Sand Equivalent Value

[singkt]

In our Specification there is a requirement which says, ‘the aggregate shall meet the sand equivalent (clay content)requirements… minimum sand equivalent percent 40 …’. If we see the sand equivalent formula, which is 100*sand reading/ clay reading, the higher the sand reading during testing means it gives higher sand equivalent value, and that means there are less clayey materials. Either clay content should be calculated as 100-sand equivalent value to set minimum requirement as above or if we use sand equivalent, what the Spe'c should set is a maximum requirement.

I read AASHTO and found that it has similar requirement to our specification.

Any ideas how the minimum requirement is meaningful; either during testing or interpretation of the test result.

 

[bmike]

Have you performed a sand equivalent test? I think if you did a few it would help your understanding. For an even better understanding, try mixing the same percentages of kaolinite and bentonite into a clean sand, and see what happens to the results.

The sand reading is the height of the sand column in the cylinder, and the clay reading is the height to the top of the flocculated clay, which sits on top of the sand. With no clay content (or non-flocculating fines) the clay and sand readings are the same, and the SE is 100%. At an SE of 40, the height of sand in the cylinder is 40% of the height to the top of the flocculated clay, and, accordingly the top of the clay floc is 2.5 (100/40) the height of the sand. For SE<40, the height of the clay column is more than 2.5 the height of sand.

Be careful not to believe that the SE is a measure of the clay content; it is an indicator only of the amount of, and plasticity of, the fines in the aggregate. Sands with the same percentage contents of clays, but with different clay minerals will give different SE values. The more plastic clay minerals are more easily flocculated and will produce lower SE values. For sands containing the same clay mineral, the higher clay contents will give lower SEs.

It is pretty hard to assess the plasticity of the fines in an aggregate with only 5% fines, but this can have a significant effect on its behaviour when wetted. The purpose of the SE test is to provide an index of this.

The methylene blue test can do the same thing, it gives a good indication of the specific surface of the fines, which in turn gives a good indication of clay mineral present, which helps give an indication of its plasticity.

Soil mechanics is a game of drawing conclusions from clues, don't get trapped in a dualist pass/fail way of thinking.