What is a Pothole?

A pothole is a bowl-shaped hole in a pavement surface where pavement material has broken away.

The FHWA and FAA documents cited here define it for asphalt surfaces, which engineers call flexible pavement. The FAA says it is a disruption in the pavement surface where a portion of the pavement material has broken away, leaving a hole.

How a pothole forms

The FAA says most potholes are caused by fatigue of the pavement surface. Fatigue cracks interlock and form alligator cracking. When sections of the cracked pavement work loose, wheel loads can pick them out of the surface and leave a pothole.

In northern climates, freeze-thaw cycles make pothole development worse. A pothole there may not relate only to traffic patterns.

Moisture also plays a part. At one Illinois test site of the FHWA pothole repair experiment, a warmer and wetter than average climate caused many new potholes. At the wet-freeze sites of the same experiment, only 48 percent of the repair patches survived. At the dry-freeze sites, 85 percent survived. FHWA warns that traffic, pavement age and subgrade support also differ between sites, so the data do not prove the effect of precipitation.

Pothole or something else

Other distresses are easy to mix up with a pothole.

• Depression. The PAVER manual defines a depression as a localized area with an elevation slightly lower than the pavement around it. Settlement of the foundation soil can cause it, and ponding water often shows it after rain. A depression keeps its surface material. A pothole has lost a portion of it.
• Raveling. Raveling is the dislodging of coarse aggregate particles from the pavement surface. It is a loss of single particles. In a pothole, a portion of the pavement material has broken away.
• Popout. A popout is a small piece of concrete surface that breaks loose. FAA AC 150/5380-6C gives a usual size of about 1 to 4 inches (2.5 to 10 cm) across. It gives a usual depth of 1/2 to 2 inches (1.3 to 5 cm). It occurs in concrete pavement, not in asphalt.
• Patch. A patch is an area where the original pavement was removed and replaced by a filler material. AC 150/5380-6C says that a patch deteriorates faster than the original pavement.

A pothole can lie in an area of alligator cracking. FHWA reduces the area of the fatigue cracking by the area of the pothole, so the survey does not count the same area twice.

How a pothole is measured

The FHWA Long-Term Pavement Performance (LTPP) manual is a highway manual, but it gives clear rules for a pothole:

• The smallest plan dimension is at least 150 mm. A 150 mm circle must fit inside an irregular hole.
• The minimum area is about 0.02 m2.
• A smaller hole is drawn on the distress map and commented on. It is not counted in the summary.
• The surveyor records the number of potholes and the area at each severity level.
• If a pothole lies in an area of fatigue cracking, the fatigue area is reduced by the area of the pothole.

The LTPP manual rates severity by the maximum depth below the pavement surface:

Potholes on an airfield

The FAA says potholes are not a common distress on airfields. The PAVER manual for asphalt-surfaced airfields lists 17 distress types for the airfield Pavement Condition Index (PCI), and a pothole is not one of them. A PCI is based on the type, severity and quantity of distress and ranges from 0 to 100.

Loose material from a distressed pavement is a hazard. The FAA says it can strike propellers or be ingested by jet engines, and that all pavement inspections should address foreign object debris (FOD). The FAA also says that distress can severely affect the structural integrity, ride quality and safety of airport pavements.

The rules on irregular surfaces are general. They do not name a pothole.

• ICAO Annex 14 Volume I, paragraph 3.1.22, is a Standard. A runway surface must have no irregularities that impair surface friction or adversely affect the take-off or landing of an aeroplane. A Note says that irregularities may cause excessive bouncing, pitching and vibration.
• Paragraph 10.2.2 is a Standard. The surface of a runway must be maintained so that harmful irregularities do not form.
• Paragraph 3.9.13 is a Recommendation. A taxiway surface should not have irregularities that damage aeroplane structures.
• In the EU, the aerodrome operator inspects the surfaces of movement areas and maintains them to avoid FOD. The operator also maintains runway, taxiway and apron surfaces to prevent harmful irregularities (ADR.OPS.C.010).

The sources checked for this page set no repair deadline or size limit for a pothole. The operator sets these in its maintenance programme. EASA guidance gives limit curves for the height of an isolated runway irregularity against its length. It says that isolated irregularities of the order of 2.5 cm to 3 cm over a 45 m distance are acceptable.

How a pothole is found

AC 150/5380-6C says that the airport sets a schedule of regular and routine pavement inspections. Severe weather can call for extra inspections. Airport staff should also take reports from airport users and make daily drive-by inspections.

An FAA study tested small drone data at six airports. It recommends a 1.5 mm per pixel colour orthophoto and a 6.0 mm per pixel elevation model to detect and rate airfield distresses. The elevation model helped to check distresses that have a height difference, such as depression and shoving. The study’s distress tables do not include a pothole, so it gives no resolution for one. At 1.5 mm per pixel, the minimum 150 mm pothole of the FHWA manual is 100 pixels wide.

How a pothole is repaired

AC 150/5380-6C says that crews should make pavement repairs as quickly as possible. They should repair at an early stage of distress, even when it is minor. A delay can let a minor distress grow into a major failure. The AC also says that pothole patches need warm, dry weather for best results. When an emergency repair has to be made in worse weather, the first repair is temporary and crews replace it when the weather permits.

Appendix A3 of the AC gives a procedure for full-depth repair of flexible pavement. Do not start it on a wet surface, or when the surface temperature of the underlying course is below 45 °F (7 °C). The steps are:

1. Saw cut or mill out the area to the full depth of the asphalt. Extend the cut at least 12 inches (30 cm) beyond the distressed area.
2. Repair and re-compact the base as necessary.
3. Apply a tack coat to the bottom and sides of the repair area.
4. Fill the area with hot mix asphalt equal to or better than the existing pavement. Compact it to the specified density.
5. Use a straightedge to check that the patch is flush with the pavement around it.
6. Clean the work area completely before aircraft use it.

The AC does not give a procedure for a pothole by name, and this one is for full-depth repairs. The AC also warns that repair work can leave FOD near the repair. All maintenance needs quality control and clean-up.

Highway cold-mix patching

FHWA tested pothole repair on highways in the SHRP H-106 experiment. It placed 1,250 patches of cold-mix material at eight sites. The experiment used four procedures:

• Throw-and-roll. Place the material in the hole without removing water and debris, then compact it with the tires of the truck. Check that the patch has a slight crown.
• Edge seal. A throw-and-roll patch, with a band of tack material and sand along its edge to limit water entry.
• Semipermanent. Remove water and debris and square up the sides so that they are vertical. Place the material and compact it with a roller or plate compactor. This is a partial-depth repair.
• Spray injection. A machine blows out the hole, sprays the sides with binder, and sprays in aggregate and binder together.

FHWA found that throw-and-roll was as effective as the semipermanent procedure where the two were compared. Throw-and-roll needs less labor and equipment, so it was more cost-effective in most cases if crews use quality materials. Spray injection performed as well as the control patches, but it depends on the skill of the operator. Overall, 56 percent of the patches survived to the last round of monitoring. FHWA says that pothole patches are intended to be temporary repairs. It recommends the best materials available. In most cases the poorer performance of cheap cold mix leads to greater overall patching costs.

This was a highway study with cold-mix materials. It is not an airfield procedure. On an airfield, AC 150/5380-6C says that patch deterioration affects ride quality and creates FOD potential.