Open-Graded MixesThe difference from the first two with the open-graded mixes is the permeability characteristic. This hot mix asphalt is designed only with crushed stone and a few grains of sand in the mix. There are two major classifications of this type of mix: Open-graded friction course - Air voids minimum requirement are 15%, and no maximum air void percentage is specified. This mix is only used for surface courses. It has a smoother surface finish than the dense-graded. Its low cost of placement counteracts the high cost of producing it. However, make sure that you are not clogging or sealing the pores as this action will dramatically reduce and degrade asphalt performance and stability. Asphalt treated permeable bases - It is used only under dense-graded, stone mix or portland cement concrete for drainage. It is used for drainage purposes below dense-graded, stone mix, or Portland cement concrete.