New test method for assessing fine aggregate

In Australia, natural sands are depleting. New deposits are difficult to establish due to environmental and regulatory factors. The decreased availability of natural sand has required higher utilisation of fines resulting from the rock crushing process to produce coarse aggregate products. These fine materials – manufactured sands – are abundant and higher utilisation is a cornerstone for improving sustainability of the construction aggregate industry.

Particle size distribution, shape/surface texture and deleterious fines are key features of manufactured sands that control their use in concrete mixes. Recent developments allow a rapid, direct measurement of deleterious clays within the fines, enabling innovative chemical approaches to make them inert. By ?cleaning up? manufactured sands through chemical treatment, their use in concrete mixes at elevated levels can be achieved.

Increased manufactured sand usage leads to less quarry waste and strategic use of natural sand deposits ? which improve the aggregate industry?s sustainability credentials. This article focuses on a new test procedure for quantitatively assessing clay contamination.

AUSTRALIAN STANDARDS
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).

SE is done by shaking fine aggregate in a clear cylinder with a flocculant and preservative solution. After shaking, the particles are allowed to sediment for 20 minutes and the SE is taken as the ratio of the height of the sand column to the height of the sand and flocculated clay multiplied by 100.

Higher percentages indicate ?cleaner? sand. SE is used in the USA, New Zealand and Europe to qualify aggregates for use in concrete and asphalt applications. Still, the test can give false negatives, whereby sand with low SE values produces concrete with acceptable performance. Conversely, sand with a high SE value can produce concrete with poor performance.

The SE test relies on apparent density differences between ?good? sand and ?bad? clay. Although a flocculant encourages separation of fine clay particles, similar-sized fine sand particles can be impacted by the flocculation process. As a result, the settling behaviours and the difference in SE may not be distinctive. For these reasons, the correlation of SE values and concrete performance can be suspect.

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.