Fine aggregates in Australia must comply with AS2758.1.1 In light of this specification, Cement Concrete Aggregates Australia (CCAA) in 2007 made efforts to determine the applicability of this standard towards manufactured sands.2-6 It recommends several useful tests, including sand equivalent (SE) and methylene blue value (MBV).
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MBV reacts deleterious clay fines with a blue dye and measures dye uptake, as a colour change, to estimate clay contamination. This method identifies the presence of clay minerals in aggregates. There are several variations in the literature based on an end point titration technique, which requires a sieving procedure to 75-micron, a slow titration process and visual determination of a blue ?halo? on filter paper. Despite these shortcomings, the test measures clay contamination and results can help predict concrete performance.
IMPROVED MBV TEST
A recent improvement on the MBV test addresses the disadvantages of the titration method. This new method uses a colorimeter, a device that measures the absorption of a given solution at a specific light wavelength.
The colorimeter removes the human interpretation of the blue halo and improves reproducibility of the results. An added benefit is that the entire sand sample can be used, not just the 75-micron fraction. The new test takes 10 minutes, providing a quick, reliable MBV test that can be measured easily in the field as well as in the laboratory. The equipment and testing sequence is shown in the accompanying images.
Compared to the standard tests, the improved MBV test provides an excellent correlation. The chart (right) shows an example correlating the new test and EN 933-9 for 26 field-obtained sands from around the world.
From the MBV, a conversion can calculate an equivalent clay amount based on a well known deleterious clay, sodium montmorillonite (Na-Mont).
This provides a unit of measure for a variety of clay systems.
This improved MBV test is used in Australia for determining equivalent clay contents of natural and manufactured sands. The test is a rapid, accurate quality control tool to determine the variability of aggregate contamination.
With knowledge of concrete mix performance, aggregate clay contamination is an important predictor of concrete behaviour and allows aggregate and concrete producers flexibility for increasing fines utilisation.
The MBV also identifies situations when high levels of clay contamination preclude increased fines use.
Leon Bablouzian and Nathan Tregger, of WR Grace & Co, are based in Cambridge, Massachusetts, USA. Tasha Eagle, of Grace Australia, is based in Brisbane.
REFERENCES
1. Standards Australia, 1998. AS2758: Aggregates and rock for engineering purposes ? Part 1: Concrete aggregates.
2. Cement Concrete Aggregates Australia (CCAA) Technical Liaison Committee, 2007. Manufactured sand: National test methods and specification values.
3. CCAA Technical Liaison Committee, 2007. Abrasion resistance and effect of manufactured sand on concrete mortar.
4. International Slurry Surfacing Association (ISSA), 2007. ISSA Technical Bulletin 145: Methylene blue absorption value in mineral aggregate filler and fines.
5. Standards Australia, 2009.
AS1289: Methods of testing soils for engineering purposes.
6. Standards Australia, 2009. AS1141: Methods for sampling/testing aggregates.