Pavement Cracking: A Failure Indicator of Your Roads
It has been almost 2-years since arriving here at the Cornell Local Roads Program (CLRP). There are many things I have learned in that time, the most important being that there is a lot more to learn. Not being satisfied with just recognizing a problem, I often find myself seeking the reason for the cause of the problem. Having participated in the reintroduction of the Road Surface Management System (RSMS) program into the curriculum of CLRP with the advent of new software, I found myself seeking a better understanding of why pavements fail.
There are many causes to why pavements fail; one of the most common is fatigue due to pavement deflection. In the CLRP Pavement Maintenance workbook, fatigue is described as the failure of a material due to repetition of many loads. In the workshop, the bending of a paper clip is used to show how the greater the movement or bending angle in the paper clip, the sooner the failure. The same is true in pavements; the greater the load on the surface, the greater the flexing of the pavement surface. This is why roads with heavier loads require different designs than lower/ lighter volume roads. Excessive flex in some paved roads can also be found to occur in the spring, when the road base material becomes saturated and soft. Because of this it is a good practice to limit heavy loads on weaker roads during the spring thaw.
It is easy to say that a road is underdesigned. The bigger issue is how do we fix the problem after it is identified? This is where a better understanding of the types of fatigue that show up in a road is important. Fatigue is typically seen as some form of cracking. Cracking found in pavements vary according to the root cause of the problem. Common types of cracking are fatigue or alligator cracking, longitudinal cracking, transverse cracking, block cracking, slippage cracking, reflective cracking and edge cracking.
One of the most important things to consider in roadway maintenance is drainage. The entrance of water into the base material is a major culprit in the acceleration of pavement failure. Water can enter into the base material in several ways. The most obvious is through the existing cracks in the road surface. Other means of water access to the base are through the piping of ground water up through the base, high exposed pavement edges, and standing water on a road without a crown.
The CLRP Pavement Maintenance workbook identifies typical types of cracking, their causes and repair options. One of the more common types of cracking is alligator cracking, also referred to as fatigue cracking. Below we will briefly discuss alligator cracking to better understand and identify the severity of this type of cracking.
Fatigue (Alligator) Cracking
Alligator cracking is characterized as a series of interconnected cracks creating small, irregular shaped pieces of pavement. The sizes of these irregular shaped pieces vary, from less than 12-inches to 24-inches. The cracking pattern is similar to the pattern found on alligator skin, and therefore the source of its name.
The cause of this type of cracking is the failure of the bound layer due to repeated traffic loading resulting in the disintegration of the surface and eventually potholes. The cracking starts out of sight, at the bottom of the surface course or in the stabilized base where the tension stresses are the highest. As the pavement flexes under repeated wheel loading the crack moves upward toward the surface eventually leading to failure. As the wheel loading continues, more cracks are formed.
Repair: The type of repair is dependent on the extent and severity of the failure area. There are three levels of severity used in rating alligator cracking.
Low Severity Cracking
Low Severity is characterized by an area of cracking with very narrow cracks with almost no deterioration of the surface. The cracking is often isolated and many times the cracking may not be interconnected to other areas and there is not much distortion. These cracks may be sealed to extend the useful life of the pavement, but the underlying cause may still lead to additional premature cracking.
Medium Severity Cracking
Medium Severity is characterized by interconnected cracking forming a small area of the typical alligator pattern. The cracks may have signs of slight spalling, with no pumping visible. Sealing of the surface cracks may be an option but should be carefully considered to avoid creating slick spots on the pavement surface. In smaller areas a proper fix would be to remove and replace the base and surface (i.e. a box out and replacement). Providing subsurface drainage may help in preventing the need for future repairs. Larger areas will require either reclamation or reconstruction.
High Severity Cracking
High Severity alligator cracking is characterized by an area of moderate to severe spalled interconnected cracks creating a full pattern of cracking. Pieces of asphalt may be loose or missing from traffic and the pumping of water or fine material from below the pavement may be present on the surface. As with medium severity, in smaller areas a proper fix would be to remove and replace the base and surface. Providing subsurface drainage may help in preventing the need for future repairs. Larger areas will require either reclamation of reconstruction.
In all cases, it is advisable to look into the drainage in these areas since this type of failure can be caused by poor drainage within the base layer. Caution should also be taken when using a crack seal on this type of failure. Excessive crack seal material create slick and potentially dangerous safety issues due to the reduced friction surface in both wet and hot weather.
Identifying the severity and extent of cracking on a road can assist in determining the cause of the failure and the appropriate repair that will provide the best long term solution to eliminating or reducing the problem. There is no one treatment fix-all when repairing roads. Without properly repairing the drainage and/or the base problem causing the alligator cracking will only prolong the return of the cracking.
An asphalt overlay will only temporarily fix an alligator cracked road. Due to the continual flex movement of the base, an overlay will eventually fail due to the same underlying problems. The typical failure rate, in this case, for an overlay is approximately 1-inch of asphalt per year. So for a 2-inch overlay, one could expect the cracking to return in 2-years. In higher traffic, the failure may return sooner. Because of this and the associated costs of road repairs, recognizing the type of asphalt surface failure and the understanding the cause of the failure is an important step in properly utilizing the ever-limited funds available to make these repairs.
Distress Identification Manual, U.S. Department of Transportation, FHWA-RD-03-031, June 2003.
Road Surface Management System, Technology Transfer Center University of New Hampshire, May 2003.
Pavement Maintenance (PDF) CLRP workshop manual