Common asphalt pavement defects and their causes
Asphalt pavement defects are a common occurrence in roads, parking lots, and other paved surfaces. Understanding the different types of cracking in asphalt pavement can help in determining the appropriate repair or maintenance strategies to ensure the longevity and safety of the pavement.
The list below covers the different types of pavement distresses that are often found on asphalt pavements.
Defects in Asphalt Pavement
Fatigue/Crocodile Cracking
This type of cracking resembles the pattern of a crocodile's skin. It is caused by the repeated application of traffic loads that exceed the structural capacity of the pavement layers.
Block Cracking
Block cracking appears in a rectangular or square pattern in the asphalt pavement. It is usually caused by temperature changes that cause the pavement to expand and contract, eventually leading to cracking.
Longitudinal Cracking
Longitudinal cracks form parallel to the direction of traffic. It can be caused by inadequate construction of the paving lane joint, excessive traffic loads, temperature changes, and a lack of pavement strength (i.e. if there are existing reflection cracks beneath the surface course). Longitudinal cracking can lead to the separation of the pavement layers and the formation of potholes.
Transverse Cracking
This is a type of cracking that appears perpendicular to the direction of traffic. It is usually caused by temperature changes that cause the pavement to expand and contract, eventually leading to cracking. Transverse cracking can also be caused by a lack of pavement strength, moisture infiltration, and excessive traffic loads.
Reflection Cracking
Reflection cracks occur when cracks from the underlying pavement layers are reflected through to the surface of a new overlay. It is caused by the movement of the underlying pavement layers due to temperature changes, moisture infiltration, and aging. Reflection cracking can be prevented by ensuring that the underlying pavement layers are in good condition before applying an overlay.
Edge Cracking
This is a type of cracking that occurs along the edge of the pavement from the lack of support on the shoulder caused by poor drainage and weak materials. Edge cracking first appear as crescent shaped cracks that eventually expand to the edges to resemble crocodile cracking.
Edge Fatigue Cracking
This kind of cracking consists of a series of interconnected cracks that form along the edges of the pavement, usually near the curb or the shoulder. Edge fatigue cracking is more common in older pavements or in areas with heavy traffic loads, and can also be exacerbated by poor construction practices, such as inadequate compaction or improper joint construction.
Slippage Cracking
Slippage cracking typically appears as crescent or circular-shaped cracks that extend parallel to the pavement's direction of travel. This type of cracking can be attributed to low strength surface mix (i.e. those with high sand content), or a lack of bonding between the surface and the layer below. The lack of adherence could be ascribed to various factors such as the existence of debris, grease, grime, moisture, and the absence of a tack coat or prime coat while applying a thin layer of asphalt on the aggregate foundation.
Corrugation and Shoving
This type of plastic movement is characterised by ripples (corrugations) or abrupt waves (shoving) that form perpendicular to the direction of traffic. Corrugation occurs in areas where traffic starts and stops, while shoving occurs where the hot mix asphalt (HMA) meets a rigid object. It is typically caused by combined effects of traffic action (starting and stopping) and an unstable HMA layer. The instability of the HMA layer can be caused by various factors, such as mix contamination, poor mix design, poor HMA manufacturing, or inadequate aeration of liquid asphalt emulsions. Additionally, too much moisture in the subgrade can negatively contribute the issue.
Rutting
This is a surface depression which usually occurs in the wheel path. Consolidation or lateral movement of materials resulting from the pressure of traffic loads is usually the cause of this issue. Rutting can happen in the subgrade or base from inadequate design thickness, lack of compaction or moisture infiltration. A weak asphalt mixture can also cause rutting.
Depressions
A depression is a specific area on the pavement that has a lower elevation than the surrounding pavement and may sometimes include cracking. It is possibly caused by traffic overloading, or by foundation soil consolidation, settlement or deterioration of the lower layers.
Swelling
This is a type of pavement distress where a localised area of the pavement is higher than the surrounding pavement, and is not caused by shoving. The swell can appear as a protrusion on the roadway, which can either be a sudden and steep rise in a small area or a gradual and elongated wave. Both cases may be accompanied by cracking. The cause can be attributed to frost action in the subgrade, or expansion in the soil.
