Crocodile cracking
Encyclopedia
Crocodile cracking, also called fatigue cracking or 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 300 millimetres (11.8 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 pothole
s or premature pavement failure.
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.
Edge cracking, is the formation of crescent-shaped cracks near the edge of a road. It is caused by lack of support of the road edge, sometimes due to poorly drained or weak shoulders. If left untreated, additional cracks will form until it resembles crocodile cracking. Like wheel-path crocodile cracking, poor drainage is a main cause of edge cracking, as it weakens the base, which hastens the deterioration of the pavement. Water ponding
(a build up of water which is also be called puddling) happens more frequently near the edge than in the center of the road path, as roads are usually sloped to prevent in-lane ponding. This leads to excess moisture in the shoulders and subbase at the road edge. Edge cracking differs from crocodile cracking in that the cracks form from the top down, where crocodile cracks usually start at the bottom and propogate to the surface.
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.
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.
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.
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
pavement. It is usually studied under the transportation section of civil engineering
Civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...
. Crocodile cracking is characterized by interconnecting or interlaced cracking in the asphalt layer resembling the hide of a crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
. Cell sizes can vary in size up to 300 millimetres (11.8 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
Drainage
Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
, 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 pothole
Pothole
A pothole is a type of disruption in the surface of a roadway where a portion of the road material has broken away, leaving a hole.- Formation :...
s or premature pavement failure.
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 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
Construction Aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
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.
Edge cracking, is the formation of crescent-shaped cracks near the edge of a road. It is caused by lack of support of the road edge, sometimes due to poorly drained or weak shoulders. If left untreated, additional cracks will form until it resembles crocodile cracking. Like wheel-path crocodile cracking, poor drainage is a main cause of edge cracking, as it weakens the base, which hastens the deterioration of the pavement. Water ponding
Ponding
Ponding is a term that refers to the unwanted pooling of water, typically on a flat roof.-Cause:Most flat roof systems are designed with a slight pitch to shed water off the sides, usually into a scupper system or into an internal drainage system. When a scupper or drain is clogged or fails for...
(a build up of water which is also be called puddling) happens more frequently near the edge than in the center of the road path, as roads are usually sloped to prevent in-lane ponding. This leads to excess moisture in the shoulders and subbase at the road edge. Edge cracking differs from crocodile cracking in that the cracks form from the top down, where crocodile cracks usually start at the bottom and propogate to the surface.
Development
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.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 and 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 IndexPavement Condition Index
The Pavement Condition Index is a numerical index between 0 and 100 and is used to indicate the condition of a roadway. It is widely used in transportation civil engineering. It is a statistical measure and requires manual survey of the pavement...
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
AASHO Road Test
The AASHO Road Test was a series of experiments carried out by the American Association of State Highway and Transportation Officials to determine how traffic contributed to the deterioration of highway pavements...
.
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
Pavement management system
Pavement management system or PMS is a term that relates to a system that utilizes the condition coding of roadways coupled with the identification of strategies to determine maintenance or re-construction activities...
, 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
Profilometer
Profilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness.While the historical notion of a profilometer was a device similar to a phonograph that measures a surface as the surface is moved relative to the contact profilometer's stylus, this...
, which is mounted on a vehicle and measures the profile of the road surface while it is moving down the roadway.
Prevention and repair
Preventing crocodile cracking can be as simple as preventing the common causes. For example, 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
Core sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...
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.