Ravelling

What is Ravelling?

Ravelling is the loss of surface aggregate from a bituminous surfacing mixture. Ravelling affects the integrity of the mix. If it is not halted it may cause deterioration to the extent where the surfacing mix no longer performs competently, resulting in a lack of waterproofing and causing an uncomfortable ride.

What Causes Ravelling?

Causes of ravelling can be dirty aggregate, segregation on the surface and poor compaction. In the case of dirty aggregate the bitumen sticks to the dust not to the stone and thus traffic can flick out the poorly held stone. In areas where the grading is deficient in fines, the coarse aggregate is effectively depleted of binder. As the binder ages the bond weakens, or stripping occurs. This rips the coarse aggregate out.

Poor compaction can also be a cause of raveling. Cohesion may be too low and the material will fret away under traffic.