Alligator Cracking

ALLIGATOR CRACKING AN INTRODUCTION :- Crocodile cracking, also called fatigue cracking or alligator cracking, is a common type of distress in asphalt pavement. 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 300 millimetres (12 in) across, but are typically less than 150 millimetres (5. 9 in) across. Crocodile cracking is generally a loading failure, but numerous factors can contribute to it. 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 and can lead to formation of potholes or premature pavement failure. CAUSES OF ALLIGATOR CRACKING :- * Crocodile cracking is an asphalt pavement distress most often instigated by failure of the surface due to traffic loading. * 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 cannot handle loads that it would normally endure. 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 weaken the base course, leading to crocodile cracking. * Stripping or raveling is another possible cause of crocodile cracking. Stripping occurs when poor adhesion between asphalt and aggregate allows the aggregate at the surface to dislodge. If left uncorrected, this reduces the thickness of the pavement, reducing the affected portion's ability to carry its designed loading.

This can cause crocodile cracking to develop rapidly, as overloading will happen with loads of less magnitude or frequency. CAUSES OF ALLIGATOR CRACKING :- Failure of surface due to loading Stripping of bitumen due to poor adhesion of bitumen ; asphalt Environmental conditions :- (a) Due to excessive rain b) hot weather (c) frost action CHARACTERISTICS or IDENTIFICATION :- * Crocodile cracking manifests itself initially as longitudinal cracking (cracks along the direction of the flow of traffic) in the top layer of the asphalt. * These cracks are initially thin and sparsely distributed. If further deterioration is allowed, these longitudinal cracks are connected by transverse cracks to form sharp sided, prismatic pieces. * This interlaced cracking pattern resembles the scales on the back of a crocodile or alligator, hence the name, alligator 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. MEASUREMENT & QUANTIFICATION :-

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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 AASHO 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 criterion 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. Severity may be rated numerically, or given a rating from "low" to "severe". The rating may be entered into a pavement management system, which will suggest a priority and method for the repair.

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. PREVENTION & REPAIR :- Preventing crocodile cracking can be as simple as preventing the common causes. Reducing overloading on an asphalt pavement or improving 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 and subbase 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, a structural asphalt overlay or complete reconstruction may be necessary. CONCLUSION When repairing pavement affected by crocodile cracking, the main cause of the distress should be determined. However, often the specific cause is fairly difficult to determine, and prevention is therefore correspondingly difficult. Any investigation should involve digging a pit or coring the pavement and sub base to determine the pavement's structural makeup as well as determining whether the moisture is a contributing factor.

Small areas may be repaired by removal of the affected area, and replacement with new base and asphalt surface. Once the damage has progressed or the affected area is large and extensive, a structural asphalt overlay or complete reconstruction is necessary to ensure structural integrity. If the government also takes necessary action by giving an yardstick of the maximum ESWL that can be allowed, then cracking can be prevented. However in practical it is not possible always. Our conciousness also plays a great role. If we can understand what can cause cracking and take necessary action, then it can be prevented.

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