Flexible (Asphalt) Pavement Failure Modes
Whether its to go get a gallon of milk, drop off the kids at soccer practice, or go to work, most of us drive a car everyday. And every day that we travel, we are likely to drive on a road that was paved with asphalt. Asphalt is a by-product of crude oil refinement and is the surface of choice for nearly 94% of the roads in America. This is because asphalt is an inexpensive, readily available, and relatively easy to use material. The following definitions will help the lay person understand the wide variety of failure modes that this type of pavement can experience.
Flexible Pavement Organizations
Bleeding - A failure mode of the pavement surface characterized by prominent black streaks on the pavement's surface typically located in the path of vehicle wheels. This form of failure is generally caused excessively high temperatures. This causes the asphalt to be 'squeezed out' of the void spaces within the mix.
Block Cracking - A failure mode similar to fatigue cracking that is characterized by large rectangular blocks on the pavement surface. This form of failure is caused by thermal stresses, aging pavements, and poor mix design.
Corrugation/Shoving - A failure mode of that is characterized by ripples or waves across the pavement surface. The ripples occur perpendicular to the direction of travel and most often occurs near intersections or adjacent to more rigid surfaces. This form of failure is caused by excessive moisture and/or poor mix design.
Depression - A failure mode of the pavement surface characterized by prominent channel-like features in the pavement. Depression results from increased wheel loads and poor subbase and subgrade construction. Depression failure is not the same as Rutting.
Fatigue Cracking - A failure mode of the pavement surface characterized by 'alligator' shaped cracks or fractures. This type of crack forms propagates upward from the bottom of the asphalt layer. This form of failure results from increased wheel loads when a flexible pavement is too rigid or the temperatures have been too cold. It can also be caused by poor design and/or construction.
Longitudinal Cracking - A failure mode consisting of long parallel cracks or fractures in the direction of vehicular traffic. This type of cracking is a form of fatigue cracking. This form of failure is caused by poor joint construction or poor mix design.
Polishing - Polishing is a failure mode of the pavement surface consisting of rough exposed aggregates. This form of failure is caused by excess repeated traffic on an aging pavement system.
Potholes/Patches - Potholes are large bowl shaped holes in the pavement surface indicative of a localized problem such as drainage or subgrade deterioration. Usually the pothole exposes the pavements subbases. Potholes usually occur after fatigue cracking has gone un-repaired for some time. Potholes are more common in thinner pavements. Areas of dense potholes may indicate an overall failure of an aging pavement. Patches are the repair that is completed to remedy the pothole. The patch is still considered a failure mode as it almost never completely meshes with the existing pavement nor is it structurally bound to it.
Pumping - A failure mode of the pavement surface characterized by upward seepage of water through the pavement layers. This form of failure is typically caused by poor drainage or a high water table.
Raveling - A failure mode characterized by complete disintegration of the pavement surface resulting in dislodgement of the aggregates. The aggregate is left behind as loose gravel on the surface of the pavement. This form of failure is caused by poor compaction during construction or poor mix design. Raveling is also known as weathering.
Rutting - A failure mode that is characterized by permanent deformation of the pavement surface in the wheel path of vehicular traffic. Rutting differs from depression is there is usually some shearing or 'uplift' of the pavement surface at the edge of the ruts. This form of failure is caused by inadequate compaction during construction and poor mix design.
Stripping - A failure mode similar to raveling that is characterized by the loss of adhesion between the asphalt binder and aggregates at the bottom of the pavement layer. This form of failure is caused by poor quality aggregates that may not be clean or contain too much moisture.
Top Down Cracking - A failure mode of the pavement surface characterized by surface cracks or fractures that have a definite change in width as the depth increases. The form of failure is caused by oxidation penetration of the asphalt mix. It can also be caused by the freeze and thaw process of water on the pavement surface.
Thermal Cracking- A failure mode of the pavement surface characterized by longitudinal cracks or fractures. At first glance it may be in-discernable from fatigue cracking. This form of failure results from thermal expansion and contraction of the pavement surface due to large and quick changes in temperature. Thermal cracking is also sometimes known as transverse cracking.