Asphalt is the preferred advanced construction material for modern construction of all forms of roadways. Asphalt continues to advance in its ability to adapt to a wide variety of loads and climatic conditions. However, since asphalt is the chosen material, it is very widely used across the nation. This results in numerous modes of failure for asphalt since every construction site is unique. Some of the forms of failure that have been classified are: fatigue cracking, bleeding, block cracking, corrugation and shoving, depression, joint reflection cracking, longitudinal cracking, patching, polished aggregate, potholes, raveling, rutting, slippage cracking, stripping, transverse (thermal) cracking, water bleeding and pumping. These failures occur for numerous conditions and can be caused due to another failure which are explained in this paper. Many of the failures occur from the same cause and create the same type of hazardous condition. However, much of the failures produce unique problems that another failure can not cause, these failures are easier to recognize but are usually less common.
What are asphalt failures? How do asphalt failures occur? And why do asphalt failures occur? Any observer of any modern day constructed roadways may ask themselves these questions. A bystander may believe that those cracks observed on roadways to be due to just traffic, however, this is not the only contributor to asphalt damage, but only one possible factor. Asphalt, or pavement failure occurs when the surface of the asphalt can no longer maintain its initial shape and develops a material stress which causes various forms of damage 1. Doing some research into asphalt failures, one can easily find that “pavement failure is caused by a number of variables including water, intrusion, stress from heavy vehicles, expansion and contraction from seasonal changes, as well as sun exposure” 1. The most common and basic forms of asphalt failure include, cracking, distortion, disintegration, skidding hazards and surface treatment distress. These asphalt failures are just a few of the many forms of failures that are affecting the commute on the roadways to date.
As presented previously, there are many forms of asphalt distress which include fatigue cracking, bleeding, block cracking, corrugation and shoving, depression, joint reflection cracking, longitudinal cracking, patching, polished aggregate, potholes, raveling, rutting, slippage cracking, stripping, transverse (thermal) cracking, water bleeding and pumping.
Fatigue, or also known as alligator, cracking is a series of interconnected cracks caused by fatigue of the hot mix asphalt (HMA) surface under a repeated traffic loading. As the asphalt reaches its maximum limit of loads, longitudinal cracks begin to form starting in the wheel paths where the pavement experiences the most friction and load. “After repeated loading, these longitudinal cracks connect forming many-sided sharp-angled pieces that develop into a pattern resembling the back of an alligator or crocodile” 2. This causes a roughness on the surface of the pavement that is an indicator of structural failure. Furthermore, these cracks also allow infiltration of moisture into the base and subgrade levels which will eventually result in other issues such as potholes, and if left untreated, pavement disintegration will occur. Some common possible causes of poor structural support are a decrease in load supporting properties such as a loss of base, subbase or subgrade support due to inadequate drainage. “Water under a pavement will generally cause the underlying materials to become weak” 2. Also, an increase in loading where the asphalt is loaded heavier than its design, inadequate structural design where the asphalt was designed too thin for the anticipated loads, or possibly poor compaction during construction are all possible causes of inadequate structural support 2. The figure below represents severe fatigue cracking near the stop line at an. intersection.
Figure 1: Fatigue (Alligator) Cracking
Another form of asphalt failure is called bleeding. Bleeding is when a film of asphalt binder creates a shiny, glass-like reflecting surface that usually becomes sticky when dry and slippery when wet due to a loss of skid resistance. Some possible causes of bleeding are when an asphalt binder fills the voids of the aggregate during either hot weather or compaction due to traffic, and then spreads onto the surface of the asphalt. Bleeding is nonreversible, so during periods of low temperatures or loading, the binder will accumulate over time on the asphalt surface. The most likely causes of bleeding include an excesses of asphalt binder in the HMA either due to a poor mix design or manufacturing issue. Another cause is a low HMA air void content where there is not enough void space for the asphalt to occupy due to a problem in the mix design 2. The figure below represents a severe case of bleeding.
