Draindown is a term used to describe the separation of the asphalt binder from the aggregate in which type of mixtures?

Draindown refers to the phenomenon where asphalt binder separates from the aggregate due to a lack of sufficient aggregate interlock or when the mixture lacks proper cohesion and stability. This issue is particularly prevalent in open and gap-graded mixtures, which are designed to have larger void spaces and are less dense compared to dense-graded mixtures.

Open and gap-graded mixtures are designed to allow for high drainage rates and are often used in applications where enhanced permeability is desirable. However, the increased voids can also lead to the potential for binders to migrate downwards under gravitational forces, especially if the mixture temperature is elevated, creating a tendency for draindown. It's crucial that these mixtures maintain the asphalt binder within the aggregate to ensure performance, which is why understanding draindown is vital for inspectors of such mixtures.

Dense graded mixtures, on the other hand, are designed to have a more uniform particle size distribution, which provides better interlocking of the aggregates and helps to retain the binder within the mix structure. This significantly reduces the likelihood of draindown occurring compared to open or gap-graded types. Furthermore, recycled and rubberized mixtures, while they may have their own set of challenges, are not typically characterized by draindown in the same way