What is Rutting in a Pavement?

Rutting

Definition

Rutting is a surface depression in the wheel path of a flexible pavement that traffic loads cause. The load consolidates or displaces the material in one or more pavement layers or in the subgrade. The deformation is permanent.

The pavement may rise along the sides of a rut. Many ruts show only after rain, when water fills the wheel paths. Significant rutting can lead to major structural failure of the pavement.

Types of rutting

A rut can start in different layers of the pavement. The layer tells you the cause and the right repair. Pavement Interactive describes three types.

Densification rutting

Traffic compacts a layer further after construction. This occurs when the construction crew did not compact the asphalt or a granular layer enough. The surface goes down in the wheel path. The material does not move sideways, so there are no raised edges.

The air voids in the mix show the risk. A Transportation Research Board paper says that a dense-graded mix should have no more than about 8 percent air voids after construction. FHWA gives the same limit. Traffic then compacts the mix further. The voids should not fall below about 3 percent, because a mix with too few voids ruts by plastic flow.

Mix rutting

The asphalt mix is not stable enough to carry the wheel loads. The mix moves down and sideways under the tires. This leaves raised edges, or humps, along both sides of the wheel path. Mix rutting is also called instability rutting or shear-flow rutting. It occurs in the asphalt layers near the surface.

Heat increases the risk. NCHRP Report 468 says that asphalt stiffness decreases in hot summer months and that the tendency to rut increases.

Subgrade rutting

The subgrade or an unbound base layer deforms under repeated loads. The whole pavement structure then sinks into the depression. The rut is wide and has no raised edges.

Pavement design limits this type. The FAA design method for flexible airport pavements (FAARFIELD) limits the vertical strain at the top of the subgrade to guard against subgrade rutting.

How to tell the types apart

NCHRP Report 468 uses the shape of the transverse profile to find the layer that rutted:

• Humps beside the rut show that the asphalt moved sideways. The rut is in the asphalt layers.
• A wide dip without humps shows that the base or the subgrade went down.

A profile does not prove the cause. A trench or cores across the rut show which layer changed thickness. The same report says that a trench is more costly and more hazardous than a profile.

Causes

AC 150/5380-6C puts rutting in the group of distortion distresses of flexible pavements. It lists these causes for the group:

• Foundation settlement.
• Insufficient compaction of the pavement courses.
• A lack of stability in the bituminous mix.
• Poor bond between the surface and the layer below.
• Swelling soils or frost action in the subgrade.

Temperature also has an effect. Hot weather makes the asphalt less stiff, so the risk of mix rutting increases in summer.

Laboratory tests for rutting resistance

Engineers test the binder and the mix before they build the pavement.

• Performance grade (PG) of the binder. The first number of a Superpave PG grade is a temperature, for example 64 in PG 64-22. It is the average seven-day maximum pavement temperature for the binder. It is a pavement temperature, not an air temperature. The dynamic shear rheometer (DSR) measures the rutting parameter G*/sin δ at 10 rad/s. For the original binder, the minimum is 1.00 kPa at the grade temperature.
• Multiple stress creep recovery (MSCR) test. AASHTO T 350 measures how much of the strain in the binder does not recover. The result, Jnr at 3.2 kPa, sets the traffic grade. The FHWA tech brief gives the maximum Jnr for each grade. It is 4.0 per kPa for S (standard) and 2.0 for H (heavy). It is 1.0 for V (very heavy) and 0.5 for E (extremely heavy).
• Hamburg wheel-track test. In AASHTO T 324, a loaded steel wheel moves back and forth over a mix specimen. The specimen is in water at 40 to 50 °C. The lab records the rut depth against the number of passes. The test shows the resistance to rutting and to moisture damage.

Stone matrix asphalt (SMA) is a mix designed for high resistance to rutting. It has a gap-graded coarse aggregate with stone-on-stone contact, which carries the load. SMA typically uses a modified binder.

Severity and measurement

The PAVER manual for asphalt airfields rates rutting by the mean rut depth:

• Low severity: 6 to 13 mm (1/4 to 1/2 in).
• Medium severity: more than 13 mm up to 25 mm (1/2 to 1 in).
• High severity: more than 25 mm (1 in).

To find the depth, lay a straightedge across the rut and measure the depth. ASTM E1703 is the test method that measures the rut depth at a point with a straightedge and a gauge. Compute the mean depth from measurements along the length of the rut. The manual records the surface area of the rut in square metres. If alligator cracking and rutting occur in the same area, the inspector records each at its own severity.

Agencies do not use one scale. TxDOT rates highway rutting in four bands:

• Shallow: 0.25 to 0.49 in.
• Deep: 0.50 to 0.99 in.
• Severe: 1.00 to 1.99 in.
• Failure: 2.00 in or more.

The FHWA Long-Term Pavement Performance manual defines no severity levels. It records the measured rut depth.

Rutting on an airfield

EASA guidance lists rutting among the physical parameters of the runway surface. The operator checks rutting visually during and after rain, and measures the degree of rutting with a straight edge. A friction test does not replace this check. Friction devices of the self-wetting type run on a dry runway, so they do not detect the effects of ponding and rutting.

Ruts hold water. FAA AC 150/5320-12C says that structural failure such as rutting can contribute to a loss of runway friction. It also says to repair depressed areas that pond in rain. Deep water in the wheel path also increases the risk of hydroplaning. NASA TN D-2056 gives a rule for the dynamic hydroplaning speed of a tire. In knots, it is about 9 times the square root of the tire pressure in psi. This applies when the water is deeper than the tread. For a tire at 200 psi, this is about 127 kt.

Rutting is one of the distress types in the PAVER rating for asphalt airfields, and it feeds the Pavement Condition Index (PCI).

Repair

The repair depends on the layer that rutted.

• Rutting in the asphalt layers. The rut may come mostly from lateral distortion of the mix. Then NCHRP Report 468 says that milling or leveling with a new wearing course, or recycling the surface, is a possible repair.
• Rutting in the base or the subgrade. The same report says that rutting from the lower layers needs more structural strength. A new surface layer alone does not add it.

AC 150/5380-6C lists rutting under surface irregularities. The repair is to remove and replace the damaged areas, or to grind or mill the surface. The probable causes are traffic and age. Before an overlay, AC 150/5320-6G says to repair rutting with profile milling and to level the surface with a suitable asphalt mixture.