Stone mastic asphalt (SMA) mixtures require a high content of binder, which results in thick binder films on the aggregate grains. To avoid the draindown effect, stabilizing additives (drainage inhibitors) are indispensable in most cases. This chapter describes the types of stabilizers and methods of testing them.
Have you ever seen fat spots on an SMA surface? Or binder running out of a truck hauling a hot SMA mixture? If you have, those troubles may have been caused by a binder or mastic draindown.
An SMA asphalt mixture has an intentional binder surplus. The specific surface of the mineral mixture is too small in relation to the designed binder volume. Under normal conditions, that binder is not bonded with the mineral mixture grains and does not remain on the grains’ surface; instead it drains-off. The draindown effect results from the separation of part (binder or mastic) of the SMA asphalt mixture. Keeping in mind that SMA has a lot of binder—in fact, SMA has a deliberate excess of binder—one should always take into account the risk of draindown.
In many countries, in the early applications of SMAs there were some cases of hot binder running out of a silo that held a hot SMA mix. Similar occurrences took place out of the backs of trucks carrying SMA to construction sites, eventually appearing as fat spots (bleeding) on the finished surfaces.
Methods to determine the amount of draindown are explained in Chapter 8 and other problems that may occur on the construction site in Chapter 11.
