Understanding asphalt pavement distresses – five distresses explained

Before the appropriate repair strategy can be applied to a distressed asphalt pavement, the type and extent of the deterioration must be understood, and the cause of the distress must be identified.

The common types of pavement distresses include: cracking; distortion; disintegration; skidding hazards; and surface treatment distresses. Some of the typical causes of pavement deterioration include: traffic loading; environment or climate influences; drainage deficiencies; materials quality problems; construction deficiencies; and external contributors, such as utility cuts.

As pavements age and experience traffic repetitions, pavement distresses begin to accumulate. Also distresses can compound themselves; for example, a crack can allow water to enter the pavement and lead to the development of a pothole or stripping. So it is important to perform timely maintenance.

Pavement distresses and associated treatments are covered in more detail in the Asphalt Institute�s manuals, MS-4, The Asphalt Handbook, and MS-16, Asphalt in Pavement Preservation and Maintenance.

#1 Cracking
Several different types of cracks can develop in asphalt pavements. The repair depends on the type of crack. Some cracks are load-related and some are attributable to temperature or environment.

Fatigue Cracking.
Fatigue cracking is sometimes called alligator cracking due to the interconnected cracks which resemble an alligator�s skin. Fatigue cracking is caused by load-related deterioration resulting from a weakened base course or subgrade, too little pavement thickness, overloading, or a combination of these factors.

Block Cracking.
Block cracking is a series of large (typically one foot or more), rectangular cracks on an asphalt pavement�s surface. This type of cracking typically covers large areas and may occur in areas where there is no traffic. Block cracking is typically caused by shrinkage of the asphalt pavement due to temperature cycles.

Edge Cracking.
Edge cracks are longitudinal cracks which develop within one or two feet of the outer edge of a pavement. These cracks form because of a lack of support at the pavement edge.

Longitudinal Cracking.
Longitudinal cracks occur parallel to the centerline of the pavement. They can be caused by: a poorly constructed joint; shrinkage of the asphalt layer; cracks reflecting up from an underlying layer; and longitudinal segregation due to improper paver operation. These cracks are not load-related.

Transverse Cracking.
Transverse cracks occur roughly perpendicular to the centerline of the pavement. They can be caused by shrinkage of the asphalt layer or reflection from an existing crack. They are not load-related.

Reflection Cracking.
Reflection cracks are cracks that form over joints or cracks in a concrete pavement or in an overlay of a deteriorated asphalt pavement. The cracks form because of movement of the old pavement.

Slippage Cracking.
Slippage cracks are crescent-shaped cracks which form because of low-strength asphalt mix or a poor bond between pavement layers. The cracks form due to the forces applied by turning or braking motion of vehicles.

#2 Distortion
Distortions in an asphalt pavement are caused by instability of an asphalt mix or weakness of the base or subgrade layers. These distresses may include rutting, shoving, depressions, swelling and patch failures.

Rutting is a linear, surface depression in the wheelpath. Rutting is caused by deformation or consolidation of any of the pavement layers or subgrade. It can be caused by insufficient pavement thickness, lack of compaction, and weak asphalt mixtures.

Shoving is the formation of ripples across a pavement. This characteristic shape is why this type of distress is sometimes called wash-boarding. Shoving occurs at locations having severe horizontal stresses, such as intersections. It is typically caused by: excess asphalt; too much fine aggregate; rounded aggregate; too soft an asphalt; or a weak granular base.

Depressions, also call bird-baths, are localized low spots in the pavement surface. These flaws can be caused by settlement or other failure in the lower pavement layers or by poor construction techniques.

Upheaval is a localized upward movement in a pavement due to swelling of the subgrade. Frost heave is an example of this type of distress.

Patch Failures.
When pavements are patched, some distress may begin to occur. The rate at which a patch deteriorates is influenced by compaction, materials selection, and the quality of the surrounding or underlying pavement.

#3 Disintegration
Disintegration is the breakup of a pavement into small pieces that are lost with time and traffic. Raveling and potholes are the most common types of disintegration.

Raveling is the wearing away of the aggregate particles from the asphalt cement. This condition indicates that the asphalt has hardened or that a poor quality mixture was used. Raveling occurs in the presence of traffic.

Potholes are bowl-shaped holes caused by the localized disintegration of the pavement surface. Potholes typically result from the continued deterioration of another type of distress. Segregation, cracks or failed patches may serve as the start of a pothole. Poor mixtures and weak spots in the base or subgrade accelerate pothole failures.

#4 Skidding Hazards
Skidding hazards are caused by water on the surface of the pavement, polished aggregates, or excess asphalt or other lubricants on the pavement�s surface. Maintaining a pavement which allows water to flow off the pavement is a basic consideration.

Polished Aggregate.
Some aggregates are susceptible to wearing to a smooth finish or texture under traffic. These aggregates should not be used in wearing courses. If the aggregate polishes, the surface roughness must be restored.

A pavement surface can become flushed with asphalt and lose its skid resistance. Bleeding can be caused by too much asphalt in the mix, excess tack or bond coat, or by poor construction of a sealcoat. Traffic can also over-compact an unstable mix and force asphalt to the surface.

#5 Surface Treatment Distress
Surface treatments can experience their own distresses. These include loss of cover aggregate and streaking.

Loss of Cover Aggregate.
Surface treatments can lose the aggregate cover prematurely. Typically this is caused by: spreading the chips too late in the construction process; using a dirty aggregate; allowing traffic to use the surface before the aggregate is seated and bonded; or applying too little asphalt to embed the aggregate.

Streaking is the presence of alternating stripes of aggregate or asphalt. Streaking is primarily caused by the uneven application of the asphalt to the pavement surface before the aggregate chips are applied.

Asphalt pavements offer incredible value. They are quick and economical to construct, have excellent ride quality and are long lasting. Still they are subject to some distresses, particularly when poor materials are used or sloppy construction techniques are followed. By understanding the types of pavement distresses and the causes of these problems, appropriate corrective action can be taken to repair the pavement and continue its service life.

Dwight Walker is a consulting engineer specializing in asphalt pavement materials and is a contributing editor for Asphalt Magazine.