When placing RAP materials, what maximum layer thickness should be obtained?

When placing RAP materials, what maximum layer thickness should be obtained?

• A. 2 inches
• B. 4 inches
• C. 6 inches
• D. 8 inches

Answer: (B)

Controlling how thick a lift of RAP is placed matters because RAP already contains aged binder and fines, which behave differently under compaction and heating than new material. If a lift is too thick, it can trap air voids, heat unevenly, and fail to densify properly under rollers, leading to weak spots, rutting, or premature cracking. Limiting each lift to four inches gives the equipment a better chance to spread, blend, and compact the material evenly in a few passes, achieving the required density and surface quality. If more RAP needs to be placed, it’s better to divide it into additional thin lifts and compact progressively, but the recommended maximum is four inches for each lift.

Controlling how thick a lift of RAP is placed matters because RAP already contains aged binder and fines, which behave differently under compaction and heating than new material. If a lift is too thick, it can trap air voids, heat unevenly, and fail to densify properly under rollers, leading to weak spots, rutting, or premature cracking. Limiting each lift to four inches gives the equipment a better chance to spread, blend, and compact the material evenly in a few passes, achieving the required density and surface quality. If more RAP needs to be placed, it’s better to divide it into additional thin lifts and compact progressively, but the recommended maximum is four inches for each lift.