Figure 2: Bleeding
Block cracks are interconnected fractures that divide the asphalt up into rectangular blocks range in size from approximately one square foot to one hundred square feet. Blocks larger than one hundred square feet are generally classified as longitudinal and transverse cracking. Although, block cracking normally occurs over a large portion of pavement area, sometimes it will occur only in non-traffic areas 2. This cracking, like fatigue cracking, allows the infiltration of moisture and causes excess roughness as well. Some possible causes of block cracking are due to HMA shrinkage and daily temperature cycling. Or more typically is caused by an inability of asphalt binder to expand and contract with temperature cycles due to aging or poor choice of binder in the initial mix design 2. The figure below represents block cracking in a parking lane that most likely experiences little traffic.
Figure 3: Block Cracking
Corrugation and Shoving
Corrugation and shoving are both other forms of asphalt failures where plastic movement occurs and is characterized by ripples, as known as corrugation, or by abrupt waves, as known as shoving, through the surface of the pavement. The deformation is typically perpendicular to the direction of traffic and occurs at points where the traffic usually begins and ends (i.e. corrugation) or areas where the HMA meets a rigid object (i.e. shoving) 2. This creates an excess of surface roughness, that can eventually lead to more major damage. Corrugation and shoving are commonly caused by actions due to traffic (e.g. starting and stopping) combined with other factors such as an unstable (i.e. low stiffness) HMA layer possibly caused by mix contamination, poor mix design, poor HMA manufacturing, or lack of aeration of liquid asphalt emulsions. Traffic can also be combined with an excess of moisture in the subgrade layer to cause corrugation and shoving 2. The figure below represents shoving and corrugation at a high-profile intersection.
Figure 4: Shoving/Corrugation
Another asphalt failure are depressions, which are localized pavement surface areas with slightly lower elevations than the surrounding surfaces. Depressions can easily be observed when precipitation has occurred, and the depressions have filled with water. Depressions can cause problems such as excess roughness, or worse, when the depressions fill with water, they can cause vehicle hydroplaning. Depressions are caused by a settlement of the subgrade layer resulting from inadequate compaction during the construction phase 2. The figure below depicts depression that has caused fatigue cracking in an access roadway most likely caused by subgrade settlement.
Figure 5: Depression
Joint Reflection Cracking
Joint reflection cracking are the cracks in a flexible overlay of rigid pavement. These cracks will occur directly over the underlying rigid pavement joints. However joint reflection cracks do not include reflection cracks that occur away from an underlying joint or from any other type of base such as cement or lime stabilized 2. Problems that arise from this failure include moisture infiltration and excess roughness. A possible reason that joint reflection cracking has occurred is due to the movement of the rigid pavement slab beneath the HMA surface due to thermal and moisture changes. Although loading does not initiate this failure, it can increase rate by which the cracking occurs 2. The figure below represents joint reflection cracking on an urban roadway.
Figure 6: Joint Reflective Cracking
Another form of fatigue cracking is classified as longitudinal cracking, where the cracks are parallel to the pavement’s centerline or laydown direction. Problems that exist due to longitudinal cracking include moisture infiltration and roughness. These issues are indicative that an onset of fatigue cracking and possibly structural failure 2. During the construction phase, if the joints are placed in the wheel path or are initially constructed poorly, they will most likely fail early. This is because the joints are typically the least dense areas of the pavement, so therefore, the joints typically constructed away or outside of the wheel paths to prevent an excess of loading. Furthermore, reflective cracking can also cause longitudinal cracks from the underlying layer along with HMA fatigue, which as mentioned before indicates an onset of future fatigue cracking, and top-down cracking 2. The figure below represents longitudinal cracking that appears to be onset of fatigue cracking and is most likely occurring on the longitudinal joints.
