The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which also includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF) or recipe.
For HMA manufacturing, target values of gradation and asphalt binder content are specified based on the JMF along with allowable specification bands to allow for inherent material and production variability (see Table 1 and Figure 1). It bears repeating that these target values and specification bands are based on the JMF and not any general HMA gradation requirements. Thus, the mix designer is allowed substantial freedom in choosing a particular gradation for the JMF and then the manufacturer is expected to adhere quite closely to this JMF gradation during production.
Table 1: Example Job Mix Formula (JMF) with Specification and Tolerance Bands
|Sieve (metric)||19.0 mm||12.5 mm||9.5 mm||2.36 mm||0.075 mm|
|Sieve Size (U.S. units)||3/4 inch||1/2 inch||3/8 inch||No. 8||No. 200|
|Gradation Control Points||100 min.||90 - 100||90 max.||28 - 58||2.0 - 7.0|
|Job Mix Formula (JMF)||100||96||75||29||4.5|
|Tolerance||99 - 100||+/- 6%||+/- 6%||+/- 4%||+/- 2.0%|
|Tolerance Limits||99 - 100||90 - 100||69 - 81||25 - 33||2.5 - 6.5|
The below animation steps through the process of getting from the general Superpave gradation requirements to the actual JMF specification band.