AS 1141.33:2015 – Methods for sampling and testing aggregates Method 33: Clay and fine silt (settling method)
AS 1141.33:2015 – Methods for sampling and testing aggregates Method 33: Clay and fine silt (settling method).
The test method causes a sample that is a mixture of sand, silt and clay-sized particles to be agitated in a column of a salt solution. The particles settle in the liquid and the silt and clay particles that require a longer time to settle out of the suspension, usually form a distinct layer above the sand-sized particles. The sodium ions in the salt solution assist in settling the clay particles by causing partial flocculation. The test result is the volume of clay and silt particles in relation to the volume of the sand particles.
In samples where there is a high volumetric proportion of clay and silt, or where the clay particles are active (i.e. have a high cationic exchange capacity) clay and silt particles may remain visible, trapped within the sand particles at the end of the test. In these instances, this standard test is not appropriate for the material in question, although the method provides some suggestions for dealing with this situation.
The test provides a volumetric measure of the clay and silt particles present in the sample. There is no consistent relationship between this test result and the result of the mass passing the 75 im test (either AS 1141.11.1 or AS 1141.12). In general, the magnitude of the result of this test will be greater than the result of the passing 75 l..im. It is probable that the greater the difference between the two tests, the more active the clay in the sample.
3 NORMATIVE REFERENCES
The following are the normative documents referenced in this Standard:
NOTE: Documents rcferenccd for informative purposes are listed in the Bibliography.
The following apparatus, complying with the requirements below and those of AS 1141.2 are required:
(a) Balance—having a limit of performance of ±0.5 g.
(b) Cylinder—of glass or transparent plastic, graduated to 250 rnL in 2 mL increments. The cylinder shall be stoppered to prevent loss of sample and test fluid when the cylinder is shaken and inverted during testing.
The reagent shall be common salt or table salt.
7 TEST PORTION
The test portion shall be representative of a sample obtained in accordance with AS 1141.3.1. The portion obtained shall he sufficient to yield about 100 mL of fine aggregate from the sample. This test portion shall he used without drying or any other treatment.
The procedure shall be as follows:
(a) Prepare a nominal 1% solution of common salt in water by adding 10 g of common or table salt to I L of water.
(b) Place about 50 mL of the salt solution in the 250 mL measuring cylinder. Pour aggregate into the cylinder until its measured volume in the cylinder is approximately 100 mL. If the volume of the aggregate and liquid is less than 150 mL, make the volume up to 150 mL by the addition of more salt solution and stopper the cylinder.
(c) Shake the mixture vigorously for approximately 30 s until adherent particles have been dispersed. Place the measuring cylinder on a level surface, and gently tap it until the surface of the aggregate is level.
(d) Allow the measuring cylinder and contents to stand for 180 +10, —0 minutes. At the end of that time carefully examine the sand layer to determine if clay and silt particles are clearly visible, trapped between the sand particles. If the separation of the clay and silt from the sand has not occurred, report the result as ‘Indeterminate’.
NOTE: For quality control purposes at individual sites, it may be possible to obtain a result by reducing the quantity of sample in Item (b) above to 50 mL instead of 100 mL, but filling the cylinder with the liquid to 150 mL. This may result in better separation of the silt and clay. However, as the relationship between results so obtained and results from this Standard are unknown, any results obtained should be reported as ‘Non-standard’.
(e) If a clear separation of the silt and clay from the sand is obtained, record the volume of the sand (S) and the volume of the settled clay and fine silt (F), in millimetres (see Figure 1).
NOTE: When using a measuring cylinder, reading the heights in millilitres is sufficient as the cross-sectional area of the cylinder is the same at any height and therefore the ratio of the layer volumes is equivalent to the ratio of the layer heights.