Types of Distress in Bituminous Pavements and their Causes
a) Cracking in Bituminous Pavements and their Causes
Cracks in bituminous pavements are caused by deflection of the surface over an unstable foundation, shrinkage of the surface, thermal expansion and contraction of the surface, poorly constructed lane joints, or reflection cracking. Five types of cracks commonly occur in these types of pavements:
(i) Longitudinal and Transverse Cracks
Longitudinal and transverse cracks often result from shrinkage or contraction of the bituminous concrete surface. Shrinkage of the surface material is caused by oxidation and age hardening of the asphalt material. Contraction is caused by thermal fluctuations. Poorly constructed lane joints may accelerate the development of longitudinal cracks.
(ii) Alligator or Fatigue Cracking
Alligator cracks refer to interconnected cracks that form a series of small blocks resembling alligator skin. They may be caused by fatigue failure of the bituminous surface under repeated loading or by excessive deflection of the asphalt surface over a weakened or under-designed foundation. The weakened support is usually the result of water saturation of the bases or subgrade.
(iii) Block Cracking
Shrinkage of the asphalt concrete and daily temperature cycling, which results in daily stress/strain cycling, causes block cracking. These are interconnected cracks that divide the pavement into approximately rectangular pieces. This type of distress usually indicates that the asphalt has hardened significantly.
Block cracking generally occurs over a large portion of the pavement area and may sometimes occur only in non-traffic areas.
(iv) Slippage Cracks
Slippage cracks appear when braking or turning wheels cause the pavement surface to slide and deform. This usually occurs when there is a low-strength surface mix or poor bond between the surface and the next layer of the pavement structure. These cracks are crescent or half-moon-shaped with the two ends pointing away from the direction of traffic.
(v) Reflection Cracking
Vertical or horizontal movements in the pavement beneath an overlay cause this type of distress. These movements may be due to expansion and contraction caused by temperature and moisture changes or traffic loads. The cracks in asphalt overlays reflect the crack pattern in the underlying pavement.
They occur most frequently in asphalt overlays on Portland cement concrete pavements. However, they may also occur on overlays of asphalt pavements wherever cracks in the old pavement have not been properly repaired.
b. Disintegration in Bituminous Pavements and their Causes
Disintegration in a bituminous pavement is caused by insufficient compaction of the surface, insufficient asphalt in the mix, loss of adhesion between the asphalt coating and aggregate particles, or overheating of the mix.
The most common type of disintegration in bituminous pavements is raveling. Raveling is the wearing away of the pavement surface caused by the dislodging of aggregate particles and the loss of asphalt binder. As the raveling continues, larger pieces are broken free, and the pavement takes on a rough and jagged appearance.
c. Distortion in Bituminous Pavements and their Causes
Distortion in bituminous pavements is caused by foundation settlement, insufficient compaction of the pavement courses, lack of stability in the bituminous mix, poor bond between the surface and the underlying layer of the pavement structure, and swelling soils or frost action in the subgrade. Four types of distortion commonly occur:
A rut is characterized by a surface depression in the wheel path. In many instances, ruts become noticeable only after a rainfall when the wheel paths fill with water. This type of distress is caused by a permanent deformation in any one of the pavement layers or subgrade, resulting from the consolidation or displacement of the materials due to traffic loads.
(ii) Corrugation and Shoving
Corrugation results from a form of plastic surface movement typified by ripples across the surface. Shoving is a form of plastic movement resulting in localized bulging of the pavement surface. Corrugation and shoving can be caused by a lack of stability in the mix and a poor bond between material layers.
Depressions are localized low areas of limited size. In many instances, light depressions become noticeable only after a rain, when ponding creates "birdbath" areas. Depressions may result from traffic heavier than that for which the pavement was designed, localized settlement of the underlying pavement layers, or poor construction methods.
An upward bulge in the pavement's surface characterizes swelling. It may occur sharply over a small area or as a longer gradual wave. Both types of swell may be accompanied by surface cracking. A swell is usually caused by frost action in the subgrade or by swelling soil.
d) Loss of Skid Resistance in Bituminous Pavements and their Causes
Factors that decrease the skid resistance of a pavement surface and can lead to hydroplaning include too much asphalt in the bituminous mix, too heavy a tack coat, poor aggregate subject to wear, and buildup of contaminants. In bituminous pavements, a loss of skid resistance may result from the following:
Bleeding is characterized by a film of bituminous material on the pavement surface that resembles a shiny, glass-like, reflecting surface that usually becomes quite sticky. It is caused by excessive amounts of asphalt cement or tars in the mix and/or low air-void content and occurs when asphalt fills the voids in the mix during hot weather and then expands out onto the surface of the pavement.
Bleeding may also result when an excessive tack coat is applied prior to placement of the asphalt surface material. Since the bleeding process is not reversible during cold weather, asphalt or tar will accumulate on the surface. Extensive bleeding may cause a severe reduction in skid resistance.
(ii) Polished Aggregate
Aggregate polishing is caused by repeated traffic applications. It occurs when the aggregate extending above the asphalt is either very small, of poor quality, or contains no rough or angular particles to provide good skid resistance.
(iii) Fuel Spillage
Continuous fuel spillage on a bituminous surface will soften the asphalt. Areas subject to only minor fuel spillage will usually heal without repair, and only minor damage will result.