Potholes
Potholes refer to bowl-shaped holes on the pavement surface that often extend from the asphalt layer down to the base course. Water, poor drainage, traffic loading and a weak layer(s) lead to the formation of potholes. Potholes occur as a consequence of other defects, such as crocodile cracking, swelling, ravelling, or failed patching.
Patches
Patches involve the removal and replacement of pavement material to repair a defect or cover a utility trench. Even though patches are a repair solution, it is still considered a defect, regardless of how well it performs. If a patch fails, it can lead to further damage to the surrounding pavement, causing roughness and lowering ride quality.
Stripping
This issue occurs when the lower layers of the asphalt deteriorate and cause damage to the upper layers. It can be challenging to identify bottom-up stripping as it often appears as other forms of pavement distress such as rutting, shoving, corrugations, ravelling, or cracking on the surface. To confirm if stripping is the cause of the damage, a core sample is usually required. The causes of stripping may include inadequate aggregate surface chemistry, moisture damage caused by water in the asphalt, and overlays on an open-graded surface course. Such overlays are prone to stripping.
Ravelling
Ravelling is a condition where the surface loses coarse aggregate particles, causing the pavement to appear rough and jagged. This distress is often found in the wheelpath. The occurrence of ravelling can be attributed to various factors, such as inadequate compaction of asphalt, constructing a thin layer during cold weather, usage of contaminated or deteriorating aggregate, insufficient amount of asphalt in the mixture, or excessive heating of the asphalt mix. In almost any instance, traffic loads and water are the two most common elements that are responsible for ravelling.
Polishing
Polishing in asphalt pavement refers to smoothed aggregate. It is caused by the wearing down of the aggregate particles and binder over time due to traffic. Polishing reduces the pavement's skid resistance and can increase the risk of accidents, particularly in wet conditions.
Bleeding
Also known as flushing, bleeding is the condition where there is excess bituminous binder on the road surface, and can often occur in the wheelpath. Bleeding can manifest in various ways, from discoloration of the surface relative to the rest of the pavement, to loss of surface texture due to excess asphalt, and even to the point where the aggregate becomes obscured by a shiny, glass-like, reflective surface that may feel tacky to the touch. Bleeding possibly occurs when asphalt binder fills the voids in the aggregate during hot weather and expands onto the pavement surface. Since it does not reverse during colder weather, the binder accumulates on the surface over time. Causes of bleeding may include excessive asphalt binder in the HMA due to mix design or manufacturing, excessive application of asphalt binder during BST application, and low air void content in the HMA, which doesn't provide sufficient space for the asphalt to expand into during hot weather.
Delamination
Delamination in asphalt pavement occurs when the bond between the asphalt layers breaks down, causing separation between the layers. It is caused by improper surface preparation or tack coat before the application of the wearing layer. Environmental factors such as ingress of water and repeated freeze-thaw cycles can also cause delamination.
What can we do about all these defects?
After considering all the above defects, you may be wondering, how do we go about repairing them? We believe that before you do any maintenance or rehabilitation work, you should first scan your roads to correctly identify and rate the severity of the defects. The ARAN LCMS is a laser profilometer that can automatically extract crack data, including crack type, and severity. The high resolution of the LCMS means it can automatically detect ruts (depth and type), macrotexture and ravelling. Click here to download a free technical brochure.
Alternatively, feel free to contact our pavement consultants to get a tailored solution for your organisation. Get in touch today!
Figure 1: Distresses in asphalt pavement
References
Indiana Department of Transportation. (2021). Causes of AC Distress. [online] Available at: https://www.in.gov/indot/div/aviation/pavement-inspection/pci-review/causes-of-distress-ac.html
The Engineering Community. (2018). Pavement Defects and Failures You Should Know! [online] Available at: https://www.theengineeringcommunity.org/pavement-defects-and-failures-you-should-know/
Pavement Interactive, (n.d.). Pavement Interactive – Free online knowledge for the paving industry. [online] Available at: https://pavementinteractive.org/