What is Edge Cracking in Asphalt Pavements?

Definition

An edge crack is a crack in an asphalt surface that meets the pavement edge and lies within 0.6 m (2 ft) of it. The Long-Term Pavement Performance (LTPP) Distress Identification Manual of the FHWA defines it as “crescent-shaped cracks or fairly continuous cracks”. The definition applies only to pavements with unpaved shoulders. It includes longitudinal cracks outside the wheel path that lie within 0.6 m of the edge. The manual lists it as distress type ACP 3.

Severity and measurement

The LTPP manual rates severity by the breakup and loss of material.

• Low: cracks with no breakup or loss of material.
• Moderate: some breakup and loss, for up to 10 percent of the length of the affected portion.
• High: considerable breakup and loss, for more than 10 percent of that length.

The surveyor records the length of edge affected, in meters, at each severity. The total cannot be more than the length of the section. If edge cracking overlaps fatigue or block cracking, the surveyor rates both.

ASTM D6433, the method for roads and parking lots, also has edge cracking. A training deck based on D6433-24 gives a measuring zone that is usually 0.3 to 0.5 m from the edge. It has Low, Medium and High severity.

Causes

The shoulder cannot support the pavement edge. A study of Yukon highways links edge cracking to lateral and vertical movement of the shoulder and side slope. Differential frost heave and moisture in the edge material cause the movement. The same study names weak edge material, differential compaction and snow that crews plough onto the shoulder. These findings come from northern low-volume highways.

Airfields

Edge cracking is not a distress type of the ASTM D5340 airfield method. The PAVER manual for asphalt airfields lists 17 types, numbered 41 to 57, and none is edge cracking. A surveyor who uses the airfield PCI method therefore does not record it under that name.

Unpaved shoulders do exist on airfields. FAA AC 150/5300-13B requires paved runway shoulders for ADG-IV and larger aircraft. For ADG-I to ADG-III it requires stabilized shoulders, which means turf or soil treatment. FAA AC 150/5380-6C tells inspectors to look for soil that builds up at pavement edges and stops runoff, and for poorly shaped or eroded shoulders.