Figure 7: Longitudinal Cracking
Another form of asphalt distress is patching. Although patching is a form of pavement repair, it actually is considered a failure. Patching is “an area of pavement that has been replaced with new material to repair the existing pavement. A patch is considered a defect no matter how well it performs” 2. Patching, amongst other issues that will eventually arise with aging and deterioration, casues an excess of roughness. This failure is caused by previous localized pavement erosion that has been cut and removed from area and replaced with fresh asphalt known as the patch. The figure below displays a large utility patch across a roadway where the edges of the patch are separated from the existing pavement.
Figure 8: Patching
Polished aggregate are areas of HMA pavement where a portion of the aggregate that is in contact with the above asphalt binder is either very small or there are no rough or angular aggregate particles. Since there is less resistance due to friction polished aggregate causes a decrease in skid resistance. As the pavement ages, the protruding rough, angular aggregate particles become polished as they are more susceptible to abrasion due to repeated traffic 2. The figure below depicts a pavement surface where the aggregate has experienced wear for about five years.
Figure 9: Polishing
Potholes are one of the more common types of asphalt failures experienced by drivers. Potholes are small to large, bowl-shaped depressions in the surface of the pavement that penetrate all the way through the HMA layer down to the base course. Typically, potholes have sharp edges and the sides are vertical near the surface of the hole where the asphalt has been stripped away. Potholes are most likely to occur on roads with a thin HMA surface of about one to two inches rather than on a road constructed with a four inch or more HMA surface 2. Potholes cause many issues such as roughness and moisture infiltration with the more serious problem being the damage caused to vehicles driving across potholes at high speeds. Furthermore, potholes are commonly caused by untreated fatigue cracking once it has become more severe and the interconnected cracks create disconnected wedges in the pavement. As vehicles drive over these small wedges, they become dislodged and the remaining hole is called the pothole 2. The figure below represents potholes that have formed on a major highway.
Figure 10: Potholes
Another asphalt failure is called raveling, which is “the progressive disintegration of an HMA layer from the surface downward as a result of the dislodgement of aggregate particles” 2. Raveling causes problems such as loose debris on the surface of the pavement, roughness, water collection in raveled locations that can result in hydroplaning, and loss of skid resistance. Raveling is a fairly common failure of asphalt and hence has many possible causes. A loss of bond between aggregate particles and the asphalt binder as a result of aging where oxidation of the asphalt binder as occurred. As the binder ages, “oxygen reacts with its constituent molecules resulting in a stiffer, more viscous material that is more likely to lose aggregates on the pavement surface as they are pulled away by traffic” 2. A loss of bond can also occur due to a dust coating on aggregate particles that causes the asphalt binder to attach to the dust particles rather than the aggregate. Aggregate segregation can also cause a loss of bond if the aggregate mixture is gap-graded and the fine particles are missing from the matrix, then the asphalt binder has no other option but to bond with the remaining coarse particles that contain few points of contact. Loss bond between the aggregate and binder due to inadequate compaction during construction. A high density is required by the HMA in order to develop a sufficient cohesion and poor compaction can actually cause rutting because once the pavement begins experiencing traffic, compaction will continue in the wheel paths due to the new loading. Raveling can also be caused by a variety of mechanical devices such as snow plows, studded tires or vehicles with tracks 2. The figure below displays a section of roadway that has experienced raveling due to segregation, temperature differentials or poor compaction.
Figure 11: Raveling
Rutting is a form of asphalt failure where surface depressions form in the wheel paths. Along the sides of the rut, a pavement uplift may occur due to shearing. Ruts are easily observed after rain and the ruts have filled with water. There are two forms of rutting, mix and subgrade. Mix rutting occurs when the subgrade does not rut yet and the pavement surface exhibits wheel path depressions due to compaction or mix design issues. On the other hand, subgrade rutting occurs when the subgrade exhibits the wheel path depressions due to traffic loading and the pavement settles into the subgrade ruts causing the surface depression 2. As mentioned in previous failures in any location where water can collect and puddle, vehicle hydroplaning can occur which is the case for rutting. Ruts also tend to pull vehicles towards the rut path as it is steered across the path causing a very hazardous condition. Rutting is generally caused by a “permanent deformation in any of a pavement’s layers or subgrade usually caused by consolidation or lateral movement of the materials due to traffic loading” 2. Some specific cases that cause rutting include inadequate compaction during construction in which compaction continues after it is opened, subgrade rutting or a defective mix design. The figure below depicts rutting at a busy intersection in region of high heat.
