User:Alec r 623/ Outline

Alec_r_623/_Alligator_CRACking

Crocodile Cracking, also called alligator cracking, is a common type of distress in asphalt pavement. It is usually studied under the transportation section of civil engineering. Crocodile Cracking is characterized by interconnecting or interlaced cracking in the asphalt layer resembling the hide of a crocodile. Cell sizes can vary in size up to 300mm across, but are typically less than 150mm across. Crocodile cracking is generally a loading failure, but it can be precipitated by numerous factors. It is often a sign of sub-base failure, poor drainage, or repeated over-loadings. It is important to prevent crocodile cracking, and repair as soon as possible, as advanced cases can be very costly to repair.

Causes
Crocodile cracking is an asphalt pavement distress most often instigated by failure of the surface due to traffic loading. However, crocodile cracking can be greatly influenced by environmental and other effects while traffic loading remains the direct cause. Frequently overloading happens because the base or subbase inadequately support the surface layer, and subsequently it can't handle loads that it could normally endure. There are many ways that the subbase or base can be weakened.

Poor drainage in the road bed is a frequent cause of this degradation of the base or subgrade. A heavy spring thaw, similarly to poor drainage, can cause a weakening of the base course, which often leads to crocodile cracking. Stripping is another possible cause of crocodile cracking. Stripping is another type of asphalt failure, which prevents the affected portion from being able to carry it's designed loading. This can cause crocodile cracking to develop rapidly, as overloading will happen with loads of less magnitude or frequency.

Edge cracking, discussed below, is caused in much the same way as in-lane crocodile cracking, but has some causes that are more prevalent. Water ponding(a build up of water which is also be called puddling)is a main cause of edge cracking, as it weakens the base which hastens the deterioration of the pavement. Ponding also happens more frequently near the edge than in the center of the road path, as roads are usually angled to prevent in-lane ponding.

Development
Crocodile cracking manifests itself initially as longitudinal cracking(cracks along the direction of the flow of traffic) in the top layer of the hot mix asphalt. These cracks are initially thin and sparsely distributed. If further deterioration is allowed, these longitudinal cracks are connected by latitudinal cracks to form sharp sided, prismatic pieces. This interlaced cracking pattern is the origin of the name "Crocodile Cracking". More severe cases involve pumping of fines, spalling, and loose pieces of pavement. The most severe cases of crocodile cracking often occur with other pavement distresses, but are exemplified by: potholes, large cracks(3/8" or larger), and severely spalled edges.

Another form of crocodile cracking is edge cracking, which is simply crocodile cracking located near the edge (within 1-2 feet) of the asphalt pavement. The failure begins at the edge of the pavement, and continues towards the center as condition worsens. Further Deterioration proceeds in much the same manner as with in-lane crocodile cracking.

Measurement/Quantification
There are many different ways to measure crocodile cracking, but in general a pavement distress manual or index will be used. For example, the Pavement Condition Index is widely used to quantify the overall level of distress and condition of a section of road. Measurement of crocodile cracking specifically(and pavement distress in general) is necessary to determine the overall condition of a road, and for determination of a time-line for rehabilitation and/or repair. There are many other rating systems, and many rating systems currently in use are based on the AASHTO Road Test.

There are two important criteria to take into effect when measuring crocodile cracking. The first is the extent of the cracking. This is the amount of road surface area which is affected by this pavement distress. The second criteria is the severity of the cracking. Severity, which has been discussed above, refers to how far the cracking has progressed, and is often directly a function of crack width.

Crocodile cracking is often given a rating, sometimes numerically. Other times it is given a rating from "low" to "severe". These levels of distress are often separated in much the same way as the levels of severity in the development section.

Systems have been developed that detect crocodile and other types of pavement distress automatically. They measure the severity and frequency of alligator cracking on the road-path. One such machine is the road surface profilometer, which is mounted on a vehicle and measures the profile of the road surface while it is moving down the roadway.

Repair/Prevention
Preventing crocodile cracking can be as simple as preventing the common causes. For example, reducing overloading on an asphalt pavement or alleviating water ponding and poor drainage can prevent crocodile cracking in many cases. However, often the specific cause is fairly difficult to determine, and prevention becomes difficult correspondingly.

A good strategy to prevent overloading, which is a main cause of crocodile cracking, is to increase the depth of the asphalt layer. According to certain researchers, pavements of a minimum strength or thickness can hypothetically handle infinitely many loads without showing structural defects, including crocodile (fatigue) cracking. These pavements are called perpetual pavements.

When repairing pavement affected by crocodile cracking, the main cause of the distress should be determined. Any investigation should involve digging a pit or coring the pavement to determine the pavement's structural makeup as well as determining whether or not subsurface moisture is a contributing factor. The repair needed also differs based on the severity and extent of the cracking.

In the early stages, sealing cracks with crack sealant limits further deterioration of the subgrade due to moisture penetration. Small areas may be repaired by removal of the affected area, and replacement with new base and asphalt surface. Once the damage has progressed from medium to sever stages, an asphalt overlay or complete reconstruction may be necessary.