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Dust samples are frequently obtained directly from structures using a hand-held rotary percussive drill (with a 10, 12 or even 20 mm bit). It may be necessary to drill a close pattern of holes to obtain sufficient material for analysis. The drill dust can be recovered using a plastic shroud around the bit or using an angled plastic tube with a sample bag on the end. It is often of interest to investigate how properties vary with the distance from the concrete surface (for example where salts may have penetrated from an external source). In this situation, dust samples are recovered in increments at different depths.
Dust samples are dissolved in acid at the laboratory and can be tested for chloride content, sulfate content or cement content by chemical means or ion-specific electrodes. Analysis of concrete is covered in BS 1881: Part 124, and a useful commentary on the tests and their interpretation is given in Concrete Society Technical Report 32.
Acknowledgement: The Concrete Society
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