Methylene Blue Test -Acceptance Criteria for fines in HMA

 

It is not common to use such a criteria for hot mix asphalt fines.  The Methylene Blue test is an old and somewhat indirect test for clays which occasionally pops its head up in the bituminous world. It measures the total exchange capacity of a clay system which is the capacity of a clay to absorb cations from solution where exchangeable cations on the clay surfaces are replaced by methylene blue cations.  The more ions the clay can exchange for methylene blue cations, the more reactive the clay, and the greater the swelling potential. It would have to be rather dirty fines or a very specialist application to need the test. Another test to use if you face such circumstances might be the Cation Exchange Capacity, which is widely used by soil scientists.

 

The use of the Methylene Blue test in bituminous surfacings is for special cationic emulsion applications such as super-quickset slurries where it is particularly important that the clay content be strictly limited or it messes up the emulsion chemistry.  Two measures have been used to control this: a sand equivalent value of 65 or higher, and a Methylene Blue test value of 5 to 11. We did some work on the Methylene Blue test over a wide range of climatic regions (because climate and geology are inter-related of course) and we found that the problem quarries with high methylene blue values were in the moisture surplus areas (Thornthwaite's Im > 0; i.e.  wet areas). I'd be surprised if this was a problem in Israel.

 

In my humble opinion, a better test for aggregate cleanness for HMA is the sand equivalent test (ASTM D2419).

Kind regards