Esteemed Researchers,

I have come across three approaches used by researchers for fabrication of asphalt mix specimens when they study modified asphalt binders with different modifier dosages (or different modifiers):

Approach 1: Determine the optimum asphalt content (OAC) for the mix with control (unmodified) asphalt binder, and use the same binder content while fabricating asphalt mix specimens with modified binders. Mix volumetrics (e.g. air voids) are kept constant for mixes with modified binders. This has the main advantage of avoiding binder content as a secondary variable when your interest is just to see how the different mixes perform under variable modifier dosages.

Approach 2: Determine the OAC for each binder obtained with different dosages of the modifier. This generally results in OAC values that are quite close (but not exactly the same) to the OAC obtained for control (unmodified) asphalt binder. But this approach introduces binder content as a secondary variable (even though the volumetrics are the same) when one is just interested to analyze the mix performance results as a function of modifier dosage. Hence, it becomes difficult to understand whether the observed trends are due to different modifier dosages or due to different binder contents.

Approach 3: Some researchers also suggest keeping a constant binder content by mix volume (instead of a constant binder content by weight of the mix). However, this approach will lead to difference in the binder content by weight of the mix. Also note that expressing binder content by weight is the most common way of specifying binder content in field and during mix design. Hence, this approach is quite rarely used in the literature.

What would you recommend as the 'correct' approach? Will really appreciate your advice!

Thank you for your valuable time!