The draindown effect is the process of the separation of liquid binder or mastic from an SMA mixture that occurs at a high temperature when the binder is still molten.
Both the binder and the mastic can separate. It is common knowledge that SMA mixes are marked by an intentional excess of binder, and the draindown effect is caused by the impossibility of maintaining such an excess of binder on grains of aggregate. That problem is most frequently solved by adding a stabilizer (or drainage inhibitor) to a mix. Its task is to absorb any excess of binder.
Excessive draindown may be caused by several factors (as described in Chapter 4). Binder or mastic draindown from a mix brings about many problems; most of them are described in Chapter 11.
Research on an SMA mix’s susceptibility to binder or mastic segregation has been conducted in virtually all the countries using this type of mix. A short review of the procedures of draindown testing and the regulations for the following methods will be discussed later:
• Using Schellenberg’s method (the original German method)
• Using the European standard EN 12697-18
• Using the U. S. standard AASHTO T 305-97
