What is Polished Aggregate?
Polished aggregate is a pavement surface distress in which traffic wear leaves too little rough aggregate above the binder to give skid resistance.
What it is
Repeated traffic applications cause polished aggregate. The PAVER manual for asphalt airfields says it is present when little aggregate sticks out of the asphalt. It is also present when no rough or angular particles remain. A low or falling skid resistance number also indicates it. The FHWA LTPP manual describes it as surface binder worn away to expose coarse aggregate.
It also occurs on concrete. The FAA says some aggregates polish quickly under traffic and create skid hazards if they are used without crushing. Crushing gives rough, angular faces.
Microtexture and skid resistance
EASA defines microtexture as the texture of the individual stones. It is hardly detectable by eye and is the main part of skid resistance at slow speeds. Polishing removes it. A runway inspector can check for it by touch, because a smooth feel means that microtexture is missing. Rubber deposits and polishing are the usual causes.
The FAA states that the skid resistance of runway pavement deteriorates mainly through mechanical wear and polishing by aircraft tires, and through contaminants. The effect depends on the volume and type of traffic. ICAO Annex 14 lists surface polishing among the factors that can degrade runway friction in a note.
The polished stone value (PSV) expresses the resistance of an aggregate to polishing. EASA guidance advises crushed material that withstands polishing.
How it is rated
| System | Number | Severity levels | Unit |
|---|---|---|---|
| PAVER, asphalt airfields | 51 | None | Square feet (square meters) |
| FHWA LTPP, asphalt concrete | ACP 12, a surface defect | Not applicable | Square meters |
PAVER counts polished aggregate only when the polishing is significant. It does not count it in an area already counted as bleeding. LTPP does not rate it on a test section where a preventive maintenance treatment covers the original surface.
Repair
Table 6-1 of the FAA maintenance guide lists “apply surface treatment” as the repair for loss of skid resistance. It names polished aggregate as a probable cause. EASA guidance says to manage a polishing trend by monitoring the loss of sharpness. It adds a retexturing or resurfacing programme.
Frequently Asked Questions
- What causes polished aggregate?
- Repeated traffic applications cause it. Tires wear the aggregate that sticks out of the asphalt until it is very small or has no rough, angular particles left. Some aggregates polish quickly, and the FAA warns that naturally polished aggregates create skid hazards if they are used without crushing.
- Does polished aggregate have severity levels?
- No. Both the PAVER manual for airfield asphalt and the FHWA LTPP manual define no severity levels. Surveyors record the area in square feet or square meters. PAVER counts it only when the polishing is significant, and does not count it in an area already counted as bleeding.
- How does a runway inspector find polished aggregate?
- By close examination of the surface, and by a low or falling skid resistance number. EASA guidance also says to check for microtexture by touching the aggregate. If the surface feels smooth, microtexture is missing, most commonly because of rubber deposits or polishing.
- How is polished aggregate repaired?
- Table 6-1 of the FAA maintenance guide lists "apply surface treatment" as the repair for loss of skid resistance. It names polished aggregate as a probable cause. EASA guidance says to manage a polishing trend by monitoring the loss of sharpness. It adds a retexturing or resurfacing programme.
References
- faa-paver-asphalt
- PAVER Distress Identification Manual for Asphalt Surfaced Airfields US Army Corps of Engineers ERDC-CERL
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
- fhwa-ltpp-dim
- Distress Identification Manual for the Long-Term Pavement Performance Program (FHWA-RD-03-031), Fourth Revised Edition FHWA
- easa-ear-aerodromes
- Easy Access Rules for Aerodromes (Regulation (EU) No 139/2014), including CS-ADR-DSN Issue 7 EASA
- icao-annex-14-vol-i
- Annex 14 to the Chicago Convention, Aerodromes, Volume I, 9th edition (July 2022) ICAO
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