Rutting

Since the advent of Superpave mix design technology and the follow-on predictive testing now used in hot mix asphalt (HMA) design evaluation, rutting has largely been eliminated as a concern when the pavement structure and HMA are properly designed.  In Washington State, the Hamburg Wheel Test device has been in use since 2010 to evaluate HMA designs for rut resistance prior to approval for use.  The exception to the rule, for both asphalt and concrete pavements, is in areas where extensive studded tire use is common.

Rutting in a busy arterial in the Greenwood area of Seattle.  Ruts can even be seen in the two-way left turn lane indicating that the rutting is not due to heavy traffic, but likely due to an improper HMA mix evaluation, poor subsurface conditions or wheel loading in excess of design parameters.

Rutting in the parking lane.  Likely, an overlay of the main road was extended beyond the original roadway out to the curb .  Extra HMA was used to make up the difference in height but was not adequately compacted because the roller bridged over the rut area (one side of the drum on the original pavement and the other on the new mix nearest the curb).  Subsequently, a large load or two (i.e., a bus) drove over the poorly compacted area and caused a large rut.

Problem

Ruts filled with water can cause vehicle hydroplaning, can be hazardous because ruts tend to pull a vehicle towards the rut path as it is steered across the rut.

Possible Causes

Permanent deformation in any of a pavement’s layers or subgrade usually caused by consolidation or lateral movement of the materials due to traffic loading.  Specific causes of rutting can be:

• Insufficient compaction of HMA layers during construction.  If it is not compacted enough initially, HMA pavement may continue to densify under traffic loads.
• Subgrade rutting (e.g., as a result of inadequate pavement structure)
• Improper mix design or manufacture (e.g., excessively high asphalt content, excessive mineral filler, insufficient amount of angular aggregate particles)

Ruts caused by studded tire wear present the same problem as the ruts described here, but they are actually a result of mechanical dislodging due to wear and not pavement deformation.

Repair

A heavily rutted pavement should be investigated to determine the root cause of failure (e.g. insufficient compaction, subgrade rutting, poor mix design or studded tire wear).  Slight ruts (< 1/3 inch deep) can generally be left untreated.  Pavement with deeper ruts should be leveled and overlayed or the deficient pavement should be milled out and replaced with a well designed, modern HMA mix.