Typical mix design for asphaltic concrete surface course

A typical mix design for an asphaltic concrete surface course (like Dense Graded Bituminous Macadam - DBM) is a precise recipe designed to create a strong, durable, and weather-resistant road layer. The design balances the proportions of its three main components using methods like the Marshall Mix Design.

A typical mix composition by weight is:

1. Aggregates (90-95%): This is the stone framework of the asphalt.
   • Coarse Aggregates: Crushed stone (e.g., 10-20mm) that provides the primary strength and stability.
   • Fine Aggregates: Crushed sand or stone dust that fills the gaps between the larger stones.
   • Filler: Very fine material like rock dust or cement (<0.075mm) that fills the remaining voids, stiffens the binder, and improves durability.
2. Bitumen Binder (4-6%): This is the glue that holds the aggregates together.A viscosity grade like VG-30 is commonly used. The exact percentage is called the Optimal Binder Content (OBC), determined during the mix design process to ensure maximum strength without bleeding (excess bitumen).
3. Air Voids (3-5%): These are tiny, empty spaces intentionally left in the mix. They allow for slight thermal expansion of the bitumen in hot weather and prevent the road from becoming unstable.