Figure 12: Rutting
Another form of asphalt failure is slippage cracking. Slippage cracks are crescent or half-moon shaped cracks typically having two ends extended unto the direction of traffic 2. Problems that arise from slippage cracking are moisture infiltration and an excess of surface roughness. Slippage cracking is commonly caused by braking and wheel turning which causes the pavement surface to slide and deform. This sliding and deformation are “caused by a low-strength surface mix or poor bonding between the surface HMA layer and the next underlying layer in the pavement structure” 2. The figure below represents a slippage crack that was most likely caused by an inadequate bonding of the tack coat.
Figure 13: Slippage Cracking
Stripping is classified as the loss of bond between aggregates and asphalt binder that commonly begins at the bottom of the HMA layer and advances upwards. Raveling as mentioned previously is when the stripping begins at the pavement surface and moves downward. Problems that persist due to stripping include decreased structural support, rutting, shoving and corrugation, raveling, fatigue and longitudinal cracking. “Bottom-up stripping is very difficult to recognize because it manifests itself on the pavement surface as other forms of distress including rutting, shoving/corrugations, raveling, or cracking” 2. Usually a sample of the core must be collected to properly identify this type of asphalt distress. Stripping is commonly caused by an inadequate aggregate surface chemistry and water in the HMA that causes moisture damage. The figure below displays small asphalt core showing stripping at the bottom of the pavement section.
Figure 14: Stripping
Transverse (Thermal) Cracking
Another asphalt failure is transverse or thermal cracking which are cracks that form perpendicular to the pavement’s centerline or laydown direction. Transverse cracking allows moisture infiltration and roughness. “Shrinkage of the HMA surface due to low temperatures or asphalt binder hardening, reflective crack caused by cracks beneath the surface HMA layer and top-down cracking” are all possible causes for thermal cracking 2. The figure below represents a transverse crack during a winter season when the temperature is very low.
Figure 15: Transverse (Thermal) Cracking
Water Bleeding and Pumping
When water seeps out of joints or cracks or though an excessively porous HMA layer that is known as water bleeding. Pumping occurs when water and fine material is ejected from underlying layers though cracks in the HMA layer or out the sides of the HMA layer under moving loads 2. Water bleeding and pumping cause issues such as decreased skid resistance and structural support for pumping while water bleeding is an indication of high pavement porosity. Possible causes for this failure include pavement containing high porosity due to poor compaction, a high-water table or poor drainage 2. The figure below depicts water bleeding/pumping though a porous pavement surface.
Figure 16: Water Bleeding/Pumping
The knowledge of all forms of asphalt failures is important to understanding how pavement reacts to different conditions. This information can help engineers to design for each unique condition to prevent the same failures from happen once they have been replaced or repaired. A successful asphalt mixture and proper regular maintenance is the key to preventing failures on new and existing roadways. With annual maintenance and inspections roadways will never experience any severe damage, however, with an ever-growing society, more mileage of asphalt is being laid everyday which is decreasing the overall quality of all roadways. This due to the fact that there is just too much roadway to constantly check and maintain. In other words, the rate of asphalt failure is greater than the rate that the asphalt is being repaired. So, when one road is repaired, most likely one or more other roads are reaching severe conditions and require repair or even complete replacement. Although an excellent asphalt design can last years, it will not last for eternity and will require maintenance and replacement many times throughout the roadway’s lifetime.