How thick should the loose soil layer mix with the 4 inch max layer of RAP?

How thick should the loose soil layer mix with the 4 inch max layer of RAP?

• A. 4-6 inches
• B. 8-10 inches
• C. 12-14 inches
• D. 2-3 inches

Answer: (B)

Mixing efficiency and a uniform finished base come from using a substantial amount of soil with the RAP. When you have a 4-inch maximum lift of RAP, you need enough loose soil to be adequately blended with it so the asphalt binder in the RAP can coat and merge with the soil particles, creating a consistent, stable base after compaction. An 8–10-inch depth of loose soil provides enough volume for the mixing equipment to effectively shear and blend the materials, resulting in a homogeneous stabilized layer. Too shallow a soil layer (4–6 inches or 2–3 inches) risks incomplete mixing and pockets of RAP, while a much deeper layer (12–14 inches) is unnecessary and harder to compact properly. So, 8–10 inches is the recommended depth.

Mixing efficiency and a uniform finished base come from using a substantial amount of soil with the RAP. When you have a 4-inch maximum lift of RAP, you need enough loose soil to be adequately blended with it so the asphalt binder in the RAP can coat and merge with the soil particles, creating a consistent, stable base after compaction. An 8–10-inch depth of loose soil provides enough volume for the mixing equipment to effectively shear and blend the materials, resulting in a homogeneous stabilized layer. Too shallow a soil layer (4–6 inches or 2–3 inches) risks incomplete mixing and pockets of RAP, while a much deeper layer (12–14 inches) is unnecessary and harder to compact properly. So, 8–10 inches is the recommended depth.