Question: The engineering firm you work for has recently been selected to design a portion of the newly proposed runway for Atlanta-Hartsfield International Airport. While
The engineering firm you work for has recently been selected to design a portion of the newly proposed runway for Atlanta-Hartsfield International Airport. While performing research prior to the design phase, your boss becomes suspicious of some of the aggregates that will be used in the mixture for the concrete pavement. The real concern is the possibility of an alkali - silica reaction (ASR) in the concrete pavement. Your boss does not fully understand this reaction. He asks you to research this type of reaction further and present a two page report on this phenomenon. Included in the report should be: Briefly describe how alkali-silica reaction occurs. Provide a detailed explanation of how ASR occurs (i.e. where does the alkali come from, where does the silica come from, what happens?) Briefly explain why it is a problem. Does this pose durability problems? What measures could you use to prevent this type of reaction from occurring and make a longer lasting concrete pavement? Explain your reasoning behind your methods. Also, provide options for remedial repair (or slowing down the reaction). Explain. Provide a one paragraph summary on a case study (failure or preventative measure) that involves alkali - silica reactivity (ASR) You may include pictures; however, your papers should not consist of only pictures. Find at least two additional sources for this article (library, books, internet, etc.). Cite your references both within the document i.e. (Tuthill, 1982) and at the end in a reference section (with the Author, Article Title, Journal/Book/Website/etc.., Publisher, Publication Date, and Page Numbers. Do not forget to document your pictures. Your boss will not only evaluate your research report on technical aspects but also your writing skills.
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1 Introduction The varying aggregate properties in particular those related to the rate of the alkali reaction make it necessary to apply standard testing methods and assessment criteria for the aggregate reactivity 1 The aim of the present paper has been to estimate alkali reactivity of selected silica aggregates with the application of a variety of stan dard testing methods In the tests carried out the ASTM methods have been used as the most complete as well as providing the starting points for developing testing methods by RILEM TC 106 2 The ASTM C 289 chemical test has been used 3 together with longterm methods of testing mor tar expansion in compliance with the ASTM C 227 4 and concrete expansion in compliance with the ASTM C 1293 5 as well as the accelerated mortar test in compliance with the ASTM C 1260 6 Tests of mor tar and concrete microstructure after the alkalisilica reac tion have been conducted by means of scanning microscopy combined with Xray analysis within the microsurface The present paper includes test results and alkali re activity assessment of the selected silica aggregates both rapid and slow alkali reactive The results of silica dis solving in a sodium hydroxide solution have been com pared as well as scale of the expansion of the mortar and concrete bars with silica aggregate and highalkali con tent cement and scale of the expansion of the mortar bars stored in a sodium hydroxide solution at 80 C Exemplary photographs showing the products mi crostructure of the selected silica aggregatesalkali reac tion have been presented 2 Properties of alkalireactive silica aggregates It is generally acknowledged that alkalireactive ag gregates are divided into two types rapid and slow alkali reactive Initially the term of alkalireactive aggregates was applied to rapid reactive porous and occasionally hydrated silica minerals such as opal or chalcedony as well as a variety of heterogenic rock types including chert flint or certain types of volcano glass which could be ex tremely reactive even during standard field performance 7 Currently reactions with the aggregate obtained from wellcrystallised higherdensity quartz rocks such as greywackes sandstones claymica slates or metamorphic rocks are observed in concrete Despite the slow reaction course in case of aggregates containing these rocks the reaction leads to delayed concrete expansion and destruc tion Also microcrystalline or imperfectly crystallised quartz stressed quartz may be the reason for alkali reac tivity in similar slow alkalireactive aggregates 8 In recent years many researchers have noted an in crease of new types of rocks which are alkalireactive in concrete DolarMantuani 9 has collected the informa tion and presented a list of potentially reactive rocks In case of potentially reactive aggregates regard to the rock type as the only criterion is inadequate in particular for assesing polycrystalline slowreactive rocks French 1 also suggests that a complete description of potentially reactive rocks is required In like manner Jensen 10 claims that alkali reactivity of slowexpansive rocks may be estimated through a microstructure analysis and well known field observations rather than basing on a list of rocks derived from the traditional classification 3 Applied methods of aggregate alkali reactivity testing Potential aggregate reactivity has been estimated by means of a quick chemical method in accordance with the ASTM C 289 3 In ground aggregate the content of silica dissolved in a sodium hydroxide solution at 80 C was determined and the decrease in the solution alkalinity was measured The interpretation of the test results is not univocal but it is usually assumed that the potentially deleterious reaction will occur if test results are contained to the right of the borderline marked in the chart The potentially deleterious aggregates represented by the points above the dashed line may be highly alkalireac tive but their field performance demonstrates a relatively low expansion 11 It is believed that this method may be not suitable for all aggregate types nevertheless it does provide an adequate indicator for certain aggregate types and continues to be used in some cases 12 Mortar bar expansion tests have been conducted in accordance with the ASTM C 227 method From the aggregate tested of recommended graining bars are made with the use of cement containing over 08 of Na2Oe The bars are stored over water at 38 C their expansion being thus accelerated and displaying a tendency to be larger than at either higher or lower temperatures The aggregate tested is reactive if expansion exceeds 01 after 6 months In order to reduce the duration of the mortar bars expansion test an accelerated method with the use of high temperature 80 and the medium of the sodium hydroxide solution the ASTM C1260 method has been devised 6 This method enables to detect the potential of the deleterious alkalisilica reaction in mortar bars within 16 days Indirectly it detects the capacity of the aggre ... 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