Failures in flexible pavements can be due to failure of its component layers which undergo distress due to various causes. Types of failures in flexible pavements and repair techniques are discussed.
- Types of Failures in Flexible Pavements and their Repair Techniques
- Types of Failures in Flexible Pavements due to Exposure
- Types of Failures in Flexible Pavements due to Distress
- Types of Failures in Flexible Pavements due to Structural Distresses
- Types of Failures in Flexible Pavements due to Environmental Distresses
Types of Failures in Flexible Pavements and their Repair Techniques
In general, the flexible pavement consists of the following component layers:
- Sub-base course
- Base Course
- Surface Course
Instability in any of the layers will result in the complete failure of the pavement system. This makes it necessary to construct each layer with utmost care and precision.
There are different types of failures in flexible pavements. Determination of this failure and its reasons is necessary to facilitate correction in mix design and construction for the future projects.
Types of Failures in Flexible Pavements due to Exposure
The exposure that affects the flexible pavements adversely are:
Change in weathers imposes distress in flexible pavements.
Use of chemical and salts in colder climates
The frost heave and the stripping of asphalt due to snow and ice will result in potholes and other distress.
The ultraviolet rays make the pavement to undergo oxidation and bring it to a brittle state. On a hot sunny day, the pavement temperature can be up to 140 degree Celsius. This is the softening point of liquid asphalt. This will make the pavement to expand and move.
The reduction of temperature will make the pavement to contract. This expansion and contraction are the main reason for initial cracking.
Water (natural rain and irrigation)
Through the cracks, water can enter to the base and the subgrade, which will result in the structural damage
Vehicle loads and petroleum
The fuel spillage coming from the vehicles deteriorates the integrity of the pavement. This increase the softening point of the binder.
A parked vehicle has chances to leak gasoline or brake fluid that make the asphalt to liquefy. This makes the binder to separate from the rock that may create softer areas. Hence sudden treatment of oil spots on parking area should not be ignored.
Aging of Flexible Pavements
Aging is a life cycle deterioration of the pavement. This results in highly accelerated oxidation and cracks formation. Small deterioration determined at the initial stages will help in reducing the intensity of the aging effect. As the exposure to temperature and ultraviolet increases the rate of deterioration.
Types of Failures in Flexible Pavements due to Distress
The distress faced by the pavement can be of two types:
- Environmental distress, and
- Structural distress
Environmental Distress in Flexible Pavements
The outside influence that affects the pavement performance are categorized under environmental factors. These include snow, the chemicals, water and problems with aging.
These types of distress are observed from the top down. The remedy for such problems is a surface application. These include crack sealing, seal coating, chip seals, skin-parching. In certain situations, a hot mixed overlay is added to the surface as part of treatment.
Structural Distress in Flexible Pavements
The structural are categorized as the physical failures that are found on the pavement and the sub-base. These structural failures are occurred due to overloading, wet subgrade, frosting effect or lower standards of design. This kind of distress is found from bottom up.
The only remedy for these is removal and their replacement, mentioned as (R & R) of the area that is affected. Or repaving that includes total removal, milling, pulverizing the area and then paving back.
Types of Failures in Flexible Pavements due to Structural Distresses
Some of the structural distresses which can cause failures in flexible pavements are:
1. Alligator Cracking of Flexible Pavements
Alligator cracks are also called as map cracking. This is a fatigue failure caused in the asphalt concrete. A series of interconnected cracks are observed due to such distress.
The tensile stress is maximum at the asphalt surface (base). This is the position where the cracks are formed, i.e. the area with maximum tensile stress. A parallel of longitudinal cracks will propagate with time and reaches the surface.
Repeated loading and stress concentration will help the individual cracks to get connected. These will resemble as a chicken wire or similar to the alligator skin. This is termed as the alligator cracking. It is also known as the crocodile cracking.
These cracking is observed only in areas that have repeated traffic loading. Alligator cracking is one of the major structural distress. This distress is later accompanied by rutting. The figure-1 below shows alligator cracks formed in the pavement.
Fig.1: Alligator Cracks Formed in Flexible Pavements
2. Depressions in Flexible Pavements
There are certain areas in the pavement that are localized and have a lower elevation compared to the surrounding pavement level. These lowering are depressions found on the pavement. They are mainly noticed only when they are filled with water (After rain).
Depressions in flexible pavements are a very common distress found in parking lot construction as well as in overlays. These depressions can be caused either by the foundation soil settlement due to continuous loading or it can be formed during the construction.
There are different severity levels that are considered for the depression in the flexible pavement that is constructed for airfield purposes.
Fig.2: Depression Distress
3. Corrugations in Flexible Pavements
The corrugations are distress seen in the pavement at regular intervals in the form of ridges and valleys. These are usually less than 5 feet, along with the direction of the pavement.
The ridges form of corrugations will be perpendicular to the traffic direction. Unstable pavement plus traffic will create such distress. Where the traffic starts and stops, this distress are observed.
A form of plastic movement that is seen in the form of the wave is called as shoving distress. These are also observed perpendicular to the direction of the traffic.
In road surfaces where a portion of the same has broken away, cause a disruption by forming a pothole. These are also called as a kettle. In the Western United States, these are known as chuckhole.
