Flexible pavement are ones that are layered over the subgrade and comprise a mixture of asphaltic or bituminous material and aggregates that have been compacted. Its construction is based on the idea that a load of any magnitude gets less as it is transferred downward from the surface. The load spreads over an expanding surface area, which conducts it through successive granular material layers deep enough into the earth. This is why materials with a high degree of strength are employed at or close to the surface without the requirement for steel reinforcing.
Types of Failures in Flexible Pavements:
The following is a description of the various flexible pavement failures:
- Subgrade failure
- Base course failure or base failure
- Failure of the surface or wearing course
1. Subgrade Failure
Subgrade failure is a flexible failure that deforms subgrade soil excessively.
One of the primary reasons flexible pavements collapse is due to this. The pavement surface will have corrugations and undulations due to this type of failure.
Causes of Subgrade Failure:
- insufficient stability
- improper road drainage
2. Base Course Failure
Base course failure refers to excessive distortion of flexible pavement’s base or foundation course.
On the pavement surface, this kind of failure results in potholes, waves, and corrugations.
- Base Course Failure Causes:
- insufficient strength
- Loss of binding action
- Inadequate wear course
- Inadequate quality control
- Base course material crushing
- The granular base course is not confined laterally.
- Inadequate road drainage.
3. Surface failure
Surface failure, or wearing course failure, refers to excessive distortion in the wearing course of a flexible pavement. This kind of failure results in ruts, patholes, fractures, and other defects in the pavement structure.
Surface Failure Causes:
- Lack of an appropriate mix design
- Making use of a low-quality binder
- Inadequate quality control.
- Volatization and oxidation of the binder
Various types of failure are encountered in flexible pavements as follows:
- Block Cracking
- Edge Cracking
- Formation of waves & corrugation
- Longitudinal Cracking
- Map Cracking
- Pot Holes
- Shear Failure Cracking
- Slippage cracks
1. Block Cracking
This is usually caused by asphalt shrinking owing to temperature variations. Using an overly dry mixture. Poor selection of asphalt binder in mix design
2. Edge Cracking
These are longitudinal cracks that develop on the asphalt’s outside margins, a foot or two apart from one another. The most common causes of these types of cracks are poor drainage around the pavement’s edges; inadequate support at the pavement’s edge; heavy vegetation along the pavement’s edge; and heavy traffic.
3. Formation of waves & corrugation
4. Longitudinal Cracking
This failure is caused by frost action, various volume changes in the subgrade, settlement of the fill material, or side slope sliding.
5. Map Cracking
This kind of flexible road collapse results from either localized base course weakness or excessive wear of the road surface.
6. Pot Holes
These often develop in all varieties of flexible pavements as a result of the breakdown of road metal or a lack of a surface course that binds the base with the surface.
Ruts are longitudinal depressions or cuts in the flexible pavement. Due to the repetitive traffic wheel loads in the same place, particularly under wheeled traffic, they are typically formed on earth or W.B.M roads of one lane width.
8. Shear Failure Cracking
Shear failure a fracture or crack is the formation of a fracture or crack as a result of the upheaval of the pavement portion followed by a depression. Localized pavement weakening is the cause of this kind of failure.
It occurs when a vehicle applies its brakes or when the pavement meets rigid objects. This may be due to the following reasons:
- Weak sub-grade conditions
- Improper rolling
- Poor mixing
- Temperature effect
- Weak bottom
10. Slippage cracks
Slippage cracks are crescent-shaped or horseshoe-shaped fractures or splits in the surface layer. It is the asphalt where the fresh material has slid over the underlying course. It is caused by a lack of connection between layers. Acceleration, deceleration, and sudden braking on the road cause slippage fractures. Most often, these fissures appear at intersections as a result of stopping.
Maintenance of Failures
- Bituminous surface treatment-sometimes known as a seal coat or chip seal, this is a thin protective wearing surface put on a pavement or foundation course.
- It is used to protect the underlying pavements and to reduce rutting.
- A BST provides preventative maintenance against the impacts of the sun and water, which can both deteriorate the pavement structure.
- Asphalt overlay is a paving technique that involves putting a fresh layer of asphalt on an aged surface.
- An asphalt overlay construction will use existing layers as the foundation for new asphalt pavement.
- Some asphalt surfaces with severe deterioration, such as rutting, potholes, massive fractures, and expansions, may require milling before applying an overlay. is the loss of the top layer of asphalt due to fractures, raveling, or other types of deformation.
Slurry Seals, Crack Seals
- Slurry seals are a homogeneous mixture of emulsified asphalt, water, fine aggregate, and mineral filler that has a creamy fluid-like appearance when applied.
- Slurry sealants are used to repair existing pavement surface defects as a prelude to other maintenance treatments or as a wearing course.
- Crack sealants are used to fill specific pavement cracks.
- It is used to keep out water and other non-compressible materials like sand, soil, boulders, or weeds.
- Crack sealant is frequently used to prevent the progression of longitudinal, transverse, reflection, and block cracks.
- Crack filling material is usually rubberized asphalt or sand slurry