The minimum content of binder in an SMA has been fixed at 6.0% (m/m), but a slightly higher quantity of binder in a mixture is advised. This is intended to provide protection from exceeding the lower production limit when producing the SMA.
To begin, an initial trial quantity of binder in mixtures should be adopted (the same for each of the three mixtures). The trial binder quantity should be adjusted depending on the density of the aggregate mixes. The reference density of an aggregate mix is typically 2.75 g/cm3; if the density differs, the quantity of binder should be adjusted in accordance with this rule: each change of density by 0.05 g/cm3 corresponds to an adjustment of binder by 0.1% (m/m). For densities less than 2.75, and for densities greater than 2.75, the adjustments are positive ( + ) or negative (-), respectively.
A series of samples should be made up of 12 specimens of the SMA asphalt mixture, four for each SMA trial design gradation. Then nine of the 12 samples will be compacted according to the selected method (three for each SMA design), while the remaining three samples will be used to determine the maximum density according to AASHTO T209.
Samples can be compacted using either the Marshall method or the Superpave gyratory compactor. Compaction parameters are as follows:
• Marshall hammer: 50 blows on each side of a sample 100 mm in diameter
• Superpave gyratory compactor: 100 revolutions[34] on samples 150 mm in diameter
Due to the risk of excessive crushing, higher compaction efforts are not recommended. The temperature of compaction samples should be determined using AASHTO T 245, which specifies that the compaction temperature is that at which the binder viscosity equals 280 ± 30 cSt, or that provided by the manufacturer of a modified binder (when applicable).