The pavement fatigue is the main reason behind the formation of potholes. The occurrence of fatigue cracking will interlock to form alligator cracking. These chunks between the cracks formed in the pavement will become loose and will be picked out under continuous loading and stresses. This will leave a pothole on the pavement.
In cold temperatures, the water trapped in the pothole will carry out the freezing and thawing action that leads to additional stresses and crack propagation.
Once the pothole is formed, the distress grows resulting in the continuous removal of pavement chunks. Water entrapped will increase this rate of expansion of distress. The pothole can expand to several feet in width. They don’t develop too much in depth. The vehicle tires are damaged due to large potholes.
Fig.4: Potholes in Pavement
6. Rutting of Flexible Pavements
The depression formed in the surface is called the rutting. This is formed in the wheel path surface. This depression will make the other sides of the wheel to undergo uplift as shown in the figure-6. This pavement uplift is also called as shearing.
These ruts like depressions are evident after rain. Where these depressions would be filled with water. There are two types of rutting that can occur;
- Pavement Rutting
- Subgrade Rutting
Fig.5: Showing the Rutting Formation under Vehicular Load
Fig.6: Real-Time Formation of Ruts
7. Swelling of Flexible Pavements
These are distress that long and gradual wave. These can be ten feet long. The swelling distress is characterized by the upward bulge in the pavement surface. Surface cracking is the next series of distress that is seen after swelling.
The main reason behind swelling in flexible pavement is the frost action in the subgrade. Where frosting results in the swelling of the soil.
Fig.7: Swelling in Road Pavements
Types of Failures in Flexible Pavements due to Environmental Distresses
1. Bleeding in Flexible Pavements
The phenomenon of formation of a film of asphalt binder over the surface of the pavement surface is called as bleeding. The occurrence of bleeding will give a shiny glass like reflecting surface. The layer will have bubbles which are seen as blisters. The asphalt binder formed will be sticky in nature.
The filling of asphalt binder into the aggregate voids during hot weather conditions and their expansion in later situations will result in bleeding. As the process of bleeding cannot be reverted in cold temperatures, they remain on the top of the pavement as such. The bleeding can be caused due to the following factors:
- Excessive asphalt binder in the mix
- Excessive application of the binder during surface treatment
- Lower air void content – no adequate voids for the bitumen to penetrate
2. Block Cracking in Flexible Pavements
This is also called as thermal cracking. The cracking is happening in the form of blocks. These cracks are interconnected making the pavement to divide into rectangular pieces (almost rectangular).
The size of each rectangle may vary from one foot by one foot to ten foot by ten foot. This is spread over a wide pavement area. But these are observed in areas of no traffic. This is an after effect of environmental exposure, hence it is called thermal cracking. The temperature effects and aging are the possible reasons.
3. Bumps and Sags
Pavement surface that is localized, small in area that has undergone an upward displacement will be named as bumps. These are caused due to the instability factor of pavement.
Several factors contribute to bumping formation. They can be caused even due to buckling or the bulging of the concrete slabs. Areas, where an asphalt pavement is laid over a concrete pavement, observes such failures.
Another contribution to bumps are the frost heaves that creates bumps due to expansion. Oxidation will result in the spelling of the crack edges. Any plant roots growing under the pavement too can cause bumps in the pavement.
The sags are mainly caused due to the settlement or the displacement of the pavement surface. Sags are small, abrupt and localized. Large or long dips in the pavement can be created by the sags.
4. Edge Cracking in Flexible Pavements
In unconfined asphalt pavements, edge cracking is found to occur. During the compaction process of the pavement, the edges will start to yield, especially when there is no sort of confinement like curbs or edge barriers.
The edges will yield with age, undergo oxidation and becomes brittle. The edge cracking is observed in the shape of ‘C’ formed along the edges of the street, parking lot or the roads.
5. Joint Reflection Cracking
These are cracks that are observed in the flexible overlay over a rigid pavement. The rigid pavement joints that are an underlying experience these cracks.
The dislodgement of aggregate particles will result in the disintegration of the hot mixed asphalt progressively from the surface to downward direction. This failure is called as raveling. This dislodgement is the loss of bonding between the aggregate particles and the asphalt binder.
The aggregates are sometimes coated with dust particles that result in lack of bonding. This will make the aggregate to bind with the dust rather than the binder.
7. Cold Joints in Flexible Pavements
These are longitudinal joints which are formed in the asphalt pavement. This failure occurs when a hot mix asphalt is poured adjacent to an existing pavement. This kind of failure is mainly common in parking lots, inverted crowns and areas with lower traffic.
The difference in temperature and the plasticity variation will bring a different between the two layers. This will cause a longitudinal joint to occur between the asphalt mats that are laid.
The longitudinal joint possesses a lesser density compared to other pavements. These longitudinal joints called the cold joint, with time will let intrusion of water. It increases the roughness and hence limits the life of the pavement.
Longitudinal and Transverse Cracking Distress
This distress can be considered as either a structural or an environmental distress. The longitudinal cracks are formed parallel to the pavement alignment or the center line of the pavement.
Fig.8: Longitudinal Cracks in Asphalt Flexible Pavement
Fig.9: Transverse Cracking in Asphalt Flexible Pavement
This is a fatigue cracking. Here, the cracking occurs in the direction of traffic flow. The transverse cracking is formed perpendicular to the pavement centerline. This is caused as a thermal cracking.