Coarse aggregates of 2/6 mm are not permitted in an SMA mixture with a gradation of 0/11 mm just to guarantee a gap gradation. The sand fraction has to consist of a minimum of 50% crushed stone, whereas the content of air voids according to Rigden and the bituminous number* should form the basis for selection of the filler. As in many other countries, the amount of stabilizer is based on draindown testing.
The road binder 70/100,* and in special cases modified binder, should be used in SMA. No Reclaimed Asphalt Pavement (RAP) is allowed in SMA in the Netherlands.
7.4.2.2 Designing an Asphalt Mixture
Reading the recommended fixed binder content in SMA from the standard marks the beginning of design. This quantity depends on the SMA gradation; the bigger the maximum aggregate size in a mix, the lower the intended amount of binder. It is necessary to stress again that the accepted amount of binder is a constant value; hence it is not subject to change in the design stage but is to be matched with an aggregate mix.
The amount of binder is fixed prior to establishing the filler content. The content of filler (particles less than 0.063 mm) must fall within the range of 6-10% (m/m). Limestone filler is preferred; since its content of Rigden air voids is known, its behavior in the mixture is somewhat predictable.8
Fixing the amount of the sand fraction is based on the indicated constant ratio of this fraction to the quantity of filler. This ratio amounts to 65:35 (m/m) if the densities are comparable with the reference density.
It has been accepted in the later stages of design that the binder content and the ratio between the filler and sand are fixed and that only the coarse aggregate content is subject to change (design). Of course, the higher the coarse aggregate content, the lower the sand-filler content. As we remember from Chapter 6, for such an assumption with a variable quantity of coarse aggregate of the same origin, the VMA will be subject to change.
Finally, the content of the coarse-aggregate fraction should amount to between 72.5 and 82.5% (m/m) according to Dutch regulations, so it is increased by 2.5% (m/m) in comparison with the most frequently used requirements (70-80% [m/m]). This is directly related to the higher requirements for the air voids content in heavy- duty pavements, at the level of 5% (v/v) (in other countries such pavements typically require 3- 4% [v/v]).
