis of the paste. By analyzing a variety of sample configurations, the spectrum of information acquired can be synergistically expanded. Optical information gained from petrographic sections can be expanded by SEM analys
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