An SMA formula has to be documented and demonstrated (declared). Any SMA mixture made according to the recipe has to meet the standard requirements determined by a given country.
14.5.1 Gradation
The fundamental rules regarding the mix design include the following:
• The gradation should be expressed in mass percentages of the total aggregate mix; the accuracy of percentages passing
• all sieves (with the exception of the 0.063 mm sieve) should be expressed to 1%.
• the 0.063 mm sieve should be expressed to 0.1%.
• The content of binder and additives should be expressed in mass percentages of the asphalt mixture, with an accuracy of 0.1%.
• The type of fine aggregate used and the adopted ratios in the case of a mix may be given in a recipe or specification.
• The gradation may be described with either “basic sieve set plus set 1[75]” or “basic sieve set plus set 2t”; a combination of sieves from set 1 to set 2 is not permissible.*
The gradation of an SMA mixture should be established with a minimum of five sieves: 0.063, 2.0, D, 1.4D, and the characteristic coarse sieve (a selected sieve between 2.0 mm and D). Basically, the gradation limits, which are given in the standard, must adhere to the rules for preparing NADs to the standard EN 13108-5. Each country, by its NAD, may determine an SMA mix’s gradation envelopes, guided by the following:
1. Overall limits on the target composition displayed in Tables 1 and 2 of the standard
2. Permissible ranges between maximum and minimum values on selected sieves (in Table 3)
The standard allows for the use of additional control sieves, called optional (characteristic) sieves, to enable a more precise description of the gradation as follows:
• A characteristic sieve for fine aggregate may be selected between the
2.0 mm and the 0.063 mm sieves; in addition, the standard stipulates the set of sieves to be chosen from 0.125, 0.25, 0.5, and 1.0 mm;
• An optional characteristic sieve for the coarse aggregate may be selected to provide one more additional sieve with a size between 2.0 mm and D.
Finally, to describe the gradation envelope, one can use the following set of sieves:
• 0.063 mm sieve (obligatory)
• Characteristic sieve for the fine aggregate (optional)—sieve between 0.063 and 2.0 mm,
• 2.0 mm sieve (required)
• Characteristic coarse sieve (required)—a selected sieve between 2.0 mm and D
• Additional characteristic coarse sieve (optional)—a selected sieve between
2.0 mm and D
• Sieve D (required)
• Sieve 1.4D (required)
It is worth noting that the freedom to select characteristic sieves gives a chance to choose those sieves that will provide the best possible control of an SMA mixture (e. g., breakpoint sieves). Figures 14.2 through 14.5 depict the position of boundary points for example mixtures SMA 8 and SMA 11 for sieve set + 1 and SMA 10 and SMA 14 for sieve set + 2. As can be seen, the scope of available solutions (positions of overall limits to a target composition) for any of the mixtures is quite broad. Additionally, in the same figures, the German (for SMA 8 and 11) and British (for SMA 10 and 14) gradation envelopes are presented as examples.
