In the 1960s—during the beginning stages of SMA manufacture—the need arose to incorporate stabilizing agents to prevent binder draindown. Such additives are called stabilizers; they stabilize or keep the binder in place. Because of these stablizers, an increase in the binder film thickness on the aggregate is possible.
The two main techniques of reducing binder draindown are as follows: [17]
Each of these methods will be discussed later in this chapter, but let us start with a definition of a stabilizer. A stabilizer or drainage inhibitor is an additive to an SMA asphalt mixture put in to prevent binder (or mastic) from draining-off. Stabilizers may be made up of various materials, including both binder absorbers (e. g., fibers) and viscosity boosters (e. g., polymers).
In addition to helping to retain binder on the aggregate, stabilizers may also improve other properties of the binder itself, the mastic, or the mixture (see Section 4.2.1.4). There are examples of such tests available in the literature—for an example, see Behbahani et al. (2009).
But are stabilizers really necessary for SMA? Research done in the United States during the 1990s revealed a 70 times higher binder draindown in a mix without any stabilizer compared with the same mix containing 0.3% of cellulose fibers (Brown and Mallick, 1994).
The author’s experience at the beginning of 1990s showed that even with using modified binders, fat spots often occurred. Then trials testing modified binders with half of the typical amount of stabilizer were conducted, and fat spots also occurred. Now, independent of binder type, at least a minimal amount of stabilizer is commonly used. So the answer to the question, should be stabilizer used or not? is evident; however, one has to note that some compositions of SMA are less susceptible to draindown.
