United States Department of Transportation
i
Treatments for clays in aggregates used to produce cement concrete, bituminous materials, and chip seals : technical report.
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2013-07-01
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[PDF-15.00 MB]
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and Performance
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Abstract:The clay contamination of coarse and fine aggregates and its effects on pavement performance of portland cement concrete, bituminous mixes and chip seals is a major concern for Texas Department of Transportation. We proposed (i) to determine what type and concentration of clay mineral will result in poor pavement performance, (ii) to identify a quick field test method to detect the presence of deleterious clay minerals in the stockpile, and (iii) suggest remedial techniques to make the clay contaminated aggregates acceptable for use.
The modified methylene blue (MMB) and X-Ray Diffraction (XRD) tests were identified as the most promising methods to identify and quantify clay mineral in aggregate fines. Nearly thirty aggregate sources have been tested using the MMB test. XRD was used to determine the content of different clay minerals present in these samples. A strong positive correlation between expansive clay content and methylene blue value (MBV) was evident which clearly indicates that the MMB test is the most reliable and rapid test method to detect clay minerals in aggregate fines. XRD is an advance research tool which was used to validate the MMB test. The correlation between MBV and concrete performance testing (both PPC and HMA) became the basis to assign a threshold MBV (corresponds to the maximum permissible clay content within the aggregate fines) and categorize aggregates fines with different ranges of MBV. A high MBV indicate increased potential for diminished aggregate performance in asphalt, concrete, and other construction applications. The Methylene Blue Test is sensitive to clays which contribute to stripping in HMA and could be used to eliminate problematic field sand sources. It appears that HMA is more robust and can tolerate higher amounts of clay contamination than PCC, mainly because water is not present in HMA.
Although the bar linear shrinkage and sand equivalent tests give good repeatability in the results, these tests fail to provide consistent and accurate indications of clay minerals present in aggregate fines. Based on MMB test, materials that are failed by the current specifications (e.g., aggregate fines with clay size non-clay mineral particles) can be allowed, which promotes sustainability and save money. Therefore, type and concentration of clay minerals present in aggregate fines are very important aspects in relation to pavement performance. The guidelines for controlled use of aggregate fines with varying level of clay contamination have been developed and further research / implementation plan has been proposed.
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